J. Phys. Chem. C 2007, 111, 7875-7878
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Preparation and Reversible Phase Transfer of CoFe2O4 Nanoparticles Shi-Yong Zhao,† Ru Qiao,‡ Xiao Li Zhang,‡ and Young Soo Kang*,‡ Industrial Materials Institute, National Research Council Canada 75, de Mortagne, BoucherVille, Que´ bec J4B 6Y4, Canada, and Department of Chemistry, Pukyong National UniVersity, 599-1 Daeyeon-3-dong, Namgu, Pusan 608-737, South Korea ReceiVed: January 19, 2007; In Final Form: April 3, 2007
In this paper, we present a chemical co-precipitation approach in aqueous solution to prepare a highly crystalline monodispersed CoFe2O4 magnetic nanoparticles coated with oleic acid. These products can be transferred into organic solvent by adjusting to pH 5 under the effect of HCl and re-transferred into water phase from organic phase completely under the effect of (CH3CH2)3N by adjusting the added amount into the water phase. X-ray diffraction, energy-dispersive spectrometry, transmission electron microscopy, and UV-vis spectra were used to characterize these nanoparticles and their reversible phase transfer. The properties of the reversible phase transfer imply the novel CoFe2O4 products have great potential applications in nanotechnology.
Introduction Many investigations in recent years can be found that focused on the magnetic nanoparticles, such as Fe3O4,1 γ-Fe2O3,2 cobalt-ferrite,3 nickel-ferrite,4 barium-ferrite,5 and so on, because they hold many novel physical and chemical properties that are different from their bulk counterparts or atoms. The magnetic nanoparticles would find wide applications in ultrahigh-density magnetic storage,6 ferrofluids,7 magnetic resonance imaging (MRI),8 and biomedical application.9 There exists a dream in the investigation of nanoparticles, which is how to transfer nanoparticles reversibly between aqueous phase and organic phase. But, there are few reports about this aspect. Recently, Kimura et al.10 reported that Au nanoparticles coated with carboxylate groups can be transferred from water into toluene with the help of tetraoctylammonium bromide (TOAB), and the remarkable result is that after transfer the Au particles can be redispersed into water by removing TOAB through adjusting the acidity of the reaction mixture under ambient condition. But the re-dispersion needs a long time (1 day), and the re-transfer was not complete; only a few Au particles came into the water phase again. Many techniques have been used to gain ordered structure of nanoparticles, including Langmuir-Blodgett (LB) technique,11 self-assembly technique,12 electrophoretic deposition,13 and magnetophoretic deposition.14 Usually the nanoparticles and their assembly were characterized by UV-vis, FT-IR, X-ray diffraction (XRD), thermogravimetric/differential thermal analyses (TGA/DTA), ζ potential, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), and scanning tunneling microscopy (STM). Especially, the magnetic properties of magnetic nanoparticles and their assembly were characterized by Mo¨ssbauer spectra, magnetization curve, ferromagnetic resonance (FMR),15 magnetoresistance (R(H)), and electron paramagnetic resonance (EPR).16 Several uniform-sized magnetic nanoparticles have been synthesized.17-20 However, relatively little work has been done * To whom correspondence should be addressed. E-mail: yskang@ pknu.ac.kr. † National Research Council Canada 75. ‡ Pukyong National University.
