Synergistic effect of adsorption and photo-catalysis in removal of various textile dyes: Excellent efficacy of molybdenum disulfide-zinc oxide hybrids

Abstract

The present work reports the development of molybdenum disulfide (MoS₂) wrapped zinc oxide nano-rods through an in-situ hydrothermal approach at a moderate temperature of 200 °C. The as-developed samples were characterized by different sophisticate techniques like X-ray diffraction (XRD), Field emission scanning electron microscopes (FESEM), Fourier Transformed Infrared (FTIR), UV-Visible (UV-Vis) and photoluminescence (PL) spectroscopy. The composition analysis was done with the help of energy-dispersive x-ray (EDX) and stability of the sample against elevated temperature was estimated with the help of thermo gravimetric as well as differential thermal analysis (TG-DTA). XRD shows that the crystallinity of MoS₂ was not very high whereas pure ZnO shows well-crystalline features. FESEM micrographs confirmed growth of fractal nature of MoS₂ over ZnO rods. FTIR showed the different bond present in the material whereas UV-Vis spectra aided to have ideas about the band gap of all the samples. PL study shows that ZnO gives the most intense luminescence signal carrying the signature of both band to band and defect induced transition. In case of hybrid sample the emission wavelength gets shifted from dark blue to cyan region. It has been shown that the MoS₂ has excellent efficacy in removing different textile dyes like Bengal rose, malachite green, methyl orange or rhodamine B and thus when it is combined with ZnO the removal efficiency of the ZnO increases significantly. It has also been shown that for pure MoS₂ adsorption is the key mechanism of the dye removal whereas for pure ZnO photo-catalysis is the major contribution for the removal. In case of the hybrid sample, thus synergistic effect of the two processes gives the best result and in most of the cases over 90 % removal was achieved within 15 minutes only. Efforts have been given to explain the result in terms of dye structure and its interaction, surface area as optical band gap of the sample.