Picosecond Laser Generation and Modification of Ag-TiO 2 Nanoparticles for Antibacterial Application

Transcription

1 Article I. Picosecond Laser Generation and Modification of Ag-TiO 2 Nanoparticles for Antibacterial Application A thesis submitted to The University of Manchester for the degree of Doctor of Philosophy (PhD) in the Faculty of Science and Engineering 2016 Abubaker Hassan Hamad School of Mechanical, Aerospace and Civil Engineering 1

2 Table of Contents Table of contents. 2 List of Figures 7 List of Tables 15 Abstract.16 Declaration Copyright statement 18 Acknowledgments List of abbreviations 20 List of publications Chapter 1. INTRODUCTION Overview Problem statement Research Aim and Objectives Scientific challenges Outline of Thesis Logical development of the research Chapter 2. Literature review Introduction to nanoparticles Overview Classification of nanoparticles Toxicity and health hazard of nanoparticles Properties of nanoparticles Nanoparticle production techniques Characterisation of nanoparticles Applications of nanoparticles Lasers, laser-material interaction and nanoparticles generation Introduction Laser and laser beam properties Ultra-short pulse lasers Ablation process Laser water interaction Laser material interaction Generation of nanoparticles in water with a laser beam

3 2.3. Effects of laser beam parameters and ablation environments on Ag and TiO 2 nanoparticle production Introduction Silver nanoparticles Titanium dioxide nanoparticles Ag-modified TiO Nanoparticle generation by laser ablation in liquid environments Antimicrobial and photocatalytic activities of Ag modified TiO 2 nanoparticles Introduction Antibacterial activity of Ag nanoparticles Photocatalytic activity of TiO 2 nanoparticles Antimicrobial activity of Ag-modified TiO Knowledge gap in the literature review Summary Chapter 3. Picosecond laser generation of Ag-TiO 2 nanoparticles with reduced energy gap by ablation in ice water and their antibacterial activities Introduction Experimental set-up Materials Nanoparticle production Sample preparation Characterisation Antibacterial function testing procedure Energy gap calculation Results The characteristics of Ag-TiO 2 nanoparticles Characteristics of TiO 2 nanoparticles Antibacterial Activity Discussion Summary Chapter 4. The Characteristics of Novel Bimodal Ag-TiO2 Nanoparticles Generated by Hybrid Laser-Ultrasonic Technique Introduction Experimental materials and procedures Materials Preparation of Ag-TiO 2 nanoparticles by hybrid ultrasonic sonication and laser ablation

4 Nanoparticle sample preparation for characterisation Characterisation Antibacterial Activity Test Procedure Results Ag-TiO 2 nanoparticle characteristics Ag-TiO 2 nanoparticles generation without ultrasonic vibration The characteristics of TiO 2 nanoparticles generated with ultrasonic vibration The characteristics of laser generated Ag nanoparticles Antibacterial characteristics Discussion Generation of Ag-TiO 2 cluster and TiO 2 Nanoparticles Ag and TiO 2 combination Antibacterial activity Summary Chapter 5. Generation of Silver Titania Nanoparticles from Ag-Ti alloy via Picosecond Laser Ablation and their Antibacterial Activities Introduction Experimental set-up Materials Ag-TiO 2 compound nanoparticle production Material Characterisation Antibacterial activity analysis Results and discussion Bulk Ti/Ag alloy characterisation Ag-TiO 2 compound nanoparticles Sedimentation and zeta-potentials of nanoparticles Antibacterial activity Summary Chapter 6. Sequential laser and ultrasonic wave generation of TiO2@Ag core-shell nanoparticles and their anti-bacterial properties Introduction Experimental set-up Materials Ag-TiO 2 compound and TiO core-shell nanoparticle production Material characterisation Antibacterial activity analysis

5 6.3. Results Ag-TiO 2 compound and TiO core-shell nanoparticle production XRD of Ag-TiO 2 compound and TiO core-shell nanoparticles Sonication of mixed and added Ag and TiO 2 nanoparticles Antibacterial activity Discussion Core-shell and compound nanoparticles Antibacterial activity Summary Chapter 7. Comparison of characteristics of Ag-TiO 2 nanoparticles produced from an Ag-Ti alloy using nano-, pico- and femtosecond lasers and their antibacterial activities Introduction Experimental materials and procedures Materials Ag-TiO 2 nanoparticles production methods Sample preparation for nanoparticle characterisation Characterisation Antibacterial test procedure Results and discussion Ag-TiO 2 nanoparticle generation by nano-, pico and femtosecond laser ablation XRD of Ag-TiO 2 compound nanoparticles FTIR spectra of Ag-TiO 2 compound nanoparticles Raman shift of Ag-TiO 2 compound nanoparticles XPS analysis of Ag-TiO 2 chemical structures Antibacterial activity Summary Chapter 8. A comparison of the characteristics of nanosecond, picosecond and femtosecond laser generated Ag, TiO2 and Au nanoparticles in deionised water Introduction Experimental Materials and Procedure Materials Lasers Nanoparticle production procedure Material characterisation methods

6 8.3. Results and discussion Optical reflectivity of Ag, Ti and Au target materials Laser ablation material removal rate in deionised water as a function of laser fluence and water level Comparison between nano- pico- and femtosecond laser generation of nanoparticles Morphology of the nanoparticles Understanding the absorption spectra of colloidal nanoparticles Ablation rates Size distribution and average nanoparticle sizes Colour of colloidal nanoparticles Ablation mechanism by the nano-, pico- and femtosecond lasers Summary Chapter 9. Comparison of characteristics of selected metallic and metal oxide nanoparticles produced by picosecond laser ablation at 532 nm and 1064 nm wavelengths Introduction Experimental Materials and Procedure Materials Nanoparticle Production Procedure Material Characterisation and Sample Preparation Procedure Results Au Nanoparticles Ag-TiO 2 Compound Nanoparticles TiO 2 Nanoparticles Iron Oxide Nanoparticles ZnO Nanoparticles Ag Nanoparticles Discussion Effects of Wavelengths on the Size of the Nanoparticles Effects of Wavelengths on the Crystallinity of Metal Oxide Nanoparticles Summary Chapter 10. A Single-step Process of Generating Hollow and Porous TiO2 Nanoparticles by Picosecond Laser Ablation in Deionised Water Introduction Experimental set-up Nanoparticle production Sample preparation

7 Characterisation Results and discussion Generation of hollow TiO 2 nanoparticles Effect of laser power on the production of hollow/ porous TiO 2 nanoparticles Effect of laser power on the optical properties of hollow/porous TiO 2 nanoparticles XRD of hollow/porous TiO 2 nanoparticles Summary Chapter 11. Conclusions Conclusions Chapter 12. Future Work Future work recommendations List of Figures Figure 2-1 Comparison of nano and micro length scales and 32 biological components [24]. Figure 2-2 The most common nanoparticle categories [28] 33 Figure 2-3 3, 2, 1 and 0 dimensions of materials and the density of 35 their states as a function of energy [35] Figure 2-4 Classification of nanomaterials on the basis of morphology 36 [24] Figure 2-5 Classification of nanomaterials on the basis of uniformity 36 and agglomeration [24]. Figure 2-6 Variation in melting point of gold nanoparticles with the 41 particles diameter [24] Figure 2-7 Methods of nanoparticle generation: break down (top 43 down) and build up (bottom-up) a [75] and b [76] Figure 2-8 Top down method for production nanoparticles [76] 44 Figure 2-9 Bottom up process for the generation of nanoparticles [76] 45 Figure 2-10 The phases and reactions of the sol-gel method [76] 47 Figure 2-11 Effect of water height on the ablation rate [92] 54 Figure 2-12 Absorption process within matter as a function of 56 increasing power [90] Figure 2-13 The interaction processes of ns, ps and fs laser pulses with 57 materials as a function of intensity [95] Figure 2-14 The physical stages of TiO nanoparticle generation [87] 58 Figure 2-15 Absorption spectra of Ag nanoparticles [51] 60 Figure 2-16 X-ray diffraction spectrum of pulsed laser deposition of a thin film of TiO 2 and the three crystal phases of TiO 2 (anatase (A), rutile (R) and brookite (B)) [106] 63 Figure 2-17 Band gap dependence on doping agent concentration [109] 65 Figure 2-18 Absorption spectra of TiO 2 NPs and Ag-deposited TiO

8 particles in water [124] Figure 2-19 The mechanism of the photocatalytic properties of TiO 2 77 [159] Figure 2-20 The mechanism of antimicrobial activity of Ag-TiO 2 79 nanocomposites (a) [115] (b) [168]. Figure 2-21 Antimicrobial activity of Ag NPs against S. aureus and E. 80 coli (A and B) and TiO 2 NPs against S. epidermidis and K. pneumonia (C and D) [136] Figure 2-22 Antibacterial activity of controlled slides and Ag-TiO 2 84 prepared by sol-gel/laser route and furnace sintering route against E. coli under UV light irradiation using the drop test method [126]. Figure 3-1 Experimental set-up to generate nanoparticles in ice 96 (frozen deionised water) using a picosecond laser; = 1064 nm, f = 200 khz, = 10 ps, and v = 250 mm/s. Figure 3-2 (a) Absorption spectra of Ag-TiO 2 colloidal nanoparticles 100 generated in frozen deionised water (ice) and unfrozen deionised water using a picosecond laser; = 1064 nm, f = 200 khz, v = 250 mm/s, = 10 ps, spot size = 125 µm, E pulse = µj and F laser = J/cm 2. (b) The bottles of Ag-TiO 2 nanoparticles generated in ice (righthand bottle) and deionised water (left-hand bottle). (c) Indirect and (d) direct band gaps of the Ag-TiO 2 nanoparticles generated in ice and deionised water. Figure 3-3 TEM images of Ag-TiO 2 nanoparticles produced in ice 101 using the picosecond laser (a,c,d,e). A histogram and lognormal size distribution of produced Ag-TiO 2 nanoparticles (b). Figure 3-4 HAADF and EDS images (line profile) of the Ag-TiO nanoparticles generated in frozen deionised water using a picosecond laser. Figure 3-5 EDS image of Ag-TiO 2 nanoparticles that shows their 103 chemical components, which include C, Cu, Ag, Ti and O. Both Cu and C are from the grid and grid coating respectively. The presence of high amounts of Ag, Ti, and O indicates the formation of Ag-TiO 2 nanoparticles in the solution. Figure 3-6 (a) Absorption spectra of TiO 2 colloidal nanoparticles 104 generated in both deionised and frozen deionised water (ice) using a picosecond laser; = 1064 nm, f = 200 khz, v = 250 mm/s, = 10 ps, spot size = 125 µm, E pulse = µj, F laser = J/cm 2 and t= 10 min. (b) The real bottles of the TiO 2 nanoparticles generated in both media. (c) Indirect and (d) Direct band gaps of the TiO 2 nanoparticles generated in ice and deionised water. Figure 3-7 (a) Number of surviving E. coli bacteria as a function of 105 concentrations of the Ag-TiO 2 and Ag nanoparticles. (b) Histogram of relative the antibacterial activity of Ag-TiO 2, TiO 2 and Ag nanoparticles at 12.5 µg/ml concentration. The test were done a day after preparation of nanoparticles. (Note; the control value, i.e. no nanoparticles in Figure (b) is 1). They were tested under standard room light. Figure 4-1 Experimental set-up for generation of Ag-TiO

9 nanoparticles in deionised water in an ultrasonic cleaning bath. Figure 4-2 The absorption spectra of (a) Ag, TiO 2 and (b) Ag-TiO 2 colloidal nanoparticles generated in deionised water in an ultrasonic tank and without ultrasonic tank by picosecond laser ablation in deionised water (P= 9.12 W, f= 200 khz and v= 250 mm/s). In the Ag-TiO 2 suspension produced in the ultrasonic tank, the amount of Ag and TiO 2 nanoparticles were 0.5 mg and 0.6 mg respectively. The ablation time for Ag, TiO 2 and Ag-TiO 2 with ultrasonic waves was 16 min, and for Ag-TiO 2 without ultrasonic waves it was 10 min. Figure 4-3 (a-d) TEM images of Ag-TiO 2 nanoparticles generated in deionised water by the picosecond laser (P = 9.12 W, f = 200 khz, v = 250 mm/s and t = 16 min.). The production process was carried out in a tank of ultrasonic cleaner with a frequency 49 khz. The ablation rate of Ag-TiO 2 nanoparticles was mg/min with a Ag:TiO 2 ratio of 1:1.2. Figure 3-e is the lognormal size distribution of Ag- TiO 2 nanoparticles. Figure 4-4 High-Angle Annular Dark-Field Microscope (HAADF) images of the Ag-TiO 2 nanoparticles. Figure 4-5 EDS - Line profile images of the Ag-TiO 2 nanoparticles synthesised by picosecond laser in deionised water supporting ultrasound waves in an ultrasonic cleaner. Figure 4-6 X-Ray Diffraction (XRD) spectrum of Ag-TiO 2 nanoparticles produced by picosecond laser in deionised water in ultrasonic vibration. Figure 4-7 TEM images of Ag-TiO 2 nanoparticles produced in deionised water using picosecond laser without using ultrasonic waves. (Wavelength 1064 nm, power 9.12 W, frequency 200 khz and scan speed 250 mm/s and t = 10 min). Figure 4-8: (a and b) TEM images of TiO 2 nanoparticles generated in deionised water in an ultrasonic cleaner tank by picosecond laser (P = 9.12 W, f = 200 khz, v = 250 mm/s and t = 15 min.). The quantity of TiO 2 nanoparticles generated in the suspension is 0.8 mg and the concentration is 53.3 µg/ml. The ablation rate of TiO 2 NPs is mg/min. (0.8 mg/15 min). (c) Histogram of the size distribution of TiO 2 nanoparticles. Figure 4-9 (a and b) TEM images of Ag nanoparticles generated by picosecond laser in deionised water without ultrasonic vibration. Laser parameters; P = 9.12 W, f = 200 khz, v = 250 mm/s. (c) Histogram of the size distribution of Ag nanoparticles. Figure 4-10 Antibacterial activity of Ag-TiO 2 nanoparticles in comparison with control sample. Equal amount of E. coli were cultured with (Ag-TiO 2 (ultrasonic wave based generation), Ag and TiO 2 ) nanoparticles or without (control) nanoparticles in LB broth for 6 hours and 10 µl of the broth culture was plated on to LB agar plate for colony formation after overnight incubation at 37 C overnight. The number

10 of E. coli colonies represents the survived E. coli after culturing with or without nanoparticles which negatively correlate to the antibacterial effect of the nanoparticles. Figure 4-11 Relationship between the number of surviving E. coli 125 bacteria as a function of the concentration of Ag-TiO 2 (US) (ultrasonic) and Ag nanoparticles generated by picosecond laser in deionised water with ultrasonic wave assisted. The antibacterial test was carried out under normal light. Figure 4-12 Image of plasma plume and nanoparticles dispersing in 127 deionised water during picosecond laser ablation of a target (without using ultrasonic waves). The photo is taken after recording the ablation process by a normal camera. Figure 5-1 Experimental set-up for generation of nanoparticles in 137 deionised water by picosecond laser. Figure 5-2 Reflectivity of the Ti/Ag alloy plate. 139 Figure 5-3 XRD of Ti/Ag alloy bulk sample. 140 Figure 5-4 XRF of the Ag/Ti alloy. 140 Figure 5-5 The absorption spectra of the Ag-TiO 2 compound 142 nanoparticles produced by picosecond laser in deionised water (a). Indirect band gap energy of the Ag-TiO 2 compound nanoparticles (b). Figure 5-6 TEM images of the Ag-TiO 2 compound nanoparticles 143 generated by picosecond laser in deionised water (a,b,c and d). Histogram of the nanoparticles lognormal size distribution (e). The ablation rate of the Ag-TiO 2 nanoparticles is mg/min. Figure 5-7 (a,b,c and d) HAADF-STEM and EDS images of the Ag- 144 TiO 2 compound nanoparticles. Figure 5-8 Line profile spectrum (a) and EDS images (b) of the Ag- 145 TiO 2 compound nanoparticles. Figure 5-9 The atomic percentage (at.%) ratio of Ag:Ti:O chemical 146 elements of a spectrum of some of the nanoparticles (or points). Figure 5-10 XRD image of the Ag-TiO 2 compound nanoparticles. 146 Figure 5-11 XPS images of the Ag-TiO 2 compound nanoparticle. Peakfitting 148 spectra at high resolution of Ag 3d (q), Ti 2p spectra (b) and O 1s spectra (c) of the Ag-TiO 2 compound nanoparticles. Figure 5-12 Sedimentation of Ag-TiO 2 compound nanoparticles (a), and 151 Ag nanoparticles (b) during time. (c) The average sizes of the nanoparticles as a function of time. The photographs show the bottles which contain the colloidal nanoparticles. Figure 5-13 The absorption spectra of Ag (a) and Ag-TiO 2 (b) 152 nanoparticles on the day of preparation as well as after 15 and 30 days. Figure 5-14 Antibacterial activity of the Ag and Ag-TiO 2 compound 154 nanoparticles at 20 µg/ml (a) and 25 µg/ml (b) compared with control sample (c) under standard room light and dark conditions after one day of generation. Figure 5-15 Number of survived E. coli colonies as a function of incubation time for Ag-TiO 2, Ag and control under standard room light. The concentration of nanoparticles was 20 µg/ml

11 Figure 5-16 Antibacterial activity of Ag-TiO 2 compound and Ag 155 nanoparticles as a function of concentration. Figure 5-17 (a) Agar plate shows antibacterial activity of TiO nanoparticles (20 µg/ml) against E. coli bacteria under both standard room light and dark conditions. (b) Antibacterial activity of laser generated Ag, Ag-TiO 2 and TiO 2 nanoparticles under standard room light at the same concentration (20 µg/ml) and incubation time (6 hours). Figure 5-18 Agar plate shows antibacterial activity of chemically 157 produced commercial Ag nanoparticles in sodium citrate (20 µg/ml concentration and about 35 nm size in diameter) against E. coli bacteria under both standard laboratory light and dark conditions. It was tested about two months after their production. Figure 6-1 Experimental set-up for (a) production of Ag-TiO compound nanoparticles by picosecond laser ablation in deionised water and (b) to produce the TiO core-shell nanoparticles by an ultrasonic cleaner (Ultraschall- Reiniger, Emmi 5 with a pulse repetition rate of 49 khz). Figure 6-2 (a) Sonication of Ag-mixed TiO 2 nanoparticles and (b) TiO added to Ag nanoparticles. Ag and TiO 2 nanoparticles were produced by picosecond laser in deionised water under the same laser and experimental conditions. Figure 6-3 (a and b) TEM images of TiO core-shell nanoparticles 168 produced by ultrasonic vibration (Ultraschall-Reiniger, Emmi 5 with a pulse repetition rate of 49 khz) from Ag-TiO 2 compound nanoparticles generated by picosecond laser in deionised water. (c and d) TEM images of the Ag-TiO 2 compound nanoparticles fabricated by a picosecond laser in deionised water. A 400 W Edgewave picosecond laser was used to generate the nanoparticles with the laser beam parameters: wavelength = 1064 nm, laser power P = 9.12 W, frequency f = 200 khz, pulse duration = 10 ps, spot size D = 125 µm, scan speed v = 250 mm/s, laser pulse energy E pulse = 45.6 µj and laser fluence F laser = J/cm 2. Figure 6-4 (a) Absorption spectra of Ag-TiO 2 compound and TiO 169 core-shell nanoparticles. The inset figure shows the match of the two spectra (b) Histogram of the size distribution of Ag-TiO 2 compound and TiO core-shell nanoparticles. Figure 6-5 TEM images of the Ag (a) and TiO 2 (b) nanoparticles and 170 their optical absorption spectra (c) produced via picosecond laser ablation in deionised water. ( = 1064 nm, f = 200 khz, v = 250 mm/s). Figure 6-6 HAADF images of TiO core-shell nanoparticles. 170 Figure 6-7 EDS line scanning images of the TiO core-shell 171 nanoparticles. Figure 6-8 HAADF image (a) and EDS image (b) of the TiO 171 core-shell nanoparticles. Figure 6-9 XRD images of the Ag-TiO 2 compound and TiO coreshell 172 nanoparticles. Figure 6-10 (a and b) TEM images of the Ag-TiO 2 nanoparticles produced by ultrasonic vibration after mixing the colloidal

12 Ag and TiO 2 nanoparticles. (c and d) TEM images of the Ag-TiO 2 nanoparticles produced by ultrasonic vibration after the addition of the colloidal Ag to TiO 2 nanoparticles during ultrasonic vibration for 6 minutes. Figure 6-11 (a) Antibacterial activity of TiO core-shell and Ag-TiO compound nanoparticles against E. coli bacteria at 20 µg/ml concentration and 6 hours incubation, under standard laboratory light conditions. (b) Agar plates with E. coli bacteria colonies treated with core-shell and compound nanoparticles. (c) Agar plate with E. coli bacteria colonies on control sample (dh 2 O). Figure 6-12 Antibacterial activity of the laser-generated TiO 2 and Ag 174 nanoparticles under standard room light. They were produced via picosecond laser ablation in deionised water. Figure 7-1 Experimental set-up. 185 Figure 7-2 TEM images of nanoparticles generated by nanosecond 188 (a), picosecond (b) and femtosecond (c) lasers in deionised water. Histogram of size distribution of the Ag-TiO 2 compound nanoparticles (d). Photograph of bottles of the Ag-TiO 2 nanoparticles (e). Figure 7-3 Absorption spectra of the Ag-TiO 2 compound nanoparticles 189 (a). Energy band gaps of the Ag-TiO 2 nanoparticles (b). Figure 7-4 XRD of Ag-TiO 2 compound nanoparticles produced by 191 nanosecond, picosecond and femtosecond laser in deionised water. Figure 7-5 FTIR spectra for the Ag-TiO 2 nanoparticles produced by 192 nanosecond, picosecond and femtosecond lasers. Figure 7-6 Raman spectra for the Ag-TiO 2 nanoparticles produced by 194 nanosecond, picosecond and femtosecond lasers. Figure 7-7 XPS images of the Ag-TiO 2 compound nanoparticles 195 produced by nano-, pico- and femtosecond laser in deionised water. Peak-fitting spectra at high resolution of Ag 3d, Ti 2p spectra and O 1s spectra of the Ag-TiO 2 compound nanoparticles. Figure 7-8 Antibacterial activity of Ag-TiO 2 compound nanoparticles 197 produced via nanosecond, picosecond and femtosecond laser in deionised water. The antibacterial activity of nanoparticles was tested against E. coli bacteria under a standard room light after one day of preparation. The concentration of nanoparticles was 20 µg/ml. Figure 7-9 TEM images of Ag-TiO 2 nanoparticles produced by 198 nanosecond laser in deionised water. Figure 8-1 Experimental set-up for nanoparticle production. 205 Figure 8-2 Optical reflectivity of Ag, Ti and Au samples as a function 206 of wavelength. Figure 8-3 The ablation rate of both Ag and Ti targets as a function of laser fluence at a fixed water level at 2 mm above the target material surface (a), and as a function of water level at a fixed laser fluence ( mj, 0.22 J/cm 2 ) (b). These tests were carried out using a picosecond laser at = 1064 nm, f = 200 khz, v = 30 mm/s, t = 1/2 h and a laser spot size = 125 µm. 207 Figure 8-4 Absorption spectra and size distribution of Ag nanoparticles

13 Figure 8-5 Figure 8-6 Figure 8-7 Figure 8-8 Figure 8-9 Figure 9-1 Figure 9-2 Figure 9-3 Figure 9-4 Figure 9-5 Figure 9-6 Figure 9-7 (a and b), TiO 2 nanoparticles (c and d) and Au nanoparticles (e and f) produced by nanosecond, picosecond and femtosecond laser in deionised water. TEM images of Ag nanoparticles generated in deionised water using nanosecond (a), picosecond (b) and femtosecond (c) lasers. TEM images of TiO 2 nanoparticles generated in deionised water using nanosecond (d), picosecond (e) and femtosecond (f) lasers. TEM images of Au nanoparticles generated in deionised water using nanosecond (g), picosecond (h) and femtosecond (i) lasers. Shifting SPR of Ag and Au nanoparticles and shifting tangent line of TiO 2 absorption spectra (a). Shift (decrease) of indirect band gap energy of TiO 2 nanoparticles (b) as a function of nanosecond, picosecond and femtosecond laser. Ablation rate of Ag, TiO 2 and Au nanoparticles produced in deionised water by NSL, PSL and FSL at laser fluence 12.2 J/cm 2, 0.37 J/cm 2 and 28.3 J/cm 2 respectively and at fixed scan speed 250 mm/s. Size distribution of Ag, TiO 2 and Au nanoparticles generated by NSL (a) PSL (b) and FSL (c) in deionised water. Average size of Ag, TiO 2 and Au nanoparticles as a function of nanosecond, picosecond and femtosecond laser produced in deionised water (d). Colloidal Ag, TiO 2 and Au nanoparticles produced by nanosecond, picosecond and femtosecond lasers in deionised water. Scheme of the picosecond laser with dual wavelength (a), experimental set-up for nanoparticle production (b) and ablation area on a sample at 532 nm and 1064 nm wavelengths (c). Optical absorption spectra (a), histogram of the size distribution (b) and TEM images (c and d) of the Au nanoparticles produced by a picosecond laser at 532 nm and 1064 nm wavelengths in deionised water. Optical absorption spectra (a), histogram of the size distribution (b) and TEM images (c and d) of the Ag-TiO 2 nanoparticles produced by a picosecond laser at 532 nm and 1064 nm wavelengths in deionised water. Optical absorption spectra (a), histogram of the size distribution (b) and TEM images (c and d) of the TiO 2 nanoparticles produced by a picosecond laser at 532 nm and 1064 nm wavelengths in deionised water. XRD images of TiO 2 nanoparticles produced by picosecond laser in deionised water with different wavelengths; (532 nm and 1064 nm). Optical absorption spectra (a), histogram of the size distribution (b) and TEM images (c and d) of the iron oxide nanoparticles produced by a picosecond laser at 532 nm and 1064 nm wavelengths in deionised water. XRD images of iron oxide nanoparticles produced by picosecond laser in deionised water with different

14 Figure 9-8 Figure 9-9 Figure 9-10 Figure 9-11 Figure 9-12 Figure 10-1 Figure 10-2 Figure 10-3 Figure 10-4 Figure 10-5 Figure 10-6 Figure 10-7 Figure 10-8 wavelengths (532 nm and 1064 nm). Optical absorption spectra (a), histogram of the size distribution (b) and TEM images (c and d) of the ZnO nanoparticles produced by a picosecond laser at 532 nm and 1064 nm wavelengths in deionised water. XRD images of ZnO nanoparticles produced by picosecond laser in deionised water with different wavelengths (532 nm and 1064 nm). Optical absorption spectra (a), histogram of the size distribution (b) and TEM images (c) of the Ag nanoparticles produced by a picosecond laser at 1064 nm wavelength in deionised water. Laser power loss at 532 nm and 1064 nm wavelengths due to water level above the sample during generation of nanoparticles. Increasing of focal length of the lens of a laser tool as a function of height above water level (a). Real and virtual depth of the target material (b). Experimental set-up to produce TiO 2 nanoparticles in deionised water via a picosecond laser; = 1064 nm, f = 200 khz, = 10 ps, and v = 250 mm/s. (a and b) TEM images of hollow TiO 2 nanoparticles produced by picosecond laser in deionised water. ( = 1064 nm, f = 200 khz, v = 250 mm/s). (c) Histogram of the size distribution of the TiO 2 nanoparticles measured from 615 TEM images of nanoparticles. TEM image of TiO 2 nanoparticles show more than one holes produced in the nanoparticles. Schematic diagram of hollow/porous TiO 2 nanoparticles formation. HAADF-STEM and EDS images of the TiO 2 nanoparticles showing the production of hollow TiO 2 nanoparticles. TEM images of TiO 2 nanoparticles produced by picosecond laser in deionised water with different laser powers; P = 3.35 W (a), P = 5.95 W (b) and P = 9.12 W (c). Figure (d) shows the approximate number of hollow TiO 2 nanoparticles in 100 TiO 2 nanoparticles produced at different laser powers by picosecond laser in deionised water. Optical absorption spectra (a), indirect (b) and direct (c) band gap transition of the TiO 2 nanoparticles produced by picosecond laser in deionised water at different laser powers. XRD images of hollow/porous TiO 2 nanoparticles produced by picosecond laser in deionised water with different laser powers; P = 3.35 W (a), P = 5.95 W (b) and P = 9.12 W (c)

15 List of Tables Table 2-1 Advantages and disadvantages of processing methods for 49 nanomaterials [80]. Table 2-2 The relation between energy and average size of obtained 72 Au NPs [141]. Table 2-3 Ag-modified TiO 2 nanoparticles or films for antimicrobial and 87 photocatalytic applications Table 3-1 The ablation information and E g of Ag-TiO 2 generated in 100 deionised and frozen deionised water. Table 3-2 The ablation rate and E g of pure TiO 2 generated by 104 picosecond laser in deionised and ice water. Table W Edgewave picosecond laser beam parameters used 114 to produce Ag-TiO 2 cluster nanoparticles. Table 4-2 Optimal laser parameters at constant laser power P(60%) = W and different scan speeds. Table 4-3 Optimal laser beam parameters at constant scan seed ( mm/s) and different laser powers. Table 7-1 Types of laser beam and parameters used to produce Ag- 186 TiO 2 compound nanoparticles. Table 8-1 Characteristics and the process parameters of nanosecond, 204 picosecond and femtosecond lasers used in this investigation. Table 8-2 Laser beam parameters 208 Table 8-3 Results of Ag, TiO 2 and Au nanoparticles produced by 220 nanosecond, picosecond and femtosecond lasers at a fixed scan speed of 250 mm/s. Table 9-1 Characteristics and process parameters of the picosecond 229 laser at both wavelengths (532 nm and 1064 nm) Table 9-2 Average size of Au, Ag-TiO 2, Ag, TiO 2, Iron oxide and ZnO 242 nanoparticles produced by picosecond laser at 532 nm and 1064 nm. Table 10-1 Picosecond laser beam parameters used to produce TiO 2 nanoparticles in deionised water

16 Abstract The University of Manchester Abubaker Hamad - PhD Mechanical Engineering 2016 Laser Generation and Modification of Nanoparticles for Antibacterial Application Despite the wide success of antibiotics against infectious diseases, bacterial infections continue to threaten mankind and animals. A major concern is that certain bacteria have developed resistance to drugs. This has been associated with the widespread use of antibiotics. Recent investigations have demonstrated that certain type of nanoparticles (NPs) can kill bacteria without causing bacterial resistance. A number of methods can be used to generate nanoparticles. One method is to use a laser. The expected advantages of laser generation of nanoparticles include high purity of the nanoparticles, special surface charge characteristics and ease of preparation in comparison with other methods particularly chemical methods. Although laser production of nanoparticles has been investigated for several decades, there is still a considerable knowledge gap. This includes the need to control nanoparticle properties such as energy gap, shape, size, surface charges, as well combination of nanoparticles. Ag nanoparticles are widely used for antibacterial purposes such as medical dressings. However, they are also highly toxic to human cells. There is a great interest to develop next generation antibacterial nanoparticles that are as effective as Ag nanoparticles for antibacterial functions, while having less toxicity to human cells. The purpose of the present study was to generate and modify novel Ag-TiO 2 nanoparticles by laser ablation in liquid and ice media and to understand their characteristics including antibacterial activities, energy gaps, and particle size distributions. Both Ag and TiO 2 nanoparticles are antibacterial. TiO 2 nanoparticles only work when exposed to ultra violet (UV) light due to their high energy gap. In this investigation, silver-titania nanoparticles (Ag-TiO 2 NPs) were synthesised by picosecond (ps) laser ablation in ice water, and bimodal Ag-TiO 2 NPs were generated in deionised water by hybrid ultrasonic vibration and ps laser ablation of Ag and Ti bulk targets. In addition, a bulk Ti/Ag alloy was used, to produce Ag-TiO 2 compound NPs using ps laser ablation in deionised water, and TiO core-shell NPs were produced via ultrasonic treatment of the compound NPs. The antibacterial activities of these new NPs were evaluated in comparison to those of standard Ag and TiO 2 NPs. The toxicity study of these nanoparticles to human cells was carried out by another PhD student and the results were not reported in this thesis. The present PhD project was focused on nanoparticle production and their basic characteristics. The characteristics of Ag-TiO 2 NPs were compared in the case of generation by nanosecond (ns), picosecond (ps) and femtosecond (fs) pulse lasers in deionised water. In this work, to show the effects of the different laser wavelengths (532 nm and 1064 nm) of a picosecond laser on the characteristics of Au, Ag, Ag-TiO 2, TiO 2, ZnO and iron oxide NPs, a comparison study was reported. The nanoparticle generation was carried out in deionised water. Finally, production of hollow and porous TiO 2 NPs in a single step via high-repetition rate ps laser ablation in deionised water is presented. The significance of this work is the development and demonstration of a number of new ways of producing nanoparticles. The benefits of these new nanoparticle manufacturing methods include a decrease in the energy gap, Eg, of TiO 2 NPs by using Ag NPs, effective combination of Ag and TiO 2 NPs and extension of optical absorption spectra of the Ag-TiO 2 NPs. Reduction of energy band gap of TiO 2 nanoparticles is important to improve their photoactivation because a smaller E g allows longer light wavelength, such as visible light, instead of UV light to initiate photoactivation. In addition, shifting the spectra to a longer wavelength would also lead to the enhancement of photoactivation of TiO 2 and Ag-TiO 2 nanoparticles. The reduction of the energy gap of nanoparticles is a major contribution from this research. 16

17 Declaration I hereby declare that no portion of the work referred to in this thesis has been submitted in support of an application for another degree or qualification of this or any other university or other institution of learning. Abubaker Hamad

18 COPYRIGHT STATEMENT I. The author of this thesis (including any appendices and/or schedules to this thesis) owns certain copyright or related rights in it (the Copyright ) and he has given The University of Manchester certain rights to use such Copyright, including for administrative purposes. II. Copies of this thesis, either in full or in extracts and whether in hard or electronic copy, may be made only in accordance with the Copyright, Designs and Patents Act 1988 (as amended) and regulations issued under it or, where appropriate, in accordance with licensing agreements which the University has from time to time. This page must form part of any such copies made. III. The ownership of certain Copyright, patents, designs, trademarks and other intellectual property (the Intellectual Property ) and any reproductions of copyright works in the thesis, for example graphs and tables ( Reproductions ), which may be described in this thesis, may not be owned by the author and may be owned by third parties. Such Intellectual Property and Reproductions cannot and must not be made available for use without written permission of the owner(s) of the relevant Intellectual Property and/or Reproductions. IV. Further information on the conditions under which disclosure, publication and commercialisation of this thesis, the Copyright and any Intellectual Property and/or Reproductions described in it may take place is available in the University IP Policy (see in any relevant Thesis restriction declarations deposited in the University Library, The University Library s regulations (see and in The University s Policy on presentation of Theses. 18

19 Acknowledgements My endless thanks to Allah Almighty for helping me to complete this work. I would also like to thank my supervisor Professor Lin Li and cosupervisor Dr Zhu Liu for being such enthusiastic and involved supervisors throughout the whole years of my study. Special thanks go to Dr Tao Wang from whom I have learned new science and who has introduced me to microbiology. Many people have helped me throughout the PhD years: the first whom I must thank is Dr David Whitehead who trained me on most of the instruments which I used in the laboratories as well as Mr. Daniel Wilson, Dr Wei Guo, Dr Alan Harvey and Ms. Xiang Li Zhong. I appreciate the help I received from my colleagues and friends (Omonigho Otanocha, Ahmad Syamaizar Bin Ahmad Sabli, Salwan Al-Saigh, Adel Salama, Hoshang Farouk and Zyad Nawaf Haji) who have helped me considerably, especially the first two people. My special thanks go to Dr Christopher Muryn who helped me with the several material characterization tools; I really appreciate his efforts. I thank my sponsor, the Ministry of Higher Education and Scientific Research Kurdistan Regional Government (MHES-KRG), for giving me this precious opportunity to learn via KRG-HCDP Scholarship program. In addition, many thanks go to all the HCDP staff. Finally, I would like to thank my wife for her moral support, love and stoicism during all years of sedulous efforts committed to this work. 19

20 List of Abbreviations AFM Atomic Force Microscope Ag Silver B. Subtilis Bacillus Subtilis CTAB Cetyltrimethylammonium Bromide CW Continuous Wave E. coli Escherichia Coli EDX Energy-dispersive x-ray spectroscopy FEG-SEM Field Emission Gun-Scanning Electron Microscope fs Femtosecond IC Indigo Carmine LB Lysogeny Broth MG Malachite Green MIC Minimal Inhibitory Concentration MO Methyl Orange MR Methyl Red NPs Nanoparticles NP Nanoparticle ns Nanosecond ps Picosecond PVP Polyvinylpyrrolidone RhB Rhodamine B SDS Sodium Dodecyl Sulfate SEM Scanning Electron Microscope TEM Transmission Electron Microscope Ti Titanium TiO 2 Titanium Dioxide UV Ultraviolet XRD X-Ray Diffraction E g Energy band gap XPS X-ray photoelectron spectroscopy XRD X-ray diffraction 20

21 List of Publications Journal publications Published papers 1. Hamad, A., Li, L., Liu, Z., Zhong, X.L. and Wang, T., Picosecond laser generation of Ag TiO 2 nanoparticles with reduced energy gap by ablation in ice water and their antibacterial activities. Applied Physics A, (4): p Hamad, A., Li, L., Liu, Z., Zhong, X.L., Burke G. and Wang T., The characteristics of novel bimodal Ag TiO 2 nanoparticles generated by hybrid laser-ultrasonic technique. Applied Physics A, (4): p Hamad, A., Li, L., Liu, Z., Zhong, X.L., Liu, H., and Wang, T., Generation of silver titania nanoparticles from an Ag Ti alloy via picosecond laser ablation and their antibacterial activities. RSC Advances, (89): p Hamad, A., Li, L., Liu, Z., Zhong, X.L., and Wang, T., Sequential laser and ultrasonic wave generation of TiO Ag core-shell nanoparticles and their anti-bacterial properties. Lasers in medical science, (2): p Hamad, A., Li, L., and Liu, Z., A comparison of the characteristics of nanosecond, picosecond and femtosecond lasers generated Ag, TiO 2 and Au nanoparticles in deionised water. Applied Physics A, (4): p Hamad, A., Li, L., and Liu, Z., Comparison of characteristics of selected metallic and metal oxide nanoparticles produced by picosecond laser ablation at 532 and 1064 nm wavelengths. Applied Physics A, (10): p Hamad A., Li, L., Liu, Z., and Zhong, X.L., A Single-step Process of Generating Hollow and Porous TiO 2 Nanoparticles by Picosecond Laser Ablation in Deionised Water. Journal of Laser Micro/Nanoengineering, (3): p Submitted 1. Hamad A., Li, L., Liu, Z., Liu, H., and Wang T., Comparison of characteristics of Ag-TiO 2 nanoparticles produced using nano-, pico- and femtosecond lasers and their antibacterial activities,

22 Chapter 1: Introduction 1 Chapter 1. INTRODUCTION Overview The use of lasers has increasingly pervaded many aspects of our lives, especially in recent decades. Perhaps one of the most important fields using lasers is that of medicine. Preparation of nanoparticles in solutions, particularly in deionised water, has received much attention from researchers because for the synthesis of nanoparticles that can be free from any contamination. Silver nanoparticles (Ag NPs) are an excellent type of the nanoparticles for a wide range of applications in various fields of industry and medicine due to their strong antimicrobial activity [1]. In addition, titanium dioxide nanoparticles (TiO 2 NPs) have considerable applications in biomedicine [2] due to their significant photocatalytic effect to kill bacteria [3-4] and photodegradation effect to degrade the organic matters under ultraviolet (UV) and visible light irradiation [5-6]. The present study provides experimental evidence to validate a hypothesis that a combination or modification of TiO 2 nanoparticles with Ag nanoparticles would produce Ag-TiO 2 nanoparticles with bactericidal activity in both light and dark conditions [7]. 22

23 Chapter 1: Introduction 1.2. Problem statement Due to the widespread use of antibiotics in the past, antibiotic resistance has become a serious global health issue. Dangerous, antibiotic resistant superbugs have developed, spreading not only among patients in hospitals, but also through food and even drinking water. Antimicrobial nanoparticles could be an alternative material to the conventional antibiotics. Silver nanoparticles have been known as an important material for antibacterial application due to their unique properties and possess broadspectrum antimicrobial, antifungal and antiviral activity. They have been shown to be effective in killing Escherichia coli, Pseudomonas aeruginosa and antibiotic-resistant bacterial strains. However, the price of Ag nanoparticles is quite high in comparison with the other materials. In order to find alternative materials Ag has been modified with TiO 2 nanoparticles. The method of Ag-TiO 2 nanoparticles production by laser ablation process in liquid environment was used to generate high purity nanoparticles for antibacterial applications. Another important justification in producing and studying Ag-TiO 2 nanoparticles is combining between the antibacterial activities of the silver nanoparticles and the photocatalytic activity of TiO 2 nanoparticles. For this reason, the antibacterial activity of the Ag-TiO 2 requires further improvement in order to increase their efficiency against superbugs (bacteria, viruses and fungi) as an alternative to ordinary antibiotics. Most current research investigating laser generation of nanoparticles is based on the use of nanosecond and femtosecond lasers. Very few investigations were reported on picosecond laser generation of nanoparticles and their characteristics Research Aim and Objectives On the basis of the problems described in the last section, the aim of this research is to use laser ablation to generate and manipulate Ag-TiO 2 nanoparticles in deionised water and investigate the optimal Ag-TiO 2 nanoparticle efficiency against the E. coli bacteria. The research objectives can be listed as follows: 23

24 Chapter 1: Introduction o To synthesise Ag-TiO 2 nanoparticles by ps laser ablation in deionised water. o To modify the Ag-TiO 2 nanoparticles properties by laser combined with other energy forms. o To modify TiO 2 nanoparticles with Ag nanoparticles using doping processes, enabling decreasing the energy gap of TiO 2 to improve antibacterial activity of the Ag-TiO 2 nanoparticles. o To attach Ag nanoparticles to TiO 2 nanoparticles to increase their antibacterial activity. o To extend the optical absorption spectra of the Ag-TiO 2 nanoparticles to longer wavelengths, i.e. extension of TiO 2 nanoparticles absorption spectra from the UV region to the visible region by modifying with noble metal Ag nanoparticles. o To generate nanoparticles using picosecond, nanosecond and femtosecond lasers, and compare their antimicrobial activities. o To determine the optimal concentration of the Ag-TiO 2 nanoparticles for antibacterial activity under normal light without using ultraviolet (UV) light. o To study single nanoparticles generation using different lasers and compare their characteristics Scientific challenges In an effort for modifying TiO 2 nanoparticles with Ag nanoparticles many scientific questions will arise. Because the energy gap of nanoparticles has a significant role in changing the optical properties of the nanoparticles the first scientific question is: Is it possible to reduce the band gap energy (E g ) of TiO 2 nanoparticles by ultrafast laser processing and what is the mechanism involved? The approach in this research to reduce energy gap of TiO 2 nanoparticles was by doping TiO 2 nanoparticles with Ag. An ice water environment was used to carry out the laser ablation process that reduced the energy gap of the titania considerably. Another aspect to improve nanoparticles properties is their morphology and form and these would affect their antibacterial activities; therefore, the second scientific question is: Is it possible to produce bimodal Ag-TiO 2 nanoparticles and what are the process mechanisms and antibacterial characteristics? In this research, for 24

25 Chapter 1: Introduction the first time, an approach to produce bimodal Ag-TiO 2 nanoparticles by pulsed laser ablation in deionised water and ultrasonic vibration assisted was explored. The third scientific question is: Is it possible to generate Ag-TiO 2 nanoparticles from a bulk Ag/Ti alloy and what are the process mechanisms and antibacterial properties? An investigation on producing nanoparticles from an Ag/Ti alloy was investigated to answer this question. Along the effect of the nanoparticles form on their properties, a coreshell form has received much attention by researcher in the production of nanoparticles due to their unique properties. Several researches have been done to produce Ag@TiO 2 core-shell nanoparticles but not TiO core-shell. The fourth scientific question is: Is it possible to generate TiO coreshell nanoparticles using laser techniques and what are the process mechanisms and antibacterial properties? In this research, for the first time, new method was investigated to produce TiO core-shell nanoparticles. After provide several new methods to produce and modify Ag-TiO 2 nanoparticles, it is time to ask a scientific question about: What is the effect of laser pulse length on the characteristics of nanoparticle production using lasers and what are the process mechanisms and antibacterial properties? In this research Ag-TiO 2 and single nanoparticles were produced by three different types of lasers (nanosecond, picosecond and femtosecond lasers) and the effect of laser processing parameters on the nanoparticle properties were investigated. The sixth scientific question is: What is the effect of laser wavelength on picosecond laser generated nanoparticles properties? To answer the question, different wavelengths (532 nm and 1064 nm) of a picosecond laser were used to generate Ag-TiO 2 nanoparticles and single nanoparticles such as Ag, Au, TiO 2, ZnO and iron oxide. The effect of laser wavelength on nanoparticle properties was investigated. Recently, it was demonstrated that Ag-doped TiO 2 nanoparticles greatly improved photocatalytic inactivation of bacteria and viruses [8-9]. In addition, Ag modified TiO 2 nanocomposites were applied as a long-term antibacterial agent to prevent the growth of bacterial cells [10]. The antimicrobial activity of the nanoparticles originates from the presence of Ag ions and reactive oxygen species (ROS) on the surface of TiO 2 [11]. As mentioned in the research 25

26 Chapter 1: Introduction objectives, the aim of this research is to improve the characteristics of Ag-TiO 2 nanoparticles using lasers and to improve their antibacterial activity. In other words, the purpose is combining the antibacterial property of Ag nanoparticles with the photocatalytic inactivation property of TiO 2 nanoparticles to produce synergistic bactericidal activity of Ag-TiO 2 nanoparticles. Photoactivation of TiO 2 nanoparticles can be improved by reducing their energy band gap transition [12] by doping with Ag nanoparticles at a specific ratio where the electrons in valence band of the TiO 2 are excited to localized energy levels and as a result of which effectively makes the band gap narrower in comparison with pure TiO 2 nanoparticles [13], and that it absorbs in wider range of UV-Vis light because of lowering the energy band gap [14]. To improve the nanoparticles properties, ice water was used to reduce the energy band gap transition of the Ag-TiO 2 nanoparticles. In chapter 3, the energy band gap of the Ag-TiO 2 nanoparticles was reduced to 1.75 ev. Another important characteristic of the Ag-TiO 2 nanoparticles to improve antimicrobial and photocatalytic activities is how strong Ag and TiO 2 nanoparticles are combined? It was concluded that the combination between Ag and TiO 2 nanoparticles by a laser is stronger than the other methods [15]. The concentration and depth of the incorporated Ag into TiO 2 can be tailored by changing the silver plasma immersion ion implantation parameters which are more effective for bio-application than pure TiO 2 nanomaterials [16]. In chapter 4, a new method is presented which allowed the attachment of smaller Ag nanoparticles onto the larger TiO 2 nanoparticles using a hybrid ultrasonic sonication and picosecond laser ablation in pure water. Another significant characteristic of the Ag-TiO 2 nanoparticles to increase antimicrobial activity is their optical absorption spectra because photoactivation of the TiO 2 nanoparticles can be improved by extending the optical absorption spectra from the ultraviolet (UV) range to the visible (Vis) range [17]. In the next chapter, Chapter 5, for the first time, a bulk Ti/Ag alloy was used to produce Ag- TiO 2 compound nanoparticles using picosecond laser ablation in deionised water. Core-shell forms of nanoparticles have unusual physical, chemical and biological properties. This form of nanoparticles enhanced their antibacterial 26

27 Chapter 1: Introduction properties, possibly due to the more active atoms in the shell surrounding core due to high surface free energy of the core surface atoms and shell thinness in the core-shell structure [18]. The aim of this research was not only to produce Ag-TiO 2 nanoparticles but also the modification of the nanoparticles. In chapter 6, novel TiO core-shell (TiO 2 -core and Ag-shell) nanoparticles were generated by ultrasonic vibration of Ag-TiO 2 compound nanoparticles. In general, the properties of the nanoparticles such as size, size distribution, morphology, structure and optical properties are affected by laser wavelength [19], laser pulse energy or laser fluence [20-21], ablation time [22], liquid media [23]. For this reason, different types of lasers were used to produce and improve the Ag-TiO 2 nanoparticles for antibacterial activity, in chapter 7, to indicate the best laser to produce the Ag-TiO 2 nanoparticles for antibacterial activity: nanosecond, picosecond and femtosecond laser ablation in deionised water were compared. In addition, in chapter 8, a comparison between nano-, pico- and femtosecond laser ablation in deionised water for the generation of Ag, TiO 2 and Au nanoparticles was made. Furthermore, In chapter 9, (Comparison of Characteristics of Selected Metallic and Metal Oxide Nanoparticles Produced by Picosecond Laser Ablation at 532 nm and 1064 nm Wavelengths), investigated the effects of two laser wavelengths (532 nm and 1064 nm) of a picosecond laser on the size and size distribution of the Ag-TiO 2, Ag, TiO 2, Au, ZnO and iron oxide nanoparticles in deionised water. During the generation of nanoparticles an interesting hollow and porous TiO 2 nanoparticle was observed which the number of the hollow and porous nanoparticles can be controlled by laser pulse energy. In chapter 10, hollow and porous crystalline TiO 2 nanoparticles produced by picosecond laser ablation in deionised water in a single step Outline of Thesis This thesis is written in an alternative format instead of the classical PhD thesis format. Thus, the core context is provided in the form of published or/and submitted research journal papers. It is worth mentioning that all cited 27

28 Chapter 1: Introduction references are compiled and grouped under References at the end of the thesis. Chapter one is a current chapter Introduction which includes Overview, problem statement, research objectives, research review and outline of thesis. Chapter two is a Literature review provides an introduction to nanoparticles, lasers, laser-material interaction and nanoparticles generation, the effects of laser parameters and process media on Ag and TiO 2 nanoparticles and the last part is antimicrobial and photocatalytic activities of Ag modified TiO 2 nanoparticles. In Chapter 3, Ag-TiO 2 nanoparticles were produced by a picosecond laser in ice water, a new method not reported before. In the next chapter, Chapter 4, a novel method is presented to produce bimodal Ag-TiO 2 nanoparticles by a picosecond laser in deionised water. In Chapter 5, a bulk Ti/Ag alloy was used, for the first time, to produce Ag-TiO 2 nanoparticles using pulsed laser ablation in deionised water. The nanoparticles have wider absorption spectra than those produced by previous methods In Chapter 6, novel TiO core-shell nanoparticles were produced two steps; firstly, Ag-TiO 2 were nanoparticles generated by a picosecond laser ablation in deionised water then the colloidal nanoparticles were put in an ultrasonic bath for about 5 mints, as a result, TiO core-shell nanoparticles were obtained Chapter 7, includes an investigation to determine the better type of laser among nanosecond, picosecond and femtosecond lasers to produce Ag-TiO 2 nanoparticles. They were characterised and their antibacterial activity was examined. In Chapter 8, a comparison between nano-, pico- and femtosecond laser ablation in deionised water for the generation of Ag, TiO 2 and Au nanoparticles was been presented. In chapter 9, Ag-TiO 2, Au, Ag, TiO 2, ZnO and iron oxide nanoparticles were produced in deionised water by a picosecond laser ablation at 532 nm and 28

29 Chapter 1: Introduction 1064 nm wavelengths. They were compared in terms of size, size distribution, absorption spectra and crystallite structure of the oxide nanoparticles. Chapter 10, discusses how in a single step, hollow and porous crystalline TiO 2 nanoparticles were produced by picosecond laser ablation in deionised water. Finally, Conclusions and Future Works provides the key scientific findings and concluding remarks from this research and discusses the possibilities for future follow-on research Logical development of the research In this work, a picosecond laser was used as the main production tool. On the other hand, nanosecond and femtosecond lasers were used for comparison. In addition, pure Ag, Au, TiO 2, ZnO and iron oxide nanoparticles were generated for comparison. The laser processing parameters and methods have significant effects on their optical, catalytic and antibacterial properties determined by their form, shape and energy band gaps. For example, the energy gap of TiO 2 nanoparticles was reduced by doping with Ag nanoparticles in ice water (Chapter 3); the form of nanoparticles was improved by producing bimodal Ag- TiO 2 nanoparticles using a laser/ultrasonic hybrid method (Chapter 4). The optical properties of the nanoparticles was improved by using a Ti/Ag alloy bulk material to produce Ag-TiO 2 nanoparticles, resulting in their optical absorption spectra having been shifted from UV range to visible range (Chapter 5). Finally novel TiO core-shell nanoparticles were produced via sonication of the Ag-TiO 2 compound nanoparticles (Chapter 6). Another part of this work is to find out the best laser tool to produce nanoparticles. A nanosecond, picosecond and femtosecond lasers were used to produce Ag-TiO 2 compound nanoparticles (Chapter 7) and a comparison was made for the production of single nanoparticles Ag, Au and TiO 2 in order to better understand the process behavior (Chapter 8). In addition, the effects of 532 nm and 1064 nm laser wavelengths of a picosecond laser on the size and 29

