Spherical micro-heterostructured Bi2MoO6/Mo–Bi2O3 with interfacial oxygen vacancies enabling efficient photocatalytic NH3 production from N2 and H2O

Abstract

Photocatalytic nitrogen fixation under mild conditions remains an open challenge due to the low carrier separation efficiency and discrete active sites of conventional photocatalysts. Defect engineering and heterojunctions have emerged as effective strategies to address these issues by enhancing charge carrier mobility and increasing the number of active sites for nitrogen activation. In this study, spherical Bi₂MoO₆/Mo–Bi₂O₃ composite photocatalysts with different Mo/Bi ratios are reported to be photocatalytically active for ammonia synthesis from N₂ and H₂O under full-spectrum light. Bi₂MoO₆/Mo–Bi₂O₃ with a Mo/Bi ratio of 24/100 shows the highest activity, with an ammonia production rate as high as 141.93 μmol g⁻¹ h⁻¹. This high photocatalytic activity is attributed to the formation of a micro-heterojunction of Bi₂MoO₆ on Mo-doped Bi₂O₃ and the presence of oxygen vacancies (O_v) at the interfacial surface between two phases. The O_v enhances the adsorption and activation of N₂ and H₂O molecules, while the micro-heterojunction improves separation efficiency of photogenerated charges. More importantly, N₂ molecules and H₂O molecules could be adsorbed at Bi³⁺ and Mo⁵⁺ sites at two sides of the O_v at the interfacial surface between Bi₂MoO₆ and Mo–Bi₂O₃ to form a N₂/Bi–O_V–Mo/H₂O cyclic transition state. Upon illumination, the photogenerated electrons and holes transfer to the Bi and Mo sites, where they are captured by N₂ and H₂O molecules, respectively. In this way, N₂ and H₂O are transformed step by step into NH₃ and O₂. This work underscores the synergistic roles of the micro-heterojunction and oxygen vacancies of Bi₂MoO₆/Bi₂O₃ for enhancing photocatalytic nitrogen fixation. The proposed mechanism offers valuable insights for designing high-performance photocatalysts for photocatalytic ammonia production.