Observations of TiO2 Surfaces Using Totally Reflected X-ray In-plane Diffraction Under UV Irradiation
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Observations of TiO2 Surfaces Using Totally Reflected X-ray In-plane Diffraction Under UV Irradiation T. Horiuchi, H. Ochi, K. Kaisei, K. Ishida, K. Matsushige Department of Electronic Science and Engineering, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501 JAPAN ABSTRACT Surface lattice displacements of titanium dioxide (TiO2: rutile) during ultra-violet (UV) light irradiation have been investigated using a total reflection x-ray diffraction, which provides a high signal to noise ratio (S/N) and superior in-plane surface diffraction. Under the environments in vapors of H2O, CH3OH, C2H5OH and C3H6OH, the photo-catalytic activities of TiO2 (110), (100) and (001) surfaces subject to UV irradiation have been measured. It is found that the diffraction peaks and their full width half maxima (FWHMs) show some peculiarities with respect to the photo-catalytic activities in both surface lattices and adsorbed molecules in vapors. Furthermore, Kelvin force microscopy (KFM) has showed that there exists a very high surface potential, probably due to surface atom displacements induced by UV irradiation. With regard to the origin of the photo-catalytic activities, the induced surface potentials are discussed. INTRODUCTION The discovery [1, 2] of photo-induced hydrophilicity of TiO2 surfaces has led to various applications of TiO2-coatings such as antifogging and self-cleaning [3] which results in clear viewing TiO2-coated mirrors and easily washable TiO2-coated substrates. One of the well-known photo-catalytic activities of TiO2 is based on its strong oxidizing power induced by the photo-induced holes drifting toward TiO2 surfaces. The oxidation of the bridging oxygen leads to the formation of a hydroxyl group, which is responsible for the highly hydrophilic surface. This is the main origin of hydrophilic conversion [4]. However, a few fundamental mechanisms such as the formation of both hydrophilic and hydrophobic domains under UV irradiation [1] have not yet been clearly understood. To clarify this point, we have investigated the relaxation of surface atoms on TiO2 (110), (100) and (001) surfaces during UV irradiation and its relation to the various adsorbed molecules using a specially designed x-ray system. This system provides very high S/N ratio by using a total reflection phenomenon, which makes it possible to detect the relaxations of lattice ions close to the surface. Also, in-plane diffraction enables us to get the scattering vectors parallel to the surfaces in order to find the relative position of adsorbed molecules to substrate. From both the diffraction peak and their widths (FWHMs) obtained simultaneously by using white x rays, we can get very important information about the lattice relaxations induced by the photo-catalytic reaction between the adsorbed molecules and the lattice atoms. In order to illustrate the mechanism of the photo-catalytic reaction, it is important to focus our interest on both hydrophilic and hydrophobic domains appearing clearly on the TiO2 (110) rutile si
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