Uncovering the Size-Dependent Thermal Solid Transformation of Akaganéite to Maghemite and Hematite

Investigating the structural evolution and phase transformation of iron oxides is crucial for gaining a deeper understanding of geological changes on diverse planets and identifying oxide materials suitable for industrial applications. In this study, we employed in-situ heating techniques in conjunction with transmission electron microscopy (TEM) observations and ex-situ characterization to thoroughly analyze Akaganeite nanowires of varying sizes. Our findings offer compelling evidence for a size-dependent morphology evolution in Akaganeite nanowires, which can be attributed to the transformation from Akaganeite to Maghemite and subsequent crystal growth. Specifically, we observed that 50 nm Akaganeite nanorods transformed into hollow polycrystalline Maghemite nanorods, which demonstrated remarkable stability without arresting crystal growth under continuous heating. In contrast, smaller Akaganeite nanowires ranging from 20 to 8 nm displayed a propensity for forming single-crystal nanowires through phase transformation and densifying. Furthermore, we successfully captured the solid transformation of Maghemite to Hematite under vacuum heating conditions, suggesting a potential influence of orientation-dependent behavior on this transformation process. These significant findings provide new insights into the size-dependent structural evolution and phase transformation of iron oxides at the nanoscale, specifically in relation to diffusion-controlled crystal growth.