Electronic Theses and Dissertations (PhDs)

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Interfacial engineering of NbSe2 and TaSe2 to enhance their electrocatalytic activity for hydrogen production
(University of the Witwatersrand, Johannesburg, 2023-07) Kolokoto, Tshwarela; Moloto, Nosipho
There has been a need to replace fossil fuels, develop sustainable energy systems, and alleviate the negative environmental effects. These effects can be alleviated by developing efficient processes such as water-splitting, which can produce hydrogen gas in an environmentally friendly manner and, in turn, use it as a clean fuel. However, this process requires an effective electrocatalyst comparable to Pt and cost-effective. Herein, we demonstrate that the electrocatalytic activity of NbSe2 and TaSe2 can be improved by metal inclusion using interfacial engineering for the hydrogen evolution reaction (HER). The readily synthesised NbSe2 was decorated with 20% wt. Ni, 20% wt. Pt, 10% wt. Pt / 10% wt. Ni using two synthetic methods, namely the ex-situ and in-situ methods. The ex-situ samples had higher HER activities than the in-situ samples. Pt/PtO2-NbSe2 (derived from Pt decorated NbSe2 using the ex-situ method) showed a significantly enhanced HER activity compared to bare NbSe2. The Pt/PtO2-NbSe2 nanomaterial had the lowest overpotential, favourable kinetics and durability in an alkaline solution of 0.1 M KOH. The trend was as follows: Pt/PtO2-NbSe2 (Pt-decorated ex situ) > PtO-NbSe2 (Pt-decorated in-situ) > PtO/NiO-NbSe2 (Pt/Ni-decorated) > Ni/NiO-NbSe2 (Ni-decorated ex-situ) > Ni0.5Se/Ni(OH)2-NbSe2 (Ni-decorated in-situ) > NbSe2. In addition, NbSe2 was further decorated with 20% wt. Co using both the ex-situ and in-situ synthetic methods, and 10% wt. Pt / 10% wt. Co using the in-situ method. The ex-situ sample resulted in a higher HER activity compared to the in-situ samples. In particular, Co/Co3O4-NbSe2 nanomaterial (Co-decorated ex-situ) had the lowest overpotential, favourable kinetics and durability in an alkaline solution of 0.1 M KOH. The resultant trend was as follows: Co/Co3O4-NbSe2 (Co-decorated ex-situ) < Co3O4/CoSe2/PtO/PtO2-NbSe2 (Pt/Co-decorated in-situ) < Co3O4/CoSe2-NbSe2 (Co-decorated in-situ) < NbSe2. Consequently, the ex-situ method was the optimum synthetic method for forming NbSe2-based nanomaterials. TaSe2-based nanomaterials were formed similarly. TaSe2-based hybrids were formed by decorating TaSe2 with 20% wt. Ni, Co and Pt using the ex-situ method. The hybrid nanomaterials resulted in higher HER activities compared to pristine TaSe2 (i.e. Pt/PtO/PtO2-TaSe2 (Pt-decorated) > Ni/Ni(OH)2-TaSe2 (Ni-decorated) > Co/Co3O4-TaSe2 (Co-decorated) > TaSe2). Pt/PtO/PtO2-TaSe2 hybrid, in particular, resulted in the lowest overpotential under alkaline solutions (0.1 M KOH). Generally observed, was NbSe2-based electrocatalysts were better than TaSe2-based catalysts. In addition, the Pt-decorated ex-situ NbSe2 and Pt-decorated TaSe2 electrocatalysts were better than the model Pt/C catalyst, with the prior being the best overall. This is attributed to the basal sites of the NbSe2 and TaSe2. The ex-situ method was better than the in-situ method and this was due to the presence of metallic particles and the minimization of oxidation compared to the latter.
