Masters Degrees (Physics)
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Browsing Masters Degrees (Physics) by Advisor "Ntwaeaborwa, O. M."
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Item Open Access Cathodoluminescence degradation and surface characterization of SrGa₂S₄:Ce³⁺ power and thin films(University of the Free State, 2011-05) Moleme, Pulane Adelaide; Ntwaeaborwa, O. M.; Swart, H. C.The structure, morphology and luminescent properties of commercial SrGa2S4:Ce3+ phosphor powder and thin films were investigated. The phosphor shows bright blue under ultraviolet (UV) excitation. Measurements were carried out using various characterization techniques such as Xray diffraction (XRD), scanning electron microcopy (SEM) and X-ray energy dispersive spectroscopy (EDS). The XRD data were collected using a D8 advance powder X-ray diffractometer with CuKα radiation. Morphology and elemental composition were done using Shimadzu Super Scan SSX-550 coupled with EDS. Photoluminescence (PL) data were collected using Varian Cary Eclipse Fluorescence Spectrophotometer with a monochromatized Xenon lamp (60-75 W) as excitation source and measurements were carried out in air at room temperature, and cathodoluminescence (CL) data were collected with S2000 Ocean Optics Spectrometer. The absorption spectra were recorded using Perkin Elmer Lambda 950 UV-VIS spectrometer. The same characterization tools were used to characterize the thin films. XRD data confirmed the orthorhombic structure of SrGa2S4 that was consistent with the standard JCPDS file no. (77-1189). The SEM images of the SrGa2S4:Ce3+ powder showed particles with irregular shapes and EDS detected presence of the major elements. Both PL and CL showed the broad emission peaks around 444 nm and 485 nm which are due to Ce3+ radiative transitions (5d (T2g) → 4f (2F5/2) and 5d (T2g) → 4f (2F7/2)). Cathodoluminescent ageing characteristics of the SrGa2S4:Ce3+ powder and thin films under prolonged electron beam bombardment were studied and presented. The cathodoluminescent intensity with increasing Coulomb loading was observed to degrade under different primary electron beam voltages for the powder. Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) were used to monitor the surface chemical changes both during electron beam bombardment and after the degradation process. Auger peak to peak heights monitored during the ageing process suggest a decrease in S and C Auger peak intensity and an initial increase in oxygen concentration on the surface. XPS results indicate the formation of an SrO overlayer due to electron stimulated surface chemical reactions (ESSCRs). For preparation of films, silicon (Si) (100) substrates were used. A pellet was prepared from the standard SrGa2S4:Ce3+ powder. The Lambda Physik EMG 203 MSC 309 nm XeCl excimer laser was used to grow the films. The films growth was carried out in a chamber which was first evacuated to a base pressure of 8 x 10-5 mbar before backfilling to pressures of 1.0 x 10-2 mbar Ar and 1.0 x 10-2 mbar O2, where Ar and O2 were used as cross pulse gases. The films were deposited at different substrate temperatures ranging from 400°C to 600°C with 28 800 and 57 600 pulses respectively. The laser beam was operated at 8 Hz repetitive rate. The substrate temperature, number of pulses and the working pressure are the parameters that were varied during the preparation of the thin films. A highly crystalline SrGa2S4 layer was obtained at the growth temperature of 400°C. XRD patterns also showed that the properties of the films were sensitive to substrate temperature. PL and CL spectra were characterized by a broad band that can be fitted by two Gaussian peaks according to the two Ce3+ radiative transitions. At high substrate temperature a shift to Ce3+ emission in SrS occurred as well as in Ar atmosphere for both UV and high energy electrons excitation. The atomic force microscopy (AFM) images before annealing exhibited smooth surface at low substrate temperature, which became rough at high substrate temperature and after annealing in vacuum at 700°C temperature. Non-uniformity in particles (big and small) of the films and smooth films were observed from the SEM images.Item Open Access Characterization of Y3(Al,Ga)5O12:Ce3+ phosphor thin films prepared by pulsed laser deposition(University of the Free