Unique Surface Chemical Species on Indium Doped TiO₂ and Their Effect on the Visible Light Photocatalytic Activity

TiO 2 doped by different contents of indium was prepared by the sol−gel method by using titanium(IV) tetrabutoxide and indium chloride as precursors. It was revealed that a unique chemical species, O−In−Cl x ( x = 1 or 2), existed on the surface of the indium doped TiO 2 . The surface state energy level attributed to the surface O−In−Cl x species was located at 0.3 eV below the conduction band of TiO 2 . The transition of electrons from the valence band of TiO 2 to the surface state energy level was responsive to visible light. The photogenerated carriers generated under visible light irradiation can be efficiently separated by the surface state energy level of the O−In−Cl x species and the valence band of TiO 2 to contribute to the photocatalytic reaction. Consequently, the indium doped TiO 2 showed improved photocatalytic activity for photodegradation of 4-chlorophenol compared to pure TiO 2 under visible light irradiation.