3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Application of ionic liquids in the leaching and extraction of precious metal from electronic waste(2018) Masilela, Mthobisi DelisaElectronic waste (e-waste) contains valuable materials such as metals, which may be recovered using different processes such as conventional pyrometallurgy or hydrometallurgy. Existing metal recovery methods have a number of limitations which include adverse environmental footprints, therefore necessitating the investigation into other methods or reagents. In this research, ionic liquids were investigated for both leaching of gold (Au) and silver (Ag) from e-waste material and their extraction from leach solutions. The ionic liquid [Bmim][HSO4] together with thiourea as a lixiviant and Fe2(SO4)3 as an oxidant was investigated for the leaching of Ag and Au from pulverized printed circuit board sample containing 162 g/t Au and 618 g/t Ag. On the other hand, [Bmim][Tf2N], [Bmim][PF6], and Cyphos 101 were investigated as extractants for Ag and Au from chloride and glycine-histidine leach liquors of the printed circuit board samples. Ag and Au dissolution in the ionic liquid [Bmim][HSO4] leaching system was observed. However, the leaching efficiencies recorded were low with Au efficiency at 24% in a leaching solution of 30% [Bmim][HSO4] and the efficiency of Ag only at 3% in the same solution. The ionic liquids [Bmim][Tf2N], [Bmim][PF6], and Cyphos 101 were efficient for gold extraction from aqua regia leach liquor with more than 90% of Au extracted. Ag extraction was lower compared to Au, with no extraction recorded for [Bmim][Tf2N] but over 90% extraction was recorded for both Cyphos 101 and [Bmim][PF6]. The ionic liquids did not extract any of the two metals from the alkaline glycine-histidine leach liquors.Item Synthesis and characterisation of metal selenide nanocrystals for use in electronic devices(2017) Airo, Mildred AwuorAdvancements in nanotechnology and nanosystems promise to extend limits of sustainable development and environment remediation in an attempt to address some of the world most challenging problems. Specifically, nanotechnology has played an important role in the design, synthesis, and characterization of various new and novel functional nanomaterials possessing extremely unique properties. For example, low dimensional nanostructures such as semiconductor nanocrystals with well controlled sizes, shapes, porosities, crystalline phases, and structures have been prepared via various synthetic methods. In addition these semiconductor nanocrystals have attracted research attention because of their fundamental role in the comprehension of the quantum size effect and great potential applications to save resources and improve the environment. Tremendous studies have established that morphological, optical, catalytic and electronic properties of semiconductor nanocrystals can be manipulated during synthesis by simply varying the growth parameters. Herein we establish the effect of different synthetic methods and several growth parameters on the properties of the as-synthesized semiconducting metal selenides nanocrystals (NixSey and InxSey) including structural, optical, electronic and catalytic properties. For example, reducing coordinating solvent oleylamine was seen to favour a particular morphologies and stoichiometries despite the duration of synthesis. In the case of InxSey nanocrystals, oleylamine favoured indium monoselenide (InSe) nanosheet formation while addition of 1-DDT as a co-surfactant to oleylamine produces In2Se3 nanowires. For NixSey nanocrystals, TOP as a co-surfactant to different ligands favoured the formation Ni3Se2 with different shapes including dots, plates, rods and wires in different solvents. Other parameters studied included the reaction time and temperature. Besides the properties, we probe the potential applications of these materials in dye sensitized solar cells as counter electrodes as well in chemical sensor as the sensing material. NixSey nanocrystals were employed as CE in DSSCs in an attempt to replace the noble expensive platinum conventionally used as CE in most DSSCs. It was established that different stoichiometry played a significant role in the catalytic reduction of I3-. Thus, different photovoltaic performance parameters were obtained with NiSe2 giving a higher PCE of 1.5 % followed Ni3Se4 then Ni3Se2. These values were however very low compared to the ones reported in literature, something that was attributed to low electron mobility, enhanced recombination and reduced catalytic performance as a result of poor device assembly and the organic ligand layer encapsulating the nanocrystal. In another scenerio, indium monoselenide nanocrystals were employed in chemiresistive sensors to detect the presence of a number of VOCs including formaldehyde, methanol, chloroform and acetone in the ambient. Indeed despite the well-known electrical, optical and structural properties previously reported in literature, metal selenides such as CdSe, PbSe and ZnSe among others present lack of investigation for gas sensing. The experimental results showed that different morphologies of InSe nanostructures interacted differently to the analyte gas suggesting difference in the electronic properties of different morphologies. The InSe nanoparticle based sensors gave a good response to HCHO and MeOH fumes and were more selective to HCHO fumes than chloroform and acetone. While those fabricated using the InSe nanosheets though responding well to HCHO recovered half way when exposed back in air and resulted in relatively high noise to signal ratio when exposed to MeOH. The operating temperature range for the InSe sensor devices were determined to be near room temperature. The sensors response was observed to decrease with increasing temperature from 30 °C to 90 °C. Evident from the results, the surface capping molecule (oleylamine) employed to stabilize the nanostructures during synthesis was responsible for the poor sensing abilities of the nanostructures.