ETD Collection

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Author: search.filters.author.Maubane, Manoko StephinaHas files: Yes

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    Nickel and copper catalysed synthesis of carbon fibers
    (2014-01-10) Maubane, Manoko Stephina
    Structured carbon nanomaterials have attracted considerable interest because of their unique structures and outstanding properties. Among other structured carbon nanomaterials, carbon nanofibers (CNFs) have been the subject of study for several decades with particular interest having been paid towards their synthesis and application. However, control over the size and shape of these materials still remains a challenge. Three main components necessary for the synthesis of CNFs are the catalyst or template, the carbon source and the source of energy/power. It has been noted that catalyst morphology and the carbon source plays an important role in controlling CNF growth and morphology. As such one of the main challenges is to produce the catalyst particles that would yield the desired CNF morphology. In this study, we investigated methods for controlling the size and morphology of CNFs by synthesizing Ni and Cu catalysts of particular morphology, while using C2H2 and trichloroethylene (TCE) as a carbon source for the synthesis of CNFs. A mixture of TCE/C2H2 was also employed as a carbon source for comparison. The catalysts and synthesized CNFs were characterized by different techniques such as TEM, XRD, TPR, TGA, Raman spectroscopy, IR spectroscopy, etc. The synthesis of Ni nanoparticles (NPs) was achieved by reduction of Ni(acetate)2 with hydrazine (35%). CNFs were synthesized by deposition of TCE, C2H2 and their mixtures using a chemical vapor deposition technique (CVD) in the temperature range 400-800 oC. N2 and CO2 were used as carrier gases. TEM analysis of the Ni particles as a function of time revealed that the Ni underwent a morphological change with time. Further, as the temperature of the reaction changed, so did the shape of the carbon materials. The shapes changed from structures showing bilateral growth at T = 400 oC to tripod-like structures and multipod-like structures at T = 450 oC and T = 500 oC respectively. Irregular shaped materials were observed at T > 500 oC. It was also found that when acetylene or an acetylene/trichloroethylene mixture was used at T = 450 oC, helical (> 80%) and linear fibers were produced respectively. It was also demonstrated that the flow rate of H2, N2 and CO2 had a dramatic influence on the morphology of CNFs. CO2/TCEwas found to produce linear fibers with controlled sizes at 800 oC. The results demonstrated that the formation of tripod CNFs only occurs in a very narrow parameter regime. Manoko S. Maubane The preheating of the TCE prior to its deposition over a Ni particle catalyst was achieved using a double stage CVD reactor. TCE was subjected to high temperatures prior to its deposition at low temperatures. Results showed that the decomposition temperature was the key parameter in the synthesis of CNFs. It was found that during the decomposition, TCE breaks down into different species/radicals which then adsorb onto the catalyst particle to give CNFs of different morphology. Raman studies revealed that the synthesized CNFs showed an increase in graphitic nature when the temperature in the first reactor of a two stage reactor was increased. Decomposition of C2H2 was also performed over Cu NPs, and Cu modified catalysts (Cu@SiO2 and Cu/SiO2) with different silica coatings at 300 oC. These catalysts were prepared by reduction of Cu(acac)2 with hydrazine (35%). TEM images revealed that coiled CNFs were only produced from Cu/SiO2 grown in the presence of H2 (> 90 %; d = 60-70 nm). IR spectra of all the CNFs indicated the presence of surface C=C, C=O, CH3 and CH2 moieties, and that the ratios of peak intensities of C=O/CHx and C=C/CHx species indicated the variable CNF surface that was produced by the gases and the Cu particles used. It was thus revealed that the CNFs produced by different Cu catalysts have different chemical and physical properties and that these properties correlate with catalyst particle size and the gas mixtures used. CuO and SrO modified Cu catalysts (with different Cu/Sr ratios) were also employed using the CVD method for the synthesis of CNFs at 300 oC. These catalysts were prepared by a coprecipitation method. The TEM images of the CNFs revealed a mixture of straight and coiled CNFs with a broad diameter distribution (50-400 nm) dependent on the Cu/Sr ratio of the catalyst used. IR and TGA analysis revealed that the chemical composition of the CNFs changed as the SrO content changed. The SrO content also affected the Cu particle size and influenced the morphology of the Cu particles from which the CNFs grew.
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    Synthesis, doping and functionalization of carbon nanotubes
    (2011-03-11) Maubane, Manoko Stephina
    This study reports the synthesis of carbon nanotubes (CNTs) incorporated into polymeric materials for potential use in photovoltaic solar cells. Both undoped (CNTs) and nitrogendoped (N-CNTs) materials were made using the chemical vapor deposition (CVD), catalytic CVD and floating catalyst CVD methods. The procedures produced CNTs with an average yield of 1151 % using a 10 % Fe/Co catalyst supported on CaCO3. This is about three times that produced using 5 % Fe/Co catalyst (average 409 %). Morphology studies showed that the synthesized materials had an average diameter of 30 nm. CNTs were successfully incorporated into polythiophenes (PTh) using an in situ chemical oxidative polymerization method. TEM images showed that the functionalized CNTs in polythiophene, f-CNT/PTh, were thicker (average 192 nm) as compared to pristine CNTs (30 nm). TGA analysis then revealed that the new materials (f-CNT/PTh) were more thermally stable as compared to the pure polymer. N-CNTs were synthesized by the floating catalyst CVD method using toluene, ferrocene and tetramethylethylenediamine. Functionalization of the N-CNTs was then achieved using 3- thiophenecarboxaldehyde and sarcosine in 1,2-dichlorobenzene (Prato reaction). Elemental analysis showed nitrogen incorporation (1.8%) into the N-CNTs and this value tripled after functionalization with the nitrogen donor reagents. Morphology studies showed that the amount of monomer used in forming the N-CNT/PTh nanostructures had an influence on the average diameters of the materials. Different ratios of nanotubes to thiophene monomer by weight were used (1:3, 1:10 and 1:20). It was found that when the amount of thiophene monomer was increased, the overall diameter of the materials increased as did the thickness of the polymer attached onto the N-CNTs. Similar studies were undertaken in order to evaluate the influence of time on the formation of f-N-CNT/PTh nanostructures. Polymerization reactions were carried out for 1 h, 12 h and 24 h and it was found that when the polymerization time increased, the average diameter of the f-N-CNT/PTh also increased, as did the thickness of the polymer attached onto the f-N-CNTs.