Free energy diagrams of photocatalytic CO₂ reduction to CH₄ for (A) the V_S-CuIn₅S₈ single-unit-cell layers and (B) the pristine CuIn₅S₈ single-unit-cell layers. The blue line represents the more facile way while the black line shows the less favorable way. Steps in the red-dashed box are determining factors of the reaction selectivity. Key steps of photocatalytic CO₂ reduction to CH₄ for (C) the V_S-CuIn₅S₈ single-unit-cell layers and (D) the pristine CuIn₅S₈ single-unit-cell layers, in which the charge-enriched Cu-In dual sites convert the endoergic protonation to an exoergic process, and hence change the reaction routes to product CH₄ instead of CO. Adapted with permission from Ref. [71], copyright 2019 Springer Nature.