School of Chemistry (ETDs)

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A comprehensive analysis of urban river pollution – the case of the Hennops river in Gauteng Province, South Africa
(University of the Witwatersrand, Johannesburg, 2023) Letseka, Thabiso Esaiah; Chimuka, L.; Richards, L.H.
The water quality of rivers is declining at an alarming rate due to pollution from anthropogenic activities associated with urbanization. To ensure ecological restoration and management of rivers, engaging in pollutant source apportionment, evaluation, and monitoring of water quality is of great significance. The study delivers a comprehensive assessment of the state of pollution in the Hennops river catchment facing pollution threats from rapid urbanization. The water quality assessment of the Hennops river was performed through chemical, microbiological, microplastics analysis and ecotoxicological approaches, spanning from upstream region in Tembisa to the downstream Hartbeespoort Dam. Standard methods were employed to assess physiochemical properties of the river’s water. Electrical conductivity and pH fell within the accepted criteria based on the standard water quality guidelines. However dissolved oxygen (DO) levels were below acceptable limits, ranging from 1.53 mg L-1 to 6.47 mg L-1. This signifies a substantial demand for oxygen in the river, likely due to the discharge of sewage from leaking pipes and wastewater treatment plants. This sewage introduces a high volume of organic matter, leading to an increased oxygen demand in the water. Microbiological pollution indicators were employed to assess the microbial water quality of the river. The study's findings revealed elevated bacterial counts, with Escherichia Coli (E. coli) reaching up to 2 250 cfu mL-1 upstream and decreasing to 30 cfu mL-1 downstream. These high counts suggest faecal contamination in the river water. Similar trends were observed with total coliform counts, high coliform counts 170 000 cfu mL-1 in the upstream which remained detectable even downstream and beyond the Hartbeespoort Dam, despite the dilution effects within the dam. The dam was identified as the primary repository for pollution originating upstream. Grab sampling followed by solid phase extraction (SPE) and the passive sampling using a Polar Organic Integrative Sampler (POCIS), were employed as sample preparation methods for preconcentration of methocarbamol, etilefrine, nevirapine, carbamazepine and venlafaxine from river water with subsequent analysis on Liquid Chromatography coupled to quadrupole time of flight mass spectrometry. Both methods yielded good figures of merit with limits of quantification in the range of 0.57 to 2.12 ng mL-1 for POCIS and 0.19 to 1.82 ng mL-1 for SPE. The compounds were detected in the water but at low levels (µgL-1 ), with detected concentrations of carbamazepine in the range 0.62 ng mL-1 – 0.32 ng mL-1 , methocarbamol detected in the range 0.11 ng mL-1 - 0.14 ng mL-1 and venlafaxine 0.50 ng mL-1 – 0.44 ng mL-1 using POCIS. The detected concentrations using SPE were in the range 0.13 ng mL-1 – 0.19 ng mL-1 for carbamazepine, while nevirapine and venlafaxine were detected although below limit of quantification. This underscores the advantage of using passive samplers, which enable the detection of fluctuating contaminant concentrations over time, in contrast to the one-time measurements obtained through grab sampling. In the case of microplastics in the water and sediment samples, five polymer types were identified: polyethylene (PE), polypropylene (PP), high density polyethylene, (HDPE), polyester and polystyrene. The predominant polymer type in surface water was PE (48.6 %), and that in sediment was PP (52.7 %). PE and PP were the most abundant polymer types in both phases, and as these also the leading polymers in plastics production. 80% of the identified microplastics were found to be fibre with most dominant sizes of 1-2 mm for sediments and 0.5-1 mm in water samples. The conducted tests deemed the river water not suitable for irrigation, drinking or recreational purposes and not capable to support aquatic life.
A study of the support effect of carbon dots-derived graphene-like sheets on the autoreduction of cobalt nanoparticles for Fischer–Tropsch synthesis
(2022) Mokoloko, Lerato Lydia
The aim of this study was to synthesize and characterize carbon dots (CDs) and to use them as a support material for cobalt (Co) based Fischer-Tropsch synthesis (FTS) reactions. The CDs were chosen for this study due to their small size (< 10 nm), easy surface functionalization and synthesis. The small size of the CDs was required for the study of inverse catalyst support effects. An inverse supported catalyst (in this case, the Co/CDs catalyst) refers to the dispersion of a support material that has a small size (d < 5 nm) onto the surface of a metal catalyst with a similar small size (d > 8 nm). The synthesis of this proposed catalyst was successful. FTS studies on the Co ‘supported’ CDs were attempted. Extremely poor FT activity was observed. Post analysis of the catalyst revealed that the CDs did not retain their quasi-spherical and small particle size morphology under the FTS reaction conditions (temperature 220 °C, 10 bar P; H2:CO ratio = 2:1). Instead, upon exposure to a heat treatment, the CDs were transformed into layered structures with a unique resemblance to graphene-based nanosheets (GNSs). This transformation impacted on the use of these catalysts in the FTS reaction. However, this result indicated an unusual transformation of the CDs into another carbon shape. In light of the fascinating transformation phenomenon, annealing studies were then conducted to investigate the effect of annealing temperatures on the CDs structural changes. The CDs (average d= ~ 2.5 nm) used in this study were obtained from the microwave-assisted carbonization of L-ascorbic acid and subjected to a heat treatment (i.e. annealing) at temperatures between 200 and 700 ℃ in a horizontal CVD apparatus under an inert nitrogen gas. It was observed that annealing transformed the CDs from 0-D qausi-spherical nanoparticles to 3- D multi-layered carbons (at 300-600 ℃) and finally 2-D layered materials (at 700 ℃). Furthermore, annealing at 700 ℃ yielded a 2-D single-layered material with comparable properties to traditionally reduced graphene oxide (rGO). A wide range of characterization techniques were used to gain an insight into the physicochemical properties of these novel CDs-derived allotropes as well as to rationalize their mechanism of formation. After evaluating the properties of these materials, it was clear that the surface oxygen functional groups, observed from XPS, 13C NMR and other studies, were responsible for the CDs to rGO transformation. It was proposed that the CDs are assembled to form rGO (and other CDs-rGO derivatives) by either the Ostwald ripening (in which the carbons agglomerated via a gas phase) or a solid phase reaction (involving reaction of CD edges). To further investigate the effect of annealing on the evolution of CDs to layered carbon structures, N-doped CDs (or NCDs) were also studied. The method used to make the pristine CDs was modified by incorporating urea as a nitrogen source to make the NCDs. Annealing the NCDs at temperatures between 200 and 700 ℃ also transformed the quasi-spherical NCDs (average d = ~ 4.1 nm) to multi-layered carbon sheets at temperature as low as 200 ℃. The CD transformation was also associated with the loss of surface functional groups, with % O and N contents of ca. 17 and 16 % (pristine NCDs) being reduced to ca. 8 and 7 % for NCDs annealed at 700 ℃. A similar mechanism for the formation of these N-doped layered carbon structures by annealing was also proposed here. For these samples, it was also observed that the N-bonds, especially the sp3 type nitrogen bonds found on the edges of the NCDs, also took part in the coalescence of the NCDs to give the layered materials. XPS data suggested that in the process, these sp3 type nitrogen bonds were transformed into sp2 pyrrolic-N, pyridinic-N and GraphiticN groups. The annealed CDs products were used to support Co (called Co3O4/T250, Co3O4/T400 and Co3O4/T700 where T is the temperature at which the CDs were annealed) for use in FT studies. Studies were conducted to evaluate the effect Co hydrogen reduction temperatures verses autoreduction temperature, catalyst thermal stability and performance in the FTS reaction at 220 °C (10 bar P; H2:CO ratio = 2:1). Upon investigation of the reduction behaviour of the Co/CDs derivative catalysts using in situ PXRD, it was found that these materials can successfully facilitate autoreduction of Co3O4 to Co face-centered-cubic (fcc) at temperatures > 400 ℃ by a reduction pathway similar to that observed using conventional H2 reduction conditions. As expected, the reduction under H2 took place at a lower activation temperature (> 250 ℃) than the autoreduction process. It was also noted that these novel carbon support derived from CDs gave reduced FTS performance compared to the unsupported Co, especially towards C5+ yields (< 30 % for all Co supported catalysts). These novel CDs-derived allotropes were found to have limited use as supports in Co-based FTS, due to Co agglomeration. These NCDs-derived allotropes (annealed at 200 ℃, 400 ℃ and 700 ℃) were incorporated as active layers in the fabrication of chemoresistive sensing device detection of volatile organic compounds (VOCs). These layered showed enhanced chemical vapour sensing properties, especially for methanol and ethanol detection at room temperature. Therefore, although there are great limitations for applications of these CDs-derived layered allotropes in FTS reaction, these materials show a much better potential for applications in facile and cost effective VOC sensors. Further studies on this will be conducted.
