PHOTOCHEMICAL MECHANISM OF SIZE-QUANTIZED VANADIUM-DOPED TIO2 PARTICLES

Transition metal ions doped into TiO2 can increase the quantum efficiency of the heterogeneous photooxidation of chlorinated hydrocarbons. In this regard, a single dopant (vanadium) has been selected for a detailed investigation to elucidate the mechanism of the dopant action on the photoreactivity of TiO2. Large polycrystalline (1-4 nm) TiO2 particles (50 mu m) that show size quantization effects due to the individual crystallites are synthesized. Doping (1 at. %) of the TiO2 crystals with vanadium reduces the photooxidation rates of 4-chlorophenol (4-CP) compared to the undoped aggregates. Under ambient conditions (25 degrees C), vanadium is found to be present primarily on TiO2 surfaces as > VO2+ (similar to 90%) (''>'' denotes a surficial moiety) and secondarily as interstitial V4+ (similar to 10%). Sintering at higher temperatures (200-400 degrees C) results in the formation of surficial islands of V2O5 on TiO2 while sintering at 600 and 800 degrees C produces nonstoichiometric solid solutions of VxTi1-xO2 Vanadium appears to reduce the photoreactivity of TiO2-25 by promoting charge-carrier recombination with electron trapping at > VO2+ whereas V(IV) impurities in surficial V2O5 islands on TiO2-200/400 promote charge-carrier recombination by hole trapping. Substitutional V(IV) in the lattice of TiO2-600/800 appears to act primarily as a charge-carrier recombination center that shunts charge carriers away from the solid-solution interface with a net reduction in photoreactivity. The complexities of the physical and electronic effects of vanadium doping are expected to be present in the mechanisms of other transition metal ions doped into TiO2.