http://hdl.handle.net/10386/8 2013-05-19T20:56:25Z http://hdl.handle.net/10386/743 Title: Computer modelling studies of gold nanoclusters, nanotubes and nanowires Authors: Mahladisa, Mokete Abram Abstract: The importance of gold for scientific uses is of fundamental importance to research and technology developments. The bulk gold shows reluctance to participate in chemical reactions, the effect which has been corrected by the change in the size towards nanoclusters. It is therefore imperative that the structure of gold nanomaterials is understood for better applications in catalysis and other developments. Molecular dynamics and the density functional theory have proven to be good tools in computational material science and have thus been used to greater lengths. Molecular dynamics simulations on different gold nanoclusters and nanotubes were successfully carried out at different thermodynamic conditions. The effect of size on the melting of materials was duly tested and our results to some extend agree with what has already been reported. Gold nanoclusters show melting below the bulk and the melting temperatures increase with cluster size. However, the Au55 cluster shows different results in that it melts above the bulk due to structural reconstruction. The structure of the clusters changes from spherical shapes to tetragonal or face centred cubic (fcc) structures. Gold nanotubes show no resistance to temperature and different configurations are obtained in different ensembles. Single wall nanotubes form spherical clusters in the NVT while the NPT conditions give patches of clusters at elevated temperatures. The multi wall nanotubes also form spherical clusters in the NVT but fcc structures are obtained in the NPT Berendsen ensemble towards melting. Ab initio calculations in DMOL3 code on different gold nanoclusters show the stability of the clusters to increase with size and the Au3 and Au8 clusters contain the most stable structures. The Au-Au bond length in the dimer was obtained to within reasonable agreement with experiments and other theoretical works. Doping of the clusters further improved their stability although different impurities give different observations. The QMERA code calculations show that a gold atom on top of the surface causes slanting of the outer MD layers. The morphology of the quantum atoms also changes as compared to the neutral surface and the results are compared by the DMOL3 code which confirms the QMERA results. Description: Thesis (Ph.D. (Physics)) --University of Limpopo, 2011 2011-01-01T00:00:00Z http://hdl.handle.net/10386/737 Title: Phase stability study of Pt-Cr and Ru-Cr binary alloys Authors: Tibane, Meriam Malebo Abstract: Planewave pseudopotential calculations were conducted to predict the energetics and phase stability of Pt-Cr and Ru-Cr binary alloys. Validation of appropriate number of k-points and planewave energy cut-off was carried out for all studied systems. At the composition of A3B and AB3 (where A = Cr and B = Pt or Ru) phases, the heats of formation determined for five different structures, L12, A15, tP16, DOC and DO′ C are almost of the same magnitude and the relaxed structures show no rotation. We observed that the cubic L12 Pt3Cr is the most stable structure in agreement with the experiments. The results for PtCr3 indicate the negative heat of formation for the A15 phase whereas all the remaining studied phases have positive heats of formation. It is clear that the PtCr3 (A15) is the most stable structure. PtCr (L10) was found to be more stable compared with PtCr (B2) phase. The L12 Pt3Cr, A15 PtCr3 and L10 PtCr phases could be considered as possible coatings to cover the engines which are exposed to aggresive environments. The heats of formation of all studied compositions and phases of Ru-Cr systems are positive, these results suggest that, generally, studied Ru-Cr phases are not stable. The effect of pressure and doping were investigated on A15 RuCr3 structure which was reported to exist at a higher temperature. Elastic constants and moduli were investigated to determine the strength of the PtCr systems. The strength of PtCr L10 is greater than that of B2 phase. The ratio of shear to bulk modulus (G/B) has been used to predict the ductility or the brittleness of the material. It was found that Pt3Cr L12 is the most ductile phase among those considered in this study. The density of states were calculated to further analyze the stability of systems. The magnetic properties of Cr were studied using VASP which predicted an anti-ferromagnetic and a non-magnetic ground state for pure Cr. We have investigated the thermal stability at 0 GPa for different phases of Pt3Cr, PtCr3, PtCr and RuCr3 A15 phase, where we detected the soft modes at X, G, M and R points of the Brillouin zone from the phonon spectra of Pt3Cr A15 phase. Pt3Cr L12 and PtCr3 A15 are predicted as dynamically stable structures. RuCr3 A15 phase was found to be dynamically stable but