Density functional theory investigation of water adsorption on the Fe3O4(001) surface

Magnetite (Fe3O4) plays a significant role in geophysics and
mineralogy and it is a potential spintronics material.
Additionally, it exhibits interesting catalytic properties. As both
in nature and technology, these catalytic reactions typically take
place at the interface with water, it is important to gain a
fundamental understanding of these processes at the atomistic
level. This work comprises the first systematic investigation of
the water adsorption on the Fe3O4(001) surface based on large scale
density functional theory (DFT) calculations. The influence of
electronic correlations was explored within the LDA/GGA+U approach.
A variety of concentrations and configurations of H2O molecules on
the surface with and without defects were studied to explore the
underlying adsorption energetics. The DFT results were extended to
finite temperatures (T) and pressures (p) of the surrounding gas
phase molecules by compiling a surface phase diagram within the
framework of ab initio atomistic thermodynamics. This phase diagram
reveals a dissociative mode of adsorption for an isolated H2O
molecule, especially at oxygen vacancies. With increasing coverage,
a crossover from dissociative to partial dissociation of H2O
molecules on the surface is predicted. This is attributed to
adsorbate-adsorbate interactions stabilized by hydrogen bond
formation between the linear chains of alternating H2O and OH
groups. This partially dissociated termination is stable across a
wide range of water vapor and oxygen partial pressures and
confirmed by a quantitative low energy electron diffraction (LEED)
analysis. In addition, the LEED pattern also indicates a lifting of
the (sqrt(2)X sqrt(2))R45 surface reconstruction. The DFT results
reveal that defects and adsorbates induce a unique charge and
orbital order (CO/OO) on the Fe3O4(001) surface. This provides a
novel way to alter the catalytic properties of the Fe3O4(001)
surface. While the CO/OO in the sub-surface layers lead to an
insulating character of the clean surface, a transition to
half-metallic behavior with the adsorption of H2O molecules is
predicted. This insulating to half-metal transition can be explored
for applications in spintronics. The calculated surface core level
shifts are used to interpret the X-ray photoemission spectroscopy
data, disclosing the major contribution of the screening effects.