Solvothermal synthesis of magnetic Fe3O4 microparticles via self-assembly of Fe3O4 nanoparticles

Particuology 9 (2011) 179–186

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Solvothermal synthesis of magnetic Fe3 O4 microparticles via self-assembly of Fe3 O4 nanoparticles Wei Zhang, Fenglei Shen, Ruoyu Hong ∗ College of Chemistry, Chemical Engineering & Materials Science, and Key Laboratory of Organic Synthesis of Jiangsu Province, Soochow University, SIP, Suzhou 215123, China

a r t i c l e

i n f o

Article history: Received 6 April 2010 Received in revised form 27 June 2010 Accepted 7 July 2010 Keywords: Iron oxides Magnetic microparticles Oriented attachment Solvothermal method

a b s t r a c t Ferromagnetic Fe3 O4 nanoparticles were synthesized and then self-assembled into microparticles via a solvothermal method, using FeCl3 ·6H2 O as the iron source, sodium oleate as the surfactant, and ethylene glycol as the reducing agent and solvent. The obtained Fe3 O4 microparticles were characterized by X-ray powder diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy and vibrating sample magnetometer (VSM). The size and morphology of the particles were examined using transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The Fe3 O4 microparticles of nearly monodisperse diameters, controllable in the range of 120–400 nm, consist of assemblies of Fe3 O4 nanoparticles with a diameter of 22 nm. The effects of reaction time, amount of surfactant and NaAc on the products were discussed. Interestingly, by using the pre-synthesized Fe3 O4 microparticles as the growth substrates, spherical and smooth-looking Fe3 O4 microparticles with average diameter of 1 ␮m were obtained. A plausible formation process was discussed. © 2011 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

1. Introduction Nanotechnology has gained tremendous success during recent decades by joining together with biology, medicine or other disciplines, and continues to receive considerable attention worldwide. Although rapid development and application of nanomaterials and nanotechnology have potential health and environmental impacts on humans, non-human biota and ecosystems (Bregoli et al., 2009; Karlsson, Gustafsson, Cronholm, & Möller, 2009; Zhu, Han, Xiao, & Jin, 2008), extensive research on nanotechnology has been focused on nanocrystalline materials, of which magnetic nanoparticles have become the recent center because they possess attractive properties which portend potential applications in biology and medicine, such as contrast agents for magnetic resonance imaging (MRI) (Qiao, Yang, & Gao, 2009), targeted drug delivery (Panyam & Labhasetwar, 2003), magnetic separation (Ansari, Grigoriev, Libor, Tothill, & Ramsden, 2009; Chiang, Sung, Wu, Chen, & Hsu, 2005; Saiyed, Bochiwal, Gorasia, Telang, & Ramchand, 2006), hyperthermia (Hosono et al., 2009; Zhao, Zeng, Xia, & Tang, 2009), removal of metal ions (Liu, Zhao, & Jiang, 2008; Zhao, Wang, et al., 2009). Among the various magnetic nanoparticles, iron oxides (such as Fe3 O4 or ␥-Fe2 O3 ), have been extensively investigated for

∗ Corresponding author. Tel.: +86 512 65882057; fax: +86 512 65882057. E-mail address: [email protected] (R. Hong).

the simple reason that they exhibit combined properties of high magnetic saturation, low cytotoxicity, good biocompatibility and stability under various physiological conditions (Wu, He, & Jiang, 2008). Many well-established methods have been developed for preparing magnetic iron oxide nanocrystals with controlled size and shape. Studies show that magnetotactic bacteria (MTB) are able to internalize Fe and convert it into magnetic nanoparticles, in the form of either magnetite (Fe3 O4 ) or greigite (Fe3 S4 ) (Xie, Chen, & Chen, 2009). However, the MTB method, harnessing such nanoparticles gestated by magnetotactic bacteria, is still in its infancy. Nowadays, Fe3 O4 nanoparticles are commonly fabricated by chemical-based synthetic approaches, including chemical coprecipitation, thermal decomposition, hydrothermal and solvothermal synthesis (Wu et al., 2008). However, most of these approaches focus on the synthesis of well-crystallized ferrite nanoparticles with small diameters (below 30 nm). In spite of their superparamagnetism, ferrite nanoparticles with diameters smaller than 30 nm have low magnetization on average. As a result, it is difficult to separate them from solutions or direct their movements effectively by applying a moderate magnetic field. Besides, studies show that the magnetic properties of Fe3 O4 nanocrystals are greatly influenced not only by their size but also by their attachment structure (Xi, Wang, & Wang, 2008). Wellcrystallized, self-assembled Fe3 O4 microparticles with size similar to protein molecules, having the advantage of strong magnetization

