Additive Manufacturing of 3D Nano-Architected Metals and Ceramics

Additive manufacturing (AM) represents a set of manufacturing processes that create complex 3D parts out of polymers, metals, and ceramics. AM of metals and ceramics is widely used to produce parts for aerospace, automotive, and medical applications. At the micro- and nano-scales, AM is poised to become the enabling technology for efficient 3D microelectromechanical systems (MEMS), 3D micro-battery electrodes, 3D electrically small antennae, micro-optical components, and photonics. Today, the minimum feature size for most commercially available metal and ceramic AM is limited to ~20-50 μm. Currently, no established processes can reliably produce complex 3D metal and ceramic parts with sub-micron features. In this thesis, we first demonstrate a nanoscale metal AM process that can produce ~300 nm features out of nanocrystalline, nanoporous nickel using synthesized hybrid organic-inorganic materials, two-photon lithography, and pyrolysis. We study microstructure and mechanical properties of as-fabricated nickel architectures and compare their structural strength to established AM processes. We then show how this process can be extended to other metals and metalloids, including Mg, Ge, Si, and Ti. This study extends further into nanoscale AM of transparent, high refractive index materials for micro-optics and photonic crystals. We develop an AM process to 3D print fully dense nanocrystalline rutile titanium dioxide (TiO₂) with feature dimensions down to ~120 nm. We carefully study and model the relationship between feature dimensions and process parameters to achieve a &#60;2% variation in critical dimensions. We then use this understanding of the process to fabricate and study 3D dielectric photonic crystals with a full photonic bandgap in the infrared. Finally, a microscale AM process of titanium dioxide is demonstrated for photocatalytic water treatment. We show how synthesized hybrid organic-inorganic materials can be applied for stereolithography to print TiO₂ architectures with 100 μm features. We use the developed 3D printing process to investigate the effect of 3D architecture on the efficiency of photocatalytic water treatment. This work establishes a versatile and efficient pathway to create three-dimensional nano-architected metals and ceramics and to investigate their properties for applications in 3D MEMS, micro-optics, photonics, and photocatalysis.</p

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

Fabrication of Semiconductor with Modified Microstructure for Efficient Photocatalytic Hydrogen Evolution Under Visible Light

Since sustainable energy and environment emerging as one of the top issues and challenges for humanity, the photocatalytic hydrogen evolution under visible light has attracted increasing attention. Basically the separation and transmission of photogenerated charge carriers are the two main steps of a photocatalytic reaction. They should be key aspects in the design of efficient photocatalysts for solar energy conversion

Synthesis and Characterization of Transition Metal Oxide and Dichalcogenide Nanomaterials for Energy and Environmental Applications

