phys. stat. sol. (c) 3, No. 5, 1302 – 1307 (2006) / DOI 10.1002/pssc.200563115
Microstructural study and size control of iron oxide nanoparticles produced by microemulsion technique T. Koutzarova*1, S. Kolev1, Ch. Ghelev1, D. Paneva2, and I. Nedkov1 1 2
Institute of Electronics, Bulg. Acad. Sci., 72 Tzarigradsko Chaussee, 1784 Sofia, Bulgaria Institute of Catalysis, Bulg. Acad. Sci., "Acad. G. Bonchev" Str., bl. 11, 1113 Sofia, Bulgaria
Received 5 July 2005, accepted 11 November 2005 Published online 17 January 2006 PACS 75.50.Tt, 81.07.Bc, 81.07.Wx, 81.16.Be, 81.20.Fw In this paper we study the possibility to control the size of iron oxide (Fe3O4) nanoparticles by the microemulsion technique. We used a water-in-oil reverse microemulsion system with n-hexadecil trimethylammonium bromide (CTAB) as a cationic surfactant, n-butanol as a co-surfactant, n-hexanol as a continuous oil phase, and aqueous phase. The magnetite nanopowders were synthesized by a single microemulsion technique in which the aqueous phase contains only metal ions (Fe2+ and Fe3+). The particle size of the powders varied in the range of 14–36 nm depending on the preparation conditions. We studied the influence of changing the water/surfactant ratio (W0 = 5, 10, 15, 20) and the metallic ion (Fe2+ and Fe3+) concentration on the particle size distribution and crystallinity of Fe3O4. © 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction Nanoparticles in the nanometer-size range have recently attracted the attention of researchers because of their unique physical and chemical properties that differ significantly from those of the bulk materials due to their extremely small size and large specific surface [1–3]. Iron oxides are important materials for many industrial applications such as pigments, magnetic materials, catalysts and sensors and also for biological applications [4–6]. They are also of great interest in fundamental science, especially for elucidating the fundamental relationships between magnetic properties and crystal chemistry and structure. The electrical and magnetic properties of these ferrites are strongly dependent on the chemical composition, cation distribution and the method of preparation in general, and on the structure in particular [7]. Various chemistry-based novel processing routes have been developed in synthesizing ultrafine ferrite powders as precipitation/coprecipitation [8–11], sol-gel processes [12] and hydrothermal processing [13]. The most commonly used method for obtaining iron oxide nanoparticles is co-precipitation. Although the co-precipitation method can vary the average size of nanomagnetic particles by adjusting the pH and the temperature of aqueous media, it has only limited control over the size distribution of the particles [14]. Recently, nanosized magnetic oxides were successfully prepared via a reverse microemulsion process [7, 15–17]. One of the advantages of this technique is the preparation of very uniform particles (< 10% variability) [14]. The microemulsion system consists of an oil phase, a surfactant phase and an aqueous phase. The reverse microemulsion system is a thermodynamically stable isotropic dispersion of the aqueous phase in the continuous oil phase [18]. It exhibits a dynamic structure of nanosized aqueous droplets which are in constant deformation, breakdown, and coalescence. The size of these aqueous droplets is in the range 5 to 100 nm depending on the water/surfactant ratio *
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Rw =
3Vaq [ H 2 O] , σ [S]
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(1)
where Rw is the water pool radius, Vaq is the volume of water molecule, σ is the area per polar head of surfactant [19]. Precipitation/coprecipitation reactions are expected to take place when aqueous droplets containing the desirable reactants collide with each other. Each of the aqueous droplets acts as a nanosized reactor for forming nanosized precipitate particles [7]. The present work aims to prepare a ferrite oxide powder of nanometer particle size via water-in-oil reverse micelles. The powders were synthesized by a single microemulsion technique, where there is only one microemulsion system whose aqueous phase contains metal ions (Fe2+ and Fe3+) only. This approach differs from the known microemulsion method used so far to prepare nanosized oxides, where the synthesis process runs via the mixing of two microemulsion systems that differ by the aqueous phase type – one containing metal ions, the other, a precipitating agent [16]. Since the conditions of synthesis have a significant influence on the chemical, structural and physical properties of iron oxides, we investigated the relation between the experimental conditions of iron oxides synthesis and their properties. We studied the influence of the stirring time, the water/surfactant ratio (W0) and metallic ion (Fe2+ and Fe3+) concentration on the particle size distribution and crystallinity of Fe3O4.
