- Email: [email protected]
Synthesis of oleate capped Cu nanoparticles by thermal decomposition
Colloids and Surfaces A: Physicochem. Eng. Aspects 284–285 (2006) 364–368
Synthesis of oleate capped Cu nanoparticles by thermal decomposition Young Hwan Kim a , Don Keun Lee a , Beong Gi Jo b , Ji Hean Jeong b , Young Soo Kang a,∗ a
Department of Chemistry, Pukyong National University, 599-1 Daeyon 3-Dong, Namgu, Pusan 608-737, South Korea b R&D Center, Coreana Cosmetics Co., Ltd., Cheonan-si 330-830, South Korea Received 25 June 2005; received in revised form 13 October 2005; accepted 28 October 2005 Available online 15 December 2005
Abstract A method has been introduced to produce monodispersed copper nanoparticle using thermal decomposition of Cu-oleate complex, which was prepared by the reaction of CuCl2 with sodium oleate in water solution. Sodium oleate acts as a stabilizer of the Cu nanoparticles and protector of oxidation of Cu nanoparticles in various concentrations. We also found that no extra inert gases for protecting the oxidation of Cu nanoparticles were necessary. By TGA measurement, thermal decompositions of Cu-oleate complexes were observed. TEM data show that at 0.05 M oleate concentration, spherical copper nanoparticles are prepared, but at high concentration (0.3 M), large lumps are formed. This method can be easily increased scale up for industrial purpose. We confirm that these nanoparticles are very pure copper. © 2005 Elsevier B.V. All rights reserved. Keywords: Cu nanoparticle; Capping agent; Thermal decomposition
1. Introduction Nanoparticles is ultra fine particles which have the diameter from 1 to 100 nm and the properties of nanoparticles are different form those of macro materials. The synthesis of nanosized particle is a growing research field in chemical science, in accordance with the extensive development of nanotechnology [1–3]. The size-induced properties of nanoparticles enable the development of new applications or the addition of flexibility to existing systems in many areas, such as catalysis, optics, microelectronics and so on [4–7]. Mono and bimetallic particles in the nanosize regime find extensive applications in catalysis, since with reduced size, surface area increases leading to enhanced catalytic activity [8]. Many efforts have been directed toward the chemical synthesis of various metal nanoparticles with a narrow size distribution, such as photoreduction, radiolytic reduction, alcohol reduction, and reduction using various reducing agents in association with protective polymers or surfactants [9–13]. To avoid oxidation, these methods were usually performed in non-aqueous media, at low precursor concentration, and under inert atmosphere. Using soluble polymers or surfactants as capping agents to prepare Cu nanoparticles in aqueous solutions are
∗
Corresponding author. Tel.: +82 51 6206379; fax: +82 51 6288147. E-mail address: [email protected] (Y.S. Kang).
0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.10.067
attractive because organic solvents are not used and the corresponding pollutants are absent. However, until now, only few works have been done because Cu is easily oxidized. Lisiecki et al. and Joshi et al. prepared Cu nanoparticles in aqueous solution of anionic sodium dodecyl sulfate, and various water-soluble polymers and surfactants under nitrogen atmosphere, respectively [14,15]. In this paper, the preparation of Cu nanoparticles in aqueous solution and without input of extra inert gases was examined using the anionic sodium oleate as a capping agents. Cu nanoparticles are synthesized by using thermal decomposition method of various Cu-oleate complex concentrations. This method has the advantages of dispersibility in organic solvent and reduction stability because of oleate coating on the surface of Cu nanoparticle. 2. Experimental 2.1. Materials and method Copper chloride (CuCl2 , +99%) and sodium oleate (98%) were purchased from Aldrich Chemical Co. and used without further purification. The aqueous solutions contained 0.05, 0.1, 0.2 and 0.3 mol sodium oleate were stirred at 20 ◦ C for 2 h and then 0.1 mol copper chloride solution was added into oleate solution, respectively. After filtering and drying, they were transferred into the pyrex tubes. Tube was made vacuous (0.3 Torr)
Y.H. Kim et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 284–285 (2006) 364–368
365
