The synergy of low S-vacancy and Cu sites in Cu-In4SnS8 promotes the favorable conversion of CO2 to ethanol with full selectivity

[Display omitted] •Cu-In double-active sites on In4SnS8 were constructed for CO2 reduction reaction.•Cu-In4SnS8 exhibits low free energy barrier to form *COOH and *COH.•Cu-In4SnS8 achieves full selectivity conversion for CO2-to-CH3CH2OH. Direct photocatalytic CO2 conversion to C2 products holds notable potential, but this technology is still limited by the high thermodynamic energy and sluggish kinetics. By introducing Cu in In4SnS8 catalyst, we here developed low S vacancies and chemical reactive sites photocatalysts Cu-In4SnS8, enabling photocatalytic CO2 to ethanol with full selectivity. Minor electron paramagnetic resonance peak intensity and large formation energy for S-vacancy suggest Cu introduction suppresses S vacancies formation. Low S vacancies photocatalysts Cu-In4SnS8 possess excellent carrier transfer and separation ability by the steady photoluminescence and transient fluorescence characteristic results. Furthermore, in situ, FTIR measurements confirmed ethanol formation via the presence of the intermediates *COOH, *CO, *OCCO(H), and *OCH2CH3. Also, theory calculations reveal that Cu efficiently reduces the energy barriers of the rate-limiting steps of *COOH and *COH, and promotes C-C coupling and multi-proton coupled electron transfer processes by co-adsorption and asymmetric charge distribution effects. As a result, the optimized sample achieves 0.73 μmol h−1 CH3CH2OH generation with full selectivity.