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A facile and novel way for the synthesis of nearly monodisperse silver nanoparticles
Materials Research Bulletin 42 (2007) 1657–1661 www.elsevier.com/locate/matresbu
A facile and novel way for the synthesis of nearly monodisperse silver nanoparticles Zhitao Chen, Lian Gao * State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China Received 16 August 2006; received in revised form 16 October 2006; accepted 22 November 2006 Available online 3 January 2007
Abstract A facile and novel way was reported for the preparation of nearly monodisperse silver nanoparticles with controlled hydrophilic or hydrophobic surface, using trioctylphosphine as the surfactant and stabilizer. The synthesized nanoparticles were characterized by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) and UV–vis spectroscopy. The monodisperse silver nanoparticles showed a strong surface plasmon resonance band at 402 nm from the UV–vis spectrum. # 2006 Elsevier Ltd. All rights reserved. Keywords: A. Metals; A. Nanostructure; B. Chemical synthesis; C. Electron microscopy; D. Optical properties
1. Introduction Researches on metal particles in nanometer order (nanoparticles) have been reported extensively from expectation to obtain novel functions arising from the ‘‘quantum size effect’’. Many preparation methods and some unique properties of metallic nanoparticles have been reported [1–3]. In comparison to Au nanocrystals, synthesis of other noble metal nanocrystals is less developed [4]. Silver nanoparticles have been studied because they play important roles in different branches of science, such as chemical catalysis (CO oxidation on Au/TiO2 composites) [5], nanoelectronics (single-electron transistors, electrical connects) [6], conductive coatings [7], chemical analysis (chemical and biological sensors) [8], etc. Most of these applications require nanoparticles with narrow size distribution whose surface need to be derivatizable with hydrophobic and hydrophilic surfactants. In the past, silver nanoparticles were synthesized mainly by two methods: the citrate method introduced by Faraday [9], and the two-phase method described by Brust et al. [10]. The citrate method produces nearly monodisperse silver nanoparticles in the size range from 2 to 100 nm. But the problems of this method are the restriction to water as a solvent and a low silver particle content of the resulting solutions. The Brust method and its variations [11] are the most popular synthetic schemes in the field now. But the size range of the Brust method is limited to about 1–5 nm and the size distribution is broad. The other drawback of this method is that the resulting nanoparticles are coated with a monolayer of strong ligands, which makes it difficult to achieve the surface functionalization and modification needed
* Corresponding author. Tel.: +86 21 52412718; fax: +86 21 52413122. E-mail address: [email protected] (L. Gao). 0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2006.11.028
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for some purposes. In recent years, there are many methods regarding the monodisperse silver nanoparticles, such as the reduction of noble metal ions by ethanol [12] or by ethylene glycol in the presence of PVP [13], etc. Moreover, organometallic strategies as an outstanding method for synthesizing monodisperse nanoparticles is applied to the preparation of metal nanoparticles extensively, including the synthesis of gold [14], silver [15–17] and cobalt [18] nanoparticles. The system of organometallic strategies are mainly made up of trioctylphosphine (TOP)– trioctylphosphine oxide (TOPO) or oleic acid–oleic amine, etc. Using the organometallic route, we can get monodisperse nanoparticles with narrow size distribution and stable property in air. However, this route requires well manipulation skill and high reaction temperature (250–300 8C). Herein, base on these research and consideration, we report a facile, reproducible, single-phase method for the preparation of nearly monodisperse silver nanoparticles. In this approach, we use the single-phase system of silver nitrate dissolved in TOP as raw materials. TOP plays the role of the reducing agent, solvent, stabilizer and surfactant. 2. Experimental The synthetic procedure is described as follows. In a cuvette of 20 mL capacity, 0.2 g of AgNO3 was transferred to 10 mL of trioctylphosphine and stirred for 24 h until the AgNO3 was dissolved. Then this solution was heated to a destined centigrade in a silicon oil bath. The destined temperature ranged from 160 to 200 8C. The color of the solution slowly turned from slight yellow to orange, then to dark brown as the reaction proceeded. When the solvent became bright yellow, the cuvette was extracted and double volume acetone was added to precipitate the silver particles from
Fig. 1. The TEM image, SAED pattern and the size distribution and HRTEM image of the sample prepared at 180 8C for 3 min.
