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Formation of silver nanoparticles in an acid-catalyzed silica colloidal solution
Applied Surface Science 233 (2004) 135–140
Formation of silver nanoparticles in an acid-catalyzed silica colloidal solution Zhong-Jie Jiang, Chun-Yan Liu*, Yun Liu Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100101, China Received in revised form 2 March 2004; accepted 9 March 2004 Available online 6 May 2004
Abstract In a weak basic, weak acidic or neutral water–alcohol solution, silver nanoparticles were generated by the reduction of Agþ ions in the present of colloidal silica. Silica as a substrate played an important role in the formation of Silver particles. The plasmon band of silver particles supported on the surface of silica was considerably shifted to longer wavelength compared with the pure silver sol. The shift in absorption spectra was explained in terms of surface effects induced by the interaction of silver and silica, as well as size effects and irregular shape. # 2004 Elsevier B.V. All rights reserved. Keywords: Silica colloidal particles; Silver nanoparticles; Plasmon absorption band
1. Introduction Recently, much effort has been devoted to studying metal/oxide systems because of their broad applications in many areas, such as heterogeneous catalysis, ceramics, thin film technology and microelectronic devices [1]. Great attention has been paid to both experimental and theoretical investigation to well understand the interaction between metals and supports [2]. It is experimentally proved that strong metal–support interaction (SMSI) exists between metal and supports [3–5] and the properties of composites is closely dependent on their long-range
*
Corresponding author. Tel.: þ86-10-64888179; fax: þ86-10-64879375. E-mail address: [email protected] (C.-Y. Liu).
correlation of positions or orientations of active constituents [6]. So far, many metal/oxide systems have been extensively investigated. Among the varying oxide substrates, TiO2, whatever occurs naturally as mineral rutile or as less common polymorphs anatase and brookite, has been one of the most investigation systems [7,8]. Currently, the system of metal/SiO2 has become a research focus, especially on the system of Au/SiO2 [9,10]. But the study on the interaction and electron transfer between metal and SiO2 is less reported. In the present work, we studied a Ag/SiO2 system. The aim of this research was to explore the formation process of Ag/SiO2 in nanoscale, microstructure, interaction between metal and semiconductor and the changes in the absorption band of silver nanoparticles supported on SiO2 with respect to microenvironments.
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.03.224
136
Z.-J. Jiang et al. / Applied Surface Science 233 (2004) 135–140
2. Experimental section 2.1. Materials and Instrumentation Ethanol, tetraethyl orthosilicate (TEOS, CR) and AgNO3 were purchased from Beijing Chemical Reagent Co. and were used as received without further purification. UV-Vis absorption spectra were conducted in the range between 250 and 700 nm using a SHIMAZDU UV-1601PC UV-Vis scanning spectrometer. The transmission electron microscopy (TEM) image was observed and recorded with JEM-100CX at an acceleration voltage of 80 kV and Tecnai 20 transmission electron microscopy at an acceleration voltage of 160 kV. The electron diffraction (ED) was detected by Tecnai 20 transmission electron microscopy. The X-ray photoelectron spectra (XPS) were recorded with an X SAM 800 electron energy spectrometer, using Al Ka radiation under high vacuum conditions. 2.2. Preparation Colloidal silica was prepared by an acid-catalyzed sol–gel reaction of TEOS. In a typical preparation, 5 mL of TEOS was added into 5 mL ethanol. Next, the mixture was injected into 30 mL of 0.1 mol/L HNO3 solution, which was kept at 60 8C for 8–10 h to ensure that the reaction was complete. Finally, a transparent colloidal solution was gained. The volume of the solution was adjusted to 50 mL with deionized water. The final concentration of silica was 26 mg/mL. The pH of the silica colloidal solution was adjusted by 0.1 mol/L NaOH to appropriate values. One milliliter of 2 104 mol/L AgNO3 was added to a certain amount of the silica colloidal solution. The color of mixture gradually evolved, indicating the formation of the Ag/SiO2 composites.
3. Results Fig. 1 showed a typical TEM image of silver obtained in the silica colloidal solution. As shown in Fig. 1, silver particles had irregular shapes with diameters range from 15 to 30 nm. However, the silica particles prepared by an acid-catalyzed sol–gel method usually induced amorphous shape [11,12].
Fig. 1. TEM micrograph of silver particles on the surface of the colloidal silica.
