Masters Degrees (Physics)

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Browsing Masters Degrees (Physics) by Advisor "Koao, L. F."

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    Novel ZnO nanostructures: synthesis, growth mechanism, and applications
    (University of the Free State (Qwaqwa Campus), 2014-12) Molefe, Fokotsa Victor; Koao, L. F.; Dejene, B. F.; Swart, H. C.
    The ZnO nanostructures were successfully synthesized by chemical bath deposition method (CBD) to study the influence of parameters such as reaction temperature, time, precursor concentration and the annealing temperature respectively. The main motivation for this thesis is to successfully synthesise novel ZnO nanostructures and understand the growth mechanism. In this work, the thermal, structural, morphology, optical, and luminescence properties of ZnO were investigated in details by means of thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometer (EDS), x-ray photoelectron spectroscopy (XPS), ultraviolet visible (UV-vis) spectroscopy and photoluminescence (PL) spectroscopy techniques. From TGA results when increasing both reaction and annealing temperature we observed the increase in thermal stability of ZnO due to the removal of adsorbed species in the material. The melting temperatures (as determined through DSC) decreased due to crystallization of ZnO with the increase in both reaction and annealing temperature. X-ray diffraction (XRD) indicated that all the ZnO nanostructures prepared at 80 ℃ crystallizes in the wurtzite structure with the mean lattice parameters a = b = 3.25 Å and c = 5.18 Å and there is an increment in the particle size resulting into the improvement of crystallinity of the material. In materials prepared at lower reaction temperature, reaction time, and precursor concentration, traces of zinc hydroxide Zn(OH)2 were observed. When Zn(OH)2 decomposes into ZnO, the entire surface morphology through the study of ZnO consisted of agglomerated nanoflakes. The EDS results confirmed the presence of Zinc (Zn) and Oxygen (O) as the major product, and the ratio of Zn to O increased as ZnO becomes more crystalline. The UV-Vis reflectance spectra showed that the absorption band edges shift to the higher wavelength with an increase in reaction time, temperature, molar concentration precursors, and annealing temperature. As a result the band gap energy of ZnO nanostructures determined using Kubelka Munk’s equation was found to decrease due to quantum confinement effects and the increase in particle size. In general, the photoluminescence (PL) analysis showed that ZnO nanoflakes prepared at different parameters have almost the same characteristics. PL measurements revealed broad emission that extends from UV region to the visible region. The luminescence intensity of this emission was quenched when increasing parameters mentioned above, and these quenching is attributed to the decrease in concentration of defect related emissions. It is well known that when using chemical reaction methods such as CBD the emission intensity quenches as Zn(OH)2 dehydrates into ZnO. The slight red-shift in the emission band is also observed which is attributed to band gap narrowing.
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    Synthesis and characterization of MBxOy:Eu (M = Ca, Sr, Ba) phosphors and TiO2 semiconductor for application in luminescence and energy materials
    (University of the Free State, 2014-11) Sithole, Thokozane Moses; Dejene, B. F.; Koao, L. F.
    MBxOy:Eu (M = Ca, Sr, Ba) phosphors with different Ca:B /Sr:B /Ba:B molar ratio and with constant Eu concentration have been synthesized by a solution combustion method at initial reaction temperature of 5000C for 15 minutes. The morphology, structure and luminescence properties of the synthesized nanostructures were investigated using X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Photoluminescence (PL) spectroscopy. The XRD spectra of all the as prepared samples show monoclinic phase for Ca:B. The XRD spectra of variation of Strontium nitrate and europium nitrate shows no effect on the structure of the sample. The estimated average grain sizes calculated using the XRD spectra were found to be between 16 - 20 nm. The grain size was found to be significantly dependent on Sr: B mole ratios and concentration of Eu. XRD patterns analysis of samples with different Ba:B mole ratio when annealed at temperature above (7000C) confirms the existence of pure BaB8O13 phase (JCPDS: 74-0674). The variation of Eu concentrations content was found to have no effects on the crystal structure. SEM micrograph show that the surface aspects are nanorod like for low Ca:B mole ratios but the grains become flake-like as the Ca:B molar ratio increased, giving rise to the increase in particle size resulting from agglomeration. SEM micrograph showed the presence of well-packed and randomly oriented platelet-like grains and the variation of Sr: B and Eu ion did not influence the surface aspect. SEM micrographs of the Ba:B phosphors at low magnification show agglomerates and high magnification depicts that particle with sizes in nano ranges are the primary sources of agglomerates. The PL result also shows that the luminescence intensity of these emission spectra increased with increase in Ca: B molar ratios. The narrow emission lines between 590-688 nm are due to transitions Eu3+. The PL emission results showed that luminescence intensity was found to increase in both Sr: B and Eu ions with no effect on the luminescence band position. The excitation spectrum showed two broad bands in the range of 200–400 nm: one was the host lattice absorption with the maxima at 320 nm and the other was Ba-O absorption overlapped with the CT band of Eu3+, which indicated that the energy of the host lattice absorption could be efficiently transferred to the Eu2+. Titanium dioxide (TiO2) powder was prepared by a sol-gel method. X-ray diffraction and Fourier transform infrared spectroscopy were used to determine film behaviour. The super-hydrophilicity was assessed by contact angle measurement. Photocatalytic properties of these films were evaluated under UV irradiation. The XRD pattern of TiO2 powder samples confirmed the presence of polycrystalline anatase phase with a crystal size of 17 nm. The results indicated that UV light irradiation had significant effect band energy and photocatalytic properties of 𝑇𝑖𝑂2 nanopowders.