- Email: [email protected]
Synthesis of ordered mesostructured silica nanotubal arrays
Microporous and Mesoporous Materials 84 (2005) 69–74 www.elsevier.com/locate/micromeso
Synthesis of ordered mesostructured silica nanotubal arrays Weiping Zhu, Yanchun Han *, Lijia An
*
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Graduate School of the Chinese Academy of Sciences, Changchun 130022, People’s Republic of China Received 24 January 2005; received in revised form 13 April 2005; accepted 17 April 2005 Available online 17 June 2005
Abstract Novel mesostructured silica nanotubes perpendicular to the substrate were successfully prepared by the use of porous alumina membranes as a template. The uniform mesopores arranged over the whole nanotube sheath were about 5.5 nm in diameter. The mesoporous channels run parallel to the longitudinal axis of the tubes. The formation mechanisms and the characteristics of the mesostructured silica nanotubes were discussed in detail. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Mesoporous silica; Nanotubes; Alumina membranes; Template synthesis
1. Introduction In 1991, Iijima first reported the discovery of carbon nanotubes [1]. It is a significant step in expanding the application of one-dimensional nanostructured materials. Since then, some other nanotubes have been developed in succession because of their potential applications for electronic devices, catalysis, separation and optical devices, etc. [2–4]. Many synthetic methods have been developed to prepare various nanotubes, such as, the surfactant mediated technique [5], the sol–gel method [6], the multi-step oxidation-etching process [7], the template method [8–18] and so on. The template method is a flexible and affordable route for a large number of nanotubes. The size and structural properties of the materials prepared are determined by the size, shape, and morphology of the template used since the growth of the materials is confined by the template. Anodized pore alumina membranes, which have nearly ordered circular * Corresponding authors. Tel.: +86 431 5262175/5262206; fax: +86 431 5262126 (Y. Han). E-mail addresses: [email protected] (Y. Han), [email protected] (L. An).
1387-1811/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.04.020
pores perpendicular to the membrane surface, are ideally suited for templates to prepare uniform 1D nano-materials, such as nanotubes or nanorods [19–22]. Mesostructured nanotubes have attracted more and more attentions because of their specific characteristics. For example, they have not only two different kinds of holes such as the nanotubes and the mesopores arranged over the whole tube sheath, but also three different kinds of surfaces such as the outer surface of the nanotube, the inner surface of the nanotube and the mesoporous surface arranged over the nanotube sheaths. They can be used to assemble differently sized nanodevices or act together in an integrated chemical system due to being functionalized differently [11]. Thus, they may find potential applications in chemical sensors [23], optical devices, surfaces for heterogeneous catalysis, especially future generations of microelectronic chips whose densities continue to increase and feature sizes continue to decrease accordingly [24]. There was a certain success in combining the advantages of mesoporous silica materials and anodized alumina membranes to form new hierarchical structures [13,25–27]. For example, 1D hierarchically mesostructured materials with ordered nanofibers or nanotubes
70
W. Zhu et al. / Microporous and Mesoporous Materials 84 (2005) 69–74
in anodic alumina membranes were prepared through the sol–gel route [13]. The mesopore silica oriented parallel to the alumina host pores was prepared by dropping a surfactant/TEOS precursor solution into the pores of the anodic alumina membrane [25]. 1D mesoporous silica nanotubes and nanowires inside porous alumina membranes were fabricated via the sol–gel rotary evaporation route [26]. The mesostructured nanotubes or nanorods prepared so far were inside the anodic alumina membranes, i.e., the alumina membranes were not removed [25,26]. Motivated by the potential application of the ordered arrangement of mesostructured nanotubes, we developed a template synthesis route for the fabrication of mesostructured nanotubal arrays. The formation mechanisms and the characteristics of the mesostructured silica nanotubes were discussed in detail.
ethoxysilane (TEOS, Aldrich) was added to this solution slowly and stirred for 2 h at the same temperature. The molar ratio was TEOS:P123:HCl:ethanol:H2O = 1:0.017:0.1:31.8:10.8. The thin films of mixtures of P123 and TEOS were prepared by spin-coating at two different rates (1000 rpm and 2000 rpm, respectively) on a silicon wafer. Then, the aluminum oxide membranes (Anodisc 25, 0.2 lm pore diameter, 60 lm thickness, Whatman, Inc.) were placed on the top of the films. The samples were heated to 100 °C. After annealing for 2 h, 5 h, 15 h and 20 h under vacuum, respectively, the samples were quenched to room temperature. The samples were subjected to thermal treatment at about 400 °C for 4 h to remove P123 immediately. The aluminum oxide membranes were removed by 30 wt% H3PO4 at about 80 °C for 30 min. Finally, the samples were washed by deionized water and dried at about 100 °C for 3 h.