thermodynamically unstable. Phonon DOS were studied to observe the modes of vibration and atoms that contribute to soft modes. Lastly we investigated the thermal expansion of Pt3Cr L12 and A15 phases. Description: Thesis (Ph.D. (Physics)) --University of Limpopo, 2011 2011-01-01T00:00:00Z http://hdl.handle.net/10386/702 Title: A Theoretical Study of Alkali Metal Intercalated Layered Metal Dichalcogenides and Chevrel Phase Molybdenum Chalcogenides Authors: Kganyago, Khomotso R. Abstract: This thesis explores the important issues associated with the insertion of Mg2+ and Li+ into the solid materials: molybdenum sulphide and titanium disulphide. This process, which is also known as intercalation, is driven by charge transfer and is the basic cell reaction of advanced batteries. We perform a systematic computational investigation of the new Chevrel phase, MgxMo6S8 for 0 ≤ x ≤ 2, a candidate for high energy density cathode in prototype rechargeable magnesium (Mg) battery systems. Mg2+ intercalation property of the Mo6S8 Chevrel phase compound and accompanied structural changes were evaluated. We conduct our study within the framework of both the local-density functional theory and the generalised gradient approximation techniques. Analysis of the calculated energetics for different magnesium positions and composition suggest a triclinic structure of MgxMo6S8 (x = 1 and 2). The results compare favourably with experimental data. Band-structure calculations imply the existence of an energy gap located ~1 eV above the Fermi level, which is a characteristic feature of the electronic structure of the Chevrel compounds. Calculations of electronic charge density suggest a charge transfer from Mg to the Mo6S8 cluster, which has a significant effect on the Mo-Mo bond length. There is relatively no theoretical work, in particular ab initio pseudopotential calculations, reported in literature on structural stability, cations "site energy" calculations, and pressure work. Structures obtained on the basis from experimental studies of other ternary molybdenum sulphides are examined with respect to pressure-induced structural transformation. We report the first bulk and linear moduli of the new Chevrel phase structures. This thesis also studies the reaction between lithium and titanium disulfide, which is the perfect intercalation reaction, with the product having the same structure over the range of reaction 0  x  1 in LixTiS2. Calculated lattice parameters, bulk moduli, linear moduli, elastic constants, density of states, and Mulliken populations are reported. Our calculations confirm that there is a single phase present with an expansion of the crystalline lattice as is typical for a solid solution, about 10% perpendicular to the basal plane layers. A slight expansion of the lattice in the basal plane is also observed due to the electron density increasing on the sulfur ions. Details on the correlation between the electronic structure and the energetic (i.e. the thermodynamics) of intercalation are obtained by establishing the connection between the charge transfer and lithium intercalation into TiS2. The theoretical determination of the densities of states for the pure TiS2 and Li1TiS2 confirms a charge transfer. Lithium charge is donated to the S (3p) and Ti (3d) orbitals. Comparison with experiment shows that the calculated optical properties for energies below 12 eV agrees well with reflectivity spectra. The structural and electronic properties of the intercalation compound LixTiS2, for x = 1/4, 3/4, and 1, are also investigated. This study indicates that the following physical changes in LixTiS2 are induced by intercalation: (1) the crystal expands uniaxially in the c-direction, (2) no staging is observed. We also focus on the intercalation voltage where the variation of the cell potential with the degree of discharge for LiTiS2 is calculated. Our results show that it can be predicted with these well-developed total energy methods. The detailed understanding of the electronic structure of the intercalation compounds provided by this method gives an approach to the interpretation of the voltage composition profiles of electrode materials, and may now clearly be used routinely to determine the contributions of the anode and cathode processes to the cell voltage. Hence becoming an important tool in the selection and design of new systems. Keywords Magnesium rechargeable battery; Chevrel, Lithium batteries; Li and Mg-ion insertion; TiS2; Mo6S8; Charge transfer; reflectivity, intercalation, elastic constants, voltage, EOS, Moduli. Description: Thesis (Ph.D. (Engineering mechanics)) --University of Limpopo, 2004 2004-01-01T00:00:00Z http://hdl.handle.net/10386/675 Title: Synthesis, characterization and kinetic studies of porphyrins with thiolated side chains for functionalization of paramagnetic iron-oxide- gold, core- shell nanoparticles Authors: Eshilokun, Adeolu O. Abstract: 5,1 0, ,15,20-Tetrakis[3-(3-thioacetoxypropoxy)phenyl]porphyrin a photosensitizer for photodynamic therapy (PDT) was synthesized from 3-hydroxybenzaldehyde. Acid and alkaline hydrolysis of the thioacetoxypropoxyphenyl porphyrin was attempted to afford a free base porphyrin, 5,10, 15,20-tetrakis[3-(3-thiolpropoxy)phenyl]porphyrin but not achieved. Spectroscopic data indicates that partial hydrolysis occurred. 