Ewald methods for polarizable surfaces with application to hydroxylation and hydrogen bonding on the (012) and (001) surfaces of alpha-Fe2O3

TL;DR

This paper introduces a faster Ewald-like method for calculating electrostatic energies in 2D periodic systems, applied to study hydroxylation and hydrogen bonding on alpha-Fe2O3 surfaces through molecular dynamics simulations.

Contribution

A new efficient Ewald-like method for 2D periodic electrostatics and its application to surface hydroxylation and hydration studies of alpha-Fe2O3.

Findings

Hydroxylation significantly alters surface structure.

Hydration has a minor effect on surface relaxation.

Greater hydroxylation predicted on the (012) surface.

Abstract

We present a clear and rigorous derivation of the Ewald-like method for calculation of the electrostatic energy of the systems infinitely periodic in two-dimensions and of finite size in the third dimension (slabs) which is significantly faster than existing methods. Molecular dynamics simulations using the transferable/polarizable model by Rustad et al. were applied to study the surface relaxation of the nonhydroxylated, hydroxylated, and solvated surfaces of alpha-Fe2O3 (hematite). We find that our nonhydroxylated structures and energies are in good agreement with previous LDA calculations on alpha-alumina by Manassidis et al. [Surf. Sci. Lett. 285, L517, 1993]. Using the results of molecular dynamics simulations of solvated interfaces, we define end-member hydroxylated-hydrated states for the surfaces which are used in energy minimization calculations. We find that hydration has a small effect on the surface structure, but that hydroxylation has a significant effect. Our calculations, both for gas-phase and solution-phase adsorption, predict a greater amount of hydroxylation for the (012) surface than for the (001) surface. Our simulations also indicate the presence of four-fold coordinated iron ions on the (001) surface.