Controlled synthesis of cobalt-doped magnetic iron oxide nanoparticles

Uniform sphere-like Co-doped iron oxide nanoparticles were synthesized by the thermolysis of iron oleate and cobalt oleate precursors in octadecene in the presence of oleic acid. The Co/Fe mole ratios in the final Co-doped iron oxide can be controlled by changing their ratios in the reaction precursors. With the increase of Co/Fe mole ratios from 0.024 to 0.156 in the final iron oxide nanoparticles, the magnetic saturation values of the nanoparticles slightly decreased from 39 to 30 emu·g ^-1, while their coercivity values increased from 0 to 190 Oe. The sizes of the Co-doped iron oxide nanoparticles increased from 7 to 14 nm when the thermolysis time was increased from 0.5 to 3 h with a thermolysis temperature 305 °C. Generally, with increasing thermolysis time the metal elements in the final nanoparticles were partially reduced. The domain polymorph of the final nanoparticles was magnetite with a very small amount of ferrous oxide for thermolysis time less than 1 h, while the phases were a mixture of magnetite and wüstite ferrous oxide for thermolysis time longer than 2 h. Further increasing thermolysis time to 3 h, except Fe ₃O ₄ and FeO, CoFe alloys appear too. It indicates that the iron (cobalt) elements were reduced from trivalence to bivalence, and finally to zero valence. The sizes of the iron oxide nanoparticles increased and more ferrous oxide appeared in the final products with increasing the thermolysis temperature.