The hematite-liquid water interface is of key importance to applications in a wide range of research areas like mineralogy, colloid chemistry and electrochemistry because hematite is abundant, cheap, stable in aqueous environments over a wide pH range. With a band gap of about 2.1 eV which is well-suited for absorption from the solar spectrum, hematite remain a material of great interest.<br/>The chemical properties of this interface and reactions such as interfacial charge transfer are fundamentally controlled by surface reactions with water and the dynamical equilibrium that emerges.  Though computationally intensive, ab initio molecular dynamics simulations are an increasingly essential tool to properly understand surface acidity arising through the coupled fluctuations in the structure of the hydroxylated hematite surface and adjacent water molecules.  However, the internal electronic structure (band gap, antiferromagnetic spin patterns and band ordering) renders the simulation highly challenging since hybrid functionals are required to adequately describe the simulation setup which tremendously increases the necessary computational effort. In fact, the calculations are only possible thanks to recent advances in computational methods and high performance computing.<br/>We present a simulation procedure that allows to quickly obtain an equilibrated configuration of the hematite-water interface which is transferable to other surfaces, as well. Based on 30ps of hybrid-functional molecular dynamics, we present the atomistic structure of the interface for different surface terminations which is of crucial importance for the interpretation of experimental results. We observe only a short-range perturbation of the water structure due to the interface but a strong interaction between the surface and the first solvation layer. This shields surface protons from solvent interaction and, thereby, impacts the acidity thereof.<br/>Differences in the structure of the terminations give rise to a significantly different hydrogen bond structure of the interface. In particular, we observe hydrogen bonding patterns that are stable on the 1ps timescale and quickly interconvert within 50fs. Their features (like hydrogen bond chains across the surface) can be explained by a combination of geometric and electronic arguments.<br/>Through thermodynamic integration, we obtain pKa estimates at GGA level which is helpful to evaluate the quality of estimates from bond valence sum considerations.

Structure and dynamics of the hematite(001)-liquid water interface: pKa estimates from thermodynamic integration and hydrogen bond patterns

255th National Meeting (2018)

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