An easier method of preparation of mesoporous anatase TiO 2 nanoparticles via ultrasonic irradiation

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1 Journal of Experimental Nanoscience ISSN: (Print) (Online) Journal homepage: An easier method of preparation of mesoporous anatase TiO 2 nanoparticles via ultrasonic irradiation Manayil Valappil Swapna & Karickal R. Haridas To cite this article: Manayil Valappil Swapna & Karickal R. Haridas (2016) An easier method of preparation of mesoporous anatase TiO 2 nanoparticles via ultrasonic irradiation, Journal of Experimental Nanoscience, 11:7, , DOI: / To link to this article: Published online: 09 Oct Submit your article to this journal Article views: 520 View related articles View Crossmark data Full Terms & Conditions of access and use can be found at Download by: [ ] Date: 04 December 2017, At: 01:20

2 JOURNAL OF EXPERIMENTAL NANOSCIENCE, 2016 VOL. 11, NO. 7, An easier method of preparation of mesoporous anatase TiO 2 nanoparticles via ultrasonic irradiation Manayil Valappil Swapna and Karickal R. Haridas School of Chemical Sciences, Kannur University, Kannur, India ABSTRACT Mesoporous anatase TiO 2 nanopowder was synthesised by the sol gel method using ultrasonic irradiation. This method is simple and faster for the synthesis of phase pure mesoporous anatase TiO 2 nanopowder. The product is characterised by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron spectroscopy (TEM), thermo gravimetric analysis, Brunauer Emmett Teller (BET) surface area, UV visible diffuse reflectance spectroscopy and Fourier transform infrared spectroscopy. Analysis of XRD patterns, SEM and TEM image shows that the average particles size is of 19.9 nm and has an anatase structure. The mesoporous nature was determined by the BET method using the Barrett Joyner Halenda (BJH) model. 1. Introduction ARTICLE HISTORY Received 9 October 2014 Accepted 8 September 2015 KEYWORDS TiO 2 ; sol gel method; mesopore; anatase; ultrasonic irradiation Nanoscience and nanotechnology have grown at an enormous rate for the last three decades, and recent advances in nanostructured materials have opened up new opportunities for different applications in electronics, catalysis, energy, materials chemistry and biology. In the recent years, scaling optical and electronic properties of nanomaterials, which become strongly size-dependant-focused attention on the preparation of nanoparticle semiconductors.[1] Titanium dioxide (TiO 2 ) is one of the most attracted materials in nanoscience and nanotechnology because it has many interesting properties from fundamental and practical point of view.[2] TiO 2 is an n type semiconductor with a wide variety of applications. It is most widely used as a white pigment [3] because of its brightness and high refractive index. Nanocrystalline titania has been widely used in many applications, including photocatalysis, adsorbents, photonic crystals, solar cells and coatings for self-cleaning surfaces. This is due to its interesting properties, such as high chemical stability, wide bandgap, high photocatalytic activity, non-toxicity, low cost and environmental safety.[4 6] The major crystalline structures of TiO 2 are anatase (tetragonal), rutile (tetragonal) and brookite (orthorhombic). Rutile phase is stable at high temperature and has high refractive index and weatherability.[7,8] Anatase phase is the most active photocatalytically due to its wide bandgap, high specific surface area and low recombination rates of electron-hole pairs.[8,9] Brookite phase has larger cell volume and is the least dense of the three forms and is not used for the experimental investigations.[10] Many methods have been used for titania synthesis, such as sol gel technique,[8] hydrothermal method,[11] chemical vapour deposition,[12] direct oxidation and others.[13] Among them, the CONTACT Karickal R. Haridas krharidas2001@gmail.com 2015 Taylor & Francis

