Masters Degrees (Physics)
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Browsing Masters Degrees (Physics) by Author "Dejene, B. F."
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Item Open Access 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.Item Open Access Synthesis and characterization of Ce³⁺ doped silica (SiO₂) ) nanophosphors co-doped with Al³⁺ or Mg²⁺ ions(University of the Free State (Qwaqwa Campus), 2009-11) Koao, Lehlohonolo Fortune; Dejene, B. F.; Swart, H. C.In recent studies, amorphous silica (SiO2) has been used as a host matrix for rare-earth ions to prepare luminescent materials that can be used in various light emitting devices. Sol-gel glasses have the potential to hold up to ≥10% dopants without losing their amorphous structure. However, before rare earth (RE) - doped sol-gel glasses can be used as luminescent material, several fluorescence quenching mechanisms must be overcome. There are several quenching mechanisms which are present in all materials that are more serious in sol-gel glasses. The first is cross relaxation which involves energy transfer between RE elements; the others are energy transfer through lattice vibrations and to hydroxyl (OH) groups which are present due to the use of water as the solvent during the preparation process. A few studies have demonstrated that the luminescence intensity of rare-earth doped silica can be improved through incorporation of co-dopants such as Al, TiO2, B and by annealing at high temperatures (e.g. > 500ºC). Following their footsteps and in order to make comparisons, we used aluminum as the codopant in some samples to investigate the effects on luminescence yield for various RE concentrations. We also investigated the effects of magnesium co-dopant and high temperature annealing on the luminescence intensity of rare-earth doped silica. In this work, the highest emission intensity was observed for the sample with a composition of 0.5 mol% Ce3+. Cerium doped silica glasses had broad blue emission corresponding to the D3/2- FJ transition at 445 nm but exhibited apparent concentration quenching after higher concentrations of 0.5 mol% Ce3+. Silica containing Mg2+ or Al3+ ions displayed an increase in luminescence intensity as the Mg2+ or Al3+ to Ce3+ ratio increases for the range investigated but significant luminescence enhancement was observed for Mg2+:Ce ratio greater than 20, while that of Al3+ co-doping had the highest luminescent intensity when the ratio of Al:Ce is 10:1. This enhanced photoluminescence was assigned to an energy transfer from the Mg nanoparticles, to result in enhanced emission from Ce3+. The Al3+ or Mg2+ ions disperses the Ce3+ clusters, enhancing 2F5/2 and 2F7/2 emissions due to increased ion-ion distances and decreased cross-relation.Item Open Access Synthesis and characterization of long afterglow phosphors (Sr Al2iO4:Ce3+ SrAl2iO4:Tb3+CaAlxOy:Tb3+, Y3Al5O12:Eu3+) using solution combustion method(University of the Free State, 2011-11) Foka, Kewele Emily; Dejene, B. F.; Swart, H. C.This work consists of several aspects of phosphor materials. Strontium, calcium and yttrium aluminate doped with rare earth (Ce, Tb and Eu) have been synthesized by solution combustion method using urea as a fuel for investigations of the luminescent, structure and morphological properties. The phosphors were characterized by several techniques such as Xray diffraction (XRD), energy dispersive electroscopy (EDS), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and Photoluminescence (PL), PL data were collected using a Cary Eclipse Photoluminescence Spectrophotometer equipped with a 150 W xenon lamp. Cerium doped strontium aluminum oxide (SrAhO4:Ce3+) were synthesizes. The effects of different concentration of cerium were investigated. X-ray diffraction results confirmed the formation of the SrAhO4 monoclinic phase (Powder Diffraction Standards (JCPDS) file N0 34-0379). The particle sizes of different peaks were estimated and the average particle size was 47 nm. SEM results showed agglomerated as well as small elongated-egg-like shape on particles when taken to higher magnification. The PL spectra show a broad emission consisting of two bands peaking at 374 and 384 nm, corresponding to the transitions