State, 2013-11) Dlamini, Sipho Thapo Solomon; Swart, H. C.; Ntwaeaborwa, O. M.The morphological and luminescent properties of Y3(Al,Ga)5O12:Ce3+ powder phosphor were investigated. Scanning Electron Microscopy (SEM) revealed the phosphor’s agglomerated particles with a size ranging from 0.4μm to 1.4μm. The X-ray diffraction (XRD) indicated a cubic polycrystalline phosphor with an average crystal size of 80 nm. Excitation peaks for the powder were obtained at 439, 349, 225 and 189 nm and emission peaks at 512 and 565 nm. Emission wavelength at 512 nm was also used to approximate the Al/Ga ratio within the crystal. Photoluminescence (PL) data also revealed that the addition of the Ga into the YAG:Ce3+ matrix caused a blue-shift in the emission spectra. The UV-VUV excitation and emission spectra of the Y3(Al,Ga)5O12:Ce3+ were also recorded and an energy diagram was constructed from the values. The phosphor powder was used as target material for Pulsed Laser Deposition (PLD). SiO2/Si(100) was used as substrates and thin films were deposited in the presence of different background gases. XRD indicated that better crystallization took place for films deposited in a 20 mTorr O2 atmosphere. Atomic force microscopy (AFM) revealed an RMS value of 0.7 nm, 2.5 nm and 4.8 nm for the films deposited in vacuum, O2 and Ar atmospheres, respectively. The highest PL intensity was observed for films deposited in the O2 atmosphere. The thickness of the films varied from 120 nm to 270 nm with films deposited in vacuum having the thin layer and those in Ar having the thick layer. The stoichiometry of the powder was maintained in the film during the deposition as confirmed by Rutherford backscattering spectroscopy (RBS). Luminescent properties of Y3(Al,Ga)5O12:Ce3+ thin films prepared by PLD at different substrate temperatures in an O2 background atmosphere were also investigated. XRD indicated that the films have the same cubic polycrystalline phase structure as the powder. AFM revealed poorly defined grain growth for films ablated at a substrate temperature of 22°C and 500°C but well defined grain growth was observed for films ablated at a 300°C substrate temperature. Auger electron spectroscopy (AES) depth profile of the film ablated at 500°C indicated that Si has diffused into the thin film. The highest PL intensity was observed for films deposited at the substrate temperature of 300°C. A slight shift in the wavelength of the PL spectra was obtained for the thin films with respect to the powder due to a change in the crystal field. The maximum PL intensity was obtained from the film deposited at the substrate temperature of 300⁰C in an O2 atmosphere. In addition, the films with well-defined grains (rougher surfaces) showed higher PL intensity compared to films with poorly-defined grains (smooth surfaces) as confirmed from AFM dataItem Open Access Luminescent properties of combustion synthesized BaAl2O4:Eu²+ and (Ba1-xSrx)Al2O4:Eu²+ phosphors co-doped with different rare earth ions(University of the Free State, 2011-11) Annah, Lephoto Mantwa; Ntwaeaborwa, O. M.; Mothudi, B. M.; Swart, H. C.A Combustion method was used to prepare all the alkaline earth aluminates (rare-earths doped BaAl2O4, BaSrAl2O4 and BaZnAl2O4) phosphor powders in this study. Measurements of these phosphor powders were carried out using various characterization techniques such as X-ray diffraction (XRD), Scanning Electron Microcopy (SEM), X-ray energy dispersive spectroscopy (EDS), X-ray Photoelectron Spectroscopy (XPS) and Fourier Transform Infrared Spectroscopy (FT-IR). The XRD data were collected using a D8 advance powder X-ray diffractometer with CuKα radiation. Morphology and elemental composition were done using JEOL- JSM 7500F Scanning Electron Microscope. The stretching mode frequencies data were collected using Perkin Elmer Spectrum 100 FTIR spectrometer and the elemental composition on the surfaces of the phosphor powders were monitored by the PHI 5400 Versaprobe scanning X-ray photoelectron spectrometer. Photoluminescence (PL) data were collected using 325nm He-Cd laser and decay data were collected using Varian Cary Eclipse Fluorescence Spectrophotometer coupled with a monochromatized Xenon lamp (60-75 W) as excitation source and measurements were carried out in air at room temperature. The thermoluminescence (TL) data were collected