Mechanistic understanding of the increased photoactivity of TiO2 nanosheets upon tantalum doping

Abstract

Anatase TiO₂ is doped with Ta cations through a hydrothermal route. Based on X-ray photoelectron spectroscopy and X-ray absorption near-edge structure spectroscopy, the Ta dopants exist in the 5+-oxidation state. The oxidation state is insensitive to the Ta loading amount. Extended X-ray absorption fine structure spectroscopy confirms that the local structure around Ta cations is not identical between the Ta-doped samples. The Ta–O distance monotonically increases with the Ta loading amount due to a gradually expanding lattice. The Ta-doped samples show higher activity than pristine TiO₂ for photomineralizing recalcitrant organics. The enhanced photocatalytic activity is proposed to be due to an enhanced population of photoexcited electrons, as probed using light-induced IR absorption spectroscopy, and an extended electron lifetime, as probed using time-resolved microwave conductivity, which are associated with the formation of Ti³⁺ defect states acting as shallow electron traps. The maximum photocatalytic activity is observed for TiO₂ doped with 2 mol% of Ta, which shows enhancement of mineralization efficiency (about 3 times) and enhancement of electron population (up to 20 times), as compared to those of pristine TiO₂. The fundamental question of why a proper metal doping into TiO₂ increases photocatalytic activity is discussed in this study.