Disorder in materials often brings in new and exotic physical properties along with it. It is thus very important to study different kinds of disorder and their implications on various material properties. In this thesis, we study selected transition metal oxide families of compounds, each being associated with a specific kind of disorder and investigate effects of that disorder on their dielectric and magnetic properties. In Chapter1, we have given brief introductions on the origin of magnetic and dielectric properties in materials and have also discussed various mechanism which give rise to multiferroism in materials which exhibit both spontaneous magnetic ordering and spontaneous electric ordering in the same phase of the material. In Chapter2,we describe the various methodologies adopted in this thesis. In Chapter3,we mainly study the effect of cationic size-disorder in deciding un-usually robust dielectric properties of Ln2CuTiO6(Ln=Y,Dy,Ho,Er,Yb) family of compounds. We discover that these materials, in addition to possessing large dielectric constant values, also exhibit exceptional stabilities of their dielectric constants with respect to large changes in temperature and frequency. We further find that this class of materials are non-ferroelectrics though its hares the same non-centrosymmetric space group, P63cm,with the well-known multiferroic YMnO3 that undergoes a ferroelectric transition at 940K. Using first principles calculations, we establish that exceptional dielectric properties result from a combination of two separate effects. Extensive size disorders at the Cu/Ti B-site suppress the expected ferroelectric transition, leading to relatively large values of the dielectric constant for every compound investigated in this series. Additionally, it is shown that the majority contribution to the dielectric constant arises from intermediate-frequency polar vibrational modes, making it relatively stable against temperature variations. In Chapter4, we study the effect of cation anti-site disorder on the magnetic, electric and dielectric properties of the solid solution series of (x)Fe2O3-(1-x)FeTiO3 for several values of x. For intermediate values of x, these solid solution members are found to be strong ferrimagnets. Anti-site cation disorder, between Fe and Ti, however strongly reduces the magnetic moment values. By tuning the degree of cation anti-site disorder, we attain multi-functionality in these samples. We have performed detailed characterizations of valence states of Fe and Ti across the solid solution series using x-ray photoelectron spectroscopy and x-ray absorption spectroscopy. Using x-ray magnetic circular dichroism, we validate the microscopic model of magnetism and suggest a microscopic picture of the anti-site cation disorder for these samples. In Chapter5,we study the effects of controlled chemical disorder in SrTiO3 lattice, by performing site-specific doping of Mn in SrTiO3. We find that site specific Mn doping has decisive influence on their dielectric properties with qualitatively and quantitatively different behaviors between these doped samples .Using electron paramagnetic resonance experiments, we establish the site specific doping of Mn in SrTiO3 lattice. We find that while Mn doping at Ti site continues to remain paraelectric, Mn doping at the Sr site becomes a relaxor ferroelectric. We find samples having Mn substituted at both Sr and Ti sites simultaneously to be also relaxor ferroelectrics. Combining experiments with first principles calculations, we understand the origin for the high temperature dielectric properties of various Mn doped SrTiO3 samples. We show that Mn ions doped at the Sr sites off-centers and gives a significant dipolar contribution to their dielectric constants. While demonstrating the superior dielectric properties of Mn doped SrTiO3 ceramics ,we also elucidate their magnetic properties in details. In Chapter6,we study the effect of cation anti-site disorder on the magnetic and dielectric properties of undoped and Lu doped La2NiMnO6 samples. Using detailed spectroscopic characterizations of these samples with x-ray absorption experiments, coupled with d.c.magnetization and a.c.susceptibility measurements, we demonstrate that while the doped samples are ferromagnetic, the undoped samples exhibit re-entrant spin-glass magnetism. We also show that the dielectric properties of undoped La2NiMnO6,crys-tallizing in monoclinic and rhombohedral phases are distinctly different and we study their dielectric relaxations in details. We also demonstrate multiferroism in Lu doped La2NiMnO6 samples. In Appendix A, we study the electronic origin of ferroelectric polarization in the spin spiral compound ,MnWO4. Using x-ray absorption spectroscopy on synthesized MnWO4 samples, coupled with configuration interaction calculations, we establish quantitatively a significant population of Mn 3d states beyond the expected half filling and provide a critical insight into the significant presence of spin-orbit coupling and consequent finite polarization in this system. In Appendix B, we study charge-transfer doping in few-layer grapheme covered with electron acceptor (Tetracyanoethylene) and donor (Tetrathiafulvalene) molecules using x-ray photoelectron spectroscopy. We give quantitative estimates of the extent of doping in these samples and thereby elucidate the origin of unusual shifts of their Raman G bands in contrast to electrochemically doping schemes. In conclusion, we investigate, in this thesis, properties of different classes of compounds in presence of distinctly different kinds of disorder and establish the critical role of disorder in each case in tuning their desirable physical properties.

