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Semiconductive metal oxides are of great importance in environmental remediation and electronics because of their ability to generate charge carriers when excited with appropriate energy. The electronic structure, light absorption and charge transport properties have made the transition metal oxides an attractive material as photocatalyst. Recently, facet-engineering by morphology control has been intensively studied as an efficient approach to further enhance their photocatalytic performance. However, various processing steps and post-treatments used in the preparation of facet-engineered particles may generate different surface active sites which may affect their photocatalysis. Moreover, many traditional techniques (PL, EPR and XPS) used for materials characterization (oxygen vacancy, hydroxyl group, cation, etc.) are not truly surface specific but analyzing a range from surface few layers to bulk. Accordingly, they can only provide very limited information on chemical states of the surface active features and their distribution among facets, causing difficulties to unambiguously correlate facet-dependent results with activity. As a result, this often leads to different interpretations amongst researchers during the past decades. As the publications of titanium and zinc ranked top two among studies of first row of transition oxides in the past decades, this thesis will firstly review on the disagreements generated among researchers when they correlated the performance of ZnO and TiO2 with their facet activities based on traditional techniques. As there are shortcomings of these techniques in producing truly facet-dependent features, some results can be misleading and with no cross-literature comparison. To address these issues, we have developed a new technique "probe-molecule-assisted NMR" which allows a genuine differentiation of surface active sites from various facets. This surface-fingerprint technique has been demonstrated to provide both qualitative (chemical shift) and quantitative (peak intensity) information on the concentration and distribution of truly surface features among facets. In light of the new technique, this thesis will revisit the facet-dependent photocatalytic properties and shed light on these issues.

The use of depleting fossil fuels for energy generation and producing various valuable chemicals is the major contributor to climate change. The clean energy sources are way forward to mitigate the devastating impacts of climate change. Hydrogen has a great potential to be a clean energy source when produced using renewable sources such as solar and wind. Another potential clean energy source is nuclear power which provides an effective source of baseline carbon-free energy. Electrochemical water splitting is the pre-eminent technology to access carbon-neutral hydrogen but is notoriously limited by reaction performance of anodic oxygen evolution reaction (OER), which often occurs at a large overpotential with poor reaction kinetics. Another strategy to make hydrogen generation more economically beneficial is to replace half-cell anodic OER with biomass representative compound 5-hydroxymethyl furfural electrooxidation (HMFOR). Among the reported transition metal based electrocatalytic nanomaterials, layered double hydroxide (LDH) based electrocatalysts are especially promising for OER and HMFOR, as the ability to modulate metal chemistries within hydroxide lattice allows for optimized structure and electronics. LDHs are also promising as adsorbent materials for removal of metals from aqueous solutions, including U removal from seawater and spent nuclear fuel. This thesis focuses on the development of LDH based engineered nanomaterials for electrochemical OER, HMFOR and recovery of uranium from seawater. The nanomaterial's structure and property relationships have been systematically established for a specific application using suite of atomic-scale characterization tools and computational approaches. First, the OER benchmark has been set using vacancy engineered Co-Ni LDH electrocatalysts. We achieved improved OER performance owing to oxygen vacancy generation due to Ce doping. Further, MgAl based LDHs have been designed for enhanced uranium recovery from seawater. Then, vacancy mediated HMF conversion mechanism has been established using defective Ce/La doped NiFe LDHs. Ce doped NiFe being rich in VO demonstrated higher FE to FDCA.

