- Email: [email protected]
A facile sol–gel method for the encapsulation of gold nanoclusters in silica gels and their optical properties
Journal of Non-Crystalline Solids 255 (1999) 254±258
www.elsevier.com/locate/jnoncrysol
Letter to the Editor
A facile sol±gel method for the encapsulation of gold nanoclusters in silica gels and their optical properties S. Tamil Selvan a,*, Masayuki Nogami a, Arao Nakamura b, Yasushi Hamanaka b a
Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan b Department of Applied Physics, Faculty of Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan Received 16 March 1999; received in revised form 9 June 1999
Abstract A novel process consisting of a synthesis of gold hydrosol using tetrakis (hydroxymethyl) phosphonium chloride (THPC) reduction and a sol±gel process using tetramethylorthosilicate (TMOS) for the encapsulation of gold nanoparticles (Au/SiO2 : 0.1±1 wt% Au) in a matrix of silica gel without the aid of any external stabilizing agent or organically modi®ed sol±gel monomers is described. The optical absorption spectra showed the typical surface plasmon resonance for Au at around 530 nm, corroborated by X-ray photoelectron spectroscopy. Transmission electron microscopy revealed the existence of spherical Au particles in the matrix. The mean diameter of Au nanoparticles in gels varied from 10 to 20 nm, supported by X-ray diraction data. The third-order optical non-linearity (v3 ) determined by the degenerate four-wave mixing (DFWM) method exhibited a value of 4.6 ´ 10ÿ11 esu for a gel with 0.1 wt% Au. Ó 1999 Elsevier Science B.V. All rights reserved.
Metal quantum dots have been shown to exhibit strong linear and non-linear optical responses [1,2]. Small metal particles embedded in glasses [3,4] and gels [5,6] have been widely studied and sparked much interest in the ®eld of photonics. The fabrication of glasses doped with small Au particles remains problematic, largely owing to the diculties of achieving higher concentration of Au particles in glasses, prepared by conventional meltquenching method. The use of the sol±gel process helps overcome these diculties because of its low temperature operation, by which the dopant concentration in glasses can be easily controlled. The existing sol±gel methods [5,6] for the incorporation of Au nanoparticles in silica gels, either by chem-
*
Corresponding author. Fax: 81-52 735 5285; e-mail: [email protected]
ical or photo-reduction, suer from a large distribution of particle sizes ranging from 50 to 170 nm. Moreover, there is a considerable agglomeration of the particles, resulting in dierent shapes especially in the formation by precipitation using UVirradiation. Several methods exist in the literature for reducing AuClÿ 4 to colloidal Au in dierent media [7,8]. Most of these methods involve either chemical or photo-reduction of HAuCl4 . Ligand-stabilized clusters [9], thiol-derivatized [10,11] and block copolymer-stabilized Au colloids [12,13] have already been described in the literature. Partially hydrolyzed tetrakis(hydroxymethyl) phosphonium chloride (THPC) has been used as a reducing agent by Du et al. [14] for making small Au nanoparticles and by Sarathy et al. [11] for thiol-derivatized Au nanoparticles. Recently, organo-functionalized silanes have been used for
0022-3093/99/$ - see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 4 1 9 - 6
S. Tamil Selvan et al. / Journal of Non-Crystalline Solids 255 (1999) 254±258
stabilizing Au nanoparticles in sol±gel ®lms [15] and coatings [16]. The preparation of gold colloids using TEOS or methyltrimethoxysilane (MTMOS) in the absence of aminosilane precursors resulted in the immediate aggregation of the Au particles during reduction with NaBH4 [15]. In order to stabilize the Au nanoparticles in silica gels either amino-functionalized sol±gel monomers or a stabilizing agent should be employed. Very recently, we have demonstrated the preparation of polymer (PVP)-protected Au clusters in silica glass [17]. Herein, we show that the Au nanoparticles can be stabilized in silica gel without the aid of any external stabilizing agent or organically modi®ed sol±gel monomers. This method of preparing Au nanoparticles (10±20 nm) isolated in silica gels is new and potentially important. It provides a simpli®ed processing method that uses tetramethoxysilane (TMOS) directly, with no cosolvent and no need for functionalized silanes. Gold±silica composites (Au/SiO2 : 0.1±1.0 wt% Au) were fabricated according to the following method. The chemicals (TMOS, THPC and HAuCl4 ) were not puri®ed and used as received. The dierent gold concentrations in the gel were obtained by varying the