A systematic study on the use of the sol-gel synthetic method for lithium manganese oxide-based cathode materials
(University of the Witwatersrand, Johannesburg, 2024-09) Muntswu, Zwivhuya; Billing, Caren; Ferg, Ernst E.; Billing, David G.
This dissertation investigated the synthesis of two lithium manganese oxide-based cathode materials (Li1.03Mn1.97O4 and LiAl0.4Mn1.6O4) using the sol-gel method and probing the phase transitions during the synthesis. The sol-gel synthetic method involved dissolving stoichiometric amounts of lithium nitrate, manganese nitrate hydrate, and citric acid in distilled water forming an aqueous solution. The starting precursor materials were dried at 140 °C which formed a crystalline phase of -Aqua-S-citrato (2-)-manganese(II) with an orthorhombic crystal system and P222 space group. The thermal behaviour of the precursor was explored to understand the effects of calcination/annealing temperatures. Thermal analysis of precursors prepared using nitrate salts with a 1:1 total metal ion to citric acid ratio displayed thermal stability to temperatures higher than 380 °C with the formation of a final metal oxide after 70% mass loss due to the decomposition of the organic and nitrate materials. However, when increasing the concentration of the complexing agent, an increase in material decomposition due to an increase in organic material is seen. The precursor materials prepared with a lower complexing agent concentration result in materials that have thermal instability when exposed to high temperatures. Thermal analysis of Li1.03Mn1.97O4 and LiAl0.4Mn1.6O4 prepared using acetate salts as starting materials shows material decomposition at high temperature of ~600 °C Calcining both undoped and Al-doped nitrate precursors at moderate temperatures (380 °C to 500 °C) resulted in the formation of Li1.03Mn1.97O4 and LiAl0.4Mn1.6O4 with a pure cubic spinel structure and an Fd-3m space group, however, increasing the calcining temperature to 800 °C for the undoped nitrate-based precursor revealed an impurity phase formation relating to dilithium manganese oxide with a monoclinic crystal system. On the other hand, calcining acetate-based precursors at moderate temperatures (380 °C to 500°C) results in metal oxides with low crystallinity compared to metal oxides prepared with nitrate-based precursors. Calcining acetate-based precursors at 800 °C was more favourable since they form the desired metal oxides without any impurities which might imply structural phase stability at high temperatures. The local and average crystallographic structures (via PDF and XRD respectively) of various nitrate-based metal oxides were investigated, where a good agreement between collected data and a calculated structural model revealed the formation of a cubic spinel structure of space group Fd-3m. Li1.03Mn1.97O4 and LiAl0.4Mn1.6O4 metal oxides were achieved from calcining precursors at moderate temperatures of 380 °C and 450 °C. The PDF high r-value signal displays a good fit which confirms to the average structure data information where the r-value signal which correspond to the local structure refinements have a minor discrepancy when fitted with a cubic spinel of space group Fd-3m.
Application of oxidative enzymes in membrane systems for the bioremediation of triazines in wastewater
(University of the Witwatersrand, Johannesburg, 2024-10) Lesaoana, Mahadi; Richards, Heidi L.; Brady, Dean
The prevalence of herbicidal pollutants present in various environmental matrices have become a global concern. The discharge and accumulation of s-triazine agrochemicals in effluents remains a major challenge, threatening the quality of freshwater resources. These are newly identified recalcitrant contaminants of concern (CECs) with complex structures, and inadvertent exposure poses deleterious ecological risks and human health-related adverse effects. Unfortunately, they have shown resistance to conventional treatment strategies, hence their persistence in wastewater treatment plant (WWTP) effluents and water bodies. Therefore, there is an urgent need for the exploration of alternative technologies for the effective eradication of such contaminants from water samples. The bioconversion of such micropollutants using oxidative enzymes like laccase is a promising research avenue, providing a sustainable, economically and ecologically benign strategy. The current research examined the potential of a hybrid biocatalytic membrane system to degrade common s-triazine agrochemical herbicides in aqueous solutions. Specifically, the use of Novoprime base 268 laccase coupled with hollow fibre polyethersulfone (PES) membranes was investigated for the bioremediation of atrazine (ATZ), ametryn (AMT), simazine (SMZ) , prometon (PMT) and terbuthylazine (TERB) in wastewater. In batch-mode reactions, major operating parameters (i.e. pH and temperature profiles, enzyme dosage and contact time) were varied for the laccase-assisted catalysis of s-triazine compounds. Optimised conditions provided highest removal efficiencies (> 88.9%) at pH 5.0, combined with a temperature of 25°C and 1.0 mg L-1 solution concentration after 24h reaction time. Through the addition of redox mediators viz. 2,2-azino-bis-3-ethylbenzothiazoline-6-sulphonic acid (ABTS), violuric acid (VA), vanillin (VA), syringaldehyde (SRA) and acetosyringone (ASR) recalcitrant triazine degradation was enhanced by 10 to 20 % at 1.50 mm. Subsequently, the performance of a standalone continuous flow-mode membrane system was evaluated firstly, using a bed adsorption column only operated under various conditions. The efficiencies were compared to batch-mode enzymatic experiments. The adsorption of triazines by PES was only weakly influenced by pH, and the optimum removal was attained at pH 5.0 (5.0 mg L-1), 2.35 g bed mass (14.0 cm height) and 24h column operation time. The overall removal percentages were 72.6%, 75.2%, 71.4%, 67.4%, and 68.2% for ATZ, AMT, SMZ, PMT and TERB, respectively. Although the results indicated satisfactory performances by both systems, their performance is limited when used as separate units (continuous membrane vs laccase reactor). A biocatalytic membrane system was achieved by integrating laccase into the dynamic packed-bed membrane column. Relevant process control design parameters of the fixed-bed biocatalytic column were carefully evaluated and recorded an optimum of 93.2 % removal efficiency as observed at a feed flow rate 2.0 mL min-1, at a bed height of 14.0 cm using an atrazine influent concentration of 5.0 mg L-1. Equilibrium dynamics of the breakthrough modelling were best fitted by Thomas model. Results attained demonstrated selectivity for triazines in matrix-matched real river water samples with remarkable recyclability after six successive operational cycles. This reflects the potential workability of the integrated system for extended enzymatic reactions evaluated under robust experimental conditions. As a benchmarking exercise, cost-analysis studies showed comparable projected scalability of our configuration at 1200 m3/d capacity at an estimated total cost of R7.036 mil.
