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Authors: Schlegel, Simon Johannes
Advisor: Bonn, Mischa
Title: Investigating the water-titanium dioxide interface using sum frequency generation spectroscopy
Online publication date: 4-May-2023
Year of first publication: 2023
Language: english
Abstract: In order to fight global warming, clean and cheap alternatives to fossil fuels are needed. A promising candidate is hydrogen, which can be obtained by photocatalytic splitting of water utilizing sunlight and titanium dioxide as photocatalyst. To make this process profitable, its fundamentals need to be understood. The research presented in this work aims at a better understanding of the structure of the water-TiO2-interface, using the surface sensitive Sum-Frequency-Generation (SFG) spectroscopy technique. The first experimental part of this work demonstrates the potency of SFG. It is used as a complementary technique to learn more about the behaviour of the surfactant CTAB on the water-air-interface in the presence of high amounts of dissolved NaCl. One part of this work was the deposition of TiO2-thin films on CaF2-windows using the magnetron-RF-sputter technique. The target was to produce samples for measurements at the TiO2-D2O-interface. The main problems of this task are presented and it is shown why no reproducible results could be obtained. The first main part of this thesis describes SFG experiments on the TiO2-D2O-interface using solutions of pD 3, 5, 7, 9 and 11. The SFG-signal shows three peaks for all pD values. The high-frequency peak originates from weakly hydrogen-bonded interfacial Ti-OD groups that are pointing away from the TiO2. The low-frequency peak is caused by D2O molecules pointing towards the semiconductor, forming strong donating hydrogen-bonds with the aforementioned Ti-OD groups. The central-frequency peak originates from D2O molecules in the second water layer. When the solution pD is below 5 (point of zero charge (pzc) of TiO2), the water molecules point away from the TiO2 due to a positive charge caused by protonation of the surface. At pD 5, the over all intensity of the SFG-spectra decreases, indicating that the orientation of the water molecules is random. With rising pD value, the intensity increases, caused by deprotonation of the surface, which results in a negative charge. At higher pD, the water molecules in the second layer are oriented with their deuterium atoms pointing towards the semiconductors surface. Additional measurements at pD 2 showed a coupling e˙ect between the low and the central frequency peaks. This is not the case for pD 11. In the second main part of this work, femtosecond time-resolved SFG-experiments are presented, conducted on the TiO2-D2O-interface with solutions of pD 3, 7 and 11. Using pump frequencies of 2550 and 2400 cm−1, it was found that with increasing pD-value the lifetime of the excited state decreases, probably caused by the stronger hydrogen-bonding network for high pD-values. It is also shown that for the 2550 cm−1 pump the coupling from the direct to the cross peak is dominated by a down-hill energy transfer, whereas the coupling for the 2400 cm−1 pump includes an instantaneous frequency shift of the cross peak. Additionally, experiments using isotopically diluted solutions of pD 3, 7 and 11 showed, that for all three pD-values the relaxation time strongly increases, due to the suppression of coupling e˙ects in the hydrogen-bonding network. For pD 11 this finding stands in contrast to results of the static SFG-experiments conducted before. This hints to an unknown coupling mechanism between the water molecules and the semiconductor.
DDC: 540 Chemie
540 Chemistry and allied sciences
Institution: Johannes Gutenberg-Universität Mainz
Department: FB 09 Chemie, Pharmazie u. Geowissensch.
Place: Mainz
URN: urn:nbn:de:hebis:77-openscience-07ee1d68-6099-4277-ae68-431e940573415
Version: Original work
Publication type: Dissertation
License: In Copyright
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Extent: IV, 119 Seiten, Illustrationen, Diagramme
Appears in collections:JGU-Publikationen

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