Thesis (M.Sc.)(Chemistry)-University of Pretoria, 2006. / Includes summary. Includes bibliographical references. Available on the Internet via the World Wide Web.

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This thesis describes the development of electrochemical analytical approaches for the investigation of sulphide detection in stagnant and fluidic environments. The project reports the use of Fourier transform large amplitude alternating current voltammetry (FTACV) as a novel analytical technique for the investigation of sulphide sensing. Novel reactor technology and FTACV measurements carried out using macro and microelectrodes in stagnant and fluidic conditions are reported for the first time. The novel strategy adopts the use of an electrocatalytic (EC') mechanism by using a redox mediator to facilitate the reaction with sulphide in aqueous solutions. In order to support the analysis of FTACV, other electrochemical analytical techniques, cyclic voltammetry (CV) and linear sweep voltammetry (LSV), were also employed to support the observations from FTACV. Chapter 3 reports the application of the CV and FTACV for the detection of sulphide in stagnant conditions at a macroscale electrode. A split wave phenomenon, which is related to the reaction with sulphide, was observed both in the CV and FTACV. By measuring the current behaviour of the split wave, sulphide content in aqueous solution can be determined. Importantly, the split wave phenomenon of the FTACV is the first documented observation using macroscale electrodes. These observations highlight the potential of FTACV to support the detection of sulphide detection. Numerical models of the system are also presented from the calculation to support the experimental interpretation of the voltammetric responses of the CV and FTACV. In Chapter 4 measurements were focused on the voltammetric response of sulphide containing aqueous solutions using microelectrodes. In conventional CV measurements, the split wave behaviour observed at macroelectode disappears from the DC signal; however, for the FTACV measurements, the split wave can still be observed in the higher harmonics providing a clear and simple strategy for detecting sulphide. The results achieved in the FTACV are the first documented observation under the steady state at microelectrodes. Again numerical simulations are reported for this case to support the experimental results. Chapter 5 extends the FTACV measurements for sulphide detection to hydrodynamic environments. The design, development and application of a microfluidic electrochemical system are reported. Split wave characteristics were for the first time detected in both dc and FTACV measurements. The results support the possibility of using dc and ac voltammetry to detect sulphide, while also being used as a guide to assess the split-wave behaviour of the EC' mechanism under fluidic conditions. Numerical models were used to support the analysis of the experimental measurements.

Using sunlight to drive chemical reactions has long been one of the goals in developing sustainable processes. Previous research has focused on solar fuel production in the form of H₂, but this thesis demonstrates that solar-to-chemicals processes can be constructed to produce more complex compounds, using hybrid systems composed of enzymes and inorganic materials. Tetrachloroethene reductive dehalogenase (PceA), an enzyme that catalyzes the conversion of tetrachloroethene (PCE) to trichloroethene (TCE) and subsequently to cis-dichloroethene (cDCE), was shown to accept electrons from both graphite and TiO₂ electrodes. Irradiation by UV light onto PceA-adsorbed TiO₂ particles led to the selective production of TCE and cDCE, which was not possible without PceA as a catalyst. Ferredoxin-NADP⁺ reductase (FNR) is a key enzyme in photosynthesis, as it receives energetic electrons from Photosystem I and produces NADPH as an energy carrier for downstream 'Dark' reactions involving CO₂ assimilation. This thesis presents the discovery of FNR activity on indium tin oxide (ITO) electrodes which led to direct electrochemical investigation of the properties of FNR, both in the absence and presence of its substrate, NADP⁺. The FNR-adsorbed electrode, termed 'the electrochemical leaf', rapidly interconverts NADP⁺/NADPH, and this was coupled to a downstream NADPH-dependent enzyme, thus demonstrating a new approach to cofactor regeneration for enzyme-catalyzed organic synthesis. The NADP⁺ reduction by FNR was also driven by light using a photoanode made of visible-light responsive semiconductors.

