Directed Charge Transfer-Driven Efficient Photocatalytic Hydrogen Production in Dual S-Scheme WS₂/Co₉S₈/ZnCdS Heterojunction

ZnCdS-based photocatalysts exhibit great potential for solar-driven hydrogen (H₂) evolution due to their tunable bandgaps and visible-light absorption. Nevertheless, rapid charge recombination and structural instability hinder their practical implementation. To overcome these challenges, this work proposes a dual S-scheme heterojunction design strategy utilizing polyoxometalates (POMs) as precursors to precisely control the heterojunction interfacial coupling. A dual S-scheme WS₂/Co₉S₈/ZnCdS system was synthesized via a precursor-guided sulfidation process, using K₇[Co₂W₁₁O₄₀H₂]·15H₂O (Co₂W₁₁) POM clusters as dual-source templates. This approach enables the simultaneous achievement of tight interfacial coupling and a simplified single-interface architecture. The charge transfer mechanism within the heterojunction was systematically investigated through analyses of the Fermi level, band structure, ultrafast timescale femtosecond transient absorption (fs-TAS), time-resolved photoluminescence (TRPL), in situ x-ray photoelectron spectroscopy (XPS), and synchrotron radiation. The dual S-scheme heterojunction not only expands the light absorption range of ZnCdS but also promotes efficient charge migration and separation. Under visible-light irradiation (λ ≥ 420 nm), this dual S-scheme heterojunction exhibits remarkable stability and achieves a hydrogen evolution rate of up to 15.66 mmol g⁻¹ h⁻¹, surpassing most reported noble metal-free ZnCdS-based photocatalysts. This research provides a robust methodology for developing dual S-scheme heterojunctions that enhance photocatalytic hydrogen evolution efficiency.