Enhanced photocatalytic nitrogen fixation on ultrathin Nb₂O₅·nH₂O nanosheets in pure water through the synergistic effect of oxygen vacancies and acid sites

Gaseous nitrogen is abundant in the atmosphere, and its efficient conversion to ammonia is vital to the future of a greener and more sustainable world. Since the N≡N covalent triple bond is difficult to break, the adsorption and activation of N₂ molecules on the photocatalyst surface are critical to improve the efficiency of photocatalytic nitrogen fixation. In this work, Nb₂O₅·nH₂O nanosheets were synthesized by a hydrothermal reduction process with a weak reducing agent of glyoxal, which created more oxygen vacancies on their surfaces. Furthermore, their surface acidity was modulated by subsequent heat treatment in an Ar atmosphere. Thus, the effects of the oxygen vacancy and surface acidity on the photocatalytic nitrogen fixation performance of these Nb₂O₅·nH₂O nanosheets could be investigated. It was found that both factors contributed to the adsorption/activation of N₂ and the charge carrier separation/transfer in these Nb₂O₅·nH₂O nanosheets. Owing to their synergistic effect, a high ammonia yield of 173.7 μmol/(g·h) was achieved by these Nb₂O₅·nH₂O nanosheets through photocatalysis in pure water under simulated solar illumination without assistance from either sacrificial agents or cocatalysts.