The aim of this PhD thesis is to undertake a comprehensive research to study the optical, electrical, surface electronic and morphologic properties, formulation and surface modification of solution processable organic-inorganic hybrid transparent electrodes as well as their applications in optoelectronic devices. In this study, MoO3 nanoparticles and graphene oxide (GO) nanosheets were incorporated into the poly(3,4-ethylenedioxythiophene) -poly(styrenesulfonate) (PEDOT:PSS) layer forming a hybrid anode interfacial layer (AIL) and subsequently a hybrid transparent electrode of AIL/silver nanowires (AgNWs), significantly improved charge injection in CdSe/ZnS-based quantum dot-light emitting diodes (QD-LEDs) and charge collection in bulk heterojunction (BHJ) organic solar cells (OSCs). The effect of oxidation behavior and charge transfer between PEDOT and MoO3, as well as PEDOT and GO, on the enhancement in conductivity of hybrid PEDOT:PSS-MoO3 and PEDOT:PSS-GO AILs was investigated systematically. The presence of a PEDOT:PSS-MoO3 AIL promotes a good interfacial contact between the hole transporting layer (HTL) and the solution-processed hybrid transparent electrode for efficient operation of QD-LEDs. This work reveals that the use of the hybrid PEDOT:PSS-MoO3 AIL benefits the performance of QD-LEDs in two ways: (1) to assist in efficient hole injection, thereby improving luminous efficiency of QD-LEDs, and (2) to improve electron-hole current balance and suppression of interfacial defects at the QD/electrode interface. The surface wettability of the PEDOT:PSS-MoO3 AIL was controlled successfully for making a good contact between the HTL and the AgNWs, enabling efficient charge injection or charge collection, and thereby improvement in the device performance. The effect of PEDOT:PSS-GO AIL on the performance of transparent QD-LEDs was also analyzed. The maximum brightness of the transparent QD-LEDs, made with a solution-processed hybrid top transparent electrode of PEDOT:PSS-GO/AgNWs, is 3633 cd/m2 at 15 V, comparable to that of a structurally identical control QD-LED made with an evaporated Ag electrode, with a brightness of 4218 cd/m2 operated under the same condition. The change in the hydrophobicity of the PEDOT:PSS-GO AIL, e.g., from the hydrophobic to hydrophilic characteristics, was observed. The interaction between PEDOT and GO nanosheets induces the transition between benzoid-quinoid structures, contributing to the enhanced charge carrier transport via the PEDOT:PSS-GO AIL. The energy level alignment at the HTL/electrode interface and the excellent electrical conductivity of PEDOT:PSS- GO/AgNWs transparent electrode result in an obvious improvement in the performance of QD-LEDs. Transparent QD-LEDs also demonstrated remarkable efficiency via cathode interfacial engineering. Two cathode interfacial modifications include incorporating (1) a hybrid bathophenanthroline (Bphen):Cs2CO3-based electron transporting buffer layer (EBL) and (2) a conjugate polymer of poly[(9,9-bis(3'-((N,N-dimethyl)-N-ethylammonium)-propyl)-2,7- fluorene)-alt-2,7-(9,9-dioctylfluorene)] (PFN-Br)-based EBL. The approach of n-doping effect in the BPhen:Cs2CO3 EBL not only modifies the surface electronic properties of the ZnO electron transporting layer (ETL) but also improves the electron injection at the QD/cathode interface. The n-doping mechanism in the Bphen:Cs2CO3 EBL was investigated. PFN-Br EBL has also been employed to tune the surface work function of ZnO ETL. It was observed that the ZnO/PFN-Br formed an interfacial dipole at the ETL/QD interface, which is suitable for efficient electron injection in the transparent QD-LEDs. In order to improve electron-hole current balance, a GO/MoO3-based multilayer AIL was adopted facilitating efficient charge transfer through improved energy level alignment at the HTL/hybrid electrode interface. Photoelectron spectroscopy revealed tuned surface work function with reduced interfacial barrier for efficient hole injection in transparent QD-LEDs. In these devices, the cathode and anode interfacial modifications have been optimized and studied. This study was also extended to investigate the effect of the organic-inorganic hybrid electrode on performance enhancement of all solution processable organic solar cells (OSCs). The reduction in series resistance and increase in shunt resistance of solution-processed OSCs originated from improved contact selectivity as well as enhanced charge collection efficiency. These properties are reflected in the significantly improved fill factor and short-circuit photocurrent density for the all solution-processed OSCs. Enhanced charge collection at the BHJ/electrode interfaces and improved process compatibility are mainly responsible for efficiency improvement in the cells. The outcomes of this work would allow further advances in device performance. This research also highlights the need to explore interfacial electronic properties and reduce energetic barrier at BHJ/electrode interfaces in fully solution-processed OSCs through photoelectron spectroscopy measurements. The results of this research demonstrate that the solution processable organic-inorganic hybrid transparent electrode developed in this work is beneficial for application in fully solution-processed optoelectronic devices.

