Abstract
Undoped and manganese doped ZnS nanocrystals encapsulated with thioglycolic acid(ZnS-TGA) were synthesized and characterized with different techniques, and finally tested in the photodegradation of a methyl orange in aqueous solution under UV and sunlight irradiations. FTIR and X-ray diffraction results confirmed the functionalization of these nanocrystal surface by thioglycolic acid and the formation of crystalline structures of ZnS and Mn-doped ZnS with cubic and hexagonal phases. Calculated average size of ZnS nanocrystals was in the range of 2-3nm. It was observed a blue shift of the absorbance threshold and the estimated bandgap energies were higher than that of Bulk ZnS thus confirming the quantum confinement effect of charge carriers. Photoluminescence spectra of ZnS nanocrystals exhibited emission in the range of 410-490nm and the appearance of an additional emission band around 580nm (2.13eV) connected to the 4T1 → 6A1 transition of the Mn2+ions. Photodegradation of methylene orange with undoped and Mn-doped ZnS-TGA nanocrystals was investigated. Dye adsorption prior to photocatalysis using nanocrystals was studied via kinetic and equilibrium experiments. The maximum dye adsorption capacity on doped ZnS-TGA was ~ 26.98mg/g. The adsorption kinetic was found to follow the pseudo-second-order kinetic model. A statistical physics model was used to analyze the equilibrium data where the calculated adsorption energy was 17-18kJ/mol. It was concluded that the dye adsorption was associated to the hydrogen interaction where the removal process was feasible and multi-molecular at 25°C. The photocatalytic activity of undoped ZnS nanoparticles under UV irradiation showed better efficiency than doped nanocrystals thus indicating that manganese doping generated a dropping of the photocatalytic degradation of the dye. Dye degradation efficiency of 81.37% using ZnS-TGA nanocrystals was achieved after 6min, which indicated that ZnMnS-TGA nanocrystals may be considered an alternative low cost and environmental friendly material for facing water pollution caused by organic compounds via photodegradation processes.
Highlights
The spectra of ZnS NCs doped with different concentrations of Mn2+ showed the presence of the same absorption bands at wavenumber values close to those recorded with pure ZnS and no other apparent bands to other vibration modes of doping impurities or other elements were observed
Bonds, respectively (Amirian et al 2018). These results clearly demonstrated that NCs were well functionalized by thioglycolic acid (TGA) ligand, which allowed the control of their growth during synthesis and prevented NCs aggregation (Singh and Chauhan 2009)
Nanocrystals of undoped and Mn-doped ZnS were successfully synthesized by colloidal precipitation reaction method and applied in the photodegradation of methylene orange (MO) dye
Summary
Semiconductors nanoparticles (NPs) have found numerous applications in different technological fields going from photovoltaic devices to medical imaging, lasers, LEDs and photocatalysis (Dao 2020; Rtimi et al 2021; Meikle et al 2020; Long et al 2020; Su et al.2020; Li et al 2019; Daskalakis et al 2020; Rtimi et al 2021) These applications are supported by their unique properties as compared to their bulk counterpart, which arise from the confinement of the charge carriers as a result of their small sizes. This makes these NPs lesser stable against aggregation and more reactive with their environment in comparison to bulk materials
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