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

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The varied electronic structure of Ce-doped TiO2 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 TiO2 and deduce the mechanism in detail by charge transfer and covalent hybridization. Oxygen vacancies would release electrons behind the Ce 4f state, inducing the mixing valence characteristic of the Ce ions (Ce3+/Ce4+) doped in TiO2. Oxygen defect state thus hybridizes with Ce 4f state (Ce3+/Ce4+) constructing an impurity band below the conduction band, causing a tailing of the conduction band minimum and narrowing of band gap of TiO2. As Ce-doped TiO2 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 vacancy is critical to mediate the impurity band formation and band gap narrowing of Ce-doped TiO2. The phenomenon should also be emphasized in TiO2 doped with other elements.
2011-11-15