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functionalized mesoporous silica by in situ formation of adsorbed silver
Materials Letters 61 (2007) 156 – 159 www.elsevier.com/locate/matlet
The preparation of Ag/mesoporous silica by direct silver reduction and Ag/functionalized mesoporous silica by in situ formation of adsorbed silver Jay-hyun Park a , Jai-koo Park a,⁎, Hee-young Shin b a
Department of Geoenvironmental System Eng., Hanyang University, Haengdangdong 17, Sungdonggu, Seoul, 133-791, Korea b Korea Institute of Geoscience and Mineral Resources, Kagungdong 30, Yuseong-gu, Taejeon, 305-350, Korea Received 4 November 2005; accepted 4 April 2006 Available online 5 June 2006
Abstract Mesoporous silica (MS) with large pores and thiol functionalized mesoporous silica (TFMS) were synthesized. 29Si MAS NMR confirmed the functionalization of MPTMS on the surface of mesoporous silica. Silver nanoparticles were prepared by two methods: (1) direct reduction of Ag+ ions with NaBH4 in aqueous AgNO3 solution containing MS, (2) in situ reduction of Ag+ ions adsorbed on TFMS with NaBH4. The characteristics of products from both methods were compared using SAXRD, TEM, and N2 adsorption–desorption. Ag nanoclusters were mostly confined and dispersed in the channels of the TFMS and their sizes were under 6 nm. However, Ag nanoparticles on the MS formed outside the mesoporous channels rather than within them. © 2006 Elsevier B.V. All rights reserved. Keywords: Mesoporous material; Silver nanoparticles; Thiol
1. Introduction The interest in nanosized noble metal particles has recently increased due to the recognition of their distinctive physicochemical properties, including optical nonlinearity and specific heat and magnetism, which make them desirable for use in optical devices, catalysis and microelectronics [1–3]. Several methods have been used for the preparation and stabilization of metal nanoparticles [4–6]. One of the most common methods reduces the metal particles in the pores of a porous template material [7– 11]. Mesoporous silica is an exceptional candidate for the support of nanostructured metals due to the easy control of its size (2– 50 nm), its narrow size distribution and its high surface area [12]. Nanosized metal particles do not become agglomerated within a template because of the inhibition of the template pore walls. Various supports can be coated with nanoparticles and agglomeration can be avoided by removing the template after the deposition of the metal nanoparticle/template particles on the supports [13]. Up to now, there have been numerous investiga-
⁎ Corresponding author. Tel.: +82 2 2296 2954; fax: +82 2 2281 7769. E-mail address: [email protected] (J. Park). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.04.118
tions of the synthesis of metal nanoparticles or nanowires by using mesoporous silica as templates [14–21]. There are several approaches to produce silver nanoparticles within mesoporous silica: (1) traditional silver impregnation on mesoporous silica by various methods of metal reduction, including thermal reduction [14,15], gamma irradiation [16], sonocation [17], and reductant (NaBH4, hydrazine) [18]; (2) one-step sol–gel synthesis achieved by adding an Ag precursor during the condensation of mesoporous silica [19]; (3) in situ reduction of an Ag precursor adsorbed on a mesoporous material with assistance from a functional group [20,21]. In this work, we report the preparation of nanostructured silver/mesoporous silica samples and compare the characteristics of silver/mesoporous silica without thiol and silver/mesoporous silica with thiol formed by the in situ adsorbed silver reduction method. 2. Experimental 2.1. Mesoporous silica synthesis 4 g of Pluronic P123 (Aldrich) was dissolved in 120 g of 2 M HCl solution and then stirred at 35 °C [22]. Then, 46 g of silicate solution, which is prepared by dissolving 1 M of amorphous silica
J. Park et al. / Materials Letters 61 (2007) 156–159
157
Fig. 1. N2 adsorption–desorption isotherm of (a) mesoporous silica (MS) and (b) thiol functionalized mesoporous silica (TFMS).
Fig. 3. Pore volume of samples as a function of Ag concentration in solution.
