abstract: Dealloying, the selective dissolution of an elemental component from an alloy, is an important corrosion mechanism and a technological significant means to fabricate nanoporous structures for a variety of applications. In noble metal alloys, dealloying proceeds above a composition dependent critical potential, and bi-continuous structure evolves "simultaneously" as a result of the interplay between percolation dissolution and surface diffusion. In contrast, dealloying in alloys that show considerable solid-state mass transport at ambient temperature is largely unexplored despite its relevance to nanoparticle catalysts and Li-ion anodes. In my dissertation, I discuss the behaviors of two alloy systems in order to elucidate the role of bulk lattice diffusion in dealloying. First, Mg-Cd alloys are chosen to show that when the dealloying is controlled by bulk diffusion, a new type of porosity - negative void dendrites will form, and the process mirrors electrodeposition. Then, Li-Sn alloys are studied with respect to the composition, particle size and dealloying rate effects on the morphology evolution. Under the right condition, dealloying of Li-Sn supported by percolation dissolution results in the same bi-continuous structure as nanoporous noble metals; whereas lattice diffusion through the otherwise "passivated" surface allows for dealloying with no porosity evolution. The interactions between bulk diffusion, surface diffusion and dissolution are revealed by chronopotentiometry and linear sweep voltammetry technics. The better understanding of dealloying from these experiments enables me to construct a brief review summarizing the electrochemistry and morphology aspects of dealloying as well as offering interpretations to new observations such as critical size effect and encased voids in nanoporous gold. At the end of the dissertation, I will describe a preliminary attempt to generalize the morphology evolution "rules of dealloying" to all solid-to-solid interfacial controlled phase transition process, demonstrating that bi-continuous morphologies can evolve regardless of the nature of parent phase. / Dissertation/Thesis / Ph.D. Materials Science and Engineering 2013

The electrochemical properties of graphene were investigated using a novel and clean method to fabricate device structures with mechanically exfoliated graphene samples. Graphene is known as being particularly sensitive to both contaminating fabrication methods and the substrate it is placed on, with these effects being detrimental to accurate research into the fundamental properties and sensing applications of graphene. This thesis presents micron scale graphene electrodes that have not been subject to polymer contamination or micro-lithography methods. The effect of utilising atomically flat hexagonal boron nitride as a substrate material was investigated, believed to be the first example of this for graphene electrochemical measurements. Cyclic voltammetry demonstrated the expected steady-state behaviour for microelectrodes in the hemispherical diffusion regime. The reduction of IrCl62- in weak KCl electrolytes was studied to investigate the electron transfer characteristics of the graphene devices and the reproducibility of the measurements. Average values of the standard rate constant, k0 and the transfer coefficient, alpha were found to be 3.04 ± 0.78 ×10-3 cms-1 and 0.272 ± 0.024 respectively. These values differ significantly from previous similar studies, with the effect of reduced charge doping from the substrate and the potential dependence of the density of electronic states thought to account for the differences. Despite the clean fabrication methods, a relatively large variation between separate devices was found, highlighting an inherent variation in the properties of graphene samples.

