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 3.0×10^-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.