3. Electronic Theses and Dissertations (ETDs) - All submissions
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Item Development of activated carbons from South African coal waste for application in natural gas storage(2019) Abdulsalam, JibrilEvery year, South African coal sector generates over 60 million tons of coal waste, which are landfilled in discard dump and slurry ponds. The stockpile of this waste “resource” poses a severe danger to public health, the environment and the socio-economic development of the coal mining region. Therefore, there is an urgent need for an innovative strategy to coal waste reuse and recovery. In this study, the potential of three South African coal waste samples (run-of-mine fines, discard and flotation slurry) were examined in synthesizing an activated carbon for application in natural gas storage. Activated carbons were prepared by KOH activation, and the impacts of KOH/sample weight ratio and temperature on the activated carbon adsorptive characteristics were examined and optimized using Response Surface Methodology (RSM). The results obtained indicated that with an increased temperature and KOH/sample weight ratio, the surface area and pore volume of the resulting activated carbon increased. The activated carbon with the highest surface area and pore volume from each of the samples were obtained at a temperature of 800 oC and KOH/sample weight ratio of 4:1. The morphology, textural characteristics and elemental composition of the activated carbons produced were compared. The synthesized activated carbons were characterized by nitrogen at 77 K adsorption – desorption isotherms and SEM/EDS characterization. Surface area of 1925.34 m2/g, 1826.41 m2/g, 1484.96 m2/g, pore volume of 1.26 cm3/g, 1.21 cm3/g, 1.03 cm3/g and pore size of 2.90 nm, 2.66 nm and 2.51 nm were obtained for activated carbon from run-of-mine fines, discard and slurry, respectively. The SEM/EDS analysis showed pore development and high carbon content. The XRD evaluation confirms the activated carbons as amorphous. The presence of a hysteresis loop in the nitrogen isotherms and the pore size distribution (PSD) confirms highly porous activated carbons consisting of micropores and mesopores. The characteristics of methane (the major constituent of natural gas) adsorption onto the activated carbons produced are measured for temperatures ranging from 0 to 50 oC, and pressures up to 40 bar. For this measurement, activated carbon with the highest surface area and pore volume from each of the coal waste samples was used. The activated carbon produced from the run-of-mine fines (ACR) offers a greater adsorption capacity due to its higher surface area and pore volume. Three adsorption isotherm models (Langmuir, Toth, and Dubinin-Astakhov) were used to validate the measured adsorption data and the Dubinin-Astakhov isotherm model was found to be the most appropriate. The model described the measured data with an average regression error of less than 1% for all the three activated carbon samples. The impact of diffusion on adsorption kinetics was determined in relation to the time taken to achieve equilibrium for the methane/activated carbon system using a mass balance equation that defines pore and surface diffusion. The findings indicate the relationship between the adsorption kinetics, diffusivity, and temperature. An increase in temperature for the methane/activated carbon system was noted to cause an increase in diffusivity, thus reducing the time taken to attain equilibrium. The study indicated that adsorption characteristics (isotherm and kinetics) are the key information in designing and analysing an adsorbed natural gas storage (ANG) system. An adsorption system using the activated carbons produced as the adsorbent bed was simulated using Aspen Adsorption Software (Adsim). In this study, adsorption capacity was found to be significantly increased by a lower flowrate. This enhances thermal stability and maximizes the quantity of gas adsorbed on the bed. The simulation shows that an ANG storage system's efficiency depends on the suitable selection of adsorbent, inlet flow conditions, and bed geometry.Item The potential of medical imaging modalities to identify a concealed energetic materials device(2018) Mngqete, SamkeloThe act of terrorism has received much attention in recent years. In this study, a technique was investigated to identify concealed energetic materials which could aid in reducing the threat of terrorism and save innocent lives. Using different X-ray source and detector technologies for imaging, a variety of medical imaging modalities in Charlotte Maxeke Academic Hospital (CMJAH) are investigated as a potential to identifying concealed materials. An inert dummy improvised explosive device (IED) was built in the form of a handheld briefcase. The IED and an Electronic Portal Imaging Device (EPID) Quality Control (QC) phantom were imaged using the Toshiba medical simulator, Orthovoltage teletherapy machine, Cobalt 60 (60Co) teletherapy machine, Linear Accelerator (Linac), and Computed Tomography (CT) scanner. Detection methods used were Kodak X-Omat V ready pack film, Fuji medical X-ray film, Gafchromic film, and an EPID. The low energy from the simulator was absorbed by the high-density materials in the briefcase. The image had high contrast, and since this was planar imaging hidden, materials were not visible. The CT scanner contained many streak