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

Abstract

• A new phosphidation route was developed for the synthesis of NiP x . • Including Ni, Ni 12 P 5 , Ni 2 P/Ni 12 P 5 , Ni/Ni 2 P and Ni 2 P were selectively synthesized. • Enhanced photocatalytic HER performance of g-C3N4 by decoration with NiP x . • Co-catalytic efficiency of NiP x is mainly determined by P content. • High separation of e − and h + and low over-potential of NiP x account for the enhanced HER performance. Ni-based phosphides (NiP x ) composed of earth-abundant elements are promising cocatalysts to replace noble metals for photocatalytic H 2 evolution reaction (HER). A safe, energy-saving, and composition-controllable synthesis of NiP x is still highly desired. A facile and mild solvothermal process was developed for the first time for selective synthesis of a series of NiP x , including Ni, Ni 12 P 5 , Ni 2 P/Ni 12 P 5 , Ni/Ni 2 P and Ni 2 P, through controlling the dosage of NaBH 4 and NaH 2 PO 2 . The phosphidation process was mainly composed of (1) a sequential reduction of Ni 2+ to Ni 0 and (H 2 PO 2 ) − to P (around the formed Ni 0 ) triggered by NaBH 4 , and (2) a final phosphidation between Ni 0 and the in situ generated P atoms. The photocatalytic HER performance of g-C 3 N 4 can be substantially improved with the decoration of NiP x (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 NiP x is mainly determined by P content. Ni 2 P 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-C 3 N 4 (35.6 µmol g −1 h −1 ). Thus, as for the cocatalyst based on Ni phosphides, Ni 2 P is the preferable crystal phase and more efforts should be devoted to Ni 2 P to further optimize its structure, texture, and morphology in future works. Control synthesis of NiP x through adjusting the dosage amount of NaBH 4 and NaH 2 PO 2 was achieved by a new phosphidation route. Ni 2 P with a high ratio of P exhibits the highest photocatalytic HER performance.