on the fabrication of monodispersed crystalline CoFe2O4 nanoparticles. In this paper a highly crystalline monodispersed CoFe2O4 magnetic nanoparticle is obtained using chemical coprecipitation in aqueous solution. Although the precipitation method is commonly done, in our paper, less toxic materials (oleic acid) and a lower temperature (80 °C) was used. The CoFe2O4 nanoparticle is coated with oleic acid and is transferred into organic solvent by adjusting pH ) 5 under the effect of HCl. By self-assembly, a monolayer of CoFe2O4 nanoparticle was obtained using the organosol. The exciting phenomenon is that this nanoparticle can be re-transferred into the water phase from the organic phase completely under the effect of (CH3CH2)3N by adjusting an added amount of it into the water phase. The motivation for the transfer of the particle from water to organic solvent is that organic sol can be used to form a selfassembled layer of nanoparticles, and that for the transfer from organic solvent to water is that the aqueous sol is suitable for biomedical applications. Experimental Section Synthesis of CoFe2O4 Nanoparticles in Aqueous Solution and Transference to Organic Solvent. All of the chemicals, including FeCl3‚6H2O (99+%), NaOH (99.9%), sodium oleate (98%), CHCl3 (HPLC grade), and CH3COCH3 (HPLC grade) were obtained from Aldrich Chemical Co. and used without further purification. Distilled water was passed through a sixcartridge Barnstead Nanopure II purification train consisting of Macropure pretreatment. The distilled water was deoxygenated by bubbling N2 gas for 1 h prior to the use, and the main synthesis steps were carried out under a N2 gas atmosphere. Typically,1 10 mL of aqueous solution, dissolving 2 mmol (0.54 g) of FeCl3‚6H2O and 1 mmol (0.238 g) of CoCl2‚6H2O, was prepared. A 1.2 g amount of NaOH was added into the 10 mL of aqueous solution under stirring for 30 min at 80 °C, and stirring was continued for another 30 min at this temperature to allow the growth of nanoparticles. After the solution was cooled to room temperature, the precipitate was isolated in a magnetic field and washed with water three times. Coating was carried out by adding 0.2 g of sodium oleate into 10 mL of aqueous solution. After stirring for 1 h, the suspension was slowly acidified with 1 M HCl until pH ) 5 and an oily black
10.1021/jp070457w CCC: $37.00 © 2007 American Chemical Society Published on Web 05/15/2007
7876 J. Phys. Chem. C, Vol. 111, No. 22, 2007
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Figure 1. (A) XRD and (B) EDS patterns of CoFe2O4 nanoparticle.
precipitate had appeared. The oily black precipitate was soluble in chloroform. Also, the nanoparticles can be transferred into chloroform phase directly by the acidification. The precipitate was dissolved into 230 mL of chloroform, obtaining a transparent solution. To remove the larger particle, 20 mL of acetone was added to the chloroform solution, and the solution became cloudy. Laying for 1 h, the larger particle became sediment in the bottom and the solution became transparent again. The transparent solution was removed to another beaker, and 230 mL of acetone was added to precipitate most of the particle; only the smaller particle existed still in the solution. The precipitate was dried in air naturally and could be soluble in chloroform readily. Reversible Transfer of CoFe2O4 from Organic Phase to Water Phase. A 5 mL aliquot of CoFe2O4 chloroform sol was added to 5 mL of aqueous solution of (CH3CH2)3N, and the mixture was shaken for 60 s and allowed to be static. Two liquid layers formed, and the color of the water phase became pale brown, meaning CoFe2O4 particles were transferred into the water phase. Characterization of CoFe2O4 Nanoparticle by XRD, EDS, TEM, and UV-Vis. The crystalline structure of synthesized nanoparticles was analyzed by XRD with a Philips X’Pert-MPD system. The average size of the crystals was estimated using Scherrer’s formula. TEM experiments were carried out on a JEOL JEM2010 transmission electron microscope, and energydispersive spectrometry (EDS) was performed with an EDAX X-ray energy-dispersive analysis system attached to the JEOL JEM2010 transmission electron microscope. TEM samples were prepared on the 400 mesh copper grid coated with carbon. A drop of the nanoparticle solution was carefully placed on the copper grid and dried in air. The size distributions of the particles were measured from enlarged photographs of the TEM images. UV-vis spectra were taken on a Hitachi model U-3210 spectrophotometer. Results and Discussion Figure 1A illustrates the XRD pattern of the CoFe2O4 nanoparticle. The discernible peaks can be indexed to (220), (311), (400), (511), (440), and (533) planes of a cubic unit cell, which correspond to that of the cubic spinel structure of cobalt
Figure 2. (A) TEM image of CoFe2O4 when a drop of the chloroform nanoparticle solution was placed on the Cu grid; (B) TEM image of CoFe2O4 nanoparticles after being transferred into the water phase.
