NiS-Decorated ZnO/ZnS Nanorod Heterostructures for Enhanced Photocatalytic Hydrogen Production: Insight into the Role of NiS

Loading cocatalysts can effectively enhance the surface hydrogen reduction in photocatalytic water splitting by introducing a positive Schottky barrier. NiS is regarded as a promising cocatalyst to replace the noble metals due to its low cost and equivalent or even better performance. However, there is a huge controversy over whether the NiS cocatalyst is used to trap electrons or holes in the photocatalytic process. Herein, a new type of NiS-decorated ZnO/ZnS (ZnOS) nanorod heterostructure photocatalysts is first designed from the corresponding bimetallic organic frameworks (ZnNi-MOFs). The Zn species in the bimetallic-MOFs can spatially separate the Ni species to restrain their aggregation, which is beneficial for the formation of NiS with a small enough size. The optimal heterostructure photocatalysts exhibit an excellent hydrogen production rate of 27 mmol g(-1) h(-1), which is about seven times higher than that of the ZnOS heterostructure. X-ray photoelectron spectroscopy and open-circuit potential characterizations disclose that NiS can effectively facilitate the migration of the electrons. Density functional theory calculations, including differential charge density, Mulliken population analyses, and d-band center, intuitively reveal that the real role of NiS in the photocatalytic process is to capture the electrons rather than the holes.