Electron directed migration cooperated with thermodynamic regulation over bimetallic NiFeP/g-C3N4 for enhanced photocatalytic hydrogen evolution

Bimetallic phosphides have attracted considerable attention in photocatalytic hydrogen evolution reaction (HER), owing to its thermodynamic feasibility. Most reports attributed the HER activity to the reduction of free energy of protonation (Delta G(H*)), however, few revealed the whereabouts of the photo-induced electrons, which makes the mechanism of HER not very clear. Herein, nickel-iron bimetallic phosphide (NiFeP) is designed onto graphitic carbon nitride (NiFeP/g-C3N4) for the photocatalytic HER, which exhibits a high hydrogen production of 3.549 mmol g(-1) h(-1). It is found that the co-catalyst of NiFeP can not only reduce the Delta G(H*), but also make the photogenerated electrons transfer directionally, which play a synergistic role in enhancing the activity of HER. The reduction of Delta G(H2O) and Delta G(H*) over NiFeP in HER can be well demonstrated by the density functional theory (DFT) calculations. Furthermore, the localized distribution of electrons on the surface of NiFeP is verified by measuring the lifetime of photo-generated carriers combined with the REDOX experiment. The shortened lifetime tau, decreased tau 1 and tau 2 of NiFeP/g-C3N4 indicate that the photoexcited electrons of g-C3N4 are directly transferred to the co-catalyst surface, contributing to the worse delocalization capacity of electrons on the cacatalyst and its easier reactions with protons trapped on the co-catalyst surface. Furthermore, the degradation with peroxymonosulfate (PMS) confirms the photogenerated holes over 5NiFeP/g-C3N4 are mainly concentrated on the surface of g-C3N4 to decompose H2O for the generation of (OH)-O-center dot, while the photogenerated electrons are directed to the NiFeP to react with the neighboring PMS or protons to form SO4 center dot- or H-2. Besides, the density of state intensity (DOS) also confirms the photogenerated electron migration pathway at the metal-semiconductor (NiFeP-g-C3N4) interface. Our research clarifies the mechanism of bimetallic phosphides co-catalytic HER system, which will provide theoretical guidance for the design and preparation of multi-metallic center photocatalytic system with a higher HER activity in the future.