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doi:10.1016/j.partic.2010.07.025

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while retaining the ferromagnetic or superparamagnetic behavior would see considerable applications in biomedical fields, especially applications in vivo. Therefore, it is of great importance to develop practical approaches to synthesize size-controlled, monodisperse Fe3 O4 sub-micrometer spheres with strongly oriented attachment structures (Ge, Hu, Biasini, Beyermann, & Yin, 2007; Xuan, Wang, Yu, & Leung, 2009). Recently, many methods have been reported to produce Fe3 O4 particles with oriented attachment structures. For example, Cheng, Tang, Qi, Sheng, and Liu (2010) succeeded in preparing hollow and core-shell Fe3 O4 spheres constructed of nanoparticles by a one-pot hydrothermal method. Yu et al. (2006) reported the oriented-assembly of Fe3 O4 nanoparticles into large Fe3 O4 microparticles and Qu, Yao, Zhou, Fu, and Huang (2010) have developed an aqueous medium method for synthesizing Fe3 O4 microparticles by using a copolymer as a capping and assembly reagent. However, all of these products reported are hollow-structured and with seriously coarse surface. Synthesis of smooth-looking Fe3 O4 microparticles via both oriented attachment and Ostwald ripening process at different stages has not been reported. In this context, we conducted a modified solvothermal procedure to synthesize a series of size-controlled Fe3 O4 microparticles which are self-assembled from small Fe3 O4 nanoparticles. The approach involves the nucleation of small Fe3 O4 nanocrystals and subsequent self-assembly of the primary small nanoparticles into Fe3 O4 microparticles with controlled size and spherical structure. FeCl3 ·6H2 O was used as the sole iron source, which was reduced by ethylene glycol (EG) in the presence of sodium acetate as an alkali source and electrostatic stabilizer, while sodium oleate acted both as a surfactant as well as electrostatic stabilizer. The microparticles have nearly monodisperse diameters that can be controlled in the range of 120–400 nm. Moreover, the experimental results demonstrated that these sphere-like Fe3 O4 microparticles with coarse surface could served as substrates to further grow into smooth-looking spherical Fe3 O4 microparticles with average size of 1 ␮m which are believed to be ideal candidate for a wide range of potential applications such as bioseparation, adsorbents and so on. 2. Experimental 2.1. Materials Ferric chloride hexahydrate (FeCl3 ·6H2 O), sodium oleate (C17 H33 COONa), sodium acetate anhydrous (CH3 COONa, NaAc), sodium hydroxide (NaOH), ammonium acetate (NH4 Ac), ethylene glycol (EG) were of analytical grade and used without further purification. Deionized water was used throughout the experiments. 2.2. Synthesis of Fe3 O4 microparticles A typical synthetic procedure was as follows: FeCl3 ·6H2 O (1.62 g), NaAc (2.4 g), sodium oleate (3.66 g) and ethylene glycol (50 mL) were added in a beaker and stirred for 30 min at room temperature using a vigorous magnetic stirrer to form an orange solution. The obtained solution was then transferred to a teflonlined stainless-steel autoclave (100 mL capacity). The autoclave was sealed, settled into a muffle furnace and the temperature was raised and fixed at 200 ◦ C for 10 h. After the reaction, the autoclave was allowed to cool down to room temperature naturally. The obtained black product was washed with deionized water and ethanol several times, and then dried under vacuum at 50 ◦ C for 24 h.

Fig. 1. XRD pattern of the Fe3 O4 microparticles obtained at 200 ◦ C after 10 h.

