| dc.description.abstract | The physical, chemical, and mechanical properties of materials often depend strongly on the functional defects of various dimensionalities, such as vacancies, interfaces, and precipitates. Therefore, defect engineering lies at the heart of materials innovations for the next-generation technologies. However, due to the enormous span in the length scale (from electrons to meters), such a bottom-up approach can be extremely challenging. This thesis aims to contribute to bridging this gap based on the following. First, by combining well-controlled synthesis, state-of-the-art defect characterization, and multiscale defect modeling, I developed an experimental and analysis framework to identify the critical functional defects during catalytic reactions and solid-state phase transformations in functional oxides. Second, I have utilized and explained the effects of external stimuli, specifically with lattice strain, oxygen chemical potential, chemical doping, and ion irradiation, in tailoring the concentration and stability of these critical defects in oxides to boost their functionalities. The findings and methodologies provided in this thesis help to establish a framework for defect genome, which can benefit materials design in a broad range of defect-sensitive applications including solid oxide fuel cells, memristors, and (electro)catalysts.
In the first study, I investigated the surface defect equilibria and their strain dependency in La₀.₆Sr₀.₄FeO₃ (LSF) during oxygen incorporation reactions. Since the surface composition and structure can deviate significantly from the bulk, the surface defect chemistry can also differ from the bulk. Here, I demonstrated for the first time that the strain-dependent surface defect equilibria of LSF can be largely captured by bulk-like defect models with shifted oxygen chemical potential.
In the second study, I investigated the role of surface defect chemistry in CeO₂ during carbon poisoning (i.e., coking) reactions, which is an undesirable process in CO₂ electrolysis. I examined the coking reaction both experimentally and computationally, and identified the surface Ce³⁺-Ce³⁺ pairs to be the critical catalytic center. Based on these insights, I successfully mitigate the coking on the CeO₂-based materials by suppressing the formation of Ce³⁺-Ce³⁺ pairs with doping.
The third to sixth studies in this thesis investigated the role of defects in controlling the solid-state phase transformations in functional oxides. Exsolution is a promising synthesis method to fabricate self-assembled nanocomposite via phase decomposition. Since defect formation in the lattice is the elementary step in exsolution, I expect that defect engineering to be the fundamental tuning knob to control and tailor exsolution. To examine this hypothesis, I employed lattice strain, facet engineering, thermal annealing, and ion beam irradiation to tailor the defect chemistry in the host oxide and investigated their impact on both the surface and bulk exsolution phenomena.
In the third study, I utilized lattice strain as a dopant-free method to tailor the concentration of point defects in LSF. As tensile strains facilitate defect formation, the tensile-strained LSF results in a higher Fe⁰ metal concentration, a larger density of nanoparticles, and a reduced particle size at its surfaces. In the fourth study, I controlled exsolution with surface orientation. Different lattice orientations give rise to different incubation time in exsolution, and hence generates exsolved particles with different sizes and densities. In the fifth study, I varied the external gas environment to control the bulk exsolution in LSF. By tuning the concentration and oxidation states of the exsolution-induced lattice defects, we succeeded to obtain a substantial increase in electrical conductivity by more than 2 orders of magnitude, and continuous modulation of magnetization between 0 and 110 emu/cm³. In the last study, we used 10 keV Ni- beam irradiation to introduce defects and dopants into the SrTi₀.₆₅Fe₀.₃₅O₃ (STF) to tailor Fe exsolution. As a result, we demonstrated that in-situ Ni bombardment can change the composition of the exsolved nanoparticles from unary Fe nanoparticles to binary Fe-Ni alloys. Moreover, we found that, compared to the thermal exsolution, the STF film after Ni irradiation also experienced significantly enhanced bulk exsolution, which is likely due to irradiation-induced defects in the STF matrix. These results not only advance the fundamental understanding of the exsolution mechanisms, but also pave the way towards utilizing external stimuli to design novel functional oxides with optimal morphology, composition, and defect chemistry. | |