30 Chapter 1: Introduction size distribution of the compound nanoparticles (Ag-TiO 2 ) and single nanoparticles (Ag, TiO 2, Au, ZnO and iron oxide) in deionised water were investigated (Chapter 9). Finally, during the generation of nanoparticles interesting hollow and porous TiO 2 nanoparticles were observed and the number of the hollow and porous nanoparticles could be controlled by laser pulse energy (Chapter 10). 30

31 Chapter 2: Literature review 2 Chapter 2. Literature review Introduction to nanoparticles Overview The term nano is originally derived from the Greek nanos meaning dwarf. Nowadays, nano is a widespread prefix, featuring in many branches of science and technology in terms such as nanometre, nanoscale, nanostructure, nanotechnology, nanoscience etc. The original idea of nanosize technology came from a speech by Richard Feyman during a meeting of the American Physical Society in December 1959, where he asked What would happen if we could arrange the atoms one by one the way we want them? The nanometre is a unit of measurement of length that is equal to one billionth (10-9 ) of a meter. The upper limit of the nanometre scale is 1000 nm, i.e. 1 µm, equal to m [24-25]. Nanoparticles can be defined as particles that have their dimensions smaller than 1 micron and possibly as small as the length of an atom or a molecule ( 0.2 nm). Figure 2-1 shows a comparison of the size of nanomaterials and micromaterials with biological components such as viruses, Deoxyribonucleic Acid DNA and proteins [24]. 31

32 Chapter 2: Literature review Figure 2-1: Comparison of nano and micro length scales and biological components [24]. The physical properties of particles at the nanoscale, such as the surface area-to-volume ratio, are different from those of larger particles. The size of the particles may even reach a scale where quantum effects appear [26]. In addition, the catalytic, mechanical and thermal properties of the materials will be changed [27]. In the nanoscale range, a large surface area is crucial for improving a particle s catalytic properties and the nanoparticle s structure. As a result of increasing the surface area-to-volume ratio, the behaviour of atoms will be observed predominantly on the surface of the particle. This phenomenon affects both individual particles and their interactions with other particles or materials [26]. The size dependent property in semiconductors is quantum confinement; in metallic magnetic nanoparticles, it is paramagnetism; and in some metallic nanoparticles, it is surface plasmon resonance (SPR). It is worth mentioning that in ultrafine nanoparticles in the range of 1 to 10 nm, superparamagnetism will appear [27]. The materials from which nanoparticles are most commonly 32

33 Chapter 2: Literature review formed are metal oxides, ceramics, metals, silicates, nano-oxide ceramics, polymer materials and compound semiconductors. The most popular or familiar materials for generating nanoparticles are shown in Figure 2-2 [28]. Figure 2-2: The most common nanoparticle categories [28]. In the study of nanoparticles the difference between two terms must be distinguished: agglomerate and aggregate. An agglomerate structure is a collection (or connection) of a number of particles with each other by weak forces including Van der Waals force and surface tension. An aggregate particle is a heterogeneous particle in which the various components are not easily broken apart [29]. Nanoparticles exist in three different forms: suspension (colloidal nanoparticles) which mostly consists of solids in liquids, aerosol which is usually a liquid or solid suspended in air; and emulsion in the case of two liquid-phase particles [30]. In their expectation of the development of nanotechnology, Morris and Willis [31] hypothesised that future generations (to be the fourth generation in 2015) will include molecule-by-molecule design and self-assembly capability. 33

34 Chapter 2: Literature review Classification of nanoparticles Nanoparticles are classified on the basis of a number of their properties such as their dimensions, morphology, uniformity, agglomeration and composition [24]. The majority of these classifications are mentioned below Size On the basis of the size of nanoparticles, they can be classified as fine or ultrafine particles if they fall between the range of nm and nm respectively. The nanoparticles are particles commonly with the physical dimensions in the range nm [25] Dimensionality On the basis of dimensions, nanoparticles can be divided into three categories: one-dimensional (1D); two-dimensional (2D); and three-dimensional (3D) nanomaterials [24]. Restriction of quasi-freely mobile electrons in a piece of bulk metal can be obtained not only by reducing the volume of metal to an ultrafine size (a zero dimensional (0D) quantum dot), but this scenario can also be created by reducing the dimensionality from 3 to 2 or 1, as in the case when the piece of metal is made thinner and thinner until the electron can only move in two dimensions instead of three [32]. Klabunde [32] performed a twodimensional (2D) quantum confinement, known as a quantum well. To bring about a further reduction in dimensionality a quantum wire (1D) was generated, which allowed the electron to move only in one direction D nanomaterials In the case of 0D nanomaterials, all dimensions are within the nanoscale range and no dimensions fall within the macroscale range such as nanospheres, nanocubes, and quantum. The electrons in 0D nanomaterials are therefore fully confined. Many physical and chemical routes have been provided to synthesize 0D nanomaterials with well-controlled dimension [33]. 34

35 Chapter 2: Literature review D nanomaterials For this type of nanomaterials, two of their three dimensions are in the range of the nanometre scale, and the third dimension falls within the macroscale. Examples of 1D nanomaterials include nanowires or nanorodes [24, 34] which are used in the electronics, and chemical industries as well as in engineering [34] D nanomaterials For 2D nanomaterials, one of their three dimensions is within the range of nanoscale, and the other two dimensions are within the macroscale range. Examples of these include surface coatings (manufactured surfaces) and thin films such as graphene [24, 34] D materials In the case of 3D materials, all dimensions are within the macroscale range. Figure 2-3 shows a schematic of the density of states as a function of dimensionality from (3D) bulk to (0D) QD systems [35]. Figure 2-3: 3, 2, 1 and 0 dimensions of materials and the density of their states as a function of energy [35]. 35

36 Chapter 2: Literature review Morphology On the basis of morphology, nanoparticles can be divided into two categories: high-aspect ratio and low-aspect ratio. Furthermore, high-aspect ratio nanoparticles can be sub-classified into nanowires, nanohelices, nanozigzags, nanopillars, nanotubes and nanobelts. Similarly, low-aspect ratio nanoparticles can be subdivided into nanospheres, nanohelices, nanopillars, nanowires, nanopyramids, nanocubes and various other, as shown in Figure 2-4 [24]. Figure 2-4: Classification of nanomaterials on the basis of morphology [24] Uniformity and agglomeration Agglomerated nanoparticles or nanoparticles dispersed in air (aerosols) and in water (colloids) exist due to their chemical and electro-magnetic properties. As shown in Figure 2-5, Nanoparticles can be divided into homogeneous dispersed or inhomogeneous dispersed particles, as well as homogeneous agglomerates and inhomogeneous agglomerates [24]. Figure 2-5: Classification of nanomaterials on the basis of uniformity and agglomeration [24]. 36

37 Chapter 2: Literature review Composition On the basis of composition, nanoparticles can be divided into single material nanoparticles and multimaterial nanoparticles. Multimaterial nanoparticles can be found in nature as agglomerations with different compositions; single nanoparticles can be generated by various means [24] Other classifications On the basis of organic and inorganic chemistry, nanoparticles can be divided into two categories; organic nanoparticles such as carbon nanoparticles (fullerenes) and inorganic nanoparticles like noble metals, semiconductors, magnetic nanoparticles [27] and quantum dots [34]. Nanoparticles may be classified into two categories; the first of these is nanoparticles of natural origin, such as DNA (2.5 nm), some types of viruses (10-60 nm), and bacteria (30 nm to 10 micron). The second category consists of nanoparticles which are of mineral or environmental origin, such as desert sand (finer fractions), smog and oil fumes [36] Toxicity and health hazard of nanoparticles Before working with nanoparticles it is crucial to understand the possible hazards of the nanoparticles in question. For this purpose, knowledge of the form of the nanoparticles is vital, as it plays a remarkable role in the risk of exposure; for instance, powdered nanoparticles are more hazardous than colloidal nanoparticles if inhaled. There are some scenarios in which exposure to nanoparticles may occur, such as working with colloidal nanoparticles without using gloves and during mixing or pouring, handling nanoparticles in powder form, cleaning spilled colloidal nanoparticles, cleaning waste materials, repairing experimental equipment, grinding and machining, as well as during production of nanoparticles by gas phase methods [37]. One of the most dangerous kinds of nanoparticle is carbon black, which is produced in many different ways in huge quantities - about 1.5 million tons per year [26]. Ostiguy et al. [36] observed from the work of Brom and Kreyling that in order to understand the effects of nanoparticles when inhaled by humans, 37

38 Chapter 2: Literature review efforts should be focused on the 5 Ds: dose, deposit, dimension, durability and defence mechanism. The dose of the nanoparticles received by the pulmonary tree determines the potential toxicity. The dose (quantity of nanoparticles) is estimated by ascertaining the concentration of nanoparticles, as well as their dimensions. Another D is the deposition of nanoparticles within the pulmonary system, which depends on the size of the deposited nanoparticles. The durability refers to the length of time for which nanoparticles remain in the lungs; this time will be greater in the case of insoluble nanoparticles. There is different defence mechanism of the respiratory system designed to eliminate undesirable particles. A higher toxicity of silver nanoparticles was observed in a solution with low ionic strength as well as a toxicity of Ag dependent upon both dissolved silver and surface coating. It was also determined that there is no relation between the toxicity of Ag and their sizes, but the toxicity of the Ag is directly proportional to the amount of dissolved silver [38]. In contrast, Williams [30] showed that when the particle size is reduced the toxicity is increased. Prathna et al. [27] suggested that nanoparticles produced using the biomimetic method are less toxic. Aruoja et al. [39] concluded that the toxicity of TiO 2 nanoparticles to the microalgae Pseudokirchneriella subcapitata are higher than bulk TiO 2 material. On the other hand, the toxicity of zinc oxide (ZnO) was reported to be equal for both its nanoparticles and bulk forms. Barcikowski et al. [40] conducted a risk assessment of the process of nanoparticle production using femtosecond laser pulses in the workplace, focusing on the mass production rate and size distribution. It was concluded that within one laser manufacturing shift, the generated nanoparticles may accumulate in the workplace to a concentration which is 200 times higher than the background concentration. The possible case of the toxicity of TiO 2 nanoparticles is oxidative stress due to generation of reactive oxygen species (ROS) and photoactivity: cell death and fibrillation due to interference in macrophage call membrane functions [41]. The TiO 2 nanoparticles have detrimental or even toxic effects on cells under UV light irradiation [42]. 38

39 Chapter 2: Literature review Properties of nanoparticles When the size of the materials is reduced from bulk to nanoscale sizes, the classical mechanics is converted to quantum mechanics [26]. Their properties are changed and quantum size effects will be produced. Some properties are affected by this conversion to nanoscale proportions, including melting point, conductivity, magnetism, colour and Mossbauer spectroscopy (nuclear resonant fluorescence) [32]. The properties of nanoparticles were first described by Michael Faraday in 1857, in his published paper, Experimental relations of gold (and other metals) to light [27] Electrical properties The electrical properties of the materials are one of the properties that are most dependent on particle size. For example, the Current-Voltage (I-V) graphs of the TiO 2 nanoparticles (commercial and prepared with Pluronic P123 and Brij 35 as surfactants) do not follow Ohm s law, I-V curves exhibit a nonlinear behaviour. In addition, the electrical conductivity of TiO 2 nanoparticles is increased due to the increase in surface area. It is worth mentioning that the factor which most affects the electrical property of TiO 2 nanoparticles is the purity of the anatase crystalline TiO 2 phase [43]. Mangrola et al. [44] have shown that the dielectric constant and dielectric loss of TiO 2 nanoparticles are affected by changes in temperature. Furthermore, it has been shown that dielectric constant decreases with increasing frequency and there is a good correlation between both the dielectric constant and dielectric loss with frequency Optical properties The first study of the optical properties of metal nanoparticles was conducted by Faraday in the mid-1800s, when he investigated colloidal gold [45]. These properties were selected in order to establish the types of nanoparticles present in a solution by taking optical absorption spectra, for example Ag nanoparticles and TiO 2 nanoparticles. In addition, optical properties are used to investigate the modification of a metal oxide nanoparticle by a noble 39

40 Chapter 2: Literature review metal nanoparticle, such as the modification of the catalytic properties of TiO 2 nanoparticles by Ag nanoparticles. This property of nanoparticles is of particular interest to the field of physical chemistry [45] Catalytic properties Catalytic properties of nanoparticles are factors which increase the speed of chemical reactions [46]. Moreover, the photocatalytic properties of nanoparticles are those which increase the rate of the chemical reactions by the photons of an electromagnetic source, such as light. These properties have been used to improve the degeneration efficiencies of TiO 2 nanoparticles for the purposes of water purification [47] and minimising air pollution Thermal properties In their research, Zhang and Li [48] showed the effects of the doping process on the thermal conductivity and thermoelectric properties of nanomaterials. It was concluded that the thermal properties of nanoscale materials are considerably different from those of bulk materials, because the phonon characteristic lengths are comparable to the characteristic length of the nanostructures. The researchers also showed that low dimensional nanomaterials are remarkable candidates for proving the theory of phonon transport. Another thermal property of nanomaterials is the melting point as a function of size. When the size of the bulk materials is reduced to nanoscale sizes, their melting points are also decreased. An empirical equation was derived to determine the melting point (T M ) of the nanoscale materials on the basis of the bulk melting point (T b ), as shown the equation below [46]. (2-1) where r o is the particle radius at liquid phase at 0 K, and r is the particle radius. The existence of a liquid like shell on the nanoparticle has been demonstrated and a good match of theory to experiment can be obtained by adjustment of the unknown layer thickness to an equation of the form [46]. 40

41 Chapter 2: Literature review (2-2) where A, B and C are all thermodynamic constants. Figure 2-6 shows the variation in melting point of gold nanoparticles as a function of the particles diameters and comparison with the bulk melting temperature [24]. Figure 2-6: Variation in melting point of gold nanoparticles with the particles diameter [24] Magnetic properties The magnetic properties of the particles in nature are dependent upon size [49]. This property also varies greatly depending on the nature of the materials; for example, Ag is diamagnetic and its magnetic molar susceptibility M is 19.5 x 10-6 emu mol -1 Oe -1, whereas Ti is paramagnetic, with M equal to 153 x 10-6 emu mol -1 Oe -1 at room temperature (20 C) Nanoparticle production techniques Many methods have been developed not only for the preparation of nanoparticles but also for controlling their sizes. Generally, the methods of nanoparticle generation can be divided into three types: chemical, physical and biological methods, amongst which are sol-gel, chemical vapour deposition (CVD), physical vapour deposition (PVD), wet chemistry, ion sputtering, pyrolysis [50], laser ablation of a solid metal in a liquid environment [51-54] and 41

42 Chapter 2: Literature review the use of Murraya Koenigii (curry leaf) extract [55]. Recent years have seen a growing interest in the use of laser ablation of a solid material target in a liquid environment as a new technique for the preparation of nanoparticles [51-52, 56], including porous nanoparticles [57], metal oxide nanoparticles [58], nanodisks [53], and for the morphological conversion of nanoparticles [59]. Some researchers prefer this new technique to other conventional methods for synthesising nanoparticles because in comparison to chemical methods, colloidal nanoparticles prepared by laser ablation in a liquid environment are free from contamination [60], surfactants and counterions [53, 61-62], and chemical reagents [63]. Furthermore, this method is a relatively simple way to prepare nanoparticles [64-65] despite the fact that research papers have also been published on preparing noble metal nanoparticles using sodium dodecyl sulfate (SDS) as a surfactant in a colloidal solution [51-52, 66-67] and a Polyvinylpyrrolidone (PVP) solution [68-69]. Phuoc et al. [64] have determined that the chemical approaches for preparation of nanoparticles must always include reduction reactions and they have a tendency to produce agglomeration followed by reduction of the potential of the high surface area of the nanoparticles. This situation has not been seen to occur in the laser ablation process in liquid media, probably because in these instances the purity of the surface of the nanoparticles, for example free from extraneous ions or other chemicals. Another drawback of chemical methods is the need for a further purification process following preparation of the nanoparticles in order to remove uncontrolled chemical byproducts [70] such as protective reagents and ions [69]. This is especially true for processes such as photocatalysis, sensing devices, and homogeneous organic catalysis and biomedical applications because they require extremely pure nanoparticles [71]. Hahn et al. [72] have also reported that chemical methods have limitations in terms of availability of materials, agglomeration, impurities and size distribution of the nanoparticles. Conversely, the laser ablation technique in an aqueous solution has the ability to generate almost unlimited varieties of metal nanoparticles and can be performed using different liquids; the facility of the process is also a distinct advantage over traditional techniques. 42

43 Chapter 2: Literature review Recently, biological techniques to bring about synthesis of nanoparticles have been developed, such as the biosynthesis of Ag nanoparticles using the fungus Trichoderma harzianum [55] and Murraya koenigii (curry leaf) [73]. The Ag nanoparticles generated were spherical and completely uniform in size and shape, ranging in size from 10 to 25 nm [73]. This new route is totally free from toxins, unlike the established chemical methods [55]. A number of methods have been devised, not only to generate a variety of nanoparticles, but also to improve their quality and quantity. These include solid state processes, vapour condensation methods, chemical synthesis/solution methods [26] and gas-phase synthesis methods [74]. In general, there are two main methods to produce nanoparticles: the break down (top down) and the build up (bottom-up) methods, shown in Figure 2-7 a [75] and b [76]. (a) (b) Figure 2-7: Methods of nanoparticle generation: break down (top down) and build up (bottomup) a [75] and b [76]. 43

44 Chapter 2: Literature review Top down (Mechanical physical) production process The mechanical physical method to produce nanoparticles is based on microsystem technology principles (see Figure 2-8) [76]. These methods are also known as solid-state methods. Figure 2-8: Top down method for production nanoparticles [76] Solid state methods These methods are also used to create nanoparticles and it is worth mentioning that these techniques are well-established [28]. The solid-state techniques for generating nanoparticles include grinding, milling [26, 28] and mechanical alloying techniques [28]. These methods are used for materials from which nanoparticles cannot easily be created by other methods [26]. The grinding method is the most popular top down method for the synthesis of nanoparticles. The nanoparticles are ground as finely as possible, a stage which is also referred to as grinding equilibrium. As the grinding time is increased, the size of the nanoparticles produced is increased due to agglomeration [75]. This method presents two problems for the production of nanoparticles: firstly, the resultant low homogeneity in the size distribution of the particles produced, as well as the generation of high volumes of waste [28]. The milling process was developed for production of metallic and ceramic nanoparticles. The metallic target is ground by high energy ball mills. This process involves thermal stress and is energy-intensive [76]. This technique also presents a disadvantage in terms of the quality of the nanoparticles produced; furthermore, the particles are inevitably contaminated 44

45 Chapter 2: Literature review by the milling process. As well as the heterogeneous size distribution and the high cost of manufacturing are two additional drawback of this method [28]. Another shortcoming of this technique is that the shape of the nanoparticles cannot be controlled [76]. Mechanical alloying techniques is another solid state method. It involves solid-state powder processing. It was originally used for generating oxide dispersion-strengthened alloys, but nowadays it is used for a variety of material processes. The method was pioneered by Benjamin and his co-workers at the International Nickel Company in the 1960s. The mechanical alloying/milling is a non-equilibrium processing technique that is analogous to rapid solidification methods for the synthesis of materials with superior homogeneity, microstructured refinement, and/or metastable crystal structure [77] Bottom up (chemo physical) production processes In terms of the phases of materials into liquid phases and gas phases, these methods may be classified into one of three categories: gas-phase synthesis, liquid-phase synthesis and vapour-phases synthesis. These techniques are based on the physico chemical principles of molecular or atomic self-organization [76]. Figure 2-9 shows a schematic diagram of the aforementioned methods. Figure 2-9: Bottom up process for the generation of nanoparticles [76]. 45

46 Chapter 2: Literature review Gas phase synthesis Gas-phase methods include chemical reactions and physical state changes. Using physical or chemical means, nanoparticles can be generated from gas-phase processes by generating vapour from the materials produced. The preliminary nanoparticles are produced in the liquid phase or solid-state phase by homogenous nucleation. Condensation then occurs after further growth of the nanoparticles, which is dependent on the preparation process. After condensation has occurred, chemical reactions, coagulation processes and coalescence (fusion) will eventually be produced using equipment such as a flame reactor, plasma reactor, laser reactor or hot wall reactors, resulting in the generation of nanoparticles such as fullerenes and carbon nanotubes. These techniques are more common than other methods in industrial-scale technologies [76]. Yokoyama and Huang [75] developed a new gas-phase route to generate nanoparticles which is based on the technique of plasma-assisted chemical reaction Liquid-phase synthesis Liquid-phase methods include techniques such as evaporation decomposition, crystallisation precipitation, the sol gel process, polymerization etc. The rate of nanoparticle production using this method is higher than for gas-phase methods, making it applicable to mass-production techniques [75]. It can therefore be said that this is a time-saving technique. These methods are also called wet chemical nanoparticle generation methods, which require lower temperatures than gas-phase production processes [76]. Precipitation processes is a liquid-phase synthesis, they are based on the reactions between salts in solvents. Firstly, a precipitation agent is poured into a container to produce the desired particle precipitation. After filtration process, the precipitated particles are thermally post-treated. This method is used for preparing metal oxides and non-metal oxides (metallic nanoparticles). In addition, this technique enables the user to control the size of the nanoparticles produced using self-assembled membranes [76]. 46

47 Chapter 2: Literature review Another important and well-known liquid-phases method to produce nanoparticles is sol gel processes. Sol gel is the process of producing a gel form powdered materials, also known as a wet chemical process. The sol term refers to the dispersion of particles ranging from 1 to 100 nm in size. This method is chiefly used to generate oxide nanoparticles, ceramic non-structural polymers and porous nanomaterials. The production processes are concluded at low temperatures and under moderate conditions. The phases of the process and reactions that occur during the sol gel process are shown in Figure 2-10 [76]. Figure 2-10: The phases and reactions of the sol-gel method [76] Vapour-phase synthesis These methods are used to synthesise nanoparticles from metal and metal oxide ceramics [26, 28]. The main disadvantage of these methods is the high cost involved [28], whereas their main advantage is the low rate of contamination produced [26]. These methods involve physical vapour deposition (PVD), chemical vapour deposition (CVD) and vacuum evaporation on running liquids (VERL). 47

48 Chapter 2: Literature review PVD entails the deposition of nanoparticles from one surface (target) to another (substrate) using vaporization in the form of atoms or molecules. This process is carried out in a vacuum or low-pressure gaseous medium. The optimal rate of deposition is between 1 10 nm/sec [78]. This method produces low volumes of material and is expensive [28]. This method of nanoparticle synthesis can be subdivided into vacuum evaporation, sputter deposition, and ion plating [78]. The high cost is the main drawback of CVD. CVD is the process of deposition of atoms and molecules via a chemical vapour reaction of a chemical vapour precursor on a heated surface. While a degree of competition exists between the CVD with PVD processes, in some aspects the two processes overlap. For example, the noble processes are a combination of plasma enhanced CVD and activated sputtering. In addition to a full understanding of CVD, a general familiarity with PVD is important [79]. CVD exists under different names, such as Vapour-Phase Epitaxy (VPE) in the case of the deposition of single crystal films, Metalorganic CVD (MOCVD) in the case of metalorganic precursor gases, Plasma-Enhanced CVD (PECVD) when plasma is used to improve the decomposition or reaction of the particles, and Low- Pressure CVD (LPCVD) when the pressure is reduced to normal pressure [78]. Camargo et al. [80] showed advantages and disadvantages of some methods to production of nanomaterials (see Table 2-1). 48

49 Chapter 2: Literature review Table 2-1. Advantages and disadvantages of processing methods for nanomaterials [80]. Methods Advantages disadvantages Simple method, low temperature, high Weak bonding, high purity products, high chemical permeability and difficult homogeneity and rigorous stoichiometry control of porosity. control. Chemical methods (Sol-Gel, Colloidal) Chemical vapour deposition (CVD) / physical vapour deposition (PVD) High Energy Ball Milling Rapid Solidification Process (RSP) RSP with ultrasonics Liquid Infiltration Spray Pyrolysis Generate high concentration and pure materials, adhesion at high deposition rates and good reproducibility Homogeneous mixing and uniform distribution. Simple and effective method. Good distribution and no agglomeration. Very short contact times between matrix and reinforcements, moulding into various and near net shapes of different stiffness, rapid solidification and both lab scale and industrial scale production. Produce ultra-fine NPs, spherical and homogeneous powders and reproductive size and quality. Optimization of many parameters, cost and relative complexity. Use only to produce metalmetal nanocomposites, produce agglomeration and non-homogeneous distribution of small NPs High temperature, separate of reinforcements, production of undesired products during processing. High cost associated with producing large quantities of uniform Characterisation of nanoparticles A number of instruments have been used to measure the number, size, surface area and structure of nanoparticles. The most popular techniques are Transmission Electron Microscopy (TEM), Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). Aitken et al. [29] showed that the mass of nanoparticles can be measured by using Size Selective Personal Sampler method and the surface area may be measured using the Epiphaniometer bulk analysis method. The number of nanoparticles can be obtained using several instruments: an Optical Particle Counter (OPC), Condensation Particle Counter (CPC), Scanning Mobility Particle Sizer (SMPS) and an Electrical Low-Pressure Impactor (ELPI). In addition, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) [29] and Atomic Force Microscopy (AFM) [81] are methods of imaging nanoparticles in order to analyse their morphology. 49

50 Chapter 2: Literature review Several methods have been devised in order to analyse the length (size), shape, mass and forces of nanoparticles, as well as their other physical properties, among which are the electron beam technique (High-Resolution Transmission Electron Microscopy - HRTEM) and scanning probe technique (Scanning Tunnelling Microscope STM, and Atomic Force Microscopy - AFM)[36] Applications of nanoparticles Although the application of nanoparticles is now widespread among the scientific community, their use is not a recent development. For example, ancient Chinese communities used nanoparticles to glaze their porcelain. In addition, gold nanoparticles were used in the era of the Roman Empire to colour the Lycurgus Cup enabling various colours depending on whether it was illuminated from which sides [26]. Furthermore, both silver and gold nanoparticles were used by the artisans of Mesopotamia to make their shimmering pots [27]. Due to their high surface reactivity and the benefits of their size, nanoparticles have been applied in numerous fields, including sensors, catalysts, cosmetics, batteries, paints, inks, films and plastics, etc. [75]. They have also proved indispensable in the delivery of drugs, as well as for bioengineering, therapeutics and discovery of therapeutics drugs. Nanoparticles have also been used in the manufacture of scratchproof eyeglasses, crackresisted paints, anti-graffiti coatings for walls, transparent sunscreens, stainrepellent fabrics, self-cleaning windows and ceramic coatings for solar cells[25]. In the specific case of the delivery of drugs, different polymers have been applied to produce nanoparticles, which have then been used for research on drug delivery methods to develop their therapeutic properties and minimise negative side effects. Further examples of the drug delivery application of nanoparticles include the brain and the gene delivery of genetic material [82]. In the present research, the benefits of applying Ag-TiO 2 nanoparticles to the field of biomedicine will be studied, such as their ability to kill certain types of bacteria. 50

51 Chapter 2: Literature review 2.2. Lasers, laser-material interaction and nanoparticles generation Introduction The word laser is an acronym for Light Amplification by Stimulated Emission of Radiation. In general, a laser consists of three main parts: an active medium, resonators, and a pumping source. The discovery of stimulated emission phenomena by Albert Einstein in 1861 underlies the basic principle of the lasing process. The stimulated emission is responsible for amplifying laser light inside the active medium. On the basis of their changing intensity with time, laser beams may be divided into continuous wave (CW) and pulse mode [83]. The power of a laser beam can be measured directly by a power meter Laser and laser beam properties The light emitted from a laser has certain unique properties, namely directionality, monochromatic and coherence that are very different from common light such as the Sun. However, both laser light and sunlight are electromagnetic radiations. The properties of both will be briefly discussed in the following sections Directionality Directionality is the focus of the laser beam (all photons) in one direction. Laser beams are well known for their low diffraction levels and their ability to convey a typical plane wave front. These two properties combine to make laser beams highly directional [84-85]. In spite of their directionality, laser beams are inevitably associated with a certain level of divergence caused by diffraction [86] Monochromatically Monochromatically of the laser beams is a property that causes them to be, or appear to be, only one wavelength. Despite appearing to be a monochromatic light source, lasers do produce a very narrow wavelength band of light. In general, the wavelength of the laser beam is represented by 51

52 Chapter 2: Literature review instead of, as the beam is not fully monochromatic. In the same way, (c= ) the frequency is written as to describe the range of frequency, where is known as bandwidth [84-85] Coherence Coherence describes the properties of two or more waves when each individual wave is in phase with others. It can be divided into two types: coherence time and coherence length [84-85] Ultra-short pulse lasers Ultra-short pulse lasers, also known as ultra-fast lasers, are lasers with beams that have pulse duration in the order of picosecond to femtoseconds. For ultra-short laser pulse durations - for example, in the order of picoseconds (ps) and femtoseconds (fs) - the peak powers of the pulses is very high. The laser pulse energy (E pulse ) can be calculated directly using the average power of the laser (P avg ) and dividing it by the laser pulse repetition rate (f laser ), as in Equation (2-3) [83]: (2-3) The pulse peak power (P pulse ) can be calculated dividing the pulse energy (E pulse ) with the laser pulse duration ( ), as in Equation (2-4): (2-4) High repetition rate (sometimes called frequency) and low average power of the beam itself are two properties of most picosecond and femtosecond lasers. The premise of ultra-short laser pulse duration is to produce an effective, powerful pulses. A typical fs laser tool has an average power of 1 W and a frequency of 1 khz. The energy density of a circular laser beam (laser fluence) at any beam size (diameter) can be calculated using Equation (2-5) bellow [83]: (2-5) 52

53 Chapter 2: Literature review Ablation process The ablation process is the removal of materials on the surface of a target material while it is irradiated by a laser beam source. For this process to be successful, the laser beam fluence must be greater than the material s threshold energy [87] or the energy of one laser pulse must be greater than the binding energy of the target atoms [88-89]. The laser beam photons interact with the electrons in the target material; as a result, the atoms are heated by energy exchange processes [88] and the materials will be ablated in the form of plasma plume [87] Laser water interaction When lasers interact with water, several phenomena occur, such as absorption of the laser energy, the formation of shock waves, generation of bubbles and a change in the laser beam s focal point, caused by changing the refractive index of the water. After irradiation of an electromagnetic wave (laser light is a form of electromagnetic wave) on the surface of a material, some of the incident photons are absorbed, some are transmitted (in transparent materials) and the rest of the photons are reflected. The absorption rate of photons by the material can be calculated by Beer Lambert s Law [90]: (2-6) where, I and I o are the intensity or power per unit area (W/m 2 ) of the incident and the transmitted light respectively, is the absorption coefficient (cm -1 ) of the target material and z is the thickness (cm) of the material. Equation (2-6) is also used to describe laser-water interaction. When lasers interact with water, some of the energy from the laser beam will be absorbed by the water; as a result, the amount of laser power transmitted will be reduced. Another important phenomenon is the formation of shock waves during the ablation process underwater at the interface of the water and the target. Tokar et al. [91] investigated the dynamics of shock wave formation during the underwater interaction of a 1064 nm laser with both Ag and Au targets. The 53

54 Chapter 2: Literature review authors used an equation by Sedov (1982) to calculate the radius of the shock wave front R s as follows: (2-7) where is the energy absorbed by the target, o is the undistributed water density and d is the time delay. Al-Mamun et al. [92] have concluded that the level of the water on the metal target during the generation of nanoparticles has a significant effect on the ablation rate. Figure 2-11 shows that the ablation rate is very high when the water is at a height of 2 mm. On the other hand, when the water level was doubled to 4 mm, the ablation rate decreased sharply. At higher water levels the ablation rate increased slightly. This experiment was carried out using a Q- switched Nd:YAG laser ( =1064 nm, =6 ns, f = 10 Hz). Figure 2-11: Effect of water height on the ablation rate [92] Laser material interaction The response of materials to light depends on the power of the laser beam; the temperature of the beam affects the reaction. Continuous laser irradiation on a material will subsequently produce consecutive reactions of heating, melting, boiling and plasma formation [90]. Han and Li [93] classified the response of materials to a laser beam in terms of thermal effects such as 54

55 Chapter 2: Literature review melting, boiling, vaporisation and phase-explosion of growth and explosion of nucleation, and some mechanical effects include deformation and stress in materials. It is worth mentioning that at higher intensities of laser beams, a large number of photons will interact with matter; as a result, nonlinear optical phenomena will be produced, such as Raman scattering, Rayleigh scattering and Brillouin scattering [90]. When a high number of photons interact with a material, the atoms vibrate due to the force induced by the incident electric field; as a result, the heating process occurs and heat will be distributed within the matter, according to Fourrier s Law of heat conduction: (2-8) where Q is the rate of conduction heat transfer (W), A is the area (m 2 ), k is the thermal conductivity (W.m -1 K -1 ) and dt/dx is the temperature gradient (K.m -1 ). Further light irradiation causes the target material to absorb enough energy to increase the vibration of the molecules, resulting in a decrease in the binding energy between the molecules, leading the material to melt. After this point, further irradiation will cause further molecular vibration, leading to further loosening of the metal s bonds and eventually evaporation will occur. Finally, in the case of a further increase in temperature, the electrons will vibrate freely, a state described as plasma formation. These stages of the absorption process are illustrated in Figure 2-12 [90]. 55

56 Chapter 2: Literature review Figure 2-12: Absorption process within matter as a function of increasing power [90]. Miklos et al. [94] demonstrated another aspect of laser-material interaction, in that after irradiation of a target using a laser beam, the laser beam photons are absorbed by the target. As a result, the internal energy levels of the target (consisting of its electric, vibrational and rotational energy) will be excited. Then, as a result of radiation (spontaneous or stimulated emission) or nonradiative deactivation which channels at least part of the absorbed energy into heat, the energy of the excited states will be lost. Rethfeld et al. [95] showed that the interaction of ultra-short laser pulses with matter is different from the interaction of longer laser pulses with matter. As shown in Figure 2-13, the interaction of each nanosecond, picosecond and femtosecond (or timescale ranges) with materials depend on the intensity of the laser beam. For example, fs laser excitation during interaction with materials consists of energy deposition, melting and ablation as the basic physical processes. Furthermore, in the ps timescale range the melting process will appear briefly. It is worth mentioning that the laser-matter interactions differ for metals, semiconductors and dielectrics. 56

57 Chapter 2: Literature review Figure 2-13: The interaction processes of ns, ps and fs laser pulses with materials as a function of intensity [95]. Unlike ns laser pulses, the energy of fs laser pulses can be localised in a specific area on the target. As a result, the damage will be reduced [96] Generation of nanoparticles in water with a laser beam When a target is irradiated by a laser beam, a plume is produced; this has a high temperature and causes the water to vaporise and ionise at the plume-water interface. Water plasma (atomic and molecular hydrogen and oxygen) is then generated; plasma from the target material will also be produced at the plume target interface. An interaction between the water plasma and the ablated target matter will subsequently take place. Herein, the ablation rate in liquid media is lower than that in a vacuum and in a controlled gas atmosphere, as the plume expands adiabatically with supersonic velocity in liquid media. In addition, a shock wave will be generated at the plume-liquid interface that oscillates within the plume. As a result of this phenomenon the physical properties of the generated plume are increased, such as its temperature, pressure and density. After this stage, the plume will begin to cool as a result of the thermodynamic instability of the ablated plume at room 57

58 Chapter 2: Literature review temperature. Once the plume has cooled in the liquid, the quenching process will begin; this follows the synthesis of the nanoparticles. It is important to note that the quenching process depends on the rate at which the plume cools in the liquid, as well as on the additional confinement of the particles [87]. The whole process is shown schematically in Figure Figure 2-14: The physical stages of TiO nanoparticle generation [87]. 58

59 Chapter 2: Literature review 2.3. Effects of laser beam parameters and ablation environments on Ag and TiO 2 nanoparticle production Introduction The specific properties of nanoparticles which are different from bulk size particles have received much attention from researchers [63] and have been used in a wide range of applications, ranging from electronic and catalytic uses to magnetic and medical functions [58]. The size and shape of nanoparticles are responsible for the different properties found in metal nanoparticles [97]. Phuoc et al. [64] preferred to work with suspended nanoparticles rather than microparticles due to many advantages afforded by their smaller size: firstly, the force of gravity prevents nanoparticles from sinking or floating, due to the surface force of nanoparticles; this means that a stable suspension of nanoparticles can be produced with a low sedimentation rate. Secondly, the size- and shape- dependent properties of nanoparticles are greater in number than those of microparticles; these include their optical, electrical, thermal, mechanical, rheological and magnetic properties Silver nanoparticles Silver is a noble metal and is resistant to both oxidation and corrosion in moist environments. Its melting point for most solutions is around 961 C [53]. Because of the possible applications of Ag nanoparticles, including potential use as antibacterial, antiviral and antifungal agents, increasing significance is being attached to them; the examination of silver nanoparticles is taking priority over other metals, such as gold and copper, because the energy of the surface plasmon resonance of silver is considerably far from the energy of interband transition [98]. The applications of Ag nanoparticles can be divided into several fields, most important application is in bioscience and biotechnology [99]. Ag nanoparticles have been synthesised using many methods, such as the photoreduction method [100] and biosynthesis [55, 73]. Furthermore, during recent decades researchers have succeeded in developing a synthesis of Ag nanoparticles using a laser ablation route in liquid environments [51-52]. 59

60 Chapter 2: Literature review In most published papers, an optical absorption spectrum has been recorded after the laser ablation process in order to confirm the existence of Ag nanoparticles in colloidal solutions. The absorption spectrum of Ag nanoparticles consists of two absorption bands: the first is the strong absorption band, in which around 400 nm forms are located, due to the plasmon band; the second band is the weak absorption band, in which between 200 nm to 250 nm forms are located, as a result of interband transitions [63]. Figure 2-15 shows the general features of the absorption spectra of silver nanoparticles synthesised using a ( =532 nm) laser ablation process on a silver metal plate in a 0.01 M SDS solution [51]. Figure 2-15: Absorption spectra of Ag nanoparticles [51] The general features of the absorption spectrum of the Ag colloidal solution were considerably different depending on the laser beam spot size used; a narrow and symmetrical plasmon peak was obtained when using a small spot size, while a wide and asymmetrical peak was obtained using a larger spot size [99]. Ganeev et al. [98] have concluded from the work of Brause et al. and their previous paper that the plasmon peak position changes depending on the method of preparation, but generally speaking, when using chemical methods the plasmon peak is located in the range of 415 to 425 nm, whereas when the laser ablation technique is used, the peak is located in the range of 400 to 410 nm. When the laser ablation technique is used, the plasmon peak is shifted to a shorter wavelength; this blue shift phenomenon occurred as a result of the decrease in the average size of the Ag nanoparticles. 60

61 Chapter 2: Literature review Tsuji et al. [101] have obtained differently shaped plasmon bands of Ag nanoparticles by using different laser beam wavelengths; this phenomenon occurred due to the generation of different sizes of nanoparticles [101], causing the plasmon peak edge of the silver nanoparticles to extend to the UV region [51-52, 102]. Phuoc et al. [64] concluded that the shapes and the intensities of the plasmon band in the absorption spectra depended on the regime of laser operation [64]. After irradiating Ag nanoparticles in a colloidal solution using an unfocused laser beam wavelength (400 nm), blue shift occurred at the plasmon peak; further to this, the width of the plasmon peak was reduced. These occurrences confirmed that the average size and size distribution of Ag nanoparticles after laser irradiation had been reduced. Additionally, the red shift phenomenon and broadening of the absorption spectrum occurred after laser irradiation using a longer laser wavelength. These cases suggest the tendency of formation of the larger Ag nanoparticles and their agglomeration [103]. As mentioned previously, the surface plasmon peak of the UV-Vis spectrum of the Ag nanoparticles was increased by increasing the laser beam frequency, leading to a red shift from 398 nm to 404 nm, which can be attributed to the increased particle size [102]. Furthermore, the optical absorption spectrum was affected by scattering, the optical path, particle size and concentration [104]. Nikov et al. [105] presented a blue shift of the plasmon peak a day after the colloidal solution was prepared; however, the researchers reported that the unchanged plasmon peak was evidence of the stability of the colloidal solution. Mafune et al. [52] have taken from Petit et al. that the interband transition of Ag nanoparticles did not change considerably with a change in particle size, but that the intensity of the band changed with the amount of Ag atoms per quantity of nanoparticles; in essence, the absorbance of the colloidal solution at the interband transition wavelength changed with the number of Ag atoms in the colloidal solution. In conclusion, if the size distribution of nanoparticles is known, the number density of nanoparticles can be calculated from the absorbance [52]. Furthermore, the interband transition can be used to indicate the increase in the formation efficiency of a colloidal solution by increasing the laser wavelength under fixed experimental conditions [63]. On the basis of interband 61

62 Chapter 2: Literature review transition intensity, Tsuji and co-workers [54] found that the formation efficiency of particles using nanosecond laser ablation was much higher than that exhibited when femtosecond laser ablation was used. The plasmon band position of an Ag colloidal solution depends on the type of liquid environment used during laser ablation; for example, in the cetyltrimethylammonium bromide CTAB solution the plasmon band was shifted to a longer wavelength than the plasmon peak in the SDS solution. This phenomenon indicates that larger Ag nanoparticles were formed in the cetyltrimethylammonium bromide CTAB solution [56]. The plasmon peak of Ag nanoparticles is not only affected by the type of liquid, but also by the chemical concentration of the liquid under the same laser beam parameters [51]; the plasmon band can also be changed by changing the wavelength of ablation laser light [63]. Dolgaev et al. [61] found that the intensity of the plasmon band of a colloidal silver solution in water was increased with exposure time (irradiation time), but that its position was not changed considerably. Tsuji et al. [97] concluded that when a 0.2 mm NaCl solution was added to pure water in dark conditions, the plasmon peak of Ag colloids was not changed significantly, but after secondary irradiation using an unfocused laser beam, at a laser intensity of 50 mj/cm 2 and an exposure time of 10 minutes, the plasmon peak was decreased and broadened in both solutions. The shape of the absorption spectrum and the peak position can be used to describe the shape and size of nanoparticles. For instance, the red shift phenomenon of the absorption spectrum of gold nanoparticles represents the formation of larger nanoparticles. In addition, single-peak and double-peak absorption spectra indicate the generation of spherical and elliptical nanoparticles respectively. On the other hand, the optical absorption spectrum of a well-distributed spectrum of sizes of nanoparticles is narrow, but the widely dispersed nanoparticle spectrum is wide. The size of nanoparticles can be determined by the peak position, and the amount of nanoparticles can be determined by the intensity of the absorption peak [60]. 62

63 Chapter 2: Literature review Titanium dioxide nanoparticles Titanium dioxide, (TiO 2 ), also known as titania, exists in nature in three different phases; anatase, rutile and brookite. The type most commonly found in nature is rutile, and the rarest is brookite [106]. The amorphous TiO 2 particles can be converted to the anatase phase after calcination at 300 C [107]. Furthermore, anatase TiO 2 can be converted to rutile TiO 2 by increasing its temperature from 673 to 1273 K [108] or by raising the substrate temperature above 600 C [109]. This conversion depends on the nature of the precursor used. Figure 2-16 shows the XRD spectrum of TiO 2 and its phases [106]. The physical and chemical properties of TiO 2 nanoparticles strongly depend on their morphology, particle size and phase structure [67]. Figure 2-16: X-ray diffraction spectrum of pulsed laser deposition of a thin film of TiO 2 and the three crystal phases of TiO 2 (anatase (A), rutile (R) and brookite (B)) [106] Many methods have been developed for the generation of Ti nanoparticles and TiO 2 nanoparticles or films, such as pulsed laser deposition to prepare brookite-rich TiO 2 thin films [106], pulsed laser ablation in a gas flow 63

64 Chapter 2: Literature review chamber [110] or in both deionised water and an SDS solution [66-67]. The optimal TiO 2 nanoparticles were prepared at 600 C, as they had the smallest range of size distribution and the smallest average size [109]. Tian et al. [111] prepared mixed-phase (anatase and rutile) TiO 2 nanoparticles using a laser ablation technique in water. The researchers improved the photocatalytic activity of TiO 2 nanoparticles under UV irradiation the resultant nanoparticles are expected to be useful for water cleaning. At low temperatures, both anatase and rutile phases were observed, but upon increasing the temperature to 1000 C, the uniphase of titanium (rutile) was observed. In addition, the rutile phase was formed before anatase, as it was located in the shell of the polycrystalline nanosphere [111]. Furthermore, the mixed phases of TiO 2 nanoparticles were prepared via continuous-wave laser ablation in a 0.01 M SDS solution with a laser power of 250 W [58]. Chang-Ning et al. [112] generated non-stoichiometric titanium oxide compounds (TiO 2, TiO and Ti 2 O 3 ) more than amorphous phase using a pulsed laser ablation technique in water at a low power density. However, more oxidised crystallites of TiO 2 were observed at higher power density rates than TiO and Ti 2 O 3 when the laser was Q-switched [112]. In addition, anatase TiO 2 nanoparticles were prepared by pulsed laser ablation in water [113] and Ti nanoparticles were formed using a femtosecond laser pulse in a liquid environment (water, ethanol and n-propanol saturated with hydrogen). Moreover, the formation of cavities was observed within Ti nanoparticles, to the extent of about 20 to 50% of the particle volume. However, the case of the nanosecond laser pulse was observed to produce faceted nanoparticles with faceted cavities inside [57]. Shah et al. [109] studied the effects of temperature deposition and size distribution on a pure TiO 2 nanostructure. It was shown that anatase nanoparticles with a polycrystalline structure were obtained when the substrate temperatures were between 350 and 600 C. The development of anatase particles was attributed to the competition between the effect of calcinations and deposition [109]. The size, shape and structural phase of TiO 2 products depend on the heating time and the amount of titanium precursor used to generate the TiO 2 [107]. 64

65 Chapter 2: Literature review Ag-modified TiO 2 The main purpose of doping titanium dioxide with other metals such as noble metals and transmission metal ions is to promote its antibacterial activity (ABA) and photocatalytic activity (PCA) [7, 109, 114]. Ag-doped/composited TiO 2 can be synthesised using several methods, including hydrothermal [114], sol-gel [ ], impregnation via photoreduction [120], photochemical [121], microemulsion [122] and a combination of two bottom-up and top-down methods [8]. Shah et al. [109] concluded that the photocatalytic activity of TiO 2 doped with the transmission metal ions Nd 3+, Fe 3+, Pd 2+ and Pt 4+ was higher than that observed in undoped TiO 2. Furthermore, it was shown that the promotion of photocatalytic activity depends on the size difference between the host ions (Ti 4+ ) and the doping substance's ionic properties. Another purpose of doping TiO 2 with other metals is to reduce the energy band gap of TiO 2 nanoparticles in order to improve the photoactivation because small E g allows the delay in recombination rate between elections and holes [123]. The reduction of the band gap is strongly dependent on the concentration of the doping agent, as shown in Figure 2-17; in this case, the maximum band gap reduction of Nd-doped TiO 2 nanoparticles was 0.55 ev for a 1.5% concentration of Nd [109]. Figure 2-17: Band gap dependence on doping agent concentration [109] Liu et al. [124] synthesised Ag-deposited TiO 2 particles via a laser ablation technique in deionised water. TiO 2 nanoparticles with sizes ranging from 20 to 30 nm were coated with Ag nanoparticles. The Ag/TiO 2 nanoparticles 65

66 Chapter 2: Literature review were then synthesised using a Nd:YAG laser with =1064 nm, =7 ns, f=10 Hz, laser spot size (in diameter) = 1 mm and laser power = 100 mj/pulse. A titanium metal plate was fixed to the bottom of a glass vessel filled with 30 ml of deionised water. During the laser ablation process (which lasted for 20 minutes) the vessel was rotated uniformly at a rate of 30 rpm. The Ti target was then removed and replaced with a piece of Ag metal plate and the laser ablation process was continued under the same conditions. Once the laser ablation process was complete, the colloidal solution of Ag/TiO 2 was then dropped onto a piece of silicon acting as a substrate. Once the water had evaporated, the sample was annealed at 400 C for 2 hours. It was concluded that the Ag/TiO 2 nanoparticles exhibited enhanced photocatalytic activity. Figure 2-18 shows the optical absorption spectra of Ag/TiO 2 and TiO 2 nanoparticles. It can be seen that the edge of the plasmon peak of the Ag/TiO 2 was extended into the visible light range. Figure 2-18: Absorption spectra of TiO 2 NPs and Ag-deposited TiO 2 particles in water [124] Whang et al. [125] generated Ag-doped TiO 2 nanoparticles via a nanosecond pulsed Nd:YAG laser ablation process in isopropanol. After preparing the AgNO 3 (99.5% purity) and TiO 2 then mixing them with different ratios (0.5, 1.0, 2.0, 5.0 and 10) weight percent of AgNO 3 with a fixed total weight of 10 g, each ratio was mixed into 10 g of isopropanol (99.9% purity). Once the prepared samples had been mixed thoroughly, they were then sonicated for half an hour. Next, a pulsed Nd:YAG laser with parameters of =532 nm, E=25 mj and f=10 Hz was used to conduct the ablation process for 66

67 Chapter 2: Literature review 1 hour. Finally, once the samples had dried in ambient air, they were annealed at 200 C for 1 hour. All of the samples were investigated using TEM, XRD, SEM, EDS and UV-Vis techniques. Upon investigation and testing the samples using the degradation of Methylene blue (MB) in aqueous solution, it was concluded that the photocatalytic activity of the samples had been enhanced. After illumination for 2 hours with a halogen lamp, the maximum that the MB had degraded was 82.3% in the case of 2 wt.% Ag. Joya et al. [126] and [127] prepared Ag-TiO 2 thin film nanostructures via dual sol-gel and laser-induced techniques, the samples were prepared by immersing the as-dried TiO 2 coated glass slides in to an AgNO 3 (0.01M) aqueous solution for 15 minutes. The samples were then washed in deionised water and dried in ambient air at room temperature. Finally, the prepared samples were irradiated using a KrF Excimer laser ( =248 nm and =13-20 ns). A mask was used to obtain a laser spot size of 0.25 cm 2. During the laser process the sample was moved uniformly to ensure that an area of cm 2 was covered. The laser was operated under the conditions f=10-15 Hz, with a fluence = 85 mj/cm 2 and a number of pulses between 50 and 200. For ease of comparison, the Ag-TiO 2 films were sintered to 700 C for 60 minutes in a furnace to produce a nanocrystalline anatase structure. In addition, a UV light source was used to illuminate the Ag-TiO 2 samples for 4 to 5 hours to reduce the Ag +2 ions to Ag metal. The samples were examined using FEG-SEM imaging and EDX analysis, TEM imaging, XRD spectrum and UV-Vis spectroscopy to reveal their surface morphology, crystalline structure, phase conversion, sample thickness and optical properties. As a part of the research, the antibacterial activities of the Ag-TiO 2 samples from both methods of preparation were evaluated against E. coli bacteria under UV irradiation. The particle sizes were measured using Scheerer s formula. (2-9) where D is the average particle size, B is the width of the peak at FWHM, is the X ray wavelength and b is the Bragg s angle in degrees. As a part of the research, the thermo-physical phenomenon of a TiO 2 film was numerically simulated, with its thickness assumed to be 350 nm on glass. 67

68 Chapter 2: Literature review TiO 2 nanocomposite was modified not only by Ag nanoparticles, but also by Au and Pt nanomaterials, for example Yoon et al. [128] prepared Pt/TiO 2 nanocomposite thin films on Indium Tin Oxide (ITO) glass substrates by pulsed laser deposition using a Nd:YAG laser with =355 nm, =7 ns, f=10 Hz, with an irradiated laser intensity of 5.7 J/cm 2 and a Pt concentration of 20 wt%. Gyorgy et al. [129] prepared Au-TiO 2 nanocomposite thin films on a quartz substrate using a dual-laser and dual-target technique. Au nanoparticles were embedded in a TiO 2 matrix using two synchronised laser sources. The first laser tool was an ArF excimer laser with = 193 nm and = 12 ns, and the second laser was a Nd:YAG with = 355 nm and = 10 ns. It was concluded that a continuous band gap can be achieved through controlling the growth parameters by comparing them with undoped TiO 2 thin films in the near UV range; as a result, the surface plasmon resonance peak position was extended to the visible light range. Sasaki et al. [66] generated Pt/TiO 2 (bi-combinant) nanocomposite particles by pulsed laser ablation in deionised water. A target consisting of a mixture of sintered pellets of Pt and TiO 2 was fixed to the bottom of a beaker filled with deionised water. A pulsed Nd:YAG laser with a third harmonic generation wavelength 355 nm, f=10 Hz and =7 ns and spot size diameter = 1 mm was then used to irradiate the target for 1 hour. It is worth mentioning that generation and modification of Ag-TiO 2 nanoparticles by picosecond laser never been reported, although there have been a few investigations based on the use of a nanosecond laser Nanoparticle generation by laser ablation in liquid environments On the whole, during the last two decades, methods of nanoparticle generation using laser ablation of a solid-state material in a vacuum chamber [ ], controlled gas atmosphere [70] and air [54, 132] have been developed most widely, but the advancement of techniques performed in liquid environments or aqueous solutions has been of the greatest interest to researchers [51-52, 56-57, 66, 98-99, 101, , 112, 133]. Tsuji et al. [54] mentioned that Henglein and co-workers were the first to use laser ablation for preparing nanoparticles in an aqueous solution. Similarly, copious research studies have been published on nanoparticle generation by laser ablation in 68

