Megan Hoover

Graduation Semester and Year

2014

Language

English

Document Type

Thesis

Degree Name

Master of Science in Physics

Department

Physics

First Advisor

Qiming Zhang

Abstract

The interaction of uranium dioxide (UO₂) with environmental elements occurs at its exposed surface and a fundamental understanding of this interaction process begins at the atomic scale. In this regard, atomic scale modeling of the properties of clean and adsorbate-covered uranium dioxide (UO₂) surfaces can be used to elucidate UO₂ surface mechanisms such as corrosion and the formation of complex species via environmental gas adsorption. In this thesis, structural and electronic properties of clean and adsorbatecovered low index UO₂ surfaces were modeled using regular and hybrid density functional theory. Specifically, the properties of the clean (111) and (110) surfaces were modeled with hybrid density functional theory. To gain some insight into the surface oxidation of UO₂, we performed preliminary modeling studies on the interaction of atomic oxygen with the UO₂ (111) surface using density functional theory and hybrid density functional theory. For the clean surface, the evolution of the work function, surface energy, incremental energy, and band gap with respect to the system size was studied. We observed that at five formula units and beyond the surface properties of UO₂ converge. The estimated work function, surface energy, and band gap of the (111) surface were 3.5 eV, 0.97 J/m2, and 1.2 eV respectively; the corresponding values for the (110) surface were 2.2 eV, 1.76 J/m2, and 0.65 eV respectively. The localization of the 5f electron states is pronounced at the top surface layer while bulk-like behavior is exhibited at and below the subsurface layer. The Mott-Hubbard type insulating behavior in the bulk is retained in the surfaces, albeit with a smaller band gap. The adsorption of O in the UO₂ (111) surface indicates that UO₂ oxidation is a stable process. The top site is the preferred adsorption site with adsorption energy of 5.37 eV. The presence of the adsorbate results in the change of the electronic work function by 2.56 eV, implying charge transfer from U to O. The analysis of the electronic density of states indicates hybridization between the O adsorbate 2p electron states and the neighboring U 5f electron states. Furthermore, the presence of the adsorbate did not alter the Mott-Hubbard insulating behavior seen in the bulk crystal and clean surface.

Disciplines

Physical Sciences and Mathematics | Physics

Comments

Degree granted by The University of Texas at Arlington

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