using a Thermoluminescence Reader (Integral-Pc Based) Nucleonix TL 1009I. BaAl2O4:Eu2+ phosphor powders co-doped with different trivalent rare-earth (RE= Dy3+, Nd3+, Gd3+, Sm3+, Ce3+, Er3+, Pr3+ and Tb3+) ions were prepared at an initiating temperature of 600oC and annealed at 1000oC for 3 hours. The X-ray diffraction (XRD) data shows hexagonal structure of BaAl2O4 for both as prepared and post annealed samples. All samples exhibited bluish-green emission associated with the 4f65d1→4f7 transitions of Eu2+ at 504 nm. The longest afterglow was observed from the BaAl2O4:Eu2+ co-doped with Nd3+. BaAl2O4:Eu2+, Nd3+, Gd3+ phosphor powders were prepared at different initiating temperatures of 400-1200oC. X-ray diffraction data show the formation of the hexagonal BaAl2O4 structure at the temperatures of 500oC-1200oC. The crystal size calculated from the phosphor powder prepared at 1200oC was found to be 63 nm. Blue-green photoluminescence with persistent/long afterglow, was observed at 502 nm and the highest PL intensity was observed from the sample prepared at 600oC. The phosphorescence decay curves showed that the rate of decay was faster in the case of the sample prepared at 600oC compared to that prepared at 1200oC. The TL glow peaks of the samples prepared at 600oC and 1200oC were both stable at 72oC suggesting that the traps responsible for the long afterglow were not affected by the temperature. Barium-substituted phosphor powders of (Ba1-xSrx)Al2O4:Eu2+;Nd3+ composition were prepared at an initiating temperature of 500oC. The X-ray diffraction with the composition of x = 0 shows the hexagonal phase of BaAl2O4 and the one for x = 1 shows the monoclinic phase of SrAl2O4. The XRD with the composition of x = 0.4, 0.5 and 0.6 shows the admixture of BaAl2O4 and SrAl2O4 structures. SEM investigations showed some changes on the surface morphology for different compositions. Photoluminescence (PL) studies showed the (Ba1-xSrx)Al2O4:Eu2+;Nd3+ (x = 0) and (Ba1-xSrx)Al2O4:Eu2+;Nd3+ (x = 1) with blue-green to bright-green emissions with peaks at 505 nm and 520 nm respectively. The mixed composition with x = 0.4, 0.5 and 0.6 showed two peaks at 447 nm and 517 nm. Phosphorescence showed higher luminescence for (Ba1-xSrx)Al2O4:Eu2+;Nd3+ at (x = 0) compared to other compositions. (Ba1-xZnx)Al2O4:Eu2+;Nd3+ phosphor powders with the compositions x = 0.2, 0.4, 0.5, 0.6, 0.8 and 1 were prepared at an initiating temperature of 500oC. The X-ray diffraction showed the cubic structure for the compositions of x = 0 and x = 1. The SEM images of the phosphor samples showed different kinds of morphologies for the compositions x = 0, 0.5 and 1. The PL emission of the phosphor powder clearly showed a shift from green to blue regions. The highest PL emission and the long afterglow ascribed to trapping and detrapping of charge carriers were observed from (Ba1-xZnx)Al2O4:Eu2+;Nd3+ with x = 0.2.Item Open Access Luminescent properties of combustion synthesized BaAl₂O₄:Eu²⁺ and (Ba₁₋xSrx)Al₂O₄:Eu²⁺ phosphors co-doped with different rare earth ions|(University of the Free State, 2011-11) Lephoto, M. A.; Ntwaeaborwa, O. M.; Mothudi, B. M.; Swart, H. C.Combustion method was used to prepare all the alkaline earth aluminates (rare-earths doped BaAl2O4, BaSrAl2O4 and BaZnAl2O4) phosphor powders in this study. Measurements of these phosphor powders were carried out using various characterization techniques such as X-ray diffraction (XRD), Scanning Electron Microcopy (SEM), X-ray energy dispersive spectroscopy (EDS), X-ray Photoelectron Spectroscopy (XPS) and Fourier Transform Infrared Spectroscopy (FT-IR). The XRD data were collected using a D8 advance powder X-ray diffractometer with CuKα radiation. Morphology and elemental composition were done using JEOL- JSM 7500F Scanning Electron Microscope. The stretching mode frequencies data were collected using Perkin Elmer Spectrum 100 FTIR spectrometer and the elemental composition on the surfaces of the phosphor powders were monitored by the PHI 5400 Versaprobe scanning X-ray photoelectron spectrometer. Photoluminescence (PL) data were collected using 325nm He-Cd laser and decay data were collected using Varian Cary Eclipse Fluorescence Spectrophotometer coupled with a monochromatized Xenon lamp (60-75 W) as excitation source and measurements were carried out in air at room temperature. The