69 Chapter 2: Literature review deionised water (or distilled water) [53-54, 61-62, 67, 97, 101], sodium dodecyl sulfate (SDS) [51-52, 56, 66-67], ethanol and ethylene chloride [61, 98, 134], acetone [103, 133], Polyvinylpyrrolidone (PVP) [68-69] and liquid nitrogen solutions (LN) [135]. For this purpose, both continuous-wave (CW) laser beams [50, 58, 64, 71, 97, 105, 124, ] and pulsed laser beams [51-53, 56, 61, 101, 135] were used. The main appeal of generating nanoparticles by laser ablation in liquid environments is the ability to control their sizes and size distributions. In order to establish this purpose, many attempts have been made by changing the laser beam parameters [101], the experimental configurations [68] or the aqueous solution [56, 61-62, 98] Effects of laser beam parameters on nanoparticles First of all, the formation of metal sols can only occur while the laser fluence is greater than the melting threshold of the ablated metal [53]. Tsuji et al. [101] reported that the efficiency of Ag NPs production depends on laser fluence: at low fluence (900 mj/cm 2 ) (unfocused laser beam) the ablation efficiency was higher at shorter wavelengths, but at high fluence (> 12 J/cm 2 ) (focused laser beam) the ablation efficiency was higher at longer wavelengths for the same exposure time. It was also found that, at a laser wavelength of 532 nm, a colloidal solution was formed more quickly than at 355 nm, so the efficiency of nanoparticle ablation was increased dramatically at 532 nm in relation to that observed at a wavelength of 355 nm [101]. This is might be due to high self-absorption at shorter wavelengths. Furthermore, the size and size distribution of nanoparticles in the colloidal solution were affected by the laser wavelength; by decreasing the laser wavelength, the particle size was decreased but their spherical shapes did not change. In summary, a smaller particle size was obtained using shorter laser wavelengths [63]. In addition, the size of the nanoparticles could be reduced further by increasing the exposure time of the suspension [139]. Tsuji et al. [63] concluded that, under their experimental conditions, that the formation of a colloidal solution of Ag nanoparticles was accelerated by 69

70 Chapter 2: Literature review increasing the laser wavelength and that the sizes of the nanoparticles can be controlled by changing the photon energy of laser (wavelength). In contrast, Simakin et al. [53] showed that laser fluence has no effect on fresh colloidal silver. Another important laser beam parameter is laser pulse width ( ); the formation of an Ag colloidal solution prepared using nanosecond (ns) pulses was considerably more efficient than femtosecond (fs) pulses. The size distribution of Ag nanoparticles in the colloidal solution prepared using nanosecond pulses was greater than in the solution prepared using femtosecond laser pulses. It was also shown that the ablation efficiency of nanosecond laser pulses was similar in both water and ambient air, but for femtosecond laser pulses, the ablation efficiency was higher in air than in water [54]. The efficiency of the ablation process was also affected by the size of the laser spot and the exposure time [99]. It was reported that the ablation efficiency in the vacuum was reduced with further increase of laser fluence, caused by increasing the duration of the atomization stage [ ]. The silver nanoparticles changed morphologically in the colloidal solution from nanodiscs to nanowires, following laser irradiation under specific laser conditions [103]. Tsuji et al. [97] reported a significant increase in the number of crystal shaped particles over time among Ag colloids containing 0.2 mm of NaCl after laser irradiation at 355 nm. Tsai et al. [110] concluded that the average size of TiO 2 particles was increased slightly by increasing the oxygen flow rate, but that this was ultimately independent on laser power. Phuoc et al. [64] generated Ag nanoparticles using a double-laser beam ablation process in deionised water. It was reported that the size, size distribution and concentration of the Ag nanoparticles depended significantly on the laser and experimental conditions such as time exposure and laser beam intensity. Laser beam energy can be modified in order to control the size of Ag nanoparticles [56]; for example, at a laser fluence of 0.09 J/cm 2 using a singlebeam method, the average size of particles was about 29 nm, but for the double-beam method the particles were found to measure about 18 nm in size. On the other hand, for a laser fluence of J/cm 2 using both single- and double-beam technique, the average sizes of particles were around 18 nm and 70

71 Chapter 2: Literature review 40 nm respectively. In summary, not only did the ablation rate increase when using a double-beam technique, but the morphology of the nanoparticles changed as well [64]. The effect of laser frequencies on the size and size distribution of Ag nanoparticles in pure water was reported by Darroud and co-workers [102]; the authors determined that the mean diameter of the Ag nanoparticles and their size distribution were increased by increasing the laser beam frequency. Noel et al. [130] concluded that the ablation rates (nm/pulse) (the ablation depth per laser pulse) of copper and gold in a vacuum chamber increased with increasing laser fluence but were almost independent of the laser pulse duration, and plume temperature was observed to be independent of the laser fluence. Hermann et al. [131] determined that the ablation depth of copper and gold in a vacuum chamber increased gradually at low laser fluence up to 0.5 J/cm 2, but that this figure increased considerably at high laser fluence. Semerok et al. [140] concluded that the ablation efficiency depends on the nature of the metal target, in that metals which have a lower melting point and harder than other metals have a higher ablation efficiency for all ranges of pulse regime (nanosecond (ns), picosecond (ps) and femtosecond (fs)) and for every laser beam wavelength. In addition, the ablation efficiency depends on the laser spot size and wavelength. It was concluded that fs laser pulses have greater ablation efficiency than ns and ps laser pulses. This is because in the case of femtosecond laser ablation, the laser pulse terminates before the energy is completely redistributed in the solid target materials. On the other hand, for both ns and ps laser pulses, the efficiency of the ablation process is higher at shorter wavelengths. Moreover, the rate of ablation is increased with laser increasing pulse duration when the latter is longer than the ideal relaxation time. Imam et al. [141] showed the effects of laser pulse energy on the average size of Au nanoparticles (see Table 2-2). 71

72 Chapter 2: Literature review Table 2-2: The relation between energy and average size of obtained Au NPs [141]. Energy (mj) Average size (nm) Effects of liquid environment on nanoparticles in laser ablation in liquid As the structure of nanoparticles is affected by the type of solvent used, the selection of an appropriate solvent is very important in order to control the characteristics of nanoparticles when generating them using a laser ablation technique in liquid environments [65]. Ruth and Young [139] showed that the ablation process did not strongly depend on the solvent. Mafune et al. [52] found that the average size of Ag nanoparticles decreased with increasing sodium dodecyl sulfate (SDS) concentration for fixed laser pulse energy and increased by increasing the laser pulse energy for fixed SDS concentrations. The researchers ultimately concluded that the formation of Ag nanoparticles depends on the type of surfactant, surfactant concentration and the type of solvent used [52]. A colloidal solution of Ag nanoparticles was prepared without any surfactants, merely consisting of a chain of aggregated nanoparticles. On the other hand, the colloidal solutions with anionic SDS and cationic Cetyltrimethylammonium Bromide (CTAB) were observed to be morphologically different from the colloidal solution that did not contain surfactants. It was also shown that the plasmon band of the Ag nanoparticles in the CTAB solution shifted to a longer wavelength (red shift) than that in the SDS solution. This phenomenon indicates that the average size of Ag nanoparticles in the CTAB solution increased. Finally, it was concluded that the colloidal solution of Ag nanoparticles prepared with SDS was more stable than the solution prepared 72

73 Chapter 2: Literature review with CTAB [56]. Tsuji et al. [97] have since promoted a new technique to control the structure of Ag nanoparticles using a minimal amount of chemical reagents. Kazakevich et al. [62] determined the effect of the type of solution on the structure of Ti nanoparticles prepared by laser ablation in a liquid environment under the same laser energy conditions. After the laser ablation process was conducted in ethanol, the structure of the Ti nanoparticles converted from a tetragonal structure to a cubic structure. This structure is known as a metastable structure, and only appears at temperatures greater than 600 C. In water, laser ablation of Ti results in the generation of non-stoichiometric oxide TiO x nanoparticles (x=1.04). Finally, in dichlorethane, titanium carbide TiC nanoparticles were synthesised. Sasaki et al. [66] prepared TiO 2 nanoparticles in a SDS solution and concluded that the concentration of SDS in the solution has a significant impact on both the crystallinity of the nanoparticles produced and their stability [66]. The largest quantity of ultrafine TiO 2 nanoparticles (anatase) was synthesised in the solution with an SDS concentration of 0.01 M; the particles were observed to have a mean size of around 3 nm [67]. Khan et al. [50] prepared NiO nanoparticles using a CW fiber laser ablation process in water, producing nanoparticles with an average size of about 12.6 nm; however, after adding a 0.01M SDS concentration to the solution, it was found that the average particle size was reduced to 10.4 nm. Not only did the increased SDS concentration modify the size of the NiO nanoparticles, but it also transformed their shapes from the spherical shape they had assumed in water to a tetragonal structure. A similar phenomenon was observed by Tsuji and co-workers [68]: the Ag nanoparticles were prepared by laser ablation in clear water, and their sizes were reduced after adding a PVP solution of up to 6 mm concentration, but the particle sizes were less affected by secondary laser irradiation in PVP solutions. Tsuji et al. [54] found that for the femtosecond laser pulse ablation regime, the ablation s efficiency in water was considerably less in comparison to the same case conducted in air [54]. The rate of nanoparticle generation in air was observed to be 100 times higher than in water [70]. 73

74 Chapter 2: Literature review 2.4. Antimicrobial and photocatalytic activities of Ag modified TiO 2 nanoparticles Introduction One of the most important properties of nanoparticles is the antimicrobial activity found in some types of nanoparticles. For example, silver nanoparticles (Ag NPs) can be used as an effective antimicrobial eliminator, making them a promising candidate for medical devices and antimicrobial control systems [136]. On the other hand, the nanostructure of titanium dioxide (TiO 2 ) can be used to promote biomedical devices, antimicrobial coating, photocatalysts and solar cells [137]. In addition, Ag-TiO 2 nanoparticles can be used for applications which require the high pure nanoparticles, such as photocatalysis, sensing devices, homogeneous organic catalysis [71], biomedical applications and water purification [138] Antibacterial activity of Ag nanoparticles Silver nanoparticles are a very promising type of nanoparticles as an antimicrobial agent for the development of new antimicrobial systems. Significant improvements in the antibacterial activity of silver nanoparticles are due to their specific effects such as an adsorption at bacterial membrane [142]. Silver nanoparticles incorporated in dressings as antimicrobial agent to reduce or prevent infections. In addition, the nanoparticles used in cotton and silk cloth due to its bactericidal effect against S. aureus [143]. Functionalized Ag NPs, such as encapsulate with a functional group rich gum, are promising candidates for various pharmaceutical, biomedical, and environmental applications [144]. Furthermore, protein-conjugated Ag nanoparticles were used to improve novel antimicrobial agents for antimicrobial packaging materials and burn wound healing and treatment [145]. Most researchers have explained the antibacterial activity of silver nanoparticles on the bases of the presence of an Ag core. They provided a mechanism that When put in contact with bacteria, Ag nanoparticles tend to accumulate at the bacterial membrane, and form aggregates. In these conditions, several authors reported the diminution of the bacterial membrane 74

75 Chapter 2: Literature review integrity, and observed its perforations leading to cellular death [ ]; however, the antibacterial activity of the nanoparticles is strongly depended upon the size of the nanoparticles [148], the different size should not interact with the bacteria in the same way and have the same action mechanisms [142]. In addition, reactive oxygen species (ROS) produced by silver nanoparticles has also been considered as a main mode of cytotoxic action of the silver nanoparticles [149]. In this case, the cells sustain a very high oxidative stress; as a result, the cellular will be inactivated. It was considered that the role of Ag nanoparticles are a catalytic role in the formation of ROS, these oxygen species are a natural by-product of the oxygenic respiration [142]. It has been shown that the antibacterial activity of silver nanoparticles is directly related to silver ions. Silver ions release from silver nanoparticles when contacted with a liquid environment. Some other mechanisms of silver interaction with bacteria give the main role to silver ions. In this case, silver ions release involves an oxidative dissolution of the Ag nanoparticle, and leads to the presence of an oxidizer [142]. A phenomenon suggests possible antibacterial mechanisms of silver ions by which they prevent bacterial growth. After the silver ion treatment, first shrinks the cytoplasm membrane and leads to separation from the cell wall. Then, the cellular contents will be released from the cell wall, finally the cell wall will be decomposed [150]. It is worth mentioning that the researchers provided different antibacterial mechanism of silver against microorganisms. For example, in the case of E. coli the mechanism is alteration of membrane permeability and respiration [151], for S. aureus is irreversible damage on bacterial cells [152], for S. epidermidis is inhibition of bacterial DNA replication, bacterial cytoplasm membranes damage, modification of intracellular ATP levels [153], and Listeria monocytogenes is due to morphological changes, separation of the cytoplasmic membrane from the cell wall [ ]. Recently Roy et al. [156] produced silver nanoparticles from aqueous solution of silver salts using yeast (Saccharomyces cerevisiae) extract. The photocatalytic study of these biogenic silver nanoparticles concludes that they have efficiency to degrade methylene blue (MB) under solar irradiation. 75

76 Chapter 2: Literature review Pal et al. [157] compared bactericidal properties of Ag nanoparticles of different shapes. It was concluded that the Ag nanoparticles undergo a shapedependent interaction with the gram-negative organism E. coli. In addition, Raza et al. [158] observed that the smallest-sized spherical silver nanoparticles have a better antibacterial activity against two types of Gram-negative bacteria, Pseudomonas aeruginosa and Escherichia coli Photocatalytic activity of TiO 2 nanoparticles Photocatalytic activity is an important trait of some types of nanoparticles, in which chemical reactions are produced by the introduction of photons to the nanoparticles. For instance, TiO 2 has photocatalytic properties. When TiO 2 is illuminated under a light source with photon energy equal to or greater than the energy band gap of TiO 2, an electron will transfer from the lower energy band (valance band) to the upper energy band (conduction band). An electron (e - ) and electric hole (h + ) will then be produced on the surface of the photocatalyst. Combining the electron (e - ) with oxygen (O 2 ) will produce the oxygen radical ions O 2-. Similarly, combining the (h + ) with water (H 2 O) will produce hydroxyl radicals OH -. The important point here is that the oxygen radical ions O 2- and hydroxyl radicals OH - are able to undergo secondary reactions because both products are unstable [ ]. As a result, when the surface of TiO 2, which is rich with both products, comes into contact with an organic compound, carbon dioxide CO 2 and water (H 2 O) will be generated [159]. See Figure 2-19 for an illustration of this process [159], and equations below [161]. 76

77 Chapter 2: Literature review Figure 2-19: The mechanism of the photocatalytic properties of TiO 2 [159] This phenomenon can be used as a self-cleaning process; in nature, a basic form of this phenomenon occurs in lotus leaves [159]. Pure TiO 2 (that has not been subject to the doping process) has a high energy band gap. In this instance, only UV light can produce electron-hole pairs; this means that ordinary light (visible light) cannot be used as a source to produce a photocatalytic reaction in TiO 2 [162], because there is a small amount of UV is available in the sun light. In order to overcome this limitation and enable TiO 2 s photocatalytic ability to operate within a visible light range, TiO 2 may be modified via a deposition or doping process with noble metals [114], transition metals ions [109] or non-metallic elements [163] Antimicrobial activity of Ag-modified TiO 2 Widespread use of antibiotics in the past has given rise to the problem of microorganisms becoming resistant to antibiotics. Researchers have consequently proposed that an alternative to antibiotics is necessary to solve this issue. The unique properties of both Ag and TiO 2 nanoparticles, especially when they are combined, have caught the attention of researchers, particularly in terms of their biological applications as a new alternative antibiotic to fight bacteria [120], viruses [115] and fungi [122, 162]. 77

78 Chapter 2: Literature review It has been demonstrated that TiO 2 exhibits strong antibacterial activity when subjected to UV light irradiation [164]; it has also been revealed that Ag nanoparticles are able to inhibit the growth and multiplication of bacteria [165]. When Ag is doped with TiO 2 nanoparticles, its antibacterial activity improves considerably [114, 120, 166]. Several methods have been devised to evaluate the antimicrobial activity of Ag-doped TiO 2 nanoparticles and thin films, amongst which are minimal inhibitory concentration (MIC) [122, 162], zone of inhibition [122], Agar disc diffusion [118, 121, 163] and the drop test [167] methods. Even the antibacterial mechanism of the nanoparticles not completely understood yet, Kim et al. [115] explained the antimicrobial activity mechanism of Ag/TiO 2 composites as follows: the contact between microorganisms and Ag/TiO 2 nanoparticles has a detrimental effect on the cellular metabolism and prevents cell development. Then the basal metabolism of electron transfer system, breathing and substrate transfer in the cell wall become suppressed. As a result, the evolution and multiplication of microorganisms are inhibited. Finally, the organic structure of the microorganism will be damaged and the microorganism will die after interaction between the Ag/TiO 2 nanocomposite and the functional group of the microbe such as -SH, -COOH, and -OH in the cell membrane of the microbe (bacteria, virus or fungi), as shown in Figure 2-20-a. On the other hand, Pan et al. [168] divided the mechanism of the bactericidal activity of Ag-TiO 2 nanocomposite samples into four steps (Figure 2-20-b). The first is the reduction of the band gap of the sample. The second step is the generation of reactive oxygen species (ROS), hydroxyl radicals (OH), superoxide anion (O 2- ) and hydrogen peroxide (H 2 O 2 ). The third step consists of the oxidation of the cell membrane (lipid peroxidation). Finally, the cell will die as a result of the inactivation of both the cell protein and its DNA. 78

79 Chapter 2: Literature review (b) Figure 2-20: The mechanism of antimicrobial activity of Ag-TiO 2 nanocomposites (a) [115] (b) [168]. Ramesh et al. [136] generated Ag and TiO 2 nanoparticles separately by sonochemical and colloidal methods respectively; the nanoparticles produced were regular shapes and their sizes fell within the range of 20 to 50 nm. The Ag and TiO 2 nanoparticles were used as antibiotic against Staphylococcus aureus, Staphylococcus epidermindis, Escherichia coli and Klebsiella pneumonia. They concluded that the antimicrobial activity of Ag nanoparticles was greater than that of TiO 2 nanoparticles. Figure 2-21 displays images of their results. 79

80 Chapter 2: Literature review Figure 2-21: Antimicrobial activity of Ag NPs against S. aureus and E. coli (A and B) and TiO 2 NPs against S. epidermidis and K. pneumonia (C and D) [136]. Kim et al. [115] examined the antibacterial and antiviral activity of Ag/TiO 2 blending, Ag/TiO 2 formulation, Ag colloidal and TiO 2 sol. The antibacterial activity depends on the properties of each material being produced. The Ag/TiO 2 formulation demonstrated the optimal antibacterial activity. The Ag/TiO 2 blending and Ag/TiO 2 formulation exhibited the maximum antibacterial efficiency against E. coli and Salmonella choleraesuis, as well as notable antiviral activity against Porcine Epidemic Diarrheal Virus (PEDV) and the Transmissible Gastro Enteritis Virus (TGEV). Skorb et al. [166] concluded that the TiO 2 /Ag photocatalysis is more efficient against P. fluorescens than L. lactis. This is due to the different cell membrane structure of gram-negative (P. fluorescens) and gram-positive (L. lactis) bacteria: their wall thicknesses are 2 nm and 40 nm respectively. Furthermore, it was shown that the rate of photoinactivation of bacteria is governed not only by the thickness of the cell membrane, but also by the cell envelope morphology and the resistance of the outer surface cell membrane to the reactive oxygen species. 80

81 Chapter 2: Literature review Liu et al. [120] concluded that the antibacterial activity of the mesoporous anatase TiO 2 films against E. coli was much greater than for commercial P25 TiO 2. This result was attributed to their smaller size, larger surface area and more active sites of mesoporous structure catalysis. Furthermore, the antibacterial activity improved considerably following the doping process. Greater antibacterial activity requires a higher crystallisation of TiO 2. It was noted that Ag ions were released from the Ag/TiO 2 film. The rate at which the Ag + ions were released in Ag/TiO 2 increased sharply during the first 5 days. This problem can be solved by modifying the water diffusion characteristics of the matrix, although the condition is exacerbated in bulk matrices. The amount of ions released by the Ag/TiO 2 film was measured to be mol after 20 days. Antibacterial activity was evaluated by the inactivation of bacteria growth of a gram-negative E. coli cell, as the outer surface membrane of the E. coli was damaged. In order to distinguish between the surviving and dead cells, confocal laser scanning microscopy (CLSM) images were taken of the E. coli on glass, and the mesoporous TiO 2 and Ag/TiO 2 composite substrates were placed under UV light irradiation for 5 minutes. The dead and surviving cells could then be completely distinguished by means of their respective red and green colours. Another antibacterial activity test on E. coli was taken for a glass substrate, P25 spinning film and a mesoporous TiO 2 and Ag/TiO 2 film with and without UV light irradiation. Firstly, the antibacterial activity on the glass substrate was observed to be very weak in all conditions. It can be concluded that weak UV light had a negligible effect on bacteria growth. Secondly, on the P25 spin film, the survival ratios of E. coli were recorded as 77.9% and 53.7% in dark conditions and under UV light irradiation respectively. However, when the UV exposure time was increased to 80 minutes, all of the cells were killed. Thirdly, on the mesoporous TiO 2 film, the survival ratio of E. coli in dark conditions was 60.1%. On the other hand, this figure was reduced dramatically to 7.6% after 5 minutes of UV light irradiation. When the light irradiation time was increased to 20 minutes, all of the cells were killed. Fourthly, on the mesoporous Ag/TiO 2 composite film, the survival ratio of the cells in dark conditions was 9.2% and the cells were completely eliminated after 5 minutes of UV irradiation. Optimal antibacterial activity was therefore observed on the mesoporous Ag/TiO 2 film in comparison with the other samples. It was therefore 81

82 Chapter 2: Literature review concluded that the Ag nanoparticles play an antibacterial role on the mesoporous structure and act as an intensifier for photocatalysis [120]. Sun et al. [167] showed that the rate of antibacterial activity of TiO 2 and Ag/TiO 2 films against the E. coli were about 48% and 99% respectively after 24 hours. On the other hand, in the case of S. aureus, the rate of antibacterial activity of both the TiO 2 film and Ag/TiO 2 film were 42% and 91% respectively after 24 hours, without UV light irradiation. The Ag/TiO 2 films exhibited the best antibacterial activity against E. coli and S. aureus in comparison with the TiO 2 films, because silver ions are released from the surface of Ag/TiO 2 thin films and these ions kill the microorganisms. Yuan et al. [163] prepared Ag-doped TiO 2, N-doped TiO 2 and Ag-N codoped TiO 2 nanoparticles. After evaluating their antibacterial activity against both E. coli and B. subtilis using the agar diffusion test method, it was shown that the pure TiO 2 nanoparticles had no effect on the bacteria but that the N- doped TiO 2 nanoparticles slightly enhanced the antibacterial activity, whereas the Ag-doped TiO 2 nanoparticles enhanced the antibacterial activity considerably. The optimal antibacterial activity was recorded for the 1% Ag-N- TiO 2 sample. Zielinska Jurek et al. [162] concluded that the Ag and Cu co-doped TiO 2 has a greater antibacterial activity against E. coli, S. aureus, the yeast Saccharomyces cerevisiae and pathogenic fungi belonging to the Candida family. It can be concluded that the Ag nanoparticles demonstrate greater antibacterial activity than the Cu NPs prepared by the same method. Zielinska et al. [122] evaluated the antibacterial activity of Ag-doped TiO 2 using both minimal inhibitory concentration (MIC) and zones of inhibition methods against E. coli, S. aureus, Saccharomyces cerevisiae and pathogenic fungi belonging to the Candida family. According to the first test, the highest bioactivity (the lower MIC value) of Ag-TiO 2 was obtained for S.cerevisiae ATCC9763, S. cerevisiae JG and JG CDR1, at 6.5% concentration. It was concluded that the Ag nanoparticles showed better antibacterial activity against E. coli (gram-positive) than against S. aureus (gram-negative) bacteria. According to the second test method, the optimal antibacterial activity was 82

83 Chapter 2: Literature review observed under conditions of 6.5mol% of the Ag and Ag-TiO 2 particles, which inhibited the growth of bacteria and yeast. Chen et al. [169] generated Ag nanoparticles (by bioaffinity adsorption) loaded on the surface of chitosan-tio 2 adsorbent (CTA) material via photocatalysis in order to produce crystal growth. Two methods were employed to investigate the antibacterial activity of the Ag-CTA samples against E. coli, S. aureus and Asper. niger. The first method was a zone of inhibition test. According to this method, the inhibition zones for the E. coli (10 5 CFU) bacteria were around 15 mm on the agar plate. On the other hand, in the case of S. aureus (10 6 CFU) the inhibition zones were nearly 8 mm. No zones of inhibition were observed for the controlled plates with CTA. According to the second test method (MIC test), the E. coli (150 mg of 1.67 wt.% Ag-CTA, cfu) was totally inhibited. In the case of S. aureus, at a low concentration of 2 mg/l Ag nanoparticles in Ag-CTA no growth was observed. In comparison with the other types of bacteria, the effects of inhibition on the Asper. niger were mild. The different responses of these three types of microorganism to Ag-CTA samples were due to the different structures of their membranes. The E. coli (gram-negative) bacterium has a thin membrane, but the S. aureus (grampositive) bacterium has a thick and rigid membrane. Moreover, Asper. niger, a fungus, has a cell wall consisting largely of chilin and other polysaccharides. Joya et al. [126] prepared Ag-TiO 2 thin film nanostructures via dual solgel and laser-induced techniques. As shown in Figure 2-22, the Ag-TiO 2 sample prepared using the SGLIT method exhibited the best antibacterial activity, killing all E. coli bacteria after 30 minutes. The sample prepared using the furnace sintering method showed less antibacterial activity than the samples prepared using the first method. In the first 30 minutes the antibacterial activity increased sharply, but after this point that it occurred very slowly. On the other hand, the controlled sample showed no antibacterial activity - the amount of E. coli bacteria killed in this instance was due to the UV light irradiation. 83

84 Chapter 2: Literature review Figure 2-22: Antibacterial activity of controlled slides and Ag-TiO 2 prepared by sol-gel/laser route and furnace sintering route against E. coli under UV light irradiation using the drop test method [126]. Sauthier et al. [170] synthesised an Ag-TiO 2 nanostructure on SiO 2 quartz substrates using pulsed laser deposition in a stainless steel chamber. The TiO 2 pellet targets were prepared by compressing TiO 2 powder under pressure (3 MPa). The targets were then sintered at a temperature of 1100 C for 4 hours. Following this, the TiO 2 and Ag targets with purities of 99.8% and 99.9% respectively were fixed within the reaction chamber. While being irradiated by a KrF Excimer laser with parameters = 248 nm, = 25 ns, laser fluence = 2 J/cm 2 and f = 10 Hz with an angle of incident of 45, they were also rotated at a uniform frequency (3 Hz) and translated via the x-y surface plane. In order to avoid piercing the SiO 2 quartz substrate fixed parallel to the target, the samples were placed at a distance of exactly 5 cm from each other and heated up to 500 C during the nanoparticle generation process laser pulses were employed to promote the growth of the thin film. The Ag-TiO 2 nanoparticles and films have been used not only for antimicrobial purposes, but also for improving photocatalytic activity. The photocatalytic activity of the Ag-TiO 2 nanoparticles and/or films were tested 84

85 Chapter 2: Literature review using degenerate Methylene Blue [125, ], Rhodamine B (RhB) [ ], Methyle Orange (MO) [114, 117, 160], Methyl Red (MR) dye [181], Malachite Green (MG) [161], C.I. Acid Red 88 (AR 88) [182], Acid Red 44 [183], Acid Orange 7 (AO 7) [184], Indigo Carmine (IC) [119], Reactive Yellow 17 [185], 3,4,6-trichlorophenol (TCP) [116], 4 chlorophenol [186], Pentachlorophenol (PCP) [15], C60 [187] and gaseous toluene [188] as well as for Epoxidation of Styrene Oxide [189] as an organic pollutant under UV and/or Visible light irradiation. Further information and published work on Ag TiO 2 preparation and the investigation of antimicrobial activity is shown in Table Knowledge gap in the literature review During the literature review, it was identified that picosecond laser was rarely used for nanoparticle production and the generation of Ag-TiO 2 compound nanoparticles by a picosecond laser was not reported. In addition, there were nanoparticles that have better antibacterial activity than pure silver nanoparticles. Also the literature review showed that it was difficult to control the nanoparticle combination and the structure of the nanoparticles. No report was found on the use of ice water in laser production of nanoparticles. No report was found on hybrid laser and ultrasonic production of nanoparticles. No report was found on the production of TiO (i.e. TiO 2 core and Ag shell) nanoparticles, although reports were found on the production of Ag@TiO 2 (i.e. Ag core and TiO 2 shell) nanoparticles. No report was found on the production of Ag-TiO 2 nanoparticles using an Ag-TiO 2 alloy bulk Summary Silver and titanium dioxide nanoparticles have been generated by pulsed laser ablation in different environments. They were used as an agent to kill bacteria and organic pollutants. Ag modified TiO 2 nanoparticles were produced by the chemical methods and rarely produced by laser ablation method. In addition, all laser produced Ag-TiO 2 nanoparticles were used two steps. During the review, it was observed that picosecond laser has not been used to 85

86 Chapter 2: Literature review generate and modify Ag-TiO 2 nanoparticles. This is a clear gap in generation of the nanoparticles. Several hypotheses or mechanisms have been provided to explain bacterial-nanoparticle interactions. The mechanism is not fully understood in the case of compound nanoparticles. 86

87 Chapter 2: Literature review Table 2-3 Ag-modified TiO 2 nanoparticles or films for antimicrobial and photocatalytic applications Form and phase Generation method and Short size of nanoparticles Results Ag-TiO 2 nanoparticles Sol-gel The nanoparticles enhanced antibacterial activity. The number of E. coli bacteria decreased as mixing time and catalyst concentration increased. The optimal amount of Ag in the Ag-TiO 2 nanoparticles was 1 mol %. Test Method: Agar disc diffusion. Ag-TiO 2 Powder One-pot sol-gel Enhanced antibacterial activity. When 3.9 wt% Ag/TiO 2 was tested on cfu/ml E. coli with 2.4 µg/ml Ag concentration, the E. coli were completely killed, whereas Phase: for the 7.4 wt%ag/tio 2 (1.2 µg/ml concentration) and 3.9 wt%agtio 2 (1.6 µg/ml Ag-TiO 2 : Anatase Ag concentration), 98.8% and 99.9% of the bacteria were killed respectively. Test Method: Spread-plate test (Lysogeny Broth) LB agar plate Ag-TiO 2 Nanocomposite Phase: Ag: fcc TiO 2 : 90% anatase 10% rutile Ag@TiO 2 nanocomposite particles Phase: Ag-TiO 2 : rutile Ag-TiO 2 NPs Phase: Ag-TiO 2 : Anatase Ag-TiO 2 Nano-composite Phase: As-prepared TiO 2 : amorphous Annealed: anatase Ag-TiO 2 Nanocomposites Phase: N/A Size: TEM Ag: about 3 nm Ag-TiO 2 : about 20 nm Chemical reduction method Size: TEM Ag-TiO 2 : nm PH dependence Photochemical Size: STEM Ag-TiO2: nm Ag: 2-20 nm Microemulsion (water/aot/ cyclohexane) Size: STEM Ag-TiO 2 : 5-10 nm One-step refluxing method using ethylene glycol EG medium under ambient conditions. Size: TEM Ag and TiO 2 : nm A facile bottom-up colloidal approach was used to fabricate nanostructured Ag- TiO 2 nanocomposite Size: TEM Ag-TiO 2 : nm Both Ag-TiO 2 and Ag NPs were completely killed E. coli under visible light irradiation. The influence of Ag- TiO 2 on E. coli bacteria is nearly 5 times higher than that of pure TiO 2 because of the wide E g of TiO 2. The durability of Ag-TiO 2 was prolonged with respect to silver metals content. Test Method: Agar disc diffusion method The Ag@TiO 2 nano-composite particles enhanced antibacterial activity. All of the E. coli bacteria were killed at 10 µg/ml of Ag concentration. Test Method: Lysogeny Broth (LB) agar plate Antimicrobial activity of the Ag-TiO 2 nanoparticles differed depending on silver content, microbial strain and a reducing agent applied during preparation of the nanoparticles. The optimal Ag concentration in Ag-TiO 2 was 6.5 mol.%. They were tested using E.coli and S. aureus, yeast S. cerevisiae and pathogenic fungi belonging to the Candida family. Test Method: MIC and ZI methods. The Ag-TiO 2 NPs annealed at 400 C showed a 100% bacterial inactivation after 90 min, while the Ag-TiO 2 NPs annealed at 500 C revealed better antibacterial efficiency, killing all bacteria only after 60 min. In the case of As -synthesized Ag-TiO 2 NPs, a 2-log decrease was observed after 120 min. While, in the case of the annealed Ag-TiO 2 NPs at 500 C, an 8-log decrease after 120 min in dark condition. The Ag-TiO 2 nanocomposites have strong antibacterial activity against E. coli bacteria because of their large reactive surface area and unique chemical reactivity. It was shown that a 5 mol% Ag results in the most decrease in E g of TiO 2. This is a cause of theoretical mechanisms and interactions between Ag-TiO 2 nanocomposites and the Bacteria, which promote photolysis of water by TiO 2 under both visible light or dark conditions. Ref. [118] [190] [168] [121] [122] [191] [192] 87

88 Chapter 2: Literature review Hybrid Ag-TiO 2 nanoparticles Phase: Mixed Anatase-rutile TiO 2 TiO NPs Phase: N/A Ag-TiO 2 NPs Phase: Ag-TiO 2 : Anatase Ag/TiO 2 core-shell NPs Phase: Ag-TiO 2 : Ag: Crystalline TiO 2 : amorphous Ag/TiO 2 NPs Phase: NPs N/A Wet-impregnation or UV photo deposition Size: TEM TiO 2 : nm Ag: 5 to 20 nm. A photochemical method Size: TEM Ag: 5 nm TiO 2 : 20 nm Sol-gel + arc discharge or combination of two bottom-up and top-down Size: XRD Ag-TiO 2 : 16.8 nm Size: BET model Ag-TiO 2 : 28 nm Colloid-seeded deposition Size: TEM Ag/TiO 2 : 15 nm Ag: 2-4 nm Turkevich Method or citrate reduction method. Size: SEM Ag-TiO 2 : nm and nm Hybrid Ag-TiO 2 NPs showed stronger antibacterial activity than UV alone, Ag/UV, or UV/TiO 2 samples. In the dark condition antibacterial activity of the Ag-TiO 2 NPs was greater than pure Ag and TiO 2 NPs. The NPs produced a synergistic antibacterial effect which unrelated to the photocatalytic activity of TiO 2 NPs. In addition, less silver ions dissolved from Ag-TiO 2 NPs than from pure Ag NPs. Test Method: well diffusion assay. The TiO nanoparticles-functionalized silk fibroin fabric (SFF) is endowed with remarkable UV protection properties and an efficient antibacterial activity against E. coli, S. aureus, and Pseudomonas aeruginosa. In addition, the nanoparticles were degraded almost all methylene orange (MO) under UV light irradiation. The functionalized nanoparticles showed self-cleaning ability. Ag-TiO 2 nanoparticles enhanced antibacterial activity. They were investigated against E. coli bacteria under both UV and visible light irradiation. Test Method: Inactivation of E. coli bacteria under UV and visible light irradiation. The Core-shell nanoparticles enhanced antibacterial activity. Samples were tested against E. coli bacteria. The antibacterial activity of the core-shell form is attributed to its structural feature of silver nanoparticles. Test Method: Inhibition test in Petri dishes. The nanoparticles showed uniform distribution of Ag on TiO 2 particles as observed by SEM-EDX and antimicrobial tests according to the Japanese norm JIS Z 2801 shows excellent antimicrobial properties. In antimicrobial test, a large amount of antimicrobial fillers showed linear increase of antimicrobial properties. [7] [193] [8] [194] [195] Ag-doped TiO 2 Phase: Ag-TiO 2 : Anatase Modified sol gel method. Size: TiO 2 : 50 nm It was shown that the antibacterial activity of Ag nanoforms in Ag-TiO 2 depends on particle size, oxidation state, porosity, Ag content, surface area to volume ratio and the crystalline form of the carrier TiO 2. In addition, the Ag showed excellent antimicrobial activity against E. coli, S. aureus, and Klebsiella pneumoniae. [196] Ag-TiO 2 NPs Phase: Ag-TiO 2 : Amorphous Sol-gel Size: TEM Ag-TiO 2 : 5-10 nm The nanoparticles enhanced antibacterial activity. The samples were tested against pathogenic bacteria such as Enteropathogenic Escherichia coli (EPEC) and Methicillin-resistant Staphylococcus aureus (MRSA), and showed sensibility in most cases. Test Method: Kirby-Bauer disk-diffusion method. [197] 88

89 Chapter 2: Literature review Ag doped TiO 2 Phase: TiO 2 : Anatase Spherical Ag@TiO 2 core shell NPs Phase: Amorphous Ag/TiO 2 composite nanoparticles Phase: N/A Ag-doped TiO 2 NPs Phase: Anatase Ag TiO 2 Composites Phase: Anatase Hybrid Ag NPs - TiO 2 Microsphere Phase: Anatase Sol-gel Size: AFM 3 15 nm One pot simultaneous reduction of AgNO 3 and hydrolysis of Ti(IV) isopropoxide. Size: TEM Ag@TiO 2 : below 50 nm Ag core: 15 nm to 20 nm. TiO 2 shell: 2 to 3 nm. CVD and plasma bombardment method. Size: SEM Ag NPs: nm. An acid-catalyzed sol gel process Size; (TEM) TiO 2 : 10 nm Annealed Ag-TiO 2 : 3% = 8 nm 7% = 5 nm An ultrasound assisted method - a one-step procedure Size: Ag: 2-3 nm TiO 2 : nm Solvothermal and wetimpregnation methods. Size: FESEM TiO 2 : about 500 nm. Ag-doped TiO 2 nanoparticles enhanced antibacterial activity. They were tested against E. coli, S. aureus, Candida albicans, B. Subtilis, and S. typhimurium. Samples were found to inhibit the growth and multiplication of the tested microorganisms. The optimal concentration of the Ag in Ag-doped TiO 2 nanoparticles was µg/ml. Test Method: Zones of inhibition, Disc diffusion method and MIC test method. Spherical Ag@TiO 2 core shell NPs showed higher antibacterial activity against the E. coli bacteria in comparison with the S. aureus bacteria. This is because the peptidoglycan layer in gram-negative (E. coli) is thinner than that in gram-positive (S. aureus) bacteria. In addition, the antibacterial activity is increased by increasing the concentration of the nanoparticles. The concentration of the Ag@TiO 2 core-shell NPs was 100 μg/ml for S.aureus and 200 μg/ml for E.coli. Test Method: well diffusion method. The Ag-TiO 2 nanoparticles have stronger antibacterial activity than pure TiO 2. In addition, Ag NPs showed significant antibacterial activity under both light and dark conditions. Test Method: The survival of bacteria was estimated by a plate count method. Enhanced photocatalytic antibacterial of TiO 2 and Ag-doped TiO 2 against S. aureus. P. aeruginosa and E. coli. It was shown that 7% doped Ag TiO 2 NPs completely killed P. aeruginosa cells at 40 mg/30 ml concentration, while 5% and 4% viabilities of S. aureus and E. coli were obtained, respectively. On the other hand, 3% Ag-doped TiO 2 NPs at 60 mg/30 ml of culture, 0% viability in the case of P. aeruginosa was recorded, while in the case of S. aureus and E. coli 7% and 3% viabilities were recorded. It was also concluded that the E g decrease with the doping of silver ions, that enhanced photocatalytic activity because small E g allows the delay in recombination rate. The E g was 3.15 ev, 2.8 ev and 2.7 ev for annealed TiO 2, Ag TiO 2 (3%) and Ag TiO 2 (7%), respectively. Test Method: % viability (% survival) of bacteria The antibacterial activity of the Ag TiO 2 composite against E. coli bacteria was 2.5 times better than the colloidal Ag solution containing the same initial amount of silver. A high bactericidal effect was found in the absence of UV light. The durability of the Ag-TiO 2 was more than a year. Test Method: Agar plates following serial dilution in saline. The hybrid Ag TiO 2 microspheres showed high antibacterial activity against P. aeruginosa with different amounts of bound extracellular polymeric substances (EPS) under both UV light irradiation and dark conditions. This is because of the synergistic effects of Ag and TiO 2 in hybrid form. Test Method: The photocatalytic inactivation kinetics described by the Chick Watson model [198] [199] [200] [123] [11] [201] 89

90 Chapter 2: Literature review 2 core shell NPs Phase: Anatase Ag-TiO 2 NPs Ag@TiO 2 core Shell Phase: NPs Ag: fcc TiO 2 : Anatase Phase: core-shell TiO 2 : Anatase+Rutile One pot simultaneous reduction of AgNO 3 and hydrolysis of Ti (IV) isopropoxide Size: HRTEM Ag@TiO 2: Ag core: nm. Ag@SiO 2: nm with an average of about 42 nm. The shell thickness is 37 nm A facile method under mild conditions hybrid Ag-TiO 2 nanostructures Size: TEM Ag-TiO 2 : nm The antibacterial activity of Ag@SiO 2 was higher than that of the Ag@TiO 2 NPs at the same concentrations against E. coli and S. aureus bacteria. This is because of the higher surface area of the Ag@SiO 2 than the other nanoparticles. In addition, positively charged Ag reacted easily with gram-negative bacteria than gram-positive bacteria. The concentration of the core-shell nanoparticles was 200 µg/ml for E. coli and 100 µg/ml for S. aureus and Ag@SiO 2 core shell NPs are 100 µg/ml for both E. coli and S. aureus. Test Method: The agar diffusion method It was concluded that the TiO nanoparticles show superior in bacterial inactivation to the Ag@TiO 2 core shell ones, but after post irradiation by a UV light, the Ag@TiO 2 core shell nanoparticles show better antibacterial activity than the TiO ones. It was also found that without UV irradiation, both samples show weak antibacterial activity. Test Method: The survival of E. coli was estimated by a plate count method. [202] [203] 90

91 3 Chapter 3. Picosecond laser generation of Ag-TiO 2 nanoparticles with reduced energy gap by ablation in ice water and their antibacterial activities Authors: Abubaker Hamad, Lin Li, Zhu Liu, Xiang Li Zhong and Tao Wang Journal: Applied Physics A: Materials Science and Processing Volume, issue and pages: 119, 4, Status: Published Note: The format of the paper is edited 91

92 Chapter 3: Picosecond laser generation of Ag-TiO 2 nanoparticles... Picosecond laser generation of Ag-TiO 2 nanoparticles with reduced energy gap by ablation in ice water and their antibacterial activities Abstract Ag-TiO 2 nanoparticles were synthesised in ice water using a picosecond laser with a 1064 nm wavelength, at a 200 khz repetition rate, a laser pulse energy of µj and laser fluences of J/cm 2, by ablation of solid Ag and Ti targets. The absorption spectra and size distribution of the colloidal nanoparticles were obtained by UV-VIS spectroscopy and transmission electron microscopy (TEM) respectively. The morphology and chemical composition of the nanoparticles were characterised using High-Angle Annular Dark-Field Scanning Transmission Electron Microscope (HAADF-STEM) and Energy Dispersive X-ray spectroscopy (EDS). The results show that the sizes of the Ag-TiO 2 nanoparticles range from less than 10 nm to 130 nm, with some large particles above 130 nm, of which the predominant size was 20 nm. A significant reduction of the energy gap of TiO 2 nanoparticles was obtained to 1.75 ev after the modification with Ag nanoparticles during co-ablation. The role of Ag nanoparticles in the reduction of the energy band gap of the TiO 2 nanoparticles can only be seen during laser ablation in an ice environment but not in deionised water at room temperature. Furthermore, the TiO 2 nanoparticles were produced in ice and deionised water under the same laser and experimental conditions; the results show that the nanoparticles in both media have the same energy gap (about 2.4 ev). The antibacterial activity of the Ag-TiO 2 nanoparticles generated was then tested against E. coli bacteria under standard laboratory light conditions. The results show that the Ag-TiO 2 nanoparticles can effectively kill E. coli bacteria much more effectively than laser generated TiO 2 nanoparticles. Keywords: Ag-TiO 2 nanoparticles; energy gap; ablation in ice water; laser ablation; antibacterial activity. 92

93 Chapter 3: Picosecond laser generation of Ag-TiO 2 nanoparticles Introduction The photocatalytic activity of TiO 2 nanoparticles is key for several practical applications such as their uses as an antibacterial agent [123], water purification [204], and self-cleaning [205], but due to their wide energy band gap the photocatalytic activity can only be activated by ultraviolet (UV) light. As a result, TiO 2 nanoparticles are not as active in sun light due to their being only a small amount of UV light available. Photocatalytic activation of the TiO 2 nanoparticles can be improved either by decreasing the band gap [12] or bringing the optical absorption spectra from the UV range to the visible (VIS) range [17]. Several attempts have been made at achieving the above goals including doping or loading nanoparticles of noble metals and non-noble metals such as Ag [206], Au [207], S [ ], Nd 3+ [109], Fe [210], Mn [211] and V [212] into TiO 2 nanoparticles to modify electronic states. Chauhan et al. [206] reduced the energy gap of TiO 2 nanoparticles from 3 ev to 2.8 ev after doping them with 5% Ag nanoparticles and followed by calcinations at 500 C. It was shown that the energy band gap was increased if 10% Ag nanoparticles were doped [206], and overloading of Ag above a specific amount led to antibacterial inactivation [213]. Umebayashi et al. [208] doped TiO 2 with S by oxidation heating of the TiS 2 powder. As a result, the TiO 2 absorption edge was shifted to lower energy. The energy gap, E g, of the TiO 2- xs x produced was 1.4 ev from 2.3 ev. These researchers found that the band gap reduction was due to the occupation of 3p states by S with the valence band gap. After S was used to dope TiO 2, the 3p states of S were delocalized, and as a result they combine to produce valence band (VB) with both O2p and Ti 3d states. Consequently, the combination of the 3p of S and VB causes VB to increase. Shah et al. [109] reduced the energy gap of TiO 2 from 3.2 ev to 2.3 ev after doping with 1.5 at% of Nd 3+. This reduction is due to the substitution of Nd 3+ ion which causes the formation of some electronic states in the TiO 2 band gap, caused by the Nd 4f electrons. A doped material absorption edge can be generated between O 2p and Nd 4f, as is the case in pure TiO 2. Other studies have shown that the energy band gap can be decreased by altering the size of the nanoparticles. Researchers studied and calculated the energy band gaps of different semiconductors, including CdS, CdSe, CdTe, 93

94 Chapter 3: Picosecond laser generation of Ag-TiO 2 nanoparticles... PbSe, InP and InAs quantum dots, using the developed potential-morphing method (PMM) within the effective-mass approximation EMA [214]. Due to the unique antimicrobial properties of certain types of materials in nanoscale compared with those in the bulk form, nanoparticles, such as Ag and Ag-TiO 2 have been used to kill microorganisms. The antibacterial activity of nanoparticles depends on their size [148] and shapes [157]. Smaller size nanoparticles are more effective than the larger sizes to eliminate bacteria [215]. This is because the smaller sizes of nanoparticles have more surface area to volume ratio than the larger nanoparticles, and the atoms located on the surfaces have lower binding energy with neighbors. As a result chemical reactivity is increased [24]. Pan et al. [168] found that the ultrafine Ag-TiO 2 nanocomposites eliminated 100% E. coli bacteria under visible light conditions. Lu et al. [148] showed that the 5 nm Ag nanoparticles have better antibacterial activity than 15 nm and 50 nm of Ag nanoparticles against S. mutans, S. sanguis, S. mitis, A. actinomycetemcomitans, F. nuceatum and E. coli. On the other hand, truncated triangular shaped Ag nanoparticles with a lattice plane (111) were observed to be more efficient to delay E. coli bacteria growth than spherical nanoparticles [157]. The reduction band gape energy of TiO 2 nanoparticles by doping with Ag nanoparticles has been found to improve antibacterial activity due to the promotion of the generation of reactive oxygen species (ROS) such as hydroxyl radicals, superoxide anion and hydrogen peroxide, under visible light exposure. As a result, ROS interacts with cell membrane leading to inactive DNA and cellular protein, and cause cell death [168]. In this work, the authors present a new experimental technique to reduce the energy band gap of TiO 2 nanoparticles. This was achieved by producing the Ag-doped TiO 2 nanoparticles in deionised ice water instead of room temperature water. The antibacterial activity of the nanoparticles was investigated. 94

95 Chapter 3: Picosecond laser generation of Ag-TiO 2 nanoparticles Experimental set-up Materials A 99.99% pure silver plate with dimensions of 25 mm 25 mm 2 mm and a % pure titanium plate with dimensions of 25 mm 25 mm 1 mm were used to generate the Ag-TiO 2 nanoparticles, TiO 2 nanoparticles (for energy band gap comparison) and Ag nanoparticles (for antibacterial activity comparison). Both sample targets were washed several times in deionised water and sonicated for 15 to 20 min in deionised water and ethanol before the ablation process Nanoparticle production Ag-TiO 2 nanoparticles were produced using the bimetallic method; for this purpose the Ag and Ti plates were placed side by side on the bottom of a glass vessel that was filled with small pieces of ice (made from frozen deionised water). To support this ice during the laser ablation process, the glass vessel was also placed inside another larger glass vessel where the space between them was occupied by crushed ice cubes. The level of the ice water above the targets was about 2 to 6 mm. In this work, a 400 W Edgewave picosecond laser was used with the following parameters: wavelength = 1064 nm, frequency f = 200 khz, pulse duration = 10 ps, spot size = 125 µm, laser pulse energy E pulse = µj and laser fluence F laser = J/cm 2. The ablation process was conducted for 10 minutes at a scan speed, v, of 250 mm/s. The experimental setup is illustrated in Figure 3-1. The effects of the ice on the laser beam energy and laser beam focal length were taken into account, but it should be noted that in the correction and calculation of the laser beam intensity or laser fluence it was assumed that the medium was water, whereas the absorption coefficient of ice is actually slightly higher than that of water. For antibacterial activity comparison, pure TiO 2 and Ag nanoparticles were also generated under the same laser beam parameters and experimental conditions but in normal deionised water. 95

96 Chapter 3: Picosecond laser generation of Ag-TiO 2 nanoparticles... Galvo scan head Picosecond laser beam Pyrex glass vessels Ag and Ti plate targets Target holder Ice (frozen deionised water) Figure 3-1: Experimental set-up to generate nanoparticles in ice (frozen deionised water) using a picosecond laser; = 1064 nm, f = 200 khz, = 10 ps, and v = 250 mm/s Sample preparation To characterise the produced nanoparticles using transmission electron microscopy, a copper microgrid mesh (Formvar / Carbon on 200 Copper mesh) was used. A drop of the colloidal nanoparticles was deposited on the copper mesh and then allowed to dry at room temperature. It was repeated several times to collect enough nanoparticles on the mesh. During drying, the samples were covered with a transparent lid to avoid contamination by air dust Characterisation The generated Ag-TiO 2 nanoparticles were characterised in terms of their light absorption spectra, size distribution and elemental compositions using a UV-VIS optical spectrometer (Analytic Jena, SPECORD 250, dual beam), a Transmission Electron Microscope (TEM) (JEOL 2000 FX AEM + EDX), a High- Angle Annular Dark-Field Microscope-Scanning Transmission Electron Microscope (HAADF-STEM) and Energy Dispersive X-ray Spectroscopy (EDS) (FEI Tecnai G 2 F30). The aforementioned equipment and techniques were used to obtain images of the Ag-TiO 2 nanoparticles and their line elemental spectrum. To measure the concentration of colloidal nanoparticles and the amount of ablated nanoparticles in the solution, a microbalance scale (Sartorius BL 210S) with a readability of d = 0.1 mg was used by weighing the bulk Ag and Ti plates before and after the laser ablation process. Before weighing the bulk 96

97 Chapter 3: Picosecond laser generation of Ag-TiO 2 nanoparticles... samples after the ablation process, they were dried using a conventional hair dryer to remove the drops of water to obtain the accurate weight Antibacterial function testing procedure The antibacterial activity of the nanoparticles was tested against E. coli bacteria (JM109 from Promega UK). A single colony of E. coli bacteria (or 10 µl of glycerol stock) was incubated in 10 ml of Lysogeny broth (LB) in a 50 ml tube and cultured at 37 C overnight with constant orbital shaking at 225 rpm. The optical density of the cultured E. coli was measured at 600 nm (OD 600 ) and diluted down to a colony forming unit (CFU)/ml of with LB. 200 µl of colloidal nanoparticles was mixed with 1.80 ml of the diluted E. coli and incubated for 6 hours at 37 C while shaking at 225 rpm. Finally, 10 µl of each dilution was spread on the LB agar plates and left at room temperature under standard room light for about two days (48 hours) followed by counting colonies on each plate. For negative control, 200 µl of nanoparticles was replaced by 200 µl dh 2 O Energy gap calculation TiO 2 is an indirect band gap transition semiconductor [111]. It has three different phases, which include anatase, rutile and brookite phases. These phases have different values of transition band gap energy [216]. Rutile has a nanorod shape, anatase has a spherical shape and brookite has a nanoplate shape [217]. The energy band gap can be calculated using several methods [218]. For example, Reyes-Coronado et al. [216] measured the direct and indirect band gap of the three phases of pure TiO 2 (rutile, anatase and brookite) by plotting graphs of ( KM h ) 2 and ( KM h ) 1/2 vs. energy (h ) in ev, respectively, where, h is the planks constant ( m 2 kg/s), is the frequency and KM is the absorption coefficient based on the Kubelka Munk formula; α KM = (1 R ) 2 /2 R where R is the reflectance as a function of wavelength. The indirect band gap of rutile, anatase and brookite was 3.01, 3.20 and 3.13 ev respectively, while the direct band gap was 3.37, 3.53 and 3.56 respectively. Reddy et al. [219] calculated the indirect gap energy, E g, of 97