2. Experimental
2.2. Analysis
2.1. Synthesis
Low angle X-ray diffraction (XRD) pattern were taken on a RINT2000 vertical goniometer equipped with a liquid nitrogen cooled germanium solid-state detector using Cu Ka radiation, k = 0.15406 nm. Field emission scanning electron microscope (FESEM) was carried out on a Philips XL-30-ESEM-FEG instrument operating at 20 kV. The specimen was sputtered with a thin film of gold.
Mesostructured silica nanotubes were synthesized as follows: 1.0 g of Pluronic 123 (P123, EO20PO70EO20, MW = 5800, Aldrich), 2.0 g of deionized water, 15 g of ethanol and 0.1 g of 37 wt% hydrochloric acid were mixed and stirred for 1 h at room temperature until a homogenous solution was formed. Then, 2.13 g of tetra-
Fig. 1. Scheme for the formation of silica nanotubes with a mesostructure using anodized pore alumina membranes as the template on a Si wafer.
W. Zhu et al. / Microporous and Mesoporous Materials 84 (2005) 69–74
71
Fig. 2. FE-SEM images of the silica nanotubes having been annealed at 100 °C for (a) 2 h, (b) 5 h, (c) 15 h, (d) 20 h after calcination and complete removal of the alumina template and (e) 20 h after complete removal of the alumina template and before calcination. The inset in (a) indicates the tubular nanostructure instead of nanorods.
Energy-dispersive X-ray spectroscopy (EDX) observation was performed on an EDAX instrument. The specimen was sputtered with a thin film of gold. Transmission electron micrographs were taken on a JEM-1011 JEOL electron microscope operating at 100 kV. Holey carbon film on a Cu grid was used for the support.
3. Results and discussion The general procedure used to fabricate the mesoporous silica nanotubes is shown schematically in Fig. 1. The thin films of TEOS and P123 were prepared by spin-coating onto the silicon wafer. Then, the aluminum oxide membrane with 0.2 lm pore diameter was placed
72
W. Zhu et al. / Microporous and Mesoporous Materials 84 (2005) 69–74
on the top of the samples. P123/TEOS mixtures entered into the pores of the alumina template after annealing at 100 °C for different time under vacuum. Then, the samples were quenched at room temperature. Finally, P123 was removed via the calcination and the aluminum oxide membrane was removed through dissolution. The tubular nanostructure is clearly visible in Fig. 2a inset after the P123 is removed by calcinations and alumina is dissolved. This indicates that a one-dimensional mesoporous material has been prepared as tubes instead of rods. An inner hole diameter of between 130 and 190 nm and a wall thickness between 50 and 70 nm can be derived from the FE-SEM images (Fig. 2a–d). Thus, the whole nanotubes have an outer diameter in the range of 180–260 nm, in agreement with the pore sizes of the alumina membrane used. This indicates that the size and structural properties of the nanotubes are controlled by the characteristics of the template. The diameters of the nanotubes obtained are not uniform because the uniformity of the commercial alumina membranes is not good. If the P123 is not removed by calcination and only alumina is dissolved (Fig. 2e), the nanotubes look like similar to that of the mesostructured nanotubes after the P123 is removed by calcination and alumina is dissolved. This confirms that the interior cavity does not originate from calcination. The annealing time had an influence on the formation and the height of the nanotubes within the limits of period of time (Fig. 2a–d). When the annealing time was 2 h, some nanotubes with a height of about 0.5 lm is formed (Fig. 2a). When the annealing time increased to 5 h, 15 h and 20 h, respectively (Fig. 2b–d), the nanotubes with the length of 1–10 lm grew on the Si wafer. Most of the nanotubes were perpendicular to the plane of the substrate. In order to confirm that silica nanotubes have been formed, chemical composition analysis was done via energy-dispersive X-ray spectroscopy (EDX). Since the samples form on the Si wafer, the EDX image (Fig. 3)
Fig. 3. EDX pattern of the silica nanotubes on a silicon wafer after complete removal of the alumina template and P123.