(Dongyang Chem. Co.) in 1 M NaOH solution, was added into HCl solution and then stirred for 20 h. The mixture was aged at 90 °C for 24 h. The precipitated product was filtered, washed, dried and calcined at 600 °C for 2 h. The thiol functionalized mesoporous silica was synthesized using 3-mercaptopropyltrimethoxysilane (MPTMS). 8 g of mesoporous silica was then added to 200 ml of n-hexane (Wako) containing MPTMS 8 g and the solution was stirred for 24 h at room temperature. The products were filtered and washed. Mesoporous TFMS (0.1 g) was poured in glass bottles containing 50 ml of AgNO3 solutions. The solutions were shaken for 6 h at 25 °C, allowing the adsorption to reach equilibrium and then Ag adsorbed TFMS was filtered, dried and suspended in an aqueous solution (50 ml). A copious amount of the NaBH4 was subsequently added drop by drop to the solutions. Immediately, the white TFMS turned brown. Ag/MS were synthesized through Ag reduction with NaBH4 in the AgNO3 solution. Mesoporous silica (0.1 g) was poured in AgNO3 solutions (50 ml) of different concentrations and the solutions were stirred vigorously for 6 h at room temperature. A copious amount of NaBH4 was added drop by drop to the solutions with mesoporous silica. The white samples turned
gray. The solutions were then stirred for 2 more hours. MS and TFMS samples formed at different Ag concentrations in solution and were denoted as Agn/MS and Agn/TFMS, where n (n = 500, 1000, 2000) was the Ag concentration (ppm) in solution. After being reduced with the NaBH4, the samples were filtered and dried for a day.
Fig. 2. 29Si MAS NMR of (a) MS and (b) TFMS samples.
2.2. Characterization The specific surface area was measured with the N2-gas adsorption method applying the BET apparatus (Nova1000, Quantachrome). 29Si MAS solid state NMR experiments were performed on a 200 MHz solid NMR spectrometer (Unity Inova 200, Varian). Small-angle X-ray diffraction patterns were collected with a RINT 2000 using CuKα radiation. Transmission electron
Fig. 4. Small angle XRD patterns of (a) MS, (b) TFMS, (c) Ag1000/MS, and (d) Ag1000/TFMS samples.
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microscope (TEM) was performed with a Hitachi H9000-NAR electron microscope operating at 200 kV. 3. Results and discussion 3.1. Characteristics of MS and TFMS Fig. 1 shows that the surface functionalization by MPTMS brought about a decrease in the nitrogen adsorption capacity, which reflects a decrease in the specific surface area of the sample. However, the position and the height of hysteresis, which are in proportion to the mesopore diameter and the mesopore volume, respectively, decreased slightly after the functionalization. The 29Si MAS NMR spectra of the mesoporous silica sample shows that the Q2 signal derived from the silandiol (Si(OH)2) group exists in small amount and that the silanol group (Si–OH, Q3) is more abundant than the siloxane (Si–O–Si, Q4) group on the silica surface (Fig. 2). The Si NMR signals of the TFMS display higher Q4 than Q3 signals and a lack of Q2 signals, which means that the MPTMS react with the active silanol and silandiol groups. Three additional peaks occurred from −50 to − 80 ppm, which correspond to the following three different environments for the siloxane groups in the TFMS: T1isolated group (Si C(OH)(OR)(OSi)), T2-terminal group (Si C(OH) (OSi)2), T3-cross linked group (Si C(OSi)3). These results clearly show the existence of MPTMS in the mesoporous silica, indicating the successful functionalization of the thiol groups in the surface of the mesoporous silica. 3.2. Characteristics of Ag/MS and Ag/TFMS
Fig. 5. Pore diameter distributions of Ag-contained samples measured by the BJH method.
The pore volumes of Ag/MS and Ag/TFMS decreased as the concentration of Ag in solution increased, because the amount of loaded
Fig. 6. TEM images (a) Ag/MS (×100 k), (b) Ag particles in Ag/MS (×100 k), (c) Ag/TFMS (× 500 k) and (d) Ag/TFMS (×200 k).
