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Formation of Titanium Oxide Nanotube Tomoko Kasuga,*,† Masayoshi Hiramatsu,† Akihiko Hoson,† Toru Sekino,‡ and Koichi Niihara‡ Electrotechnology Applications R & D Center, Chubu Electric Power Co. Inc., Oodaka, Midori-ku, Nagoya 459, Japan, and The Institute of Scientific and Industrial Research, Osaka University, 8-1 Mihogaoka, Ibaraki, Osaka 567, Japan
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Received December 16, 1997. In Final Form: April 8, 1998 Nanotubes composed of various materials such as carbon, boron nitride, and oxides have been studied recently. In this report, the discovery of a new route for the synthesis of a nanotube made of titanium oxide is presented. Needle-shaped TiO2 crystals (anatase phase) with a diameter of ≈8 nm and a length of ≈100 nm were obtained when sol-gel-derived fine TiO2-based powders were treated chemically (e.g., for 20 h at 110 °C) with a 5-10 M NaOH aqueous solution. It was found by observation using a transmission electron microscope that the needle-shaped products have a tube structure. The TiO2 nanotubes have a large specific surface area of ≈400 m2‚g-1. TiO2 nanotubes obtained in the present work are anticipated to have great potential for use in the preparation of catalysts, adsorbants, and deodorants with high activities, because their specific surface area is greatly increased. If metallic-, inorganic-, or organic-based materials can be inserted into the TiO2 nanotubes, novel characteristics such as electric, electromagnetic, or chemical properties may be induced in the TiO2 materials.
Introduction Carbon nanotubes, which were found by Iijima et al., have great potential as materials with novel properties that are not found in conventional graphite or carbon fullerene.1-3 Much research has been conducted on developing nanotubes with novel properties by controlling the nanostructure. Recently, pure boron nitride (BN) nanotubes with the inner diameter 1-3 nm and the length e200 nm were produced in a carbon-free plasma discharge between a BN-packed tungsten rod and a cooled electrode.4 Nanotubes made of oxides such as SiO2, Al2O3, V2O5, and MoO3, with diameters of about several tens of nanometers have been prepared by templating carbon nanotubes and subsequently removing the template.5 In the templating method, carbon nanotubes are indispensable for producing oxide nanotubes. Needle-shaped TiO2 having the diameter 0.1 ( 0.05 µm and the aspect ratio 40-100 has also been reported.6 They are expected to be applied as reinforcing materials or fillers for plastics and ceramics. Recently, TiO2 nanotubes with diameters of 70-100 nm were reported to be produced using a replication method.7 Since, in this method, the dimension of the tubes is controlled by the pore size of the mold prepared using porous materials such as alumina, it is not easy to obtain nanotubes with smaller diameters. We prepared TiO2 nanotubes with the diameter ≈8 nm using a simple method different from templating or replication. TiO2 shows a shielding effect against ultraviolet rays.8 TiO2 has been used in various applications such as in paint, paper, cloth fibers and cosmetics. Since recently, † ‡
Chubu Electric Power Co. Inc. Osaka University.
(1) Iijima, S. Nature 1991, 354, 56. (2) Iijima, S; Ichihashi, T. Nature 1993, 363, 603. (3) Hamada, N., Sawada, S; Oshiyama, A. Phys. Rev. Lett. 1992, 68, 1579. (4) Chopra, N. G.; Luyken, R. J.; Cherry, K.; Crespi, V. H.; Cohen, M. L.; Louie, S. G; Zettl, A. Science 1995, 269, 966. (5) Satishkumar, B. C.; Govindaraj, A.; Voli, E. M.; Basumallic, L.; Rao, C. N. R. J. Mater. Res. 1997, 12, 604. (6) Nakamura, H.; Matsui, Y.; J. Am. Chem. Soc. 1995, 117, 2651. (7) Hoyer, P. Langmuir 1996, 12, 141. (8) For example: Hird, M. J. J. Coatings Technol.1976, 48, 75.
