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.

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TiO2 nanoparticles doped with different concentrations of Zr4+ ions were prepared by the sol–gel method and annealed at different temperatures. X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and high resolution transmission electron microscopy (HRTEM) techniques were used to investigate the existing states and doping mechanism of dopants as well as the phase transition of the Zr4+-doped TiO2 samples. It was revealed that the doping behavior of introduced Zr4+ ions was closely related to the doping concentration. The Zr4+ ions would replace the lattice Ti4+ ions directly in substitutional mode at a certain annealing temperature. Moreover, if the concentration of doped Zr4+ ions is high enough, excess Zr4+ ions would form ZrTiO4 on the surface of TiO2. In addition, the phase transition temperature from anatase to rutile increases significantly after doping Zr4+ ions, due to their larger electropositivity and radius than those of Ti4+ ions. Our results may afford a better understanding on the doping mechanism and aid in the preparation of Zr-doped TiO2 with high photoelectric performance.