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.