Lee, Roxy; (2024) Photoemission Spectroscopy for the Electronic Structure Determination of Energy Materials. Doctoral thesis (Eng.D), UCL (University College London).

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Abstract

Accurate surface characterisation is essential for the research and development of new energy materials and technologies, which are required to transition the current energy system to a low carbon energy mix. For many functional materials, their application is dominated by processes occurring at the surface, thus a major challenge lies in the characterisation of the surface structure. X-ray photoemission spectroscopy (XPS) is a powerful tool to analyse the top ~ 5 nm of a sample, providing elemental quantification and chemical state information. In XPS measurements, core and valence electrons are ejected from the sample surface, giving rise to core-level and valence band (VB) spectra, respectively. Analysis of core-level spectra to determine peak positions and intensities is the basis of most routine XPS characterization. However, VB spectra have complex spectral features that arise from the overlap of several orbital contributions from different atoms, which makes analysis more challenging. Whilst centred around XPS, a major theme in this thesis is the integration of other scientific practices with the technique, thus emulating the interdisciplinary nature of materials science. The aim is to extend the conventional applications of XPS and develop new methods in materials analysis and can be divided into four main parts. Firstly, this work combines XPS with theoretical photoemission spectra simulated within the framework of density functional theory (DFT) to develop a refinement method, offering new insights into VB electronic structure and complex spectral features in metals and vacancy-ordered double perovskites, Cs2TeX6 (X = Cl, Br, I). This refinement method is then used in the second part to probe the vibrational broadening in materials with a range of conductivities, including photocatalysts bismuth oxyhalides, semiconductors Nb-doped strontium titanate and alkali halides. In the third part, a joint theoretical/experimental VB mapping method is developed and combined with photocatalytic activity mapping, to quantify phases in heterogeneous surfaces, applied to mixed-phase TiO2. Lastly, electrochemical measurements are performed in tandem with XPS data collection during the operando monitoring of Na ion behaviour and ionic liquids electrolytes in a 3-electrode electrochemical cell.

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