thermoluminescence (TL) data were collected using a Thermoluminescence Reader (Integral-Pc Based) Nucleonix TL 1009I. BaAl2O4:Eu2+ phosphor powders co-doped with different trivalent rare-earth (RE= Dy3+, Nd3+, Gd3+, Sm3+, Ce3+, Er3+, Pr3+ and Tb3+) ions were prepared at an initiating temperature of 600oC and annealed at 1000oC for 3 hours. The X-ray diffraction (XRD) data shows hexagonal structure of BaAl2O4 for both as prepared and post annealed samples. All samples exhibited bluish-green emission associated with the 4f65d1→4f7 transitions of Eu2+ at 504 nm. The longest afterglow was observed from the BaAl2O4:Eu2+ co-doped with Nd3+. BaAl2O4:Eu2+, Nd3+, Gd3+ phosphor powders were prepared at different initiating temperatures of 400-1200oC. X-ray diffraction data show the formation of the hexagonal BaAl2O4 structure at the temperatures of 500oC-1200oC. The crystal size calculated from the phosphor powder prepared at 1200oC was found to be 63 nm. Blue-green photoluminescence with persistent/long afterglow, was observed at 502 nm and the highest PL intensity was observed from the sample prepared at 600oC. The phosphorescence decay curves showed that the rate of decay was faster in the case of the sample prepared at 600oC compared to that prepared at 1200oC. The TL glow v peaks of the samples prepared at 600oC and 1200oC were both stable at 72oC suggesting that the traps responsible for the long afterglow were not affected by the temperature. Barium-substituted phosphor powders of (Ba1-xSrx)Al2O4:Eu2+;Nd3+ composition were prepared at an initiating temperature of 500oC. The X-ray diffraction with the composition of x = 0 shows the hexagonal phase of BaAl2O4 and the one for x = 1 shows the monoclinic phase of SrAl2O4. The XRD with the composition of x = 0.4, 0.5 and 0.6 shows the admixture of BaAl2O4 and SrAl2O4 structures. SEM investigations showed some changes on the surface morphology for different compositions. Photoluminescence (PL) studies showed the (Ba1-xSrx)Al2O4:Eu2+;Nd3+ (x = 0) and (Ba1-xSrx)Al2O4:Eu2+;Nd3+ (x = 1) with blue-green to bright-green emissions with peaks at 505 nm and 520 nm respectively. The mixed composition with x = 0.4, 0.5 and 0.6 showed two peaks at 447 nm and 517 nm. Phosphorescence showed higher luminescence for (Ba1-xSrx)Al2O4:Eu2+;Nd3+ at (x = 0) compared to other compositions. (Ba1-xZnx)Al2O4:Eu2+;Nd3+ phosphor powders with the compositions x = 0.2, 0.4, 0.5, 0.6, 0.8 and 1 were prepared at an initiating temperature of 500oC. The X-ray diffraction showed the cubic structure for the compositions of x = 0 and x = 1. The SEM images of the phosphor samples showed different kinds of morphologies for the compositions x = 0, 0.5 and 1. The PL emission of the phosphor powder clearly showed a shift from green to blue regions. The highest PL emission and the long afterglow ascribed to trapping and detrapping of charge carriers were observed from (Ba1-xZnx)Al2O4:Eu2+;Nd3+ with x = 0.2.Item Open Access Narrowband Ultraviolet B emission from gadolinium and praseodymium co-activated calcium phosphate phosphors for phototherapy lamps(University of the Free State, 2014-01) Mokoena, Puseletso Pricilla; Ntwaeaborwa, O. M.; Swart, H. C.Different phases of calcium phosphates co-doped with gadolinium and praseodymium were prepared by co-precipitation, urea combustion, citrate-gel combustion and microwave-assisted methods. Ca5(PO4)3OH:Gd3+,Pr3+ phosphors were prepared by the co-precipitation and citrate-gel methods, and were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Scanning electron microscopy (SEM), High resolution transmission electron microscopy (HRTEM), Energy dispersive x-ray spectrometer (EDS) and photoluminescence (PL) spectroscopy. The XRD pattern was consistent with the hexagonal phase of Ca5(PO4)3OH referenced in JCPDS Card Number 73-0293. The XPS data indicated that Ca2+ occupied two different lattice sites referred to as Ca1 and Ca2. The P5+ is surrounded by O2- ions in the tetrahedral arrangements. Each tetrahedron contains oxygen atoms designated as O1, O2, and O3. The particle morphology was analyzed using SEM and HRTEM. SEM shows that the powder was composed of an agglomeration of irregular particles. HRTEM revealed faceted edges forming a hexagonal shape. PL data exhibited a narrowband emission located at 313 nm, which is associated with the 6P7/2→8S7/2 