J. MATER. CHEM., 1994, 4( lo), 1619-1620 y-Radiation Sol-Gel Synthesis of Glass-Metal Nanocomposites Yingjie Zhu,*a Yitai Qian,a Manwei Zhang," Zuyao Chen" and Guien Zhoub a Department of Applied Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China Structure Research Laboratory, University of Science and Technology of China, Hefei, Anhui 230026, P.R. China Glass-metal nanocomposites of nanocrystalline silver in a silica glass matrix have been successfully synthesized by y-radiation combined with the sol-gel method. Most research on nanocrystalline metals has concentrated on the fabrication and characterization of single-phase materials. More recently the field of interest has broadened to include nanocomposite materials, because of their interesting electrical and optical properties,' their possible commercial e~ploitation~,~and their importance in providing models for understanding the physics of ultrafine particles.435 Different techniques have been used to prepare such materials, the most popular being the gas e~aporation,~,' RF sputtering3** and ion exchange and redu~tion.'"~ Sol-gel techniques for prepar- ing nanocomposites have also been rep~rted'l-'~ in which heat treatment or hydrogen reduction is needed after the sol-gel process.Recently, we developed a y-radiation method for preparing nanocrystalline metal^.'^,^^ In order to prepare glass-metal nanocomposites under ambient pressure and at room tem- perature, we have explored the possibility of using y-radiation combined with the sol-gel process, and the results are reported here.Experimental Solutions were prepared by dissolving analytically pure AgNO, in distilled water and adding sodium dodecyl sulfate as a surfactant, and isopropyl alcohol as a scavenger for hydroxyl radicals. The solutions were bubbled with pure nitrogen for 1 h to remove oxygen, then irradiated in the field of a 2.59 x 10" Bq 6oCo pray source with different doses. After the solutions had been y-irradiated, solutions of colloidal silver were obtained. Then the sol-gel method was used to prepare silica-silver nanocoposites. Tetraethoxysilane [(C,H,O),Si, 5 ml] was dissolved in isopropyl alcohol (10 ml) with water (5 ml), and a few drops of concentrated nitric acid were added to keep the pH at ca.2. After continuous stirring for 1.5 h, different volumes of irradiated solutions were added so that the final product would have different Ag :silica ratios. For gelation the pH of the solution was increased to 8 by addition of aqueous ammonia under mild stirring. The hydro- gels obtained were dried overnight in air. The gels were ground, washed with distilled water and aqueous ammonia solution to remove impurities and were dried again. Finally, powders of silica-silver nanocomposites were obtained. The nanocomposites were characterized by powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). XRD analysis was performed using a Rigaku Dmax yA X-ray diffractometer at a scanning rate of 0.05" s-' in the 26' range from 25" to 85", using graphite-monochromated Cu-Kx radiation.TEM images were recorded with a Hitachi H-800 transmission electron microscope, using an acceler-ating voltage of 200 kV. The amounts of silver present as metallic species in the nanocomposites were determined by atomic absorption spectroscopy using an IL-95 1 spectrometer at 2 =328 nm. Absorption spectra of colloidal silver .;elutions produced by y-irradiation were recorded on a UV-240 UV-VIS spectrophotometer using quartz cells. Results and Discussion The radiation reduction of Ag' ions in solution leads to a yellow solution of colloidal silver. This process can be written in a simplified way: Ag' +eaq- +Ago (reduction) nAg" -+Ag,, (aggregation) The primary reduction product is silver atoms produced by the reaction of Ag+ ions with hydrated electrons fxmed in solution during y-radiation. These silver atoms undergo further aggregation to progressively larger clusters, leading to the formation of colloidal silver with an intense optical absorption at ca.400 nm. Our experiments show that colloidal silver is formed at the beginning of the radiation and that the 400 nm band intensity increases with increasing radiation dose (Fig. 1). This implies that the concentration of colloidal silver increases with the radiation dose. Fig. 2 gives the XRD pattern of a typical sample containing metallic silver particles prepared by ?