| Author: | Xu, Zhihang |
| Title: | Unravelling structure evolution of metal oxides during intercalation and encapsulation reactions using transmission electron microscopy |
| Advisors: | Zhu, Ye (AP) |
| Degree: | Ph.D. |
| Year: | 2024 |
| Department: | Department of Applied Physics |
| Pages: | xvi, 105 pages : color illustrations |
| Language: | English |
| Abstract: | Nanoscale structural changes in materials have profound impacts on their properties, leading to exciting advancements in various scientific and technological fields. The ability to decipher and understand the structure of materials at the nanoscale level opens up new possibilities for tailoring the properties of materials to meet specific application requirements. Transmission electron microscopy (TEM) has emerged as a powerful tool for exploring and understanding nanoscale structures in diverse fields of science and technology. On the one hand, TEM contributes to our understanding of fundamental material characteristics with its exceptional spatial resolution and ability to probe nanoscale features of materials. On the other hand, TEM plays a crucial role in investigating dynamic structural changes and transformations that occur at the nanoscale. Real-time observation and tracking of structural transformations such as diffusion, nucleation, growth and phase transitions through advanced in-situ microscopy techniques provide valuable insights into the kinetics, mechanisms, and pathways of these processes. In this thesis, the nanoscale structure evolution induced by (de)protonation and encapsulation is investigated by TEM characterization and in-situ microscopy. Protonation is a fundamental chemical reaction with broad application prospects in the fields of energy, environment, and memory devices. Probing the protonation mechanism, however, presents a formidable challenge owing to the elusiveness of intercalated protons. We directly imaged the proton intercalation pathway in α-MoO3 induced by UV illumination, using atomic-scale structure changes detected along cross-sectional samples by high-resolution TEM and electronic structure changes reflected from the optical absorptance properties. We reveal the anisotropic intercalation behaviour which is initiated by photocatalyzed water dissociation preferentially at the (001) edges and then propagates along the c axis, transforming α-MoO3 into HxMoO3 to realize photochromism. The observed anisotropic behaviour can be attributed to the intrinsically low energy barriers for both proton migration along the c axis and water dissociation at (001) edges. The identified proton intercalation behaviour inspires us to further investigate proton deintercalation behaviour by heating HxMoO3. Combining in-situ/ex-situ TEM with optical microscopy reveals the proton deintercalation behaviour in air, Ar and vacuum environments. Upton heating in air, proton deintercalation shows anisotropic behaviour with the preferential diffusion along the c axis, consistent with the proton intercalation, whereas under heating in an anaerobic environment, the anisotropy disappears and random deintercalation occurs. We demonstrate that the deintercalation anisotropy is attributed to oxidation reaction occurring at the (001) edges and its resulting recovery of α-MoO3. Core-shell metal catalysts have shown great capabilities in the modern chemical industry because their strong overlayer prevents severe ripening and coalescence of metal nanoparticles while becoming a heterogeneous phase to regulate the active sites on the metal surface. A new core-shell catalyst Co@BaAl2O4 was prepared by thermal reduction of Co3O4/BaCO3/Al2O3 mixture, which showed excellent catalytic performance in NH3 decomposition. The synthesis method effectively avoids metal coalescence induced by thermal activation, showing great potential to construct core-shell catalyst in low-melting point metals such as Co, Ni and Cu. However, the reason for its encapsulation is not yet clear. In order to explore the encapsulation mechanism, we performed in-situ atmospheric STEM to reveal the morphological changes and phase transitions during the entire reaction process from Co3O4/BaCO3/Al2O3 precursor to Co@BaAl2O4 core-shell catalyst. A series of physical and chemical reactions were demonstrated, including BaCO3 diffusion, Co3O4 reduction and BaAl2O4 formation. Based on the in-situ findings, a general and straightforward approach was proposed for achieving thermal-induced carbonate encapsulation on metal nanoparticles. |
| Rights: | All rights reserved |
| Access: | open access |
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