Biocatalytic oxidative conversion of valencene to nootkatone mediated by lipoxygenase and cytochrome P450
(University of the Witwatersrand, Johannesburg, 2024-03) Raboya, Christopher; Ngwira, Kennedy; Brady, Dean
Nootkatone (NK) is an oxygen-containing sesquiterpene with a significant grapefruit aroma and plays an important role in the flavour and fragrance industry. The natural production of NK through extraction produces trace amounts and is therefore not a viable option to meet industrial needs. The chemical synthesis often utilises reagents harmful to the environment. The purpose of this research was to explore the use of crude lipoxygenase (LOX) enzyme extracted from soya beans, commercial mutants of CYP450 as well as laccase enzymes for the conversion of valencene (VL), the aroma components of citrus fruits to NK. For the LOX reactions, a conversion of 28.79% (mol/mol) was obtained when the reactions were performed with LOX only. The best conversion of 74.46 % was realised when FeSO4.7H2O and MnSO4 were added to the reaction. In the temperature studies, 70 ˚C was shown to be the optimal temperature for the conversion. In addition, we observed that vegetable oils provided sufficient unsaturated fatty acids to facilitate the conversion of VL to NK with sunflower oil being the best. In exploring the potential of LOX to oxidise other organic molecules, caryophyllene was oxidised to novel caryophyllene oxide, and styrene was oxidised to benzoic acid, 1-phenylethane-1,2-diol, and 2-hydroxyl-2-phenylethyl benzoate. This is the first time that such oxidations are reported, and this underlines the potential of LOX in biotransformation and organic synthesis. For CYP450 reactions, the best conversion of 16.70 % was obtained using a variant sourced from Prozomix. Evaluation of parameters such as temperature, pH (7.0 to 7.5), using buffer solutions should be explored to optimise the activity of the enzyme. Laccase from Novoprime Base 268 showed no activity for the conversion of VL to NK despite making use of mediators. Therefore, we should explore changing the reaction conditions, varying the pH of the reaction, buffer strength or mediator and laccases from other sources such as Cerrena unicolor and Trametes versicolor, as well as a fresh batch of laccase from Trichoderma (Merck) should be assessed in the conversion of VL to NK.
Biophysical studies of metal chelate binding by HSA: Towards an understanding of metallodrug transport
(University of the Witwatersrand, Johannesburg, 2023) Sookai, Sheldon; Munro, Orde
Human serum albumin (HSA) is the most abundant blood protein, transporting many exogenous compounds including clinically deployed and investigational drugs that are generally organic in nature. HSA may largely influence the pharmacokinetics and pharmacodynamics of these drugs. Therefore, studying their interactions with HSA is vital in progressing drug development. In this thesis we present work on the synthesis and characterisation of five Schiff base bis(pyrrolide-imine) ligands that were metalated with either Au(III) (Chapters 2 and 3) or Pt(II) (Chapters 4 and 5). One of the ligands H2L1 was further metalated with Ni(II) and Pd(II) (Chapter 6). In Chapters 2 and 3 focus on a patented class of anti-cancer bis(pyrrolide-imine) Au(III) Schiff base chelates. Three Au(III) chelates were synthesized in Chapter 2 and underwent National Cancer Institute (NCI)-60 cytotoxic screening. Among them, AuL1 and AuL3 underwent full-five dose testing and recorded GI50 values of 7.3 µM and 11.5 µM, and IC50 values of 15.7 µM and 30.9 µM, respectively. AuL1 was tested further and found to be an interfacial poison of topoisomerase II at 0.5–5 µM and a catalytic inhibitor at 50 µM. In Chapter 3, two chiral tetradentate cyclohexane-1,2-diamine-bridged bis(pyrrole-imine) Au(III) complexes were reported, both of which were found to be cytotoxic in the NCI-60 screen. The chiral Au(III) chelates had a different mode of action compared to AuL1. Hierarchical cluster analysis suggest that their mode of action is similar to that of taxol. All five Au(III) chelates bound to HSA with moderate affinity (104–105 M–1) and minimally perturbed the structure of the protein. This highlights the potential for the Au(III) complexes to be transported by the HSA-mediated pathway. Chapters 4 and 5 focused on the synthesis of novel and previously reported Pt(II) Schiff base chelates to spectroscopically and computationally study their interaction with HSA and elucidate if the chelates could act as theranostic agents. It was found that switching the linking bis(imine) carbon linkage altered the binding affinity of the complex. However, the Pt(II) ion ensured that all three Pt(II) chelates preferred binding to Sudlow’s site II of HSA. The data was corroborated by molecular docking simulations and ONIOM calculations. Only 2 was found to be cytotoxic when irradiated with UV light but was found to act as a photosensitizer rather than a theranostic agent. Chapter 6 investigated the influence of d8 metal ions (Ni(II), Pd(II) and Pt(II) within the same ligand scaffold (H2PrPyrr) binding to HAS, which was investigated by steady state fluorescence quenching. The affinity constants, Ka, ranged from -3.5 -103 M−1 to-1- 106 M–1 at 37 C, following the order Pd(PrPyrr) > Pt(PrPyrr) > Ni(PrPyrr) >H2PrPyrr. The Pd(II) chelate was prone to hydrolysis and had a unique binding mode which we attribute to the unusually high binding affinity. The complexes uptake is enthalpically driven, hinging mainly on London dispersion forces. In summation, twelve metal complexes were successfully synthesized, of which 11 bound to HSA with a moderate binding affinity. The Au(III) chelates preferred Sudlow’s site I, while the Pt(II) chelates preferred Sudlow’s site II. Overall, the metal complexes bound fully intact to HSA.
