Transmission electron microscopic studies on noble metal nanoparticles synthesized by pulsed laser ablation in liquid M.I. Mendivil 1, S. Shaji 1,2,3, G.A. Castillo 1 and B. Krishnan 1,2 1 Facultad de Ingeniería Mecánica y Eléctrica, Universidad Autónoma de Nuevo León, San Nicolás de los Garza, Nuevo León, Mexico, 66450. 2 CIIDIT- Universidad Autónoma de Nuevo León, Apodaca, Nuevo León, Mexico. 3 Corresponding author e-mail: sadasivan.shaji@uanl.edu.mx Nanoscience and Nanotechnology are tremendously gaining importance in the field of research and development that aims at producing, characterizing and understanding novel materials with sizes ranging from 1 nm to several microns. The developments in the innovation of material science and technology require a deep understanding of their physical and chemical properties which arise from confinement effects intimately depend on their morphological properties such as their shapes, sizes and spatial orientation. Transmission electron microscopy (TEM) is an indispensable characterization technique for exploring the morphological properties of nanomaterials. In the present chapter, we describe TEM studies on noble metal nanoparticles (Ag, Au and Pd) synthesized in our laboratory by pulsed laser ablation in liquid (PLAL) technique. PLAL is a versatile technique in the synthesis of a variety of nanomaterials with some advantages over chemical methods as higher purity of nanomaterials due to absence of reducing agents. Detailed analysis of the nanoparticles synthesized at various conditions (wavelength, energy and liquid medium) using high-resolution transmission electron microscopy, selected area electron diffraction, energy dispersive X-ray spectrometry and high-angle annular dark-field imaging (Z-contrast) are presented. Further, the identification of chemical states using X-ray photoelectron spectroscopy and the optical properties using UV-Visible absorption spectroscopy are also included. The results illustrated that the metal nanoparticles of Ag and Au obtained in colloidal solution in general presented a spherical morphology, well dispersed and stable for some months without the use of surfactants. Also, the analysis on the effect of in-situ and post irradiation of a continuous wave laser on the ablation process for Ag and Au nanoparticles is added in this chapter. Keywords: Pulsed laser ablation in liquid; metal nanoparticles; UV-Vis spectroscopy; TEM; HRTEM; SAED; XPS 1. Introduction Nanotechnology involves the characterization, fabrication and/or manipulation of structures, devices or materials that have at least one dimension (or contain components with at least one dimension) approximately 1 100 nm in length. When particle size is reduced to this threshold, the resulting material exhibits physical and chemical properties that are significantly different from the properties of macroscale materials composed of the same substance. Advanced microscopic tools facilitated more on investigation, characterization and studies of nanomaterials, nanostructures and nanodevices. The recent advances in this technology created an array of engineered nanomaterials (ENMs) that have unique physical properties like size, shape, crystallinity, surface charge etc. and chemical properties like surface coating, elemental composition, solubility, reaction kinetics etc. Enhanced surface activities of the nanoparticles (NPs) provide conjugation and interaction of them with different biomolecules. Nanoparticles of silver and gold can function as useful antimicrobial agents. Since the evolution of nanotechnology, nanomaterials have a long list of applications in improving human life and environment. Nanomaterials are essential building blocks of a vast range of scientific and technical applications in materials science, chemistry, biology and medicine. Some of these applications are in optoelectronic devices, nanophotonic devices, solar cells, surface enhanced Raman spectroscopy, sensors, inkjet printers, photo-induced thermal therapy, biochemical sensors, carriers of drug delivery, bio-sensing in vivo and in vitro, diabetic healing, cooling systems, antibacterial agents, in cancer treatments, in catalysis, imaging and sensing. These nanomaterials include metal nanoparticles, metal oxide