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

Lead halide perovskites demonstrate outstanding luminescent characteristics. However, the inclusion of lead components restricts their extensive utilization. Halide perovskite materials, formulated as A₂M(III)M(I)X₆ or A₂M(IV)X₆, 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 CO₂ reduction driven by sunlight, with a CH₄ 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.