Synthesis of carbon nanodots-peptide conjugates decorated with germanium for bioimaging
(University of the Witwatersrand, Johannesburg, 2023-10) Machumele, Khanani Peggy; Makatini, Maya Mellisa; Maubane-Nkadimkeng, Manoko
The World Health Organization Global Cancer Observatory estimates that cancer caused 9.96 million deaths worldwide in 2020, making early detection crucial for diagnosis and treatment. Accurate identification of cancer plays a crucial role in the diagnosis and treatment process. It allows for customized and efficient therapies, minimizes unnecessary procedures and adverse effects, and improves the prognostic insights for patients and healthcare providers alike. The challenges in diagnosis include overdiagnosis, false positives/negative outcomes, and limited sensitivity. Advanced technologies are needed for better imaging accuracy and minimizing harm. This study aims to fabricate carbon dot-peptide conjugates to enhance bio-imaging capacity and selectivity. The peptides used are derived from the GKPILFF cell-penetrating peptide sequence and the RLRLRIGRR peptide, which is selective to cancerous cells. The Carbon dots were used to provide the photoluminescent properties required for bio-imaging of cancerous cells. Carbon dots (CDs) were synthesized using iso-ascorbic acid as the source of carbon using a microwave-assisted method. The nitrogen and germanium-modified carbon dots (Iso-N-Ge-CDs) demonstrated the highest photoluminescent properties compared to the unmodified CDs (Iso-CDs) and those with either N (Iso-N-CDs) or Ge (Iso-Ge-CDs). Photoluminescence emissions of longer wavelengths suitable for cell imaging were observed for the CDs, and the Iso-N-Ge-CDs demonstrated excitation-dependent emission wavelength behavior, pH sensitivity, and Fe3+ sensitivity. The 13 peptides derived from the peptide accelerating sequence GKPILFF and the cancer-selective peptide RLRLRIGRR were successfully synthesized. The peptides were characterized using Liquid Chromatography Mass Spectrometry (LCMS) and purified using preparative High-Pressure Liquid Chromatography (prep-HPLC). The secondary structure of the L-GKPILFF penetration acceleration peptide sequence (Pas) adopted a helical secondary structure. The D-GKPILFF derivative was found to adopt a random coil structure. These were confirmed using Nuclear Magnetic Resonance (NMR) techniques such as Total Correlation Spectroscopy (TOCSY) and Rotating Frame Overhauser Enhancement Spectroscopy (ROESY) NMR. The CDs-peptide conjugates were successfully synthesized, and the confirmation of conjugation involved multiple methods, including UV-Vis and PL techniques. To the best of our knowledge, the thesis incorporates the first study to demonstrate long-range interactions through ROESY NMR. The NMR analysis indicated that the helical structure of the peptide could be affected after conjugation, leading to notable peak shifts. Since the helical structure is crucial for the peptide's bioactivity and stability enhancement, NMR spectra with fewer structural changes in the peptide region may improve its biological properties. The research contained valuable information for scientists aiming to design and characterize Carbon dot-peptide conjugates with enhanced permeability and selectivity that can effectively deliver materials into cytosolic space.
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.
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.
