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(2021)ThesisArtificial micro/nanomotors (MNMs), inspired by mobile biomolecular entities, have demonstrated great potential as miniaturized robots performing diverse tasks from environmental remediation to biological treatment owing to their great mobility and versatility. The reported MNMs can be propelled using various power sources, including magnetic field, electric field, ultrasound, light, and chemical reaction. MNMs that operate on chemical reactions are usually equipped with higher velocity due to the superior energy conversion efficiency, which dominantly present as bubble propelled systems. However, the majority of the bubble propelled MNMs utilize bubble ejection and detachment force, which result in swarming and linear motion for Janus and tubular motors, respectively. It is still challenging for chemical propelled MNMs to have absolute control in direction without external field. A crafty design to circumvent this limitation is to develop biocatalytic MNMs with bubble buoyancy propulsion. This thesis focuses on the design, fabrication, and applications of submarine-like buoyancy-propelled MNMs that move in the vertical direction. I fabricated buoyancy propelled nanomotors with one-pot synthesis and provided the first work characterizing detailed motion behavior with electrochemistry. With coupled biocatalytic cascade reaction converting glucose as the fuel to oxygen bubbles, the nanomotor was propelled by buoyancy, which dominated the initiative collision at the electrode surface. Four representative electrical impact signals were observed and corresponded to four types of motion patterns. The corresponding relationship was confirmed with a numerical simulation. The integration of MNMs and electrochemistry provided a new dimension to characterize and understand the complex dynamics of the self-propelled nanoparticles. I further investigated the buoyancy propelled MNMs in biomedical applications. The pH-sensitive polymer incorporated micromotor exhibited regulated vertical motion via hydrophilic/hydrophobic phase shifting in different pH environments, and the system was proved to be applicable for anti-cancer drug delivery in a proof-of-concept three-dimensional cell culture. The proposed micromotor opens up new avenues in autonomous robotic fabrication for in vivo drug delivery in complex media. I also investigated the buoyancy propelled MNMs in water remediation. The buoyancy propelled nanomotor exhibited reversible vertical motion in low concentrations of H2O2, which induced the convection of the micro-environment and increased the pollutants to get in contact with the absorbents. The proposed nanomotor showed efficient removal of both inorganic heavy metal ions and organic per- and poly-fluoroalkyl substances (PFAS) in complex environments. At last, the buoyancy propelled MNMs were studied for the vertically spatial separation of targeted cancer cells in mixed samples. With the aid of antibody surface modification, the buoyancy-propelled nanomotors can autonomously attach to the targeted cells and endow the cancer cells with vertical motion. With a customized glass tube, the floated cells can be easily separated. The proposed nanomotor exhibited great isolating efficiency with facile operations, which broadened the development of cell separation methods towards biocompatible nanostructures. The findings presented in this thesis open up new avenues for the development of buoyancy propelled MNMs in diverse applications.
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(2021)ThesisIn recent years, there has been an increased interest in utilising light and heat from the sun to produce solar fuels. Photo-assisted conversion of CO2 to CH4 is an effective and straightforward approach to produce solar fuels. The role of light, coupled with heat, for the methanation of carbon dioxide was investigated using different transition metal catalysts. The synergistic effect of heat and light was probed for the methanation of CO2 using Co, Cu and Ni supported on CeO2 and Al2O3. The role of light was explored using different wavelengths and intensities. Illuminating Co/CeO2 and Ni/CeO2 catalysts with blue light (450-460 nm) improved catalytic activity and selectivity. This improvement was attributed to the direct photoactivation of reaction intermediates on the catalyst surface by photogenerated hot electrons and the presence of intrinsic oxygen vacancies on CeO2. The absence of considerable light enhancement for Co/Al2O3 validated the importance of surface oxygen vacancies in accessing light enhancement. Based on the findings from the Co-based catalysis study, a detailed understanding of the reaction system was then developed by probing the role of surface basicity under light for CO2 methanation. The modification of commercial TiO2 with different loadings of La was investigated for a Co/La-TiO2-based catalyst. It was shown that homogeneously dispersing lanthanum on the surface of the TiO2 support boosted the catalytic performance of Co deposits under both non-illuminated (dark) and illuminated conditions. It was revealed that La promotion increased the surface basicity which facilitated CO2 adsorption and activation and allowed access to light enhancement. Subsequently, to gain further insights into the support properties which enable light enhancement during CO2 methanation, a Co/Al2O3 catalyst was modified with La and Pd via a double flame spray pyrolysis synthesis approach. La addition to the Al2O3 support played a crucial role, introducing actives site for CO2 adsorption and its potential transformation into an intermediate product which is more responsive under visible light illumination. In accompaniment to its plasmonic behavior, Pd metal addition to the Co/La-Al2O3 system facilitated H2 activation and provided further enhancement to the activity and CH4 yield. Overall, the research has demonstrated that through the rational design of transition metals loaded on a suitably active support the benefits of visible light illumination can be harnessed and can reduce the activation energy for thermal catalytic reactions.
