Graduation Semester and Year

2015

Language

English

Document Type

Dissertation

Degree Name

Doctor of Philosophy in Physics and Applied Physics

Department

Physics

First Advisor

Muhammad N Huda

Abstract

Phase stability and charge transport of pristine and transition metal alloyed bismuth titanate (Bi2Ti2O7, a.k.a. BTO), a photocatalytic water splitter, has been studied using the generalized gradient approximated density functional theory (GGA-DFT). The primary goals of this work were to predict the effective conditions for pure phase synthesis of the modified ternary multi-metal oxide and to determine the most suitable modifications for enhancing its photocatalytic properties. To understand the details of phase stability and photoconduction, we have derived the formation enthalpies, defect formation energies, electronic structures, spectral absorptions and polaron activation energies for pristine and transition metal doped bismuth titanate (Bi2Ti2O7, a.k.a. BTO). Implantation of the localized 3d electrons is a primary band engineering technique for extending the spectral absorptions of metal oxides into the visible range. However, localized states typically increase charge trapping that reduces crucial photocurrent for the photocatalytic process. Therefore one objective here is to understand the extent to which localization plays a role in electron transfer and which mode of conduction, band or polaron hopping, is dominantly effected.As predicting the effective conditions for pure phase stability and modeling electron transport of multi metal oxide materials is still in development as a whole, we have benchmarked our methods by reproducing relative quantities of world class metal oxide photocatalyst, rutile TiO2 and monoclinic scheelite BiVO4. In recognition of our methods, our results have been used to enhance H2 production of a facile hydrothermal synthesized Fe-doped BTO. Furthermore, we demonstrate with results for Cr- and Mn-doped BTO how experimental characterization can also be enhanced.For each transition metal ion (M = Cr, Mn, and Fe), pure phase stability has a unique association with the presence or absence of O defects. Band modifications vary with impurity d electron configuration and the polaron activation energies are increased with accompanying oxygen interstitials or vacancies. Hence, the ideal doping promotes the desired band gap reduction while maintaining the underlying stoichiometry. Thus, the key mechanism for phase stability and optimized photocurrent is the O chemical potential which is limited by dopant inspired phases (DIPs) instead of the host material.As predicting the effective conditions for pure phase stability and modeling electron transport of multi metal oxide materials is still in development as a whole, we have benchmarked our methods by reproducing relative quantities of world class metal oxide photocatalyst, rutile TiO2 and monoclinic scheelite BiVO4. In recognition of our methods, our results have been used to enhance H2 production of a facile hydrothermal synthesized Fe-doped BTO. Furthermore, we demonstrate with results for Cr- and Mn-doped BTO how experimental characterization can also be enhanced.For each transition metal ion (M = Cr, Mn, and Fe) case, pure phase stability has a unique association with the presence or absence of O defects. Band modifications vary with impurity d electron configuration and the polaron activation energies are increased with accompanying oxygen interstitials or vacancies. Hence, the ideal doping promotes the desired band gap reduction while maintaining the underlying stoichiometry. Thus, the key mechanism for phase stability and optimized photocurrent is the O chemical potential which is limited by dopant inspired phases (DIPs) instead of the host material.

Disciplines

Physical Sciences and Mathematics | Physics

Comments

Degree granted by The University of Texas at Arlington

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