Characterization of nanocrystalline Ag/SiO 2 nanocomposites and synthesis by wet chemical method

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1 Indian Journal of Pure & Applied Physics Vol. 48, April 2010, pp Characterization of nanocrystalline Ag/SiO 2 nanocomposites and synthesis by wet chemical method Surender Duhan 1*, Sunita Devi 2,3 & M Srivastava 3 1 *Department of Physics, Govt S S S Sanghi (Rohtak), Haryana 3 Department of Chemistry, Lajpat Rai College, Sahibabad (Ghaziabad), Uttar Pradesh * surender6561@yahoo.co.in Received 22 June 2009; revised 17 November 2009; accepted 11 January 2010 The silver/silica nanocomposites have been successfully prepared by the sol-gel process. After drying in air at 70 C for 30 min, samples are heat treated in air at 150, 300 and 500 C. Characterizations have been made by ultraviolet-visible (UV-VIS-NIR), X-ray diffraction (XRD), Differential scanning calorimetry (DSC), Fourier transform infrared spectroscopy (FTIR) and Scanning electron microscopy (SEM). Mechanisms of silver clusters formation in the densified silica matrix with respect to thermal treatment have been studied. Keywords: Silver nitrate, Doping effect, Sol-gel, Nanocomposites 1 Introduction Preparation and processing of nano-composites have been studied by many researchers 1-5. Among various methods, solgel has been largely used for the preparation of inorganic oxide, due to its versatility and low cost. The sol-gel technique 6-8 is a promising low-temperature route that provides good homogeneity for the preparation of metallic nanoparticles in thin coatings in a thermally stable dielectric solid oxide matrix such as SiO 2. Concentration of metals in these thin coatings can be controlled with ease. Another salient feature of this method over others like impregnation and inert gas evaporation is the chemical purity of resulting nano-composite samples. Metallic nano-particles and matrix develop simultaneously in this process that lead to uniform particle size distribution. Particles agglomeration is inhibited in the network matrix. This method provides an ability to prepare nano-composite materials with interesting optical properties Annealing of metal-composites is equally important since it is responsible for the formation of metallic nano-particles. Particle size, their distribution and number density are dependent on annealing temperature and time in oxidative and/or reductive environments. Growth mechanism of the particles takes place in second step of the synthesis. Optimization of these crucial factors is always an issue of research and discussion 15. The focus of the present work is to develop transparent and chemically durable Ag nano-composite thin films on glass substrate by sol-gel method. These metal-silica gel coated samples were oxidized in air and subsequently reduced in pure hydrogen environment at three temperatures. Influence of annealing temperature on the size, distribution, morphology and number density of Ag nano-particles was studied. In the present study, the structure of silver doped silica gel is prepared by sol-gel method and characterized the samples after heat treatment. It is found that the amorphous structure allows silver to diffuse more freely and therefore to form larger silver particles. 2 Experimental Details Silica gel containing silver (Ag) were prepared from tetraethyl orthosilicate (TEOS), silver nitrate (AgNO 3 ), water (H 2 O), nitric acid (HNO 3 ), and ethanol (C 2 H 5 OH). Molar ratio of Si(OC 2 H 5 ) 4 :AgNO 3 : H 2 O:NHO 3 :C 2 H 5 OH was 1:0.14:14:0.3:8. About 70% of the total water was used for hydrolysis and condensation reaction and rest for the dissolution of AgNO 3. The resultant homogeneous solutions were filled in a mold and placed in drying oven type GFL 7105 at 70 C. The gelation acts after 9 days. After gelation, the samples were still left inside the oven for 15 days for ageing (which result in a further shrinkage and stiffening of the gel), until no shrinkage appear. Densification of the prepared samples was obtained by annealing in air at different temperatures (150, 300, and 500 C) and times.

2 272 INDIAN J PURE & APPL PHYS, VOL 48, APRIL 2010 The optical absorption spectra in the wavelength range nm were obtained at room temperature using a dual beam spectrometer (Perkin-Elmer, model lambda-19). A silica glass was used as a reference sample. To understand the approximate size and distribution of the particles, the optical spectra were analyzed by Mie theory fitting with variation of the particle radii in order to obtain best fits to the experimental curves. The X-ray diffraction (XRD) patterns of the prepared samples were recorded with a Philips X-ray diffractometer PW/1710; with Ni filter, using monochromatised CuKα radiation of wavelength Å at 40 KV and 30 ma. Using XRD data, approximate crystalline size was also determined via Scherrer formula: Dhkl = kλ / βcosθ (1) Fig. 1 Absorption spectra of a silver doped silica matrix heated at (1) 300 C, (2) 400 C and (3) 500 C. where k is a constant taken as 0.9 for calculation, β is full width at half maximum (FWHM) in radians, θ is the Bragg angle at which the peak maximum occurs and λ is the wavelength of X-ray radiation used for the study. Differential scanning calorimetry (DSC) analyses were performed in air (Metler TC 11). Infrared spectra were collected from with a Perkin- Elmer 1600 (spectrophotometer) in cm 1 range. Scanning electron microscopy (SEM) of the samples was done with JEOL-JSM-T330-A 35 CF microscope at an accelerating voltage of 20 KV. 3 Results 3.1 Optical Figure 1 shows the evolution of the spectrum of a typical 70 C dried gel sample heat treated at different temperatures in a cumulative manner. Heating at about 300 C induced the evolution of a peak at around 410 nm along with an additional peak at around 350 nm, attributed to atomic silver 16. These two peaks gradually weakened upon heat treatment and vanished at about 400 C (Fig. 1) and the sample became transparent. Since longer wavelengths are connected to absorption by larger clusters, coarsening of bigger ones occur. 3.2 XRD The presence of silver metal in the silica matrix was confirmed by XRD analysis of bulk samples. The XRD pattern of the 150 C dried samples (Fig. 2) showed a typical amorphous spectrum, and the spectra of all other samples starting from as 300 C had sharp peaks that are characteristic of the silver Fig. 2 XRD pattern of the silver doped samples annealed at 150 C, 300 C, 500 C crystals 17. The average crystalline size was found to be nm. 3.3 DSC To understand the formation and decomposition of the silver silicate network, DSC studies were performed for the silver doped bulk samples after drying at 70 C. For a comparative study, an undoped gel was also measured. Figure 3 represents the DSC spectra of doped and undoped bulk samples. The major differences between the two spectra are the absence of an endothermic peak centered at about 175 C in the case of undoped gel and a broad exothermic peak (in the range C) in the case of silver doped silica, due to the elimination of water formed by silanol condensation 18. The aforementioned endothermic peak at about 175 C for silver doped silica is connected to the initial development of silver silicates.

