- Email: [email protected]
Structural characterization of mesoporous silica nanowire arrays grown in porous alumina templates
Chemical Physics Letters 409 (2005) 172–176 www.elsevier.com/locate/cplett
Structural characterization of mesoporous silica nanowire arrays grown in porous alumina templates Kewang Jin a, Baodian Yao a
a,b
, Ning Wang
a,*
Department of Physics, and Institute of Nano Science and Technology, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, China b Department of Chemistry, Fudan University, Handan Road 220, Shanghai 200433, China Received 13 April 2005 Available online 3 June 2005
Abstract Highly ordered mesoporous silica nanowire (NW) arrays were fabricated inside the pores of porous anodic aluminum oxide (AAO) templates using a simple sol–gel method at different aging conditions. High-resolution transmission electron microscopy investigation shows that the structure of silica NWs is sensitive to the template structure and aging environment. For the samples aged with the presence of water, all the nanochannels in the silica NWs were found to be circular around the NW axes. However, for those samples aged without water, most silica NWs have a tube-like structure due to the confinement effect of the AAO pore-walls. Those tube-like silica NWs formed in large AAO pores generally consist of a core (hexagonally arranged nanochannels parallel to the NW axis) wrapped by shells. The formation mechanism for these novel structures of mesoporous silica NWs were proposed and discussed. Ó 2005 Elsevier B.V. All rights reserved.
The importance of the alignment and size distribution of one-dimensional (1D) nanostructures in various potential applications has propelled extensive research on the syntheses of nanostructure arrays via different techniques. Mesoporous silica [1–5] and porous anodic aluminum oxide (AAO) membranes [6–10] are two kinds of templates widely used in synthesizing highly ordered arrays of nanotubes and nanowires (NWs). Mesoporous silica has ultrasmall nanochannels with diameters around one to several nanometers. The nanostructures formed in these ultrasmall nanochannels can offer the quantum size effects required for fabricating future nanoscale devices. However, it is still a challenging issue to grow uniform mesoporous silica with highly ordered nanochannels over a large area on most substrates. Large AAO templates containing unidirectional pores with pore sizes in the order of 10–300 nm can be easily *
Corresponding author. Fax: +852 2358 1652. E-mail address: [email protected] (N. Wang).
0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.05.002
prepared by anodization. They are thermally and chemically stable, with their pore size (hence the diameter of the nanostructures formed in the pores) that can be fully controlled. Due to the relatively large size of the pores in AAO templates, various materials can be easily introduced into the AAO pores, which makes AAO films ideal templates for the synthesis of highly ordered nanostructures. One of the strategies to fabricate highly ordered mesoporous silica templates is to use AAO as the host and grow mesoporous silica NWs inside the pores of AAO to form new hierarchical structures with ultrasmall nanochannels in the silica NWs parallel to the direction of the pores in AAO. 1D hierarchically mesostructured silica materials and their arrays prepared in AAO membranes have been reported by Yang et al. [11] and all the nanochannels of the mesoporous silica were found to be circular. Silicasurfactant nanocomposite was prepared inside AAO membranes with the direction of the nanochannels parallel to the alumina pores by Yamaguchi et al. [12].
K. Jin et al. / Chemical Physics Letters 409 (2005) 172–176
However, the arrangement of the mesopores was slightly random inside the NWs. A systematic study of the confined assembly of silica-surfactant composite mesostructures within cylindrical pores of varying diameters was carried out by Wu et al. [13]. Again, the architecture of the nanochannels were found to be circular with a single channel in the direction parallel to the pores of AAO. It is really a challenge to form large areas of hexagonally arranged nanochannels inside the pores of AAO. The formation process of silica NWs is complicated, and the structures are very sensitive to the synthesis processes and the subsequent treatments. In this Letter, we report structural characterization of highly ordered mesoporous silica NWs prepared inside the pores of AAO templates using a simple sol–gel method. We found that the structures of mesoporous silica NWs were influenced not only by the AAO channels but also by the aging conditions. To fabricate mesoporous silica NWs, tetraethoxysilane (TEOS) and poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) triblock copolymer surfactants (BASF, Pluronic P123) were used as the silica source and the structure-directing agent, respectively. The AAO templates used in the present work were commercially available from Whatman (Anodisc 25) which contain uniform channels (200 nm in diameter). Fig. 1 schematically shows the sample preparation process. In brief, TEOS (10.4 g), absolute ethanol (25 g), and HCl (1 g of 1 M HCl solution) were mixed together in the presence of P123 (5 g). The resultant mixture was stirred at 37 °C for 10 min to form a clear solution. The alumina templates were immersed in the as-prepared sol and all of the volatile solvent was evaporated by a rotary evaporator in vacuum condition at 40 °C for approximately 25 min. After sealing the container and aging at 60 °C for 12 h, we obtained sample A. By adding some water before sealing and aging the container at the same condition, sample B was obtained. All samples were subsequently calcined at 500 °C for 6 h. Samples treated at different conditions have been investigated. In this Letter, detailed character-
Fig. 1. The preparation process of mesoporous silica NWs inside AAO templates.