J. Park et al. / Materials Letters 61 (2007) 156–159
metal Ag on the support increased (Fig. 3). The small angle X-ray diffraction (SAXRD) patterns of samples are shown in Fig. 4. Samples used as a support (Fig. 4a, b) display the characteristics of a well ordered hexagonal structure. After the incorporation of the Ag with mesoporous samples, the peak intensity for the low angle reflections decreased (Fig. 4c, d). However, one major peak and two smaller peaks occurred at about 0.8° of 2è of the Ag incorporated sample, suggesting that the hexagonal pore structure of the mesoporous sample is retained after the reduction of Ag. The pore size distributions of the Ag/MS and the Ag/TFMS were identified by the Barrett, Joyner and Hallenda (BJH) method [23] to investigate the influence of Ag loading on the mesopore structure of samples (Fig. 5). The modal pore diameters of the mesoporous silica (Fig. 5a) and TFMS (Fig. 5f) host materials were 6.4 nm, indicating that pore sizes are decreased slightly by MPTMS functionalization of the inner pore surface. As the loaded Ag amount increased, the pore volumes of both the Agn/MS and Agm/TFMS samples decreased. After the Ag loading, the modal pore diameter (about 6.4 nm) of the Agn/MS sample was not changed, even though the Ag loading amount increased (Fig. 5 b–e). From these results, it is considered that the Ag clusters of the Agn/MS were formed on the exterior surface of the mesopore. This implies that it is difficult to prepare the Ag cluster within the mesopore by the general NaBH4 reduction method. On the contrary, the modal pore diameters of the Agm/TFMS (Fig. 5 g–j) are in the region of 4– 6 nm and have a tendency to be 4 nm when the Ag loading amount increases because Ag clusters were partially filled in pores. The TEM images of the Ag/MS and Ag/TFMS samples are shown in Fig. 6. Ag particles in the Ag/MS sample were found to be separate from the mesoporous silica and sized between 50–300 nm (Fig. 6a, b). The TEM images of the Ag/TFMS showed that Ag nanoparticles were formed inside the mesopore channel (Fig. 6d). Ag particles in the Ag/ TFMS were well dispersed with Ag particle diameters under 6 nm, which occurred because the Ag particle growth was restricted by the dimensions of the channel space (Fig. 6c).
4. Conclusions The growth of silver nanoparticles within the pores of mesoporous silica has been achieved by reducing adsorbed silver cations on a thiol functionalized surface of mesopores. Silver nanoparticles could not be grown within the pore channels without help of thiol substituents. The silver nanoparticles prepared on thiol functionalized mesoporous silica maintained
159
their dispersed state and diameters under 6 nm, which is smaller than the pore size. The process with TFMS is considered to be a fast and effective method for the preparation of nanoparticles with a narrow size distribution, and the method could be extended to prepare other nanosized metals. References [1] G.R. Stewart, Phys. Rev., B 15 (1997) 1143. [2] G. Schmid, M. Baumle, M. Geerkens, I. Heim, C. Osemann, T. Sawitowski, Chem. Soc. Rev. 28 (1999) 179. [3] K. Uchida, S. Kaneko, S. Omi, C. Hata, H. Tanji, Y. Asahara, A.J. Ikushima, T. Tokizaki, A.J. Nakamura, Opt. Soc. Am. B 11 (1994) 1236. [4] M.P. Zach, K.H. Ng, R.M. Penner, Science 290 (2000) 2120. [5] M.H.V. Werts, M. Lambert, J.P. Bourgoin, M. Brust, Nano Lett. 2 (2002) 43. [6] M.E.T. Molares, V. Buschmann, D. Dobrev, R. Neumann, R. Scholz, I.U. Schuchert, J. Vetter, Adv. Mater. 13 (2001) 62. [7] A.J. Yin, J. Li, W. Jin, A.J. Bennett, J. Xu, Appl. Phys. Lett. 79 (2001) 1039. [8] W.H. Zhang, J.L. Shi, H.R. Chen, Z.L. Hua, D.S. Yan, Chem. Mater. 