3 DUHAN et al.: CHARACTERIZATION OF NANOCRYSTALLINE NANOCOMPOSITES 273 Fig. 3 DSC spectra of silver doped (1) and undoped (2) bulk samples 3.4 FTIR Figure 4 shows the FTIR spectra of the samples annealed at different temperatures in the region cm 1. In all of the IR spectra, water bands observed at around 3400 and 1640 cm 1 corresponding to stretching and bending vibrations indicate hygroscopic character of the powdered samples 19. The spectra of samples heat treated at 150 C show the deconvoluted bond-stretching vibration of the Si-O-Si units Strong bands observed at 1082 and 965, 798 and 468 cm -1 have been attributed to Si-O-Si and Si-OH absorptions 22. However, when the sample was sintered for 300 C, discrete bands appeared between to 800 cm 1. Small absorption due to metaloxygen (M-O) stretching is also seen at around 600 cm 1. As the heat treatment time increases for the, specimens with equal amount of Ag, the sharpening of bands and slight shift to higher frequency have been observed. This exhibits the occurrence of the band shortening and reduction in the mean Si-O-Si bond angle with increasing heat treatment time. However, when the sample was sintered for 500 C, elimination of bands corresponding to water molecules, Si-OH and volatiles in FTIR spectra, confirm densification of the binary system 23 and also Fig. 4 Infrared spectra of a silver doped silica matrix heated at (1) 150 C, (2) 300 C and (3) 500 C makes the system almost transparent for spatial frequency 2700 to 4000 cm 1 (Ref. 24). During preparation of the samples it was noted that the sols as well as gels remain colourless transparent materials. However, after drying of the aged gels, there were visible changes in colour along with lowering of transparency. It was found that this change of colour was predominant in the humid condition even at room temperature if the samples were exposed to air for a few days. However, if the samples with changed colours are heat treated above 300 C the samples regain the transparency. Alternate exposure to humid air and heat treatment repeat the cycle. Such activities are believed to be due to the presence of oxide compounds of Ag which are opaque in the visible band, in present samples, as have been found by analysis of X-ray diffraction. 3.5 SEM Figure 5 shows microstructure of C Ag substituted silica samples. The morphology of the

4 274 INDIAN J PURE & APPL PHYS, VOL 48, APRIL 2010 sample calcined at 150 C (i.e. amorphous Nd-doped silica). When the sample annealed at 300 C change in grain sizes is observed considerably, image shows broad size distribution of particles having diameter in the range of less than 10 µm. The microstructure changes from coarse scale to good ones by increasing the sintering temperature to 500 C of sample. The SEM observation confirmed the advantages of the sol-gel process for obtaining amorphous materials at lower temperature while, dense and more homogeneous materials were obtained at higher temperature. When the sample annealed at higher temperature, the silver nanoparticles can be clearly seen embedded in the SiO 2 matrix. Their dispersion in glass matrix is high and homogeneous. The particles are well separated from each other and their shape is essentially semispherical rather than spherical. Occasionally, ellipsoidal shape particles are also observed by SEM technique. The sintering of the thick semispherical structures was intended to roughly spherical shape with size between nm. 4 Discussion Composite silver-silicate nanoparticles were successfully synthesized by sol-gel method using hydrochloric acid as catalyst. Reduction process leads to the decomposition of metal-oxides and formation of non-agglomerated spherical nanoparticles in the composite. The heat treatment forms Ag/SiO 2 nanoclusters in the range nm in the sample. Moreover, XRD, DSC, FTIR and SEM analyses confirmed the formation of silver nanocomposites at about 300 C, a temperature much lower than that for the AgNO 3 decomposition. The largest part of silver remains in an atomic state, with the increase of temperature, the densification of silicate network followed by elimination of residual organics mainly carbon and water formed by condensation reaction. It is also noticed that silver clusters formed at and above 500 C are stable and do not show any degradation of their optical properties with aging. Work is in progress on the characterization of the non-linear optical properties of the obtained samples. Fig. 5 Scanning Electron Micrograph of a silver doped silica matrix heated at (1) 150 C, (2) 300 C and (3) 500 C 5 Conclusions Silver-doped silica samples were successfully prepared by the sol-gel process. The samples were characterized by UV-VIS, XRD, FTIR and SEM to confirm the formation of Ag/SiO 2 nanocomposites. It is found that the evolution of the system as a function

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