nanoparticles, semiconductor nanoparticles, alloy nanoparticles, ceramic nanoparticles, magnetic nanoparticles etc. In applications of all these nanomaterials, the size dependent properties of nanomaterials, surface plasmon resonance of metal nanoparticles as well as their large surface area to volume are made use of. Transmission electron microscopy (TEM) offers a broad range of characterization techniques with high spatial and analytical resolution, coupled with a completely quantitative understanding. As applications of nanomaterials and related areas attain more importance, TEM is the central tool for complete characterization of nanoscale materials and devices. TEM is well suited in observing and characterizing nanoscale materials with specific dimensional limits in 1D, 2D, or 3D precisely because of these limits. 1.1 Noble metal nanoparticles by pulsed laser ablation in liquid. Noble metal nanoparticles have obtained a lot of attention from researchers because of their unique size and shape dependent optical properties. Nanoparticles of gold (Au), silver (Ag) and palladium (Pd) have various applications in 911
the fields of material science, optics, chemistry, biology and medicine. The surface plasmon resonance (SPR) absorption of these noble metal NPs were dependent on their composition, size and shape. They have tunable optical properties which are due to their size confinement effects. Among noble metal nanoparticles, silver and gold nanoparticles have considerable attentions due to their interesting physicochemical properties. These noble metal nanoparticles are very reactive and they can inhibit microbial respiration and metabolism and they cause physical damage. The biocompatibility of these nanoparticles can be improved by reducing their particle size that makes them an efficient and reliable tool in antibacterial activities. Once these NPs are obtained in smaller size, they have extremely large relative surface areas which increase their contact with bacteria or fungi, providing a significant improvement in their bactericidal and fungicidal effectiveness. Gold nanoparticles and nanoclusters have been used as materials for surface-enhanced Raman scattering, the building blocks of nanostructures, catalysts for CO oxidation, ultrasensitive bio- and medical sensors, cancer diagnostics and therapies. Palladium (Pd) is well known for its excellent catalytic properties. It is also highly resistant to wear and tarnish, resistance to chemical attack, excellent high temperature characteristics and stable electrical properties. One very important application of Pd nanoparticles is in their use as hydrogen storage materials in for instance fuel cells, batteries and supercapacitors for the future production of clean energy. Pulsed laser ablation of metal targets in liquid is a simple technique compared to other physical and chemical methods for fabrication of nanomaterials (1-5). In such experiments, the output of a pulsed laser (nano, pico or femto second) is focused on the surface of the target material immersed in the liquid. Local melting and evaporation of the target takes place and the adjacent liquid layer absorbs part of the heat energy and attains a higher temperature and pressure. The plasma plume formed at the point of ablation can expand and result in release of nanoparticles. The ablation can be non-reactive like in the case of noble metal nanoparticles and also reactive as in formation of oxide nanomaterials. The products obtained by laser ablation in liquid depend on many parameters of the input laser pulses as well as the nature of the surrounding media. The laser wavelength, pulse duration, fluence, beam waist, repetition rate and number of pulses incident per unit area are the laser parameters that affects the nanomaterial products of ablation. Pulsed laser ablation in liquid (PLAL) is widely used for the fabrication of several kinds of nanoparticles such as noble metals, alloys, oxides and semiconductors. Due to the high energy state of the ablation species, one of the main characteristics of PLAL is the production of well-defined crystalline nanoparticles through a single step process without any posterior thermal treatment. So, it is possible to produce pure nanoparticle colloidal solutions without the formation of by-products. We have successfully synthesized and characterized nanoparticles of metal, metal oxide and ceramic nanoparticles by PLAL technique (6-8). A transmission electron microscope (TEM) can be used in various modes of operations like imaging, diffraction and spectroscopic anlysis (9, 10). In our nanomaterial characterizations, we used bright field imaging mode, high resolution TEM, selected area electron diffraction (SAED), scanning transmission electron microscopy (STEM), high angle annular dark-field imaging (HAADF) and energy dispersive X-ray analysis (EDX). 