The experiments showed that in the absence of silica, the reduction of Agþ ions in the basic alcoholic solution was a fast process with alcohol as a reductant, and a brown precipitate appeared in the solution. The reduction of Agþ ions by alcohol molecules could not occur in the weak acid or neutral alcoholic solution without silica. However, in the water–alcohol mixed solution containing silica colloid prepared here, the reduction of Agþ ions slowly proceeded and formed silver nanoparticles on the surface of silica, whatever the pH value of the colloidal solution was weak acid, neutral or weak basic. During the reaction, the system color gradually changed from a colorless to bright yellow. The evolution of absorption spectra with the reaction time was presented in Fig. 2. A broad adsorption band centered at 415–445 cm was developed and shifted continuously to a longer wavelength during the reaction. This is the plasmon band of silver particles. The pH value of the solution could influence the position of the plasmon band of silver nanoparticles deposited on silica. As shown in Fig. 3, the absorption band became stronger and the absorption maximum shifted to longer wavelength with the increasing pH values. When a pH value reached a certain value, the intensity of the absorption maximum began to decrease and the position of absorption band continuously shifted to longer wavelength.
Z.-J. Jiang et al. / Applied Surface Science 233 (2004) 135–140
Fig. 2. Evolution of the UV-Vis absorption spectra with the reaction time. ½SiO2 ¼ 0:145 mol/L, ½Ag ¼ 3:3 104 mol/L, pH ¼ 7:6.
Fig. 3. UV-Vis absorption spectra of silver nanoparitcles on the silica at different pH, ½SiO2 ¼ 0:145 mol/L, ½Ag ¼ 3:3 104 mol/L.
137
particles of Ag and Au have been prepared using this procedure [14,15]. In the reactions, the hydroxide ions played a crucial role in the reaction of metal ions with alcohol [14]. The experiments showed here, however, in the water–alcohol solution containing the colloidal silica, the reduction of Agþ ions could be carried out not only in basic, but also in a weak basic or neutral solution, even in an acidic solution (Fig. 3). This result indicated the colloidal silica prepared by the acidcatalyzed sol–gel method had a significantly impact on the reduction of Agþ ions, which provided a twodimensional microenvironment for reactant molecules and promoted the reaction. Johnston [16] and Zhang et al. [17] have demonstrated that the reduction of Agþ ions occurs preferentially on the surface of semiconductors. As shown in Fig. 3, when the pH value of the solution was less than 2, the reduction of Agþ ions was slower, and with the increasing pH, the reaction rate speeded up. This made clear that the pH value of the solution and the surface property of colloidal silica were important to the formation of Ag particles on the surface of silica. As is well know, the isoelectric point (iep) of silica is at about pH 2–3 [12]. Above the iep, the density of negative charge on the surface of silica increased with the increasing pH, which availed to the adsorption of Agþ ions by electrostatic interactions. Fig. 4 showed the acid–base titration curves of the colloidal silica solution and the acidic alcohol solution. Because OH ions partly adsorbed onto the surface of silica, the concentration of hydroxyl ions in the solution decreased, while the negative charge
According to the experiments, when the size of silica nanoparticles increased, for example bigger than 40 nm, the above noted reduction of Agþ became difficult. The processes about the adsorption, reduction and nucleation of Agþ ions on the surface of silica nanoparticles bigger than 40 nm have been discussed elsewhere [13].
4. Discussion The metal particles could be prepared in basic alcohols, which is based on the instability of some metal complexes in the basic alcohols and resulted in spontaneous reduction of metal ions. Small colloidal
Fig. 4. Titration curves of neutralization: A, the acidic silica colloidal solution containing alcohol; B, the same solution as A without silica.