Carbon nitride-based catalysts for thermal carbon monoxide oxidation: Does phase matter?
(University of the Witwatersrand, Johannesburg, 2023-06) Mohamed, Ahmed Gamal Abdelmoneim; Ozoemena, Kenneth Ikechukwu; Abdullah, Aboubakr M.; Eid, Kamel
Carbon monoxide (CO) has a poisonous effect on all living organisms as it binds to the hemoglobin of blood cells, preventing oxygen uptake. Thus, the conversion of CO to less dangerous gas such as CO2 is an essential process. This work presented the utilization of carbon nitrides (C3Nx) in different phases (βgC3N4, βC3N5, βC3N6) for thermal carbon monoxide (CO) oxidation. Herein, gC3N4, C3N5, and C3N6 were prepared by pyrolysis of their amine precursors, which were doped with Fe by two distinct methods; mechanical mixing (Fe/C3Nx-M) and polymerization (Fe/C3Nx-P). The controlled preparation of Fe/gC3N4-P allowed the formation of hierarchical porous structures with high surface area (219 m2/g) compared to the Fe/gC3N4-M (77 m2/g). This enabled the ease of reactants diffusion, enhanced the electron transfer, and maximized the atomic utilization. Accordingly, Fe/gC3N4-P (T100= 245 °C) presented higher catalytic activity than Fe/gC3N4-M (T100= 291 °C). In addition, bimetallic FeTi/gC3N4-P and trimetallic FeTiCu/gC3N4-P catalysts achieved the complete conversion of carbon monoxide (CO) at lower temperatures; 175 and 147 °C, respectively, which was attributed to the enhanced reducibility, and synergistic effect of Ti and Cu. Besides, FeTi/gC3N4-P and FeTiCu/gC3N4-P showed higher catalytic activity than Pd/C commercial catalyst (T100= 198 °C). In addition, the trimetallic FeTiCu/gC3N4-P showed a stable catalytic behavior without any deactivation for more than ten hours. This study showed that the C3Nx phases worked successfully in the thermal catalytic CO oxidation. However, the gC3N4 phase is the most active one when doped with metal(s), as it offered higher crystallinity, graphitization, and thermal stability than C3N5 and C3N6. This study also paves the way for the utilization of gC3N4 as a support for different metals to be used efficiently in various thermal catalytic applications, not only CO Oxidation.
Characterization, quantification, and recovery of rare earth elements(rees) in South African coal fly ash samples
(University of the Witwatersrand, Johannesburg, 2024) Rampfumedzi, Tshilidzi Michael; Chimuka, Luke
Rare earth elements (REEs) are naturally distributed throughout the Earth's crust, typically in low concentrations. They are not typically found in isolation but are rather present in various minerals, often in amounts too minute for cost-effective extraction. Fly ash is among the sources that are deemed economically viable for extracting REEs. The objective of this study was to create environmentally sustainable approaches for measuring and reclaiming rare earth elements (REEs) in coal fly ash (CAF) samples. The study involved analyzing fly ash samples collected from various coal power stations using a range of standard and advanced techniques, including X-ray fluorescence (XRF), X-ray diffraction(XRD), scanning electron microscopy (SEM), and inductively coupled plasma mass spectrometry (ICP-MS) and inductively coupled plasma optical emission spectrometry (ICP-OES). The XRF only shows the presence of REEs from all three fly ash samples with a range of 40 to 100 ppm and mineral oxide ranging from 0.1 to 50 %. The XRD results show that fly ash sample is a siliceous-rich sample with abundant minerals such as quartz (SiO2), magnetite (Fe3O4), and mullite (Al4.52Si1.48O9.74). The SEM analysis of the sample confirmed the presence of rare earth minerals, including monazite which is a light atomic mass (LREE), xenotime, a heavy atomic mass (HREE), and perrierite-bearing minerals. The results obtained from the instrumental analysis show that the ICP-MS instrument is the more effective analytical technique for REE analysis in this context as compared to ICP-OES. Using certified reference materials, the results obtained by two acids digestion technique, acids digestion and sodium peroxide fusion in, CGL 111, CGL 124, and AMISO276, were compared to validate whether the methods are reliable. The acid digestion approach demonstrated greater effectiveness in comparison to the sodium peroxide fusion method. The recovery percentage (%) from ICP‒MS showed an excellent percentage yield (80 – 120%) compared to the ICP‒OES instrument (50 –120%). The ICP‒MS data indicate that all fly ash samples have a high concentration of LREEs and a lower concentration of HREEs. Excellent recovery was obtained by ICP‒MS in a developed microwave acid digestion method. The concentration of REEs obtained from ICP - MS and OES in fly ash samples ranged from 50 ppm to 200 ppm for light rare earth elements and 0.5 ppm to 20 ppm for heavy rare earth elements. The total REE (TREE) concentrations in all fly ash samples range from 400 ppm to 600 ppm
Colloidal synthesis and characterization of molybdenum and tungsten-based phosphide electrocatalysts for hydrogen evolution reaction
(2022) Nkabinde, Siyabonga Sipho; Moloto , Nosipho
The production of hydrogen gas via hydrogen evolution reaction (HER) in acidic media has become an important area of research in light of the increasing demand for sustainable and environmentally friendly sources of energy. However, its large-scale production is currently being hindered by the lack of inexpensive and highly efficient non-noble electrocatalysts. Transition metal phosphides (TMPs) have transpired as favourable catalysts that can be prepared from cheap and readily available sources. Up to now, TMPs have been commonly prepared using solid-state and solid-gas reactions, which rely on the use of high temperatures and hence generate inhomogeneity in the prepared materials. Inhomogeneous materials are unattractive as catalysts because the correlation between a catalyst and its structural features cannot be systematically studied. For this reason, colloidal synthesis has emerged as a powerful method in the synthesis of TMPs as it allows for control over the resulting physical features (i.e. size, morphology, crystal phase, crystallinity etc.). The ability to tailor these physical properties provides room for improving the catalytic activity. By using the colloidal synthesis method, we have successfully prepared molybdenum and tungsten-based phosphide nanoparticles and studied the effect of their physical features on HER activity. In chapter 3, we report a facile colloidal synthesis method to produce an amorphous phase of molybdenum phosphide (MoP) by using trioctylphosphine (TOP) as a phosphorus source, molybdenum pentachloride (MoCl5) as a metal source and 1-octadecene (1-ODE) as a solvent/reducing agent. The use of the forementioned precursors promoted the formation of very small, shape controlled and well dispersed amorphous molybdenum phosphide (MoP) nanoparticles. Annealing (800 °C) of the amorphous MoP nanoparticles resulted in the formation of a crystalline MoP phase with a slightly bigger size but retained its dispersity and morphology upon exposure to high temperature. The amorphous and crystalline MoP phases were compared as HER electrocatalysts. HER results indicated that the amorphous MoP phase exhibited enhanced catalytic activity in hydrogen evolution reaction compared to the crystalline MoP phase. The high activity displayed by the amorphous MoP was attributed to the small sizes and the high density of unsaturated active sites characteristic of nanoparticles lacking long range crystalline order.