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(2021)ThesisImprovements in liquid lithium-ion battery electrolytes using of metal organic frameworks (MOFs) as a functional decoration on polymer membrane separators were investigated using a combination of experimental and theoretical methods. Zirconium-based MOF UiO-66 was introduced to the polymer support using the mixed matrix membrane (MMM) method. The method allowed the one-step manufacture of a robust, mechanically pliable polymer-MOF membrane composite of high MOF loading. MOF-MMMs imparted improved electrochemical behaviours such as a widened potential operating window, near-unity transference number, and increased presence of solid electrolyte interphase (SEI) components crucial to battery performance. Density functional theory (DFT) calculations were also performed to provide insights regarding electrolyte solvation in the presence of MOF. A simple dip-coating technique was utilised to modify the surface of the MOF-MMMs with polydopamine (PDA) for further improvement of the electrochemical properties. Increased transference numbers, as well as stability during rate cycling, were observed with the resulting PDA-MMM owing to the improved electrode/electrolyte interface. However, surface analyses using x-ray photoelectron spectroscopy (XPS) showed that there are reduced amounts vital SEI components compared to the original MOF-MMM support. The last section further explores the versatility of UiO-66 and tackled the preparation of gel polymer electrolytes (GPEs) decorated with UiO-66 via phase inversion technique. Using the phase inversion method, the fabricated GPE contained pores from both polymer substrate and the intrinsic pores of the 3D nanomaterial for improvement of electrochemical properties. It was demonstrated in this work that the MOF GPE is equally inert and suitable in ether or carbonate-based electrolytes. Overall, this study demonstrated the versatility of UiO-66 metal organic frameworks for use as a functional nanofiller for electrolyte membranes. With the use of inexpensive membrane fabrication methods, the composites obtained are viable for lithium-metal battery applications. Similarly, insights drawn can provide a springboard towards future study of MOF-based electrolytes.
