XAS (XANES and EXAFS) Investigations of Nanoparticulate Ferrites Synthesized Continuously in Near Critical and Supercritical Water

Nanoparticulate ferrites (39−105 nm), including magnetite (Fe3O4) and materials containing additional metals (cobalt, zinc, and nickel), have been synthesized continuously in near-critical and supercritical water. For comparison, a cobalt ferrite (particle size, 16 nm) was synthesized by a conventional hydrothermal procedure. The local metal environments of the iron atoms and the additional metal in the ferrites have been studied by X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure (EXAFS) in order to determine the type of spinel structure. Previously reported results for the bulk-phase materials show that Fe3O4 (magnetite), CoFe2O4, and NiFe2O4 exhibit inverse spinel structure, while ZnFe2O4 is a normal spinel. Our results show that the inverse spinel structures extend to the nanoregime. For the ZnFe2O4 material, XANES and EXAFS show that the sample adopts the normal structure in the same size regime. All of the materials are very polydisperse. In the case of CoFe2O4 synthesized under supercritical conditions the EXAFS clearly indicates that the particle sizes are weighted heavily toward 10 nm. To interpret the EXAFS, it was necessary to calculate actual or apparent multiplicities of the different interactions. These are based on 1/3 of the metal atoms occupying the tetrahedral positions and 2/3 the octahedral positions of the spinel structure.