Elsevier

Particuology

Volume 9, Issue 2, April 2011, Pages 179-186
Particuology

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

https://doi.org/10.1016/j.partic.2010.07.025Get rights and content

Abstract

Ferromagnetic Fe3O4 nanoparticles were synthesized and then self-assembled into microparticles via a solvothermal method, using FeCl3·6H2O as the iron source, sodium oleate as the surfactant, and ethylene glycol as the reducing agent and solvent. The obtained Fe3O4 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 Fe3O4 microparticles of nearly monodisperse diameters, controllable in the range of 120–400 nm, consist of assemblies of Fe3O4 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 Fe3O4 microparticles as the growth substrates, spherical and smooth-looking Fe3O4 microparticles with average diameter of 1 μm were obtained. A plausible formation process was discussed.

Graphical abstract

Ferromagnetic Fe3O4 nanoparticles were synthesized and then self-assembled into microparticles via a solvothermal method. The Fe3O4 microparticles of nearly monodisperse diameters consist of assemblies of Fe3O4 nanoparticles of diameter 22 nm. Interestingly, by using the pre-synthesized Fe3O4 microparticles as the growth substrates, spherical and smooth-looking Fe3O4 microparticles with average diameter of 1 μm were obtained.

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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 et al., 2009, Zhu et al., 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 et al., 2009, Chiang et al., 2005, Saiyed et al., 2006), hyperthermia (Hosono et al., 2009, Zhao et al., 2009a), removal of metal ions (Liu et al., 2008, Zhao et al., 2009b). Among the various magnetic nanoparticles, iron oxides (such as Fe3O4 or γ-Fe2O3), have been extensively investigated for 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 (Fe3O4) or greigite (Fe3S4) (Xie, Chen, & Chen, 2009). However, the MTB method, harnessing such nanoparticles gestated by magnetotactic bacteria, is still in its infancy. Nowadays, Fe3O4 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 Fe3O4 nanocrystals are greatly influenced not only by their size but also by their attachment structure (Xi, Wang, & Wang, 2008). Well-crystallized, self-assembled Fe3O4 microparticles with size similar to protein molecules, having the advantage of strong magnetization 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 Fe3O4 sub-micrometer spheres with strongly oriented attachment structures (Ge et al., 2007, Xuan et al., 2009). Recently, many methods have been reported to produce Fe3O4 particles with oriented attachment structures. For example, Cheng, Tang, Qi, Sheng, and Liu (2010) succeeded in preparing hollow and core-shell Fe3O4 spheres constructed of nanoparticles by a one-pot hydrothermal method. Yu et al. (2006) reported the oriented-assembly of Fe3O4 nanoparticles into large Fe3O4 microparticles and Qu, Yao, Zhou, Fu, and Huang (2010) have developed an aqueous medium method for synthesizing Fe3O4 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 Fe3O4 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 Fe3O4 microparticles which are self-assembled from small Fe3O4 nanoparticles. The approach involves the nucleation of small Fe3O4 nanocrystals and subsequent self-assembly of the primary small nanoparticles into Fe3O4 microparticles with controlled size and spherical structure. FeCl3·6H2O 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 Fe3O4 microparticles with coarse surface could served as substrates to further grow into smooth-looking spherical Fe3O4 microparticles with average size of 1 μm which are believed to be ideal candidate for a wide range of potential applications such as bioseparation, adsorbents and so on.

Section snippets

Materials

Ferric chloride hexahydrate (FeCl3·6H2O), sodium oleate (C17H33COONa), sodium acetate anhydrous (CH3COONa, NaAc), sodium hydroxide (NaOH), ammonium acetate (NH4Ac), ethylene glycol (EG) were of analytical grade and used without further purification. Deionized water was used throughout the experiments.

Synthesis of Fe3O4 microparticles

A typical synthetic procedure was as follows: FeCl3·6H2O (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

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 Fe3O4 (JCPDS 75-1609), without any of other crystalline materials. According to the Scherrer equation, the average crystallite size calculated on the basis of XRD pattern is about 22 nm, which is consistent with the results of TEM analysis below. Since γ-Fe2O3 (JCPDS no. 39-1346) has a similar XRD pattern, the XRD patterns could not be used to exactly distinguish

Conclusions

In summary, ferromagnetic Fe3O4 microparticles self-assembled from small Fe3O4 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 Fe3O4 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

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),

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