Photocurrent Generation of a TiO ₂ Electrode Modified with Platinum or Vanadium Ions under Visible Light Irradiation─Comparison of Doping and Loading

Many studies have addressed that doping or loading of transition metal ions enhances the photocatalytic activities of TiO2 under visible light (VL) irradiation. Systematic studies are needed to determine which of these two methods is more effective. TiO2 doped with Pt4+ or V3+ (Pt- or V-TiO2) prepared by conducting a dialysis in the sol–gel method acts as a VL-responsive photocatalyst. In this study, four types of Pt- or V-TiO2 electrodes were fabricated using the precursor sol after the dialysis and compared with the TiO2 electrode loaded with Pt or V ions prepared by a wet-impregnation method. All electrodes were transparent to transmit most of the incident light. We have demonstrated that metal-ion doping causes two opposing effects: enhancing the VL absorption and trapping the photogenerated electrons by dopants. The former is dominant for Pt-TiO2, whereas the latter prevails for V-TiO2 to decrease the photocurrent. The TiO2 electrode loaded with Pt or V ions exhibited higher photocurrents compared with their doping. This is reasonable because most of the loaded ions exist on the surface. However, the photocurrent and the V-content on the surface decreased with increasing sintering temperature, indicating that some V ions are doped inside TiO2. For the synthesis of VL-responsive photocatalysts using metal ions, the loading is more beneficial than the doping, but the sintering temperature should be carefully controlled.