3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item The synthesis of nitrogen doped carbon spheres as supports for palladium catalysts in the hydrogenation of cinnamaldehyde(2016) Manikai, Sibongile Mary-AnneThe selective hydrogenation of cinnamaldehyde (CALD), an α,β-unsaturated aldehyde, at the carbonyl (C=O) and olefinic (C=C) groups is an important reaction since its products mainly cinnamyl alcohol (CA) and hydrocinnamaldehyde (HCALD) are important intermediates for the production of many chemicals in a wide range of industries (pharmaceuticals, flavouring, agrochemicals, perfume). In this study the synthesis of nitrogen doped carbon spheres (NCSs) as catalyst supports for the hydrogenation of CALD is reported. At the heart of the hydrogenation of CALD is the catalyst, since it provides the surface for the various reactions to take place. In this study, an in-depth study was conducted on the NCSs support, by varying pyrolysis time, pyrolysis temperatures and flow rates of gases to determine the physical and chemical properties. The effects of chemically modifying the surfaces of the NCS supports by functionalization with acid and doping with carbon were also investigated. NCSs which had undergone different pre-treatments procedures were then deposited with Pd nanoparticles using different metal deposition methods and the resultant catalysts tested for the hydrogenation of CALD.Item Dynamic electrical transport in carbon nanotubes and nanodiamond films(2014) Chimowa, GeorgeA comprehensive experimental study on alternating current (AC) electrical transport in the three forms of carbon nanotubes (CNTs) and nanodiamond films is presented. It is termed dynamic electrical transport to differentiate it from direct current measurements, which may be referred as static transport. The results and analysis are based on the scattering parameter measurements of a few horizontally aligned single, double, multi-walled carbon nanotubes and nanodiamond films. Which were measured in the frequency range 10 MHz to 65 GHz, at room and cryogenic temperatures using a vector network analyser. The work is motivated by the fact that AC transport in 1D systems has not been fully studied and is not well understood. From direct current measurements, it is known that one dimensional (1D) electrical transport is very different from its two or three dimensional counterpart. This is because adding an electron to a 1D system tends to affect the whole system in ways which to date cannot be fully explained theoretically. CNTs present an ideal platform to study the AC or dynamic transport behaviour of 1D systems because of the high mobility and electrical conductivity at nano-scale. Therefore from the AC complex impedance and conductance, this work demonstrates quantum effects of collectively excited strongly interacting electrons (Luttinger Liquid), which had been predicted theoretically but not observed experimentally using this technique. Ballistic transport at room temperature is also demonstrated by setting the stimulus frequency higher than the scattering rate in the CNTs. A crossover from capacitive to inductive behaviour in the imaginary component of impedance has been shown by improving the CNT-electrode coupling. Furthermore the effect of metal contacts on microwave/ radio frequency transmission is also demonstrated. The results are consolidated by RF simulations, as strong conclusions are drawn. Studies on the dynamic transport in nanodiamond films revealed a crossover from the insulating to semi-metallic regime by nitrogen incorporation. The crossover is explained by considering the changes of the grain boundary morphology. This work shows that AC transport in polycrystalline nanodiamond films is similar to DC transport.Item A comparative study of micro & nanocarbon reinforced synthetic rubber composites(2014-09-01) Maifadi, JamesThis study concentrated on two main thrusts: 1) the optimal synthesis and characterisation of various micro- and nanosized carbon materials and 2) a comparative investigation of the capabilities of these carbonaceous materials to reinforce a locally available styrene butadiene rubber (SBR), which was commonly used to make car tyres. In the former case, a range of carbon materials including nitrogen doped/undoped carbon nanotubes as well as carbon microspheres (CMSs) were successfully synthesized by two different techniques (i.e. chemical vapour deposition (CVD) and hydrothermal synthesis). These were then fully characterised by numerous techniques which included: TEM, TGA, FTIR, PXRD, laser Raman spectroscopy, Zeta potential measurements and BET surface area analysis. In the latter case, these micro and nanocarbon materials were systematically added to SBR at various loadings (ranging from 0.125–0.500% (m/m)). Here the tensile strengths of the resultant composites, loaded with these various micro and nanocarbon materials, were measured for comparison to establish which (if any) was the best reinforcement material for SBR. Results obtained from the tensile strength measurements of the variously loaded SBR composites, showed that irrespective of the method of synthesis (i.e. CVD or hydrothermal synthesis) carbon microspheres (undoped, doped, functionalised or unfuntionalised) performed more poorly as fillers than carbon nanotubes. Furthermore the results obtained, from the various characterisation techniques mentioned previously, indicated that the lower performance of these microspheres as fillers may have been due to their: