FAST MICROWAVE SYNTHESIS OF Fe3O4 AND Fe3O4/Ag MAGNETIC NANOPARTICLES USING Fe2+ AS PRECURSOR
© 2010 Baozhan Zheng*, Minghui Zhang*, Dan Xiao***, Yong Jin***, Martin M. F. Choi****
*School of Chemical Engineering, Sichuan University, Chengdu, 610065, China **School of Chemistry, Sichuan University, Chengdu, 610065, China ***College of Materials Science and Engineering, Sichuan University, Chengdu, 610065, China ****Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong SAR, China
e-mail:xiaodan@scu.edu.cn (Dan Xiao) e-mail: mfchoi@hkbu.edu.hk (Martin M.F. Choi), Received 06.03.2010
A simple and quick microwave method to prepare high performance magnetic nanoparticles (Fe3O4 NPs) directly from Fe2+ has been developed. The as-prepared Fe3O4 NPs product was fully characterized by X-ray diffraction, transmission electron microscopy and scanning electron microscopy. The results show that the as-prepared Fe3O4 NPs are quite monodisperse with an average core size of 80 ± 5 nm. The microwave synthesis technique can be easily modified to prepare Fe3O4/Ag NPs and these NPs possess good magnetic properties. The formation mechanisms of the NPs are also discussed. Our proposed synthesis procedure is quick and simple, and shows potential for large-scale production and applications for catalysis and biomedical/bi-ological uses.
INTRODUCTION
Magnetic materials, especially Fe3O4, have attracted a great deal of attention for their fascinating properties and wide range ofpotential applications in ferrofluids [1], data storage [2], catalysis [3], and environmental analysis [4]. Properly modified magnetic nanoparticles (NPs) can also be applied in the fields of medicine and biology, such as targeted drug delivery [5], immunoassays [6], and protein/enzyme immobilization [7]. The main advantage of magnetic NPs is that the particles can be easily and rapidly separated from their matrix by an external magnetic field. Therefore, the synthesis of monodisperse magnetic NPs ofdesired sizes and distributions has long been a major basic scientific and technological challenge.
The studies of the magnetic materials not only provide information about the structure and magnetic properties of the materials but also improve understanding of the synthesis technique [8]. A number of different methods have been used to synthesize magnetic NPs such as co-precipitation [9, 10], microemulsions [11], sol-gel techniques [12], hydrothermal synthesis [13], electrochemical methods [14] and so on. Aqueous solution methods including co-precipitation or hydrothermal synthesis have also been developed in recent years but only a few could prepare NPs with satisfactory size distribution [15]. Organic solution phase decomposition routes can produce high quality monodisperse NPs and has been widely used [16]; however, this route usually requires relatively high reaction temperature and pressure as well as complicated operation procedure, which there-
by limits their routine applications. As such, it is still necessary to develop a simpler and quicker route for synthesizing size-controlled monodisperse magnetic NPs.
It is well-known that microwave irradiation is not only a special form of heat but also enhances the reactivity of reaction. A wide variety of chemical reactions accelerated by microwave irradiation on reactants have been observed [17, 18]. In comparison with conventional heating method, reactions under microwave irradiation have the significant advantages of higher reaction rates and product yields in a shorter period of time. Microwave can also boost the chemical reactivity of reagents [19]. Therefore, the use of microwave energy for organic and inorganic syntheses has recently aroused intensive research interests [20, 21].
In this work, a simple and facile microwave method to prepare relatively monodisperse magnetic Fe3O4 and Fe3O4/Ag NPs is proposed. Our synthetic protocol is based on other previous preliminary findings with some modifications [22]. Different from other microwave-assisted methods that employ both Fe2+ and Fe3+ as precursors [23], this work uses Fe2+ only; hence, the synthetic condition is much simpler and the quality of the as-prepared magnetic NPs will not be affected much by the initial stoichiometric ratio ofFe2+/Fe3+. Since Ag is easier to manipulate and functionalize with many useful ligands, our synthesized Fe3O4 magnetic NPs can be easily modified to dope with some Ag NPs which should have potential applications in biomedical, biosensor and chemosensor, and catalysis fields.
I, a.u.
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29, deg
Fig. 1. XRD pattern of Fe3O4 nanoparticles synthesized by
the microwave method.
EXPERIMENTAL
Chemicals and Instruments. All chemical reagents used in the experiments were of analytical grade and without further purification. Distilled water was used throughout. The microwave oven used in the experiment was a domestic microwave oven (LG WP-700) purchased from Tianjin Lejin Development Co. Ltd. The microwave oven could be operated at various output power levels. In this work, the maximum output power of 700 W was used. The reaction mixtures were irradiated for about 30—60 seconds depending on the amount of the reaction mixture.
