Progress in Nanoporous Templates: Beyond Anodic Aluminum Oxide and Towards Functional Complex Materials

Successful synthesis of ordered porous, multi-component complex materials requires a series of coordinated processes, typically including fabrication of a master template, deposition of materials within the pores to form a negative structure, and a third deposition or etching process to create the final, functional template. Translating the utility and the simplicity of the ordered nanoporous geometry of binary oxide templates to those comprising complex functional oxides used in energy, electronic, and biology applications has been met with numerous critical challenges. This review surveys the current state of commonly used complex material nanoporous template synthesis techniques derived from the base anodic aluminum oxide (AAO) geometry

Multifunctional Lightweight Structures of Silicon Carbide Nanowires

Silicon carbide (SiC) as a type of ceramic material possesses unique properties such as high hardness, good high temperature strength, and excellent oxidation resistance. However, the intrinsic shortcomings of ceramic-based materials, such as high brittleness, low recoverable compressibility, and low fatigue resistance, prevent their utilisations as structural or functional components. To overcome these issues, highly porous lightweight and flexible SiC ceramics constructed by nanowires are promising alternatives for advanced engineering applications. The aim of this thesis is therefore to fabricate highly porous lightweight and flexible SiC nanowire structures by three novel approaches: (1) in-situ chemical-blowing; (2) melamine foam-based replica template; (3) electrospinning and explore their properties towards different applications. The overview, including the aims and objectives of this thesis is outlined in Chapter 1. The existing knowledge about lightweight SiCNW structures including crystallography, synthesis approaches, physical properties (mechanical strength, thermal conductivity, high temperature stability), and well-developed energy and environment-related applications (piezoresistive sensors, catalyst support, absorbers, and filters) is documented in Chapter 2. The generic information of the starting materials, synthesis techniques, equipment, and method used for the fabrication of 3D SiCNW structures, characterisation of their microstructural features, and evaluation of the various aspects of their multifunctionalities is descripted in Chapter 3. To identify suitable techniques to assemble SiC nanowires (SiCNWs) into 3D architectures, Chapter 4 provides a selection of advanced manufacturing approaches for lightweight SiCNW structures with easy and precise control of the overall shape and growth of SiCNWs. Followed with the demonstration of the exciting properties of the as-obtained three SiCNW structures including mechanical properties, thermal insulation performance, thermo-oxidation resistance, and fire-retardance in Chapter 5. Finally, based on their own characteristics, the applications of the SiCNW structures such as piezoresistive sensors, catalyst support, and efficient absorbents for oil and organic solvents are present in Chapter 6. A guidance in the manufacturing of advanced ceramic nanowire structures with desired microstructures and properties tailored for specific applications will be eventually provided. I first demonstrated the creation of SiCNW sponges by a facile template/catalyst-free sugar-blowing technique, by reacting SiO2 with sustainable kitchen sugar, using NH4Cl as a blowing agent. The as-grown, highly porous SiCNW sponges exhibited a core-shell structure, the core part with a density of 115-125 mg cm-3 was comprised of short and tangled SiC whiskers with SiC flakes embedded, while the shell layer with an ultralow density of ~25 mg cm-3 consisted of numerous smooth SiCNWs. These sponges exhibited a compressive modulus of ~389 kPa, recoverability under cyclic compression loading for 100 cycles at a strain of 20% and a thermal conductivity of 42-92 mW m-1K-1. Secondly, I reported the fabrication of SiCNW scaffolds with tuneable microstructures, densities, and therefore properties, by regulating the solid loading content in the reticulated melamine foam (MF) template. The resulting samples exhibited high strength (modulus up to ~167.3 kPa), good recoverability (11% residual strain and 72% maximum stress after 100 compressive cycles at a ε = 20%), and low thermal conductivity of 32-54 mW m-1K-1. Finally, I successfully created 3D SiCNW aerogels by using a Mille crêpe stacking and sintering of the electrospun PAN/SiO2 fibres for the first time. The resulting aerogels made of interconnected SiCNWs displayed an ultralight density of 29 mg cm-3, excellent compressive recoverability and fatigue resistance. Meanwhile, the SiCNW aerogels exhibited a thermal conductivity of 24 mW m-1K-1, even lower than that of the air, suggesting its superinsulation capability. Benefitting from intrinsic properties of SiC, experimental results have shown that all the as-obtained SiCNW structures exhibited good thermal insulation performance, exceptional high-temperature stability, fire-retardance, and temperature-invariant elasticity. Furthermore, I have explored the best-suited functional applications for each SiCNW structure. The SiCNW sponges and aerogels with better compressive recoverability and mechanical stability exhibited interesting electromechanical sensing capability. The sponge-based sensor exhibited a gauge factor up to 87 and stable wide-range compression-resistance responses. Whilst the aerogel-based strain sensor with higher recoverable strains presented stable sensing behaviour at different strains, frequencies, elevated temperatures over 200 °C and excellent repeatability over 2000 cycles. Owing to the cellular structure with the co-existence of SiC nanowires and struts, good interconnectivity, and competent mechanical strength and stability, the SiCNW scaffolds demonstrated the exclusive suitability as excellent support for MOF-derived TiO2-C catalyst, with ~35% enhanced in-situ loading of the catalyst, enabling a superior photocatalytic performance and good repeatability for at least 3 cycles. I further examined the SiCNW structures as organic solvent/oil absorbent. They exhibited rapid absorption of various organic solvents and oils. Typically, the SiCNW aerogels possess the highest absorption capacity of 32-86 g g-1, as well as robust recoverability. Meanwhile, the absorbed content can be easily removed by squeezing, distillation, and combustion, while the SiCNW structures remain unchanged. These features have shown that the SiCNW structures are promising for applications for the potential removal of chemical spills and oil leakage, with the advantage of easy recycling. All these remarkable findings will not only provide an important opportunity to advance the understanding of lightweight SiCNWs structures and make original contributions to utilise them as multifunctional devices, but also bring us the new ways to reshape the manufacturing of porous ceramics for future energy and environment-related applications

