Surface Oxygen Vacancies on Copper-Doped Titanium Dioxide for Photocatalytic Nitrate-to-Ammonia Reduction

Photocatalytic transformation of nitrate (NO₃^–) in wastewater into ammonia (NH₃) is a challenge in the detoxification and recycling of limited nitrogen resources. In particular, previously reported photocatalysts cannot promote the reaction using water as an electron donor. Herein, we report that copper-doped titanium dioxide (Cu-TiO₂) powders, prepared via the sol–gel method and subsequent calcination, promote NO₃^–-to-NH₃ reduction in water. The Cu²⁺ doping into TiO₂ creates a large number of surface oxygen vacancies (OV_surf), which are stable even under aerated conditions. The Ti³⁺ and Cu²⁺ atoms adjacent to OV_surf behave as active sites for the NO₃^–-to-NH₃ reduction. Doping with an appropriate amount of Cu²⁺ and calcination at an appropriate temperature produce the catalysts with a large number of OV_surf, while maintaining a high conductivity, and exhibit a high photocatalytic activity.