Sponge microspheres of wurtzite-ZnS with a defunctionalized surface of [(ENH)2]2+ and CO32- species for the enhanced hydrogen evolution rate.

ZnS materials in two distinct phases, sphalerite and wurtzite, are typically tested for their H2 evolution rates, but their photocatalytic efficiencies remain controversial because of differences in optoelectronic properties, morphology, and surface functionalization. These properties depend heavily on multiple synthesis parameters, and a defunctionalized surface should also be developed. This study presents the targeted synthesis of zinc sulfide (ZnS) in two distinct phases and morphologies-irregular sphalerite and sponge-like wurtzite microspheres-utilizing a synthetic and commercial hydrozincite ((Zn5CO3)2(OH)6) precursor. The functionalization with ENH+, OH- and CO32- species was controlled by tuning the ethylenediamine (ENH+) concentration in binary water-alcohol (butanol or ethanol) solvent mixtures. Comprehensive effects of the ENH+/Zn2+ and alcohol/water ratios on the physicochemical properties of ZnS materials, studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and FTIR analysis, revealed that the choice of both alcohol-solvent and hydrozincite precursor profoundly influences surface chemical composition, specifically, increasing the retention of ENH+, OH- and CO32- groups at high ENH+/Zn2+ ratios. High dispersibility of the functionalized ZnS powders, optimized in a water-ethanol mixture, was achieved with an ideal transport coefficient for the photocatalytic hydrogen evolution test. The sponge-like wurtzite microspheres achieved a high intrinsic H2 evolution rate (HER) of 10 mmol h-1 g-1 under UV-C irradiation, outperforming a commercial hydrozincite-derived control. Crucially, high photoactivity was also achieved under simulated sunlight (visible light of 315-500 nm) with an HER value of 8.4 mmol h-1 g-1, being 1.8 times more active than irregular sphalerite-ZnS (4.7 mmol h-1 g-1). Ex situ defunctionalization of the best wurtzite-ZnS photocatalyst substantially changes the surface chemical composition by removing the ENH+ and CO32- species, without affecting the active sites, and maintains the HER value with excellent stability over six cycles (36 h). The photoactivity is well correlated with the photocurrent density (9-19 μA cm-2) and small charge transfer resistance. Ultimately, mechanistic models for both the in situ functionalization and ex situ defunctionalization on the surface of ZnS materials are proposed, establishing a robust framework for designing highly efficient, visible-light-responsive semiconductor photocatalysts.