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

Abstract

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 μmolꞏh⁻¹ꞏg⁻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 cocatalyst 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 (E_f) 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 (ΔG_H*) 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.

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