Synthesis of cuinse2/zns quantum dots and investigation of dye adsorption properties for waste treatment

Abstract

In recent years, rapidly increasing industrial growth, specifically in the paper and textile sectors, has led to a significant environmental threat to water resources. In addition, organic dyes released as a result of these industrial activities and frequently found in water sources carry a serious risk to both the environment and human health. Research on the removal of organic dyes is gaining importance due to their harmful effects on human health, such as the risk of cancer as well as their toxicity and potential to remain in the environment for long periods. Compared to various traditional methods for removing the dyes, the adsorption method emerges as a promising approach in terms of efficiency, as it is both more cost-effective and offers easy application. In this thesis, the potential of quantum dots for adsorption studies against organic dyes was investigated. Quantum dots, one of the most recent developments in the field of nanotechnology, are widely used in many areas due to their outstanding optical and photophysical properties. In this study, their dye removal performance was examined through the adsorption studies conducted and the underlying mechanism, particularly due to their surface advantage. In the experimental part, CuInSe2/ZnS core/shell quantum dots which containing no heavy metals such as cadmium and lead were synthesized and capped with three different ligands: L-glutathione, N-acetyl-L-cysteine, and 3-mercaptopropionic acid. The synthesis of core/shell CuInSe2/ZnS quantum dots was carried out in one pot, in aqueous medium, under reflux and nitrogen gas for a total reaction time of 1.5 hours. In this process, syntheses were also conducted by doubling the amounts of copper nitrate hemi-pentahydrate as copper source and indium acetate as indium source, while keeping all other parameters constant, to examine the effect of metal stoichiometry on the structure and optical properties. In the second stage, the structural properties of the synthesized quantum dots were characterized using XRD, FTIR, and Raman spectroscopy. The XRD spectrum confirmed the crystal structure, and the analysis of the structure verified the formation of a core/shell structure and confirmed that the material is quantum dot. A common finding observed in the FTIR spectra of all synthesized quantum dots is the disappearance of thiol peaks typically seen around 2500 cm-1 on all ligand surfaces. This result indicates that all three selected ligands have successfully bonded to the quantum dot via sulfur atoms. To characterize their optical properties, the solubility of the quantum dots was also investigated in a total of seven pH solutions ranging from pH 5 to pH 11 using photoluminescence and UV spectroscopy. Since no conclusive results were obtained regarding the optimal pH at which they dissolve, they were considered unsuitable for use in pH sensor applications. However, it was clearly observed that the L-glutathione capped CuInSe2/ZnS QDs dissolved at pH 11. The investigation of such pH-dependent solubility provides valuable insight for adsorption studies with dyes. Following the characterization of structural and optical properties, adsorption studies were conducted using three different dyes. These dyes were methylene blue, rhodamine B and methyl orange. After these three dyes were interacted with quantum dots, the adsorption mechanism was investigated using UV spectroscopy and FTIR methods. According to the UV results, methylene blue exhibits the best interaction with the quantum dots, and the highest removal efficiency of 99.1% was observed with N-acetyl-L-cysteine capped Cu(x2)InSe2/ZnS QDs. Although the primary adsorption mechanism here is explained by electrostatic interactions arising from the positive and negative charges on the surfaces of the dye and quantum dots, FTIR spectra of the adsorption products suggest that hydrogen bonding and hydrophobic interactions also play a critical role. Another important factor is the type of ligand that is used in the synthesis of QDs. For instance, since Lglutathione is a larger ligand with greater steric hindrance compared to others, L-GSH capped QDs' dye adsorption performance is lower. Desorption experiments were conducted to investigate the potential for recovering the adsorbed dye. During this study, two different approaches were employed: a solvent based approach and a thermal based approach. In the solvent based approach, methanol was added to the quantum dots that had interacted with the dyes and analyzed via UV spectroscopy. In the thermal based approach, distilled water was added to the quantum dots adsorbing the dye, followed by a shaking in a water bath at 50°C. According to the results obtained from UV spectroscopy after the experiments, no dye recovery was observed following thermal treatment, whereas the experiments conducted with methanol, characteristic spectrum of the pristine dyes were observed, particularly for the quantum dots that had interacted with rhodamine B and methylene blue. In summary, the potential of QDs developed for environmental applications was thoroughly investigated in adsorption and desorption processes. Analytical results and characterization results showed that the QDs were capable of adsorbing different dye molecules, showing the highest adsorption efficiency toward methylene blue.