transition of the Gd3+ ion. This emission is classified as ultraviolet B (UVB) and it is suitable for use in phototherapy lamps to treat various skin diseases. The PL intensity of the 313 nm emission was enhanced considerably by Pr3+ co-doping. The crystallographic structure of Ca5(PO4)3OH:Gd3+,Pr3+ and possible mechanism of energy transfer from Pr3+ to Gd3+ are discussed. Ca5(PO4)3OH:Gd3+,Pr3+ phosphor exhibited a single thermoluminescence peak between 339-363 K. The peak shifted towards high temperature with an increase in dose. The shift shows that the trap system is more complicated than a single trap obeying first order kinetics. The calculated activation energy (EA) was found to be 0.91 eV when the using initial rise method. The activation energy values were further calculated using the peak shape method. The calculated activation energies for , and , were 0.75, 1.03, and 0.42 eV respectively. There was a peak shifting to higher temperatures with an increase in heating rate which is attributed to recombination that is slowing down due to electron-phonon interactions. The peak intensity increased with an increase in heating rate from 0.6 to 2.0 °C.s-1 and started to decrease from 3.0 to 5.0 °C.s-1, the decrease maybe due to thermal quenching as the peak shift to higher temperatures. The calculated activation energy by heating rate method was found to be 0.60 eV. This value is comparable to other calculated values of activation energies by various methods mentioned above. Ca3(PO4)2:Gd3+,Pr3+ phosphors with different concentrations of Gd3+ and Pr3+ were successfully prepared by urea combustion process using metal nitrates as precursors and urea as fuel and also by the microwave assisted method. XRD exhibited a rhombohedral phase of Ca3(PO4)2 referenced in JCPDS Card No. 70-2065. The PL excitation spectra of Ca3(PO4)2:Gd3+ and Ca3(PO4)2:Pr3+exhibited peaks at 220-280 nm and 300-490 nm associated with the f-f transitions of Gd3+and Pr3+ respectively. The UVB emission resulting from the 6P7/2→8S7/2 transition of Gd3+ was observed at 313 nm when the Ca3(PO4)2:Gd3+ phosphor was excited at a wavelength of 274 nm using a monochromatized xenon lamp. Upon Pr3+ co-doping, the excitation peaks due to Gd3+ and Pr3+ f-f transitions were suppressed and an intense broad excitation peak ascribed to the 4f4f5d transitions of Pr3+ was observed at 227 nm. The peak intensity of the UVB emission at 313 nm was shown to improve considerably when the Gd3+ and Pr3+ co-doped systems were excited at the wavelength of 227 nm suggesting that the Pr3+ is a good sensitizer of the 313 nm narrow line UVB emission from Gd3+.Item Open Access Room temperature gas sensing characteristics of titanium dioxide nanostructures: effects of hydrochloric acid on the structure and magnetic properties(University of the Free State, 2016-01) Tshabalala, Zamaswazi Portia; Motaung, D. E.; Mhlongo, G. H.; Ntwaeaborwa, O. M.Monitoring and detection of toxic and combustible gases such as methane in underground mining, carbon monoxide from burning coal in our homes and odourless gases such as nitrogen dioxide and sulphur dioxide in industries has become the subject of extensive scientific and technological research and this has been motivated by their harmful impact on the environment and human health. Early detection of these gases can help prevent fatal incidences such as fire, suffocation and death. Development of portable gas sensors with higher sensitivity and selectivity, fast response and recovery times, low detection limit and capability of operating at room temperature is one of the challenges facing researchers world-wide. Various materials such as semiconductor metal oxides (MOX), polymers, and carbon nanotubes have been used for gas sensing application. Among all the various materials, MOX such TiO2, ZnO, SnO2, Fe2O3, WO3 are the most preferred materials for gas sensing application due to their noticeable response to any change in electrical resistance when exposed to either reducing or oxidizing gas and also due to their unique properties such as high stability and easy to synthesize. However, among the range of MOX semiconductors mentioned above, TiO2 has emerged as the preferred MOX semiconductor for gas sensing application due to its remarkable features such as nontoxicity, biocompatibility, high photocatalytic activity and