Mechanistic insights into the nitrogen photofixation reaction by BiOBr-based heterojunctions

Photocatalytic nitrogen fixation (PNF) offers a green route to ammonia synthesis under ambient conditions. We present a scalable synthesis of BiOBr/g-C3N4 heterojunctions and identify that the composition with 10 wt% BiOBr achieves ∼20 µmol g−1 h−1 ammonia production—outperforming pristine materials of the heterojunction. Mechanistic investigations reveal that enhanced activity stems from efficient charge separation, supported by time-resolved spectroscopy showing extended carrier lifetimes. DFT calculations reveal that the catalytically active (010) surface of BiOBr exhibits favorable band alignment with g-C3N4 and enables downhill electron transfer to the N2/NH3 redox level. Crucially, this surface hosts localized electron polarons, which act as reactive sites for nitrogen reduction. In contrast, the (001) surface lacks such features, explaining the reduced performance at higher BiOBr loadings. These findings establish a direct link among surface structure, charge dynamics, and catalytic functions, offering design principles for next-generation photocatalysts for sustainable ammonia production.