Browsing by Author "Mbuyisa, Puleng Nontobeko"

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    Growth and characterization of carbon nanostructures on zinc oxide nanostructures
    (University of Zululand, 2012) Mbuyisa, Puleng Nontobeko; Ndwandwe, M.O.; Cepek, C.
    The research work of this thesis is aimed to explore the possibility to synthesize different hybrid materials based on ZnO and carbon hierarchical nanostructures, and to test their performances when used as chemiresistors to detect low doses (~ ppm) of polluting gases, like ammonia and acetone. Nanostructures of ZnO and carbon, including carbon nanotubes, exhibit on their own exceptional qualities, which can be enhanced when they are combined in hybrids, extending their possible practical applications further. In this thesis it is shown that, it is possible to grow different CNs/ZnO hybrid nanostructures which present higher sensitivity to ammonia if compared to other carbon-based and ZnO-based chemiresistors. The hybrids have been grown by combining different growth techniques, like magnetron sputtering, hydrothermal process, electron beam deposition and chemical vapour deposition. The grown samples were studied by using a variety of experimental techniques, also in-situ, which allowed us to deeply understand their structural and chemical properties. In particular the chemical analysis was done by ultra violet and X-ray photoelectron spectroscopy (UPS and XPS, respectively, also using synchrotron radiation), Raman spectroscopy and energy dispersive X-ray spectroscopy (EDX). The morphology of the samples have been analysed, ex-situ, by scanning electron microscopy (SEM) and their fine atomic structure by high resolution transmission electron microscopy (HRTEM). The synthesis of aligned ZnO nanorods was done following the procedure below. Zinc oxide films were first deposited on Si <100> substrates using DC magnetron 4 sputtering at different chamber pressures, using oxygen to fully oxidize the films. Vertically oriented ZnO nanorods were then grown using the hydrothermal method on the resultant films. The pristine ZnO films were characterized by SEM, AFM, EDX and XRD. The films transparency and grain size were found to be pressure dependent: as the pressure was increased from the recommended sputtering pressure of 3x10-3 to 6x10-2 Torr the films became more transparent due to the decreases in the films grain size and thickness. The resultant nanorods were found to be highly dense with nonuniform dimensions due to the different grain sizes within the individual film samples. At a chamber pressure of 9x10-3 Torr we were able to grow c-axis oriented and crystallized miniature rods directly using DC sputtering, of approximately 250 ± 10 nm in length. This method of growing the nanorods directly from sputtering may lead to a simple way of growing ZnO nanorods, by eliminating the hydrothermal growth step and allowing in-situ growth in cases where ZnO rods are used as a substrate. Our work show that chemical vapour deposition (CVD) done on the so grown vertically aligned ZnO nanorods can synthesize different carbon nanostructures (CNs), whose morphology is driven by the ZnO nanorods and whose dimensions and structure change as a function of the process temperature. The grown CNs range from amorphous carbon cups, completely covering the nanorods, to highly dense one dimensional carbon nanodendrites (CNDs), which start to appear like short hairs on the ZnO nanorods. The nanorods are partially etched when the process is done at 630- 740 °C, while they are completely etched at high temperatures (> ~ 800 °C). In the later case the CNDs are preferentially aligned along the location of the pristine nanorods and emerge from a porous carbon sponge formed at the substrate interface. When the CVD process was done on Fe coated ZnO NRs at 580-630 °C, in addition 5 to the previously described CNs, we observed randomly oriented CNTs. The CNs/ZnO hybrids were characterised by ex-situ SEM, TEM and Raman spectroscopy, and in-situ XPS and UPS. We further found that, when used as a chemiresistor, the CND/ZnO nanostructures have a higher sensitivity to ammonia compared to chemiresistors made from the bare ZnO nanorods, other one-dimensional CNs, like carbon nanotubes or other metal/metal-oxides hybrid CNs. The absorption and desorption of ammonia gas on the bare ZnO NRs and the CNs/ZnO hybrids were studied by fast acquisition XPS using synchrotron radiation. Ammonia gas was found to chemisorb on the hybrid structure by forming amine groups, while on the NRs physisorbed on the NRs surface. The hybrid showed a ~ 4.5 higher sensitivity to ammonia as compared to the ZnO NRs sensor but a slower recovery time. The enhanced response and slow desorption of the CNs/ZnO hybrid can be attributed to the strong interaction of the hybrid with ammonia gas i.e. the different adsorption surface chemistry of C (chemisorption) and ZnO (physisorption) and also to the increased surface to volume ratio of the CNDs. The ZnO NRs were sensitive to both ammonia and acetone while the hybrid was ammonia selective.
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    Synthesis and characterization of graphene: a raman study of the effect of electromagnetic and proton irradiations on graphene
    (2009) Mbuyisa, Puleng Nontobeko; Maaza, M.; Ndwandwe, O.M.
    Graphene, a single atomic layer of hexagonally arranged sp2–hybridized carbon atoms, with a thickness of only 0.34 nm, exhibits unique properties. The current interest in graphene can be attributed to three main reasons. Firstly, various forms of graphite, nanotubes, buckyballs and others can all be viewed as derivatives of graphene. Secondly, the scalability of graphene devices to nano-dimensions makes it a promising candidate for applications in nano-devices. Thirdly, its electron transport properties are described by the Dirac equation which allows access to quantum electrodynamics in a simple condensed matter experiment. The methods for obtaining individual graphene sheets have progressed, from ripping it with adhesive tape, or gently pushing small graphite crystals along a hard surface to produce high quality graphene by cleaving of graphite, to oxidation of graphite using the modified Hummers method developed by M. Hirata et al. (Carbon 42(2004)). In this research the graphene was synthesized using the modified Hummers method which resulted in a suspension of graphene oxide flakes in distilled water. The graphene oxide (GO) was then chemically reduced to produce graphene or reduced graphene oxide (rGO). The physical properties of the resulting graphene films were characterized and the effects of irradiation by an excimer pulsed laser (UV radiations), visible light (green) and proton irradiation were investigated. During the irradiations the dose of the radiation was varied in order to track the changes in the properties of the materials as a function of the flux. The different spots were then characterized using Raman spectroscopy to measure the created disorder. The Raman spectra of the samples irradiated by light displayed the D and G mode. The Raman spectra characteristics for the UV irradiated sample were similar to that of the proton irradiated sample. In both cases there was a splitting of the G mode. It was then concluded that the splitting of the G band similar to that found in semiconducting Single Walled Nanotubes is an indication that the samples are semiconducting. However the G line shape is highly sensitive to whether the SWNT is metallic or semiconducting and in the UV irradiated sample there was a transition from semiconducting to more metallic as the irradiation was increased.