2.3. Synthesis of smooth Fe3 O4 microparticles The above-synthesized Fe3 O4 microparticles together with FeCl3 ·6H2 O (1 g), NaAc (2.4 g), sodium oleate (3.66 g) and ethylene glycol (40 mL) were placed in a three-neck flask and stirred vigorously to form a yellow-green suspension. The obtained suspension was then transferred to a Teflon-lined stainless-steel autoclave (100 mL capacity) which was then sealed, heated in a muffle furnace at 200 ◦ C for 10 h, and cooled to room temperature naturally after reaction. The resulting black product was washed several times with deionized water and ethanol, and then dried under vacuum at 50 ◦ C for 24 h. 2.4. Characterization Fourier transform infrared spectroscopy (FTIR) measurement was carried out using a Nicolet Avatar 360 FTIR by applying the KBr method. The FTIR spectra of Fe3 O4 microparticles were obtained by FTIR analysis. TEM images were obtained using Hitachi H-600II transmission electron microscope, which shows the average particle size, size distribution and morphology of the prepared samples. The samples were prepared by placing a drop of welldispersed microparticle aqueous suspension onto a carbon-coated copper grid and allowing the solvent to evaporate naturally at room temperature. X-ray photoelectron spectroscopy (XPS) was performed on a Kratos Corporation’s Axis Utltradld, by using nonmonochromatized Al K␣ radiation as the excitation source. Raman spectra were recorded at room temperature on a Jy HR800 Raman spectrometer with an argon-ion laser at an excitation wavelength of 514 nm. XRD data were collected by an XRD (D/Max-IIIC, Japan) using Cu K␣ radiation ( = 1.5406 Å), to characterize the crystal structures of the as-prepared sample. SEM images that contain information about the surface topography, composition and particle size of the as-prepared samples were obtained using SEM (Hitachi S-4700, Japan). 3. Results and discussion 3.1. Characterization of a typical product Fig. 1 shows the XRD pattern of the crystal phase of a sample prepared at 200 ◦ C for 10 h under solvothermal conditions: typical of Fe3 O4 (JCPDS 75-1609), without any of other crystalline materials. According to the Scherrer equation, the average crystallite size

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Fig. 2. Raman spectra of (a) the as-synthesized Fe3 O4 microparticles, (b) the Fe3 O4 microparticles irradiated over a period of time and (c) the commercial ␥-Fe2 O3 powders. Fig. 3. XPS spectrum of the as-synthesized Fe3 O4 microparticles.

calculated on the basis of XRD pattern is about 22 nm, which is consistent with the results of TEM analysis below. Since ␥-Fe2 O3 (JCPDS no. 39-1346) has a similar XRD pattern, the XRD patterns could not be used to exactly distinguish these two substances (Liu, Hu, Fang, Wu, & Xie, 2009). To further identify the sample, the Raman spectroscopy and XPS were carried out to better distinguish iron oxides with different structural phases (Shebanova & Lazor, 2003). For Fe3 O4 nanocrystals, the Raman spectrum contains a strong band centered at 668 cm−1 , while ␥-Fe2 O3 nanocrystals show a spectrum consisting of three strong bands around 700, 500, and 350 cm−1 . Curve a in Fig. 2 represents the Raman spectrum of the sample, showing a sharp peak centered at 667.85 cm−1 . In contrast, three main peaks at around 688.14, 497.96 and 337.72 cm−1 are observed in the Raman spectrum of commercial ␥-Fe2 O3 shown as curve c in Fig. 2. Curve b in Fig. 2 shows the Raman spectrum of the sample with an increased laser irradiation time, which is similar to curve c rather than curve a. These facts indicate that the sample is composed of Fe3 O4 nanocrystals and the transformation of Fe3 O4 nanocrystals into ␥-Fe2 O3 nanocrystals after prolonged laser irradiation (Shebanova & Lazor, 2003). Fig. 3 shows the XPS spectrum of the sample, in which binding energies of 710.9 and 724.01 eV, corresponding to the known values of the Fe2p3/2 and Fe2p1/2 oxidation states, respectively, are quite consistent with the values of Fe3 O4 in the handbook