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density on the surface of silica increased and two curves were distinctly different. As a result, the reduction of Agþ could take place, even if the solution was weak acid. Huang et al. [14] and Lee et al. [18] have investigated the reduction Agþ ions in the alcohol solutions. They believed that the Agþ ions in the basic alcoholic solution could be reduced and formed metal particles, and meanwhile, the alcohol molecules were oxidized to form corresponding aldehyde and acid. In the present work, the reduction of Agþ ions was related with OH adsorbed on the surface of silica. Firstly, Agþ ions diffused onto the surface of silica and reacted with OH adsorbed on the surface of silica to form Ag2O particles. Subsequently, the Ag2O particles were reduced by ethanol and deposited on the silica. Concretely, the mechanism could be represented by the following reaction: 2Agþ þ 2OH ads
adsorption
! Ag2 Oads
SiO2
(1)
Ag2 Oads þ CH3 CH2 OH ! CH3 CHO þ 2Agads þ H2 O
(2)
Ag2 Oads þ CH3 CHO ! CH3 COO þ 2Agads þ Hþ
(3)
Hþ þ OH ads ! H2 O
(4)
The overall reaction : 4Agþ þ 5OH ads þ CH3 CH2 OH ! 4Agads þ CH3 COO þ 2H2 O
Where OH ads represented the hydroxide ions adsorbed on the surface of silica, Ag2Oads was Ag2O particles deposited on the surface of silica, and Agads corresponded to silver nanoparticles supported on the surface of silica. The electrode poten0 0 tials of ECH , EAg and þ 3 CHOþ2H =CH3 CH2 OH 2 O=AgþOH 0 ECH3 COO þ2Hþ =CH3 CHOþH2 O were þ0.34, 0.2 and 0.4, respectively. Evidently, the reactions described above were reasonable. To well understand that the silver particles were formed on the surface of the silica prepared here, an XPS experiment was performed (Fig. 5). As shown in Fig. 5, the electron transfer from the silver particles to the silica was confirmed. In the absence of silver, the binding energies of Si 2p and O 1s of SiO2 particles were 103.75 and 533.25 eV, respectively. For the silica particles deposited silver particles, the binding energies for Si 2p and O 1s were 103.05 and 532.60 eV, respectively. Evidently, the binding energies of Si 2p and O 1s negatively shifted, which indicated the electron transfer from Ag particles to silica. As we know, the silica particles would be a semiconductor when the size of particles was small, which might be responsible for the electron transfer between the silica and silver particle. This behavior is very different from the characteristic of silica in bulk phase. Compared with Ag particles in the silver sol [19– 21], the plasmon band of silver particles supported on the surface of silica was significantly shifted to longer wavelength. The shifts in absorption spectra could be explained in terms of surface effects induced by the interaction of silver and silica. As is well known, the
Fig. 5. Binding energy of Si 2p and O 1s for the colloidal silica determined by XPS: (a) silica colloid in the absence of silver particles; (b) silica colloidal deposited silver particles.
Z.-J. Jiang et al. / Applied Surface Science 233 (2004) 135–140
shape and position of the plasmon absorption band is very sensitive to microenvironment around silver particles. For example, a red shift of the absorption band would be observed [19,21], when Cd2þ, Pb2þ, and In3þ ions were adsorbed onto the surface of a silver particle, which could be explained by a change of electron density on the surface of the silver particles due to the adsorption of metal ions or deposition of metal clusters [14,15,22]. In the present work, the electron transfer from Ag particles to silica took place, which resulted in a decrease in electron density on the surface of silver particles and a red shift of the silver plasmon band. Red shift of the plasmon band might be also a result of size effects. Size effects are pronounced for particles bigger than 20 nm [15]. With increasing a size, the absorption band of silver can gradually shift to longer wavelength. As was seen from Fig. 2, with increasing the reaction time, the size of silver particles increased, and the plasmon band shifted to longer wavelength. Similarly, the change of pH would influence the reduction reaction of Agþ, so that the size of Ag particles increased and the plasmon band also shifted to longer wavelength (as shown in Fig. 3). The irregular shape is also an important factor to influence the position of plasmon band of silver particles. The plasmon band of a metal particle is very sensitive to particles shape [23–25]. As shown in Fig. 1, silver particles prepared in the present work were irregularly shaped. Such irregular shape would result in significant red shift of plasmon band. In addition, the position of band was also effected by other factors, such as media constant of a solution, refraction coefficient, etc. [26,27].
5. Conclusion In conclusion, in the present of the colloidal silica prepared by the acid-catalyzed sol–gel method, silver nanoparticles could be generated not only in the basic, but also in the weak acid, weak basic or neutral water– alcohol mixed solution. The plasmon band of silver particles supported on the surface of silica was considerably shifted to a longer wavelength compared with a pure silver sol. The shift in the absorption band basically resulted from surface effects, size effects and irregular shape. XPS showed that the binding energy
139
for Si 2p and O 1s of silica loaded silver nanoparticle was negatively shifted, which indicated that the formation of small silver particles on the surface of silica would result in electronic interactions between the two types of particles, which was very different from the behavior of bulk silica.
