Layer-controllable WS₂-reduced graphene oxide hybrid nanosheets with high electrocatalytic activity for hydrogen evolution

In this study, an efficient poly(vinylpyrrolidone) (PVP)-assisted hydrothermal method for the in situ growth of WS₂ nanosheets with layer-controllability on reduced graphene oxide (rGO) is reported. The number of layers (from a monolayer to ∼25 layers) of the exfoliated WS₂ can be accurately controlled by adjusting the amount of PVP. The layer structure and the morphology of the as-prepared hybrids are confirmed by field emission scanning electron microscopy and high-resolution transmission microscopy. The X-ray diffraction, Raman, and X-ray photoemission spectroscopy of the obtained WS₂-rGO hybrid nanosheets indicate highly crystallized structures, a clear Raman shift and a stoichiometry, which is dependent on the number of layers. Furthermore, these highly active and durable catalysts exhibit an electrocatalytic current density of 10 mA cm^-2 at a small hydrogen evolution reaction (HER) overpotential (-170 mV) and a Tafel slope of 52 mV dec^-1 with an excellent electrocatalytic stability (after 6 months storage).