13 (2001) 648. [9] Y.S. Seo, K.S. Kim, A. Galambos, R.G.H. Lammertink, G.J. Vancso, J. Sokolov, M. Rafailovich, Nano Lett. 4 (2004) 483. [10] T. Thurn-Albrecht, J. Schotter, G.A. Kastle, N. Emley, T. Shibauchi, L. Krusin-Elbaum, K. Guarini, M.T. Tuominen, T.P. Russell, Science 290 (2000) 2126. [11] L. Gao, E.B. Wang, S.Y. Lian, Z.H. Kang, Y. Lan, D. Wu, Solid State Commun. 130 (2004) 309. [12] M.H. Huang, A. Choudrey, P.D. Yang, Chem. Commun. (2000) 1063. [13] W. Zhu, Y. Han, L. An, Microporous Mesoporous Mater. 80 (2005) 221. [14] H. Bi, W. Cai, H. Shi, X. Liu, Chem. Phys. Lett. 357 (2002) 249. [15] W. Chen, J. Zhang, Scr. Mater. 49 (2003) 321. [16] A.L. Pan, H.G. Zheng, Z.P. Yang, F.X. Liu, Z.J. Ding, Y.T. Qian, Mater. Res., B 38 (2003) 789–796. [17] W. Chen, Junying Zhang, Weiping Cai, Scr. Mater. 48 (2003) 1061. [18] R. Patakfalvi, I. Dékány, Appl. Clay Sci. 25 (2004) 149. [19] T. Hayakawa, S.T. Selvan, M. Nigami, Appl. Phys. Lett. 74 (1999) 1513. [20] X.G. Zhao, J.L. Shi, B. Hu, L.X. Zhang, Z.L. Hua, Mater. Lett. 58 (2004) 2152. [21] Y. Guari, C. Thieuleux, A. Mehdi, C. Reye, R.J.P. Corriu, S.G. Gallardo, K. Philippot, B. Chaudret, R. Dutartre, Chem. Commun (2001) 1374. [22] D. Zhao, Q. Huo, J. Feng, B.F. Chmelka, G.D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024. [23] S. Lowell, J.E. Shields (Eds.), Powder and Surface Area and Porosity, 3rd ed., Chapman & Hall, London, 1991.
The preparation of Ag/mesoporous silica by direct silver reduction and Ag/functionalized mesoporous silica by in situ formation of adsorbed silver Jay-hyun Park a , Jai-koo Park a,⁎, Hee-young Shin b a
Department of Geoenvironmental System Eng., Hanyang University, Haengdangdong 17, Sungdonggu, Seoul, 133-791, Korea b Korea Institute of Geoscience and Mineral Resources, Kagungdong 30, Yuseong-gu, Taejeon, 305-350, Korea Received 4 November 2005; accepted 4 April 2006 Available online 5 June 2006
Abstract Mesoporous silica (MS) with large pores and thiol functionalized mesoporous silica (TFMS) were synthesized. 29Si MAS NMR confirmed the functionalization of MPTMS on the surface of mesoporous silica. Silver nanoparticles were prepared by two methods: (1) direct reduction of Ag+ ions with NaBH4 in aqueous AgNO3 solution containing MS, (2) in situ reduction of Ag+ ions adsorbed on TFMS with NaBH4. The characteristics of products from both methods were compared using SAXRD, TEM, and N2 adsorption–desorption. Ag nanoclusters were mostly confined and dispersed in the channels of the TFMS and their sizes were under 6 nm. However, Ag nanoparticles on the MS formed outside the mesoporous channels rather than within them. © 2006 Elsevier B.V. All rights reserved. Keywords: Mesoporous material; Silver nanoparticles; Thiol
1. Introduction The interest in nanosized noble metal particles has recently increased due to the recognition of their distinctive physicochemical properties, including optical nonlinearity and specific heat and magnetism, which make them desirable for use in optical devices, catalysis and microelectronics [1–3]. Several methods have been used for the preparation and stabilization of metal nanoparticles [4–6]. One of the most common methods reduces the metal particles in the pores of a porous template material [7– 11]. Mesoporous silica is an exceptional candidate for the support of nanostructured metals due to the easy control of its size (2– 50 nm), its narrow size distribution and its high surface area [12]. Nanosized metal particles do not become agglomerated within a template because of the inhibition of the template pore walls. Various supports can be coated with nanoparticles and agglomeration can be avoided by removing the template after the deposition of the metal nanoparticle/template particles on the supports [13]. Up to now, there have been numerous investiga-
