Bavis, Natalie
(2023)
Utilising Atomic Layer Deposition (ALD) to Develop Efficient Hematite-based Photoanodes for Photocatalytic Water-Splitting.
PhD thesis, University of Liverpool.
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Abstract
It is a scientific consensus that the consumption of fossil fuels result in carbon dioxide (CO2) emissions that negatively impact the climate, in a phenomenon referred to as ‘global warming’ or ‘climate change’.1 To address this problem, novel fuels are required that have a lowered carbon footprint, with hydrogen often hailed as future fuel of combustion vehicles.2 However, a low carbon, renewable means of hydrogen production is required to fully address this problem.3 Water-splitting using solar energy offers a route to renewable, clean hydrogen production. Hematite, α-Fe2O3, is an n-type semiconductor that shows promise as a photoanode in a water-splitting photoelectrochemical cell. Cheap, stable, abundant and with a suitable bandgap, α-Fe2O3 possesses many of the desirable criteria for use in this context.4 However, it is also plagued with electronic issues such as poor charge transport, short carrier lifetimes and low conductivity. 5 This work attempts to address these problems by using plasma enhanced atomic layer deposition (PEALD) to produce nanostructured hematite films with improved properties. Using a ferrocene precursor in a vapour push setup, a process was designed with a pulse purge sequence of 2-5-5-4s (precursor-purge-coreactant-purge) and 0.05nm/cycle growth rate. The performance of the films was then assessed using photoelectrochemical techniques, with film modifications, including Al2O3 underlayers and interlayer doping, explored as a means of improving performance. Specifically, a 0.5nm underlayer was found to improve photocurrent dramatically, exhibiting a peak performance of 1.29 mA cm-2 at 0.7 VAg/AgCl, coupled with a ~250mV cathodic shift in onset potential from the non-modified Fe2O3. Layer or delta doping with Al2O3 offered further improvements in onset potential, with the addition of discrete, 1.2nm thick Al2O3 interlayers generating an impressive VOnset of 0.73 VRHE. This was further lowered to 0.61 VRHE with the addition of a CoPi surface layer. Finally, the effects of heat exposure are addressed, with low energy ion scattering (LEIS) spectroscopy employed to discern compositional changes in the films following heating. Key observations here were the heat-induced migration of aluminium from the underlayers, as well as the suppression of Sn migration from the FTO substrate. In particular, these suppression effects were pronounced for the ALD films compared to other methods of deposition. This provided insight into the mechanisms behind the photoelectrochemical improvements witnessed, while raising questions on the effects of heat exposure on hematite/FTO based photoanodes.
| Item Type: | Thesis (PhD) |
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| Divisions: | Faculty of Science and Engineering > School of Engineering |
| Depositing User: | Symplectic Admin |
| Date Deposited: | 19 Sep 2023 10:37 |
| Last Modified: | 19 Sep 2023 10:38 |
| DOI: | 10.17638/03172660 |
| Supervisors: |
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| URI: | https://livrepository.liverpool.ac.uk/id/eprint/3172660 |
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