Acknowledgements The authors thank the support of the National Natural Science Foundation of China (90306003).
References [1] (a) M. Sambi, E. Pin, G. Sangiovanni, L. Zaratin, G. Gronozzi, F. Parmigiani, Surf. Sci. 349 (1996) L169; (b) K. Sunada, T. Watanabe, K. Hashimoto, Environ. Sci. Technol. 37 (2003) 4785. [2] A.Y. Stakheev, M.L. Kustov, Appl. Catal. A 188 (1999) 3. [3] S.J. Tauster, S.C. Fung, R.L. Garten, J. Am. Chem. Soc. 100 (1978) 170. [4] S.C. Fung, J. Catal. 76 (1982) 225. [5] R.H. Sadeghi, V.E. Henrich, J. Catal. 109 (1988) 1. [6] F. Caruso, M. Spasova, V. Saigueirino-Maceira, L.M. LizMarzan, Adv. Mater. 14 (2001) 1090. [7] S. Munnix, M. Schmeits, Phys. Rev. B 30 (1984) 2202. [8] S. Munnix, M. Schmeits, Phys. Rev. B 31 (1985) 3369. [9] S.L. Westcott, S.J. Oldenburg, T.R. Lee, N.J. Halas, Langmuir 14 (1998) 5396. [10] S.J. Oldenburg, R.D. Averitt, S.L. Westcott, N.J. Halas, Chem. Phys. Lett. 288 (1998) 143. [11] J. Zarzychi, M. Prassas, J. Phalippou, J. Mater. Sci. 17 (1982) 3379. [12] R.K. Iler, The Chemistry of Silica, Wiley/Interscience, New York, 1979. [13] (a) Z.-J. Jiang, C.-Y. Liu, Y. Liu, Z.-Y. Zhang, Y.-J. Li, Chem. Lett. 32 (2003) 668; (b) Z.-J. Jiang, C.-Y. Liu, J. Phys. Chem. B 107 (2003) 12411. [14] Z.-Y. Huang, G. Mills, B. Hajek, J. Phys. Chem. 97 (1993) 11542. [15] M. Quinn, G. Mills, J. Phys. Chem. 98 (1994) 9840. [16] F. Johnston, J. Radiat. Res. 75 (1978) 286. [17] D.B. Zhang, H.M. Cheng, J.M. Ma, J. Mater. Sci. Lett. 20 (2001) 439. [18] C.-L. Lee, C.-C. Wan, Y.-Y. Wang, Adv. Funct. Mater. 11 (2001) 334. [19] A. Henglein, P. Mulvaney, T. Linnert, A. Holzwarth, J. Phys. Chem. 96 (1992) 2411. [20] C.P. Lee, D. Meisel, J. Catal. 70 (1981) 160. [21] A. Henglein, P. Mulvaney, A. Holzwarth, T.E. Sosebee, Ber. Bunsen-Ges. Phys. Chem. 96 (1992) 754.
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[22] A. Henglein, J. Phys. Chem. 97 (1993) 5457. [23] (a) S. Link, M.B. Mohamed, M.A. El-Sayed, J. Phys. Chem. B 103 (1999) 3073; (b) S. Link, M.A. El-Sayed, J. Phys. Chem. B 103 (1999) 8410. [24] T. Wenzel, J. Bosbach, F. Stietz, F. Trager, Surf. Sci. 432 (1999) 257.
[25] S. Chen, Z. Fan, S.L. Carroll, J. Phys. Chem. B 106 (2002) 10777. [26] C.-Y. Wang, C.-Y. Liu, J.-J. He, X.-B. Yan, M.-H. Zhang, T. Shen, J. Photochem. Photobiochem. A: Chem. 104 (1997) 159. [27] C.-Y. Wang, C.-Y. Liu, Y. Liu, Z.-Y. Zhang, Appl. Surf. Sci. 147 (1999) 52.