In recent years, the urgent need for efficient energy technologies such as electrocatalysis and energy storage has attracted great attention around the world. Among them, the electrocatalytic is one of the most promising green hydrogen production approaches. For consideration of limited earth reserves, exploring the earth-abundant non-noble metal catalysts for highly efficient is necessary. Manganese-based electrocatalysts including manganese oxides and manganese sulphides are promising candidates owing to their high natural abundance and cheap price. However, some key factors such as exploring facile and effective methods to synthesize Mn-based catalysts, optimize the phase and morphology, enrich the active sites and vaccines, and control the electronic structure still need deep study in the current stage. Compared to the hydrogen evolution reaction (HER) process, the oxygen evolution reaction (OER) process requires the transfer of four electrons with a higher reaction energy barrier. It is very important to investigate the highly efficient OER catalysts to break through the technology of hydrogen production from overall electrocatalytic. Hence, in this work, the controllable phases of MnO2 catalysts for bifunctional water splitting, the modified MnO2 catalyst with rich oxygen vacancies for OER, Co doped MnO catalyst for OER and MnS/Ni3S2 catalyst for OER have been designed and deeply studied. The as prepared samples show superior catalytic performance with low overpotential, small Tafel slopes, enhanced conductivity, and outstanding durability. The mechanisms of enhanced electrocatalytic performance have been investigated as well. It is found that both crystalline structures and electronic structures have a strong effect on the catalytic performance of Mn-based catalysts. Firstly, the phases and morphologies directly affect the exposure of the active sites. Secondly, the high content of Mn3+ sites and oxygen vacancies could be highly improved the water splitting performance. Thirdly, superior band alignment tunability allows for efficient improvement of the electrocatalytic performance. Finally, the specific element doping results in the lower work function could accelerate the charge transfer process of the OER activity. This work has demonstrated feasible ways to fabricate efficient electrocatalysts, which may provide key strategies to design advanced metal oxides for practical water splitting devices in the future.

This thesis studied an emerging electronic device, the memristor, to gain a fundamental understanding of the switching characteristics of different device structures. Memristors with a thin film hafnium oxide (HfO2) switching layer and a phase change material (PCM), germanium telluride (GeTe), thin film switching layer are studied. This work investigated a variety of electrical characterization experiments to determine the core functionally, robustness, and neuromorphic attributes of the two different memristor devices. The electrical biasing comprised of endurance, stability, plasticity, multi-state, and synaptic plasticity characterization. The HfO2 based memristors were determined to have multiple stable resistance states when restricting the applied current at different values. These limits were 10 µA, 15 µA, 30 µA, 50 µA, and 300 µA and as the allowed current increased the lower the measured resistance would be. This study also explored transmission electron microscopy (TEM) to determine structural changes of the GeTe memristors due to electrical and thermal stimuli. The TEM results for the (PCM) showed similar structural changes near the GeTe and top electrode interface when comparing the results from both stimuli.

In this dissertation, there are three topics including nanoscale engineering effects on cathodes for intermediate-temperature solid oxide fuel cells, effects of mechanical and electrical application on atomic force microscopy, and ionic diffusivity and mobility study of oxygen reduction/evolution reaction by nanoscale in situ characterization via atomic force microscopy. Their abstracts are listed below. Topic I: The impact of various infiltrates on the kinetics and rate-limiting step of oxygen reduction reaction (ORR) is examined with LaNi0.6Fe0.4O3-δ (LNF) as the cathode backbone of solid oxide fuel cells (SOFC). Multiple materials were infiltrated on the backbone. The dependencies of electrode polarization resistance on the precursor concentration, temperature, and oxygen partial pressure are presented, and related discussions are made to interpret the differences in ORR kinetics and the rate-determining step for ORR. Topic II: Conductive atomic force microscopy (CAFM) has been widely employed to study the localized electrical properties of a wide range of substrates in non-vacuum conditions by the use of noble metal-coated tips. In this topic, the impact of mechanical and electrical stimuli on the apex geometry of gold-coated tips and electrical conduction properties at the tip-substrate contact is discussed by choosing gold and highly-ordered pyrolytic graphite as the representative tip and substrate materials, respectively. Topic III: The ionic transport in solid oxides after becoming thermally activated is understood as hopping to point defects (i.e. oxygen vacancies), and the kinetics behind it are highly dependent upon the bonding state of the ionic species with their surrounding lattice environment. In this study we report the findings of our recent efforts to directly observe ionic transport in SrTiO3 (STO) and Y2O3-stabilized ZrO2 (YSZ)–the most widely used cathode material in intermediate temperature SOFC–by use of Kelvin probe force microscopy (KPFM) and conductive atomic force microscopy (CAFM).

