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

TiO2 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 TiO2. 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 TiO2. The transition of electrons from the valence band of TiO2 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 TiO2 to contribute to the photocatalytic reaction. Consequently, the indium doped TiO2 showed improved photocatalytic activity for photodegradation of 4-chlorophenol compared to pure TiO2 under visible light irradiation.