Study on Nano-Engineering of High-Capacity Anode Materials for High-Power Energy Storage System

Department of Energy Engineering(Battery Science and Technology)Nano-engineering and nanotechnology issue in various industry fields such as semiconductor, chemistry, energy solution, material science, and medicine. A definition of nanotechnology includes quantum mechanics, molecular chemistry, biology, and atomic level behaviors. Also, nanostructured materials (e.g., nanoparticle, nanorod, nanotube, nanowire, hollow, and yolk-shell) improve properties of materials for performance enhancement of devices. These nanomaterials have been synthesized using bottom-up and top-down approaches. In the early 2000s, many researchers garnered information and experiences about the nanotechnology that led to innovation and progress in industry and academy of science. As a result, many electronic devices were developed for a convenience of our life. Especially, significant advances of devices lead to the development of another new device with more improved performances including faster processing ability, longer working time, light weight, and easy transportation. In this regard, gradual development of energy storage system must need to satisfy this demand for new electric device (e.g. electric vehicle (EV), energy storage system (ESS), even drone) As one of the powerful energy storage systems, lithium-ion batteries (LIBs) are critically important to operate portable electronic devices. However, they cannot meet requirements for more advanced applications, like electric vehicles and energy storage systems due to limitations of conventional cathode/anode materials in high power and high energy density. To overcome these limitations, several strategies have been developed, including nanostructured design of electrode materials, coating of active materials with electrically conductive layers, and control of electrode architectures. Herein, we study on a simple, cost effective and unique synthesis method of various shaped functional materials by nano-engineering process in an each chapter. Also, we conduct research about a mechanism of reaction, key for synthesizing good materials, change of chemical reaction in experiment. So, the developed materials appear outstanding properties such as structural stability, chemical stability in electrochemical test, and mainly used energy storage system like LIBs. In chapter III, we demonstrate a simple route for fabricating trench-type copper patterns by combining a photo-lithography with a wet etching process. Nanostructured CuO was grown on the patterned Cu current collectors via a simple solution immersion process. And silicon nanoparticles were filled into the patterned Cu current collectors. The strongly immobilized CuO on the patterned Cu exhibited high electrochemical performance, including a high reversible capacity and a high rate capability. In chapter IV, we demonstrate multi-scale patterned electrodes that provide surface-area enhancement and strong adhesion between electrode materials and current collector. The combination of multi-scale structured current collector and active materials (cathode and anode) enables us to make high-performance Li-ion batteries (LIBs). When LiFePO4 (LFP) cathode and Li4Ti5O12 (LTO) anode materials are combined with patterned current collectors, their electrochemical performances are significantly improved, including a high rate capability (LFP : 100 mAhg-1, LTO : 60 mAhg-1 at 100 C rate) and highly stable cycling. Moreover, we successfully fabricate full cell system consisting of patterned LFP cathode and patterned LTO anode, exhibiting high-power battery performances. We extend this idea to Si anode that exhibits a large volume change during lithiation/delithiation process. The patterned Si electrodes show significantly enhanced electrochemical performances, including a high specific capacity (825 mAhg-1) at high rate of 5 C and a stable cycling retention. In chapter V, Chemical reduction of micro-assembled CNT@TiO2@SiO2 leads to the formation of titanium silicide-containing Si nanotubular structures. The