1HNMR, 13CNMR, UV-Visible and Infra-red spectrometers were used to characterize all the compounds. The synthesized porphyrin was immobilized on gold coated superparamagnetic iron oxide (Fe304@Au) nanoparticles for possible use as a carrier or drug delivery system for the porphyrin to cancer sites. The use of magnetic nanoparticles for biomedical applications have been proposed to a large extent for several years. In recent years nanotechnology has developed to a stage that makes it possible to produce, characterize and specifically tailor the functional properties of nanoparticles for clinical applications. This has led to various opportunities such as improving the quality of magnetic resonance imaging, hyperthermia treatment for malignant cells, site-specific drug delivery and the manipulation of cell membranes. To this end a variety of iron oxide nanoparticles have been synthesized. Using co-precipitation method, nanoparticles comprised of gold shell and magnetite/maghemite were synthesized by overgrowing the gold shell onto the magnetic seeds using sodium citrates as a reducing agent. Oxidized magnetites (Fe304) fabricated by co-precipitation of Fe2+ and Fe3+ in strong alkaline solution were used as magnetic cores. These magnetic nanoparticles were characterized by Transmission Electron Microscopy (TEM), High Resolution Transmission Electron Microscopy (HRTEM), and X-ray diffraction spectroscopy (XRD). The transmission electron microscopy (TEM) image indicated that the particles were well dispersed nd spherical in shape with an average size of 34 - 58 nm with the mean particle diameter of 42.41 nm :!: 12.03 nm and standard deviation of (a) 0.27 nm indicating broad size distribution for the superparamagnetic iron oxide nanoparticles (SPION), the particle diameters for the gold coated SPION are in the range 34-54 nm while the average particle size as determined from the TEM image for the selected fraction is 44.26 nm :t 6.93 nm. The relative standard deviation of the sizes of the nanoparticles is 0.16 nm and the TEM for the functionalized gold coated SPION shows monodispersed particles which appear in spherical, cubic, irregular hexagon and oblong shapes with an average diameter of 44.65 nm. The HRTEM image, shows the existence of lattice planes which confirms the crystallinity of the material. The spacing between adjacent lattice planes is about 0.23 nm which corresponds to the (311) interplanar distance of Fe304 nanocrystals. Atomic force microscopy (ATM) was used to confirm the transmission electron microscopy (TEM) and HRTEM findings. The UV-Visible absorption spectra of the deionised water used in the procedure of preparation of the as-synthesized nanoparticles showed no absorption peak in a wavelength range of 350-750 nm, but after introducing the suspension of the Au-coated magnetic nanoparticles, an absorption peak located at 526 nm appeared which is ascribed to the surface plasmon of nanosized Fe304@Au nanoparticles. The kinetic studies using UV-visible spectrometer and the HRTEM images confirmed the adsorption of the porphyrin macromolecules as a coat on the magnetites particles. At a 5000 revolution per second (rps), aliquot sample of the aqueous phase of the suspension of 0.002 mM Fe304@Au in contact with the organic phase solution of the porphyrin taken for absorbance measurement at every 600 s showed a decrease in the concentration of the suspension while a noticeable and appreciable increase was also noted in the concentration of the porphyrin macromolecules measured at a A max of 420 nm representing the functionalized sample. A new absorption peak of 535 nm was recorded for the nanoparticle/porphyrin phase. A clear magnetic behaviour was established from the two different samples (coated and functionalised samples) using superconducting quantum interference device (SQUID) magnetometer. No long-range cooperative behaviour is evident anywhere from room temperature down to 2 K, which attests to a characteristic length of the scale of the Fe-magnetic agglomeratios on the nano-scale. Being especially prone to intrinsic ferromagnetic ordering, any impurity form of Fe or macro-scale Fe aggregates would impart a very large and easily identifiable magnetic signal. Such a reponse was completely absent from these stud ies and hence testifies to the homogeneity quality of the materials under investigation. KEY WORDS: photodynamic therapy, photosensitizer, porphyrins, magnetites, maghemite, crystalinity, magnetic seeds, nanoparticles, Description: Thesis (PhD (Organic Chemistry) -- University of Limpopo, 2011. 2011-01-01T00:00:00Z