Construction of S-scheme heterostructure via anion and cation vacancy mediated electrostatically driven self-assembly in Cd1-xS-CdS1-x for robust photocatalytic CO2 reduction

Although vacancy engineering strategies have been extensively developed for single semiconductor photocatalysts, the exploration of vacancy roles and their intrinsic mechanisms in heterojunction construction remains limited. Herein, a novel S-scheme system was elaborately fabricated through synergistic engineering of anion and cation vacancies in two distinct defective semiconductors: S-vacancy-rich CdS_1-x and Cd-vacancy-rich Cd_1-xS nanoparticles. Interestingly, the formation of Cd_1-xS-CdS_1-x heterostructure was spontaneously driven by electrostatic interactions between inherently positively charged CdS_1-x and negatively charged Cd_1-xS. Remarkably, the S-scheme Cd_1-xS-CdS_1-x system exhibited optimal performance for photocatalytic CO₂ reduction reaction (CO₂RR) in solid-gas mode, achieving CO production rates 60-fold higher than pristine CdS, 30-fold greater than Cd_1-xS, and 12-fold superior to CdS_1-x. Combined experimental characterization and theoretical simulations revealed that the dual-vacancy engineering induced: (i) optimized staggered band alignment, (ii) accelerated interfacial charge transfer, and (iii) enhanced adsorption of CO₂ and H₂O reactants. Particularly, pre-adsorbed H₂O molecules on Cd_1-xS were found to facilitate subsequent CO₂ adsorption, effectively lowering the energy barrier for CO₂-to-COOH conversion during CO generation. The proposed vacancy co-engineering strategy opens more opportunities for novel S-scheme system design.