Abstract

Abstract Photocatalytic conversion of CO 2 to methane faces challenges due to the stability of CO 2 , unpredictable intermediates, and complex electron transfer steps. Herein, a spatial In 2 S 3 /In 2 O 3 heterojunction with abundant S vacancies (ISIO(V S )) is obtained through facile Polyvinylpyrrolidone (PVP) treatment to reach a methane yield of 16.52 µmol·g −1 ·h −1 with a selectivity of 95.93%, which is the highest among reported In 2 S 3 and In 2 O 3 based catalysts. The work function ( W f ), differential charge density, and Kelvin Probe Force Microscopy (KPFM) results confirm that S vacancies strengthen the built‐in electric field (BEF) of In 2 S 3 /In 2 O 3 (ISIO) heterojunctions, improving carrier separation. Density functional theory (DFT) calculations reveal that S vacancies induce electron redistribution, facilitating adsorption and activation of CO 2 and *CO intermediate, thus promoting hydrogenation to yield *CHO. The reaction pathway of photocatalytic CO 2 reduction is revealed by in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and Gibbs free energy (Δ G ). The S vacancies modify electronic orbitals and the highest occupied molecular orbital (HOMO) of In atom, resulting in a stronger interaction between the catalyst and *CHO, which reduces Δ G *CHO and regulates the selectivity of CH 4 . This study paves a new avenue for the design of photocatalysts with highly selective reduction of CO 2 to CH 4 through defect engineering.