The Role of Surface Chemistry of a Model Metal Oxide for Photochemical and Biomaterial Applications

UoM administered thesis: Phd

  • Authors:
  • Michael Wagstaffe

Abstract

The functionality of devices that utilise TiO2 is heavily influenced by the interactions which occur at the surface and interface. Developing a full mechanistic understanding of these interactions is essential, and the focus of the work presented in this thesis. The interactions of phenylphosphonic acid (PPA) and [C4C1Im][BF4] with the surface of anatase(101) have been investigated using X-ray photoelectron spectroscopy (XPS) and near edge X-ray absorption fine structure (NEXAFS) spectroscopy because of their potential applications in dye-sensitised solar cells. XPS data suggests that at 0.15 monolayer (ML) coverage PPA adsorbs in a bidentate geometry following the deprotonation of both P-OH groups. At 0.85 ML coverage there is a shift to a mixed bidentate/monodentate binding mode. NEXAFS spectroscopy data shows that the phenyl ring is oriented 65 ±4 away from the surface plane and twisted 57 ±11 away from the [101] azimuth. [C4C1Im][BF4] is found to order at the surface of anatase(101) via electrostatic attraction only at low coverages, with the imidazolium ring oriented 32 ± 4 from the surface. XPS data suggests that the anion undergoes a surface induced degradation to form BF3, resulting in incorporation of F into O vacancies. The behaviour of metallic clusters supported on metal oxide supports and the nature of the crystal structure modification of TiO2 following doping with impurity ions both have important implications in catalysis. XPS and scanning tunnelling microscopy (STM) data show that the interaction of Ag with the anatase(101) surface leads to a metal-surface charge transfer, as evidenced by the reduction of Ti4+ to Ti3+, and the formation of 3D size limiting clusters. Under exposure to 3 mbar CO/H2O, partial oxidation of the Ag clusters in conjunction with the formation of a transient Ag-carbonyl species is observed. Upon returning to ultra-high vacuum these species disappear. XPS and photoelectron diffraction (PhD) have been employed and show that nitrogen doping results in an additional Ti species due to TiO2-N. XPS suggests that nitrogen exists in substitutional sites, a conclusion which is reinforced by the PhD data where the N 1s and O 1s modulation functions exhibit similarities. Theoretical modulation functions are yet to be calculated and future work will aim to provide quantitative information regarding the substitutional nature of this system. The effect of adventitious carbon on Ti implants has been modelled by studying peptide adsorption on single crystal TiO2, both as-prepared and pyrocatechol capped. XPS shows that pyrocatechol successfully prevents the adsorption of adventitious carbon species from air and leads to a greater uptake of RGD, which bonds to Ti atoms via the deprotonated carboxylate groups. An increased uptake of RGD is also observed on the SLActive surface and we conclude that the chemistry of the hydrocarbon overlayer governs the uptake of RGD, and may affect osseointegration.

Details

Original languageEnglish
Awarding Institution
Supervisors/Advisors
Award date1 Aug 2018