Defect–engineered lithium titanate anode materials for lithium–ion batteries
(University of the Witwatersrand, Johannesburg, 2023-10) Podile, Seromo; Haruna, Aderemi Bashiru; Ozoemena, Kenneth Ikechukwu
Energy is one of the aspects that plays a central role in moving society forward since it is one of the most important agenda of global economic and energy forums. There is an urgent need to move to clean energy given the environmental and the health benefits resulting from implementing energy systems that utilize green energy. The proposed energy sources in these systems are primarily natural (e.g. wind and solar), which means they are beyond human control and would work better if coupled with energy storage devices (ESDs). From this emerge the importance of energy storage systems (ESSs) which mostly perform based on the materials utilized to assemble the devices. In this study, we seek to enhance the power and energy densities of two of the prominent energy storage systems, namely lithium-ion batteries (LIBs) and lithium-ion capacitors (LICs), using modified commercial lithium titanate (LTO) materials as anodes. The materials consist of the pristine LTO (LTO-p), LTO coated with cerium fluoride (LTO-CeF3) and dry and wet irradiated pristine LTO (LTO-p-md and LTO-p-mw) and LTO coated with cerium fluoride (LTO-CeF3-md and LTO-CeF3-mw). Microwave irradiation was used to study the possible defect that the radiation can bring to the materials and possibly use the microwave effect to improve their electrochemistry. The effects of the coated layer of CeF3 and microwave irradiation on the structure of the commercial LTO were extensively studied using powder X-ray diffractometer (XRD), Raman spectroscopy, transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA) and Brunauer-Emmett-Teller (BET) theory. Structural investigations of TEM micrographs revealed CeF3 was present on the surface of the coated LTO materials. Further analysis did show that some portion of the CeF3 coating layer was co-doped into the LTO nanostructures. The findings from XRD and XPS analyses showed that co doping promoted a mixed state of Ti3+ and Ti4+ resulting from charge compensation when Ce3+ and F possibly substituted Ti4+ and O2-. This mixed state of titanium ions allowed the materials to have high electric conductivity than the pristine LTO (LTO-p). The spectrographs obtained from XPS analysis also showed that LTO microwave irradiated materials without coating experienced the same mixed states, which may have originated from oxygen vacancies that allowed for charge compensation when some of the LTO-p Ti4+ had to turn to Ti3+. The HRTEM analysis demonstrated changes to the lattice planes spacing of the modified LTO materials and these changes supported the (111) lattice plane shifts observed in the XRD analysis. The electrochemistry of the lithium-ion battery was studied on all the materials using cycling voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) techniques. From GCD technique, the results indicated that the modified materials had higher specific capacities than LTO-p at all rates under rate capability studies. All as prepared materials were stable with coulombic efficiency of almost 100% for 100 cycles using 1 C, with modified materials surpassing LTO-p specific capacity. Cycling at a high rate (5 C) for 1000 cycles, saw LTO-md, LTO-mw, LTO-CeF3 and LTO-CeF3-md having high-capacity degradation after 500 cycles. It was also observed that initial capacities were retained up to the 1000th cycle for LTO-CeF3-mw and LTO-p. With an aim of comparing LTO-p and LTO-CeF3 half cells, these two materials were further studied for lithium-ion capacitors (LICs). This choice was based on LTO-CeF3 having a better charge-discharge profile, high specific capacity and coulombic efficiency than LTO-p. The improvements resulted from the coating layer (leveraging the high ionic conductivity and chemically inert properties of CeF3) and the observed defects seen on the XPS studies created by the coating synthesis process. The LTO-CeF3 anode was paired with a carbon black cathode to construct a full LIC cell. The assembled cell produced a maximal energy density of 107.4 Wh kg 1 with an accompanying power density of 2000 W kg-1, and it yielded a power density of 10 000 W kg -1 with an energy density of 61.8 Wh kg -1.
Design and synthesis of chronic wound healing collagen peptide mimics
(University of the Witwatersrand, Johannesburg, 2024) Lesotho, Ntlama Francis
The South African wound care management market is expecting a compound annual growth rate (CAGR) of 6.75%. The numbers are expected to further increase because South Africa has the highest number (4.6 million) of people living with diabetes in Africa. Annually approximately 2% of patients with diabetes develop diabetic foot ulcers and hence chronic wounds. Many chronic wound patients must deal with the financial burden, as many current wound treatment options are expensive, ineffective, and inconvenient. Intervention in the form of synthetic collagen mimetic peptides has been limited due to cytotoxicity and susceptibility to protease degradation. These challenges have, for an ardent time affected the clinical and commercial development of synthetic wound healing peptides. The aim of the current study is to develop novel wound healing peptides by derivatizing bioactive peptides into selective and protease stable peptidomimetics. All the synthesized peptides are meant to mimic the function of collagen type I. Thus, the designed peptides comprise of the retro- integrin binding type I collagen motif, -GFOGER-, the DGD tripeptide for attraction of growth factors, the retro- tripeptides Thr-Thr-Lys (TTK), Gly-His-Lys (GHK), Gln-Pro-Arg (QPR) and Glu-Glu-Met (EEM) to stimulate collagen production. The importance of collagen is evidenced by the fact that it features in all four stages of wound healing. This therefore means, its inclusion in any biomaterial meant to curb chronicity in wound healing is indispensable. With this approach, the biomaterial would overcome the challenge of excess matrix metalloproteinases (MMPs), which degrade both viable and nonviable collagen used in the wound healing process. It would further provide a collagen-based wound scaffold that compensates for the loss of collagen required for proper tissue regeneration. The applications of collagens in wound healing are immense. Due to its material properties, and apparent effectiveness, collagen has the potential to be utilized as an unprecedented treatment protocol for chronic, slow-healing wounds. Sixteen palmitate and adamantane collagen mimetic peptides were designed and successfully synthesized using the solid-phase peptide synthesis strategy. Eight of the sixteen peptidescomprise of lipophilic moieties (adamantane and palmitic acid) for improved membrane permeability and different collagen inducing retro-tripeptides namely, TTK, GHK, QPR and EEM (retro-DGD-GG-GFOGER-GG-TTK-Adamantane (NL010)/palmitate (NL009), retro-DGD- GG-GFOGER-GG-GHK-Adamantane/palmitate, retro-DGD-GG-GFOGER-GG-QPR- Adamantane/palmitate and retro-DGD-GG-GFOGER-GG-EEM-Adamantane/palmitate). Another eight are control peptides without the retro-tripeptides (retro-DGRGOF- Adamantane/palmitate, retro-GOP-GFOGER-GOP-Adamantane/palmitate, retro-GG- GFOGER-GG-Adamantane/palmitate and retro-DGD-GG-GFOGER-GG-Adamantane (NL008)/palmitate). The tertiary structure and secondary features (folding patterns) of the peptides were determined using the Nuclear Magnetic Resonance (NMR) and Circular Dichroism (CD). From NMR experiments, medium-range couplings were detected for NL010 and NL009, suggesting a possibility of alpha helices. Temperature 1H NMR experiment for the peptide DGRGOF- Adamantane proved the presence of cis and trans geometric isomers. CD experiments revealed that NL009 mainly has α-helix while NL010 mainly consists of a parallel conformation. Synthesis of adamantane and palmitate peptides with enhanced integrin binding was accomplished by incorporation of para-fluorophenylalanine in place of phenylalanine in the peptide retro-GG-GFOGER-GG-Adamantane/palmitate. The peptides were obtained in low yields but with increased hydrophobicity. Structural features for the improvement of the stability of the peptides against protease degradation were accomplished by the synthesis of peptoids and N-methylated peptides. The peptoids were synthesized in low yields but with increased hydrophobicity. The efficacy of NL009 and NL010 in wound healing was tested both in vitro and in vivo. In the former, the efficiency of both NL009 and NL010 in inducing migration of cells in a scratch wound was accentuated by hyaluronic acid. In in vivo studies, NL010 performed better than NL009. However, NL010 was outperformed by a comparator, Puramatrix® The peptides have the ability to induce migration of cells and therefore have an ability to create an environment needed for proper wound healing. The peptides could be used in place of native collagen and bring about proper healing of wounds
Design and synthesis of triazine derivatives as non-nucleoside reverse transcriptase inhibitors
(University of the Witwatersrand, Johannesburg, 2024) Munetsi, Wendy; Bode, Moira; Ngwira, Kennedy
This research work was carried out to investigate the properties of different groups that can be used to modify the triazine core with the aim of designing a new library of possible HIV non- nucleoside reverse transcriptase inhibitors (NNRTIs). Triazine derivatives have been used extensively in the synthesis of numerous classes of drugs due to their significant biological activity. In this project, the specific focus was to synthesize 1,3,5-triazine derivatives by successive nucleophilic substitution reactions of the Cl atoms from cyanuric chloride. In the first step of the substitution reactions, 2,4,6-trichloro-1,3,5-triazine was reacted with various anilines, phenols and thiophenols which acted as nucleophiles to displace one of the Cl atoms upon reaction completion. The yields varied from 28% -90% with the best yields being observed when the anilines were used as a nucleophile and most of the substituents in this first step were anilines. The substituents used at each step of the substitution were vital in terms of determining the order of the reaction to enable a successful reaction. The introduction of different linkers to the triazine core such as -NH, -S, -O yielded compounds with different properties expected to provide significant interactions in the NNRTI binding pocket. We expected better binding properties from the -NH bearing compounds due to hydrogen bond formations with amino acid residues inside the allosteric binding pocket of the HIV-1 RT. The success of the second step of the substitution reactions was identified to be dependent on the substituent attached to the triazine ring from the first step. Some reactions were not successful when a stronger nucleophile was used in the first step and a weaker nucleophile was being used as the incoming nucleophile substituting the second Cl atom. Therefore, these reactions were repeated and the order of the reaction rearranged. Temperatures were increased and reaction times were increased at this stage as the reactivity of the triazine ring was reduced and therefore higher kinetic energy was required for successful reactions. In general, the synthesized triazine derivatives bearing two aromatic substituents exhibited the most significant presence of tautomers. The final stage in the synthesis of the trisubstituted triazine derivatives was relatively complex and required much higher temperatures and longer reaction times. The reactions were also performed at smaller scales and difficulties with the purification processes also contributed to the loss of product thereby resulting in lower yields, with one of the compounds giving a yield of 11%. The results obtained from the anti-HIV assay studies from the selected compounds tested, showed that antiviral activity was observed in triazine derivatives with electron withdrawing groups attached to the aromatic substituent as well as -NH and -O linkers at the right and left wing of the triazine core, respectively.
Determination of U and Th Radioisotopes in environmental samples by ICP-QMS
(University of the Witwatersrand, Johannesburg, 2023-07) Rikhotso, Xikhongelo Valentia; Sehata, James; Chimuka, Luke
Not Available
Development of a Commercial Manufacturing Process of 9-[(R)-2- (phosphonomethoxy)propyl] adenine (PMPA): A Key Intermediate for the Production of Tenofovir-based HIV Medicines
(University of the Witwatersrand, Johannesburg, 2023) Mbutho, Banele; Gohain, Mukut; De Koning, Charles
South Africa runs the largest antiretroviral (ARV) program in the world and yet 99% of the active pharmaceutical ingredients (APIs) used to make ARVs are imported from China. Dependence on imported APIs has major cost implications and influences the medication’s security of supply. This project was concerned with making it possible to produce the APIs tenofovir, a precursor for tenofovir disoproxil fumarate and tenofovir alafenamide locally and at a lower cost. A new synthetic route recently introduced by Medicines 4 All (M4ALL) was studied and used in this dissertation. The four-step process that produces an adenine derivative was optimized and scaled into a commercial industrial process producing tenofovir intermediates in repeatable yield and purity. This route was determined to be the most cost-effective since it utilized low cost and commercially available diaminomaleonitrile and triethyl orthoformate as starting materials—contrary to the synthetic routes currently used by the 17 largest tenofovir manufacturers. Key process improvements included a decrease in the number of solvents used and the minimization of by-product formation. Results showed that high yields of tenofovir intermediates were successfully synthesized using this new route. As such, the chemical company we conducted this research in, Chemical Process Technology Pharma will be able to employ this synthetic methodology to affordably produce the APIs used in the manufacturing of ARVs locally improving access to affordable medication.
Development of eco-friendly building bricks derived from carbon nanotube-reinforced coal ash and low-density polyethylene waste materials
(University of the Witwatersrand, Johannesburg, 2024) Makgabutlane, Boitumelo; Maubane-Nkadimeng, M.S.; Coville, N.J.
