Hargreaves, Laura; (2024) Computational Modelling of Molecular Adsorption on Oxide Surfaces at Different Coverages. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The primary focus of this thesis is the modelling of molecular adsorption on oxide surfaces at different coverage levels. The influence of adsorbate concentration on the geometries and the comprehensive electronic structure at the interface are investigated by employing classical, semi-empirical, and ab initio approaches. The variety of adsorbates examined extends from organic molecules to atomic entities across a spectrum of substrates, spanning from insulators to metals. Given the extensive array of materials investigated and multiple objectives, this thesis is divided into two sections: In Part I, the adsorption of carboxylic acids on the surfaces of rutile titania is studied. The first chapter discusses the results obtained from a semi-empirical method, DFTB, that are compared to the results of hybrid DFT calculations. The results from the DFTB calculations qualitatively corresponded to the trends observed from the results from the hybrid DFT calculations. The next chapter investigates molecular overlayers of acetic acid on the TiO₂ (100)-1×1 surface, and compares them to overlayers studied with scanning tunnelling microscopy images and overlayer films on the TiO₂ (110)-1×1 surface. It was found that due to smaller intermolecular distances and Coulombic repulsion, acetic acid arranges itself in a less dense packing arrangement on the (100)-1×1 surface compared to the adsorption on the (110)-1×1 surface. In Part II, the focus of this work concerns the mechanism relating to the function of a hydrogen gas sensor. Hydrogen adsorption is thought to induce a dipole effect, which then affects the current through a doped silicon layer. The sensor consists of a Pd-SiO₂-Si interface that operates under ambient conditions. Experimental results show that humidity degrades sensor performance; therefore, the initial two chapters discuss the influence of water at different levels of coverage on hydroxylated SiO₂. Classical methods are used to explore the average geometry and dynamics close to the interface. It was found that with an increasing amount of water, molecules are on average further from the surface and have faster dynamics compared to low coverage. DFT calculations are then discussed with respect to the electronic structure of the H₂O-SiO₂ interface. Changes in electron density are found to be restricted to the first layer of water molecules and silanol groups. The last chapter investigates the effect of H adsorption on Pd nanoparticles on the total dipole moment. H adsorption on Pd nanoparticles results in a small amount of charge transfer. There is an increase in the total dipole moment and work function with increasing H adsorption at coverage.

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