Structurally ordered porous materials from bottom-up self-assembly and transient Joule heating
Highly crystalline and well-ordered mesoporous metal oxide and carbon structures possess many desirable functional attributes, including high specific surface area, regular pore size and accessible pore networks, high mechanical and thermal stabilities, as well as tunable chemical reactivity. These...
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| Format: | Thesis-Doctor of Philosophy |
| Language: | English |
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Nanyang Technological University
2023
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| Online Access: | https://hdl.handle.net/10356/172852 |
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| Institution: | Nanyang Technological University |
| Language: | English |
| Summary: | Highly crystalline and well-ordered mesoporous metal oxide and carbon structures possess many desirable functional attributes, including high specific surface area, regular pore size and accessible pore networks, high mechanical and thermal stabilities, as well as tunable chemical reactivity. These diverse materials properties increase their overall appeal for a wide range of sustainable fields and applications, particularly, energy storage, energy conversion, and carbon sequestration. Utilizing block copolymers as a structure directing agent for organic/inorganic additive provides a direct and scalable route to design highly ordered mesoporous organic/inorganic materials with different periodic 2D/3D morphologies, specific properties and functionalities.
However, significant challenges persist, specifically in terms of optimizing the interactions between the block copolymers and additives, chemical reaction rates, as well as developing suitable post-process steps to remove the copolymers and induce crystallization of additives to obtain the desired functionalities. This thesis aims to explore new chemistries of block copolymer-additive systems and to develop a novel transient Joule heating approach that facilitates the formation of highly crystalline and ordered mesoporous transitional metal oxide structures within seconds.
The first study introduces a new self-assembly mixing protocol that expands the F127-Al2O3 mesotructural morphology space and allows the formation of new ordered mesophases, including the 2D lamellar and hierarchically ordered aluminum oxide (Al2O3) structures with interconnected macroscale pores and ordered hexagonal mesopore channels. This advancement is made possible via the independent preparation of stable aluminum alkoxide-derived sol precursors before combining with the structure-directing F127 triblock copolymer, thereby enhancing the overall control and facilitation of the self-assembly process. Subsequent furnace annealing at 900 °C yields well-ordered mesoporous Al2O3 materials that exhibit excellent performance in carbon dioxide adsorption and Congo red dye separation.
However, furnace annealing at high temperatures over extended durations is generally inefficient and can result in mesostructural collapse and loss of textural properties. Hence, I develop a novel general Joule heating-based approach termed as self-assembly and nonequilibrium annealing phase transformation (SNAP) to quickly anneal free-standing monoliths of well-ordered and highly crystalline mesoporous oxide and carbon materials. The SNAP method is highly versatile and enables formation of various block copolymer-directed metal oxide and carbon systems, including γ-Al2O3, γ-Al2O3/MgO, anatase-TiO2, α-Fe2O3 and graphite carbon structures within a matter of second time frames. The resultant SNAP-derived crystalline mesoporous inorganic materials have shown excellent performance in proof-of-concept mesoporous TiO2/Li-ion batteries and carbon dioxide adsorption applications.
Finally, I present a comprehensive account of the structural evolution of ordered mesoporous γ-Al2O3 and MgxAlyO4 spinel structures under transient Joule heating. Quantitative analysis of the ex situ X-ray diffraction data sets that fit well with the Johnson-Mehl-Avrami (JMA) model and Arrhenius equation reveal an accelerated surface-nucleation and crystallite growth mechanism. In particular, the SNAP method promotes the formation of non-stoichiometric, yet well-ordered, mesoporous MgxAlyO4 spinel structures.
The development of these new soft matter-directed self-assembly synthesis strategies, coupled with nonequilibrium Joule heating, along with advancements in our fundamental understanding of structure formation thermodynamics and kinetics, may unlock new opportunities for scalable production of high-quality novel functional nanostructures suitable for emerging applications and various fields, including sustainable manufacturing and environmental remediation. |
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