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(2021)ThesisPolymer brushes are arrays of densely surface-tethered polymer chains, and are of interest for two reasons. Firstly, they possess interfacial characteristics, such as antifouling and lubrication, that are desirable in many applications. Secondly, they are model systems that can provide additional insight into polymer behaviour due to their unique geometry. Observing the interfacial structure of these brush layers is critically important for understanding both their properties and the mechanisms driving the polymer behaviour. To date, neutron reflectometry (NR) is the only technique that can demonstrably resolve the nanoscale structure of polymer brushes. However, these diffuse interfaces produce subtle features in the reflectometry data that challenge interpretation, with typical analyses failing to quantify the derived structure's uncertainty. Furthermore, the experimental potential of this technique for the study of brushes is only just being realised. This Thesis advances NR as a tool for studying polymer brush systems by establishing a robust analysis methodology that overcomes previous hurdles and demonstrating novel experimental techniques. In both cases, poly(N-isopropylacrylamide) (PNIPAM) brushes are used as model systems. First, the polymer system is characterised through the novel observation of surface-initiated ARGET ATRP using time-resolved NR, and a study of the dry brush as a function of humidity and temperature. Second, methodologies are developed that allow for robust determination of both solvated and confined brush structures. Lastly, NR is used to elucidate the behaviour of PNIPAM brushes in complex environments. A novel confinement apparatus is used to investigate the structure of a PNIPAM brush under mechanical confinement and contrast-variation provides unparalleled insight into PNIPAM–surfactant systems. In each case, complementary techniques are essential in guiding reflectometry experiments and fully understanding the polymer system. This work develops and demonstrates techniques that enhance the study of diffuse interfaces with the NR technique. Moreover, the holistic structural examination of PNIPAM undertaken sheds new light on the phase behaviour of this ostensibly well-understood polymer and highlights its rich interaction with surfactants.
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(2020)ThesisThe rapid development of technology has made the design, monitoring and data storage of large-scale, complex interconnected systems possible. These efficient and economical interconnected systems come with a price: the complex dynamics due to convoluted interconnections make the effective control of such a system incredibly difficult. The behaviour of the subsystems in a network is vastly different than that when it is not, and the inherent uncertainties due to modelling errors may be amplified as a result of the strong interactions. Furthermore, the ability to collect and process large amount of data leads to the paradigm shift from model-centric description to data-centric description or hybrid model/data description of a system. These challenges necessitate the need for a unified foundation for the control of complex systems that is able to admit descriptions of systems not only limited to the conventional differential/difference models. Motivated by these challenges, this thesis aims to develop such a framework for the distributed control of an interconnected system using the behavioural systems theory. As a theory that focuses on analysing the dynamics of the external variables and places the trajectories admissible within the system as the central role of describing a dynamical system, it is perfect for the construction of a platform that unifies various classes of systems and is effective in the analysis of interconnections. The framework is eventually set up as a completely representation-free structure, allowing for free choice of representations for the systems according to the specific needs. Algorithms for several representation structures are also provided. For the case where the subsystems are represented as linear time-invariant differential systems while the global requirements are specified as H-infinity type conditions, the control design follows a two-step algorithm. Firstly, the behaviours of the subsystems, the (to-be-designed) controllers as well as the global requirements are all represented as dissipative dynamical systems with quadratic supply rates, from which the (to-be-determined) controller supply rates can be found. Secondly, parametrisations of the supply rates are carried out to search for linear time-invariant representations for the controllers. Algorithms for subsystems with various types of parametric uncertainties are given to add robustness to the controllers. The resulting framework deals with interconnections, uncertainties in the subsystems and disturbance attenuation simultaneously. For the general framework, neither the subsystems nor the controllers have prescribed representations. The behaviours of the subsystems are denoted by their respective sets of trajectories and interconnections are interpreted entirely as variable sharing instead of signal flows. Furthermore, the network of an interconnected system is also defined as a dynamical system with its own behaviour, leading to a generic, scalable and flexible representation of the interconnected behaviour. From this structure, necessary and sufficient conditions for the existence of the controller behaviours can be given and all distributed controller behaviours can be constructed explicitly. This framework unites various representations and descriptions of the features of dynamical systems as behaviours, thereby allowing for the formation of a hybrid platform for the analysis and distributed control generically and systematically.
