Composition-Controlled Photocatalytic and Antibacterial Performance of ZnO-ZnS Nanocomposite Catalysts Synthesized by Solid-State Ion Exchange

Zinc oxide (ZnO) and zinc sulfide (ZnS) nanocomposites represent promising multifunctional photocatalysts due to their complementary band structures and synergistic charge separation. ZnO-ZnS nanocomposites with varied ZnS content were synthesized to elucidate the composition-structure-property relationships governing their multifunctional performance. Structural characterization using XRD, SEM/EDS, Raman spectroscopy, and XPS confirmed the coexistence of wurtzite crystalline phases of ZnO and ZnS. SEM analysis revealed ZnS fine deposition on the ZnO surface. XPS measurements showed a gradual increase in the amount of ZnS on the ZnO surface with increasing sulfide content and a shift in the valence band maximum from 2.32 eV (pure ZnO) to 0.77 eV (pure ZnS). Optical measurements (IR, UV-Vis diffuse reflectance, photoluminescence) demonstrated that, despite the evolution of vibrational and luminescence features characteristic of ZnS, the apparent band gap remained nearly constant at 3.16-3.18 eV across the series. Photocatalytic methylene blue (MB) degradation followed pseudo-first-order kinetics, peaking for ZN_2 (1% ZnS, k_app = 103 × 10^-3 min^-1), which is 1.7 times higher than for pure ZnO. This enhanced performance is consistent with an S-scheme-like heterojunction that facilitates electron migration to the ZnS conduction band while retaining ZnO valence band holes for oxidation. Scavenging experiments confirmed that electrons dominate MB degradation (k_app up to 185.1 × 10^-3 min^-1 with EDTA/t-BuOH/Ar), outperforming hole-mediated pathways. Antibacterial assays against <i>Staphylococcus aureus</i> revealed good antimicrobial activity for all nanoparticles. The nanocomposite's antibacterial activity was similar across all samples and was only slightly lower than that of pure ZnS and ZnO.