to avoid the explosion of pyrex tubes, sealed and immediately treated by heating to 290 ◦ C at 2 ◦ C/min for 2 h and more heating at 290 ◦ C for 2 h, and then cooled at room temperature. Cu nanoparticles which have different diameters corresponding to different sodium oleate concentrations were obtained. The decomposition of Cu-oleate complex was analyzed with thermogravimetric analysis (TGA) and the structural characterization of the product was done with transmission electron microscopy (TEM). We selected the optimized sodium oleate concentration from TEM images and measured with energy dispersive X-ray (EDX) and X-ray powder diffraction (XRD) instruments. 2.2. Analytic apparatus TEM examinations of the samples were carried out on a HITACHI H-7500 (low-resolution) and a JEOL JEM 2010 (highresolution) TEM. TEM samples were prepared on the 400 mesh copper grid coated with carbon. The structure of synthesized nanoparticles was analyzed with XRD (Philips X’pert-MPD system) with a Cu K␣ radiation source (λ = 0.154056 nm). EDX and TGA study of the sample were carried out on a Scanning Electron Microscope HITACHI S-2400 and PERKIN-ELMER TGA 7, respectively. 3. Results and discussion Fig. 1 shows weight loss of Cu-oleate complex during heat treatment in a nitrogen atmosphere. Very strong endothermic peaks were observed at about 290 and 460 ◦ C in Cu-oleate complex. Fig. 1 illustrates that most of the mass loss occurs around 290 ◦ C, and then a small amount is subsequently lost around 460 ◦ C. The major loss commences near 180 ◦ C and is complete around 370 ◦ C. During the interval, the actual mass loss amounts to 75%. On the basis of the formula weight of copper oleate, if the organic moiety is completely lost, the mass loss has to amount to 82% in total. There is 7% mass difference, and this is attributed to the formation of oleate capped copper nanoparticles [16,17]. Fig. 2 shows the formation of Cu-oleate complex. In this study, we used sodium oleate as capping agents to protect oxidation of Cu nanoparticles because oleate has a C18 (oleic) tail with a cis-double bond in the middle, forming a kink. Such kinks have been postulated as necessary for effective stabilization, and indeed stearic acid (CH3 (CH2 )16 COOH) with no double-bond in its C18 tail, cannot stabilize suspensions [18]. Fig. 3 shows TEM images of nanocrystallite of copper at different oleate concentrations. At the oleate concentration of 0.05 M (a), the mean diameter was 12.7 nm, with a standard deviation of 3.8 nm. Within this concentration, the particles are well separated and spherical. In the oleate concentration of 0.1 M and 0.2 M, the diameters of Cu nanoparticles increase slightly with increasing oleate concentration. In these two oleate concentrations, Cu nanoparticles are separated, although a few lumps are present. These lumps of coalesced nanoparticles may partly have been formed on the grid. The mean diameter of Cu nanoparticle at 0.1 M (b) and 0.2 M (c) oleate concentration are 19.2 nm with a standard deviation of 4.3 and 19.5 nm with a standard deviation of 4.4 nm, respectively. At 0.3 M (d), oleate concen-
Fig. 1. TGA and its first derivative traces of Cu-oleate complex during heat treatment in a nitrogen atmosphere.
tration, the mean diameter of Cu nanoparticles is determined as 25.9 nm and a large standard deviation of 7.2 nm is observed. At high oleate concentration, large lumps are formed because of the coalescence of destabilized nanoparticles [19]. Fig. 4 illustrates the XRD pattern of aging of Cu-oleate (0.1 M) complex at 290 ◦ C. The signature of copper has been observed in the aging of Cu-oleate complex at 290 ◦ C. Peaks are very sharp due to the high nanocrystalline nature of copper. Three peaks at 2θ values
Fig. 2. The formation of Cu-oleate complex.
366
Y.H. Kim et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 284–285 (2006) 364–368
Fig. 3. Transmission electron micrographs of copper nanoparticles at different oleate concentrations; 0.05 M (a), 0.1 M (b), 0.2 M (c) and 0.3 M (d) oleate concentration.
of 43.3, 50.4 and 74.1◦ corresponding to (1 1 1), (2 0 0) and (2 2 0) planes of copper, respectively. (JCPDS, copper file no. 04–0836). No impurity peak is observed in the X-ray diffraction pattern. Fig. 5 shows the EDX spectra of copper nanoparticles excited by an electron beam (20 kV). Peaks for the elements of Cu, C and O were observed at 0.5249 (Oka ), 0.9297 (CuLa ), 0.9498 (CuLb ), 8.0477 (Cuka ), 8.905 (Cukb ), and 0.2774 (Cka ).
