Quantification of Partial Oxidation of Magnetite to Maghemite in Aqueous Solution Using XRD

Abstract In aqueous solutions, magnetite (Fe3O4) nanoparticles (NPs) easily oxidize into a material with crystalline characteristics in between magnetite and maghemite (γ-Fe2O3), inevitably modifying their properties. The routine quantification and characterization of this transformation were previously hampered by the structural similarity of magnetite and maghemite. Here, we compared X-ray Absorption Fine Structure (XAFS) characterization to laboratory-based X-ray Diffraction (XRD) pattern analysis, such as Rietveld refinement and peak position shifts of the (311), (400), (511), and (440) reflections, to determine if XRD can be used to determine magnetite oxidation by itself. We analyzed the transformation extent of nanoparticles (10 nm) reacting under different concentrations of oxidizing agents such as O2 and H2O2. We showed that XRD methods─Rietveld refinements and single peak, such as (511)-based─can within error reproduce XAFS results. Our results document a more accessible characterization method for the quantification of magnetite NPs partial oxidation in solution.