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The research for developing sustainable and clean energy conversion and storage technologies (such as rechargeable metal−air batteries, fuel cells, etc.) has attracted tremendous attention over past decades. Among which, the development of cost-effective and high-performance electrocatalysts for the sluggish oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is crucial but still challenging for practical applications. Therefore, the primary objective of this thesis was to rationally design carbon-based ORR and OER electrocatalysts for applications in zinc-air batteries by a cost-effective strategy. The research background and theoretical knowledge are summarized in Chapter 1 and Chapter 2. Several novel strategies for the preparation of oxygen electrocatalysts and air-cathodes in zinc-air batteries are proposed and presented in the following Chapters. Firstly, considering intrinsic microporosity, high specific surface area and nitrogen rich ligands in ZIF-67 (zeolitic imidazolate framework-67, is a sub-family of metal-organic frameworks with zeolite-like structure, consist of cobalt metal centres and imidazolate linkers), and the hierarchical pore structure with lower oxygen content in thermal-shock exfoliated graphene oxide (EGO), a series of ZIF-67@EGO hybrid nanostructures were successfully synthesised via a highly controllable and facile method. The influence of ZIF-67 loading ratio in the hybrids and the carbonization temperature for the preparation of the final oxygen electrocatalysts was systematically investigated, an optimal hybrid nanostructure was achieved to produce highly efficient bifunctional oxygen electrocatalyst. The zinc-air battery based on this catalyst exhibits a high peak power density of 175 mW cm-2 and specific capacity of 767 mAh g-1, as well as superior long-term cycling stability (Chapter 3). Subsequently, inspired by the first study, a further low-temperature thermolysis strategy was developed to produce ultra-small Co3O4/Co nanoparticles in nitrogen-doped hyperporous graphenic networks (Co3O4/Co@N-G). By utilising the residual oxygen functionalities of the EGO and low-gasification point (≈350 °C) of the ZIF-67, the catalyst, Co3O4/Co@N-G, was developed at moderate conditions, ≈450 °C in nitrogen only atmosphere. The as-synthesised catalyst without any further acid washing or oxidation process, exhibits excellent ORR performance. In addition, this low-temperature route yields a high amount of the catalyst (>65 wt%), which is far higher than the commonly reported (<30 wt%) high-temperature derivatives. This study shows not only a new method of producing high-performance ORR catalyst with high yield and low energy consumption, but also an effective way of controlling metal aggregation and in-situ oxidation of MOF (ZIF) structures (Chapter 4). Moreover, a self-activation phenomenon of carbon paper cathode substrates in zinc-air batteries was discovered and systematically explored. It was found that the air-cathodes generated from electrochemical activation (during the galvanostatic discharge/charge process) of normal carbon paper substrates without any additional electrocatalysts can be directly used in zinc-air batteries. Therefore, this new method for making air-cathodes is scalable, extremely facile and low-cost. The self-activated carbon paper substrate exhibits an impressive cycling stability (more than 165 hours for 1,000 cycles) and a small discharge-charge voltage gap. After the activation, the maximum power density and electrochemical surface area were increased by over 40 and 1920 times, respectively. The mechanism behind this enhancement was revealed by multiscale simulations and comprehensive characterizations (Chapter 5). Overall, step-by-step research was performed to develop new strategies and new nanomaterials with reduced cost for application in oxygen electrocatalysis. The achievements in this thesis pave a novel and cost-effective pathway for rational design of carbon-based heterostructures as high-performance oxygen electrocatalysts and can be further applied in diverse energy conversion and storage technologies.

This thesis presents an investigation of the hydrogen transfer reactions between dihydrofolate (H2F) and NADPH that are catalysed by the dihydrofolate reductase (DHFR) isolated from Geobacillus stearothermophilus (BsDHFR) as well as an artificial hybrid originating from the DHFRs from mesophilic Escherichia coli (EcDHFR) and hyperthermophilic Thermotoga maritima (TmDHFR). A broad spectrum of studies, focusing on the relationship between structure, thermostability and kinetics, showed that the catalytic behaviour of BsDHFR is generally similar to other monomeric DHFRs, including ones found in the mesophile Escherichia coli and the psychrophile Moritella profunda, but significantly different from the dimeric TmDHFR. The fact that all monomeric DHFRs display similar catalytic behaviour, regardless of their widely different optimal temperatures, suggests that thermostability does not directly relate to catalytic efficiency. The biophysical differences between monomeric DHFRs and TmDHFR are likely derived from the dimeric nature of the hyperthermophilic enzyme. An artificial dimeric variant of EcDHFR, Xet-3, was prepared by introducing residues at the dimer interface of TmDHFR. While thermostability of this variant is enhanced, it showed a great decrease in its steady-state and pre-steady-state rate constants. Given that the corresponding rate constants did not increase when the loops are released in the monomeric variant of TmDHFR, the lowered catalytic ability in Xet-3 is likely a consequence of geometric distortion of the active site and loss of loop flexibility that is catalytically important in EcDHFR. In contrast, the relatively poor activity of TmDHFR is not simply a consequence of reduced loop flexibility; the dimer interface of TmDHFR plays a rather complicated role in catalysis.