The oxidative damage deriving from reactive oxygen species (ROS) is the primary cause or aggravation of most human pathologies, including cancer. Accordingly, considerable efforts have been undertaken in order to develop relevant antioxidant therapies to protect cells from oxidative damage and treat the resultant pathologies. To these ends, cerium oxide (CeO2) has attracted considerable attention because it can act as a biocompatible antioxidant and prooxidant that can eliminate or create ROS under basic and acidic conditions, respectively, thereby offering the potential to regulate redox metabolism. The relevant variables of pH conditions, chemistry of physiological environment, and exposed crystallographic planes of CeO2 can determine the nature of these redox reactions. In the present work, density functional theory (DFT) calculations are used to interpret, at the atomic scale, the pH-driven mechanisms for the switchable biocatalytic properties of CeO2. Analyses of surface-adsorbed media (principally H2O, H3O+, and OH⁻) and ROS (superoxide and peroxide) reveal that, under basic conditions, the capacity for the reversible Ce3+ and Ce4+ switching and the associated formation and annihilation of oxygen vacancies is preserved owing to complete filling of oxygen vacancies by the abundant OH⁻ ions. Consequently, the superoxide dismutase (SOD) and catalase (CAT) biomimetic reactions can be performed cyclically, thereby scavenging and decomposing ROS into harmless products, thus making CeO2 an excellent antioxidant. However, under acidic conditions, this reversible switching is hindered owing to incomplete filling of oxygen vacancies by H2O molecules owing to their larger size. Consequently, a Fenton biomimetic reaction occurs simultaneously with the SOD and CAT biomimetic reactions, resulting in the formation of destructive hydroxyl radicals, thus making CeO2 a cytotoxic prooxidant. Furthermore, the influence of the exposed crystallographic planes on the design of biocompatible CeO2 is demonstrated through the elucidation of the effects of the three most stable low-index crystallographic surfaces ({100}, {110}, and {111}) on surface silanisation. The results reveal that the cubic {100} surface yields the most favourable morphology for surface silanisation owing to its lower steric hindrance and higher density of surface-active sites.

Emerging pollutants generated from pharmaceuticals and personal care products and their potential effect on public health have become a growing concern in recent years. In contrast to conventional treatment techniques, catalytic ozonation is an effective method for removing these pollutants from wastewater. The main aim of this thesis is to investigate the capacity of oxygen vacancies and their electron density to boost the generation of hydroxyl radicals and reactive oxygen species (ROSs) from ozone and consequently improve the mineralisation of organic pollutants. This study chose Cerium oxide as the primary catalyst due to its redox properties and surface oxygen vacancy capacity. Three different approaches are employed in this thesis to regulate the surface oxygen vacancies and explore their effect on catalytic ozonation performance. As the first approach, UV light pre-treatment was used to boost surface oxygen vacancies. Spectroscopic analysis showed that UV light pre-treatment could promote the surface oxygen vacancies and consequently enhance the catalyst performance. A mechanistic study revealed that molecular ozone plays the main role in the initial degradation of aromatic organics and breaking of the benzene ring, while hydroxyl radicals are essential in the mineralisation of later intermediates to CO2 and H2O. To understand the influence of different types of oxygen vacancies (based on their charge state) on catalyst performance, CeO2-SiO2 binary oxide composites (CexSi1-xO2-) were fabricated as the second approach. Experimental and theoretical analyses revealed that oxygen vacancies with two electrons (OV0), existing as peroxide species, are more effective than other types of oxygen vacancies for activating oxygen and ozone to form ROSs. The results also demonstrated that OV0 clusters were more favourable than single OV0 sites for mineralising organics. Key active species were explored in the presence of several radical scavengers, showing that the contribution of •OH for catalytic ozonation is greater than •O2 and surface OH groups. Accordingly, the generation of H2O2 from O2 via peroxide species is proposed as the primary source of •OH generation through ozone molecules. To gain further insight into the role of oxygen vacancies and electron flow on the surface of the catalyst, CeO2 was dispersed on the surface of carbon fibres (CFs). In this approach, the surface chemistry of the CFs was modified using K2S2O8 and H2O2 to promote the electron flow and localised electrons on oxygen vacancies. CF was selected as a support due to its potential for exposing more active sites and providing more localised -electrons. It was found that modifying the surface of CFs to obtain structurally ordered support with surface oxygen functional groups offered better electron transfer capabilities and generated more OV0. Overall, this study demonstrated that high dispersity and greater free-electron availability (in the form of two electron-containing oxygen vacancies) can promote the capability of CeO2 as a catalyst for the catalytic ozonation reaction. The new knowledge enables the development of efficient and low-cost catalysts for catalytic ozonation and other oxidation reactions.