artefacts, producing poor 3D images of the briefcase. At high energies the Linac and 60Co teletherapy machine penetrated through the thick materials, displaying underlying and overlying materials on the image. The modality which had the best balance between contrast and spatial resolution was the orthovoltage teletherapy machine. The orthovoltage teletherapy machine, being the only modality to display all the materials of the IED has the potential to be used as a modality to identify a concealed energetic materials device.Item The synthesis and characterization of ZnS nanoparticles from zinc-based thiourea derivative complexes for potential use in photocatalysis(2017) Lethobane, Manthako HycinthNanotechnology has been instrumental in finding strategies of combating some of the world’s grand challenges. Water scarcity and the growing industrialization have made it an imperative to find ways of cleaning water. Photocatalysis is a promising method for water purification personified by the use of solar energy as well as nanomaterials with tailored properties. Colloidal synthesis has made it possible to synthesize new materials with tailored properties, in particular the single-source precursor method has been found to be a useful method in synthesizing nanomaterials with high purity. In the synthesis of metal chalcogenides, the single-source precursor method has an advantage of the precursor having the desired metal-chalcogenide bond hence eliminating the possible formation of side products particularly metal oxides. Herein, acylthiourea (ATU) and thiourea (TU) zinc complexes were used as precursors for the synthesis of ZnS nanoparticles. Bis(N,N-diethyl-N’-benzoylthiourea)Zn(II) [Zn(ATU)2] and bis(diaminomethylthio)Zn(II) chloride [Zn(TU)2Cl2] complexes were synthesized using a conventional method and characterized with elemental analysis, 1H NMR , 2D NMR, COSY, FTIR, mass spectrometry and X-Ray crystallography. The resultant precursors, Zn(ATU)2 and Zn(TU)2Cl2 complexes were then thermolyzed to yield ZnS nanocrystals and characterized fully. Reaction parameters that included the synthetic time, temperature, concentration and capping agents were optimized for each single-source precursor in an attempt to control the nanoparticles yielded hence their properties. Time and temperature studies generally demonstrated the most pronounced effect and with an increase, they showed increasing particle sizes through the Ostwald ripening effect. Also visible from the TEM was that the temperature had an effect on the morphology of the nanoparticles. Increasing the precursor concentration resulted in the agglomeration of nanoparticles, while using different capping agents yielded similar nanoparticles with different degrees of agglomeration. Evident from the results the ATU precursor behaved similar to the TU precursor and generally the particles obtained from the two precursors regardless of the reaction condition were very small. Preliminary investigations into the use of the synthesized nanoparticles obtained from the two precursors revealed potential in photocatalytic degradation of Rhodamine B (RhB) dye in water. While smaller particles were obtained from the synthesized nanoparticles, the degradation efficiencies were lower than the commercial ZnO and TiO2. This is due to the presence of the long-chained capping agents on the synthesized particles blocking the interaction of the core ZnS and the light.Item Synthesis and characterisation of metal selenide nanocrystals for use in electronic devices(2017) Airo, Mildred AwuorAdvancements in nanotechnology and nanosystems promise to extend limits of sustainable development and environment remediation in an attempt to address some of the world most challenging problems. Specifically, nanotechnology has played an important role in the design, synthesis, and characterization of various new and novel functional nanomaterials possessing extremely unique properties. For example, low dimensional nanostructures such as semiconductor nanocrystals with well controlled sizes, shapes, porosities, crystalline phases, and structures have been prepared via various synthetic methods. In addition these semiconductor nanocrystals have attracted research attention because of their fundamental role in the comprehension of the quantum size effect and great potential applications to save resources and improve the environment. Tremendous studies have established that morphological, optical, catalytic and electronic properties of semiconductor nanocrystals can be manipulated during synthesis by simply varying the growth parameters. Herein we establish the effect of different synthetic methods and several growth parameters on the properties of the as-synthesized semiconducting metal selenides nanocrystals (NixSey and InxSey) including structural, optical, electronic and catalytic properties. For example, reducing coordinating solvent oleylamine was seen to favour a particular morphologies and stoichiometries despite the duration of synthesis. In the case of InxSey nanocrystals, oleylamine favoured indium monoselenide (InSe) nanosheet formation while addition of 1-DDT as a co-surfactant to oleylamine produces In2Se3 nanowires. For NixSey nanocrystals, TOP as a co-surfactant to different ligands favoured the formation Ni3Se2 with different shapes including dots, plates, rods and wires in different solvents. Other parameters studied included the reaction time and