3 JOURNAL OF EXPERIMENTAL NANOSCIENCE 541 sol gel technique is the most used methods due to its possibility of deriving unique metastable structure at low reaction temperatures and excellent chemical homogeneity.[14] Mesoporous TiO 2 has better properties than normal TiO 2 because of its high specific surface area and uniform pore diameter. Mesoporous TiO 2 was synthesised by sol gel routes using alkyl phosphates as surfactant templates,[15] hydrothermal process using cetyltrimethylammonium bromide as a template agent,[16] the use of block copolymers,[17] the hydrolysis of TiOCl 2 aqueous solution at low temperature using octyl polyethylene oxide as the templating agent [18] and so on. In this paper, we report the synthesis of mesoporous TiO 2 by the sol gel method using titanium tetra isopropoxide as the precursor and acetic acid is used as a complexing as well as a capping agent. We have made an attempt to prepare tetragonal pure anatase phase mesoporous TiO 2 nanoparticles in an easier way by using ultrasound irradiation and successfully synthesised it. The utilisation of high intensity ultrasound offers a facile, versatile synthetic tool for nanostructured materials that are often unavailable by conventional methods.[19] Main advantages of ultrasonic irradiation over the conventional methods are shortening of the sol gel reaction time, uniform size distribution and improved phase purity. The present work focuses the synthesise of nanoporous anatase titania particles by the sol gel method and ultrasonic irradiation. The prepared sample characterised using techniques like X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron spectroscopy (TEM), Brunauer Emmett Teller (BET), thermo gravimetric analysis (TGA), UV visible diffuse reflectance spectroscopy and Fourier transform infrared spectroscopy (FT-IR). 2. Methods and materials 2.1. Materials Titanium tetra isopropoxide (Merck), isoproponol (Merck) and acetic acid (Merck) have been used in the synthesis of nanoporous titania particles. All the reagents used were of analytical grade and no further purification was done before use. 3. Experimental section Mesoporous TiO 2 nanopowders were prepared via the sol gel method using titanium tetra isopropoxide, isoprponol and acetic acid as the starting materials. The sol gel synthesised TiO 2 was obtained from titanium tetra isopropoxide; 0.01 mole of the compound was dissolved in 0.26 mole isoproponol by a 10-minute sonication. To the clear solution, mole acetic acid was added and the sonication continued for another 30 minutes. After the required time, the mixture was converted to a sol. It was kept static for 24 hours for the conversion of the sol into gel. The gel was then dried in oven 80 C overnight to evaporate the organic materials to the maximum extent. The dried crystals were calcined at 500 C for 4 hours. 4. Instrumentation Thermal decomposition evolution of the sample was investigated by thermogravimetric analyser TGA 4000, conducted in N 2 atmosphere at a heating rate of 20 C/min. XRD patterns for the phase analysis were obtained with a Rigaku Miniflex II diffractometer, using Cu Ka radiation. The morphology of the sample was characterised by SEM using JEOL 5600 SL microscope. The microstructure was observed by TEM by FEI tecnai-g2 30S transmission electron microscope. N 2 adsorption desorption measurements were done on a volumetric Micrometrics Tristar apparatus at liquid N 2 temperature, K. The sample was treated at 300 C before measurement. On an average, 33 points were taken for the sample and the average mass of the sample was 0.3 g. Pore size distributions were calculated using the BJH method and surface area were calculated from the adsorption isotherm by the BET method. The average pore size was calculated from the t-plot method. The

4 542 M. V. SWAPNA AND K. R. HARIDAS Figure 1. TGA graph of synthesised mesoporous anatase TiO 2 nanopowder. UV visible absorption diffuse reflectance spectra were measured on a Jasco V-550 spectrometer using BaSO 4 as reference. Infrared spectra were measured on a Shimadzu FT-IR S spectrometer as potassium bromide disc. 5. Results and discussion 5.1. Thermo gravimetric analysis (TGA) Figure 1 shows the TGA graph of the synthesised titania sample before calcination. The degradation occurred in two steps. The first weight loss is due to desorption of physisorbed water and alcohol on the external surface of the crystallites or occluded in the mesopores present between the crystallites aggregates. The initial weight loss started at 60 C and is attributed to the loss of alcohol and water from the surface and the mass loss is about 4.7%. The second degradation is from 300 C to 440 C, which results in the loss of organic residues. In this region, 6.36% mass reduction has been observed. Above 600 C, an essentially constant mass (73% sample) has been found indicating the thermal stability of the sample. Figure 2 shows the TGA graph of the synthesised mesoporous anatase TiO 2 nanopowder calcinated 500 C for 4 hours and the graph indicates the removal of organic residues from the material. Figure 2. TGA graph of the synthesised mesoporous anatase TiO 2 nanopowder after 500 C calcination.

