The Functionalisation and Defects of Titania: A Photoemission Study

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Abstract

This thesis consists of photoemission studies of the rutile and anatase polymorphs of TiO2. It is split into two sections. In the first part, studies of the functionalisation of the rutile TiO2 (110) and the anatase TiO2 (101) surfaces with small organic molecules are presented. In the second part, studies of defects at the anatase TiO2 (101) surface are presented. Four organic molecules are investigated, p-aminobenzoic acid (pABA) and 3-fluoroaniline (3-FA) on anatase TiO2 (101), and dopamine and malonic acid (MA) on rutile TiO2 (110). The four adsorbates studied all have potential as dyes for dye-sensitised solar cells (DSSCS), and dopamine and pABA have the potential to be used as linkers for light sensitisers for photocatalytic applications such as DSSCs. Near-edge X-ray fine structure (NEXAFS) spectroscopy was used to determine the orientation of the molecules on the surface, and X-ray photoemission (XPS) spectroscopy was used to determine the chemical environment of the molecule on the surface. On the anatase TiO2 (101) surface, for pABA, the plane of the ring sits upright, normal to the surface, bonding through the carboxylic acid group after deprotonation. 3-FA also bonds upright, normal to the surface. It bonds through the amine group to the surface. It is unclear whether the molecule bonds dissociatively (the amine group deprotonating). On the rutile TiO2 (110) surface, dopamine is found to sit just off normal at ~ 78 º. It bonds through the catechol moiety after dissociation of the hydrogens. Both of the MA carboxylic acid protons dissociate and both moieties bond to the surface. MA is unstable under irradiation from insertion device beamlines. The defects in the anatase TiO2 (101) surface layers are investigated by XPS. Defects in the form of oxygen vacancies are observed in the band gap region of the photoelectron spectrum at around 1 eV binding energy (BE). Understanding the nature of these defects, intrinsic dopants in n-type TiO2 is imperative for photocatalytic applications. The effect of water adsorption on the stoichiometric and Ar cluster-ion-source-sputtered anatase TiO2 (101) surface is investigated at near-ambient pressures (NAP) and high vacuum. High vacuum studies show the anatase surface oxygen vacancies migrate to the subsurface region over time, as has been postulated with scanning tunnelling microscopy and density functional theory analysis. NAP-XPS is a developing technology allowing the analysis of surfaces and interfaces in “real-life” conditions. Results show water adsorbs in a similar fashion on both surfaces and reaches a saturation point between 0.6 and 1.8 mbar at room temperature, meaning there is little difference in reactivity between the stoichiometric and reduced surfaces. Oxygen vacancies are created at the TiO2 surface by insertion-device synchrotron radiation beamlines and the creation of these defects is monitored in situ with XPS. The bandgap state is made up of three components, relating to oxygen vacancies at 1.0 eV BE and two related components at 0.2 and 1.7 eV BE that are due to a surface 2D electron gas, created by electron doping via Nb impurities and synchrotron radiation-induced oxygen vacancies.

Layman's Abstract

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Keyword(s)

TiO2; XPS; photoemission

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