BiOI-SnO₂ Heterojunction Design to Boost Visible-Light-Driven Photocatalytic NO Purification

The efficient, stable, and selective photocatalytic conversion of nitric oxide (NO) into harmless products such as nitrate (NO₃^-) is greatly desired but remains an enormous challenge. In this work, a series of BiOI/SnO₂ heterojunctions (denoted as X%B-S, where X% is the mass portion of BiOI compared with the mass of SnO₂) were synthesized for the efficient transformation of NO into harmless NO₃^-. The best performance was achieved by the 30%B-S catalyst, whose NO removal efficiency was 96.3% and 47.2% higher than that of 15%B-S and 75%B-S, respectively. Moreover, 30%B-S also exhibited good stability and recyclability. This enhanced performance was mainly caused by the heterojunction structure, which facilitated charge transport and electron-hole separation. Under visible light irradiation, the electrons gathered in SnO₂ transformed O₂ to ·O₂^- and ·OH, while the holes generated in BiOI oxidized H₂O to produce ·OH. The abundantly generated ·OH, ·O₂^-, and ¹O₂ species effectively converted NO to NO^- and NO₂^-, thus promoting the oxidation of NO to NO₃^-. Overall, the heterojunction formation between p-type BiOI and n-type SnO₂ significantly reduced the recombination of photo-induced electron-hole pairs and promoted the photocatalytic activity. This work reveals the critical role of heterojunctions during photocatalytic degradation and provides some insight into NO removal.