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(2020)ThesisPhoto-controlled polymerisation uses a photo-excited photocatalyst (PC) to reversibly and deactivate the propagating species. Under regulation by light, photo-controlled polymerisation features temporal control, spatial control, sequence control and high level of selectivity/orthogonality between different systems, leading to a range of applications in advanced macromolecular synthesis such as surface patterning, 3D/4D printing, polymeric micelles, multiblock antimicrobial polymers with precise sequences and architectures. All these unique features of photo-controlled polymerisation are largely dependent on properties and functionalities of PCs. Traditionally, the selection and discovery of appropriate PCs rely heavily on a trail-and-error approach, where extensive experimental screening is needed to identify desired candidates. To reduce costs and circumvent the challenges in laborious experimental work, a rational design strategy emerged where a new PC in application to a photo-controlled polymerisation system can be designed based on understanding of the structure-property-performance relationships. This dissertation aims to enable and streamline a general fully computer-guided rational strategy of designing an efficient and functional PC for a commonly used photocontrolled polymerisation technique, namely photoinduced electron/energy reversible addition-fragmentation chain transfer (PET-RAFT) polymerisation. This thesis starts from using naturally evolved Chl a with various functional substituents and investigating its photocatalytic performance and functionalities in PET-RAFT polymerisation. General orientations for the design of PET-RAFT PCs were inspired from this natural design. On top of this, comprehensive structure-property-performance relationships were established at the quantum chemical level as guiding principles for rational PC design of PET-RAFT polymerisation, by combining experimental and computational studies on a library of halogenated xanthene dyes. Finally, by implementing the most cutting-edged quantum chemical software packages, the fully computer-guided strategy of functional PC design for PET-RAFT polymerisation was enabled based on broadened structure-property-performance relationships. As an example, an efficient pH-switchable organic PC was designed. Application of this rationally designed PC in PET-RAFT polymerisation resulted in the first organocatalysed pH and light dual-gated controlled polymerisation.
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(2020)ThesisSolar energy conversion efficiency can be determined by the properties of a photocatalyst. Recently, bismuth vanadate (BiVO4) has emerged as a potential photocatalyst due to its suitable band structure and superior visible light response. However, severe charge recombination, poor electron transportation and sluggish surface reaction kinetics prevent BiVO4 from reaching its potential. Herein, different tactics regarding its modification were employed to improve the photocatalytic capability of BiVO4. The research initially investigated the interfacial contact between BiVO4 and reduced graphene oxide (rGO). Two BiVO4/rGO composites with BiVO4 particles extensively or insufficiently contacted with rGO sheets were fabricated. An enlarged interfacial contact greatly improved charge separation and electron transfer efficiency as well as band alignment within the BiVO4/rGO composite, hence delivering a greater photocurrent during photoelectrochemical (PEC) testing. However, an excessive rGO coverage also resulted in higher hydrophobicity, which reduced the water miscibility and lowered the capability toward photocatalytic O2 production. Consequently, the balance between electron shuttling and the water miscibility should be considered depending on the photoreaction mode. Subsequently, a CoOx co-catalyst was loaded onto a dual-faceted BiVO4 which possessed distinct levels of exposed {010} and {110} facets. Enhancement provided by the co-catalyst was depended on both its deposit location and the ratio of {010}/{110} facet exposure. When selectively deposited on the hole-accumulating {110} facet or loaded on {010}-dominant BiVO4 particles, CoOx functioned more effectively in alleviating surface hole trapping and reducing the charge recombination, thus delivering a higher photocatalytic activity. Finally, the bimetallic phosphide, NiFePx, was studied as a BiVO4 co-catalyst for PEC performance. NiFePx was immobilized onto a BiVO4 photoanode via a hydrothermal method followed by a phosphorization process. The NiFePx passivated the surface states of BiVO4 and provided a greater driving force to initiate water oxidation. Accordingly, surface charge recombination was suppressed, and charge transfer and charge injection efficiency were promoted. In summary, surface modification strategies can endow BiVO4-based photocatalysts with a good capacity for water splitting with rational manipulation of their interfacial properties providing even greater benefits.