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(2022)ThesisThe polydisperse solid-liquid system has been practised in many chemical engineering applications. A fundamental understanding of complex multi-phase flow with a wide particle size distribution (PSD) in the system is beneficial for process control and reactor optimisation, yet the currently existing numerical models, including conventional computational fluid dynamics - discrete element method (CFD-DEM), fail to capture the cross-scale inter-phase/particle interactions. Accordingly, multi-resolution models are developed in this thesis for the high-fidelity simulation of polydisperse solid-liquid systems. 1) A smoothed volume distribution model (SVDM) is first developed based on the unresolved CFD-DEM framework, with the capability of simulating the polydisperse solid-liquid system with a coarse-to-fine particle size ratio of up to 20. Via studying the migration of fine particles in suspension flow through a packed bed of coarse particles, the migration mechanism of fine particles is proposed and the inherent fundamental of clusters are elucidated. Via investigating the bed hydrodynamics in a bi-disperse solid-liquid fluidised bed (SLFB), the segregation and mixing mechanisms of particles in solid-liquid systems are illuminated. Via quantifying the solid transportation behaviours during the rapid filtration of dual-media filters, a probabilistic model is derived and verified for predicting clogging performance. This work establishes an effective framework to handle complex polydisperse solid-liquid systems. 2) Two acceleration methods (i.e., coarse-grained method and machine learning method) are studied, with the capability of simulating solid-liquid systems with improved computational efficiency at spatial and temporal scales, respectively. The coarse-grained method is employed to simulate large-scale particulate systems for unveiling the sedimentation mechanism of particles in water. The machine learning method is used to predict mixing and segregation behaviours in a solid-liquid system. This work provides an efficient method to predict granular flow behaviours in solid-liquid systems. 3) Further, a hybrid CFD-DEM model combining the resolved and unresolved CFD-DEM frameworks is originally developed, with the capability of simulating the polydisperse solid-liquid system with unlimited coarse-to-fine particle size ratios, for the first time. A resolved part obtains the fluid details around each coarse particle without extra models using fine grids (i.e., grid size to particle diameter ratio, lm/dp < 1/10), an unresolved part describes the fluid-fine particle interactions with empirical correlations using coarse grids (lm/dp > 3), and a semi-resolved part denotes the medium particle behaviours with a kernel-based approximation using medium grids (1/10 < lm/dp < 3). This work delivers a novel idea for modelling cross-scale solid-liquid flow and has the potential application to any polydisperse solid-liquid systems. This thesis represents collection of a suite of innovative numerical works of polydisperse particulate flows in solid-liquid systems and provides a range of numerical tools for understanding and optimising polydisperse solid-liquid flow systems.
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(2022)ThesisUrban stormwater runoff possesses the properties of intermittent occurrence, unexpectable volume and variable pollution which lead to different environmental issues, including flooding and waterlogging, pollution transportation, damage to downstream and contamination of the receiving waters. On the other hand, the low-level contamination (relative to sewerage) and large volume supply of stormwater makes it suitable as an alternative water resource to relieve the water shortage in the urban areas. Stormwater harvesting is under the concept of Water Sensitive Urban Design (WSUD) trying to treat stormwater properly for the different end-uses (like irrigation, toilet flushing and even for the uses close to human contact). Several treatment technologies (e.g., biofilters, constructed wetlands) have already been implemented to purify the stormwater with effective performance prior to reuse. However, the refractory organic micropollutants (especially herbicides) presented resistance to these nature-based solutions by showing variable treatment outcomes. In order to provide harvested stormwater for end-uses with high quality requirement (e.g., close to human contact recreational waters), a reliable treatment technology for organic micropollutants is desired as a post-treatment method in the stormwater harvesting system. This thesis aims to develop advanced oxidation processes (AOPs), in particular, photoelectrochemical oxidation (PECO), as the post-WSUD treatment approach for stormwater using its oxidation capacity towards the refractory organic micropollutants. Following the technology development procedure, three steps have been conducted: (1) testing the feasibility of AOPs for stormwater herbicides treatment; (2) investigating the intrinsic mechanism in the stormwater herbicides degradation process; and (3) assessing the operation conditions impact towards PECO stormwater treatment system. Boron-doped diamond (BDD) anode was used in the preliminary