concentration of the gold sol. First, Au hydrosol was prepared by mixing X cm3 (X 0.4, 2 and 4 for 0.1, 0.5 and 1 wt% Au, respectively) of a 25 mM HAuCl4 aqueous solution with partially hydrolyzed THPC. The latter was prepared by adding 1 ml of a fresh 50 mM solution of THPC in water to 47 ml of 6.38 mM NaOH solution [11]. TMOS (5 ml) was slowly dropped into the homogeneous Au sol with vigorous stirring, which was continued for 1 h and then 1.5 g of 0.15 M ammonia was added. The solution was stirred heavily for 5±10 min and the resulting homogeneous solution was transferred into polystyrene containers. A wet gel (light brown in color) formed within 20 min of ammonia addition. Drying at room temperature resulted in sti gels after 3±4 weeks. The shrinkage of the gels was about 45± 50%. The pore volume and surface area estimated from B.E.T measurements for a typical sample of 0.1 wt% Au were found to be 0.311 ml/g and 480.56 m2 /g, respectively. The porosity was 53%. The absorption spectra of Au-doped silica gels (Fig. 1(A)) show the typical surface plasmon res-
255
onance (SPR) for colloidal Au at around 530 nm. In general, an increase in particle size is either characterized by a red shift or a sharper peak in the absorption spectra. The latter is apparent in our samples with a higher concentration of Au (trace c). On the other hand, trace a shows a broader surface plasmon resonance which is typical of smaller size clusters. Fig. 1(B) depicts the DTA and TG curves for Au/SiO2 gel with 1 wt% Au. The DTA endothermic behavior is noticed from ca. 28°C to 175°C. The TG curve
Fig. 1. (A) UV-VIS spectra of Au-doped silica gels: (a) 0.1 wt% Au; (b) 0.5 wt% Au; (c) 1 wt% Au. The surface plasmon resonance is noted at around 520 nm. (B) DTA and TG curves of Au/SiO2 gel with 1 wt% Au.
256
S. Tamil Selvan et al. / Journal of Non-Crystalline Solids 255 (1999) 254±258
illustrates the fact that the weight of the silica gel decreases with an increase in temperature until about 160°C. The weight loss is ca. 5% at 160°C which is primarily due to the removal of water and methanol, resulting in condensation to a denser gel. X-ray photoelectron spectroscopic studies of Au/SiO2 gels with 1 wt% Au showed a binding energy at ca. 83 eV, characteristic of Au (4f7=2 ) corresponding to metallic gold (Au° state). It is obvious that the peak position of Au (4f) is shifted to lower energy in comparison to that of bulk Au (84 and 87 eV respectively, for 4f7=2 and 4f5=2 ), indicating that the Au clusters are larger in size [18]. Furthermore, Si (2p) and O (1s) electron spectra showed the characteristic binding energies at 102 and 532 eV, respectively, which con®rmed the formation of silica. TEM images were acquired with an electron microscope operating at an accelerating voltage of 80 kV. The samples were prepared by mixing the gel with ethanol and dropping the solution onto a carbon-coated Cu grid with an underlying tissue paper, leaving behind a thin ®lm. Shown in Fig. 2(a), are the TEM images of 0.1 and 0.5 wt% Audoped silica gels. As can be seen, the particle size distribution is wide in Fig. 2(a) and the average particle size of Au is estimated to be about 11 nm. Spherical particles of a relatively narrow size distribution are evident in Fig. 2(B) and the average particle size is found to be 18 nm. The inset shows the enlarged version of uniform size particles. Importantly, slightly elongated particles with a mean diameter of 21 nm appear in Fig. 3(a), at a higher concentration of Au (1 wt%). These changes are attributed to a higher concentration of Au sol. It is to be noted that the particles were different in the starting Au sol. There appears to be no signi®cant change in the particle size at concentrations from 0.5 to 1 wt%. Although the particle size distribution is not narrow (Fig. 3(b)), the agglomeration phenomenon is weak. This is quite interesting because there is no external stabilizing agent employed. It is noteworthy that the sodium citrate reduction method used with a similar sol± gel process yielded ca. 50 nm Au particles in silica gels [6]. Furthermore, the photo-reduction of AuClÿ 4 ions in sol±gel derived silica gels yielded particles with a larger size distribution and the
Fig. 2. (a) Transmission electron micrograph of Au-doped silica gel (Au/SiO2 : 0.1 wt%). The mean diameter of spherical Au particles is 11 nm. The grayish area represents the SiO2 matrix. (b) Transmission electron micrograph of Au-doped silica gel (Au/ SiO2 : 0.5 wt%). The mean diameter of spherical Au particles is 18 nm. The grayish area represents the SiO2 matrix. The particles of narrow size distribution are shown in the inset as an enlarged view.
