Revealing Insight into Defect-Mediated Charge Dynamics and Reaction Intermediates Stabilization for CO2 Photoreduction on S-scheme Catalyst

Abstract

Heterostructure photocatalysts with well-defined charge-transfer pathways offer strong potential for CO2 photoreduction, yet designing systems that achieve efficient separation and directional carrier flow remains difficult. This study presents an S-scheme heterojunction constructed from sulfur-deficient Cu3SnS4 (CTS) and oxygen-deficient carbon-doped WO3 (CW), coupled through reduced graphene oxide (rGO). Density Functional Theory (DFT) calculations demonstrate that sulfur vacancies in CTS reduce the free energy difference for the CO2→COOH* reaction by approximately 0.05 eV, while oxygen vacancies in CW enhance H2O adsorption and proton availability. Carbon doping forms W–O–C covalent linkages that act as rapid electron-transport channels, further strengthening carrier separation. Electron localization function and differential charge density analyses reveal substantial electron accumulation near the vacancies (~1.52 e⁻ at Cu sites and ~4.0 e⁻ at W sites), which strengthens CO2 adsorption and promotes the stabilization and transformation of reaction intermediates. Transient absorption and time-resolved photoluminescence spectroscopy highlight the critical role of the rGO interface in regulating charge flow, extending the carrier lifetime to 0.58 ns-2.32 times longer than that of pristine CW. In situ XPS and EPR confirm that sulfur and oxygen vacancies remain intact during reaction, preserving the material’s photoresponse. Together, these cooperative effects deliver markedly improved CO2 photoreduction, with CO and CH4 generation rates increased by 5.8- and 5.5-fold over CW. This work highlights that an S-scheme heterojunction with functionally differentiated yet electronically coupled defect sites enables a continuous CO2 reduction pathway, offering a new strategy for designing efficient photocatalysts.