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the solution. The products were separated by centrifugation, and washed three times with 30 mL of acetone to remove unreacted starting materials. These Ag nanoparticles can be easily dispersed into toluene. Just added a little toluene (0.2–1 mL), the solution will become yellow. The morphology, microstructure and composition of the samples were determined by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) using a field emission transmission electron microscope (JEM-2100F). UV–vis absorption spectra analysis was performed on a Shimadzu UV-3101PC spectrophotometer with liquid samples at room temperature. 3. Result and discussion The TEM, HRTEM image and SAED pattern of a typical sample prepared at 180 8C for 3 min are shown in Fig. 1, meanwhile the size distribution of this sample was given via the TEM photograph. Fig. 1(a) shows that the diameter of the particles is uniform, which is corresponded with Fig. 1(c). In Fig. 1(b), the SAED pattern is corresponding to the fcc Ag. The strongest peak is homologous to (1 1 1) facet of silver. The HRTEM image (Fig. 1(d)) reveals that the particle is single crystal. The fringe spacing between two crystal layers of silver is about 0.23 nm, which is consistent with the (1 1 1) facet of fcc Ag. Fig. 2 is the TEM images of the silver particles at various reaction conditions. At different reaction conditions, the spherical nanocrystals of Ag have an average diameter from 6 to 10 nm. The particles are well dispersed. Fig. 1(a) is
Fig. 2. Representative TEM images of silver nanoparticles synthesized at various temperature and reaction time: (a) 180 8C, 3 min; (b) 180 8C, 5 min; (c) 190 8C, 3 min; (d) 200 8C, 3 min.
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Fig. 3. UV–vis absorption spectrum of the typical sample prepared at 180 8C for 3 min.
the high magnification micrograph of the sample prepared at 180 8C for 3 min. The average diameter of the four samples is 6.8, 7.8, 8.5 and 9.3 nm, respectively. With the increase of the reaction temperature, the diameters of the particle increase obviously and the aggregations of particle begin to appear, as shown in Fig. 2(d). The proper temperature and reaction time are the most important conditions for the synthesis of nearly monodisperse nanoparticles [19,20]. In the experimental process, silver nanoparticles could not be obtained when the reaction temperature is below 160 8C even that the holding time is prolonged to 30 min. The initially formed silver atoms selfnucleate to form a fixed number of seeds during the first stage of the reaction, and particles then continue to grow by diffusion-driven deposition of silver atoms onto these existing seeds [21–23]. The UV–vis spectrum of the silver nanoparticles dispersed in acetone is shown in Fig. 3. Silver nanoparticles have a surface plasmon band at 402 nm, consistent with other reports [15,16]. The other samples dispersed in acetone or toluene have a very similar absorption curve. Xia and co-wokers [24,25] and Qi and co-wokers [26] reported the silver nanoparticles and nanowires plasmon band at 410 nm with the diameter of 20–30 nm. Research has shown that the energy of the surface plasmon band is sensitive to various factors, including particle shape, size, surrounding media and interparticle interactions. The stability of TOPO-capping silver nanoparticles in toluene was tested for a period of a few weeks by measuring their absorption spectra and any change in intensity or location of the surface plasmon peak was not observed, confirming that the particles in organic solvent were stable even lasted for a long time. TOP has been used widely for the synthesis of semiconductor nanomaterials as a high efficient surfactant, such as CdSe, CdS, etc. [27,28]. In our experiment, TOP was used as reducing agent and stabilizer preventing the particles from aggregation. The surface of the nanoparticles is hydrophobic owing to the adsorbed TOP on the surface of the product. The starting product is capping with ligands of trioctyl, which is a hydrophobic functional group, so these silver nanoparticles have hydrophobic surface and can disperse into organic solvent easily. When these product mixed with amphiphilic molecule, such as amino-hexanol or triethylamine, their surface will possess a hydrophilic group and result in that they can dispersed into water well. Adding 2 mL of amino-hexanol (or triethylamine) into one cuvette, and then loading the mixture into a vacuum oven at 60 8C for several minutes, the surface group of Ag nanoparticles is changed into hydrophilic quickly. This solution can be mixed with ethanol and water, which can be stable up to 3 h. This solution can also be mixed with ethylene glycol, but it is just stable for several minutes. The reason for these phenomena has not been well known. This changeable surface group property of silver particles can make them promising materials to modify and functionalize some traditional organic materials by capping with them.
