The effects upon the photophysical and photocatalytic properties of the codoping of rhodium and antimony in rutile-type TiO2 were investigated. TiO2 codoped with rhodium and antimony (TiO 2:Rh/Sb) had an absorption band in the visible light region. Its energy gap was 2.1 eV. The TiO2:Rh/Sb powder showed photocatalytic activity for the oxidation of water to form O2 in the presence of Ag+ ions as an electron acceptor under visible light irradiation (λ > 440 nm), whereas TiO2 doped with only rhodium showed no activity. The photocatalytic activity strongly depended on the amount and ratio of the Sb/Rh dopant. Electron spin resonance, diffuse reflection spectroscopy, and X-ray absorption near-edge structure analyses revealed that the doped rhodium exists in a trivalent oxidation state in the active photocatalyst. The photocatalytic activity was decreased remarkably even by a small amount of Rh4+ ions. Antimony played an important role in suppressing the formation of Rh4+ ions by charge compensation. Thus, it has been clarified that Rh3+ is the species responsible for the photocatalytic activity under visible light irradiation in the TiO 2:Rh/Sb photocatalyst.
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