affordability. TiO2 occurs in three crystalline forms namely: anatase, rutile and brookite. Anatase and rutile polymorphs are widely studied for technological applications. Therefore, in this study, we investigated the gas sensing properties of TiO2 nanoparticles annealed at 450 C, and that annealed at various temperatures, as well as those doped with various concentrations of Mn. The undoped TiO2 nanoparticles were synthesized from P25 Degussa via a simple hydrothermal method in an aqueous solution of sodium hydroxide (NaOH). The samples were washed with distilled water (H2O) and different concentrations of hydrochloric acid (0.25, 0.5 and 1.0 M) which acted as the morphological controlling agent. TiO2 doped with various concentration of Mn2+ were washed using 1.0 M HCl. To investigate the effect of hydrochloric acid (HCl) as a washing agent on the structure, morphology, optical, magnetic and gas sensing properties, x-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM), photoluminescence (PL) spectroscopy, electron paramagnetic resonance (EPR) spectroscopy, X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET) and Kinosistec gas sensor testing system were used for characterization. Microscopy analysis showed that the sizes of the pure TiO2 nanoparticles were reduced when increasing the HCl concentration indicating that the particle sizes could be easily tailored and tuned by adjusting the HCl concentration. Structural analyses revealed a phase transformation from a mixture of anatase and rutile phases to pure anatase phase at higher HCl concentration. The PL, XPS, EPR and BET analyses disclosed that the 1.0 M sample contained relatively high concentration of oxygen vacancy, Ti4+ and Ti3+ interstitial defects and it also had higher surface area which played an important role in transforming the sensing properties, resulting in higher sensing response, sensitivity and selectivity to NO2 at room temperature. Furthermore, the effect of thermal annealing was investigated on the structural and gas sensing properties of the pure TiO2 nanoparticles washed with H2O and HCl, and annealed at different temperatures (300, 450, 700 °C) in air. Surface morphology analyses revealed that the nanoparticles transformed to nanorods after annealing at 700 C. The results showed that the sensing properties are dependent on annealing temperature. The 1.0 M TiO2 nanostructures annealed at higher temperatures (700 C) revealed improved sensing response to CH4 gas at room temperature due to higher surface area of 180.51 m2g-1 and point defects related to Ti3+ observed from EPR and PL analyses. In addition, the 1.0 M TiO2 sensing material annealed at 700 C also revealed an interstitial defect states which played a vital role in modulating the sensing properties. To improve sensitivity, selectivity and stability of the gas sensing materials, various concentrations (1.0, 1.5, 2.0, 2.5 and 3.0 mol % denoted as S1, S2, S3, S4 and S5, respectively) of Mn-ions were loaded on the TiO2 particles. The nanoparticles were characterised in detail using various analytical techniques. XRD analysis showed that the structure of both pure and Mn-doped TiO2 was tetragonal and no peaks corresponding to Mn or impurities were observed. Raman spectroscopy revealed quenching and peak broadening due to lattice disorder with increasing concentration of Mn. Optical studies revealed that the Mn loaded TiO2 nanoparticles have enhanced UV-Vis emission and a broad shoulder at 540 nm denoting defects induced by substitution of Ti4+ ions by Mn2+. The XPS and the EPR results revealed the presence of Ti4+, Ti3+ and single ionised oxygen vacancies in both pure and Mn loaded nanoparticles. Additionally, a hyperfine split due to Mn2+ ferromagnetic ordering was observed confirming incorporation of Mn ions into the lattice. Gas sensing studies revealed that Mn2+ loaded TiO2 surface improved the NO2 and NH3 sensing performances in terms of response and selectivity. The S1 (1.0 mol. % Mn) demonstrated an improved sensitivity of approximaterly 85.39 ppm-1 at 20 ppm NH3 gas at room temperature. Our findings showed that, the thermal annealing and Mn doping improve the sensitivity and selectivity and stability of the gas sensing materials. The results also validated that our sensing materials are highly sensitive and selective to CH4, NO2 and NH3 at room