The electrochemistry of planar and porous lead dioxide electrodes has been investigated using the techniques: linear sweep voltammetry, potentiostatic pulse experiments and Faradaic (a.c.) impedance studies. The first two techniques were digitally controlled and the data were acquired at high speed and could be easily retrieved after storage. A morphological examination on the electrodes was also carried out using scanning electron microscopy. The electrochemical behaviour of flat lead in sulphuric acid at concentrations in excess of 5 mol dm-3 has been studied. Various solid lead alloys in different sulphuric acid concentrations were investigated together with porous PbO2 on different lead alloy supports. It was found that the acid concentration had a marked effect on the electrochemistry of the electrodes. The current transients obtained from potentiostatic step experiments with the porous PbO2 electrodes were nearly all of a complex nature and some attempt was made to match the data with established mathematical relationships for electracrystallisation processes. The alloys used were of industrial importance and they were lead-antimonyp lead-tin-calcium and lead-tin-bismuth-calcium. Also various automotive positive pastes were employed to form the porous PbO2 structure. A.C. impedance studies were carried out on the porous PbO2 electrodes. An analogue was found which described the discharge process of Pb02.

Magister Scientiae - MSc / The objective of this research was to develop highly efficient and yet effective MnO2 electrode materials for supercapacitors applications. Most attention had focussed on MnO2 as a candidate for pseudo-capacitor, due to the low cost of the raw material and the fact that manganese is more environmental friendly than any other transition metal oxide system. The surface area and pore distribution of MnO2 can be controlled by adjusting the reaction time. The MnO2 synthesised under optimum conditions display high capacitance, and exhibit good cycle profile. This work investigates the ways in which different morphological structures and pore sizes can affect the effective capacitance. Various -MnO2 were successfully synthesised under low temperature conditions of 70 oC and hydrothermal conditions at 120 oC. The reaction time was varied from 1 to 6 hours to optimise the conditions. KMnO4 was reduced by MnCl.H2O under low temperature, whereas MnSO4.4H2O, (NH4)2S2O8 and (NH4)2SO4 were co-precipitated under hydrothermal conditions in a taflon autoclave to synthesise various -MnO2 nano-structures. / South Africa