Fig. 4. TEM image of the calcined mesoporous silica after complete removal of the alumina template.
shows a high silicon content. The fact that the composition is composed of oxygen and silica indicates that the majority of P123 and aluminum oxide have been removed and that the nanotubes existing on the Si substrate should be silicon dioxide. Further structure characterization of the silica nanotubes was performed by TEM (Fig. 4). The TEM image showed that the outer diameter of the nanotube was about 185 nm, which corresponded to the pore size of the template used. The image also showed the ordered hexagonal mesochannels arranged over the whole nanotube sheath with a channel size of about 5.5 nm, which is the typical pore size of mesoporous silica. In addition, the mesopore channels are parallel to each other and run parallel to the longitudinal axis of the tubes. It indicates that the mesochannels arranged over the nanotube sheaths are also vertical to the substrate. Based on these experimental facts, the formation mechanism of the long uniform silica nanotubes with ordered mesoporous arrays arranged the nanotube sheaths is as follows: After the precursor solution containing pre-hydrolyzed TEOS and P123 were deposited on a silicon wafer by spin-coating, solvent evaporation resulted in spherical micelles to begin with. As solvent evaporation continued and the P123 concentration increased, spherical micelles changed to rod-shaped micelles. The micellar exteriors were surrounded by a silica source at the same time. Finally, hexagonal silica-P123 mesophases formed on the Si wafer with micelles oriented parallel to the substrate surface, according to the principle of the lowest energy. The XRD pattern of the as-coated silicaP123 thin film (Fig. 5) indicated that the (1 0 0) family of
W. Zhu et al. / Microporous and Mesoporous Materials 84 (2005) 69–74
73
arranged nanotubes perpendicular to the substrates can be obtained.
Intensity
4. Conclusions
2
3
4 2 Theta/°
5
6
Fig. 5. Low angle XRD pattern of the as-coated mesoporous silica thin film.
planes of the hexagonal unit cell was oriented parallel to the surface of the silicon substrate due to the absence of the (1 1 0) [28,29]. When the aluminum oxide membrane was placed on the top of the samples and heated to the temperature above the samplesÕ glass transition temperature, P123/TEOS composites entered into the pores of the membrane due to capillary forces which originate from a reduction in free energy. This occurs by replacing the air/wall interface with P123/TEOS mixtures at the inner pore wall interface of the aluminum oxide membrane and wet the inner walls of the pores on the basis of a preferential wetting mechanism [8,9,21,22]. At the same time, this process will induce the realignment of block copolymer micelles and the volume contraction caused by the silica moieties condensation. The realignment makes the cylindrical (P123) micelles run parallel to the inner wall of the pore of the membrane instead of running parallel to the silicon substrate, according to the principle of the lowest energy. Under heating conditions, the hydrolysis and condensation of TEOS will generate ethanol and water, which will evaporate. Thus, it will result in a contraction of the silica moieties volume. The shrinkage occurs internally due to a strong interaction of the silica moieties with the alumina pore wall during condensation of the silica moieties. P123 micelles will move to the inner walls of the alumina pore since P123 is adsorbed easily at the hydrophilic alumina wall. Therefore, this indicates that the P123/TEOS mixture volume within the alumina pores has undergone a contraction from the pore wall center outward during heating. Wetting, realignment and volume contraction happen at the same time in the inner walls of the pores. This process needs some time to finish. The process eventually results in an axial nanotube growth. Moreover, the nanotubes prepared attach to the substrate steadily. If the template is removed, the ordered