temperature. The observed room temperature response in this work, suggests that these TiO2 nanostructures are possible candidates for gas sensing application in work places, mining sectors, etc. Moreover, the findings in this work give a possible solution to the issue of energy consumption of metal oxide gas sensors.Item Open Access Study of the structure, particle morphology and optical properties of mixed metal oxides(University of the Free State, 2017-01) Mokoena, Pulane; Ntwaeaborwa, O. M.; Kroon, R. E.The structure, morphology and optical properties of metal oxides (ZnO, MgO and SrO), their composites (MgO-ZnO, SrO-ZnO) and systems with different x molar concentration values (0.2, 0.4, 0.5, 0.6, 0.7, 0.8) of MgxZn1-xO and SrxZn1-xO, were synthesized via solution combustion method at initial reaction temperature of 600 ˚C for 15 minutes. These properties of the synthesized nanostructures were investigated using X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), High resolution transmission electron microscope (HR-TEM) and Photoluminescence (PL) spectroscopy. The ZnO, MgO and SrO phosphors were successfully synthesized via solution combustion method and their crystallization was confirmed by XRD analysis. The ZnO powder crystallized in the hexagonal phase. The diffraction patterns of the ZnO samples became sharper and more intense when synthesis temperature was increased from 600 ˚C to 700 ˚C indicating improvement of crystallinity and an increase in crystallite sizes from 23.3 nm to 30.06 nm of the as-prepared undoped ZnO phosphor powder. The MgO powder had cubic crystal structure with Fm-3m space group and crystallized in rocksalt/sodium chloride (NaCl) type cubic structure and the SrO sample indicated the presence of three well-defined crystalline phases which are SrO, Sr(OH)2 and Sr(CO3)2, with Sr(OH)2 appearing as the most prominent phase. With respect to the following systems: MgxZn1-xO and SrxZn1-,xO and their composites, their XRD patterns revealed the presence of two well-defined crystalline phases, namely MgO or SrO and ZnO, the most prominent phase being ZnO. The SEM images of ZnO showed agglomeration of small particles and flower-like morphology. The HR-TEM images showed that the nanoparticles (NPs) were hexagonally shaped and aggregated into clusters. The SEM images of MgO showed spherical cube-like morphology with the appearance of closely-packed or attached particles in all the SEM micrographs. The HR-TEM images show that the NPs were cubic-spherically shaped and aggregated into clusters. For the SrO sample small and coagulated particles of irregular shapes and different sizes were observed. Pores of different sizes were also observed from the solution combustion synthesis. This is due to the outgassing of the gaseous products, namely N2 and CO2, of this synthesis method. The HR-TEM images showed that the NPs were spherically shaped and aggregated into clusters. The selected area electron diffraction pattern confirmed the observation of a large number of nanoparticles and hence there were many spots within each ring. In the case of the MgxZn1-xO system SEM observations revealed different kinds of particle morphologies such as pyramids clustered together to form flowers with spherical particles grouped together on the sides, triangles grouped together in the shape of a cauliflower, tetragonally shaped particles with some degree of faceting and for the SrxZn1-xO system, flower-like structures, oval-shaped particles and elongated rod-like structures. The photoluminescence results of ZnO exhibits two characteristic peaks: one narrow in the ultraviolet (UV) region at 380 nm which comes from recombination of free excitons, and one broad in the visible region at 639 nm for ZnO synthesized at 600 ˚C and 626 nm for ZnO synthesized at 700 ˚C, which were attributed to electron mediated defect levels in the bandgap. The MgO sample showed three PL emission peaks at approximately 419, 432 and 465 nm and a minute emission peak at 663 nm. The SrO PL spectrum exhibited UV and deep level emission peaks. In addition, there was a narrow peak in the UV region at 397 nm and a broad peak in the visible region at 750 nm. With regards to the MgxZn1-xO system with x ranging from 0.2, 0.4, 0.5, 0.6 and 0.7, a red shift in the emission peaks from 602 to 610 nm was observed for the 0.2 and 0.4 molar