(Moulder, Stickle, Sobol, & Bomben, 1992). On the basis of XRD, Raman spectroscopy, and XPS analyses, it can be concluded that highly crystalline Fe3 O4 has been prepared by the present modified solvothermal reduction reaction. Fig. 4a shows an SEM image of the sample, an assembly of densely packed uniform spherical Fe3 O4 microparticles with coarse surface. Fig. 4b shows a TEM image of the as-prepared Fe3 O4 microparticles, indicating that each microparticle is composed of a large number of small nanoparticles with an average size of 22 nm (Lee, Ribeiro, Longo, & Leite, 2005). 3.2. Effect of reaction time on products To investigate the formation process of the Fe3 O4 microparticles, time-dependant experiments were carried out. After 1 h of solvothermal treatment, only a small amount of brown precipitate was obtained in an orange suspension, indicating that Fe3 O4 nanoparticles have not been formed. When the time was increased to more than 5 h, Fe3 O4 microparticles of various dimensions and structures were obtained, as shown in Fig. 5, at 200 ◦ C for 5–24 h. The results indicate that microparticles grow larger and more sphere-like as reaction time increases. After 5 h of reaction, such a large amount of Fe3 O4 nanoparticles had accu-

Fig. 4. SEM (a) and TEM (b) of the same sample of self-assembled Fe3 O4 mircoparticles synthesized at 200 ◦ C for 8 h.

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Fig. 5. TEM images of the Fe3 O4 microparticles obtained at different reaction times while all other parameters remain unchanged: (a) 5 h, (b) 8 h, (c) 15 h and (d) 24 h.

Fig. 6. TEM images of Fe3 O4 microparticles synthesized with different mass ratios of sodium oleate to FeCl3 ·6H2 O: (a) 2:3, (b) 3:3, (c) 4:3, (d) 5:3 and (e) 6:3, while keeping other parameters unchanged.

mulated to form Fe3 O4 microparticles each 60 nm in size (Fig. 5a). After 8 h of solvothermal treatment, the Fe3 O4 microparticles had grown to 120 nm in size as shown in Fig. 5b, gaining the shape of a pomegranate (Liu, Sun, et al., 2009). After 15 h, more Fe3 O4 nanoparticles had aggregated as shown in Fig. 5c, until finally at 24 h, the texture of the Fe3 O4 microparticles had grown to be so dense as to appear almost opaque under the TEM, as shown in Fig. 5d. 3.3. Effect of amount of sodium oleate on products Ethylene glycol and NaAc are believed to play important roles in the solvothermal process (Deng et al., 2005), the former as both strong reducing agent and solvent, and the latter as both electrostatic stabilizer and alkali. Surfactant is added to prevent particles agglomeration and to ensure monodispersion of the Fe3 O4 microparticles. Sodium oleate, as an amphiphilic surfactant, displays higher affinity towards the surface of magnetite particles as compared to other surfactants (Korolev, Ramazanova, & Blinov, 2002; Liu et al., 2006), though not much different from oleic acid (Xi et al., 2008). When sodium oleate is added to the ethylene glycol solution, it dissociates into C17 H33 COO− and Na+ . The former anions are prone to assemble on the surface of the initially generated magnetite particles, forming a protective outer micelle which prevents the magnetic particles from agglomerating and confines the random Brownian motion of the particles suspended in the solution. This results in the formation of highly ordered Fe3 O4 microparticles through spontaneous assembly of the small Fe3 O4 nanoparticles that are coated with an oleate layer, while the Na+ ions are attached to the exterior of the micelle. The protecting ability of the obtained micelle, which determines the diameter and structure of the products, is thought to depend on the type of surfactant used. In our case, we found it quite easy to obtain

secondary-structured Fe3 O4 microparticles with size ranging from 120 to 400 nm, though quite difficult to synthesize Fe3 O4 microparticles smaller than 100 nm. By contrast, Yan et al. (2008) prepared size-controlled Fe3 O4 microparticles varying from 15 to 190 nm using the mixed surfactant of sodium dodecyl sulfate (SDS) and polyethylene glycol (PEG). In order to investigate the effect of sodium oleate on the products, a set of experiments using different mass ratios of sodium oleate to ferric chloride hexahydrate (FeCl3 ·6H2 O) were carried out. Fig. 6 shows that, by keeping other reaction parameters unchanged and as sodium oleate to FeCl3 ·6H2 O mass ratio was increased from

Fig. 7. FTIR spectra of Fe3 O4 microparticles synthesized with different mass ratios of sodium oleate to FeCl3 ·6H2 O: (a) 3:3, (b) 4:3, (c) 5:3 and (d) 6:3, while keeping other parameters unchanged: FeCl3 ·6H2 O = 1.62 g, EG = 50 mL, temperature = 200 ◦ C and reaction time = 10 h.