The success of carbon nanotubes and graphene drives people to continuously search for novel low dimensional materials. Among the various discovered two-dimensional (2D) materials, 2D semiconductors (2DSCs) has been most extensively studied due to their unique electronic, optoelectronic, and mechanical properties. In terms of fundamental semiconducting performance, the ultra-thin body of 2DSCs enable strong electrostatic gate control, which is favored for high output current and ultra-low leakage current for field effect transistors. Meanwhile, the atomic thin film of 2DSCs is free of dangling bonds due to its layered structure, which contributes to low interfacial density of state and sharp turn on of density of states at the band edges. Those electronic properties are specially desired for short channel field effect transistor and tunneling transistor. Besides, the layer dependent band structure offers tremendous options for opto-excitation detection and heterostructures. However, the research of 2DSCs is limited on transition metal dichalcogenides (TMDCs) such as MoS2 and WSe2. Experimental studies of 2DSCs on other group compounds has little been reported. Besides, detailed studies of charge transport in 2DSCs transistor in both lateral and vertical direction requires further understanding; studies on metal/semiconductor contact and interface engineering in 2DSCs heterostructure potentially enable fascinating electronic functions. In this thesis, I will present my effort on uncovering a new group of 2DSCs, charge transport in 2DSCs transistor and their heterostructure with either other 2DSCs or metals. I will include the work from two of my first author publication. I will first present my research studies on characterizing a new group of 2DSCs from IV-V group compound, 2D GeAs for the first time. By examining the semiconducting properties of few-layer GeAs transistors, we achieve high ON-OFF ratio of 105 and maximum hole field-effect mobility of 99 cm2 V-1s-1at room temperature. Due to highly anisotropic crystal structure, highly angle dependent optical and electronic transport is also observed. To reveal the intrinsic carrier transport, we carried out systematic studies on Schottky barrier and contact resistance. The intrinsic mobility proves charge impurity scattering limited transport of GeAs transistor. At last, we demonstrate mid-infrared photodetector by utilizing the band gap of GeAs crystal. Then I will show our investigation of charge transport in multilayer MoS2 transistor with optimized van der Waals contact. For the first time we demonstrate a vertical built-in potential in 2DSCs that induces negative transconductance (NTC) behavior. By varying measurement temperature, body thickness and terminal numbers, we reveal that the NTC originates from the vertical barrier due to distinct doping type and carrier distribution. We further demonstrate a frequency doublers and phase shift keying circuits based on single transistor by utilizing the NTC phenomena. Furthermore, I will present a study on tunneling transistor based on 2DSC SnSe/GeAs heterostructure. By stacking two flexible n-type and p-type semiconductor, we acquire a hetero-pn junction. The cryo-temperature study of the heterostructure proves a type III band alignment and Esaki type diode, which indicates intrinsic tunneling current at low bias. The subthreshold swing and gate coupling efficiency has also been discussed. For the final part, we will discuss about a novel approach to enable memristive switching behavior on metal and 2DSCs interface. With alignment transfer technique, we can integrate active metal with fragile 2DSC without introducing any destruction on 2DSC. Due to gentle interdiffusion of active metal and native oxide on 2DSC surface, a resistive switching layer forms and enable switching with ON/OFF ratio up to three orders. Resistive random-access memory and synaptic modulation has been realized with our metal/oxide/2DSC memristor. Moreover, we examine the working principle of our memristor and propose the formation and dissolution of metal oxide as the working mechanism. These findings and engineer could provide exciting opportunities for 2DSCs for novel electronics and optoelectronics.