1.2 Gold (Au) nanoparticles 1.2.1 Synthesis of gold (Au) nanoparticles by pulsed laser ablation in liquid Gold nanoparticles were synthesized by laser ablation of an ultra-pure gold plate (99.99% pure) in distilled water. The target was placed at the bottom of a glass vessel filled with 10 ml of distilled water. The Au plate was irradiated with a focused second harmonic (532 nm) output from a nanosecond pulsed Nd:YAG laser (SolarLS, LQ929, operated at 10 Hz, 10 nanosecond pulse width). A convex lens of 20 cm focal length was used as shown in the figure 1. The ablation time was 10 minutes. During the irradiation of the laser output, the solution was gradually turned to reddish brown (like the color of agua de jamaica ). To study the effect of in-situ irradiation of a continuous wave (CW) laser (457 nm / 532 nm), the glass vessel containing the target and liquid was irradiated using an expanded beam of 457 and 532 nm output from continuous laser having 4 watts of laser power before expansion. The experimental schematic is shown in figure 2. 912
Fig. 1 Exprimental set up for (from left) single 532 nm pulsed laser ablation, in-situ CW laser (457/532 nm) irradiation and post irradiation by CW laser. Fig. 2 Photograph of the colloidal Au nanoparticle solution obtained by (from left) single pulsed laser ablation, in-situ CW laser irradiation during ablation as well as CW laser post irradiated Au nanocolloids. 1.2.2 Characterization using Transmission Electron Microscopy (TEM) Au nanoparticles synthesized by PLAL were analyzed using TEM to characterize their size, morphology, structure (SAED) and elemental composition (EDX). Drops from each of these Au nanocolloids were taken on carbon coated copper grids and dried at ambient conditions for this analysis. TEM analysis was carried using a FEI Titan 200 TEM. Micrographs were collected from different zones in the grid containing Au nanoparticles. The micrographs corresponding to gold nanoparticles obtained in single 532 nm ablation, in-situ irradiation using 457 nm continuous wave laser during ablation and post irradiated by 457 nm CW laser Au nanocolloids are shown in figure 3. This analysis was carried out using the bright field imaging mode. The size distribution histograms for each of micrographs are also included. These gold nanoparticles presented spherical morphologies, well dispersed with average sizes of 8.92 ± 2 nm, 9.51± 1.8nm and 9.02±1.43 nm respectively. Fig. 3 TEM images for Au nanoparticles synthesized by pulsed laser ablation in distilled water and their size histograms. From left: ablation using single pulsed 532 nm only, pulsed 532 nm with in-situ CW 457 nm irradiation during ablation and post irradiated Au nanocolloids (synthesized by single pulsed 532 nm ablation) using CW 457 nm. 913
Figure 4 shows the TEM images of Au nanoparticles obtained in single 532 nm ablation, in-situ irradiation using 532 nm continuous wave laser during ablation and post irradiated Au nanocolloids. Well dispersed spherical gold nanoparticles are obtained through PLAL. The size distribution histograms for gold nanoparticles in each case are also included. The average sizes of the gold nanoparticles were 8.92 ± 2 nm, 8.48± 1.68 nm and 9. 