Synthesis of Fe3O4 nanoparticles. The magnetitc monodisperse iron oxides (Fe3O4) NPs were synthesized by microwave irradiation of FeSO4 • 7H2O in an alkaline medium. In a typical synthesis, 2.0 mmol FeSO4 • 7H2O was dissolved in 100 mL distilled water with continuous stirring. The pH of the solution was adjusted to 11 by ammonia with an instantaneous formation of black precipitate. The suspension was then transferred into the microwave oven under microwave radiation at high power for one minute. Afterwards, the Fe3O4 NPs product was separated from the reaction medium with a permanent magnet and washed by distilled water and ethanol for several
times to remove the excess SO2-, NH+ and other ions.
Synthesis of Ag-doped Fe3O4 nanoparticles. Ag-doped Fe3O4 NPs were prepared by a co-precipitation method. An appropriate amount of [Ag(NH3)2]+ solution was added into a FeSO4 solution with constant stirring. The pH of the mixture solution was quickly adjusted to about 11 with ammonia. The reaction mixture was then exposed to the same microwave irradiation in the microwave oven. The separation and wash procedures are the same as that of Fe3O4 NPs. The resulting Ag-doped Fe3O4 (Fe3O4/Ag) NPs product was obtained and subject to further characterization.
Characterization. X-ray powder diffraction (XRD) was performed on an X-ray diffractometer (XK-1000, Dandong, China) with Cu^a radiation. Transmission electron microscopic (TEM) images were captured on a Hitachi model H-800 transmission electron microscope using an accelerating voltage of200 kY The morphology of the NPs sample was examined by scanning electron microscopy (SEM, S-4800, Hitachi, Japan). Their magnetic properties were evaluated on a superconducting quantum interference device (SQUID) magnetometer (MPMS-7, Quantum Design, USA) at 300 K.
RESULTS AND DISCUSSION
The crystalline structure of our as-synthesized Fe3O4 magnetic NPs was characterized by the powder XRD and depicted in Fig. 1. Magnetite is believed to be the major crystalline phase as identified by the diffraction peaks at 30.28°, 35.64°, 37.32°, 43.32°, 53.72°, 57.24°, and 62.84°, corresponding to the seven indexed planes (220), (311), (222), (400), (422), (511), and (440) reflections of magnetite, respectively (JCPDS card No. 19-0629). These results indicate that the Fe3O4 NPs have cubic spinel structure [24]. The average core size of the Fe3O4 NPs deduced from Sherrer's formula is about 80 nm, which is in complete agreement with the TEM results (vide infra). No impurity peak was detected, demonstrating the high purity of Fe3O4 magnetic NPs product. Figure 2 shows the TEM and SEM images of the magnetic NPs synthesized from our microwave method. The TEM micrographs show that the as-prepared Fe3O4 NPs are nearly spherical with an average core size of 80 ± 5 nm. The SEM image of the Fe3O4 magnetic NPs is also depicted in Fig. 2b. To investigate the magnetic properties of Fe3O4 magnetic NPs, the hysteresis loops of NPs were studied by the SQUID. Figure 3 displays the magnetic hysteresis curves of the Fe3O4 magnetic NPs at 300 K which demonstrates the excellent ferromagnetic behavior of Fe3O4 NPs.
When ammonia is added to the FeSO4 solution, Fe(OH)2 is initially formed (Equ. 1) which is then oxidized to Fe3O4 under microwave irradiation (Equ. 2). The formation mechanisms of Fe3O4 magnetic NPs under microwave irradiation can be summarized as follows:
Fe2+ + 2NH3 + H2O ^ Fe(OH)2 + 2NH+, (1)
3 Fe(OH)2 +1/2 O2
microwave
^Fe3O4 + 3 H2O. (2)
The major attribute of our microwave technique is the direct oxidation of Fe2+ to Fe3+ in the reaction vessel without adding Fe3+. In essence, no Fe3+ precursor is needed in this work as compared to other hydrothermal synthesis methods [23]; thus, this can simplify the NPs synthetic method. In addition, our method avoids the
Fig. 2. TEM (a) and SEM (b) images of Fe3O4 nanoparticles.
consideration of the use of a fixed stoichiometric ratio of Fe3+/Fe2+ so that the quality of Fe3O4 NPs can be easier to manupulate. As such, it is a more convenient method to synthesize magnetic NPs.
It is well-known that Ag NPs can interact with proteins and biomolecules which have potential uses in bio-analysis. In order to extend our magnetic NPs for future biological application, the Fe3O4 NPs are doped with Ag NPs using a co-precipitation method. A known aliquot of[Ag(NH3)2]+ solution was added into a FeSO4 solution with constant stirring. The solution was then adjusted to pH 11 with ammonia. Afterwards, the reaction mixture was subject to microwave irradiation for 1.0 min to obtain Fe3O4/Ag NPs. The XRD pattern of the resulting Fe3O4/Ag NPs product is shown in Fig. 4. It can be seen that the magnetic
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