Surface-Charge-Controlled Synthesis of ZnIn2S4 Nanosheet-Based Materials for Selective Adsorption of Organic Dyes

In recent years, controlling and manipulating the materials' surface charges have gained significant attention because of efficient application in various fields such as catalysis, dye adsorption, chromatography, etc. In this study, we have controlled the surface charge of ZnIn2S4 nanomaterials (NMs) by loading Zn2+ ions on the surface of materials. The zeta-potential studies revealed that the average surface charge of the NMs was successfully controlled from -30.3 to +31.4 mV. The X-ray diffraction patterns suggested that the phase of the NMs is changed from hexagonal ZnIn2S4 to cubic ZnIn2S4 with slight trigonal ZnS moieties. Further, the electron microscopy results displayed that the morphology of the NMs gradually changed from a sheetlike structure to a hybrid of sphere/petals and finally a very small flakelike structure with an increasing loading concentration of Zn2+ ions. Moreover, the nanosheet micrograph showed the different numbers of sheet layers, which are composed of 2-6 layers. The Brunauer-Emmett-Teller analysis indicated that the specific surface area is increased from 48.2 to 118.6 m(2) g(-1) after the loading of Zn2+ ions. The obtained NMs displayed a high adsorption selectivity performance for organic dyes. A separation factor (alpha(1)(2)) of 95% was obtained for anionic Congo red (CR) over positively charged nanoflakes and 85% for cationic methylene blue (MB) dyes on negatively charged nanosheet. The as-prepared nanosheet and nanoflakes also showed efficient adsorption behavior toward two different kinds of antibiotics such as tetracycline and ciprofloxacin. The adsorption process followed the Langmuir isotherm model for the MB, CR, and ciprofloxacin, whereas tetracycline adsorption obeyed the Freundlich isotherm model. The adsorbents displayed a pseudo-second-order rate kinetic model. The recyclability of the best adsorbents was found up to five times without significant loss of removal efficiency.