Transition metal oxides (TMOs) and transition metal dichalcogenides (TMDs) have gained immense interest recently for energy and environmental applications due to their exceptional structural, electronic, and optical properties. For example, titanium dioxide (TiO2) as one of the TMO photocatalysts has been widely studied due to its stability, non-toxicity, wide availability, and high efficiency. However, its wide bandgap significantly limits its use under visible light or solar light. Recent studies also show that semiconducting TMDs could be used as potential supercapacitor electrode materials and platinum (Pt)-free electrocatalysts for economical utilization of renewable energy, because the high cost and scarcity of Pt have impeded the large-scale commercialization of many green technologies. In this dissertation study, various novel TMO and TMD nanomaterials are designed and synthesized, and their catalytic performance is further investigated. First, a facile route for the controllable synthesis of modified TiO2 is designed to improve its photocatalytic efficiency under the visible/solar light. The resulting Ti3+-doped TiO2 with tunable photocatalytic properties using a hydrothermal method with varying amounts of reductant, i.e., sodium borohydride (NaBH4), showed color changes from light yellow, light grey, to dark grey with the increasing amount of NaBH4. The present method can controllably and effectively reduce Ti4+ on the surface of TiO2 and induce partial transformation of anatase TiO2 to rutile TiO2, with the evolution of nanoparticles into hierarchical structures attributing to the high pressure and strong alkali environment in the synthesis atmosphere; in this way, the photocatalytic activity of Ti3+-doped TiO2 under visible-light can be tuned. The band gap of Ti3+-doped TiO2 based on the Kubelka-Munk function is 3.1 eV, which is smaller than that of pristine TiO2 (3.28 eV), confirming that adding NaBH4 as a reductant causes the absorption edge of TiO2 to shift to a lower energy region. After 20 min of simulated sunlight irradiation of photocatalytic reactions for the degradation of methylene blue (MB) aqueous solution, nearly 97.2% of MB was degraded by the sample TiO2-4 (reduced by 12 g of NaBH4 in the hydrothermal reaction), compared with the degradation efficiency of the pristine TiO2 (23.5%). The as-developed strategy may open up a new avenue for designing and functionalizing TiO2 materials with enhanced visible light absorption, narrowed band gap, and improved photocatalytic activity. Second, cobalt sulfide-based (CoSx) nanostructures as one of the TMDs are competitive candidates for fabrication of supercapacitor electrodes due to their high specific surface area, high electrical conductivity, and redox-active structures. However, CoSx materials still suffer from relatively low specific capacitances, degradation of performance over long cycling duration, and tedious synthesis and assembly methods. Hence, metallic vertically-aligned cobalt pyrite (CoS2) nanowires (NWs) are prepared directly on current collecting electrodes, e.g., carbon cloth or graphite disc, for high-performance supercapacitors. These vertically-aligned CoS2 NWs have a variety of advantages for supercapacitor applications. Because the metallic CoS2 NWs are synthesized directly on the current collector, the good electrical connection enables efficient charge transfer between the active CoS2 materials and the current collector. In addition, the open spaces between the vertical NWs lead to a large accessible surface area and afford rapid mass transport. Moreover, the robust CoS2 NW structure results in high stability of the active materials during long-term operation. Electrochemical characterization reveals that the CoS2 NWs enable a large specific capacitance (828.2 F/g at a scan rate of 0.01 V/s) and excellent long-term cycling stability (0-2.5% capacity loss after 4,250 cycles at 5 A/g) for pseudocapacitors. This example of vertically-aligned metallic CoS2 NWs for supercapacitor applications expands the opportunities for transition metal sulfide-based nanostructures in emerging energy storage applications. Third, to combine the advantages of TMOs and TMDs, an aerosol processing method is developed for the facile and green synthesis of reduced graphene oxide (rGO)/tungsten disulfide (WS2)/tungsten trioxide (WO3) ternary nanohybrids, because both TMOs and TMDs are promising candidates for platinum-free electrocatalysts in renewable energy applications. The resulting hybrid material has a spherical structure constructed of crumpled graphene and WS2/WO3 nanorods. The crumpled graphene/WS2/WO3 (CGTH) catalyst showed a superior electrocatalytic activity in the hydrogen evolution reaction (HER), with a Tafel slope of 37 mV/dec and an onset potential of 96 mV. Compared with reported MoS2/WS2-based electrocatalysts, this hybrid material shows one of the highest catalytic activities in HER. The environmentally-friendly synthesis and outstanding performance suggest a great potential of CGTH for noble metal-free electrocatalysts in water splitting. Next, in order to improve the specific capacity of lithium-ion batteries (LIBs)/ potassium-ion batteries (PIBs) and relieve volume expansion of nanoparticles to fulfill the urgent need of reliable energy storage applications, TMD nanomaterials especially MoS2 quantum dots (QDs) have been considered promising anode materials for LIBs owing to their higher theoretical capacity and better rate capability compared with commercial graphite anodes. An exfoliated mesoporous MoS2 QDs-graphite composite anode was designed and investigated. The MoS2 QDs are located in the void spaces between graphite particles, thereby preventing the graphite particles from losing electrical contact with the current collector and enhancing the cycling performance of the MoS2/graphite composite anode. The optimized MoS2 QDs with graphite composites displayed good charge/discharge characteristics and the capacity maintained at 449.8 mAh g-1 after 300 charge/discharge cycles for LIBs. And the MoS2 QDs for PIB cells exhibited a stable capacity of approximately 409 mAh g-1 for 17 cycles. Finally, metal-organic frameworks (MOFs) have attracted substantial research attention owing to their tunable pore size, high pore volume, high specific surface area, and highly ordered crystalline porous networks. Previous studies have mostly focused on sensing, drug delivery, batteries, and selective catalysis; however, their application as photocatalysts has not been thoroughly reported. It is well known that bulk MoS2 is unsuitable for photocatalytic applications due to the insufficient reduction and oxidation ability for the photocatalysis. However, exfoliated MoS2 exhibits a direct band gap of 2.8 eV resulting from quantum confinement, which enables it to possess suitable band positions and to retain good visible-light absorption ability. As a result, it is considered to be a promising candidate for photocatalytic applications. Encapsulating exfoliated MoS2 into MOF exhibits enhanced absorption in the visible light range compared with pure MOF and the highest hydrogen production rate could reach 68.4 μmol h-1g-1, which is much higher than that on pure MOF. With suitable band structure and improved light-harvesting ability, exfoliated MoS2@MOF can be a potential photocatalyst for hydrogen production. This dissertation study suggests that modified TiO2 and exfoliated MoS2@MOF can be efficient photocatalysts with enhanced visible light absorption ability; metallic CoS2 NWs could be active materials with a large specific capacitance and excellent stability; reduced graphene oxide (rGO)/tungsten disulfide (WS2)/tungsten trioxide (WO3) as a ternary nanohybrid offers advantages of TMOs and TMDs, making it an outstanding noble-metal free electrocatalyst in water splitting; and MoS2 QDs with relieved volume expansion are promising anode materials for LIBs/PIBs. The study provides a scientific foundation to design and discover low-cost, efficient and stable TMOs and TMDs candidates for suitable energy and environmental applications

Fabrication of Bi2 WO6 photoelectrodes with enhanced photoelectrochemical and photocatalytic performance

