The accumulation of solid waste due to rapid industrialization has a negative impact on the environment. In this study, ZnS-containing waste from the mining-metallurgy industry was utilized for the synthesis of trigonal ZnIn2S4 layered crystals by a flux method using various binary fluxes: CaCl2:InCl3, SrCl2:InCl3, BaCl2:InCl3, NaCl:InCl3, KCl:InCl3, and CsCl:InCl3. Among the binary fluxes used, KCl:InCl3 was found to be the most favorable for the synthesis of phase-pure trigonal ZnIn2S4 layered crystals. The XRD and SEM results revealed that the flux-grown ZnIn2S4 crystals have a trigonal structure and a morphology composed of large stacked layers. Unexpectedly, the UV-vis diffuse reflectance spectrum exhibited the onset of the absorption edge at approximately 700 nm for trigonal ZnIn2S4 crystals. The photocatalytic activities for H2 production of the Pt-photodeposited ZnIn2S4 samples grown using CaCl2:InCl3, NaCl:InCl3, and KCl:InCl3 fluxes were evaluated. Trigonal ZnIn2S4 crystals grown using the KCl:InCl3 flux in this study exhibited higher photocatalytic activity for H2 evolution (132 μmol h-1) than previously reported hexagonal ZnIn2S4 synthesized by a hydrothermal method due to the decreased defect density and higher crystallinity achieved by the binary flux method. The presence of secondary crystalline phases (ZnS and In2S3) in ZnIn2S4 crystals grown using NaCl:InCl3 and CaCl2:InCl3 fluxes positively impacted the photocatalytic activity and exhibited photocatalytic H2 evolution rates of 188 and 232 μmol h-1, respectively, because of the efficient separation and transfer of photogenerated charge carriers. The decay of the transient absorption of electrons in three samples at 2000 cm-1 was monitored by transient absorption spectroscopy, confirming that the lifetime of free electrons becomes longer depending on the binary flux used: KCl:InCl3 < NaCl:InCl3 < CaCl2:InCl3. The binary flux method applied in this study demonstrates that accumulated solid industrial wastes can be turned into beneficial photocatalytic materials.
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