2 Experimental A microemulsion system with cetyltrimethylammonium bromide (CTAB) as the cationic surfactant; nbutanol as the co-surfactant; n-hexanol as a continuous oil phase and an aqueous phase of metal ions (Fe2+ and Fe3+) was chosen. CTAB (with purity 99 %), FeCl3·6H2O (99 %) and FeCl2·4H2O (99 %) were purchased from Merck. The microemulsion composition consisted of 50 ml n-hexanol, 20 ml n-butanol and 40 ml aqueous phase, with the water/surfactant ratio being varied, namely (W0 = 5, 10, 15, 20). The metallic ion (Fe2+ and Fe3+) concentration in the aqueous phase was 0.5 M and 0.05 M for W0 = 5, 10, 15, 20. The magnetite synthesis occurred in-situ in the following process. The reagents FeCl2·4H2O and FeCl3·6H2O were mixed in an aqueous solution with the ratio of Fe2+ to Fe3+ being 1:2. CTAB and nbutanol were added into the solution of ferrous and ferric ions. After homogenization of the solution, nhexanol was added to obtain a microemulsion system. The magnetite was precipitated by adding aqueous solution of ammonia under constant stirring and the reaction mixture was kept at pH = 10. We found the optimal stirring time to be 20 min. Since the vesicle wall is permeable to the hydroxyl ions, they diffuse down their chemical potential gradient into the intravesicular space and react with the available ferrous and ferric ions to form an intravesicular product of magnetite. The black precipitate was separated in a centrifuge at 4000 rpm for 10 min. After the magnetite nanoparticles were isolated, the precipitate was washed with water and solution of chloroform and methanol (50 v.% and 50 v.%) to remove the excess surfactant. The final magnetite nanoparticle powder was obtained after drying the sample at 80 °C. The structural properties of the magnetite powders produced were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR) and Mössbauer spectroscopy.
3 Results and discussion Phase analysis of the powders was carried out by X-ray diffraction (XRD) on a TUR-M6 diffractometer with Bragg-Brentano geometry at room temperature using Co Kα radiation. The scanning step was 0.05°. The exact position of the lines and their widths were determined by computer software (“FIT” program) for deconvolution and profile analysis of diffraction patterns. The XRD data exhibited consistently a single-phase spinel structure for all types of samples. A typical XRD pattern is shown in Fig. 1.
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T. Koutzarova et al.: Microstructural study and size control of iron oxide nanoparticles 2.525
800
Fe3O4
700
100
10 8 6
4
2
1.481
1.199
1.714
200
1.361 1.323 1.278
300
1.612
400
2.093
2.958
500
4.862
Intensity, imp/s
600
Fig. 1 X-ray powder diffraction patterns of magnetite measured at room temperature.
d, A
The magnetite unit cell parameter was determined by the “PowderCell” software. Its value for the powders obtained by us using the single microemulsion technique is in the range 0.8371 nm–0.8374 nm. A comparison with the standard values for Fe3O4 (PDF 82-1533) a = 0.8397 nm and γ-Fe2O3 (PDF 391346) a = 0.8352 nm indicates that the magnetite powders obtained have a strongly defective structure. Such structural defects on the particles surface are typical for nanosized Fe3O4 powders [20] prepared by coprecipitation in air at room temperature. Using the XRD patterns, the approximate average size of nanoparticles can be estimated by applying the Scherrer formula [8]: D = Kλ/βcos(θ),
(2)
where λ is the X-ray wavelength (nm), θ is the Bragg angle, β is the excess line broadening (radians), K is the constant. We thus estimated the mean magnetite particles size in the powders obtained by microemulsion with low concentration of ferric and ferrous ions in aqueous phase to be about 14 nm. No difference was observed in the particle size that could be related to the water/surfactant ratio. Figure 2 presents a SEM image of magnetite powders obtained by microemulsion with low concentration of ferric and ferrous ions in aqueous phase. As can be seen the particles are spherical with uniform particle-size distribution. It can also be seen that the particles are strongly aggregated, which is typical for particles with a size less than 50 nm.
