published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / Physics / Master / Master of Philosophy

published_or_final_version / Physics / Doctoral / Doctor of Philosophy

Graphene is a one-atom thick sheet of graphite which made of carbon atoms arranged in a hexagonal lattice. Carbon nanotubes and graphene nanoribbons can be viewed as single molecules in nanometer scale. Carbon nanotubes are usually labeled in terms of the chiral vectors which are also the directions that graphene sheets are rolled up. Due to their small scale and special structures, carbon nanotubes present interesting electrical, optical, mechanical, thermal, and toxic properties. Graphene nanoribbons can be viewed as strips cut from infinite graphene. Graphene nanoribbons can be either metallic or semiconducting depending on their edge structures. These are robust materials with excellent electrical conduction properties and have the potential for device applications. In this research project, we present a theoretical study of electrical properties of the carbon structures. Electronic band structures, density of states, and conductance are calculated. The theoretical models include a tight-binding model, a Green’s function methodology, and the Landauer formalism. We have investigated the effects of vacancy and weak disorder on the conductance of zigzag carbon nanoribbons. The resulting local density of states (LDOS) and conductance bands show that electron transport has interesting behavior in the presence of any disorder. In general, the presence of any disorder in the GNRs causes a decrease in conductance. In the presence of a vacancy at the edge site, a maximum decrease in conductance has been observed which is due to the presence of quasi-localized states. / Theory -- Band structure and density of states of carbon nanotubes -- Band structure and density of states of graphene nanoribbons -- Quantum conductance of zigzag graphene nanoribbons -- Quantum conductance of a zigzag graphene nanoribbon with defects. / Department of Physics and Astronomy

The results presented in this thesis demonstrate the first synthesis of several nanostructured transition metal oxides and lithium containing transition metal oxides. Their uses in lithium-ion batteries and/or as magnetic materials have been investigated. The first example of two and three dimensional mesoporous Fe₂O₃ has been prepared by using the soft templating (surfactant) method. The materials have amorphous walls and exhibit superparamagnetic behaviour. By using a hard template route, a mesoporous α-Fe₂O₃ with highly crystalline walls has been synthesized. Its unique magnetic behaviour, distinct from bulk α-Fe₂O₃, nanoparticulate α-Fe₂O₃, and mesoporous Fe₂O₃ with disordered walls, has been demonstrated. The hard template method was also used to prepare nanowire and mesoporous Co₃O₄, β-MnO₂ and MnO₃ with crystalline walls. Their electrochemical properties as electrodes in Li-ion batteries have been investigated. Mesoporous β-MnO₂ can accommodate 0.9 Li/Mn in stark contrast to bulk β-MnO₂ which cannot accommodate Li. To prepare mesoporous materials which cannot be obtained directly by the hard template method, a post-templating route has been developed. Mesoporous Fe₃O₄, γ-Fe₂O₃, and Mn3O4 with ordered mesostructures and highly crystalline walls have been obtained by post-synthesis reduction/oxidation treatments. All the materials show unique magnetic properties compared with nanoparticulate and bulk materials. Also, the first example of lithium containing mesoporous material, LT-LiCoO₂, was synthesized by first preparing mesoporous Co₃O₄, then reacting this with LiOH to form LT-LiCoO₂, with retention of the ordered nanostructure. The nanostructured LT-LiCoO₂ compounds demonstrate superior performance compared with normal or nanoparticulate LT-LiCoO₂, when used as intercalation electrodes in lithium batteries. Finally, monodispersed Mn₃O₄ nanoparticles (diameter ~ 8 nm) with a core-shell structure (a highly crystalline Mn₃O₄ core encased in a thin MnO₂ shell) have been prepared for the first time. Ordered three-dimensional arrays form by spontaneous self-assembly. Magnetic measurements demonstrated that the self-assembled three-dimensional arrays exhibit spin-glass behaviour, rather than the anticipated superparamagnetic behaviour for isolated nanoparticles. Such behaviour is interpreted as arising from strong interactions between the core (crystallized Mn₃O₄) and shell (MnO₂).

