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 α-Fe₂O₃ → γ-Fe₂O₃ phase transformation of one part of the bicrystal, resulting in the formation of a two-phase structure of α-Fe₂O₃ and γ-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 γ-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.