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Abstract Lead‐free halide double perovskites (DPs) have attracted great attention due to their stability, nontoxicity, and good photophysical properties. Herein, environmentally friendly small‐bandgap Cs2CuSbCl6 and Cs4CuSb2Cl12 DPs microcrystals (MCs) are prepared via a solvent synthesis technique. This study also reports a bottom‐up synthetic protocol for obtaining evenly distributed Cs2CuSbCl6 and Cs4CuSb2Cl12 quantum dots (QDs) utilizing a modified hot injection approach. These MCs and QDs have strong absorption throughout the visible region, indicating that they could be promising light‐absorbing materials as sunlight‐driven photocatalysts. For the first time, the efficient sunlight‐driven photocatalytic CO2 reduction activity of these Pb‐free “green” MCs and QDs has been revealed, and a product yield of 307 μmol g−1 and an electron consumption of 1053 μmol g−1 are achieved by the Cs4CuSb2Cl12 QDs under 3 h irradiation. Our results highlight the potential of this family of materials for photocatalytic applications.

Summary: Lead halide perovskites are prospective candidates for CO2 photoconversion. Herein, we report copper-doped lead-free Cs2AgSbCl6 double perovskite microcrystals (MCs) for gas-solid phase photocatalytic CO2 reduction. The 0.2Cu@Cs2AgSbCl6 double perovskite MCs display unprecedented CO2 photoreduction capability with CO and CH4 yields of 412 and 128 μmol g−1, respectively. The ultrafast transient absorption spectroscopy reveals the enhanced separation of photoexcited carriers in copper-doped Cs2AgSbCl6 MCs. The active sites and reaction intermediates on the surface of the doped Cs2AgSbCl6 are dynamically monitored and precisely unraveled based on the in-situ Fourier transform infrared spectroscopy investigation. In combination with density functional theory calculations, it is revealed that the copper-doped Cs2AgSbCl6 MCs facilitate sturdy CO2 adsorption and activation and strikingly enhance the photocatalytic performance. This work offers an in-depth interpretation of the photocatalytic mechanism of Cs2AgSbCl6 doped with copper, which may provide guidance for future design of high-performance photocatalysts for solar fuel production.