5 JOURNAL OF EXPERIMENTAL NANOSCIENCE 543 Figure 3. X-ray diffraction pattern of the synthesised mesoporous anatase TiO 2 nanopwder X-ray diffraction (XRD) analysis Figure 3 shows the XRD patterns of calcined titania particles. Based on the XRD results, the crystallite size and phase percentage of anatase and rutile can be determined.[20,21] Peaks at 25,38,48, 54,55,62,69,71 and 75 (JCPDS ) were observed which confirms the formation of anatase phase of titania. The crystallite size (D) can be calculated by the Scherrer formula [22] as expressed in Equation (1), D D kλ b cosu ; (1) where D is the crystallite size, λ is the wavelength of X-ray radiation, k is a constant taken as 0.89, b is the line width at half maximum height of the peak and u is the diffraction angle. The crystallite size of the titania from the present sol gel study was found to be 19.9 nm Scanning electron microscopy (SEM) analysis SEM of the nanoporous TiO 2 was carried out to estimate the surface morphology. XRD and SEM together give the exact knowledge about the particle size and characteristics of the synthesised sample. Figure 4(a) and 4(b) show the SEM image of the synthesised sample under different magnification. The pure TiO 2 nanoparticles exhibited spherical morphology with clumped distributions is visible through the SEM analysis Transmission electron microscopy (TEM) analysis TEM images of sol gel-derived titania are shown in Figure 5. Figure 5(a) and 5(b) show the TEM image of the synthesised sample under different magnification. Nanoparticles are packed randomly and pore channels were formed from the particle packing. The particles are of spherical structure and the shadow region indicates that the nanoparticles have agglomerated BET analysis Specific surface area is a significant microstructural parameter of materials particles, which depends on the geometrical shape and porosity. The microstructural characteristics of the

6 544 M. V. SWAPNA AND K. R. HARIDAS Figure 4. (a, b) SEM image of the synthesised mesoporous anatase TiO 2 nanopowder under different magnification. nanoparticles are investigated with the N 2 adsorption desorption analysis. Figure 6 shows the N 2 adsorption desorption isotherm. It exhibits a type IV isotherm typical for mesoporous materials [23] with a hysteresis loop. This hysteresis is an intermediate between typical H1 and H2 type hysteresis loop in the relative pressure range (p/p 0 ) suggesting large uniform mesopores with a cage-like pore structure connected by windows with a small size.[24 26] The specific surface area is determined from the isotherms to be m 2 /g based on the BET model. The pore size distribution is shown in Figure 7. The size of most of the pores concentrate between 5 and 20 nm with an average pore size of 10.4 nm estimated with the BJH method.

7 JOURNAL OF EXPERIMENTAL NANOSCIENCE 545 Figure 5. (a, b) TEM image of the synthesised mesoporous anatase TiO 2 nanopowder under different magnification. The BET surface area and the pore volume were determined to be m 2 /g and cm 3 /g, respectively UV visible diffuse reflectance spectroscopy The optical properties of TiO 2 nanoparticles were investigated by the diffuse reflectance measurement. The absorbance and % reflectance spectra of the synthesised nano titania are shown in Figure 8(a) and 8(b). In order to estimate the bandgap energy of nano titania, we have used the Kubelka Munk function. The Kubelka Munk function F(R) can be calculated by the

8 546 M. V. SWAPNA AND K. R. HARIDAS Figure 6. Nitrogen adsorption isotherm pattern of the synthesised mesoporous anatase TiO 2 nanopowder. equation, F(R) D (1 R) 2 /2R where R is the diffuse reflectance. Figure 8(c) shows the plot of Kubelka Munk function F(R) against wavelength. The bandgap (Eg) of the prepared TiO 2 nanoparticles was estimated by extrapolation of the linear portion of (F(R) hn) 2 versus photon energy (hn) plots. The bandgap of mesoporous nano titania obtained, Figure 8(d), is 3.25 ev. The optical bandgap value determined (3.25 ev) was slightly higher than the ones usually found in the literature (around 3.2 ev).[27] Thisblueshiftmaybeduetosmallparticlesizeof the titania powder. Figure 7. The pore size distribution of the synthesised mesoporous anatase TiO 2 nanopowder.

9 JOURNAL OF EXPERIMENTAL NANOSCIENCE 547 (a) 1.0 (b) Absorbance (a.u.) (c) Wavelength (nm) 3 % Reflectance (d) Wavelength (nm) Bandgap= 3.25 ev F(R) Wavelength (nm) 5.7. Fourier transform infrared spectroscopy (FT-IR) FT-IR spectra of synthesised titania sample is shown in Figure 9. Peaks at 3460 cm 1 correspond to the stretching vibration of water. The peak at 1683 cm 1 corresponds to the stretching vibration of C=O bond. The peak at 1500 cm 1 corresponds to the O H in-plane bend vibration. The (F(R)hν) 2 (a.u.) Bandgap (ev) Figure 8. (a) Absorbance spectra of the synthesised mesoporous anatase TiO 2 nanopowder. (b) % Reflectance spectra of the synthesised mesoporous anatase TiO 2 nanopowder. (c) Kubelka Munk function against wavelength spectra of the synthesised mesoporous anatase TiO 2 nanopowder. (d) (F(R)hn) 2 versus hn plot of the synthesised mesoporous anatase TiO 2 nanopowder. Figure 9. FT-IR spectra of the synthesised mesoporous anatase TiO 2 nanopowder.

