Prof, Angelos Michaelides
Research Interests Our research aims at understanding important phenomena in surface- materials- and nano-science. Using concepts from quantum mechanics and statistical mechanics, we apply and develop methods and computer simulations to study processes of relevance to catalysis - such as the properties of metal surfaces and chemical reactions at surfaces - and processes of environmental relevance - such as the nucleation of ice or the dissolution of salts. Water and ice are major focuses of our work. Recent Publications
Abstract: Heterogeneous ice nucleation plays a key role in fields as diverse as atmospheric chemistry and biology. Ice nucleation on metal surfaces affords an opportunity to watch this process unfold at the molecular-scale on a well-defined, planar interface. A common feature of structural models for such films is that they are built from hexagonal arrangements of molecules. Here we show, through a combination of scanning tunneling microscopy, infra-red spectroscopy, and density-functional theory, that ca. one nanometer wide ice chains that nucleate on Cu(110) are not built from hexagons, but instead are built from a face sharing arrangement of water pentagons. The pentagon structure is favored over others because it maximizes the water-metal bonding whilst at the same time maintaining a strong hydrogen bonding network. It reveals an unanticipated structural adaptability of water-ice films, demonstrating that the presence of the substrate can be sufficient to favor non-hexagonal structural units.
We all know that ice becomes slippery some 20-40 K below its bulk melting point. Less is known, however, about the surface of ice at lower temperatures such as those experienced by ice crystals in the upper atmosphere. Here we show through first principles electronic structure simulations that although bulk ice is a proton disordered solid, at the surface, protons order. Electrostatic repulsion between the protons at the surface cause them to line up, effectively making the surface superchilled. This insight into the ice surface is likely to have implications for the equilibrium crystal shape of ice crystals or catalytic reactions which take place on their surfaces.
The salt-water interface is one of the most important and common on earth, playing a prominent role in disciplines such as atmospheric science and biology. Despite the apparent simplicity of such interfaces, arguably the most fundamental question of what the nature and structure of the liquid water/salt interface is under ambient conditions remains unclear. Here we address this issue with an ab initio molecular dynamics simulation of a nanoscale liquid water film on NaCl. A pronounced layering is observed in the film, with the density exhibiting a damped oscillatory behavior in the direction of the surface normal. In addition, water molecules in the contact layer are preferentially adsorbed at specific adsorption sites, involved in about 20% fewer hydrogen bonds with each other, and carry considerably reduced dipole moments compared to bulk liquid water. Biography
A recent highlight of our work in the area of water-metal interfaces was the identification and characterization of the so-called “smallest particle of ice”: the water hexamer. In this project the cyclic water hexamer and a family of hydrated hexamer-like intermediates, were observed with scanning tunneling microscopy by Karina Morgenstern at the University of Hannover and characterized by us with density functional theory. The experimental STM image and the structure of the adsorbed hexamer predicted by theory are given below. In addition to the characterization of the water hexamers we also learned something new about the nature of interfacial hydrogen bonds: specifically we identified a hitherto unknown competition between the ability of water molecules to simultaneously form water-metal bonds and to accept H bonds. A. Michaelides and K. Morgenstern, Nature Mater. 6, 597 (2007) [associated press release]
The “smallest particle of ice” – a water hexamer as seen by STM (left) and quantum mechanics (right) |
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