⁎ Corresponding author. Tel.: +82 2 2296 2954; fax: +82 2 2281 7769. E-mail address: [email protected] (J. Park). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.04.118
tions of the synthesis of metal nanoparticles or nanowires by using mesoporous silica as templates [14–21]. There are several approaches to produce silver nanoparticles within mesoporous silica: (1) traditional silver impregnation on mesoporous silica by various methods of metal reduction, including thermal reduction [14,15], gamma irradiation [16], sonocation [17], and reductant (NaBH4, hydrazine) [18]; (2) one-step sol–gel synthesis achieved by adding an Ag precursor during the condensation of mesoporous silica [19]; (3) in situ reduction of an Ag precursor adsorbed on a mesoporous material with assistance from a functional group [20,21]. In this work, we report the preparation of nanostructured silver/mesoporous silica samples and compare the characteristics of silver/mesoporous silica without thiol and silver/mesoporous silica with thiol formed by the in situ adsorbed silver reduction method. 2. Experimental 2.1. Mesoporous silica synthesis 4 g of Pluronic P123 (Aldrich) was dissolved in 120 g of 2 M HCl solution and then stirred at 35 °C [22]. Then, 46 g of silicate solution, which is prepared by dissolving 1 M of amorphous silica
J. Park et al. / Materials Letters 61 (2007) 156–159
157
Fig. 1. N2 adsorption–desorption isotherm of (a) mesoporous silica (MS) and (b) thiol functionalized mesoporous silica (TFMS).
Fig. 3. Pore volume of samples as a function of Ag concentration in solution.
(Dongyang Chem. Co.) in 1 M NaOH solution, was added into HCl solution and then stirred for 20 h. The mixture was aged at 90 °C for 24 h. The precipitated product was filtered, washed, dried and calcined at 600 °C for 2 h. The thiol functionalized mesoporous silica was synthesized using 3-mercaptopropyltrimethoxysilane (MPTMS). 8 g of mesoporous silica was then added to 200 ml of n-hexane (Wako) containing MPTMS 8 g and the solution was stirred for 24 h at room temperature. The products were filtered and washed. Mesoporous TFMS (0.1 g) was poured in glass bottles containing 50 ml of AgNO3 solutions. The solutions were shaken for 6 h at 25 °C, allowing the adsorption to reach equilibrium and then Ag adsorbed TFMS was filtered, dried and suspended in an aqueous solution (50 ml). A copious amount of the NaBH4 was subsequently added drop by drop to the solutions. Immediately, the white TFMS turned brown. Ag/MS were synthesized through Ag reduction with NaBH4 in the AgNO3 solution. Mesoporous silica (0.1 g) was poured in AgNO3 solutions (50 ml) of different concentrations and the solutions were stirred vigorously for 6 h at room temperature. A copious amount of NaBH4 was added drop by drop to the solutions with mesoporous silica. The white samples turned
gray. The solutions were then stirred for 2 more hours. MS and TFMS samples formed at different Ag concentrations in solution and were denoted as Agn/MS and Agn/TFMS, where n (n = 500, 1000, 2000) was the Ag concentration (ppm) in solution. After being reduced with the NaBH4, the samples were filtered and dried for a day.
Fig. 2. 29Si MAS NMR of (a) MS and (b) TFMS samples.
2.2. Characterization The specific surface area was measured with the N2-gas adsorption method applying the BET apparatus (Nova1000, Quantachrome). 29Si MAS solid state NMR experiments were performed on a 200 MHz solid NMR spectrometer (Unity Inova 200, Varian). Small-angle X-ray diffraction patterns were collected with a RINT 2000 using CuKα radiation. Transmission electron
Fig. 4. Small angle XRD patterns of (a) MS, (b) TFMS, (c) Ag1000/MS, and (d) Ag1000/TFMS samples.