S. Tamil Selvan et al. / Journal of Non-Crystalline Solids 255 (1999) 254±258
257
Fig. 4. X-ray diraction patterns of Au-doped silica gels: (a) Au/SiO2 : 0.5 wt%. The broad amorphous pattern at around 23° con®rms SiO2 ; (b) Au/SiO2 : 1 wt%. The characteristic (1 1 1) and (2 0 0) planes of cubic Au are noted.
Fig. 3. (a) Transmission electron micrograph of Au-doped silica gel (Au/SiO2 : 1 wt%) exhibiting slightly elongated Au particles, whose mean diameter is 21 nm. The coalescence of two such particles is seen in one place. The grayish area represents the SiO2 . (b) Histogram depicting the particle size distribution.
particle sizes ranged from 25 to 170 nm for Au/ SiO2 gel with 1 wt% Au [5]. The novelty of our work is the relatively small particle size and the ability to increase concentration up to 1 wt% without signi®cant aggregation. The reason for the
lack of agglomeration is presumably simply a consequence of eective encapsulation of the Au particles in the silica matrix. The powder XRD patterns of Au/SiO2 gels at two dierent Au weight percents are shown in Fig. 4. The broad amorphous halo pattern at around 2h 23° con®rms the formation of silica. Sharp crystalline peaks are noticed at ca. 2h 38° and 44°. These re¯ections are assigned, respectively, to (1 1 1) and (2 0 0) planes of cubic Au [5]. The Au particle size is estimated using Scherrer's equation and found to be about 21 nm from the diraction band at 2h 38° in the case of 1 wt% Au. This XRD pattern indicates that a cubic-Au nanocomposite with silica can be formed by a simple method. Preliminary measurements of third order optical non-linearity (v3 ) by the degenerate fourwave mixing method (DWFM) for gel samples with 0.1 wt% Au showed a value of 4.6 ´ 10ÿ11 esu. This value is one order of magnitude greater than those of silica gels doped with Au, reported by Yazawa et al., by a similar sol±gel method [6]. At higher concentration of Au, v3 increases further, revealing the fact that a larger size induces a larger eect. Measurements of v3 for dierent concentrations of Au and for dierent host glass samples are underway. In conclusion, we have demonstrated the encapsulation of spherical Au nanoparticles in silica
258
S. Tamil Selvan et al. / Journal of Non-Crystalline Solids 255 (1999) 254±258
gels, by a facile sol±gel process, without the aid of any external stabilizing agent or organically modi®ed sol±gel monomers. This method, by its versatility and ease in preparation, could circumvent the diculties in incorporating higher concentrations of Au nanoparticles (up to 1 wt%) into a silica matrix. Preliminary v3 measurements show a higher value for particles produced by this method. In addition, this simple approach should allow the extension of this technique to other noble metals and their encapsulation in silica gels and glasses. Acknowledgements One of us (S.T.S) gratefully acknowledges the Japan Society for the Promotion of Science (JSPS), Tokyo, for the award of a postdoctoral fellowship. The technical assistance of Mr K. Yamamoto is thankfully acknowledged. References [1] C.P. Collier, R.J. Saykally, J.J. Shiang, S.E. Henrichs, J.R. Heath, Science 277 (1997) 1978.
