2D black phosphorus as a phosphide source for the formation of mixed cobalt phosphide clusters active in photocatalytic hydrogen evolution

The use of solar energy to produce valuable feedstocks and energy vectors, such as hydrogen, is a fascinating research field of utmost practical importance. Herein, we report a facile strategy to functionalize 2D black phosphorus nanosheets (BPNS) with mixed cobalt phosphide clusters of different stoichiometry, Co₂P and CoP, with sub-nanometric sizes. The in-depth study at the atomic level by X-ray absorption spectroscopy and electron microscopy confirmed the nature and structure of the phosphides and revealed that Co₂P/CoP functionalization effectively hindered the spontaneous degradation of BP under ambient conditions. Under simulated solar-light irradiation, the hydrogen evolution rate of BP@Co_xP integrated with TiO₂ reached 5.3 mmol g^-1 h^-1, almost 30-fold higher than that of TiO₂. Electrochemical and photoluminescence investigations elucidated the electronic properties of the photocatalyst, showing that the mixed Co_xP phases led to increased charge density across the heterostructure, a lower conduction band energy, which infers more reducing power, and the generation of multiple interfaces, which increase charge transfer and reduce electron-hole recombination.