Formation of silver nanoparticles in an acid-catalyzed silica colloidal solution Zhong-Jie Jiang, Chun-Yan Liu*, Yun Liu Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100101, China Received in revised form 2 March 2004; accepted 9 March 2004 Available online 6 May 2004
Abstract In a weak basic, weak acidic or neutral water–alcohol solution, silver nanoparticles were generated by the reduction of Agþ ions in the present of colloidal silica. Silica as a substrate played an important role in the formation of Silver particles. The plasmon band of silver particles supported on the surface of silica was considerably shifted to longer wavelength compared with the pure silver sol. The shift in absorption spectra was explained in terms of surface effects induced by the interaction of silver and silica, as well as size effects and irregular shape. # 2004 Elsevier B.V. All rights reserved. Keywords: Silica colloidal particles; Silver nanoparticles; Plasmon absorption band
1. Introduction Recently, much effort has been devoted to studying metal/oxide systems because of their broad applications in many areas, such as heterogeneous catalysis, ceramics, thin film technology and microelectronic devices [1]. Great attention has been paid to both experimental and theoretical investigation to well understand the interaction between metals and supports [2]. It is experimentally proved that strong metal–support interaction (SMSI) exists between metal and supports [3–5] and the properties of composites is closely dependent on their long-range
*
Corresponding author. Tel.: þ86-10-64888179; fax: þ86-10-64879375. E-mail address: [email protected] (C.-Y. Liu).
correlation of positions or orientations of active constituents [6]. So far, many metal/oxide systems have been extensively investigated. Among the varying oxide substrates, TiO2, whatever occurs naturally as mineral rutile or as less common polymorphs anatase and brookite, has been one of the most investigation systems [7,8]. Currently, the system of metal/SiO2 has become a research focus, especially on the system of Au/SiO2 [9,10]. But the study on the interaction and electron transfer between metal and SiO2 is less reported. In the present work, we studied a Ag/SiO2 system. The aim of this research was to explore the formation process of Ag/SiO2 in nanoscale, microstructure, interaction between metal and semiconductor and the changes in the absorption band of silver nanoparticles supported on SiO2 with respect to microenvironments.
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.03.224
136
Z.-J. Jiang et al. / Applied Surface Science 233 (2004) 135–140
2. Experimental section 2.1. Materials and Instrumentation Ethanol, tetraethyl orthosilicate (TEOS, CR) and AgNO3 were purchased from Beijing Chemical Reagent Co. and were used as received without further purification. UV-Vis absorption spectra were conducted in the range between 250 and 700 nm using a SHIMAZDU UV-1601PC UV-Vis scanning spectrometer. The transmission electron microscopy (TEM) image was observed and recorded with JEM-100CX at an acceleration voltage of 80 kV and Tecnai 20 transmission electron microscopy at an acceleration voltage of 160 kV. The electron diffraction (ED) was detected by Tecnai 20 transmission electron microscopy. The X-ray photoelectron spectra (XPS) were recorded with an X SAM 800 electron energy spectrometer, using Al Ka radiation under high vacuum conditions. 2.2. Preparation Colloidal silica was prepared by an acid-catalyzed sol–gel reaction of TEOS. In a typical preparation, 5 mL of TEOS was added into 5 mL ethanol. Next, the mixture was injected into 30 mL of 0.1 mol/L HNO3 solution, which was kept at 60 8C for 8–10 h to ensure that the reaction was complete. Finally, a transparent colloidal solution was gained. The volume of the solution was adjusted to 50 mL with deionized water. The final concentration of silica was 26 mg/mL. The pH of the silica colloidal solution was adjusted by 0.1 mol/L NaOH to appropriate values. One milliliter of 2 104 mol/L AgNO3 was added to a certain amount of the silica colloidal solution. The color of mixture gradually evolved, indicating the formation of the Ag/SiO2 composites.
3. Results Fig. 1 showed a typical TEM image of silver obtained in the silica colloidal solution. As shown in Fig. 1, silver particles had irregular shapes with diameters range from 15 to 30 nm. However, the silica particles prepared by an acid-catalyzed sol–gel method usually induced amorphous shape [11,12].
Fig. 1. TEM micrograph of silver particles on the surface of the colloidal silica.
