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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.

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This work describes, for the first time, the electrochemical and electrocatalytic properties of self-assembled layers of single-walled carbon nanotubes (SWCNTs) intergrated with selected organo-iron complexes and Cysteamine (Cys) forming a base on gold electrodes. The organo-iron complexes selected for this study were octa(hydroxyethylthio)phthalocyaninatoiron(II) (FeOHETPc), tetraaminophthalocyninatoiron(II) (FeTAPc), tetraaminophthalocyninatocobalt(II) (CoTAPc), ferrocene monocarboxylic acid (FMCA), ferrocene dicarboxylic acid (FDCA) or a mixture of SWCNT and FMCA or FDCA. The successful fabrication of these electrodes were established using atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and electrochemical techniques such as cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), square wave voltammetry (SWV) and chronoamperometry (CA) The Au-Cys-SWCNT-FeOHETPc electrode exhibited strong dependence on the reaction of the head groups and the pH of the working electrolytes. The high electron transfer capability of the Au-Cys-SWCNT-FeOHETPc electrode over other electrodes as the Au-Cys-SWCNT or the Au-Cys-FeOHETPc or the Au-FeOHETPc suggests that SWCNT greatly improves the electronic communication between FeOHETPc and the bare gold electrode. The electron transfer rate constant (kapp) of Au-Cys-SWCNT-FeOHETPc in pH 4.8 conditions (~1.7 x 10-2 cm s-1) over that of the electrode obtained from SWCNT integrated with tetraaminophthalocyninatocobalt(II) (Au-Cys-SWCNT-CoTAPc) (5.1 x 10-3 cm s-1) is attributed to the possible effect of both the central metal on the phthalocyanine core and subsituents on the peripheral positions of the phthalocyanine rings. This work clearly proved that the aligned SWCNTs arrays exhibit much faster electron transfer kinetics to redox-active species in solutions compared to the randomly dispersed (drop-dried) SWCNTs. The advantageous electron transfer properties of the aligned Au-Cys-SWCNT-FeOHETPc electrode, coupled with its ease of fabrication and electrochemical stability, could be found useful in electrochemical sensing and catalysis. Thiocyanate ion was used as an analytical probe to examine the electrocatalytic properties of these modified gold electrodes. This work shows that SWCNT-FeOHETPc hybrid exhibits excellent sensitivity towards the detection of thiocyanate compared to electrodes containing SWCNT or FeTAPc or FeOHETPc only, indicating the ability of the SWCNTs to function as effective conductive nanowires for the detection of this important analyte. The electrochemical response of the FeOHETPc based electrodes was greater than their FeTAPc-based electrode counterparts, indicative of the impact of peripheral substituents on the phthalocyanine core towards electrocatalytic behaviour of these types of hybrids. Nevertheless, the exchange of the central metal as seen with the case of CoTAPc (see chapter 3), provide useful comparative electrochemical activity of this complexes versus FeTAPc with the same chemical environment with an indication of iron being the best as metal centre. FMCA or FDCA were covalently attached to the base Cysteamine monolayer to form the Au-Cys-FMCA and Au-Cys-FDCA, respectively. The same covalent attachment strategy was used to form the mixed SWCNTs and ferrocene-terminated layers (i.e., Au-Cys-SWCNT/FMCA and Au-Cys-SWCNT/FDCA). The impact of neighbouring SWCNTs on the electron transfer dynamics of the ferrocene molecular assemblies in acidic medium (0.5 M H2SO4) and in a solution (pH 7.2) of an outer-sphere redox probe ([Fe(CN)6]4-/ [Fe(CN) 6]3-) was explored. The electron transfer rate constants in both solution media essentially decrease as Au-Cys-FMCA > Au-Cys-SWCNT/FDCA > Au-Cys-FDCA > Au-Cys-SWCNT/FMCA. This trend has been interpreted in terms of several factors such as the locations of the ferrocene species in a range of environments with a range of potentials, the proximity /interactions of the ferrocenes with one another, and electrostatic interaction or repulsion existing between the negatively-charged redox probe and the modified electrodes. Square wave voltammetry was used to examine the catalytic behaviour of the electrodes. Au-Cys-SWCNT/FDCA proved to be the best electrode, possibly due to the repulsive interactions between the negatively charged SCN- and high number of surface –COOH species at the SWCNT/FDCA. This novel study has provided some useful insights as to how CNTs co-assembled with ferrocene-terminated thiols could impact on the heterogeneous electron transfer kinetics as well as the electrocatalytic detection of the self-assembled ferrocene layers. / Thesis (PhD)--University of Pretoria, 2010. / Chemistry / unrestricted

