Magnetic iron oxide nanoparticles were successfully prepared by a novel reverse precipitation method with the irradiation of ultrasound. TEM, XRD and SQUID analyses showed that the formed particles were magnetite (Fe3O4) with about 10 nm in their diameter. The magnetite nanoparticles exhibited superparamagnetism above 200 K, and the saturation magnetization was 32.8 emu/g at 300 K. The sizes and size distributions could be controlled by the feeding conditions of FeSO4 · 7H2O aqueous solution, and slower feeding rate and lower concentration lead to smaller and more uniform magnetite nanoparticles. The mechanisms of sonochemical oxidation were also discussed. The analyses of sonochemically produced oxidants in the presence of various gases suggested that besides sonochemically formed hydrogen peroxide, nitrite and nitrate ions contributed to Fe(II) ion oxidation.
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