4. Conclusions In summary, nearly monodisperse silver nanoparticles with narrow size distribution and changeable surface characteristic have been synthesized successfully via a facile and novel organic route. The proper reaction temperature and appropriate heating time is the key factor to synthesize nearly monodisperse silver nanoparticles. The UV–vis
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curve of the silver nanoparticles indicates that it can be a promising candidate as useful functional units in nanoelectronic devices based on the optical property. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
B.L. Cushing, V.L. Kolesnichenko, C.J. O’Connor, Chem. Rev. 104 (2004) 3893. M.C. Daniel, D. Astruc, Chem. Rev. 104 (2004) 293. B.L.V. Prasad, S.K. Arumugam, T. Bala, M. Sastry, Langmuir 21 (2005) 822. Filankembo, S. Giorgio, I. Lisiecki, M.P. Pileni, J. Phys. Chem. B 107 (2003) 7492. J.J. Pietron, R.M. Stroud, D.R. Rolison, Nano Lett. 2 (2002) 545. T. Sato, H. Ahemd, D. Brown, B.F.G. Johnson, J. Appl. Phys. 82 (1997) 696. W.P. Wuelfing, F.P. Zamborini, A.C. Templeton, X.G. Ween, H. Yoon, R.W. Murray, Chem. Mater. 13 (2001) 87. W.P. Wuelfing, R.W. Murray, J. Phys. Chem. B 106 (2002) 3139. M. Faraday, Philos. Trans. R. Soc. Lond. (1857) 145. M. Brust, M. Walker, D. Bethell, D.J. Schiffrin, R. Whyman, Chem. Commun. (1994) 801. D.E. Cliffel, F.P. Zamborini, S.M. Gross, R.W. Murray, Langmuir 16 (2000) 9699. X. Wang, J. Zhuang, Q. Peng, Y. Li, Nature 437 (2005) 121. Y. Sun, Y. Xia, Science 298 (2002) 2176. M. Green, P. O’Brien, Chem. Commun. (2000) 183. M. Green, N. Allsop, G. Wakefield, P.J. Dobson, J.L. Hutchison, J. Mater. Chem. 12 (2002) 2671. S.W. Kim, J. Paark, Y. Jang, Y. Chung, S. Hwang, T. Hyeon, Nano Lett. 3 (2003) 1289. S.D. Bunge, T.J. Boyle, T.J. Headley, Nano Lett. 3 (2003) 901. A. Holzwarth, J. Tou, T.A. Hatton, P.E. Laibinis, Ind. Eng. Chem. Res. 37 (1998) 2701. T. Yonezawa, S. Onoue, N. Kimizuka, Langmuir 16 (2000) 5218. H. Hiramatsu, F.E. Osteroh, Chem. Mater. 16 (2004) 2509. Z.M. Sui, X. Chen, L.Y. Wang, Y.C. Chai, C.J. Yang, J.K. Zhao, Chem. Lett. 34 (2005) 100. G. Wang, Y. Zhang, Y.P. Cui, M.Y. Duan, M. Liu, J. Phys. Chem. B 109 (2005) 1067. N.R. Jana, X.G. Peng, J. Am. Chem. Soc. 125 (2003) 14280. Y.G. Sun, B. Gates, B. Mayers, Y.N. Xia, Nano Lett. 2 (2002) 165. Y.G. Sun, Y.N. Xia, Adv. Mater. 14 (2002) 833. D.B. Zhang, L.M. Qi, J.H. Yang, J.M. Ma, H.M. Cheng, L. Huang, Chem. Mater. 16 (2004) 872. L. Manna, E.C. Scher, A.P. Alivisatos, J. Am. Chem. Soc. 122 (2000) 12700. X.G. Peng, J. Wickham, A.P. Alivisatos, J. Am. Chem. Soc. 120 (1998) 5343.
