Ferromagnetic Fe3O4 nanofibers: Electrospinning synthesis and characterization

Ferromagnetic Fe3O4 nanofibers: Electrospinning synthesis and characterization

Journal of Alloys and Compounds 577 (2013) 192–194 Contents lists available at SciVerse ScienceDirect Journal of Alloys and Compounds journal homepa...

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Journal of Alloys and Compounds 577 (2013) 192–194

Contents lists available at SciVerse ScienceDirect

Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom

Letter

Ferromagnetic Fe3O4 nanofibers: Electrospinning synthesis and characterization Weiwei Pan, Rui Han, Xiao Chi, Qingfang Liu, Jianbo Wang ⇑ Institute of Applied Magnetics, Key Laboratory of Magnetism and Magnetic Materials of Ministry of Education, Lanzhou University, Lanzhou 730000, PR China

a r t i c l e

i n f o

Article history: Received 5 November 2012 Received in revised form 3 February 2013 Accepted 29 April 2013 Available online 7 May 2013 Keywords: Electrospinning Nanofibers Ferromagnetic materials Mössbauer spectra

a b s t r a c t In this paper, Fe3O4 nanofibers with average diameters of 100 nm were successfully synthesized by electrospinning of PVP/Fe(NO3)9H2O composite solution followed by calcinations and reduction. The resulting Fe3O4 nanofibers were characterized with X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron microscope (TEM), transmission Mössbauer spectra (MS), and vibrating sample magnetometer (VSM). These nanofibers showed a cubic structure and uniform and continuous morphology. The Fe3O4 nanofibers possessed a saturation magnetization Ms of 57.6 emu/g. A large coercivity Hc of 188.4 Oe together with a high remanence ratio Mr/Ms = 0.28 is observed, which result from the shape anisotropy. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction In the past decade, considerable attention has been drawn towards one-dimensional (1D) nanostructured materials, including nanotubes, nanorods, nanowires, and nanofibers, because they exhibit novel physical and chemical properties, differing from those of their bulk counterparts, due to their reduced size and large surface-to-volume ratios. To date, various approaches have been developed for preparation of 1D magnetic materials, for example, electrospinning is a simple and versatile method for generating ultrathin nanofibers made of various materials [1]. The nanofibers produced by electrospinning have several remarkable advantages including: small diameter, high aspect ratio, large specific surface area, diverse in composition [2,3]. Among magnetic materials, iron oxides, such as Fe2O3 and Fe3O4, are the most popular materials and possess many advantages in technological applications. Recently, numbers of facile methods have been paid to fabricate of Fe2O3 and composite nanostructures, and their gas-sensing properties were studied. For instance, Yan et al. synthesized porous flower-like a-Fe2O3 and they exhibited short response/recovery time within 6/3 s and a much higher gas response than a-Fe2O3 nanoparticles [4]. Huang et al. synthesized ZnO/a-Fe2O3 hierarchical nanostructures and studied their morphologies and gas-sensing properties. The results showed that amount of a-Fe2O3 added significantly affected

⇑ Corresponding author. Tel.: +86 0931 8914171; fax: +86 0931 8914160. E-mail address: [email protected] (J. Wang). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.04.199

overall morphology, sensors based on ZnO/a-Fe2O3 hierarchical nanostructures exhibited a much higher sensitivity to ethanol vapor and the 2% a-Fe2O3 added ZnO/a-Fe2O3 nanostructures sensors showed the highest sensitivity [5]. Yan et al. Synthesized spindlelike a-Fe2O3/ZnO core–shell structures, which displayed better photocatalytic activities than those of pure a-Fe2O3 under ultraviolet irradiation [6]. For Fe3O4 materials, The compound exhibited unique electric and magnetic properties due to the transfer of electrons between Fe2+ and Fe3+ ions in the octahedral sites. Fe3O4 has an inverse spinel structure with a face-centered cubic unit cell of 32 O2 ions, in which one Fe3+ ion occupy the tetrahedral sites, while the other Fe3+ and one Fe2+ ions occupy both the octahedral sites [7]. Over the past decades, researchers have proposed several synthesis methods for preparing Fe3O4 with different nanostructured morphologies, such as nanoparticles, nanorods, nanowires, nanotubes, and spheres [8–10]. However, it is a great challenge to obtain Fe3O4 nanofibers fabricated by electrospinning due to the mix values of Fe ions in octahedral sites. Previous studies have shown that the electrospinning process has been utilized to fabricate Fe3O4 nanoparticles inside polymer nanofibers, which including PEO, PVA, PAN, PANCAA, etc. [10–14]. To the best of our knowledge, Fe3O4 nanofibers fabricated by electrospinning have not been reported in the literature. In this paper, we firstly electrospun a precursor Fe(NO3)39H2O and then obtain Fe3O4 nanofibers after heat and reduce treated the precursor membranes in proper conditions. We reported the structure, morphological characteristics and magnetic properties of the resulting nanofibers by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), transmission electron

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F30, FEI). Transmission MS were recorded at room temperature using a conventional constant acceleration spectrometer with a c-ray source of 25 mCi57Co in a palladium matrix. The isomer shifts quoted in this work are relative to that of aFe2O3. The magnetic properties of the nanofibers were measured at 300 K using a VSM 7403 from Lakeshore Corporation. The hysteresis loops of the nanofibers were recorded.

