Binary flux-promoted formation of trigonal ZnIn2S4 layered crystals using ZnS-containing industrial waste and their photocatalytic performance for H2 production

Abstract

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.