Guided by increasing legislation, the analysis of food borne toxins, including mycotoxins, seeks to address market related demands for the development of analytical systems to monitor this threat to food security and human health. This Thesis is directed at the assessment of the application of electrochemistry for direct electroanalysis and characterisation of the mycotoxin citrinin (CIT) in aqueous media as well as fundamental investigations of the surface of polished and oxidised glassy carbon electrodes (GCE). This study provides the first known account of CIT detection through electrochemical methods. Although electrochemically active, CIT current responses (Ip) were highly irreproducible at polished GCE with a coefficient of variation (C.V.) of 20.16 %. As stability of Ip across multiple electrode preparations is a key requirement in electroanalysis, investigations were directed at attaining stability in CIT Ip. Achieving stability in CIT Ip was investigated via two approaches, including: accounting for Ip variability between electrode preparations as a result of variable GCE surface conditions as a post-data-acquisition analysis and secondly, removing Ip variability through modification of GCE. Accounting for variability in Ip was investigated through the application of double layer capacitance as an indicator of the activity of an electrode, and in so doing serving as a relative mediator of Ip responses between electrodes. Application of this procedure dropped CIT C.V. to a third of starting value across polished GCE (C.V. = 7.18 %), chemically oxidised GCE (Pi-GCE, C.V = 8.47 %) and functionalised multi-walled carbon nanotube modified GCE (fMWCNT, C.V. = 25.79 %) and was effective with analysis of structurally distinct molecules, 2,4-dimethylaniline (2,4-DMA) and 1,2,4-trihydroxybenzene (Triol). Furthermore, it afforded the ability to determine discreet solution overlapping data sets of Ip. Stabilising Ip through GCE surface modification was achieved by anodic electro-oxidation of GCE and allowed for direct electroanalysis of CIT and subsequent characterisation and analysis of CIT in complex media as it reduced C.V. of CIT Ip to 0.73 %. Fundamental investigations of the electrode surface condition are described such that the source of variability could be identified and the interactions of CIT with the electrode understood. Two surface oxidation techniques were applied in modification of GCE; anodic electro-oxidation (EOx GCE) and chemical oxidation using piranha solution (Pi-GCE), analysis of which has previously not been reported. Fundamental analyses to determine surface morphology and chemistry of Pi-GCE, EOx-GCE and polished GCE were conducted using high resolution scanning electron microscopy (HRSEM), scanning electrochemical microscopy (SECM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), fourier transform infrared spectroscopy (FTIR) and via electroanalytical methods. These studies showed that both oxidation procedures introduced a variety of oxide species at GCE surface, and further that the extent of those species was similar with total % O being 27.67 % and 33.47 % at Pi-GCE and EOx-GCE respectively. Although chemically similar, each surface was morphologically distinct. Electrochemical analyses at the surfaces revealed Pi-GCE to behave more similarly to polished GCE than EOx-GCE. As CIT responses were found to be stable at EOx-GCE (C.V. = 0.73 %) as opposed to Pi-GCE (C.V. = 22.87 %), stability of CIT Ip was likely to be as a result of a physical interaction with electrode morphology rather than interaction on a chemical basis. Morphological analyses revealed polished GCE and Pi-GCE to be highly morphologically irregular at the micro-scale. Although comparatively smooth, the surface morphology of EOx-GCE does not account for the stability of Ip. This study thus proposed a theory to describe the mechanism by which the limited conductivity and porosity of EOx-GCE allow for it to provide a relatively stable surface area within the oxide layer, adjacent to the electrode surface, and thus provided a stable platform for electroanalysis. Voltammetric characterization of CIT at EOx-GCE revealed that anodic oxidation in aqueous media involved an uneven number of electrons to protons via an ECE mechanism. This was illustrated to be nt = 2e- accompanied by the transfer of 1H⁺ per molecule oxidised. A proposed reaction scheme for the initial stages of CIT oxidation was suggested to involve both hydroxyl and carboxyl moieties of the CIT molecule. CIT oxidation was shown to arise as a result of a relatively complex mass transport regime which included both adsorptive and diffusive derived Ip₁. The LOD in buffered aqueous media was found to be 16 nM, a highly competitive result in relation to chromatographic techniques. Further application of EOx-GCE in complex media illustrated that CIT associates non-specifically with the components of food samples, primarily proteins. As a result of this, extraction of CIT from such media is mandatory. Liquid-liquid extraction illustrated a recovery in CIT Ip₁ and in so doing provided a means of accurately and sensitively detecting CIT from food samples with an LOD of 20 nM. These responses were corroborated by HPLC analyses on the same extractions and illustrate the applicability of electroanalysis as an analytical technique.

Studies were conducted into several aspects of the recovery of a spark gap consisting of 6 mm. diameter, flat tungsten electrodes, after an initial discharge of 32 kamp. maximum, and duration 11 sec. The controlling effect of electrode heat transport on the intermediate recovery of the spark gap was demonstrated using air at 760 mm. Hg as a discharge medium, and gap lengths of 3, 4, and 5 mm. In the intermediate stages of recovery, the increase in breakdown strength of the gap was found to be proportional to t½, and to be nearly independent of gap length. These results are explained on the basis of uniaxular heat flow, through the electrodes. A previously observed long-time or delayed recovery in a hydrogen spark gap between clean tungsten electrodes was investigated to establish the possible role of hydrogen-tungsten adsorption in this effect. The temperatures of the electrodes were varied by means of small heating coils, and the recovery characteristics for heated electrodes measured. By comparing these results to the normal recovery characteristic, the phenomana of delayed recovery was shown to be at least partially attributable to the adsorption of hydrogen onto the electrode surfaces. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

A spark gap with mercury jets as electrodes has been developed. Investigations show that the gap can be triggered reliably by photons from a trigger spark with a jitter in the formative time lags less than 0.2 µsec. To overcome uncertainties in the breakdown conditions produced by surface waves on the jet, it has been found necessary to trigger the spark no later than 0.5 sec. after starting the jets„ By suitably designing the geometry of the jets undesirable surface instabilities excited by electric fields have been eliminated. Since the spark gap is isolated electrically from the triggering source very little noise is produced in neighbouring measuring circuits when the main spark is fired. The continual regeneration of the electrode surfaces also eliminates the erratic features observed in normal spark gaps which are fitted with solid metal electrodes. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

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