Experimental and theoretical investigation on facet-dependent MoO2/BiOBr Z-scheme heterojunction photocatalytic nitrogen reduction: modulation of bulk charge separation efficiency by built-in electric field intensity

Abstract

Based on facet engineering and Z-scheme heterojunctions, a series of MoO₂/BiOBr Z-scheme heterojunctions with different facet ratios of (102)/(001) were prepared for photocatalytic nitrogen reduction. The performance of nitrogen reduction is greatly improved after constructing heterojunctions, and the activity increases rapidly with the increase of the (102)/(001) ratio of BiOBr in the heterojunction. MoO₂/BiOBr-0 with a (102)/(001) ratio of 0.167 exhibits the highest activity, reaching 176.66 μmol g⁻¹ h⁻¹, which is 4–5 times higher than that of pristine MoO₂ and BiOBr. Based on the built-in electric field (BIEF) strength, bulk charge separation (BCS) efficiency, and theoretical calculation of the materials, it is believed that due to the increase in the (102)/(001) facet ratio, the BIEF strength between the two phases of the heterojunction is enhanced, resulting in a high BCS efficiency. This promotes more surface enriched photo-generated electrons to act on nitrogen reduction, thereby achieving efficient photocatalytic ammonia synthesis. According to DFT, compared to MoO₂ and BiOBr, MoO₂/BiOBr not only adsorbs N₂ more strongly than H, but also ΔG_max in the potential energy determination step (PDS) is lower, thus exhibiting superior NRR activity. The calculation is completely consistent with the experimental results, further confirming that the construction of Z-scheme heterojunctions with a high proportion of (102) facets greatly promotes the performance of MoO₂ and BiOBr in catalyzing nitrogen reduction.