Development of Visible-Light Active S cation-doped TiO2 Photocatalyst

Titanium dioxide photocatalysts having anatase and rutile phases are a promising substrate for photodegradation of pollutants in water and air. However, their photocatalytic activities show only under ultraviolet (UV) light. In order to utilize a wide range of incident light such as solar light, development of the photocatalysts whose activities show under visible light is one of the most important strategies. We and other researchers have reported chemically modified titanium dioxide photocatalysts in which S (S4+ or S2-) substitutes for some of the lattice titanium atoms. They show rather strong absorption for visible light and high activities for degradation of organic compounds under visible light irradiation. The oxidation state of the S atoms incorporated into the TiO2 particles is determined to be mainly S4+ from X-ray photoelectron spectra (XPS). In addition, a new method to adsorb Fe3+ ions only onto the surface of S cationdoped TiO2 is proposed. The photocatalytic activities of Fe3+ ions adsorbed on S cation-doped TiO2 photocatalysts for oxidation of organic compounds are markedly improved compared to those of S cation-doped TiO2 without treatment of Fe3+ ions under a wide range of incident light wavelengths, including UV light and visible light. Furthermore, further improvement in their photocatalytic activities of S cation-doped TiO2 photocatalysts with Fe3+ treated with NaBH4 was observed. After the treatment, the crystal structure of Fe3+ compounds on S cation-doped TiO2 was changed. We also discussed the photocatalytic activity of S cation-doped titania nanotube (TNT) site-selectively loaded with Fe3+ compounds under visible light.