size, shape and low surface areas. By contrast when the tensile strengths of SBR reinforced with either CNTs or NCNTs were measured, the former outperformed the latter as fillers. It was speculated, based upon the data obtained, that NCNTs were poorer fillers than CNTs due to their higher negative surface charges, larger diameters and lower crystallinity. Hence this study has shown that low loadings (i.e. 0.250 % (m/m)) of the correctly matched type of carbonaceous material can significantly enhance the tensile strength and Young’s modulus of a locally available styrene butadiene rubber.Item Electrical transport properties of nitrogen doped carbon microspheres(2014-07-22) Wright, William PatrickA suite of four samples of nitrogen doped carbon microspheres, each with a di erent level of nitrogen dopant, was synthesised in a horizontal chemical vapour deposition reaction. The samples were characterized using scanning electron microscopy, Raman spectroscopy and electron paramagnetic resonance spectroscopy. Scanning electron microscopy showed that microspheres were produced by the reaction. Raman spectroscopy con rmed the graphitic nature of the samples. Electron paramagnetic resonance spectroscopy determined that nitrogen was present in the graphitic lattice and was used as a non-destructive technique to measure the amount of substitutional nitrogen present in the samples. In order to perform electrical transport measurements an automated magneto-transport measurement station was developed in the laboratory. This transport station was computer controlled and contained all of the necessary hardware and software required to perform magneto-electrical transport measurements. Variable temperature electrical transport measurements were performed on all samples to determine their conductive properties. Resistance measurements showed that two of the samples were semiconductors while the other two samples displayed a transition to metallic behaviour at higher temperatures. This transition can be ascribed to the thermal desorption of nitrogen dopant. Models were tted to the data and the semiconducting behaviour is best explained by a model of uctuation induced tunnelling while the metallic behaviour is best explained by a quasi-1 dimensional metallic term based on electron-phonon interactions. The IV characteristics of two of the samples display increasing non-linearity of the current's voltage dependence with decreasing temperature. The other two samples exhibit this behaviour at lower temperatures while higher temperature IV data displays a current saturation with increasing voltage. The same models used to explain the resistance measurements can be used to explain the IV characteristics data extremely well. The magnetoresistance data taken with the direction of current ow orientated both parallel and perpendicular to the eld, show a transition from negative to positive magnetoresistance with decreasing temperature. The results of these experiments are inconclusive, as a theoretical model of magnetoresistance in systems that conduct via uctuation induced tunnelling is not well de ned. A comparison between the resistance measurements of all four samples was made to determine the e ect of nitrogen doping on the samples' electronic transport properties. The result of this comparison was indeterminate. This was due to samples with identical nitrogen dopant levels displaying vastly di erent conductive properties and indicates that very strict synthesis conditions need to be adhered to in order to ensure sample quality. Resistance measurements were rerun on the two samples that displayed purely semiconducting behaviour to investigate the possibility of atmospheric doping. It was found that the samples now displayed a transition to metallic behaviour and a reduced resistance. These results are suggestive of atmospheric doping by oxygen and water vapour.Item Electronic properties of single walled carbon nanotubes synthesized by laser ablation(2014-07-21) Ncube, SiphephileCurrent research in the field of nano-electronics is directed towards device miniaturization in order to find ways to increase the speed of electronic devices. The work presented in this dissertation is on the electronic transport properties of single walled carbon nanotube (SWNT) ropes synthesized by laser ablation. The measurements were performed on devices with different geometries; namely SWNT mats, metal incorporated (aligned individual and bundled) SWNTs and lastly on aligned pure SWNTs from low temperatures up to room temperature. The work was performed so as to gain an understanding on how best to utilize SWNTs in the semiconductor industry towards miniaturization. Such an understanding would ultimately highlight if SWNTs can be considered as a viable alternative to the current silicon-based technology, which seems to be approaching its physical limit. For a mat of SWNTs, 3D-Variable range hopping is the principal conduction mechanism from 2 K – 300 K. The magneto-resistance was found to be predominantly negative with a parabolic nature which converts to a linear nature as the temperature is increased. The negative MR is a consequence of quantum interference and the positive upturn is attributed to wave function shrinkage at low temperatures as described by the Efros-Shklovskii model. The hopping ranges of the electrons for a SWNT mat increases as the temperature decreases due to manifestation of quantum effects and reduced scattering. It was also found that metal incorporation does not