Theses and Dissertations

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Browsing Theses and Dissertations by Author "Dima, Ratshilumela Steve"

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    Density functional theory study of TiO2 Brookite (100), (110) and (210) surfaces doped with ruthenium (RU) and platinum (Pt) for application in dye sensitized solar cell
    (2018-05-18) Dima, Ratshilumela Steve; Maluta, E.N.; Maphanga, R. R.
    Since the discovery of water photolysis on a TiO2 electrode by Fujishima and Honda in 1972, TiO2 has attracted extensive attention as an ideal photocatalytic material because of its excellent properties such as high activity, good stability, nontoxicity and low cost. Hence, it has been widely used in the fields of renewable energy and ecological environmental protection. However, as a wide band gap oxide semiconductor (Eg = 3.14 eV), brookite TiO2 can only show photocatalytic activity under UV light irradiation (λ < 387.5 nm) that accounts for only a small portion of solar energy (approximately 5 %), in contrast to visible light for a major part of solar energy (approximately 45 %). Therefore, effectively utilizing sunlight is the most challenging subject for the extensive application of TiO2 as a photocatalyst. Due to the unique d electronic configuration and spectral characteristics of transition metals, transition metal doping is one of the most effective approaches to extend the absorption edge of TiO2 to the visible light region. This method of doping either inserts a new band into the original band gap or modifies either the conduction band or valence band, improving the photocatalytic activity of TiO2 to some degree. In this work, the structural, electronic and optical properties of doped and undoped TiO2 (100), (110) and (210) surfaces were performed using first principle calculations based on DFT using a plane-wave pseudopotential method. The generalized gradient approximation was used in the scheme of Perdew-Burke-Ernzerhof to describe the exchangecorrelation functional as implemented in the Cambridge Sequential Total Energy Package code in the Materials Studio of BIOVIA. The metal dopants shift the absorption to longer wavelengths and improves optical absorbance in visible and near- IR region. The un-doped (210) surface showed some activity in the visible and near IR region.
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    Multiscale modeling of sodium-iron battery materials
    (2025-05-16) Dima, Ratshilumela Steve; Maphanga, R. R.; Maluta, E. N.
    In recent years, there has been a growing interest in alternative energy storage technologies as a result of the diminishing reserves of fossil fuels. The development of these technologies requires a careful evaluation of factors such as energy storage and conversion, implementation costs, and environmental impact. Rechargeable batteries are expected to become crucial energy storage devices and promote a more sustainable energy ecosystem. Battery technology has the potential to become cost competitive, especially for portable applications, and exhibits exceptional efficiency, exceeding 90% in electrical efficiency. Sodium ion batteries are considered to be cost-effective and economically feasible alternatives. This work used multiscale computer modelling techniques to understand, control, and improve the intrinsic properties of NaxMnPO4, an electrode material that undergoes Na intercalation and de-intercalation processes. This work aims to promote a more sustainable energy ecosystem. Firstly, we examine the structural and electrochemical performance of NaxMnPO4 using the first-principle density functional theory method. Comparison of the exchange correlation functionals PBE, PBEsol, and PBE+U was conducted, and the results showed that the PBE+U replicated the structural parameters and the energy band gap values well and was used to further analyse the electrochemical performance of the de-intercalated systems. The effect of Na atom de-intercalation on the structural, electronic, mechanical, and thermodynamic properties of both maricite and olivine polymorphs of NaMnPO4 has been investigated by first-principle calculations. The calculated values for the formation energy were found to be negative for all NaMnPO4 systems, hence the solid solution is predicted for states of de-intercalation. The analysis of the electronic density of states indicated that, during the Na removal stages, the material exhibited a rise in its metallic properties between the first and third stages. On the contrary, in the fourth stage, the material displayed semiconductor behaviour, characterised by a band gap of 0.194 eV. A voltage range of 3.997 to 3.848 V was observed, and the computed formation energy values of the de-intercalated systems were determined to be negative, indicating the anticipated presence of a solid in the material. Secondly, the ab initio molecular dynamics method was used to simulate the dynamic properties of NaxMnPO4 materials at different temperatures. The results showed an increasing mean-square displacement gradient as the number of de-intercalated Na atoms increased. The Na-ion diffusion coefficients for olivine and maricite NaMnPO4 were calculated at 100 K and 300 K. Both polymorphs had low diffusion rates at 100 K but increased at 300 K, suggesting faster ion movement. These findings are crucial for understanding the behavior of NaxMnPO4 materials and their potential applications, as diffusion rates can affect processes such as charge / discharge rates in batteries and ion transport in solid-state electrolytes. Controlling temperature and understanding its influence on diffusion coefficients can optimize the performance of NaxMnPO4 materials. Lastly, the cluster expansion (CE) method was introduced as a multiscale pipelining method, establishing a connection between first-principles calculation and large-scale atomistic simulations, as well as Monte Carlo simulation. CE was used to examine the phase stabilities of Na concentrations in relation to vacancies. The stability of the predicted structures on the isotopically optimized volume binary diagram was assessed by calculating their mechanical, electronic, and dynamic properties. Structures that underwent isotropic volume optimisation yielded a cross-validation score of 1.1 meV. This score suggests that the cluster expansion is of good quality, as it falls below the threshold of 5 meV per active position. Based on the analysis of the electronic structure, it is observed that both parent structures (MnPO4 and NaMnPO4) exhibit semiconducting behaviour, while the remaining structures (Na1MnPO4, Na0.825MnPO4, Na0.75MnPO4, Na0.625MnPO4, and Na0.25MnPO4) have semi-metallic characteristics. The mechanical stability of NaMnPO4 was shown by the estimated elastic constants, since the stability conditions were met for all intercalated systems, except for the parent structure MnPO4. Based on the Pugh criterion pertaining to the properties of ductility and brittleness, the structures of Na1MnPO4, Na0.825MnPO4, Na0.75MnPO4, Na0.625MnPO4, and Na0.25MnPO4 exhibit ductile characteristics, while the structures of Na0.5MnPO4 and MnPO4 display brittleness. In addition, MD simulations were performed, revealing that the mean square displacement slope is influenced by the concentration of sodium ions, whereas the diffusion coefficients of sodium ions are influenced by the temperature. These findings suggest that the addition of sodium ions improves the ductility of Na1-xMnPO4 structures. The higher concentration of sodium ions leads to increased ductility, as evidenced by the ductile characteristics observed in Na1MnPO4 and Na0.825MnPO4. However, as the concentration of sodium ions decreases, the structures become more brittle, as seen in Na0.5MnPO4 and MnPO4. Furthermore, the MD simulations indicate that the movement of sodium ions within the structures is influenced by both the concentration of sodium ions and the temperature, highlighting the complex relationship between the composition and mechanical properties in these materials.