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

Abstract

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 CO₃^2- 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 CO₃^2- 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 CO₃^2- 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.