Ultrathin molybdenum phosphide films as high-efficiency electrocatalysts for hydrogen evolution reaction

Nanostructured molybdenum phosphide (MoP) has been considered as a kind of high-efficient electrochemical catalysts, which has a huge potential to replace noble metal Pt-based electrocatalysts in hydrogen evolution reaction (HER). Accordingly, ultrathin two-dimensional (2D) metal phosphides are expected to have higher HER activities owing to being supposed to have both maximizing surface active site densities and fast electron transport. However, the synthesis of ultrathin MoP films has experimentally not been still realized. Here we report the controllable synthesis of the ultrathin MoP film via direct phosphorization of pre-annealed molybdenum foil surfaces at elevated temperature. Especially, the ultrathin MoP film of 4.87 nm in thickness can be grown by reacting the Mo foil surfaces with a phosphorus vapour at 900 degrees C within 30 min. Furthermore, the nanofilm shows a superior electrochemical stability in acidic solution and has a small Tafel slope of 56 mV/dec which is smaller than that of the corresponding bulk. The metallicity of the MoP nanofilm predicted by the first-principles calculations, can be partly responsible for the robust electrocatalytic HER performance, which facilitates fast electron transport along the electrocatalytic active edges of the nanofilm for improving the HER activity.