Enhanced Visible Light Response of TiO₂ Codoped with Cr and Ta Photocatalysts by Electron Doping

The importance of electron concentration in rutile TiO₂ photocatalysts motivated us to apply H₂ reduction treatment to visible-light-responsive TiO₂ photocatalysts sensitized by chromium ion doping. We found that H₂ reduction treatment of rutile TiO₂ particles codoped with Ta and Cr (TiO₂:Ta/Cr) enhanced photocatalytic activity for O₂ evolution by water oxidation under visible irradiation (>2.2 eV). The enhanced visible light activity of H₂-treated TiO₂:Ta/Cr was attributed to the increase of electron concentration, which was confirmed by UV-vis diffuse reflectance and electron spin resonance (ESR) spectroscopy. The H₂-treated TiO₂:Ta/Cr photocatalyst was repeatedly used in aqueous media in spite of the presence of doped electrons. Photoluminescence and transient absorption spectroscopies revealed that electron doping with H₂ treatment decreased the midgap states working as deep traps of photoexcited electrons and increased the accumulation of the photoexcited electrons in the conduction band. The optimized H₂ reduction temperature was decreased with an increase in the amount of higher valence Ta⁵⁺ used as a donor-type dopant. This study shows that the precise control of the bulk electronic structure of rutile TiO₂ by a combination of codoping and H₂ reduction treatment improves the visible-light-driven photocatalytic activity because of the decrease of the trapping sites at deep energy levels and the recombination between deeply trapped electrons and valence band holes.