iron oxide (JCPDS card, no.22-1086). The crystal sizes determined by Debye-Scherre equation with XRD data have been found to be 14.8 nm, which is close to the particle sizes calculated from TEM images (14.6 nm). This indicates that CoFe2O4 is nanocrystalline. The other evidence for the formation of CoFe2O4 nanoparticles was identified with EDS. As shown in Figure 1B, the particles contain two elements of Fe and Co. The peaks attributed to Cu were caused by copper grid. Figure 2A is the TEM image of the CoFe2O4 nanoparticles monolayer formed by self-assembly using chloroform sol. Most of the CoFe2O4 particles are spherical-like, although some particles are irregular, as if they were formed by aggregation of two or three virgin particles. A monolayer of nanoparticle is observed from the image with almost no multilayer in it. The self-assembled monolayer filled the whole mesh of the copper grid observed from the TEM image with lower magnification. The insertion in Figure 2A is the histogram of the size distribution of CoFe2O4 nanoparticles obtained from the enlarged image of Figure 2A. The mean size of CoFe2O4 nanoparticles is 14.6 nm with a standard deviation of 3.8 nm. The monodispersivity of the particle could be improved further by optimizing the experimental conditions and separating out the bigger and smaller particles once again using chloroform and acetone. The UV-vis spectra of the CoFe2O4 nanoparticle hydrosol as-prepared are given in Figure 3A. Examination of the spectra shows that the hydrosol has absorption in the entire range of the UV-vis spectrum, and the absorption increases gradually with the decrease of wavelength. This is in good agreement with previous results.21 Markovich et al.1 studied CoFe2O4 nanoparticles coated with oleic acid with XPS, and they concluded that in the solution a
Reversible Phase Transfer of CoFe2O4 Nanoparticles
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Figure 3. UV-vis spectra of CoFe2O4 nanoparticle (A) hydrosols, (a) as-prepared and after reversible phase transference with (b) 1, (c) 0.5, (d) 0.2, (e) 0.1, (f) 0.05, and (g) 0 mL of inducer (CH3CH2)3N; and (B) organosols, (a) before and after reversible phase transference with (b) 0, (c) 0.05, and (d) 0.1 mL of (CH3CH2)3N. A 5 mL aliquot of CoFe2O4 chloroform sol and 5 mL of water were used for phase transference.
major part of the oleic acid is free in the solvent. The oleic acid coating on the surface of CoFe2O4 nanoparticles is weakly adsorbed, possibly held by electrostatic forces or hydrogen bonds, unlike thiol-coated Au and Ag nanoparticles,22 in which the ligands are chemically bound to the metal surface. So the oleic acid could be removed from the surface of the particles when the concentration of the free oleic acid in solution is lowered. This phenomenon is observed in our reversible phase transfer of CoFe2O4 nanoparticles between the water and organic solvent. The reversible transfer of CoFe2O4 nanoparticles between aqueous phase and organic phase is schematically shown in Figure 4. The CoFe2O4 nanoparticle coated with sodium oleate is prepared in aqueous solution and is transferred into organic solvent under the effect of HCl. This enables the organosol of the CoFe2O4 nanoparticles to be obtained. Reversible phase transfer of CoFe2O4 nanoparticles from organosol to water phase was carried out under the effect of (CH3CH2)3N. A 5 mL aliquot of CoFe2O4 chloroform sol was added into 5 mL of water, when no (CH3CH2)3N was used, and the mixture was shaken for 60 s and allowed to be static; no CoFe2O4 nanoparticle was transferred into the water phase because the water phase was still colorless as shown in Figure 3A(g). But the color of the chloroform sol became pale, as shown in Figure 3B(b), meaning that the concentration of CoFe2O4 nanoparticles became low. Observing the chloroform phase carefully, a few little black precipitate particles can be found, meaning that some CoFe2O4 nanoparticles deposited out from the chloroform sol. This can be explained as follows: when water was added into the CoFe2O4 chloroform sol and shaken, some of the free oleic acid in the chloroform sol was transferred into the water, so the concentration of free oleic acid in the chloroform sol was lowered, and the oleic acid could be removed from the surface of the CoFe2O4 particles. This caused the aggregation of CoFe2O4 particles. The explanation is supported further by the following experiment: Oleic acid was added into the above chloroform sol containing the little black precipitate. After stirring for 30 min and laying undisturbed for 12 h, the little
Figure 4. Schematic drawing on the phase transfer of CoFe2O4 between organic and aqueous phases.