Nanomaterials, in particular carbon nanotubes have been shown to exhibit favourable properties for the enhancement of electrochemical detection of target analytes in complex matrices. There is however scope for improvement in terms of the optimization thereof in electrochemical sensors surface modification. The aim of this thesis was to examine methods that would result in increased current response, lowered passivation and application of such modified surfaces with application to pharmaceutically and industrially relevant analytes. Current methods for enhancing the performance of carbon nanotubes include acid functionalization which not only increases the hydrophilicity of the nanotubes, and consequently their ability to provide stable (aqueous) suspensions, but also introduces electrochemically active sites. This particular approach is however not normalized in the literature. Over-exposure to acid treatment results in loss of structural integrity of the carbon nanotubes, and as such a fine balance exists between achieving these dual outcomes. Guided by high resolution scanning electron microscopy, atomic force microscopy, voltammetric and impedance studies, this thesis examined the role of the length of time of the acid functionalization process as well as the impact of activation of carbon nanotubes and fullerenes on electrochemical sensor performance. Based on desired charge transfer resistances, rate transfer coefficients and sensitivity towards redox probes the optimal length of acid functionalization for multiwalled carbon nanotubes was 9 hours and 4 hours for single-walled carbon nanotubes. Further improvements in the desired outcomes were achieved through electrochemical activation of the modified electrode surface by cycling in the presence of catechol, in a novel approach. By employing electrochemical impedance spectroscopy it was observed that catechol activation resulted in lowered charge transfer resistance, before and after activation, with functionalized multi-walled carbon nanotubes (9 hours) exhibiting the greatest decrease of 90 % and functionalized single-walled carbon nanotubes (4 hours), a 50 % decrease. Corresponding increases in the heterologous rate transfer coefficient showed a 770 % increase for functionalized multi-walled carbon nanotubes (9 hours), following catechol activation. Comparative observations for fullerenes following partial reduction in potassium hydroxide yielded a 30 % decrease in charge transfer resistance, with an increased heterologous rate transfer coefficient at a fullerene modified surface The performance of the nanomaterial modified electrodes was applied to the detection of wortmannin with applications in bioprocess control and in the pharmaceutical sector as well as to the detection and monitoring of the industrial dye Reactive red. Of particular relevance to these analytes was the assessment of the nanomaterial modified electrodes for enhanced stability, reproducibility, sensitivity and decreased passivation effects. In this study the first known account of wortmannin detection through electrochemical methods is reported. Voltammetric characterization of wortmannin revealed an irreversible cathodic process with a total number of 4 electrons and a diffusion coefficient of 1.19 x 10-7 cm².s⁻¹. At a functionalized multiwalled carbon nanotubes modified glassy carbon electrode a limit of detection of 0.128 nmol.cm⁻³ was obtained, and with limited surface passivation the detection scheme afforded pertinent analyses in biological media representing a substantial improvement over chromatographic detection methods. This study also provided the first account of the voltammetric detection of reactive red, competing favourably with traditional spectroscopic methods for monitoring biodegradation of this compound in real time.

This work expands the investigation into the behaviour of verdazyl radicals and N-alkylated tetrazines as ligands. These new ligands were coordinated to various metals as a means of exploring new properties in the metal-verdazyl and metal-tetrazine products. The synthesis of N,N’-diphenyl Kuhn and 6-oxo verdazyl radicals bearing a 2-pyridyl group at the C3 position was accomplished. Palladium(II) dichloride complexes of each of these radicals were prepared in order to study the differences in the structural, electronic, and electrochemical properties compared to corresponding complexes of the previously reported N,N’-dialkyl-6-oxoverdazyl ligands. The N,N’-diphenyl verdazyl ligands are structurally bulkier than their dialkyl counterparts resulting in increased interaction between the ligand and palladium as observed in the solid state. The radical complexes were investigated by EPR and shown to exhibit a small amount of spin density on the palladium atoms with most of the spin density remaining on the ligands. The UV-Visible spectra had a noticeable red-shift in the absorbance maxima of the complexes compared to the free ligands. The electrochemistry of the new palladium-verdazyl complexes showed that there was a positive increase to the reduction and oxidation potentials when compared to the free ligands. An N-benzyl tetrazine and its Ru(hfac)2 complex were synthesized from their corresponding radical species utilizing Mn2(CO)10 to photogenerate benzyl radicals. This method was found to give high yields of the tetrazine and its metal complex. Spectroscopic, structural, and electrochemical properties of the tetrazine and its Ru(hfac)2 complex are reported. These compounds were investigated in regards to the activation energy associated with the homolytic cleavage of the C-N bond in the inert solvent, tert-butylbenzene. The activation energy of C-N bond of the tetrazine was 155 kJmol-1 while its Ru(hfac)2 complex was 138 kJmol-1; this resulted in the rate of dissociation being a factor of ~40 greater for the Ru(hfac)2 complex at 393 K. This work presents the potential of coordination compounds in tuning the properties of molecules associated with the stable free radical polymerization process. / Graduate / 0488 / 0485 / cooper_johnston@hotmail.com