Si-based nanotube anodes exhibit a high capacity (>1850 mAh g-1) and an excellent cycling performance (capacity retention of >99% after 80 cycles). In chapter VI, we revisit the metallothermic reduction process to synthesize shape-preserving macro-/nano-porous Si particles via aluminothermic and subsequent magnesiotheric reaction of porous silica particles. This process enables us to control the specific capacity and volume expansion of shape-preserving porous Si-based anodes. Two step metallothermic reactions have several advantages including a successful synthesis of shape-preserving Si particles, tunable specific capacity of as-synthesized Si anode, accommodation of a large volume change of Si by porous nature and alumina layers, and a scalable synthesis (hundreds of gram per batch). An optimized macroporous Si/Al2O3 composite anode exhibits a reversible capacity of ~1500 mAh g-1 after 100 cycles at 0.2 C and a volume expansion of ~34% even after 100 cycles. In chapter VII, we report a redox-transmetalation reaction-based route for the large-scale synthesis of mesoporous germanium particles from germanium oxide at temperatures of 420 ~ 600 oC. We could confirm that a unique redox-transmetalation reaction occurs between Zn0 and Ge4+ at approximately 420 oC using temperature-dependent in situ X-ray absorption fine structure analysis. This reaction has several advantages, which include (i) the successful synthesis of germanium particles at a low temperature (∼450 oC), (ii) the accommodation of large volume changes, owing to the mesoporous structure of the germanium particles, and (iii) the ability to synthesize the particles in a cost-effective and scalable manner, as inexpensive metal oxides are used as the starting materials. The optimized mesoporous germanium anode exhibits a reversible capacity of∼1400 mA h g-1 after 300 cycles at a rate of 0.5 C (corresponding to the capacity retention of 99.5%), as well as stable cycling in a full cell containing a LiCoO2 cathode with a high energy density. In chapter VIII, we report a unique synthesis of redox-responsive assembled carbon-sheathed germanium coaxial nanowire heterostructures without a need of metal catalyst. In our approach, germanium nanowires are grown by reduction of germanium oxide particles and subsequent self-catalytic growth mechanism during thermal decomposition of natural gas, and simultaneously, carbon sheath layers are uniformly coated on the germanium nanowire surface. This process is a simple (one-step process), reproducible, easy size-controllable and cost-effective (mass production) process which total mass of metal oxides can be transformed into nanowires. Furthermore, the germanium nanowires exhibit outstanding electrochemical performance including capacity retention of ~96% after 1000 cycles at 1C rate as lithium-ion battery anode.ope

Bio-inspired dewetted surfaces based on SiC/Si interlocked structures for enhanced-underwater stability and regenerative-drag reduction capability

Drag reduction has become a serious issue in recent years in terms of energy conservation and environmental protection. Among diverse approaches for drag reduction, superhydrophobic surfaces have been mainly researched due to their high drag reducing efficiency. However, due to limited lifetime of plastron (i.e., air pockets) on superhydrophobic surfaces in underwater, the instability of dewetted surfaces has been a sticking point for practical applications. This work presents a breakthrough in improving the underwater stability of superhydrophobic surfaces by optimizing nanoscale surface structures using SiC/Si interlocked structures. These structures have an unequaled stability of underwater superhydrophobicity and enhance drag reduction capabilities, with a lifetime of plastron over 18 days and maximum velocity reduction ratio of 56%. Furthermore, through photoelectrochemical water splitting on a hierarchical SiC/Si nanostructure surface, the limited lifetime problem of air pockets was overcome by refilling the escaping gas layer, which also provides continuous drag reduction effects.119Ysciescopu