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For the first time, new metallophthalocyanine (MPc) complexes: (i) nanostructured MPc (nanoMPc, where M = iron or cobalt); (ii) octabutylsulphonylphthalocyanine (MOBSPc, where M = iron or cobalt); and (iii) iron (II) tetrakis(diaquaplatinum)octacarboxyphthalocyanine (PtFeOCPc) were synthesized and characterized using advanced microscopic and spectroscopic techniques such as MS, AFM, HRTEM, FESEM, and EDX. Electrochemical techniques such as cyclic voltammetry, square wave voltammetry, chronoamperometry, rotating disk electrode, and electrochemical impedance spectroscopy, were used to explore the redox chemistry, heterogeneous electron transfer kinetics (HET), and electrocatalytic properties of these MPc complexes towards oxygen reduction reaction (ORR), oxidation of formic acid, thiocyanate and nitrite on a edge plane pyrolytic graphite electrode (EPPGE) platform pre-modified with or without acid functionalized multi-walled carbon nanotubes (MWCNTs). The MWCNT-MPc platforms exhibit enhanced electrochemical response in terms of (i) HET towards an outer-sphere redox probe ([Fe(CN)6]3-/[Fe(CN) 6]4-), and (ii) catalytic activities towards the investigated analytes. The MWCNTnanoMPc electrode exhibits faster HET constant (kapp ≈ 30 – 56 x 10-2 cms-1 compared to their bulk MPc counterparts (≈ 4 – 25 x 10-2 cms-1). The MWCNT-nanoMPc exhibited enhanced electrocatalytic properties (in terms of sensitivity and limit of detection, LoD) towards the detection of thiocyanate and nitrite in aqueous solutions. ORR was a 4- electron process with very low onset potential (-5 mV vs. Ag|AgCl saturated KCl). HET and ORR at MOBSPc complexes supported on MWCNTs showed that the MWCNT–MOBSPc exhibited larger Faradaic current responses than the electrodes without MWCNTs. The rate constant at the MWCNT-MOBSPc electrodes (kapp ≈ (22 – 37) x 10-2 cms-1) is about a magnitude higher than the electrodes without MWCNT (kapp ≈ (0.2 – 93) x 10-3 cms-1). The MWCNT–FeOBSPc electrode gave the best ORR activity involving a direct 4-electron mechanism with low onset potential (0.0 mV vs. Ag|AgCl saturated KCl). The onset potential is comparable and even much lower than recent reports. The HET and electrocatalytic properties of PtFeOCPc supported on a MWCNT platform (MWCNT-PtFeOCPc) gave enhanced electrochemical response in terms of (i) HET (kapp ≈ 78 x 10-2 cms-1), (ii) catalytic rate constant (kcat ≈ 41 cm3mol-1s-1) and (iii) tolerance towards CO poisoning during formic acid oxidation. The ORR activity is a direct 4-electron transfer process at a rate constant of 2.78 x 10-2 cms-1; with a very low onset potential approximately 0.0 mV vs. Ag|AgCl saturated KCl. The electrooxidation of formic acid at MWCNT-PtFeOCPc follows the preferred ‘direct pathway’. This work clearly proves that the MWCNT-MPcs hybrid exhibit enhanced electrochemical and electrocatalytic activities towards the selected analytes compared to the MPcs alone. Considering the ease of fabrication of these electrodes (drop-dry method), these nanocomposite materials are promising platform for potential application in sensing and cataly. / Thesis (PhD)--University of Pretoria, 2011. / Chemistry / unrestricted