Inclusion of nano-silver compounds in RO membranes as solutions to fouling by microbes and natural organic matter during seawater desalination
(University of the Witwatersrand, Johannesburg, 2023-08) Nchoe, Obakeng Boikanyo; Moloto, Nosipho; Sikhwivhilu, Keneiloe; Tetyana, Phumlani
The access to safe and potable water has become a salient discussion for governments across the globe. This is due to pronounced levels of the decline in volumes of available freshwater. Attributions to this phenomenon are mainly climate change, eutrophication, discharge of untreated effluent, heightened irrigation, and industrialization. Currently exploited freshwater sources are rivers, lakes, dams, glaciers, and aquifers. However, inconsistent rainfall patterns have rendered some of these sources as ‘stressed’, which is exacerbated by exponential population growth and misallocation of available freshwater. In hindsight, seawater was identified as a possible source of potable water. However, the high levels of salinity and miscellaneous contaminants (i.e., pathogens and natural organic matter) necessitates treatment of seawater prior its usage. Therefore, the purpose of this work is to develop rugged polyamide thin film nanocomposite (TFN) reverse osmosis (RO) membranes with antifouling properties for seawater desalination. TFN were fabricated by the inclusion of silver-based (i.e., silver sulfide) nanoparticles during interfacial polymerization of the polyamide active layer. Silver compounds are known to have superior antibacterial and photocatalytic properties, due to plasmonic and photo absorption properties. For this reason, silver oxide (Ag2O), silver sulfide (Ag2S), and silver chloride (AgCl) nanoparticles (NPs) were colloidally synthesized. These were then characterized and evaluated in photocatalytic and antibacterial applications. Cytotoxicity studies were also done to determine which of these NPs pose less risk to human health. The consolidation of data from these applications advised which of these NPs would be suitable for incorporation into the polyamide layer to produce fouling resistant TFN. Microscopic analysis depicted well-defined shapes, with average sizes of 23.0±5.7 (Ag2O), 30.6±7.4 (Ag2S), and 10.6±7.2 nm (AgCl). X-ray diffraction determined Ag2O, Ag2S, and AgCl NPs to have cubic, monoclinic, and cubic lattices, respectively. Optical spectroscopy determined Ag2O, Ag2S, and AgCl NPs to have band gap energies of 2.97, 3.11, and 3.05 eV, respectively. These observations inferred that crystalline NPs that exhibit surface plasmon resonance (SPR) in the visible region were successfully synthesized. SPR is a desired characteristic for photocatalysts, and indeed Ag2O, Ag2S, and AgCl NPs achieved humic acid degradation (HA) efficiencies of 86.2, 88.1, and 76.5%, respectively. In antibacterial studies, the broth micro-dilution method indicated that the minimum inhibitory concentration (MIC) values against Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) for Ag2O, Ag2S and AgCl NPs were 0.03125, 0.125, and 0.125 mg/mL, respectively. The well-diffusion tests showed that Ag2O NPs had the largest zones of inhibition (ZOI), followed by Ag2S, then AgCl NPs. These observations demonstrated the concentration-dependent mitigation of bacterial cell proliferation. The NPs were further tested for cytotoxicity against human embryotic kidney 293 (HEK 293) cells. It was found that the cytotoxic concentration that rendered 50 % viability (CC50) were 0.0302, 0.3606 and 0.3419, and were obtained for Ag2O, Ag2S and AgCl NPs, respectively. This data implied that Ag2O NPs were the most toxic, while Ag2S and AgCl NPs were least toxic. In light of the above, Ag2S NPs were selected to be incorporated into TFN RO membranes. TFN RO membranes were fabricated by the addition of three different concentrations of Ag2S NPs in the aqueous phase to form the active polyamide (PA) layer on a polysulphone (PSF) support, namely 20, 30, and 50 mg. Fourier transform infrared (FTIR) spectroscopy detected vibrational peaks at 1659 cm-1 (amide I C=O stretch), 1542 cm-1 (amide II C-N stretch) 1481 cm-1 (C-H bend), 1385 cm-1 (C-O stretch), 1242 cm-1 (C-N stretch), and 779cm-1 (aromatic C-H and C=C wagging). The presence of aromatic and amide functional groups corroborated the formation of the TFN active layer, which is responsible for RO filtration of dissolved ions in water. Moreover, atomic force microscopy (AFM) revealed that average surface roughness decreased with increased Ag2S NP loading. TFN loaded with 20, 30, and 50 mg Ag2S NPs recorded water contact angles (WCA) of 54.1, 45.4, and 43.3°, respectively. The WCA of thin film composite membranes (TFC) without Ag2S NPs was recorded to be 73.5°. This demonstrated that the inclusion of Ag2S NPs increased surface hydrophilicity. In addition, salt rejection and water flux were higher for 30 mg loaded TFN (98 % and 32.7 L/m2h) compared to those of TFC (97% and 24.8 L/m2h). The bacterial growth inhibition was observed to be significantly high for 30 mg loaded TFN (80 %) compared to that of TFC (38 %). These observations indicate that the inclusion of Ag2S NPs significantly enhanced the performance of RO membranes and cost effectiveness of desalination.