concentrations while their luminescence intensity decreased. For a molar concentration 0.5 there was a blue shift in the emission peak from 610 to 551 nm together with luminescence quenching. From molar concentration 0.5 to 0.6 there was a blue shift in the emission peaks from 551 to 539 nm with a luminescence enhancement, but when the molar concentration was 0.7 there was a slight red shift in the emission peak located from 539 to 549 nm together with a luminescence enhancement. With regards to the MgO-ZnO composite sample there was only one broad emission peak at 559 nm in the visible region and luminescence intensity increased significantly. For molar concentrations 0.2 and 0.4 there were emission peaks at 383, 540 and 760 nm. For molar concentration 0.5 there were emission peaks at 383, 514 and 760 nm. For molar concentration 0.6 there were emission peaks at 383 nm, minor humps at 413, 435 and 760 nm and a broad peak at 514 nm. For molar concentration 0.7 there were emission peaks at 383, 514 and 760 nm and for molar concentration 0.8 there were emission peaks at 383, 514 and 760 nm. The emission peak in the UV region (383 nm) was narrow and this was ascribed to recombination of free excitons, while the broad emission peaks at 514 and 540 nm were attributed to electron mediated defect levels in the bandgap.Item Open Access Tunable multicolour emission from dysprosium-doped mixed rare-earths oxyorthosilicate nanophosphors for application in ultraviolet-pumped multicolour and white light emitting diodes(University of the Free State, 2015-02) Ogugua, Simon Nnalue; Ntwaeaborwa, O. M.; Swart, H. C.Phosphors have many uses today in applications such as electronic information displays, solid state lighting, solar cells, advertising and theft prevention. By using urea-assisted solution combustion method, we prepared tunable multicolour and white light emitting dysprosium (Dy3+) doped rare earth oxyorthosilicate (R2SiO5) (R = La, Y, Gd) powder phosphors. We prepared five sets of powder samples, namely LaYSiO5:Dy3+, LaGdSiO5:Dy3+, GdYSiO5:Dy3+, La2-xGdxSiO5:Dy3+ (x = 0, 0.5, 1.0, 1.5 and 2.0) and LaGdSiO5:Dy3+ x mol% (x = 0.05, 0.1, 0.25, 0.75, 1.0, 1.5, 2.0, 3.0 and 5.0). The structure and the stretching modes of vibration of the phosphors were analyzed using X-ray diffractometer (XRD) and Fourier transform infrared (FT-IR) spectrometer respectively while the morphologies and the elemental composition of the phosphors were analyzed using, respectively, field emission scanning electron spectroscopy (FE-SEM) and energy dispersive X-ray spectroscopy (EDS). In addition, X-ray photoelectron spectroscopy (XPS) was also used to analyze the elemental composition, chemical and electronic states of the phosphors while the distribution of atomic and molecular ionic species on the surface region of the samples was studied using the time-of-flight secondary ion mass spectroscopy (TOF-SIMS). The elemental composition analysis indicated that there was a correlation among the EDS, XPS and TOF-SIMS data. The TOF-SIMS overlay images suggested that the dopant ions were evenly distributed and co-localized with major ionic species on the surface. The crystallite sizes calculated from the X-ray diffraction peaks using Williamson-Hall equation were in the range of 8.0 to 21.0 nm. The band gaps of the phosphors determined from the diffuse reflectance data using Tauc plot were found to vary from 5.0 to 4.45 eV. The photoluminescence spectra recorded when the samples were excited using the 325 nm He-Cd laser consisted of broad band and line emission peaks which we assigned respectively to self-trapped excitons (STE) in SiO2 and 4F9/2→6H15/2 and 4F9/2→6H13/2 transitions of Dy3+. The peak intensities of the emission bands were shown to depend on the molar ratios of La to Gd, La to Y and Gd to Y on the mixed rare-earths oxyorthosilicate hosts. The colour purity of the bands estimated using CIE coordinates confirmed that our samples were emitting tunable multicolour and white light. These results suggest that our material can be used as single host phosphors in energy efficient UV-pumped multicolour and white light emitting diodes (LED). The structure, particle morphology, surface chemical composition and electronic states, photoluminescent properties and possible applications of these materials in UV-pumped LEDs were investigated.