2±1.57 nm respectively. HRTEM images and SAED patterns for Au NPs are shown in figure 5. HRTEM images showed that these Au NPs were crystalline and the crystal planes corresponding to [200] and [111] orientations were identified. SAED analysis confirmed the cubic crystalline structure with planes corresponding to (111), (200), (220) and (311) for crystalline gold. They were identified using the standard data PDF. No. 04-0784 (JCPDS) for cubic structure of gold. Fig. 4 TEM images for Au nanoparticles synthesized by pulsed laser ablation in distilled water and their size histograms. (from left) Ablation using single pulsed 532 nm only, pulsed 532 nm with in-situ CW 532 nm irradiation during ablation and post irradiated Au nanocolloids (synthesized by single pulsed 532 nm ablation) using CW 532 nm. Fig. 5 HRTEM images for Au nanoparticles synthesized by pulsed laser ablation in distilled water (from left): ablation using single pulsed 532 nm only, pulsed 532 nm with in-situ CW 532 nm irradiation during ablation and post irradiated Au nanocolloids (synthesized by single pulsed 532 nm ablation) using CW 532 nm. At the bottom, SAED (selected area electron diffraction) pattern for the Au nanoparticle. 1.2.3 Optical Properties Gold NP colloids were analyzed using UV-Visible absorption spectra to study their optical properties. Absorption spectra of the gold nanocolloids are shown in figure 6. All these nanocolloids showed good absorption in the visible region with their surface plasmon resonance (SPR) around 519 nm which is in agreement with the SPR absorption for gold nanoparticles synthesized by other methods also. These Au nanocolloids were analyzed again for their SPR absorption after one month and still with prominent SPR absorption peaks were detected. This time the SPR absorption was noted at 526 nm with an increase in the peak width. It refers to some agglomeration of Au NPs in solution that 914
caused this absorption peak red shift with a higher full width at half maximum (FWHM). It is interesting to note that the Au NPs synthesized by in-situ irradiation of continuous wave 532 nm shows better absorption and stability. Fig. 6 UV-Visible absorption spectra for Au nanocolloids prepared by (left) single pulsed laser ablation and in-situ CW laser irradiation during ablation as well as CW laser post irradiated Au nanocolloids and (right) the absorption spectra after one month. 1.2.4 X-ray photoelectron spectroscopy (XPS) characterization Drops of gold nanocolloids were taken on conductive copper tape and dried well at ambient conditions. This sample is subjected to X-ray photoelectron spectroscopy (XPS) analysis in a Thermo Scientific K-Alpha XPS instrument. The samples were excited by a monochromatized Al K α X-ray radiation of energy 1486.6 ev. All the spectral peaks were recorded with reference to C 1s peak (284.6 ev). The high resolution photoelectron spectrum for Au 4f is shown in figure 7. The XPS analysis confirmed that the gold nanoparticles obtained by PLAL were in their elemental state. Fig. 7 High resolution XPS analysis of Au 1.3 Silver (Ag) nanoparticles 1.3.1 Synthesis of silver (Ag) nanoparticles by pulsed laser ablation in liquid Silver nanoparticles were synthesized by pulsed laser ablation of an ultra-pure silver target (99.99% pure) in distilled water. The target was kept at the bottom of a glass vessel filled with 10 ml of distilled water. The ablation experiment was as described in section 1.2.1. During the laser irradiation, the solution was gradually turned to yellowish (figure 8). To study the effect of in-situ irradiation of a continuous laser (457 nm / 532 nm), the glass vessel containing the target and liquid was irradiated using an expanded beam of 457 and 532 nm output from continuous laser having 4 watts of laser power before expansion as in the case of gold nanoparticles. Then, the silver colloidal nanoparticle solution prepared by single pulsed laser ablation (532 nm) was post irradiated in the same configuration to analyze its effect on the nanoparticles in colloidal solution. Fig. 8 Photograph of the colloidal Ag nanoparticle solution obtained by (from left) single pulsed laser ablation and in-situ CW laser irradiation during ablation as well as CW laser post irradiated Ag nanocolloids. 915