Le sujet de la topologie dans les oxydes, en particulier à la surface des oxydes de pérovskite comme SrTiO₃, ou à l'interface de LaA1O₃ / SrTiO₃ sera étudié dans cette thèse. Les deux matériaux, à leurs surfaces orientées (001), contiennent un état métallique limité à quelques nanomètres de la surface. De plus, nous montrerons qu'il existe certains croisements de trois bandes autour desquelles des perturbations vont provoquer l'apparition d'un spectre de bandes inversées et gappées. Ceux-ci conduiront à des états de bord topologiques qui peuvent être détectés via la supraconductivité induite comme dans le cas des puits quantiques topologiques ou des nanofils des supraconducteurs-semi-conducteurs. Ensuite, la surface orientée (111) de LaA1O₃ / SrTiO₃ sera étudiée lorsque les mesures de transport de Hall révèlent une transition de un à deux porteurs par dopage électrostatique. Une explication basée sur un modèle de liaisons fortes incluant des corrélations U de Hubbard sera proposée, ce qui donnera lieu à des croisements de bandes entre les sous-bandes favorisant les états topologiques. Enfin, une étude ab-initio de CaTiO₃ sera effectuée pour expliquer l'état métallique qui existe à sa surface (001) orientée et pour prédire le magnétisme dans le système. CaTiO₃ est différent des autres composés étudiés précédemment, en raison de la grande rotation et de l'inclinaison des octaèdres d'oxygène entourant le Ti, ce qui complique les faits. La structure avec et sans lacunes d'oxygène sera étudiée en profondeur pour fournir des détails sur la bande de conduction et leurs caractères orbitaux. / The subject of topology in oxides, in particular at the surfaces of perovskite oxides like SrTiO₃, or at the interface of LaA1O₃/SrTiO₃ will be investigated in this thesis. Both compounds, at their (001) oriented surfaces, contain a metallic state confined to a few nanometers at the surface. In addition, we will show that there exist certain three band crossings around which perturbations will cause an inverted and gapped band spectrum to appear. These will lead to topological edge states which can be detected via induced superconductivity as in the case of topological quantum wells or superconductor-semiconductor nanowires. Next, the (111) oriented surface of LaA1O₃/SrTiO₃ will be studied where Hall transport measurements reveal a one to two carrier transition via electrostatic doping. An explanation based on a tight binding modelling including Hubbard U correlations, will be proposed which will give rise to band crossings between sub-bands promoting topological states. Finally, an ab-initio study of CaTiO₃ will be performed to explain the metallic state which exists at its (001) oriented surface and to predict magnetism in the system. CaTiO₃ is different from the other compounds studied previously, due to the large rotation and tilting of the oxygen octahedra surrounding the Ti, which complicates the picture. The structure with and without oxygen vacancies will be studied in-depth to provide details about the conduction band and their orbital characters.

Tin dioxide (SnO2) is considered one of the most promising anode materials for Lithium ion batteries (LIBs), due to its large theoretical capacity and natural abundance. However, its low electronic/ionic conductivities, large volume change during lithiation/delithiation and agglomeration prevent it from further commercial applications. In this thesis, we investigate modified SnO2 as a high energy density anode material for LIBs. Specifically two approaches are presented to improve battery performances. Firstly, SnO2 electrochemical performances were improved by surface modification using Atomic Layer Deposition (ALD). Ultrathin Al2O3 or HfO2 were coated on SnO2 electrodes. It was found that electrochemical performances had been enhanced after ALD deposition. In a second approach, we implemented a layer-by-layer (LBL) assembled graphene/carbon-coated hollow SnO2 spheres as anode material for LIBs. Our results indicated that the LBL assembled electrodes had high reversible lithium storage capacities even at high current densities. These superior electrochemical performances are attributed to the enhanced electronic conductivity and effective lithium diffusion, because of the interconnected graphene/carbon networks among nanoparticles of the hollow SnO2 spheres.