from the lowest 5d excited state to the 2F512 and 2F712 states. The excitation and emission peak position shifted with varying the cerium concentration. This maybe due to uncontrollable electrospinning conditions like air and wetness, which influence the cristal field that surround Ce3+.SrAhO4:Tb3+ XRD peaks confirmed the formation of the SrAhO4 monoclinic phase and some impurities were also observed. The photoluminescence characteristics show the emission peaks at 415, 436 and 459 nm which correspond to the 5D3 to 7F1 (1=5 , 4, and 3) level and 489, 543, 585, and 622 nm corresponding to 5D4 to 7F 1 (J= 6, 5, 4, 3) under excitation at 229 nm and the terbium concentration was varied. The elements of the phosphor SrAhO4:Tb3+ were shown by energy dispersive spectroscopy. The decay curves were also observed and the decay constants show a higher value at a concentration of 0.25 mol% and lower value at a concentration of 2 mol¾. CaAlxOy:Tb3+ green phosphors were obtained at low temperature (500 °C) by a solutioncombustion method. The structural analysis revealed the presence of both monoclinic CaA4O1 and CaAhO4. The main parent structure of CaAhO4 monoclinic was revealed when varying the concentration of terbium. The characteristic luminescence properties were investigated using emission spectra. The emission peaks are from transition of the 5D4 state to the 7F1 (J = 6, 5, 4, 3) state. The optimal intensity was obtained when the concentration of Tb3+ was increased to 2 mol¾. FTIR was used to identify all the chemical bands. Absorption bands of the condensed matter AlO4 located in the range of700 cm·1-900 cm·1 and condensed matter AlO6 at 500 cm·1-680 cm·1 are attributed to AlO4 liberation at 600 cm·1-900 cm·1 The decay curves of the phosphor were investigated and showed a higher intensity and longer afterglow time at higher concentration of terbium 2 mol%.Y3Al5O12 known as Yttrium aluminum garnet (Y AG) phosphor doped with different concentration of Eu was synthesized by the solution combustion method. The crystalline structure, morphology and luminescent properties of the phosphors were studied. The SEM revealed the agglomerated morphology containing small spherical particles around the pores. FTIR spectra reveal all bonds that exist in the phosphor. The emission spectra revealed three major emission peaks at 592, 615, and 628 nm, corresponding to the 5D0-7F1 (592 nm), 5D0-7F2 (615 nm) and 5D0-7F3 (628 nm) transitions respectively. The luminescence intensity increased with an increase in Eu concentration at 0. 7 mol¾ and then decreases with an increasing of concentration further.Item Open Access Synthesis and characterization of long afterglow phosphors (SrAl2O4:Ce³+, SrAl2O4:Tb³+, CaAlxOy:Tb³+, Y3Al5O12:Eu³+) using solution combustion method(University of the Free State, 2011-11) Foka, Kewele Emily; Dejene, B. F.; Swart, H. C.This work consists of several aspects of phosphor materials. Strontium, calcium and yttrium aluminate doped with rare earth (Ce, Tb and Eu) have been synthesized by solution combustion method using urea as a fuel for investigations of the luminescent, structure and morphological properties. The phosphors were characterized by several techniques such as X-ray diffraction (XRD), energy dispersive electroscopy (EDS), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR) and Photoluminescence (PL), PL data were collected using a Cary Eclipse Photoluminescence Spectrophotometer equipped with a 150 W xenon lamp. Cerium doped strontium aluminum oxide (SrAl2O4:Ce3+) were synthesizes. The effects of different concentration of cerium were investigated. X-ray diffraction results confirmed the formation of the SrAl2O4 monoclinic phase (Powder Diffraction Standards (JCPDS) file No 34-0379). The particle sizes of different peaks were estimated and the average particle size was 47 nm. SEM results showed agglomerated as well as small elongated-egg-like shape on particles when taken to higher magnification. The PL spectra show a broad emission consisting of two bands peaking at 374 and 384 nm, corresponding to the transitions from the lowest 5d excited state to the 2F5/2 and 2F7/2 states. The excitation and emission peak position shifted with varying the cerium concentration. This maybe due