Morphology-phase coevolution driven by oxygen chemical potential in Fe₃O₄/α-Fe₂O₃ nanosheets

Studies on the morphology and phase of two-dimensional (2D) non-van der Waals iron oxides are often carried out independently, leaving their coupling relationship unexplored. Herein, we report an oxygen-potential-driven chemical vapor deposition (CVD) route that couples morphology control with phase selection in the same system. We vary only the quartz-tube outer diameter to modulate the oxygen flux, which tunes the oxygen chemical potential (μ_O) without requiring any sophisticated oxygen supplying equipment and transforms the product from magnetite (Fe₃O₄) to hematite (α-Fe₂O₃). With increasing μ_O, Fe₃O₄ evolves from sharp triangular to truncated triangular nanosheets. However, regular hexagonal α-Fe₂O₃ nanosheets are obtained above this threshold, indicating morphology-phase coevolution. Opposite curvatures of Fe₃O₄ {111} A/B edges create a surface chemical-potential difference (Δμ) that drives the evolution into nanosheets bounded solely by A-type edges. DFT calculations show that increasing μ_O reduces Δμ, enabling the retention of the B edge and yielding truncation. Correlative magnetic imaging further reveals morphology-domain coupling: a vortex state in sharp triangles, a weakened vortex upon truncation, multidomain characteristics when approaching the hexagonal shape, and weak-ferromagnetic α-Fe₂O₃ with no magnetic phase contrast. This facile regulation method synchronizes phase selection and morphological engineering by regulating μ_O, laying the groundwork for structure-dependent spintronic device design.