much attention is being paid to TiO2 crystals (anatase phase) having photocatalytic activities,9 because they have great potential for such applications as environmental purification, decomposition of carbonic acid gas, and generation of hydrogen gas.10,11 Much effort has been directed at obtaining high-performance TiO2 powders with a large surface area. In general, TiO2 powders are prepared by vapor deposition12 or by precipitation in solution.13 For preparing very fine TiO2 powders, the solgel method14 using a metal alkoxide is well-known. These fabrication methods yield TiO2 particles smaller than several hundreds of nanometers in size. It has been reported15,16 that TiO2-based powders doped with a small amount of SiO2 prepared by the sol-gel method with subsequent heat treatment have large specific surface areas. After the powders were treated with a strong aqueous solution of caustic soda, they showed very high photocatalytic activity in the decomposition of acetic acid.15,16 In the present work, TiO2 nanotubes with the diameter ≈8 nm were found to be obtained by controlling the chemical treatment conditions. TiO2-SiO2 powders were prepared using a sol-gel method by the following procedure, as described in our earlier work.15,16 Commercial reagents such as Ti(Oi-C3H7)4 (>99%, Kishida Regents Chemicals) and Si(OEt)4 (>99%, Kishida Regents Chemicals) were mixed with EtOH (>99.5%, Kanto Chemicals), HCl (35-37%, (9) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (10) Yamashita, H.; Ichihashi, Y.; Anpo, M. Hyoumen-Kagaku 1995, 16, 194. (11) Perlizzetti, E; Minero, C. Electrochim. Acta 1993, 38, 47. (12) For example: Manocha, R. Trans. Ind. Inst. Met. 1953, 7, 95. Patel, C. C; Jere, G. V. Trans. Am. Inst. Min., Metall. Pet. Eng. 1960, 218, 219. (13) For example: Prasad, S; Tripathi, J. B. P. J. Ind. Appl. Chem. 1958, 21, 162. Feng, A. A; Hanna, T. R. TAPPI Proc. Plas. Laminates Symp. 1994, 71. (14) For example: Prassas, M; Hench, L. L. Ultrastructure Processing of Ceramics, Glasses and Composites; John Wiley and Sons: New York, 1984; p 101. Livage, J. Mater. Res. Soc. Symp. 1986, 73, 717. (15) Kasuga, T.; Hiramatsu, M.; Hirano, M.; Hoson, A.; Oyamada, K. J. Mater. Res. 1997, 12, 607. (16) Kasuga, T.; Hiramatsu, M.; Hoson, A.; Oyamada, K. Proceedings of the 2nd International Meeting of the Pacific Rim Ceramic Society, Cairns, 1996, in press.
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Letters
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Figure 1. SEM photographs of (a) 80TiO2‚20SiO2 (in mol %) powders prepared by heating at 600 °C and (b) the powders obtained by treating the powders in part a chemically with 10 M NaOH aqueous solution for 20 h at 110 °C. Numerous needle-shaped products of ≈100 nm in length are seen.
Figure 3. Selected-area electron diffraction (SAED) pattern from Figure 2.
Figure 2. TEM photograph of 80TiO2‚20SiO2 (in mol %) powders treated with 10 M NaOH aqueous solution for 20 h at 110 °C.
Kishida Regents Chemicals) and H2O so as to obtain nominal compositions of 80TiO2‚20SiO2 in mole percent; 23.5 mL of Ti(Oi-C3H7)4 and 4.5 mL of Si(OEt)4 were added to 23 mL of ethanol, respectively, and subsequently, the resulting solution was stirred for 15 min at room temperature. A mixed solution of 23 mL of ethanol and 18 g of 4.4 M HCl aqueous solution was added slowly to the mixed solution of Ti(Oi-C3H7)4, Si(OEt)4, and EtOH. The mixture was held for hydrolysis and gelled in an incubator at 40 °C at a humidity of 70% for 5 days. The obtained
gel was heated to 600 °C and held for 2 h, resulting in the precipitation of fine TiO2 crystals (anatase phase). No SiO2-containing crystalline phase was precipitated. Amorphous SiO2-related phases are also present. The crystalline size was estimated to be 6 nm using XRD patterns and Sherrer’s equation. The crystallized product was pulverized into powders of