This work describes metal (M) and metal oxides (MO) films (where M = Ni, Co and Fe) obtained by electrosynthesis and chemical synthesis, and modified with carbon nanotubes (CNTs) on edged plane pyrolytic graphite electrode (EPPGE). The MO nanoparticles investigated are nickel oxide (NiO), cobalt oxide Co3O4) and iron oxide (Fe2O3). Successful modification of the electrodes with the M or MO/CNT nanocomposite was confirmed by field emission scanning electron microscopy (FESEM), high resolution scanning electron microscopy (HRSEM), high resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM), x-ray diffraction spectroscopy (XRD), x-ray photoelectron spectroscopy (XPS), electron dispersive x-ray spectroscopy (EDX), fourier transformed infra-red spectroscopy (FTIR) and ultraviolet-visible (UV-vis) spectroscopy. Electron transport (ET) properties of the modified electrodes was explored using cyclic voltammetry (CV) and electrochemical impedance spectroscopic techniques (EIS) with ferricyanide/ferrocyanide ([Fe(CN) 6]3-/4-) as the redox probe. The electron transfer constant (k0) differs in terms of materials, method of synthesis and electrical equivalent circuits used in the fitting or modelling process. Generally, the k0 values are in the 10-3 – 10-2 cms-1 with Ni nanoparticles having the highest k0 or fastest electron transport. The presence of CNTs also enhances the ET compared with electrodes without CNTs. The electrocatalytic properties of the modified electrodes were explored using the following analytical probes: diethylaminoethanethiol (DEAET), hydrazine, nitrite and dopamine. The study showed that the electrocatalytic oxidation of DEAET and hydrazine was favoured on electrode modified with Ni nanoparticles; nitrite and dopamine were best catalysed by the Co and Fe2O3 nanoparticles, respectively. Electroanalysis results (using chronoamperometry, square wave voltammetry and linear sweep voltammetry) indicated some level of adsorption of DEAET, hydrazine and nitrite on the modified electrode, while dopamine electrocatalytic oxidation and detection followed a simple diffusion controlled process. The adsorption process was found to be physically induced and could be eliminated by repetitive cycling of the electrode in the aqueous electrolyte solution. Electrodes modified with chemically-synthesised material (particularly nickel) were less adsorptive towards DEAET and hydrazine detection, and gave sensitivity and limit of detection values that compared with data obtained using electrochemical deposition / synthesis. The chemical stability and reproducibility of the modified electrodes were determined and discussed. Finally, electrochemical properties were studied to help screen these electrode materials in supercapacitors. CNT-NiO nanocomposites exhibit remarkable super capacitive behaviour in neutral and acidic media compared to the other CNT-MO nanocomposites investigated. Interestingly, the capacitive behaviour of the CNT-NiO was more enhanced in H2SO4 solution than in Na2SO4, possibly due to the high conductivity of the former. The CNT-NiO electrode maintained good stability with only about 5% loss of its specific capacitance after 1000 cycle life. / Thesis (PhD)--University of Pretoria, 2011. / Chemistry / unrestricted

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第21248号 / 理博第4418号 / 新制||理||1634(附属図書館) / 京都大学大学院理学研究科化学専攻 / (主査)教授 北川 宏, 教授 吉村 一良, 教授 竹腰 清乃理 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DGAM

Due to their small size and large surface area-to-volume ratios, nanoparticles (particles with limiting dimensions smaller than 100 nm) have been widely applied as catalysts. Metal nanoparticles are typically produced in suspensions from metal ion precursors, reducing agents, and organic ligands called capping agents. Capping agents help prevent particle agglomeration, fix nanoparticle size, and promote monodispersity. However, capping agents also affect the morphology and the physico-chemical surface properties of nanoparticles, which can influence catalytic properties in unpredictable ways. While there have been extensive studies focused on examining the relationship between nanoparticle size, shape, composition and catalytic activity, relatively few have investigated the effects of capping agent properties on catalysis, and most studies involving nanoparticle catalysts have been conducted on collections, ensembles, or arrays of particles rather than single nanoparticles. Results obtained for systems composed of multiple nanoparticles dispersed on solid surfaces can be difficult to interpret due to variations in particle loading and interparticle distance, which are often challenging or impossible to control and characterize. The complexity of these unavoidable experimental variables may explain some of the seemingly inconsistent conclusions that have been drawn between nanoparticle properties and catalytic activity in recent reports. Single nanoparticle studies should help overcome limitations associated with investigations based on collections of nanoparticles by helping uncover direct relationships between nanoparticle size, surface properties, and catalytic activity that are unobscured by complex factors such as interparticle distance and particle loading. In this work, we aim to use nitrided carbon fiber ultramicroelectrodes to examine electrocatalytic properties of bare (uncapped) and capped gold nanoparticles at the single nanoparticle level.

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