black precipitate disappeared and the color of the sol became deep, indicating that the aggregated CoFe2O4 particles redispersed into the chloroform solution under the effect of oleic acid. The addition of (CH3CH2)3N induces the reversible phase transfer of CoFe2O4 nanoparticles from the organic phase to the water phase. This can be seen from the change of color of the water phase. The UV-vis spectra of the water phase after phase transfer with various amount of (CH3CH2)3N are shown in Figure 3A(b-g). Like that of the CoFe2O4 nanoparticle hydrosol as shown in Figure 3A(a), the absorption in the range of the UV-vis spectrum increases gradually with the decrease of wavelength. With the increasing of amount of (CH3CH2)3N, the optical absorption density also increases. And when 1 mL of (CH3CH2)3N was used, the absorption band of the water phase was the same as that of the original chloroform sol as given in Figure 3B(a), meaning that most of the CoFe2O4 nanoparticles had been transferred from organosol to the water phase. Figure 3B shows the UV-vis spectra of the chloroform sol before and after phase transfer with various amounts of (CH3CH2)3N. When no (CH3CH2)3N was used, after shaking with the water phase, the absorption density of the chloroform sol became lower because some of the CoFe2O4 nanoparticles deposited out from the chloroform sol as mentioned above. When (CH3CH2)3N was used, phase transfer took place from the chloroform sol to water, and a lower absorption of the chloroform sol was observed, as shown in Figure 3B(c, -d). The large absorption peak around 260 nm is contributed to due to light scattering from (CH3CH2)3N.
7878 J. Phys. Chem. C, Vol. 111, No. 22, 2007 Figure 2B shows the TEM image of CoFe2O4 nanoparticles after being transferred into the water phase. Compared with Figure 2A, no apparent size change of the particles is found; only the aggregation is observed, and this is attributed to the property of hydrosol. Conclusions In summary, CoFe2O4 nanoparticle has been synthesized by chemical co-precipitation method in aqueous solution and coated with oleic acid. These nanoparticles can be transferred into organic solution, and the self-assembled monolayer film of these nanoparticles was formed using the organic solution. The nanoparticles are spherical and have narrow size distribution. The particles are nanocrystalline. Particularly, the CoFe2O4 nanoparticle can be re-transferred into the water phase from the organic phase completely under the effect of (CH3CH2)3N. Acknowledgment. This work is financially supported by the Brain Korea 21 program and the Functional Chemicals Development Program. References and Notes (1) Fried, T.; Shemer, G.; Markovich, G. AdV. Mater. 2001, 13, 1158. (2) Cannas, C.; Gatteschi, D.; Musinu, A.; Piccaluga, G.; Sangregorio, C. J. Phys. Chem. B 1998, 102, 7721. (3) de Vicente, J.; Delgado, A. V.; Plaza, R. C.; Duran, J. D. G.; Gonzalez-Caballero, F. Langmuir 2000, 16, 7954.
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