Engineering Oxygen Vacancies in MoO ₂ for Enhanced Photocatalytic Nitrogen Reduction to Ammonia

Photocatalytic ammonia synthesis is considered one of the most promising fields as the sustainable, which will attract increasing attention. In this work, grayish‐white MoO ₃ was first synthesized using one‐step hydrothermal calcination method under an inert atmosphere. It was then subjected to secondary calcination in pure hydrogen, ultimately yielding black MoO ₂ photocatalysts with abundant oxygen vacancies (OVs). The reduced MoO ₂ achieves a maximum photocatalytic ammonia synthesis rate of 46.29 μmol·h ⁻¹ ·g ⁻¹ . Electron paramagnetic resonance and X‐ray photoelectron spectroscopy characterization confirmed the formation of OVs. Photoelectrochemical testing demonstrated that compared to MoO ₃ , MoO ₂ enhanced light absorption and suppressed carrier recombination, thereby improving nitrogen fixation efficiency. Density functional theory calculations show a lower adsorption energy for MoO ₂ (0.16 eV) than for MoO ₃ (0.95 eV), highlighting the crucial role of OVs in tuning the reaction kinetics. The dynamic role of OVs in catalytic reactions is demonstrated in this work.