173
ization and comparison of these two kinds of samples are shown. The morphologies and mesostructures of silica NWs were investigated by scanning electron microscopy (SEM) (Philips XL30 microscope) and transmission electron microscopy (TEM) (Philips CM120 microscope operating at 120 kV and JEOL 2011 microscope operating at 200 kV). Plan-view TEM samples were prepared by mechanical polishing followed by Ar ion milling at room temperature. For the preparation of side view TEM samples, the AAO templates containing silica NWs were first dissolved using 5 M HCl solution and then the silica NWs were collected by filtration. Fig. 2 illustrates the typical morphologies of samples A and B viewed by SEM. Silica NWs in these two samples uniformly formed inside each pore of AAO templates and did not show significant differences. Because the silica gel within the alumina pores has undergone contraction during the drying and calcination processes, small gaps formed at the interfaces between silica NWs and the walls of columnar pores of AAO. Such a contraction in silica NWs became more obvious when the sample was exposed to the electron beam irradiation during the TEM observation. This is because the AAO matrix is a poor electrical and heat conductor and the heating effect always occurs in AAO due to the electron beam irradiation. This results in a large temperature increase in silica NWs, and hence the contraction occurs. As shown in Figs. 3a and 4a, all silica NWs previously fully filled in the AAO pores shrink axially. Therefore, the cross-sectional shape of a shrunk silica NW is very similar to that of the AAO channel host. In sample A, two kinds of silica NWs have been observed. Most silica NWs consist of pure tube-like shell structure. However, some silica NWs formed in relatively large AAO pores have hexagonally arranged nanochannels at their interior regions (Figs. 3d,e). Figs. 3b and c are plan-view high-resolution TEM images showing the cross-sectional feature of silica NWs with no nanochannels that: (i) the spacing between adjacent layers is 2.0 ± 0.4 nm; (ii) the layer-to-layer distance is 8.6 ± 0.3 nm. Figs. 3d,e are plan-view high-resolution TEM images showing the silica NWs with nanochannels at the interior regions that: (i) the nanochannels have a well hexagonal arrangement; (ii) the diameter of the nanochannels is 5.0 ± 0.4 nm, and the channelto-channel distance is 11.5 ± 0.5 nm; (iii) the nanochannels were wrapped by a layered structure, while the layer-tolayer distance is different from place to place due to different degree of contraction. There are some particles attached to the surfaces of silica NWs and the inner walls of the pores which have been identified to be copper (contamination introduced sample preparation by Ar ion milling) by both X-ray energy dispersive spectroscopy (EDS) and electron diffraction. Figs. 3f,g are high-resolution TEM images taken along the side view
174
K. Jin et al. / Chemical Physics Letters 409 (2005) 172–176
Fig. 2. (a) Top-view and (b) side-view SEM images of sample A; (c) top-view and (d) side-view SEM images of sample B. Scale bar: 1 lm.
