Insights into the photocatalytic mechanism of S-scheme g-C3N4/BiOBr heterojunction

The energy band alignment and photocatalytic performance of g-C3N4/(0 0 1)-BiOBr heterojunction are studied based on the first-principles HSE06 hybrid density functional method. Our results show that g-C3N4 and BiOBr are staggered near the forbidden band of the heterojunction. The built-in electric field (BEF) is formed with the direction from the g-C3N4 surface to the BiOBr surface. Under visible light irradiation, the existence of BEF makes the photogenerated electrons in the conduction band minimum (CBM) of BiOBr migrate across the interface region of g-C3N4/(0 0 1)-BiOBr heterojunction to reach the valence band maximum (VBM) of g-C3N4 to achieve the effective separation of photogenerated electron-hole pairs. g-C3N4/(0 0 1)-BiOBr heterojunction is an S(step)-scheme photocatalytic mechanism and a large number of photogenerated electrons come from the BiOBr will greatly improve the hydrogen production activity in the g-C3N4 surface, which is consistent with the previous experimental observations. Moreover, we predict that g-C3N4/(0 0 1)-BiOBr heterojunction has better hydrogen production efficiency and photocatalytic performance than g-C3N4/(0 0 1)-BiOCl heterojunction. Our work can provide an effective scheme and theoretical supports for the design of novel S-scheme heterojunction photocatalysts.