There is no impurity atom in the nanoparticles except oleate capped copper. Accordingly, from the EDX spectra we could confirm that the nanoparticles in TEM images are pure copper [20,21]. The lattice of Cu nanoparticle is shown in Fig. 6. ˚ is consistent The lattice spacing in the HRTEM image of 1.99 A with the distance for 1 1 1 lattice spacing in Cu. The UV–vis absorption spectrum of Cu nanoparticles was shown in Fig. 7.
Y.H. Kim et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 284–285 (2006) 364–368
367
Fig. 7. The UV–vis absorption spectra of Cu nanoparticle.
Fig. 4. X-ray diffraction pattern (Cu K␣ radiation) of Cu-oleate complex at 290 ◦ C.
Cu nanoparticles display an optical absorption band peaked at 587 nm and no characteristic absorption band for copper oxide around 800 nm was observed [14]. 4. Conclusion
Fig. 5. Energy-dispersive X-ray spectra of copper nanocrystallites.
A method has been introduced to produce monodispersed copper nanoparticle using thermal decomposition of Cu-oleate complex, which was prepared by the reaction of CuCl2 with sodium oleate in water solution. We also introduce that no extra inert gases protecting the oxidation of Cu nanoparticles were necessary. By TGA measurement, thermal decompositions of Cu-oleate complex was observed at about 290 ◦ C. Transmission electron micrographs show that at 0.05 M oleate concentration, spherical copper nanoparticles are prepared, but at high concentration (0.3 M), large lumps are formed. This method can be easily increased scale up for industrial purpose. We confirm that these nanoparticles are very pure copper. Acknowledgments This work was financially supported by Functional Chemicals Development Program and Coreana Cosmetics Co. Ltd., and Y.H. Kim would like to thank the financial support by Brain Busan 21. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Fig. 6. The lattice spacing of Cu nanoparticle.
K.E. Drexler, Sci. Am. 74 (2001) 285. D. Spurgeon, Nature 412 (2001) 846. D.R. Forrest, IEEE Instrum. Meas. Mag. 4 (2001) 11. S. Forster, M. Antonietti, Adv. Mater. 10 (1998) 195. M. Moffit, A. Eisenberg, Chem. Mater. 7 (1995) 1178. K. Ghosh, S.N. Maiti, J. Appl. Polym. Sci. 60 (1996) 323. R.P. Andres, J.D. Bielfeld, J.I. Henderson, Science 273 (1996) 1960. N. Toshima, Y. Wang, Langmuir 10 (1994) 4574. S. Remita, M. Mostafavi, M.O. Delcour, Radiat. Phys. Chem. 47 (1996) 275. [10] J.H. Hodak, A. Henglein, M. Giersig, G.V. Hartland, J. Phys. Chem. B 104 (2000) 11708. [11] H.H. Huang, X.P. Ni, G.L. Loy, C.H. Chew, K.L. Tan, G.C. Loh, J.G. Deng, G.Q. Xu, Langmuir 12 (1996) 909.
368 [12] [13] [14] [15]
Y.H. Kim et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 284–285 (2006) 364–368
A.B.R. Mayer, J.E. Mark, Polymer 41 (2000) 1627. D.H. Chen, C.H. Hsieh, J. Mater. Chem. 12 (2002) 2412. I. Lisiecki, F. Billoudet, M.P. Pileni, J. Phys. Chem. 100 (1996) 4160. S.S. Joshi, S.F. Patil, V. Iyer, S. Mahumuni, Nanostruct. Mater. 10 (1998) 1135. [16] S.H. Wu, D.H. Chen, J. Colloid Interface Sci. 273 (2004) 165. [17] S.J. Lee, S.W. Han, H.J. Choi, K. Kim, J. Phys. Chem. B 106 (2003) 2892.
[18] R. Tadmor, R.E. Rosensweig, J. Frey, J. Klein, Langmuir 16 (2000) 9117. [19] A. Henglein, M. Giersig, J. Phys. Chem. B 103 (1999) 9533. [20] E.N. Kang, J.N. Park, Y.S. Hwang, M.S. Kang, J.G. Park, T.H. Hyeon, J. Phys. Chem. B 108 (2004) 13932. [21] D.K. Lee, Y.H. Kim, Y.S. Kang, P. Stroeve, J. Phys. Chem. B 109 (2005) 14939.