98 Chapter 3: Picosecond laser generation of Ag-TiO 2 nanoparticles... anatase TiO 2 nanoparticles by plotting α 1/2 against E phot and the direct band gap by plotting (αe phot ) 2 against E phot, where α is the absorption coefficient and E phot is the photon energy (E phot = (1239/λ (nm) ) ev). The forbidden band gap was calculated after extrapolating E photo to α=0. Verma et al. [220] measured the energy gap of nanoparticles by. This equation was derived from the relationship between the energy and wavelength,, where c is the speed of light ( m/s) and is the wavelength of the used laser beam. Some papers directly used absorbance as a function of energy to find the energy gap [221] using the Kubelka Munk function (F(R ) = (1-R ) 2 /2R ) as the equivalent of absorbance [222]. Tsukamoto et al. [223] drew upon the energy band gap scheme of the Ag-TiO 2 nanoparticles. It was suggested that the low photocatalytic activity of the Ag-TiO 2 was due to the rapid recombination of electron hole pairs. This fast recombination is due to the high Schottky barrier at the Ag-TiO 2 junction (about 0.2 ev). This was calculated from the difference between the work function ( ) (4 ev) and the electron affinity ( ) (3.8 ev) of silver and titania respectively. In the present work the authors calculated the transition energy gap using the following equations [206, 224]: (3-1) (3-2) where is the absorption coefficient, h is the laser photon energy, E g is the energy band gap of the sample, A is a constant and n is a constant depending on the type of band transition. The exponent n has different values depending on the transition type, such as n=1/2 for allowed direct transition, n=2 for allowed indirect transition, n=3/2 for forbidden direct transition and n=3 for forbidden indirect transition band gap. The value of the energy band gap can be determined by finding the intersection of extrapolating a straight line with the x-axis or when or. The absorption coefficient is given by, where k is the absorbance or absorption index and is the laser wavelength. Here in the authors used n=1/2 as indirect and n=2 as direct band gaps of the TiO 2 and Ag-TiO 2 nanoparticles [216]. 98

99 Chapter 3: Picosecond laser generation of Ag-TiO 2 nanoparticles Results The characteristics of Ag-TiO 2 nanoparticles Figure 3-2-a compares the absorption spectra of Ag-TiO 2 colloidal nanoparticles produced in deionised water at room temperature and in frozen deionised water (ice) using the picosecond laser with the laser parameters = 1064 nm, f = 200 khz, E pulse = µj/pulse, F laser = J/cm 2, v = 250 mm/s and t =10 min. Both spectra have the same behavior in the visible light wavelength range, but in the UV range the two spectra are totally different. At 200 nm wavelength, in deionised water without ice the absorption gradually decreases until 260 nm, then decreases sharply to 330 nm, whereas the spectrum obtained in ice water decreased sharply to 240 nm, then decreased moderately until 360 nm. The higher intensity of the Ag-TiO 2 spectra in deionised water is due to their high concentration in comparison with the ice environments. The blue shift phenomenon is evident for the nanoparticles produced in ice water, in which the maximum absorption peak or the surface Plasmon resonance peak is located at 400 nm. On the other hand, in the deionised water the strong peak is at 405 nm wavelength. The images of bottles of the Ag-TiO 2 nanoparticles generated in ice and deionised water are shown in Figure 3-2-b. Their different colours are due to their different concentrations and size distribution. Figure 3-2-c shows the graph of (αhν) 1/2 versus E g (ev) to find the indirect energy gap of the Ag-TiO 2 nanoparticles produced in both deionised water and frozen deionised water. The E g of the Ag-TiO 2 nanoparticles generated in ice is about 1.75 ev, significantly lower than that of the nanoparticles synthesised in unfrozen deionised water, which is about 2.58 ev. Figure 3-2-d is the graph of (αhν) 2 versus E g (ev) that shows the direct band-gap energy of the Ag-TiO 2 nanoparticles produced in both aforementioned media. The indirect band gaps for both ice and deionised water are quite similar (about ev). Table 3-1 shows information about the energy gaps, ablation rate and ratio of Ag-TiO 2 colloidal nanoparticles in both media. 99

100 Absorbance [A] Chapter 3: Picosecond laser generation of Ag-TiO 2 nanoparticles (a) water ICE (b) ( h ) 1/2 ev 1/2 nm -1/ water Ag-TiO ice DIW ICE wavelength (nm) (c) water ice water ice (d) water ice 0.4 h ev 2 nm water ice Photon Energy (ev) Photon Energy (ev) Figure 3-2: (a) Absorption spectra of Ag-TiO 2 colloidal nanoparticles generated in frozen deionised water (ice) and unfrozen deionised water using a picosecond laser;. = 1064 nm, f = 200 khz, v = 250 mm/s, = 10 ps, spot size = 125 µm, E pulse = µj and F laser = J/cm 2. (b) The bottles of Ag-TiO 2 nanoparticles generated in ice (right-hand bottle) and deionised water (left-hand bottle). (c) Indirect and (d) direct band gaps of the Ag-TiO 2 nanoparticles generated in ice and deionised water. Table 3-1: The ablation information and E g of Ag-TiO 2 generated in deionised and frozen deionised water. Ablation medium Ratio Ag:TiO 2 Ablation rate (mg/min) Energy gap (ev) Direct Indirect Deionised water 1: Ice water 1: Figure 3-3-a, c, d and e show the TEM images of Ag-TiO 2 nanoparticles produced in frozen deionised water using a picosecond laser. The nanoparticles are spherical in shape; most of the largest particles are TiO 2 nanoparticles, and most of the smaller nanoparticles are silver. After measuring 490 particles, it was ascertained that their sizes were distributed from a few nanometers to 140 nm, with a few large particles up to 200 nm being observed. The predominant 100

101 Number of nanoparticles Chapter 3: Picosecond laser generation of Ag-TiO 2 nanoparticles... size among them was from 11 to 20 nm. It can be seen from Figure 3-3-b, showing the size distribution of Ag-TiO 2 nanoparticles, that more than half of the measured particles were in the range of a few nanometers to 30 nm, but that the average size of measured nanoparticles was nm. In general, the size distribution of the nanoparticles can be considered lognormal. (a) (b) No. of NPs 490 Average. Size (nm) nm Size of Ag-TiO2 nanoparticles (nm) (c) (d) (e) Figure 3-3: TEM images of Ag-TiO 2 nanoparticles produced in ice using the picosecond laser (a,c,d,e). A histogram and lognormal size distribution of produced Ag-TiO 2 nanoparticles (b). Figure 3-4-a is a HAADF image of the Ag-TiO 2 nanoparticles produced using the picosecond laser in ice. Significant unification of the TiO 2 and Ag nanoparticles can be observed. The TiO 2 particles appear darker than the Ag nanoparticles, so the bright nanoparticles on the surface of the TiO 2 are Ag nanoparticles. The EDS line profile images (Figure 3-4, silver (c), Titanium (d) and oxygen (e)) show that the big particles are the TiO 2 particles and the small particles combined with the big particles are the Ag nanoparticles. It can be noted that the amount of Ti generated is higher than that of Ti and O. 101

102 Chapter 3: Picosecond laser generation of Ag-TiO 2 nanoparticles... (a) (b) 50 nm 100 nm Figure 3-4: HAADF and EDS images (line profile) of the Ag-TiO 2 nanoparticles generated in frozen deionised water using a picosecond laser. Figure 3-5 shows the EDS image of Ag-TiO 2 nanoparticles; this image confirms the formation of Ag and TiO 2 nanoparticles in the solution with smaller Ag nanoparticles attached to larger TiO 2. The chemical elements Ti, Ag, O, Cu, C and Si appear in the image. The Ti and Ag were formed due to the sample targets; in addition, O was formed after Ti interacted with deionised water in the glass vessel. The existence of both Cu and C are due to the test grid and grid coating respectively, which were used for sample preparation for the TEM and EDS analysis. One more chemical element (Si) was observed in the EDS image, which formed due to the glassware of the experimental equipment [192], such as the glass vessel used for ablation and the glass tube used for storage and holding the colloidal nanoparticles. It can be noted from the graph that the amount of Ti produced is greater than the quantities of the other two elements, Ag and O. 102

103 Chapter 3: Picosecond laser generation of Ag-TiO 2 nanoparticles... Figure 3-5: EDS image of Ag-TiO 2 nanoparticles that shows their chemical components, which include C, Cu, Ag, Ti and O. Both Cu and C are from the grid and grid coating respectively. The presence of high amounts of Ag, Ti, and O indicates the formation of Ag-TiO 2 nanoparticles in the solution Characteristics of TiO 2 nanoparticles In order to investigate whether the reduction in the band gap of the Ag- TiO 2 nanoparticles is due to Ag nanoparticles or not, TiO 2 nanoparticles were generated in both deionised and frozen deionised water using a picosecond laser with the same laser and experimental conditions used for the production of Ag-TiO 2 nanoparticles in both ice and deionised water. As shown in Figure 3-6- a, the optical absorption spectra of colloidal TiO 2 nanoparticles generated in both ice and deionised water were recorded as from 200 to 900 nm. The intensity in ice decreased rapidly (from 200 nm to 240 nm), then decreased slowly until 300 nm; after this, it decreased exponentially to within the visible range. On the other hand, the spectrum intensity in deionised water decreased slowly (from the starting point to 260 nm), then decreased exponentially. Figure 3-6-b shows the images of the bottles of TiO 2 colloidal nanoparticles generated in both ice and the deionised water by the laser ablation process (right and left images respectively). Their different colours relate to their different concentrations. Figure 3-6-c indicates the indirect band gap of both samples, which have the same energy gaps (about 2.4 ev). However, their direct energy gaps, shown in Figure 3-6-d, are larger (about 3.6 ev). Table 3-2 shows information about the energy gaps and ablation rate of TiO 2 nanoparticles in deionised water and ice water. 103

104 Chapter 3: Picosecond laser generation of Ag-TiO 2 nanoparticles... (b) DIW TiO 2 ICE ( h ) 1/2 ev 1/2 nm -1/2 Absorbance [A] (a) Water ice (c) Wavelength (nm) Water ice water ice water (d) Water Ice water 0.6 ice ( h ) 2 ev 2 nm ice Photon Energy (ev) Photon Energy (ev) Figure 3-6: (a) Absorption spectra of TiO 2 colloidal nanoparticles generated in both deionised and frozen deionised water (ice) using a picosecond laser;. = 1064 nm, f = 200 khz, v = 250 mm/s, = 10 ps, spot size = 125 µm, E pulse = µj, F laser = J/cm 2 and t= 10 min. (b) The real bottles of the TiO 2 nanoparticles generated in both media. (c) Indirect and (d) Direct band gaps of the TiO 2 nanoparticles generated in ice and deionised water. Table 3-2: The ablation rate and E g of pure TiO 2 generated by picosecond laser in deionised and ice water. Ablation medium Ablation rate (mg/min) Direct Energy gap (ev) Indirect Deionised water Ice water Antibacterial Activity The antibacterial activity of Ag-TiO 2 nanoparticles generated in ice using a picosecond laser was tested against E. coli bacteria under standard room light then compared with pure Ag and TiO 2 nanoparticles generated in deionised water (not in ice water) using the same laser and experimental conditions used to produce the Ag-TiO 2 nanoparticles. Figure 3-7-a shows a single measurement of the relationship between the concentration of the Ag-TiO 2 104

105 Number of survived E. coli colonies Chapter 3: Picosecond laser generation of Ag-TiO 2 nanoparticles... nanoparticles and the number of surviving E. coli bacteria; the effect of different concentrations of pure Ag nanoparticles for elimination of E. coli bacteria is shown for comparison. The Ag-TiO 2 nanoparticles demonstrate significant antibacterial activity, although not as good as pure Ag nanoparticles; however, the amount of doped silver in Ag-TiO 2 is half in comparison with pure Ag. Figure 3-7-b shows a single measurement of the antibacterial activity of Ag-TiO 2, TiO 2 and Ag nanoparticles at 12.5 µg/ml concentrations (a) Ag-TiO2 NPs Ag NPs (b) E. coli viability C/C o Concentration ( g/ml) Ag-TiO2 (ICE) Ag TiO2 Figure 3-7: (a) Number of surviving E. coli bacteria as a function of concentrations of the Ag- TiO 2 and Ag nanoparticles. (b) Histogram of relative the antibacterial activity of Ag-TiO 2, TiO 2 and Ag nanoparticles at 12.5 µg/ml concentration. The test were done a day after preparation of nanoparticles. (Note; the control value, i.e. no nanoparticles in Figure (b) is 1). They were tested under standard room light. 0.0 Nanoparticles As shown in Figure 3-7-b, the antibacterial activity of laser generated TiO 2 nanoparticles, produced in normal deionised water not in ice water, were tested at the same concentration (12.5 µg/ml). The results show that the antibacterial activity of Ag-TiO 2 nanoparticles and pure Ag nanoparticles are much better than pure TiO 2 nanoparticles Discussion Production of nanoparticles in the deionised ice water leads to a decrease in the indirect band gap energy of TiO 2 nanoparticles after modification by Ag nanoparticles. The ice environment has two advantages for the production of nanoparticles; the first is the confinement or restriction of the movement of the produced particles in ice, which leads to the combination of Ag and TiO 2 nanoparticles (see Figure 3-4-a). Another benefit of ice is the cooling 105

106 Chapter 3: Picosecond laser generation of Ag-TiO 2 nanoparticles... of the plasma plume during the laser ablation process, as the additional confinement and cooling of the plasma plume from the liquid leads to a shortening of the quenching time of the plume [87]; this situation is more likely to occur in ice. This case may cause a reduction in the transition band gap of TiO 2 nanoparticles from about 2.58 to 1.75 ev after the modification with the Ag nanoparticles. The blue shift phenomena, or a decrease in the strong peak intensity from 400 nm in ice to 405 nm in normal deionised water, can also be noted in Figure 3-2; this indicates that the nanoparticles in ice are smaller than those produced in standard deionised water. This is due to the faster combination of the Ag and TiO 2 nanoparticles in frozen deionised water. It may be possible to say that the rapid quenching of the plasma plume composites lead to further interaction of Ag nanoparticles with TiO 2 nanoparticles, producing an interband transition and a reduction in forbidden transition band gap energy. Chang-Ning et al. [112] predicted that pulse laser ablation in a liquid environment would yield a faster cooling rate to quench TiO 2 nanocondensates as amorphous. Further scientific analysis and explanation of energy gap decreasing in ice environments is required to give full understanding of this phenomenon. In spite of modifying the TiO 2 with Ag nanoparticles in deionised water, their energy gap (2.58 ev) was higher than the energy gap of pure TiO 2 nanoparticles (2.4 ev) produced in deionised water under the same laser and experimental conditions. This may be due to the different sizes produced, as a result of the change of beam size on the target. The latter plays a crucial role in controlling the size of the nanoparticles during the laser ablation process [63, 101]. In this work, it can be seen from the EDS images that the rate of oxygen produced by the TiO 2 nanoparticles is high, due to the oxidisation of ablated species in water as a result of their fast reactive quenching with the medium [66], so the ice environment has greater probability for rapid quenching than water. In general, when the state of a matter is changed, some of its physical properties will also be altered, such as optical properties. As a result of this fact, as water becomes ice, its optical properties will be changed; for example, its absorption coefficient is changed by seven orders of magnitude in the range of wavelength starting from 0.4 to 2.5µm, so the real part of the refractive index (n) 106

107 Chapter 3: Picosecond laser generation of Ag-TiO 2 nanoparticles... of ice is lower than water [225]. However, at 1064 nm wavelength, absorption coefficient of water at 1064 nm is 12.2 m -1 [226], and ice absorption coefficient is close to that of water. The ablation rate of nanoparticles or nanoparticle productivity in ice is lower than that in water patricianly due to the higher attenuation to the laser beam in icy environments. In addition, the absorptivity of the frozen deionised water (ice) is higher than for unfrozen deionised water. In the Ag-TiO 2 TEM images, the Ag and TiO 2 nanoparticles can be distinguished due to their different colours. The bright nanoparticles are Ag nanoparticles and the dark particles are TiO 2 nanoparticles. The different appearances of these two nanoparticles are due to their different physical properties. The density of Ag is higher than that of Ti, leading to more light absorption within Ag while imaging by TEM; as a result, the Ag nanoparticles appear brighter. When the concentration of the nanoparticles in the solution was increased, their ability to eliminate the E. coli bacteria increased; this is because at higher concentrations more nanoparticles exist in the solution, and as a result the probability of nanoparticles coming into contact with the microorganism (E. coli) increased. This situation leads to the killing of more E. coli bacteria at higher concentrations in comparison with low concentrations. The ability of Ag- TiO 2 nanoparticles to kill E. coli is not as high as that of pure Ag nanoparticles, but it is worth mentioning that the amount of Ag nanoparticles in 20 µg/ml of Ag- TiO 2 nanoparticles with the ratio 1:1 is half in comparison with pure Ag nanoparticles. The ratio of Ag at Ag-TiO 2 and pure Ag is therefore 1: Summary Spherical Ag-TiO 2 nanoparticles were generated using a picosecond laser in an ice environment (frozen deionised water). The sizes of the particles were observed by TEM, and ranged from a few nanometers to140 nm, with 20 nm being their predominant size. The indirect energy gap of the TiO 2 nanoparticles was reduced significantly after the modification by the Ag nanoparticles in ice in comparison with the Ag-TiO 2 nanoparticles produced in 107

108 Chapter 3: Picosecond laser generation of Ag-TiO 2 nanoparticles... deionised water. The indirect forbidden band gap of Ag-TiO 2 nanoparticles in ice was recorded as 1.75 ev, while in standard room temperature deionised water this figure was 2.58 ev. The energy gap of unmodified TiO 2 nanoparticles in both ice and deionised water were observed to be the same (about 2.4 ev). This method could be used to reduce the forbidden band gap energy of other semiconductor nanoparticles with different noble metals in icy environments. The Ag-TiO 2 nanoparticles have good antibacterial activity against E. coli bacteria under standard room light. 108

109 4 Chapter 4. The Characteristics of Novel Bimodal Ag-TiO2 Nanoparticles Generated by Hybrid Laser-Ultrasonic Technique Authors: Abubaker Hamad, Lin Li, Zhu Liu, Xiang Li Zhong, Grace Burke and Tao Wang Journal: Applied Physics A: Materials Science and Processing Volume, issue and pages: 122, 4, 1-12 Article id: 275 Status: Published Note: The format of the paper is edited 109

110 Chapter 4: The characteristics of Novel Bimodal Ag-TiO 2 Nanoparticles... The Characteristics of Novel Bimodal Ag-TiO 2 Nanoparticles Generated by Hybrid Laser-Ultrasonic Technique Abstract Silver-titania (Ag-TiO 2 ) nanoparticles with smaller Ag nanoparticles attached to larger TiO 2 nanoparticles were generated by hybrid ultrasonic vibration and picosecond laser ablation of Ag and Ti bulk targets in deionised water, for the first time. The laser has a wavelength of 1064 nm and a pulse duration of 10 ps. It was observed that without the ultrasonic vibration, Ag and TiO 2 nanoparticles did not combine, thus the role of ultrasonic vibration is essential. In addition, colloidal TiO 2 and Ag nanoparticles were generated separately for comparison under the same laser beam characteristics and process conditions. The absorption spectra of colloidal Ag-TiO 2 cluster nanoparticles were examined by UV-VIS spectroscopy, and size distribution was characterised using Transmission Electron Microscopy (TEM). The morphology and composition of Ag-TiO 2 nanoparticles were examined using Scanning Transmission Electron Microscopy (STEM) in High-Angle Annular Dark-Field (HAADF), and Energy Dispersive X-ray Spectroscopy (EDS). The crystalline structures were investigated by X-Ray Diffraction (XRD). The size of larger TiO 2 particles was in the range nm and the smaller sized Ag nanoparticles attached to the TiO 2 was mainly in the range of nm. The yield is more than 50% with the remaining nanoparticles in the form of uncombined Ag and TiO 2. The nanoparticles generated had strong antibacterial effects as tested against E. coli. A discussion is given on the role of ultrasonic vibration in the formation of Ag-TiO 2 hybrid nanoparticles by picosecond laser ablation. Keywords: nanoparticles, Ag-TiO 2 cluster, ultrasonic waves, picosecond laser, laser ablation, bacteria. 110

111 Chapter 4: The characteristics of Novel Bimodal Ag-TiO 2 Nanoparticles Introduction The specific properties of nanoparticles which are different from bulk materials have received much attention over the last 2 decades [63] and nanoparticles have been used in a wide range of applications from electronic and catalytic uses to magnetic and medical fields [58]. The phases, size and shape of nanoparticles are responsible for the specific properties found in metal and metal oxide nanoparticles [97]. Both the hydrophilic and photocatalytic properties of TiO 2 nanoparticles are important for numerous applications, such as antimicrobial activity [123] and self-cleaning [205]. The photocatalytic activity of TiO 2 requires the activation with a UV light, which limits its applications. To overcome this limitation, the widely adopted method is to dope TiO 2 nanoparticles with other materials, such as transition metals [227] and non-metals [228]. Many methods have been developed not only for the preparation of nanoparticles but also to control their sizes. In general, the methods of nanoparticle generation can be divided into three types: chemical, physical and biological, amongst which are sol-gel, chemical vapour deposition (CVD), physical vapour deposition (PVD), wet chemistry, ion sputtering, plasma or flame pyrolysis [50], laser ablation of a solid metal in a liquid environment [51-54] and the use of Murraya Koenigii (curry leaf) extract [55]. Several methods and experimental setups have been proposed to combine Ag and TiO 2 nanoparticles and improve their properties; Guo et al. [229] prepared Cage-bell hybrid Ag- modified TiO 2 nanoparticles by means of an environmental template-free (alcoholysis) route followed by a facile impregnation (calcination). The photocatalytic activity of the particles was improved. Yang et al. [12] prepared hybrid Ag-TiO 2 core-shell nanoparticles by a chemical method which enhanced their photocatalytic activity in comparison with commercial TiO 2 and Ag-doped TiO 2 nanocomposite structures. Petronella et al. [230] prepared a composite of TiO 2 nanorods and Ag nanoparticles and succeeded in achieving the degradation of antibiotic nalidixic acid and a recalcitrant pollutant. Su et al. [231] combined the electrospinning technique with a solvo-thermal process to generate Ag/TiO 2 nanoheterostructures (Ag nanocrystals / TiO 2 nanofiber mats 111

112 Chapter 4: The characteristics of Novel Bimodal Ag-TiO 2 Nanoparticles... based), and showed an improvement in the properties of the particle s nanostructure. The product exhibits excellent photocatalytic activity for the degeneration of Rhodamine B dye under visible light irradiation. Zhang et al. [15] used a laser beam to combine Ag/TiO 2 nanoparticles by laser ablation in liquid media and subsequent hydrothermal treatment without using any organic additive or chemical reduction agent. Firstly, colloidal TiO x nanoparticles were produced from a bulk target by laser ablation in liquid (LAL) then they were mixed with different concentrations of AgNO 3 solution, and then it was put and kept in an electric oven at 180 C for 1 day (24 h). It was claimed that the combination of Ag and TiO 2 using this method was stronger than other methods, as during the crystallization process of TiO x, silver ions were reduced and then deposited on the surface of TiO 2 nanoparticles. In our previous work, Hamad et al. [232], Ag- TiO 2 compound nanoparticles were produced via laser ablation in ice water. The energy gap of the TiO 2 nanoparticles was reduced significantly due to ice environment after doping with Ag nanoparticles. Although a combination between TiO 2 and Ag nanoparticles was observed, the yield was very low (approximately 5%). Recent decades have seen a growing interest in the use of laser ablation of a solid material target in a liquid environment as a technique for the preparation of nanoparticles [51-52, 56], including porous nanoparticles [57], metal oxide nanoparticles [58], nanodisks [53], and for the morphological conversion of nanoparticles [59]. In comparison to chemical methods, colloidal nanoparticles prepared by laser ablation in a liquid environment are free from contamination [60], surfactants, counter-ions [53, 61-62], and chemical reagents [63]. Furthermore, this method is a relatively simple way to prepare nanoparticles [64-65] without the need for the removal of unwanted chemicals. As well as the generation of nanoparticles in deionised water, work has also been reported on preparing noble metal nanoparticles using sodium dodecyl sulfate (SDS) as a surfactant in a colloidal solution [51-52, 66-67] and a Polyvinylpyrrolidone (PVP) solution [68-69]. Nanoparticle clusters with smaller conducting nanoparticles attached to larger dielectric nanoparticles are desirable for many applications including antibacterial functions, light scattering control, local plasmonic tuning and enhanced efficiency in thin film solar cells. In the present research, the authors 112

113 Chapter 4: The characteristics of Novel Bimodal Ag-TiO 2 Nanoparticles... present a new technique to combine two different nanoparticles using picosecond laser ablation of Ag and Ti solid targets in ultrasonic assisted deionised water to achieve a single step high yield in generating bimodal Ag and TiO 2 nanoparticle clusters Experimental materials and procedures Materials An Ag target plate with the dimensions of 25 mm (length) 25 mm (width) 2 mm (thickness) and a purity of 99.99%, and a Ti target plate with the dimensions 25 mm (length) 25 mm (width) 1 mm (thickness) and a purity of % were used to generate the Ag-TiO 2 nanoparticles. Both targets were washed in autoclaved deionised water as well as in ethanol. The samples were then sonicated for minutes in deionised water and ethanol before laser irradiation. They were placed in deionised water side by side (see Figure 4-1) during the laser ablation Preparation of Ag-TiO 2 nanoparticles by hybrid ultrasonic sonication and laser ablation Ag-TiO 2 cluster nanoparticles were produced using the picosecond laser ablation in deionised water in an ultrasonic tank (Ultraschall-Reiniger, Emmi 5 with a power of 50 W at a pulse repetition rate of 49 khz, tank volume 500 ml, Digital timer 1-9 minutes adjustable). Ag and Ti plates were put on a sample holder, beside each other, in a glass vessel containing about 20 ml of deionised water. Then the glass vessel was put in an ultrasonic tank filled with deionised water. The deionised water in the glass vessel was poured to a level of about 2 mm above the samples. The laser ablation experimental set-up is shown in Figure 4-1. It should be noted that the effect of the water level on the laser beam focal length was taken into account. Under the same laser conditions and experimental setup (with ultrasonic assisted production) TiO 2 nanoparticles generated to show whether single nanoparticles would combine or not. For comparison, Ag and Ag-TiO 2 nanoparticles were generated under the same 113

114 Chapter 4: The characteristics of Novel Bimodal Ag-TiO 2 Nanoparticles... laser beam parameters and experimental conditions without ultrasonic vibration. Table 4-1 shows laser beam parameters used in this work. Galvo scan head Laser beam Ultrasonic tank Glass vessel Deionised water Stainless steel holder Ag and Ti targets Figure 4-1: Experimental set-up for generation of Ag-TiO 2 nanoparticles in deionised water in an ultrasonic cleaning bath. Table 4-1: 400 W Edgewave picosecond laser beam parameters used to produce Ag-TiO 2 cluster nanoparticles. Parameters Value Wavelength ( ) 1064 nm Frequency (f) 200 khz Laser beam power (P) 9.12 W Scan speed (v) 250 mm/s Laser pulse duration ( ) 10 ps Laser post size in diameter (D) 125 µm Laser pulse energy (E pulse ) 45.6 µj Laser fluence (F laser ) 0.37 J/cm 2 To identify the optimal laser beam parameters to produce nanoparticles in deionised water, several sets of laser beam powers and scan speeds were examined. The optimal laser parameters for preparation of Ag-TiO 2 nanoparticles are shown in Table 4-2 and Table 4-3. Table 4-2: Optimal laser parameters at constant laser power P(60%) = 9.12 W and different scan speeds. Scan speed (mm/s) Ablation rate (mg/min) Size distribution (nm) Average size (nm)

115 Chapter 4: The characteristics of Novel Bimodal Ag-TiO 2 Nanoparticles... Table 4-3: Optimal laser beam parameters at constant scan seed (250 mm/s) and different laser powers. Laser power (W) Ablation rate (mg/min) Size distribution (nm) Average size (nm) The morphology of nanoparticles in all cases was spherical. The number of larger nanoparticles at the 50 mm/s scan speed was a little higher in comparison at 250 mm/s and the ablation rate at 9.12 W was higher than that at 5.9 W. As shown in Tables 2 and 3, the smallest average nanoparticles were obtained at 9.12 W average power and 250 mm/s scan speed. Although smaller nanoparticles of less than 10 nm were also observed, these were in very small quantities Nanoparticle sample preparation for characterisation For TEM analysis, a drop of the colloidal suspension of nanoparticles was deposited onto a copper microgrid mesh (A Formvar / Carbon on 200 Copper mesh) and then allowed to dry in ambient air temperature. This process was repeated three to six times to ensure that the sufficient amount of nanoparticles was collected on the copper mesh substrate. The concentration of the colloidal nanoparticle samples was obtained by measuring the weight of the targets before and after the nanoparticle generation process using a microbalance scale. The X-Ray diffraction sample was prepared on a glass slide; firstly the produced colloidal nanoparticles were centrifuged for 10 to 15 minutes using a microcentrifuge machine, then they were dropped onto the glass slide and allowed to dry in ambient air. This process was repeated several times to deposit a sufficient amount of nanoparticles on the slide Characterisation In order to characterise the nanoparticles, a UV-Vis spectrometer (Analytic Jena, SPECORD 250, dual beam) was used for examining the absorption spectra of the colloidal nanoparticles. A Transmission Electron 115

116 Chapter 4: The characteristics of Novel Bimodal Ag-TiO 2 Nanoparticles... Microscope (TEM) (JEOL 2000 FX AEM) was used to examine the morphology of the nanoparticles and measure their sizes and size distribution. Images and composition of Ag-TiO 2 nanoparticles were obtained using Scanning Transmission Electron Microscope (STEM) where High-Angle Annular Dark- Field (HAADF) imaging was employed for the examination of the morphology of the nanoparticles and Energy Dispersive X-ray Spectroscopy (EDS) line scan was used to analyses the chemical composition. The morphological characterisations were examined by a Field-Emission Gun Transmission Electron Microscope (FEI Tecnai G 2 F30). In addition, X-Ray Diffraction (XRD) was used for the investigation of crystalline material structures. A microbalance scale (Sartorius BL 210S ) with a resolution of 0.1 mg was used to measure the weight of the samples before and after ablation process, and the concentration of the colloidal nanoparticles was calculated after measuring the amount of deionised water Antibacterial Activity Test Procedure The antibacterial activities of the nanoparticles were examined against E. coli bacteria (JM109 from Promega UK). A single colony of E. coli (or 10 µl of glycerol stock) was incubated in 10 ml of Lysogeny broth (LB broth) in a 50 ml tube and cultured at 37 C overnight with constant shaking at 225 rpm. The optical density of the cultured E. coli was measured at 600 nm (OD 600 ) and diluted down to a colony forming unit (CFU)/ml of about with LB. 200 µl of colloidal nanoparticles was mixed with 1.80 ml of the diluted E. coli and incubated for 6 hours at 37 C while shaking at 225 rpm. Finally, 10 µl of each dilution was spread on the LB agar plates and left at room temperature for about 48 hours under normal light followed by counting colonies on each plate. For negative control, 200 µl of nanoparticles was replaced by 200 µl dh 2 O. 116

117 Chapter 4: The characteristics of Novel Bimodal Ag-TiO 2 Nanoparticles Results Ag-TiO 2 nanoparticle characteristics Figure 4-2 shows the optical absorption spectra of the Ag, TiO 2 and Ag- TiO 2 nanoparticles (with and without ultrasonic vibration) generated by a picosecond laser in deionised water. Two different materials, Ag and TiO 2, contributed to the formation of the Ag-TiO 2 spectrum. The Ag nanoparticles spectrum has two peaks; the first is a weak absorption peak in the range of nm, which is formed due to inter-band transition, and the second is a strong transition peak located around 410 nm formed due to surface plasmon resonance [63]. The optical absorption spectra of the Ag-TiO 2 nanoparticles produced with and without ultrasonic vibration (Figure 4-2-b) indicate that there is no shift of the surface plasmon peak position both at 404 nm. Another spectrum in the figure is the TiO 2 absorption spectrum that can be divided into two regions: the UV region, which has a higher absorption rate than the visible region. The absorption rate in the UV region decreases sharply when approaching the visible region. As shown in Figure 4-2, the Ag-TiO 2 absorption spectra have a strong peak in the UV range, which is almost exactly the same as with the TiO 2 band, and the second peak in the visible range is formed due to the surface plasmon resonance of the Ag nanoparticles because pure Ag has a strong peak at this wavelength which about 400 nm. The general shape of Ag- TiO 2 nanoparticle optical absorption spectra depends on the ratio of ablated Ag and TiO 2 nanoparticles in the solution. For example, when the amount of ablated Ag nanoparticles is increased, the surface plasmon resonance (SPR) peak will become higher, which is an indication that the Ag concentration in the solution has increased. However, when the amount of TiO 2 nanoparticles is increased a strong peak at nm appears. It was noted that the amount of ablated nanoparticles in the solution rose with prolonged irradiation time. 117

118 Absorbance [A] (Ag) Absorbance [A] (TiO 2 ) Absorbance [A] (Ag-TiO 2 with ultrasonic) Absorbance [A] (Ag-TiO 2 without Ultrasonic) Chapter 4: The characteristics of Novel Bimodal Ag-TiO 2 Nanoparticles (a) Ag Ag TiO TiO (b) Ag-TiO 2 (with ultrasonic) Ag-TiO 2 (with ultrasonic) Ag-TiO 2 (without ultrasonic) Ag-TiO 2 (without ultrasonic) Wavelength (nm) Wavelength (nm) 0.0 Figure 4-2: The absorption spectra of (a) Ag, TiO 2 and (b) Ag-TiO 2 colloidal nanoparticles generated in deionised water in an ultrasonic tank and without ultrasonic tank by picosecond laser ablation in deionised water (P= 9.12 W, f= 200 khz and v= 250 mm/s). In the Ag-TiO 2 suspension produced in the ultrasonic tank, the amount of Ag and TiO 2 nanoparticles were 0.5 mg and 0.6 mg respectively. The ablation time for Ag, TiO 2 and Ag-TiO 2 with ultrasonic waves was 16 min, and for Ag-TiO 2 without ultrasonic waves it was 10 min. In terms of morphology, size and size distribution, there is no significant difference between the Ag-TiO 2 nanoparticles produced with and without ultrasonic vibration except that combination of Ag and TiO 2 nanoparticles (i.e. smaller Ag nanoparticles were attached to the larger TiO 2 nanoparticles) occurred in the case of having the ultrasonic vibrations in the nanoparticle production process. Figure 4-3 a-d show the TEM images of the Ag-TiO 2 nanoparticles generated in deionised water in an ultrasonic bath. Spherical Ag-TiO 2 nanoparticles of different sizes are produced. The weight ratio of Ag-TiO 2 nanoparticles was 1:1.2, so the amount of TiO 2 nanoparticles was 20% higher than the amount of Ag nanoparticles. Figure 4-3-e shows a histogram of the size distribution of 612 of the produced Ag-TiO 2 nanoparticles; the distribution is between few nano-meters to 150 nm with an average size of 39 nm and a small amount of particles in the range of >150 nm up to 190 nm. The size distribution shape is lognormal distribution. Almost all of the large particles were TiO 2 particles and most of the small nanoparticles were Ag. As can be seen in the figure, a combination of the TiO 2 and Ag nanoparticles was formed. It is worth mentioning that the combination of two different nanoparticles by pure laser technique without using any chemical assistants such as organic additives or reduction agents or additional heating was not observed in the previous studies. The yield of the bimodal Ag-TiO 2 cluster was about 50%. 118

119 Number of nanoparticles Chapter 4: The characteristics of Novel Bimodal Ag-TiO 2 Nanoparticles... (a) (b) (c) (d) 200 (e) Number of NPs: 612 Ave. size: nm Size of Ag-TiO 2 nanoparticles (nm) Figure 4-3: (a-d) TEM images of Ag-TiO 2 nanoparticles generated in deionised water by the picosecond laser (P= 9.12 W, f= 200 khz, v= 250 mm/s and t=16 min.). The production process was carried out in a tank of ultrasonic cleaner with a frequency 49 khz. The ablation rate of Ag- TiO 2 nanoparticles was mg/min with a Ag:TiO 2 ratio of 1:1.2. Figure 3-e is the lognormal size distribution of Ag-TiO 2 nanoparticles. Figure 4-4 shows the images of the bimodal Ag-TiO 2 nanoparticles recorded by the High-Angle Annular Dark-Field Microscope (HAADF). The combination of the Ag and TiO 2 nanoparticles appears clearly. Small silver nanoparticles in the range of 3-15 nm were deposited on the surface of larger ( nm) TiO 2 particles. In addition, Figure 4-4-b reveals the existence of ultrafine nanoparticles on the surface of the TiO 2 particles, measuring a few nanometres. 119

120 Chapter 4: The characteristics of Novel Bimodal Ag-TiO 2 Nanoparticles... (a) 100 nm 50 nm (b) 20 nm 50 nm Figure 4-4: High-Angle Annular Dark-Field Microscope (HAADF) images of the Ag-TiO 2 nanoparticles. Figure 4-5 shows the line scanning of the Energy Dispersive X-ray Spectroscopy (EDS) images of the Ag-TiO 2 nanoparticles formed by the picosecond laser ablation in deionised water with ultrasonic vibration. These images confirm the production of Ag and TiO 2 nanoparticles, which shows that the large particles are TiO 2 particles and the small nanoparticles are Ag. It can be concluded that the amount of Ti is higher than the amount of Ag and O compositions. 120

121 Chapter 4: The characteristics of Novel Bimodal Ag-TiO 2 Nanoparticles nm Figure 4-5: EDS - Line profile images of the Ag-TiO 2 nanoparticles synthesised by picosecond laser in deionised water supporting ultrasound waves in an ultrasonic cleaner. X-ray diffraction analysis was used for the determination of both the structure and crystal phase. Figure 4-6 shows the X-Ray diffraction patterns of both the Ag and TiO 2 (or Ag-TiO 2 ) nanoparticles. The group of diffraction peaks, which include 2 = 38, 44.4, 52, 76.75, 93.4 and 99 correspond to the Ag nanoparticles, and another group that includes 2 = 32, 42, 48.2, 64 correspond to the rutile structure of the TiO 2 nanoparticles. In addition, a diffraction peak at 2 =37.5 corresponds to the existence of AgO. A previously published paper indicated that the peaks at 2 = 37.5 and 48.1 were related to the existence of anatase TiO 2 nanoparticles (annealed at 500 C), which correspond to the 004 and 200 crystalline plains respectively [233]. 121

122 Chapter 4: The characteristics of Novel Bimodal Ag-TiO 2 Nanoparticles... Figure 4-6: X-Ray Diffraction (XRD) spectrum of Ag-TiO 2 nanoparticles produced by picosecond laser in deionised water in ultrasonic vibration Ag-TiO 2 nanoparticles generation without ultrasonic vibration Figure 4-7 show the TEM images of Ag and-tio 2 nanoparticles fabricated by laser ablation in deionised water without using ultrasonic vibration. It was generated under the same laser beam conditions and experimental setup but without ultrasonic vibration. It can be seen there is no clear combination between Ag and TiO 2 nanoparticles. Figure 4-7: TEM images of Ag-TiO 2 nanoparticles produced in deionised water using picosecond laser without using ultrasonic waves. (Wavelength 1064 nm, power 9.12 W, frequency 200 khz and scan speed 250 mm/s and t=10 min) The characteristics of TiO 2 nanoparticles generated with ultrasonic vibration In order to show whether this combination of nanoparticles would occur between the nanoparticles of single material or not, the TiO 2 nanoparticles were synthesised in deionised water in an ultrasonic bath. As shown in Figure 4-8-a 122

123 Number of Nanoparticles Chapter 4: The characteristics of Novel Bimodal Ag-TiO 2 Nanoparticles... and b, the TiO 2 nanoparticles did not combine. This is a valid proof that the combined particles that form the Ag-TiO 2 nanoparticles are the Ag and TiO 2 particles. The histogram image in, Figure 4-8-c, shows the size distribution of the TiO 2 nanoparticles, which indicates that the size distribution of the TiO 2 nanoparticles is wider than that of the Ag-TiO 2 nanoparticles; in other words, the large nanoparticles in the Ag-TiO 2 nanoparticles are TiO 2. The size distribution ranges from 10 nm to 140 nm and a few other large particles above 140 nm are also evident. (a) (b) (C) Number of NPs: 594 Ave. size: nm Size of TiO2 nanoparticles (nm) Figure 4-8: (a and b) TEM images of TiO 2 nanoparticles generated in deionised water in an ultrasonic cleaner tank by picosecond laser (P= 9.12 W, f= 200 khz, v= 250 mm/s and t=15 min.). The quantity of TiO 2 nanoparticles generated in the suspension is 0.8 mg and the concentration is 53.3 µg/ml. The ablation rate of TiO 2 NPs is mg/min. (0.8 mg/15 min). (c) Histogram of the size distribution of TiO 2 nanoparticles The characteristics of laser generated Ag nanoparticles Ag nanoparticles synthesised by picosecond laser ablation in deionised water without ultrasonic vibration. As shown in Figure 4-9-a and b semispherical Ag nanoparticles were produced. The particle size histogram graph (Figure 4-9-c) shows the size distribution of silver nanoparticles ranged from about 2 nm to about 100 nm. The average size of 660 counted nanoparticles was 32.5 nm. It was fabricated to show and compare their antibacterial activity with ultrasonic assisted Ag-TiO 2 hybrid nanoparticles. 123

124 Number of survived E. coli colonies Number of nanoparticles Chapter 4: The characteristics of Novel Bimodal Ag-TiO 2 Nanoparticles... (a) (b) (C) Number of NPs: 660 Ave. size: 32.5 nm Size of Ag nanoparticles (nm) Figure 4-9: (a and b) TEM images of Ag nanoparticles generated by picosecond laser in deionised water without ultrasonic vibration. Laser parameters; P= 9.12 W, f= 200 khz, v= 250 mm/s. (c) Histogram of the size distribution of Ag nanoparticles Antibacterial characteristics Figure 4-10 shows the antibacterial activity of Ag-TiO 2 nanoparticles (produced with ultrasonic vibration) under standard room light with the weight ratio 1:1.2 which were tested against E. coli bacteria (JM109 from Promega UK) and compared with Ag and TiO 2 nanoparticles, and with non-nanoparticle control. The nanoparticles at a concentration 20 µg/ml demonstrate significant antibacterial activity Ag-TiO2 (US) Ag TiO2 Control Materials Figure 4-10: Antibacterial activity of Ag-TiO 2 nanoparticles in comparison with control sample. Equal amount of E. coli were cultured with (Ag-TiO 2 (ultrasonic wave based generation), Ag and TiO 2 ) nanoparticles or without (control) nanoparticles in LB broth for 6 hours and 10 µl of the broth culture was plated on to LB agar plate for colony formation after overnight incubation at 37 C overnight. The number of E. coli colonies represents the survived E. coli after culturing with or without nanoparticles which negatively correlate to the antibacterial effect of the nanoparticles. 124

125 Number of survived E. coli colonies Chapter 4: The characteristics of Novel Bimodal Ag-TiO 2 Nanoparticles... It is worth noting that, the Ag concentration within the bimodal Ag-TiO 2 nanoparticles is much less than that in pure Ag nanoparticle solution, while the antibacterial activity is similar, indicating that TiO 2 in this combined form has played a role in antibacterial activity. The antibacterial activity of TiO 2 nanoparticles produced by the picosecond laser in deionised water without using ultrasonic vibration was examined against E. coli bacteria with the same concentration 20 µg/ml. The results show that the antibacterial activity of the Ag-TiO 2 nanoparticles is much better than that of the TiO 2 nanoparticles. Figure 4-11 shows the relationship between the concentration of the Ag- TiO 2 cluster nanoparticles (generated with ultrasonic assisted) and the number of survived bacteria after co-culturing with Ag-TiO 2 nanoparticles. The results were also compared with the effect of Ag nanoparticles. It can be seen that the number of surviving bacteria is decreased by increasing the concentration of the nanoparticles for both the Ag-TiO 2 and Ag nanoparticles. At each nanoparticle concentration, the number of survived E. coli in Ag nanoparticles was significantly lower (higher bacterial killing effect) than that of Ag-TiO 2 nanoparticles but the ratio of the Ag nanoparticles to Ag-TiO 2 nanoparticles is 1:1.2 which is lower than the amount of Ag nanoparticles. Pure Ag nanoparticles could eliminate all bacteria at 20 µg/ml. It can be expected that the Ag-TiO 2 nanoparticles will be able to kill a full E. coli bacteria colony at a higher concentration Ag Ag-TiO2 (US) Conentration ( g/ml) Figure 4-11: Relationship between the number of surviving E. coli bacteria as a function of the concentration of Ag-TiO 2 (US) (ultrasonic) and Ag nanoparticles generated by picosecond laser in deionised water with ultrasonic wave assisted. The antibacterial test was carried out under normal light. 125

126 Chapter 4: The characteristics of Novel Bimodal Ag-TiO 2 Nanoparticles Discussion Generation of Ag-TiO 2 cluster and TiO 2 Nanoparticles The first sign of the generation of Ag-TiO 2 nanoparticles is a change in the colour of the deionised water or the contamination of the deionised water by the generated nanoparticles. The second sign of the presence of these nanoparticles can be observed by noting the absorption spectra of the generated colloidal nanoparticles. When the laser beam is incident on the surface of the target, firstly, the laser beam interacts with the deionised water. Because water has a significant absorption coefficient some power of the laser beam will be lost. The amount of power absorbed by the water can be calculated according to Lambert s Law. (4-1) where I and I o are the transmitted and incident intensity of the laser beam respectively, α is the absorption coefficient of the deionised water and is the height of the water level above the samples. The water level covering the target material has an effect not only on the laser beam power or intensity but also on the laser beam focal length [234]. For example, the amount of laser power loss for a 2 mm of water level above the target is about 4%; this amount will be increased to 8% when the water level is 4 mm above the bulk target, so there is a linear relationship between the water height and laser power loss. As well as for a 2 mm water level, the distance between the target and the focusing lens should be increased by an extra 0.5 mm and for 4 mm of water an extra 1 mm [ ]. In this work, the effects of both cases were considered. It is worth mentioning that some water will be lost through evaporation during the generation of nanoparticles by ablation. The amount of evaporated water on a sunny day on which the maximum temperature is 20 C and the minimum is 9 C and using a picosecond laser under the same laser beam parameters which are used for the generation of nanoparticles can be found using the following equation: (4-2) 126

127 Chapter 4: The characteristics of Novel Bimodal Ag-TiO 2 Nanoparticles... where y is the amount of evaporated deionised water and t is time. For example after 10 minutes the amount of evaporated water will be 0.55 g/10 min. (0.055 g/min.). After laser-water interaction, laser-target and water-target interactions will occur when the laser beam strikes the surface of the target or at the watertarget interface. At this point, shock waves in the target material and another shock wave in the water will be produced and propagated [236]. This is may be due to Newton s third law (action and reaction). A spark plume was produced at the water-matter interface; the plume was shiny and looks like a small flame. When the laser energy is equal to or greater than the ablation threshold of the target material, the ablation process will begin, followed by the production of nanoparticles through the condensation of the target material vapour. After observing the nanoparticle production process using a digital camera, it was discovered that the generated nanoparticles were propagated along the laser beam axis from the target surface to the water-air interface inside the solution. This occurred due to a decrease in the density of the solution along the beam path (see Figure 4-12). Water air interface Deionised water Dispersing nanoparticles Plasma plume Target material Figure 4-12: Image of plasma plume and nanoparticles dispersing in deionised water during picosecond laser ablation of a target (without using ultrasonic waves). The photo is taken after recording the ablation process by a normal camera. TiO 2 nanoparticles were generated immediately after the production of Ti vapour in the solution and then interacted with water. As a result, Titanium (IV) Hydroxide (Ti(OH) 4 ) was produced, followed by the production of TiO 2 nanoparticles according to the equations (3) and (4) [237]. 127

128 Chapter 4: The characteristics of Novel Bimodal Ag-TiO 2 Nanoparticles... Ti(clusters) + H 2 O Ti(OH) 4 + H 2 (4-3) Ti(OH) 4 TiO 2 + H 2 O (4-4) As shown in Table 4-2, the ablation rate of the nanoparticles is decreased with an increase scan speed at the lower scan speeds (50, 150 and 250 mm/s) because when the nanoparticles were dispersed in the solution they would travel towards the laser (see Figure 4-12). As a result, they would scatter the laser beam and prevent the laser from reaching the target. This would lead to less nanoparticles produced from the target material. Although, the ablation rate at a higher scan speed (2000 mm/s) is increased slightly in comparison at 50 mm/s scan speed, this is because at higher scan speed more laser power will reach the target per unit time in comparison with that at a low scan speed. Although the minimum size of the nanoparticles was observed at the 250 mm/s scan speed the fluctuation of the size and size distribution was observed. This may be due to the present of a liquid level on the target material since large size particles would be generate due to liquid layer ejection and fragmentation [238]. As such the actual water level on the target in one experiment may not be exactly the same as in another experiment. In general, the optical absorption spectra of the Ag-TiO 2 nanoparticles produced with and without ultrasonic vibration have the same features. The surface plasmon resonance is higher than that produced without ultrasonic waves. This is because the amount of the Ag nanoparticles is higher with the ultrasonic waves due to longer exposure time Ag and TiO 2 combination In the present research, hybrid ultrasonic vibration and laser ablation was used to generate and modify Ag -TiO 2 cluster nanoparticles for the first time. Ultrasound waves have been used in many applications such as measuring distance, defect recognition within an object, detecting the presence of objects, imaging (sonography) and cleaning devices (ultrasonic cleaners). Ultrasonic cleaning devices can be classified into traditional ultrasonic and megasonic frequencies, as their working frequencies are in the range of 20 khz 500 khz and 0.5 MHz 5 MHz respectively. When ultrasonic waves disperse into the solution, they create compression waves, consequently rupturing the solution 128

129 Chapter 4: The characteristics of Novel Bimodal Ag-TiO 2 Nanoparticles... and generating an enormous number of voids or cavities. These huge quantities of bubbles collapse and produce very high temperature and pressure; 5000 K and lbs/inch 2 respectively [ ]. These very high temperatures increase the Brownian motion and mobility of the surface atoms; as a result, collisions between nanoparticles will increase, followed by adhesion and coalescence between them [191]. The combination between Ag and TiO 2 nanoparticles was not observed while producing them without ultrasonic vibration because in the case of absent ultrasonic vibration the high pressure and temperature will not be generated. Several mechanisms have been provided to explain production of nanoparticles by laser ablation in deionised water [87, 238]. In general, generation of nanoparticles in a liquid environment by laser ablation can be divided into two steps: firstly, generation of nanoparticles from the target material and secondly, dispersion of nanoparticles in the liquid media. As observed experimentally, the size and size distribution of the Ag-TiO 2 nanoparticles were quite similar when they produced with and without ultrasonic vibration. The only difference between them is the combination of the Ag with the TiO 2 nanoparticles. The ultrasound interaction with matter depends on the characteristic of the ultrasonic wave, type of the material and the physical properties of the material such as acoustic impedance. Acoustic impedance measure the resistance of the material to mechanical vibration [241]. In addition, higher ultrasonic power density or higher frequency leads to the increased effects of the ultrasonic waves on the materials. At a fixed ultrasonic power and frequency, the denser nanoparticles are more affected by ultrasonic waves. Here, Ag nanoparticles were more affected by ultrasonic power than the lower density TiO 2. As a result, the Ag nanoparticles were directed or moved to attach the lighter TiO 2 nanoparticles. The ultrasonic waves act as a driving force to combine different nanoparticles in liquid media because of the acoustic wave properties such as density, sound speed and specific acoustic impedance are different in air, water and even in different materials. In addition, for a sinusoidal ultrasonic wave of rms acoustic pressure amplitude at 100 Pa, the particle velocity amplitude, displacement amplitude and acceleration amplitude are different at different frequencies [242]. When ultrasonic wave interacts with a 129

