Conduction-Band Tailing of Ce-Doped TiO2 Visible-Light-Driven Photocatalyst: The Essential Role of Oxygen Vacancy

The varied electronic structure of Ce-doped TiO 2 was probed with x-ray absorption spectra at O K -, Ce L -, Ti L -edge, and Ti K -edges. We expose the essential role of existed oxygen vacancies in band gap narrowing of Ce-doped TiO 2 and deduce the mechanism in detail by charge transfer and covalent hybridization. Oxygen vacancies would release electrons behind the Ce 4 f state, inducing the mixing valence characteristic of the Ce ions (Ce 3+ /Ce 4+ ) doped in TiO 2 . Oxygen defect state thus hybridizes with Ce 4 f state (Ce 3+ /Ce 4+ ) constructing an impurity band below the conduction band, causing a tailing of the conduction band minimum and narrowing of band gap of TiO 2 . As Ce-doped TiO 2 annealed in an oxidized atmosphere, filling up oxygen vacancies, the absorption edge in the O K -edge x-ray absorption spectrum is shifted to higher energy as compared with that for the un-treated sample, indicating the drastic diminution of band-gap-narrowing effect. This study evidences the existed oxygen vacanc...