Abstract
Atomic-scale structural dynamics and phase transformation pathways were probed, in situ, during the hydrogen-induced reduction of Fe(2)O(3) nanostructure bicrystals using an environmental transmission electron microscope. Reduction commenced with the α-Fe(2)O(3) → γ-Fe(2)O(3) phase transformation of one part of the bicrystal, resulting in the formation of a two-phase structure of α-Fe(2)O(3) and γ-Fe(2)O(3). The progression of the phase transformation into the other half of the bicrystalline Fe(2)O(3) across the bicrystalline boundary led to the formation of a single-crystal phase of γ-Fe(2)O(3) with concomitant oxygen-vacancy ordering on every third {422} plane, followed by transformation into Fe(3)O(4). Further reduction resulted in the coexistence of Fe(3)O(4), FeO, and Fe via the transformation pathway Fe(3)O(4) → 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.