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Chem. Mater. 2006, 18, 996-1000
Synthesis of SiO2 Nanotubes and Their Application as Nanoscale Reactors Hitoshi Ogihara,*,† Sakae Takenaka,† Ichiro Yamanaka,† Eishi Tanabe,‡ Akira Genseki,§ and Kiyoshi Otsuka† Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8552, Japan, Western Hiroshima Prefecture Industrial Institute, Kagamiyama, Higashi-Hiroshima, Hiroshima 739-0046, Japan, and Center for AdVanced Materials Analysis, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8552, Japan ReceiVed August 3, 2005. ReVised Manuscript ReceiVed NoVember 15, 2005
SiO2 nanotubes could be synthesized by hydrolysis of tetraethyl orthosilicate (TEOS) on the surface of carbon nanofibers (CNF), followed by removal of CNF. In the synthesis of SiO2 nanotubes, CNF acted as templates. The wall thickness and the internal diameter of SiO2 nanotubes were changed by hydrolysis time of TEOS and the diameter of CNF templates, respectively. The catalysts for the hydrolysis of TEOS were functional groups on the surface of CNF. The methane decomposition was carried out over SiO2 nanotubes containing Ni metal particles, so that carbon nanotubes with uniform diameter (ca. 8 nm) were formed at the inside of SiO2 nanotubes. This result strongly indicated that SiO2 nanotubes worked as nanoscale reactors.
Introduction Since the discovery of carbon nanotubes,1 one-dimensional nanostructured materials such as nanotubes, nanowires, nanobelts, and nanorods have been drawing attention due to their specific mechanical, electrical, and chemical properties.2-4 Many nanostructured materials have been synthesized by using templates such as carbon nanotubes.5-8 In particular, the template synthesis of SiO2 nanotubes has been investigated vigorously. They were generally synthesized by hydrolysis of TEOS on the surface of various templates, followed by removal of templates. Anodic aluminum oxide membranes,9 organic gelators,10 cholesterol nanotubes,11 * To whom correspondence should be addressed. Present address: Hitoshi Ogihara, Catalysis Research Center, Hokkaido University, N21-W10 Kita-ku, Sapporo 001-0021, Japan. Tel.: +81 11 706 9165. Fax: +81 11 706 9163. E-mail:
[email protected]. † Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology. ‡ Western Hiroshima Prefecture Industrial Institute. § Center for Advanced Materials Analysis, Tokyo Institute of Technology.
(1) Iijima, S. Nature 1991, 354, 56. (2) Patzke, G. R.; Krumeich, F.; Nesper, R. Angew. Chem., Int. Ed. 2002, 41, 2446. (3) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353. (4) Caruso, R. A.; Antonietti, M. Chem. Mater. 2001, 13, 3272. (5) Ajayan, P. M.; Stephan, O.; Redlich, P.; Colliex, C. Nature 1995, 375, 564. (6) Keller, N.; Pham-Huu, C.; Estourne`s, C.; Grene`che, J.-M.; Ehret, G.; Ledoux, M. J. Carbon 2004, 42, 1395. (7) Artyukhin, A. B.; Bakajin, O.; Stroeve, P.; Noy, A. Langmuir 2004, 20, 1442. (8) Zhu, Y. Q.; Hsu, W. K.; Kroto, H. W.; Walton, D. R. M. J. Phys. Chem. B 2002, 106, 7623. (9) Mitchell, D. T.; Lee, S. B.; Trofin, L.; Li, N.; Nevanen, T. K.; So¨derlund, H.; Martin, C. R. J. Am. Chem. Soc. 2002, 124, 11864. (10) Ono, Y.; Nakashima, K.; Sano, M.; Kanekiyo, Y.; Inoue, K.; Hojo, J.; Shinkai, S. Chem. Commun. 1998, 1477. (11) Jung, J. H.; Lee, S.-H.; Yoo, J. S.; Yoshida, K.; Shimizu, T.; Shinkai, S. Chem. Eur. J. 2003, 9, 5307.