temperature. Besides the properties, we probe the potential applications of these materials in dye sensitized solar cells as counter electrodes as well in chemical sensor as the sensing material. NixSey nanocrystals were employed as CE in DSSCs in an attempt to replace the noble expensive platinum conventionally used as CE in most DSSCs. It was established that different stoichiometry played a significant role in the catalytic reduction of I3-. Thus, different photovoltaic performance parameters were obtained with NiSe2 giving a higher PCE of 1.5 % followed Ni3Se4 then Ni3Se2. These values were however very low compared to the ones reported in literature, something that was attributed to low electron mobility, enhanced recombination and reduced catalytic performance as a result of poor device assembly and the organic ligand layer encapsulating the nanocrystal. In another scenerio, indium monoselenide nanocrystals were employed in chemiresistive sensors to detect the presence of a number of VOCs including formaldehyde, methanol, chloroform and acetone in the ambient. Indeed despite the well-known electrical, optical and structural properties previously reported in literature, metal selenides such as CdSe, PbSe and ZnSe among others present lack of investigation for gas sensing. The experimental results showed that different morphologies of InSe nanostructures interacted differently to the analyte gas suggesting difference in the electronic properties of different morphologies. The InSe nanoparticle based sensors gave a good response to HCHO and MeOH fumes and were more selective to HCHO fumes than chloroform and acetone. While those fabricated using the InSe nanosheets though responding well to HCHO recovered half way when exposed back in air and resulted in relatively high noise to signal ratio when exposed to MeOH. The operating temperature range for the InSe sensor devices were determined to be near room temperature. The sensors response was observed to decrease with increasing temperature from 30 °C to 90 °C. Evident from the results, the surface capping molecule (oleylamine) employed to stabilize the nanostructures during synthesis was responsible for the poor sensing abilities of the nanostructures.Item Synthesis of cation substituted LiMn2-xMxO4 (M=A1,Ni) cathode materials for a lithium ion battery: improving energy storage, capacity retention, and lithium transport(2017) Kunjuzwa, NikiweSpinel LiMn2O4 cathode materials continues attracting the attention of researchers globally due to its economic, environmental and electrochemical benefits it provides in the lithium ion battery field. Obviously, it also experiences drawbacks in terms of its poor cycle life due to severe capacity fading. In this study, the necessary effort to improve the capacity retention, the Li+ ion diffusion and increasing the energy density by increasing the voltage of the spinel LiMn2O4 cathode materials. We have used small amounts of aluminium and then with nickel, and succeeded in following an effective doping method which uses wet chemistry synthesis techniques namely solution combustion, and aqueous reduction methods. We further explored the utilization of the South African manganese precursor, electrolytic manganese dioxide (EMD from a South African supplier). In chapter 3, we were able to enhance the capacity retention of LiMn2O4 by aluminium doping. We have synthesized LiAlxMn2-xO4 (x = 0, 0.125, 0.25, 0.375, and 0.5) cathode materials for Li ion batteries using metal nitrates and urea as precursors by a solution-combustion method. The first cycle discharge capacity of LiAl0.125Mn1.875O4 is comparable to that of the pristine LiMn2O4, just as the values of their lattice parameter are essentially the same. In addition, the LiAl0.375Mn1.625O4 sample exhibited a more stable discharge capacity than the other samples. The variation in lattice parameter as a result of Al doping correlated with the greatly enhanced cyclability of the discharge capacity of the LiMn2O4 spinel. In chapter 4, we studied the electrochemical performance of LiNixMn2-xO4 (x=0, 0.1, 0.2) spinel cathode material synthesized by solution combustion method. The nickel substituted samples exhibited excellent capacity retention (> 99%) and the use of a low amount of Ni adopted to eliminate the Jahn-Teller effects of the LMO; and enhanced lithium ion transport compared to the LMO. Based on the promising results achieved in this work we have decided to attempt to get similar results by employing the South African manganese resource, electrolytic manganese dioxide (EMD), rather than using manganese nitrate from Sigma Aldrich. In chapter 5, we employed electrolytic manganese dioxide (EMD) as manganese precursor and a low temperature aqueous reduction synthesis technique to successfully prepare a low content nickel substituted spinel LiNixMn2-xO4 cathode for a lithium ion battery by using NiSO4·6H2O as nickel source. All nickel substituted samples LiNixMn2-xO4 (x = 0.05, 0.1, 0.2) exhibited superior capacity retention as compared to pristine LiMn2O4. In chapter 6, we successfully synthesized nickel substituted spinel LiMn2-xNixO4 cathode materials for lithium ion battery by using Ni(NO3)2.6H2O as nickel source using electrolytic manganese dioxide (EMD). All nickel substituted samples LiMn2-xNixO4 (x = 0.05, 0.1, 0.2) exhibited superior capacity retention when compared to pristine LiMn2O4. The results confirmed that a Ni(NO3)2·6H2O