A facile and novel way for the synthesis of nearly monodisperse silver nanoparticles Zhitao Chen, Lian Gao * State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China Received 16 August 2006; received in revised form 16 October 2006; accepted 22 November 2006 Available online 3 January 2007
Abstract A facile and novel way was reported for the preparation of nearly monodisperse silver nanoparticles with controlled hydrophilic or hydrophobic surface, using trioctylphosphine as the surfactant and stabilizer. The synthesized nanoparticles were characterized by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED) and UV–vis spectroscopy. The monodisperse silver nanoparticles showed a strong surface plasmon resonance band at 402 nm from the UV–vis spectrum. # 2006 Elsevier Ltd. All rights reserved. Keywords: A. Metals; A. Nanostructure; B. Chemical synthesis; C. Electron microscopy; D. Optical properties
1. Introduction Researches on metal particles in nanometer order (nanoparticles) have been reported extensively from expectation to obtain novel functions arising from the ‘‘quantum size effect’’. Many preparation methods and some unique properties of metallic nanoparticles have been reported [1–3]. In comparison to Au nanocrystals, synthesis of other noble metal nanocrystals is less developed [4]. Silver nanoparticles have been studied because they play important roles in different branches of science, such as chemical catalysis (CO oxidation on Au/TiO2 composites) [5], nanoelectronics (single-electron transistors, electrical connects) [6], conductive coatings [7], chemical analysis (chemical and biological sensors) [8], etc. Most of these applications require nanoparticles with narrow size distribution whose surface need to be derivatizable with hydrophobic and hydrophilic surfactants. In the past, silver nanoparticles were synthesized mainly by two methods: the citrate method introduced by Faraday [9], and the two-phase method described by Brust et al. [10]. The citrate method produces nearly monodisperse silver nanoparticles in the size range from 2 to 100 nm. But the problems of this method are the restriction to water as a solvent and a low silver particle content of the resulting solutions. The Brust method and its variations [11] are the most popular synthetic schemes in the field now. But the size range of the Brust method is limited to about 1–5 nm and the size distribution is broad. The other drawback of this method is that the resulting nanoparticles are coated with a monolayer of strong ligands, which makes it difficult to achieve the surface functionalization and modification needed
* Corresponding author. Tel.: +86 21 52412718; fax: +86 21 52413122. E-mail address: [email protected] (L. Gao). 0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2006.11.028
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for some purposes. In recent years, there are many methods regarding the monodisperse silver nanoparticles, such as the reduction of noble metal ions by ethanol [12] or by ethylene glycol in the presence of PVP [13], etc. Moreover, organometallic strategies as an outstanding method for synthesizing monodisperse nanoparticles is applied to the preparation of metal nanoparticles extensively, including the synthesis of gold [14], silver [15–17] and cobalt [18] nanoparticles. The system of organometallic strategies are mainly made up of trioctylphosphine (TOP)– trioctylphosphine oxide (TOPO) or oleic acid–oleic amine, etc. Using the organometallic route, we can get monodisperse nanoparticles with narrow size distribution and stable property in air. However, this route requires well manipulation skill and high reaction temperature (250–300 8C). Herein, base on these research and consideration, we report a facile, reproducible, single-phase method for the preparation of nearly monodisperse silver nanoparticles. In this approach, we use the single-phase system of silver nitrate dissolved in TOP as raw materials. TOP plays the role of the reducing agent, solvent, stabilizer and surfactant. 2. Experimental The synthetic procedure is described as follows. In a cuvette of 20 mL capacity, 0.2 g of AgNO3 was transferred to 10 mL of trioctylphosphine and stirred for 24 h until the AgNO3 was dissolved. Then this solution was heated to a destined centigrade in a silicon oil bath. The destined temperature ranged from 160 to 200 8C. The color of the solution slowly turned from slight yellow to orange, then to dark brown as the reaction proceeded. When the solvent became bright yellow, the cuvette was extracted and double volume acetone was added to precipitate the silver particles from
Fig. 1. The TEM image, SAED pattern and the size distribution and HRTEM image of the sample prepared at 180 8C for 3 min.
