Fast sonochemically-assisted synthesis of pure and doped zinc sulfide quantum dots and their applicability in organic dye removal from aqueous media.

In this research, a sono-assisted chemical precipitation method was developed for a quick and efficient water-based synthesis of zinc sulfide (ZnS) quantum dots (QDs), in room temperature. ZnS QDs as pure and doped with some transition metal ions (such as Cu2+, Mn2+, Ag+) were prepared in the presence of l-cysteine, as capping agent. In comparison with the previous conventional methods, applying the ultrasonic irradiation provides a significant decrease in the reaction time. In comparison with previous reported conventional chemical QD synthesis methods, the required time for a complete synthesis in the presence of ultrasonic irradiations was significantly reduced to a period as short as 15 min, in a temperature of 40 °C. The sono-synthesized QDs were characterized with various techniques such as colorimetry, UV-Vis absorption spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM). A broad absorption band between 270 and 290 nm was observed for colloidal ZnS QDs with the optical band gap larger than 3.68 eV. From the XRD and TEM results, the average particle size below 5 nm was estimated for the prepared QDs. After characterization, the samples were employed as nanophotocatalysts during a UV-derivate photocatalytic process for the degradation of Victoria blue R (VBR), as a pollutant dye. To obtain the optimal operating conditions, the influence of the affecting parameters such as the initial pH of the dye solution, irradiation time, initial dye concentration, type, electrolyte salt concentration and dosage of the nanophotocatalyst samples on the degradation efficiency (DE % was comprehensively studied, all in batch mode. Taking the optimum conditions into account, the maximum removal of VBR dye (>95%) can be obtained by spectrophotometric measurements at alkaline pH of 10.5, in the presence of low amounts of QDs (3 mg) and after 30 min of being exposed to UV irradiations. The results demonstrate that despite the removal of VBR, the QDs prepared by sonochemically method can be reused for at least six more times; without a significant decrease in their photocatalytic properties.