carbon nanotubes,12 crystalline fibers,13-15 and silicon nanowires16 were used as templates for SiO2 nanotube syntheses. Carbon nanofibers (CNF) are well-known as nanoscale carbonaceous materials.17 CNF are synthesized by the decomposition of hydrocarbons over metal catalysts such as Fe, Co, and Ni. Large amounts of CNF are easily formed at mild reaction conditions. It was reported that the morphology of CNF strongly depended on the kind of metal catalysts, carbon sources, and reaction conditions.18-20 For example, straight, bent, helical, or branched CNF could be synthesized by selecting proper reaction conditions or catalysts. In addition, the diameters of CNF can be controlled by the particle size of metal catalysts. If these CNF are used as templates for the synthesis of metal-oxide nanotubes, metaloxide nanotubes with various shapes would be formed. CNF can be easily removed by oxidation or hydrogenation because CNF are reactive with O2, CO2, or H2.21,22 These properties of CNF show that CNF are promising templates for the synthesis of metal-oxide nanotubes. However, there are few studies in the utilization of CNF as templates. Ermakova et (12) Satishkumar, B. C.; Govindaraj, A.; Vogl, E. M.; Basumallick, L.; Rao, C. N. R. J. Mater. Res. 1997, 12, 604. (13) Naito, S.; Ue, M.; Sakai, S.; Miyao, T. Chem. Commun. 2005, 1563. (14) Hippe, C.; Wark, M.; Lork, E.; Schulz-Ekloff, G. Microporous Mesoporous Mater. 1999, 31, 235. (15) Zygmunt, J.; Krumeich, F.; Nesper, R. AdV. Mater. 2003, 15, 1538. (16) Fan, R.; Wu, Y.; Li, D.; Yue, M.; Majumdar, A.; Yang; P. J. Am. Chem. Soc. 2003, 125, 5254. (17) Jong de, K. P.; Geus, J. W. Catal. ReV.-Sci. Eng. 2000, 42, 481. (18) Takenaka, S.; Shigeta, Y.; Tanabe, E.; Otsuka, K. J. Phys. Chem. B 2004, 108, 7656. (19) Yang, S.; Chen, X.; Kusunoki, M.; Yamamoto, K.; Iwanaga, H.; Motojima, S. Carbon 2005, 43, 916. (20) Otsuka, K.; Kobayashi, S.; Takenaka, S. Appl. Catal. A 2001, 210, 371. (21) Takenaka, S.; Kato, E.; Tomikubo, Y.; Otsuka, K. J. Catal. 2003, 219, 176. (22) Otsuka, K.; Takenaka, S.; Ohtsuki, H. Appl. Catal. A 2004, 273, 113.
10.1021/cm051727v CCC: $33.50 © 2006 American Chemical Society Published on Web 01/19/2006
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al. synthesized SiO2 by the sol-gel method on CNF.23 Tubular SiO2 with a high surface area (up to 1255 m2/g) was obtained, but a clear nanotube structure was not formed because its wall thickness was very thin. Therefore, we examined synthesis of SiO2 nanotubes by using CNF as templates. Recently, the formation of carbon nanotubes on various SiO2 materials such as mesoporous silica,24-26 silica glass fiber,27 porous glass,28 polyaniline-silica substrates,29 and macroporous silica30 has been reported. In the present study, the formation of carbon nanotubes at the inner walls of SiO2 nanotubes, that is, application of SiO2 nanotubes as nanoscale reactors was also examined.
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Figure 1. TEM image of CNF formed by methane decomposition over Ni/SiO2 catalysts at 823 K. Inset shows a Ni metal particle at the tip of CNF.
Experimental Section A. Preparation of CNF Templates. CNF were synthesized by decomposition of methane over Ni(40 wt %)/SiO2 catalysts at 823 K or over Pd(20 wt %)/SiO2 catalysts at 973 K.31 Decomposition of methane was carried out with a conventional gas-flow system at atmospheric pressure. Prior to using CNF as templates, they were treated with an aqueous HNO3 solution at 353 K for 4 h. CNF thus treated were washed with ion-exchanged water at room temperature and were dried at 373 K for 10 h. B. Synthesis of SiO2 Nanotubes. CNF (0.2 g) was impregnated with 5 mL of tetraethyl orthosilicate (TEOS). To hydrolyze TEOS moderately, CNF and TEOS were stirred in the presence of water vapor with saturated partial pressures at 313 K. After