A closer look at the interface of Cs3Bi2Br9 lead free perovskite in photocatalytic process

Cs3Bi2Br9 (CBB) is emerging as a highly promising lead-free halide perovskite photocatalyst for environmental remediation thanks to its optimal bandgap and notable stability across various solvents. The impact on CBB electronic properties due to interaction with target molecules and solvents and how these changes may affect the photocatalytic performance, remains, however, largely unexplored. Here, we synthesized CBB powders using both solvent-based chemical co-precipitation and solvent-free mechanical ball mailing methods. Our observations revealed that under specific conditions, these perovskite powders systematically transform into the non-perovskite bismuth oxobromide BiOBr (BOB) phase. Rather than being merely a photocatalytic investigation, comparing the efficiency of both the intrinsic perovskite CBB and non-perovskite BOB phases in the decolouration of two target dyes (methylene blue, a cationic dye, and methyl orange, an anionic dye), in this study photocatalytic experiments and theoretical modelling of the semiconductor/dye interacting systems are applied to rationalize the complex interplay of electrostatic interaction, band alignment and material passivation affecting charge transfer at the interface and kinetics of decolouration process. When these dyes contact the perovskite, interband energy states are formed within the energy band gap, acting as hole scavenger triggering the photocatalytic process. This work provides a comprehensive mechanistic insight into the photocatalytic behavior of perovskite and the interaction of CBB with solvent and dyes. Our findings, based on CBB and BOB analysis, highlight the importance of considering several factors when designing photocatalytic materials and for a specific organic transformation.