PHOTOCHEMICAL MECHANISM OF SIZE-QUANTIZED VANADIUM-DOPED TIO2 PARTICLES

Transition metal ions doped into TiO2 can increase the quantum efficie ncy of the heterogeneous photooxidation of chlorinated hydrocarbons. I n this regard, a single dopant (vanadium) has been selected for a deta iled investigation to elucidate the mechanism of the dopant action on the photoreactivity of TiO2. Large polycrystalline (1-4 nm) TiO2 parti cles (50 mu m) that show size quantization effects due to the individu al crystallites are synthesized. Doping (1 at. %) of the TiO2 crystals with vanadium reduces the photooxidation rates of 4-chlorophenol (4-C P) compared to the undoped aggregates. Under ambient conditions (25 de grees C), vanadium is found to be present primarily on TiO2 surfaces a s > VO2+ (similar to 90%) (''>'' denotes a surficial moiety) and secon darily as interstitial V4+ (similar to 10%). Sintering at higher tempe ratures (200-400 degrees C) results in the formation of surficial isla nds of V2O5 on TiO2 while sintering at 600 and 800 degrees C produces nonstoichiometric solid solutions of VxTi1-xO2 Vanadium appears to red uce the photoreactivity of TiO2-25 by promoting charge-carrier recombi nation with electron trapping at > VO2+ whereas V(IV) impurities in su rficial V2O5 islands on TiO2-200/400 promote charge-carrier recombinat ion 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 wit h 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.