Hollow NiCo<sub>2</sub>S<sub>4</sub> Nanospheres as a Cocatalyst to Support ZnIn<sub>2</sub>S<sub>4</sub> Nanosheets for Visible-Light-Driven Hydrogen Production

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.