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the solution. The products were separated by centrifugation, and washed three times with 30 mL of acetone to remove unreacted starting materials. These Ag nanoparticles can be easily dispersed into toluene. Just added a little toluene (0.2–1 mL), the solution will become yellow. The morphology, microstructure and composition of the samples were determined by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) using a field emission transmission electron microscope (JEM-2100F). UV–vis absorption spectra analysis was performed on a Shimadzu UV-3101PC spectrophotometer with liquid samples at room temperature. 3. Result and discussion The TEM, HRTEM image and SAED pattern of a typical sample prepared at 180 8C for 3 min are shown in Fig. 1, meanwhile the size distribution of this sample was given via the TEM photograph. Fig. 1(a) shows that the diameter of the particles is uniform, which is corresponded with Fig. 1(c). In Fig. 1(b), the SAED pattern is corresponding to the fcc Ag. The strongest peak is homologous to (1 1 1) facet of silver. The HRTEM image (Fig. 1(d)) reveals that the particle is single crystal. The fringe spacing between two crystal layers of silver is about 0.23 nm, which is consistent with the (1 1 1) facet of fcc Ag. Fig. 2 is the TEM images of the silver particles at various reaction conditions. At different reaction conditions, the spherical nanocrystals of Ag have an average diameter from 6 to 10 nm. The particles are well dispersed. Fig. 1(a) is
Fig. 2. Representative TEM images of silver nanoparticles synthesized at various temperature and reaction time: (a) 180 8C, 3 min; (b) 180 8C, 5 min; (c) 190 8C, 3 min; (d) 200 8C, 3 min.
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Fig. 3. UV–vis absorption spectrum of the typical sample prepared at 180 8C for 3 min.
the high magnification micrograph of the sample prepared at 180 8C for 3 min. The average diameter of the four samples is 6.8, 7.8, 8.5 and 9.3 nm, respectively. With the increase of the reaction temperature, the diameters of the particle increase obviously and the aggregations of particle begin to appear, as shown in Fig. 2(d). The proper temperature and reaction time are the most important conditions for the synthesis of nearly monodisperse nanoparticles [19,20]. In the experimental process, silver nanoparticles could not be obtained when the reaction temperature is below 160 8C even that the holding time is prolonged to 30 min. The initially formed silver atoms selfnucleate to form a fixed number of seeds during the first stage of the reaction, and particles then continue to grow by diffusion-driven deposition of silver atoms onto these existing seeds [21–23]. The UV–vis spectrum of the silver nanoparticles dispersed in acetone is shown in Fig. 3. Silver nanoparticles have a surface plasmon band at 402 nm, consistent with other reports [15,16]. The other samples dispersed in acetone or toluene have a very similar absorption curve. Xia and co-wokers [24,25] and Qi and co-wokers [26] reported the silver nanoparticles and nanowires plasmon band at 410 nm with the diameter of 20–30 nm. Research has shown that the energy of the surface plasmon band is sensitive to various factors, including particle shape, size, surrounding media and interparticle interactions. The stability of TOPO-capping silver nanoparticles in toluene was tested for a period of a few weeks by measuring their absorption spectra and any change in intensity or location of the surface plasmon peak was not observed, confirming that the particles in organic solvent were stable even lasted for a long time. TOP has been used widely for the synthesis of semiconductor nanomaterials as a high efficient surfactant, such as CdSe, CdS, etc. [27,28]. In our experiment, TOP was used as reducing agent and stabilizer preventing the particles from aggregation. The surface of the nanoparticles is hydrophobic owing to the adsorbed TOP on the surface of the product. The starting product is capping with ligands of trioctyl, which is a hydrophobic functional group, so these silver nanoparticles have hydrophobic surface and can disperse into organic solvent easily. When these product mixed with amphiphilic molecule, such as amino-hexanol or triethylamine, their surface will possess a hydrophilic group and result in that they can dispersed into water well. Adding 2 mL of amino-hexanol (or triethylamine) into one cuvette, and then loading the mixture into a vacuum oven at 60 8C for several minutes, the surface group of Ag nanoparticles is changed into hydrophilic quickly. This solution can be mixed with ethanol and water, which can be stable up to 3 h. This solution can also be mixed with ethylene glycol, but it is just stable for several minutes. The reason for these phenomena has not been well known. This changeable surface group property of silver particles can make them promising materials to modify and functionalize some traditional organic materials by capping with them.