The experiments showed that in the absence of silica, the reduction of Agþ ions in the basic alcoholic solution was a fast process with alcohol as a reductant, and a brown precipitate appeared in the solution. The reduction of Agþ ions by alcohol molecules could not occur in the weak acid or neutral alcoholic solution without silica. However, in the water–alcohol mixed solution containing silica colloid prepared here, the reduction of Agþ ions slowly proceeded and formed silver nanoparticles on the surface of silica, whatever the pH value of the colloidal solution was weak acid, neutral or weak basic. During the reaction, the system color gradually changed from a colorless to bright yellow. The evolution of absorption spectra with the reaction time was presented in Fig. 2. A broad adsorption band centered at 415–445 cm was developed and shifted continuously to a longer wavelength during the reaction. This is the plasmon band of silver particles. The pH value of the solution could influence the position of the plasmon band of silver nanoparticles deposited on silica. As shown in Fig. 3, the absorption band became stronger and the absorption maximum shifted to longer wavelength with the increasing pH values. When a pH value reached a certain value, the intensity of the absorption maximum began to decrease and the position of absorption band continuously shifted to longer wavelength.
Z.-J. Jiang et al. / Applied Surface Science 233 (2004) 135–140
Fig. 2. Evolution of the UV-Vis absorption spectra with the reaction time. ½SiO2 ¼ 0:145 mol/L, ½Ag ¼ 3:3 104 mol/L, pH ¼ 7:6.
Fig. 3. UV-Vis absorption spectra of silver nanoparitcles on the silica at different pH, ½SiO2 ¼ 0:145 mol/L, ½Ag ¼ 3:3 104 mol/L.
137
particles of Ag and Au have been prepared using this procedure [14,15]. In the reactions, the hydroxide ions played a crucial role in the reaction of metal ions with alcohol [14]. The experiments showed here, however, in the water–alcohol solution containing the colloidal silica, the reduction of Agþ ions could be carried out not only in basic, but also in a weak basic or neutral solution, even in an acidic solution (Fig. 3). This result indicated the colloidal silica prepared by the acidcatalyzed sol–gel method had a significantly impact on the reduction of Agþ ions, which provided a twodimensional microenvironment for reactant molecules and promoted the reaction. Johnston [16] and Zhang et al. [17] have demonstrated that the reduction of Agþ ions occurs preferentially on the surface of semiconductors. As shown in Fig. 3, when the pH value of the solution was less than 2, the reduction of Agþ ions was slower, and with the increasing pH, the reaction rate speeded up. This made clear that the pH value of the solution and the surface property of colloidal silica were important to the formation of Ag particles on the surface of silica. As is well know, the isoelectric point (iep) of silica is at about pH 2–3 [12]. Above the iep, the density of negative charge on the surface of silica increased with the increasing pH, which availed to the adsorption of Agþ ions by electrostatic interactions. Fig. 4 showed the acid–base titration curves of the colloidal silica solution and the acidic alcohol solution. Because OH ions partly adsorbed onto the surface of silica, the concentration of hydroxyl ions in the solution decreased, while the negative charge
According to the experiments, when the size of silica nanoparticles increased, for example bigger than 40 nm, the above noted reduction of Agþ became difficult. The processes about the adsorption, reduction and nucleation of Agþ ions on the surface of silica nanoparticles bigger than 40 nm have been discussed elsewhere [13].
4. Discussion The metal particles could be prepared in basic alcohols, which is based on the instability of some metal complexes in the basic alcohols and resulted in spontaneous reduction of metal ions. Small colloidal
Fig. 4. Titration curves of neutralization: A, the acidic silica colloidal solution containing alcohol; B, the same solution as A without silica.