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
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
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
Synthesis, characterization and investigation of the mode of action in the anticancer activity of novel platinum complexes
(University of the Witwatersrand, Johannesburg, 2024) Peega, Tebogo; Harmse, Leonie; Kotzé, Izak. A.
Cancer remains a global health concern, causing approximately 10 million deaths in 2020. Lung cancer, accounting for 18% of cancer-related deaths, and colorectal cancer, contributing 9.4%, are major contributors to this alarming statistic, emphasizing the urgent need for innovative and effective treatment options. Despite the success of platinum-based drugs such as cisplatin, carboplatin, and oxaliplatin, their limitations and severe adverse effects necessitate the exploration of alternative chemotherapeutic agents. This research project focused on synthesizing and characterizing square planar platinum(II) complexes bearing variations of two bidentate coordinating ligands; disubstituted acylthiourea and diimine ligands, each possessing unique physical and chemical properties. A series of cationic [Pt(diimine)(Ln-κO,S)]Cl complexes were successfully synthesized and characterized using nuclear magnetic resonance spectroscopy, infrared spectroscopy, mass spectrometry, and elemental analysis. The anticancer activity of these complexes was evaluated against two lung cancer cell lines, A549 and H1975, and a colorectal cancer cell line, HT-29. In vitro cytotoxicity studies included the determination of IC50 values of active complexes and assessing their cell death mechanisms through multiple biochemical marker assays. These included annexin-V binding, caspase-3/7 and caspase-8 activity, mitochondrial membrane potential (MMP), reactive oxygen species (ROS) and immunofluorescence for the expression of key proteins involved in the DNA damage response and oxidative stress response, such as p21 and haemoxygenase-1 (HO-1). A proteome array was employed to investigate the effects on apoptosis-associated proteins. The results indicated that these platinum complexes were more cytotoxic than cisplatin with IC50 values ranging between 0.68 μM and 2.28 μM. Further investigation showed that the platinum complexes induced cell stress, chromatin condensation, nuclear fragmentation, increased phosphatidylserine (PS) on the outer cell membranes and activated caspase-3/7. Platinum complexes induced intrinsic apoptosis in cancer cells, as evidenced by the loss of mitochondrial membrane potential and the absence of caspase-8 activity. Elevated ROS levels, increased HO-1 expression and increased expression of p21 suggested oxidative stress and DNA damage as the trigger source for intrinsic apoptotic cell death. The active complexes downregulated pro-survival proteins (IGFs) in lung cancer cells and anti-apoptotic proteins (survivin and HSP70) and upregulated pro-apoptotic proteins (p21, TRAIL R2), across the three cancer cell lines, indicating potential dual activation of apoptotic pathways. DNA binding studies indicated groove binding and intercalation as the mode of interaction with DNA. The findings highlight the potential of these platinum complexes as promising candidates for further development as cancer therapeutics.
Structural Characterization of Bimetal-Phosphate Based Solid-State Electrolytes: A PXRD, PDF and XAS Study
(University of the Witwatersrand, Johannesburg, 2024) Nkala, Gugulethu Charmaine; Billing, David G.; Billing, Caren; Vila, Fernando D.; Forbes, Roy P.
