Payne, DT; (2017) Photoexcitation at Titania Surfaces. Doctoral thesis , UCL (University College London).

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Abstract

This thesis employs energy- & time-resolved photoemission spectroscopy to examine two surfaces of titanium dioxide (TiO2) single crystals. Both surfaces investigated, the rutile TiO2(110) and anatase TiO2(101) surfaces, are pertinent to future energy research. Resonant two-photon photoexcitation at the reduced rutile TiO2(110) surface is found to involve the Ti 3d band gap states and energy levels of t2g or eg symmetry in the conduction band. The unoccupied state is determined to be centred ~2.7 eV above the Fermi level (EF), with a lifetime of less than 15 fs. Adsorption of bridging hydroxyls (OHb) is shown to enhance the intensity of the observed resonance. The signal intensity reaches a maximum under a monolayer of water, which is attributed to partial dissociation of adsorbed water. Electron bombardment of the anatase TiO2(101) surface is shown to increase the reduction state of the surface in ultra-violet photoemission spectra, which is attributed to the formation of oxygen vacancies. However, these point defects are known to migrate towards the bulk at temperatures above 200 K, restoring a thermally-equilibrated concentration. Bombardment also results in the appearance of new features in the valence band spectral region, which we associate with the 3σ and 1π molecular orbitals of OHb. Since excess electrons mediate redox reactions at the surface, their greater concentration may increase the potential for these processes to occur at the hydroxylated surface. Finally, a femtosecond-resolved, extreme ultra-violet photoemission study of the reduced rutile TiO2(110) surface is presented. Under infra-red photoexcitation, an electron trapping time of 40 fs is obtained, which coincides with the period of the material’s longitudinal phonon mode. Hence, interaction between electrons and this phonon mode is inferred to facilitate polaron formation. An additional, slow decay component, observed only under ultra-violet irradiation, is assigned to electron-hole recombination on the pico- to nanosecond timescale. The unexpectedly slow recovery of the Ti 3d band gap states population is attributed to trap-assisted recombination processes.

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