Browsing by Author "Masemola, Khanyisile"

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    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.