A new phosphidation route for the synthesis of NiP and their cocatalytic performances for photocatalytic hydrogen evolution over g-C3N4

Ni-based phosphides (NiPx) composed of earth-abundant elements are promising cocatalysts to replace noble metals for photocatalytic H2 evolution reaction (HER). A safe, energy-saving, and composition-controllable synthesis of NiPx is still highly desired. A facile and mild solvothermal process was developed for the first time for selective synthesis of a series of NiPx, including Ni, Ni12P5, Ni2P/Ni12P5, Ni/Ni2P and Ni2P, through controlling the dosage of NaBH4 and NaH2PO2. The phosphidation process was mainly composed of (1) a sequential reduction of Ni2+ to Ni0 and (H2PO2)− to P (around the formed Ni0) triggered by NaBH4, and (2) a final phosphidation between Ni0 and the in situ generated P atoms. The photocatalytic HER performance of g-C3N4 can be substantially improved with the decoration of NiPx (3 wt%) as the separation of photoinduced charge carriers can be promoted and some active sites with low over-potential for HER can be introduced. The cocatalytic efficiency of NiPx is mainly determined by P content. Ni2P with a high ratio of P consequently exhibits the highest HER performance (215.1 µmol g−1 h−1), which is almost six times higher than that of the pristine g-C3N4 (35.6 µmol g−1 h−1). Thus, as for the cocatalyst based on Ni phosphides, Ni2P is the preferable crystal phase and more efforts should be devoted to Ni2P to further optimize its structure, texture, and morphology in future works.