Facile route to γ-Fe2O3/SiO2 nanocomposite used as a precursor of magnetic fluid

https://doi.org/10.1016/j.matchemphys.2007.09.014Get rights and content

Abstract

Maghemite (γ-Fe2O3) nanoparticles (NPs) homogeneously dispersed in silica matrix were obtained via a three-step chemical procedure at mild conditions. The acid-treated solids were obtained using silica NPs, Fe(NO3)3·9H2O and acetic acid. The nanocomposites with various contents of maghemite were prepared by heating the acid-treated solids at 80 °C for 2 h and then at 400 °C for 1 h. The acid-treated solids were studied by means of Fourier Transform infrared spectroscopy (FT-IR) and thermogravimetry (TG). The morphology and particle size of the magnetic nanocrystallites were evaluated by the transmission electron microscopy (TEM) technique, while the nature of the obtained nanocomposites was studied using X-ray powder diffraction (XRD) and vibrating sample magnetometer (VSM), showing that the acid treatment played a critical role for the magnetic-phase formation and the maximum saturation magnetization (Ms) of the obtained nanocomposite was 37.78 emu g−1. However, when FeCl3·6H2O was used as precursor instead of Fe(NO3)3·9H2O, pure hematite (α-Fe2O3) particles dispersed in silica matrix were obtained. Magnetic fluid (MF) was prepared using the maghemite/silica nanocomposites by high-energy ball milling and was characterized by UV–vis, Gouy magnetic balance and rotating rheometer.

Introduction

The synthesis of magnetic nanomaterials is a subject of great interest due to the broad applications of these materials, including data storage [1], magnetic resonance contrast-enhancing media [2], gas sensors [3], [4], magnetic fluids (MFs) [5], magnetic refrigeration [6], magneto-optics [7], [8], bioprocessing [9], [10], and so on. Among a variety of magnetic materials, maghemite powder (γ-Fe2O3, the cubic form of iron (III) oxide) has received much attention due to the excellent ferrimagnetic and anti-oxidation properties, and has been used widely for the production of magnetic materials and catalysts.

Various methods had been reported in literature for preparing iron oxide NPs, such as wet chemical synthesis [11], sol–gel processing [12], chemical vapor deposition [13], electrochemical preparation [14], pyrolysis techniques [15], [16], and so on. Generally, these methods encountered difficulties in large-scale production of magnetic NPs with controllable particle size and single crystalline structure. The preparation of pure γ-Fe2O3 NPs also presents some difficulties partially arising from the different states of metal oxides, which can lead to the simultaneous presence of various oxides (FeO, Fe2O3, and Fe3O4) during the heating treatment. Moreover, Fe2O3, besides the maghemite phase, also exhibits amorphous or other crystalline phase, among which hematite (α-Fe2O3) is the one that is thermodynamically stable. γ-Fe2O3 transforms to α-Fe2O3 at a temperature above 380 °C. Moreover, the performance of the materials made of γ-Fe2O3 depends strongly on the particle size and particle size distribution (PSD). Magnetic NPs with a uniformity size distribution were difficult to be obtained using conventional methods. The aggregation of NPs induced the loss of some excellent performance of nanomaterials. One way to stabilize the magnetic NPs and also to prevent the magnetic NPs from aggregation is uniformly dispersing the iron oxide particles in some matrix, such as glass [17], [18], silica matrix [8], [19], [20], ceramic [21], or polymer [7], [22]. Consequently, pure iron oxide NPs homogeneously dispersed in some matrix could demonstrate satisfactory properties, though the dispersion process might be rather complicated.

Magnetic fluid (MF) is a colloidal suspension of magnetic NPs dispersed in a solvent (water or organic solvent) by some surfactants. The performance and stability of the MFs are controlled by the nature of the particle–particle interactions which in turn are dictated by the surfactant layers on the particles. Based on the abroad applications (such as in some loudspeakers and computer hard discs for dynamic sealing), research and development on the preparation and characterization of MFs have been very active since the inception of MFs. Laibinis and co-workers [23] successfully prepared stable and water-based MFs consisting of iron oxide NPs, stabilized by self-associated bilayers of two alkanoic acids. Sousa and co-workers [24] prepared biocompatible MFs based on γ-Fe2O3 NPs, which were successfully grafted by antibodies to transport the NPs into different human tissues.

In this paper, to prepare the maghemite NPs with high purity and narrow PSD, the precursor was mixed with silica NPs and treated with acetic acid first, and then followed with heating treatment. The γ-Fe2O3 NPs were homogeneously dispersed in the silica matrix. Stable MF was obtained using the nanocomposites by ball milling. The magnetic properties and rheological behavior of the MF were investigated.

Section snippets

Materials

Silica NPs (M-5, Cabot GmbH, about 30 nm, 200 m2 g−1) were obtained from Deutschland Cabot GmbH. Ferric nitrate (Fe(NO3)3·9H2O), ferric chloride (FeCl3·6H2O), ethyl alcohol and acetic acid were all analytic grade, purchased from the commercial market, and used without further purification before utilization. Oleate sodium and polyethylene glycol 4000 (PEG-4000) were chemical grade. Deionized water was used throughout the experiments.

Particle preparation

The silica–maghemite composites with various maghemite contents

FT-IR spectra

The IR spectra of the solids prepared with different procedures were displayed in Fig. 1, Fig. 2, Fig. 3. In these figures, the absorptions around 1100, 810 and 470 cm−1 are the characteristic absorption of the silica, and the peaks around 3380 cm−1 is the characteristic absorption of hydroxyl groups existed on the particle surface. Fig. 1 shows the IR spectra of the solids before calcination but after heating treatment at 80 °C for 2 h using ferric nitrate (Fe(NO3)3·9H2O) as precursor, in which

Conclusions

In summary, silica–maghemite nanocomposites were obtained through a three-step chemical procedure under mild conditions. The acid treatment in the first preparing stage played a critical role in the magnetic-phase formation. The ferrite, γ-FeOOH and β-FeOOH corresponding to Fe(NO3)3·9H2O and FeCl3·6H2O as precursor, were obtained after acid treatment. γ-Fe2O3 or α-Fe2O3 was obtained after calcining such ferrite, respectively. The decomposing temperature of the γ-FeOOH was around 350 °C in

Acknowledgments

The project was supported by the National Natural Science Foundation of China (NNSFC, No. 20476065), the Scientific Research Foundation for the ROCs of State Education Ministry (SRF for ROCS, SEM), the Key Lab. of Multiphase Reaction of the Chinese Academy of Science (No. 2003-5), the Key Lab. of Organic Synthesis of Jiangsu Prov., the Chemical Experiment Center of Soochow Univ. and R&D Foundation of Nanjing Medical Univ. (NY0586).

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