Fig. 3. TEM images of sample A: (a) low magnification plan-view TEM image; (b,c) high-resolution cross-sectional TEM images of silica nanowires without nanochannels in the interior region; (d,e) high-resolution cross-sectional TEM images of silica NWs with nanochannels in the interior region; (f,g) high-resolution side-view TEM images of silica NWs showing the tube-like structure. Scale bar: 100 nm.
direction of an individual silica NW. The tube-like feature from the exterior regions as observed from the plan-view images is clearly visible, which also confirms that the direction of the nanochannels in the interior re-
gion of the NWs is along the axis of the pores in AAO templates. Similar to sample A, silica NWs in sample B also shrink axially as shown in Fig. 4. Plan-view TEM
K. Jin et al. / Chemical Physics Letters 409 (2005) 172–176
175
Fig. 4. TEM images of sample B: (a) low magnification plan-view TEM image; (b,c) high-resolution plan-view TEM images; (d,e) high-resolution side-view TEM images. Scale bar: 100 nm.
images shown in Figs. 4b,c are very similar to that of tube-like silica NWs in sample A. The spacing between adjacent layers is 1.8 ± 0.2 nm and the layerto-layer distance is 8.6 ± 0.3 nm which is almost the same with sample A. Compared to sample A, the significant difference is that the architecture of the nanochannels in the NWs in sample B is circular instead of longitudinal, which is clearly seen from Figs. 4d and e. The diameter of the nanochannels is 3.3 ± 0.4 nm, and the channel-to-channel distance is 7.4 ± 0.2 nm. Disordered nanochannels can be found at the interior regions in some NWs (Fig. 4c), which may be due to the contraction during the calcination process. Mesoporous silica nanofibers with controlled pore architectures have been synthesized in dilute solutions of cationic surfactants and silica species under strongly acidic conditions [14]. Generally, silica nanofibers with longitudinal pore architecture can be produced at low temperature conditions, and silica nanofibers with circular pore architecture form at high temperature conditions. It is interesting to note that the arrangement and orientation of the nanochannels in the silica NWs synthesized by the present method are sensitive to the aging conditions, particularly the presence of water during the formation of silica NWs. Based on our observa-
tion, we propose that the rate of the solvent evaporation influences the structure of silica NWs. For the sample aged without the presence of water (sample A), the rate of the solvent evaporation is much faster than that of the sample aged with water (sample B), which makes the silica NWs not to have enough time to reach a stable phase. The first stage of filling silica sol into the pores of AAO may result in the formation of the tube-like silica NWs. On the one hand, AAO pore walls confine the structural orientation of silica NWs, i.e., the tube-like NWs replicate the shape of the AAO pores. This is because the nucleation and growth of silica should initiate at AAO pore walls. For a small size AAO pore, the shell structure easily occurs in the entire silica NW as observed by TEM. On the other hand, if the size of the AAO pore is large enough, such a confinement effect will be weak, which may give rise to the structural change of silica NWs in the interior region, for example, the hexagonally arranged nanochannels as observed in the interior regions of silica NWs in the large pores of AAO in sample A. With the presence of water, however, the rate of solvent evaporation is much slower, which allows the silica NWs to have enough time to reach an achievable energy minimum [15]. Mesoporous silica NW arrays with different architectures formed in AAO templates are
176
K. Jin et al. / Chemical Physics Letters 409 (2005) 172–176
highly relevant to fabricating nano-sized electronic and optoelectronic devices, and therefore may provide exciting possibilities for fundamental research and technological applications. In summary, mesoporous silica NWs have been fabricated inside porous alumina membranes via a simple sol–gel rotary evaporation method. The orientation of the nanochannels in silica NWs can be readily controlled by varying the aging environment. For the sample aged with the presence of water, all the nanochannels in the silica NWs were found to be circular around the NW axes, while for the sample aged without water, most silica NWs were pure tube-like structure. Tube-like silica NWs formed in large AAO channels contained hexagonally arranged nanochannels at the interior regions. Different from the NWs treated with the presence of water, all these nanochannels were found to be parallel to the NW axes.
Acknowledgment This work was financially supported by Research Grant Council of Hong Kong (Project No. 603804 and CityU 3/04C).