3. Results and discussion

Fig. 1. Typical XRD patterns of the as-prepared Fe2O3 and Fe3O4 nanofibers.

microscope (TEM), transmission Mössbauer spectra (MS), and vibrating sample magnetometer (VSM). 2. Experimental In this paper, Fe(NO3)39H2O (99.99% purity), polyvinyl pyrrolidone (PVP, Mw = 1,300,000), N,N-Dimetylformamide (DMF, 99.8% purity), and ethanol (100% purity) were used as the starting chemicals. In a typical procedure, Fe(NO3)39H2O was dissolved in a mixed solvent containing ethanol and DMF with a weight ratio of 1:1, followed by magnetic stirring for 2 h. Then a appropriate amount of PVP were added into the above solution and further magnetically stirred for 1–2 h at room temperature to form a homogeneous solution with PVP concentration of 8 wt.%. The viscous solution was loaded into a plastic syringe with a stainless steel needle. The applied voltage for electrospinning was 14 kV and the distance between the needle tip and the collector was 14.0 cm. The collected Fe(NO3)39H2O/PVP precursor nanofibers were calcined at 550 °C for 2 h in air followed by reducing at 250 °C for 2 h in a hydrogen atmosphere to obtain Fe3O4 nanofibers with a heating rate of 1 °C/min. XRD analysis of the Fe3O4 nanofibers was performed using an X-ray diffractometer (PANalytical diffractometer with Cu Ka radiation). The morphologies of the nanofibers were characterized on Hitachi S-4800 FE-SEM and TEM (Tecnai™ G2

Annealing the Fe(NO3)39H2O/PVP precursor nanofibers in air resulted in a formation of Fe2O3 nanofibers, as shown in Fig. 1. Fe3O4 nanofibers could be obtained by very carefully deoxygenizing Fe2O3 nanofibers in a hydrogen atmosphere at 250 °C for 2 h. A typical XRD pattern of as-synthesized Fe3O4 products is shown in Fig. 1. All the diffraction peaks can be indexed to cubic structured Fe3O4, with cell constant a = 8.393 Å, which is in good agreement with the value in the literate (JCPDS card No. 65-3107, a = 8.391 Å). The main diffractions peaks correspond to (3 1 1), (4 0 0), (5 1 1), and (4 4 0) crystal phase. No obvious impurity phase can be detected. Thereby, Fe3O4 nanofibers formation processes contain calcinations and reduction treatment in this paper. Fig. 2a and b shows the SEM images of as-synthesized Fe2O3 and Fe3O4 nanofibers, it can be seen that the diameters of nanofibers are almost uniform (100 nm) and the surfaces are fairly smooth. This indicates that the nanofibers keep original shape after reduction treatment. Fig. 2c shows the TEM imagne of Fe3O4 nanofibers, sample exhibited fibrous morphology with a good dispersity, which is in consistent with the result of SEM. The upper inset shows that a nanofibers is composed of fine nanoparticles. The high-resolution TEM image presented in Fig. 2d, the lattice fringe is of 0.29 nm, agreeing with that of the (2 2 0) lattice plane regarding Fe3O4 sample. Transmission MS was carried out to further confirm the formation of Fe3O4 and a typical result is shown in Fig. 3. The spectrum consists of two hyperfine magnetic sextets (Hhf: 48.7 T and 45.7 T), one for the Fe3+ tetrahedral sites and the other for the mixed valence Fe2.5+ octahedral sites. From the different values of the Hhf, Fe2O3 and Fe3O4 can be identified because the values of the

Fig. 2. SEM images of the as-prepared nanofibers, (a) Fe2O3, (b) Fe3O4; (c) TEM image of Fe3O4 nanofibers, the upper inset shows high-magnification TEM image; (d) highresolution TEM image of Fe3O4 nanofibers.

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but is smaller than the Fe3O4 nanoparticles [8–10]. Although the source of low Ms is not clear, the main reasons may be come from the high shape anisotropy of the nanofibers, preventing them from magnetizing in directions other than along their easy magnetic axes. The coercivity (Hc) and remanence ratio (Mr/Ms) are 188.4 Oe and 0.28, which much higher than that of bulk Fe3O4 [16]. The high Hc and Mr/Ms may result from the shape anisotropy, forcing the magnetic moments to mostly align along the axis of the nanofibers. 4. Conclusion

Fig. 3. Mössbauer spectra of Fe3O4 nanofibers measured at room temperature.

In conclusion, electrospinning is a simple and essential method for fabricating functional nanofibers. We successfully prepared Fe3O4 nanofibers by electrospinning method, followed by calcinations in air and reduction in hydrogen atmosphere. The XRD patterns showed that calcined Fe2O3 sample after being deoxidized at 250 °C in hydrogen atmosphere for 2 h were Fe3O4 annofibers. From the MS the larger difference of Hhf corresponding to A and B sites indicated that the sample is Fe3O4 rather than c-Fe2O3. The high Hc and Mr/Ms result from the shape anisotropy of nanofibers. These novel magnetic nanofibers can potentially be used in high-density magnetic recording and magnetic sensor. Acknowledgments This paper was supported by National Science Fund of China (11074101, 51171075), and the Fundamental Research Funds for the Central Universities (Lzujbky-2011-54). References [1] [2] [3] [4]

Fig. 4. Magnetization hysteresis curve measured at room temperature for the Fe3O4 nanofibers.

hyperfine fields of A- and B-sites for Fe3O4 have an obvious difference; however, for c-Fe2O3 the values are almost the same [15]. The values of Hhf of these nanofibers are small than that of the bulk Fe3O4 materials mainly due to the existence of collective magnetic excitation caused by the size distribution of the crystallites. The magnetic properties of the as-prepared Fe3O4 nanofibers were evaluated by VSM at room temperature as shown in Fig. 4. The hysteresis loop shows a ferrimagnetic behavior. The nanofibers exhibits a saturation magnetization (Ms) of 57.6 emu/g, much smaller than that of bulk Fe3O4 materials [16]. The value of Ms is in agreement with the Fe3O4 nanowires, nanorods and nanotube,

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