title: Combined theoretical and experimental study of the core-level electronic structure of ionic liquids
Creators: name: Gousseva, Ekaterina
Publisher: name: University of Reading
Year: 2023

description: Ionic liquids (ILs) are a group of materials, composed solely of ions, which exist in their liquid state at functional temperatures i.e., ≤100 ℃. ILs are novel fluids which can exhibit unique combinations of desirable properties, compared to traditional molecular liquids. This ability means there is much interest in their use for a vast range of applications, from the most specialised to the most common, such as solvents, catalysis and energy storage or conversion. To employ a new material in any application, a thorough understanding of its nanoscopic and macroscopic behaviours is imperative. In ILs, the nanoscopic environment is far more complex than a molecular liquid. These materials consist of two different ions at minimum. To be liquid at low temperatures, the ions tend to be bulky or asymmetric. Furthermore, an extensive choice of ions produces an immense estimate of 106 potential ILs. Together, these attributes make the fundamental study of ILs and their electronic behaviours more intriguing than that of a typical liquid. One prominent experimental method used to study these behaviours is X-ray photoelectron spectroscopy (XPS). The core levels can be investigated to examine binding energy (EB) shifts and peak widths to gather information on the system such as composition and chemical states. Further information, such as bonding, local geometric structures or oxidation states can only be hypothesised. In this thesis, a combined theoretical and experimental approach is used to investigate the nature of EB shifts of ILs. This combined approach allows a more comprehensive analysis than would be possible with either method alone. In particular, an approach employing density functional theory (DFT) enables effective electronic simulation to complement the experimental data. ILs are studied in the bulk form and at an interface with a titanium dioxide (TiO2) surface. It is first determined that the initial, ground state, of the IL system is the dominant influence in measured XPS peaks. The complex interplay of the various types of interactions in the IL system is carefully studied, to demonstrate that when simulating these complicated systems, long-range interactions are critical to the electronic behaviour and a basic gas phase, lone ion pair approach is insufficient. The method developed to simulate EB is applied to a further four ILs to demonstrate its validity in predicting component EB separations. This method is also shown to reproduce peak EB shifts observed in experiments, establishing for the first time that these shifts originate in the initial state rather than the final state. New XPS and normal incidence X-ray standing wavefield (NIXSW) data is presented for an IL monolayer on a rutile (110) TiO2 surface. Excellent agreement is found between the experimental and the calculated geometric adsorption structure. Calculations on TiO2 are further evaluated at a range of scales, from lone ion pair to bulk models, to find that the ions have contrasting preferred adsorption geometries based on the local liquid (or lack thereof) environment. Calculated bond length differences are observed between geometry optimisation calculations at 0 K and ab initio molecular dynamics (AIMD) simulations at 298 K and 398 K.