Insights into the photocatalytic mechanism of g-C3N4/Cs2BBr6 (B = Pt, Sn, Ti) heterojunction photocatalysts by density functional theory calculations

Among all well-studied photocatalysts, g-C3N4 has attracted significant research interest in various fields. However, the recombination rate of photogenerated electron–hole pairs in unmodified g-C3N4 is high, leading to a decrease in photocatalytic efficiency. In practical applications, it is often necessary to introduce appropriate amounts and types of surface cocatalysts to amplify the photocatalytic activity of g-C3N4. The heterojunctions between g-C3N4 and perovskite materials can facilitate efficient charge separation, leading to improved photocatalytic performance while maintaining the stability of the photocatalyst. In recent years, lead-free halide double perovskites, such as A2BX6, have been widely applied in the field of photocatalysis. In this study, we conducted systematic investigations on the band structures and charge transfer of g-C3N4/Cs2BBr6 (B = Pt, Sn, Ti) heterojunctions using density functional theory (DFT) calculations, and explored the interaction between the Cs2BBr6(001) surface and the g-C3N4. The results show that the g-C3N4/Cs2PtBr6 as well as g-C3N4/Cs2SnBr6 heterojunctions exhibit staggered band alignment, which facilitates the migration of photogenerated charge carriers and enhances catalytic activity. Besides, the g-C3N4/Cs2TiBr6 heterojunction exhibited a straddling gap. Furthermore, the analysis of density of states, charge density differences, and Bader charges reveals that the presence of an internal electric field promotes the partition of electron–hole pairs at the heterojunction interface, effectively suppressing the recombination of charge carriers. Therefore, depending on the specific metal ions at the B site in the g-C3N4/Cs2BBr6 structure, the resulting heterojunctions will exhibit different band alignments and photocatalytic performances. This work contributes to providing theoretical insights for the design of novel high activity heterojunction photocatalysts.