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 compositioncontrollable 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.