Identification of Halogen-Associated Active Sites on Bismuth-Based Perovskite Quantum Dots for Efficient and Selective CO2-to-CO Photoreduction.

All-inorganic Pb-free bismuth (Bi) halogen perovskite quantum dots (PQDs) with distinct structural and photoelectric properties provide plenty of room for selective photoreduction of CO2. However, the efficient conversion of CO2-to-CO with high selectivity on Bi-based PQDs driven by solar light remains unachieved, and the precise reaction path/mechanism promoted by the surface halogen-associated active sites is still poorly understood. Herein, we screen a series of nontoxic and stable Cs3Bi2X9 (X = Cl, Br, I) PQDs for selective photocatalytic reduction of CO2-to-CO at the gas-solid interface. Among all the reported pure-phase PQDs, the as-synthesized Cs3Bi2Br9 PQDs exhibited the highest CO2-to-CO conversion efficiency generating 134.76 μmol g-1 of CO yield with 98.7% selectivity under AM 1.5G simulated solar illumination. The surface halogen-associated active sites and reaction intermediates were dynamically monitored and precisely unraveled based on in situ DRIFTS investigation. In combination with the DFT calculation, it was revealed that the surface Br sites allow for optimizing the coordination modes of surface-bound intermediate species and reducing the reaction energy of the rate-limiting step of COOH- intermediate formation from •CO2-. This work presents a mechanistic insight into the halogen-involved catalytic reaction mechanism in solar fuel production.