nickel source performed electrochemically better than a NiSO4·6H2O nickel source. In chapter 7, we examined the impedance and Li+ ion diffusion properties of as-synthesized nickel substituted LiNixMn2-xO4 cathodes from two kinds of nickel sources. The nickel nitrate source gives more suppressed impedance as compared with nickel sulfate.Item Optical properties of vanadium oxide nanostructures synthesized by laser pyrolysis(2012-02-28) Shikwambana, Lerato DavidIn this work, the primary investigation has been on the development of the laser pyrolysis setup and its optimization for the synthesis of nano-size VO2-x films. More specifically the focus was on making VO2-x depositions using various laser pyrolysis parameters and establish in this way (1) an optimum laser wavelength threshold for the photon induced dissociation of the molecular precursors while the thermal contribution was kept minimal by using low power density (laser energy of 30 W) and (2) the lower threshold for pure thermal contributions by working with wavelengths far from resonance in order to minimize pure photon induced contributions. The interest in synthesizing nano-size VO2-x materials stems from the low metal-insulator transition temperature at near room temperature with opto-electronic and thermo-electronic properties that can be used in specialised applications. A large number of samples were synthesized under various conditions and annealed under argon atmosphere for 17 hours. XRD analysis identified the VO2 (B) and/or β-V2O5 vanadium oxide phases characteristic for certain samples grown under optimum conditions. Raman spectroscopy also confirmed these vanadium oxide phases with bands observed at 175, 228, 261, 303, 422 and 532 cm-1. SEM analysis revealed a plethora of different nanostructures of various size and shapes. The particles have a range of sizes between 55 nm to 185 nm in diameter. The particles showed morphologies which included nano-rods, nanospheres and nano-slabs. An interesting phenomenon was observed on the samples synthesized with high power density, which was observed and reported by Donev iii et al. EDS analysis on the particles was also used to probe the elemental composition of the sample. Optical studies were performed on the samples which showed transitions in the visible and infrared region in accordance with the ones observed in the international literature using different nano-synthesis methods.Item Synthesis of carbon nano-structured materials(2012-02-24) Shaikjee, AhmedABSTRACT Page | iiAbstract i The deposition of carbon during catalytic reactions has a long history, with major efforts initially focused towards their prevention rather than synthesis. However the discovery of fullerenes and later that of carbon nanotubes by Iijima, shifted scientific focus towards the synthesis, characterization and application of carbon deposits. This renewed interest in carbon based materials, has revealed a universe of extraordinarily shaped carbon materials (SCMs) in the nano and micro range, from tubes and helices to horns and most recently graphene. It has been noted that there exists a relationship between the morphology of the carbon material and its inherent properties, making them highly prized for numerous technological applications. However before these carbon materials can be effectively exploited control over their selective synthesis is necessary, a problem that has been solved with only limited success. As such, there still exists a need to develop synthetic strategies that would yield shaped carbon materials selectively. More importantly, it is essential that a better understanding of the growth factors that lead to differently SCMs is obtained. In this study we have highlighted the parametric conditions for optimum growth of carbon helices, as well as that of carbon fibers with unique structure. We have found that catalyst morphology and the carbon source are key aspects, which control carbon material growth and morphology. The synthesis of carbon materials using bi and tri-metallic supported catalyst systems revealed that Cu was an effective promoter for obtaining helices, particularly at low temperatures (≤ 550 ˚C). On further investigation, Cu was shown to exhibit incredible carbon deposition capabilities at temperatures as low as 200 ˚C. Adjustments of the catalyst preparation conditions (support, metal counter ion, solvent and reduction temperature) and synthesis temperature, revealed that the yield and morphology of the carbon deposit could be altered to selectively produce both straight and helical carbon fibers. A TEM tomography study revealed that the copper particles that gave distorted decahedra formed helical fibers, while trigonal bi-pyramidal particles gave linear fibers. Various plate-like particles revealed that as the number of sides of a catalyst particle varied (3, 4, 5 or 6) there was a corresponding change in the Abstract Page | iv carbon fiber helicity. A relationship between catalyst particle morphology and fiber morphology was thus established. TEM analysis also revealed that catalyst particles underwent rapid reconstruction during carbon fiber synthesis, and that the carbon source (gas environment) was influential in this reconstruction event. A NiOx (unsupported) catalyst was prepared and reactions with various substituted alkyne hydrocarbons were undertaken. Analysis revealed that different alkynes produced carbon fibers with varying