Highly Ordered Nanostructured Magnetite and Maghemite of Micrometric Sized and Rhombohedral Habit

M.L. Kiss, A. Ercuta, M. Chirita, C. Savii, A. Ieta
SUNY Oswego, US

Keywords: siderite, iron oxide, single crystalline, mesocrystalline, topotactic

Summary:

Despite some reports regarding the obtaining of magnetite or maghemite by thermal treatment of siderite, none of them refer to porosity developed in these iron oxides after the thermal treatment. We present here a case of porous magnetite and vacancy-ordered maghemite rhombohedron-shaped crystals reaching 150μm in size having remarkably large specific surface area of 88.55 m2/g for magnetite, and 40.14 m2/g for maghemite obtained via topotactical conversion, starting from FeCO3 single crystals. For synthesizing single crystalline FeCO3, we developed a new procedure by hydrothermal decomposition of the Fe(III)-EDTA complex in the presence of urea, starting from Ferric Ammonium Sulfate and Na4EDTA as main precursors. Single phase FeCO3 microcrystals with size in the range of 150µm-200µm have been obtained after high pressure-temperature treatment time between 15 hours and 26 hours at 230ºC and 250ºC. We observed an increase in density (from 3.9 g/cm3 for siderite to 5.24 g/cm3 for magnetite and 4.9 g/cm3 for maghemite) caused quasi-ordered internal pores-grains pattern, with mesocrystalline appearance. These results demonstrate the possibility of obtaining highly ordered nanostructured porous rhombohedral submillimeter-sized magnetite and maghemite crystals with remarkably large specific surface area usually associated with nanometric crystallites. These properties could be used in various catalytic applications.