[2] D. Ricard, P. Roussignol, C. Flytzanis, Opt. Lett. 10 (1985) 511. [3] M. Nogami, Y. Abe, A. Nakamura, J. Mater. Res. 10 (1995) 2648. [4] R.K. Jain, R.C. Lind, J. Opt. Soc. Am. 73 (1983) 647. [5] I. Tanahashi, T. Tohda, J. Am. Ceram. Soc. 79 (1996) 796. [6] T. Yazawa, K. Kadono, H. Tanaka, T. Sakaguchi, S. Tsubota, K. Kuraoka, M. Miya, J. Non-Cryst. Solids 170 (1994) 105. [7] K. Esumi, N. Sato, K. Torigoe, K. Meguro, J. Colloid Interf. Sci. 149 (1992) 295. [8] K. Kurihara, J. Kizling, P. Stenius, J.H. Fendler, J. Am. Chem. Soc. 105 (1983) 2574. [9] G. Schmid, Chem. Rev. 92 (1992) 1709. [10] M. Brust, M. Walker, D. Bethel, D.J. Schirin, R. Whyman, J. Chem. Soc. Chem. Commun. (1994) 801. [11] K.V. Sarathy, G. Raina, R.T. Yadav, G.U. Kulkarni, C.N.R. Rao, J. Phys. Chem. B 101 (1997) 9876. [12] S.T. Selvan, J.P. Spatz, H.-A. Klok, M. Moeller, Adv. Mater. 10 (1998) 132. [13] S.T. Selvan, Chem. Commun. (1998) 351. [14] D.G. Du, A. Baiker, P.P. Edwards, Chem. Commun. (1993) 96. [15] S. Bharathi, O. Lev, Chem. Commun. (1997) 2303. [16] B. Kutsch, O. Lyon, M. Schmitt, M. Mennig, H. Schmidt, J. Non-Cryst. Solids 217 (1997) 143. [17] S.T. Selvan, Y. Ono, M. Nogami, Mater. Lett. 37 (1998) 156. [18] M.G. Mason, Phys. Rev. B 27 (1983) 748.
www.elsevier.com/locate/jnoncrysol
Letter to the Editor
A facile sol±gel method for the encapsulation of gold nanoclusters in silica gels and their optical properties S. Tamil Selvan a,*, Masayuki Nogami a, Arao Nakamura b, Yasushi Hamanaka b a
Department of Materials Science and Engineering, Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan b Department of Applied Physics, Faculty of Engineering, Nagoya University, Chikusa-ku, Nagoya 464-8603, Japan Received 16 March 1999; received in revised form 9 June 1999
Abstract A novel process consisting of a synthesis of gold hydrosol using tetrakis (hydroxymethyl) phosphonium chloride (THPC) reduction and a sol±gel process using tetramethylorthosilicate (TMOS) for the encapsulation of gold nanoparticles (Au/SiO2 : 0.1±1 wt% Au) in a matrix of silica gel without the aid of any external stabilizing agent or organically modi®ed sol±gel monomers is described. The optical absorption spectra showed the typical surface plasmon resonance for Au at around 530 nm, corroborated by X-ray photoelectron spectroscopy. Transmission electron microscopy revealed the existence of spherical Au particles in the matrix. The mean diameter of Au nanoparticles in gels varied from 10 to 20 nm, supported by X-ray diraction data. The third-order optical non-linearity (v3 ) determined by the degenerate four-wave mixing (DFWM) method exhibited a value of 4.6 ´ 10ÿ11 esu for a gel with 0.1 wt% Au. Ó 1999 Elsevier Science B.V. All rights reserved.
