Surface atomic, compositional, and electronic structures play decisive roles in governing the performance of catalysts during electrochemical reactions. Nevertheless, for efficient and cheap transition-metal phosphides used for water splitting, such atomic-scale structural information is largely missing. Despite much effort being made so far, there is still a long way to go for establishing a precise structure-activity relationship. Here, in combination with electron-beam bombardment and compositional analysis, our atomic-scale transmission electron microscopy study on Ni5P4 nanosheets, with a preferential (001) orientation, directly reveals the coverage of a self-epitaxial Ni2P nanolayer on the phosphide surface. Apart from the presence of nickel vacancies in the Ni5P4 phase, our quantum-mechanical image simulations also suggest the existence of an additional NiPx (0 < x < 0.5) nanolayer, characteristic of complex surface atom reconstruction, on the outermost surface of the phosphides. The surface chemical gradient and the core-shell scenario, probably responsible for the passivated catalytic activity, provide a novel insight to understand the catalytic performance of transition-metal catalysts used for electrochemical energy conversion.
Keywords: core−shell; electron-beam bombardment; nickel phosphide; surface reconstruction; transmission electron microscopy; water splitting.