In summary, we have successfully developed a simple and effective method for preparing mesostructured silica nanotubes by using a porous alumina membrane as a template. Annealing time had an influence on the formation and growth of the nanotubes. The nanotubes were perpendicular to the substrate and the formed mesopore channels arranged over the whole nanotube sheath were parallel to each other. The nanotubes prepared could remain their sizes confined within the pores of alumina membranes and still attached to their growth substrate after the template has been removed. It is reasonable to expect that the aligned mesostructured silica nanotubes may find applications in the future. Acknowledgements This work is subsidized by the National Natural Science Foundation of China (50125311, 20334010, 20274050, 50027001, 50373044, 20074037, 50390090, 20490220, 20474065, 50403007, 50373041), the Ministry of Science and Technology of China (2003CB615601), the Chinese Academy of Sciences (Distinguished Talents Program, KJCX2-SW-H07), and the Jilin Distinguished Young Scholars Program (20010101). References [1] S. Iijima, Nature 354 (1991) 56. [2] S.B. Lee, D.T. Mitchell, L. Trofin, T.K. Nevanen, H. So¨derlund, C.R. Martin, Science 296 (2002) 2198. [3] D.T. Mitchell, S.B. Lee, T. Trofin, N.C. Li, T.K. Nevanen, H. Soderlund, C.R. Martin, J. Am. Chem. Soc. 124 (2002) 11864. [4] S. Banerjee, S.S. Wong, J. Am. Chem. Soc. 124 (2002) 8940. [5] Z.L. Wang, R.P. Gao, J.L. Gole, J.D. Stout, Adv. Mater. 12 (2000) 1938. [6] N.I. Kovtyukhova, T.E. Mallouk, T.S. Mayer, Adv. Mater. 15 (2003) 780. [7] R. Fan, Y. Wu, D. Li, M. Yue, A. Majumdar, P. Yang, J. Am. Chem. Soc. 125 (2003) 5254. [8] M. Steinhart, J.H. Wendorff, A. Greiner, R.B. Wehrspohn, K. Nielsch, J. Schilling, Science 296 (2002) 1997. [9] S.I. Moon, T.J. McCarthy, Macromolecules 36 (2003) 4253. [10] F. Krumeich, M. Wark, L. Ren, H.J. Muhr, R.Z. Nesper, Z. Anorg. Allg. Chem. 630 (2004) 1054. [11] F. Kleitz, U. Wilczok, F. Schu¨th, F. Marlow, Phys. Chem. Chem. Phys. 3 (2001) 3486. [12] P. Hoyer, Langmuir 12 (1996) 1411. [13] Z.L. Yang, Z.W. Niu, X.Y. Gao, Z.Z. Yang, Y.F. Lu, Z.B. Hu, C.C. Han, Angew. Chem. Int. Ed. 42 (2003) 4201. [14] C.R. Martin, Chem. Mater. 8 (1996) 1739. [15] J.H. Jung, S. Shinkai, T. Shimizu, Nano Lett. 2 (2002) 17. [16] S.O. Obare, N.R. Jana, C.J. Murphy, Nano Lett. 1 (2001) 601. [17] M. Zhang, E. Ciocan, Y. Bando, K. Wada, L.L. Cheng, P. Pirouz, Appl. Phys. Lett. 80 (2002) 491.
74
W. Zhu et al. / Microporous and Mesoporous Materials 84 (2005) 69–74
[18] Y.D. Yin, Y. Lu, Y.G. Sun, Y.N. Xia, Nano Lett. 2 (2002) 427. [19] J.C. Hulteen, C.R. Martin, J. Mater. Chem. 7 (1997) 1075. [20] H.Q. Xiang, K. Shin, T. Kim, S.I. Moon, T.J. Mccarthy, T.P. Russell, Macromolecules 37 (2004) 5660. [21] J.S. Jie, G.Z. Wang, Q.T. Wang, Y.M. Chen, X.H. Han, X.P. Wang, J.G. Hou, J. Phys. Chem. B 108 (2004) 11976. [22] K.L. Hobbs, P.R. Larson, G.D. Lian, J.C. Keay, M.B. Johnson, Nano Lett. 4 (2004) 167. [23] H. Yang, N. Coombs, I. Sokolov, G.A. Ozin, J. Mater. Chem. 7 (1997) 1285.
[24] A.E. Braun, Semicond. Int. 56 (1999) 25. [25] A. Yamaguchi, F. Uejo, T. Yoda, T. Uchida, Y. Tanamura, T. Yamashita, N. Teramae, Nat. Mater. 3 (2004) 337. [26] B.D. Yao, D. Fleming, M.A. Morris, S.E. Lawrence, Chem. Mater. 16 (2004) 4851. [27] Q.Y. Lu, F. Gao, S. Komarneni, T.E. Mallouk, J. Am. Chem. Soc. 126 (2004) 8650. [28] M. Ogawa, N. Masukawa, Micropor. Mesopor. Mater. 38 (2000) 35. [29] D.Y. Zhao, P.D. Yang, N. Melosh, J.L. Feng, B.F. Chmelka, G.D. Stucky, Adv. Mater. 10 (1998) 1380.