alter the properties of the SWNT significantly. SWNT ropes aligned by di-electrophoresis across a 1 micron gap between gold micro-electrodes, exhibit Tomonaga-Luttinger liquid (TLL) like behaviour, within the 80 K – 300 K temperature range. The effects of confinement and electron-electron interaction unique to one dimension were identified in electronic transport as a non-universal power law dependence of the differential conductance on temperature and source-drain voltage. Ballistic conductance at room temperature was confirmed from the high frequency transport of the SWNT devices. The complex impedance showed some oscillatory behaviour in the frequency range 6 to 30 GHz, as has been predicted theoretically in the Tomonaga-Luttinger Liquid model. The observation of Luttinger Liquid behaviour demonstrates the outstanding nature of these one-dimensional molecular systems. In these devices the charging Coulomb energy of a single particle played a critical role in the overall device performance. This study can be used to understand the nature of dynamics of plasmons which are the charge carriers in a TLL system and how Coulomb interactions can be used to design highly tuneable systems for fabrication of single molecule devices. The incorporation of metal onto individual SWNT ropes does not alter its electronic properties significantly but the properties of the bundled metal incorporated SWNT ropes are altered. This study has found that under optimized conditions SWNTs might be a viable option for incorporation in nano electronics devices. Individual SWNT ropes promise better devices compared to SWNT mats and further work should be done on individual SWNTs.Item Synthesis of carbon nanofibers and their subsequent use as catalyst supports for Fischer-Tropsch synthesis(2014-07-07) Phaahlamohlaka, Tumelo NathanielIn this study the synthesis and use of carbon nanofibers (CNFs) as catalysts supports for Fischer Tropsch synthesis is reported. The synthesis of carbon nanofibers with two distinct morphologies was optimized based on the reports in the literature that the straight (SCNF) and helical (CCNF) carbon nanofibers grow on Cu catalysts with different particle sizes. To selectively grow CNFs with a single morphology Cu catalysts were designed using different synthesis procedures (by using unsupported, coated and silica supported catalysts). The prepared copper oxide (CuO) nanoparticles were characterized by techniques such as TEM, XRD and nitrous oxide chemisorption. These techniques showed that the unsupported and coated CuO catalyst precursors has large particle sizes (range 100-300 nm) and thus had low Cu atomic surface area, while the supported CuO catalysts displayed low particles sizes in the nanoscale regime (<20 nm) and hence had high atomic surface area. Preparation of CNFs was carried out 300 using acetylene (C2H2) gas as the carbon source. Cu catalysts with large particle sizes resulted in straight CNFs and the small supported Cu nanoparticles grew helical CNFs because of the change in the nanoparticle surface energy during adsorption of the acetylene gas and the silica (SiO2) support effects that limited Cu nanoparticles from sintering (i.e. final particles size 60 nm). Soxhlet extraction proved to be an invaluable step in removing adsorbed polycyclic aromatic hydrocarbons. Because of the low thermal stability of these CNFs the materials were then annealed at higher temperatures ranging from 500-1400 in an inert environment (passing N2 gas). The helical CNFs snapped under high temperature annealing ( 900 ) resulting in shorter lengths in comparison to the straight CNFs. BET analysis of the annealed CNFs indicated that the CNFs annealed at 500 and 900 have increased surface area and have a mesoporous pore structure with the surface area ranging from 200-350 m2/g. Raman and Fourier transform IR spectroscopy indicated that the CNFs annealed at 500 and 900 , (which were the main material of interest because of their high surface area and thermal stability) had different hybridized carbon content. CNFs annealed at 500 contained both sp2 and sp3 hybridized carbon while annealing the CNFs at 900 resulted in a complete rehybridization of the carbon content to sp2. The carbon sp3 content in the CNFs annealed at 500 therefore implied that CNFs annealed at this temperature are more defective in comparison to the CNFs annealed at 900 . Since it is well known that material functionalities are affected by the amount of defects present inside the different CNFs were then applied as catalyst supports for Fischer Tropsch synthesis (FTS) to compare the support effects on cobalt active sites. The CNF surfaces were first modified by functionalization using concentrated HNO3 solution. The preparation of the catalyst systems was performed by a simple HDP method using urea. The CNFs and the FT catalysts were characterized using different techniques such as XRD, TEM, BET, TPR and Raman spectroscopy. Reactor studies performed at 220 (P = 8 bar, GHSV= 1200 mL.h-1. ) showed the catalysts had activities with CO conversion ranging from 25-45%. It was observed that catalysts supported on CNFs annealed at 500 displayed higher average activities of about 15% (based on the CO conversions) in relation to the catalysts supported on CNFs annealed at 900 . Catalysts showed minimal water gas shift reaction and high methane selectivity (i.e. 20-30%) which can be attributed to the small Co crystallite sizes and low pressure reaction conditions.Item Chemical vapor growth of nitrogen doped carbon nanotube and graphene materials for application in organic photovoltaic devices.