Field-Effect Transistors for Gas Sensing

This chapter reviews gas-sensitive field-effect transistors (FETs) for gas sensing. Although various types of gas sensors have been reported, this review focuses on FET-based sensors such as catalytic-gate FETs, solid electrolyte-based FETs, suspended-gate FETs, and nanomaterial-based FETs. For recognition of analytes in the gas phase, the combination of cross-reactive gas sensor arrays with pattern recognition methods is promising. Cross-reactive sensor arrays consist of gas sensors that have broad and differential sensitivity. Signals from the cross-reactive sensor array are processed using pattern recognition methods. Reports of FET-based sensor arrays combined with pattern recognition methods are briefly reviewed

Caracterización estructural y funcional de películas delgadas nanoporosas mediante microscopías electrónicas de transmisiónbarrido y espectroscopías ópticas

Nano-structuration of materials at the mesoscale to give rise to porosity-controlled coatings represents an important breakthrough in the area of Materials Science and Engineering, offering new and enhanced functionalities of interest in fields such as optics, optronics and optoelectronics. In order to optimize their performances, in-depth analyses are required: local information about the morphology, composition and atomic structure, the compactness distribution, but also layer homogeneity, interface and interpenetration between stacked layers or oxidation are extremely important factors that can ruin their way of operation. In this particular context, the objective of the present PhD Thesis is to make significant contributions to the study and development of multifunctional porous nanostructured systems, from their design and elaboration, to the maximum knowledge of their structure and properties, through advanced (S)TEM methods, including 3D reconstructions, elemental analyses at the nanoscale and atomic-scale imaging, combined with optical spectroscopy techniques. In the first instance, given the great potential of the slanted nanostructures generated by means of oblique angle depositions, in which the refractive index gradient can be tuned by the columns tilt and density imposed via the growth angles and parameters, OAD broadband antireflective coatings based on Si, Ge or SiO2 OAD films have been designed, manufactured, and extensively characterized with the aim of maximizing the performance of the optical elements in the vis-IR wavelength range. This same approach has also been implemented to enhance the antireflective capabilities of transparent conductive ITO thin films in the near-IR window without compromising too much their electrical response. On the other hand, the advanced structural and functional characterization of porosity-controlled GaN NW arrays grown by plasma-assisted MBE through (S)TEM methods and vis-IR SE elliposmetry, has helped not only to improve growth processes but also to optimize their resulting optical and electrical properties. Finally, the knowledge and methodologies acquired during the study and optimization of the previous porous systems have been transferred to the development of a two-step procedure, based on the deposition and the subsequent fast oxidation of vanadium-based OAD films in open air atmosphere, for the synthesis of thermochromic VO2 coatings of tunable metal-to-insulator response and controlled grain sizes and crystallinities

Fabrication and characterization of Silicon Nanowires

In this project work Si nanowires were fabricated on the Si substrate by aqueous method. In this aqueous method Ag is used for electroless chemical etching. The precursors those were taken are AgNO3, HF and H2O2. Si nanowires are fabricated at 55⁰C. The samples were characterized by X-ray diffraction and scanning electron microscope. Result shows morphology of the Si nanowires by scanning electron microscope. X-ray diffraction confirms the phase Si. The XRD analysis confirms the phase of silicon and crystallinity nature of silicon .It is found to be single crystalline with plane (1 0 0).The SEM study shows that the particles were uniform and afterwards the non uniformity arises. At 60 second of electroless deposition, the particles shape became anisotropic. Some of the particles have grown vertically. This kind of non uniform pattern can cause a non uniform distribution of Silicon nanowires. It is confirmed that the the morphology of the nanowires also depends on the resistivity of the wafers. The magnified HRTEM image shows the well-resolved lattice spacing of the silicon nanowire, which depicts the crystalline nature of the silicon nanowires