Photocatalytic Hydrogen Evolution Activity of Nitrogen/Fluorine-Codoped Rutile TiO2.

The development of a photocatalyst capable of evolving H2 from water under visible light is important. Here, the photocatalytic activity of N/F-codoped rutile TiO2 (TiO2:N,F) for H2 evolution was examined with respect to metal cocatalyst loading and irradiation conditions. Among the metal species examined, Pd was the best-performing cocatalyst for TiO2:N,F under UV-vis irradiation (λ > 350 nm), producing H2 from an aqueous methanol solution. The H2 evolution activity was also dependent on the state of the loaded Pd species on the TiO2:N,F, which varied depending on the preparation conditions. Pd/TiO2:N,F prepared by an impregnation-H2 reduction method, showed the highest performance. However, the activity of the optimized Pd/TiO2:N,F toward H2 evolution from an aqueous methanol solution was negligibly small under visible-light irradiation (λ > 400 nm), although the use of an ethylenediaminetetraacetic acid disodium salt as an electron donor resulted in observable H2 evolution. Transient absorption spectroscopy revealed that although a relatively large population of reactive electrons was generated in the TiO2:N,F under 355 nm UV-pulse photoexcitation, the density of reactive electrons generated under 480 nm visible light was lower. This wavelength-dependent behavior in photogenerated charge carrier dynamics could explain the different photocatalytic activities of the TiO2:N,F catalysts under different irradiation conditions.