Synthesis of Fe3O4 nanoparticles at 100 °C and its magnetic characterization
Introduction
Transition metal oxides have been of scientific and technological interest for many decades due to their interesting optical, magnetic, electrical and catalytical properties. Among these, magnetite (Fe3O4) is a common magnetic iron oxide that has a cubic inverse spinel structure with fcc close packed oxygens and Fe cations occupying interstitial tetrahedral and octahedral sites [1]. Magnetite unit cell can be represented with the formula (Fe83+)A[Fe40/33+Fe8/32+]BO32, where A and B indicates tetrahedral and octahedral positioning, respectively. The electrons can hop between Fe2+ and Fe3+ ions in the octahedral sites at room temperature, rendering magnetite an important class of half-metallic materials [2], [3]. Its particle dispersions are widely used as ferrofluid in, for example, rotary shaft sealing, oscillation damping, and position sensing [4], [5]. The use of surface functionalized aqueous suspension of magnetite nanoparticles in clinical medicine has also intensified [6], [7], [8]. Magnetic nanoparticles are also used extensively in the field of biomagnetics for a broad range of applications, such as drug delivery [9], [10], [11], cell labeling and sorting [12], magnetic resonance imaging, sensing [13], [14] as well as therapeutic applications [15], [16] such as and AC magnetic field-assisted cancer therapy, i.e. hyperthermia, PDT [17]. All these technological and medical applications require that the nanoparticles are superparamagnetic with sizes smaller than 20 nm with narrow size distribution to have uniform physical and chemical properties. Their reduced size and large surface-to-volume ratio leads to distinct magnetic [18], electronic [19], and optical [20] properties which are different from those of their bulk counterparts. However, producing magnetite particles with the desired size and acceptable size distribution without particle aggregation has consistently been a problem.
Wet-chemical syntheses, such as coprecipitation, sol–gel, hydrothermal and micro-emulsion techniques effectively control the morphology and chemical composition of prepared powder. Particle synthesis by gel to crystalline conversion method helps to obtain final products at temperatures around 100 °C [21], [22], [23]. This method differs from the traditional sol–gel technique in two aspects: (i) no expensive alkoxide reactants are required, and (ii) no need of higher temperature calcinations to produce final product. A metal hydroxide gel is formed by the addition of a strong base to ferrous chloride solution. Metal hydroxide gels are in general polymeric chains forming an entangled network in which solvent is entrapped. The gel is stable due to osmotic pressure, which is the sum of the rubber elasticity, polymer–polymer affinity and hydrogen ion pressure, and if any one of these factors is altered, the gel collapses irreversibly. In the gel to crystalline method for particle synthesis, a continuous influx of the solvent breaks the gel network and creates small crystalline iron-oxide regions. This crystallization is favored by the reduction in the free energy, resulting in the formation of crystalline phase at around 100 °C [24]. Though gel to crystalline synthesis of nanocrystalline TiO2 [25], SnO2 [26], Ce0.75Zr0.25O2 [27], and Mn3O4 [28] are reported, this is the first report of synthesis of magnetic Fe3O4 nanoparticles by this novel method. Other advantages of this method includes, use of ferrous chloride as the sole iron precursor, cost-effective, and lower synthesis temperatures.
Section snippets
Instrumentation
X-ray powder diffraction (XRD) analysis was conducted on a Huber JSO-DEBYEFLEX 1001 Diffractometer operated at 40 kV and 35 mA using Cu Kα radiation.
FTIR transmission spectra were taken on Mattson Satellite Infrared Spectrometer in the range of 4000–400 cm−1. Samples for FTIR characterization was prepared by mixing 3 mg of sample powder with 100 mg of KBr, which were ground and pressed into a transparent pellet with a diameter of 1 cm.
Magnetic measurements were carried out with the Quantum Design
Results and discussion
Alkalization reaction of ferrous ions has been extensively studied by Refait and Olowe [29], [30] and they proposed the following reactions for the mechanism of formation of Fe3O4:Fe2+ + 2OH− → Fe(OH)23Fe(OH)2 + 1/2O2 → Fe(OH)2 + 2FeOOH + H2OFe(OH)2 + 2FeOOH → Fe3O4 + 2H2O
Thus, in the synthesis with ferrous ions alone, as in our case, Fe3O4 is formed as a result of the dehydration reaction of ferrous hydroxide and ferric oxyhydroxide (reaction (III)) in which the latter compound is produced by the partial
Conclusion
Superparamagnetic iron oxide nanoparticles were successfully synthesized by a simple, novel, and cost-effective gel-to-crystalline method starting from a single component and hydrolyzing at 80–100 °C under refluxing conditions. XRD analysis suggested non-stoichiometric magnetite with lattice parameter between magnetite and maghemite. The average crystallite size for this non-stoichiometric magnetite is calculated as 11 nm from XRD peak broadening, and 11.4 nm from TEM micrographs. Magnetic domain
Acknowledgements
The authors are thankful to the Fatih University Research Project Foundation (contract no.: P50020602) and the Ministry of Industry of Turkey (San_tez project no. 00185.STZ.2007-2) for financial support of this study. The fellowship for Dr. M. S. Toprak from Knut and Alice Wallenbergs Foundation (no.: UAW2004.0224) is also thankfully acknowledged. Authors thank to Mr. Ali C. Başaran for his invaluable help with magnetic measurements and to Mr. Abhilash Sugunan for the language revision of the
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