Synthesis of oleate capped Cu nanoparticles by thermal decomposition Young Hwan Kim a , Don Keun Lee a , Beong Gi Jo b , Ji Hean Jeong b , Young Soo Kang a,∗ a
Department of Chemistry, Pukyong National University, 599-1 Daeyon 3-Dong, Namgu, Pusan 608-737, South Korea b R&D Center, Coreana Cosmetics Co., Ltd., Cheonan-si 330-830, South Korea Received 25 June 2005; received in revised form 13 October 2005; accepted 28 October 2005 Available online 15 December 2005
Abstract A method has been introduced to produce monodispersed copper nanoparticle using thermal decomposition of Cu-oleate complex, which was prepared by the reaction of CuCl2 with sodium oleate in water solution. Sodium oleate acts as a stabilizer of the Cu nanoparticles and protector of oxidation of Cu nanoparticles in various concentrations. We also found that no extra inert gases for protecting the oxidation of Cu nanoparticles were necessary. By TGA measurement, thermal decompositions of Cu-oleate complexes were observed. TEM data show that at 0.05 M oleate concentration, spherical copper nanoparticles are prepared, but at high concentration (0.3 M), large lumps are formed. This method can be easily increased scale up for industrial purpose. We confirm that these nanoparticles are very pure copper. © 2005 Elsevier B.V. All rights reserved. Keywords: Cu nanoparticle; Capping agent; Thermal decomposition
1. Introduction Nanoparticles is ultra fine particles which have the diameter from 1 to 100 nm and the properties of nanoparticles are different form those of macro materials. The synthesis of nanosized particle is a growing research field in chemical science, in accordance with the extensive development of nanotechnology [1–3]. The size-induced properties of nanoparticles enable the development of new applications or the addition of flexibility to existing systems in many areas, such as catalysis, optics, microelectronics and so on [4–7]. Mono and bimetallic particles in the nanosize regime find extensive applications in catalysis, since with reduced size, surface area increases leading to enhanced catalytic activity [8]. Many efforts have been directed toward the chemical synthesis of various metal nanoparticles with a narrow size distribution, such as photoreduction, radiolytic reduction, alcohol reduction, and reduction using various reducing agents in association with protective polymers or surfactants [9–13]. To avoid oxidation, these methods were usually performed in non-aqueous media, at low precursor concentration, and under inert atmosphere. Using soluble polymers or surfactants as capping agents to prepare Cu nanoparticles in aqueous solutions are
∗
Corresponding author. Tel.: +82 51 6206379; fax: +82 51 6288147. E-mail address: [email protected] (Y.S. Kang).
0927-7757/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2005.10.067
attractive because organic solvents are not used and the corresponding pollutants are absent. However, until now, only few works have been done because Cu is easily oxidized. Lisiecki et al. and Joshi et al. prepared Cu nanoparticles in aqueous solution of anionic sodium dodecyl sulfate, and various water-soluble polymers and surfactants under nitrogen atmosphere, respectively [14,15]. In this paper, the preparation of Cu nanoparticles in aqueous solution and without input of extra inert gases was examined using the anionic sodium oleate as a capping agents. Cu nanoparticles are synthesized by using thermal decomposition method of various Cu-oleate complex concentrations. This method has the advantages of dispersibility in organic solvent and reduction stability because of oleate coating on the surface of Cu nanoparticle. 2. Experimental 2.1. Materials and method Copper chloride (CuCl2 , +99%) and sodium oleate (98%) were purchased from Aldrich Chemical Co. and used without further purification. The aqueous solutions contained 0.05, 0.1, 0.2 and 0.3 mol sodium oleate were stirred at 20 ◦ C for 2 h and then 0.1 mol copper chloride solution was added into oleate solution, respectively. After filtering and drying, they were transferred into the pyrex tubes. Tube was made vacuous (0.3 Torr)
Y.H. Kim et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 284–285 (2006) 364–368
365
