Photocatalytic O₂ evolution of rhodium and antimony-codoped rutile-type TiO₂ under visible light irradiation

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.