Atomic Structural Evolution during the Reduction of α-FeO Nanowires.

The atomic-scale reduction mechanism of α-FeO nanowires by H was followed using transmission electron microscopy to reveal the evolution of atomic structures and the associated transformation pathways for different iron oxides. The reduction commences with the generation of oxygen vacancies that order onto every 10 [Formula: see text] plane. This vacancy ordering is followed by an allotropic transformation of α-FeO → γ-FeO along with the formation of FeO nanoparticles on the surface of the γ-FeO nanowire by a topotactic transformation process, which shows 3D correspondence between the structures of the product and its host. These observations demonstrate that the partial reduction of α-FeO nanowires results in the formation of a unique hierarchical structure of hybrid oxides consisting of the parent oxide phase, γ-FeO, as the one-dimensional wire and the FeO in the form of nanoparticles decorated on the parent oxide skeleton. We show that the proposed mechanism is consistent with previously published and our density functional theory results on the thermodynamics of surface termination and oxygen vacancy formation in α-FeO. Compared to previous reports of α-FeO directly transformed to FeO, our work provides a more in-depth understanding with substeps of reduction, i.e., the whole reduction process follows: α-FeO → α-FeO superlattice → γ-FeO + FeO→ FeO.