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 ZnIn 2 S 4 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/ZnIn 2 S 4 heterostructures feature intimate contact interfaces that promote charge transfer, facilitating their use as efficient photocatalysts for hydrogen evolution. The 38Ni-CNT/ZnIn 2 S 4 sample exhibits a high H 2 production rate (12267 μmolꞏh −1 ꞏg −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 ZnIn 2 S 4 . The results of X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) corroborate the observations on Ni-CNT/ZnIn 2 S 4 heterostructures . Electrochemical measurements reveal that the combination of the Ni-CNT and ZnIn 2 S 4 facilitates the transfer of photogenerated electrons and effectively prevents rapid recombination of photocarriers, thus improving the hydrogen evolution performance of ZnIn 2 S 4 . Electron spin resonance (ESR) results further prove that cocatalyst Ni-CNTs are beneficial for prolonging the lifetimes of ZnIn 2 S 4 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 ZnIn 2 S 4 bends downward, which is favorable for photogenerated electron flow from ZnIn 2 S 4 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 ZnIn 2 S 4 nanosheets , indicating the formation of a photogenerated electron transfer pathway of ZnIn 2 S 4 → CNT → Ni. Furthermore, Gibbs free energy of H* adsorption (Δ G H* ) and crystal orbital Hamilton population (COHP) analysis indicate that Ni nanoparticles can serve as active sites, promoting H 2 evolution. Thus, the present study formulates a new strategy for developing low-cost, high-efficiency, non-noble-metal co-catalysts for photocatalytic hydrogen production . Download: Download high-res image (88KB) Download: Download full-size image

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