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(2020)ThesisPhotocatalytic water splitting to produce H2 fuel has drawn considerable attention due to ever-increasing environmental concerns and the rising global energy demand for renewable and clean energy formation. Since last century, the semiconductor photocatalysts have been studying to explore their photocatalytic properties for solar-driven H2 evolution from water. Among the various H2-evolving photocatalysts, CdS with excellent visible-light absorption and suitable band potentials is one of the most studied photocatalysts. Although cocatalyst-decorated CdS nanorods (NRs) offer a promising H2 evolution performance, further benefits invoked by spatial cocatalyst decoration are worth exploring. Given that noble metal cocatalysts were most deposited on the tips of CdS NRs, the tip deposition of non-noble metal cocatalysts on CdS NRs was explored in the first stage of PhD research. For the first time, it was found that amorphous MoOxSy preferentially photodeposited on CdS NR tips, which enhanced photocatalytic H2 evolution. Detailed characterization and studies determined the composition and possible formation pathways of the amorphous MoOxSy on the CdS tips. The enhancement was ascribed to the effective trapping of photoinduced electrons by the MoOxSy resulting from its lower surface work function compared to CdS. However, amorphous MoOxSy is unstable in most hole scavengers except methanol. Consequently, the MoOxSy was vulcanized into MoS2, with MoS2 presenting either on the only tips, on the tips and random walls, or having an overall coating. MoS2-tipped CdS NRs exhibited a better H2 evolution activity compared to other MoS2-loaded CdS NRs. Kelvin probe force microscopy and time-resolved photoluminescence spectroscopy were used to study spatial charge separation, transfer, and lifetime. The superior photoactivity of MoS2-tipped CdS NRs was assigned to spatially intensified charge separation along the long axis of NRs, thereby lowering the charge recombination rate. To further understand the effects of MoS2 distribution on CdS NRs, MoS2 was parallelly or perpendicularly placed on polar CdS (001) surfaces (representing CdS NR tips) and non-polar CdS (110) surface (representing CdS NR wall) to construct different CdS/MoS2 models for DFT calculations. It was verified that CdS NR is capable of the longitudinal charge transfer due to charge polarization on polar (001) surfaces. The distinct electronic properties (i.e. shift in core-level energies of Cd and S orbitals, interfacial charge accumulation and depletion) of different CdS/MoS2 heterostructures, induced by the different interfacial bonding arrangements and MoS2 orientation, were observed and analyzed. In addition, two types of MoS2-loaded CdS NRs (MoS2 being only on the tips or only on the walls of CdS NRs) were fabricated to obtain their BEs of Cd 3d and S 2p by XPS for comparison, which is consistent with the expected DFT results. The band edges of CdS component in the as-prepared samples were also determined by combining Tauc plots and VB XPS measurements; the band shifts in experiments approximately correspond to DFT results. As a result, the MoS2 distribution and orientation affect the electronic properties of CdS/MoS2 heterostructures and thereby giving a different photoactivity. Overall, this thesis reveals that the origins of photocatalytic H2 evolution advantages induced by spatially located cocatalyst and gives insights into the activity enhancement mechanism for tip-decorated CdS NRs, which guides the construction of more efficient CdS-based photocatalysts.