Metal quantum dots have been shown to exhibit strong linear and non-linear optical responses [1,2]. Small metal particles embedded in glasses [3,4] and gels [5,6] have been widely studied and sparked much interest in the ®eld of photonics. The fabrication of glasses doped with small Au particles remains problematic, largely owing to the diculties of achieving higher concentration of Au particles in glasses, prepared by conventional meltquenching method. The use of the sol±gel process helps overcome these diculties because of its low temperature operation, by which the dopant concentration in glasses can be easily controlled. The existing sol±gel methods [5,6] for the incorporation of Au nanoparticles in silica gels, either by chem-
*
Corresponding author. Fax: 81-52 735 5285; e-mail: [email protected]
ical or photo-reduction, suer from a large distribution of particle sizes ranging from 50 to 170 nm. Moreover, there is a considerable agglomeration of the particles, resulting in dierent shapes especially in the formation by precipitation using UVirradiation. Several methods exist in the literature for reducing AuClÿ 4 to colloidal Au in dierent media [7,8]. Most of these methods involve either chemical or photo-reduction of HAuCl4 . Ligand-stabilized clusters [9], thiol-derivatized [10,11] and block copolymer-stabilized Au colloids [12,13] have already been described in the literature. Partially hydrolyzed tetrakis(hydroxymethyl) phosphonium chloride (THPC) has been used as a reducing agent by Du et al. [14] for making small Au nanoparticles and by Sarathy et al. [11] for thiol-derivatized Au nanoparticles. Recently, organo-functionalized silanes have been used for
0022-3093/99/$ - see front matter Ó 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 4 1 9 - 6
S. Tamil Selvan et al. / Journal of Non-Crystalline Solids 255 (1999) 254±258
stabilizing Au nanoparticles in sol±gel ®lms [15] and coatings [16]. The preparation of gold colloids using TEOS or methyltrimethoxysilane (MTMOS) in the absence of aminosilane precursors resulted in the immediate aggregation of the Au particles during reduction with NaBH4 [15]. In order to stabilize the Au nanoparticles in silica gels either amino-functionalized sol±gel monomers or a stabilizing agent should be employed. Very recently, we have demonstrated the preparation of polymer (PVP)-protected Au clusters in silica glass [17]. Herein, we show that the Au nanoparticles can be stabilized in silica gel without the aid of any external stabilizing agent or organically modi®ed sol±gel monomers. This method of preparing Au nanoparticles (10±20 nm) isolated in silica gels is new and potentially important. It provides a simpli®ed processing method that uses tetramethoxysilane (TMOS) directly, with no cosolvent and no need for functionalized silanes. Gold±silica composites (Au/SiO2 : 0.1±1.0 wt% Au) were fabricated according to the following method. The chemicals (TMOS, THPC and HAuCl4 ) were not puri®ed and used as received. The dierent gold concentrations in the gel were obtained by varying the concentration of the gold sol. First, Au hydrosol was prepared by mixing X cm3 (X 0.4, 2 and 4 for 0.1, 0.5 and 1 wt% Au, respectively) of a 25 mM HAuCl4 aqueous solution with partially hydrolyzed THPC. The latter was prepared by adding 1 ml of a fresh 50 mM solution of THPC in water to 47 ml of 6.38 mM NaOH solution [11]. TMOS (5 ml) was slowly dropped into the homogeneous Au sol with vigorous stirring, which was continued for 1 h and then 1.5 g of 0.15 M ammonia was added. The solution was stirred heavily for 5±10 min and the resulting homogeneous solution was transferred into polystyrene containers. A wet gel (light brown in color) formed within 20 min of ammonia addition. Drying at room temperature resulted in sti gels after 3±4 weeks. The shrinkage of the gels was about 45± 50%. The pore volume and surface area estimated from B.E.T measurements for a typical sample of 0.1 wt% Au were found to be 0.311 ml/g and 480.56 m2 /g, respectively. The porosity was 53%. The absorption spectra of Au-doped silica gels (Fig. 1(A)) show the typical surface plasmon res-
255
onance (SPR) for colloidal Au at around 530 nm. In general, an increase in particle size is either characterized by a red shift or a sharper peak in the absorption spectra. The latter is apparent in our samples with a higher concentration of Au (trace c). On the other hand, trace a shows a broader surface plasmon resonance which is typical of smaller size clusters. Fig. 1(B) depicts the DTA and TG curves for Au/SiO2 gel with 1 wt% Au. The DTA endothermic behavior is noticed from ca. 28°C to 175°C. The TG curve
Fig. 1. (A) UV-VIS spectra of Au-doped silica gels: (a) 0.1 wt% Au; (b) 0.5 wt% Au; (c) 1 wt% Au. The surface plasmon resonance is noted at around 520 nm. (B) DTA and TG curves of Au/SiO2 gel with 1 wt% Au.
