Interfacial chemistry and electroactivity of black phosphorus decorated with transition metals

Black phosphorus (BP) exhibits a significant chemical reactivity toward transition metals, which is strongly dependent on its electronic and morphological features. Using as a case study metal Co atoms deposited by physical vapor deposition on chemically exfoliated BP thin films, we document that environmentally oxidized BP edges react immediately with the formation of Co(II)-POx species, but later also the basal plane strongly reacts, already at room temperature, to form an interfacial phosphide. Atomically resolved scanning tunneling microscopy data reveal the formation of a Co2P(112) layer, upon which the further growth of metal Co nanoparticles takes place. X-ray photoemission spectroscopy data show similar interfacial reactions also for atomic W, suggesting a quite general behavior. The electrochemical and photoelectrocatalytic properties of the resulting metal-BP nanohybrids are investigated by linear sweep voltammetry and electrochemical impedance spectroscopy. The Co-BP nanohybrid shows a marked stability in acid media and a significant increase of the electrocatalytic activity for the hydrogen evolution reaction with respect to bare BP, due to the formation of a very stable phosphide phase, and even an interesting photoactivity connected to the formation of p-n junction between BP and Co2P. On the other hand, the W-BPnanohybrid exhibits a modest performance in the electrocatalytic production of hydrogen because of the formation of less active oxide phases as a consequence of environmental oxidation.