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(2020)ThesisElectrochemical energy conversion systems are capable of storing renewable energy in chemical forms which could be subsequently used as fuels or chemical feedstocks. Electrocatalytic reactions are cornerstones of these electrochemical processes and play pivotal roles in determining the overall efficiency of various electrolysis systems. For instance, the oxygen evolution reaction (OER) liberates the electrons and protons for the generation of clean hydrogen (H2) fuel through the hydrogen evolution reaction (HER) in an electrolytic water splitting system integrated with renewable power supplies. In addition, the oxygen reduction reaction (ORR) enables the energy and chemical conversions in fuel cells, metal-air batteries and electro-synthesis of hydrogen peroxide, satisfying our pursuit of a green chemistry. The main obstacle for the large-scale implementation of these energy conversion systems is the sluggish reaction kinetics on both the anodes and the cathodes, including anodic OER, cathodic HER and ORR, which require noble-metal-based electrocatalysts to expedite the reaction at a desirable rate and selectivity (e.g. IrOx and RuO2 for OER, Pt/C for ORR and PtHg for H2O2 production). However, the high cost and poor stability of these noble-metal-containing materials severely hamper their commercial viability. In these regards, the rational design of non-noble-metal electrocatalysts with high activity, good durability, desirable selectivity and low cost is highly sought after. This thesis focuses on the screening, design, synthesis and characterization of first-row transition-metal-based materials as efficient electrocatalysts for various electrolysis systems. To resolve the activity and stability issues haunted by water electrolysis system, two kinds of nanostructured bifunctional HER-OER electrocatalyst are fabricated including (1) ultrathin Fe-N-C nanosheets coordinated Fe-doped CoNi alloy nanoparticles and (2) defective FeCoNi (oxy)hydroxide atomic layers, which aim at an improved catalytic performance and long-term durability. Besides, ORR is the bottleneck of not only fuel cells and metal-air batteries but also other devices targeting at a continuous production of H2O2. To this end, another two novel electrocatalysts are synthesized: (1) epoxy-modified carbon nanotubes with isolated Co atomic sites and (2) thin Cu layer decorated Co nanoparticles encapsulated by a Co-Nx/Cu-Nx co-doped carbon, both achieving a high selectivity in different pathways with excellent activity. New preparation techniques are developed to fabricate these catalysts, and their catalytic properties are evaluated by electrochemical measurements. The structural and chemical properties of catalysts down to atomic scale are also investigated with a variety of physical characterizations (e.g. X-ray adsorption spectroscopy, X-ray photoelectron spectroscopy, Raman spectroscopy, transmission electron microscopy). The correlations established between activity and structural properties give insights into the origin of activity/selectivity within these composites. Last but not the least, the results indicated that, through rational design, transition-metal-based electrocatalysts with high activity, long-term durability and desirable selectivity even superior to the noble metal benchmarks can be achieved with an apparently lower manufacturing cost.
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(2021)ThesisThis thesis investigated the ecology and metabolism of microorganisms, especially yeasts, during the wet fermentation of Australian coffee beans, and their contribution to coffee quality. Pulped coffee beans were fermented underwater for 36 h where yeast growth was suppressed by the addition of Natamycin at 300 mg/L. Spontaneous fermentation without the addition of Natamycin was conducted as control. The growth and diversity of microorganisms during fermentation were monitored by both culture dependent and independent methods. Major non-volatile metabolites during fermentation were monitored by high performance liquid chromatography (HPLC) and volatiles in the green and roasted beans were measured by solid phase microextraction coupled with gas chromatography tandem mass spectrometry (SPME GC-MS). Both bacteria and yeasts grew significantly during spontaneous fermentation while yeast growth was restricted in the Natamycin treated fermentation without significant impact on bacterial growth. The bacterial community was dominated by Citrobacter sp., Gluconobacter cerinus, Leuconostoc mesenteroides and Lactococcus lactis with maximum populations between 4-7.2 log CFU/g, while Hanseniaspora uvarum and Pichia kudriavzevii were the predominant yeasts at 4.5-5 CFU/g. During fermentation, the microflora utilized sugars in the mucilage and produced mannitol, glycerol and essential volatiles, mainly alcohols, esters, aldehydes and organic acids, with their concentrations generally lower in beans fermented with yeast suppression. Coffee produced from yeast suppressed fermentation received lower sensory scores in flavour and aroma and overall quality by 3 Q-Grade coffee masters. When H. uvarum and P. kudriavzevii were inoculated individually and in combination, they dominated the fermentation by growing to 9-10 log CFU/ml, and produced greater amounts of glycerol and flavour volatiles in the green beans which remained in higher levels after roasting compared with the control. Coffee brewed from these beans received significantly high scores of flavour, aroma, acidity and overall quality. Mucilage degradation seems to be initiated by endogenous enzymes and microbial contributions to the process occurred subsequently either enzymatically or by acidification. These findings demonstrated the crucial contribution of yeasts to successful coffee fermentation and high-quality coffee, and the potential of developing the two yeasts into starter cultures for coffee fermentation.