256
S. Tamil Selvan et al. / Journal of Non-Crystalline Solids 255 (1999) 254±258
illustrates the fact that the weight of the silica gel decreases with an increase in temperature until about 160°C. The weight loss is ca. 5% at 160°C which is primarily due to the removal of water and methanol, resulting in condensation to a denser gel. X-ray photoelectron spectroscopic studies of Au/SiO2 gels with 1 wt% Au showed a binding energy at ca. 83 eV, characteristic of Au (4f7=2 ) corresponding to metallic gold (Au° state). It is obvious that the peak position of Au (4f) is shifted to lower energy in comparison to that of bulk Au (84 and 87 eV respectively, for 4f7=2 and 4f5=2 ), indicating that the Au clusters are larger in size [18]. Furthermore, Si (2p) and O (1s) electron spectra showed the characteristic binding energies at 102 and 532 eV, respectively, which con®rmed the formation of silica. TEM images were acquired with an electron microscope operating at an accelerating voltage of 80 kV. The samples were prepared by mixing the gel with ethanol and dropping the solution onto a carbon-coated Cu grid with an underlying tissue paper, leaving behind a thin ®lm. Shown in Fig. 2(a), are the TEM images of 0.1 and 0.5 wt% Audoped silica gels. As can be seen, the particle size distribution is wide in Fig. 2(a) and the average particle size of Au is estimated to be about 11 nm. Spherical particles of a relatively narrow size distribution are evident in Fig. 2(B) and the average particle size is found to be 18 nm. The inset shows the enlarged version of uniform size particles. Importantly, slightly elongated particles with a mean diameter of 21 nm appear in Fig. 3(a), at a higher concentration of Au (1 wt%). These changes are attributed to a higher concentration of Au sol. It is to be noted that the particles were different in the starting Au sol. There appears to be no signi®cant change in the particle size at concentrations from 0.5 to 1 wt%. Although the particle size distribution is not narrow (Fig. 3(b)), the agglomeration phenomenon is weak. This is quite interesting because there is no external stabilizing agent employed. It is noteworthy that the sodium citrate reduction method used with a similar sol± gel process yielded ca. 50 nm Au particles in silica gels [6]. Furthermore, the photo-reduction of AuClÿ 4 ions in sol±gel derived silica gels yielded particles with a larger size distribution and the
Fig. 2. (a) Transmission electron micrograph of Au-doped silica gel (Au/SiO2 : 0.1 wt%). The mean diameter of spherical Au particles is 11 nm. The grayish area represents the SiO2 matrix. (b) Transmission electron micrograph of Au-doped silica gel (Au/ SiO2 : 0.5 wt%). The mean diameter of spherical Au particles is 18 nm. The grayish area represents the SiO2 matrix. The particles of narrow size distribution are shown in the inset as an enlarged view.
S. Tamil Selvan et al. / Journal of Non-Crystalline Solids 255 (1999) 254±258
257
Fig. 4. X-ray diraction patterns of Au-doped silica gels: (a) Au/SiO2 : 0.5 wt%. The broad amorphous pattern at around 23° con®rms SiO2 ; (b) Au/SiO2 : 1 wt%. The characteristic (1 1 1) and (2 0 0) planes of cubic Au are noted.
Fig. 3. (a) Transmission electron micrograph of Au-doped silica gel (Au/SiO2 : 1 wt%) exhibiting slightly elongated Au particles, whose mean diameter is 21 nm. The coalescence of two such particles is seen in one place. The grayish area represents the SiO2 . (b) Histogram depicting the particle size distribution.
