Resolving Iron(II) Sorption and Oxidative Growth on Hematite (001) Using Atom Probe Tomography

The distribution of Fe resulting from the autocatalytic interaction of aqueous Fe(II) with the hematite (α-Fe 2 O 3 ) (001) surface was directly mapped in three dimensions (3D) for the first time, using Fe isotopic labeling and atom probe tomography (APT). Micrometer-sized hematite platelets were reacted with aqueous Fe(II) enriched in 57 Fe and prepared for APT using conventional focused ion beam lift-out techniques. Mass spectrum analyses show that specific Fe-ionic species (i.e., Fe 2+ and FeO + ) accurately reproduce isotopic ratios within natural abundance in the hematite bulk, and thus were utilized to characterize the distribution of 57 Fe and quantify Fe isotopic concentrations. 3D reconstructions of Fe isotopic positions along the surface normal direction showed a zone enriched in 57 Fe, consistent with oxidative adsorption of Fe(II) and growth at the relict hematite surface reacted with 57 Fe(II) aq . An average net adsorption of 3.2–4.3 57 Fe atoms nm –2 was estimated using Gibbsian interfacial excess principles. Statistical, grid-based frequency distribution analyses show a heterogeneous, nonrandom distribution of 57 Fe across the 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 57 Fe atoms and the extent of atomic segregation on the hematite surface. This new ability to spatially map growth on specific crystal faces will potentially enable resolution of long-standing unanswered questions about underlying mechanisms for electron transfer and atom exchange involved in redox-catalyzed processes at this archetypal and broadly relevant interface.