morphologies. Using different alkynes in a sequential manner led to the formation of ‘co-block’ carbon fibers with an A-B-A-B... or A-B-C... morphology. Using different alkynes followed by acetylene led to the selective synthesis of straight, Y-junction or irregular carbon fibers. Accompanying these results was the observation that in each case the catalyst particle morphology was unique. Reaction of NiOx with trichloroethylene, in which trichloroethylene acted as a source of carbon for fiber growth, also restructured the Ni catalyst into a tetrahedral shape that gave tripod-like carbon growth. It was found that, substituted alkynes (and alkenes) provided a means for controlling catalyst particle morphology and hence carbon fiber morphology. The study has highlighted the relationship that exists between catalyst and SCM morphology, as well as the effect of hydrocarbons, not only as a source of carbon for SCM growth but also as a means of controlling catalyst morphology and SCM structures.Item Effect of gold nanoparticles on the activity of perovskites for CO oxidation(2011-11-18) Mokoena, Lebohang VivaciousGold has for many years been regarded as being inert and catalytically inactive compared to the PGMs (platinum group metals). However, in the past decade it has attracted a lot of interest as both a heterogeneous and a homogenous catalyst and has been shown to catalyse a wide range of reactions e.g. oxidation, hydrogenation and reduction among others. Highly dispersed gold nanoparticles on metal oxides, like titanium oxide (Degussa, P25) have predominantly been studied because they yield some of the most active and stable catalysts. Modification of the catalysts and/or supports has been shown to affect their catalytic properties. Likewise, perovskites, which can be manipulated by partial substitution, are reported to be active supports for CO oxidation, but only at high temperatures with no activity shown for temperatures below 200°C. In this study, these perovskites were investigated at low temperatures (below 100°C) with improved activity found upon gold deposition. The presence of gold nanoparticles therefore significantly enhanced the catalytic activity, while the support itself was suspected to be involved in the reaction mechanism. A series of perovskites of the type ABO3 (LaMnO3, LaFeO3, LaCoO3 and LaCuO3) were prepared using the citrate method, while the gold was deposited on them using the deposition-precipitation method. The supports were calcined at different temperatures for optimisation. The catalysts were tested for carbon monoxide oxidation and the active catalysts characterised by XRF, XPS, XRD, Raman spectroscopy and BET surface area measurements. With the support calcined at 800ºC, the best catalyst was then modified and compared with the unmodified catalyst. The 1-wt%Au supported on LaFeO3 was found to give the best catalytic performance. This support was then modified with various weight loadings of calcium to determine the effect of calcium on the catalytic activity. Calcium-doped materials showed decreased surface area, poorer crystallinity and a drop in catalytic activity relative to the Au-LaFeO3 which indicated the best results for CO oxidation. In addition, Au-LaFeO3 showed online stability over 21 hours. Calcining the support improved the incorporation of gold nanoparticles into the perovskite lattice, resulting in superior catalytic activity. Nevertheless, at higher calcination temperatures, the catalytic activity of Au-CaTiO3 was depressed while that of Au-LaFeO3 was enhanced. The activity of perovskites increased upon gold deposition. XPS, revealed that in the active catalysts, both cationic and metallic gold co-existed, whilst in the inactive catalysts the gold existed predominantly either as cationic or metallic gold.Item The role of the university in the field of nanotechnology : the case of the University of the Witwatersrand.(2008-12-19T07:00:27Z) Iyuke, Patience OdiriThis study examines the role of the university in the domain of nanotechnology research and training using the University of the Witwatersrand (Wits) as a case study. It focused on the Faculties of Science and Engineering, given their involvement in the field. It is essentially a qualitative study based on documentary analysis and semi-structured interviews with academic staff members. It shows how the University of Witwatersrand has responded to the South African National Nanotechnology Strategy set by the South African government to enhance the country’s global competitiveness and sustainable economic growth in strategic areas. The study reveals that Wits has selectively by firmly engaged in the domain of nanotechnology and has laid the foundations for a comprehensive programme in both research and training. However, its success in this direction will largely depend upon the ways it maximises the use of the increasing opportunities offered by globalization and it manages the constraints associated with it. By opportunities here I refer to the multiplicity of research sites outside the narrow academic domain and the increasing interest displayed by government, the private sector and relevant international agencies in the field. The constraints are connected to the fact that the field of nanotechnology remains incipient and suffers from the uncertainties surrounding a relatively young field of enquiry in universities in South Africa (financial shortages, lack of skills etc).