Research Article Phase Transformation of Iron Oxide Nanoparticles by Varying the Molar Ratio of Fe 2+ :Fe 3+ Co-precipitation from a solution of ferrous/ferric mixed salt with the ratio of Fe 2+ :Fe 3+ = 1:2 in air atmosphere is not a reliable method to synthesize magnetite (Fe 3 O 4 ) nanoparticles because of the fact that Fe 2+ oxidizes to Fe 3+ and the molar ratio of Fe 2+ :Fe 3+ changes. Therefore, the phase composition changes from mag- netite to maghemite (c-Fe 2 O 3 ). The influence of the initial molar ratio of Fe 2+ :Fe 3+ on the phase composition of nanoparticles, their crystallinity and mag- netic properties was studied. Experimental data from XRD, FTIR, SEM, and VSM reveal that the appropriate method to synthesize magnetite nanoparticles is re- verse precipitation from only ferrous salt. It is found that by decreasing the synth- esis temperature and by increasing the concentration of alkaline solution and the ratio of Fe 2+ :Fe 3+ the crystallinity and the specific saturation magnetization (r s ) are increased. Keywords: Iron oxide, Maghemite, Magnetite, Nanoparticles, Phase transformation Received: February 14, 2008; revised: July 31, 2008; accepted: July 29, 2008 DOI: 10.1002/ceat.200800093 1 Introduction Recently, the synthesis and characterization of magnetic nano- particles have attracted much attention, inasmuch as they ex- hibit interesting magnetic properties which can be different from those of the bulk material. Magnetite (Fe 3 O 4 ) and mag- hemite (c-Fe 2 O 3 ) nanoparticles which are mostly used as mag- netic particles, have found extensive applications such as ferro- fluids because of their high magnetic susceptibility and high saturation magnetization. However, magnetite nanoparticles are preferred magnetic particles in ferrofluids due to their greater saturation magnetization [1–3]. Magnetite, a member of spinel-type ferrite, is an important magnetic material. Magnetite nanoparticles have been applied in different fields of applications, such as ferrofluids, recording materials, magnetic refrigeration, magnetic resonance imaging (MRI), cancer therapy, and catalysis [4–9]. Various methods have been reported for the synthesis of Fe 3 O 4 nanoparticles, such as sol-gel, reduction of hematite by CO/CO 2 or H 2 , c-ray radiation, hydrothermal technique, forced hydrolysis, and co- precipitation from a solution of ferrous/ferric mixed salt [10– 16]. Maghemite has a structure similar to that of magnetite. It differs from magnetite in that all or most Fe is in the trivalent state. There are many reports on applications of maghemite nanoparticles including magnetic recording media, ferrofluids, magnetic resonance imaging contrast enhancement, controlled drug delivery, and medical diagnosis [17–21]. Maghemite nanoparticles can be synthesized from several methods, such as sol-gel, microemulsions, high-temperature decomposition of organic precursors, oxidization of magnetite nanoparticles, hydrothermal technique, and co-precipitation [22–27]. Co-precipitation from a solution of ferrous/ferric mixed salt has been widely used to produce magnetite nanoparticles due to its ease, large volume capability, and economy. There are two methods of adding precursors in the technique of co-pre- cipitation from a solution of ferrous/ferric mixed salt to synthesize magnetite: (i) normal co-precipitation and (ii) re- verse co-precipitation. In the first case, the pH value gradually increases, because an alkali solution is dropped into the mixed metal solution. In the second case, the mixed metal solution is directly dropped into an alkaline solution [28]. Consequently, the pH which is a critical factor in synthesis of magnetite could be easily controlled at high values. In addition, in the co-precipitation method according to re- action (1), the initial molar ratio of Fe 2+ :Fe 3+ is considered to be 1:2, but because of the fact that Fe 2+ oxidizes to Fe 3+ in the air (reaction 2) and this oxidation rate is almost independent of the Fe 2+ concentration [29], the Fe 2+ :Fe 3+ ratio of the system reduces from the initial value (1:2). © 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim http://www.cet-journal.com Samaneh Alibeigi 1 Mohammad Reza Vaezi 1 1 Research Center of Advanced Materials, Materials and Energy Research Center, Karaj, Iran. – Correspondence: Dr. M. R. Vaezi (vaezi9016@yahoo.com), Research Center of Research Center of Advanced Materials, Materials and Energy Research Center, Mshkin Dasht, Imam Khomeini Blvd., P.O. Box 31787/ 316, Karaj, Iran. Chem. Eng. Technol. 2008, 31, No. 11, 1591–1596 1591