Electronic Theses and Dissertations (Masters)
Permanent URI for this collectionhttps://hdl.handle.net/10539/38003
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Item Synthesis and characterization of onion-like carbons for adsorption of tartrazine dye in water(University of the Witwatersrand, Johannesburg, 2024-08) Cwayi, Herbert Qaqambile; Maubane-Nkadimeng, Manoko S.; Coville, Neil J.; Maboya, Winny K.Industrial effluent often can contain a significant amount of synthetic dyes. The discharge of wastewater containing dyes into water streams without proper treatment consequently enters the soil and disturbs the aquatic and terrestrial life. Several wastewater treatment technologies have been proposed that can efficiently reduce the amount of synthetic dyes from the environment, in particular azo dyes. Among all the existing technologies for wastewater treatment, physical adsorption is a popular technology because it is inexpensive, simple, and efficient. The aim of this study is to synthesize, modify, and characterize onion-like carbons (OLCs) derived from four different waste oils for the adsorption of tartrazine dye in water. The OLCs derived from different carbon precursors (waste household oil, restaurant waste oil, engine waste oil, and paraffin oil bath waste) were synthesized using a flame pyrolysis method. The synthesized materials were doped with nitrogen using a chemical vapor deposition technique using 10% ammonia gas as a source of nitrogen. The N-doped OLCs were attached with hydroxyl groups through oxidation reactions to improve their solubility and adsorption efficacy. According to the high-resolution transmission electron microscopy and scanning electron microscopy images, the OLCs from all four-carbon precursor were quasi-spherical, agglomerated, and presented a chain-like structures of multi-layers. The distance between the graphitic layers was found to be 0.32 nm. The average particle size of OLCs was calculated to be 40.2 ± 2.5 nm. Adsorption studies revealed that the initial dye concentration, contact time, and pH of the dye solutions influenced the adsorption capacity of the tetrazine. Nitrogen doping of OLCs increased its capacity to adsorb the tartrazine dye. The nitrogen doped OLCs from household waste oil (H-N-OLCs) and engine waste oil (E-N- OLCs) were used in equilibrium adsorption studies in this work. For a concentration of 20 mg/L of tartrazine dye, an adsorption capacity of 28.9 mg/g was achieved using the N- doped OLCs from household waste oil. The adsorption process follows the pseudo second- order kinetic model. The adsorption isotherm is best fitted to the Freundlich mathematical model. The results obtained show that, the source of oil did not have major effect on the physicochemical properties of OLCs and that incorporation of nitrogen onto carbon matrix enhanced the adsorption of the anionic tartrazine dye in aqueous solution.Item The microwave assisted synthesis of doped carbon dot/carbon nano-onion composites: A novel all-carbon counter electrode for dye-sensitized solar cell(University of the Witwatersrand, Johannesburg, 2023) Masemola, Khanyisile; Moloto, Nosipho; Maubane-Nkadimeng, Manoko S.; Coville, Neil J.Human society's development is heavily reliant on stable energy supply, and fossil energy sources have long been a very reliable energy source for this objective. However, being a non-renewable energy source, fossil fuel depletion is unavoidable and impending in this or future generations. To solve this issue, renewable energy, particularly solar energy, has received a lot of attention since it directly turns solar energy into electrical power with no environmental consequences. Various photovoltaic technologies based on organic, inorganic, and hybrid solar cells have been successfully manufactured to date. However, much study has been concentrated on organic solar cells for household and other commercial uses due to its inherent cheap module cost and ease of production. But dye-sensitized solar cells (DSSCs) have been reported to be the most efficient and simplest applied organic solar cell technology. In this study, carbon dot: onion-like carbon nanomaterial composites (Cdots: OLCNs) were synthesized for possible future application in electronic devices with particular attention to dye-sensitized solar cells. The nitrogen-doped carbon dots (NCdots) and functionalized onion-like carbon nanomaterials (OLCNs) were synthesized using a one-step hydrothermal microwave assisted irradiation method and flame pyrolysis method using liquid fuels, respectively. The as-synthesized OLCNs where purified and washed using an organic solvent n-pentane to obtain pristine OLCNs (p-OLCNs) which were further functionalized with N2 gas to obtained nitrogen-doped CNOs (N- OLCNs) and H2O2 to give oxygenated OLCNs (ox-OLCNs). For the synthesis of NCdots, various precursors (ethylenediamine, urea and fumaronitrile) were used to evaluate the effect of different nitrogen sources on the properties of these materials. Photoluminescence spectroscopy showed that the resulting NCdots exhibited the conventional excitation-dependency behavior. The NCdots which presented with the highest fluorescence quantum yield (made from ethylenediamine) were used to make the subsequent NCdot: OLCNs composites. The as-prepared p-/ox-/N-OLCNs all showed similar morphologies typical of chain-like carbon nanostructures, according to transmission and scanning electron microscopy studies, but with varying particle sizes of 42 nm, 125nm and 85 nm, respectively. The corresponding nanocomposites were used as counter electrode materials in DSSCs. The application of all the nanocomposites in the DSSCs resulted in cell efficiencies, current densities, open circuit voltages and fill factors that were lower than that of a conventional platinum (Pt) electrode. All nanocomposites tested presented with cell efficiencies <1%. Furthermore, the cells displayed some photovoltaic effect of minimal activity in the absence of light, under dark field conditions implying it is still a photovoltaic material. This photocurrent generated by the cell in the dark is suggested to be a dominant contributor to the low performance of the cells. However, what was remarkable was that this photovoltaic effect, primarily due to the thermal activity from the long lasting glow of the NCdots specifically, was found to be stable and efficient in response as infrared radiation even without being illuminated with light for 5 minutes. This suggests that the NCdots: OLCNs composites have potential application, possibly as efficient diodes rather than for use in DSSCs.