Ahmed, M H Mesbah
(2023)
Soft X-ray core-level spectroscopy studies of molecules on TiO2 surfaces in the context of light harvesting.
PhD thesis, University of Nottingham.
Abstract
Studying the interaction between the dye and the surface as well as how the system reacts to light is important for understanding the electrical and chemical characteristics of dye sensitised solar cells (DSSC). In this thesis, several model systems relevant to dye sensitised solar cells have been investigated for further understanding. But, this thesis starts with the investigation on the popular semiconductor for DSSC application, rutile TiO2(110). At first, the annealing induced migration of defects in single crystals of rutile titanium dioxide have been investigated, and then the next chapter, investigates how defects interact with the molecules of oxygen, water, and air.
Firstly, this thesis present an in-situ study of defect migration in rutile TiO2(110) conducted using X-ray Photoelectron Spectroscopy (XPS). The ability of titanium dioixide to split water into OH- and H+ species is heavily dependent on the behaviour of defects in the crystal structure at or near the surface. First, surface and sub-surface defects were created in the crystal by argon ion sputtering. Subsequent in-situ exposure of the defective crystal to liquid water healed the surface defects whilst the sub-surface remained defective. The sample was then annealed whilst XPS was used to monitor the concentration of titanium defects. At low annealing temperatures Ti3+ was observed to migrate from the sub-surface to the surface. Further annealing gradually restored the surface and sub-surface to the defect-free Ti4+ form, during which the changes in abundance of Ti1+, Ti2+ and Ti3+ defects are discussed.
In the second experimental chapter, water, oxygen, air, and ambient pressure were applied to defective rutile TiO2(110) surfaces. Before each exposure, defective surfaces were created using Ar+ sputtering to investigate the effect of the corresponding exposed molecules. When exposed to 0.2 mbar water, 1 mbar oxygen, 5 mbar air, ambient pressure at room temperature, 323, 373, 423, 573, and 723 K respectively, the intensity of the oxygen vacancy gradually decreased. Water exposure heals defects by water dissociation whereas during oxygen exposure oxygen dissociates into an oxygen atom and oxygen adatom, where the oxygen atom fills the vacancy and the oxygen adatom dissociates further water and produces two identical OH groups. Further depletion of the defect was observed after exposure to ambient pressure. During annealing in ambient pressure, a rapid depletion of Ti intermediate oxidation state was observed as temperature increased. This is more likely due to the bulk assistance (through titanium interstitial and oxygen vacancy diffusion mechanisms) oxidation mechanism's progressive conversion of Ti1+ defects to Ti4+ via intermediate oxidation states .
In the next experimental chapter, bi-isonicotionic acid has been deposited onto rutile, anatase and mesoporous TiO2. X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS) have been used to obtain chemical information and map out the density of states for the system. We find that the LUMO lies entirely within the band gap of the rutile, anatase and mesoporous titania, as a consequence, the electron transfer between LUMO and substrates is electronically forbidden. But the LUMO+1 and LUMO+2 states of the core excited molecules are overlapped with the unoccupied states in the substrate. So, it is possible to transfer charge from molecules to substrate. The core-hole clock is implemented using resonant photoelectron spectroscopy (RPES) data. The charge transfer time of bi-isonicotinic acid on rutile and anatase titania is found to be comparable. While the obtained charge transfer time for bi-isonicotinic acid on mesoporous titania is longer than the rutile and anatase.
Finally, the chemisorbed and physisorbed coverage of N3 dye complex on mesoporous TiO2 has been investigated. X-ray photoemission spectroscopy (XPS) spectra reveal the presence of protonated carboxylic groups, which might be evident of the absorption of N3 on mesoporous substrate bonded through a mixture of bidentate and monodentate bonding geometries. From the density of states for the system we found that the LUMO lies entirely within the band gap of the mesoporous titania, and the electron transfer between LUMO and substrate is electronically forbidden. The intense features and vibronic coupling observed in RIXS and RPES have been discussed thoroughly. The RIXS, RPES and calculated density functional theory (DFT) have been combined to provide an understanding of the electronic structure of complex molecules. The orbital contributions of the bi-isonicotinic acid and thiocyanate ligands were used to interpret the obtained RIXS and RPES data.
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