Mobile devices for the personalized detection of health and environmental hazards are becoming the basis for futuristic sensing technologies. In recent decades, air and environmental pollution levels have risen globally. Therefore, environmental protection must be strengthened by developing sensors that detect pollutants. The monitoring of these pollutants with high spatial coverage requires inexpensive electronic gas sensors and self sustainable sensing systems that can be deployed everywhere. This dissertation reports on technological developments to provide solutions for inexpensive, compact, power efficient, and easily deployable toxic gas sensors and integrated systems using semiconducting metal-oxide thin-film transistors (TFTs). The first part of the dissertation introduces the fabrication and characterization of an amorphous indium gallium zinc oxide (IGZO) TFT as a toxic gas sensor. In contrast to existing metal-oxide gas sensors, which are active either with light activation or at high temperature, the developed IGZO TFT sensors are operable at room temperature and require only visible light activation to revive them after exposure to NO2. IGZO TFT sensors exhibited remarkable selectivity and sensitivity to low concentrations of nitrogen dioxide (NO2). The second part of the dissertation introduces the design and realization of the IGZO-based fully integrated gas detectors. Unlike existing gas-sensing systems, which have discrete hardware for signal conditioning, read-out, and data acquisition, the developed integrated detectors constitute thesemodules integrated using IGZO TFT technology. The integrated detectors detect ambient NO2 gas and generate a digital output that is proportional to the ambient gas concentrations. Two types of integrated gas detectors were developed that differ in their mode of operation and circuitry design. These detectors are scalable and pave the way for portable systems to realize various gas-sensing applications, including smart cities and sustainable ecosystems. The success of personalized monitoring devices relies on the following factors: minimum power consumption, selectivity, and stability under extreme conditions that determine overall performance. One of the best solutions to minimize power consumption in these devices is to have a complementary energy-harvesting feature. Hence, the dissertation concludes with the design of self-powered sensors, which are IGZO sensors with self-powering capabilities. Self-powered sensors are p-n heterojunction sensors, developed using IGZO and hybrid-perovskites.

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Thesis (M.Sc. (Chemistry)) -- University of Limpopo, 2022 / Heart and Stroke Foundation South Africa (HSFSA) reports that about 17.3% of deaths in the country are associated with heart-related diseases and this rate is expected to increase to 41% by the year 2030. This severe increase in death cases is related to diseases caused by consumption of meat (i.e., pork, fish, red meat, and poultry) with high levels of hypoxanthine. Therefore, this raises the need to investigate and detect hypoxanthine levels in the meat. This study aimed at developing a highly stable and sensitive biosensor for the detection of hypoxanthine in fish meat using the glassy carbon electrode (GCE) modified with carbon nanocomposites materials (consisting of metal oxides doped multi-walled carbon nanotubes (MO-MWCNTs) that are treated with amine groups) and an enzyme, xanthine oxidase (XOD) as a catalyst. The sol gel method was used to prepare the metal oxides including zinc oxide (ZnO), zirconium dioxide (ZrO2), manganese (MnO2), cobalt oxide (Co3O4), and titanium dioxide (TiO2). The in-situ method of functionalisation of MWCNTs was employed to increase their current outputs/sensitivity using selected amines, namely, methylenediamine, hydrazine, ethylenediamine (EDA), and triethylenetetramine (TETA). The electrochemical properties of the metal oxides and amine functionalised MWCNTs were studied using both cyclic and differential pulse voltammetry. Fourier transform infrared spectroscopy (FTIR) confirmed the presence of carboxyl (COOH), hydroxyl (OH), and amino (NH2) groups on the surface of the modified MWCNTs; as well as formation of stretching vibrations which appear at lower wavelengths due to the metallic species within the nanocomposite. Thermal gravimetric analyser (TGA) was employed to determine the thermal stability of the nanocomposite. Scanning electron microscopy (SEM) was used to confirm the composite structure and correct deposition of the metal oxides on the walls of MWCNTs. XRD was used to confirm correct structure formation, the crystallinity, and the purity of the nanocomposite. Optimum conditions of the developed biosensor were determined, and the application of the developed biosensor was undertaken on fish meat bought at the local supermarket using the Cyclic and Differential pulse voltammetric techniques. vi Two