Uncovering the Size‐Dependent Thermal Solid Transformation of Akaganéite

Abstract Investigating the structural evolution and phase transformation of iron oxides is crucial for gaining a deeper understanding of geological changes on diverse planets and preparing oxide materials suitable for industrial applications. In this study, in‐situ heating techniques are employed in conjunction with transmission electron microscopy (TEM) observations and ex‐situ characterization to thoroughly analyze the thermal solid‐phase transformation of akaganéite 1D nanostructures with varying diameters. These findings offer compelling evidence for a size‐dependent morphology evolution in akaganéite 1D nanostructures, which can be attributed to the transformation from akaganéite to maghemite (γ‐Fe₂O₃) and subsequent crystal growth. Specifically, it is observed that akaganéite nanorods with a diameter of ∼50 nm transformed into hollow polycrystalline maghemite nanorods, which demonstrated remarkable stability without arresting crystal growth under continuous heating. In contrast, smaller akaganéite nanoneedles or nanowires with a diameter ranging from 20 to 8 nm displayed a propensity for forming single‐crystal nanoneedles or nanowires through phase transformation and densification. By manipulating the size of the precursors, a straightforward method is developed for the synthesis of single‐crystal and polycrystalline maghemite nanowires through solid‐phase transformation. These significant findings provide new insights into the size‐dependent structural evolution and phase transformation of iron oxides at the nanoscale.