Synthesis of ordered mesostructured silica nanotubal arrays Weiping Zhu, Yanchun Han *, Lijia An
*
State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Graduate School of the Chinese Academy of Sciences, Changchun 130022, People’s Republic of China Received 24 January 2005; received in revised form 13 April 2005; accepted 17 April 2005 Available online 17 June 2005
Abstract Novel mesostructured silica nanotubes perpendicular to the substrate were successfully prepared by the use of porous alumina membranes as a template. The uniform mesopores arranged over the whole nanotube sheath were about 5.5 nm in diameter. The mesoporous channels run parallel to the longitudinal axis of the tubes. The formation mechanisms and the characteristics of the mesostructured silica nanotubes were discussed in detail. Ó 2005 Elsevier Inc. All rights reserved. Keywords: Mesoporous silica; Nanotubes; Alumina membranes; Template synthesis
1. Introduction In 1991, Iijima first reported the discovery of carbon nanotubes [1]. It is a significant step in expanding the application of one-dimensional nanostructured materials. Since then, some other nanotubes have been developed in succession because of their potential applications for electronic devices, catalysis, separation and optical devices, etc. [2–4]. Many synthetic methods have been developed to prepare various nanotubes, such as, the surfactant mediated technique [5], the sol–gel method [6], the multi-step oxidation-etching process [7], the template method [8–18] and so on. The template method is a flexible and affordable route for a large number of nanotubes. The size and structural properties of the materials prepared are determined by the size, shape, and morphology of the template used since the growth of the materials is confined by the template. Anodized pore alumina membranes, which have nearly ordered circular * Corresponding authors. Tel.: +86 431 5262175/5262206; fax: +86 431 5262126 (Y. Han). E-mail addresses: [email protected] (Y. Han), [email protected] (L. An).
1387-1811/$ - see front matter Ó 2005 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2005.04.020
pores perpendicular to the membrane surface, are ideally suited for templates to prepare uniform 1D nano-materials, such as nanotubes or nanorods [19–22]. Mesostructured nanotubes have attracted more and more attentions because of their specific characteristics. For example, they have not only two different kinds of holes such as the nanotubes and the mesopores arranged over the whole tube sheath, but also three different kinds of surfaces such as the outer surface of the nanotube, the inner surface of the nanotube and the mesoporous surface arranged over the nanotube sheaths. They can be used to assemble differently sized nanodevices or act together in an integrated chemical system due to being functionalized differently [11]. Thus, they may find potential applications in chemical sensors [23], optical devices, surfaces for heterogeneous catalysis, especially future generations of microelectronic chips whose densities continue to increase and feature sizes continue to decrease accordingly [24]. There was a certain success in combining the advantages of mesoporous silica materials and anodized alumina membranes to form new hierarchical structures [13,25–27]. For example, 1D hierarchically mesostructured materials with ordered nanofibers or nanotubes
70
W. Zhu et al. / Microporous and Mesoporous Materials 84 (2005) 69–74
in anodic alumina membranes were prepared through the sol–gel route [13]. The mesopore silica oriented parallel to the alumina host pores was prepared by dropping a surfactant/TEOS precursor solution into the pores of the anodic alumina membrane [25]. 1D mesoporous silica nanotubes and nanowires inside porous alumina membranes were fabricated via the sol–gel rotary evaporation route [26]. The mesostructured nanotubes or nanorods prepared so far were inside the anodic alumina membranes, i.e., the alumina membranes were not removed [25,26]. Motivated by the potential application of the ordered arrangement of mesostructured nanotubes, we developed a template synthesis route for the fabrication of mesostructured nanotubal arrays. The formation mechanisms and the characteristics of the mesostructured silica nanotubes were discussed in detail.
ethoxysilane (TEOS, Aldrich) was added to this solution slowly and stirred for 2 h at the same temperature. The molar ratio was TEOS:P123:HCl:ethanol:H2O = 1:0.017:0.1:31.8:10.8. The thin films of mixtures of P123 and TEOS were prepared by spin-coating at two different rates (1000 rpm and 2000 rpm, respectively) on a silicon wafer. Then, the aluminum oxide membranes (Anodisc 25, 0.2 lm pore diameter, 60 lm thickness, Whatman, Inc.) were placed on the top of the films. The samples were heated to 100 °C. After annealing for 2 h, 5 h, 15 h and 20 h under vacuum, respectively, the samples were quenched to room temperature. The samples were subjected to thermal treatment at about 400 °C for 4 h to remove P123 immediately. The aluminum oxide membranes were removed by 30 wt% H3PO4 at about 80 °C for 30 min. Finally, the samples were washed by deionized water and dried at about 100 °C for 3 h.
