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(2021)ThesisThe electrochemical water splitting reaction, which consists of hydrogen reduction at the cathode and oxygen evolution (OER) at the anode, is one of the core processes for the utilization of sustainable and green energy sources. However, the sluggish kinetics of the oxygen evolution reaction requires a higher overpotential than the theoretical potential (1.23 V). Engineering a high-performance electrocatalyst is an avenue to improve the reaction kinetics for OER. Bimetallic branched nanoparticles offer substantial benefits for OER electrocatalysts; which include a greatly increased exposed surface area, highly crystalline hcp branches and stable surfaces. This thesis aims to design branched nanoparticles as electrocatalysts for enhanced OER in the following ways: (i) extending the cubic-core hexagonal-branch growth approach for a new bimetallic (Co, Au) system, (ii) leveraging the advantages of the Ru-Pd branched nanoparticles by tuning the surface facets and branch number, and (iii) making branched nanoparticles consisting of a cubic core (Pd) and alloyed branches (RuCo). Chapter one discusses the literature on the oxygen evolution reaction and Co- and Ru-based electrocatalysts for OER as well as the organic solution-phase synthesis method. The limiting factors of Co- and Ru- based catalysts and the strategies for improving their catalytic performance are also summarized. Also outlined is the fundamental understanding for synthesizing metallic nanoparticles using a seed-mediated growth approach in an organic solution phase and controlling the shape and size of the final products. Chapter two describes the synthetic methodology, sample purification, ink preparation for electrochemical measurements and characterization techniques in more detail. Chapter three provides the synthetic approaches and challenges in making Co and Ru branched nanoparticles. Chapter four compares the OER catalytic activity and stability of the Co-Au branched nanoparticles with the Co-Au core-shell and Co3O4 nanoparticles in alkaline media. The improved catalytic performance of the branched nanoparticles can be attributed to the formation of an active and stable oxide layer on the branch surface. Chapter five investigates the effect of branch number, and surface facets on the catalytic properties of the Ru-Pd branched nanoparticles with tunable branch number and surface facets. It is found that tuning surface facets and branch length is essential for enhancing catalytic performance by increasing the exposure of more active sites and improving the accessibility of the catalytic surface to the catalytic reaction. Chapter six explores alloyed branched nanoparticles consisting of a Pd core and RuCo branches and assesses their catalytic activity for OER electrocatalysts. It is demonstrated that Co leaching during catalytic activation in acid solution increases the exposure of highly catalytically active sites on the branch surface resulting in enhanced catalytic activity. Chapter seven concludes the overall results and achievements of this thesis and also discusses future opportunities.
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(2021)ThesisWith a theoretical capacity of 1672 mA h g-1, more than five times higher than any commercially available lithium-ion (Li-ion) cell systems, the lithium-sulfur (Li-S) cell is an attractive candidate for next generation energy storage. Despite this high theoretical capacity, Li-S cells generally suffer from poor capacity retention and working lifetimes that prevent them from mass commercialisation. This is mainly due to current limitations in managing the inherent Li-S redox reactions which involve diffusion and migration of electrochemically active polysulfides. One approach to prevent polysulfide migration is by rational design of the sulfur electrode framework. The aim of this research is to investigate the electrochemical implications of using different frameworks for entrapment of redox active species, mainly designed for the Li-S cell system. The two types of frameworks investigated are: (1) mixed-morphology carbon feeds derived from waste sources wherein the intention is for the carbon to purely act as a structural framework to trap lithium polysulfides, and (2) sulfur-rich copolymers wherein redox active sulfur is covalently bound within the framework. More specifically, the goals involve determining: (1) whether carbon acts purely as a structural framework to trap redox active species during electrochemistry, and (2) whether sulfur-rich copolymers act purely as a sulfur feed. Achieving these goals requires a thorough understanding of what properties in each framework are ideal for the Li-S cell. The main conclusion drawn from this work is that neither of the materials studied behaved as pure structural or covalent frameworks partaking in various side processes. Using specialised techniques such as X-ray powder diffraction, solid-state NMR, and X-ray absorption near-edge structure spectroscopy, the beneficial and parasitic side processes involved in each framework are able to be determined. Overall, a significantly enhanced understanding of the Li S cell chemistry when using these materials is presented in this work.
