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Mulakaluri, Narasimham (2010): Density functional theory investigation of water adsorption on the Fe3O4(001) surface. Dissertation, LMU München: Faculty of Geosciences
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Abstract

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.