Abstract: This research aims at treating mature fine tailings (MFT) with a novel pH-responsive copolymer poly (N,N-dimethylaminoethyl methacrylate-co-acrylamide-co-acrylic acid) p(DMAEMA-co-AM-co-AA). Compared with conventional polyacrylamide (PAM), the conformation and property of this novel copolymer can be switched by pH variation, enhancing the compaction of settled/flocculated tailings furthermore. The copolymer was synthesized with 16, 20 and 24 hours respectively. The characterization, by mass spectrometry, Fourier Transform Infrared spectroscopy (FTIR), Nuclear Magnetic Resonance (NMR), turbidity and viscosity measurements, of this novel copolymer were conducted. Mechanism of conformational change by pH-switching was proposed. The flocculation of this copolymer was investigated to determine the optimal dosage. Moreover, the compaction of settled/flocculated tailings was achieved by changing the pH due to the change of conformation. Initial settling rate, supernatant turbidity, solid content, yield stress and water drainage of the sediment were examined to study the optimal condition of pH-responsive flocculant for treating MFT. QCM-D was used to study the adsorption of this copolymer on the silica surface and the copolymer’s conformational change at varied pHs.

The first part of the dissertation introduces several new methods for estimating the structure of graphical models. Firstly, we consider estimating graphical models with discrete variables, including nominal variables and ordinal variables. For the nominal variables, we prove the asymptotic properties of the joint neighborhood selection method proposed by Hoefling and Tibshirani (2009) and Wang et al. (2009), which is used to fit high-dimensional graphical models with binary random variables. We show that this method is consistent in terms of both parameter estimation and structure estimation and extend it to general nominal variables. For ordinal variables, we introduce a new graphical model, which assumes that the ordinal variables are generated by discretizing marginal distributions of a latent multivariate Gaussian distribution and the relationships of these ordinal variables are described by the underlying Gaussian graphical model. We develop an EM-like algorithm to estimate the underlying latent network and apply the mean field theory to improve computational efficiency. We also consider the problem of jointly estimating multiple graphical models which share the variables but come from different categories. Compared with separate estimation for each category, the proposed joint estimation method significantly improves performance when graphical models in different categories have some similarities. We develop joint estimation methods both for Gaussian graphical models and for graphical models for categorical variables. In the second part of the dissertation, we develop two methods to improve interpretability of high-dimensional unsupervised learning methods. First, we introduce a pairwise variable selection method for high-dimensional model-based clustering. Unlike existing variable selection methods for clustering problems, the proposed method not only selects the informative variables, but also identifies which pairs of clusters are separable by each informative variable. We also propose a new method to identify both sparse structures and “block” structures in factor loadings in principal component analysis. This is achieved by forcing highly correlated variables to have identical factor loadings via a regularization penalty.

The phenomenon that amorphous polymers are not in thermodynamic equilibrium at temperatures below their glass transition was observed by several scientists. Being in unstable non-equilibrium states, the amorphous solids undergo slow processes which attempt to establish equilibrium. This gradual approach to equilibrium is termed physical aging. Physical aging will affect all those temperature-dependent properties which change drastically and abruptly at T$\sb{\rm g}$, because it is a gradual continuation of the glass formation that sets in around T$\sb{\rm g}$. When the polymer is cooled to some temperature below T$\sb{\rm g}$, the transport mobility will be small but not zero. Since at this stage the free volume is greater than it would be at equilibrium, the volume will continue to decrease slowly. This contraction will be accompanied by a decrease in the transport mobility with concomitant changes in all those properties of the glassy polymer which depend on it. In the pharmaceutical film coating process, some low-molecular-weight diluents, plasticizers, are added to polymers to modify their physical properties and to improve their film forming characteristics. Aqueous film coating techniques have obtained a lot of attention in the pharmaceutical industry in recent years, since the aqueous, collidal latex or pseudolatex dispersions provide revolutionary coating capabilities for totally water-based systems at a time when solvent coating has become increasingly unattractive from cost, environmental, and safety standpoints. Therefore, the specific aims of this research are to: (1) study the effect of physical aging on the dissolutions of hydroxypropyl methylcellulose phthalate film-coated tablets and granules and correlate it with the mechanical behavior of free films; (2) investigate the effects of physical aging and plasticizers on the water permeability of cellulose acetate and polyethylene glycols film-coated tablets and correlate with the mechanical behavior of free films; (3) investigate the effects of plasticizer and physical aging on the mechanical and mobility properties of ethylcellulose organic solvent and aqueous based membranes and the effect of film forming temperature on the properties of latex films, and (4) correlate the effects of physical aging on the dissolution and permeation properties with changes in the mechanical properties of these polymer films and estimate long-term aging effects on the dissolution rates and water permeabilities of the controlled release polymer film coatings.