Synergistic Electron‐Proton Transfer Over In ₂ O ₃ /CuGa 0.5 S Z‐Scheme Heterojunction for Highly Selective CO ₂ ‐to‐CH ₄ Photoconversion

Solar‐driven photoconversion of CO 2 and H 2 O into value‐added chemicals such as CH 4 remains a promising yet challenging strategy, hindered by inefficient charge separation and sluggish proton migration kinetics. Herein, an interfacial‐engineered Z‐scheme In 2 O 3 /CuGa 0.5 S heterojunction is designed to synchronize electron transfer with proton delivery. Utilizing Kelvin probe force microscopy, a direct Z‐scheme charge‐transfer pathway with electron accumulation on CuGa 0.5 S is identified. Electronic‐state modulation facilitating CO 2 activation is revealed by quasi‐ in situ XANES and operando XPS, while water dissociation is promoted by In–O sites on In 2 O 3 for continuous reactive H* supply. Preferential H* relocation to CuGa 0.5 S and coupling with *COOH intermediates are demonstrated by in situ DRIFTS and DFT calculations, through which continuous hydrogenation toward CH 4 is driven. Key intermediates are stabilized by synergistic interactions between adjacent components, resulting in significantly enhanced CH 4 selectivity and effectively suppressed competing H 2 evolution. Consequently, a CH 4 evolution rate of 319.2 µmol g −1 h −1 with approximately 100% selectivity is achieved over optimized In 2 O 3 /0.5CuGa 0.5 S in pure water. A mechanistic understanding of electron‐coupled proton transfer in photocatalytic CO 2 reduction is provided, offering an efficient pathway for advancing solar‐driven hydrocarbon production technologies.