Identification of Charge Transfer Pathways in Metal-Organic Framework- Derived Ni-CNT/ZnIn2S4 Heterojunctions for Photocatalytic Hydrogen Evolution

Hydrogen is an important zero-pollution green energy source with potential for alleviating environmental contamination and energy shortages. Hydrogen evolution via solar-energy-induced semiconducting water splitting is among the most environmentally friendly methods available to date. In this study, a metal-organic-framework-derived, Ni-decorated carbon nanotube (Ni-CNT) is used as a non-noble co-catalyst. This Ni-CNT is grown in situ on ZnIn2S4 nanosheets using a simple one-step oil bath strategy, wherein Ni nanoparticles are wrapped around the top and cross sections of the nanotubes, preventing their agglomeration. Notably, Ni-CNT/ZnIn2S4 heterostructures feature intimate contact interfaces that promote charge transfer, facilitating their use as efficient photocatalysts for hydrogen evolution. The 38Ni-CNT/ZnIn2S4 sample exhibits a high H2 production rate (12267 mu mol center dot h-1 center dot g-1), with an apparent quantum efficiency (AQE) of 11.3% under 420 nm monochromatic light irradiation, which is nearly 6.4 times that of pure ZnIn2S4. The results of X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) corroborate the observations on Ni-CNT/ZnIn2S4 heterostructures. Electrochemical measurements reveal that the combination of the Ni-CNT and ZnIn2S4 facilitates the transfer of photogenerated electrons and effectively prevents rapid recombination of photocarriers, thus improving the hydrogen evolution performance of ZnIn2S4. Electron spin resonance (ESR) results further prove that co catalyst Ni-CNTs are beneficial for prolonging the lifetimes of ZnIn2S4 photogenerated electrons, thereby achieving effective charge separation. A charge transfer pathway in the heterojunction interfaces is further explored and confirmed by density functional theory (DFT) calculations. The difference in the Fermi level energy (Ef) contributes to both charge migration and the generation of a built-in electronic field (BEF), indicating that the energy band of ZnIn2S4 bends downward, which is favorable for photogenerated electron flow from ZnIn2S4 to the Ni-CNT electron acceptor. The results of planar averaged electron density difference analysis confirm that the hot electrons are transferred from Ni nanoparticles to the CNT and then to the ZnIn2S4 nanosheets, indicating the formation of a photogenerated electron transfer pathway of ZnIn2S4 -> CNT -> Ni. Furthermore, Gibbs free energy of H* adsorption (Delta GH*) and crystal orbital Hamilton population (COHP) analysis indicate that Ni nanoparticles can serve as active sites, promoting H2 evolution. Thus, the present study formulates a new strategy for developing low-cost, high-efficiency, non-noble-metal co-catalysts for photocatalytic hydrogen production.