Water at surfaces and interfaces from first-principles studies
Water is ubiquitous in nature and ordinary to us. However, no matter how common it is, water is unique, mysterious, and has numerous unusual properties, many of which still lack a scientific interpretation. Water at surfaces and interfaces is central to a broad class of phenomena in physics, chemistry and biology, such as lubrication, corrosion, solar cells and fuel cells, photoreaction, hydrogen production, wetting, cleaning, biomedical implantation, food production and preservation. During the past three decades or so, water on solid surfaces has been intensively investigated in surface science laboratories. Beyond its potential applications, it offers a model system for fundamental research on adsorbate-surface interactions and on water in complex processes in general.
This thesis studies water, in particular its various forms of nanostructures and overlayers on solid surfaces, using first-principles total-energy calculations and molecular dynamics simulations. The focus of this work is on the following four aspects of surface water: (i) The structure of adsorbed water and its vibrational recognition. Water nanoclusters, chains, and bilayers on metal surfaces are determined. The vibrational spectrum of such water, in particular its OH stretch modes, is found to be sensitive to the hydrogen-bonding environment in the surroundings. By an interplay with experimental spectra, our theory offers a promising tool for vibrational recognition of H-bonded water on surfaces. More specifically, the calculated vibrational spectra enable us to distinguish between dissociated water and molecular water, as in D2O/Ru(0001), and the shell structures of the hydrated K and Na ions on graphite. (ii) The nature of chemical bonding between water and surfaces. Water forms localized chemical bonds with surfaces via its lone pairs. Meanwhile, the intermolecular H-bonds are generally enhanced upon adsorption. In the water bilayer, two types of H-bonds with different strengths can be clearly identified. (iii) Phase transition in thin confined water films. The ice-to-water phase transition has been observed at a pressure of roughly 0.5 GPa in such thin ice films. And finally, (iv) water in the ion hydration shells at a hydrophobic graphite surface, a system of biological relevance. Water around K and Na ions is found to form two-dimensional (2D) shell structures, due to the 2D confinement and surface charge transfer. These results are important for the understanding of water at the surfaces investigated. They also have implications to confined and interface water in biological systems.