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Structure and Chemistry of Iron-Oxide Surfaces

Iron oxides are omnipresent in the natural environment on Earth, and have been found to exist in large amounts on our Moon as well as Mars. Reactions happening at mineral surfaces play an important role in geo- and atmospheric chemistry, and we have found many opportunities to use the reactivity of these materials in catalysis. The seminar will focus on atomic-scale studies of processes occurring at the surfaces of hematite (α-Fe2O3) and magnetite (Fe3O4) single crystals (1). Single crystal samples are prepared in vacuum such that the structure is precisely known (2, 3), and adsorption/reactivity is probed via a combination of scanning probe microscopies and electron spectroscopies. This provides an ideal complement to density functional theory (DFT) calculations, and allows a complete picture to be developed. In recent times, we have extended our work to include studies in ambient pressure and ultra-pure liquid water environments (4), and this will be the primary topic of the presentation. We find that partially dissociated water agglomerates are the most stable species on both the Fe2O3(012) and Fe3O4(001) surfaces in UHV (5-7), and that significant oxygen exchange occurs during water desorption from the hematite surface. Such a process does not occur on magnetite, but Fe3O4(001) undergoes a self-limited transformation to an (oxy)hydroxide phase at pH7 (8), and dissolves if CO2 is permitted to acidify the water during the experiment (9).

1.  G. S. Parkinson, Iron oxide surfaces. Surf. Sci. Rep. 71, 272-365 (2016).

2.  R. Bliem et al., Subsurface cation vacancy stabilization of the magnetite (001) surface. Science 346, 1215-1218 (2014).

3.  F. Kraushofer et al., Atomic-Scale Structure of the Hematite α-Fe2O3(11̅02) “R-Cut” Surface. J. Phys. Chem. C 122, 1657-1669 (2018).

4.  J. Balajka et al., High-affinity adsorption leads to molecularly ordered interfaces on TiO2 in air and solution. Science 361, 786 (2018).

5.  M. Meier et al., Water agglomerates on Fe3O4(001). Proc. Natl. Acad. Sci. U.S.A. 115, E5642- E5650 (2018).

6.  Z. Jakub et al., Partially Dissociated Water Dimers at the Water-Hematite Interface. ACS Energy Letters, (2019).

7.  E. Zaki et al., Water Ordering on the Magnetite Fe3O4 Surfaces. J. Phys. Chem. Lett. 10, 2487- 2492 (2019).

8.  F. Kraushofer et al., Self-limited growth of an oxyhydroxide phase at the Fe3O4(001) surface in liquid and ambient pressure water. J. Chem. Phys. 151, 154702 (2019).

9.  F. Mirabella et al., Atomic-Scale Studies of Fe3O4(001) and TiO2(110) Surfaces Following Immersion in CO2-Acidified Water. ChemPhysChem 21, 1788-1796 (2020).