In this work, NASICON-type lithium titanium phosphate (LiTi2(PO4)3, LTP) was synthesized following the conventional solid-state reaction methodology. Single and double-doped formulations of LTP were made, with the primary objective of improving the room-temperature ionic conductivity, for their application as potential solid-state electrolytes for all-solid-state Li ion batteries. The primary characterization technique applied was ambient-temperature powder X-ray diffraction (PXRD) at both laboratory and synchrotron experimental conditions. The Rietveld refinement approach was used to determine the qualitative and quantitative phase compositions of each sample, revealing the rhombohedral (R-3c, space group #167) main phase, with phosphate-based secondary phases. Total scattering data, through the pair distribution function (PDF) was applied, revealing lattice site preference during the substitution of Ti with Al, Sn and Dy at the 12c site. Further analysis through small-box modelling indicated the local structure deviation below 10 Å, from rhombohedral (R-3c) to monoclinic (P21/n, space group #14). The application of experimental X-ray absorption spectroscopy (XAS) revealed a stable 4+ oxidation state for Ti regardless of doping. However, the extended X-ray absorption fine structure (EXAFS) data showed that the replacement of Ti with Sn results in heavy disorder and subsequent changes in the PO4 tetrahedra, corroborating the findings from Raman spectroscopy. Theoretical XAS spectra were computed using FEFF, providing insights into the origins of experimentally observed XAS features from first-principles. Applying electrochemical impedance spectroscopy (EIS) to assess the ambient-temperature ionic conductivity, co-doped systems showed an improvement in the conductivity. The application of characterization techniques at various length scales has been demonstrated to provide insights into the mechanisms governing the performance of the solid-state electrolytes.
Quantitative analysis of gold in low-grade tailings from different matrices, coupled with a study into the associated uncertainties
(University of the Witwatersrand, Johannesburg, 2023) Mashale, Kedibone Nicholine; Tshilongo, James; Chimuka, Luke
Gold is one of the precious group elements that is used for various purposes, such as jewellery, auto catalysts and as a form of investment. Various countries have gold reserves, with South Africa being the leading gold producer between 1980 and 2007. However, as of 2022, it is ranked as the eighth largest producer of gold, contributing 3% to the global contribution. The majority of gold is mainly mined from the Witwatersrand Basin in Johannesburg. It is well known that mining has been ongoing for decades, which means that a significant amount of land has been mined across the country. During gold mining, a large proportion of the ore material from which the gold is extracted is waste, together with the chemicals that were used, and this waste is termed mine tailings. This implies that based on the years that gold mining has occurred for and the depth of mining, a significant amount of the tailings have been deposited into free land around the mines, some of which are close to communities. The tailings consist of traces of gold that were left due to inefficient extraction processes and other components, such as base metals. The disadvantage of this is that due to the other chemical composition of these tailings, they have the potential to be dangerous to the environment. Some tailings contain minerals such as jarosite (KFe2(SO4)2(OH)6) that cause acid mine drainage, while heavy metals such as lead, mercury, arsenic and chromium can leach into surface and ground waters, causing pollution. Furthermore, they pose a danger if the dams that they are stored in collapse, which was recently witnessed in South Africa. Because of these factors, there have been various advances made towards the beneficiation of tailings, such as utilizing them to make glass or bricks for construction. A major advancement was the reprocessing of these mine tailings to recover or extract the remaining gold, which benefits both the environment and the mining houses. Therefore, in a move to support this initiative, scientists have taken to the laboratory to develop new or optimize existing methods for the extraction and quantification of gold, which is expected to be of a low grade over time. Various methods can be used for the quantification of gold, including the conventional fire assay, wet and dry chlorination and acid digestion. Most of these are suitable for medium- to high-grade gold ores but are known to experience challenges in regard to low-grade ores. The aim of this research was therefore to find the optimum method for the quantification of gold from mine tailings emanating from the Ventersdorp Contact Reef (VCR) and Barberton Greenstone Belt (GBS). Subsequent to chemical analysis, the samples were characterized for mineralogy using X-ray diffraction (XRD) and Brunauer‒Emmett‒Teller (BET) surface area
