Photochemical Mechanism of Size-Quantized Vanadium-Doped TiO2 Particles

Article abstract of DOI:10.1021/j100102a041

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 μm) that show size quantization effects due to the individual crystallites are synthesized.Doping (1 at. percent) of the TiO2 crystals with vanadium reduces the photooxidation rates of 4-chlorophenol (4-CP) compared to the undoped aggregates.Under ambient conditions (25 deg C), vanadium is found to be present primarily on TiO2 surfaces as >VO2⁺ (ca. 90percent) (">" denotes a surficial moiety) and secondarily as interstitial V⁴⁺ (ca. 10percent).Sintering at higher temperatures (200-400 deg C) results in the formation of surficial islands of V2O5 on TiO2 while sintering at 600 and 800 deg C produces nonstochiometric solid solutions of V_xTi_1-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.