(2014-03-05) Bepete, GeorgeApplication of carbon nanomaterials like fullerene, carbon nanotubes, and graphene in solar cells using solution processable methods presents a great potential to reduce the cost of producing electricity from solar energy. However, carbon nanotubes and graphene materials are predominantly metallic and this limits their function in organic photovoltaic devices (OPVs) where semiconducting behavior is required. Doping of carbon nanomaterials is a well-known method for making them semiconducting. Doping of carbon nanomaterials with nitrogen and boron can tune their properties to suit the requirements for use in photovoltaic applications as n-type and p-type semiconducting materials, respectively. Indeed, the use of nitrogen doped and boron doped carbon nanotubes in organic solar cells together with fullerene acceptors can improve the current density of the OPV devices. Nitrogen doping of carbon nanotubes can be achieved by using nitrogen-containing precursor materials during chemical vapor deposition. However the doping of carbon nanotubes with nitrogen does not automatically make them n-type materials; they remain metallic unless a large amount of quaternary type nitrogen is incorporated in the carbon nanotubes. In this work we have developed a method to control the type of nitrogen that is incorporated in CNTs by using an appropriate synthesis temperature and use of oxygen-containing carbon precursors during the chemical deposition of carbon nanotubes. Quaternary N was incorporated in a CVD process when high temperatures and a high concentration of O in the precursor materials were used. We also showed that the type and amount of N can be changed from pyrrolic and pyridinic-N-oxide to pyridinic N and quaternary N by annealing N doped carbon nanotubes at temperatures above 400°C. At temperatures above 800°C most of the nitrogen is converted to quaternary nitrogen. N-CNT thin films were used in OPVs so as to modify the ITO electrode and transform it into a 3D electrode. The resulting effect was an improved short circuit current density in the devices containing an N-CNT thin film that was placed on top of the ITO electrode. A reduction in efficiency losses in OPVs at increasing light intensity was observed in the NCNT ITO modified electrode OPVs. This is a remarkable finding when considering that one of the main problems hindering commercialization of OPVs is the loss of efficiency at high light intensities. We related these effects to the efficient charge collection by the modified ITO electrode. Incorporation of N-CNTs in the bulk heterojunction layer of the OPV device resulted in poor performance when compared to an OPV device made without N-CNTs. This effect is caused by shorting of the OPVs. We used a method of incorporating N-CNTs whilst minimizing shorting and this showed potential for better performance. A study on the attempted doping of graphene with B to make it a p-type material showed that in the presence of a nitrogen carrier gas, BN instead of B was incorporated in graphene. This remarkable finding enabled us to grow a p-type graphene with a possible a band gap opening. This was corroborated by XPS and Raman spectroscopy studies of the material. This BN doped graphene material showed potential as a possible replacement of PEDOT:PSS as a hole transport material in OPVs. The BN doped graphene material can match the performance of PEDOT:PSS when the level of BN doping in graphene is increased.Item Quantum transport through impurity clusters in carbon nano-materials(2014-02-07) McIntosh, Ross WilliamModified graphene and low dimensional carbon nano-electronic devices have the potential to supersede current technologies in many respects although manufacturing and understanding these materials poses a significant challenge which requires an incremental approach. Doping of graphene, a prerequisite for modifying the electronic properties, is still poorly understood.Band-modulation is therefore difficult to control. Resonant tunneling induced through the incorporation of impurity clusters has not yet been addressed. On the other hand electronspin correlations in modified graphenes have hardly been studied. In this work we address these issues through a tandem approach of theoretical and experimental studies. This work begins with an ab-initio study of the electronic properties of bilayer graphene and the modifications induced through the substitutional incorporation of isolated nitrogen impurities.Nitrogen modification results in a change from a zero-gap semiconductor to a metal as a result of nitrogen incorporation while charge density calculations show the localization of charge in the vicinity of the impurity. This work on isolated impurities was then extended to impurity clusters. The quantum transport properties of impurity clusters distributed within a high bandgap material were then studied. Different geometrical configurations of the impurity clusters were studied to tune quantum interference to control the carrier lifetime. The effects of randomly distributed clusters were also studied to interpret the effects of disorder. These studies provide insight into the transport properties of naturally grown quantum dot systems such as reduced graphene oxide which consists of low defect density graphene nano-islands randomly distributed in oxygen and free radical functionalized graphene which was studied experimentally. Resistance