Defects in Anatase Titanium Dioxide

Transition metal oxides have been attracting fundamental and technological interests because their various properties are influenced by many factors such as the number of d-electrons on transition metal ions, crystalline structures, oxygen defects and doped impurities. Elucidation of influence on properties from the factors will lead us to discovery of novel materials. From this standpoint of view, titanium dioxide is one of the prototypes. Titanium dioxide has no d-electron by itself, so that the number of d-electron can be controllable by doping of transition metals. Three crystalline modifications of titanium dioxide, rutile, anatase, and brookite are known well, so that structural dependence on properties can be discussed. It is expected that defects and impurities are also dominant over optical and electrical responses, such as transmittance, photoluminescence and conductivity, which is the same as conventional semiconductors. Titanium dioxide, moreover, is a material which has been used for a long time in a wide range of common and high technique applications because of its moderate price, chemical stability and nontoxicity. Recent topical application is a photocatalyst [1]. Photocatalytic reaction of titanium dioxide is a redox reaction of reactants adsorbed on the surface and it involves photogeneration, migration, and trapping of charge carriers. In these processes, the photogeneration and the migration of carriers are crucial processes to govern inherent activity of the material as a photocatalyst. It can be easily imagined that the factors in nanoscale size have some influence on the behavior of the carriers and the catalytic activity. In fact, anatase has higher photocatalytic activity than rutile because of a difference in Fermi energy [2] and the charge carrier in anatase thin film has a higher mobility than that of rutile [3]. Many approaches to raise photocatalytic activity under visible light have been done; for example transition metal doping [4,5], nitrogen doping [6], and oxygen defect [7]. Their results indicate that optical absorption in the visible region is controllable by doping of impurities. Among three crystalline modifications of titanium dioxide, only rutile crystals have been obtained by crystallization of the substance from its own melt or from a solution in a melt [8]. In contrast to extensive studies for rutile, fundamental optical and electronic properties of anatase which is low temperature modification have not been well understood. To reveal fundamental properties of anatase titanium dioxide, it is indispensable to investigate defect states in it. Recently, anatase titanium dioxide single crystals can be grown by gas phase reaction [9]. As-grown anatase crystals generally exhibit pale blue color in spite of the wide bandgap of about 3.3eV. This suggests the presence of some defects in the as-grown crystals. On this report, it is shown that several colors in anatase can be available by defects controlled in nanoscale, and some electrical properties are controllable by photoirradiation, which implies the possibility of nanoscale doping.