Ahart, Christian;
(2022)
Charge transport in bulk hematite and at the hematite/water interface.
Doctoral thesis (Ph.D), UCL (University College London).
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Abstract
Transition metal oxide materials have attracted much attention for photoelectrochemical water splitting, but problems remain, e.g. the sluggish transport of excess charge carriers in these materials, which is not well understood. In this thesis I will show how periodic, spin-constrained and gap-optimised hybrid density functional theory can be used to uncover the nature and transport mechanisms of excess electrons and electron holes in a widely used water splitting material, hematite (α-Fe2O3). I will show that upon ionisation the electron hole relaxes from a delocalized band state to a polaron localised on a single iron atom with localisation induced by tetragonal distortion of the 6 surrounding iron-oxygen bonds. This distortion is responsible for the sluggish hopping transport in bulk hematite, characterised by an activation energy of 70 meV and a hole mobility of 0.031 cm2/Vs. By contrast, the excess electron induces a smaller distortion of the iron-oxygen bonds resulting in delocalisation over two neighbouring Fe units. I will show that 2-site delocalisation is advantageous for charge transport due to the larger spatial displacements per transfer step. As a result, the electron mobility is predicted to be a factor of 3 higher than the hole mobility, 0.098 cm2/Vs, in qualitative agreement with experimental observations. Extending this analysis to the hematite/liquid water interface, I will show that both excess electrons and electron holes localise at the interface with qualitatively similar structures to bulk hematite. However, the presence of the interface breaks the symmetry present in the bulk crystal and as a result the hole mobility is expected to be greatly reduced. These calculations provide new fundamental insights essential for a better understanding of rate-limiting transport processes governing photocatalytic water splitting efficiency at the hematite/liquid water interface.
| Type: | Thesis (Doctoral) |
|---|---|
| Qualification: | Ph.D |
| Title: | Charge transport in bulk hematite and at the hematite/water interface |
| Open access status: | An open access version is available from UCL Discovery |
| Language: | English |
| Additional information: | Copyright © The Author 2022. Original content in this thesis is licensed under the terms of the Creative Commons Attribution-NonCommercial 4.0 International (CC BY-NC 4.0) Licence (https://creativecommons.org/licenses/by-nc/4.0/). Any third-party copyright material present remains the property of its respective owner(s) and is licensed under its existing terms. Access may initially be restricted at the author's request. |
| UCL classification: | UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences UCL > Provost and Vice Provost Offices > UCL BEAMS > Faculty of Maths and Physical Sciences > Dept of Physics and Astronomy UCL > Provost and Vice Provost Offices > UCL BEAMS UCL |
| URI: | https://discovery-pp.ucl.ac.uk/id/eprint/10153463 |
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