2. Experimental
2.2. Analysis
2.1. Synthesis
Low angle X-ray diffraction (XRD) pattern were taken on a RINT2000 vertical goniometer equipped with a liquid nitrogen cooled germanium solid-state detector using Cu Ka radiation, k = 0.15406 nm. Field emission scanning electron microscope (FESEM) was carried out on a Philips XL-30-ESEM-FEG instrument operating at 20 kV. The specimen was sputtered with a thin film of gold.
Mesostructured silica nanotubes were synthesized as follows: 1.0 g of Pluronic 123 (P123, EO20PO70EO20, MW = 5800, Aldrich), 2.0 g of deionized water, 15 g of ethanol and 0.1 g of 37 wt% hydrochloric acid were mixed and stirred for 1 h at room temperature until a homogenous solution was formed. Then, 2.13 g of tetra-
Fig. 1. Scheme for the formation of silica nanotubes with a mesostructure using anodized pore alumina membranes as the template on a Si wafer.
W. Zhu et al. / Microporous and Mesoporous Materials 84 (2005) 69–74
71
Fig. 2. FE-SEM images of the silica nanotubes having been annealed at 100 °C for (a) 2 h, (b) 5 h, (c) 15 h, (d) 20 h after calcination and complete removal of the alumina template and (e) 20 h after complete removal of the alumina template and before calcination. The inset in (a) indicates the tubular nanostructure instead of nanorods.
Energy-dispersive X-ray spectroscopy (EDX) observation was performed on an EDAX instrument. The specimen was sputtered with a thin film of gold. Transmission electron micrographs were taken on a JEM-1011 JEOL electron microscope operating at 100 kV. Holey carbon film on a Cu grid was used for the support.
3. Results and discussion The general procedure used to fabricate the mesoporous silica nanotubes is shown schematically in Fig. 1. The thin films of TEOS and P123 were prepared by spin-coating onto the silicon wafer. Then, the aluminum oxide membrane with 0.2 lm pore diameter was placed
72
W. Zhu et al. / Microporous and Mesoporous Materials 84 (2005) 69–74
on the top of the samples. P123/TEOS mixtures entered into the pores of the alumina template after annealing at 100 °C for different time under vacuum. Then, the samples were quenched at room temperature. Finally, P123 was removed via the calcination and the aluminum oxide membrane was removed through dissolution. The tubular nanostructure is clearly visible in Fig. 2a inset after the P123 is removed by calcinations and alumina is dissolved. This indicates that a one-dimensional mesoporous material has been prepared as tubes instead of rods. An inner hole diameter of between 130 and 190 nm and a wall thickness between 50 and 70 nm can be derived from the FE-SEM images (Fig. 2a–d). Thus, the whole nanotubes have an outer diameter in the range of 180–260 nm, in agreement with the pore sizes of the alumina membrane used. This indicates that the size and structural properties of the nanotubes are controlled by the characteristics of the template. The diameters of the nanotubes obtained are not uniform because the uniformity of the commercial alumina membranes is not good. If the P123 is not removed by calcination and only alumina is dissolved (Fig. 2e), the nanotubes look like similar to that of the mesostructured nanotubes after the P123 is removed by calcination and alumina is dissolved. This confirms that the interior cavity does not originate from calcination. The annealing time had an influence on the formation and the height of the nanotubes within the limits of period of time (Fig. 2a–d). When the annealing time was 2 h, some nanotubes with a height of about 0.5 lm is formed (Fig. 2a). When the annealing time increased to 5 h, 15 h and 20 h, respectively (Fig. 2b–d), the nanotubes with the length of 1–10 lm grew on the Si wafer. Most of the nanotubes were perpendicular to the plane of the substrate. In order to confirm that silica nanotubes have been formed, chemical composition analysis was done via energy-dispersive X-ray spectroscopy (EDX). Since the samples form on the Si wafer, the EDX image (Fig. 3)
Fig. 3. EDX pattern of the silica nanotubes on a silicon wafer after complete removal of the alumina template and P123.