This dissertation focuses on nanoscale phase manipulations of Vanadium Dioxide. Nanoscale control of material properties is a current obstacle for the next generation of optoelectronic and photonic devices. Vanadium Dioxide is a strongly correlated material with an insulator-metal phase transition at approximately 345 K that generates dramatic electronic and optical property changes. However, the development of industry device application based on this phenomenon has been limited thus far due to the macroscopic scale and the volatile nature of the phase transition. In this work these limitations are assessed and circumvented. A home-built, variable temperature, scanning near-field optical microscope was engineered for Vanadium Dioxide manipulations and detections. Using this instrument, various scanning probe lithography based methods are implemented to induce new nanoscale phases. Three new phase transitions are discovered; a monoclinic metallic at the nanoscale, a rutile metallic metastable phase, and a van der Waals layered insulator. These new phases are studied and characterized to further understand phase manipulations in strongly correlated materials. One of the new phase transitions, monoclinic metallic, showcases plasmonic excitations. This phenomenon is used to demonstrate various nanoplasmonic devices such as rewritable waveguides, spatially modulated resonators, and reconfigurable planar optics. Finally, Oxygen Vacancy diffusion of the monoclinic structure is monitored to determine the temporal limitation for device applications. The discovery, demonstration, and study of these phases clearly shows the ability to manipulate Vanadium Dioxide on the nanoscale for the first time. Phase control is accomplished under ambient conditions and is stable over long periods of time. This technology opens the door for multifunctional device application using strongly correlated materials.

To maintain the fast growth of the economy worldwide, it is urgent to explore renewable energy sources, such as tidal, wind and solar energy. The current generated from these renewable energy sources needs to be stored for resupplying which requires advanced electrochemical energy storage and conversion systems. Developing electrode materials with high electrochemical performance is key to achieve efficient energy storage and energy conversion. Tungsten oxide, one of the most abundant metal oxides in the earth's crust, has attracted wide attention in electrochemical applications. In the first part of the research, the vacancy-rich tungsten oxide is tailored by the nitrogen heteroatoms. Consequently, the as-prepared tungsten oxide will N-filled oxygen vacancies exhibited significantly enhanced lithium-ion storage performance compared to the vacancy-rich tungsten oxide and tungsten trioxide. It has been found that the nitrogen heteroatoms play a crucial role in facilitating the charge transfer and maintain the internal channels for fast lithium-ion diffusion. The next part of the research involves designing the heterostructure of surface-confined amorphous tungsten oxide on tungsten carbide (a-WOx/WC), as an efficient catalyst for hydrogen evolution reaction (HER). The a-WOx/WC exhibited a significantly enhanced HER activity compared to pure tungsten oxide and pure tungsten carbide. Furthermore, the active sites were examined by in situ X-ray absorption spectroscopy (XAS) and density-functional theory (DFT) calculations. It has been found that the surface-confined a-WOx plays a key role in tuning the electronic properties of tungsten carbide, which enables the formation of favorable catalytic state under negative potential for efficient HER. In the last part of the research, Pt nanoparticles anchored on the vacancy-rich tungsten oxide substrate (WOx-Pt) is prepared in comparison to the pure carbon substrate. Different from other traditional methods, it is easier to achieve the strong interaction between substrate and Pt nanoparticles via the in situ electrochemical depositions. As a result, the WOx-Pt shows an enhanced HER activity compared to Pt nanoparticles deposited on the carbon substrate in both acidic and neutral conditions. Vacancy-rich tungsten oxide, as a co-catalyst, can help to cleave the H−OH bond, which facilitates the water dissociation process during HER in the neutral condition.