M.Sc. (Nanoscience) / Please refer to full text to view abstract

Molecular fluorophores and semiconductor quantum dots (QDs) have been used as cellular imaging agents for biomedical research, but each class has challenges associated with their use, including poor photostability or toxicity. Silicon is a semiconductor material that is inexpensive and relatively environmental benign in comparison to heavy metal-containing quantum dots. Thus, red-emitting silicon nanoparticles (Si NPs) are desirable to prepare for cellular imaging application to be used in place of more toxic QDs. However, Si NPs currently suffer poorly understood photoinstability, and furthermore, the origin of the PL remains under debate. This dissertation first describes the use of diatomaceous earth as a new precursor for the synthesis of photoluminescent Si NPs. Second, the stabilization of red PL from Si NPs in aqueous solution via micellar encapsulation is reported. Thirdly, red to blue PL conversion of decane-terminated Si NPs in alcohol dispersions is described and the origins (i.e., color centers) of the emission events were studied with a comprehensive characterization suite including FT-IR, UV-vis, photoluminescence excitation, and time-resolved photoluminescence spectroscopies in order to determine size or chemical changes underlying the PL color change. In this study, the red and blue PL was determined to result from intrinsic and surface states, respectively. Lastly, we determined that the blue emission band assigned to a surface state can be introduced by base addition in originally red-emitting silicon nanoparticles, and that red PL can be restored by subsequent acid addition. This experimentally demonstrates blue PL is surface state related and can overcome the intrinsic state related excitonic recombination pathway in red PL event. Based on all the data collected and analyzed, we present a simple energy level diagram detailing the multiple origins of Si NP PL, which are related to both size and surface chemistry.

Cette thèse s'inscrit dans le cadre du projet ANR « PROCOM » du programme Matériaux et Procédés dont le coordinateur est EADS IW. Elle a eu pour objectif la mise en place et le développement d’un procédé industrialisable consistant en la fabrication de renforts fibreux pour les composites de haute performance. Elle intègre des nouveaux concepts à nano/micro-échelles et un traitement de surface par voie électrochimique. Les travaux de cette thèse présentent la synthèse d’un polymère conducteur électronique, le polypyrrole, par voie électrochimique, effectuée à la surface de renforts hybrides qui sont des fibres de carbone greffées de nanotubes de carbone (NTCs). Dans un premier temps il s’agit d’optimiser la synthèse du polymère et d’observer l’influence des différents paramètres liés à l’électrochimie sur le taux de dopage et l’épaisseur du film polymère. Les paramètres étudiés étant le potentiel appliqué, le temps de polymérisation, la nature de l’électrolyte et le dopant. Puis il s’agit d’évaluer l’influence du dépôt de polymère sur trois paramètres très importants. Ces paramètres sont la conductivité électrique, les propriétés mécaniques et l’accrochage des NTCs à la surface de la fibre de carbone. Les propriétés électriques sont importantes car ces composites seront utilisés pour le fuselage d’aéronefs qui doivent pouvoir dissiper le courant en cas de foudre. Les bonnes propriétés électriques intrinsèques des NTCs ainsi que l’utilisation d’un polymère conducteur ont permis d’améliorer les propriétés conductrices du renfort. Le polymère joue également le rôle d’interface entre le renfort qui est la fibre hybride et la matrice dans le but d’améliorer les propriétés mécaniques du matériau final. Cependant pour améliorer l’interface différents pré-traitements ont été effectués, tels qu’un traitement thermique, une fonctionnalisation de surface par plasma et le greffage d’une couche d’accroche. Enfin, le polymère joue un rôle protecteur au niveau de la dissémination des NTCs dans l’atmosphère afin d’éviter tout risque sanitaire. Dans un deuxième temps, un système permettant l’électropolymérisation des fibres hybrides en continu a été mis au point en vue de la réalisation d’un procédé pilote par les partenaires industriels du projet. / This thesis is part of the ANR project “PROCOM” from the Mat&Pro program whose coordinator is EADS IW. The aim of the project is the development of a process likely to be scaled up industrially to produce fibrous reinforcements for high performance composites. The project incorporates new concepts in nano / micro-scale and an electrochemical surface treatment. This PhD work presents the synthesis of an electronically conductive polymer (polypyrrole) by an electrochemical route, on the surface of hybrid reinforcements which are carbon fibers grafted by carbon nanotubes (CNTs). At first, the polymer synthesis has been optimized and the influence of different electrochemical parameters on the doping level and the thickness of the polymer layer was investigated. The parameters studied were the applied potential, the polymerization time, the nature of the electrolyte and the dopant. Then, the influence of polymer deposition on three very important parameters was considered. These parameters are the electrical conductivity, the mechanical properties and the adhesion of CNTs on the surface of the carbon fibers. The electrical properties are important because these composites are intended to be used for the fuselage of aircraft that must be able to dissipate the current from lightning. Good intrinsic electrical properties of CNTs and the use of a conductive polymer have improved the conductive properties of reinforcements. The polymer, which is at the interface between the reinforcing hybrid fibers and the matrix, is expected to improve the mechanical properties of the final material. However, to improve this interface different pre-treatments were carried out, such as heat treatment, plasma surface functionalization and incorporation of a grafting layer. Finally, the polymer plays a protective role in the dissemination of CNTs in the atmosphere in order to avoid any health risk. In a second step, a system for the electropolymerization of hybrid fibers in continuous was implemented with the aim of developing an industrially scalable process.