130 Chapter 4: The characteristics of Novel Bimodal Ag-TiO 2 Nanoparticles... solid material its amplitude will be decreased in the material due to absorption or scattering which it means that the some energy will be absorbed by nanoparticles which leads to a change in their direction acts as a driving force [243]. Here, it can be said that the ultrasonic waves might have not effected the generation mechanism of nanoparticles such as phase transition, thermodynamic and kinetic growth, but they have an effect on the combination of two different types of nanoparticles while they are still hot (during the laser ablation) that have different physical properties. In addition, ultrasonic waves were used to dismantle large-sized particles to small-sized particles [244] and to disperse agglomerated nanoparticles in deionised water [245] Antibacterial activity The antibacterial activity of nanoparticles depends on the types of nanoparticles, such as Ag, Au, or TiO 2. In addition, the concentration and the amount of nanoparticles used for antibacterial tests are two important factors for eliminating microorganisms. It is worth mentioning that the antimicrobial efficiency will be increased by increasing the concentration and the amount of nanoparticles in the solution to a certain extent. This increases the efficacy of the solution, because in both cases the number of nanoparticles in the solution will be increased, and as a result the chance of bacteria coming into contact with the nanoparticles will be increased leading to killing more numbers of bacteria. It is worth mentioning that the concentration of both Ag and Ag-TiO 2 cluster nanoparticles was 20 µg/ml, but the amount Ag in the Ag-TiO 2 cluster nanoparticles was less than half (Ag-TiO 2 weight ratio of 1:1.2) in comparison with the pure Ag nanoparticles, expecting less toxicity to human cells (our separate work, to be published, on the comparison of toxicity of Ag-TiO 2 cluster and Ag nanoparticles shows that Ag nanoparticles tend to penetrate into the human cells while the Ag-TiO 2 clusters tend to remain outside the human cells.), despite that the antibacterial activity of pure Ag nanoparticles was a little higher than Ag-TiO 2 cluster nanoparticles. 130

131 Chapter 4: The characteristics of Novel Bimodal Ag-TiO 2 Nanoparticles Summary Bimodal Ag-TiO 2 cluster nanoparticles were generated using a hybrid ultrasonic sonication and picosecond laser ablation in deionised water. The method has allowed the attachment of smaller Ag particles onto the larger TiO 2 nanoparticles with a yield of approximately 50%. This method has been demonstrated for the first time. The results show that there is no combination between Ag and TiO 2 nanoparticles produced in deionised water without ultrasonic vibration. In addition, the combination is not obtained between TiO 2 nanoparticles upon placing them in the ultrasonic machine. This method is a new way of combining two different nanoparticles. The Ag-TiO 2 cluster nanoparticles have good antibacterial activity and this activity is enhanced by increasing the concentration and amount of nanoparticles in the solution. 131

132 5 Chapter 5. Generation of Silver Titania Nanoparticles from Ag- Ti alloy via Picosecond Laser Ablation and their Antibacterial Activities Authors: Abubaker Hamad, Lin Li, Zhu Liu, Xiang Li Zhong, Hong Liu and Tao Wang Journal: RSC Advances Volume, issue and pages: 5, 89, Status: Published Note: The format of the paper is edited 132

133 Chapter 5: Generation of Silver Titania Nanoparticles from Ag-Ti alloy... Generation of Silver Titania Nanoparticles from Ag-Ti alloy via Picosecond Laser Ablation and their Antibacterial Activities Abstract In this work, a bulk Ti/Ag alloy was used, for the first time, to produce Ag- TiO 2 compound nanoparticles using picosecond laser ablation in deionised water. Spherical Ag-TiO 2 compound nanoparticles with an average size of 31 nm were produced. They were characterised using a UV-VIS spectrometer, transmission electron microscopy (TEM), High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM), Energy Dispersive X-ray Spectroscopy (EDS), X-ray Photoelectron Spectroscopy (XPS) and X-ray Diffraction (XRD) methods to identify the nanoparticle size distribution, morphology, chemical composition, phase and surface properties. It was found that Ag-doped TiO 2 nanoparticles were produced. The optical absorption spectra of the Ag-TiO 2 compound nanoparticles shifted to longer wavelengths. The antibacterial activity of the Ag-TiO 2 compound nanoparticles against gram-negative Escherichia coli (E. coli) bacteria was examined and compared with those using laser generated Ag and TiO 2 nanoparticles and distilled deionised water. It was found that the antibacterial activity of the Ag- TiO 2 compound nanoparticles was better than laser generated TiO 2 nanoparticles and chemically produced Ag nanoparticles, and was almost as good as laser generated Ag nanoparticles while Ag-TiO 2 having used at much lower Ag concentration. The reason behind this is discussed. Keyword: Ti/Ag alloy, Ag-TiO 2 nanoparticles, laser ablation, antibacterial activity. 133

134 Chapter 5: Generation of Silver Titania Nanoparticles from Ag-Ti alloy Introduction One of the biggest threats to humanity is the drug resistant by bacteria to antibiotics. Over the past decade, many potential antibacterial agents including noble metallic nanoparticles and nano metal-oxides have been found to be toxic against bacteria. Due to the specific properties of Ag and TiO 2 nanoparticles for different applications, particularly in bioscience for antimicrobial functions, they have been produced and modified using various methods, including chemical and physical methods. Silver and titania nanoparticles have been produced for their antibacterial functions, both individually and by way of modification with each other to produce bimetallic particles. On the other hand, due to the increasing use of silver and titanium dioxide nanoparticles in the medical field and everyday products, some researchers have expressed their concerns over the environmental impacts of nanoparticles. The risks of using these nanoparticles have been evaluated in terms of their toxicity for human or animal cells such as red blood cells [246], human lung cancer cell lines, A549 [247], and human and rat liver cells [248]. To produce more effective nanoparticles, bulk alloy and composite materials were used to prepare alloy and compound nanoparticles. For this purpose laser ablation techniques have been used by some researchers; for example, Lee et al. [249] and Kuladeep et al. [250] prepared Au-Ag alloy nanoparticles using pulsed laser ablation in water and a polyvinyl alcohol (PVA) solution respectively. They generated a tunable localized surface plasmon resonance (SPR) frequency using different ratios of Au:Ag, including100:0, 75:25, 50:50, 25:75 and 0:100 % [250] and 25:75, 50:50 and 75:25 % [249]. It was also concluded that the size distribution of the alloy nanoparticles produced can be controlled using suitable laser pulse energy and time exposure [249]. Grade et al. [251] generated homogenous Au-Ag nanoalloys via laser ablation in liquid environments. In order to stabilize the produced nanoparticles in biological media, they were conjugated ex-situ using bovine serum albumin (BSA). It was observed that a higher ratio of Au in the Au-Ag nanoalloys led to a reduction in antibacterial activity and cytotoxicity. It was found that 12 µg/cm 3 of pure Ag nanoparticles prevented bacterial growth, but 50 µg/cm 3 of Ag-Au alloy nanoparticles with 50% Au was ineffective in inhibiting the bacteria growth. This 134

135 Chapter 5: Generation of Silver Titania Nanoparticles from Ag-Ti alloy... reduction of the antibacterial activity of the alloy nanoparticles is due to the effect of Au ratio on the release of silver ions [251]. Ag-modified TiO 2 have been used for antibacterial functions, for instance; Ramesh et al. [136] generated Ag and TiO 2 nanoparticles separately by sonochemical and colloidal methods respectively; the nanoparticles produced were spherical in shape and their sizes fell within the range of 20 to 50 nm. The Ag and TiO 2 nanoparticles were used as antibiotic against S. aureus, S. epidermindis, E. coli and K. pneumoniae. They concluded that the antimicrobial activity of Ag nanoparticles was greater than that of TiO 2 nanoparticles. Pan et al. [168] concluded that the Ag-TiO 2 nanocomposite and Ag nanoparticles completely killed E. coli under visible light irradiation. The researchers also reported that the effect of an Ag-modified TiO 2 nanocomposite on E. coli is nearly 5 times more potent than that of TiO 2 and the durability of the Ag-TiO 2 nanocomposite was extended further compared with Ag. Recently Gupta et al. [123] prepared both Ag-TiO 2 and TiO 2 via acid catalyzed sol gel method and tested their antimicrobial activity against S. aureus, P. aeruginosa and E. coli bacteria under visible light irradiation. It was concluded that the antimicrobial activity of the doped TiO 2 is considerably higher than that un-doped TiO 2 nanoparticles. For example, in the case of P. aeruginosa bacteria, they were completely killed by doped TiO 2 nanoparticles with 7% Ag at 40 mg/ml culture, but for S. aureus and E. coli that amount is raised to 60 mg/ml, while at 3% Ag doping the viability of all types of bacterial culture was decreased to zero at 80 mg/30 ml culture. Barudin et al. [252] generated and tested the antibacterial activity of Ag-TiO 2 nanoparticles, with different Ag concentrations, against E. coli bacteria under fluorescence light irradiation by using cotton diffusion test. The optimal antibacterial activity against E. coli was observed at 0.06 mol% Ag in Ag-TiO 2 nanoparticles which they have 38 mm inhibition zone at 2.0 M concentration. With a proper ratio of Ag and TiO 2 in the Ag-TiO 2 nanoparticles, the compound nanoparticles are expected to have a stronger antibacterial activity in comparison with Ag and TiO 2 nanoparticles, because in the compound nanoparticles can combine two factors to kill bacteria; the first one is the antibacterial activity of the Ag nanoparticles and the second one is the photocatalytic activity of the TiO 2 nanoparticles. 135

136 Chapter 5: Generation of Silver Titania Nanoparticles from Ag-Ti alloy... In this work, the authors demonstrate the generation of Ag-TiO 2 nanocompound particles by laser ablation of Ti/Ag bulk alloy in deionised water. Their antibacterial characteristics were investigated. The difference between this work from the previous works is the use of Ti/Ag alloy as a precursor to produce Ag-TiO 2 nanoparticles for the first time. As a result, the shifting optical absorption spectra was observed. In addition, the Ag-TiO 2 nanoparticles, at much reduced Ag content compared with pure Ag, were found to be much better in their antibacterial capability than TiO 2 nanoparticles and comparable with pure Ag nanoparticles, leading potentially reduced use of Ag Experimental set-up Materials A Ti/Ag bulk alloy plate with a ratio of 3:1 at.%, supplied by Cathay Advanced Materials Limited, was used as the precursor material for the production of Ag-TiO 2 compound nanoparticles. The purity of the Ag and Ti alloy components used in this work were 99.95% and 99.7% respectively. The size of the bulk alloy was 25 mm 25 mm 1 mm. For comparison with Ag- TiO 2 nanoparticles, Ag nanoparticles were also generated from a pure Ag bulk plate with purity of 99.99% and the dimensions 25 mm 25 mm 2 mm. TiO 2 nanoparticles were also produced for comparison. A pure Ti bulk plate with a purity of 99.99% and the dimensions of 25 mm 25 mm 1 mm were used. Chemically produced commercial Ag nanoparticles in sodium citrate as stabiliser (20 µg/ml concentration and about 35 nm average size) were acquired for comparison Ag-TiO 2 compound nanoparticle production Ag-TiO 2 nanoparticles were produced by placing the Ti/Ag alloy plate at the bottom of a 70 ml glass vessel, containing about 20 ml of deionised water. Picosecond laser ablation was used to generate nanoparticles in the water; this process lasted for 10 minutes. An Edgewave picosecond laser was used to produce the nanoparticles with the following parameters: wavelength = 1064 nm, frequency f = 200 khz, laser power P = 9.12 W, pulse width = 10 ps, spot 136

137 Chapter 5: Generation of Silver Titania Nanoparticles from Ag-Ti alloy... size D = 125 µm, scan speed v = 250 mm/s, laser pulse energy E pulse = 45.6 µj and laser fluence F laser = J/cm 2. The water level above the sample target was about 2 mm and its effects of reducing the laser beam intensity and increasing focal length were considered. The experimental setup is shown in Figure 5-1. In addition, both the Ag and TiO 2 nanoparticles were fabricated under the same experimental conditions and laser beam parameters for antibacterial activity comparison. Figure 5-1: Experimental set-up for generation of nanoparticles in deionised water by picosecond laser Material Characterisation The bulk alloy was characterised by X-ray diffraction (XRD) and X-ray fluorescence (XRF). The Ag-TiO 2 colloidal nanoparticles were characterised using a UV-VIS spectrometer (Analytic Jena, SPECORD 250, dual beam) and the nanoparticles were analysed using XRD ((BrukerD8-Discover) (Step Size [ 2 ] = )), Transmission Electron Microscopy (TEM) (JEOL 2000 FX AEM + EDX model), High-Angle Annular Dark-Field - Scanning Transmission Electron Microscopy (HAADF-STEM), X-ray Photoelectron Spectroscopy (XPS) and Energy-Dispersive X-ray Spectroscopy (EDS) (FEI Tecnai G2 F30). A copper micro-grid mesh was used for sample preparation for the TEM and HAADF-STEM analyses. After depositing a drop of colloidal nanoparticles onto the mesh, the substrate was allowed to dry at room temperature. This process was repeated several times to deposit sufficient amount of nanoparticles on the copper microgrid mesh. The sample used for the XPS analysis was prepared on 137

138 Chapter 5: Generation of Silver Titania Nanoparticles from Ag-Ti alloy... an aluminum plate, by dropping the Ag-TiO 2 colloidal nanoparticles onto it, followed by drying with a hair dryer. The sample used for the XRD analysis was prepared on a glass slide, by dropping the Ag-TiO 2 colloidal nanoparticles onto it then drying at room temperature. A substantially larger amount of nanoparticles was required for this process. To obtain more nanoparticles, the colloidal nanoparticles were centrifuged for 15 minutes at 10,000 rpm, then the deposition process onto the glass slide was repeated several times in a 2 cm 2 cm area. A microbalance scale (Sartorius BL 210S, with readability d = 0.1 mg) was used to determine the concentration of the colloidal nanoparticles by weighing the bulk Ti/Ag alloy and Ag before and after the nanoparticle production process. The samples were dried using a hair dryer after the ablation process to record the weight of the ablated materials with greater accuracy. The zeta-potential of laser produced Ag, Ag-TiO 2 compound nanoparticles and commercial Ag nanoparticles was measured using Zetasizer Nano Series (Nano-ZS) Malvern Instruments to understand the stability and charge characteristics of the nano-particle (zeta-potential absolute value < 30 mv indicates that the nanoparticle are unstable and can easily coagulate, a zetapotential absolute value > 40 mv indicates good stability) Antibacterial activity analysis E. coli bacteria (JM109 Promega UK) were used to test the antibacterial activity of the Ag-TiO 2 compound nanoparticles. In addition, the antibacterial activity of laser generated Ag and TiO 2 nanoparticles was tested for comparison. E. coli were cultured from a single colony in LB (Lysogeny Broth) broth overnight at 37 C with 225 rpm shaking. Ag-TiO 2 and Ag nanoparticles at a concentration 20 µg/ml were co-cultured with E. coli respectively in LB broth for 6 hours at 37 C with 225 rpm shaking. 10 µl of the culture were plated on to a 10 cm LB agar petri dish and incubated at room temperature for 48 hours. The numbers of bacteria colonies were then counted. The optical density of the cultured E. coli was measured at 600 nm (OD600) and diluted down to a colony forming unit (CFU)/ml about to with LB. The nanoparticles antibacterial activities were tested under two conditions: under standard room light and in dark conditions. To achieve dark conditions the plates were wrapped with aluminum foil to prevent light reaching the specimens. 138

139 Reflectivity [R] Chapter 5: Generation of Silver Titania Nanoparticles from Ag-Ti alloy Results and discussion Bulk Ti/Ag alloy characterisation Before generation of the nanoparticles, the bulk Ti/Ag alloy plate was characterised Reflectivity of Ti/Ag alloy (bulk) Figure 5-2 shows the reflectivity of the bulk Ti/Ag alloy. It is observed that the reflectivity of the bulk ally is increased slightly from visible light range to infrared wavelengths, indicating that the reflectivity is increased slowly with increasing wavelength. As shown in the figure, the reflectivity of the alloy at 1064 nm wavelength is 56% Wavelength Reflectivity [R] 1064 nm Wavelength (nm) Figure 5-2: Reflectivity of the Ti/Ag alloy plate Phase analysis by X-ray diffraction Figure 5-3 shows the X-ray diffraction of the Ti/Ag alloy (bulk sample). As shown in the inset table, the sample consists of three compounds: -Ti, AgTi 3 and Ag syn. The peak positions 2 = 40.93, 44.33, 46.95, , 74.46, and represents the alpha-titanium ( -Ti), while the peaks at 2 = 34.83, 36.27, 50.46, 53.83, 57.82, 76.83, and indicate the existence of silver-titanium (AgTi 3 ) and 2 = 44.5, and represent the silver Syn. (Ag syn). 139

140 Chapter 5: Generation of Silver Titania Nanoparticles from Ag-Ti alloy... Figure 5-3: XRD of Ti/Ag alloy bulk sample X-ray florescence Figure 5-4 is an XRF image of the Ti/Ag alloy plate which confirms the existence of Ag and Ti elements in the alloy plate. The Ag shows the transitions of L, L 1 and L 2 and the Ti has K and K transitions (L : transition from M shell to L shell, L transition from N shell to L shell, K transition from L shell to K shell and K transition from M shell to K shell). The intensity of the Ti is higher than the Ag because the atomic weight of Ti in the sample is higher than that of Ag in the alloy sample. Figure 5-4: XRF of the Ag/Ti alloy. 140

141 Chapter 5: Generation of Silver Titania Nanoparticles from Ag-Ti alloy... The XRD pattern of the Ti/Ag alloy plate shows that the alloy consists of -Ti, AgTi 3 and Ag syn appears in the bulk. Further analysis using XRF reveals that the alloy consists of pure Ag and Ti materials. The intensity of the Ti is stronger than the Ag because the atomic ratio of the Ti is three times higher than Ag in the alloy sample. In this work, the sample used has 25% Ag and the remaining quantity is Ti, which consists of -Ti, AgTi 3 and Ag syn. The production of AgTi 3 corresponds to the ratio of bulk sample Ag-Ti 1:3 at.%. In summary, in pure bulk titanium and 5%-20% Ag only -Ti was observed, but at 22.5% and 25% Ag the alloy sample consists of both -Titanium and the intermetallic compounds Ti 2 Ag and Ti 2 Ag+TiAg [253]. Consequently, the production of -Ti, gives almost pure Ti material with the possibility of trace amounts of Ag material being found in the alloy Ag-TiO 2 compound nanoparticles Spherical Ag-TiO 2 compound nanoparticles were generated by picosecond laser ablation (calibrated to = 1064 nm, f = 200 khz, P = 9.12 W and v = 250 mm/s) in deionised water. The UV-VIS spectrum measured for the as-prepared Ag-TiO 2 compound nanoparticles (see Figure 5-5-a) show that the surface plasmon resonance of Ag is shifted to the longer visible light range of 500 nm, which begins at about 400 nm. The strong band appears in the UV range due to the addition of the strong band of Ti nanoparticles with their high ratio of Ti (75%) in the Ti/Ag alloy and the inter-band transition of the Ag nanoparticles at 200 and 250 nm [63]. A high concentration of ablated nanoparticles in the solution leads to the production of the strong band or high intensity spectrum. Because TiO 2 has an indirect band gap transition [111], the indirect transition energy band gap of the compound nanoparticles was measured, as shown in Figure 5-5-b. It can be noted that the forbidden energy band gap is reduced significantly from original TiO 2 band gap (about 3 ev) to 2.35 ev. The reduction of the energy gap may be due to the extension of the absorption spectra to the visible light range. 141

142 Absorbance [A] ( E) 1/2 ev 1/2 nm -1/2 Absorbance [A] Chapter 5: Generation of Silver Titania Nanoparticles from Ag-Ti alloy (a) (b) Wavelength (nm) Wavelength (nm) Photon energy (ev) Figure 5-5: The absorption spectra of the Ag-TiO 2 compound nanoparticles produced by picosecond laser in deionised water (a). Indirect band gap energy of the Ag-TiO 2 compound nanoparticles (b). Figure 5-6-a, b, c and d show the TEM images of the Ag-TiO 2 compound nanoparticles; their average size in diameter is 31 nm of 1354 measured particles. The figures exhibit different sizes of spherical nanoparticles. The TEM images show some small nanoparticles were attached to big nanoparticles, which have been combined together. The size distribution (shown in Figure 5-6- e) of the nanoparticles has lognormal probability distribution function fitting. The minimum and maximum sizes of the compound nanoparticles are 2 nm and 222 nm respectively. It can be seen that almost all nanoparticles are in the range of a few nanometers to 90 nm, and few of the particles were observed to be above 100 nm. 142

143 Number of Nanoparticles Chapter 5: Generation of Silver Titania Nanoparticles from Ag-Ti alloy... (a) (b) (c) (d) (e) Number of NPs 1354 Average Size 31 nm Minimum 2 nm Maximum 222 nm Size of Ag-TiO 2 compound nanoparticles (nm) Figure 5-6: TEM images of the Ag-TiO 2 compound nanoparticles generated by picosecond laser in deionised water (a,b,c and d). Histogram of the nanoparticles lognormal size distribution (e). The ablation rate of the Ag-TiO 2 nanoparticles is mg/min. Figure 5-7 (a, b, c and d) shows the HAADF-STEM and EDS images of the Ag-TiO 2 nanoparticles. In the electron image, three different points on the nanoparticles were selected and characterised. The nanoparticles were about 50 nm in diameter. Some small nanoparticles were also observed; their size ranged from 5 nm to 20 nm. As shown in Figure 5-7, the weight ratio (wt.%) Ag:Ti:O shown in Figure 5-7-a is 48.54:19.65:31.80 wt.%, 8.45:41.88:49.67 wt.% and 15.07:35.58:49.35 wt.%, shown in Figure 7-b, c and d respectively, indicating that Ag is doped in TiO

144 Chapter 5: Generation of Silver Titania Nanoparticles from Ag-Ti alloy... (a) (d) (c) (b) Figure 5-7: (a,b,c and d) HAADF-STEM and EDS images of the Ag-TiO 2 compound nanoparticles. 144

145 Chapter 5: Generation of Silver Titania Nanoparticles from Ag-Ti alloy... Figure 5-8 shows the line profile spectrum and an EDS image of a nanoparticle. It can be seen that this nanoparticle is an Ag-TiO 2 compound nanoparticle, in which the amount of Ag is higher than both the Ti and O. In Figure 5-8-a and b, some other elements were observed; the C and Cu peaks were observed owing to the Formvar / Carbon on 200 copper mesh which was used for the sample preparation for the EDS and TEM analysis. In addition, negligible amount of Si and S appear in the image, which is probably formed due to the glassware used for the nanoparticle production process and storage on colloidal nanoparticles. (b) Figure 5-8: Line profile spectrum (a) and EDS images (b) of the Ag-TiO 2 compound nanoparticles. Figure 5-9 shows the atomic percentage (at.%) ratio of Ag:Ti:O of a spectrum of 17 points (nanoparticles) counted by the EDS spectrum. The ratios of chemical elements are different; one group consists of Ti and O with a small amount of Ag, whereas in the second group Ag is predominant. Is spite of some of the nanoparticles having a balanced amount of the chemical elements, a few nanoparticles are pure TiO 2 and completely free from Ag. 145

146 Atomic percentage (at.%) Chapter 5: Generation of Silver Titania Nanoparticles from Ag-Ti alloy Ag Ti O Figure 5-9: The atomic percentage (at.%) ratio of Ag:Ti:O chemical elements of a spectrum of some of the nanoparticles (or points). Figure 5-10 shows the X-ray diffraction of the Ag-TiO 2 compound nanoparticles. As shown in the inset table, the compounds of the nanoparticles are Ag and TiO 2 with some silver oxides. The crystal phase of the TiO 2 produced is rutile. The position peaks 2 = 38.10, 44.41, and represent the Ag elements and the positions 2 = 35.77, and represent the existence of rutile TiO 2. In addition, the peak positions at and represent AgO Counted nanoparticles Figure 5-10: XRD image of the Ag-TiO 2 compound nanoparticles. 146

147 Chapter 5: Generation of Silver Titania Nanoparticles from Ag-Ti alloy... The absorption spectra of the colloidal Ag-TiO 2 compound nanoparticles show the effect of the surface plasmon resonance (SPR) of the Ag from a wavelength of 400 nm to 500 nm. Shifting the spectra to a longer wavelength leads to a decrease in the transition energy band gap of the nanoparticles to 2.35 ev. These nanoparticles are expected to be effective with photocatalytic activities not only under UV light irradiation but also when exposed to visible light. This phenomenon had not been previously observed. Pure TiO 2 nanoparticles have a higher energy band gap about 3, 3.13 and 3.21 ev from rutile to anatase [216]. After doping with Ag nanoparticles it was reduced to 2.35 ev. This is may be due to the use of the Ag-TiO 2 alloy as the precursor material which makes nanoparticles more combined than physically mixing Ag and TiO 2 nanoparticles. As a result the optical absorption spectra shifted to longer wavelengths and the energy gap was reduced. Figure 5-11 shows the XPS analysis of the Ag-TiO 2 compound nanoparticle. The results show that the Ag, Ti and O are three main elements in the sample. The Ag 3 d spectrum (Figure 5-11-a) consists of two pairs of doublet peaks of Ag 3d 3/2 and Ag 3d 5/2. The two big peaks located at ev and ev represent silver metal (Ag o ) and the two small peaks, located at ev and ev represent silver oxide (Ag 2 O) [254]. Pham and Lee [255] indicated that the peaks (at ev and ev) represent the silver ions (Ag + ) and the two peaks (at ev and ev) represent the silver metal (Ag o ). In addition, Teng et al. [256] indicated Ag o peaks occur at ev and ev, and Ag + peaks occur at ev and ev. The Ti 2P spectra (Figure 5-11-b) also consists of two peaks Ti 2p 1/2 and Ti 2p 3/2. Each of them can be fitted by two spectra. The peaks located at ev and ev represent Ti 3+, and the peaks located at ev and ev, represent existence Ti 4+ [255, ]. The location of the bare TiO 2 of 3d 3/2 level is at ev [231], in this work Ag-TiO 2 is shifted to higher binding energy at ev at the same level. This indicates a strong interaction between Ag and TiO 2 materials and confirms a lower electron density of the TiO 2 surface after Ag nanocrystals deposition [231]. Figure 5-11-c shows three wide asymmetric spectra of the O 1s at ev, and ev. The spectra at ev is due to the lattice oxygen of O Ti-O and the other two spectra at ev and are due to the existence of O species such as 147

148 CPS CPS CPS Chapter 5: Generation of Silver Titania Nanoparticles from Ag-Ti alloy... hydroxyl groups (O-H) and water (H-O-H) molecules on the surface of the sample [ ]. Zhao et al. [257] concluded the existence of lattice oxygen (O Ti O ) at ev and surface hydroxyl groups (O O H ) at ev. In addition, Chu et al. [260] indicated a hydroxyl species at ev and Ramasamy et al. [261] indicated adsorbed O 2 at ev. 3x10 4 2x10 4 (a) Ag 3d 5/2 Ag o Ag 3d 3/2 2.5x x10 4 (b) 2p 3/2 Ti x x x10 4 (c) O 1s 2x10 4 Ag + 2.5x10 4 1x x10 4 Ti 3+ 2p 1/2 2.0x10 4 5x x x x x Binding Energy (ev) Binding Energy (ev) 5.0x Binding Energy (ev) Figure 5-11: XPS images of the Ag-TiO 2 compound nanoparticle. Peak-fitting spectra at high resolution of Ag 3d (q), Ti 2p spectra (b) and O 1s spectra (c) of the Ag-TiO 2 compound nanoparticles. Spherical Ag-TiO 2 nanoparticles with different sizes and lognormal size distribution function were produced. The lognormal size distribution means that the nanoparticles are not uniformly distributed, as the uniform size distribution of nanoparticles usually has a Gaussian distribution form. The EDS images show that some pure Ag and TiO 2 nanoparticles were also observed because both Ag and Ti have different physical properties, such as melting point, reflectivity, work function, heat of evaporation and ablation threshold. Their different properties cause the elements to respond differently to the laser beam energy; as a result they release from the surface of the bulk with different kinetic energy [262]. An irregular ratio (disproportionation of the compound nanoparticles) of Ag and Ti in the Ag-TiO 2 compound nanoparticles (not as the ratio of 3:1 at.% Ti/Ag alloy) was observed. The production of nearly pure Ag and TiO 2 nanoparticles is due to the separation of ablated species of the elements in the plasma plume because of their different kinetic energies after ejection from the target. This process is due to their confinement in a liquid environment [ ]. The different kinetic energies of the ablated species of the elements are due to the different heats of evaporation, which leads to the production and ejection of one of the elements faster than the other [264]. The 148

149 Chapter 5: Generation of Silver Titania Nanoparticles from Ag-Ti alloy... heat of evaporations of Ag and Ti are 254 kj/mol and 425 kj/mol, respectively. They have different heats of evaporation by a factor of 1.67, so more Ag should be produced and at a faster rate than the other component. This hypothesis corresponds with the results of the experiment, as more Ag was produced in the compound nanoparticles (see Figure 5-9). Furthermore, laser ablation of the Ti/Ag alloy led to the production of larger TiO 2 nanoparticles (above 100 nm) and smaller Ag nanoparticles (below 100 nm); this is due to the different heats of vaporization. The higher heat of vaporization of the Ti would generate bigger nanoparticles. Similar findings were observed for the brass alloy ablation process by laser, where large Cu nanoparticles than the Zn were produced which their heats of evaporation differed by a factor of 2.52 [264]. Moreover, laser ablation of the brass alloy, revealed that the ratio of Zn and Cu in the particles altered when their diameters were changed. Zn was predominant in the small particles, whereas Cu was predominant in the bigger particles [265]. Their conclusions correlate with the results obtained in this study. The XRD image shows that the rutile phase of TiO 2 in the Ag-TiO 2 nanoparticles was produced because the laser ablation process was accompanied by a high temperature within the plasma plume. This should be equal to or higher than the melting points of the target composites, which for Ag is 962 C and is 1668 C for Ti [266]. So the plasma plume temperature reaches 6000 K [267]. These high temperatures bring about a rutile phase (this will be produced when the substrate temperature rises above 600 C [109]), while the anatase phase occurs at lower temperatures (around 300 C) [107]. It is worth mentioning that mixed anatase-rutile phases of TiO 2 were produced at high temperatures (around 800 C) [268]. A low temperature nanoparticle production process is required for the anatase phases of TiO 2 to occur, such as in the solgel method [269]. In spite of the fact that rutile is more thermodynamically stable than the other two phases of titania [206], Nasr-Esfahani. and Habibi [117] reported that the photocatalytic activity of anatase is more effective than rutile due to its larger transition band gap energy which causes a delay in the electron-hole recombination (or an increase in the charge carrier lifetime) and an increase in the surface redox potential [117]. On the contrary, Li et al. [ ] showed that an amorphous TiO 2 nanocolumn array has better photocatalytic activity than an anatase nanocolumn array because amorphous 149

150 Chapter 5: Generation of Silver Titania Nanoparticles from Ag-Ti alloy... structure has large surface area and special microstructures. In addition, the nanoparticles showed excellent superhydrophilicity with zero contact angle, for self-cleaning surfaces [ ]. It is worth noting that the production of rutile TiO 2 nanoparticles in this work is due to the high amount of Ag in the Ag-TiO 2 nanoparticles, as Cao et al. [272] showed that doping TiO 2 nanoparticles with greater quantities of Ag leads to the production of a more extended rutile phase of titania. In highly absorbent materials, for ultra-short pulses, the ablation mechanism is governed by spallation, phase explosion or fragmentation. For longer pulses the ablation is due to trivial fragmentation [273]. In addition, an increase in laser fluence or exposure time would change the morphology of nanoparticles from octahedron to spherical shape [274] Sedimentation and zeta-potentials of nanoparticles Figure 5-12-a and b show the sedimentation of the Ag-TiO 2 compound and pure Ag nanoparticles after 15 and 30 days. On the day of preparation, some nanoparticles were extracted from the center of the bottle of colloidal nanoparticles (see Figure 5-12-d). Then their sizes were measured using a TEM. The bottles were left for 15 days. Then some of the colloidal nanoparticles were extracted from the middle of the bottle. The extraction process was repeated after 30 days. The results show that heavier (larger) nanoparticles were deposited at the bottom of the bottles and the small nanoparticles were suspended in the middle. As shown in Figure 5-12-c, the average sizes of the Ag-TiO 2 nanoparticles were reduced from 47 nm to 29 and 14 nm after 15 and 30 days respectively. Moreover, the average sizes of the Ag nanoparticles decreased from 38 nm to 28 and 26 nm respectively. The figures confirm that more small nanoparticles were observed after 30 days; this indicates that the larger particles were sinking to the bottom of the bottle. 150

151 Average Size of Nanoparticles (nm) Frequency Frequency Chapter 5: Generation of Silver Titania Nanoparticles from Ag-Ti alloy (a) Ag-TiO 2 (0 day) Ag-TiO 2 (15 days) Ag-TiO 2 (30 days) 150 (b) Ag (0 day) Ag (15 days) Ag (30 days) days No. of NPs Ave. size (nm) days No. of NPs Ave. size (nm) Size of Ag-TiO 2 NPs (nm) (c) Ave. Size (Ag) Ave. SIze (Ag-TiO 2 ) Number of days Size of Ag NPs (nm) Figure 5-12: Sedimentation of Ag-TiO 2 compound nanoparticles (a), and Ag nanoparticles (b) during time. (c) The average sizes of the nanoparticles as a function of time. The photographs show the bottles which contain the colloidal nanoparticles. (d) Figure 5-13 shows the absorption spectra of the colloidal pure Ag and Ag-TiO 2 compound nanoparticles on the day of preparation and after 15 and 30 days. The size distribution of the single nanoparticles (Ag nanoparticles) is more uniform than that of the compound nanoparticles (Ag-TiO 2 ), but the average size of the compound nanoparticles after 30 days is smaller than the pure Ag nanoparticles. It can be seen from the images of the bottles that the colour of the colloidal nanoparticles became lighter and lighter after 15 and 30 days, as the concentration of the suspension decreased. 151

152 Absorbance [A] Absorbance [A] Chapter 5: Generation of Silver Titania Nanoparticles from Ag-Ti alloy (a) 0 day 15 days 30 days (b) 0 day 15 days 30 days Wavelength (nm) Wavelength (nm) Figure 5-13: The absorption spectra of Ag (a) and Ag-TiO 2 (b) nanoparticles on the day of preparation as well as after 15 and 30 days. The zeta-potentials of laser generated Ag, Ag-TiO 2 compound nanoparticles and commercial Ag nanoparticles were measured at 25 C using Zetasizer Nano Series (Nano-ZS) Malvern Instruments. The results show that the laser generated Ag nanoparticles have zeta-potentials of about mv (Zeta deviation= 7.65 mv), commercial (chemically produced) Ag nanoparticles have zeta-potentials of about mv (Zeta deviation = 5.93 mv) and the laser produced Ag-TiO 2 nanoparticles have zeta-potentials of about mv (Zeta deviation= 8.88 mv). According to these results, pure Ag nanoparticles are slightly more stable than the Ag-TiO 2 composite nanoparticles. A balance between the Van der Waals and Coulomb repulsion are responsible for the stability of particles. Rapid coagulation of the colloidal suspension can be achieved by improving the Van der Waals attraction force. As a result, more charged particles will be induced due to the oxidative state of the liquid medium and the particles will be coagulated [275]. The sizes of the Ag nanoparticles on the day of generation were between 4 and 200 nm; however, after 15 days of preparation these ranged from 5 to 10 nm [98]. Precipitation of nanoparticles in solutions is related to the type of solution and the size of the nanoparticles; for example, producing nanoparticles in acetone (with a mean size 5 nm) is better than conducting the process in both deionised water (a mean size of 13 nm) and ethanol (a mean size of 22 nm) because the colloidal Ag nanoparticles precipitated after about two weeks and 48 hours in both deionised water and ethanol respectively [276]. So bigger nanoparticles had more probability to sink in the solution due to the greater effect of gravitational force on the bigger particles (F = mg). Nishi et al. [104] prepared a high concentration solution with small silver nanoparticles (20 mg/h and 2 nm) in 152

153 Chapter 5: Generation of Silver Titania Nanoparticles from Ag-Ti alloy... deionised water using laser ablation. The nanoparticles was stable for more than 6 months [104], indicating that the concentration of nanoparticles in a solution has an effect on the rate of precipitation. Stable silver nanoparticles (about 5 nm) were prepared in citrate as a stabiliser for one year [277]. The sedimentation and aggregation of silver nanoparticles is probably the fundamental cause of the changing the optical and non-linear optical properties of the nanoparticles [263]. An important property which influences nanoparticle stability is zetapotential. Some researchers investigated nanoparticle stability in terms of this [278]. Nanoparticles with higher zeta-potentials are more stable than those with lower zeta-potentials, because high value zeta-potential causes the nanoparticles to repel each other, preventing agglomeration and accumulation. For example, the zeta-potential of TiO 2 nanoparticles produced by pulsed laser ablation in an SDS solution is between mv and mv, creating stable nanoparticles with a very small agglomeration[279]. In order to produce stable nanoparticles their zeta-potential should be greater in absolute value than 30mV [280]. The main requirement to produce stable colloidal nanoparticles is the use of a surface-stabilizing agent [281]. In the present work, the results show that the laser generated Ag nanoparticles are slightly more stable than the laser produced Ag-TiO 2 compound nanoparticles. Although, the commercial Ag nanoparticles are slightly more stable than the laser produced Ag and Ag-TiO 2 compound nanoparticles. In this work, in spite of high zeta-potential of the nanoparticles, the larger nanoparticles were sedimented after 15 and 30 days. It may be because the gravitational force is more predominant for the larger nanoparticles than the zeta-potential. The mean hydrodynamic diameter of the nanoparticles is an important factor to obtain colloidal stable nanoparticles [282]. In addition, the zeta-potential may be decreased due to increase the PH of the solution [283]. Furthermore, dispersions with a low zeta-potential will be aggregated due to Van Der Waal inter-particle attractions. Our results correspond to the above analysis, because, as shown in Figure a and b, the rate of sedimentation of Ag-TiO 2 nanoparticles is higher than that of pure silver nanoparticles. This is more apparent after 30 days sedimentation of the nanoparticles in the deionised water. 153

154 Chapter 5: Generation of Silver Titania Nanoparticles from Ag-Ti alloy Antibacterial activity Figure 5-14 shows the antibacterial activity of both pure Ag nanoparticles and Ag-TiO 2 compound nanoparticles compared with the control sample. The tests were carried out under normal room light and in dark conditions for comparison. The antibacterial activity of the nanoparticles was tested after one day of preparation. The concentration of both types of nanoparticles was 20 and 25 µg/ml. After 6 hours incubation, the E. coli bacteria were killed by nanoparticles, not only under normal light exposure but also in dark conditions. Figure 5-18 shows the comparison with the control test. (a) (b) (c) Ag Ag-TiO2 Light (20 µg/ml) Ag Ag-TiO2 Light (25 µg/ml) Control Light Ag Ag-TiO2 Dark (20 µg/ml) Ag Ag-TiO2 Dark (25 µg/ml) Control Dark Figure 5-14: Antibacterial activity of the Ag and Ag-TiO 2 compound nanoparticles at 20 µg/ml (a) and 25 µg/ml (b) compared with control sample (c) under standard room light and dark conditions after one day of generation. Figure 5-15 shows the effect of incubation time on the bacteria growth in laser generated Ag and Ag-TiO 2 NPs and in the controlled samples. As the incubation time for growing bacteria increased, the number of bacteria increased in the controlled sample. However, the E. coli bacteria could not grow in either the Ag-TiO 2 or Ag nanoparticle solutions. Growth of some bacteria was observed during the first few hours. 154

155 Number of survived E. coli colonies Number of survived E. coli colonies Chapter 5: Generation of Silver Titania Nanoparticles from Ag-Ti alloy Ag Ag-TiO 2 Control Incubation time (h) Figure 5-15: Number of survived E. coli colonies as a function of incubation time for Ag-TiO 2, Ag and control under standard room light. The concentration of nanoparticles was 20 µg/ml. Figure 5-16 shows the relationship between the concentrations of Ag- TiO 2 compound and pure Ag nanoparticles with the number of survived E. coli bacteria. As shown in the figure, the increase in the nanoparticle concentration leads to more bacteria being killed. The ability of the Ag-TiO 2 compound nanoparticles was not as good as the pure Ag nanoparticles at low concentrations, but their efficiency became more similar at higher concentrations. At a concentration of 20 µg/ml, the E. coli were eliminated to almost 0 % by the silver nanoparticles, although some live bacteria were still observed on the Ag-TiO 2 agar plate but at 25 µg/ml concentration the E. coli bacteria were eliminated completely for both types of nanoparticles. 150 Ag Ag-TiO Concentration ( g/ml) Figure 5-16: Antibacterial activity of Ag-TiO 2 compound and Ag nanoparticles as a function of concentration. 155

156 Chapter 5: Generation of Silver Titania Nanoparticles from Ag-Ti alloy... Figure 5-17 show the antibacterial activity of pure TiO 2 nanoparticles against E. coli bacteria under both light and dark conditions. The nanoparticles produced by picosecond laser with the same laser and experimental conditions which were used to produce Ag and Ag-TiO 2 compound nanoparticles. In addition the same concentration was used to antibacterial activity test. It can be seen that TiO 2 nanoparticles could not kill bacteria in either cases, although both Ag and Ag-TiO 2 nanoparticles eliminated almost all cultured bacteria on the agar plate, Figure 5-17-b shows the comparison between them under standard room light. (a) 1.0 (b) 0.8 E. coli viability C/C o TiO 2 (light) TiO 2 (Dark) 0.0 Ag Ag-TiO2 2 compound TiO2 2 Nanoparticles Figure 5-17: (a) Agar plate shows antibacterial activity of TiO 2 nanoparticles (20 µg/ml) against E. coli bacteria under both standard room light and dark conditions. (b) Antibacterial activity of laser generated Ag, Ag-TiO 2 and TiO 2 nanoparticles under standard room light at the same concentration (20 µg/ml) and incubation time (6 hours). Figure 5-18 shows the antibacterial activity of chemically produced commercial Ag nanoparticles in sodium citrate (20 µg/ml concentration) against E. coli bacteria under both standard laboratory light and dark conditions. It can be seen that commercial Ag nanoparticles could not kill bacteria in either conditions even their size are about 35 nm in diameter. 156

157 Chapter 5: Generation of Silver Titania Nanoparticles from Ag-Ti alloy... Commercial Ag (light) Control (light) Commercial Ag (Dark) Control (Dark) Figure 5-18: Agar plate shows antibacterial activity of chemically produced commercial Ag nanoparticles in sodium citrate (20 µg/ml concentration and about 35 nm size in diameter) against E. coli bacteria under both standard laboratory light and dark conditions. It was tested about two months after their production. Several properties of nanomaterials impact their antibacterial activity, such as their type, amount, size, shape and stability. Herein, the authors used Ag-TiO 2 nanoparticles to kill E. coli bacteria because Ag has strong antimicrobial activity [136] and TiO 2 has photocatalytic activity [137] and can be used as an antimicrobial operator due to its high oxidation potential and superhydrophilicity [284]. The main purpose of using Ag-TiO 2 nanoparticles is to combine the antibacterial activity of Ag and photocatalytic activity of TiO 2 [213] in the Ag-TiO 2 nanoparticles which have a synergistic antimicrobial activity unconcerned by the effects of photoactivity and which have stronger antibacterial activity than both pure Ag and TiO 2 nanoparticles [7], and a smaller energy band gap than pure TiO 2 [123]. In general, because the antibacterial activity of the nanoparticles depended on the concentration of nanoparticles used (which can be referred as a concentration-dependent manner ) [285], the antibacterial activity of the nanoparticles against the E. coli bacteria increased as the concentration increased, because at higher concentrations the number of nanoparticles in the solution rose. As a result, the probability of the nanoparticles coming into contact with the E. coli bacteria increased. When the Ag-TiO 2 nanoparticles make contact with the bacteria, the nanoparticles penetrate the membrane of the bacteria after interacting with the functional group of the microbe, such as - SH, -COOH, and OH, found in the cell membrane [115]. The inactivation of 157

158 Chapter 5: Generation of Silver Titania Nanoparticles from Ag-Ti alloy... both the cell protein and its DNA lead to the death of the bacteria[168], especially the Ag nanosize particles, which react strongly with proteins [159]. The antibacterial activity of the Ag and Ag-TiO 2 nanoparticles against the E. coli bacteria can be explained on the basis of a physical phenomenon; the Ag nanoparticles release Ag + [286], or positively charged ions, and metal oxides such as TiO 2 carry a positive charge [287]. On the other hand, E. coli is a gramnegative bacteria and carries negative charges; as a result, attraction forces between the nanoparticles and the microorganism will be produced [287] which leads to contact between them. This is the first step of the antibacterial activity of the nanoparticles. The SEM and STEM images showed that the production of pits on the cell membrane of the E. coli after contact with Ag nanoparticles leads to the death of the bacteria [288]. This is different for gram-positive and gram-negative bacteria because they have different membrane structures, particularly with regard to the thickness of the peptidoglycan layer [285], in that gram-negative bacteria have a thinner peptidoglycan cell membrane than grampositive bacteria [289] Summary Ag-TiO 2 compound nanoparticles were fabricated using a picosecond laser on the surface of the cast Ti/Ag alloy target in deionised water. - The XRD image shows that the phase of the TiO 2 nanoparticles in the compound nanoparticles is rutile. - Spherical nanoparticles, ranging from a few nanometers to about 90 nm in size, were produced, in addition to a few larger nanoparticles. The average size of the nanoparticles was 31 nm. - The red shift phenomenon was observed in the absorption spectra of the colloidal Ag-TiO 2 compound nanoparticles, so the spectrum was shifted to a longer wavelength of about 500 nm. - The compound nanoparticles demonstrated strong antibacterial activity against E. coli bacteria, but it was found to be slightly lower than for the pure Ag nanoparticles. 158

159 Chapter 5: Generation of Silver Titania Nanoparticles from Ag-Ti alloy... - The efficacy of the antibacterial activity of the nanoparticles was found to be strongly concentration-dependent. - Optimal antibacterial activity of the Ag-TiO 2 compound nanoparticles can be obtained by controlling the correct amount of release of the Ag ions from the Ag-TiO 2 nanoparticles. 159

160 6 Chapter 6. Sequential laser and ultrasonic wave generation of core-shell nanoparticles and their anti-bacterial properties Authors: Abubaker Hamad, Lin Li, Zhu Liu, Xiang Li Zhong, and Tao Wang Journal: Lasers in Medical Science Volume, issue and pages: 31, 2, Status: Published Note: The format of the paper is edited 160

161 Chapter 6: Sequential laser and ultrasonic wave generation of TiO core-shell... Sequential laser and ultrasonic wave generation of TiO coreshell nanoparticles and their anti-bacterial properties Abstract Core-shell nanoparticles have unusual physical, chemical and biological properties. Until now, for the Ag and TiO 2 combination, only Ag-core and TiO 2 - shell nanoparticles have been practically demonstrated. In this investigation, novel TiO core-shell (TiO 2 -core and Ag-shell) nanoparticles were produced via ultrasonic vibration of Ag-TiO 2 compound nanoparticles. A bulk Ti/Ag alloy plate was used to generate colloidal Ag-TiO 2 compound nanoparticles via picosecond laser ablation in deionised water. The colloidal nanoparticles were then sonicated in an ultrasonic bath to generate TiO core-shell nanoparticles. They were characterised using a UV-VIS spectrometer, Transmission Electron Microscopy (TEM), High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (HAADF-STEM), Energy-dispersive X-ray Spectroscopy (EDS) and X-ray diffraction (XRD). The Ag-TiO 2 compound and the TiO core-shell nanoparticles were examined for their antibacterial activity against Escherichia coli (E. coli) JM109 strain bacteria and compared with those of Ag and TiO 2 nanoparticles. The antibacterial activity of the core-shell nanoparticles was slightly better than that of the compound nanoparticles at the same concentration under standard laboratory light conditions and both were better than the TiO 2 nanoparticles but not as good as the Ag nanoparticles. Keywords: TiO core-shell nanoparticles, Ag-TiO 2 compound nanoparticles, laser ablation, antibacterial activity. 161

162 Chapter 6: Sequential laser and ultrasonic wave generation of TiO core-shell Introduction Core-shell nanoparticles consist of a core, an inner material, and a shell, an outer material. Core-shell nanoparticles have been produced and used due to their unique optical, electronic and catalytic properties [290]. They have received much attention over recent decades due to their wide range of applications in biomedical drug delivery, giant magnetoresistance (GMR) sensing, catalysis and environmental remediation [291]. Particularly in the bioapplications, core-shell nanoparticles have been found better than the normal nanoparticles because they are less toxic to human cells and exhibit increases in dispersibility, bio-compatibility and cyto-compatibility, as well as high conjugation ability with other types of bioactive molecules and high chemical and thermal stability [292]. Ag@TiO 2 core-shell (Ag-core and TiO 2 -shell) nanoparticles have been produced to improve their photocatalytic and antibacterial activities. For example, Lin et al. [293] prepared Ag@TiO 2 core-shell nanoparticles via hydrolysis using Cetyl trimethylammonium bromide (CTAB) solution as a stabiliser. The size of the nanoparticles was about 15 nm. The inhibition zone test of the core-shell nanoparticles showed excellent antibacterial activity against E. coli bacteria. Dhanalekshmi et al. [199, 202] prepared Ag@TiO 2 core-shell nanoparticles by the reduction of AgNO 3 and hydrolysis of Titanium (IV) isopropoxide simultaneously. The TiO 2 shell had an anatase phase. The size of the core-shell nanoparticles was below 50 nm. It was shown that the nanoparticles have significant antibacterial activity against both E. coli and S. aureus. They also produced Ag@SiO 2 core-shell nanoparticles (under 50 nm) using Stober s method. The agar diffusion method was used to test the antibacterial activity; the results showed that the antibacterial activity of Ag@SiO 2 was higher than that of the Ag@TiO 2 nanoparticles (anatase TiO 2 ) at the same concentrations against E. coli and S. aureus bacteria. This is because the antibacterial activities of nanoparticles are highly dependent on the particles surface area, and the surface area of the Ag@SiO 2 nanoparticles was higher than that of the Ag@TiO 2 nanoparticles. 162

163 Chapter 6: Sequential laser and ultrasonic wave generation of TiO core-shell... Ag@TiO 2 core-shell nanoparticles have also been used to improve their photocatalytic activity. For instance, Wang et al. [294] produced Ag@(anatase)TiO 2 core-shell nanoparticles using a solvothermal process. It was concluded that the core-shell nanoparticles showed 50% higher photocatalytic activity than Degussa P25 to degenerate Alizarin Red (AR). Cheng et al. [177] prepared Ag@TiO 2 core-shell nanocomposite nanowires using a vapour-thermal method. They showed that their photocatalytic activity was higher than that of Ag, TiO 2 (anatase phase) and commercial P25 TiO 2 (anatase and rutile) to decolourise Rhodamine B (Rh B) under UV light exposure at room temperature. Yang et al. [12] produced hybrid Ag@TiO 2 coreshell nanoparticles that have much higher photocatalytic activity to degenerate organic dye molecules under UV light irradiation, in comparison with Ag-TiO 2 nanocomposites and commercial TiO 2 (P25). It was also shown that this method could produce tunable shell thickness, low temperature, high production rate and good reproducibility. Chen and Lee [295] prepared Ag@TiO 2 core-shell nanoparticles via a sol-gel method. The size of the Ag-core was 5-10 nm and the size of the TiO 2 -shell was nm. It was concluded that the photocatalytic activity of the core-shell nanoparticles was much better than that of pure TiO 2 nanoparticles to degrade Methylene Blue under UV light. It was reported that the core-shell structure had an advantage to prevent aggregation of Ag nanoparticles. It is worth mentioning that in core-shell nanoparticles, a double shell layer (two different materials) was produced to enhance photocatalytic activity, such as the Ag@SiO 2 triplex core-shell nanoparticles produced by sol-gel process. The thicknesses of the SiO 2 and TiO 2 layers were 2 nm and 20 nm respectively. This structure enhanced photocatalytic activity about 31 times in comparison with the commercial TiO 2 (P25) nanoparticles under visible light conditions. The improved photocatalytic activity is due to the scattering light caused by Ag-core nanoparticles. In addition, the plasmon resonance energy transfer (PRET) ( PRET is the transfer of energy from a plasmon to a nearby absorber such as a semiconductor ) that occurs is important to promote the intensity of the electrical field in a small position of the TiO 2, leading to an increase in power at this position [296]. Previous work on Ag@TiO 2 has focused almost entirely on Ag-core and TiO 2 -shell structures. In this work, the authors demonstrate the preparation of a 163

164 Chapter 6: Sequential laser and ultrasonic wave generation of TiO core-shell... novel nanoparticle of TiO core-shell (TiO 2 -core and Ag-shell) structure by ultrasonic post-treatment of Ag-TiO 2 compound nanoparticles generated by picosecond laser ablation of Ag-TiO 2 alloy bulk in deionised water. This was then compared with ultrasonic post-treatment of picosecond laser generated Ag and TiO 2 nanoparticles generated separately that failed to produce the coreshell structure. The specific physical and antibacterial properties of the coreshell nanoparticles were investigated Experimental set-up Materials A 3:1 at.% of Ti/Ag bulk alloy with the dimensions 25 mm 25 mm 2 mm and the purity of the Ti and Ag alloy components were 99.7% and 99.95% respectively, was used to generate Ag-TiO 2 compound nanoparticles using picosecond laser ablation in water. Then the produced colloidal Ag-TiO 2 compound nanoparticles were used to produce TiO core-shell nanoparticles via ultrasonic vibration. An Ag target plate with the dimensions of 25 mm 25 mm 2 mm and a purity of 99.99% was used to produce Ag nanoparticles for comparison. In addition, a Ti target plate with the dimensions 25 mm 25 mm 1 mm and a purity of % was used to produce TiO 2 nanoparticles for comparison Ag-TiO 2 compound and TiO core-shell nanoparticle production A Ti/Ag alloy plate was placed at the bottom of a 100 ml glass vessel, containing about ml of deionised water (see Figure 6-1-a). To produce Ag-TiO 2 compound nanoparticles, a picosecond laser ablation (A 400 W Edgewave picosecond laser, parameters: wavelength = 1064 nm, laser power P = 9.12 W, frequency f = 200 khz, pulse width = 10 ps, spot size D = 125 µm, scan speed v = 250 mm/s, laser pulse energy E pulse = 45.6 µj and laser fluence F laser = J/cm 2 ) was used; the laser ablation process was continued for 10 minutes. The water level above the bulk alloy target was about 2 mm. It is worth mentioning that the effects of reducing the laser beam intensity and increasing the focal length were taken into account. The produced Ag-TiO 2 compound 164

