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Crystallite Size Dependency of the Pressure and Temperature Response in Nanoparticles of Ceria and Other Oxides

Rodenbough, Philip Porter

The short title of this dissertation is Size Matters. And it really does. Before diving into the original findings of this dissertation, this abstract starts by contextualizing their significance. To that end, recall that some of the earliest concepts learned by sophomore organic chemistry students include explaining physical properties based on carbon chain length, for example, and polymer length has enormous influence on macroscopic material properties. In the 1980s it was found that the electronic properties of small inorganic semiconductor crystallites can be rigorously tied to the physical size of the crystallites, and this understanding has led directly to the successful integration of so-called quantum dots into readily available technologies today, including flat screen televisions, as well as emerging technologies, such as quantum dot solar cells. Oxides, for their part, are important components of many technologies, from paints and cosmetics to microelectronics and catalytic converters. The crystallite size dependency of fundamental mechanical properties of oxides is the topic of this dissertation.
First, this dissertation reports that consistent preparation methods were used to produce batches of specific crystallite sizes for a diverse family of five cubic oxides: CeO2 (ceria), MgO (magnesia), Cu2O (cuprite), Fe3O4 (magnetite), and Co3O4. The size-based lattice changes for small crystallites was carefully measured with X-ray diffraction. Expanded lattice parameters were found in small crystallites of all five oxides (notably for the first time in Fe3O4). This behavior is rationalized with an atomic model reliant on differing coordination levels of atoms at the surface, and fundamental calculations of physical properties including surface stress and expansion energy are derived from the measured lattice expansion for these oxides.
Then, the size dependency of the pressure response in ceria nanoparticles was measured using diamond anvil cells and synchrotron radiation. In a study unmatched in its comprehensiveness, it was found that the bulk modulus of ceria peaked at an intermediate crystallite size of 33 nm. This is rationalized with a core-shell model with a size dependent shell compressibility whose influence naturally grows as crystallite size shrinks. Complimentary thermal expansion measurements were carried out to probe the structural response of crystallites to heat. Overall, the thermal expansion of ceria decreased with crystallite size. Through careful heating cycles, it was possible to separate out quantitatively the two primary factors contributing to negative surface stress in ceria: ambient surface adsorbents and surface non-stoichiometry. These may be the first instances of such a calculation that provides this insight into the surface stress of oxide nanoparticles.
Next, pressure and temperature studies parallel to those in ceria were carried out on magnesia as well. Magnesia is an important oxide to compare to ceria because it does not share ceria's tendency to form oxygen vacancy defects with cation charge variances. Nonetheless, magneisa was shown to possess a peak (albeit a less dramatic peak) in bulk modulus at an intermediate crystallite size, about 14 nm. Magnesia, like ceria, also had decreased thermal expansion at smaller crystallite sizes.
Finally, experiments on molecular oxygen exchange properties of a series of oxides were carried out using a thermocycling reactor system designed and built in-house, with the aim of developing materials to convert carbon dioxide to carbon monoxide. Experiments were carried out under 1200C, much lower than the 1500C typically required for ceria oxygen exchange. It is thought that crystallite morphology could play an important role in dictating the effectiveness of this catalytic process. The increased understanding of fundamental physical properties of oxide nanoparticles, as explored here, may lead to their more rational integration into such emerging technologies.

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More About This Work

Academic Units
Chemistry
Thesis Advisors
Chan, Siu-Wai
Degree
Ph.D., Columbia University
Published Here
June 27, 2016
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