Atomic-Scale Insights into the FeO-Mediated Oxide Growth during Iron Oxidation

Iron (Fe) oxidation represents a prototypical system for understanding gas-solid reactions, directly impacting corrosion science, heterogeneous catalysis, and oxide-heterostructure design. Although the oxide configurations and oxidation pathways have been extensively documented, the atomic-scale structural dynamics that couple gas-surface reactions to solid-state phase transitions remain limited. Here, using in situ atomic-resolution environmental scanning transmission electron microscopy (ESTEM), we prepare atomically flat metal α-Fe(001) surfaces via thermal annealing and reveal a fundamentally different oxidation pathway from Fe to Fe₃O₄ during cooling under the ESTEM base vacuum. We demonstrate that metastable FeO plays a dynamically sustained role in two intermediate states. First, FeO nucleates on Fe(001) as distinct nanoscale islands below 300 °C, exhibiting a truncated square-pyramidal morphology. Second, FeO is retained as capping layers on the oxide during the transformation of the inner FeO into Fe₃O₄, creating an inverted phase hierarchy (FeO/Fe₃O₄/Fe). The FeO layers provide an active pathway for Fe diffusion, with their growth and transition dominating the subsequent oxide expansion. Despite the distinct growth behaviors of FeO on the (001) and {111} surfaces of oxide, their phase transitions are driven by a common basic unit. Structurally, the FeO-to-Fe₃O₄ transition is achieved via the translocation of octahedral Fe to tetrahedral sites, accompanied by outward diffusion. This work unveils the atomic-scale dynamics governing the coupled surface and interfacial reactions during the Fe oxidation via an island growth mode and establishes a new paradigm for understanding the unconventional oxide configurations in multilayer oxide-forming metals.