165 Chapter 6: Sequential laser and ultrasonic wave generation of TiO core-shell... nanoparticles were poured into a small beaker then put in an ultrasonic tank (Ultraschall-Reiniger, Emmi 5 with a pulse repetition rate of 49 khz) (see Figure 6-1-b) and then sonicated for 5 minutes (minute by minute). As a result, TiO core-shell nanoparticles were generated. Ag-TiO 2 compound nanoparticle (a) (b) Figure 6-1: Experimental set-up for (a) production of Ag-TiO 2 compound nanoparticles by picosecond laser ablation in deionised water and (b) to produce the TiO core-shell nanoparticles by an ultrasonic cleaner (Ultraschall-Reiniger, Emmi 5 with a pulse repetition rate of 49 khz). To investigate whether the core-shell form of nanoparticles could be obtained by ultrasonic vibration between Ag and TiO 2 nanoparticles produced separately from Ti and Ag bulk materials, Ag and TiO 2 nanoparticles were generated separately by picosecond laser in deionised water. These nanoparticles were then mixed (see Figure 6-2-a) and added (see Figure 6-2-b) at the same concentrations and sonicated for 5 minutes. Colloidal TiO 2 nanoparticles were added to colloidal Ag nanoparticles by syringe during the sonication process for 5 minutes. 165

166 Chapter 6: Sequential laser and ultrasonic wave generation of TiO core-shell... (a) (b) Figure 6-2: (a) Sonication of Ag-mixed TiO 2 nanoparticles and (b) TiO 2 added to Ag nanoparticles. Ag and TiO 2 nanoparticles were produced by picosecond laser in deionised water under the same laser and experimental conditions Material characterisation Both the Ag-TiO 2 compound and TiO core-shell colloidal nanoparticles were characterised using a UV-VIS spectrometer (Analytic Jena, SPECORD 250, dual beam) and the nanoparticles were analysed using XRD (BrukerD8-Discover, step size [ 2 ] = ), a Transmission Electron Microscope (TEM) (JEOL 2000 FX AEM + EDS), a High-Angle Annular Dark- Field - Scanning Transmission Electron Microscope (HAADF-STEM) and Energy-Dispersive X-ray Spectroscopy (EDS) (FEI Tecnai G2 F30). For the TEM and HAADF-STEM analyses a copper micro-grid mesh (Formvar / Carbon on 200 Copper mesh) was used for the sample preparation. After depositing a drop of colloidal nanoparticles onto the mesh, the substrate was allowed to dry at room temperature. This process was repeated 3 to 5 times to deposit enough nanoparticles on the mesh. The XRD sample was prepared on a glass slide by dropping the colloidal nanoparticles onto the mesh then allowing them to dry at room temperature. To obtain more nanoparticles, the colloidal nanoparticles were centrifuged for 15 minutes at 10,000 rpm. Then the deposition onto the glass slide was repeated several times in a 2 cm 2 cm area. To determine the concentration of the colloidal nanoparticles, a microbalance scale (Sartorius BL 210S) with a resolution of 0.1 mg was used by weighing the bulk target before and after the nanoparticle production process. Before weighing, the target was 166

167 Chapter 6: Sequential laser and ultrasonic wave generation of TiO core-shell... dried using a hair dryer to record the weight of the ablated materials with a high level of accuracy Antibacterial activity analysis E. coli bacteria (JM109 Promega UK) were used to test the antibacterial activity of both the Ag-TiO 2 compound nanoparticles and TiO core-shell nanoparticles under standard room light. In addition, the antibacterial activity of both pure TiO 2 and Ag nanoparticles was tested for comparison. The bacteria were cultured from a single colony in LB (Lysogeny Broth) overnight at 37 C with 225 rpm orbital shaking. The nanoparticles at a 20 µg/ml concentration were co-cultured with E. coli bacteria in LB for 6 hours at 37 C with 225 rpm orbital shaking. 10 µl of the cultured bacteria with nanoparticles were plated on to a 10 cm LB agar petri dish and then incubated at room temperature for about two days. The numbers of E. coli bacteria colonies on the plates were then counted. The optical density at 600 nm (OD 600 ) of the cultured E. coli was measured and diluted down to a colony forming unit (CFU)/ml of about to with LB Results Ag-TiO 2 compound and TiO core-shell nanoparticle production Figure 6-3-a and b show TEM images of the TiO core-shell nanoparticles produced by ultrasonic vibration after generation of Ag-TiO 2 compound nanoparticles by picosecond laser in deionised water (see Figures 3- c and d). As shown in the TEM images (Figure 6-3-a and b), the core-shell nanoparticles were produced whose core nanoparticles have different sizes and the shell was generated with an average thickness of about ± 1.28 nm. The large nanoparticles are darker than the shell. Figure 6-3-c and d show the Ag-TiO 2 compound nanoparticles before the ultrasonic treatment. The compound nanoparticles were fabricated using a picosecond laser in deionised water at =1064 nm, P = 9.12 W, f= 200 khz, v = 250 mm/s. The Ag-TiO 2 compound nanoparticles were synthesised with different sizes. 167

168 Chapter 6: Sequential laser and ultrasonic wave generation of TiO core-shell... (a) (b) (c) (d) Figure 6-3: (a and b) TEM images of TiO core-shell nanoparticles produced by ultrasonic vibration (Ultraschall-Reiniger, Emmi 5 with a pulse repetition rate of 49 khz) from Ag-TiO 2 compound nanoparticles generated by picosecond laser in deionised water. (c and d) TEM images of the Ag-TiO 2 compound nanoparticles fabricated by a picosecond laser in deionised water. A 400 W Edgewave picosecond laser was used to generate the nanoparticles with the laser beam parameters: wavelength = 1064 nm, laser power P = 9.12 W, frequency f = 200 khz, pulse duration = 10 ps, spot size D = 125 µm, scan speed v = 250 mm/s, laser pulse energy E pulse = 45.6 µj and laser fluence F laser = J/cm 2. Figure 6-4-a shows the UV-VIS spectrum of Ag-TiO 2 compound and TiO core-shell nanoparticles. They have similar absorption spectra along all measured wavelengths with a strong and wide surface plasmon resonance from about 400 nm to about 500 nm. Figure 6-4-b shows the histogram of the size distribution of the compound and core-shell nanoparticles. The size of the compound nanoparticles ranges from less than 10 nm to about 180 nm with an average size of 30 nm, and the size of the core-shell has a similar range, with an average size of about 27 nm. On average, the size of the core-shell nanoparticles was smaller than that of the nanoparticles that were produced in 168

169 Absorbance [A] Number of Nanoparticles Chapter 6: Sequential laser and ultrasonic wave generation of TiO core-shell... Ag-TiO 2 compound form. It can be seen from the histogram (Figure 6-4-b) that the more number of smaller and larger core-shell nanoparticles was observed in comparison with the compound nanoparticles (a) (b) Compound Core-shell Nanoparticles No. of NPs Ave. Size Compound nm Core-shell nm Core-shell Compound Wavelength (nm) Size of Nanoparticles (nm) Figure 6-4: (a) Absorption spectra of Ag-TiO 2 compound and TiO core-shell nanoparticles. The inset figure shows the match of the two spectra (b) Histogram of the size distribution of Ag-TiO 2 compound and TiO core-shell nanoparticles. Figure 6-5 shows TEM images of the Ag (a) and TiO 2 (b) nanoparticles produced separately by picosecond laser in deionised water and their optical absorption spectra (c). In general, the TiO 2 nanoparticles are larger and more spherical than the Ag nanoparticles. The Ag nanoparticles have strong absorption spectra at 400 nm, it is the surface plasmon resonance peak of Ag nanoparticles. In addition, a weak peak is observed at 250 nm. Conversely, the optical absorption spectra of the TiO 2 nanoparticles has a strong peak at UV light range from 200 to 300 nm, which then decreases sharply. 169

170 Absorbance [A] Chapter 6: Sequential laser and ultrasonic wave generation of TiO core-shell... (a) (b) 3 (c) TiO 2 Ag TiO Ag Wavelength (nm) Figure 6-5: TEM images of the Ag (a) and TiO 2 (b) nanoparticles and their optical absorption spectra (c) produced via picosecond laser ablation in deionised water. ( = 1064 nm, f = 200 khz, v = 250 mm/s). Figure 6-6 shows the HAADF images of the TiO core-shell nanoparticles. The images show that different core sizes and thin shells were produced. The shell thickness is around 10 nm. Figure 6-6: HAADF images of TiO core-shell nanoparticles. 170

171 Chapter 6: Sequential laser and ultrasonic wave generation of TiO core-shell... Figure 6-7 shows the EDS line scanning images of the TiO coreshell nanoparticles. Elemental analysis of the samples shows that the core consists of Ti and O elements; in other words, the core is composed of TiO 2 nanoparticles and the shell is Ag nanoparticles. In addition, the O was observed in the shell structure which might suggest the existence of some silver oxide in the shell. Figure 6-7: EDS line scanning images of the TiO core-shell nanoparticles. Figure 6-8 shows EDS spectra of two dot profiles or two points on the core and shell of the TiO core-shell nanoparticles. The core-point spectrum shows that the core consists of Ti and O; in addition, Cu, C and Si elements are apparent. The formation of Cu and C is due to the mesh substrate and the presence of Si is due to the glass-ware used to produce, hold and store the colloidal nanoparticles. The shell consists of a small amount of Ag. In addition, both O are seen in the spectrum. Ag can be seen on the core-scan profile because the shell covered the whole core. (b) (a) Figure 6-8: HAADF image (a) and EDS image (b) of the TiO core-shell nanoparticles. 171

172 Chapter 6: Sequential laser and ultrasonic wave generation of TiO core-shell XRD of Ag-TiO 2 compound and TiO core-shell nanoparticles Figure 6-9 shows the XRD of Ag-TiO 2 compound nanoparticles and TiO core-shell nanoparticles. Both types of nanoparticles have the same composition, except a silver peak at of the Ag-TiO 2 compound nanoparticles was not observed in the core-shell nanoparticles. The crystal phase of TiO 2 in compound and core-shell nanoparticles is rutile. The peaks at 2 = 37.59, 44.49, 44.45, 63.76, 63.87, and represent Ag elements and the peaks at 2 = 41.9, 41.87, 47.94, 48.02, and represent the existence of rutile TiO 2. In addition, the peaks at 93.2 and represent Ti. Figure 6-9: XRD images of the Ag-TiO 2 compound and TiO core-shell nanoparticles Sonication of mixed and added Ag and TiO 2 nanoparticles For comparison, both Ag and TiO 2 nanoparticles were separately synthesised via the picosecond laser ablation in deionised water under the same laser and experimental conditions. Figure 6-10 shows TEM images of Ag and -TiO 2 nanoparticles produced by sonication of Ag and TiO 2 nanoparticles by mixing (Figure 6-10-a and b) and adding (Figure 6-10-c and d) for 5 minutes. The results show that the core-shell nanoparticles were not generated in either condition. 172

173 Chapter 6: Sequential laser and ultrasonic wave generation of TiO core-shell... (a) (b) (c) (d) Figure 6-10: (a and b) TEM images of the Ag-TiO 2 nanoparticles produced by ultrasonic vibration after mixing the colloidal Ag and TiO 2 nanoparticles. (c and d) TEM images of the Ag- TiO 2 nanoparticles produced by ultrasonic vibration after the addition of the colloidal Ag to TiO 2 nanoparticles during ultrasonic vibration for 6 minutes Antibacterial activity Figure 6-11-a shows the antibacterial activity of the TiO core-shell and Ag-TiO 2 compound nanoparticles against E. coli bacteria (Bacterial strain JM109 Promega UK) under standard laboratory light conditions for about 36 hours. Figure 6-11-b and c show the agar plates of the core-shell and compound nanoparticles compared with the control. Both the compound and core-shell nanoparticles were tested at 20 µg/ml concentration after 6 hours incubation at 37 C and 225 rpm shaking. In general, they have similar antibacterial activity against E. coli gram negative bacteria, but it can be noted that the antibacterial activity of the core-shell nanoparticles is a little higher than that of the compound nanoparticles. In other words, fewer E. coli bacteria colonies can be seen on the agar plates with core-shell nanoparticles (see Figure 6-11-b). 173

174 Chapter 6: Sequential laser and ultrasonic wave generation of TiO core-shell (a) (b) (c) E. coli viability C/C o Compound Core-shell Core-shell Compound Control Ag-TiO 2 Nanoparticles Figure 6-11: (a) Antibacterial activity of TiO core-shell and Ag-TiO 2 compound nanoparticles against E. coli bacteria at 20 µg/ml concentration and 6 hours incubation, under standard laboratory light conditions. (b) Agar plates with E. coli bacteria colonies treated with core-shell and compound nanoparticles. (c) Agar plate with E. coli bacteria colonies on control sample (dh 2 O). Figure 6-12 shows the antibacterial activity of the laser generated TiO 2 and Ag nanoparticles via picosecond laser in deionised water. The antibacterial activities of the core-shell and compound nanoparticles are better than that of the TiO 2 nanoparticles and not as good as Ag nanoparticles E. coli viability C/C o Ag Nanoparticles TiO2 Figure 6-12: Antibacterial activity of the laser-generated TiO 2 and Ag nanoparticles under standard room light. They were produced via picosecond laser ablation in deionised water. 174

175 Chapter 6: Sequential laser and ultrasonic wave generation of TiO core-shell Discussion Core-shell and compound nanoparticles The TEM images show that the Ag-TiO 2 compound nanoparticles were produced as two groups of nanoparticles: large particles and smaller particles. Some of the larger particles are embedded in the smaller nanoparticles. Ultrasonic waves can dismantle large-sized particles to produce small-sized particles [244]. When ultrasonic waves disperse in a liquid environment an enormous number of voids or cavities are produced due to ultrasonic wave vibration in the water medium, followed by producing a high temperature ( K) and high pressure of up to Ibs/inch 2 (1380 bar) [ ]. The high pressure can break the particles and disintegrate the agglomerated particles in the solution [297]. It is worth mentioning that almost all of the broken particles were Ag nanoparticles, not TiO 2 nanoparticles, because the heat of vaporization of Ag is lower than that of TiO 2 nanoparticles, 254 kj/mol and 425 kj/mol respectively. In addition, the melting point of TiO 2 is higher than that of Ag by a factor of 1.7. The high temperatures cause the more active Brownian motions, and mobility of the surface atoms leads to an increase in collisions between the nanoparticles followed by adhesion and coalescence between TiO 2 and a cloud of Ag nanoparticles [191]. The compound and core-shell nanoparticles have the similar absorption spectra but they have a slightly different size distribution. As shown in the inset of Figure 6-4-a, the two spectra match. At lower wavelengths, the spectrum of the compound nanoparticles is slightly shifted towards longer wavelengths than that of the core-shell nanoparticles. On the contrary, at longer wavelengths, the spectrum of the compound nanoparticles shifts to shorter wavelengths. This is because the number of smaller compound nanoparticles (between 10 nm and 40 nm) is more than that of the core-shell nanoparticles but the number of coreshell nanoparticles of a larger size (in the range of nm) is more than that of the compound nanoparticles. In addition, the peak of the compound spectra (from 400 nm to 500 nm) is a little higher than the core-shell absorption spectra peak due to the higher number of smaller compound nanoparticles (see Figure 6-4-b). The average size of the core-shell nanoparticles is smaller than that of the compound nanoparticles, and a large number of small and large core-shell nanoparticles were generated in comparison with compound 175

176 Chapter 6: Sequential laser and ultrasonic wave generation of TiO core-shell... nanoparticles. The smaller size of the core-shell nanoparticles is due to the ultrasonic vibration because ultrasonic waves are able to disperse agglomerated nanoparticles in deionised water [245]. On the other hand, the larger size of the core-shell nanoparticles is due to the combination of the smaller nanoparticles (almost all Ag nanoparticles) with the larger nanoparticles, (should be TiO 2 nanoparticles), to form core-shell nanoparticles. The size distribution of core-shell nanoparticles can be classified into two parts: small nanoparticles, most of which are Ag nanoparticles that have crashed due to ultrasonic vibrations, and the largest nanoparticles are TiO 2 nanoparticles combined with Ag-shell nanoparticles. It appears that the TiO 2 nanoparticles were not affected by ultrasonic waves. In general, the width of the optical absorption spectra of a colloidal nanoparticle represents the size distribution of the nanoparticles in a solution, so narrow absorption spectra mean small size distribution of the nanoparticles. Here, in spite of the small size distribution of the core-shell nanoparticles in comparison with the compound nanoparticles, both have the same optical absorption spectra. This might be due to a slight difference between the size distribution of the core-shell and compound nanoparticles, or it might be due to the fact that both types of nanoparticle generally have the same size distribution. The red shift phenomena or shifting the optical absorption spectra to a longer wavelength from 400 to 500 nm, of both types of nanoparticles would be due to the specific interface structure between TiO 2 and Ag particles and the higher refractive index of the TiO 2 compared to the Ag and deionised water [298]. The strong interface structure is produced because the nanoparticles were originally produced from a Ti/Ag alloy material. The shell is a thin layer of approximately 10 nm, which is important in the core-shell form to allow light to reach the TiO 2 -core to produce photocatalytic activity. In the case of the Ag@TiO 2 core-shell nanoparticles produced previously, the shell should be thin or porous to release Ag ions form the Agcore to outside the core-shell nanoparticles [293], to produce antimicrobial activity. It can be concluded that a thin or porous shell is an important condition to produce antimicrobial activity of the core-shell nanoparticles against microorganisms and superbugs. In this work, a thin shell was obtained due to the small atomic ratio of Ag materials (3:1 at.% Ti/Ag bulk alloy) in comparison 176

177 Chapter 6: Sequential laser and ultrasonic wave generation of TiO core-shell... with the atomic ratio of the TiO 2 materials in the bulk material or target. Another reason behind the production of a thin shell is that Ag material is more vulnerable to ultrasonic waves than TiO 2 material due to lower heat of vaporization and melting point of the Ag than the TiO 2 (see Figure 6-6). In addition, the shell thickness has an effect on the optical resonance, so the optical absorption of thinner shell nanoparticles shifted to longer wavelengths, such as SiO 2 /Au core-shell nanoparticles [299]. This phenomenon did not occur in the present research, which may be explained in two ways. Firstly, because the optical absorption spectra of the colloidal Ag-TiO 2 nanoparticles are shifted to longer wavelengths from 400 nm to 500 nm, while the surface plasmon peak of the spherical Ag nanoparticles produced by laser ablation in a liquid environment is located at about 400 nm [63]. Secondly, plasmonic Ag was formed as the shell structure, but in almost all other core-shell Ag-TiO 2 forms plasmonic Ag nanoparticles were the core component of the nanoparticles. The XRD image shows that both the compound and core-shell nanoparticles have the same phase properties, so the ultrasonic vibration has no effect on the structure of the Ag-TiO 2 nanoparticles. Both types of nanoparticles have rutile phase structure but some of the core-shell peaks shifted to a little longer position (2 ) in comparison with the peaks of the compound nanoparticles. This might be due to the different thicknesses while preparing the samples for XRD analysis. In addition, the peaks of the Ag-TiO 2 compound nanoparticles are higher or more intense than the peaks of the TiO core-shell nanoparticles. This is because the thickness of the compound nanoparticles on the glass slides prepared for XRD characterisation was a little thicker than that of the core-shell nanoparticles. According to the optical absorption spectra, the core-shell form does not affect the energy gap of the TiO 2 nanoparticles because the compound and core-shell nanoparticles have the same optical properties along all wavelengths. Recently, the effects of laser beam irradiation (266 nm) of the chlorpromazine aqueous solutions to generate photochemical breakdown and photoproducts formation was reported. It was concluded that the photoproducts generation rate is affected by the generation of fluorescence emission and singlet oxygen by chlorpromazine. Producing two new bands in the optical absorption spectra indicates the generation of a new compound [ ]. In 177

178 Chapter 6: Sequential laser and ultrasonic wave generation of TiO core-shell... our case, the absorption spectra have the same feature but they are slightly different due to the different size of the compound and core-shell nanoparticles. This effect occurred due to the ultrasonic waves used to produce the core-shell nanoparticles. The effects of the laser irradiation on the produced nanoparticles during the ablation process are the same for both types of nanoparticles because the core-shell nanoparticles were produced from the compound nanoparticles (two step production). The effect of post-irradiation on the structure of the new form of nanoparticles is shown in the XRD spectra with a missing peak at for the core-shell nanoparticles. This is a silver peak which might be missed due to the low thickness and porous Ag-shell in the TiO core-shell in comparison with the solid and larger size in diameter of the Ag in the Ag-TiO 2 compound nanoparticles Antibacterial activity Both the Ag-TiO 2 compound and TiO core-shell nanoparticles at 20 µg/ml concentrations have strong antibacterial activity against E. coli bacteria under the standard laboratory light conditions. The antimicrobial activity of the core-shell nanoparticles is a little higher than that of the compound nanoparticles under the same test conditions. A reason for the higher antibacterial activity of the core-shell nanoparticles produced by ultrasonic vibration than the compound nanoparticles is the deagglomeration of the nanoparticles in deionised water by ultrasonic vibration. Ultrasonic vibration is known to be able to separate or disperse agglomerated nanoparticles in liquid solution, as a result of which smaller nanoparticles will be produced. It has been reported that smaller nanoparticles are more efficient for antibacterial activity than larger ones [215]. This is due to more surface area to volume ratio of the smaller nanoparticles, as a result of which more nanoparticles will make contact with the bacteria and kill them. In spite of the fact that the compound and the core-shell nanoparticles have the same optical absorption spectra; they respond differently to interaction with light. So, the slightly higher antibacterial activity of the core-shell nanoparticles to that of the compound nanoparticles is due to their structure. In the case of the compound nanoparticles, when the electrons transfer from the valance band to 178

179 Chapter 6: Sequential laser and ultrasonic wave generation of TiO core-shell... the conduction band in TiO 2 nanoparticles, the chance of the electrons to combine or interact with the silver nanoparticles is less in comparison with the core-shell nanoparticles, because in core-shell nanoparticles, silver forms a shell on the surface of the TiO 2, but in compound nanoparticles silver nanoparticles cover a smaller surface area of the TiO 2 nanoparticles. The antimicrobial activity of the Ag-TiO 2 nanoparticles strongly depends on the contact surface area and on the electronic performance of the noble metal nanoparticles such as Ag [302]. The close interconnection between the core and shell materials support the electron transfer and separation of photogenerated electrons from the TiO 2 to the Ag [177]. In addition, in the core-shell structure, silver will interact with the microorganisms more than in the compound form. It is worth mentioning that the hydroxyl groups (oxygen vacancies (O )) on the surface of TiO 2 nanoparticles do not have enough antibacterial power to kill microorganisms alone [302], while Hydroxyl radical (OH ) and superoxide ions (O - 2 ) were responsible for inactivating the biocells [252]. There are two mechanisms to kill microorganisms by Ag-TiO 2 nanoparticles: direct and indirect effects. The direct effect is attaching the nanoparticles to the membrane of microorganisms, which may cause functional damage or structural changes to the cell membrane [190]. The indirect effect involves releasing silver ions and the formation of reactive oxygen species (ROS). For example, hydrogen peroxide, hydroxyl radicals and superoxide anion under normal light irradiation. As a result, reactive oxygen species groups will interact with the cell wall membrane, leading to inert DNA and cellular protein functions, causing cell death or differentiation [168]. Another important factor to improve antibacterial activity of the Ag-TiO 2 nanoparticles is the structure of the TiO 2 nanoparticles; the phased structure of TiO 2 such as anatase is known to be better than its amorphous structure. For example, amorphous TiO 2 in Ag-TiO 2 nanoparticles has been found to kill or decrease 2-log of bacteria in 2 hours, but anatase TiO 2 killed all bacteria in 90 minutes under visible light conditions [191]. In addition, the photocatalytic activity was observed to be greater for anatase Ag-TiO 2 than for rutile Ag-TiO 2 [161]. In spite of the well-known fact that the anatase phase is more effective than the rutile phase for antibacterial activity, it has been reported recently that rutile has higher antimicrobial activity than anatase against E. coli bacteria 179

180 Chapter 6: Sequential laser and ultrasonic wave generation of TiO core-shell... [303]. In the present case, the TiO 2 nanoparticles have a rutile structure. Moreover, the photocatalytic activity of TiO 2 nanoparticles depends upon their crystallinity, crystallite size and crystal phases. For example, TiO 2 anatase phase structures with high crystallinity and smaller crystalline size have higher photocatalytic activity [191]. For the Ag@TiO 2 core-shell form nanoparticles, enhanced antibacterial activity is due to preventing the aggregation of nanoparticles by depositing of TiO 2 on the surface of Ag nanoparticles [202]. This indicates that, core-shell nanoparticles are a better form of nanoparticles than the simple nanoparticles for various bioapplications [292] Summary Novel structured TiO core-shell nanoparticles were produced using a novel preparation method based on ultrasonication of Ag-TiO 2 compound nanoparticles generated by a picosecond laser in deionised water. The ultrasonic waves acted as a catalyst to produce core-shell nanoparticles. The structure of the core-shell nanoparticles consisted of a TiO 2 -core with different sizes and an Ag-shell with a thickness of about 10 nm. A Ti/Ag alloy can produce core-shell form nanoparticles but pure Ag and TiO 2 cannot generate core-shell form nanoparticles with ultrasonic vibration. The rutile structure of the Ag-TiO 2 materials did not change after ultrasonic vibration. The antibacterial activity of the TiO core-shell nanoparticles was found to be slightly higher than that of the Ag-TiO 2 compound nanoparticles. The antibacterial activity of both the core-shell and compound nanoparticles yielded better performance than the pure TiO 2 nanoparticles, but not as good as the pure Ag nanoparticles. Other properties of this novel nanomaterial are to be further explored. 180

181 7 Chapter 7. Comparison of characteristics of Ag-TiO 2 nanoparticles produced from an Ag-Ti alloy using nano-, picoand femtosecond lasers and their antibacterial activities Authors: Abubaker Hamad, Lin Li, Zhu Liu, Hong Liu, and Tao Wang Status: Submitted 181

182 Chapter 7: Comparison of characteristics of Ag-TiO 2 nanoparticles... Comparison of characteristics of Ag-TiO 2 nanoparticles produced from an Ag-Ti alloy using nano-, pico- and femtosecond lasers and their antibacterial activities Abstract The characteristics of hybrid Ag-TiO 2 nanoparticles produced from an Ag-Ti alloy by nanosecond, picosecond and femtosecond laser ablation in deionised water were compared. The production of Ag-TiO 2 nanoparticles from an Ag-Ti alloy using ns and fs lasers are reported for the first time. The optical absorption spectra of the nanoparticles were measured using UV-VIS spectroscopy, and their size distribution was characterised using Transmission Electron Microscopy (TEM). The crystalline material structures were investigated using X-Ray Diffraction (XRD). Fourier transform infrared spectroscopy (FTIR) was used to obtain infrared spectra of the samples. Raman spectroscopy was used to provide the Raman shift characteristics of the nanoparticles. X-ray Photoelectron Spectroscopy (XPS) was used to provide quantitative chemical state information. The surface charges of the nanoparticles were determined using Z-potential measurements. The zeta potentials were 24.1 mv, mv and mv for ns, ps and fs laser produced nanoparticles respectively. The average sizes of the compound nanoparticles produced by ns, ps and fs laser were 22 nm, 30 nm and 20 nm respectively. The femtosecond laser produced smaller and more uniform Ag nanoparticles attached to the surface of larger TiO 2 nanoparticles. The XRD analyses showed that energy difference between the peaks of metallic silver and silver ions were 2 ev, 1 ev and 0.6 ev for the samples produced by the ns, ps and fs lasers respectively. The antibacterial activity of the nanoparticles was tested against E. coli bacteria under normal room fluorescent light (low pressure mercury-vapor gas-discharge lamps that use fluorescence to produce visible light without UV wavelengths). The results show that the nanoparticles produced by the picosecond and femtosecond lasers have higher antibacterial activity than those produced by the nanosecond laser. The reason behind this is discussed. Key words: Ag-TiO 2 compound nanoparticles, ns laser, ps laser and fs laser, antibacterial activity, E. coli. 182

183 Chapter 7: Comparison of characteristics of Ag-TiO 2 nanoparticles Introduction The applications of nanoparticles in science and technology have been increasing rapidly over the last decade. Several techniques including top-down and bottom-up processes have been used to produce nanoparticles. Laser ablation is an important method for the generation of nanoparticles. Different types of laser have been used, not only to synthesise nanoparticles, but also to modify their morphology, size, optical and electrical properties. Various laser types and beam parameters have been shown to have different effects on the nanoparticles produced; researchers have used nanosecond, picosecond and femtosecond lasers to generate nanoparticles. Several papers have been published on the preparation of Ag-TiO 2 nanoparticles. For example, Liu et al. [124] synthesised Ag-coated TiO 2 particles via a nanosecond laser ablation technique (Nd:YAG laser with =1064 nm, =7 ns, f=10 Hz, D=1 mm and E pulse = 100 mj) in deionised water. TiO 2 nanoparticles with sizes ranging from 20 to 30 nm were coated with Ag nanoparticles. It was concluded that the Ag/TiO 2 nanoparticles exhibited enhanced photocatalytic activity and the plasmon peaks of the Ag coated TiO 2 were extended into the visible light range. Zhang et al. [15] prepared Ag cluster-doped TiO 2 nanoparticles by nanosecond laser ablation in liquid media and hydrothermal treatment without using any organic additives or chemical reduction agent. Firstly, colloidal TiO x nanoparticles were produced from a bulk target by laser ablation in liquid (LAL, Nd:YAG pulsed laser; = 1064 nm, f = 10 Hz, = 8 ns, D = 1.5 mm and E pulse = 120 mj). These were mixed with different concentrations of AgNO 3 solution, and then the solution was kept in an electric oven at 180 C for 24 hours. It was claimed that the combination of Ag and TiO 2 using this method was stronger than that produced by other methods, as during the crystallisation process of TiO x silver ions were reduced and then deposited on the surface of the TiO 2 nanoparticles. Several chemical methods have been used to produce Ag-TiO 2 nanoparticles. Torkian et al. [304] prepared an Ag-doped TiO 2 nanocomposite (1% (w/w)) by a sol-gel method with the aim to improve the photocatalytic activity, which was investigated against the azo dye orange gelb (OG). The red shift phenomenon was observed in the band gap transition after the doping 183

184 Chapter 7: Comparison of characteristics of Ag-TiO 2 nanoparticles... process had been completed. It was concluded that for the photocatalytic decomposition of orange gelb (OG), the Ag-TiO 2 nanocomposite based on NaX zeolite was very efficient. Seery et al. [305] enhanced optical spectrum Ag doped TiO 2 nanomaterials to within the visible range by two different methods; the first involved incorporating Ag by irradiating the reaction mixture during preparation for reducing Ag ions to Ag metal, and the second method used direct calcination process of the sol-gel material to decompose AgNO 3 to Ag. The second method was found to be more effective for enhancing the photocatalytic activity of the Ag-TiO 2. Behpour et al. [306] produced Ag doped TiO 2 nanoparticles by a chemical method which demonstrated strong ability to degrade Methyl Orange (MO) in comparison with pure TiO 2 nanoparticles. It was shown that the degradation rate of MO depends on the amount of OH - radicals generated on the surface of the nanoparticles. Recently, Hamad et al. produced Ag TiO 2 nanoparticles with reduced energy gap by picosecond laser ablation in ice water [232], TiO core-shell nanoparticles in deionised water by sequential laser and ultrasonic wave generation [307] and novel bimodal Ag- TiO 2 nanoparticles generation in deionised water by a hybrid laser-ultrasonic technique [308]. Although there are many publications related to laser production of Ag- TiO 2 nanoparticles using different types of lasers. They were reported separately with different production methods and characterisation of these nanoparticles were not in a consistent manner, thus difficult to compare them. Furthermore, the previous publications in the production of Ag-TiO 2 nanoparticles from an Ag-Ti alloy were only from the authors of this paper and only based on ps laser ablation. Femtosecond and nanosecond laser production of Ag-TiO 2 nanoparticles from an Ag-Ti alloy have not been reported before. In this work, Ag-TiO 2 compound nanoparticles were generated from an Ag-Ti bulk alloy by nanosecond, picosecond and femtosecond laser ablation in deionised water. They were then compared in terms of size, size distribution, morphology, chemical structures, crystalline structures, (optical absorption spectrum (both visible and infrared), surface charges, Raman spectrum shifts and antibacterial activity. The main novelty and key scientific contribution of this work is a systematic comparison of the nanoparticle characteristics under similar production and characterisation conditions. The XPS, XRD, Raman 184

185 Chapter 7: Comparison of characteristics of Ag-TiO 2 nanoparticles... spectra, FTIR, zeta potential characterization were compared for the first time and antibacterial activity characterisation were compared under the same conditions Experimental materials and procedures Materials A Ti/Ag alloy target plate with the dimensions of 25 mm 25 mm 1 mm and purity of the Ag and Ti materials of 99.95% and 99.7%, respectively, was used to produce Ag-TiO 2 compound nanoparticles. Before laser ablation, the target was washed in both deionised water and ethanol, then sonicated for minutes in deionised water and ethanol Ag-TiO 2 nanoparticles production methods Nanoparticles were produced using nanosecond (Laserline-Laserval Violino), picosecond (Edgewave 400 W) and femtosecond (Coherent Spitfire Ti Sapphire) laser ablation technique in deionised water. As shown in Figure 7-1, a Ti/Ag alloy plate target was put on a stainless steel sample holder in a glass vessel containing about 20 ml of deionised water. The water level above the target material was about 2 mm. The laser ablation experimental set-up is shown in Figure 7-1. In turn, the nanosecond, picosecond and femtosecond lasers were all subjected to the target material for about 10 minutes. The laser beam parameters used to produce the Ag-TiO 2 compound nanoparticles are shown in Table 7-1. Figure 7-1: Experimental set-up 185

186 Chapter 7: Comparison of characteristics of Ag-TiO 2 nanoparticles... Table 7-1: Types of laser beam and parameters used to produce Ag-TiO 2 compound nanoparticles. Laser parameters Nanosecond laser (Laserline-Laserval Violino) Picosecond laser (400 W Edgewave) Femtosecond laser (Ti Sapphire) Average power (P) 7.22 W 9.12 W 0.8 Wavelength ( ) 532 nm 1064 nm 800 nm Frequency (f) 30 khz 200 khz 1 khz Laser pulse duration ( ) 5 ns 10 ps < 100 fs Spot size (D) 50 µm 125 µm 60 µm Scan speed (v) 250 mm/s 250 mm/s 250 mm/s Laser pulse energy (E pulse ) 241 µj 45.6 µj 800 µj Laser fluence (F laser ) 12 J/cm J/cm 2 28 J/cm Sample preparation for nanoparticle characterisation For TEM analysis, a drop of the colloidal Ag-TiO 2 compound nanoparticles was deposited onto a copper microgrid mesh (A Formvar / Carbon on 200 Copper mesh). This process was repeated three to six times to collect a sufficient amount of the nanoparticles on the mesh. The sample was allowed to dry under a normal room light (fluorescent light tubes without UV wavelengths) and at room temperature. For FTIR, Raman Spectroscopy and XRD analysis, some of the colloidal nanoparticles were deposited on a glass slide and allowed to dry at room temperature. The process was repeated several times to deposit enough nanoparticles on the glass slide for characterisation. The concentration of the colloidal nanoparticle samples for the antibacterial activity test was obtained by measuring the weight of the alloy target before and after the nanoparticle production process using a microbalance scale. In this case, a Sartorius BL 210S scale with a readability of d=0.1 mg was used to measure the weight of the samples before and after the ablation process, and then the concentration of the colloidal nanoparticles was calculated after measuring the amount of deionised water. The amount of colloidal nanoparticles was measured after laser ablation to avoid water loss due to vaporisation during the laser ablation Characterisation Ag-TiO 2 nanoparticles were characterised using a UV-Vis spectrometer (Analytic Jena, SPECORD 250, dual beam). This equipment was used to 186

187 Chapter 7: Comparison of characteristics of Ag-TiO 2 nanoparticles... measure the absorption spectra of the colloidal Ag-TiO 2 nanoparticles in the form of aqueous suspension. A Transmission Electron Microscope (TEM) (JEOL 2000 FX AEM + EDX) was used to examine the morphology of the Ag- TiO 2 nanoparticles and measure their sizes and size distribution. X-Ray Diffraction (XRD) (Bruker D8-Discover with a step size of [ 2θ] = ) was used to investigate the crystalline structures of the samples. Fourier Transform Infrared Spectroscopy (FTIR) (Shimadzu ) was used to obtain infrared spectra of the transmission of the samples. Raman spectroscopy (Renishaw Raman system with a Argon-ion 514 nm) was used to provide the Raman spectrum characteristics of the samples and X-ray Photoelectron Spectroscopy (XPS) (Kratos Axis Ultra) which is also known as Electron Spectroscopy for Chemical Analysis (ESCA) was used to provide quantitative chemical state information from the surface of the samples. A Zetasizer-Nano Series (Nano-ZS Malvern Instruments) was used to measure the zeta-potentials of the nanoparticles at 25 C Antibacterial test procedure The antibacterial activity of the Ag-TiO 2 nanoparticles was tested against E. coli bacteria (JM109 from Promega UK). A single colony of E. coli was incubated in 10 ml of LB broth in a 50 ml sterilised tube and cultured at 37 C overnight with constant orbital shaking at 225 rpm. The optical density of the cultured E. coli was measured at 600 nm (OD 600 ) and diluted down to a colony forming unit (CFU)/ml in the range of to with LB. 0.2 ml of colloidal nanoparticles was mixed with 1.80 ml of the diluted E. coli and incubated for six hours at 37 C while shaking at 225 rpm. Finally, 10 µl of each dilution was spread on LB agar plates and left at room temperature for about 48 hours under normal room light (low pressure mercury-vapor gas-discharge lamps that use fluorescence to produce visible light without UV wavelengths); the colonies on each plate were then counted. For the negative control experiment, 0.2 ml of the nanoparticles was replaced with 0.2 ml of deionised water - dh 2 O. 187

188 Number of Nanoparticles Chapter 7: Comparison of characteristics of Ag-TiO 2 nanoparticles Results and discussion Ag-TiO 2 nanoparticle generation by nano-, pico and femtosecond laser ablation Figure 7-2 shows the TEM images of the nanoparticles produced using all three types of lasers. Size distribution histograms of the nanoparticles (Figure 7-2-d) show that the nanoparticles produced with the ns pulsed laser were in the range of a few nanometers to 70 nm, with an average size of about 22 nm. The range of nanoparticles produced by, picosecond and femtosecond lasers, varied from a few nanometers up to about 120 nm with an average size of 30 nm and 20 nm, respectively; few nanoparticles were observed over 200 nm. As shown in Figure 7-2-e, the colours of the Ag-TiO 2 nanoparticles are different and their zeta-potentials are different. Nanoparticles produced with the nanosecond laser had a z-potential of mv compared with those produced with the picosecond (-32.2 mv) and femtosecond (-31.5 mv) lasers. (a) NSL (b) PSL (c) FSL 300 (d) Ag-TiO2 (NSL) Ag-TiO2 (PSL) Ag-TiO2 (FSL) 200 Ag-TiO2 NPs No. of NPs Ave. Size (nm) NSL PSL FSL Size of Ag-TiO 2 NPs (nm) Figure 7-2: TEM images of nanoparticles generated by nanosecond (a), picosecond (b) and femtosecond (c) lasers in deionised water. Histogram of size distribution of the Ag-TiO 2 compound nanoparticles (d). Photograph of bottles of the Ag-TiO 2 nanoparticles (e). 188

189 Absorbance [A] Chapter 7: Comparison of characteristics of Ag-TiO 2 nanoparticles... As shown in Figure 7-3-a, the absorption spectra of the colloidal nanoparticles show a strong absorption peak in the UV range and a peak in the visible range from 400 nm to 500 nm. The absorption spectra of the nanoparticles produced by the nanosecond laser are weak in comparison with the other two spectra. The highest peak can be seen at 403 nm, 415 nm and 413 nm for the nanosecond, picosecond and femtosecond lasers respectively. The spectra of the nanoparticles produced with shorter pulse lengths show two different strong peaks, one at 410 and the other at 490 nm. The energy gaps of the nanoparticles were different; those produced by the picosecond laser have a smaller energy gap (2.3 ev) than the others, but the nanoparticles produced by nanosecond laser have higher energy gaps (3.10 ev) (see Figure 7-3-b) (a) Ag-TiO2 (NSL) Ag-TiO2 (PSL) Ag-TiO2 (FSL) 1.00 Ag-TiO2 (NSL) Ag-TiO2 (PSL) Ag-TiO2 (FSL) (b) ev 1/2 nm -1/ Lasers Energy gap NS 3.10 ev PS 2.30 ev FS 2.45 ev Wavelength (nm) Photon energy (ev) Figure 7-3: Absorption spectra of the Ag-TiO 2 compound nanoparticles (a). Energy band gaps of the Ag-TiO 2 nanoparticles (b). The absorption spectrum produced by the nanosecond laser is quite different from those produced using shorter pulsed lasers; surface plasmon resonance was not significantly produced with nanosecond pulse generated nanoparticles. The shape and intensity of the surface plasmon resonance (SPR) depend on the laser beam conditions [64]. For Ag-TiO 2, this also depends on the ratio of ablated Ag-TiO 2 particles. The absorption spectra of the nanoparticles generated using the pico- and femtosecond lasers are similar. For nanoparticles with a radius in the range of 2-10 nm, extinction is independent of the size, but for nanoparticles with a radius greater than 10 nm the absorption peak (SPR) would be expected to produce a red shift. In addition, the width of the SPR strongly depends on the shape and size of the nanoparticles because 189

190 Chapter 7: Comparison of characteristics of Ag-TiO 2 nanoparticles... the particle boundary is limited and confines the conduction electrons and their mean free paths [64]. The SPR peaks can be seen at 403 nm, 415 nm and 413 nm for the nano-, pico- and femtosecond lasers produced nanoparticles respectively. This absorption shift may be caused by the Ag nanoparticles attached to the TiO 2 nanoparticles [309]. It has been shown that the plasmon peak is approximately directly or linearly proportional to the solvent refractive index for colloidal gold particles [32]. In other words, a shift to a longer wavelength might be an evidence of the interaction of the metal (Ag) and the semiconductor (TiO 2 ) [309]. The effects of the different types of laser on the process of producing nanoparticles are shown elsewhere [310]. The TEM image (Figure 7-2-a) shows that Ag-TiO 2 nanoparticles were produced, but the XRD (see section 3.2) and XPS (see section 3.5) analyses showed that for the nanosecond laser production method, mainly silver ions were obtained. However, the presented energy band gap analysis confirmed also the production of silver nanoparticles. Therefore, it would be a mixture of Ag ions and Ag nanoparticles. The zeta-potential of the Ag-TiO 2 compound nanoparticles varied after they were re-measured after a period of time. They changed from mv, mv and mv to mv, mv and mv after one month for the nanosecond, picosecond and femtosecond laser generated nanoparticles respectively. This might be due to agglomeration of the nanoparticles that was observed after this period of time, or changing the PH values of the colloidal nanoparticles because for a stable PH value there should not be much change in the zeta-potential value [311]. The PH values of the fresh colloidal nanoparticles were about 7.35, but after a month they were about 9. In addition, Mulvihill et al. [312] showed that the zeta-potential of nano-rods were higher than that of spherical nanoparticles produced under the same conditions. They also showed that the most important factor in predicting the stability of nanoparticles was their surface area for kinetically stable systems. It was shown that stable nanoparticles in an aqueous solution were obtained when the zetapotential absolute value >40 mv, and unstable nanoparticles were obtained when zeta-potential absolute value < 30 mv [313]. 190

191 Chapter 7: Comparison of characteristics of Ag-TiO 2 nanoparticles XRD of Ag-TiO 2 compound nanoparticles Figure 7-4 shows the XRD of the nanoparticles produced by the nanosecond, picosecond and femtosecond lasers in deionised water. In all three samples the position of peaks at 2 = 38.10, 44.41, and represent the Ag elements and the positions at 2 = 35.77, and represent the rutile TiO 2. In addition, the peak positions at and represent silver oxide AgO [313]. A Rutile TiO 2 peak was observed at 40 deg [ ]. In the picosecond laser spectra a titanium oxide peak was observed at 48 deg. which might be an anatase-tio 2 [231, ] which was not observed in the spectra of nanoparticles produced by the ns and fs lasers. The peaks of the nanoparticles produced via the nanosecond laser were higher and thinner than those generated via picosecond and femtosecond lasers. It is worth mentioning that some peaks were observed in the picosecond laser spectra at 2 = and 63.76, but these were not observed in the femtosecond and nanosecond laser spectra. The silver oxide peaks produced by the femtosecond laser were weak in comparison with those produced with ns and ps lasers, which confirms that more metallic silver nanoparticles were produced by the femtosecond laser. Figure 7-4: XRD of Ag-TiO 2 compound nanoparticles produced by nanosecond, picosecond and femtosecond laser in deionised water. 191

192 Transmittance (%) Chapter 7: Comparison of characteristics of Ag-TiO 2 nanoparticles... The rutile phase formation of the TiO 2 nanoparticles by laser ablation in liquid would be due to the high temperature produced by the laser beam, which causes the phase to transform from anatase (produced at low temperature and metastable) to rutile (produced at high temperature and thermodynamically stable) [220]. Most peaks are available in all spectra of the nanoparticles produced by ns-, ps- and fs lasers. According to the results obtained by Ocwelwang and Tichagwa [318], these peaks should be visible in the spectra of pure TiO 2 nanoparticles when Ag is doped in TiO 2 nanoparticles because of Ag nanoparticles are well scattered on the surface of the metal oxide (TiO 2 ) nanoparticles. It is worth mentioning that although Ag has a large radius to be incorporated into the lattice structure, it can attach well to the TiO 2 surface [318] FTIR spectra of Ag-TiO 2 compound nanoparticles Figure 7-5 shows the FTIR spectra of the Ag-TiO 2 nanoparticles produced by the nanosecond, picosecond and femtosecond lasers. It can be seen that the transmission peaks produced by the femtosecond laser are sharper than those produced by the nanosecond and picosecond laser. This may be because the Ag-TiO 2 nanoparticles produced by femtosecond laser have more small nanoparticles on the surface of the larger particles in comparison with those produced by the nanosecond and picosecond lasers. Some peaks are observed at , 2922, 2956, 3446 cm -1 for the nanosecond laser, at 2481, 2849, 2919, 2955 and 3565 cm -1 for the picosecond laser, and at 2512, 2853, 2923, 2953 and 3565 cm -1 for the femtosecond laser. 30 NSL (a) PSL (b) FSL (c) 20 (c) 10 (b) (a) Wavenumber (cm -1 ) Figure 7-5: FTIR spectra for the Ag-TiO 2 nanoparticles produced by nanosecond, picosecond and femtosecond lasers. 192

193 Chapter 7: Comparison of characteristics of Ag-TiO 2 nanoparticles... The peaks produced at 2922 cm -1 and 2850 cm -1 indicate the C-H vibration mode of the CH 2 groups; the peaks correspond to the antisymmetric and symmetric modes respectively. The wide peak at 3453cm -1 denotes the vibration of hydroxyl groups in water [319]; O-H vibrations are produced due to the hydroxyl ions. This wide peak also corresponds to the O-H stretch. These represent interactions between the hydroxyl groups of TiO 2. Regarding photocatalytic activity, the adsorbed OH ions can trap charge carriers to generate reactive OH radicals that act as a driving force for the photocatalytic activity of TiO 2 nanoparticles. As such, they act as adsorbents and active sites for the reactant molecules or the pollutant [318]. The broad band produced at around 3427 cm -1 represent the existence of O H in the Ti OH stretching vibration [320]. The peak at 3411 cm -1 of Ag-doped TiO 2 is slightly smaller than the bare TiO 2 due to the interaction between the Ag and TiO 2 nanoparticles [320] Raman shift of Ag-TiO 2 compound nanoparticles Figure 7-6 shows the Raman spectra of the Ag-TiO 2 nanoparticles produced by nanosecond, picosecond and femtosecond lasers. Peaks were produced at 132 cm -1 and 759 cm -1 for the nanosecond laser, at 138 cm -1 and 730 cm -1 for the picosecond laser and at 134 cm -1 and 757 cm -1 for the femtosecond laser. Two more peaks around 230 cm -1 and 585 cm -1 were observed. It can be seen that the transmission peaks produced by the femtosecond laser have slightly greater intensity than the spectra produced by the other two lasers; this is because the nanoparticles produced by the femtosecond laser are smaller in size [302]. Although the ns laser produced small size nanoparticles, their transmission peaks are weak because of rod-like particles produced by this laser. This may be due to a volume contraction and radial pressure increasing, which consequently changes the force constant and vibrational amplitude of the nearest neighborbond [321]. This may also be due to a change in the interatomic distances of the TiO 2 nanoparticles with increasing the surface pressure to a greater proportion of Ag nanoparticles deposited on the surface of the TiO 2 nanoparticles [302]. 193

194 Intensity Chapter 7: Comparison of characteristics of Ag-TiO 2 nanoparticles NSL (a) PSL (b) FSL (c) (c) (b) (a) Raman shift (cm -1 ) Figure 7-6: Raman spectra for the Ag-TiO 2 nanoparticles produced by nanosecond, picosecond and femtosecond lasers. As shown in Figure 7-6, the peaks of the Ag-TiO 2 nanoparticles produced by the picosecond and femtosecond lasers are a little higher and narrower than those produced by the nanosecond laser; this spectral characteristic the surface enhanced Raman scattering [309]. The shifting of bands towards lower wave numbers might be due to compressive stresses between the Ag and TiO 2 surfaces. Lubas et al. [322] observed bands at 604 cm -1, 438 cm -1 and 231 cm -1 of the rutile TiO 2 phase, and the strongest and most prominent peak was observed at 143 cm -1, which is typical of TiO 2 anatase. In addition, Malagutti et al. [323] observed a major Raman band at 144 cm -1, which indicates anatase-tio 2. The peaks at 132 cm -1, 138 cm -1 and 134 cm -1 represent the energy gap (E g ) of the nanoparticles. The small peaks at 230 cm -1 and 730 cm -1 confirm the deposition of Ag nanoparticles on the TiO 2 particles [302] XPS analysis of Ag-TiO 2 chemical structures Figure 7-7 shows the XPS images of the Ag-TiO 2 compound nanoparticles produced via nanosecond, picosecond and femtosecond lasers in deionised water. High-resolution peak-fitting spectra of Ag 3d, Ti 2p spectra and O 1s spectra of the Ag-TiO 2 compound nanoparticles were produced by all three types of laser. The results show that the main elements of all samples are Ag, Ti and O. The Ag spectrum (Figure 7-7-a, d and g), consists of double 194

195 CPS CPS CPS CPS CPS CPS CPS CPS CPS Chapter 7: Comparison of characteristics of Ag-TiO 2 nanoparticles... peaks of 3d 3/2 and 3d 5/2 which represent silver metal Ag o (big peaks) and silver ions Ag + (small peaks). The Ti spectrum (Figure 7-7-b, e and h), also consists of two, 2p 1/2 and 2p 3/2 which represent Ti 4+ (big peak) and Ti 3+ (small peaks). The O spectrum (Figure 7-7-c, f and i) consists of three sub-peaks [313]. The main difference between them is the width of the peaks, which decrease as the laser pulse width is decreased. The peaks of the Ag-TiO 2 compound nanoparticles produced by femtosecond laser are sharper than those produced by nanosecond and picosecond lasers (a) Ag 3d 5/2 Ag o Ag 3d 3/2 Nanosecond laser (b) Ti 2p 3/2 Ti 4+ Nanosecond laser 8000 Ag Ti 3+ Ti 2p 1/ (d) Picosecond laser (e) Picosecond laser Ag o Ti Ag Ti (g) Femtosecond laser (h) Femtosecond laser Ag o Ti Ag Ti Binding energy (ev) Binding energy (ev) (c) O 1s Nanosecond laser (f) Picosecond laser (i) Femtosecond laser Binding energy (ev) Figure 7-7: XPS images of the Ag-TiO 2 compound nanoparticles produced by nano-, pico- and femtosecond laser in deionised water. Peak-fitting spectra at high resolution of Ag 3d, Ti 2p spectra and O 1s spectra of the Ag-TiO 2 compound nanoparticles. 195

