Von Rudorff, Guido Falk; (2017) Structure and Dynamics of the Hematite/Liquid Water Interface. Doctoral thesis , UCL (University College London).

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Abstract

The hematite/liquid water interface is of interest for a wide range of applications. This work investigates structural and dynamic properties of the interface for several experimentally relevant combinations of crystal cuts, termination and protonation schemes. To this end, molecular dynamics simulations at hybrid functional DFT level have been performed for both neutral and charged systems. For bulk hematite, this work shows that the commonly used GGA+U level of theory gives good results for geometric properties, but cannot capture electronic structure equally well. However, a modified hybrid functional based on HSE06 is able to describe both geometric and electronic properties with sufficient accuracy. The two most common crystal surfaces and their most common terminations have been investigated and compared to experiment where possible. The resulting structural information is in good agreement with experiment, but highlights the importance of dynamic equilibria for the solvation structure. This work also shows that classical force fields cannot readily describe the surface protonation structure and dynamics. Besides an atomistic view of the surface structure, protonation dynamics, surface restructuring mechanisms and interconversion of surface aquo groups to bulk solvent are discussed. Two new methods are suggested in this work: (i) a screening method for Hartree-Fock exchange forces that significantly accelerates hybrid functional-based molecular dynamics calculation and (ii) a guided thermodynamic integration scheme for free energy estimation from short trajectories. The implementation thereof is tested and benchmarked and applied to the hematite/liquid water interface. To accelerate the calculation of HFX forces in a molecular dynamics force screening scheme is proposed. For actual systems of interest, this can speed up the whole molecular dynamics run by a factor of three. The method is assessed on a wide range of materials and for various properties including energy conservation. Finally, the pKa value of a surface protonation is calculated. By means of thermodynamic integration, the free energy difference between a proton at the surface of hematite and a solvated proton in bulk water is quantified. Various integration schemes are evaluated and a new analysis method is proposed to reduce human bias in the analysis and to automatically detect convergence of the vertical energy gap time series.

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