Photoemission Study of Polaronic Defect States in TiO2

This thesis focuses on the behaviour of defects and polarons in TiO2, which is a model catalyst widely used due to its low cost, versatility and stability. TiO2 exists as two main polymorphs, anatase and rutile. Although these polymorphs have the same chemical structure, the manner in which defects exist and behave is distinctly different. Photoemission spectroscopies are suitable techniques for studying polarons. In the first results chapter, bulk defects in rutile TiO2 are investigated with two-photon photoemission and supported with density functional theory. It is found that bulk polarons are less bound than their surface analogues and create an alternative photoexcitation vector. The second results chapter describes the effect of formic and acetic acid adsorption on polaron behaviour in rutile TiO2. Carboxylic acids alter the local crystal field of polarons. Two-photon photoemission spectroscopy is used to demonstrate how this leads to adsorbate specific transitions which is dictated by the electronegativity of the adsorbate. Lastly, in the final results chapter, formic acid adsorption is also considered on the anatase surface. The valence band structure is studied with ultraviolet photoemission spectroscopy to show that oxygen vacancies migrate to the surface upon dissociative adsorption, leading to the stabilisation and diffusion of the associated polarons