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J. Park et al. / Materials Letters 61 (2007) 156–159
microscope (TEM) was performed with a Hitachi H9000-NAR electron microscope operating at 200 kV. 3. Results and discussion 3.1. Characteristics of MS and TFMS Fig. 1 shows that the surface functionalization by MPTMS brought about a decrease in the nitrogen adsorption capacity, which reflects a decrease in the specific surface area of the sample. However, the position and the height of hysteresis, which are in proportion to the mesopore diameter and the mesopore volume, respectively, decreased slightly after the functionalization. The 29Si MAS NMR spectra of the mesoporous silica sample shows that the Q2 signal derived from the silandiol (Si(OH)2) group exists in small amount and that the silanol group (Si–OH, Q3) is more abundant than the siloxane (Si–O–Si, Q4) group on the silica surface (Fig. 2). The Si NMR signals of the TFMS display higher Q4 than Q3 signals and a lack of Q2 signals, which means that the MPTMS react with the active silanol and silandiol groups. Three additional peaks occurred from −50 to − 80 ppm, which correspond to the following three different environments for the siloxane groups in the TFMS: T1isolated group (Si C(OH)(OR)(OSi)), T2-terminal group (Si C(OH) (OSi)2), T3-cross linked group (Si C(OSi)3). These results clearly show the existence of MPTMS in the mesoporous silica, indicating the successful functionalization of the thiol groups in the surface of the mesoporous silica. 3.2. Characteristics of Ag/MS and Ag/TFMS
Fig. 5. Pore diameter distributions of Ag-contained samples measured by the BJH method.
The pore volumes of Ag/MS and Ag/TFMS decreased as the concentration of Ag in solution increased, because the amount of loaded
Fig. 6. TEM images (a) Ag/MS (×100 k), (b) Ag particles in Ag/MS (×100 k), (c) Ag/TFMS (× 500 k) and (d) Ag/TFMS (×200 k).
J. Park et al. / Materials Letters 61 (2007) 156–159
metal Ag on the support increased (Fig. 3). The small angle X-ray diffraction (SAXRD) patterns of samples are shown in Fig. 4. Samples used as a support (Fig. 4a, b) display the characteristics of a well ordered hexagonal structure. After the incorporation of the Ag with mesoporous samples, the peak intensity for the low angle reflections decreased (Fig. 4c, d). However, one major peak and two smaller peaks occurred at about 0.8° of 2è of the Ag incorporated sample, suggesting that the hexagonal pore structure of the mesoporous sample is retained after the reduction of Ag. The pore size distributions of the Ag/MS and the Ag/TFMS were identified by the Barrett, Joyner and Hallenda (BJH) method [23] to investigate the influence of Ag loading on the mesopore structure of samples (Fig. 5). The modal pore diameters of the mesoporous silica (Fig. 5a) and TFMS (Fig. 5f) host materials were 6.4 nm, indicating that pore sizes are decreased slightly by MPTMS functionalization of the inner pore surface. As the loaded Ag amount increased, the pore volumes of both the Agn/MS and Agm/TFMS samples decreased. After the Ag loading, the modal pore diameter (about 6.4 nm) of the Agn/MS sample was not changed, even though the Ag loading amount increased (Fig. 5 b–e). From these results, it is considered that the Ag clusters of the Agn/MS were formed on the exterior surface of the mesopore. This implies that it is difficult to prepare the Ag cluster within the mesopore by the general NaBH4 reduction method. On the contrary, the modal pore diameters of the Agm/TFMS (Fig. 5 g–j) are in the region of 4– 6 nm and have a tendency to be 4 nm when the Ag loading amount increases because Ag clusters were partially filled in pores. The TEM images of the Ag/MS and Ag/TFMS samples are shown in Fig. 6. Ag particles in the Ag/MS sample were found to be separate from the mesoporous silica and sized between 50–300 nm (Fig. 6a, b). The TEM images of the Ag/TFMS showed that Ag nanoparticles were formed inside the mesopore channel (Fig. 6d). Ag particles in the Ag/ TFMS were well dispersed with Ag particle diameters under 6 nm, which occurred because the Ag particle growth was restricted by the dimensions of the channel space (Fig. 6c).
