Iron oxide nanoparticles (NPs) are promising materials for use in many applications, including new cancer treatments and in cleaning water, because they exhibit size-dependent magnetic and absorptive properties. NP properties are caused by structural attributes of the NPs, like surface disorder and cation vacancies. However, NP synthetic methods also impact structure, therefore properties, of NPs. Furthermore, the synthetic method is often changed in order to change the core diameter of NPs. Determining if properties are caused by the dimensions of the NP is impossible if there are also structural features present in the NP caused by the synthetic method, like grain boundaries or polycrystalline shells. In Chapter II of this dissertation, we show a new continuous growth synthesis of spinel iron oxide where the diameter of NPs is changed by the amount of precursor added to the reaction, meaning the only structural feature changing between the NPs is size. Continuous growth, therefore, can be used to probe the impact that size has on NP structure and properties. We report that saturation magnetization of NPs produced from continuous growth is size-dependent and higher in magnitude than NPs of the same core diameter made by other syntheses. In chapter III of this dissertation we determine nanoscale structure by Pair Distribution Function (PDF) analysis of Total X-ray Scattering data of NPs isolated from the reaction with core diameters between 3-10 nm. In Chapter IV of this dissertation we monitored the growth of NPs in situ with Total X-ray Scattering to gain insight on the structures of NPs while forming. In situ measurements of Total X-ray Scattering data gave insights into how precursor oxidation state influences the structures formed during formation of NPs, with more oxidized precursor giving a more oxidized product and a reduced precursor yielding a more reduced product even though the NPs formed by either method are indistinguishable by ex situ analysis. This dissertation includes previously published and unpublished co-authored material. / 2021-04-30

京都大学 / 新制・課程博士 / 博士(人間・環境学) / 甲第23287号 / 人博第1002号 / 新制||人||236(附属図書館) / 2020||人博||1002(吉田南総合図書館) / 京都大学大学院人間・環境学研究科相関環境学専攻 / (主査)教授 内本 喜晴, 教授 田部 勢津久, 准教授 戸﨑 充男 / 学位規則第4条第1項該当 / Doctor of Human and Environmental Studies / Kyoto University / DFAM

Mesoporous alumina (Al2O3) in the gamma (γ) phase is widely used as a support in catalytic applications because of its high surface area, large pore volume, acid-base characteristics, and thermal stability. To improve the thermal stability of gamma alumina, dopants such as lanthanum, magnesium, zirconia, and silica are often introduced. Current laboratory-based methods for synthesizing gamma alumina generally involve 10-15 steps and/or use toxic, expensive surfactants and solvents. Industrial methods, while simpler, lack control of pore properties and surface chemistry. In contrast, we have developed an innovative solvent deficient, one-step method that is able to synthesize a wide range of pure and silica-doped aluminas with high surface areas, pore volumes from 0.3 to 1.8 cm3/g, and pore diameters from 5 to 40 nm. More significantly, our silica-doped aluminas are stable up to temperatures as high as 1300<°>C, which is 200<°>C higher than other pure and doped gamma alumina materials.The usefulness of gamma-alumina as a catalyst support is dependent on its favorable combination of textural, thermal, structural, and chemical properties, yet the relationship between structure and these other properties is still not clearly understood due to the poorly crystallized nature of the material. In particular, the mechanism by which the gamma structure is stabilized thermally by so many dopants is still not well understood. Based on our previous PDF experiments on pure and La-doped alumina, we have developed a hypothesis regarding the mechanism by which dopants increase thermal stability. To validate or refute this hypothesis, we collected PDF data on a wider range of laboratory and industrial alumina samples. Herein, we have utilized PDF analysis to study the local to intermediate-range structure of a series of our pure and silica-doped aluminas calcined at 50<°>C intervals between 50 and 1300<°>C as well as pure and silica-doped aluminas from commercial sources and other synthetic methods. This thorough study of alumina local structure will allow us to separate general trends in the local structure from idiosyncrasies based on synthetic method/conditions, and it will help us identify the structural features responsible for improved thermal stability. Having access to these PDF experiments, we have validated our current hypothesis on the nature of stabilization afforded by dopants and, more generally, developed a better understanding of the role structure plays in the properties of aluminas.