  Among transition metal dichalcogenides (TMdCs) as alternatives for Pt-based
catalysts, metallic-TMdCs catalysts have highly reactive basal-plane but are
unstable. Meanwhile, chemically stable semiconducting-TMdCs show limiting
catalytic activity due to their inactive basal-plane. Here, we propose metallic
vanadium sulfide (VSn) nanodispersed in a semiconducting MoS2 film (V-MoS2) as
an efficient catalyst. During synthesis, vanadium atoms are substituted into
hexagonal monolayer MoS2 to form randomly distributed VSn units. The V-MoS2
film on a Cu electrode exhibits Pt-scalable catalytic performance; current
density of 1000 mA cm-2 at 0.6 V, overpotential of -0.06 V at a current density
of 10 mA cm-2 and exchange current density of 0.65 mA cm-2 at 0 V with
excellent cycle stability for hydrogen-evolution-reaction (HER). The high
intrinsic HER performance of V-MoS2 is explained by the efficient electron
transfer from the Cu electrode to chalcogen vacancies near vanadium sites with
optimal Gibbs free energy (-0.02 eV). This study adds insight into ways to
engineer TMdCs at the atomic-level to boost intrinsic catalytic activity for
hydrogen evolution.
