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.