lab-scale tests for the feasibility study of AOPs towards stormwater organic micropollutants (two representative herbicides, diuron and atrazine - selected as the target pollutants in the study). The results showed that the effective herbicides degradation could be achieved by PECO process under 5 V operation (which was regarded as the optimal voltage in the system). The positive impact coming from voltage increase has been found in the study. BDD showed a remarkably durable property with stable removal performance under challenging voltage application (9 V) without observed deterioration. The catalysts loading showed negligible effect in removal performance. While the thermal effect was observed as a supporting factor for the process (higher temperature supported the oxidation process). Since BDD is not a perfect choice for scaled-up implementation due to its high manufacture cost, carbon fiber anode was chosen in the following studies and operated under low voltage (2 V) to avoid the possible anode deterioration under high voltage application. In the mechanism investigation, the superoxide radicals were found to be the major reactive species in PECO process. Meanwhile, hydroxyl radicals and free chlorine also demonstrated supporting impact for the oxidation process. With the identified intermediate products, degradation pathways of diuron and atrazine were proposed for the first time for three AOPs (PECO, electrochemical oxidation (ECO) and photocatalytic oxidation (PCO)) in stormwater herbicides degradation process. PECO was certified to be the preferrable stormwater treatment technology with the ability for further oxidation reactions towards herbicides degradation compared with ECO and PCO. In the third study, a flow reactor was designed and used to test the impacts of operational conditions (flow rate, light intensity, and initial pollutant concentration) for PECO process. An obvious improvement was observed for flow rate towards removal performance, while the light intensity was found to influence atrazine removal only. The initial pollutant concentration study demonstrated the robust performance of PECO flow reactor towards herbicides removal under challenging (240 μg L-1) pollutant concentration condition. The real stormwater experiments suggested the possible impacts coming from the stormwater chemistry towards PECO process. Further based on the energy consumption analysis, high flow rate (610 mL min-1) and normal light intensity (100 mW cm-2) were regarded as the optimal operational conditions for flow reactor system. Also, the effective PECO degradation performance of herbicides under the real stormwater environment has been verified by using the stormwater collected from field as supporting electrolyte in the experiments. Overall, this thesis confirms PECO as a promising stormwater herbicides treatment technology (potentially for all organic micropollutants) to provide further purification for stormwater high-quality targets. It also discusses the implications for the practical implementation and points out the future research directions for the system optimization.
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(2022)ThesisZinc-azole-based metal organic frameworks (MOFs) have been demonstrated to exist in a wide variety of structural states, with applications in different fields such as gas separation. In this dissertation, we explore the phase control and dynamics of zinc-azolebased MOFs in crystalline, liquid, and glassy states. We first study ZIF-7 phase control using mechanochemical synthesis. Ammonium nitrate was found to be a good catalyst in mechanochemical ZIF formation, with the usage of DMF and H2O favouring ZIF-7-I and ZIF-7-III formation, respectively. New phases of ZIF-7 variations not accessible using the solvothermal method were also obtained mechanochemically indicating the possibility of a new mechanochemical synthesis route. The mechanochemical ZIF-7 mixed matrix membrane (MMM) demonstrates good CO2- based selectivity improvements. Next, we demonstrate the formation of a new meltable zinc-azole framework (ZnCP) with liquid crystal behaviour by the addition of orthophosphate. ZnCP was able to melt at a low temperature while retaining and orienting its crystallinity into transparent liquid, thus showing promising use in optical-based applications. This material can also be obtained using a top-down approach by adding phosphoric acid to ZIF-7. Controlling phosphoric acid incorporation results in different melted ZnCP particle ratios, which was explored as a gas separation membrane. We then explore the effect of silver (Ag) composite presence on the thermal dynamics of another zinc-azole framework (ZIF-62). The benzimidazole amount within the Ag-doped ZIF-62 structure affects its thermal conversion, forming either Ag-doped ZIF-zni or Agdoped ZIF glass. The thermal dynamics of Ag-doped structures were explored using both in-situ (thermal) and ex-situ techniques. Both Ag-doped phases were demonstrated to have good MMM separation improvements for CO2 and light hydrocarbon, indicating the accessibility of the silver composite. Lastly, a quick demonstration of new methods (dip and spin coating) to process ZIF-62 and ZIF-62 composite successfully forms continuous particle dispersion, allowing the formation of a continuous glass layer. Different compositions such as sandwiched structure and layer by layer were explored, with advantages outlined. The novelty of this dissertation lies within the exploration of new synthetic methods and thermal dynamics to form structurally diverse zinc-azole MOFs which will be beneficial in the understanding of phase transformations in MOFs
