Breaking the Symmetry of Sulfur Defect State via Atomic Substitution for Enhanced CO2 Photoreduction

Conventional sulfur vacancy, characterized by a symmetric coordination of metal cations (M1-SV-M1), typically serves as a catalytic site for CO2 chemisorption. However, the symmetric SV site, with a uniform charge distribution across adjacent metal sites, enables 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 Cu dopant into SV-rich SnS2 nanosheets (Cu-SnS2-SV) to construct the asymmetric Cu-SV-Sn sites, which effectively steer CO2 photoreduction into CO with a production rate of 48.6 μmol g-1 h-1 in the absence of photosensitizer and scavenger, 18-fold higher than SnS2-SV with symmetric Sn-SV-Sn sites. The experimental investigations combined with theoretical simulations reveal that asymmetric Cu-SV-Sn structure, compared with symmetric Sn-SV-Sn structure, allows an upshift of the defect-band center, which significantly mitigates the energy loss associated with the electron relaxation from conduction band to defect band. Moreover, the advantages of the Cu-SV-Sn sites over the Sn-SV-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 the COOH* intermediates, which lowers the activation energy barrier of the rate-determining step.