Fig. 4. TEM image of the calcined mesoporous silica after complete removal of the alumina template.
shows a high silicon content. The fact that the composition is composed of oxygen and silica indicates that the majority of P123 and aluminum oxide have been removed and that the nanotubes existing on the Si substrate should be silicon dioxide. Further structure characterization of the silica nanotubes was performed by TEM (Fig. 4). The TEM image showed that the outer diameter of the nanotube was about 185 nm, which corresponded to the pore size of the template used. The image also showed the ordered hexagonal mesochannels arranged over the whole nanotube sheath with a channel size of about 5.5 nm, which is the typical pore size of mesoporous silica. In addition, the mesopore channels are parallel to each other and run parallel to the longitudinal axis of the tubes. It indicates that the mesochannels arranged over the nanotube sheaths are also vertical to the substrate. Based on these experimental facts, the formation mechanism of the long uniform silica nanotubes with ordered mesoporous arrays arranged the nanotube sheaths is as follows: After the precursor solution containing pre-hydrolyzed TEOS and P123 were deposited on a silicon wafer by spin-coating, solvent evaporation resulted in spherical micelles to begin with. As solvent evaporation continued and the P123 concentration increased, spherical micelles changed to rod-shaped micelles. The micellar exteriors were surrounded by a silica source at the same time. Finally, hexagonal silica-P123 mesophases formed on the Si wafer with micelles oriented parallel to the substrate surface, according to the principle of the lowest energy. The XRD pattern of the as-coated silicaP123 thin film (Fig. 5) indicated that the (1 0 0) family of
W. Zhu et al. / Microporous and Mesoporous Materials 84 (2005) 69–74
73
arranged nanotubes perpendicular to the substrates can be obtained.
Intensity
4. Conclusions
2
3
4 2 Theta/°
5
6
Fig. 5. Low angle XRD pattern of the as-coated mesoporous silica thin film.
planes of the hexagonal unit cell was oriented parallel to the surface of the silicon substrate due to the absence of the (1 1 0) [28,29]. When the aluminum oxide membrane was placed on the top of the samples and heated to the temperature above the samplesÕ glass transition temperature, P123/TEOS composites entered into the pores of the membrane due to capillary forces which originate from a reduction in free energy. This occurs by replacing the air/wall interface with P123/TEOS mixtures at the inner pore wall interface of the aluminum oxide membrane and wet the inner walls of the pores on the basis of a preferential wetting mechanism [8,9,21,22]. At the same time, this process will induce the realignment of block copolymer micelles and the volume contraction caused by the silica moieties condensation. The realignment makes the cylindrical (P123) micelles run parallel to the inner wall of the pore of the membrane instead of running parallel to the silicon substrate, according to the principle of the lowest energy. Under heating conditions, the hydrolysis and condensation of TEOS will generate ethanol and water, which will evaporate. Thus, it will result in a contraction of the silica moieties volume. The shrinkage occurs internally due to a strong interaction of the silica moieties with the alumina pore wall during condensation of the silica moieties. P123 micelles will move to the inner walls of the alumina pore since P123 is adsorbed easily at the hydrophilic alumina wall. Therefore, this indicates that the P123/TEOS mixture volume within the alumina pores has undergone a contraction from the pore wall center outward during heating. Wetting, realignment and volume contraction happen at the same time in the inner walls of the pores. This process needs some time to finish. The process eventually results in an axial nanotube growth. Moreover, the nanotubes prepared attach to the substrate steadily. If the template is removed, the ordered
