Author

Jiao An

ORCID Identifier(s)

0000-0002-7811-0870

Graduation Semester and Year

2019

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Physics and Applied Physics

Department

Physics

First Advisor

Qiming Zhang

Second Advisor

Alex Weiss

Abstract

Transition metal oxides have drawn lots of attention from experimental and theoretical research due to their unique physical and chemical properties. Among them, the semiconducting α-Fe2O3 and TiO2 has been investigated for their potential use in a wide range of applications, such as photocatalysis, electrochemistry, and solar cells. Hematite α-Fe2O3 is a potential low-cost, earth-abundant, and environment-friendly semiconducting material. It was found that the isovalent sulfur-doping could reduce its band gap. To study the interaction of a sulfur atom with the surface of α-Fe2O3, a model of α-Fe2O3 (0001) film is made. From the optimized bulk structure, the clean hematite α-Fe2O3 (0001) film is built of 12 atomic layers with two different types of termination. And the most stable film is terminated by Fe atoms with an anti-ferromagnetic arrangement. For a sulfur atom adsorption on the suface of this film, the results suggest that the sulfur atom prefers to stay at the surface. There is a high barrier about 2 eV for the sulfur atom penetrating into the film. This excludes the possibility of tuning the band gap of α-Fe2O3 through the sulfurization of the oxide surfaces. The geometric and electronic properties of the TiO2 single-walled and double-walled nanotubes have been investigated. The stabilities of the nanotubes have been studied. For the single-wall nanotube (SWNT), the strain energy is decreased with the increase of the radius of the nanotube. The band gap energy is increased with the increase of the radius, approaching to the band gap energy of the HexABC sheet (~3.54 eV). Especially, the band gap of the (6,0) SWNT is reduced to about 2.86 eV, due to its reconstruction. The isovalent sulfur atom (S) doping with nanotubes has also been studied. The band gap values of double-wall nanotubes (DWNT) are decreased significantly compared with that of single-wall nanotubes due to the offset of the bands of the two constitutive single-wall nanotubes. It shows that the TiO2 DWNTs with smaller innershell radii form a type II band alignment, the staggered gap. And the band gap of the NTs studied could cover the redox potentials of water splitting, by comparing the band gap position of the bulk anatase with respect to the redox potentials of water splitting.

Keywords

PV cell, Photocatalysis

Disciplines

Physical Sciences and Mathematics | Physics

Comments

Degree granted by The University of Texas at Arlington

Included in

Physics Commons

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