References [1] B. Tian, X. Liu, H. Yang, S. Xie, C. Yu, B. Tu, D. Zhao, Adv. Mater. 15 (2003) 1370. [2] M.H. Huang, A. Choudrey, P. Yang, Chem. Commun. (2000) 1063. [3] T.A. Crowley, K.J. Ziegler, D.M. Lyons, D. Erts, H. Olin, M.A. Morris, J.D. Holmes, Chem. Mater. 15 (2003) 3518. [4] N.R.B. Coleman, K.M. Ryan, T.R. Spalding, J.D. Holmes, M.A. Morris, Chem. Phys. Lett. 343 (2001) 1. [5] K.M. Ryan, D. Erts, H. Olin, M.A. Morris, J.D. Holmes, J. Am. Chem. Soc. 125 (2003) 6284. [6] E.J. Bae, W.B. Choi, K.S. Jeong, J.U. Chu, G.S. Park, S. Song, I.K. Yoo, Adv. Mater. 14 (2002) 277. [7] W.C. Yoo, J.K. Lee, Adv. Mater. 16 (2004) 1097. [8] B.D. Yao, N. Wang, J. Phys. Chem. B 105 (2001) 11395. [9] J. Choi, G. Sauer, P. Go¨ring, K. Nielsch, R.B. Wehrspohn, U. Go¨sele, J. Mater. Chem. 13 (2003) 1100. [10] G. Schmid, J. Mater. Chem. 12 (2002) 1231. [11] Z. Yang, Z. Niu, X. Cao, Z. Yang, Y. Lu, Z. Hu, C.C. Han, Angew. Chem., Int. Ed. 42 (2002) 4201. [12] A. Yamaguchi, F. Uejo, T. Yoda, T. Uchida, Y. Tanamura, T. Yamashita, N. Teramae, Nat. Mater. 3 (2004) 337. [13] Y. Wu, G. Cheng, K. Katsov, S.W. Sides, J. Wang, J. Tang, G.H. Fredrichson, M. Moskovits, G.D. Stucky, Nat. Mater. 3 (2004) 816. [14] J. Wang, C.K. Tsung, W. Hong, Y. Wu, J. Tang, G.D. Stucky, Chem. Mater. 16 (2004) 5169. [15] F. Marlow, F. Kleitz, Micropor. Mesopor. Mater. 44–45 (2001) 671.
Structural characterization of mesoporous silica nanowire arrays grown in porous alumina templates Kewang Jin a, Baodian Yao a
a,b
, Ning Wang
a,*
Department of Physics, and Institute of Nano Science and Technology, The Hong Kong University of Science and Technology, Clear Water Bay, Hong Kong, China b Department of Chemistry, Fudan University, Handan Road 220, Shanghai 200433, China Received 13 April 2005 Available online 3 June 2005
Abstract Highly ordered mesoporous silica nanowire (NW) arrays were fabricated inside the pores of porous anodic aluminum oxide (AAO) templates using a simple sol–gel method at different aging conditions. High-resolution transmission electron microscopy investigation shows that the structure of silica NWs is sensitive to the template structure and aging environment. For the samples aged with the presence of water, all the nanochannels in the silica NWs were found to be circular around the NW axes. However, for those samples aged without water, most silica NWs have a tube-like structure due to the confinement effect of the AAO pore-walls. Those tube-like silica NWs formed in large AAO pores generally consist of a core (hexagonally arranged nanochannels parallel to the NW axis) wrapped by shells. The formation mechanism for these novel structures of mesoporous silica NWs were proposed and discussed. Ó 2005 Elsevier B.V. All rights reserved.
The importance of the alignment and size distribution of one-dimensional (1D) nanostructures in various potential applications has propelled extensive research on the syntheses of nanostructure arrays via different techniques. Mesoporous silica [1–5] and porous anodic aluminum oxide (AAO) membranes [6–10] are two kinds of templates widely used in synthesizing highly ordered arrays of nanotubes and nanowires (NWs). Mesoporous silica has ultrasmall nanochannels with diameters around one to several nanometers. The nanostructures formed in these ultrasmall nanochannels can offer the quantum size effects required for fabricating future nanoscale devices. However, it is still a challenging issue to grow uniform mesoporous silica with highly ordered nanochannels over a large area on most substrates. Large AAO templates containing unidirectional pores with pore sizes in the order of 10–300 nm can be easily *
Corresponding author. Fax: +852 2358 1652. E-mail address: [email protected] (N. Wang).