Microscopy: advances in scientific research and education (A. Méndez-Vilas, Ed.) 1.3.2 Characterization using Transmission Electron Microscopy (TEM) Samples of Ag nanocolloids were dried on carbon coated copper grids for TEM analysis. TEM micrographs for Ag nanoparticles synthesized by single 532 nm pulsed laser ablation were spherical in size as shown in figure 9, an average size of 4.54±0.81 nm with a cubic crystalline structure as identified by SAED analysis (shown in the inset). During the in-situ irradiation of 457 nm, the Ag nanoparticles obtained were larger (8.03± 2.63 nm) while post irradiation by 457 nm, smaller (3.72 ±1.08 nm) silver NPs were detected. TEM micrographs of silver nanoparticles, their size histograms and SAED patterns corresponding to single pulsed laser ablation, in-situ and post irradiation using 532 nm CW laser are shown in figure 10. While the in-situ 532 nm irradiation, the average size for silver nanoparticles obtained were 2.1±0.99 nm and the post-irradiation resulted in 2.24±0.99 nm. Due to irradiation using 532 nm, smaller silver nanoparticles were obtained. All these were crystalline with a cubic structure (PDF No. 04-0783) as resulted in SAED analysis. Fig. 9 TEM images for Ag nanoparticles synthesized by pulsed laser ablation in distilled water and their size histograms. From left: ablation using single pulsed 532 nm only, pulsed 532 nm with in-situ CW 457 nm irradiation during ablation and post irradiated Au nanocolloids (synthesized by single pulsed 532 nm ablation) using CW 457 nm. SAED pattern for each of them in shown in the inset. Fig. 10 TEM images for Ag nanoparticles synthesized by pulsed laser ablation in distilled water and their size histograms. From left: ablation using single pulsed 532 nm only, pulsed 532 nm with in-situ CW 532 nm irradiation during ablation and post irradiated Au nanocolloids (synthesized by single pulsed 532 nm ablation) using CW 532 nm. SAED pattern for each of them in shown in the inset. 916
Microscopy: advances in scientific research and education (A. Méndez-Vilas, Ed.) HRTEM images (figure 11) show that Ag NPs are crystalline and planes with interplanar distances of 2.03 Å and 2.33 Å are identified. An EDX analysis in the STEM mode as shown in figure 12 confirms that these nanoparticles are of silver. Fig. 11 HRTEM of silver nanoparticle obtained by PLAL in distilled water. Fig. 12 STEM image of silver nanoparticle obtained by PLAL in distilled water and EDX elemental analysis. 1.3.3 Optical Properties Optical properties silver nanoparticle colloids were analyzed using UV-Visible absorption spectra. All these nanocolloids showed their surface plasmon resonance (SPR) around 400 nm as shown in figure 13 which is in agreement with the SPR absorption for silver nanoparticles synthesized by other methods. To study their stability in colloids, these Ag nanocolloids were analyzed again for their SPR absorption after three months and still prominent SPR absorption peaks were detected. This time the SPR absorption was noted at 410 nm with an increase in the peak width. There was some agglomeration of Ag NPs in solution that caused this absorption peak red shift with an increased full width at half maximum (FWHM). In this case, Ag NPs synthesized by single pulsed 532 nm presented better absorption and stability. Fig. 13 UV-Visible absorption spectra for Ag nanocolloids prepared by (left) single pulsed laser ablation and in-situ CW laser irradiation during ablation as well as CW laser post irradiated Ag nanocolloids and (right) the absorption spectra after three months. 917
1.3.4 X-ray photoelectron spectroscopy (XPS) characterization Drops of silver nanocolloids were taken on conductive copper tape and dried well at ambient conditions and subjected to X-ray photoelectron spectroscopy (XPS) analysis. All the spectral peaks were recorded with reference to C 1s peak (284.6 ev). The high resolution photoelectron spectrum for Ag 3d is shown in figure 14. The peak binding energy values (B.E) confirm that the silver nanoparticles are in their elemental state. Fig. 14 High resolution XPS analysis of Ag 3d spectrum. 1.4 Palladium (Pd) nanoparticles 1.4.1 Synthesis of palladium nanoparticles by pulsed laser ablation in liquid Nanoparticles of palladium were successfully obtained by pulsed laser ablation using 1064 nm output from the Nd:YAG laser. An ultra-pure (99.99%) palladium target was kept in distilled water and ablation experiments were done in a vertical configuration for 10 minutes using three different laser fluencies. The colloidal solutions obtained were stable (figure 15). Fig. 15 Photograph of the colloidal Pd nanoparticle solution obained by pulsed laser ablation using 1064 nm at three energy fluences in distilled water (from left) 46 J/cm 2, 17 J/cm 2 and 9 J/cm 2. 1.4.2 Characterization using Transmission Electron Microscopy (TEM) TEM micrographs of palladium nanoparticles and their corresponding size histograms are included in figure 16. Well dispersed spherical palladium nanoparticles were obtained at the three laser fluencies used. The average sizes of these nanoparticles were in 18 29 nm range. Fig. 16 TEM images for Pd nanoparticles synthesized by pulsed laser ablation in distilled water and their size histograms for three different fluencies 918
