Ramy Nashed

Abstract

The demand on energy is now increasing at an unprecedented rate due to the high technology revolution. Unfortunately, we can no longer depend on the current energy resources, which is mainly fossil fuels, since they are limited and have dangerous impacts on the environment. Hydrogen has recently received a great attention as an alternative fuel because it is a renewable, clean fuel and its energy content is three times that of gasoline. Photoelectrochemical water splitting is a very attractive method of producing hydrogen due to its simplicity and low cost. However, the semiconductor material used as the photoanode still needs to be optimized. Ta2O5 is considered a very promising semiconductor material for water photolysis as its conduction band minimum and valence band maximum are suitable for water splitting beside being highly stable in aqueous solutions. Unfortunately, the material’s bandgap is ~3.9 eV, which limits its absorption spectrum to the ultraviolet region. However, mixing Ta2O5 with WO3 (2.7 eV) is expected to red shifts its absorption to the visible region. We used Density Functional Theory (DFT) to study the electronic and optical properties of Ta-W-O system. Unfortunately, the reported calculations so far failed to estimate the bandgap with an acceptable accuracy that enables the understanding of the optoelectronic properties of the material. Herein, we proposed a new crystal structure and showed that the use of PBE0 hybrid functional reduced the error in bandgap estimation from 95% to 5% resulting in a calculated bandgap of 3.7 eV. This bridges the gap further between ab-initio DFT calculations and experiments. Using the proposed structure for Ta2O5, we calculated the band structure and the hole effective mass for Ta-W-O system. The bandgap calculations showed a large and composition-dependent bowing parameter. The electron excitation from the Ta2O5 valence band to WO3 conduction band at high W content may contribute to the pronounced decrease in the conduction band energy. The staggered bandgap type between Ta2O5 and WO3, as revealed from the energy band diagram, resulted in efficient charge carriers separation. The minimum effective mass occurs along the y-direction and decrease monotonically with increasing W content. Based on the DFT calculations, preliminary experimental work was carried out on low concentration W alloys, namely 2.5% and 10%W. Diffuse reflectance measurements show that the bandgap decreases with increasing W content. This suggests that alloys with high W content are able to harvest a wider range of the solar spectrum and hence higher photo-conversion efficiency. Moreover, XRD analysis showed that the alloys maintained the orthorhombic structure of pristine Ta2O5. However, the lattice parameters expanded as the W content increased owing to larger atomic radius of W. Furthermore, XPS analysis asserts the charge transfer model that was drawn from DFT calculations in which the charge carriers are transferred from the valence band of Ta2O5 to the conduction band of WO3. Finally, the photocurrent of 10%W alloy was increased by about 100x compared to pristine Ta2O5.

Department

Electronics & Communications Engineering Department

Degree Name

MS in Electronics & Communication Engineering

Graduation Date

2-1-2014

Submission Date

November 2013

First Advisor

Ismail, Yehea

Second Advisor

Allam, geh

Committee Member 1

Yehea, Ismail

Committee Member 2

Allam, geh

Extent

91 p.

Document Type

Master's Thesis

Library of Congress Subject Heading 1

nostructured materials.

Library of Congress Subject Heading 2

Solar energy -- Research.

Rights

The author retains all rights with regard to copyright. The author certifies that written permission from the owner(s) of third-party copyrighted matter included in the thesis, dissertation, paper, or record of study has been obtained. The author further certifies that IRB approval has been obtained for this thesis, or that IRB approval is not necessary for this thesis. Insofar as this thesis, dissertation, paper, or record of study is an educational record as defined in the Family Educational Rights and Privacy Act (FERPA) (20 USC 1232g), the author has granted consent to disclosure of it to anyone who requests a copy.

Institutional Review Board (IRB) Approval

Approval has been obtained for this item

Comments

This work would not have been possible without the support and valuable inputs of many people to whom I am really grateful. First of all, I would like to express my deepest gratitude to my parents and my sister for their continuous encouragement and support. They bore a lot especially in my hard moments. It is also a great honor for me to thank Dr geh Allam, my co-supervisor, who guided me throughout the development of this thesis and who taught me how to carry out cutting edge research and be part of the research community. He has just been an amazing supervisor. Not forgetting Dr Yehea Ismail, my supervisor, who provided for me a great research environment and didn’t hesitate to provide me with all the materials I needed. Additiolly, I can’t express my thanks to Dr. Mostafa El Sayed, from Georgia Institute of Technology, who hosted me in his lab to conduct the experimental part of this work. Also I would like to thank Dr Faisal Alamgir and Dr Hong Li, from Georgia Institute of Technology, for their fruitful conversations and discussions and their insightful suggestions. Special thanks shall also go to Dr Walid Hassan who helped me understand Density Functiol Theory and guided me throughout the whole computatiol section. And last but my no means least, I would like to thank all the group members, whether at AUC or at Georgia Institute of Technology, for being such a great company and giving me help whenever I need it.

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