Conjugated p-p bonded polymers offer a wide range of new electronic devices which have developed as a unique niche in the marketplace with an ever-growing need for integration. In particular, polymer-based tunnel diodes (PTDs) which exhibit negative differential resistance (NDR) at room temperature can be integrated with other novel components to realize memory and logic cells for highly manufacturable, roll-to-roll (R2R) printed electronics. The research presented here focuses primarily on the fabrication and operating principles behind PTDs. By incorporating an ultra-thin TiO2 interfacial tunneling barrier into a modified organic light emitting diode (OLED) structure, reproducible NDR can be realized. By varying the properties of the interfacial tunneling oxide, characteristics of NDR such as the peak-valley-current-ratio (PVCR), peak current density (J_peak), and the voltage at the peak density (V_peak) can be improved for memory and logic.This work successfully demonstrates room temperature NDR in PTDs using ultra-thin TiO2 interfacial tunneling barriers grown via atomic layer deposition (ALD). The intention of this work is to present a viable prototype PTD using ALD to deposit the tunneling barrier. By taking a look at the physical and electrical behavior behind the ALD deposited films, a better understanding can be gained on the nature of interfacial layer.It is suggested that localized defect states caused by oxygen vacancies induced during oxide growth is behind the tunneling behavior observed in the PTDs. By controlling the oxide growth, the crystal structure can be altered in order modify the oxygen vacancy concentration and therefore improve PVCR. Therefore, a key aspect of this thesis will be to observe how morphology, realized through varying temperature of ALD growth, can affect device characteristics. Additionally, to fully classify these devices, the physics behind the electrical operation needs to be further evaluated. Mapping the properties of the various materials through experimentation and modeling will serve as the starting place for future work to come.Finally, this thesis is part of an ongoing exploration for low-power, low-cost printed electronics. Therefore, a key aspect of this work is to present an argument for a printable process on a flexible, plastic substrate, and as such the requirement for a low temperature deposition is imperative. Low temperature ALD can come in the form of alternative precursors or alternative tunneling oxides. Moreover, by choosing to use alternative oxides, lower power NDR appearing at lower voltages may be realized. This study begins the work on finding alternative tunneling oxides that demonstrate similar oxygen vacancies observed in TiO2 films. In this case, Ta2O5 replaces TiO2 as the tunneling barrier. The initial data is promising, demonstrating a drop in the NDR voltage by approximately half compared to its TiO2 counterpart. Moreover, this work bolsters the claim that NDR is the result of a trap-based tunneling event through a defined defect band in the ALD deposited tunneling oxides. Though this thesis focuses solely on PTDs, the materials and processes demonstrated can be applied towards research interested in the conductivity properties of metal-oxides in addition to being useful for further work performed in the field of plastic electronics.

Resistive random access memory (RRAM), based on a two-terminal resistive switching device with a switching element sandwiched between two electrodes, has been an attractive candidate to replace flash memory owing to its simple structure, excellent scaling potential, low power consumption, high switching speed, and good retention and endurance properties. However, due to the current limited understanding of the device mechanism, RRAMs research are still facing several issues and challenges including instability of operation parameters, the relatively high reset current, the limited retention and the unsatisfactory endurance. In this study, we investigated the switching mechanisms, conditions and applications of oxygen vacancy (Vo) filament formation in resistive memories. By studying the behavior of conductive Vo nanofilaments in several metal/oxide/metal resistive devices of various thicknesses of oxides, a resulting model supported by the data postulates that there are two distinct modes of creating oxygen vacancies: i) a conventional bulk mode creation, and ii) surface mode of creating oxygen vacancies at the active metal-dielectric interface. A further investigation of conduction mechanism for the Vo CF only based memories is conducted through insertion of a thin layer of titanium into a Pt/ Ta2O5/Pt structure to form a Pt/Ti/ Ta2O5/Pt device. A space charge limited (SCL) conduction model is used to explain the experimental data regarding SET process at low voltage ranges. The evidence for existence of composite copper/oxygen vacancy nanofilaments is presented. The innovative use of hybrid Vo/Cu nanofilament will potentially overcome high forming voltage and gas accumulation issues. A resistive floating electrode device (RFED) is designed to allow the generation of current/voltage pulses that can be controlled by three independent technology parameters. Our recent research has demonstrated that in a Cu/TaOx/Pt resistive device multiple Cu conductive nanofilaments can be formed and ruptured successively. Near the end of the study, quantized and partial quantized conductance is observed at room temperature in metal-insulator-metal structures with graphene submicron-sized nanoplatelets embedded in a 3-hexylthiophene (P3HT) polymer layer. As an organic memory, the device exhibits reliable memory operation with an ON/OFF ratio of more than 10.