*Electronic Theses and Dissertations (Masters)
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Item Fabrication of polyaniline/indium oxide /onion-like carbon ternary nanocomposite for room tempera ture gas sensing applications(University of the Witwatersrand, Johannesburg, 2022-08) Mathe, Boipelo Nicholette; Linganiso, E.C; Tetana, Z; Moma, JMonitoring and documenting chemical stimuli or environmental fluctuations is vital to daily health care and environmental monitoring. This objective can be accomplished through the development of high-performance sensors able to detect toxic gases such as ammonia, volatile organic compounds (VOCs) and many more. The modification of carbon nano-onions with metal oxides/conducting polymer could enhance sensing performances at room temperature. This research focuses on the development of a flexible room temperature gas sensor for ammonia sensing with a sensing layer composed of indium oxide (In2O3)/onion-like carbons (OLCs)/ polyaniline (PANI). The current sensors were tested at a 40-45 percentage humidity. Polyaniline was produced utilizing the rapid polymerization technique with aniline and ammonium persulfate as precursors. Carbon nano-onions were obtained by the flame pyrolysis process with candle wax as the carbon source. The present study compared two microwave-assisted solution-phase methods for the synthesis of indium oxides. The first methods produced indium hydroxide (In(OH)3) followed by its conversion to In2O3 through annealing at 400 oC, and the second used a one-step method where ethanol was used as a solvent instead of water. Different reaction times were used to determine the effect of microwave power on the indium oxide formed through a solution-phase method, and several characterizations techniques were performed to characterize the products, including transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy, scanning electron microscopy, Brunauer-Emmett-Teller, X-ray photoelectron spectroscopy and Ultraviolet-visible spectroscopy. The ternary In2O3/PANI/OLCs nanocomposite was fabricated using physical mixing by adding varying amounts of In2O3 to fixed quantities of PANI and OLCs. Using gold-plated interdigitated electrodes (IDEs) embedded on a printed circuit board (PCB) substrate, inexpensive and room temperature functional sensors based on plain PANI, OLCs, OLCs/PANI, and OLCs/PANI/In2O3 were developed. The sensors based on ternary composites outperformed of sensors based on pure PANI, OLCs, and PANI/OLCs, due synergic effect of PANI, OLCs and In2O3 when combined. The sensor with the highest response among the sensors with the ternary nanocomposite as the sensing layer, was chosen for further evaluations of recovery time, reaction time, repeatability, and selectivity. The sensor containing (4.6 mg) B-In2O3/PANI/OLCs was particularly responsive to ammonia in comparison to other analytes (hexane, isopropanol, acetone), with the response and recovery durations of 2.2 minutes and 4.3 minutes, respectively, spanning a concentration range of 25 ppm to 125 ppm. Current results showed that In2O3 materials can be successfully applied in room temperature gas sensing application and further enhance the sensing response to levels that cannot be obtained when using PANI or OLC individually.Item Synthesis and characterization of nitrogen-doped hollow carbon spheres/ Cu2S composites for potential application as counter electrodes in dye sensitized solar cells(2022) Majola, ThelmaCurrent solar cells have disadvantages that include weather dependency and high manufacturing costs. Dye-sensitized solar cells (DSSCs) have emerged as possible solutions for these drawbacks. Since it is predicted that the global warming effect will result in the sun emitting less energy. DSSCs have the added advantage of working in low-light conditions, and the ability to harness light energy from other devices. Although DSSCs have low production costs, their low efficiency due to the platinum (Pt) electrocatalyst limits their commercial implementation. DSSCs are made up of many components including dye, and counter electrode (CE). Modifications can be made on these components to improve efficiency and make the DSSC more eco-friendly. For instance, CE research has focused on finding substitute electrocatalysts. In this study copper sulfide (Cu2S) and hollow carbon spheres (HCSs) have been considered as viable substitutes for Pt since they exhibit good electrocatalytic properties. Carbon materials have corrosion resistance towards iodine, and Cu2S has superior oxidation resistance. Therefore, they can be used for electrolyte redox reactions at the CE in a DSSC. The properties of HCSs can be enhanced by making the carbon shell porous, increasing the number of carbon shells, or by doping the carbon shell with heteroatoms such as nitrogen (N). These modifications can improve the conductivity, surface area, adsorption, and electronic properties. For this research porous nitrogen doped single-shelled and double-shelled HCSs were synthesized. To prepare the HCSs, a carbon shell was coated on the surface of the mesoporous silica spheres using the chemical vapour deposition method (CVD) with acetylene or toluene as carbon precursors at 900 °C for 1 h. Thereafter, the silica template was etched using 10 % HF at room temperature for 24 h. It was found that toluene produced HCSs with a higher surface area. Unexpectedly, the double-shelled HCSs were found to exhibit low surface areas than the single-shelled HCSs. Nitrogen doping improved the properties of both the single and double-shelled HCSs. The nitrogen-doped, single-shelled mesoporous hollow carbon spheres (N-HCS) showed better properties for electrocatalysis than the double-shelled HCSs. The Cu2S nanoparticles were prepared using colloidal synthesis at 230 °C for 1 h with oleylamine (OLA) as the surfactant and 1-dodecanethiol (1-DDT) as the sulfur source. Different copper precursors were used which gave Cu2S with varying compositions. Both precursors produced particles with a hexagonal morphology, however, copper (II) acetylacetonate (Cu(acac)2) produced smaller particles (30 nm) compared to large particles (479 nm) from copper chloride (CuCl). A time study on the different copper sources showed a variance in nucleation and particle growth. The Cu2S-HCS composites were prepared using OLA as the surfactant, with different percentages of Cu2S and HCSs. The different composition mixtures were heated to 100 °C and then washed to remove the unreacted elements. The synthesized materials were analyzed using various techniques. TEM showed the spherical morphology of HCSs, the hexagonal morphology of Cu2S, and the successful formation of the composites. The surface area and porosity of the materials were measured using the BET technique. Graphitization of carbon and the phase composition of Cu2S was analyzed using PXRD. The successful incorporation of N within the structure of the N-HCSs was confirmed using XPS. The electrocatalytic activities of the composites were investigated using CV,EIS, Tafel polarization, I-V and compared to Pt under similar conditions. The 75 wt% N-HCS: 25 wt% Cu2S composite had higher current density, lower peak-to-peak separation, and high electrochemical double layer capacitive current, good adhesion to the FTO glass and great photovoltaic performance compared to Pt. This suggests that it was the better performing composite and has the potential to substitute Pt- CE in DSSCs.