This is the final version. Available from Elsevier via the DOI in this record.Visible light active semiconductor Bi 2 WO 6 photoelectrodes with desired physical and chemical properties are sought for solar energy conversion and photocatalytic applications. The porous nanostructured Bi 2 WO 6 photoelectrodes are prepared by Spray Pyrolysis (SP). A detail study has been conducted to correlate the annealing temperature, morphology and crystallographic orientation with the photoelectrochemical (PEC), electrochemical and photocatalytic properties. The photoelectrodes possess an optical bandgap of 2.82 eV and exhibit anodic photocurrent. The current-voltage characterization of Bi 2 WO 6 photoelectrodes reveals that the photocurrent density and photocurrent onset potential is strongly dependent on the deposition parameters. The PEC study shows that the photoelectrode annealed at 525 °C has photocurrent density of 42 μAcm −2 at 0.23 V (vs Ag/AgCl/3M KCl) under AM1.5 illumination and exhibit superior photocatalytic activity for Rhodamine B (RhB) degradation. The electrochemical study shows that the photoelectrode has flatband potential of 2.85 V which is in good agreement with photocurrent onset potential. This finding will have a significant influence on further exploitation of Bi 2 WO 6 as a potential semiconductor material in solar energy conversion and photocatalytic applications.The Saudi Arabian Cultural BureauEngineering and Physical Sciences Research Council (EPSRC

Tantalate-based Perovskite for Solar Energy Applications

To realize a sustainable society in the near future, the development of clean, renewable, cheap and sustainable resources and the remediation of environmental pollutions using solar energy as the driving force would be important. During the past few decades, plenty of efforts have been focused on this area to develop solar light active materials to meet the increased energy and environmental crisis. Owning to the unique perovskite-type structure, tantalate-based semiconductors with unable chemical composition show high activities toward the conversion of solar radiation into chemical energy. Moreover, various engineering strategies, including crystal structure engineering, electronic structure engineering, surface/interface engineering, co-catalyst engineering and so on, have been developed in order to modulate the charge separation and transfer efficiency, optical absorption, band gap position and photochemical and photophysical stability, which would open a realm of new possibilities for exploring novel materials for solar energy applications

Synthesis, Chracterization and Applications of Coated Composite Materials for Energy Applications

The formulation of coated composite materials is an important field of research around the world today. Coated composite materials include inhomogeneous and anisotropic materials. These materials are formulated by an amalgamate minimum of two or more materials that accommodate different properties. These materials have a vast field of appealing applications that encourage scientists to work on them. Due to their unique properties, such as their strength, liability, swiftness, and low cost, they are used as promising candidates for reliable applications in various fields, such as biomedical, engineering, energy devices, wastewater treatment, and agriculture. Different types of composite materials have had a noticeable impact in these fields already, such as glass, plastic, and, most promisingly, metal oxide nanoparticles

Self-Supported Three-Dimensional Quantum Dot Aerogels as a Promising Photocatalyst for CO2 Reduction

With the merits of quantum dots (QDs) (e.g., high molar extinction coefficient, strong visible light absorption, large specific surface area, and abundant functional surface active sites) and aerogels (e.g., self-supported architectures, porous network), semiconductor QD aerogels show great prospect in photocatalytic applications. However, typical gelation methods rely on oxidative treatments of QDs. Moreover, the remaining organic ligands (e.g., mercaptoacids) are still present on the surface of gels. Both these factors inhibit the activity of such photocatalysts, hampering their widespread use. Herein, we present a facile 3D assembly of II–VI semiconductor QDs capped with inorganic (NH4)2S ligands into aerogels using H2O as a dispersion solvent. Without any sacrificial agents, the resulting CdSe QD aerogels achieve a high CO generation rate of 15 μmol g–1 h–1, which is 12-fold higher than that of pristine-aggregated QD powders. Our work not only provides a facile strategy to fabricate QD aerogels but also offers a platform for designing advanced aerogel-based photocatalysts

A Review on Photocatalytic Glass Ceramics: Fundamentals, Preparation, Performance Enhancement and Future Development

Photocatalytic technology is considered as one of the most attractive and promising technologies to directly harvest, convert and store renewable solar energy for generating sustainable and green energy and a broad range of environmental applications. However, the use of a photocatalyst in powder or coating forms restricts its applications due to its disadvantages, such as difficulty in recovery of nano-powder, secondary pollution, low adhesion between photocatalytic coating and substrate material, short service life of photocatalytic film and so on. The investigation and application of photocatalytic glass-ceramics (PGCs) in water purification, bacterial disinfection, self-cleaning and hydrogen evolution have received extensive attention due to their inherent advantages of low cost, easy fabrication, transparency, chemical and mechanical stability. Real-time solutions to energy shortage and environmental pollution faced by the development of human society can be provided by rationally designing the chemical composition and preparation methods of glass ceramics (GCs). This review introduces the concept and crystallization mechanism of PGCs and expounds on the basic mechanism of photocatalysis. Then, the key point difficulties of GCs’ design are discussed, mainly including the methods of obtaining transparency and controlling crystallization technologies. Different modification strategies to achieve better photocatalytic activity are highlighted. Finally, we look forward to further in-depth exploration and research on more efficient PGCs suitable for various applications