From wustite to hematite: thermal transformation of differently sized iron oxide nanoparticles in air

We have investigated the oxidation behavior of iron oxide nanoparticles in air at elevated temperatures. By wet chemical synthesis under reducing conditions, polycrystalline iron oxide nanoparticles of different sizes were produced. The samples were characterized by x-ray diffraction and transmission electron microscopy. The freshly prepared particles show dominantly the wustite phase and, in addition, one or both of the spinel-like phases maghemite or magnetite. The hematite phase is absent. By annealing under air at different temperatures, we observe a successive transformation of the initial phases to phases of higher oxidation state, until the samples consist completely of the hematite phase. During this transformation, the relative amount and the evolution of the crystallite sizes of the different phases are in the focus of the investigation. We found that the maximum temperature required for a full conversion into the hematite phase depends on the particle size and increases for the larger particles. At the same time, the average crystallite size of the large particles decreases during the initial annealing procedure, passing through a minimum, before increasing again until single crystalline particles are formed.