Breaking the symmetry of sulfur defect states via atomic substitution for enhanced CO2 photoreduction†

Conventional sulfur vacancies, characterized by the symmetric coordination of metal cations (M₁–S_V–M₁), typically serve as catalytic sites for CO₂ chemisorption. However, symmetric S_V sites, with a uniform charge distribution across adjacent metal sites, enable sluggish electron transfer kinetics for CO₂ activation and dissociation, as well as a low defect-band center that renders photoexcited electrons less energetic. Herein, we introduced a Cu dopant into S_V-rich SnS₂ nanosheets (Cu–SnS₂–S_V) to construct asymmetric Cu–S_V–Sn sites, which steer CO₂ photoreduction to CO with a production rate of 48.6 μmol g⁻¹ h⁻¹ in the absence of a photosensitizer and scavenger, 18-fold higher than that of SnS₂–S_V with symmetric Sn–S_V–Sn sites. Experimental investigations combined with theoretical simulations reveal that an asymmetric Cu–S_V–Sn structure, compared with a symmetric Sn–S_V–Sn structure, allows an upshift of the defect-band center, which significantly mitigates the energy loss associated with electron relaxation from the conduction band to the defect band. Moreover, the advantages of the Cu–S_V–Sn sites over the Sn–S_V–Sn sites are demonstrated not only by the increased Sn–S covalency, which facilitates electron transfer from catalysts to adsorbates, but also by the improved ability to stabilize COOH* intermediates, which lowers the activation energy barrier of the rate-determining step.