10 548 M. V. SWAPNA AND K. R. HARIDAS peak at 815 cm 1 corresponds to the O H out-of-plane bend. Peak observed at 426 cm 1 corresponds to the Ti O vibration.[28 31] 6. Conclusion The mesoporous titania nanoparticles were successfully synthesised using simple and cost-effective sol gel technique by ultrasonic irradiation. Ultrasonic irradiation reduced the sol gel reaction time. The TGA analysis confirms the thermal stability of titania nanoparticles. Nearly 73% remained after 600 C. XRD shows the formation of high purity anatase titania nanoparticles and the size is 19.9 nm. SEM analysis reveals the agglomeration of the titania nanoparticles. TEM analysis also confirms the nanosize of the titania. The synthesised titania shows an indirect bandgap of 3.25 ev for the anatase phase. Mesoporous structure of the synthesised titania nanoparticle was confirmed by the BET analysis and the TEM method. Due to the presence of mesopores, the synthesised titania can be used as a semiconductor in dye-sensitised solar cells. Acknowledgements The authors acknowledge the Council of Scientific and Industrial Research (CSIR), India, for the financial support. Disclosure statement No potential conflict of interest was reported by the authors. References [1] Tomkiiewicz M. Scaling properties in photocatalysis. Catal Today. 2000;58: [2] Castillo N, Olguin D, Conde-Gallardo A, et al. Structural and morphological properties of TiO 2 thin films prepared by spray pyrolysis. Rev Mex Fis. 2004; 50(4): [3] Pfaff G, Roynders P. Angle dependent optical effects deriving from submicron structures of films and pigments. Chem Rev. 1999;99: [4] Hafizah N, Sopyan I. Nanosized TiO 2 photocatalyst powder via sol gel method: effect of hydrolysis degree on powder properties. Int J Photoenergy doi: /2009/ [5] Benkacem T, Agoudjil N. Synthesis of mesoporous titania with surfactant and its characterization. Am J Appl Sci. 2008;5: [6] Lin X, Rong F, Ji X, et al. Carbon doped mesoporous TiO 2 film and its photocatalytic activity. Micropor Mesopor Mat. 2011;142: [7] Perego C, Revel R, Durupthy O, et al. Thermal stability on TiO 2 anatase: impact of nanoparticles morphology on kinetic phase transformation. Solid State Sci. 2010;12: [8] Mahshid S, Askari M, Ghamsari MS, et al. Mixed phase TiO 2 nanoparticle preparation using sol gel method. J Alloys Compd. 2009;478: [9] Fujishima A, Zhang X, Tryk DA. Photocatalysis and related surface phenomena. Surf Sci Rep. 2008;63: [10] Thompson TL, Yates JT Jr. Surface science studies of the photoactivation TiO 2 - new photochemical process. Chem Rev 2006;106: [11] Wang CC, Ying JY. Sol gel synthesis and hydrothermal processing of anatase and rutile titania nanocrystals. Chem Mater. 1999;11: [12] Liu H, Yang W, Ma Y, et al. Synthesis and characterization of titania prepared by using a photoassisted sol gel method. Langmuir. 2003;19: [13] Byun D, Jin Y, Kim B, et al. Photocatalytic TiO 2 deposition by chemical vapour deposition. J Hazard Mater. 2000;73: [14] zhang J, Xiao X, Nan J. Hydrothermal- hydrolysis synthesis and photocatalytic properties of nano-tio 2 with an adjustable crystalline structure. J Hazard Mater. 2010;176: [15] Antonelli DM, Ying JY. Synthesis of hexagonally packed mesoporous TiO 2 by a modified sol gel method. Angew Chem Int Ed. 1995;34: [16] Peng TY, Zhao D, Dai K, et al. Synthesis of titanium dioxide nanoparticles with mesoporous anatase wall and high photocatalytic activity. Phys Chem B. 2005;109:

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