The Design, Synthesis and Structure-Activity Relationship of Antitubercular Lassomycin Derivatives
(University of the Witwatersrand, Johannesburg, 2023) Ngqinayo, Ntombizanele; Makatini, Maya
Tuberculosis (TB) is a potentially fatal infectious disease caused by Mycobacterium tuberculosis (Mtb) and is a global health risk responsible for over 1.5 million deaths worldwide annually. Tuberculosis is treated with a combinatory regimen of approved first- line drugs such as rifampicin and isoniazid as well as second-line anti-TB drugs such as fluoroquinolones, most of which use similar mechanisms to cause cell death. The formation of multidrug resistance (MDR) TB strains, biofilms, and dormant persister cells (non- replicating cells) are some factors that prolong TB treatment and hence the need for developing novel antitubercular agents with a different mode of action. Furthermore, the emergence of multidrug resistance TB poses a challenge in controlling and eradicating tuberculosis. Lassomycin is a novel antimicrobial peptide (AMP) that has garnered much interest across various research groups due to its ability to effectively target and kill Mtb, including MDR strains and latent TB, with a potency that is similar to that of rifampicin. Lassomycin is highly basic and targets the highly acidic N-terminal domain (NTD) of the caseinolytic enzyme that forms part of the caseinolytic protease crucial for Mtb cell survival. Lassomycin has an unusual mode of action that causes Mtb cell death by disrupting the highly controlled and tightly regulated proteolysis by inhibiting proteolytic activities as well as increasing unfoldase activities. Thus, lassomycin shows great potential as a candidate for drug development. This study aimed to design lassomycin derivatives with improved stability and potency; and synthesize them using shorter and cost-effective synthetic routes. Peptide modifications includes (i) replacing the macrolactam ring in the peptide sequence with a disulfide bridge via a simpler ring-formation method resulting in an enlarged cyclic ring; (ii) replacing ‘difficult’ arginine residues with less basic lysine residues; (iii) forming cationic derivatives by increasing the number of basic lysine residues to enhance selectivity for the bacterial membrane; (iv) conjugating peptide derivatives to lipophilic molecules including palmitic acid and 1-adamantane carboxylic acid to improve bacterial cell penetration and binding; (v) conjugating the peptides to silver nanoparticles for improved drug delivery and antimicrobial effect; (vi) incorporating N-methylated residues to improve peptide stability; (vii) making non-polar peptide derivatives by replacing all basic amino acids with alanine to investigate the importance of the basic residues and study structure activity relationship (SAR) (viii) synthesizing linear derivatives in order to investigate the effect of the ring and (ix) shortening the peptide sequences to include only the cyclic ring or the tail sequence portions in order to shorten the synthetic route. Peptides were synthesized via the Fluorenylmethyloxycarbonyl (Fmoc) solid phase peptide synthesis strategy (SPPS) and purified using a semi-preparative High-Performance Liquid Chromatogram (prep-HPLC). They were then analysed using High-Performance Liquid Chromatography Mass Spectrometry (HPLC-MS), Circular Dichroism (CD), and nuclear magnetic resonance (NMR) spectroscopy. Silver nanoparticles and the peptide conjugates were characterized using ultraviolet-visible (UV-Vis) spectrophotometry and transmission electron microscopy (TEM) imaging. Two-dimensional (2D) Nuclear magnetic resonance (NMR) spectroscopy, including [1H, 1H] COSY, [1H, 1H] TOCSY, [1H, 13C] HSQC, [1H, 1H] HMBC and [1H, 1H] ROESY were used to determine the structural conformation of Pep-2- NN, a lassomycin derivative that has activity against tuberculosis. Furthermore, the secondary structure of selected derivatives was examined using circular dichroism (CD) spectroscopy. Computational studies were utilized to determine the structure of the active lassomycin derivatives, Pep-2-NN and Pep-2-NNA. All the peptide derivatives were successfully synthesized, including non-polar, short-chained, and those conjugated to silver nanoparticles