PNNL

Abstract

The distribution of iron resulting from the autocatalytic interaction of aqueous Fe(II) with the hematite (001) surface was directly mapped in three dimensions (3D) for the first time, using iron isotopic labelling and atom probe tomography (APT). Analyses of the mass spectrum showed that natural abundance ratios in 56Fe-dominant hematite are recovered at depth with good accuracy, whereas at the relict interface with 57Fe(II) solution evidence for hematite growth by oxidative adsorption of Fe(II) was found. 3D reconstructions of the isotope positions along the surface normal direction showed a zone enriched in 57Fe, which was consistent with an average net adsorption of 3.2 – 4.3 57Fe atoms nm–2. Statistical analyses utilizing grid-based frequency distribution analyses show a heterogeneous, non-random distribution of oxidized Fe on the (001) surface, consistent with Volmer-Weber-like island growth. The unique 3D nature of the APT data provides an unprecedented means to quantify the atomic-scale distribution of sorbed 57Fe atoms and the extent of segregation on the hematite surface. This new ability to spatially map growth on single crystal faces at the atomic scale will enable resolution to long-standing unanswered questions about the underlying mechanisms for electron and atom exchange involved in a wide variety of redox-catalyzed processes at this archetypal and broadly relevant interface.