196 Chapter 7: Comparison of characteristics of Ag-TiO 2 nanoparticles... The energy difference between Ag (3d 3/2 ) and Ag (3d 5/2 ) is 6 ev for all samples, which indicates the presence of metallic silver nanoparticles (Ag o ) [ ]. On the other hand, silver ions (Ag + ) were also formed. It can be noted that the amount of silver ions such as Ag + and Ag 2+ is greater in the ns laser generated nano-materials than by the ps and fs lasers. The energy difference between the peaks of metallic silver and silver ions are 2 ev, 1 ev and 0.6 ev of the samples produced by ns, ps and fs lasers respectively. It can be concluded that the fs laser is a more suitable tool to form metallic Ag nanoparticles in water than the ns and ps lasers. In addition, the peaks with a low amount of ionic silver are narrower. The peak positions of the metallic silver and ionic silver may depend on the size, temperature and preparation method. For example, Liang et al. [326] produced Ag, Ag 2 O and AgO at 368.2, and 367 ev respectively, while Akhavan and Ghaderi [ ] prepared them at 368.6, and ev respectively. The energy difference between Ti (2p 1/2 ) and Ti (2p 3/2 ) which refers to a normal state of Ti 4+ is around 6, 6 and 5.8 ev for nanosecond, picosecond and femtosecond lasers respectively. Curve fitting shows that each peak consisted of two Gaussian peaks, showing the amount of Ti 3+ present. This can be formed due to the incorporation of silver and silver oxides on the TiO 2 [ ]. In the same way as Ag, the energy difference between the peaks of Ti 4+ and Ti 3+ are 2.3 ev, 1.8 ev and 0.6 ev of the samples produced by ns, ps and fs lasers respectively. These results indicate that the amount of Ti 3+ formed in the sample produced by ps laser is less than that produced by ns and fs lasers. The O 1s peak consists of three peaks at ev, ev and ev for the ns laser produced nanoparticles, ev, ev and ev for ps laser produced nanoparticles and ev, ev and ev for the fs laser produced nanoparticles. These peaks correspond to the lattice oxygen of the TiO 2, the oxygen of surface OH bound in Ti(OH) O Ti (OH), and the oxygen in the water molecules respectively. Hamad et al. [313] and Akhavan and Ghaderi [ ] observed these at ev, ev and ev. 196

197 Chapter 7: Comparison of characteristics of Ag-TiO 2 nanoparticles Antibacterial activity Figure 7-8 shows the antibacterial activity of the Ag-TiO 2 compound nanoparticles produced by nanosecond, picosecond and femtosecond laser in deionised water. The antibacterial activities were carried out against gramnegative E. coli bacteria under a normal room light (low pressure mercuryvapor gas-discharge lamps that use fluorescence to produce visible light without UV wavelengths) E. coli viability C/C o Ag-TiO2 (NSL) Ag-TiO2 (PSL) Ag-TiO2 (FSL) Control Nanoparticles Figure 7-8: Antibacterial activity of Ag-TiO 2 compound nanoparticles produced via nanosecond, picosecond and femtosecond laser in deionised water. The antibacterial activity of nanoparticles was tested against E. coli bacteria under a standard room light after one day of preparation. The concentration of nanoparticles was 20 µg/ml. The results show that the Ag-TiO 2 nanoparticles produced by the picosecond and femtosecond lasers have better antibacterial activity than those produced by nanosecond laser. This may be because the Ag-TiO 2 nanoparticles produced by picosecond and femtosecond lasers have higher surface charges and more metallic Ag nanoparticles and have lower energy gaps. Metallic silver nanoparticles (non-oxidised) have a tendency to be lightly bound to oxygen and act as a catalyst to bring about oxidation [325]. In the case of antibacterial activity of the Ag-TiO 2 nanoparticles, metallic silver nanoparticles are more effective at absorbing the photoexcited electrons from the TiO 2, produced from photocatalytic activity, than the ionic silver nanoparticles. In addition, metallic Ag nanoparticles delay electron-hole recombination, leading to increased photocatalytic activity [325]. Although the Ag-TiO 2 nanoparticles produced by the nanosecond laser are more ionic nanoparticles they have inferior antibacterial activity in comparison with the 197

198 Chapter 7: Comparison of characteristics of Ag-TiO 2 nanoparticles... nanoparticles produced by the ps and fs lasers. This is because some of the nanoparticles produced by the ns laser have rod-like shapes (See Figure 7-9). It has been reported that the antibacterial activity of the nanoparticles are shape-dependent. Pal et al. [157] showed that the truncated triangular particles almost complete inhibited bacterial growth and had higher antibacterial activity than the spherical nanoparticles. On the other hand, rod-shaped nanoparticles showed inferior performance antibacterial activity. Figure 7-9: TEM images of Ag-TiO 2 nanoparticles produced by nanosecond laser in deionised water. Another reason behind the higher antibacterial activity of the nanoparticles produced by shorter pulse duration (ps and fs lasers) is their higher zeta-potentials in comparison to those produced by long pulse duration (NSL). It has been shown that the nanoparticles with higher zeta-potential are more stable than those with lower zeta-potential, thus higher colloidal stability is more active in killing microorganisms because high zeta-potential prevents the nanoparticles from agglomeration and accumulation by repelling the nanoparticles from each other [313]. Another reason behind the enhanced antibacterial activity of the Ag-TiO 2 nanoparticles produced by picosecond and femtosecond lasers would be their lower energy gaps (2.30 ev and 2.45 ev, respectively), in comparison with the nanoparticles produced by the nanosecond laser (3.10 ev). The low energy gap Ag-TiO 2 nanoparticles exhibit more effective photocatalytic activity then the high energy gap nanoparticles [12, 232]. Quiñones-Jurado et al. [302] showed that the antibacterial activity limitation of the Ag-TiO 2 nanoparticles under visible light or dark conditions might be improved by reaching a minimum Ag nanoparticle content of 8.5% w.t.: This indicates that the Ag/TiO 2 toxicity was efficiently controlled by the activity 198

199 Chapter 7: Comparison of characteristics of Ag-TiO 2 nanoparticles... of Ag NPs [302]. Effects of the antibacterial activities of gram-negative bacteria, such as E. coli, are more easily achieved, probably due to the electrostatic factor over any other possible oxidising species released by titania nanoparticles [302], In addition, this results may be produced because silver nanoparticles are unique among metal nanoparticles in their affinity for oxygen Summary Ag-TiO 2 compound nanoparticles (Ag doped and bimodal nanoparticles with smaller Ag attached to larger TiO 2 ) were produced from an Ag-Ti alloy by nano-, pico- and femtosecond lasers in deionised water. Femtosecond and nanosecond laser production of Ag-TiO 2 nanoparticles from an Ag-Ti alloy have been reported for the first time. Although this work is not a tit-for-tat comparison of the effects of pulse width under the same operating experimental conditions such as focused beam size and repetition rates limited by the operating range of the lasers used, this research work would be a fair comparison between ns, ps and fs commercial lasers operating at their usual operating conditions. The results show that the femtosecond laser produced more small Ag nanoparticles attached to the surface of the TiO 2 nanoparticles. In addition, more pure metallic silver nanoparticles were produced by the femtosecond laser than the other lasers. The antibacterial activities of the nanoparticles produced by the picosecond and femtosecond lasers are higher than that of the nanoparticles produced by the nanosecond laser. The nanosecond pulsed laser produced Ag- TiO 2 nanoparticles have much lower zeta-potential and higher energy gaps compared with those produced by the ps and fs lasers. It has been observed that the antibacterial activities of the nanoparticles produced by picosecond and femtosecond lasers are approximately similar. 199

200 8 Chapter 8. A comparison of the characteristics of nanosecond, picosecond and femtosecond laser generated Ag, TiO2 and Au nanoparticles in deionised water Authors: Abubaker Hamad, Lin Li, Zhu Liu, Hong Liu, and Tao Wang Journal: Applied Physics A: Materials Science and processing Volume, issue and pages: 120, 4, Status: published Note: The format of the paper is edited 200

201 Chapter 8: A comparison of the characteristics of nanosecond, picosecond and femtosecond laser generated Ag, TiO2 and Au nanoparticles in deionised water A comparison of the characteristics of nanosecond, picosecond and femtosecond laser generated Ag, TiO 2 and Au nanoparticles in deionised water Abstract Although there have been large quantities of published work in laser generation of nanoparticles, it is still unclear on the comparative role of laser wavelengths and pulse widths in controlling the nanoparticle sizes, morphology and production rate. In this investigation, Ag, Au and TiO 2 nanoparticles were synthesised by nanosecond ( =532 nm and = 5 ns), picosecond ( =1064 nm, =10 ps) and femtosecond ( = 800 nm, = < 100 fs) pulse lasers in deionised water. They are compared, in terms of their optical absorption spectra, morphology, size distribution and production rates, characterised by UV-VIS spectroscopy and transmission electron microscopy (TEM). The ablation rates of both Ag and Ti samples were shown as a function of laser pulse energy and water level above the samples. The average size of nanoparticles (10-50 nm) was found to be smaller for the shorter wavelength (532 nm) nanosecond pulsed laser compared with those of picosecond and femtosecond lasers, demonstrating a more dominating role of laser wavelength than pulse width in particle size control. The ps laser generated more spherical Ag nanoparticles than those with the ns and fs lasers. Under the same laser processing conditions, Au nanoparticles are smaller than Ag and TiO 2, with the latter, the largest. The nanoparticle production rate is relatively independent upon laser types, wavelengths and pulse lengths, but largely determined by the laser fluence and energy deposited. Keywords: Nanoparticles, ablation rate, nanosecond laser, picosecond laser, femtosecond laser, Ag. 201

202 Chapter 8: A comparison of the characteristics of nanosecond, picosecond and femtosecond laser generated Ag, TiO2 and Au nanoparticles in deionised water 8.1. Introduction The novel properties of nanoparticles have received much attention over the last 20 years with wide applications such as in electrochemical sensors and biosensors [327], biology and medicine [328], waste water treatment [329] and agriculture [330]. Nanoparticles have been fabricated as a single material such as Ag [165], Au [331] and Si [332], compound/composite nanoparticles such as Ag-TiO 2 [161], Mg-ZnO [221], bi-metallic and tri-metallic materials such Au-Ag [250], Au-Ag-Pt [333] and Au-PdPt core-shell [334] nanoparticles. Different methods have been reported for the production and modification of nanoparticles. These methods include sol-gel [335], chemical vapour deposition (CVD) [336], spark discharge [337] and pulsed laser ablations [338]. Among these methods, pulsed laser ablation in liquid environment has been recognised as one of most convenient methods with minimum material contamination by other species [60]. Sukhov et al. [339] produced brass (27% Zn + 73% Cu) and bronze (13% Sn + 87% Cu) nanoparticles with a ytterbium fiber laser (wavelength = nm, pulse length = 80 ns, repetition rate f=20 khz and beam spot diameter D = 35 µm), a Neodymium:YAG laser ( =1064 nm, = 10 ps, f = 50 khz and D = 10µm), and a femtosecond Ti:Sapphire laser ( =800 nm, =200 fs, f= 1 khz and D = 10 µm) in liquid ethanol and added Polyvilenepyrrolidone (PVP) as a surface active substance. It was concluded that the nanosecond laser pulses produced smaller bronze nanoparticles and narrower size distribution [339]. With the same laser wavelengths (355 nm) Ag nanoparticles produced by the nanosecond laser (10 ns) is smaller than those produced by the picosecond laser (10 ps) [340]. Dolgaev et al. [61] produced Ag, Ti, Si and Au nanoparticles by nanosecond laser ablation of solid targets in water, dichloroethane and ethanol. It was concluded that the size of nanoparticles were affected by the laser fluence (energy density) and the type of the liquid solution. For example, laser ablation of Ti in water, ethanol and dichloroethane produced non-stoichiometric oxide TiO x (x=1.04) with the mean size about 35 nm, Ti nanostructure with a size in the range of 25 to 150 nm and titanium carbide TiC with a mean size about 35 nm, respectively. The plasmon resonance of Ag in ethanol and dichloroethane was shifted to UV range in 202

203 Chapter 8: A comparison of the characteristics of nanosecond, picosecond and femtosecond laser generated Ag, TiO2 and Au nanoparticles in deionised water comparison in water. An increase in laser fluence slightly increased particle size. Nikov et al. [341] used a nanosecond pulsed laser ablation method in double distilled water to produce both Ag and Au nanoparticles. It was concluded that the laser fluence at 1064 nm laser wavelength has no effect on the average size of nanoparticles, but at the 532 nm wavelength, the average size increased considerably with increasing laser fluence. This is because of a greater separation between fundamental laser beam wavelength (1064 nm) and surface plasmon resonance peak position in comparison with that of the second harmonic laser wavelength (532 nm) with plasmon peak position. Solati and Dorranian [342] compared the generation of Ag and Au nanoparticles in deionised water using pulsed laser ablation ( = 1064 nm, = 7 ns and f = 10 Hz). It was concluded that the ablation rate of Au nanoparticles was higher than that for Ag nanoparticles. However, until now there has been no reported work to compare ns, ps and fs laser generation of Au, Ag and TiO 2 nanoparticles. Although it is generally known that laser wavelengths and pulse width have an effect on nanoparticle sizes and nanoparticle production rates, it is not yet known which factor plays a more dominating role and why. Whether their effects are dependent on the type of nanoparticles is also not yet clear. The present study aims to answer these questions. In this work, a comparison is made between ns, ps and fs laser generation of Ag, TiO 2 and Au nanoparticles. The basic characteristics of particle size distribution, morphology and ablation rate were analysed and the mechanisms in their differences due to different laser pulse length regions are discussed Experimental Materials and Procedure Materials An Ag target plate with the dimensions of 25 mm 25 mm 2 mm and a purity of 99.99%, a Ti target plate with the dimensions 25 mm 25 mm 1mm, and a purity of % and an Au target foil with the dimensions of 65 mm 10 mm 0.2 mm with a purity of 99.99% were used to generate the Ag, TiO 2 203

204 Chapter 8: A comparison of the characteristics of nanosecond, picosecond and femtosecond laser generated Ag, TiO2 and Au nanoparticles in deionised water and Au nanoparticles respectively in deionised water. All target materials were washed in deionised water and ethanol prior to use Lasers In this work, three different types of laser were used for generation of the nanoparticles. These are nanosecond, picosecond and femtosecond lasers. The details of the laser beam characteristics are summarized in Table 8-1. The laser beam scanning system was via x-y Galvo scanners in all laser types. Table 8-1: Characteristics and the process parameters of nanosecond, picosecond and femtosecond lasers used in this investigation. Laser parameters Nanosecond laser (Laserline- Laserval Violino) Picosecond laser (Edgewave-PX-1) Femtosecond laser (Coherent Ti- Sapphire) Average power (P) 7.22 W 9.12 W 0.8 W Wavelength ( ) 532 nm 1064 nm 800 nm Frequency (f) 30 khz 200 khz 1 khz Laser pulse duration ( ) 5 ns 10 ps 100 fs Spot size (D) 50 µm 125 µm 60 µm Scan speed (v) 250 mm/s 250 mm/s 250 mm/s Laser pulse energy (E pulse ) 241 µj 46 µj 800 µj Laser fluence (F laser ) 12.2 J/cm J/cm J/cm Nanoparticle production procedure Ag, TiO 2 and Au nanoparticles were generated using the nanosecond, picosecond and femtosecond lasers in deionised water. The target materials were put individually on a stainless steel holder in a Pyrex glass vessel which contained about ml of deionised water or distilled water. An examination of the effect of water level from 2 mm to 8 mm was carried out to understand its effect. Subsequently, the water level on the surface of the targets was fixed at 2 mm above the target surface. The experimental set-up is shown in Figure 8-1. The ablation area on the surface of the samples was 5 mm 5 mm. 204

205 Chapter 8: A comparison of the characteristics of nanosecond, picosecond and femtosecond laser generated Ag, TiO2 and Au nanoparticles in deionised water Figure 8-1: Experimental set-up for nanoparticle production Material characterisation methods To characterise the Ag, TiO 2 and Au nanoparticles, a UV-VIS spectrometer (Analytic Jena, SPECORD 250, dual beam) was used for measuring the absorption spectra of the colloidal nanoparticles. A transmission electron microscope (TEM) (Philips CM kv TEM (LaB6)) was used to examine the morphology and size distribution of the nanoparticles. The amount of ablated materials in deionised water was determined by weighing the target materials before and after the ablation process using a microbalance scale Sartorius BL 210S, with an accuracy of 0.1 mg Results and discussion Optical reflectivity of Ag, Ti and Au target materials Figure 8-2 shows the optical reflectivity of Ag, Ti and Au plate materials as a function of wavelengths. The reflectivity of Au is low at shorter wavelengths and high at longer wavelengths. It decreased from 350 to 500 nm then increased rapidly from 600 nm to 700 nm. The reflectivity of Ti is much lower than those of Ag and Au. The Ag has low reflectivity at shorter wavelengths and increases with increasing laser wavelength, but it is not as high as the Au reflectivity at longer wavelengths. 205

206 Reflectivity (%) Chapter 8: A comparison of the characteristics of nanosecond, picosecond and femtosecond laser generated Ag, TiO2 and Au nanoparticles in deionised water 100 Ag Ti Au = 532 nm = 800 nm = 1064 nm Wavelength (nm) Figure 8-2: Optical reflectivity of Ag, Ti and Au samples as a function of wavelength Laser ablation material removal rate in deionised water as a function of laser fluence and water level Figure 8-3-a shows the relationship between ablation rates of Ag and Ti with increasing laser pulse energy, at a fixed water level on the material samples, using picosecond laser; wavelength ( ) = 1064 nm, repetition rate (f) = 200 khz, scanning speed (v) = 30 mm/s, ablation time (t) = 30 minutes and water level = 2 mm above the target surface. At lower laser pulse energies, the ablation rates of both samples increased slightly at the same rate, but at higher laser pulse energies only the Ag increased, albeit slowly. On the other hand, the rate of ablation of Ti decreases considerably as the laser fluence increases above 0.15 J/cm 2. This may be due to the higher plasma/plume density that increased the laser beam absorption by plasma/plume and beam scattering loss by the plume, reducing the amount of laser energy reaching the target surface. The reason that such a phenomenon is possible for Ti instead of Ag would be due to the higher beam absorption of Ti than that of Ag. The ablation rate of the Ag and Ti targets decreased as the water level above them increased. As shown in Figure 8-3-b, the ablation rate at the 2 mm water level is higher. At the 4 mm water level the ablation rate decreases considerably, especially in the case of the Ag target. The nanoparticle 206

207 Ablation rate (mg/hh) Ablation rate (mg/hh) Chapter 8: A comparison of the characteristics of nanosecond, picosecond and femtosecond laser generated Ag, TiO2 and Au nanoparticles in deionised water production rate decreased slowly until the water level reaches 6 mm, and then decreased rapidly for both targets. On the whole, the amount of Ag nanoparticles generated was greater than the amount of Ti nanoparticles (a) Ag Ti Laser fluence (J/cm 2 ) (b) Ti Ag Water height (mm) Figure 8-3: The ablation rate of both Ag and Ti targets as a function of laser fluence at a fixed water level at 2 mm above the target material surface (a), and as a function of water level at a fixed laser fluence ( mj, 0.22 J/cm 2 ) (b). These tests were carried out using a picosecond laser at =1064 nm, f=200 khz, v=30 mm/s, t=1/2 h and a laser spot size = 125 µm. Effects of laser powers and water levels have been studied at laser ablation process in liquid environment. For example, Sajti et al. [343] and Al- Mamun et al. [92] produced Al 2 O 3 nanoparticles by a nanosecond laser in deionised water (see Table 8-2). It was shown that the ablation rate of the nanoparticles was increased continually with increase laser power. The decreasing ablation rate as a function of water level is in agreement with our results. 207

208 Chapter 8: A comparison of the characteristics of nanosecond, picosecond and femtosecond laser generated Ag, TiO2 and Au nanoparticles in deionised water Table 8-2: Laser beam parameters NPs Wavelength ( ) Pulse duration ( ) Frequency (f) Scan speed (v) Spot size (D) Al 2 O nm ns 4 khz 120 mm/s 80 µm Sajti et al. [343] Al 2 O nm 6 ns 10 Hz 40 rpm 250 µm Al-Mamun et al. [92] Ag and TiO nm 10 ps 200 khz 30 mm/s 125 µm Ref. This work Comparison between nano- pico- and femtosecond laser generation of nanoparticles Silver nanoparticles Figure 8-4-a shows the optical absorption spectra of the Ag colloidal nanoparticles synthesised in deionised water using nanosecond, picosecond and femtosecond lasers. Colloidal silver nanoparticles have two absorption peaks; a weak absorption peak located at the UV light range of 200 to 250 nm, due to interband transition and a strong absorption peak is located at 400 nm wavelength due to surface Plasmon resonance [63]. Figure 8-4-b shows the size distribution of Ag nanoparticles produced in deionised water using all three types of laser. The peak positions of surface plasmon resonance (SPR) of Ag nanoparticles produced by nanosecond, picosecond and femtosecond laser ablation located at; 399 nm, 414 nm and 420 nm respectively. The mean sizes in diameter of Ag nanoparticles produced by nanosecond, picosecond and femtosecond lasers are 23 nm, 32 nm and 36 nm respectively. There is a similarity among the shapes of absorption spectra and the size distribution of the Ag nanoparticles. 208

209 Absorbance [A] Number of nanoparticles Absorbance [A] Number of nanoparticles Absorbance [A] Number of nanoparticles Chapter 8: A comparison of the characteristics of nanosecond, picosecond and femtosecond laser generated Ag, TiO2 and Au nanoparticles in deionised water (a) Ag (NSL) Ag (PSL) Ag (FSL) (b) Ag NPs (NSL) Ag NPs (PSL) Ag NPs (FSL) Ag NPs No. of NPs Ave. Size (nm) NSL PSL FSL (c) Wavelength (nm) TiO2 (NSL) TiO2 (PSL) TiO2 (FSL) (d) Size of nanoparticles (nm) TiO2 NPs (NSL) TiO2 NPs (PSL) TiO2 NPS (FSL) TiO2 NPs No. of NPs Ave. Size (nm) NSL PSL FSL Wavelength (nm) Size of TiO 2 nanoparticles (nm) 0.6 (e) Au (NSL) Au (PSL) Au (FSL) (f) Au NPs (NSL) Au NPs (PSL) Au NPs (FSL) Au NPs No. of NPs Ave. Size (nm) NSL PSL FSL Wavelength (nm) Size of Au nanoparticles (nm) Figure 8-4: Absorption spectra and size distribution of Ag nanoparticles (a and b), TiO 2 nanoparticles (c and d) and Au nanoparticles (e and f) produced by nanosecond, picosecond and femtosecond laser in deionised water. As shown in Figure 8-5-a, b and c, the TEM images show that the shapes of the Ag nanoparticles produced by the picosecond laser is more spherical than those generated by either nanosecond or femtosecond lasers. The smallest nanoparticles were synthesised by the nanosecond laser. 209

210 Chapter 8: A comparison of the characteristics of nanosecond, picosecond and femtosecond laser generated Ag, TiO2 and Au nanoparticles in deionised water (a) (b) (c) (d) (e) (f) (g) (h) (i) Figure 8-5: TEM images of Ag nanoparticles generated in deionised water using nanosecond (a), picosecond (b) and femtosecond (c) lasers. TEM images of TiO 2 nanoparticles generated in deionised water using nanosecond (d), picosecond (e) and femtosecond (f) lasers. TEM images of Au nanoparticles generated in deionised water using nanosecond (g), picosecond (h) and femtosecond (i) lasers Titanium dioxide nanoparticles Figure 8-4-c shows the absorption spectra of TiO 2 nanoparticles produced by the nanosecond, picosecond and femtosecond lasers in deionised water. The absorption spectrum of colloidal TiO 2 nanoparticles has a strong absorption at UV range. It is reduced sharply from 200 to 350 for the TiO 2 produced by the nanosecond laser and to 450 for those generated by picosecond and femtosecond lasers. The spectra of nanoparticles produced by the nanosecond laser reduced more sharply than the nanoparticles generated by the other two lasers because the size of nanoparticles is smaller than that generated by the other two lasers. As shown in Figure 8-4-d the size distribution of TiO 2 nanoparticles produced by the nanosecond laser is narrower than that 210

211 Chapter 8: A comparison of the characteristics of nanosecond, picosecond and femtosecond laser generated Ag, TiO2 and Au nanoparticles in deionised water produced by the picosecond and femtosecond laser. As shown in the inset table the mean size of nanoparticles generated by nanosecond laser (30 nm) is smaller than those produced by the picosecond laser (37 nm) and femtosecond laser (47 nm). There is also a similarity among the size distribution and the absorption spectra of TiO 2 nanoparticles. As shown in Figure 8-5-d, e and f, the TiO 2 nanoparticles have similar spherical morphology for all three lasers. Smaller sizes of TiO 2 nanoparticles were also synthesised by the nanosecond laser. It is worth mentioning that some TiO 2 nanoparticles with their sizes above 500 nm were observed among those nanoparticles produced via femtosecond laser in deionised water Gold nanoparticles Figure 8-4-e shows the absorption spectra of colloidal Au nanoparticles fabricated in deionised water using the nano- pico- and femtosecond lasers. The absorption spectrum of the colloidal Au nanoparticles consists of two absorption peaks; the small absorption peak is located in the range of nm wavelengths and a resonance peak located at nm wavelengths. In general, similar absorption spectra of colloidal Au nanoparticles were observed at all types of lasers, but the surface plasmon resonance (SPR) peak position of Au nanoparticles produced by the nanosecond laser ablation and was located at shorter wavelengths (252 nm and 525 nm) than those produced by the picosecond (262 nm and 529 nm) and the femtosecond laser. Histogram image of size distribution of Au nanoparticles show (Figure 8-4-f) that the average size of nanoparticles produced by nanosecond (17.4 nm) is smaller than that generated by picosecond and femtosecond laser, 22.9 and 23.7 nm respectively. As shown in Figure 8-5-g, h and i, most of the Au nanoparticles were produced as chain nanoparticles. Almost all larger nanoparticles have spherical shapes. Also the nanosecond laser produced the smaller Au nanoparticles in comparison with other two lasers. 211

212 Chapter 8: A comparison of the characteristics of nanosecond, picosecond and femtosecond laser generated Ag, TiO2 and Au nanoparticles in deionised water Morphology of the nanoparticles The shapes of nanoparticles strongly depended on the type of the materials. The TiO 2 and Au nanoparticles produced by nanosecond, picosecond and femtosecond lasers have spherical shapes, although some Ag nanoparticles have semispherical shapes particularly those that produced by the nanosecond and femtosecond laser. The size and shape of nanoparticles strongly depended on the thermodynamic properties of nanoparticles. Nonspherical nanoparticles such as prism are produced by melting processes. Sar et al. [344] reported a thermodynamic model to understand the melting process of prism-shaped indium nanoparticles. The model was based on a simple classical thermodynamic model assuming equilibrium between solid and melted particles. In addition, the size of nanoparticles depended on the thermodynamic properties of nanoparticles [345]. Herein, some Ag nanoparticles produced were semispherical. This is may be because the heat of vaporization of Ag nanoparticles (254 kj mol 1 ) is lower than both TiO 2 and Au nanoparticles, (425 kj mol 1 ) and (342 kj mol 1 ), respectively. As a result, Ag nanoparticles can be produced faster by melting. It can be noted that the nanosecond laser ( = 532 nm) produced smaller nanoparticles in comparison with the picosecond laser ( = 1064 nm) and femtosecond laser ( = 800 nm). This may not because of the pulse lengths, but mainly because it has a shorter wavelength [63]. According to the energy equation of photon (E=hc/ ), it can be noted, a short wavelength increases the photon energy as a result it increase the kinetic energy imparted to electrons. These electrons ejected from the nanostructure surface of the target and the electrons produced by effects of ionization will participate in producing a plasma plume in the liquid environment [346]. The TEM images show the distinguishable individual Ag and TiO 2 nanoparticles, while Au nanoparticles produced as a chain of nanoparticles. All these features of nanoparticles are related to physical properties of materials because similar for all types of lasers. In is worth mentioning that the nanoparticle morphology depends on the laser pulse duration such as nanosecond, picosecond and femtosecond regime and the nature of material [57]. 212

213 Wavelength (nm) at max. absorbance of Ag and Au Chapter 8: A comparison of the characteristics of nanosecond, picosecond and femtosecond laser generated Ag, TiO2 and Au nanoparticles in deionised water Understanding the absorption spectra of colloidal nanoparticles Because surface plasmon resonance is the collective excitation of the electrons of a metal in the conduction band, it depends on the size and shape of nanoparticles; herein smaller Ag nanoparticles produced by the nanosecond laser have the narrower surface plasmon resonance (FWHM is about 75 nm). The highest intensity of SPR was not observed for the smaller nanoparticles. Surface plasmon resonance (SPR) of Ag nanoparticles shifted to longer wavelength (from nanosecond laser to femtosecond laser) because their size is increased. For the larger nanoparticles red shift was produced in SPR (see Figure 8-6-a). When the diameters of nanoparticles is in the range 2-10 nm, extinction is independent on the size, but for sizes greater than 10 nm the red shift will be produced. The width of the SPR strongly depended on the shape and size of nanoparticles due to confinement and the limit of the electrons in conduction band and their mean free paths by the particle boundary [64]. As shown in Figure 8-6-a, the tangent of TiO 2 absorption spectra was shifted to longer wavelength from nanosecond to femtosecond laser because the size of the nanoparticles were increased by decreasing the timescale of the laser pulses. In other words, by increasing the size of the nanoparticles the energy gap was decreased. See Figure 8-6-b. Wavelength (nm) at tangent line intesection of TiO (a) TiO2 NPs Ag NPs Au NPs NSL PSL FSL Laser types ev 1/2 nm -1/ (b) NSL PSL FSL Photon Energy (ev) Laser Type Eg of TiO2 NP NSL 3.2 PSL 2.35 FSL 2 Figure 8-6: Shifting SPR of Ag and Au nanoparticles and shifting tangent line of TiO 2 absorption spectra (a). Shift (decrease) of indirect band gap energy of TiO 2 nanoparticles (b) as a function of nanosecond, picosecond and femtosecond lasers. It is worth mentioning that sometimes the broadening of SPR of colloidal Ag and Au nanoparticles is due to aggregation of nanoparticles in the solution, especially those nanoparticles produced in deionised water. On the other hand, 213

214 Ablation rate (mg/min) Chapter 8: A comparison of the characteristics of nanosecond, picosecond and femtosecond laser generated Ag, TiO2 and Au nanoparticles in deionised water the width of SPR peak decreases when the size particles distribution became homogenous [63] Ablation rates Figure 8-7 shows the ablation rates of Ag, Ti and Au nanoparticles produced by nano- pico- and femtosecond laser in deionised water, at 2 mm water level above the targets. In general, the ablation rate of nanoparticles generated by nanosecond laser is higher than that produced by picosecond and femtosecond lasers particularly for Ag and Ti materials NSL PSL FSL Ag Ti Au Materials Figure 8-7: Ablation rate of Ag, TiO 2 and Au nanoparticles produced in deionised water by NSL, PSL and FSL at laser fluence 12.2 J/cm 2, 0.37 J/cm 2 and 28.3 J/cm 2 respectively and at fixed scan speed 250 mm/s. The ablation rate of nanoparticles depends on the type of material used, heat of vaporization, laser power used and scan speed. In deionised water, picosecond laser ablation, at a low laser fluence, the ablation rate of Ti is higher than that of Ag. In general, the ablation efficiency is higher for targets material which have lower meting point and hardness [140]. The Mohs hardness of Ti is 6, higher than that of Ag is 2.5. Sola and Peña [347] reported that the lower depth and removed volume obtained from the harder materials. At higher laser fluences, the relationship is the opposite because TiO 2 nanoparticles absorb more laser beam energy in comparison with Ag nanoparticles and the size of 214

215 Chapter 8: A comparison of the characteristics of nanosecond, picosecond and femtosecond laser generated Ag, TiO2 and Au nanoparticles in deionised water TiO 2 nanoparticles is larger than the size of Ag nanoparticles. As a result, less energy will be delivered to the Ti target leading to less ablation efficiency. As shown in Figure 8-3-a, at a specific laser fluence (about J/cm 2 ) the ablation rate of both materials are equal (about 2.05 mg/hh). At a low fluence the ablation rate increase because low energy produce low amount of nanoparticles, so there is sufficient time to disperse generated nanoparticles from the ablation area on the target into the solution. In this case the laser beam does not interact with the large amount of nanoparticles and the power is not significantly lost by nanoparticle scattering of light. At high fluences, higher amount of nanoparticles was produced in short time. So they are not dispersed in the solution quickly. As a result laser beam energy is absorbed by generated nanoparticles in the solution leading to reduced ablation efficiency. The same results were observed by Barcikowski et al. [348] for Ag and Co materials ablation in water. The water level on the sample target is another factor affecting the ablation rate of nanoparticles. In the case of picosecond laser ( =1064 nm) the ablation rate was decreased with increasing water level. The reduction rate of Ag is higher than Ti, particularly at low water levels. Light absorption by water is a function of wavelength, for example the water has strong absorption coefficient at the 1064 nm wavelength. However, at 800 nm and 532 nm water has a very low absorption to light. According to Lambert-Beer law the laser beam energy exponentially decrease with the thickness of the water. The ablation rates at high water level (6 mm), both Ag and Ti have the same ablation reduction rates. The ablation rate in water may be up to 50 times less than that in air [348]. In this work, the ablation rate of Au is higher than of Ag and TiO 2 nanoparticles in all NSL, PSL and FSL. So the ablation rate does not depend on the melting point. In other words, in spite of Ag having a lower melting point ( C) than Au ( C), Ag has lower ablation efficiency. Larger pulse energy such as nanosecond pulses is more effective for the ablation process than the shorter pulses such as picosecond and femtosecond pulses. This might be because longer laser pulses, such as nanosecond pulses, remain longer time on the target in comparison with shorter laser pulses, such as femtosecond laser, causing more materials to be ablated. Another reason to 215

216 Chapter 8: A comparison of the characteristics of nanosecond, picosecond and femtosecond laser generated Ag, TiO2 and Au nanoparticles in deionised water higher efficiency of the nanosecond laser in ablation rate in this particular investigation is the shorter (532 nm). For all the materials studied in this investigation the light absorption is better at the 532 nm wavelength than at the 800 nm and 1064 nm wavelengths of picosecond and femtosecond laser. See Table 8-3. It is contrary with Hahn et al. [70] where they observed that the laser pulse energy has a stronger influence on higher reflectivity materials. It can be noted that the ablation rate of materials per unit energy input is not affected by the laser pulse duration or types of laser used in this work. It was also reported by Noel et al. [130], that the ablation rate was independent on the laser pulse duration. In spite of laser ablation of nanoparticles in a water environment depends on the type of material and laser beam parameters, there are other phenomena or events that can affect on the ablation rate of nanoparticles in a liquid medium such as cavitation bubble lifetime, inter-pulse-distance and thermal circumstances and re-irradiation, absorption, scattering and reflection of the laser beam, and generation of shock waves. Different physical and chemical properties of the materials have different effects on the ablation rate of nanoparticles. For example, different thermal properties cause the target materials to respond differently to the laser beam parameters; as a result the ablated species of the elements from target materials will release with various kinetic energy [262], due to the different heats of evaporation, which cause the production and ejection of one of the elements faster than the other from the surface of the material to the liquid solution [264]. Therefore, a faster ejection would produce more amount of nanoparticles in the liquid media. Thermal circumstance is another factor that has an effect on the laser ablation. Cooling of the plasma plume by a liquid medium during the laser ablation in water leads to the shortening of the quenching time of the plasma plume [87]. It has an effect on the structure and the size distribution of nanoparticles. In addition, plasma plume effect is more predominant in the picosecond laser ablation than femtosecond laser ablation. Another important factor on the ablation efficiency during laser-materials interaction is the generation of the cavitation bubbles. Bubbles are produce due to the production of a thin layer of vapour with a high temperature and high 216

217 Chapter 8: A comparison of the characteristics of nanosecond, picosecond and femtosecond laser generated Ag, TiO2 and Au nanoparticles in deionised water pressure around the plasma plume in the water medium, followed by expanding against the surrounding water, leading to the production of cavitation bubbles [349]. The cavitation bubble lifetime depends on the type of laser used for the ablation, and it is in the range 200 to 300 µs or higher [343, 350]. During this time, the ablation process is not efficient because of light scattering by the bubbles. As a result, less elemental materials are ejected from the target, which leads to the production of less nanoparticles. To avoid this problem, it is important to select a specific repetition rate of the laser pulses to make a suitable inter-pulse distance. Because at a suitable laser pulse repletion rate, inter-pulse distance (the time duration of sequence laser pulses) is enough to collapse bubbles. In other words, the second pulse will not fire until the bubble shrinks. As a result no interaction takes place between the laser pulses and the bubbles. This will lead to an increase in the production rate of nanoparticles. For example, at a low repetition rate (10 Hz) no interaction will take place between cavitation bubbles and the laser pulses because as the inter-pulse distance is 0.1 s that is much longer than the cavitation bubble lifetime. It is worth mentioning that the lifetime of cavitation bubbles for a nanosecond laser ablation in water is longer than 100 µs and their physical size in the range of 50 to 200 µm [343, 350]. Another way to avoid the effects of cavitation bubbles on the ablation rate of nanoparticles at liquid-phase laser ablation of a solid material, particularly at high repetition rate, is using a higher scan speed of laser beam. For example, at a 5 khz repetition rate, the inter-pulse distance is 200 µs, so for 100 µm size of bubbles with the duration around 100 µs, a 500 mm/s scanning speed is required to avoid laser-bubble interaction [350]. In our work, the nanosecond laser operated at a scan speed of 250 mm/s at a laser pulse repetition rate of 30 khz. Therefore the inter-pulse duration was 33.3 s. For the picosecond laser operated at a 250 mm/s speed and a 200 khz repetition rate, the inter-pulse duration was only 5 s. For the femtosecond laser, operated at a 250 mm/s speed with a 1 khz repetition rate, the inter-pulse duration was 1000 s. It can be seen that only the femtosecond laser avoided the laser-bubble interaction. Sajti et al. [343] produced a higher ablation rate (1265 mg/h) of Al 2 O 3 nanoparticle after adjusting of inter-pulse distance and laser beam scan 217

218 Chapter 8: A comparison of the characteristics of nanosecond, picosecond and femtosecond laser generated Ag, TiO2 and Au nanoparticles in deionised water speed at 125 µm and 500 mm/s, respectively. This improvement is about 300% is higher than the non-optimised scan speed such as at 100 mm/s. Liquid flow system on the surface of the target has been used to avoid the production of cavitation bubbles during the laser ablation in water environment. For example, a 190 ml liquid flow in one minute enhanced 20% productivity of nanoparticles in comparison with that under the stationary liquid [343, 350]. Sometimes the production of shock waves due to laser beam incident on the target material in the solution can burst the cavitation bubbles. Another important phenomenon during laser ablation in liquid media is re-irradiation of produced nanoparticles by laser ablation. When the nanoparticles are produced from the surface of a target, they interact with the laser beam in the solution. As a result, some power of the laser beam will be reduced due absorption, scattering and reflection by the embryo nanoparticles. In this case, less photon energy will reach to the target material leading to reduced ablation rate of nanoparticles. This is an avoidable problem but might be solved by a water flow system. It is worth mentioning that, the re-irradiation process of the colloidal nanoparticles can be used to reduce the size and change the morphology of the nanoparticles Size distribution and average nanoparticle sizes Figure 8-8 (a, b and c) show the size distribution of Ag, TiO 2 and Au nanoparticles generated by the nanosecond, picosecond and femtosecond lasers. The size of the nanoparticles produced by the nanosecond laser has a narrow size distribution, with its shape a near Gaussian distribution. The nanoparticles distributed ranged from few nanometers to 50 nm and some bigger nanoparticles, greater than 50 nm up to 200 nm, were also observed. The number of small nanoparticles (from few nm to 10 nm) is low in comparison with the middle sizes nanoparticles (from 11 nm to about 20 nm). However, the size distributions of these nanoparticles produced by the picosecond and femtosecond laser ablation were distributed as a log-normal size distribution function. The number of smaller and larger nanoparticles produced by the ps and fs lasers is higher than that produced by the nanosecond laser. 218

219 Number of nanoparticles Average Size (nm) Number of Nanoparticles Number of Nanoparticles Chapter 8: A comparison of the characteristics of nanosecond, picosecond and femtosecond laser generated Ag, TiO2 and Au nanoparticles in deionised water (a) Nanosecond laser Ag NPs TiO2 NPs Au NPs Materials No. of NPs Ave. Size (nm) Ag TiO Au (b) Picosecond laser Ag NPs TiO2 NPs Au NPs Materials No. of NPs Ave. Size (nm) Ag TiO Au Size of Nanoparticles (nm) Size of Nanoparticles (nm) (c) Femtosecond laser Ag NPs TiO2 NPs Au NPs Mateials No. of NPs Ave. size (nm) Ag TiO Au (d) Ag TiO2 Au Size of nanoparticles (nm) 10 Nanosecond Picosecond Femtosecond Laser Types Figure 8-8: Size distribution of Ag, TiO 2 and Au nanoparticles generated by NSL (a) PSL (b) and FSL (c) in deionised water. Average size of Ag, TiO 2 and Au nanoparticles as a function of nanosecond, picosecond and femtosecond laser produced in deionised water (d). Figure 8-8-d shows the average size of Ag, TiO 2 and Au nanoparticles produced by nano-, pico- and femtosecond lasers in deionised water. The average size of the nanoparticles was increased by decreasing laser pulse duration. It can be seen that the nanosecond laser is a good laser to produce smaller size of nanoparticles. It is worth mentioning that the spot size of nanosecond laser beam (50 µm) was smaller than picosecond laser (125 µm) and femtosecond laser (60 µm). Size distribution of nanoparticles can also be estimated from the absorption spectra of the colloidal nanoparticles. It can be observed clearly at longer wavelengths, lager particle sizes were affected by longer wavelengths (See Figure 8-4 (a and b) and (c and d)). Higher absorption spectra at longer wavelengths mean the existence higher number of larger size nanoparticles, or 219

220 Chapter 8: A comparison of the characteristics of nanosecond, picosecond and femtosecond laser generated Ag, TiO2 and Au nanoparticles in deionised water agglomerated nanoparticles. The size distribution of the Ag, TiO 2 and Au nanoparticles produced by the nanosecond laser has a Gaussian distribution form, but the picosecond and femtosecond lasers produced nanoparticles have a log normal size distribution form. The Gaussian size distribution indicates that the nanoparticles are uniformly distributed but lognormal size distribution means that the nanoparticles are non-uniformly distributed in the solution. In spite of the optimal ablation efficiency of materials obtained with femtosecond laser [140], the nanosecond pulsed laser produced smaller size nanoparticles and narrower size distribution in comparison with those by the picosecond and femtosecond lasers. The average size of Ag, TiO 2 and Au nanoparticles were increased by decreasing the pulse duration from the nanosecond to the femtosecond regime. In other words, the size and size distribution of nanoparticles generated by femtosecond laser are larger and wider respectively, because of the higher laser pulse energy (for femtosecond laser = 800 µj, nanosecond laser = 241 µj and picosecond laser = 45.6 µj) producing larger size of nanoparticles and wider size distribution. A similar observation was made by Barcikowski et al. [348]. As shown in Table 8-3, smaller spot size of laser beam does not necessarily produce smaller nanoparticles. This effect was observed for all the materials investigated: Ag, TiO 2 and Au. Table 8-3: Results of Ag, TiO 2 and Au nanoparticles produced by nanosecond, picosecond and femtosecond lasers at a fixed scan speed of 250 mm/s. Laser type Wavelength (nm) Laser pulse energy (Epulse) µj Laser Fluence (Flaser) J/cm 2 Laser pulse duration ( ) Spot size (D) (µm) Ag nanoparticles TiO2 nanoparticles Au nanoparticles Average Size (nm) Ablation rate (mg/min) Averag e Size (nm) Ablation rate (mg/min) Average Size (nm) Ablation rate (mg/min) NSL ns PSL ps FSL fs Contrary to this work Tsuji et al. [63] have shown that the size and size distribution of nanoparticles in the colloidal solution were affected by the laser wavelength. By decreasing the laser wavelength the particle size was decreased but their spherical shapes did not change. In other words, the smaller particle size was obtained by shorter laser wavelengths [63]. In addition, the size of the nanoparticles could be reduced by increasing the time exposure of suspension [139]. Darroudi et al. [351] showed that the mean diameter of Ag 220

221 Chapter 8: A comparison of the characteristics of nanosecond, picosecond and femtosecond laser generated Ag, TiO2 and Au nanoparticles in deionised water nanoparticles prepared by NSL in an aqueous gelatin solution was decreased with increasing ablation time Colour of colloidal nanoparticles In general, the colour of the colloidal Ag, TiO 2 and Au nanoparticles were yellow, gray and red, respectively (See Figure 8-9). The colour of a specific material nanoparticle is different when it is produced by different types of lasers because the reflected light in the colloidal of nanoparticles is affected by size and concentrations. So, different types of laser produced different types of morphology for the same type of nanoparticles. The colour of a specific colloidal nanoparticle varies with different types of laser tools because their sizes were changed. The colour of colloidal Ag nanoparticles produced by the nanosecond laser is bright yellow but those produced by picosecond and femtosecond lasers are dark yellow because silver size produced by nanosecond laser is smaller than that produced by other lasers. The colour of the colloidal silver nanoparticles produced by the femtosecond laser is darker than that generated by the picosecond laser because their size is larger. The same characteristic was observed for the colloidal titania and gold nanoparticles. The colour or the optical absorption spectra of colloidal Ag nanoparticles can be controlled by the excitation via the irradiation with different wavelengths of LEDs sources [352]. In this work, different colours of the same type of colloidal nanoparticles were produced by different laser types. In general, the colour of the colloidal nanoparticles depends on the size, concentration, purity of materials and the type of the solution. Figure 8-9: Colloidal Ag, TiO 2 and Au nanoparticles produced by nanosecond, picosecond and femtosecond lasers in deionised water. 221

222 Chapter 8: A comparison of the characteristics of nanosecond, picosecond and femtosecond laser generated Ag, TiO2 and Au nanoparticles in deionised water Ablation mechanism by the nano-, pico- and femtosecond lasers The difference in the pulse length of the three different lasers at nanosecond, picosecond and femtosecond pulses determines the beam/material interaction times. The nanosecond laser remains longer than the picosecond and femtosecond pulses. As a result, a thermal wave will propagate into deeper part of the sample target leading to melting a thicker layer into the target [353]. In addition nanoparticles are produced from condensation [354]. In the case of picosecond and femtosecond lasers the pulses do not have sufficient time on the target to propagate thermal wavers into the deeper part of the material, so the energy is localised on a specific area on the target surface and the energy does not transfer to wider neighbouring area. As a result, the laser ablation process with ultra-short laser pulses is a direct solid-vapour or solid-plasma transition. In other words, at ultra-short laser pulses and highest peak power the irradiated region on the target does not lose much energy as a result vaporisation and ionisation will produce before thermal diffusion heat transfer [355]. The picosecond pulses can heat the lattice materials leading to the production of vapour and plasma state as a result rapid expansion will produce in vacuum [353]. Furthermore, the picosecond laser effects on the metal surface are the ejection of electrons and mass. After the thermal boiling process emit droplets and particles, a plasma plume will be produced due to a cloud of ejected electrons, ions and particles from the surface of the material [354]. For the femtosecond laser pulses the fundamental physical processes such as energy deposition, melting, and ablation are attribute for excitation but for picosecond pulses regime melting process will be observed roughly [95]. Boulais et al. [346] showed that the electronic heating and plasma production occurs for femtosecond (fs) laser ablation. For the picosecond (ps) pulses, lattice heating, medium heating will occur. For nanosecond (ns) pulses shockwaves and cavitation bubbles may be generated. On the other hand, it was shown, macroscopically, that the nanosecond and picosecond laser interactions with materials demonstrate the similar results for both laser pulses regimes to the identical temperature distribution within the target materials, due to the short thermal diffusion length and long absorption length [340]. Different laser pulses have been used for the production and modification of nanoparticles because they have different effects. In other words, the 222

223 Chapter 8: A comparison of the characteristics of nanosecond, picosecond and femtosecond laser generated Ag, TiO2 and Au nanoparticles in deionised water produced nanoparticles in a solution undergo the second laser particles interaction during the laser path, this interaction is different with different laser pulse regimes. For ultrashort pulses such as fs laser the interaction is via interpulse absorption ( absorption from particles of laser pulses subsequent to the ones by which the particles were created ), although in the case of longer laser pulses, such as ps and ns lasers their interaction is via intrapulse absorption ( absorption from particles of part of the same laser pulse from which the particles were created ) [54, 87] Summary A comparison between nanosecond, picosecond and femtosecond laser ablation in deionised water for the production of Ag, Au and TiO 2 nanoparticles has been made. Although it is not a like-for-like comparison of the effects of wavelength and pulse width under exactly the same operating conditions such as repetition rates, focused beam size, limited by the operating range of the lasers used, the experimental work can be considered as a fare comparison between the three different types of commercial lasers operating at their usual operating conditions. The main findings are: the laser wavelength may have a stronger influence on the particles sizes than the pulse lengths, shorter wavelengths give smaller particles size. The nanosecond laser at a 532 nm wavelength produced smaller nanoparticles, although it is generally perceived that longer pulsed lasers produce larger nanoparticle sizes. A smaller beam spot size does not lead to the production of smaller nanoparticles. The nanoparticle production rate is relatively independent on laser types and pulse lengths. The size distribution of the Ag and TiO 2 nanoparticles can be estimated from their optical absorption spectra. The energy band gap of TiO 2 nanoparticles was decreased by increasing their sizes. In other words, the energy gap of smaller TiO 2 nanoparticles was higher in comparison with the larger nanoparticles. 223

224 9 Chapter 9. Comparison of characteristics of selected metallic and metal oxide nanoparticles produced by picosecond laser ablation at 532 nm and 1064 nm wavelengths Authors: Abubaker Hamad, Lin Li, Zhu Liu, Hong Liu, and Tao Wang Journal: Applied Physics A: Materials Science and processing Volume, issue and pages: 122, 904, 1-15 Status: published Note: The format of the paper is edited 224

225 Chapter 9: Comparison of characteristics of selected metallic and metal oxide... Comparison of characteristics of selected metallic and metal oxide nanoparticles produced by picosecond laser ablation at 532 nm and 1064 nm wavelengths Abstract Picosecond laser generation of nanoparticles was only recently reported. The effect of laser wavelength in picosecond laser generation of nanoparticles is not yet fully understood. This investigation reports new findings comparing the characteristics of Au, Ag, Ag-TiO 2, TiO 2, ZnO and iron oxide nanoparticles generated by picosecond laser ablation in deionised water at 532 nm and 1064 nm laser wavelengths. The laser ablation was carried out at a fixed pulse width of 10 ps, a repetition rate of 400 khz and a scan speed of 250 mm/s. The nanoparticles were characterised by UV Vis optical spectroscopy, transmission electron microscopy (TEM) and X-ray diffraction (XRD). The work shows that there is no noticeable difference in the size of the metal-oxide nanoparticles produced at 532 nm and 1064 nm, especially for the TiO 2 and ZnO nanoparticles; however, a considerable size difference can be seen for metallic (e.g. Au) and metallic compound (e.g. Ag-TiO 2 ) nanoparticles at the two wavelengths. It demonstrates that noble metals are more profoundly affected by laser wavelengths. The reasons behind these results are discussed. In addition, the work shows that there are different crystalline structures of the TiO 2 nanoparticles at 1064 nm and 532 nm wavelengths. Keywords: Nanoparticles, picosecond laser, size distribution, wavelength effect, metal oxide nanoparticles. 225

226 Chapter 9: Comparison of characteristics of selected metallic and metal oxide Introduction Laser ablation in deionised water has been used widely to produce metallic, bi-metallic and tri-metallic nanoparticles as well as metal oxides [356]. An advantage of laser production of nanoparticles is the high purity of the nanoparticles produced, special surface charge characteristics and easy preparation in comparison to chemical methods. It was found that various laser parameters such as laser power, energy density (fluence), beam spot size, scanning speed, and media under which ablation takes place and wavelength [141, 357], may be used to manipulate the size distribution and shape of the nanoparticles. Nanosecond and picosecond lasers with different wavelengths have been used to produce different types of nanoparticles, although very few studies used picosecond lasers. For example, Baladi and Mamoory [358] used a nanosecond laser to produce Al nanoparticles at 1064 nm and 533 nm wavelengths in ethanol. It was shown that finer spherical particles and a higher rate of production of the nanoparticles can be produced at 1064 nm than at 533 nm. In addition, it was shown that higher productivity and larger nanoparticles were produced at higher laser energy. Mortazavi et al. [359] produced smallersized Palladium (Pd) nanoparticles at a greater production rate with a nanosecond laser in deionised water at 1064 nm (IR-Nd:YAG laser) than at 193 nm (UV-ArF Excimer laser). It was also shown that the plasma temperature of the longer wavelength was considerably higher than that of the shorter wavelength because of the higher inverse Bremsstrahlung reaction rate to heat the induced plasma. In addition, the blue shift phenomena that occurred at the longer laser wavelength is evidence that the plasma produced at 1064 nm is more energetic than that produced at 193 nm. Furthermore, Kim et al. [360] studied the effects of using a nanosecond laser of different laser wavelengths and laser fluence on the production of palladium (Pd) nanoparticles in distilled water. It was found that the Pd nanoparticles produced at 532 and 355 nm wavelengths are more homogenous and smaller in size than those produced at 1064 nm. It was concluded that increasing laser fluence lead to an increase in the size of the nanoparticles at all wavelengths. 226