highly electrochemical metal oxides among others were TiO2 and Co3O4. The modified MWCNTs containing TETA possess good electrochemical properties with improved sensitivity and selectivity towards hypoxanthine. The presence of metal oxides on MWCNTs and their treatments with amines as confirmed by techniques such as TGA, SEM, XRD, and FTIR have provided a suitable matrix for the immobilisation of the enzyme, namely, xanthine oxidase at 0.5 unit (U). TGA results showed that the unmodified MWCNTs decompose at around 600 0C, but when they are modified with acids and amine decomposition starts at 230 0C, proving that functionalisation of MWCNTs tempers with their thermal stability. Based on the SEM morphological results, attachment of the amines and metal oxides on MWCNTs was seen at x60 000 magnification. Morphology of acid treated MWCNTs appeared thinner, revealing that acids tends to deteriorate the MWCNTs, while the amino treated MWCNTs appeared well modified with less damage on the MWCNTs. XRD confirmed the successful purification of MWCNTs with the intense diffraction peak at 260 that can be assigned to the (002) reflection of graphite. The strong diffraction peak at 250o and a broad peak at 450 indicate that the titania nanoparticles are pure and in the anatase phase. They also show successful deposition of the titanium dioxide onto the surface of the MWCNTs. However, on the formation of cobalt oxide two phases were observed which were CoO, and Co3O4, and on bimetallic nanocomposite (cobalt titanium oxide) also two phases were observed which were CoTiO3, and Co2TiO4. It was found that the sensor performs better at 25 oC at a pH of 7.5 in a phosphate buffer at concentration of 5 mM. The limit of detection of the biosensor was found to be 0.16 nM. The highly electroconductive electrode was XOD/3%Co2TiO4-MWCNTs-TETA/GCE, which was selected for analysis of fish meat. The biosensor has shown low interfering values with high stability, good reusability retaining 73.4% of its initial performance after 50 days of continuous study. The excellent results were obtained on fish meat analysis using cyclic and differential pulse voltammetry revealed that even meat which is deep frozen can also deteriorate as time passes by. Altogether, the findings from this study suggest that the developed biosensor is a reliable analytical tool for the determination of freshness of fish meat using hypoxanthine levels as a marker. / NRF Sasol Inzalo Foundation

In this research we investigated the generation of methanol and the utilization of CO2 using heterogeneous catalysts. Heterogeneous catalysts are frequently used in industry due to their multiple benefits, which include long-term thermal and mechanical stability, as well as reusability. Our research has demonstrated a variety of heterogeneous catalysts for sustainable methanol production and CO2 utilization, including the novel dendritic mesoporous metal oxides support. We have also designed and screened multiple active metals on the dendritic mesoporous metal oxide catalysts, modified active metal dispersion, and further reduced metal oxides to utilize silica-based catalysts, among other things. Comprehensive characterization of the final products was performed using N2 adsorption and desorption, XRD, HR-TEM, SEM, ICP-OES, XPS, H2-TPD, CO2-TPD, Raman spectroscopy, pulse-chemosorption and DRIFT, in order to determine the chemical and physical properties of the catalysts. The catalysts were found to have the following characteristics. We obtained a CO2 conversion of 25.5 % and a MeOH yield of 6.4 % after at least three cycles of usage in an avantium fixed bed reactor system with a PdCu/CZ-3 catalyst. Additionally, continuous methanol production with a higher yield (6.9 %) has been achieved using our PdZn/CZ-3 catalysts, and the best ultra-dispersed Pd nanoparticles over CZZ catalyst produces more than 12 % methanol yield with constant selectivity to methanol even after a lengthy catalytic test (more than 100 h), demonstrating their industrial viability. Additionally, our PdZn/CeTi-DMSN exhibits a high methanol production of up to 10% and better long-term stability with lower metal oxides content. The adsorption and activation of CO2 to react with the spilled over hydrogen to generate methanol has been researched for the CO2 hydrogenation and utilization reaction. Catalysts' redox, acidic, and basic characteristics all play a crucial part in this reaction and in the formation of the various products. With 2.0 percent Pd, the supported dendritic CeZrZn catalyst exhibits the highest catalytic performance (29.1% conversion and 40.6% MeOH selectivity). Comprehensive analysis revealed in this research not only identified effective catalysts with high activity for a variety of applications, but also established a link between catalytic performance and the material's nature. These discoveries may also aid the researcher in the near future in resolving global environmental problems.