Molecular Adsorption and Charge Transfer Dynamics at Dye−TiO2 Interfaces: An XPS, NEXAFS, and XSW Study

Abstract

Dye-oxide interfaces provide a standard model system for investigating charge transfer, with dye-sensitised solar cell (DSSC) representing one of the most widely studied examples. Ruthenium dyes are among the most efficient sensitisers, as their electronic properties strongly favour binding to oxide surfaces in DSSC applications. N3 is one of the most extensively studied dyes, while D5 is an organic dye based on triphenylamine. In this thesis, these dyes are investigated on various titanium dioxide (TiO₂) surfaces. The primary objective of this study is to explore interfacial and surface chemistry central to DSSC operation, with a particular focus on the bonding and adsorption geometry of D5 and N3 on TiO₂. For sample preparation, D5 was deposited on single-crystal rutile TiO₂(110) using electrostatic spray deposition (ESD) and on TiO₂ nanoparticles by drop-casting. In contrast, N3 was deposited on TiO₂ nanoparticles using ultra-high-vacuum electrospray deposition (UHV-ESD) to achieve the thinnest coverage, while drop-casting was used to obtain thin, medium, and thick films. The interfaces were characterised using X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure (NEXAFS), resonant photoemission spectroscopy (RPES), and normal-incidence X-ray standing wave (NIXSW). Chapter 3 shows that the N3 dye, a ruthenium-based complex, is deposited onto TiO₂(110) using an ultra-high-vacuum electrospray system and subsequently binds to TiO₂(110), mesoporous TiO₂, and TiO₂ nanoparticles primarily via carboxylic acid groups. In addition, hydrogen atoms from the carboxylic groups attach to bridging oxygen sites on the titanium surface. The sulphur atoms in the thiocyanate ligands exhibit multiple bonding configurations, interacting either with lattice oxygen on TiO₂ surfaces or directly with metallic substrates such as Au(111) and Ti. N 1s NEXAFS measurements show that the LUMO and partial LUMO+1 lie within the band gap of mesoporous TiO₂ and TiO₂ nanoparticles, resulting in weak electronic coupling with the substrate. RPES reveals a reduced participant channel intensity, indicating rapid charge transfer to the surface within the core-hole lifetime. Charge-transfer times from the LUMO+3 state are determined to be 3±0.02 fs for bulk N3 at the thinnest coverage, 10.9±0.02 fs for thin N3 at thick coverage, and 18.7±0.02 fs for monolayer and multilayer N3 on mesoporous TiO₂. Chapter 4 provides a detailed investigation of D5 adsorbed on rutile TiO₂(110), mesoporous TiO₂, and TiO₂ nanoparticles using XPS, NEXAFS, and RPES. The results show that D5 bonds to TiO₂ via both the COOH group and C–OH hydrogen bonding. Hard X-ray photoelectron spectroscopy (HAXPS) reveals no significant difference in XPS peak intensities between monolayer and multilayer coverages, while the nitrile N≡C peak shifts to lower binding energy in multilayers. Measurements on TiO₂ nanoparticles exhibit similar bonding behaviour, except that complete deprotonation of C–OH occurs at thick coverage. Density functional theory (DFT) calculations support the observed electronic structure, with the HOMO localised on the triphenylamine unit and the LUMO on the thiophene group. The core-hole lifetime for LUMO+3 is calculated to be 0.5 fs. Chapter 5 presents the adsorption geometry of N3 on rutile TiO₂(110) determined using the NIXSW technique. The vertical positions of selected atoms within N3, including the central ruthenium atom and carbon atoms in the carboxylate and thiocyanate groups, are measured relative to the TiO₂(110) surface. XPS identifies three components suitable for XSW analysis: Ru 3d₅/₂, carbon in the COOH group, and carbon in the N=C=S group of thiocyanate. Analysis of the TiO₂(220) reflection shows that the Ru atom is located at the centre of the molecule, 8.3±0.07 Å above the TiO₂(110) surface, corresponding to a position between the fourth and fifth lattice planes (n = 4–5). Both carboxylic and thiocyanate groups exhibit two distinct height environments. The carboxylic groups display a low-lying component at 2.8 ± 0.12 Å, consistent with 2-M bidentate bonding in which each carboxylate bridges two surface Ti atoms, together with higher components at n = 6 and n = 8 above the Ru centre. The thiocyanate groups are located at n = 4 (7.7 ± 0.11 Å) below Ru and at n = 6 (10.8 ± 0.11 Å) above Ru. These results confirm that Ru occupies a central position within N3 and that N3 anchors to TiO₂(110) predominantly via a 2-M bidentate geometry, in good agreement with DFT predictions.