Structure, Magnetism, and the Interaction of Water with Ti-Doped Fe₃O₄ Surfaces

The functionality of magnetite, Fe₃O₄, for catalysis and spintronics applications is dependent on the molar ratio of Fe²⁺ and Fe³⁺ and their distribution at the surface. In turn, this depends on a poorly understood interplay between crystallographic orientation, dopants, and the reactive adsorption of atmospheric species such as water. Here, (100)-, (110)-, and (111)-oriented films of titano-magnetite, Fe_(3-x)Ti_xO₄, were grown by pulsed laser deposition and their composition, valence distribution, magnetism, and interaction with water were studied by ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and X-ray magnetic circular dichroism. Although the bulk compositions match the desired stoichiometry, the surfaces were found to be enriched in Ti⁴⁺, especially the top 1 nm. The highest surface energy (110) film was the most reduced, tied to local Ti enrichment, and a corresponding decreased magnetic moment. AP-XPS showed that incorporation of x = 0.25 Ti dramatically lowered the propensity to form hydroxyl species at a given relative humidity, and also that hydroxylation is relatively invariant with orientation. In contrast, the affinity for water is similar across orientations, regardless of Ti incorporation, suggesting that relative humidity controls its uptake. The findings may help demystify the interactions that lead to specific distributions of Fe²⁺ and Fe³⁺ at magnetite surfaces, toward design of more deliberately active catalysts and magnetic devices.