to uncontrollable electrospinning conditions like air and wetness, which influence the cristal field that surround Ce3+. SrAl2O4:Tb3+ XRD peaks confirmed the formation of the SrAl2O4 monoclinic phase and some impurities were also observed. The photoluminescence characteristics show the emission peaks at 415, 436 and 459 nm which correspond to the 5D3 to 7FJ (J=5, 4, and 3) level and 489, 543, 585, and 622 nm corresponding to 5D4 to 7FJ (J= 6, 5, 4, 3) under excitation at 229 nm and the terbium concentration was varied. The elements of the phosphor SrAl2O4:Tb3+ were shown by energy dispersive spectroscopy. The decay curves were also observed and the decay constants show a higher value at a concentration of 0.25 mol% and lower value at a concentration of 2 mol%. CaAlxOy:Tb3+ green phosphors were obtained at low temperature (500 oC) by a solution- combustion method. The structural analysis revealed the presence of both monoclinic CaAl4O7 and CaAl2O4. The main parent structure of CaAl2O4 monoclinic was revealed when varying the concentration of terbium. The characteristic luminescence properties were investigated using emission spectra. The emission peaks are from transition of the 5D4 state to the 7FJ (J = 6, 5, 4, 3) state. The optimal intensity was obtained when the concentration of Tb3+ was increased to 2 mol%. FTIR was used to identify all the chemical bands. Absorption bands of the condensed matter AlO4 located in the range of 700 cm-1-900 cm-1 and condensed matter AlO6 at 500 cm-1-680 cm-1 are attributed to AlO4 liberation at 600 cm-1-900 cm-1. The decay curves of the phosphor were investigated and showed a higher intensity and longer afterglow time at higher concentration of terbium 2 mol%.Y3Al5O12 known as Yttrium aluminum garnet (YAG) phosphor doped with different concentration of Eu was synthesized by the solution combustion method. The crystalline structure, morphology and luminescent properties of the phosphors were studied. The SEM revealed the agglomerated morphology containing small spherical particles around the pores. FTIR spectra reveal all bonds that exist in the phosphor. The emission spectra revealed three major emission peaks at 592, 615, and 628 nm, corresponding to the 5D0→7F1 (592 nm), 5D0→7F2 (615 nm) and 5D0→7F3 (628 nm) transitions respectively. The luminescence intensity increased with an increase in Eu concentration at 0.7 mol% and then decreases with an increasing of concentration further.Item Open Access 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.Item Open Access The synthesis and characterization of the ZnO nanoparticles(University of the Free State (Qwaqwa Campus), 2011-11) Tshabalala, Modiehi Amelia; Dejene, B. F.; Swart, H. C.ZnO has been by far the most interesting semiconductor because of its properties. The ZnO nanostructures were synthesized by a sol-gel method and the samples were annealed in air at various temperatures capped with polymers PVP (Polyvinyl Pyrrolidone) and PEG (Polyethylene glycol). Again the ZnO was synthesized using different solvents; ethanol, methanol or water at various temperatures. Characterizations of the powders were carried out using different techniques. The structure and the particle size of the samples were obtained using the XRD (x-ray diffraction). The morphology was determined by the SEM (scanning electron microscopy) and the chemical composition was analyzed using the EDS (energy x-ray dispersed spectroscopy). The PL (photoluminescence) data were collected using the He- Cd (Helium-Cadmium) laser and also using the Cary Eclipse fluorescence spectroscopy at room temperature. The absorption spectra were analyzed using the UV-Vis spectroscopy. The PL spectra for the ZnO nanostructures capped and prepared using polymers showed broad emissions in the visible range. The broad emission in the visible range with maximum intensity peaks at 449 nm and at 530 nm for the PVP capped ZnO nanoparticles were observed annealed at 150°C. This was influenced by the addition of various molar masses of PVP on the Zn(Ac)2. The green emission band at 560 nm and a blue emission at 450 nm were obtained for the PEG encapsulated ZnO nanostructure. The PL of the ZnO nanoparticles prepared using various solvent was shown, the different shifts