to avoid the explosion of pyrex tubes, sealed and immediately treated by heating to 290 ◦ C at 2 ◦ C/min for 2 h and more heating at 290 ◦ C for 2 h, and then cooled at room temperature. Cu nanoparticles which have different diameters corresponding to different sodium oleate concentrations were obtained. The decomposition of Cu-oleate complex was analyzed with thermogravimetric analysis (TGA) and the structural characterization of the product was done with transmission electron microscopy (TEM). We selected the optimized sodium oleate concentration from TEM images and measured with energy dispersive X-ray (EDX) and X-ray powder diffraction (XRD) instruments. 2.2. Analytic apparatus TEM examinations of the samples were carried out on a HITACHI H-7500 (low-resolution) and a JEOL JEM 2010 (highresolution) TEM. TEM samples were prepared on the 400 mesh copper grid coated with carbon. The structure of synthesized nanoparticles was analyzed with XRD (Philips X’pert-MPD system) with a Cu K␣ radiation source (λ = 0.154056 nm). EDX and TGA study of the sample were carried out on a Scanning Electron Microscope HITACHI S-2400 and PERKIN-ELMER TGA 7, respectively. 3. Results and discussion Fig. 1 shows weight loss of Cu-oleate complex during heat treatment in a nitrogen atmosphere. Very strong endothermic peaks were observed at about 290 and 460 ◦ C in Cu-oleate complex. Fig. 1 illustrates that most of the mass loss occurs around 290 ◦ C, and then a small amount is subsequently lost around 460 ◦ C. The major loss commences near 180 ◦ C and is complete around 370 ◦ C. During the interval, the actual mass loss amounts to 75%. On the basis of the formula weight of copper oleate, if the organic moiety is completely lost, the mass loss has to amount to 82% in total. There is 7% mass difference, and this is attributed to the formation of oleate capped copper nanoparticles [16,17]. Fig. 2 shows the formation of Cu-oleate complex. In this study, we used sodium oleate as capping agents to protect oxidation of Cu nanoparticles because oleate has a C18 (oleic) tail with a cis-double bond in the middle, forming a kink. Such kinks have been postulated as necessary for effective stabilization, and indeed stearic acid (CH3 (CH2 )16 COOH) with no double-bond in its C18 tail, cannot stabilize suspensions [18]. Fig. 3 shows TEM images of nanocrystallite of copper at different oleate concentrations. At the oleate concentration of 0.05 M (a), the mean diameter was 12.7 nm, with a standard deviation of 3.8 nm. Within this concentration, the particles are well separated and spherical. In the oleate concentration of 0.1 M and 0.2 M, the diameters of Cu nanoparticles increase slightly with increasing oleate concentration. In these two oleate concentrations, Cu nanoparticles are separated, although a few lumps are present. These lumps of coalesced nanoparticles may partly have been formed on the grid. The mean diameter of Cu nanoparticle at 0.1 M (b) and 0.2 M (c) oleate concentration are 19.2 nm with a standard deviation of 4.3 and 19.5 nm with a standard deviation of 4.4 nm, respectively. At 0.3 M (d), oleate concen-
Fig. 1. TGA and its first derivative traces of Cu-oleate complex during heat treatment in a nitrogen atmosphere.
tration, the mean diameter of Cu nanoparticles is determined as 25.9 nm and a large standard deviation of 7.2 nm is observed. At high oleate concentration, large lumps are formed because of the coalescence of destabilized nanoparticles [19]. Fig. 4 illustrates the XRD pattern of aging of Cu-oleate (0.1 M) complex at 290 ◦ C. The signature of copper has been observed in the aging of Cu-oleate complex at 290 ◦ C. Peaks are very sharp due to the high nanocrystalline nature of copper. Three peaks at 2θ values
Fig. 2. The formation of Cu-oleate complex.
366
Y.H. Kim et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 284–285 (2006) 364–368
Fig. 3. Transmission electron micrographs of copper nanoparticles at different oleate concentrations; 0.05 M (a), 0.1 M (b), 0.2 M (c) and 0.3 M (d) oleate concentration.
of 43.3, 50.4 and 74.1◦ corresponding to (1 1 1), (2 0 0) and (2 2 0) planes of copper, respectively. (JCPDS, copper file no. 04–0836). No impurity peak is observed in the X-ray diffraction pattern. Fig. 5 shows the EDX spectra of copper nanoparticles excited by an electron beam (20 kV). Peaks for the elements of Cu, C and O were observed at 0.5249 (Oka ), 0.9297 (CuLa ), 0.9498 (CuLb ), 8.0477 (Cuka ), 8.905 (Cukb ), and 0.2774 (Cka ).
