Variable Valence Ce‐Based Cs2CeAgBr6 Perovskite Nanocrystals for Highly Selective Photoconversion of CO2 to CH4

Abstract Lead halide perovskites demonstrate outstanding luminescent characteristics. However, the inclusion of lead components restricts their extensive utilization. Halide perovskite materials, formulated as A 2 M(III)M(I)X 6 or A 2 M(IV)X 6 , possess the potential to serve as stable and eco‐friendly substitutes for optoelectronic applications. Nevertheless, their wide bandgap (>3 eV) hinders the practical implementation across various domains. Here, the variable valence Ce‐based Cs₂CeAgBr₆ perovskite nanocrystals (NCs) are first synthesized with a bandgap of 2.65 eV. Intriguingly, the coexistence of trivalent and tetravalent Ce can cause localized spin of the f‐layer electrons of Ce, leading to Ce 3+/4+ (the Ce valence state ranges between III and IV) defects. By manipulating trivalent and tetravalent Ce source proportions, a dual Ce‐based perovskite achieves a minimal Ce 3+/4+ defect content of 1.4%. The as‐prepared Cs₂CeAgBr₆ NCs exhibit exceptional efficiency in CO 2 reduction driven by sunlight, with a CH 4 selectivity greater than 70% and a super high charge transfer rate of 802.5 µmol·g −1 h −1 , far surpassing previously reported findings. Additionally, theoretical calculations have elucidated the photocatalytic mechanism involved in CO₂ reduction. The outcomes of this investigation are expected to stimulate design and fabrication of novel lead‐free perovskite nanocrystals.