Electronic structure and charge ordering in magnetite: implications for the Fe3O4 (001)–water interface

Abstract

Electronic structure calculations on atomistic models of magnetite (Fe3O4) provide valuable insight into the structure and properties that dictate the behaviour of magnetite under environmental conditions. The charge ordering in the bulk oxide controls the reactivity of the exposed surfaces, but it has been difficult to measure experimentally or predict theoretically. We use spin-polarised density functional theory to calculate the structure of bulk Fe3O4 and its (001) and (111) surface terminations. We then present an ab initio thermodynamics approach to determine the most energetically favourable clean and hydroxylated surface terminations of Fe3O4 (001). We present results on molecular water adsorption and heterolytic dissociation at both quarter- and half-monolayer coverage for Fe3O4 (001). Our calculations suggest that the tetrahedral (001) surface termination is the most energetically stable across all oxygen chemical potentials, and that water molecules preferentially dissociate at the surface. The calculated hydroxylated and hydrated surface terminations, as obtained from careful consideration of the charge ordering in the bulk, serve as a strong basis for future studies of the electrical double layer at the mineral–water interface.