227 Chapter 9: Comparison of characteristics of selected metallic and metal oxide... Chewchinda et al. [19] prepared the characteristics of spherical Si nanoparticles with nanosecond laser ablation in ethanol (10 ns and 10 Hz) at 532 nm and 1064 nm laser wavelengths under the same energy density (1.2 J/cm 2 ). It was found that smaller nanoparticles and a narrower size distribution of Si nanoparticles were produced at 532 nm than those at 1064 nm; the average sizes were about 3 and 7 nm respectively. In addition, a higher concentration of nanoparticles was obtained at shorter laser wavelengths. Imam et al. [141] found that smaller Au nanoparticles and a higher rate of productivity were generated by nanosecond laser ablation in water at 532 nm than those produced at 1064 nm wavelength. He et al. [361] produced ZnO nanoparticles by nanosecond laser ablation in a liquid medium at a wavelength of 355 nm. Intartaglia et al. [362] produced Au-Ag nanoparticles by picosecond laser ablation in a liquid environment from an Ag target placed in a colloidal Au nanoparticles irradiated at different laser wavelengths. It was shown that the average size of the nanoparticles did not depend on the laser wavelength; their average sizes were 5 nm, 15 nm and 8.5 nm at 355 nm, 532 nm and 1064 nm laser wavelengths, respectively. The picosecond laser operated at a laser pulse width of 60 ps and a repetition rate of 20 Hz with maximum pulse energy of 115 mj at 1064 nm, 55 mj at 532 nm and 35 mj at 355 nm wavelengths. Giorgetti et al. [363] prepared nanoparticles by picosecond laser ablation of metallic (Au and Ag) and semiconductor (CdSe) target materials in water at fundamental (1064 nm - 25 ps), second harmonic (532 nm - 20 ps) and third harmonic (355 nm - 15 ps) laser wavelengths. Different sizes of Au nanoparticles were produced: 5.2 nm, 3.2 nm and 2.5 nm at 1064 nm, 532 nm and 355 nm wavelengths respectively. Hamad et al. [313] produced Ag-TiO 2 compound nanoparticles by picosecond laser ablation of an Ag/Ti alloy target in deionised water at a wavelength of 1064 nm [313]. Silver nanoparticles have been produced by a picosecond laser in a liquid environment at wavelengths of 1064 nm [310], 1030 nm and 515 nm [364]. Giorgetti et al. [363] produced Ag nanoparticles at 1064 nm and post-irradiated them at a wavelength of 532 nm, and Nouneh et al. [365] used the 532 nm wavelength of a picosecond laser to post-irradiate the Ag nanoparticles on an indium tin oxide (ITO) substrate in an attempt to manipulate their size and change their optical absorption properties. Schwenke et al. [364] produced Ag nanoparticles in polyurethane-doped 227

228 Chapter 9: Comparison of characteristics of selected metallic and metal oxide... tetrahydrofuran at 1030 nm and 515 nm wavelengths of a picosecond laser (7 ps) and Menéndez-Manjón et al. [366] produced Au, Ag and AuAg alloy nanoparticles using a picosecond-pulsed laser (7 ps) at a wavelength of 515 nm in liquid monomer. Liu et al. [367] produced FeO nanoparticles by a picosecond laser in poly(vinylpyrrolidone) (PVP) solution at a wavelength of 1064 nm. They showed that the stability and particle size of the colloidal nanoparticles can be controlled via the PVP concentration because of the repulsive interaction and capping effects of PVP solution. Despite the previous studies on laser wavelength effects in laser production of nanoparticles, the effect of laser wavelength in picosecond laser production of nanoparticles is not yet clear and the mechanisms involved are not clear. In this work, picosecond laser wavelengths of 532 nm and 1064 nm were used to produce Au, Ag, Ag-TiO 2, TiO 2, ZnO and iron oxide nanoparticles in deionised water. The nanoparticles were then compared in terms of their sizes and morphology Experimental Materials and Procedure Materials An Au target foil with dimensions of 65 mm 10 mm 0.2 mm with a purity of 99.99%, an Ag target plate with dimensions of 25 mm 25 mm 2 mm and a purity of 99.99%, a Ti/Ag alloy plate (3:1 at.%) with the dimensions 25 mm 25 mm 1 mm and Ag and Ti alloy components with purity levels of 99.95% and 99.7% respectively, a Ti target plate with the dimensions 25 mm 25 mm 1mm and a purity of %, a Fe target foil with the dimensions of 30 mm 30 mm 0.3 mm with a purity of 99.95%, and a Zn target plate with the dimensions of 25 mm 25 mm 2 mm with a purity of % were used to generate the Au, Ag, Ag-TiO 2, TiO 2, ZnO and iron oxide nanoparticles in deionised water. 228

229 Chapter 9: Comparison of characteristics of selected metallic and metal oxide Nanoparticle Production Procedure In this work, Au, Ag, Ag-TiO 2, TiO 2, ZnO and iron oxide nanoparticles were produced via a dual wavelength picosecond laser with wavelengths of 532 nm and 1064 nm (see Figure 9-1-a) respectively in distilled water. The liquid solution was in the stationary condition. The target materials were placed individually on a stainless steel holder in a 100 ml Pyrex glass vessel which contained ml of distilled water. The water level above the targets was fixed at around 2 mm in all experiments. The experimental set-up is shown in Figure 9-1-b. The ablation area was 5 mm 5 mm with a galvo scanning computer program, but on the surface of the samples it was approximately 3 mm 3 mm at the wavelength of 532 nm and 4.65 mm 4.65 mm at 1064 nm. (see Figure 9-1-c). Further details of the laser beam characteristics at both wavelengths are summarised in Table 9-1. Galvo scanners (SCANLAB - hurry SCAN20) were used to scan the laser beams over the target area in a raster pattern at both wavelengths. Table 9-1: Characteristics and process parameters of the picosecond laser at both wavelengths (532 nm and 1064 nm) Laser parameters Wavelengths 532 nm 1064 nm Average power (P) 16.7 W 27.5 W Frequency (f) 400 khz 400 khz Laser pulse duration ( ) 10 ps 10 ps Beam diameter at the laser 3 mm 3 mm output window Focal length 100 mm 163 mm Spot size (D) 32.5 m 106 m Scan speed (v) 250 mm/s 250 mm/s Laser pulse energy (E pulse ) 42 µj 69 µj Laser fluence (F laser ) 5 J/cm J/cm 2 229

230 Chapter 9: Comparison of characteristics of selected metallic and metal oxide... (a) Galvo scanner Mirrors 1064 nm 532 nm Mirrors Picosecond laser (b) (c) Laser beam Galvo scanner Laser beam Pyrex glass beaker Deionised water Nanoparticles Target material Ablation area at 532 nm Ablation area at 1064 nm Figure 9-1: Scheme of the picosecond laser with dual wavelength (a), experimental set-up for nanoparticle production (b) and ablation area on a sample at 532 nm and 1064 nm wavelengths (c) Material Characterisation and Sample Preparation Procedure The nanoparticles produced were characterised with a UV-Vis spectrometer (Analytic Jena, SPECORD 250, dual beam) in order to measure the absorption spectra of the colloidal nanoparticles. A transmission electron microscope (TEM) (Philips CM kv TEM (LaB6)) was used to examine the morphology and size distribution of the nanoparticles. X-ray diffraction (XRD) (BrukerD8-Discover, step size [ 2θ] = ) was used to investigate the crystalline structures of the nanoparticles. A copper micro-grid mesh (200 mesh) was used to prepare samples for the TEM analysis. The mesh was put on a glass slide on a hot plate. After depositing a drop of colloidal nanoparticles onto the mesh, the hot plate was turned on and the temperature was raised to 230

231 Chapter 9: Comparison of characteristics of selected metallic and metal oxide... about 45 C. Before drying the drop completely, the mesh was wiped across the drop to collect more nanoparticles and disperse the nanoparticles uniformly, and then the drop was allowed to dry completely. This process was repeated twice to deposit a sufficient amount of nanoparticles on the copper microgrid mesh. The X-ray diffraction samples were prepared on glass slides; first, the colloidal nanoparticles were centrifuged for about 15 min using a microcentrifuge machine, then they were dropped onto a glass slide and allowed to dry on a hot plate at about 40 C. This process was repeated several times to deposit a sufficient amount of nanoparticles on the slide Results Au Nanoparticles Figure 9-2-a shows the optical absorption spectra of Au nanoparticles produced at 532 nm and 1064 nm in deionised water. The surface plasmon resonance of the nanoparticles was observed at 520 nm and 530 nm at the laser wavelengths of 532 nm and 1064 nm respectively. For the Au nanoparticles produced at the 532 nm laser wavelength, the intensity of optical absorption spectrum of the Au nanoparticles was reduced sharply, while at 1064 nm it increased and then decreased slowly. Figure 9-2-b shows the histogram of the size distribution. It can be noted that the Au nanoparticles produced at 532 nm are smaller in size than those produced at 1064 nm, measuring 9 nm and 31 nm respectively, despite the fact that the laser fluence at 532 nm was much higher than that at 1064 nm showing more dominating effect of laser wavelength. Only about 4% of the Au NPs (generated at 532 nm) are larger than 20 nm. As shown in Figures 9-2-c and d, the Au nanoparticles generated at the shorter wavelength are more uniformly dispersed than those generated at the longer wavelength. The longer wavelength produced a chain of nanoparticles, with most of them agglomerated together. Meanwhile, the nanoparticles produced at the shorter wavelength can be seen individually. 231

232 Absorbance [A] Number of nanoparticles Chapter 9: Comparison of characteristics of selected metallic and metal oxide (a) 532 nm 1064 nm 250 (b) wavelength = 532 nm wavelength = 1064 nm nm Wavelengths No. of NPs Ave. Size 532 nm nm 1064 nm nm nm Wavelength (nm) Size of Au nanoparticles (nm) (c) (d) Figure 9-2: Optical absorption spectra (a), histogram of the size distribution (b) and TEM images (c and d) of the Au nanoparticles produced by a picosecond laser at 532 nm and 1064 nm wavelengths in deionised water Ag-TiO 2 Compound Nanoparticles As shown in Figure 9-3-a, in general, the optical absorption spectra of Ag-TiO 2 compound nanoparticles produced by picosecond laser ablation in deionised water at 532 nm and 1064 nm were similar, but the nanoparticles produced at the shorter wavelength had a slightly sharper plasmon peak (at 400 nm) than that produced at the longer wavelength. The surface plasmon resonance of the nanoparticles at 532 nm and 1064 nm started at 410 nm and 415 nm respectively, extending to about 500 nm in the visible range. The histogram of the size distribution in Figure 9-3-b shows that the 532 nm wavelength produced nanoparticles with a smaller average size (14 nm) in comparison with those produced at 1064 nm (21 nm). As shown in the TEM images (Figures 9-3-c and d), spherical Ag-TiO 2 nanoparticles were produced at both wavelengths. 232

233 Absorbance [A] Number of Nanoparticles Chapter 9: Comparison of characteristics of selected metallic and metal oxide (a) Wavelength = 532 nm Wavelength = 1064 nm 80 (b) Wavelength = 532 nm Wavelength = 1064 nm Wavelengths No. of NPs Ave. Size 532 nm nm 1064 nm nm nm 532 nm Wavelength (nm) Size of Ag-TiO 2 nanoparticles (nm) (c) (d) Figure 9-3: Optical absorption spectra (a), histogram of the size distribution (b) and TEM images (c and d) of the Ag-TiO 2 nanoparticles produced by a picosecond laser at 532 nm and 1064 nm wavelengths in deionised water TiO 2 Nanoparticles Figure 9-4-a shows the optical absorption spectra of TiO 2 nanoparticles produced at 532 nm and 1064 nm in deionised water. The difference between the optical absorption spectra of the colloidal TiO 2 nanoparticles generated at both wavelengths can be seen in the UV range. For nanoparticles produced at the 532 nm wavelength, the spectrum intensity dropped sharply at the UV wavelength earlier than those produce at 1064 nm wavelength. Both spectra have the same characteristics in the visible range. In spite of their different optical absorption spectra, the histogram of the size distribution (Figure 9-4-b) shows that there is a similarity between the size distributions of the TiO 2 nanoparticles generated at both laser wavelengths, in that the average size and size distribution are close to each other. The average sizes of the nanoparticles 233

234 Absorbance [A] Number of nanoparticles Chapter 9: Comparison of characteristics of selected metallic and metal oxide... produced at the 532 nm and 1064 nm were 34 nm and 32 nm, respectively. The TEM images (Figures 9-4-c and d) show that spherical TiO 2 nanoparticles were produced at both wavelengths (a) 1064 nm 532 nm 532 nm 1064 nm (b) wavelength = 532 nm wavelength = 1064 nm Wavelengths No. of NPs Ave. Size 532 nm nm 1064 nm nm Wavelength (nm) (c) Size of TiO 2 nanoparticles (nm) (d) Figure 9-4: Optical absorption spectra (a), histogram of the size distribution (b) and TEM images (c and d) of the TiO 2 nanoparticles produced by a picosecond laser at 532 nm and 1064 nm wavelengths in deionised water. Figure 9-5 shows the XRD pattern of the TiO 2 nanoparticles produced at different wavelengths. The nanoparticles produced at 532 nm wavelength are crystalline with mixed anatase, rutile and brookite phases, but the nanoparticles produced at 1064 nm wavelength are crystalline with mixed rutile and brookite. In general, the same peaks were not observed in both samples, thus the samples do not have the same crystalline phases. In other words, the change in laser wavelengths has a noticeable effect on the phase of the TiO 2 nanoparticles. The peak positions 2 = 37.5 and 63.3 are attributed to the anatase TiO 2 phases (004) and (204) respectively [ ]. The peak positions 2 = 234

235 Intensity (counts) Chapter 9: Comparison of characteristics of selected metallic and metal oxide... 36, 41, 54 and are attributed to the rutile TiO 2 phases (101), (111), (211) and (310) respectively [368, 370]. In addition, the peaks at 2 = 32.5 is indicative of the brookite phase of the TiO 2 nanoparticles [ ]. A strong rutile peak was observed for the 532 nm laser wavelength at 27.5 [123]. In addition, small peaks at 48 and 68.3 were observed at 532 nm which indicate the anatase phase [ ], and the peaks at and 70 represent the rutile phase [261, 373] of the TiO 2 nanoparticles R A : Anatase R : Rutile B : Brookite B B R A R R 900 R R A R R A R R A R Theta ( ) Figure 9-5: XRD images of TiO 2 nanoparticles produced by picosecond laser in deionised water with different wavelengths; (532 nm and 1064 nm) Iron Oxide Nanoparticles Figure 9-6-a shows the optical absorption spectra of iron oxide nanoparticles produced at 532 nm and 1064 nm in deionised water. Both sets of nanoparticles have almost exactly the same characteristics. The histogram of the size distribution (see Figure 9-6-b) shows that smaller iron oxide nanoparticles were produced at 1064 nm, measuring about 21 nm in average size in comparison with those produced at 532 nm (about 28 nm in average size). It can be observed that more than 50% of the measured nanoparticles were under 20 nm. The TEM images (Figure 9-6-c and d) show that spherical iron oxide nanoparticles were produced at both wavelengths. The nanoparticles are magnetic. 235

236 Absorbance [A] Number of nanoparticles Chapter 9: Comparison of characteristics of selected metallic and metal oxide (a) 532 nm 1064 nm 150 (b) wavelength = 532 nm wavelength = 1064 nm nm 100 Wavelengths No. of NPs Ave. Size 532 nm nm 1064 nm nm 532 nm Wavelength (nm) (c) Size of FeO nanoparticles (nm) (d) Figure 9-6: Optical absorption spectra (a), histogram of the size distribution (b) and TEM images (c and d) of the iron oxide nanoparticles produced by a picosecond laser at 532 nm and 1064 nm wavelengths in deionised water. Figure 9-7 shows the X-ray diffraction of the iron oxide nanoparticles produced by picosecond laser in deionised water with different wavelengths (532 nm and 1064 nm). The nanoparticles are crystalline with mixed Fe, FeO and Fe 3 O 4. The peak positions of 2 = 18.28, 30, 35.42, 43, 53.4, 56.92, 62.5 and are attributed to the anatase Fe 3 O 4 phases (111), (220), (311), (400), (422), (511), (440) and (533) respectively [ ]. The peak positions 2 = 61 (at the 532 nm wavelength), and 73.1 are attributed to the FeO phases (220), and (311) respectively [ ]. In addition, the peaks at 2 = 44.67, 65 and are indicative of the Fe phases (110), (200) and (211) respectively [379]. The diffraction peaks at planes (111), (220), (311), (400), (422), (511) and (440) are the characteristic peaks of Fe 3 O 4 crystals with a cubic spinel structure and corresponded to a cubic unit cell [375, 380], or the inverse-spinel structure (Fe 3 O 4 ) [381]. 236

237 < < Intensity (counts) < Chapter 9: Comparison of characteristics of selected metallic and metal oxide # * : Fe 3O 4 : FeO # : Fe 1600 * * * * * * * * # * # Theta ( ) Figure 9-7: XRD images of iron oxide nanoparticles produced by picosecond laser in deionised water with different wavelengths (532 nm and 1064 nm) ZnO Nanoparticles ZnO nanoparticles were produced by the picosecond laser at the two wavelengths noted above. It can be seen from Figure 8-a that their optical absorption spectra are similar, except in the range of 200 nm to 250 nm. The nanoparticles produced at 532 nm initially exhibited higher absorbance, which then decreased sharply. On the other hand, the intensity of absorption spectrum produced at 1064 nm was initially lower. The histogram of the size distribution in Figure 9-8-b shows that there is no significant effect of the wavelengths on the size of the nanoparticles. The TEM images (Figures 9-8-c and d) show that the ZnO nanoparticles have irregular shapes, but the majority are pin-shaped. 237

238 Absorbance [A] Number of nanoparticles Chapter 9: Comparison of characteristics of selected metallic and metal oxide (a) 532 nm 1064 nm (b) wavelength = 532 nm wavelength = 1064 nm Wavelengths No. of NPs Ave. Size 532 nm nm 1064 nm nm nm 532 nm Wavelength (nm) Size of ZnO nanoparticles (nm) (c) (d) Figure 9-8: Optical absorption spectra (a), histogram of the size distribution (b) and TEM images (c and d) of the ZnO nanoparticles produced by a picosecond laser at 532 nm and 1064 nm wavelengths in deionised water. Figure 9-9 shows the XRD of the ZnO nanoparticles produced by picosecond laser in deionised water with different wavelengths (532 nm and 1064 nm). A number of Bragg reflections with 2θ values of 31.76, 34.4, 36.25, 47.55, 56.59, 62.85, 66.37, 67.94, 69, 72.56, and are observed corresponding to (100), (002), (101), (102), (110), (103), (200), (112), (201), (004), (202) and (104) planes respectively, they showing a typical XRD pattern of ZnO nanoparticles. (ICPDS card No ) [382]. Almost all the peaks fully matching the ZnO hexagonal phase of JCPDF No [383]. Very similar values have also been reported by Arefi and Saeed [384], Dutta and Bichitra [385] and Prabhu et al. [383]. A peak at 60 was observed at both wavelengths; this might be due to the glass substrate. Here, both spectra had exactly the same characteristics, which indicates that the changing 238

239 Intensity (counts) Chapter 9: Comparison of characteristics of selected metallic and metal oxide... wavelengths have no effect on the crystalline structures of the ZnO nanoparticles Theta ( ) Figure 9-9: XRD images of ZnO nanoparticles produced by picosecond laser in deionised water with different wavelengths (532 nm and 1064 nm) Ag Nanoparticles An unexpected finding of this work is the inability to produce silver nanoparticles from a pure silver plate material at the 532 nm wavelength. As shown in Figure 9-10-a, when attempting the above experiment, the optical absorption spectra of the Ag nanoparticles did not show any peak. This indicates that no silver nanoparticles were present in the solution. Even when the ablation process was continued for about 30 minutes and carried out at different repetition rates, Ag nanoparticles were not produced and the colour of the solution (deionised water) did not change. In contrast, the optical absorption spectra of the Ag nanoparticles produced at 1064 nm wavelength showed a strong surface plasmon resonance peak at a wavelength of 405 nm and the colour of the colloidal nanoparticles was yellow. The average size of the Ag nanoparticles produced at the 1064 nm wavelength was 29 nm (see Figure 9-10-b). As shown in Figure 9-10-c, the TEM images of the Ag nanoparticles produced at 1064 nm are semispherical in shape, but some have pentagonal and hexagonal shapes. 239

240 Absorbance [A] Number of nanoparticles Chapter 9: Comparison of characteristics of selected metallic and metal oxide (a) 532 nm 1064 nm 100 (b) wavelength = 532 nm wavelength = 1064 nm Wavelengths No. of NPs Ave. Size 532 nm 1064 nm nm nm nm Wavelength (nm) Size of Ag nanoparticles (nm) (c) Figure 9-10: Optical absorption spectra (a), histogram of the size distribution (b) and TEM images (c) of the Ag nanoparticles produced by a picosecond laser at 1064 nm wavelength in deionised water Discussion Effects of Wavelengths on the Size of the Nanoparticles Au, Ag, Ag-TiO 2, TiO 2, ZnO and iron oxide nanoparticles were produced at both laser wavelengths 532 nm and 1064 nm, with the exception of Ag nanoparticles that could not be produced at the wavelength of 532 nm. The TEM images and the size distribution histogram show that significantly smaller Au and Ag-TiO 2 nanoparticles were produced at 532 nm, although slightly smaller TiO 2, ZnO and iron oxide nanoparticles were produced at 1064 nm. As shown in Table 9-2, there is no noticeable difference in the size of the metaloxide nanoparticles produced at 532 nm and 1064 nm, especially for the TiO 2 and ZnO nanoparticles; however, a considerable size difference can be seen for the Au and Ag-TiO 2 nanoparticles. It can therefore be ascertained that noble 240

241 Chapter 9: Comparison of characteristics of selected metallic and metal oxide... metals are more profoundly affected by laser wavelengths, as almost all of the small Ag-TiO 2 nanoparticles are Ag [308]. According to the photon energy equation E=hc/, the shorter the photon s wavelengths, the higher its energy. At a wavelength of 532 nm, green laser pulses have higher photon energy (2.33 ev) that reaches the target material in comparison with the 1064 nm wavelength, while the IR laser pulses of which have lower photon energy (1.16 ev) reaching the target material. In general, the 532 nm wavelength is more effective at producing smaller Au and Ag nanoparticles than the 1064 nm wavelength. This is because the fragmentation by 1064 nm photons only affects the larger nanoparticles, which have a high extinction coefficient in the near-infrared region. In contrast, the fragmentation produced at 532 nm leads to a reduction in the size of the nanoparticles in the colloidal solution because this wavelength (532 nm) is in the range of the surface plasmon peak position of the Au and Ag nanoparticles [357]. Au nanoparticles are particularly affected by the 532 nm wavelength, because its surface plasmon resonance is in the range of nm, very close to the 532 nm wavelength. It is worth mentioning that the Ag nanoparticles were produced with a Ti/Ag alloy material target, but no Ag nanoparticles were produced with pure Ag target material in deionised water at 532 nm wavelength. Even when the laser parameters such as power and frequency were changed, the Ag nanoparticles were not generated when using pure Ag target plate. The reason behind this is the changing physical properties of the Ag in Ti/Ag alloy material, because the grindability of some Ti/Ag alloy materials is superior to pure titanium, which leads to a decrease in the elongation caused by the precipitation of small amounts of fine intermetallic compounds [386]. The grindability may act as a driving force on the Ag material, causing it to be released from the target material, or the ablation of both Ag and Ti as an intermetallic compound such as AgTi 3, as it is a composition of the Ti/Ag alloy sample target [313]. Here the Ag-TiO 2 nanoparticles are also smaller because they have strong wide peak spectra, extending from 400 nm to 500 nm (see Figure 9-3-a). Ag nanoparticles were not produced at the 532 nm wavelength with the picosecond laser (10 ps). This may be a result of the strong absorption of silver due to their surface plasmon resonance, as Schwenke et al. [364] showed that the productivity of silver nanoparticles at a 515 nm wavelength of a picosecond 241

242 Chapter 9: Comparison of characteristics of selected metallic and metal oxide... laser decreases over the ablation process time, while at 1030 nm the productivity is constant. Another reason behind the failure to produce Ag nanoparticles at the 532 nm wavelength might be due to the saturable and reverse saturable absorption in silver nanoparticles at 532 nm using picosecond laser pulses; this phenomenon was observed by Gurudas et al. [387] in Ag nanodots prepared using the pulsed laser deposition method. Table 9-2: Average size of Au, Ag-TiO 2, Ag, TiO 2, Iron oxide and ZnO nanoparticles produced by picosecond laser at 532 nm and 1064 nm. As wavelength increased the NP Nanoparticles Average size at: size is decreased or increased by 532 nm 1064 nm Decreased Increased Au 9 nm 31 nm 22 nm Ag-TiO 2 14 nm 21 nm 7 nm Ag 29 nm TiO 2 34 nm 32 nm 2 nm Iron oxide 28 nm 21 nm 7 nm ZnO 30 nm 28 nm 2 nm Au nanoparticles are more affected by the wavelengths than the other types. A red shift can be seen in the optical absorption spectra of the Au nanoparticles as the nanoparticle production laser wavelength is increased from 532 nm to 1064 nm. This means that smaller Au nanoparticles are produced at the shorter wavelength. In addition, the shorter wavelength produced a narrower size distribution with separate nanoparticles in comparison with those produced at the longer wavelength which formed particle chains. In addition to the proximity of the 532 nm wavelength to the surface plasmon resonance peak of Au nanoparticles, another reason behind the smaller size of these nanoparticles at this wavelength might be that the shorter wavelength disperses over a smaller, thinner area on the target, which leads to the ablation of a smaller amount of materials; as a result, smaller nanoparticles are generated in the solution. In contrast, the longer wavelength disperses more widely in the target material, leading to the ablation of more materials, which in turn generates larger nanoparticles. Furthermore, the inter-particle separation can be seen clearly among the Au nanoparticles produced at 532 nm, whereas almost all of the Au nanoparticles produced at 1064 nm are stuck together, forming chains. Rong et al. [ ] showed that if the inter-particle 242

243 Chapter 9: Comparison of characteristics of selected metallic and metal oxide... separations ( ) are larger than the particle diameter (D) ( > D) the near-field interaction between the particles is small and the resonance wavelength λ res is that of an individual particle, but if the inter-particle separations are smaller than the particle diameter ( < D) the plasmons in the individual particles couple and the resonance wavelength λ res red-shifts with decreasing separation. They observed the red-shift phenomena while increasing the intensity ratio (R = I 580nm /I 530nm ). Here red-shift phenomena of the Au nanoparticles optical absorption spectra can be seen with the longer laser wavelength because the nanoparticles produced at the longer wavelength are joined together and there is no distance between them. Giorgetti et al. [363] attributed the different wavelengths to the production of different sizes of Au nanoparticles by picosecond laser ablation in water, as well as the different ablation mechanisms and suggested that at higher energy per pulse, and correspondingly higher fluence, material extraction by multiphoton absorption dominates over heating effects. Haustrup and O Connor [390] identified the existence of a linear relationship between the size of the Au nanoparticles and the size of the grain produced at both 343 nm and 515 nm wavelengths, whereas there is no relationship at 1030 nm wavelength ablation. This is due to the inter-band absorption effects at the shorter wavelengths, which results in faster electronphonon coupling and higher electron temperatures that process produces thermoelastic stresses in the target material. On the other hand, no inter-band absorption occurs at a wavelength of 1030 nm, thus the absorption occurs by intra-band absorption and the ablation process takes place over longer timescales, resulting in the removal of the grain boundaries prior to production of the nanoparticles [390]. A problem with comparing the effect of different wavelengths in producing nanoparticles in a liquid solution is the different beam absorption characteristices of the nanoparticles at different laser wavelengths. For example, Ag nanoparticles are more efficient for self-absorption at the 532 nm wavelength than at the 1064 nm wavelength because of the surface plasmon bands that form at around 400 nm [357]. In addition, the absorption coefficient is a function of the wavelength. Different laser wavelengths also have different responses to energy loss within the water above the target materials. As a result different size of the 243

244 Laser power loss % (at 1064 nm) Laser power loss % (at 532 nm) Chapter 9: Comparison of characteristics of selected metallic and metal oxide... nanoparticles would be produced, if the energy loss was not corrected. As shown in Figure 9-11, it can be seen that the water level has a strong effect on the laser power at the 1064 nm wavelength, while it has a negligible effect at the 532 nm wavelength. For example, at a 2 mm water level, about 5.5% and 0.004% of the laser power will be lost at 1064 nm and 532 nm wavelengths respectively. This is because water has a strong absorption coefficient at 1064 nm and a weak absorption coefficient at 532 nm. The relationship between laser power loss (expressed as a percentage) and water level is linear for both wavelengths nm 532 nm Water level (mm) Figure 9-11: Laser power loss at 532 nm and 1064 nm wavelengths due to water level above the sample during generation of nanoparticles. The laser beam focal length will be increased in water. As shown in Figure 9-12-a, the amount of increased focal length is directly proportional to the water height. In water the focal length will be increased when it passes through a liquid medium. This effect is increased during ablation under water at ultra-short wavelengths such as picosecond and femtosecond lasers due to refraction and self-focusing inside the liquid medium [ ]. 244

245 f (mm) Chapter 9: Comparison of characteristics of selected metallic and metal oxide Water level (mm) Laser beam Pyrex glass vessel Deionised water Water level above the target Target at a virtual position Target at a real position Figure 9-12: Increasing of focal length of the lens of a laser tool as a function of height above water level (a). Real and virtual depth of the target material (b). The focal length will increase according to the following equation [234]: (9-1) In the above equation is the amount by which the focal length increases in water, is the water level above the target material and n is the refractive index of the liquid medium. This fraction of focal length must be compensated while focusing the laser beam on the target material in the water; in other words, the distance between the lens and the target should be increased depending on the height of the liquid above the sample. As shown in Figure 9-12-a, the amount of increased focal length is directly proportional to the water height. This effect is the same at both 532 nm and 1064 nm because focal length does not depend on the wavelength. While focusing the laser beam on the target sample in water, the distance between the scan head and the target material should be increased (see Figure 9-12-b). For example, at a water level of 2 mm above the sample, the focal distance should be increased by about 0.5 mm and for 4 mm this distance should be 1 mm. It is worth mentioning that the focal length of the laser-focusing lens and the position of the target material have a significant effect on the size distribution and the shape of the nanoparticles produced [141]. Refractive index declines with the increase in wavelength, so a smaller refractive index means higher laser beam refraction in the media lead to change 245

246 Chapter 9: Comparison of characteristics of selected metallic and metal oxide... the ablation area (or marking area) in comparison with the program made marking area. This could explain why a smaller scanned area was produced on the target surface in comparison with the CNC program made. Here, at 532 nm, the laser produced smaller scan area than the 1064 nm wavelength. Here, 1064 nm produced larger scan area than the 532 nm wavelength because the longer wavelength gives smaller change in refractive index. Due to this reason longer laser wavelength leads to considerably higher ablation efficiency [391] Effects of Wavelengths on the Crystallinity of Metal Oxide Nanoparticles XRD patterns show that the iron oxide nanoparticles are crystalline with mixed Fe, FeO and Fe 3 O 4. FeO may be produced according to equation (9-2) and (9-3) [367]. The interaction between the laser beam and the Fe target material produce Fe clusters. Then they will interact with the liquid solution leading to the production of Fe(OH) 2 and hydrogen release. Due to the high temperature and high pressure at the target-liquid interface, FeO would be produced [367]. Because Fe exists in a wide range of oxidation states, redox reactions must be taken into account. The existence of the different types iron oxides dependent upon to different redox reactions [392]. As shown in equation (9-5), Fe 3 O 4 can be produced by controlled oxidation of Fe 2+ in solution: Zero-valent iron ( ) can be obtained according to equation (9-6) [393]: During laser interaction with the Zn target material in deionised water, ZnO nanoparticles could be produced according to the following equations [394]. 246

247 Chapter 9: Comparison of characteristics of selected metallic and metal oxide... After laser interaction with the Ti target, Ti cluster or Ti vapour will be produced, then they interacted with water to produce titanium (IV) hydroxide Ti(OH) 4. Finally, TiO 2 nanoparticles will be produced according to Equations (9-10) and (9-11) [395]. XRD characterisation shows that the crystallinity of titanium dioxide is strongly dependent upon the laser wavelengths, as the formation of the titanium dioxide phases (anatase, rutile and brookite) depend on temperature. Iron oxide nanoparticles are slightly dependent upon the laser wavelength, but the wavelength has no effect on crystalline ZnO nanoparticles. In other words, the oxide materials with more crystalline phases are affected by laser wavelength. XRD patterns show that the TiO 2 nanoparticles produced at a wavelength of 532 nm are more crystalline than those produced at 1064 nm. The wavelength of 532 nm can produce mixed anatase, rutile and brookite phases of TiO 2 nanoparticles, but the wavelength of 1064 nm can only produce rutile and brookite phases. A strong rutile peak was observed at optical absorption spectra of 532 nm at In addition, some small peaks of the anatase and rutile phases were observed at the 532 nm wavelength. These small peaks were produced when the sample was annealed at 500 C and 800 C for 2 h. According to the last case, the 532 nm laser wavelength may produce a higher temperature than the 1064 nm. Crystalline phases are easily produced within materials that have a low phase-transition temperature [396]. Phase transition of the TiO 2 is not only affected by wavelength, but also by the laser beam mode such as pulsed or continuous wave (CW). Singlephase rutile TiO 2 was obtained at C using a CW CO 2 laser at 1064 nm wavelength, while the rutile changed to anatase with increasing deposition temperature from 579 to 957 C using a Nd:YAG laser at 1064 nm [397]. 247

248 Chapter 9: Comparison of characteristics of selected metallic and metal oxide... The laser power and the type of target material are important in order to control the uniformity of the iron oxide phases. Furthermore, liquid environments have an effect on the crystallinity of the iron oxide phases; for example, in this work, crystalline maghemite (α-fe 2 O 3 ) nanoparticles were not produced because of the use of deionised water. Conversely, laser ablation of an iron oxide target material in acetone and ethanol produces crystalline maghemite ( - Fe 2 O 3 ) nanoparticles [398]. The crystallinity of the iron oxide nanoparticles increases with increasing laser energy [399]. However, for better understanding, further study of this topic is required Summary This study has investigated the effects of two laser wavelengths (532 nm and 1064 nm) of a picosecond laser on the size and size distribution of the Au, Ag, Ag-TiO 2, TiO 2, ZnO and iron oxide nanoparticles in deionised water. The results show that slightly smaller metal-oxide nanoparticles (TiO 2, ZnO and iron oxide) were produced at the 1064 nm wavelength. In spite of higher laser fluence at 532 nm than that at 1064 nm, significant reduction of particle size was seen for the production of pure Au and Ag-TiO 2 nanoparticles at the 532 nm wavelength showing more dominating effect of laser wavelength. Pure Ag nanoparticles could not be produced at the 532 nm laser wavelength of the picosecond laser, but Ag-TiO 2 nanoparticles were produced by using a Ti/Ag alloy as a target material. It was found that the water level above the target material does not have a significant effect on the loss of laser intensity at the 532 nm wavelength, but at 1064 nm wavelength the laser intensity was significantly reduced. It was also found that the formation of crystalline TiO 2 nanoparticles was strongly dependent upon the wavelength. 248

249 10 Chapter 10. A Single-step Process of Generating Hollow and Porous TiO2 Nanoparticles by Picosecond Laser Ablation in Deionised Water Authors: Abubaker Hamad, Lin Li, Zhu Liu, Hong Liu, and Tao Wang Journal: Journal of Laser Micro/Nanoengineering (JLMN) Volume, issue and pages: 11, 3, Status: published Note: The format of the paper is edited 249

250 Chapter 10: A single-step Process of Generation Hollow and Porous TiO 2 Nanoparticles... A Single-step Process of Generating Hollow and Porous TiO 2 Nanoparticles by Picosecond Laser Ablation in Deionised Water Abstract Over the last two decades, hollow nanoparticles have received considerable attention from researchers due to their specific properties that do not appear in other forms of nanoparticles. It has been recognised that TiO 2 hollow nanoparticles have superior photocatalytic properties than solid TiO 2 nanoparticles. Previous methods of producing hollow TiO 2 nanoparticles typically involve multiple-steps. In this paper, we report the production of hollow and porous TiO 2 nanoparticles in a single step via high-repetition rate picosecond laser ablation in deionised water. The absorption spectra of the colloidal nanoparticles were obtained by UV-VIS spectroscopy. The size distribution and morphology were characterised by transmission electron microscopy (TEM). The morphology and chemical composition of the nanoparticles were characterised using a High-Angle Annular Dark-Field Scanning Transmission Electron Microscope (HAADF-STEM) and Energy Dispersive X-ray Spectroscopy (EDS). In addition, the crystalline structures were investigated using X-ray diffraction (XRD). The results show that a higher ratio of crystalline hollow and porous TiO 2 nanoparticles ( nm in size with majority of hollow nanoparticles at 20 nm and an average size of 37 nm) of mixed anatase, rutile and brookite phases, was produced at lower laser energy; the yield was increased from 8% to 25% by reducing the laser power from 9.12 W to 3.35 W respectively. High laser powers result in reduced energy gap in the nanoparticles produced. The work shows that laser power can be used to control the yield of hollow nanoparticles. Keywords: hollow and porous nanoparticles; laser ablation; picosecond laser; TiO 2 nanoparticles, laser power. 250

251 Chapter 10: A single-step Process of Generation Hollow and Porous TiO 2 Nanoparticles Introduction Laser ablation in liquids has advantages including high material purity and being environmentally clean. In addition, using a suitable liquid (e.g. inorganic salts) or applying an electrical field, the size, shape and phase of the nanocrystals can be controlled [400]. Nanoparticles can be produced in an ambient condition without the need to provide high temperature (T) and pressures (P) [401]. Hollow nanoparticles have found a wide range of applications in the materials community and in cosmetics, coatings, composite materials, dyes, ink, artificial cells, microencapsulates for drug delivery [402], photodegradation of organic pollutants [403], cancer imaging and therapy [404]. Their popularity is due to their specific properties such as large specific areas [403], low density, good mechanical and thermal stabilities, and surface permeability [402]. Yang et al. [405] produced porous hollow TiO 2 nano-sphere particles using a facile hydrothermal method. It was concluded that the hollow nanoparticles show strong adsorption efficiency for organic dyestuff. Furthermore, they have optimal sensitivity to formaldehyde (HCHO) gas of the TiO 2 film sensor, enhanced at a comparatively low operating temperature (200 C). Pang et al. [406] produced spherical sub-micrometer sized hollow TiO 2 particles by self-assembling directly from commercial TiO 2 nanoparticles. The hollow particles show good visible light scattering match that significantly improve the photoconversion efficiency. Katagiri et al. [407] generated spherical hollow TiO 2 nanoparticles (with well-defined diameters) of the desired polymorphs via layer-by-layer formation of a water-soluble Ti complex on colloid complex and hydrothermal treatment. Tsai et al. [408] produced spherical hollow TiO 2 particles via a self-sacrificing template method. They concluded that the small hollow sphere particles have higher absorption ability than larger hollow particles because the smaller particles have thinner shells. The small hollow TiO 2 particles exhibited enhanced photocatalytic activity against photodegradation of methylene blue (MB). Hollow nanoparticles are of particular interest to materials science and catalytic applications. Chaudhuri and Paria, [403] prepared hollow TiO 2 nanoparticles during the production of sulphur-doped TiO 2 nanoparticles. It was shown that the hollow TiO 2 nanoparticles had greater photocatalytic activity 251

252 Chapter 10: A single-step Process of Generation Hollow and Porous TiO 2 Nanoparticles... under normal light against methylene blue in comparison with the standard Degussa P25 TiO 2 nanoparticle. This is because hollow TiO 2 nanoparticles have a lower energy band gap and a higher specific surface area, 2.5 ev and m 2 g -1 respectively, in comparison with solid TiO 2 nanoparticles (3.2 ev and m 2 g -1 respectively). Wang et al. [409] observed the production of hollow TiO 2 nanoparticles while generating polystyrene/titanium dioxide (PSt/TiO 2 ) core-shell composite nanoparticles in a mixed solution (ethanol and water) using the hydrolysis method. Hollow TiO 2 nanoparticles were produced after calcinations of the PSt/TiO 2 core-shell particles to remove or burn off the PSt core or using tetrahydrofuran (THF) to decay the core. The results show that the hollow TiO 2 nanoparticles produced after calcinations had the highest photocatalytic efficiency against degradation of Rhodamine B (RB). In addition to hollow TiO 2 nanoparticles, some other types of hollow nanoparticles have also been produced for use in different fields; Smovzh, [410] produced hollow Al 2 O 3 (γ-phase Al) nanoparticles via electric-arc sputtering of a composite electrode in helium gas followed by annealing in oxygen or oxidation. The nanoparticles produced were 6-12 nm in size, with a thickness of approximately 2-3 nm. Khanal et al. [402] produced hollow silica nanoparticles via a template of triblock copolymer micelle with a core-shell-corona architecture. The method can produce tuneable thickness of nanoparticles by changing the concentration of the inorganic precursors. Ming et al. [411] produced dense hollow porous Co 3 O 4, FeO x, NiO and MnO x metal oxide nanoparticles with a size of less than 100 nm by a hard-template method. These types of nanoparticles enhanced the active surface area; for example, Co 3 O 4 nanoparticles are useful for promoting electrochemical reactions in lithium-ion batteries, leading to greater productivity. A high discharge voltage and low charge potential were also observed (about 2.74 V and 4.0 V respectively). In addition, a long cycle ability (more than 100 cycles) at a delivered capacity of 2000 ma h g -1 was also noted. Niu et al. [412] produced metal oxide and sulphide hollow nanoparticles by laser ablation in a liquid environment. The hollow nanoparticles were produced by the following two steps based on the Kirkendall effect and IR laser vaporisation; In the first step, the Kirkendall effect was used to produce hollow nanoparticles with fast metal diffusivity in the metal oxide shell such as oxide/sulphide shell. In the second step, an IR laser was used to vaporise the metal core with a slow metal 252

253 Chapter 10: A single-step Process of Generation Hollow and Porous TiO 2 Nanoparticles... diffusivity to the shell of the nanoparticles. ZnS hollow nanoparticles enhanced gas sensors. It was also reported that the nanostructures, including heterostructure, compound nanospheres and spherical core-shell form can be enhanced via suitable selection of liquid environment and target material. Lee et al. [413] prepared double-shell SiO 2 /TiO 2 hollow nanoparticles by a chemical method. It was concluded that the double-shell hollow nanoparticles based on Electrorheological (ER) fluids showed promising enhanced ER qualities in comparison with the single-shell hollow nanoparticles based on ER fluids. ER performance is increased by decreasing the particle size due to increasing surface area. Yang et al. [414] produced nano-crystalline diamond, having a hexagonal lattice or cubic lattice, by high-power pulsed laser ablation of a graphite target in water. It demonstrated that the compounds that would require extreme conditions general, can be generated at normal temperature and pressure by using the pulsed-laser-induced reactive quenching (PLIRQ) method. Yang and Wang [415] produced carbon nitride nanocrystals by pulsed laser induced liquid solid interfacial reaction (PLIIR). They observed a cubic-c 3 N 4 phase in carbon nitride nanocrystals. From the above review, it is clear that the production of hollow and porous nanoparticles mainly involves multiple steps and the proportion of porous or hollow nanoparticles is not generally controllable. In this work, the authors demonstrate a controllable one-step synthesis of hollow TiO 2 nanoparticles via a picosecond laser ablation process in deionised water Experimental set-up Nanoparticle production A % pure titanium (Ti) plate with dimensions of 25 mm 25 mm 1 mm was used to synthesise TiO 2 nanoparticles by a picosecond laser in deionised water. A Ti plate was placed at the bottom of a glass vessel on a stainless steel substrate (see Figure 10-1). The water level above the target was approximately 2 mm. A 400 W Edgewave pulsed laser (picosecond laser: parameters are shown in Table 10-1) was used for the ablation process. The laser ablation process in deionised water was conducted continually for

254 Chapter 10: A single-step Process of Generation Hollow and Porous TiO 2 Nanoparticles... minutes. Because water has a significant absorption at a wavelength of 1064 nm, the effects of the water level on the laser beam energy and laser beam focal length were taken into account and recalculated. Table 10-1: Picosecond laser beam parameters used to produce TiO 2 nanoparticles in deionised water. Parameters Value Wavelength ( ) 1064 nm Frequency (f) 200 khz Pulse duration ( ) 10 ps Spot size (D) 125 µm Scan speed (v) 250 mm/s Laser pulse energy (E pulse ) 45.6 µj Laser fluence (F laser ) 0.37 J/cm 2 Figure 10-1: Experimental set-up to produce TiO 2 nanoparticles in deionised water via a picosecond laser; = 1064 nm, f = 200 khz, = 10 ps, and v = 250 mm/s Sample preparation To characterise the nanoparticles produced using transmission electron microscopy (TEM), a copper microgrid mesh (Formvar / carbon on 200 copper mesh) was used. About three drops of the colloidal nanoparticles were deposited on the copper mesh and then allowed to dry at room temperature. During drying, the samples were covered with a transparent lid to avoid contamination by airborne dust Characterisation A Transmission Electron Microscope (TEM) (JEOL 2000 FX AEM + EDX), a High-Angle Annular Dark-Field Microscope-Scanning Transmission Electron Microscope (HAADF-STEM) and Energy Dispersive X-ray Spectroscopy (EDS) (FEI Tecnai G 2 F30) were used to characterise the nanoparticles in terms of their size, size distribution and chemical compound elements. The aforementioned equipment and techniques were used to obtain 254

255 Number of Nanoparticles Chapter 10: A single-step Process of Generation Hollow and Porous TiO 2 Nanoparticles... images of the TiO 2 nanoparticles and their line elemental spectrum. A UV Vis optical spectrometer (Analytic Jena, SPECORD 250, dual-beam) was used to obtain the optical absorption spectra of the nanoparticles. In addition, X-ray diffraction (XRD) (BrukerD8-Discover, step size [ 2θ] = ) was used for the investigation of material crystalline structures Results and discussion Generation of hollow TiO 2 nanoparticles Figure 10-2-a and b shows the TEM images of TiO 2 nanoparticles produced by picosecond laser ablation in deionised water. Some hollow nanoparticles can be observed among the solid TiO 2 nanoparticles. They were produced in the different sizes with clear boundaries. Figure 10-2-c shows a histogram of the size distribution of the nanoparticles. Their sizes range from less than 10 nm to about 160 nm, with an average size of 37 nm. The ablation rate of the nanoparticles changed when the laser power was altered. In other words, the ablation rate was increased by increasing the laser beam power. (a) (b) 160 (c) 120 Number of NPs: 615 Average Size: 37 nm Size of TiO 2 Nanoparticles (nm) Figure 10-2: (a and b) TEM images of hollow TiO 2 nanoparticles produced by picosecond laser in deionised water. ( = 1064 nm, f= 200 khz, v = 250 mm/s). (c) Histogram of the size distribution of the TiO 2 nanoparticles measured from 615 TEM images of nanoparticles. As shown in Figure 10-3, a range of single holes, double holes and multiple-holes were observed in the nanoparticles. In addition, some very small holes were produced in some nanoparticles, causing the nanoparticles to appear porous. It can be noted in Figure 10-2 and Figure 10-3 that almost all of the holes are spherical with different sizes in diameter. 255

256 Chapter 10: A single-step Process of Generation Hollow and Porous TiO 2 Nanoparticles... Figure 10-3: TEM image of TiO 2 nanoparticles show more than one holes produced in the nanoparticles. Picosecond laser ablation of a solid material in water is commonly accompanied by a strong plasma plume, which cools and is then condensed [416]. Condensation of the plasma plume in water leads to the generation of Ti nanoparticles. When the concentration of the nanoparticles reaches a specific value, the highly active solid Ti nanoparticles oxidise in the liquid environment, leading to the formation of TiO 2 nanoparticles in the solution. The appearance of holes in the nanoparticles may be due to the production of gas bubbles in the liquid in the vicinity of the laser focal spot on the target after laser-target interaction. This phenomenon is explained by supposing vaporised plasma plume nucleation and a preferential condensation between gas bubbles and their surroundings (water) to minimise the surface tension [87, 417]. In addition to this, hollow TiO 2 nanoparticles are generated in deionised water due to the considerable increase of H 2 in TiO 2, exceeding its melting point, and the subsequent release of H 2 due to the solidification of the nanoparticles. The production of hollow nanoparticles was also observed in the generation of Ti 256

257 Chapter 10: A single-step Process of Generation Hollow and Porous TiO 2 Nanoparticles... nanoparticles in hydrogen-saturated and non-saturated ethanol due to the production of H 2 under pyrolysis of an ethanol solution [57]. The size and number of the holes in the nanoparticles depended on the amount of H in the solution, and the quantity of H depended on the permeability (P) according to Equation (1) [57]: P = DS (10-1) where D and S are the diffusion coefficient and the solubility of the hydrogen. The mechanism of the formation of porous hollow nanoparticles is different for different methods of preparation: for example, for a hydrothermal method, the production is due to the Ostwald ripening process. This happens by changing the morphology of the material products in the solution at the reaction stages [405]. For the Kirkendall and Diffusion processes, the mechanism is based on an idealised model and a normal steady-state diffusion governed by Fick s first law of diffusion. Briefly, this entails mass diffusion and the vacancies induced by atomic concentration differences [418]. An explanation of hollow and porous nanoparticles production by laser ablation in water is due to bubble formation [419] at water-target interface during laser-target interactions. Laser ablation in a confining liquid solution can lead to a dense plasma which induces the cavitation bubbles [420]. Some of the released materials or nanoparticles from the target surface to the solution would interact with a bubble or a number of bubbles. Then as a result of condensation due to the cooling in the water environment a hole or some holes (pores) are produced in the nanoparticles. Moreover, the bubble surface pinning may occur due to ejected clusters or particles [421] (See Figure 10-4), and the material detachment could be observed after collapse the cavitation bubbles in the solution [422]. In addition, holes could be produced due to the releasing of dissolved gas in molten nanoparticles while the cooling [234]. This would happen because the bubbles are produced in the water environment while the materials are ejected from the target material. In addition, during the production of nanoparticles; after shrinking of the cavitation bubbles, the material would be ejected, so the size distribution of the laser induced nanoparticles is strongly depended on the dynamics of the cavitation bubbles [422]. 257

258 Chapter 10: A single-step Process of Generation Hollow and Porous TiO 2 Nanoparticles... The porous nanoparticles might be produced due to burst of the bubbles forming high pressure jets. It is worth mentioning that hollow and porous nanoparticles are not produced in air during laser ablation [419]. During the production of nanoparticles; after shrinking of the cavitation bubbles the material would be ejected, so the size distribution of the laser induced nanoparticles is strongly depended on the dynamics of the cavitation bubbles [422]. The number of the hollow and porous structures in nanoparticles depended on the laser power because the size, lifetime and expansion speed of the cavitation bubble depended upon the laser beam parameters and nature of the liquid solution [420]. Laser-target interaction Bubble formation and ejection materials Condensation and hollow/porous formation Figure 10-4: Schematic diagram of hollow/porous TiO 2 nanoparticles formation. Figure 10-5 shows the HAADF-STEM and EDS images of the TiO 2 nanoparticles. The figures show that the amount of Ti and O material in the centre of the hole is less than those at the edges. This serves as evidence of the holes produced in the TiO 2 nanoparticles. The TEM images and linescanning profile show the holes in the TiO 2 nanoparticles. 258

259 Chapter 10: A single-step Process of Generation Hollow and Porous TiO 2 Nanoparticles... Figure 10-5: HAADF-STEM and EDS images of the TiO 2 nanoparticles showing the production of hollow TiO 2 nanoparticles. The TEM images show that the hollow or porous nanoparticles have lower material density in the middle in comparison with the edges. In addition, the holes appear lighter and the edges darker. The bright parts of the nanoparticles can be attributed to lower laser beam absorption due to a lack of the nanoparticles composite materials. As the hollow nanoparticles are lack of materials in comparison with solid nanoparticles, they are lighter than solid nanoparticles of the same size and shape. On the basis of this fact, hollow nanoparticles are expected to have more colloidal stability than non-hollow colloidal nanoparticles due to their lower density; as a result, gravitational force has a reduced effect on the hollow nanoparticles. 259

260 Number of Hollow NPs in 100 NPs Chapter 10: A single-step Process of Generation Hollow and Porous TiO 2 Nanoparticles Effect of laser power on the production of hollow/ porous TiO 2 nanoparticles Figure 10-6 shows TiO 2 nanoparticles produced via picosecond laser in deionised water with different laser powers: P = 3.35 W (a), P = 5.95 W (b) and P = 9.12 W (c). It can be noted that more hollow nanoparticles were generated at a low laser power (see Figure 10-6 a), in comparison with the nanoparticles produced at a higher laser power (see Figure 10-6-b and c). Figure 10-6-d shows the number of hollow or porous nanoparticles produced per 100 nanoparticles. The number of hollow nanoparticles was roughly counted. It can be noted that the lower laser powers produced more hollow nanoparticles than at higher laser powers. (a) (b) (c) (d) Laser Power (W) Figure 10-6: TEM images of TiO 2 nanoparticles produced by picosecond laser in deionised water with different laser powers; P = 3.35 W (a), P = 5.95 W (b) and P = 9.12 W (c). Figure (d) shows the approximate number of hollow TiO 2 nanoparticles in 100 TiO 2 nanoparticles produced at different laser powers by picosecond laser in deionised water. Laser power has a strong effect on the production of hollow nanoparticles. A lower laser power leads to a lower temperature on the target material compares with that at a higher laser power. This leads to increased 260