and lipophilic molecules. The disulfide bridge was successfully added to replace the lactam bridge of the parent lassomycin peptide by oxidising sidechain thiol groups of two cysteine residues inserted at appropriate positions in the sequences. All the peptides were purified to varying degrees of success, and their behaviour was analysed to investigate structure-activity relationships. The silver nanoparticles were successfully synthesized in-house and conjugated to Pep-2-NN. Transmission Electron Microscopy (TEM) imaging revealed that the silver nanoparticles have a spherical morphology at sizes that ranged between 7 nm and 9 nm whilst peptide conjugated nanoparticles were between 9 – 12 nm. Caseinolytic protease (ClpP1P2 or ClpP) assay studies revealed that the peptides display inhibiting and activating properties when screened against the protease, including lassomycin derivatives with shortened chains such as Ring-2-NNA-Ada, Ring-2-NN, and Tail-2-NN. The secondary structure of selected lassomycin derivatives was studied using circular dichroism (CD), revealing that the structures are comprised of anti-parallel beta- (β) sheets at slightly higher proportions followed by alpha- (α) helix and, to some extent, β-turn motif. Computational studies were conducted on selected derivatives to predict their secondary structure and revealed that the peptides form stable α-helical conformations. NMR revealed that Pep-2-NN formed a ‘knotted’ structure, where the tail sequence was threaded inside the cyclic ring with a curved loop, and certain residues in the ring acted as ‘steric plugs’ to prevent unthreading. In conclusion, the insertion of the disulfide bridge remains an effective alternative to the lactam bond found originally on lassomycin and can result in the formation of biologically active derivatives with the desired stable ‘lasso’ conformation.
Trimetallic nanoparticles immobilised on polymeric membranes for the degradation of organic pollutants in water
(University of the Witwatersrand, Johannesburg, 2021) Kgatle, Masaku; Moloto, Nosipho; Sikhwivhilu, Keneiloe; Ndlovu, Gebhu
Water is one of the most essential resources in the world, but its scarcity has become an issue of global concern. The scarcity of water is largely the result of climate change, water pollution and increasing population growth which limits the availability of water resources. Moreover, South Africa has been making headlines since 2010 due to water shortages experienced. It is, therefore crucial to find cost-effective ways to expand the water supply and address the issue of water pollution. This study seeks to tackle the problem of water pollution emanating from textile industries. Over the last few years, nanotechnology and membrane technology have appeared as some of the most widely used methods for the mitigation of water pollution problems. Particularly, nanoscale zerovalent iron (nZVI) has emerged as one of the most broadly used nanoparticles in wastewater treatment and remediation owing to its low-cost and high effectiveness. However, because of its ease of aggregation and consequent loss of reactivity, nZVI is coupled with one or more transition metals to produce multimetallic systems. Nanoparticles alone quickly agglomerate and form large micro-scale particles owing to the magnetic forces thus losing their mobility and chemical reactivity. To avoid these issues, the nanoparticles are stabilized on polymeric membranes. In this study, two trimetallic nanoparticle systems were synthesized, characterized and tested for catalytic activity. The polyvinylpyrrolidone (PVP)-stabilized Fe/Cu/Ag nanoparticles were synthesized by the sodium borohydride chemical reduction method. These nanoparticles were characterized using XRD, XPS, EDX and TEM. The XRD, EDX and XPS techniques showed the presence of all three metals, including iron oxides due to the oxidation of iron in air. The obtained TEM images showed the characteristic core-shell morphology of the nZVI-based nanoparticles. The evaluation of the catalytic activity of the nanoparticles was conducted using methyl orange (MO) dye as the model pollutant and this showed a remarkable degradation efficiency within few minutes. The effect of parameters such as MO solution pH, initial MO dye concentration and nanoparticle dosage in MO degradation was investigated. The nanoparticles were found to have performed better at lower pH, lower initial MO dye concentration and higher nanoparticle dosage. The degradation of MO dye was monitored using UV-Vis analysis and occurred within 1 min. The degradation was found to follow a pseudo first-order kinetic model and was vastly influenced