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Fig. 8. TEM images of Fe3 O4 microparticles obtained at 200 ◦ C for 10 h with varied amounts of NaAc as protecting agent: (a) 1.5 g and (b) 3 g.

2:3 through 6:3, the average diameter of the resulting microparticles gradually increased. It was also observed that increasing the concentration of sodium oleate led to an shape evolution of Fe3 O4 microparticles from hollow to solid microparticles (Yu et al., 2006). Fig. 7 shows the FTIR spectra of as-synthesized Fe3 O4 microparticles. The strong and broad absorption band centered at 583.6 cm−1 corresponds to the Fe–O bond of Fe3 O4 . The two peaks located at 1528 and 1404.1 cm−1 correspond respectively to the COO−1 antisymmetrical vibration and COO−1 symmetrical vibration. The absorption bands with peaks around 2923.8 and 2844.9 cm−1 correspond respectively to asymmetrical and symmetrical stretching of C–H vibration of long alkane chain of oleate. The strong absorption with peak at 1641 cm−1 corresponds to the stretching vibration of the C C bond of oleate. The O–H stretching mode at 3400 cm−1 and the O–H bending mode at 1650 cm−1 represent the characteristic stretching vibration bands of EG molecules. The FT-IR spectra indicate that an organic adsorption layer composed of oleate and EG molecules remained attached to the surface of magnetite microparticles. NaAc plays an important role in the formation of spherical product acting both as an alkaline (Cheng et al., 2010) and as

an electrostatic stabilizing agent to prevent particle agglomeration (Wang et al., 2009). Fig. 8 shows typical TEM images of the products synthesized with different amounts of NaAc, while other conditions remained the same: Fig. 8a for Fe3 O4 microparticles synthesized using 1.5 g NaAc showing Fe3 O4 microparticles with average diameter of 150 nm aggregating into irregular aggregates ranged from 200 to 400 nm, while in Fig. 8b, for NaAc increased to 3 g, the Fe3 O4 microparticles became much more irregular and more seriously aggregated, with diameters ranging from 120 to 600 nm. In Fig. 4b, using 2.4 g NaAc, nearly monodisperse secondary-structured Fe3 O4 microparticles with average size of 150 nm were obtained. In order to further investigate the influence of NaAc, NaAc was replaced with NaOH and NH4 Ac while the other experimental conditions were kept unchanged, leading to the TEM images of the products as shown in Fig. 9. Fig. 9a illustrates that using NaOH as a protecting agent led to irregularly cuboid-shaped nanoparticles with an average size of as small as 50 nm leading hardly to any self-assembled structure. The particles synthesized with NH4 Ac as protecting agent are shown in Fig. 9b, in which self-assembled particles are observed, the products are highly polydispersed in size,

Fig. 9. TEM images of Fe3 O4 products obtained at 200 ◦ C for 10 h using different stabilizing agents: (a) NaOH and (b) NH4 Ac.

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Fig. 11. XRD pattern of as-synthesized Fe3 O4 microparticles obtained by using Fe3 O4 microparticles as growth substrates.

Fig. 10. (a) Magnetization curve measured at room temperature for the Fe3 O4 microparticles with average diameter of around 200 nm. (b) Detailed view of x and y intercepts of the graph.

shape and seriously aggregated. These facts suggest how important a role NaAc plays in the growth process of Fe3 O4 microparticles (Wang et al., 2009). 3.4. Magnetic measurement of products Fig. 10a shows a representative magnetization curve measured at room temperature for as-synthesized Fe3 O4 microparticles with diameters of about 200 nm, showing a saturation magnetization (Ms) value of 78.03 emu/g. Fig. 10b shows a remanent magnetization (Mr) value of about 4.06 emu/g and a coercivity value (Hc) of 62.07 Oe. It has been reported that magnetic properties of Fe3 O4 microparticles strongly depend on their grain size and structure (Dyakonov et al., 2009; Xuan et al., 2009). In the present study, the relatively high Ms value with relatively low Mr and Hc values may be attributed to the small primary nanoparticles of about 20 nm and the oriented attachment structure of the much larger microparticles of about 200 nm. 3.5. Synthesis of Fe3 O4 microparticles with average diameter of 1 m By using these Fe3 O4 microparticles as growth substrates, nearly monodisperse Fe3 O4 microparticles with diameters of around 1 ␮m were obtained. Fig. 11 shows the XRD pattern of the obtained Fe3 O4 microparticles, typically that of Fe3 O4 (JCPDS 75-1609). Fig. 12a and