-irradiating a solution containing 0.01 rnol I-' AgNO,, 0.01 mol I-' C,,H,,NaSO, and 2.0 mol 1-1 (CH,), CHOH with a dose of 8.1 r< lo3 Gy.This shows that the sample consists of two phases, i.c,. metallic silver and non-crystalline silica. The broadening of the ( 111) r Unm Fig. 1 Absorption spectrum of a solution after various doses of y-irradiation. Solution: 0.01 mol 1-' AgNO,. 0.01 mol 1-' C~2H,,NaS0,, 0.5 mol 1-' (CH,),CHOH; dose rate: 1.0 x 10' Gy min-'; radiation time: (a) not irradiated, (b)5 min, (c) 10 min. J. MATER. CHEhl., 1994, VOL. 4 Table 1 Effect of experimental parameters on the size of silver particles dispersed in the silica glass matrix sample irradiation dose/ silver particle no. solution 10-3 GY size/nm 1 2 0.01 rnol I-' 0.01 mol I-' AgNO,-0.01 mol I-' CI2H,,NaSO,-2.0 AgN0,-2.0 mol 1-' (CH,),CHOH mol I-' (CH,),CHOH 8.1 8.1 6 40 3 0.05 mol I-' AgN0,-0.01 mol 1-' C,,H,,NaS0,--2.0 rnol 1-' (CH,),CHOH 8.1 10 4 0.01 mol 1-' AgN03-0.01 mol 1-' C1,H,,NaS0,-2.0 mol 1-' (CH,),CHOH 30 15 I 1 15 25 35 45 55 28ldegrees Fig.2 XRD pattern of the sample prepared by the y-radiation sol-gel method. Solution: 0.01 mol 1-' AgNO,, 0.01 mol I-' C,,H,,NaSO,, 2.0 rnol I-' (CH,),CHOH. Dose: 8.1 x lo3 Gy. Fig. 3 TEM micrograph of the sample in Fig. 2 diffraction peak of silver implies that the silver particles dispersed in the silica matrix are very small. The average particle size of silver is 6nm, as estimated by the Scherrer formula.16 The amount of silver present as metallic species in the composite glass is 1.24%, as measured by atomic absorption spectroscopy. A TEM micrograph of the sample prepared under the same experimental conditions as described in Fig.2 is shown in Fig. 3. The silica glass contains a dispersion of fine metallic silver grains which are quasi-spherical and well separated. The selected-area diffraction pattern confirms that these par- ticles consist of metallic silver. The diameters of the silver particles range from 4 to 20 nm; the average particle size is 7 nm obtained from the photographic image microstructure densitometry analysis method, which is in agreement with that calculated by the Scherrer formula (6 nm). The size of the silver particles dispersed in the silica matrix is dependent on several factors, such as the surfactant used, the concentration of Ag' ions and the y-radiation dose.The experimental results are listed in Table 1. Table 1 shows that the silver particle size decreases as the concentration of sodium dodecyl sulfate increases. When no sodium dodecyl sulfate is used, the silver particle size is as large as 40 nm (sample 2). If the concentration of sodium dodecyl sulfate is increased to 0.01 mol 1-', the silver particle size decreases to 6 nm (sample 1).This shows that an appropriate surfactant can limit the aggregation of silver particles in solution. Using a lower concentration of silver nitrate favours the production of smaller silver particles. This may be due to the lower rate of reduction of Ag' ions.For example, when the concentration of silver nitrate is increased from 0.01 to 0.05 mol l-', the silver particle size in the silica matrix increases from 6 nm (sample 1) to 10 nm (sample 3). The influence of the y-radiation dose on the silver particle size was also studied. When the dose was increased from 8.1 x lo3to 3.0 x lo4Gy, the silver particle size increased from 6 nm (sample 1) to 15 nm (sample 4). Therefore we preferred to use relatively small radiation doses in the preparation of colloidal silver. Conclusion ?-Radiation combined with the sol-gel method has been successfully used to prepare silica glass-silver nanocomposites with a narrow distribution of silver particle sizes. By appro-priate control of the conditions, this method may be extended to the preparation of other glass-metal nanocomposites as well as ceramic-metal nanocomposites.Financial support from the Chinese National Science Research Foundation and the Doctoral Fund of the Chinese Education Commission are gratefully acknowledged. The authors thank Professor Zhang Zhicheng for helpful discussions and Mr. Sun Tingheng and Mr. Quan Yucai for their help with the y-radiation experiments. References 1 D. Chakravorty, Bull. Mater. Sci., 1984, 6, 193. 2 J. H. Sinfelt,Science, 1977, 195, 641. 3 A. Anderson, 0. Hunderi and C. G. Granqvist, J. Appl. Phys., 1980,57, 757. 4 Aerosol Microphysics II ed. W. H. Marlow, Springer, Berlin, 1982. 5 Physics of Finely Diuided Mutter ed. N. Boccara and M. Daoud, Springer, Berlin, 1985. 6 C. G. Granqvist and 0.Hunderi, Phys. Rev. B, 1977,16,3513. 7 G. A. Niklasson, J. Appl. Phys., 1987,62,258. 8 B. Abeles, P. Sheng, M. D. Coutts and Y. Arie, Ah. 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