0009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2005.05.002
prepared by anodization. They are thermally and chemically stable, with their pore size (hence the diameter of the nanostructures formed in the pores) that can be fully controlled. Due to the relatively large size of the pores in AAO templates, various materials can be easily introduced into the AAO pores, which makes AAO films ideal templates for the synthesis of highly ordered nanostructures. One of the strategies to fabricate highly ordered mesoporous silica templates is to use AAO as the host and grow mesoporous silica NWs inside the pores of AAO to form new hierarchical structures with ultrasmall nanochannels in the silica NWs parallel to the direction of the pores in AAO. 1D hierarchically mesostructured silica materials and their arrays prepared in AAO membranes have been reported by Yang et al. [11] and all the nanochannels of the mesoporous silica were found to be circular. Silicasurfactant nanocomposite was prepared inside AAO membranes with the direction of the nanochannels parallel to the alumina pores by Yamaguchi et al. [12].
K. Jin et al. / Chemical Physics Letters 409 (2005) 172–176
However, the arrangement of the mesopores was slightly random inside the NWs. A systematic study of the confined assembly of silica-surfactant composite mesostructures within cylindrical pores of varying diameters was carried out by Wu et al. [13]. Again, the architecture of the nanochannels were found to be circular with a single channel in the direction parallel to the pores of AAO. It is really a challenge to form large areas of hexagonally arranged nanochannels inside the pores of AAO. The formation process of silica NWs is complicated, and the structures are very sensitive to the synthesis processes and the subsequent treatments. In this Letter, we report structural characterization of highly ordered mesoporous silica NWs prepared inside the pores of AAO templates using a simple sol–gel method. We found that the structures of mesoporous silica NWs were influenced not only by the AAO channels but also by the aging conditions. To fabricate mesoporous silica NWs, tetraethoxysilane (TEOS) and poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) triblock copolymer surfactants (BASF, Pluronic P123) were used as the silica source and the structure-directing agent, respectively. The AAO templates used in the present work were commercially available from Whatman (Anodisc 25) which contain uniform channels (200 nm in diameter). Fig. 1 schematically shows the sample preparation process. In brief, TEOS (10.4 g), absolute ethanol (25 g), and HCl (1 g of 1 M HCl solution) were mixed together in the presence of P123 (5 g). The resultant mixture was stirred at 37 °C for 10 min to form a clear solution. The alumina templates were immersed in the as-prepared sol and all of the volatile solvent was evaporated by a rotary evaporator in vacuum condition at 40 °C for approximately 25 min. After sealing the container and aging at 60 °C for 12 h, we obtained sample A. By adding some water before sealing and aging the container at the same condition, sample B was obtained. All samples were subsequently calcined at 500 °C for 6 h. Samples treated at different conditions have been investigated. In this Letter, detailed character-
Fig. 1. The preparation process of mesoporous silica NWs inside AAO templates.
