Herein, we investigate the effect of Bi particle size in BiOI/Bi junctions on their photocatalytic function towards NO gas. BiOI microflowers (BiOI) and BiOI microflowers decorated with micron-sized Bi particles (BiOI/Bi MPs) were produced by a solvothermal method. BiOI decorated with nano-sized Bi particles (BiOI/Bi NPs) were produced by a reduction process. All samples were physically characterised by XRD, FT-IR, SEM, HR-TEM coupled with EDX analysis, DR-UV–visible and PL spectroscopy and functionally characterised by photocatalytic testing towards NO gas, TAS and EPR spectroscopy.

Their photocatalytic activity towards NO gas was measured following ISO protocol (ISO 22197–1:2016). The best performing BiOI-based sample was BiOI/Bi NPs, showing NO and NOx conversion efficiencies of ∼33 and ∼11% under UVA light, and ∼26 and ∼8.1% under visible light, respectively. The BiOI and BiOI/Bi MPs samples showed significantly lower activities, displaying overall NOx conversion efficiencies of ∼3.5 and ∼0.8% under UVA light, respectively. Importantly, the best performing BiOI/Bi NPs samples showed visible light activity that was at least 6 times higher than that of a commercial TiO2 benchmark (CristalACTiVTM PC-S7). TAS measurements showed that charge carriers were significantly longer lived in the BiOI/Bi NPs sample (t50% from 10 μs of ∼90 μs) than the BiOI and BiOI/Bi MPs samples (t50% from 10 μs of ∼50 μs). This was attributed to the significant degree of interfacial contact formed between Bi and BiOI in the BiOI/Bi NPs sample, which enhanced charge carrier separation. EPR studies showed that this interfacial contact between BiOI and Bi likely promoted the formation of VO, which may have contributed to enhancement seen in photocatalytic activity in the BiOI/Bi junction.