In summary, we have successfully developed a simple and effective method for preparing mesostructured silica nanotubes by using a porous alumina membrane as a template. Annealing time had an influence on the formation and growth of the nanotubes. The nanotubes were perpendicular to the substrate and the formed mesopore channels arranged over the whole nanotube sheath were parallel to each other. The nanotubes prepared could remain their sizes confined within the pores of alumina membranes and still attached to their growth substrate after the template has been removed. It is reasonable to expect that the aligned mesostructured silica nanotubes may find applications in the future. Acknowledgements This work is subsidized by the National Natural Science Foundation of China (50125311, 20334010, 20274050, 50027001, 50373044, 20074037, 50390090, 20490220, 20474065, 50403007, 50373041), the Ministry of Science and Technology of China (2003CB615601), the Chinese Academy of Sciences (Distinguished Talents Program, KJCX2-SW-H07), and the Jilin Distinguished Young Scholars Program (20010101). References [1] S. Iijima, Nature 354 (1991) 56. [2] S.B. Lee, D.T. Mitchell, L. Trofin, T.K. Nevanen, H. So¨derlund, C.R. Martin, Science 296 (2002) 2198. [3] D.T. Mitchell, S.B. Lee, T. Trofin, N.C. Li, T.K. Nevanen, H. Soderlund, C.R. Martin, J. Am. Chem. Soc. 124 (2002) 11864. [4] S. Banerjee, S.S. Wong, J. Am. Chem. Soc. 124 (2002) 8940. [5] Z.L. Wang, R.P. Gao, J.L. Gole, J.D. Stout, Adv. Mater. 12 (2000) 1938. [6] N.I. Kovtyukhova, T.E. Mallouk, T.S. Mayer, Adv. Mater. 15 (2003) 780. [7] R. Fan, Y. Wu, D. Li, M. Yue, A. Majumdar, P. Yang, J. Am. Chem. Soc. 125 (2003) 5254. [8] M. Steinhart, J.H. Wendorff, A. Greiner, R.B. Wehrspohn, K. Nielsch, J. Schilling, Science 296 (2002) 1997. [9] S.I. Moon, T.J. McCarthy, Macromolecules 36 (2003) 4253. [10] F. Krumeich, M. Wark, L. Ren, H.J. Muhr, R.Z. Nesper, Z. Anorg. Allg. Chem. 630 (2004) 1054. [11] F. Kleitz, U. Wilczok, F. Schu¨th, F. Marlow, Phys. Chem. Chem. Phys. 3 (2001) 3486. [12] P. Hoyer, Langmuir 12 (1996) 1411. [13] Z.L. Yang, Z.W. Niu, X.Y. Gao, Z.Z. Yang, Y.F. Lu, Z.B. Hu, C.C. Han, Angew. Chem. Int. Ed. 42 (2003) 4201. [14] C.R. Martin, Chem. Mater. 8 (1996) 1739. [15] J.H. Jung, S. Shinkai, T. Shimizu, Nano Lett. 2 (2002) 17. [16] S.O. Obare, N.R. Jana, C.J. Murphy, Nano Lett. 1 (2001) 601. [17] M. Zhang, E. Ciocan, Y. Bando, K. Wada, L.L. Cheng, P. Pirouz, Appl. Phys. Lett. 80 (2002) 491.
74
W. Zhu et al. / Microporous and Mesoporous Materials 84 (2005) 69–74
[18] Y.D. Yin, Y. Lu, Y.G. Sun, Y.N. Xia, Nano Lett. 2 (2002) 427. [19] J.C. Hulteen, C.R. Martin, J. Mater. Chem. 7 (1997) 1075. [20] H.Q. Xiang, K. Shin, T. Kim, S.I. Moon, T.J. Mccarthy, T.P. Russell, Macromolecules 37 (2004) 5660. [21] J.S. Jie, G.Z. Wang, Q.T. Wang, Y.M. Chen, X.H. Han, X.P. Wang, J.G. Hou, J. Phys. Chem. B 108 (2004) 11976. [22] K.L. Hobbs, P.R. Larson, G.D. Lian, J.C. Keay, M.B. Johnson, Nano Lett. 4 (2004) 167. [23] H. Yang, N. Coombs, I. Sokolov, G.A. Ozin, J. Mater. Chem. 7 (1997) 1285.
[24] A.E. Braun, Semicond. Int. 56 (1999) 25. [25] A. Yamaguchi, F. Uejo, T. Yoda, T. Uchida, Y. Tanamura, T. Yamashita, N. Teramae, Nat. Mater. 3 (2004) 337. [26] B.D. Yao, D. Fleming, M.A. Morris, S.E. Lawrence, Chem. Mater. 16 (2004) 4851. [27] Q.Y. Lu, F. Gao, S. Komarneni, T.E. Mallouk, J. Am. Chem. Soc. 126 (2004) 8650. [28] M. Ogawa, N. Masukawa, Micropor. Mesopor. Mater. 38 (2000) 35. [29] D.Y. Zhao, P.D. Yang, N. Melosh, J.L. Feng, B.F. Chmelka, G.D. Stucky, Adv. Mater. 10 (1998) 1380.