Fig. 2 SEM image of magnetite particles obtained by the single microemulsion technique.
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The amount of surfactant affects substantially the size of the particles obtained, when the ferric and ferrous ion concentration in the aqueous phase is high. As the CTAB concentration increases, the particles size decreases. The particle size was estimated to be about 15 nm for the highest CTAB concentration and around 36 nm for lowest one (estimated by the Scherrer equation). The values for the particles size thus obtained, together with the line intensities in the XRD patterns, point to a relatively narrow particles size distribution. Infrared spectroscopy is an important tool in exploring various ordering phenomena and can yield information not only about the position of the ions in the crystal but also about their vibrational modes [21]. The Fourier transform infrared (FT-IR) spectra of specimens pressed into KBr discs were recorded at RT using a Nicolet-320 FTIR spectrometer in the range 4000–400 cm–1. The spectra were processed by means of the IR Data Manager program.
a
Transmittance (a. u.)
b
c
CTAB
4000 3500 3000 2500 2000 1500 1000 500 -1
Wavenumber (cm )
Fig. 3 FTIR spectra of magnetite powders and CTAB.
The FTIR spectra are shown in Fig. 3. Figure 3 (a) is the spectrum of a sample obtained by microemulsion with low concentration of ferric and ferrous ions in aqueous phase. Figure 3 (b, c) presents spectra of samples with high concentration of ferric and ferrous ions in aqueous phase for W0 = 5 and 20, respectively. Figure 3 (d) shows the infrared spectrum of CTAB. All samples exhibited absorption in the region 3400, 1620, 1050 and 574 cm–1. The general range of 3600–3100 cm–1 is related to antisymmetric and symmetric OH stretching. The IR bands in region 1670–1600 cm–1 are due to bending vibrations of OH in the water molecule. The simultaneous presence of bands in these two ranges (3600–3100 cm–1 and 1670–1600 cm–1), which are associated with the lattice of water molecule, is indicative of water crystallization in the powders [6]. The absorption in the range 800 to 1200 cm–1 is characteristic OH deformation in hydroxides [22]. The absence of absorption in this range reveals the lack of iron hydroxides in the powders obtained. Based on the above, and on the presence of characteristic hydroxyl peaks of water at 3600–3100 cm–1 and 1670–1600 cm–1, we concluded that the water is absorbed in the surface of nanoparticles.
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Bands at 576 and 574 cm–1 are also observed in the samples due to the metal-oxygen stretching vibration modes, which correspond to Fe3O4 [21]. These bands are sharp and are of strong intensity, which demonstrate the high degeree of crystallinity of the samples. Their position depends on the history of the sample and the particle size. Mössbauer spectroscopy is a specifically useful technique in the investigation of magnetic oxides. Figure 4 presents a spectrum of magnetite prepared at iron ions 0.5 M and W0 = 20. The sextet lines are broadened and the thermal relaxation effects are clearly observed. The broad and asymmetrical lines can be assigned to the presence of surface disorder and fine crystalline particles. The existence of a central doublet indicates that the magnetite-containing sample is close to a superparamagnetic state. In order to clarify the role of the surface structure defects, we plan to carry out Mössbauer measurements at low temperatures in the presence of external magnetic fields.
Fig. 4 Mössbauer spectrum of magnetite at room temperature.