b are the SEM images of Fe3 O4 microparticles used as growth substrates for the second stage synthesis, showing their formidably rough surface derived from the relatively small Fe3 O4 primary nanoparticles as building blocks. However, the Fe3 O4 microparticles obtained from these Fe3 O4 microparticles as growth substrates exhibit distinctive characteristics, as shown in Fig. 13: much larger (around 1 ␮m) than the substrate microparticles (around 300 nm), nearly uniform in size, and with smooth surface. This investigation proposes a two-stage growth process for the Fe3 O4 microparticles. The formation process could be divided into the formation of Fe3 O4 microparticles by dominant oriented attachment process (Zhang, Liu, & Yu, 2009) and the growth of Fe3 O4 microparticles by the dominant Ostwald ripening process. The formation process is illustrated in Fig. 14. In the first stage, the formation of Fe3 O4 microparticles may involve the following reactions. Nucleation is the first step that occurs in the solution forming small Fe3 O4 nanocrystals. At high temperature, a portion of Fe3+ ions will be reduced to Fe2+ by ethylene glycol with the assistance of sodium oleate. At the mean time, the hydrolysis of NaAc provides an alkaline atmosphere for the reaction aqueous solution. The alkaline condition gives rise to the formation of Fe(OH)3 and Fe(OH)2 because of the forced hydrolysis of Fe3+ and Fe2+ ions, which will be transformed to Fe3 O4 nanocrystals after dehydration (Cheng et al., 2010). It has also been reported that EG could first coordinate with FeCl3 to produce iron alkoxide, which precipitated to become nuclei and finally grew into the primary Fe3 O4 nanoparticles (Zhong et al., 2006). Secondly, once supersaturation for the nucleation is achieved, obtaining a stable surface energy, these primary Fe3 O4 nanocrystals tend to aggregate into larger secondary microparticles at high temperature (Ge et al., 2007). Although the primary Fe3 O4 nanocrystals may grow via Oswald ripening, the first-stage growth is dominated by oriented attachment, thus obtaining rough-surfaced Fe3 O4 microparticles with the shape of a pomegranate. Regarding the growing process, it is believed that sodium oleate plays a crucial role in the formation of secondary Fe3 O4 microparticles. As a surfactant, oleate can attach to the surface of primary nanoparticles via carboxylate-Fe, thus stabilizing these particles (Korolev et al., 2002). On the other hand, addition of sodium oleate may give rise to the increase of solution viscosity, slowing down the reaction rate and the motion of primary nanoparticles, thus allowing primary nanoparticles to aggregate into large and regular microparticles. This is consistent with our

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Fig. 12. SEM images of Fe3 O4 microparticles used as growth substrates: (a) low and (b) high magnification.

Fig. 13. SEM images of as-synthesized Fe3 O4 microparticles by using Fe3 O4 microparticles as growth substrates: (a) high and (b) low magnification.

Fig. 14. A schematic illustration of the formation process of two types of Fe3 O4 microparticles.

experiment results. From Fig. 13, we found that the product surface of the second stage was very smooth with a few tiny nanoparticles spreading on the surface of Fe3 O4 microparticles obtained. It is obvious that the formation of Fe3 O4 microparticles was driven by the Ostwald ripening process in which, over time, small crystals dissolved in solution, and the resolved species redeposited on the surfaces of larger crystals (Cheng et al., 2010). The result was consistent with the report by Penn, Tanaka, and Erbs (2007), in which they demonstrated that as the average crystal size increases, the contribution to growth by oriented aggregation decreases. In our experiment, when the size of Fe3 O4 microparticles used as

growth substrates was as large as around 300 nm, the second stage growth was dominated by Ostwald ripening process, thus resulting in larger, spherical and smooth-looking Fe3 O4 microparticles. 4. Conclusions In summary, ferromagnetic Fe3 O4 microparticles selfassembled from small Fe3 O4 nanoparticles have been synthesized by using FeCl3 as iron source, sodium oleate as surfactant and ethylene glycol as reduction agent, while NaAc serves as a sort of alkaline source and electrostatic stabilizer. The as-synthesized

