Correlating defect structure to the functional properties of Fe1-xO Wüstite

The aim of this project is to correlate the point defect structure of Fe1-xO to its mechanical, electrical and catalytic properties. Systematic stoichiometric variation of magnetron-sputtered Fe1-xO thin films are investigated regarding structural analysis by transition electron microscopy (TEM) and X-ray absorption near edge structure spectroscopy (XANES), which can reveal the defect point defect structure caused by chemical variation. Following this, the defect structure can be correlated to mechanical properties such as fracture toughness, electrical resistivity, and the catalytic properties for possible future water-splitting applications.

For the future hydrogen economy Fe1-xO (wüstite) is a critical phase for a number of applications. In the CO2-free H2-based iron ore reduction, for example, Fe1-xO is the rate-determining intermediate phase in the production of iron for the steel industry. While for H2 production by water-splitting, Fe1-xO is a potential photochemical material. Since Fe1-xO is non-stoichiometric, defects are inherent to its cubic, rocksalt-type structure. Defect clusters may lead to the formation of defect phases or to a gradual structural change from Fe1-xO to magnetite Fe3O4 [1]. Correlating the defect structure with mechanical and functional properties will allow for improving the efficiency of both CO2-free iron production and water splitting, as pertinent examples.

Fe1-xO thin films are synthesized by reactive magnetron sputtering with a systematic variation of oxygen partial pressure and substrate temperature. For structural analysis, grazing incidence X-ray diffraction as well as selected area electron diffraction in TEM is correlated with spectroscopic techniques such as XANES and photoelectron spectroscopy to reveal the short-range order of Fe2+ and Fe3+ ions present in Fe1-xO. Hardness, Young’s modulus and fracture toughness will be obtained by nanoindentation and micromechanical cantilever bending experiments.

Fig. 1: a) In-plane bright-field TEM image showing nanometer-sized grains of a Fe1-xO thin film with the corresponding selected area electron diffraction (SAED) pattern shown in b). Azimuthal integration of the SAED pattern reveals Fe1-xO to be the dominant phase (c).
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