This study reports on the incorporation of carbon nanotubes (CNTs) into the all-waste derived building bricks. The focus was on waste management and beneficiation of plastic waste and coal ash, which are generated in large volumes without sufficient recycling. The waste materials were characterized using a range of techniques to ascertain their properties for application. Multiwalled carbon nanotubes (MWCNTs) were synthesized using a facile floating chemical vapour deposition method (CVD) and their physicochemical properties were tested. Bricks with dimensions of 220 x 105 x 70mm were developed with an optimum 85:15 coal ash to plastic waste ratio respectively using a specialized reactor. The bricks were tested for compressive strength, split tensile strength, water absorption, strain, thermal stability and durability using oxygen permeability index, chloride conductivity index and water sorptivity index as indicators. Furthermore, environmental and financial sustainability and ecotoxicology were tested. At optimum conditions, high quality MWCNTs with a diameter of 83 nm, length of 414 μm and a carbon yield of 73% were obtained. The ID/IG ratio of 0.44, an oxidation temperature of 649 °C, a purity of 94% and surface area of 50.9 m2/g were achieved. Coal fly ash with a spherical shape, particle size of below 10 micron and a thermal stability of 680 °C was used as an aggregate for the bricks. The bricks (without CNTs) developed their maximum compressive strength of 11.9 MPa at 14 days. The incorporation of the CNTs improved the microstructure of the bricks by filling the voids and providing a bridging effect as reinforcement mechanisms. The optimum CNT loading of 0.05 wt.% produced bricks with a compressive strength of 22 MPa and tensile strength of 8.7 MPa, which exceeded the South African National Standards (SANS227:2007) requirements for building bricks by 450% and 625% respectively. The durability properties were improved as the CNT dosage was increased from 0-10 wt.%. The 0.05 wt.% bricks were categorized as “good” for all the durability indexes. The CNT containing bricks showed improved thermal stability and maintained their structural integrity. The chemical resistance also improved and the efflorescence was minimal on all the bricks. The utilization of waste in the bricks enabled resource conservation, reduced pollution and reduced cost compared to conventional bricks. When only considering the raw materials used, the cost of production per brick was $0.091. The ecotoxicology of the powdered brick samples was tested on Raphidocelis subcapitata (microalga) and Daphnia magna (aquatic organism) using leachates from neutral, acidic and basic mediums. Some heavy metals were leached above the threshold limit especially in acidic medium. The leachates were toxic to the test species at low concentrations and resulted in growth inhibition of the microalga and immobization of the aquatic organisms. The toxicity of the CNTs was inconclusive and dedicated tests are required to study their effect. With appropriate treatment of CFA, the waste derived CNT bricks have a great potential of being a sustainable alternative to the conventional bricks based on cost, properties and environmental impact
Diastereoselective conjugate addition reactions using diverse nucleophiles on a variety of Morita-Baylis-Hillman (MBH) adducts
(University of the Witwatersrand, Johannesburg, 2023-09) Bhom, Nafisa; Bode, Moira L.
The Morita-Baylis-Hillman (MBH) reaction involves the formation of a new carbon-carbon bond, generating an MBH adduct. These MBH adducts are multi-functional molecules, which can be used as synthons for the generation of complex and diverse compounds. The first part of the work described here involved the synthesis of a series of diverse ester and nitrile MBH adducts obtained as racemic mixtures. The MBH adducts were protected using different protecting groups, which could potentially control the diastereoselectivity and the formation of alternative products in the subsequent conjugate addition reaction. Conjugate addition reactions were performed on the protected MBH adducts using different nucleophiles to obtain the product as diastereomers. These reactions were monitored to detect whether diastereomers were obtained or not. The diastereomeric ratios obtained using different substrates, protecting groups and nucleophiles were determined. The best diastereomeric ratio was 3:1, obtained for the piperidine and benzylamine addition on the TBDMS protected nitrile adducts 192a/b and 196a/b. The addition of sulfur nucleophiles gave the conjugate addition product only and the addition of nitrogen nucleophiles gave both conjugate addition and allylic substitution products. It was found that the protecting groups did not have an effect on the diastereomeric ratio obtained, nor on the formation of alternative products. The last step performed in the sequence was the deprotection of the conjugate addition products. The configuration of the major and the minor diastereomers were determined, the major product was assigned as the syn diastereomer. The major:minor diastereomeric ratio for compound 208a/b was 3:1 and for compound 209a/b, a ratio of 2:1 was obtained. The next part of the work involved the synthesis of MBH adducts with amide as the electron withdrawing group. The originally proposed route involved the synthesis of MBH esters and their conversion into amides. The conjugate addition reactions were attempted on these amide adducts, but were unsuccessful. A number of alternative routes were attempted for the synthesis of amide adducts and conjugate addition products resulting from these adducts. From all the alternative routes, the best route was the originally proposed route.
Dissolution of non-functionalized and functionalized nanomaterials in simulated biological and environmental fluids
(University of the Witwatersrand, Johannesburg, 2023-06) Mbanga, Odwa; Gulumian, Mary; Cukrowska, Ewa
The incorporation of nanoparticles in consumer products is exponentially high, however, research into their behaviour in biological and environmental surroundings is still very limited. In the present study, the static system and the continuous flow-through dissolution protocols were utilized to evaluate and elucidate the dissolution behaviour of gold, silver, and titanium dioxide nanoparticles. The behaviour of these particles was studied in a range of artificial physiological fluids and environmental media, to obtain a more precise comprehension of how they would react in the human body and the environment. The biodurability and persistence were estimated by calculating the dissolution kinetics of the nanoparticles in artificial physiological fluids and environmental media. The details of the current research are described as follows: An investigation into the dissolution of non-functionalized and functionalized gold nanoparticles was conducted as the first component of the research, examining the effect of surface functionalization on dissolution. The study determined the dissolution rates of functionalized and non-functionalized gold nanoparticles. Dissolution was observed to be significantly higher in acidic media than in alkaline media. The nanoparticle surface modification, particle aggregation, and chemical composition of the simulated fluid significantly affected the dissolution rate. It was concluded that gold nanoparticles are biodurable and have the potential to cause long-term health effect as well as high environmental persistency. This work has been published in the Journal of Nanoparticle Research and is presented in this thesis as Paper 1. Silver nanoparticles were also included in this study because they have many applications and industrial purposes. Therefore, their risk assessment was also of utmost importance. The results indicated that silver nanoparticle solubility was influenced by the alkalinity and acidity of artificial media. Low pH values and high ionic strength encouraged silver nanoparticle dissolution and accelerated the dissolution rate. The agglomeration state and reactivity of the particles changed upon exposure to simulated fluids, though their shape remained the same. The fast dissolution rates in most fluids indicated that the release of silver ions would cause short-term effects. This work has been published in Toxicology Reports and has been presented in this thesis as Paper 2. Although titanium dioxide nanoparticles are insoluble and undergo negligible dissolution, it was of utmost importance to investigate their behaviour in biological and environmental surroundings. This is as a result of the incorporation of these particles in everyday consumer products, in the nanosized range which raises concerns about their safety. Therefore, in Paper 3 presented in this thesis the dissolution kinetics of titanium dioxide nanoparticles in simulated body fluids representative of the lungs, stomach, blood plasma and media representing the aquatic ecosystem were investigated to anticipate how they behave in vivo. This work has been published in Toxicology In Vitro and presented in this thesis as Paper 3. The results indicated that titanium dioxide nanoparticles were very insoluble, and their dissolution was limited in all simulated fluids. Acidic media such as the synthetic stomach fluids were most successful in dissolving the particles, while alkaline media had lower dissolution. High ionic strength seawater also had a higher dissolution rate than freshwater. The dissolution rates of the particles were low, and their half-times were long. The results indicated that these particles could potentially cause health issues in the long term, as well as remain unchanged in the environment. This work has been published in Toxicology In Vitro and presented in this thesis as Paper 3. The last component of the research compared the dissolution kinetics of gold, silver and titanium dioxide nanoparticles through the use of the continuous flow-through system. The findings indicated that titanium dioxide nanoparticles were the most biodurable and persistent, followed by gold and silver nanoparticles. Therefore, it was suggested that product developers should use the OECD's guidelines for testing before releasing their product to the market to ensure its safety. This work has been published in Nanomaterials MDPI and presented in this thesis as Paper 4.