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(2022)ThesisSinglet fission is a photo-physical process that generates two triplet excitons from one singlet exciton and can potentially enhance efficiency in photovoltaic systems. The combination of photovoltaics and singlet fission is a novel field for solar energy conversion when there is much interest in renewable, non-destructive, and continuously available energy sources. Singlet fission can also overcome thermalization losses in photovoltaics, which happens in traditional cells when the incident photon energy is higher than the silicon bandgap energy, using a carrier multiplication mechanism. This thesis will design, construct, and characterize photovoltaic devices incorporating singlet fission materials to study singlet fission in practical application. The research focuses on materials characterization, spin dynamics, and electron transfers between acene and the semiconductor layer in Au/TiO2 ballistic cells, and the incorporation of singlet fission layers on silicon-based cell structures. In detail, a set of investigations was developed and summarized by implementing singlet fission materials into a state-of-the-art ballistic photovoltaic device and silicon-based solar cell. The studies demonstrate proof of concept and rationally explain the process. The first part of the thesis investigates thin films of pentacene, TIPS-pentacene, and tetracene via crystallinity, morphology, absorption, and thickness characterization. Additionally, Au and TiO2 layers in Schottky device structures were optimized to achieve the best performance for energy transfer from an applied dye layer (merbromin). The drop-casted dye layer influences the device performance by increasing short-circuit current and open-circuit voltage, demonstrating the ability of charge transfer between the device and the applied film. This device structure provides a test bed for studying charge and energy transfer from singlet fission films. The latter part of the thesis describes several investigations to understand singlet fission in a thin film using this architecture. Magneto-photoconductivity measurements were primarily used to observe the spin dynamics via photoconductivity under an external magnetic field. Control experiments with bare Au/TiO2 devices showed no observable magneto-photoconductivity signal. In contrast, devices with pentacene and tetracene singlet fission layers showed a strong magnetoconductivity effect caused by ballistic electron transfer from the singlet fission layer into the TiO2 n-type semiconductor through an ultra-thin gold layer inserted between the layers. A qualitatively different behavior is seen between the pentacene and tetracene, which reveals that the energy alignment plays a crucial part in the charge transfer between the singlet fission layer and the device. The last section investigates the application of pentacene and tetracene evaporated thin-films as sensitizer layers to a silicon-based solar cell. The optimized Si cell structure with the annealing treatment improved the cell's performance by increasing short-circuit current and open-circuit voltage. The deposition of pentacene and tetracene as sensitizer layers into the device showed some results but posed several challenges that need to be addressed. As the current-voltage and external quantum efficiency measurements were taken, it was observed that material interfaces need to be designed to fully achieve the singlet fission of the acene layer into the Si devices.
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(2019)ThesisThis PhD thesis describes the discovery of synthetic strategies to target novel heterocycles and fused ring systems. The primary aim of the research was to develop novel heterocycles as analogous systems to the antimalarial natural product dependensin as well as to explore the chemistry of these previously unreported classes of compounds. The secondary aim of this PhD project was to explore the chemistry and develop efficient synthetic routes to novel fused heterocyclic systems containing the benzazepine moiety. Previously, extensive research had been conducted on new antimalarial compounds, focusing on the flavonoid systems closely related to dependensin. However, analogous systems in which the heterocyclic oxygen atoms of dependensin are replaced by other heterocyclic atoms, generating the 5,6- dihydrodibenzo[b,h][1,6]naphthyridine, chromeno[4,3-b]quinoline and thiochromeno[4,3-b]quinoline derivatives, had been relatively unexplored. This thesis describes the efficient synthesis of a range of dihydrodibenzo[b,h][1,6]naphthyridine, chromeno[4,3-b]quinoline thiochromeno[4,3-b]quinoline derivatives using an inexpensive and versatile Friedlaender coupling methodology which allows for the generation of diverse analogues, related to the dependensin natural product. Additionally, a robust and simple synthetic pathway was developed to access novel fused heterocyclic ring systems via an initial addition-oxidation-ring cleavage cascade reaction under basic conditions in the presence of NaOH in DMSO to give a versatile 1,4-diketone intermediate. Subsequent cyclisation reactions gave the azepine moiety fused with either quinoline or indole ring systems with high levels of substitution possible. The synthesis of two novel classes of heterocycles, namely the dihydrobenzo[6,7]azepino[3,2-c]quinolinones and 11-phenylbenzo[6,7]azepino[3,2- b]indolones was achieved. This work considerably expands the number of examples of structures incorporating the dihydrobenzazepine scaffold. The range and diversity of the developed fused heterocyclic systems have resulted in four publications to date.