There is no impurity atom in the nanoparticles except oleate capped copper. Accordingly, from the EDX spectra we could confirm that the nanoparticles in TEM images are pure copper [20,21]. The lattice of Cu nanoparticle is shown in Fig. 6. ˚ is consistent The lattice spacing in the HRTEM image of 1.99 A with the distance for 1 1 1 lattice spacing in Cu. The UV–vis absorption spectrum of Cu nanoparticles was shown in Fig. 7.
Y.H. Kim et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 284–285 (2006) 364–368
367
Fig. 7. The UV–vis absorption spectra of Cu nanoparticle.
Fig. 4. X-ray diffraction pattern (Cu K␣ radiation) of Cu-oleate complex at 290 ◦ C.
Cu nanoparticles display an optical absorption band peaked at 587 nm and no characteristic absorption band for copper oxide around 800 nm was observed [14]. 4. Conclusion
Fig. 5. Energy-dispersive X-ray spectra of copper nanocrystallites.
A method has been introduced to produce monodispersed copper nanoparticle using thermal decomposition of Cu-oleate complex, which was prepared by the reaction of CuCl2 with sodium oleate in water solution. We also introduce that no extra inert gases protecting the oxidation of Cu nanoparticles were necessary. By TGA measurement, thermal decompositions of Cu-oleate complex was observed at about 290 ◦ C. Transmission electron micrographs show that at 0.05 M oleate concentration, spherical copper nanoparticles are prepared, but at high concentration (0.3 M), large lumps are formed. This method can be easily increased scale up for industrial purpose. We confirm that these nanoparticles are very pure copper. Acknowledgments This work was financially supported by Functional Chemicals Development Program and Coreana Cosmetics Co. Ltd., and Y.H. Kim would like to thank the financial support by Brain Busan 21. References [1] [2] [3] [4] [5] [6] [7] [8] [9]
Fig. 6. The lattice spacing of Cu nanoparticle.
K.E. Drexler, Sci. Am. 74 (2001) 285. D. Spurgeon, Nature 412 (2001) 846. D.R. Forrest, IEEE Instrum. Meas. Mag. 4 (2001) 11. S. Forster, M. Antonietti, Adv. Mater. 10 (1998) 195. M. Moffit, A. Eisenberg, Chem. Mater. 7 (1995) 1178. K. Ghosh, S.N. Maiti, J. Appl. Polym. Sci. 60 (1996) 323. R.P. Andres, J.D. Bielfeld, J.I. Henderson, Science 273 (1996) 1960. N. Toshima, Y. Wang, Langmuir 10 (1994) 4574. S. Remita, M. Mostafavi, M.O. Delcour, Radiat. Phys. Chem. 47 (1996) 275. [10] J.H. Hodak, A. Henglein, M. Giersig, G.V. Hartland, J. Phys. Chem. B 104 (2000) 11708. [11] H.H. Huang, X.P. Ni, G.L. Loy, C.H. Chew, K.L. Tan, G.C. Loh, J.G. Deng, G.Q. Xu, Langmuir 12 (1996) 909.
368 [12] [13] [14] [15]
Y.H. Kim et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 284–285 (2006) 364–368
A.B.R. Mayer, J.E. Mark, Polymer 41 (2000) 1627. D.H. Chen, C.H. Hsieh, J. Mater. Chem. 12 (2002) 2412. I. Lisiecki, F. Billoudet, M.P. Pileni, J. Phys. Chem. 100 (1996) 4160. S.S. Joshi, S.F. Patil, V. Iyer, S. Mahumuni, Nanostruct. Mater. 10 (1998) 1135. [16] S.H. Wu, D.H. Chen, J. Colloid Interface Sci. 273 (2004) 165. [17] S.J. Lee, S.W. Han, H.J. Choi, K. Kim, J. Phys. Chem. B 106 (2003) 2892.
[18] R. Tadmor, R.E. Rosensweig, J. Frey, J. Klein, Langmuir 16 (2000) 9117. [19] A. Henglein, M. Giersig, J. Phys. Chem. B 103 (1999) 9533. [20] E.N. Kang, J.N. Park, Y.S. Hwang, M.S. Kang, J.G. Park, T.H. Hyeon, J. Phys. Chem. B 108 (2004) 13932. [21] D.K. Lee, Y.H. Kim, Y.S. Kang, P. Stroeve, J. Phys. Chem. B 109 (2005) 14939.