Iron oxide nanoparticles are used extensively in applications in biomedicine, magnetic storage media, water treatment and catalysis. The most ubiquitous synthetic method to date is the co-precipitation method, which is carried out in aqueous solutions containing ferric (Fe3+) and ferrous (Fe2+) salts as precursors, to which a base is added at moderate temperatures (<100 °C).
Even though the co-precipitation reaction is widely used, the mechanism of the reaction is still poorly understood, due to a severe lack of information on how intermediates are formed.
Synchrotron X-ray diffraction revealed the initial formation of crystalline iron hydroxide carbonate (green rust) plates occurringbefore the Fe3O4/γ-Fe2O3 appeared. The ferrihydrite particles increase in size over time as the proportion of iron hydroxide carbonate plates are re-dissolved into solution, until the ferrihydrite particles crystallise into Fe3O4/γ-Fe2O3.
A new particle formation mechanism is detailed,

differing drastically from the most commonly discussed classical nucleation and growth process. Such comprehensive understanding of the mechanism is essential not only for robust syntheses, but also for the tuning of particle properties for the targeted applications. In particular, the growth mechanism shows a separation of the nucleation and growth stages in the reaction which will allow chemists greater control over the size of the nanoparticles that can be readily synthesised via co-precipitation.