Abstract:

With rapid industrialization, issues pertaining to the environment and energy have become an alarming concern. Photocatalytic water splitting is considered one of the most promising green technologies capable of resolving these issues, as it can convert solar energy into chemical energy and have a positive impact on the realization of "carbon neutrality". Current research focuses on the development of highly efficient catalysts to improve the photocatalytic H₂-production activity. Transition metal phosphides and sulfides are often used as photocatalysts owing to their low H₂-evolution overpotential and excellent electrical conductivity. Among them, Ni₂P and NiS have generally been used independently during photocatalytic H₂ production; however, it is necessary to study the synergistic effect when they are combined as a dual cocatalyst. In this work, we successfully prepared a Ni₂P-NiS dual cocatalyst for the first time via a simple hydrothermal method using red phosphorus (RP) and thioacetamide (C₂H₅NS) as the sources of P and S. Ni₂P-NiS was then introduced to the surface of g-C₃N₄ nanosheets through solvent evaporation to create a Ni₂P-NiS/g-C₃N₄ heterojunction. Furthermore, X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), ultraviolet-visible spectrophotometry (UV-Vis), X-ray photoelectron spectroscopy (XPS), photoluminescence (PL), linear sweep voltammetry (LSV), Mott-Schottky (M-S), and electrochemical impedance spectroscopy (EIS) were used to reveal the crystal structures, morphologies, element compositions, and photoelectric characteristics of the samples; thus, it was demonstrated that Ni₂P-NiS was successfully deposited on the surface of g-C₃N₄ and that together they exhibited better activity than their monomers. Moreover, the optimized 15% Ni₂P-NiS/g-C₃N₄ composite exhibits a H₂ generation rate of 6892.7 μmol·g^-1·h^-1, which is about 46.1, 7.5 and 4.4 times higher than that of g-C₃N₄ (150 μmol·g^-1·h^-1), 15% NiS/g-C₃N₄ (914.5 μmol·g^-1·h^-1), and 15% Ni₂P/g-C₃N₄ (1565.9 μmol·g^-1·h^-1), respectively. In addition, photoelectric performance tests show that Ni₂P-NiS/g-C₃N₄ has a stronger photocurrent intensity, smaller charge-transfer resistance, more positive H₂-evolution overpotential, and better charge-separation ability than the individual components (i.e., Ni₂P and NiS), suggesting that the coexistence of Ni₂P and NiS can further boost the activity of g-C₃N₄ during H₂ evolution compared with their monomers. This is mainly due to the Schottky barrier effect between Ni₂P-NiS nanoparticles and g-C₃N₄ nanosheets, which can greatly promote charge separation and charge transfer at their interface. Additionally, Ni₂P-NiS can reduce the H₂-evolution overpotential, leading to the increased surface kinetics of H₂ evolution. This work offers a promising approach to obtaining a highly active and stable noble-metal-free dual cocatalyst for photocatalytic H₂ production.

Key words: Ni₂P-NiS, g-C₃N₄, Cocatalyst, Schottky barrier, Photocatalytic water splitting