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(2023)ThesisFrom the millions of gaseous species present on Earth’s atmosphere, those containing the carbonyl group (C=O) are some of the most atmospherically important. While all organic species react with OH and other radicals, the C=O bond in carbonyls provides a chromophore for photolysis reactions. Consequently, carbonyls react quickly and are an important source of atmospheric radicals. Recent laboratory findings on the photochemistry of several aldehydes have yielded new photochemical and photophysical mechanisms that are of unknown importance in atmospheric chemistry, which paves the way for this thesis. Here, two atmospheric models, GEOS-Chem and AtChem2, were used to understand the implications and relevance of these untested and new carbonyl photochemistry findings on atmospheric sources and composition. The results of implementing the two models to explore carbonyl photochemistry are described in this thesis in five chapters. Two chapters explore the previously untested primary production of molecular hydrogen (H2) from a range of aldehydes. Both in AtChem2 and in GEOS-Chem, a 1% quantum yield for H2 was tested, based on experimental data for acetaldehyde photolysis. Globally, and under three different atmospheric settings (urban, pristine oceanic and pristine forested), both models showed that biogenic-related aldehydes (e.g., methylgyoxal and glycolaldehyde) make an important contribution to the chemical H2 production; GEOS-Chem showed that it is up to 10% throughout the troposphere (most influential in tropical regions). Results showed that including the previously unaccounted photolytic sources of H2 does not improve model biases but calls for more experimental research into the true H2channel quantum yields for the most important aldehydes. Following, in a collaborative project with the New South Wales state government framed within Australia’s potential migration to a H2 economy, we implemented the AtChem2 model to test the sensitivity of radicals (OH and HO2) to changes in H2 levels. Radicals and other gases (e.g., CH4, CH3OH) displayed sensitivity to increases in H2 levels, highlighting the necessity of careful consideration of a potential new source of H2 to the Australian atmosphere. In the next chapter, the atmospheric fate of trifluoroacetaldehyde (CF3CHO), a fluorinated carbonyl produced from the oxidation of a fourth-generation refrigerant (HFO-1234ze, CF3CF=CF2), was modelled. CF3CHO photolysis has been shown to be a source of fluoroform (HFC-23) in small yields. The degradation and deposition pathways of CF3CHO were modelled using AtChem2 to provide a range of values on the production of HFC-23. The modelling estimates show that CF3CHO could contribute up to 14.7% to the current HFC-23 global growth rate. Finally, a new mechanism called photophysical oxidation (PPO) in carbonyls was also studied using acetaldehyde as a benchmark compound. In acetaldehyde, PPO forms the peroxyacetyl radical, which affects peroxyacetylnintrate (PAN) formation. Results using GEOS-Chem showed that PPO in acetaldehyde could be responsible for 3% of the global chemical production of PAN. However, PPO is unlikely to be important when photolysis is not the dominant carbonyl degradation pathway. This thesis provides new insight into carbonyl photochemistry through modelling applications: A new H2 simulation in GEOS-Chem was developed; in which further research related to H2 can be performed, the possible generation of HFC-23 from a new secondary source was explored, and a new photophysical mechanism for carbonyls was assessed. Ultimately, the results from exploring carbonyl photochemistry throughout this PhD are intended to be used as guide for new laboratory research.
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(2023)ThesisIncreased access to renewable energy is vital for continued human existence and development while minimising the impact of climate change. Over the past several decades photovoltaic (PV) technology has evolved to produce a variety of functional organic and inorganic PVs. Single junction silicon PV devices have come to dominate solar energy conversion due to their, efficiency, relative simplicity, and affordability. However, the efficiency of silicon PV is approaching the Shockley-Quiesser limit of efficiency for single junction PV devices. To exceed this limit and increase the portion of energy generated, new device architectures are required that can exceed the aforementioned limit. Singlet Fission (SF) is a spin-allowed process of exciton fission that transforms one spin-singlet excited state into two spin-triplet excited states localised on separate molecules. SF can potentially be harnessed to increase the efficiency of silicon photovoltaics by converting one absorbed high energy photon into two smaller energy excitons. The transformation of exciton energy provides a promising pathway for reducing losses via thermalisation which account for a large intrinsic loss mechanism of single junction PV. Chapter 2 of this thesis investigates the mechanisms of SF in solution to understand the role of intermediate states such that SF efficiency can be maximised in PV applications. Solutions were studied to better control concentrations and slow down reaction time scales compared to the solid state. This chapter specifically challenges the idea of red-shifted excimer intermediates in SF that facilitate high triplet yields. Solubilised acenes are probed with multiple spectroscopic techniques including transient absorption (TA) and time and spectrally resolved