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(2022)ThesisA series of magnesium-based metal-organic frameworks (Mg-MOFs) were prepared and decorated with metal nanoparticles to develop hybrid sorbent/catalysts for carbon dioxide (CO2) capture and conversion to chemical fuels. A straightforward thermal treatment approach was first established to construct hybrid sorbent/catalysts. By exploiting the disparity in Ni-O and Mg-O coordination bond strength within mixed-metal NiMg-MOF-74, Ni2+ was selectively reduced to generate dispersed Ni nanoclusters constrained by the initial MOF pore diameter. By modifying the Ni:Mg ratio in the parent MOF, accessible surface area and framework crystallinity were tuned upon thermal treatment, influencing CO2 uptake and hydrogenation selectivity. The dispersed Ni nanoclusters proved to be an active catalyst towards CO2 hydrogenation, while the preserved section of Mg-MOF retained a portion of its CO2 adsorption capacity. To gain further insight into factors governing CO2 uptake and hydrogenation, the catalyst loading method was expanded to Mg-CUK-1. The optimised NiMg-CUK-1 was deployed within a dual-mode reactor system constructed for temperature swing CO2 capture and conversion to methane (CH4). Upon repeated cycles, the CH4 generated surpassed similar sorbent-based catalysts reliant on the chemical looping of metal oxides and carbonates. The approach highlighted the significant role of physisorption-based materials in facilitating low-temperature CO2 desorption, extending the working capacity of hybrid sorbent/catalysts. The dual-mode reactor system was extended towards industrially relevant CO2 capture conditions. Low catalyst loadings of Ru and Ni within stable Mg-CUK-1 enhanced overall system performance by preserving additional CO2 uptake. Simulated dry flue gas was deployed to probe the influence of oxygen exposure on catalyst performance. Ru-loaded Mg-CUK-1 sustained CH4 generation over ten cycles, while Ni-loaded Mg-CUK-1 registered a decrease in performance attributed to Ni passivation. When a portion of Ni was replaced with Ru, the resulting dual-metal catalyst emulated the performance of the monometallic Ru hybrid sorbent/catalyst. Ru was found to aid the re-reduction of surface Ni2+ to Ni0 upon oxygen exposure, which resulted in sustained CH4 generation. The strategy developed provided an approach to foster resilient hydrogenation catalysts capable of withstanding reactive species when exposed to point source CO2 emissions.
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(2022)ThesisTwo-dimensional transition metal dichalcogenide (TMD) nanocrystals (NCs) exhibit unique optical and electrocatalytic properties. However, the growth of uniform and high-quality NCs of monolayer TMD remains a challenge. Until now, most of them are synthesized via solution-based hydrothermal process or ultrasonic exfoliation method, in which the capping ligands introduced from organic solution often quench the optical and electrocatalytic properties of TMD NCs. Moreover, it is difficult to homogeneously disperse the solution-based TMD NCs on a substrate for device fabrication since the dispersed NCs can easily aggregate. Here, we put forward a novel CVD method to grow closely-spaced TMD NCs and explored the growth mechanism and attempts on the size control. Their applications acting as electrocatalysts and adhesion layer for Au film deposition have been also well displayed. Through the whole chapters of this thesis, the following aspects are highlighted: 1. MoS2 and other TMD nanocrystals have been grown on the c-plane sapphire. The surface oxygen vacancies determine the density of TMD nanocrystals. The MoS2 nanocrystals demonstrate excellent hydrogen evolution reaction and surface-enhanced Raman scattering performance owing to the abundant edges. 2. Deep insights into the growth of MoS2 nanograins have been explored. The surface step edges and lattice structures of the underlying sapphire substrates have a significant influence on the growth behaviors. The step edges could modulate the aggregation of MoS2 nanograins to form unidirectional triangular islands. The Raman spectra of MoS2 demonstrate a linear relationship with the crystal size of MoS2. 3. The orientation of sapphire substrate has an of importance effect on the critical size of MoS2 nanocrystals. The MoS2 nanocrystals have the smallest size on the r-plane sapphire, besides, the MoS2 on r-plane sapphire demonstrates the sintering-resistance feature, which is attributed to the edge-pinning effect when MoS2 edges are anchored on the sapphire surface. 4. The MoS2 nanocrystalline layer was utilized as the adhesion layer for Au film depositing on a sapphire substrate. The Au films on MoS2 displayed superior transmittance and electrical conductivity as well as outstanding thermal stability, which lay in the strong binding of Au film with MoS2 nanocrystalline layer.
