Surface doping is more beneficial than bulk doping to the photocatalytic activity of vanadium-doped TiO2

In this study, we examined the effects of surface and bulk doping on the photocatalytic activity of vanadium (V)-doped TiO2 on the basis of charge trapping, separation, and interfacial transfer. Moreover, we characterized the microstructures and electronic structures of the two types of doped TiO2 and the chemical states of the doped V ions to elucidate the causes of the different physicochemical properties. The photocatalytic activity of the doped TiO2 was enhanced by 1.9 times when the V ions were doped only in the surface lattice at a surface V/Ti molar ratio of 3.0 x 10(-1), in contrast, bulk doping caused detrimental effects. Reduced species, including V3+ and V4+ ions, were formed in the interstitial anatase lattice. These impurities introduced occupied and unoccupied energy levels, which were close to and in the conduction band of TiO2, respectively. The V3+/V4+ ions within the TiO2 lattice inhibited charge diffusion to the surface through deep trapping. In contrast, the impurities present only in the surface lattice increased the number of surface trapped holes and facilitated the interfacial charge transfer from the photoactivated TiO2 to adsorbates. Heavy doping at the surface resulted in the formation of V2O5. The p-n junction between the TiO2 and V2O5 separated charge carriers and additionally promoted the photocatalytic activity of the surface-doped TiO2. (c) 2010 Elsevier B.V. All rights reserved.