Oxygen-Doped Ta₃N₅ Nanoparticles for Enhanced Z-Scheme Carbon Dioxide Reduction with a Binuclear Ruthenium(II) Complex under Visible Light

Metal-complex/semiconductor hybrids are promising photocatalysts for visible-light CO₂ reduction with high selectivity for the formation of a desired product. Herein we applied nanoparticulate Ta₃N₅/SiO₂ as the semiconductor component of a hybrid system with the aid of a binuclear ruthenium(II) complex. The Ta₃N₅/SiO₂ material was prepared by the nitridation of nanoparticulate Ta₂O₅/SiO₂ (which had been previously synthesized by a sol-gel method) under a flow of NH₃ gas at 973–1223 K. The synthesized hybrid reduced CO₂ into formate with very high selectivity (>99 %) under irradiation with visible light (λ>480 nm). The activity increased with nitridation temperature up to 1023 K, beyond which it began to drop. The optimized photocatalyst, which consisted of oxygen-doped Ta₃N₅ as revealed by UV-visible diffuse reflectance spectroscopy and X-ray photoelectron spectroscopy, exhibited 6 times higher activity than that for an analogous hybrid constructed with bulk Ta₃N₅. Physicochemical analyses indicated that reducing the defect density by high-temperature nitridation contributed to the suppression of charge recombination, which resulted in higher activity of Ta₃N₅/SiO₂, while a nitridation temperature that was too high was undesirable because of a decrease in the reactivity of photogenerated holes due to a loss of the doped oxygen content in Ta₃N₅.