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(2022)ThesisTo improve the imaging specificity for accurate diagnosis, nanoparticle-based contrast agents have been developed for magnetic bioimaging modalities including magnetic resonance imaging (MRI) and magnetic particle imaging (MPI). MPI, providing positive contrast, low tissue background (high signal-to-noise ratio) and unlimited tissue penetration depth, has great promise to become clinical imaging tool. Constructing the nanoprobe tailored for MPI based on its inherent properties is critical to achieve its potential. Tumour-targeted delivery of imaging nanoprobes also provides a versatile approach for precision diagnosis of diseases. MRI, owing to its excellent soft tissue sensitivity, high spatial resolution, and lack of ionizing radiation, has been considered as one of the imaging modalities to explore the imaging specificity. Our group has previously developed a tumour microenvironment-sensitive MRI contrast agent that can be further improved by increasing tumour cell targeting and tumour tissue penetration. Surface engineering of nanoparticles offers a critical strategy to improve tumour-targeting capacities of nanoprobes. Improvements to the efficacy of targeted nanoprobes have been intensively explored and much of this work centres on developing reliable and efficient surface functionalization strategies. Herein, in this thesis, various nanoparticles and surface modification approaches have been developed to improve imaging specificity for precision imaging diagnosis. Developing novel and promising nanoprobes based on their inherent properties and different surface modification strategies highlighting on nanoparticle engineering and emerging coating techniques has been described and discussed in this work.
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(2022)ThesisThermodynamic and kinetic limitations can restrict the feasibility and scalability of conventional thermal catalytic CO2 methanation. Due to non-equilibrium nature, nonthermal plasma (NTP) has potential to overcome reaction barriers and attain the substantial conversion even at low temperatures. However, the current understanding of the fundamental chemical and physical behaviours in the hybrid plasma-catalytic interactions are insufficient. This work focused on gaining insights into catalyst behaviour under plasma-driven CO2 methanation. The mutual interactions were systematically investigated through activity tests, characterization and in situ plasma diagnostics. To understand the role of Ni in plasma-driven methanation of CO2, a custom-designed plasma system was employed which exhibited the effective dissociation of CO2 into CO. Incorporation of Ni shifted the selectivity towards CH4 that can be attributed to plasma-catalyst synergy, where the formation of formate led the possible pathway for low temperature conversion. The optimum metal loading (10% Ni) demonstrated the conversion (60%) and selectivity (97%) at 150 oC. Moreover, the reciprocating effects of plasma influenced physical properties of catalysts; however, the catalysts exhibited high stability during the reaction. A comparative study for Ni supported catalysts was conducted with four different supports (Al2O3, CeO2, BaTiO3 and SrTiO3) to examine the influence of material properties on catalytic behaviour under plasma. This study elucidated the enhancement in the catalytic performance of the CeO2 and SrTiO3; where the basicity, metal support interaction, metal dispersion and the surface oxygen vacancies played a vital role in catalysts performance. Bimetallic nickel-iron catalysts were evaluated for plasma-catalytic CO2 methanation, which exhibited improved catalytic performance compared to their monometallic counterparts. The introduction of iron into nickel catalysts attributed to structural modification, modified redox properties, improved dispersion, and increased active sites. Overall, this study revealed the activation of reactant gases in plasma discharge, terminated in the dissociation of CO2 into CO. Ni based catalysts were studied and found viable for selective CO2 methanation. This work contributed to establishing the fundamental aspects of hybrid plasma-catalytic CO2 methanation whilst providing the new insights into plasma-catalyst interaction and methanation route under plasma.