Factors affecting photocatalytic activity of visible light-responsive titanium dioxide doped with chromium ions

Abstract

Titanium dioxide doped with Cr ions (Cr–TiO₂) was synthesized by a sol–gel method, with only water as the solvent, and dialysis. For Cr–TiO₂ sintered at 200 °C, ca. 90% of 4-chlorophenol (4-CP) was degraded at Cr doping amounts of 0.68–1.3 atom% under visible light irradiation. At doping amounts above 1.7 atom%, the 4-CP degradation ratio increased for Cr–TiO₂ sintered at 300–500 °C. X-ray photoelectron spectroscopy and X-ray absorption near edge structure measurements showed that only Cr(III) was present on the surfaces of Cr–TiO₂ samples sintered at 200 or 400 °C and some of the Cr(III) in the bulk was oxidized to Cr(VI) by sintering at 400 °C. The 4-CP degradation was enhanced with increasing Cr(VI)/Cr(III) ratio in the bulk, suggesting the spatial separation of Cr(III) on the surface under visible light irradiation: electrons are excited from Cr(III) to the conduction band of TiO₂ and then trapped at the doped Cr(VI) whereas the photogenerated Cr(IV) oxidizes 4-CP. Time-resolved diffuse reflectance spectroscopy was used to evaluate the lifetimes of the photogenerated electrons. The highest photocatalytic activity was obtained with ca. 0.8 atom% Cr–TiO₂ sintered at 200 °C. This high activity is ascribed to the presence of oxygen vacancies, which act as electron-trapping sites.