4. Conclusions The growth of silver nanoparticles within the pores of mesoporous silica has been achieved by reducing adsorbed silver cations on a thiol functionalized surface of mesopores. Silver nanoparticles could not be grown within the pore channels without help of thiol substituents. The silver nanoparticles prepared on thiol functionalized mesoporous silica maintained
159
their dispersed state and diameters under 6 nm, which is smaller than the pore size. The process with TFMS is considered to be a fast and effective method for the preparation of nanoparticles with a narrow size distribution, and the method could be extended to prepare other nanosized metals. References [1] G.R. Stewart, Phys. Rev., B 15 (1997) 1143. [2] G. Schmid, M. Baumle, M. Geerkens, I. Heim, C. Osemann, T. Sawitowski, Chem. Soc. Rev. 28 (1999) 179. [3] K. Uchida, S. Kaneko, S. Omi, C. Hata, H. Tanji, Y. Asahara, A.J. Ikushima, T. Tokizaki, A.J. Nakamura, Opt. Soc. Am. B 11 (1994) 1236. [4] M.P. Zach, K.H. Ng, R.M. Penner, Science 290 (2000) 2120. [5] M.H.V. Werts, M. Lambert, J.P. Bourgoin, M. Brust, Nano Lett. 2 (2002) 43. [6] M.E.T. Molares, V. Buschmann, D. Dobrev, R. Neumann, R. Scholz, I.U. Schuchert, J. Vetter, Adv. Mater. 13 (2001) 62. [7] A.J. Yin, J. Li, W. Jin, A.J. Bennett, J. Xu, Appl. Phys. Lett. 79 (2001) 1039. [8] W.H. Zhang, J.L. Shi, H.R. Chen, Z.L. Hua, D.S. Yan, Chem. Mater. 13 (2001) 648. [9] Y.S. Seo, K.S. Kim, A. Galambos, R.G.H. Lammertink, G.J. Vancso, J. Sokolov, M. Rafailovich, Nano Lett. 4 (2004) 483. [10] T. Thurn-Albrecht, J. Schotter, G.A. Kastle, N. Emley, T. Shibauchi, L. Krusin-Elbaum, K. Guarini, M.T. Tuominen, T.P. Russell, Science 290 (2000) 2126. [11] L. Gao, E.B. Wang, S.Y. Lian, Z.H. Kang, Y. Lan, D. Wu, Solid State Commun. 130 (2004) 309. [12] M.H. Huang, A. Choudrey, P.D. Yang, Chem. Commun. (2000) 1063. [13] W. Zhu, Y. Han, L. An, Microporous Mesoporous Mater. 80 (2005) 221. [14] H. Bi, W. Cai, H. Shi, X. Liu, Chem. Phys. Lett. 357 (2002) 249. [15] W. Chen, J. Zhang, Scr. Mater. 49 (2003) 321. [16] A.L. Pan, H.G. Zheng, Z.P. Yang, F.X. Liu, Z.J. Ding, Y.T. Qian, Mater. Res., B 38 (2003) 789–796. [17] W. Chen, Junying Zhang, Weiping Cai, Scr. Mater. 48 (2003) 1061. [18] R. Patakfalvi, I. Dékány, Appl. Clay Sci. 25 (2004) 149. [19] T. Hayakawa, S.T. Selvan, M. Nigami, Appl. Phys. Lett. 74 (1999) 1513. [20] X.G. Zhao, J.L. Shi, B. Hu, L.X. Zhang, Z.L. Hua, Mater. Lett. 58 (2004) 2152. [21] Y. Guari, C. Thieuleux, A. Mehdi, C. Reye, R.J.P. Corriu, S.G. Gallardo, K. Philippot, B. Chaudret, R. Dutartre, Chem. Commun (2001) 1374. [22] D. Zhao, Q. Huo, J. Feng, B.F. Chmelka, G.D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024. [23] S. Lowell, J.E. Shields (Eds.), Powder and Surface Area and Porosity, 3rd ed., Chapman & Hall, London, 1991.