173
ization and comparison of these two kinds of samples are shown. The morphologies and mesostructures of silica NWs were investigated by scanning electron microscopy (SEM) (Philips XL30 microscope) and transmission electron microscopy (TEM) (Philips CM120 microscope operating at 120 kV and JEOL 2011 microscope operating at 200 kV). Plan-view TEM samples were prepared by mechanical polishing followed by Ar ion milling at room temperature. For the preparation of side view TEM samples, the AAO templates containing silica NWs were first dissolved using 5 M HCl solution and then the silica NWs were collected by filtration. Fig. 2 illustrates the typical morphologies of samples A and B viewed by SEM. Silica NWs in these two samples uniformly formed inside each pore of AAO templates and did not show significant differences. Because the silica gel within the alumina pores has undergone contraction during the drying and calcination processes, small gaps formed at the interfaces between silica NWs and the walls of columnar pores of AAO. Such a contraction in silica NWs became more obvious when the sample was exposed to the electron beam irradiation during the TEM observation. This is because the AAO matrix is a poor electrical and heat conductor and the heating effect always occurs in AAO due to the electron beam irradiation. This results in a large temperature increase in silica NWs, and hence the contraction occurs. As shown in Figs. 3a and 4a, all silica NWs previously fully filled in the AAO pores shrink axially. Therefore, the cross-sectional shape of a shrunk silica NW is very similar to that of the AAO channel host. In sample A, two kinds of silica NWs have been observed. Most silica NWs consist of pure tube-like shell structure. However, some silica NWs formed in relatively large AAO pores have hexagonally arranged nanochannels at their interior regions (Figs. 3d,e). Figs. 3b and c are plan-view high-resolution TEM images showing the cross-sectional feature of silica NWs with no nanochannels that: (i) the spacing between adjacent layers is 2.0 ± 0.4 nm; (ii) the layer-to-layer distance is 8.6 ± 0.3 nm. Figs. 3d,e are plan-view high-resolution TEM images showing the silica NWs with nanochannels at the interior regions that: (i) the nanochannels have a well hexagonal arrangement; (ii) the diameter of the nanochannels is 5.0 ± 0.4 nm, and the channelto-channel distance is 11.5 ± 0.5 nm; (iii) the nanochannels were wrapped by a layered structure, while the layer-tolayer distance is different from place to place due to different degree of contraction. There are some particles attached to the surfaces of silica NWs and the inner walls of the pores which have been identified to be copper (contamination introduced sample preparation by Ar ion milling) by both X-ray energy dispersive spectroscopy (EDS) and electron diffraction. Figs. 3f,g are high-resolution TEM images taken along the side view
174
K. Jin et al. / Chemical Physics Letters 409 (2005) 172–176
Fig. 2. (a) Top-view and (b) side-view SEM images of sample A; (c) top-view and (d) side-view SEM images of sample B. Scale bar: 1 lm.
Fig. 3. TEM images of sample A: (a) low magnification plan-view TEM image; (b,c) high-resolution cross-sectional TEM images of silica nanowires without nanochannels in the interior region; (d,e) high-resolution cross-sectional TEM images of silica NWs with nanochannels in the interior region; (f,g) high-resolution side-view TEM images of silica NWs showing the tube-like structure. Scale bar: 100 nm.
direction of an individual silica NW. The tube-like feature from the exterior regions as observed from the plan-view images is clearly visible, which also confirms that the direction of the nanochannels in the interior re-
gion of the NWs is along the axis of the pores in AAO templates. Similar to sample A, silica NWs in sample B also shrink axially as shown in Fig. 4. Plan-view TEM
K. Jin et al. / Chemical Physics Letters 409 (2005) 172–176
175
Fig. 4. TEM images of sample B: (a) low magnification plan-view TEM image; (b,c) high-resolution plan-view TEM images; (d,e) high-resolution side-view TEM images. Scale bar: 100 nm.