138
Z.-J. Jiang et al. / Applied Surface Science 233 (2004) 135–140
density on the surface of silica increased and two curves were distinctly different. As a result, the reduction of Agþ could take place, even if the solution was weak acid. Huang et al. [14] and Lee et al. [18] have investigated the reduction Agþ ions in the alcohol solutions. They believed that the Agþ ions in the basic alcoholic solution could be reduced and formed metal particles, and meanwhile, the alcohol molecules were oxidized to form corresponding aldehyde and acid. In the present work, the reduction of Agþ ions was related with OH adsorbed on the surface of silica. Firstly, Agþ ions diffused onto the surface of silica and reacted with OH adsorbed on the surface of silica to form Ag2O particles. Subsequently, the Ag2O particles were reduced by ethanol and deposited on the silica. Concretely, the mechanism could be represented by the following reaction: 2Agþ þ 2OH ads
adsorption
! Ag2 Oads
SiO2
(1)
Ag2 Oads þ CH3 CH2 OH ! CH3 CHO þ 2Agads þ H2 O
(2)
Ag2 Oads þ CH3 CHO ! CH3 COO þ 2Agads þ Hþ
(3)
Hþ þ OH ads ! H2 O
(4)
The overall reaction : 4Agþ þ 5OH ads þ CH3 CH2 OH ! 4Agads þ CH3 COO þ 2H2 O
Where OH ads represented the hydroxide ions adsorbed on the surface of silica, Ag2Oads was Ag2O particles deposited on the surface of silica, and Agads corresponded to silver nanoparticles supported on the surface of silica. The electrode poten0 0 tials of ECH , EAg and þ 3 CHOþ2H =CH3 CH2 OH 2 O=AgþOH 0 ECH3 COO þ2Hþ =CH3 CHOþH2 O were þ0.34, 0.2 and 0.4, respectively. Evidently, the reactions described above were reasonable. To well understand that the silver particles were formed on the surface of the silica prepared here, an XPS experiment was performed (Fig. 5). As shown in Fig. 5, the electron transfer from the silver particles to the silica was confirmed. In the absence of silver, the binding energies of Si 2p and O 1s of SiO2 particles were 103.75 and 533.25 eV, respectively. For the silica particles deposited silver particles, the binding energies for Si 2p and O 1s were 103.05 and 532.60 eV, respectively. Evidently, the binding energies of Si 2p and O 1s negatively shifted, which indicated the electron transfer from Ag particles to silica. As we know, the silica particles would be a semiconductor when the size of particles was small, which might be responsible for the electron transfer between the silica and silver particle. This behavior is very different from the characteristic of silica in bulk phase. Compared with Ag particles in the silver sol [19– 21], the plasmon band of silver particles supported on the surface of silica was significantly shifted to longer wavelength. The shifts in absorption spectra could be explained in terms of surface effects induced by the interaction of silver and silica. As is well known, the
Fig. 5. Binding energy of Si 2p and O 1s for the colloidal silica determined by XPS: (a) silica colloid in the absence of silver particles; (b) silica colloidal deposited silver particles.
Z.-J. Jiang et al. / Applied Surface Science 233 (2004) 135–140
shape and position of the plasmon absorption band is very sensitive to microenvironment around silver particles. For example, a red shift of the absorption band would be observed [19,21], when Cd2þ, Pb2þ, and In3þ ions were adsorbed onto the surface of a silver particle, which could be explained by a change of electron density on the surface of the silver particles due to the adsorption of metal ions or deposition of metal clusters [14,15,22]. In the present work, the electron transfer from Ag particles to silica took place, which resulted in a decrease in electron density on the surface of silver particles and a red shift of the silver plasmon band. Red shift of the plasmon band might be also a result of size effects. Size effects are pronounced for particles bigger than 20 nm [15]. With increasing a size, the absorption band of silver can gradually shift to longer wavelength. As was seen from Fig. 2, with increasing the reaction time, the size of silver particles increased, and the plasmon band shifted to longer wavelength. Similarly, the change of pH would influence the reduction reaction of Agþ, so that the size of Ag particles increased and the plasmon band also shifted to longer wavelength (as shown in Fig. 3). The irregular shape is also an important factor to influence the position of plasmon band of silver particles. The plasmon band of a metal particle is very sensitive to particles shape [23–25]. As shown in Fig. 1, silver particles prepared in the present work were irregularly shaped. Such irregular shape would result in significant red shift of plasmon band. In addition, the position of band was also effected by other factors, such as media constant of a solution, refraction coefficient, etc. [26,27].
5. Conclusion In conclusion, in the present of the colloidal silica prepared by the acid-catalyzed sol–gel method, silver nanoparticles could be generated not only in the basic, but also in the weak acid, weak basic or neutral water– alcohol mixed solution. The plasmon band of silver particles supported on the surface of silica was considerably shifted to a longer wavelength compared with a pure silver sol. The shift in the absorption band basically resulted from surface effects, size effects and irregular shape. XPS showed that the binding energy
139
for Si 2p and O 1s of silica loaded silver nanoparticle was negatively shifted, which indicated that the formation of small silver particles on the surface of silica would result in electronic interactions between the two types of particles, which was very different from the behavior of bulk silica.
Acknowledgements The authors thank the support of the National Natural Science Foundation of China (90306003).
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