from the emission peaks were observed and the fluctuation of the intensity which was attributed to an increase and a decrease on the annealing temperatures. The effect of pH values on the ZnO prepared using different solvents. The PL on these samples exhibited a strong broad blue emission, for all the ZnO prepared using ethanol, methanol or water as solvents. The intensities differed with the amount of NaOH which was added onto the Zn(Ac)2 solution. The XRD pattern for all the prepared ZnO nanostructures exhibited the peaks corresponding to that of various planes of ZnO wurtzite structure with the JCPDS (Joint Committee on Powder Diffraction Standards) file no. (13-1451). The absorption spectra of the PVP capped ZnO nanostructures did not show any shifts while the absorption spectra for the PEG encapsulated ZnO nanostructures showed a shifts with an addition of the molar masses of the PEG. The UV-Vis spectroscopy for the ZnO prepared with ethanol, methanol or water as solvents at various temperatures gave the absorption edges and also the blue shifts that occurred with and increase on the annealing temperatures 300, 400, 500 and 600°C. It was observed from the UV absorption of the ZnO using different solvents with various pH values that the band gaps for all the samples were determined to be larger than that of ZnO bulk. The NaOH solution which was slowly added on the Zn(Ac)2 solution took control over the surface of the ZnO surfaces.Item Open Access Synthesis and characterization of undoped and doped ZnO nanoparticles prepared by sol-gel process(University of the Free State, 2009-11) Ali, Abdub Guyo; Dejene, B. F.; Swart, H. C.Nanotechnology is an emerging field of engineering that has intrigued a tremendous potential to change today's lives. Nanoparticles represent the transition region between individual atom/molecule and bulk materials have been attracting considerable research input, owing to some unique physical and chemical properties they exhibit. They are already being used in many applications. This is due to their potential useful size and shape dependant. They are provided with a number of quantum size effects that determine specific electro-physical performance. This is one of the reasons why the preparation and study of nanomaterials are important area of research. This work focuses on the synthesis and characterization of semiconducting metal oxides, the most widely used materials in powder and films forms. The aim is to compare the optical performances of zinc-oxides nanostructures obtained by means of sol-gel (SG) method. The microstructure quality (concentration and nature of defects) of materials which is a fundamental parameter to evaluate if the material is suitable for any applications are usually correlated with optical properties (intensity and spectral emission range) of materials. The idea of this work was to carry a basic characterization of the structural (by X-ray diffraction technique and scanning electron microscopy) and optical (photoluminescence measurements) properties of ZnO, ZnO in SiO2 matrix and manganese doped ZnO nanoparticles samples prepared by sol-gel process. Initially, several samples of ZnO nanoparticles were successfully synthesised by varying the pH of the precursors. Secondly, manganese doped ZnO samples were synthesised by varying the concentration of manganese. The analysis of ZnO nanoparticles prepared under varying pH has displayed a dependence of structural quality, morphology as well as optical properties on the pH of the precursors. SEM images of manganese doped ZnO samples reveal nanorods of micrometer-size. XRD patterns confirm polycrystalline wurtzite structure of ZnO and particle sizes of about 7nm. High optical transmittance greater than 80% was achieved in the visible spectral wavelengths with UV-Vis optical absorption and transmission measurements. Photoluminescence spectra of the pure ZnO are composed of two main emission bands peaked at 388nm and 569nm. Both the blue (388nm) and the green (560nm) emissions in ZnO nanostructures were quenched, although to different extent, when doped with Mn2+. Mn-doped ZnO nanorods have several applications as the most suited materials for sensors, spintronics, better insulation materials etc.