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–BP nanohybrid 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.