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Fe3 O4 microparticles have almost monodisperse diameters that can be adjusted in the range of 120–400 nm. The effects of experimental parameters such as reaction time, amount of surfactant and NaAc on the size and morphology of the final products were investigated. The as-synthesized Fe3 O4 microparticles show high saturation magnetization, therefore may have a wide range of potential applications in magnetic technology and biomedical fields such as magnetic separation and metal ion removal. Furthermore, by using the pre-synthesized Fe3 O4 microparticles as the growth substrates, spherical and smooth-looking Fe3 O4 microparticles with average diameter of around 1 ␮m were obtained. A possible formation process of the two types of Fe3 O4 microparticles was discussed. Acknowledgments The project was supported by the National Natural Science Foundation of China (NSFC, nos. 20876100 and 20736004), the National Basic Research Program of China (973 Program, no. 2009CB219904), the State Key Lab of Multiphase Complex Systems of the Chinese Academy of Sciences (no. 2006-5), the Key Lab of Organic Synthesis of Jiangsu Prov., R&D Foundation of Nanjing Medical Univ. (NY0586), Post-doctoral Science Foundation of Jiangsu Prov., National Post-doctoral Science Foundation (20090451176), the Commission of Science and Technology of Suzhou Municipality (YJS0917, SG0978), Technology Innovation Foundation of Suzhou New District and MOST, and Minjiang Scholarship of Fujian Prov. References Ansari, F., Grigoriev, P., Libor, S., Tothill, I. E., & Ramsden, J. J. (2009). DBT degradation enhancement by decorating Rhodococcus erythropolis IGST8 with magnetic Fe3 O4 nanoparticles. Biotechnology and Bioengineering, 102, 1505–1512. Bregoli, L., Chiarini, F., Gambarelli, A., Sighinolfi, G., Gatti, A. M., Santi, P., et al. (2009). Toxicity of antimony trioxide nanoparticles on human hematopoietic progenitor cells and comparison to cell lines. Toxicology, 262, 121–129. Cheng, W., Tang, K., Qi, Y., Sheng, J., & Liu, Z. (2010). One-step synthesis of superparamagnetic monodisperse porous Fe3 O4 hollow and core-shell spheres. Journal of Materials Chemistry, 20, 1799–1805. Chiang, C. L., Sung, C. S., Wu, T. F., Chen, C. Y., & Hsu, C. Y. (2005). Application of superparamagnetic nanoparticles in purification of plasmid DNA from bacterial cells. Journal of Chromatography B, 822, 54–60. Deng, H., Li, X., Peng, Q., Wang, X., Chen, J., & Li, Y. (2005). Monodisperse magnetic single-crystal ferrite microspheres. Angewandte Chemie International Edition, 44, 2782–2785. ´ Dyakonov, V., Slawska-Waniewska, A., Kazmierczak, J., Piotrowski, K., Iesenchuk, O., Szymczak, H., et al. (2009). Nanoparticle size effect on the magnetic and transport properties of (La0.7 Sr0.3 )0.9 Mn1.1 O3 manganites. Low Temperature Physics, 35, 568–577. Ge, J. P., Hu, Y. X., Biasini, M., Beyermann, W. P., & Yin, Y. D. (2007). Superparamagnetic magnetite colloidal nanocrystal clusters. Angewandte Chemie-International Edition, 46, 4342–4345. Hosono, T., Takahashi, H., Fujita, A., Joseyphus, R. J., Tohji, K., & Jeyadevan, B. (2009). Synthesis of magnetite nanoparticles for AC magnetic heating. Journal of Magnetism and Magnetic Materials, 321, 3019–3023. Karlsson, H. L., Gustafsson, J., Cronholm, P., & Möller, L. (2009). Size-dependent toxicity of metal oxide particles—A comparison between nano- and micrometer size. Toxicology Letters, 188, 112–118. Korolev, V. V., Ramazanova, A. G., & Blinov, A. V. (2002). Adsorption of surfactants on superfine magnetite. Russian Chemical Bulletin, 51, 2044–2049. Lee, E. J. H., Ribeiro, C., Longo, E., & Leite, E. R. (2005). Oriented attachment: An effective mechanism in the formation of anisotropic nanocrystals. The Journal of Physical Chemistry B, 109, 20842–20846.

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