Experimental Validation of First Principles Simulations and Observation of Diffuse Ion Profiles at Solid/Water Interfaces

Harmon, Katherine

doi:https://doi.org/10.21985/n2-jdr1-d634

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Experimental Validation of First Principles Simulations and Observation of Diffuse Ion Profiles at Solid/Water Interfaces

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Physiochemical phenomena in aqueous systems, such as corrosion, catalysis, and energy storage, are driven by the molecular-scale interactions of ionic species with charged solid surfaces. In particular, an electrical double layer (EDL) of ions forms within nanometers of a charged surface. The properties of the EDL have been explored from both a theoretical perspective and an experimental perspective for over a century, but a precise description of its structure remains elusive. For example, classical theories of the EDL developed over a century ago consider interactions between charged species in solution and a charged interface but neglect solvation effects. Furthermore, we now understand that interfacial phenomena deviate significantly from their bulk counterparts—though the precise nature of these differences depends on the ions, the solvent, and the solid surface itself. This presents a significant barrier to obtaining a comprehensive picture of interfacial phenomena. Complementary efforts from theory and experiment are needed to tackle this challenge, which demands high accuracy from both directions. X-ray reflectivity provides an exceptionally sensitive probe of interfacial structures and has been used to measure interfacial water and adsorbed ion structures at mineral/water interfaces with atomic resolution. Several recent studies have also used X-ray reflectivity as a validation tool to evaluate the accuracy of simulated structures of solid/water interfaces at different levels of theory. As a natural extension, this thesis uses X-ray reflectivity to explore in detail several different theoretical and numerical approximations required to carry out first principles molecular dynamics calculations with a focus on the alumina(001)/water interface. The sensitivity of X-ray reflectivity to atomic and electronic displacements provides a direct pathway to assess the strengths and weaknesses of different approximations and identify areas to further improve the accuracy of these simulations. In addition, this thesis uses X-ray reflectivity with and without element specificity to probe the water and EDL structures at a graphene electrode surface with potentiostatic control. The hydrophobic nature of graphene leads to a confinement-induced ordering of water at the solid surface that may affect ion adsorption. I then use resonant X-ray reflectivity to directly probe the EDL structure. I compare the observe resonance spectra to the ion distribution models derived from Gouy-Chapman theory and find a significant deviation from the classical theory under the experimental conditions employed. The discrepancy between the observed ion structure and classical models can be explained by incorporating the effects of the interfacial water. The results also suggest that the properties of the electrode play a role in establishing the double layer, which is not considered in the classical theories.

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