images shown in Figs. 4b,c are very similar to that of tube-like silica NWs in sample A. The spacing between adjacent layers is 1.8 ± 0.2 nm and the layerto-layer distance is 8.6 ± 0.3 nm which is almost the same with sample A. Compared to sample A, the significant difference is that the architecture of the nanochannels in the NWs in sample B is circular instead of longitudinal, which is clearly seen from Figs. 4d and e. The diameter of the nanochannels is 3.3 ± 0.4 nm, and the channel-to-channel distance is 7.4 ± 0.2 nm. Disordered nanochannels can be found at the interior regions in some NWs (Fig. 4c), which may be due to the contraction during the calcination process. Mesoporous silica nanofibers with controlled pore architectures have been synthesized in dilute solutions of cationic surfactants and silica species under strongly acidic conditions [14]. Generally, silica nanofibers with longitudinal pore architecture can be produced at low temperature conditions, and silica nanofibers with circular pore architecture form at high temperature conditions. It is interesting to note that the arrangement and orientation of the nanochannels in the silica NWs synthesized by the present method are sensitive to the aging conditions, particularly the presence of water during the formation of silica NWs. Based on our observa-
tion, we propose that the rate of the solvent evaporation influences the structure of silica NWs. For the sample aged without the presence of water (sample A), the rate of the solvent evaporation is much faster than that of the sample aged with water (sample B), which makes the silica NWs not to have enough time to reach a stable phase. The first stage of filling silica sol into the pores of AAO may result in the formation of the tube-like silica NWs. On the one hand, AAO pore walls confine the structural orientation of silica NWs, i.e., the tube-like NWs replicate the shape of the AAO pores. This is because the nucleation and growth of silica should initiate at AAO pore walls. For a small size AAO pore, the shell structure easily occurs in the entire silica NW as observed by TEM. On the other hand, if the size of the AAO pore is large enough, such a confinement effect will be weak, which may give rise to the structural change of silica NWs in the interior region, for example, the hexagonally arranged nanochannels as observed in the interior regions of silica NWs in the large pores of AAO in sample A. With the presence of water, however, the rate of solvent evaporation is much slower, which allows the silica NWs to have enough time to reach an achievable energy minimum [15]. Mesoporous silica NW arrays with different architectures formed in AAO templates are
176
K. Jin et al. / Chemical Physics Letters 409 (2005) 172–176
highly relevant to fabricating nano-sized electronic and optoelectronic devices, and therefore may provide exciting possibilities for fundamental research and technological applications. In summary, mesoporous silica NWs have been fabricated inside porous alumina membranes via a simple sol–gel rotary evaporation method. The orientation of the nanochannels in silica NWs can be readily controlled by varying the aging environment. For the sample aged with the presence of water, all the nanochannels in the silica NWs were found to be circular around the NW axes, while for the sample aged without water, most silica NWs were pure tube-like structure. Tube-like silica NWs formed in large AAO channels contained hexagonally arranged nanochannels at the interior regions. Different from the NWs treated with the presence of water, all these nanochannels were found to be parallel to the NW axes.
Acknowledgment This work was financially supported by Research Grant Council of Hong Kong (Project No. 603804 and CityU 3/04C).
References [1] B. Tian, X. Liu, H. Yang, S. Xie, C. Yu, B. Tu, D. Zhao, Adv. Mater. 15 (2003) 1370. [2] M.H. Huang, A. Choudrey, P. Yang, Chem. Commun. (2000) 1063. [3] T.A. Crowley, K.J. Ziegler, D.M. Lyons, D. Erts, H. Olin, M.A. Morris, J.D. Holmes, Chem. Mater. 15 (2003) 3518. [4] N.R.B. Coleman, K.M. Ryan, T.R. Spalding, J.D. Holmes, M.A. Morris, Chem. Phys. Lett. 343 (2001) 1. [5] K.M. Ryan, D. Erts, H. Olin, M.A. Morris, J.D. Holmes, J. Am. Chem. Soc. 125 (2003) 6284. [6] E.J. Bae, W.B. Choi, K.S. Jeong, J.U. Chu, G.S. Park, S. Song, I.K. Yoo, Adv. Mater. 14 (2002) 277. [7] W.C. Yoo, J.K. Lee, Adv. Mater. 16 (2004) 1097. [8] B.D. Yao, N. Wang, J. Phys. Chem. B 105 (2001) 11395. [9] J. Choi, G. Sauer, P. Go¨ring, K. Nielsch, R.B. Wehrspohn, U. Go¨sele, J. Mater. Chem. 13 (2003) 1100. [10] G. Schmid, J. Mater. Chem. 12 (2002) 1231. [11] Z. Yang, Z. Niu, X. Cao, Z. Yang, Y. Lu, Z. Hu, C.C. Han, Angew. Chem., Int. Ed. 42 (2002) 4201. [12] A. Yamaguchi, F. Uejo, T. Yoda, T. Uchida, Y. Tanamura, T. Yamashita, N. Teramae, Nat. Mater. 3 (2004) 337. [13] Y. Wu, G. Cheng, K. Katsov, S.W. Sides, J. Wang, J. Tang, G.H. Fredrichson, M. Moskovits, G.D. Stucky, Nat. Mater. 3 (2004) 816. [14] J. Wang, C.K. Tsung, W. Hong, Y. Wu, J. Tang, G.D. Stucky, Chem. Mater. 16 (2004) 5169. [15] F. Marlow, F. Kleitz, Micropor. Mesopor. Mater. 44–45 (2001) 671.