photoluminescence measurements (TSRPL). A kinetic model of SF in solubilised tetracene is constructed from data obtained from TSRPL and is used to show that triplets form directly from singlets and vice versa. Rigorous analysis of TA data of solubilised tetracene, showing the singlet, triplet, and red-shifted/excimer states, explains how the concomitant rise of the red-shifted/excimer state and triplet state can confuse the assignment of spectral features to each state. The red-shifted/excimer state is shown to have a featureless spectrum, in contrast to other reports. TA analysis of SF in solubilised pentacene fails to reveal an intermediate state. High concentration absorption spectra of solubilised pentacene shows that there is no significant ground state aggregation in high concentration solutions. These results are consistent with the lack of red-shifted/excimer intermediate shown in solubilised tetracene. This thorough investigation reveals the hindrance of an red-shifted/excimer state on the overall SF efficiency in solubilised tetracene, and an apparent absence of it in solubilised pentacene. Therefore, it was shown that the red-shifted/excime state hinders or is not required for efficient SF. It is suggested that intermolecular orientation should be controlled to minimise this lossy state. To develop a silicon/SF hybrid PV device, a new interface between organic SF material and silicon must be created. Chapter 3 of this thesis describes the synthesis of a series of silicon wafer and organic material interfaces based on 1,8-nonadiyne monolayer formation on silicon via thermal hydrosilylation. These interfaces are studied with surface sensitive techniques, such as X-ray photoelectron spectroscopy (XPS) and surface photovoltage spectroscopy (SPV). It is shown that organic functionalisation of silicon surface with solubilised tetracene via [1,2,3]-triazole species is possible, without silicon oxide formation. SPV shows that this interface yields promising signs of improved energy transfer kinetics between the organic layer and silicon. Although, ultraviolet photoelectron spectroscopy shows that the use of a 1,8-nonadiyne monolayer imbues the interface with insulating qualities. Therefore, aromatic monolayers of 1,4-diethynylbenzene are also explored as an interface that would be less insulating. Importantly, low temperature silicon photoluminescence (PL) measurements shows that organic passivation of silicon wafers with 1,8-nonadiyne does not posses the passivation quality to prevent large amounts of non-radiative exciton recombination at room temperature, evident by a lack of room temperature silicon PL. This is not conducive to efficient PV. It is suggested that the inability of organic passivation of silicon to bond to every surface silicon bond is the causes of this substandard passivation. Surface functionalisation techniques that could address this problem are discussed. In summary, this thesis presents valuable insights into the desirable properties of SF molecules to be incorporated into PV devices as well as the research direction for the development of SF/silicon hybrid interfaces and PV.
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(2023)ThesisPluripotent stem cell-derived cardiomyocytes (hPSC–CM) have great importance for predicting safety parameters for pharmaceutical compounds and models of healthy versus disease states of the human heart. In recent years, there has been an insistence that all new pharmaceutical products are tested on in vitro models for potential proarrhythmic effects and the increased demand for improved biomimetic hPSC-CM in pharmaceutical safety assays such as the Comprehensive in vitro Proarrhythmic Assay (CiPA). In addition, hPSC-CM are being utilised in cell therapies to treat and reverse the effects of ischaemic heart disease, offering potential cures for cardiovascular diseases instead of treatments for delaying progressive heart failure. In the first part of this thesis, I will examine how purified extracellular matrix proteins (ECMPs) can influence pluripotent stem cell (PSC) behaviour and how we may use this to precondition cardiac progenitor lineage specifications. I use array-based techniques to investigate how protein combinations affect proliferation, pluripotency, germ layer, and cardiac progenitors. This method allows us to visualise how individual proteins can affect cells' behaviour in a larger array whilst highlighting how specific combinations can precondition pluripotent cells towards a cardiomyocyte lineage. This combinatorial approach led to the identification of several unique matrices that promote differentiation, which will aid efforts at producing therapeutically useful cell types with greater efficiency. In the second part of this thesis, I demonstrate a novel bioreactor that attenuates a magnetic field to dynamically modulate the stiffness of magnetoactive hydrogel to look at how biomimetic dynamic stiffening of a substrate can influence cardiomyocyte lineage specification. We investigate how biomimetic in vivo mechanics may influence cell fate by following the expression profiles of cells in different dynamic environments. Non-invasive electromagnetic signals affect substrate stiffness when combined with magnetic particles and magnetic fibres and how this can help direct cell orientation and accompanying lineage specification Finally, I investigate how variability in cell phenotypes and expression patterns are influenced by biomimetic cues and how these variabilities could be utilised in future safety assessment protocols and cell therapy treatments for cardiovascular disease.