Coverage-Dependent Adsorption Mode of Water on Fe3O4(001): Insights from First Principles Calculations

Abstract

Using density functional theory calculations together with an on-site Coulomb repulsion term (GGA+U), we investigate the adsorption of water on Fe3O4(001). Starting from a single water molecule per (� 2 ×� 2)R45° unit cell, we vary the concentration and configuration of water and hydroxyl groups. Isolated water molecules on the clean surface tend to dissociate heterolytically with an OH group adsorbed on top of an octahedral iron and a proton donated to a surface oxygen. Furthermore, oxygen defects are found to promote strongly water dissociation. The released protons bind to distant surface oxygen to minimize the repulsive interaction between the surface OH groups. At higher coverages, the interplay between adsorbate-adsorbate and adsorbate-substrate interactions and the formation of hydrogen bonds between the surface species result in a crossover to a mixed adsorption mode where every second molecule is dissociated. The energetic trends are related to the underlying electronic mechanisms.