filtration, the filtrates were dried at 353 K in air for 6 h and were treated at 773 K under a He stream for 1 h. Finally, SiO2 nanotubes were obtained after the removal of CNF templates by calcination in air at 923 K for 5 h. C. Application of SiO2 Nanotubes to Catalytic Reactions. Methane decomposition over SiO2 nanotubes was carried out with a conventional gas-flow system at atmospheric pressure. Prior to the reaction, SiO2 nanotube (0.010 g) was treated with hydrogen (50.7 kPa) at 823 K for 1 h. Decomposition of methane was initiated by contacting a mixture gas of CH4 (90 mL‚min-1) and H2 (10 mL‚min-1) with SiO2 nanotubes. Ni catalysts supported on the outer walls of SiO2 nanotubes were prepared by the following procedure. SiO2 nanotubes were immersed in an aqueous Ni(NO3)2 solution (5 wt % as Ni) for 12 h and filtrated. The filtrate was washed with water in order to remove Ni(NO3)2 that did not adsorb on SiO2 nanotubes. Dried samples were calcined in air at 873 K for 5 h and reduced with H2 at 773 K for 1 h. D. Characterization of CNF and SiO2 Nanotubes. SEM images were measured by using an Hitachi FE-SEM S-800 operated at 15 (23) Ermakova, M. A.; Ermakov, D. Yu.; Kuvshinov, G. G. Kinet. Catal. 2002, 43, 427. (24) Amama, P. B.; Lim, S.; Ciuparu, D.; Yang, Y.; Pfefferle, L.; Haller, G. L. J. Phys. Chem. B 2005, 109, 2645. (25) Huang, L.; Wind, S. J.; O’Brien, S. P. Nano Lett. 2003, 3, 299. (26) Yoon, S. B.; Kim, J. Y.; Kooli, F.; Lee, C. W.; Yu, J.-S. Chem. Commun. 2003, 1740. (27) Ismagilov, Z. R.; Shikina, N. V.; Kruchinin, V. N.; Rudina, N. A.; Ushakov, V. A.; Vasenin, N. T.; Veringa, H. J. Catal. Today 2005, 102, 85. (28) Aoki, Y.; Suzuki, S.; Okubo, S.; Kataura, H.; Nagasawa, H.; Achiba, Y. Chem. Lett. 2005, 34, 562. (29) Wang, Z.; Huang, Y.; Bai, X.; Song, W.; Wang, C. Diamond Relat. Mater. 2005, 14, 1411. (30) Park, S. M.; Li, H.; Sheldon, B.; Du, H. J. Mater. Res. 2005, 20, 2498. (31) Takenaka, S.; Kobayashi, S.; Ogihara, H.; Otsuka, K. J. Catal. 2003, 217, 79.
Figure 2. SEM images of (a) CNF formed by methane decomposition over Ni/SiO2 catalysts at 823 K and (b) SiO2 formed by using CNF as templates.
kV. TEM images were measured by using JEOL JEM-2010F and JEM-3000F operated at 200 kV. The X-ray fluorescence analysis was performed by Philips PW2404.
Results and Discussion A. Synthesis of CNF Templates. A TEM image of CNF formed at the initial stage of methane decomposition over Ni/SiO2 catalysts at 823 K is shown in Figure 1. CNF shown in Figure 1 were not treated with an aqueous HNO3 solution. The TEM image indicated that CNF grew in many directions and their shapes were bent. The diameters of the CNF ranged from several tens to one hundred nanometers. The darker spots in the TEM image indicated the positions of Ni metal catalysts. The inset in Figure 1 showed that a Ni metal particle was present at the tip of the CNF and its particle size was the same as the diameter of the CNF. The Ni metal particles decomposed methane to grow CNF. B. TEM and SEM Measurements of SiO2 Nanotubes. SEM images of (a) CNF formed by methane decomposition over Ni/SiO2 catalysts at 823 K and (b) SiO2 formed by using CNF as templates are shown in Figure 2. Decomposition of methane was carried out until the deactivation of catalysts. CNF shown in Figure 2a were not treated with HNO3 (aq). When CNF were used as templates for the synthesis of SiO2, they were treated with HNO3 (aq). Large amounts of CNF were observed in a SEM image (a). The shapes of CNF were bent and their diameters ranged from 40 to 100 nm. Fibrous SiO2 nanotubes could be observed in Figure 2b. The shapes
998 Chem. Mater., Vol. 18, No. 4, 2006
Figure 3. TEM image of SiO2 nanotubes formed by using CNF as templates. Inset shows SAED pattern of SiO2 nanotubes.
Figure 4. Effect of hydrolysis time of TEOS on the amount of formed SiO2 nanotubes.
