New insights over the phase evolution during the colloidal synthesis of iron oxide nanocrystals

Albert Ruiz-Blavia, Andreu Cabot, and Pablo Guardia

E-mail: pguardia@icmab.es

For the last 10 years, magnetite nanocrystals (NCs) have demonstrated to be key actors in nanomedicine.^1,2 Nevertheless, its deployment in real applications is facing some issues such as the colloidal stability for large NCs or the production of high quality NCs. The latter is of paramount importance for some applications such as magnetic fluid hyperthermia where NCs have to show the highest magnetic properties for an efficient and safe treatment. High quality magnetite NCs are extremely complicated to obtain by means of feasible, reproducible, and large-scale syntheses. However, there are several reported large-scale routes leading to highly monodisperse NCs but with rather poor magnetic properties. One representative example of this is the thermal decomposition of an iron oxide-hydroxide precursor in the presence of oleic acid and 1-octadecene under inert conditions proposed by Colvin et al.³ Noteworthy, NCs showed an inverse spinel structure yet the magnetic properties were significantly poor. In this work, iron oxide NCs with size from 7 up to 37 nm were produced using different ligand to precursor molar ratios, but also replacing 1-octadecene by squalane as solvent. In addition, XRD patterns of as-synthesized NCs showed a combination of different iron oxide phases (i.e. magnetite, wüstite and hematite) as a function of size. The crystal structure seems to depend on the processing route (the degassing condition, reaction temperature) and, as mentioned, on the NCs size. In this regard, results suggest that, regardless of the size or conditions, as-synthesized NCs show a wüstite structure when produced. Afterwards, uncontrolled post-synthesis oxidation results in the evolution of the wüstite structure towards a magnetite one. Depending on the oxidizing conditions and the size of the NCs, XRD can be indexed to a magnetite structure, or to a mixture of wüstite and magnetite. The last case was observed for large NCs (above 18 nm) which highlights that small NCs get fully oxidized under mild oxidation conditions while large NCs are partially oxidized leading to a wüstite-magnetite core-shell structure. The results here reported provide insights about the low performance of some magnetite NCs reported with different colloidal approaches and highlight the need for additives to be involved during the synthesis to ensure the direct formation of a magnetite structure.