Phase Transformations and Growth Mechanisms of Iron-Based Nanoparticles during Oxidation in Air

摘要

Iron nanoparticles were successfully synthesized by CVC process using iron pentcarbonyl as a precursor under Ar or He atmosphere. The spherical nanoparticles of the mean diameter of 6 ― 25nm comprise the metal core and oxide shell. Average particle size increases and size distribution becomes wider and more asymmetric with increasing the decomposition temperature. Particles produced by CVC have larger size if argon as carrier gas was used. The increase of lattice parameter of metallic core with the decreasing particle size can be explained by the interaction between metal cores and oxide shells. Oxidation heat treatment leads to successive appearance of oxide phases. At the temperature of about 300°C internal voids starts to form in particles structure. Those phenomena cause initial growth of particles. Process of particles coagulation starts at the temperatures less than 400°C and leads to the disappearance of the separated particles and the formation of continuous rope-like structure. Particles coagulation process associated with the maghemite-hematite transformation. New large hematite particles of 30 ― 50nm in diameter form at the temperature of 600°C and do not grow significantly as the temperature increases. The mean size of hematite particles depends on initial size of iron particles. Low temperature heat treatment in argon atmosphere with controlled oxygen can improve magnetic properties of nanoparticles.