4. Conclusions In summary, nearly monodisperse silver nanoparticles with narrow size distribution and changeable surface characteristic have been synthesized successfully via a facile and novel organic route. The proper reaction temperature and appropriate heating time is the key factor to synthesize nearly monodisperse silver nanoparticles. The UV–vis
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curve of the silver nanoparticles indicates that it can be a promising candidate as useful functional units in nanoelectronic devices based on the optical property. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
B.L. Cushing, V.L. Kolesnichenko, C.J. O’Connor, Chem. Rev. 104 (2004) 3893. M.C. Daniel, D. Astruc, Chem. Rev. 104 (2004) 293. B.L.V. Prasad, S.K. Arumugam, T. Bala, M. Sastry, Langmuir 21 (2005) 822. Filankembo, S. Giorgio, I. Lisiecki, M.P. Pileni, J. Phys. Chem. B 107 (2003) 7492. J.J. Pietron, R.M. Stroud, D.R. Rolison, Nano Lett. 2 (2002) 545. T. Sato, H. Ahemd, D. Brown, B.F.G. Johnson, J. Appl. Phys. 82 (1997) 696. W.P. Wuelfing, F.P. Zamborini, A.C. Templeton, X.G. Ween, H. Yoon, R.W. Murray, Chem. Mater. 13 (2001) 87. W.P. Wuelfing, R.W. Murray, J. Phys. Chem. B 106 (2002) 3139. M. Faraday, Philos. Trans. R. Soc. Lond. (1857) 145. M. Brust, M. Walker, D. Bethell, D.J. Schiffrin, R. Whyman, Chem. Commun. (1994) 801. D.E. Cliffel, F.P. Zamborini, S.M. Gross, R.W. Murray, Langmuir 16 (2000) 9699. X. Wang, J. Zhuang, Q. Peng, Y. Li, Nature 437 (2005) 121. Y. Sun, Y. Xia, Science 298 (2002) 2176. M. Green, P. O’Brien, Chem. Commun. (2000) 183. M. Green, N. Allsop, G. Wakefield, P.J. Dobson, J.L. Hutchison, J. Mater. Chem. 12 (2002) 2671. S.W. Kim, J. Paark, Y. Jang, Y. Chung, S. Hwang, T. Hyeon, Nano Lett. 3 (2003) 1289. S.D. Bunge, T.J. Boyle, T.J. Headley, Nano Lett. 3 (2003) 901. A. Holzwarth, J. Tou, T.A. Hatton, P.E. Laibinis, Ind. Eng. Chem. Res. 37 (1998) 2701. T. Yonezawa, S. Onoue, N. Kimizuka, Langmuir 16 (2000) 5218. H. Hiramatsu, F.E. Osteroh, Chem. Mater. 16 (2004) 2509. Z.M. Sui, X. Chen, L.Y. Wang, Y.C. Chai, C.J. Yang, J.K. Zhao, Chem. Lett. 34 (2005) 100. G. Wang, Y. Zhang, Y.P. Cui, M.Y. Duan, M. Liu, J. Phys. Chem. B 109 (2005) 1067. N.R. Jana, X.G. Peng, J. Am. Chem. Soc. 125 (2003) 14280. Y.G. Sun, B. Gates, B. Mayers, Y.N. Xia, Nano Lett. 2 (2002) 165. Y.G. Sun, Y.N. Xia, Adv. Mater. 14 (2002) 833. D.B. Zhang, L.M. Qi, J.H. Yang, J.M. Ma, H.M. Cheng, L. Huang, Chem. Mater. 16 (2004) 872. L. Manna, E.C. Scher, A.P. Alivisatos, J. Am. Chem. Soc. 122 (2000) 12700. X.G. Peng, J. Wickham, A.P. Alivisatos, J. Am. Chem. Soc. 120 (1998) 5343.