In Situ Atomic-Scale Probing of the Reduction Dynamics of Two-Dimensional Fe2O3 Nanostructures

Atomic-scale structural dynamics and phase transformation pathways were probed, in situ, during the hydrogen-induced reduction of Fe2O3 nanostructure bicrystals using an environmental transmission electron microscope. Reduction commenced with the alpha-Fe2O3 -> gamma-Fe2O3 phase transformation of one part of the bicrystal, resulting in the formation of a two-phase structure of alpha-Fe2O3 and gamma-Fe2O3. The progression of the phase transformation into the other half of the bicrystalline Fe2O3 across the bicrystalline boundary led to the formation of a single-crystal phase of gamma-Fe2O3 with concomitant oxygen-vacancy ordering on every third {422} plane, followed by transformation into Fe3O4. Further reduction resulted in the coexistence of Fe3O4, FeO, and Fe via the transformation pathway Fe3O4 -> FeO -> Fe. The series of phase transformations was accompanied by the formation of a Swiss-cheese-like structure, induced by the significant volume shrinkage occurring upon reduction. These results elucidated the atomistic mechanism of the reduction of Fe oxides and demonstrated formation of hybrid structures of Fe oxides via tuning the phase transformation pathway.