Abstract:

The rational interface tailoring of nanosheets on hollow spheres is a promising strategy to develop efficient photocatalysts for hydrogen production with solar energy. Among the various photocatalyst materials, metal sulfides have been extensively researched because of their relatively narrow band gap and superior visible-light response. ZnIn₂S₄ is a layered ternary chalcogenide semiconductor photocatalyst with a tunable band gap energy (approximately 2.4 eV). Among various metal sulfide photocatalysts, ZnIn₂S₄ has gained considerable attention. However, intrinsic ZnIn₂S₄ only exhibits a relatively moderate photocatalytic activity, which is mainly owing to the high recombination and low migration rate of photocarriers. Loading cocatalysts onto semiconductor photocatalysts is an effective way to improve the performance of photocatalysts, because it can not only facilitate the separation of electron-hole pairs, but also reduce the activation energy for proton reduction. As a ternary transition metal sulfide, NiCo₂S₄ features a high electrical conductivity, low electronegativity, excellent redox properties, and outstanding electrocatalytic activity. Such favorable characteristics suggest that NiCo₂S₄ can expedite charge separation and transfer, thereby promoting photocatalytic H₂ production by serving as a cocatalyst. Moreover, both NiCo₂S₄ and ZnIn₂S₄ possess the ternary spinel crystal structure, which may facilitate the construction of NiCo₂S₄/ZnIn₂S₄ hybrids with tight interfacial contact for an enhanced photocatalytic performance. Herein, ultrathin ZnIn₂S₄ nanosheets were grown in situ on a non-noble-metal cocatalyst, namely NiCo₂S₄ hollow spheres, to form hierarchical NiCo₂S₄@ZnIn₂S₄ hollow heterostructured photocatalysts with an intimately coupled interface and strong visible light absorption extending to ca. 583 nm. The optimized NiCo₂S₄@ZnIn₂S₄ hybrid with a NiCo₂S₄ content of ca. 3.1% exhibited a high hydrogen evolution rate of 78 μmol·h^-1, which was approximately 9 times higher than that of bare ZnIn₂S₄ and 3 times higher than that of 1% (w, mass fraction) Pt/ZnIn₂S₄. Additionally, the hybrid photocatalysts displayed good stability in the reaction. Photoluminescence and electrochemical analysis results indicated that NiCo₂S₄ hollow spheres served as an efficient cocatalyst for facilitating the separation and transport of light-induced charge carriers as well as reducing the hydrogen evolution reaction barrier. Finally, a possible reaction mechanism for the photocatalytic hydrogen evolution was proposed. In the NiCo₂S₄@ZnIn₂S₄ composite photocatalyst, the NiCo₂S₄ cocatalyst with high electrical conductivity favorably accepts the photoinduced electrons transferred from ZnIn₂S₄ and then employs the electrons to reduce protons for H₂ production on the reactive sites. Concurrently, the photogenerated holes are trapped by TEOA that acts as a hole scavenger to accomplish the photoredox cycle. This study provides guidance for the fabrication of hierarchical hollow heterostructures based on nanosheet semiconductor subunits as remarkable photocatalysts for hydrogen production.

Key words: Photocatalysis, H₂ production, Cocatalyst, Metal sulfides