by the studied parameters. The analysis of by-products and reaction pathway were done using LC-MS and this further confirmed that the degradation of MO was indeed rapid. The Fe/Cu/Ag trimetallic nanoparticles were demonstrated as suitable and effectual alternative for the remediation of textile dye wastewater. For the second trimetallic system, three different trimetallic nanoparticles (Fe/(Zn/Ag), Fe/Zn/Ag and Fe/ Ag/Zn) with different metal addition sequences were synthesized. The prepared nanoparticles were characterized using XRD, EDX and TEM analyses. The techniques proved successful synthesis of the nanoparticles and XRD and EDX showed the presence of the three metals together with the oxides. The evaluation of the catalytic reactivity of the nanoparticles was conducted in a series of batch experiments using MO dye as the model pollutant. About 100% of the MO dye was degraded by Fe/ Ag/Zn trimetallic nanoparticles within 1 min and the second-order rate constant obtained was 0.0744 ppm- 1min-\ the rate of reaction was higher than that of the other trimetallic systems. Using Fe/ Ag/Zn trimetallic nanoparticles, parametric tests were conducted at different MO solution pH, initial MO concentration and nanoparticle dosage. The results showed that the reactivity of the Fe/Ag/Zn trimetallic nanoparticles was highly dependent on the aforementioned parameters. Like the Fe/Cu/Ag system, the Fe/Ag/Zn performed better at lower pH, lower initial MO dye concentration and higher nanoparticle dosage. The overall kinetic study showed the removal of MO using Fe/Ag/Zn system to follow a second-order kinetic model. The elucidation of the degradation pathway and MO by-products identification were done using LC-MS and the mechanism of degradation displayed the degradation of methyl orange to proceed via azo-bond cleavage. Moreover, the Fe/ Ag/Zn nanoparticles proved to be effective at degrading methyl orange dye and can be used to treat azo-dye wastewater from textile industries. The Fe/Cu/ Ag trimetallic nanoparticle system was immobilized on a polymethacrylic acid grafted polyethersulfone (PMAA-g-PES) membrane to minimize the issue of recoverability and nanoparticle agglomeration. The nanocomposite membranes were prepared by loading different quantities of Fe/Cu/Ag trimetallic nanoparticles onto the PMAA-g-PES membrane for optimization purposes. Characterization was performed using FTIR, NMR, XPS, SEM/EDS and AFM analyses. The PMAA g-PES and nanocomposite membranes were found to have a porous top layer and a rough surface. Moreover, the addition of nanoparticles did not cause any significant changes in the membrane structure, however, further addition of nano particles led to the blockage of pores. The performance of the synthesized membranes was tested using pure water flux and MO (anionic dye) and methylene blue (MB) (cationic dye) dye removal capacity. The negatively charged membranes were found to have more affinity for MB dye than the MO dye and this was ascribed to the charge interaction between the membrane surface and the dyes. The nanocomposite with 5% Fe/Cu/Ag trimetallic nanoparticle loading on PMAA-g-PES membrane (M4-5% membrane) was found to have the best adsorption capacity with about 60% MB dye removal efficiency. Furthermore, the effect of process parameters such as pH, temperature and H2O2 concentration on the removal of MB was studied. The removal efficiency was found to be higher at higher pH and lower temperature. About 100% removal efficiency was obtained when the process was performed at pH 9 in the presence of H2O2 via adsorption and Fenton degradation. This showed that a hybrid of processes was convenient for the removal of MB dye by adsorption (primarily) and degradation using the nanocomposite membrane. Adsorption equilibrium data were assessed using the Langmuir, Freundlich and Temkin models; the Temkin model was the most convenient to explain the adsorption of MB onto M4-5% membrane. Moreover, lcinetic studies were performed on four kinetic models: pseudo first-order, pseudo second order, intraparticle diffusion and elovich models. The pseudo second-order was found to be the best suitable to explain the adsorption of MB onto M4-5% membrane. Thus, the adsorption of MB onto the nanocomposite membrane is an exothermic chemical process that occurs on a heterogeneous surface. Therefore, the nanocomposite membrane has the prospective to be applied in the removal of cationic textile dyes in the presence of an oxidiser.
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.
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.

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