Self-Confined Nucleation of Iron Oxide Nanoparticles in a Nanostructured Amorphous Precursor

International audience; Crystallization from solution is commonly described by classical nucleation theory, although this ignores that crystals often form via disordered nanostructures. As an alternative, the classical theory remains widely used in a “multi-step” variant, where the intermediate nanostructures merely introduce additional thermodynamic parameters. But this variant still requires validation by experiments addressing indeed proper time and spatial scales (ms, nm). Here, we used in situ X-ray scattering to determine the mechanism of magnetite crystallization and in particular how nucleation propagates at the nanometer scale within amorphous precursors. We find that the self-confinement by an amorphous precursor slows down crystal growth by two orders of magnitude once the crystal size reaches the amorphous particle size (c.a. 3 nm). Thus, not only the thermodynamic properties of transient amorphous nanostructures, but also their spatial distribution determine crystal nucleation.