Core-Level Spectroscopy of Photoactive Materials

Hart, Jack (2023) Core-Level Spectroscopy of Photoactive Materials. PhD thesis, University of Nottingham.

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In order for next generation solar technologies to continue to improve, detailed chemical and electronic information is essential. Core-level spectroscopies, such as X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and resonant photoelectron and Auger spectroscopies are capable of providing this information. These techniques have been used for decades across many systems but are limited to ultra-high vacuum pressures, however more recently measurements at ambient pressures have become possible. The thesis presented herein applies core-level spectroscopies to two systems: dye sensitised solar cells and solar water splitting. In the former, ultra-high vacuum core-level spectroscopies are employed to probe the chemical and electronic coupling between the dye and substrate. For the latter, ambient-pressure X-ray photoelectron spectroscopy allows the oxidation of a potential solar water splitting catalyst to be monitored.

To begin, a relatively simple dye, C343, is investigated on different mesoporous substrates; TiO2 and SrTiO3. These samples were prepared ex-situ and, in the case of TiO2, compared to samples prepared by thermally evaporating C343 in-situ. By also comparing to a multilayer of dye, a detailed description of how the dye is bonded to the substrates is presented and the density of states probed. To assist in the dye’s analysis, a similar dye, C334, is also investigated.

Next, core-level spectroscopies are applied to a more complicated dye-TiO2 system involving the well-known dye N3 as the sensitiser. Here, the bonding to the surface is confirmed by X-ray photoelectron spectroscopy. In addition, resonant Auger electron spectroscopy is used to quantify both charge delocalisation within the molecule as well as injection into the substrate conduction band. Hard X-rays are used to study excitations from both the S 1s and Ru 2p3/2 core levels. The effect of the substrate is seen on the calculated charge transfer dynamics, and a picture of charge transfer from the thiocyanate ligand discussed.

Two further dye-TiO2 systems are investigated using core level spectroscopies; RuP and RuC. These dyes are qualitatively very similar, differing only in their anchoring group. Resonant Auger electron spectroscopy is again employed, alongside resonant photoelectron spectroscopy, to quantify the charge injection and delocalisation times of both dyes. In this context, the measurements are a probe of how the anchoring group influences the injection dynamics of dye-sensitised solar cells. It is found that both dyes are capable of sub- femtosecond charge injection in the Ru 2p3/2 excited regime, however that RuP is favoured for its slower relaxation.

Finally, ambient pressure X-ray photoelectron spectroscopy is used to study the oxidation chemistry of a prospective solar water splitting catalyst: TiSi2. Both powder and thin film samples are investigated, where the latter were employed in order to remove the natural oxidation caused by air. To do so, different passivation layers were investigated to preserve the TiSi2 layer. From the measurements, it is suggested that the reported catalytic activity of TiSi2 is related to the growth of a sub- stoichiometric silicon oxide, and the minimisation of SiO2 at the TiSi2 surface.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: O'Shea, J. N.
Campion, R.
Keywords: X-ray spectroscopy, solar water splitting, dye-sensitised solar cells, NAP-XPS, XPS, X-ray photoelectron spectroscopy, XAS, X-ray absorption spectroscopy, RPES, Resonant Auger Electron Spectroscopy, RAES, Resonant Auger Electron Spectroscopy
Subjects: Q Science > QC Physics > QC350 Optics. Light, including spectroscopy
T Technology > TK Electrical engineering. Electronics Nuclear engineering
Faculties/Schools: UK Campuses > Faculty of Science > School of Physics and Astronomy
Item ID: 73881
Depositing User: Hart, Jack
Date Deposited: 26 Jul 2023 04:40
Last Modified: 26 Jul 2023 04:40

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