Simultaneous Electronic and Crystallographic Modulation of Cs ₃ Bi ₂ Br ₉ by Single‐Element Sn Substitution for Enhanced CO ₂ Photoreduction

ABSTRACT The photocatalytic conversion of CO 2 into value‐added fuels offers a sustainable route to mitigate carbon emissions while storing solar energy. Lead‐free halide perovskites have recently emerged as promising photocatalysts due to their highly tunable electronic structures and strong light‐harvesting capabilities. However, their performance is often limited by inefficient charge separation and transfer. Herein, we demonstrate that aliovalent Sn 2+ substitution into layered Cs 3 Bi 2 Br 9 enables simultaneous modulation of the crystallographic and electronic structure. Combined experimental characterizations and simulation studies reveal that Sn 2+ substitution induces lattice contraction and promotes bromine vacancy formation as a charge‐compensation mechanism. These concurrent structural changes drive electronic redistribution and introduce shallow trap states, thereby improving charge carrier dynamics. As a result of these synergistic effects, the optimized Sn 2+ ‐doped Cs 3 Bi 2 Br 9 exhibits a 3.4‐fold enhancement in CH 4 yield compared to the pristine material, achieving a yield of 8.91 µmol·g −1 under continuous visible‐light irradiation for 4 h. This work demonstrates that aliovalent B‐site substitution is an effective strategy for engineering lattice distortion and vacancy chemistry to enhance charge utilization and photocatalytic selectivity in lead‐free halide perovskites.