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(2020)ThesisAlthough conventional thermal processing ensures the microbial safety of liquid foods, it adversely affects the nutritional and organoleptic properties of these food products. Hence, the application of radio frequency electric fields (RFEF) processing, as a non-thermal alternative, has been investigated in the past few decades for liquid foods. This research was conducted to study the effect of RFEF processing on liquid foods in three main aspects. In the engineering aspect, Computational Fluid Dynamics (CFD) models were utilised to assess the effect of inserting stainless steel mesh in a co-linear chamber on process homogeneity, in comparison to a no-mesh co-linear configuration. The results indicated that the insertion of a stainless steel mesh in a co-linear configuration enhanced the homogeneity of the process by improving the electric field, temperature, and velocity profiles within the chamber. In the microbiological aspect, the effect of various RFEF processing parameters, such as electric field, temperature, frequency, and treatment time, on microbial inactivation was studied. In cranberry juice, an E.coli inactivation level of 6.57±0.02 logCFU mL-1 was achieved by multiple-stage RFEF processing with an electric field, outlet temperature, and treatment time of 13.2 kV cm-1, 40 °C, and 3240 μs, respectively. Also, based on inactivation results, a kinetic inactivation model as a function of the electric field, temperature, and treatment time was developed. In orange juice, multiple-stage RFEF processing with an electric field, temperature, treatment time of 11.7 kV cm-1, 42 °C, and 1.17×10-3 seconds, respectively, extended the shelf-life compared to the control sample. Lowering the frequency from 20 to 10 kHz increased the inactivation of E.coli in saline water with a peak at 12.5 kHz, and a further decrease in frequencies below 10 kHz reduced the inactivation levels due to electrolysis. A synergistic effect on E.coli inactivation was achieved when low frequency (LF) electric fields processing of above 9.6 kV cm-1 was combined with high frequency (HF) processing with outlet temperature and power above 55 °C and 739 W, respectively. In the nutritional aspect, a comparison study between multiple-stage RFEF processing with an electric field, temperature, and treatment time of 11.7 kV cm-1, 42 °C, and 1.17×10-3 s, and a heat processing with similar inactivation level referred to as thermal processing (81.5 °C for 10 s), and an industrial level heat processing, referred to as pasteurisation processing (90 °C for 30 s), during 45 days of storage at 4 °C, was conducted. The results obtained after the storage indicated that although a higher antioxidant capacity and vitamin C content were achieved in orange juice processed by RFEF compared to pasteurisation processing, the thermal processing resulted in the highest level of antioxidant capacity and vitamin C content in orange juice. Furthermore, regarding total phenolic compounds (TPC), orange juice processed by RFEF has the lowest content of TPC compared to both heat treatments. In conclusion, the RFEF processing demonstrated considerable potential as an alternative to conventional heat treatment, especially in the microbiological aspect. However, further investigations on the effect of RFEF processing parameters, such as electric field and temperature, on nutritional and organoleptic properties of liquid foods are required. These investigations can be complemented by cost analysis studies, which can help to advance RFEF processing towards commercialisation.