Oxidation is a fundamental process in the fabrication of microelectronic devices and in order to promote the incorporation of SiGe into Si technology we have investigated the thermal oxidation of Si1-yGey alloy (y≈0.5) at high temperatures (mainly 900°C and 1000°C) using Rutherford backscattering spectroscopy, infrared transmission spectroscopy and X-ray photoelectron spectroscopy. It has been observed that three distinct regions form during oxidation of Si0.5Ge0.5 alloy, which are (I) a mixed oxide layer Si0.5Geg0.5O2, (II) a pure SiO2 layer and (III) a Si1-yGey (y≠0.5) alloy layer. These are formed during both wet and dry oxidation when the sample is not preheated, whilst only two regions (II) and (III) form when the sample is preheated in a non oxygen ambient up to the oxidation temperature prior to oxidation. Enhancement of the rate of wet oxidation of SiGe compared with bulk Si and the accumulation of Ge in region III just below SiO2 layer (region II) has also been observed. This behaviour is in good agreement with other results, however, some abnormal behaviour during wet and dry oxidation has been observed. The rate of oxidation during short wet oxidations (15 minutes) of Si0.5 Ge0.5 decreases as the oxidation temperature increases from 800°C to 1000°C for 15 minutes, which has not been previously reported, A reduced rate of oxidation of Si1-yGey (y≈0.5) in a dry environment has been observed and is discussed. In order to investigate the thermodynamics of the oxidation process new experiments have been carried out which involve the synthesis of a buried oxide layer, by O+ implantation followed by a high temperature anneal. The implanted oxygen atoms preferentially bond to silicon atoms and the oxygen atoms are found to bond to germanium atoms only after all of the silicon atoms are fully oxidised. Germanium tends to be rejected from the growing oxide during a subsequent higher temperature ( > 900°C) anneal. The different behaviour of the Si and Ge atoms during both thermal and internal oxidations is described in terms of the thermodynamics and kinetics of the SiGe alloy system. There are three important conclusions which emerge from these analyses: (i) in order to adequately control the composition of thermal oxides (T > 800°C) grown on Si0.5Ge0.5 material it is necessary to preheat the samples in a non oxidising atmosphere prior to oxidation. By so doing the entrapment of Ge in the near surface layer (region I) is inhibited; (ii) during the initial stage of dry oxidation the process is described by the reaction Si+O2?Si2 and we suggest that the rate is controlled by the availability of oxygen atoms and is numerically the same as for bulk Si; (iii) in contrast, during the initial stage of wet oxidation, the process is described by the reaction Si+2OH→SiO2+H2 and the rate is controlled by the areal density of Si atoms, which we propose is high due to the weak Si-Ge binding energy and thus an enhanced oxidation rate occurs.

This thesis is mainly concerned with the preparation of a range of novel poly(thiophene)s incorporating fused redox active substituents. Confirmation of the structure of the monomer units was carried out by NMR ([1]H and [13]C), mass spectral analysis, C,H,N analysis and where applicable, single crystal X-ray structural determination. Electrochemical studies on a number of the systems synthesised was also investigated, and voltammetric measurements were made using cyclic voltammetry. The novel fused TTF-thiophene monomer 2,5-dibromothieno[3,4-d]-1,3-dithiole-2-ylidine-4,5-bis(hexylsulfanyl)-1,3-dithiole has been prepared and polymerised using Yamamoto methodology. The resulting polymer exhibits significant electrical conductivity in the neutral state and is the first example of an annelated TTF-thiophene polymer. The development of terthiophene and bithienyl furan derivatives bearing the synthetically versatile 1,3-dithiole-2-thione functionality is also described, and conversion to a tetrathiafulvalene derivative via a phosphite mediated coupling reaction has been investigated. However, electropolymerisation of this system was unsuccessful which highlights unfavourable interactions arising between the two redox units. Synthesis of analogous terthiophene derivatives bearing a fused dithiino spacer group between the polythiophene chain and the 1,3-dithiole-2-thione moiety is also discussed. A series of bis-substituted alkylsulfanyl terthiophene derivatives have been synthesised which can be electrochemically polymerised; a hexylsulfanyl derivative can also be polymerised chemically. Preliminary photoinduced absorption studies show that an ethylene bridged derivative exhibits interesting properties, which has been studied in collaboration with the Johannes Keppler University, Linz, Austria. A polythiophene bearing a fused quinoxaline moiety has been prepared and electropolymerised to form polymer films which are capable of molecular recognition of transition metal cations. This is the first example of a conducting polymer which is able to detect selectively and reversibly Hg[2+] and Ag[+] ions in aqueous and organic media.

All solid state electrochemical cells capable of producing beams of lithium, sodium and potassium in ultrahigh vacuum have been developed and investigated. The evolution of alkali metal vapour has been demonstrated by deposition of the metal on a substrate during polarisation of the cell followed by ex-situ analysis of the metal using laser ionisation mass analysis (LIMA). The electrochemistry of alkali metal evolution from these unusual solid state cells has been investigated using cyclic voltammetry, chronoamperometry and AC impedance measurements at pressures of 10−3 mbar and 10−8 mbar (UHV). It has been found for all three sources that the mechanism at relatively high pressure involves the nucleation and growth of liquid alkali metals or compounds containing alkali metals on the working electrode prior to their evaporation. In UHV the mechanism for potassium and sodium emission appears to involve the transfer of atoms directly into the gas phase whereas lithium exhibits nucleation and growth. In order to obtain a more complete characterization of the electrochemical mechanisms a spectro-electrochemical technique involving the simultaneous mass spectrometric analysis of the evolved vapour under UHV conditions along with cyclic voltammetry was developed. The formation of p-type ZnSe is essential to the fabrication of blue light emitting diodes and semiconductor lasers but has long represented a major problem in optoelectronics. This work shows that the potassium source can be used to p-dope ZnSe during growth of the material by molecular beam epitaxy (MBE). Efforts directed to the preparation of n-type diamond using a lithium source in microwave enhanced chemical vapour deposition (MWECVD) apparatus have demonstrated that the source can introduce lithium to diamond, although full semiconductor characterization of this material has yet to be made.

