Simultaneous Electronic and Crystallographic Modulation of Cs₃Bi₂Br₉ by Single-Element Sn Substitution for Enhanced CO₂ Photoreduction

The photocatalytic conversion of CO₂ 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 Sn²⁺ substitution into layered Cs₃Bi₂Br₉ enables simultaneous modulation of the crystallographic and electronic structure. Combined experimental characterizations and simulation studies reveal that Sn²⁺ 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 Sn²⁺-doped Cs₃Bi₂Br₉ exhibits a 3.4-fold enhancement in CH₄ 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. © 2026 The Author(s). Advanced Energy Materials published by Wiley-VCH GmbH.