particle sizes ranged from 25 to 170 nm for Au/ SiO2 gel with 1 wt% Au [5]. The novelty of our work is the relatively small particle size and the ability to increase concentration up to 1 wt% without signi®cant aggregation. The reason for the
lack of agglomeration is presumably simply a consequence of eective encapsulation of the Au particles in the silica matrix. The powder XRD patterns of Au/SiO2 gels at two dierent Au weight percents are shown in Fig. 4. The broad amorphous halo pattern at around 2h 23° con®rms the formation of silica. Sharp crystalline peaks are noticed at ca. 2h 38° and 44°. These re¯ections are assigned, respectively, to (1 1 1) and (2 0 0) planes of cubic Au [5]. The Au particle size is estimated using Scherrer's equation and found to be about 21 nm from the diraction band at 2h 38° in the case of 1 wt% Au. This XRD pattern indicates that a cubic-Au nanocomposite with silica can be formed by a simple method. Preliminary measurements of third order optical non-linearity (v3 ) by the degenerate fourwave mixing method (DWFM) for gel samples with 0.1 wt% Au showed a value of 4.6 ´ 10ÿ11 esu. This value is one order of magnitude greater than those of silica gels doped with Au, reported by Yazawa et al., by a similar sol±gel method [6]. At higher concentration of Au, v3 increases further, revealing the fact that a larger size induces a larger eect. Measurements of v3 for dierent concentrations of Au and for dierent host glass samples are underway. In conclusion, we have demonstrated the encapsulation of spherical Au nanoparticles in silica
258
S. Tamil Selvan et al. / Journal of Non-Crystalline Solids 255 (1999) 254±258
gels, by a facile sol±gel process, without the aid of any external stabilizing agent or organically modi®ed sol±gel monomers. This method, by its versatility and ease in preparation, could circumvent the diculties in incorporating higher concentrations of Au nanoparticles (up to 1 wt%) into a silica matrix. Preliminary v3 measurements show a higher value for particles produced by this method. In addition, this simple approach should allow the extension of this technique to other noble metals and their encapsulation in silica gels and glasses. Acknowledgements One of us (S.T.S) gratefully acknowledges the Japan Society for the Promotion of Science (JSPS), Tokyo, for the award of a postdoctoral fellowship. The technical assistance of Mr K. Yamamoto is thankfully acknowledged. References [1] C.P. Collier, R.J. Saykally, J.J. Shiang, S.E. Henrichs, J.R. Heath, Science 277 (1997) 1978.
[2] D. Ricard, P. Roussignol, C. Flytzanis, Opt. Lett. 10 (1985) 511. [3] M. Nogami, Y. Abe, A. Nakamura, J. Mater. Res. 10 (1995) 2648. [4] R.K. Jain, R.C. Lind, J. Opt. Soc. Am. 73 (1983) 647. [5] I. Tanahashi, T. Tohda, J. Am. Ceram. Soc. 79 (1996) 796. [6] T. Yazawa, K. Kadono, H. Tanaka, T. Sakaguchi, S. Tsubota, K. Kuraoka, M. Miya, J. Non-Cryst. Solids 170 (1994) 105. [7] K. Esumi, N. Sato, K. Torigoe, K. Meguro, J. Colloid Interf. Sci. 149 (1992) 295. [8] K. Kurihara, J. Kizling, P. Stenius, J.H. Fendler, J. Am. Chem. Soc. 105 (1983) 2574. [9] G. Schmid, Chem. Rev. 92 (1992) 1709. [10] M. Brust, M. Walker, D. Bethel, D.J. Schirin, R. Whyman, J. Chem. Soc. Chem. Commun. (1994) 801. [11] K.V. Sarathy, G. Raina, R.T. Yadav, G.U. Kulkarni, C.N.R. Rao, J. Phys. Chem. B 101 (1997) 9876. [12] S.T. Selvan, J.P. Spatz, H.-A. Klok, M. Moeller, Adv. Mater. 10 (1998) 132. [13] S.T. Selvan, Chem. Commun. (1998) 351. [14] D.G. Du, A. Baiker, P.P. Edwards, Chem. Commun. (1993) 96. [15] S. Bharathi, O. Lev, Chem. Commun. (1997) 2303. [16] B. Kutsch, O. Lyon, M. Schmitt, M. Mennig, H. Schmidt, J. Non-Cryst. Solids 217 (1997) 143. [17] S.T. Selvan, Y. Ono, M. Nogami, Mater. Lett. 37 (1998) 156. [18] M.G. Mason, Phys. Rev. B 27 (1983) 748.