Ph. D., Faculty of Science, University of the Witwatersrand, 2011 / Different cadmium, cobalt and zinc complexes of substituted thioureas, dithiocarbamates and thiuram di/monosulfides were synthesized using ethanol or water as solvents. The synthesis of dithiocarbamates complexes were performed at room temperature while the rest were refluxed at 70 oC. The complexes were easy to synthesize, of low cost and stable in air and were obtained in good yields. The complexes were characterized using various instruments, such as infrared (FT-IR) and proton nuclear magnetic resonance (1H NMR) spectroscopy, elemental analyzer, thermogravimetric analysis (TGA) and X-ray crystallography. The complexes were found to coordinate the ligands through sulphur atom, instead of nitrogen atom. This was concluded after shifts to higher or lower wavenumbers were observed from the infrared spectra of the complexes as compared to their free ligands. The 1H NMR also depicted formation of the complexes, with complexes peaks shifting to downfield as compared to the free ligands. There were also signs of broad NH peaks especially for substituted thiourea complexes. The crystals grown from complex II (diphenylthiourea cadmium complex) depicted a tetrahedral geometry, with two sulphur and two chlorine atoms binding to the central atom which is cadmium. The easily synthesized complexes were thermolysed in HDA, TOPO or a mixture of the two to form metal sulphide nanoparticles. The role of the above capping agents or ligands was to control particles growth and prevent them from aggregation. A single source precursor route was employed in synthesizing hydrophobic semiconductor nanoparticles, which are also known as (QDs) quantum dots. Various shapes, which are rods (mono-, bi- and tripods), spheres and hexagonal were revealed through transmission electron microscope (TEM). The sizes of these particles ranged from 1 to 12 nm in diameter. Other instruments used for characterising the as-synthesized semiconductor nanoparticles include X-ray diffractometer (XRD), UV-Visible and Photoluminescence spectroscopy. The optical properties of the particles as determined by the UV-Visible spectroscopy revealed some differences as compared to the bulk materials. All the absorption spectra were blue shifted to the bulk materials signifying finite size of the particles. The XRD peaks observed were broad as compared to the bulk ones, which also signified small particles size. Two phases, which are hexagonal and cubic, were revealed from the XRD.
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The hydrophobic semiconductor nanoparticles or quantum dots synthesized were then transferred into water soluble using ligand exchange method. The chloroform and pyridine routes were used to synthesize hydrophilic semiconductor nanoparticles, with pyridine route being preferred. The shape and size of the particles were not influenced by the transfer into water soluble since the experiments were performed at room temperature. This was confirmed by TEM analysis. The capping agents used after displacing water insoluble capping agents were sugars, which were soluble in water. The XRD pattern of the semiconductor nanoparticles/QDs (CdS) capped by sugars after ligand exchange through pyridine yielded multiple peaks which were difficult to assign. The attempt to employ ligand exchange method in transferring hydrophobic CoxSy and ZnS nanoparticles to hydrophilic CoxSy and ZnS nanoparticles proved unsuccessful. When the materials were centrifuged after the sugars were introduced as capping agents, some solid material settled at the bottom, with some floating on top of the solution. This was an indication that the materials were not miscible.
The hydrophilic CdS, CoxSy and ZnS nanoparticles were also synthesized using direct method. In this method, the metal sources and capping (sugars) were dissolved in ethylene glycol at 100 oC. The sulphur sources were also dissolved separately in the same solvent. Upon completion, the latter solution was added to the former one. The particles were grown at 160 oC for an hour with ethylene glycol as a solvent. The morphology of the particles dominated through this method was spherical-like in shape. The crystallinity of CdS and ZnS nanoparticles depicted hexagonal and cubic phases depending on the complexes used. The XRD indicated the armophous nature of the cobalt sulphide nanoparticles, irrespective of the precursor used.
Due to the toxicity problem of the quantum dots, especially CdS, the water soluble CdS capped by glucuronic acid, glucose and sucrose after ligand exchange were chosen for that study. However, results showed that the CdS used were not toxic. It was measured or deduced by checking the viability which remained above 90%. Add a bit of deductions about toxicity study here, just some of the general trends.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:wits/oai:wiredspace.wits.ac.za:10539/10411 |
| Date | 14 September 2011 |
| Creators | Shumbula, Poslet Morgan |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Format | application/pdf |
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