Mechanisms for Iron Oxide Formation under Hydrothermal Conditions: An in Situ Total Scattering Study

The formation and growth of maghemite (γ-Fe2O3) nanoparticles from ammonium iron(III) citrate solutions (C6O7H6·xFe3+·yNH4) in hydrothermal synthesis conditions have been studied by in situ total scattering. The local structure of the precursor in solution is similar to that of the crystalline coordination polymer [Fe(H2cit(H2O)]n, where corner-sharing [FeO6] octahedra are linked by citrate. As hydrothermal treatment of the solution is initiated, clusters of edge-sharing [FeO6] units form (with extent of the structural order <5 Å). Tetrahedrally coordinated iron subsequently appears, and as the synthesis continues, the clusters slowly assemble into crystalline maghemite, giving rise to clear Bragg peaks after 90 s at 320 °C. The primary transformation from amorphous clusters to nanocrystallites takes place by condensation of the clusters along the corner-sharing tetrahedral iron units. The crystallization process is related to large changes in the local structure as the interatomic distances in the clusters change dramatically with cluster growth. The local atomic structure is size dependent, and particles smaller than 6 nm are highly disordered. The final crystallite size (<10 nm) is dependent on both synthesis temperature and precursor concentration