Atomic Structural Evolution during the Reduction of α-Fe₂O₃ Nanowires

The atomic-scale reduction mechanism of α-Fe₂O₃ nanowires by H₂ 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^th [Formula: see text] plane. This vacancy ordering is followed by an allotropic transformation of α-Fe₂O₃ → γ-Fe₂O₃ along with the formation of Fe₃O₄ nanoparticles on the surface of the γ-Fe₂O₃ 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-Fe₂O₃ nanowires results in the formation of a unique hierarchical structure of hybrid oxides consisting of the parent oxide phase, γ-Fe₂O₃, as the one-dimensional wire and the Fe₃O₄ 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-Fe₂O₃. Compared to previous reports of α-Fe₂O₃ directly transformed to Fe₃O₄, our work provides a more in-depth understanding with substeps of reduction, i.e., the whole reduction process follows: α-Fe₂O₃ → α-Fe₂O₃ superlattice → γ-Fe₂O₃ + Fe₃O₄→ Fe₃O₄.