HRTEM and SAED analysis (figure 17) shows that these Pd nanoparticles are crystalline in agreement with PDF. No. 46-1043 for palladium. An EDX analysis of Pd NPs in STEM mode is shown in figure 18 that confirmed the NPs were of palladium. Fig. 17 SAED and HRTEM images for Pd nanoparticles synthesized by pulsed laser ablation in distilled water for three different fluencies. The planes were identified using PDF no. 46-1043 for metallic palladium. Fig. 18 STEM image and EDX analysis for Pd nanoparticles synthesized by pulsed laser ablation. 1.4.3 Optical Properties Optical properties of the palladium nanoparticles obtained in distilled water and methanol were analyzed by UV-Visible absorption spectra. The UV-Vis absorption spectra for palladium nanocolloids are shown in figure 19, displaying a surface plasmon resonance absorption around 200 nm. Fig. 19 UV-Visible absorption spectra of colloidal Pd nanoparticle solution obained by pulsed laser ablation using 1064 nm at three energy fluences in distilled water 46 J/cm 2, 17 J/cm 2 and 9 J/cm 2. 919
1.4.4 X-ray photoelectron spectroscopy (XPS) characterization The XPS high resolution spectra for palladium are shown in figure 20. The peak binding energy values confirmed the chemical state of the palladium nanoparticle as elemental Pd. Fig. 20 High resolution XPS analysis of Pd 3d spectrum for palladium nanoparticles obtained by pulsed laser ablation of palladium in distilled water at three fluencies (from left : 9, 17 and 46 J/cm 2 ) Acknowledgements The authors are thankful to SEP-CONACYT-Mexico (Project No. 106955), PROMEP-Mexico and PAICYT 2013-UANL, Mexico for the financial assistance through project funding. M.I. Mendivil is grateful to CONACYT-Mexico for providing a doctoral research fellowship. Special thanks to Dr. Domingo Garcia and Dr. Alejandro Torres (CIIDIT-FIME) for their training and support in using TEM. References [1] Yan Z, Chrisey DB. Pulsed laser ablation in liquid for micro-/nanostructure generation. Journal of Photochemistry and Photobiology C: Photochemistry Reviews. 2012;13(3):204-23. [2] Yang GW. Laser ablation in liquids: Applications in the synthesis of nanocrystals. Progress in Materials Science. 2007;52(4):648-98. [3] Zeng H, Du X-W, Singh SC, Kulinich SA, Yang S, He J, et al. Nanomaterials via Laser Ablation/Irradiation in Liquid: A Review. Advanced Functional Materials. 2012;22(7):1333-53. [4] Rao SV, Podagatlapalli GK, Hamad S. Ultrafast Laser Ablation in Liquids for Nanomaterials and Applications. Journal of Nanoscience and Nanotechnology. 2014;14(2):1364-88. [5] Laser Ablation in Liquids: Principles and Applications in the Preparation of Nanomaterials. In: Yang G, editor. Laser Ablation in Liquids. Singapore: Pan Stanford Publishing; 2012. p. i-xxiv. [6] Mendivil MI, Krishnan B, Sanchez FA, Martinez S, Aguilar-Martinez JA, Castillo GA, et al. Synthesis of silver nanoparticles and antimony oxide nanocrystals by pulsed laser ablation in liquid media. Applied Physics A. 2012;110(4):809-16. [7] Garza D, Grisel García G, Mendivil Palma MI, Avellaneda D, Castillo GA, Roy TK, et al. Nanoparticles of antimony sulfide by pulsed laser ablation in liquid media. Journal of Materials Science. 2013;48(18):6445-53. [8] Castillo Rodriguez GA, Guillen GG, Mendivil Palma MI, Das Roy TK, Guzman Hernandez AM, Krishnan B, et al. Synthesis and Characterization of Hercynite Nanoparticles by Pulsed Laser Ablation in Liquid Technique. International Journal of Applied Ceramic Technology. 2014:n/a-n/a. [9] Yao N, Wang, Zhong Lin. Handbook of Microscopy for Nanotechnology: Springer US; 2005. [10] Williams DB, Carter CB. Transmission Electron Microscopy: A Textbook for Materials Science: Spinger; 2009. 920