Magister Scientiae - MSc / The particles of the Pt metal were well dispersed on carbon nanotubes when EG was used and in isopropanol poor dispersion was observed and no further investigation was done on them. The platinum wt% on the supports observed from EDS was 21.8, 19.10 and 16.74wt% for Pt/EMWCNT, Pt/LPGCNT and Pt/ commercial CNT respectively. Pt/LPGMWCNT showed high electro-catalytic activity of 2.48 mA and active surface area of 76 m2/g, toward oxygen reduction, observed from cyclic voltammogram in iv sulfuric acid. Pt/LPGMWCNT also showed better tolerance toward methanol, however it was not highly active towards methanol, and hence the methanol oxidation peak current observed between 0.75 and 08 potential was the smallest. In this study a wide range of instruments was used to characterize the properties and behavior of Platinum nanoparticles on multi-wall carbon nanotubes. To add to the already mentioned, Scanning electrochemical microscopy (SEM), proton induced x-ray emission (PIXE), scanning electrochemical microscopy (SECM) and Brunauer-Emmett Tellar (BET) were also used. / South Africa

In this study, two kinds of mesoporous materials were prepared. The first was a silica mesostructure grown within a porous aluminium oxide membrane columnar material (hybrid-AOM). This was prepared using a sol-gel technique with Pluronic P123 triblock copolymer as the structure-directing agent and tetraethyl orthosilicate as the inorganic source. The hybrid-AOM had a similar pore size distribution to that of as-prepared SBA-15 but showed an amorphous character, as demonstrated by nitrogen adsorption and SAXRD. The second type of material was a continuous mesoporous silica thin film, prepared by the dip-coating technique using Pluronic F127 triblock copolymer as the structure-directing agent and the same silica source as hybrid-AOM. The film, which was self-assembled on substrates such as indium tin oxide (ITO), glass and gold, exhibited long-range ordered mesostructures after several treatments and aging. Grazing incidence small-angle X-ray scattering method (GISAXS) showed that the thin film contracted in a direction perpendicular to the substrate after drying and surfactant removal.Removal of the surfactant template from both materials in order to create porous silica was achieved by calcination, ethanol extraction and peroxide-Fe treatments. Calcination was found to be the best method to remove surfactant from both mesostructures (hybrid-AOM and thin film). However, this was found to cause cracking and crumpling of the hybrid-AOM with the evaporated gold being easily peeled off after calcination. Ethanol extraction was thus applied where calcination was not suitable. The surfactant removal was confirmed using an infrared spectroscopy and the structure was confirmed after extraction using 1D X-ray diffraction (XRD). The surface morphology, porosity and crystallinity of the mesostructures prepared were characterized by nitrogen adsorption, scanning electron microscopy and small angle XRD. To form modified electrodes, the hybrid-AOM template was coated by evaporation with pure gold on one side, whilst the mesostructured thin film was grown on either gold or ITO. The permeability of the void space for both hybrid and thin film samples was calculated from the cyclic voltammetry response of a neutral probe (FcMeOH). Cationic ([Ru(bpy)3]2+) and anionic (I-) electroactive species were used to observe the electrochemical response under different pH regimes. FcMeOH was also used to study the effect of KCl concentration on the silica surface charge. Gold and platinum were electrochemically deposited using mesoporous silica as a template.

No description available.