was recorded as a function of temperature for graphene oxide and reduced graphene oxide two terminal devices. Evidence of mesoscopic resistance fluctuations, charge carrier activation and enhanced elastic scattering was found while the magnetic properties of reduced graphene oxide showed a phase transition from ferromagnetism at low temperatures to diamagnetism at higher temperatures. Finally, the Kondo effect was demonstrated in reduced graphene oxide through transport and magnetoresistance measurements which were interpreted within the Fermi liquid description of the Kondo effect. These effects were explained through the microstructure of reduced graphene oxide and illustrate the significance of spin in reduced graphene oxide. These studies will inform the design of functionalized graphene spin-polarized devices and spin valves.Item Examining the effect of pH on the structure and stability of CLIC1 with E228L and E85L CLIC1 variants(2013-08-01) Cross, Megan OliviaThe chloride intracellular channel CLIC1 is an anion channel protein that has been implicated in a number of physiological processes. It is fascinating in that it is synthesised as a soluble monomer that is able to reversibly bind membranes without the aid of a membrane-targeting tag or receptor. CLIC1 membrane binding is promoted by low pH and involves separation of the N- and C-domains and subsequent refolding of the N-domain, which traverses the membrane as an α-helix. At the low pH of a membrane surface, pH 5.5, soluble CLIC1 demonstrates decreased conformational stability and forms a partially unfolded intermediate state under mild denaturing conditions. In this study, these pH-effects are proposed to occur as a result of low pH-induced protonation of two conserved glutamate residues, Glu85 and Glu228. Both are involved in domain-maintaining interactions and are proposed to form part of an electrostatic network of pH-sensitive residues. At low pH, protonation of these glutamates would break their electrostatic interactions, allowing separation of the domains. To investigate this possibility, Glu228 and Glu85 were mutated to leucine residues. Each variant protein was then investigated at pH 7.0 and pH 5.5 and results were compared to the wild-type. Secondary and tertiary structures were examined using far-UV circular dichroism and fluorescence spectroscopy, respectively. Conformational flexibility was investigated with limited thermolysin proteolysis. Stability was studied using thermal and urea-induced equilibrium unfolding. The unfolding intermediate state was detected using ANS binding and its structure was characterised. While neither residue substitution caused global structural perturbations, both destabilised the structure and promoted intermediate formation at pH 5.5. This was particularly evident for the E85L variant, which also formed a significant intermediate population at pH 7.0. It was concluded that the interactions of Glu228 and Glu85 are involved in maintaining the CLIC1 native state. Additionally, the lack of pH-dependence of intermediate formation in the E85L variant suggested that Glu85 is likely to function as a pH-sensor. It is thus involved in the „priming‟ of the CLIC1 structure for the conformational changes that may lead to membrane binding.Item Magnetic properties of nitrogen- doped carbon nanospheres(2013-03-07) Dubazane, Makhosonke BerthwellElectron spin resonance (ESR) was used to characterize a suite of carbon nanospheres (CNS) samples with varying nitrogen concentrations at room temperature. The CNS were produced using two different reactors (vertical and horizontal) under different preparatory conditions. Resonance spectra of samples produced from the vertical reactor showed resonance lines- a narrow paramagnetic component, and broader component. They were attributed to nitrogen paramagnetic impurities and carrier spins, respectively. Samples produced in the horizontal reactor revealed stronger line spectra that were narrower and Dysonian in shape. The nitrogen content of the samples produced by the horizontal reactor was determined through ESR analysis which involves integration of the resonance peak, and normalizing to the mass of the sample. The relative g-shift was also measured by using a DPPH reference sample. Room temperature power saturation experiments were performed on samples produced from the horizontal reactor with the aim of estimating the spin relaxation times. Two samples from the horizontal reactor were further investigated at low temperatures (4 K- 320 K) at a constant microwave power. The resonance parameters investigated were linewidth, asymmetry ratio and amplitude, and possible spin-lattice relaxation mechanisms were investigated. The variation of the amplitude with temperature was investigated using two models: (1) a model based on lattice vibrations, and (2) a model based on nanographites assembly (considered interaction between carrier and localized spins). At low temperatures both models have amplitude that changes inversely with temperature in accordance with Curie law. At high temperatures (T > 200 K) a model based on nanographites assembly provide an alternative; it describes the rise in the signal amplitude in terms of thermally activated paramagnetic electrons from non-magnetic ground state to excited state at energy . Analysis of linewidth and asymmetry ratio data confirmed that the spin-lattice relaxation governed by thermal activated electrons is a dominant relaxation mechanism at high temperatures.