Surface Electrical Measurements of Photo-catalysis on Rutile TiO 2 (11O)
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Surface Electrical Measurements of Photo-catalysis on Rutile TiO2(11O) Kiyohiro Kaisei, Kenji Ishida, Toshihisa Horiuchi, and Kazumi Matsushige Kyoto University, Dept of Electronic Science and Engineering, Kyoto, Japan,
ABSTRACT We have studied the electronic properties of single crystal rutile TiO2(110) with surface electrical techniques; impedance spectroscopy and high sensitivity DC measurements. In the impedance spectroscopy we observed a spectrum involved in the dissociation of pure water on the TiO2 surface under UV irradiation. In the high sensitivity DC measurements we observed oscillations of the surface photo-current with three different long periodicities, which might have originated from the oxidation and reduction processes of water on TiO2 via photo-catalytic reactions induced by UV irradiation.
INTRODUCTION In recent years titanium dioxides (TiO2; anatase and rutile) have attracted considerable attention. For instance, self-cleaning building materials, antifogging mirrors with amphiphilic surfaces in photo-catalytic activities, and Grätzel solar cells as molecular dye-sensitized electrochemical devices have all been investigated and developed for industrial use. Although the photo-catalysis of TiO2 has been studied from various standpoints, the fundamental mechanism has not yet been clarified. In order to study the mechanism, it is more meaningful to focus on both hydrophilic and hydrophobic domains appearing on the surface of a TiO2 (110) rutile single crystal, after and during UV irradiation as mentioned elsewhere [1]. It was also reported that rectangular hydrophilic domains aligned along the direction of the TiO2 surface showed amphiphilic characteristics when investigated with friction force microscopy. Moreover, small changes in the lattice spacing of TiO2 (110) which accompanied expansion/shrinkage of the crystal lattice due to UV irradiation were observed by total-reflection x-ray diffraction (TXRD). Additionally, surface positive potentials were detected on the surface of TiO2 (110) during UV irradiation using Kelvin probe force microscopy (KFM). In this paper we report studies on a single crystal of rutile TiO2 to elucidate the electrochemical reactions, which are closely involved in active species on the surface [2,3], using surface electrical measurements, that is impedance spectroscopy and high sensitivity DC measurements with interdigitated electrodes.
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EXPERIMENTAL DETAILS Scheme of Surface Electrical Measurements The scheme of the surface electrical measurements is shown in figure 1. On the surface of TiO2 it is known that hydroxyl groups [4] and oxygen vacancies involved in photo-catalytic reactions exist; super-hydrophilicity or water dissociation [5]. In the impedance spectroscopy it is expected that the behavior of the adsorbents such as adhesive water or a hydroxyl group adsorbed to a site, V2+ in the surface, can be detected. If in the impedance spectroscopy of the surface of TiO2 with water some peaks except for water are detected, it could suggest the mecha
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