The syntheses of octasubstituted and unsusbstitituted Gallium(III) chloride and indium(III) chloride phthalocyanines (GaPc and InPc), their photophysical, photochemical and nonlinear optical parameters are hereby presented. The photocatalytic oxidation of 1-hexene using the synthesized GaPc and InPc complexes as well as electrochemical characterization is also presented in this thesis. Fluorescence quantum yields do not vary much among the four Ga complexes, except for complex 21c; therefore it was concluded that the effect of substituents is not significant among them. Solvents however, had an effect on the results. Lower Φ[subscript F] values were obtained in low viscosity solvents like toluene, relative to highly viscous solvents, such as DMSO. The triplet quantum yields were found to be lower in DMSO than in DMF and toluene. The rate constants for fluorescence, intersystem crossing and internal conversion as well as fluorescence and triplet lifetimes are reported. Photodegradation and singlet oxygen quantum yields have also been reported. There was no clear correlation between the latter parameters. It was however established that the four gallium MPcs were stable, within the allowed stability range for phthalocyanines. High quantum yields of triplet state (Φ[subscript T] ranging from 0.70 to 0.91 in dimethysulfoxide, DMSO) and singlet oxygen generation (Φ[subscript greek capital letter delta], ranging from 0.61 to 0.79 in DMSO) were obtained. Short triplet lifetimes 50 to 60 μs were obtained in DMSO). Calculated non-linear parameters of these complexes are compared with those of the corresponding GaPc derivatives and tetrasubstituted GaPc and InPc complexes. The optical limiting threshold intensity (I[subscript lim]) values for the InPc and GaPc derivatives were calculated and compared with those of corresponding tetrasubstituted InPc and GaPc complexes. The octasubstituted were found to be better optical limiters. Photocatalytic oxidation of 1-hexene by GaPc (21a-c) and InPc (22a-c) derivatives is also presented. The photocatalytic oxidation products for 1-hexene were 1,2- epoxyhexane and 1-hexen-3-ol. The % conversion values of 1-hexene and % selectivity of 1,2-epoxyhexane were generally higher for InPc derivatives. Even though InPc derivatives showed better photocatalytic results than GaPc derivatives, the former were less stable relative to the latter. Both type I and type II mechanism were implicated in the photocatalysis mechanism.

Neural communication is imperative for physical and mental health. Dysfunction in either ionic signaling or chemical neurotransmission can cause debilitating disorders. Thus, study of neurotransmission is critical not only to answer important fundamental questions regarding learning, decision making, and behavior but also to gain information that can provide insight into the neurochemistry of neurological disorders and lead to improved treatments. The work presented herein describes the development of techniques and instrumentation to enable advancements in neuroscientific inquiry. The effect of different temporal patterns and durations of simulation of the prefrontal cortex on dopamine release in the nucleus accumbens was examined and revealed a complex interaction that can help improve deep brain stimulation therapies. A measurement platform that combines electrophysiological and electrochemical techniques is described. The instrumentation is capable of concurrent monitoring of neural activity and dopamine release in vivo and in freely moving rodents. Analysis techniques to allow absolute quantification of tonic dopamine concentrations in vivo are detailed and the temporal resolution of the technique was vastly improved from ten minutes to forty seconds. An instrument that can simultaneously probe both dopamine and serotonin dynamics in either of their two temporal modes of signaling (tonic and phasic) using either fast-scan cyclic voltammetry or fast-scan controlled-adsorption voltammetry at two individually addressable microelectrodes is described. Together these new tools represent a significant step forward in the field of neuroanalytical chemistry by enable multiple brain regions, signaling modes (ionic flux in addition to both tonic and phasic neurotransmission), neurochemicals, and to be measured together.

The corrosion behaviour of zirconium based alloys has been primarily investigated by electrochemical impedance spectroscopy (EIS). In-situ autoclave EIS experiments were performed in simulated primary coolant conditions in order to study the high temperature water corrosion of zirconium alloys in PWRs. In-situ impedance response of the corroding material was recorded throughout first kinetic transition. A physical model of the zirconium oxide was proposed in accordance with the microstructural observation' made by SEM analysis. Electrical properties of the oxide was evaluated with equivalent circuit model (ECM) which was constructed according to the physical oxide model. Evolution of various oxide parameters obtained from ECM was analysed in accordance with the microstructure observation made by SEM. A two layer structure consists of a outer porous oxide and an inner barrier oxide, was found to be the most accurate description for the autoclave formed oxide. Supporting evidence from the SEM cross-section and surface analysis of the oxide had shown cracks and pores that were linked and connected with the environment. This observation is also confirmed by the in-situ EIS measurement which has shown porous electrode behaviour throughout the course of oxidation. The porous oxide behaviour was also confirmed by the ex-situ soaking experiment on samples with incremental exposure time. Evolution of inner barrier layer oxide thickness was found to be correlated with kinetic transition which was determined from weight gain measurement. This indicated that barrier layer maybe the oxidation rate controlling layer and its thickness maybe reduced during transition. Thus, a thinner barrier layer would resulted in a rapid corrosion of zirconium alloys. Furthermore, maintaining the barrier layer thickness maybe the possible route to improve zirconium alloy corrosion resistance.