Layered titanate thin film as an electrode material
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Yasumichi Matsumoto Department of Applied Chemistry, Faculty of Engineering, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-0862, Japan
Kazunari Domena) Chemical Resources Laboratory, Tokyo Institute of Technology, Nagatsuta 4259, Midori-ku, Yokohama 226-8503, Japan; and Core Research for Evolutional Science and Technology (CREST), Japan Science and Technology Co. (JST), 2-1-13 Higashiueno, Taito-ku, Tokyo 110-0015, Japan (Received 4 July 2003; accepted 20 November 2003)
A thin film composed of an ion-exchangeable layered oxide HxTi(2-x/4)䊐x/4O4 (HTiO) (where the open square, 䊐, is vacancy) is studied as an electrode material. The thin film prepared on an indium tin oxide substrate functions as an electrode for the redox reaction of Ru(bpy)32+/Ru(bpy)33+ without noticeable degradation. Lack of reaction in the presence of Fe(CN)64−/Fe(CN)63− indicates that reaction on this electrode occurs in the interlayers between titanate sheets. A Ru(bpy)32+-intercalated form of the electrode generates photocurrent under visible light irradiation.
I. INTRODUCTION
Thin films of inorganic materials are generally prepared by techniques such as chemical vapor deposition, sputtering, or molecular-beam epitaxy. These methods, however, require high technologies and severe conditions such as high temperature or ultrahigh vacuum. Methods that do not require such extreme conditions have also been investigated. Soft chemical techniques such as hydrothermal-electrochemical processes are attractive for the formation of complex oxide thin films, and polycrystalline thin films of cubic BaTiO3 on Ti substrates have successfully been prepared by this method,1 as have other titanates such as SrTiO3.2 The exfoliation of layered compounds has also been investigated as a means of producing thin films, and layered materials such as layered titanates and niobates have reportedly been exfoliated in aqueous solutions containing amine surfactants.3–6 Ultrathin films based on exfoliated layers originating from ␣-Zr(HPO4)2 and K4Nb6O17 have also been successfully synthesized.5,6 We have developed a method for the preparation of thin films by a soft chemical route.7–11 K4Nb6O17 and CsxTi(2-x/4)䊐x/4O4 (where the open square, 䊐, is vacancy) are ion-exchangeable oxides12,13 and are representative photocatalytic materials with layered structures. The two-dimensional sheets forming the layered a)
Address all correspondence to this author. e-mail: [email protected] J. Mater. Res., Vol. 19, No. 2, Feb 2004
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structure of these compounds can be exfoliated by a soft chemical method, and the thin films reconstructed from the exfoliated sheets maintain the layered structure and ion-exchange properties. In this paper, CsxTi(2-x/4)䊐x/4O4 thin films are prepared on indium tin oxide (ITO) substrates, and the electrochemical properties of the films are investigated with respect to application as an electrode material. II. EXPERIMENTAL A. Synthesis of HxTi(2-x/4)䊐x/4O4 thin films
Figure 1 shows the schem
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