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(2021)ThesisCO2 hydrogenation represents one of the most practical solutions for mitigating anthropogenic carbon into chemicals/fuels. The overall reactivity in these reactions is often linked to the nature of kinetics relevant surface intermediary steps. Herein, light-induced photoactivation and materials engineering strategies were employed to particularly tailor the sluggish reaction steps and improve the catalytic performance of two CO2 hydrogenation reactions, namely CO2 methanation and methanol synthesis. The light-assisted CO2 methanation over NiOx/La2O3@TiO2 (NLT) was studied using isotopic-assisted in-situ diffuse reflectance infrared transform spectroscopic (DRIFTS) technique. It was shown that one of the important surface reactions – formate (HCO2*) conversion - can be promoted under visible light illumination. The La2O3 promoter and the broad-band localised surface plasma resonance (LSPR) property of Ni nanostructure, which acts as a vital adsorption site for CO2 activation and enables the generation of high-energy electrons, respectively, could play a primary role in this promotion. Next, the effect of photoexcitation on light-assisted methanol production from CO2 hydrogenation over a Cu/ZnO/Al2O3 (CZA) catalyst, where HCO2* conversion also plays an important role, was studied. Interestingly, significant methanol enhancement was only observed under the photoexcitation of both Cu and ZnO, whilst CO production was improved irrespective of the spectral range. This was revealed to be governed by the photo-generated electrons and their subsequent interfacial transfer which were responsible for the simultaneous transformations in surface chemistry and catalytic reactions. Further, CeO2 was studied as a promoter for Cu/ZnO based catalyst where enhanced methanol production was observed at dark condition. Lastly, a dopant strategy (Mg/La doping in ZnO) was adopted to electronically promote the Cu-ZnO interaction to testify its role in light-assisted methanol production. The photothermal catalytic methanol production over promoted CZA was improved benefiting from the enriched interfacial oxygen vacancies which could help channel the photo-generated electrons to the adsorbed HCO2* species. Overall, the study demonstrated the potential of photoexcitation and materials engineering strategy in improving CO2 hydrogenation reaction over oxide supported Ni and Cu catalysts. Significantly, the mechanistic understanding at the molecular level is critical for the design of catalysts that can better harness the potential of photoexcitation.
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(2020)ThesisCoordination nanoparticles, especially layered double hydroxide and metal-organic frameworks, have attracted great attention for biomedical applications. These categories of nanomaterials stand among various conventional reagents for several outstanding features, their versatile composition allows the flexible construction or substitution of the core elements, subsequently enable the integration of multiple functions. These nanohybrids can often be derived from facile and controllable syntheses, which will result in a tunable size/morphology, hence the resultants can be tailored with specificity and serve for particular purposes. Moreover, by virtue of the metal elements within the structure, hydroxide bones or coordination bonds can be readily degraded while subjected to certain biological conditions, e.g., mild acidity in the solid tumor. It indicates a potential satisfactory biodegradability, which is critical and desirable for biological applications. Most importantly, potent catalytic reactivity relies upon rational design. The nanohybrid can be engineered with a trigger that responds sensitively to tumor microenvironments, e.g., elevated hydrogen peroxide level. This thesis has firstly summarized current prevalent catalytic nanomedicines and the corresponding paradigms achieved by layered double hydroxide and metal-organic frameworks, current challenges and perspectives were also proposed. In Chapter 3, a novel approach was used to enhance nanoparticle colloidal stability. Various reaction conditions have been investigated to optimize synthesis. Characterizations illustrated the fundamental mechanism behind the PEGylation. Further tests also revealed that the PEGylated nanoparticles had a decreased protein adsorption, enhanced cellular uptake, and negligible cytotoxicity. In the following chapter, an iron(II)-incorporated nanocatalyst was then synthesized with this PEGylation strategy. It was observed that the resultant demonstrated an ultrathin two-dimensional nanosheet morphology. The as-synthesized PEG/Fe-LDH displayed extremely high affinity and reactivity regarding the catalytic decomposition of H2O2, and abundant amount of •OH was detected. More importantly, desirable biodegradability was observed on PEG/Fe-LDH. While proceeded to in vitro and in vivo studies, significant cancer cell growth suppression was observed, which can be attributed to the high responsiveness to H2O2. Bearing H2O2-responsive catalytic reaction in mind, an Mn-based MOF was designed and tested in Chapter 5. The resultant significantly alleviated tumor hypoxia, and an enhanced photodynamic therapy has been successfully established.