of SiO2 fibers were similar to those of CNF, implying that CNF worked as templates in SiO2 synthesis. To examine the structure of SiO2 fibers in detail, TEM images of the SiO2 fibers were measured. The TEM image show that most of the formed SiO2 were nanotube structures, as described in Figure 3. Their shapes and diameters were similar to those of CNF. These results strongly suggested that CNF worked as templates for the synthesis of SiO2 nanotubes. The inset in Figure 3 shows a selected area electron diffraction (SAED) pattern of SiO2 nanotubes. In the SAED pattern, a halo pattern originating from the amorphous SiO2 was observed. In addition, the X-ray diffraction pattern of SiO2 nanotubes also showed the halo pattern of SiO2. These results suggest that SiO2 walls of nanotubes have amorphous structure. We estimated the weight of SiO2 nanotubes by subtraction of the weight of CNF templates used from that of CNF coated with SiO2 after heat treatment at 773 K. The weight of SiO2 nanotubes formed was influenced by the hydrolysis time of TEOS. The weight of formed SiO2 nanotubes per the weight of CNF templates increased linearly with the hydrolysis time of TEOS until 20 h, as described in Figure 4. The increase in the weight of SiO2 nanotubes became moderate after 20 h, indicating that the hydrolysis rate of TEOS became slow after 20 h. TEM images showed that SiO2 nanotubes with different wall thickness could be formed by changing the hydrolysis time: 20 nm thick nanotubes
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are created by 12 h TEOS hydrolysis and 30 nm nanotubes by 36 h hydrolysis, as described in Figures 5a and 5b, respectively. These TEM images suggested that the wall thickness of SiO2 nanotubes could be controlled by the hydrolysis time of TEOS. (a) The SEM image of CNF formed by methane decomposition over Pd(20 wt %)/SiO2 at 973 K and (b) the TEM image of SiO2 nanotubes synthesized by using these CNF as templates are shown in Figure 6. The diameters of CNF ranged from several hundred nanometers to one micrometer. In addition, carbons without fibrous structure were also formed through methane decomposition over Pd catalysts. The TEM image (b) showed that the diameter of formed SiO2 nanotubes was ca. 400 nm, and their wall thickness was ca. 40 nm. A large amount of SiO2 which did not form nanotube structure was also observed. It is possible that carbons without fibrous structure were related to the formation of SiO2 which did not form nanotube structure. As described in Figure 2, SiO2 nanotubes of 40-100 nm in diameter could be synthesized by using CNF that Ni/SiO2 catalysts grew. Therefore, the results in Figures 2 and 6 indicate that the diameters of SiO2 nanotubes changed with the diameters of CNF templates. C. Role of Functional Groups in the Formation of SiO2 Nanotubes. In the present study, SiO2 nanotubes would be formed according to eqs 1-3, Si(OEt)4 + 4H2O f Si(OH)4 + 4EtOH
(1)
Si(OH)4 f SiO2 + 2H2O
(2)
C + O2 f CO2
(3)
CNF are covered with silicon-hydroxide gels which are formed through the hydrolysis of TEOS (eq 1). Siliconhydroxide gels are transformed into SiO2 by heat treatment under a He stream at 773 K (eq 2), which results in coverage of CNF with SiO2. SiO2 nanotubes with similar shapes to that of CNF are formed after removal of CNF by calcination in air (eq 3). Generally, acid or basic catalysts such as HCl (aq) or NH3 (aq) are added in order to promote the hydrolysis of TEOS since the rate of hydrolysis of TEOS at room temperature is very slow in the absence of the catalysts.32-35 However, in the present study, SiO2 nanotubes were formed in the absence of acidic or basic aqueous solution. It is wellknown that functional groups (-COOH, -OH, dO) are introduced on the carbon surfaces by the treatment of carbons with an aqueous HNO3 solution at ca. 373 K.36 Therefore, surface functional groups should be introduced on the surface of CNF because CNF were treated with HNO3 (aq). These functional groups show acidic properties. If functional groups, especially -COOH, on CNF surfaces acted as acid (32) Bommel, K. J. C.; Shinkai, S. Langmuir 2002, 18, 4344. (33) Bommel, K. J. C.; Friggeri, A.; Shinkai, S. Angew. Chem., Int. Ed. 2003, 42, 980. (34) Brinker, C. J.; Keefer, K. D.; Schaefer, D. W.; Ashley, C. S. NonCryst. Solids 1982, 48, 47. (35) Brinker, C. J.; Keefer, K. D.; Schaefer, D. W.; Assink, R. A.; Kay, B. D.; Ashley, C. S. Non-Cryst. Solids 1984, 63, 45. (36) Pittman, C. U., Jr.; He, G.-R.; Wu, B.; Gardner, S. D. Carbon 1997, 35, 317.
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Figure 5. TEM images of SiO2 nanotubes formed by hydrolysis of TEOS for (a) 12 h and (b) 36 h. Insets show magnified images of each SiO2 nanotubes.
Figure 6. (a) SEM image of CNF formed by methane decomposition over Pd/SiO2 catalyst at 973 K. (b) TEM image of SiO2 nanotubes prepared by using CNF shown in (a).
catalysts for the hydrolysis of TEOS, SiO2 nanotubes could be formed without the addition of acidic or basic aqueous solution. When CNFs not treated with HNO3 (aq) were utilized as templates for synthesis of SiO2, the amount of SiO2 was very small and their shapes were granular. These results indicate that functional groups on CNF catalyze hydrolysis of TEOS to form SiO2 nanotubes. D. Application of SiO2 Nanotubes to Catalytic Reactions. The X-ray fluorescence analysis for SiO2 nanotubes showed that SiO2 nanotubes contained a small amount of Ni (0.4 wt %). In addition, TEM measurement and energydispersive X-ray (EDX) spectroscopy of SiO2 nanotubes after reduction with hydrogen indicated the presence of small Ni metal particles (
