Effect of Mixed Valence States of Platinum Ion Dopants on the Photocatalytic Activity of Titanium Dioxide under Visible Light Irradiation

Titanium dioxide doped with the Pt ion (Pt-TiO2) was synthesized by a sol-gel method using only water as the solvent and conducting dialysis. The photocatalytic activity for the degradation of 4-chlorophenol (4-CP) on Pt-TiO2 was not affected by the Brunauer-Emmett-Teller specific surface area under visible light (VL) irradiation. Xray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure measurements revealed that only the Pt(IV) ion existed in the TiO2 bulk and both Pt(II) and Pt(IV) were present near the Pt-TiO2 surface. Pt(IV) is most likely substituted in the Ti(IV) site of the TiO2 lattice because of their similar ionic sizes. Quantitative analysis of Pt(II) was performed in the XPS measurements, indicating that the amount of Pt(II) on the surface increased with an increase in the doping amount from 0.2 to 1.0 atom %. We synthesized 0.5 atom % Pt-TiO2 with various Pt(II)/Pt(IV) ratios by changing the Ti(OC3H7)(4) concentration used in the sol-gel synthesis. The 4-CP conversion on Pt-TiO2 increased linearly with an increase in the Pt(II)/Pt(IV) ratios. A similar relationship was obtained with Pt-TiO2, which was prepared by a conventional sol-gel method in ethanol-water mixed solvent. Therefore, the Pt(II)/Pt(IV) ratio is a key factor affecting the photocatalytic activity of Pt-TiO2 under VL irradiation. Our results indicate that controlling the mixed valence states of the doped metal ions is a new strategy to developing highly active metal-ion-doped TiO2 under VL irradiation.