PhD. (Chemistry) / Pre-synthesised gadolinium oxide decorated multiwalled carbon nanotubes (MWCNT-Gd) were coupled with titania to form nanocomposite photocatalysts (MWCNT-Gd/TiO2) using a sol-gel method. Rare earth metal ions (Eu, Nd and Gd), nitrogen and sulphur tridoped titania were decorated on MWCNT-Gd to yield composite photocatalysts (MWCNT-Gd/Eu/Nd/Gd/N,S-TiO2) by a similar method, using thiourea as nitrogen and sulphur source. Different carbon nanomaterials were incorporated into tridoped titania to form various composite photocatalysts (MWCNT/Gd,N,S-TiO2, MWCNT/Nd,N,S-TiO2, SWCNT (single walled carbon nanotube)/Nd,N,S-TiO2 and rGO (reduced graphene oxide)/Nd,N,S-TiO2) via the sol-gel method. Likewise, gadolinium doped graphitic carbon nitride (g-C3N4-Gd3+) was obtained by heating a mixture of gadolinium nitrate hexahydrate and cyanoguanidine and subsequently hybridised with MWCNT/TiO2 using the sol-gel method to yield composite photocatalysts with varying g-C3N4-Gd3+ loadings. All the prepared photocatalysts were characterised by microscopic tools (FE/FIB-SEM-EDX, TEM), crystallographic technique (XRD), spectroscopic tools (UV-Vis, Raman and FT-IR) and nitrogen sorption technique (BET).

A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the requirements for the degree of Masters of Science. Johannesburg, 2016 / Gold nanoparticles are ideally suited as catalysts for selected low temperature reactions such as CO oxidation for catalytic convertors in the motor industry due to their high activity. But they are prone to sintering at high temperatures. Platinum-group-metal based catalysts are efficient at elevated temperatures and generally inactive at lower temperatures. This study explored the CO oxidation efficiency of gold nanoparticles and of a combination of gold and rhodium nanoparticles. Variables such as pH, loading concentration and type of support were varied to control the final properties of the Au based catalysts. Possible bimetallic systems of gold and rhodium were explored for wider temperature range activity than gold alone. All catalysts were characterised using Transmission Electron Microscopy (TEM), Energy Dispersive X-ray Spectroscopy (EDS) and X-Ray Diffraction (XRD). Activity was measured using a temperature controlled, custom-built reactor linked to a gas chromatograph. The conditions yielding the smallest gold nanoparticles were established by adding 5, 8 or 10 wt.% loadings of chloroauric acid to aqueous suspensions of either TiO2 or SiO2 at pH 5, 7 or 9 and at 70-75 °C over 60 minutes. Each preparation was sealed in parafilm, aged in the dark at room temperature for 3 days, vacuum-filtered and subsequently calcined at 300 °C. Gold nanoparticles were smallest when deposited onto TiO2 instead of SiO2, at pH 7 and at a loading of 5 wt. %. A combination of gold and rhodium catalysts were subsequently prepared using these conditions, with the simultaneous addition of rhodium at 1, 3, 5 or 10 wt. % loading. Hydrolysis of gold is highly dependent upon pH, resulting in the synthesis of smaller particles under alkaline conditions. Catalytic activity of samples analysed at 70 and 150 °C was highest for gold nanoparticles below 5 nm, in agreement with previous studies. In the proposed bimetallic catalysts, it was difficult to distinguish gold and rhodium nanoparticles in TEM images, although EDS confirmed their combined presence on the TiO2 support. Particle sizes remained below 5 nm, appearing monodispersed on the TiO2 support except at 10 % rhodium loading where some nanoparticle aggregation was observed. CO oxidation activity showed an apparent temperaturedependent shift in the optimal rhodium loading. Au-TiO2 catalysts with a 5% loading showed the highest activity up to 350 °C for a period of 10 hours and the catalyst deactivated due to sintering. At 150 and 200 °C the Au/Rh-TiO2 catalyst remained active for more than 12 hours. It was concluded that the inclusion of rhodium is a potentially-favourable method for stabilising the activity of gold catalysts.