Using a combination of solid-state NMR techniques including Tip measurements, dynamic line shape analyses and 2D EXSY data, a variety of intramolecular motions including rotations of methyl, t-butyl, t-amyl, phenyl, trimethylammonium and trimethylphosphonium groups in a series of quaternary ammonium and phosphonium salts have been investigated. Where possible, activation parameters Ea, △G+/-, △H+/- and △S+/- have been derived. A range of values was recorded, especially for △S+/- which ranged from +75 JK−1 mol−1 for a phenyl group to -57 JK−1 mol−1for a t-butyl group. It was shown that 13p and 31p T1p measurements from CP/MAS spectra can give quantitative information on the kinetics of intramolecular motions that agree with line shape analysis. Recent work using X-ray crystallography has suggested that some derivatives of bicycle [3.3.1] nonane show evidence of conformational equilibria in the solid state. Using 13CP/MAS NMR, 22 derivatives of bicycle [3.3.1] nonane were studied, some at variable temperature. No evidence of conformational equilibria was observed in the chosen compounds. Solid-state NMR has been used to follow the kinetics of ring- chain tautomerism in a bicyclic tetrahydro-1,3-oxazine derivative. This was found to form initially the metastable chain on crystallisation, which then cyclises. The kinetics of the cyclisation were followed and the activation energy for the solid-state reaction was derived. In contrast to this, a related pyrimidine derivative has been found to form initially the metastable ring on cyclisation which quickly ring opens to the chain.

The preparation and attempted preparation of diaminomethylene-sulphuranes (thiouronium ylides) by various routes is described. The principal route chosen was by proton abstraction from the conjugate acid (thiouronium salt) using a suitable non-hydroxylic base, usually phenyl lithium or triethylamine. In the range of compounds examined where the carbanionic substituents contained electron withdrawing groups such as carbonyl, sulphonyl, nitrile and nitro groups, considerable difficulty was encountered in the preparation of the precursor salts. Of the few salts in this category that were obtainable, only one (disulphonyl substituted) gave rise to a stable ylide. On the other hand, many salts containing the cyclopentadiene ring gave rise to ylides. Some of these, notably the fluorenylides, could be isolated and characterised, while the properties of others could only be observed in solution and they decomposed or rearranged on attempted isolation. A second preparative route was briefly investigated, namely the thermal decomposition of 9-diazofluorene in the presence of arylthioureas, but in several cases this led to unexpected products. Comparative studies of the reactivities of the fluorenylides towards carbonyl compounds and hydrolysis have been made, and some other reactions of these ylides are also described. The 2,3,4-triphenylcyclopentadienylides have been found to undergo novel cyclisation reactions with aldehydes and nitrosobenzene. A product from the former reaction was dehydrogenated to give a stable heterocycle with a 14Π electron periphery. An attempt to prepare a selenium analogue of one of the thiouronium fluorenylides is described. The attempted preparation of guanidinium fluorenylides by various methods is described, principally the 'salt' and 'diazo' methods. The former method was more suitable for alkyl substituted guanidines, whereas the latter method was of benefit with aryl substituents. Proton abstraction from ono of the alkylguanidinium salts gave rise to a product which showed some evidence for ylide character. On the other hand, the aryl substituted products appeared to show no ylide-like properties, A further point of interest arose in the diazo reaction, whore the fluorene nucleus was shown to have substituted on the more sterically hindered position, which was verified later by synthesis.

The influence of pre-metallisation surface preparation on the structural, chemical, and electrical properties of metal-nGaN interfaces has been investigated by X-ray Photoemission Spectroscopy (XPS), current-voltage measurement (I-V) and cross section Transmission Electron Microscopy (TEM). XPS analysis showed that the three GaN substrate treatments investigated, ex-situ HF etch, in-situ anneal in Ultra-High-Vacuum (UHV), and in-situ Ga reflux cleaning in UHV result in surfaces increasingly free of contaminants. Additionally, the three treatments are found to induce increasingly larger upward band bending. Ag-nGaN contacts formed after Ga reflux cleaning exhibit a Schottky barrier height of 0.80 eV and an ideality factor of 1.56, as determined by I-V.XPS and TEM characterisation of Au-nGaN formed after the three pre-metallisation surface treatments show that HF etching and UHV annealing produce abrupt, well-defined interfaces. Conversely, GaN substrate cleaning in a Ga flux results in Au/GaN intermixing. I-V characterisation of Au-nGaN contacts yield a Schottky barrier height of 1.25 eV with very low ideality factor and very good contact uniformity for the pre-metallisation UHV anneal while the Ga reflux cleaning result in a much lower barrier (0.85 eV), with poor ideality and uniformity. I-V and XPS results suggest a high density of acceptor states at the surface, which is further enhanced by UHV annealing. The mechanisms of Ga-nGaN, Ag-nGaN and Au-nGaN Schottky barrier formation are discussed in the context of the Metal-Induced Gap States model (MIGS) Unified Defect Model (UDM) and Cowley-Sze model.