Band alignment modulation of g-C₃N₄ by tuning structural defects for selective ammonia photosynthesis from nitrate reduction under visible light irradiation

Photocatalytic nitrate reduction to ammonia (PcNRA) not only tackles nitrate pollution in wastewater but also transforms it into valuable ammonia, attracting attention as an eco-friendly and carbon-free ammonia synthesis technology. However, it still suffers from insufficient ammonia selectivity due to active side reactions such as nitrogen gas formation and hydrogen production. In this work, we synthesized B-doped and N-deficient g-C₃N₄ (NVCN) by thermal treatment using NaBH₄ as a reduction reagent for selective photocatalytic nitrate-ammonia conversion. The simultaneous introduction of B dopants and nitrogen vacancies (NVs) into the g-C₃N₄ (CN) framework modulated the band structure: the narrowed band gap and the generated mid-gap states suppressed charge carrier recombination and allowed more electrons to participate in the reduction reaction, while the reduced conduction band energy effectively inhibited hydrogen evolution, defining possible reaction pathways to nitrate reduction. Moreover, nitrate species were strongly adsorbed and activated on the catalyst surface in the co-presence of B dopants and NVs, which consequently facilitated selective and active nitrate-to-ammonia conversion. The optimal catalyst, NVCN475, achieved exceptional ammonia selectivity (96.9 %) and production activity (8.83 μmol h⁻¹) with negligible H₂ evolution (0.52 μmol h⁻¹) under visible light irradiation.