Green synthesis of ultrathin WS₂ nanosheets for efficient hydrogen evolution reaction

Transition metal dichalcogenide (TMD) nanostructures have emerged as promising electrocatalysts for the hydrogen evolution reaction (HER) as there is an increasing demand for cost-effective and sustainable hydrogen production. Despite significant progress, there is still a critical need for developing facile and green methods for synthesizing ultrathin TMD nanostructures. In this study, we introduce a green, top-down synthesis method to produce highly exfoliated WS₂ nanosheets. The process combines the ultrasonication of bulk WS₂ in a binary water-ethanol solvent with a solvothermal treatment. The resulting ultrathin WS₂ nanosheets exhibit clean surfaces free of surface ligands and impurities, high crystallinity in the semiconducting hexagonal phase, and outstanding electrochemical activity for HER. Key performance metrics include a low onset potential of -0.32 V (vs. reversible hydrogen electrode (RHE)) at 10 mA cm^-2 and a low Tafel slope of 160 mV dec^-1 with a catalyst loading of 0.76 mg cm^-2. The promising HER performance is attributed to (1) a high density of exposed edges and defects, (2) enhanced charge transport due to high crystallinity, and (3) clean surfaces enabling efficient interfacial electron transfer. Furthermore, operando Raman spectroscopy using a 3D-printed electrochemical cell identifies the catalytically active sites on WS₂ nanosheets for HER. This work provides a green route to high-performance, low-dimensional electrocatalysts for sustainable hydrogen production.