Transition in the nanoporous structure of iron oxides during the oxidation of iron nanoparticles and nanowires

Changes in morphology during the oxidation of iron nanoparticles and nanowires in the temperature range 473-873 K have been studied by transmission electron microscopy. Iron nanoparticles and wires become hollow nanoparticles and nanotubes of magnetite at temperatures below 673 K as a result of vacancy aggregation during the oxidation process, resulting from the outward diffusion of iron ions through the magnetite layer. On the other hand, the hollow magnetite transforms into duplex porous structures with an interior nanopore and additional nanovoids at higher temperatures above 673 K, where the inner and outer diameters of magnetite nanotubes shrink and the phase transformation from magnetite to maghemite occurs. Transition in the porous structure seems to be related to the outward diffusion of vacancies from interior pore and the phase transformation from magnetite to maghemite. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.