Energy storage properties of carbon onion-carbon nanofibre composites containing transition metal compounds
(University of the Witwatersrand, Johannesburg, 2022) Khawula, Tobile Nokuphiwa Yollanda; Ozoemena, K. I.
The quest for electrical energy storage has been a key driver for researchers to come up with more effective means of storing this form of energy due to the intermittent nature of renewable energy sources. Several countries have swiftly adopted the transformative potential of renewables, in particular solar energy, while others have delayed the implementation due to complex policies surrounding renewable energy projects. A way forward would be innovative regulatory approaches that encourage the pairing of solar systems with other generation technologies, and with storage, to offer a “round the clock” supply. Rechargeable batteries and supercapacitors are widely employed energy storage systems. A rechargeable battery system offers high energy density, with lithium-ion batteries (LIBs) being the most widely used. For some applications, it is imperative that energy is delivered at a much faster rate. This characteristic feature is known as power density, and supercapacitors have proven to be much better than batteries in this case. The large-scale commercialization and adoption of a supercapacitor are hindered by its low energy density. The electrode material is a major determinant of the success of supercapacitors. Generally, these are supported on high surface area carbon materials. This study focused on the development of electrospun polyacrylonitrile (PAN) fibres embedded with onion- like carbon (OLC) and iron (II) phthalocyanine (FePc) particles, and encapsulation of the fibres with Molybdenum disulphide (MoS2). Furthermore, composite fibres were either integrated with manganese (III) oxide (Mn2O3) or engineered with defects for enhanced performance in symmetric supercapacitors. The synthesis of electrode materials was divided into four phases; In the first phase (1), OLC nanoparticles were embedded in electrospun PAN fibres and decorated with the Mn2O3 and evaluated as supercapacitor electrode materials. For enhanced interfacial electrochemistry and overall capacitance, the electrode material in (1) was encapsulated with MoS2 in phase (2). In phase (3) FePc embedded in the PAN electrospun fibres were evaluated for supercapacitor applications. Limited specific capacitance and poor cycling stability were observed, thus suggesting integrating OLC and further encapsulation with MoS2, in phase (4). The morphology of the fibres was vii engineered with defects in the form of Fe2+ vacancies to maximize the electrochemical reactions of the OLC/MoS2 fibre composite. The electrochemical properties of the fibre composite materials were investigated and OLC/Mn2O3-CNF exhibited a specific capacitance, energy and power density of electrodes were 200 F g-1, 44.63 Wh kg-1 and 3 235 W kg-1, respectively with excellent capacitance retention. While the MoS2 encapsulated and Mn2O3 decorated fibre composite, OLC/MoS2@Mn2O3 displayed a specific capacitance, energy and power density of 348 Fg-1 18.42 Wh kg-1 and 5 095 W kg-1, respectively. It is pertinent to note that the capacitance of the electrodes was retained throughout the 5 000 cycles of the charge-discharge test. Upon thermal treatment at 600 °C, FePc-PAN transformed into FeN4-CMF and exhibited a specific capacitance, energy and power density of 147 F g-1, 12.48 Wh kg-1 and 4 320 W kg-1, respectively. The vacancy-rich (FeN4)d-OLC- CNF@MoS2 composite obtained by the removal of Fe2+ atoms, showed a specific capacitance, energy density and power density of 481 F g-1, 76 Wh kg-1 5833 W kg-1, respectively. This study underscores strategic processes that can be adapted in the design, synthesis and optimization of supercapacitors-based electrodes for enhanced performance.
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, J
Monitoring 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.
Fast oxide ion conductors for solid oxide fuel cells: average and local structure – property correlations in solid solutions of bi2o3
(University of the Witwatersrand, Johannesburg, 2023) Masina,Sikhumbuzo Mfanawemphi; Billing, David Gordon
In this thesis, substituted Bi2O3 systems were fabricated and characterized. W, Dy, Erand Nb were used as substituents in a goal to stabilise the highly conductive δ-Bi2O3like phases (hence forth referred to as the δ-phases) to ambient temperatures. Changes in both the average and local structures of the substituted Bi2O3 systems were correlated with the physical property conductivity. In the first part of the thesis, powder X-ray diffraction and Raman spectroscopy were used to show that WO3 on its own did not stabilise the δ-phase at ambient temperatures. The true equilibrium phase in the Bi2O3- WO3 system was a mixture of two tetragonal phases 7Bi2O3·2WO3 and 7Bi2O3·WO3. The co-doping strategy was used to fabricate the Bi2O3-Dy2O3-WO3 system (DWSB, where D =Dy, SB = stabilised Bi2O3). The δ-phase was stabilised with a minimum of 15 mol% total substituent concentration. Powder X-ray diffraction indicated that the δ- phases obtained in this system were metastable and degraded after isothermal annealing at ~ 500 °C for 100 hours. Addition of Er to the DWSB system to create the novel system Bi2O3-Dy2O3-Er2O3-WO3 (DEWSB, where E=Er) was found to significantly improve the stability of the δ-phase when annealed at virtually identical conditions as DWSB. The rest of the thesis is focused on the effect of each substituent cation on phase stability, local structure and the ageing phenomenon–the decrease in ionic conductivity upon isothermal annealing without any observable changes in average structure under powder X-ray diffraction. X-ray pair distribution function, X-ray absorption spectroscopy and photoluminescence were used to probe the local structure around the host Bi cations and some of the substituent cations (Dy, Er, W). The results indicated that some of the Bi cations are displaced away from the 4a site of the defect fluorite structure (Fm-3m) and that at the local level, the Bi cations assume an arrangement similar to that found in the monoclinic α-Bi2O3 phase. Dy and Er were also found to prefer local environments similar to those in their parent oxides. The resemblance increased as the material aged and might explain why the conductivity decreases upon ageing