Photochemical Mechanism of Size-Quantized Vanadium-Doped TiO2 Particles

Abstract

Transition metal ions doped into TiO_2 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 TiO_2. Large polycrystalline (1-4 nm) TiO_2 particles (50 μm) that show size quantization effects due to the individual crystallites are synthesized. Doping (1 at. % ) of the TiO_2 crystals with vanadium reduces the photooxidation rates of 4-chlorophenol (4-CP) compared to the undoped aggregates. Under ambient conditions (25 °C), vanadium is found to be present primarily on TiO_2 surfaces as >VO_2^+ (~90%) (">" denotes a surficial moiety) and secondarily as interstitial V^(4+) (~10% ). Sintering at higher temperatures (200-400 °C) results in the formation of surficial islands of V_2O_5 on TiO_2 while sintering at 600 and 800 °C produces nonstoichiometric solid solutions of V_xTi_1-xO_2. Vanadium appears to reduce the photoreactivity of TiO_2-25 by promoting charge-carrier recombination with electron trapping at > VO_2^+ whereas V (IV) impurities in surficial V_2O_5 islands on TiO_2-200/400 promote charge-carrier recombination by hole trapping. Substitutional V(IV) in the lattice of TiO_2-600/800 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 TiO_2.