4 Conclusion Magnetite nanoparticles were successfully synthesized by using a single microemulsion technique, where the aqueous phase of the microemulsion system contains only Fe2+ and Fe3+ ions. One of the advantages of the single microemulsion technique is that it is much less expensive than the other microemulsion methods. The powders structural and composition characteristics show that the samples are composed of magnetite particles with defective surface structure. The powders obtained have consistently narrow particle size distribution and the shape of the particles is spherical. It was also established that at low metal ion concentration (0.05 M) in the aqueous phase, variation of the water/surfactant ratio does not influence the average size of the nanoparticle obtained. As it is typical for nanosized magnetic powders, the particles are strongly aggregated. At higher Fe2+ and Fe3+ ion concentration (0.5 M) in the aqueous phase, the particles size is reduced as the CTAB concentration is lowered. In summary, this method is an effective way of synthesizing magnetite nanoparticles with narrow size distribution. Acknowledgements The work was supported in part by the Bulgarian National Council for Scientific Research under grants TH-1/01 and MUF1301. Dr. Koutzarova was supported by NATO Reintegration Grant (EAP.RIG.981472).
References [1] [2] [3] [4] [5]
M. Pikethly, Materials Today 7 (Suppl.), 20 (2004). M. Moumen, P. Bonville, and M.P. Pileni, J. Phys. Chem. 100, 14410 (1996). T. Li, Y. Deng, X. Song, Z. Jin, and Y. Zhang, Bull. Korean Chem. Soc. 24, 957 (2003). I. Safarik and M. Safarikova, Chem. Monthly 133, 737 (2002). I. Altman, Y. Jang, I. Agranovski, S. Yang, and M. Choi, J. Nanoparticle Res. 6, 633 (2005).
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.pss-c.com
phys. stat. sol. (c) 3, No. 5 (2006) [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
1307
Y. He, S. Wang, C. Li, Y. Miao, Z. Wu, and B. Zou, J. Phys. D: Appl. Phys. 38, 1342 (2005). J. Wang, P.F. Chong, S.C. Ng, and L.M. Gan, Mater. Lett. 30, 217 (1997). I. Rhee and Ch. Kim, J. Magn. Magn. Mater. 261, 410 (2003). S. Music, G. Santana, G. Smit, and V. Garg, Croat. Chem. Acta 72, 87 (1999). I. Nedkov, T. Merodiiska, S. Kolev, K. Krezhov, D. Niarchos, E. Moraitakis, Y. Kusano, and J. Takada, Chem. Monthly 133, 823 (2002). I. Nedkov, R.E. Vanderbefghe, G. Visokov, T. Merodiiska, S. Kolev, and K. Krezhov, phys. stat. sol. (a) 201, 1001 (2004). M. Vasquez-Mansilla, R.D. Zysler, C. Arciprete, M.I. Dimitrijewits, C. Saragovi, and J.M. Greneche, J. Magn. Magn. Mater. 204, 29 (1999). J.H. Lee, D.Y. Maeng, Y.S. Kim, and C.W. Won, J. Mater. Sci. Lett. 18, 1029 (1999). L. LaConte, N. Nitin, and G. Bao, Materials Today 8 (Suppl.), 32 (2005). P. Dresco, V. Zaitsev, R. Gambino, and B. Chu, Langmuir 15, 1945 (1999). V. Pillai, P. Kumar, M.J. Hou, P. Ayyub, and D.O. Shah, Adv. Colloid Interface Sci. 55, 241 (1995). E. Choi, Y. Ahn, S. Kim, D. An, K. Kang, B. Lee, K. Baek, and H. Oak, J. Magn. Magn. Mater. 262, L198 (2003). M. Giustini, G. Palazzo, G. Conafemmina, M. Monica, M. Giomini, and A. Ceglie, J. Phys. Chem. 100, 3190 (1996). M.P. Pileni, J. Phys. Chem. 97, 6961 (1993). I. Nedkov, T. Merodiiska, L. Slavov, Ch. Ghelev, R.E. Vandenberghe, and K. Zadro, in: I. Nedkov and Ph. Tailhades (Eds.), Nanoscale Magnetic Oxides and Bio-word (Heron Press Ltd., Sofia, 2004), pp. 29–37. R.D. Waldron, Phys. Rev. 99, 1727 (1955). J. Coey and D. Khalafalla, phys. stat. sol. (a) 11, 229 (1972).
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