Materials Letters 65 (2011) 1775–1777
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Materials Letters j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / m a t l e t
Preparation of pure iron nanoﬁbers via electrospinning Hao Shao, Xuebin Zhang ⁎, Shasha Liu, Fanyan Chen, Jie Xu, Yi Feng School of Materials Science and Engineering, Hefei University of Technology, Anhui, 230000, PR China
a r t i c l e
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Article history: Received 21 January 2011 Accepted 14 March 2011 Available online 21 March 2011 Keywords: Iron nanoﬁber Electrospinning Microstructure FTIR
a b s t r a c t In this paper, iron nanoﬁbers were successfully produced by electrospinning of ferric nitrate–PVA solutions to corresponding PVA/Fe(NO3)3 composite nanoﬁbers followed by calcinations and reduction. The thermal stability of the as-prepared PVA/Fe(NO3)3 composite nanoﬁbers were investigated by TG–DSC. The morphologies and structures of the as-prepared samples were also characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The results obtained indicated that: the thermal transformation of ferric nitrate and the decomposition of PVA in PVA/Fe(NO3)3 composite nanoﬁbers ended at 310 °C and 460 °C respectively. The as-prepared iron nanoﬁbers showed a continuous morphology and a very high degree of crystallization. Furthermore, the average diameter was about 180 nm. © 2011 Elsevier B.V. All rights reserved.
In the past 10 years, magnetic nanoﬁbers including nanoﬁbers, nanotubes, nanowires and nanobelts have received much attention due to their excellent properties and compelling applications in many areas [1,2], such as high-density magnetic recording, photonic and magnetic devices used for magnetic ﬁlters , spintronic devices, magnetic induction , information storage and magnetic sensors . Compared to other methods developed to fabricate nanoﬁbers, such as template , phase separation, self-assembly and melt-blown , electrospinning is more simple and more effectively in fabricating continuous nanoﬁbers. Besides, the ﬁber diameter can also be adjusted from nanometers to microns by changing the parameters of electrospinning [8,9]. Iron ﬁbers and their products have been developed into new materials in modern science, due to their high-performance that is caused by high speciﬁc surface area. The traditional mature methods to fabricate iron ﬁbers are drawing method, scraping method and fusing drawing method. These methods can only fabricate micrometer ﬁbers; it is very difﬁcult to fabricate ﬁner iron ﬁbers. As a convenient method, electrospinning is associated to fabricate iron ﬁbers [10,11]. In this paper, long iron nanoﬁbers are obtained by electrospinning ferric nitrate-polyvinyl alcohol (PVA) solutions corresponding composite ﬁbers followed by burning out the organic components from these composite ﬁbers and deoxidizing the ﬁbers.
2.1. Preparation Polyvinyl alcohol (PVA 1788, Chengdu Kelong Chem. Co. Ltd., China) was used for ﬁber preparation by electrospinning. Ferric nitrate, Fe(NO3)3·9H2O (AR, Aladdin Chem. Co. Ltd., China) was used as the ferric oxide precursor. In this experiment, an appropriate PVA aqueous solution (20 wt.%) was prepared in a typical procedure where 20 g PVA powder was added into 80 ml distilled water, with water-bathing at 85 °C and stirring for 2 h. This was then mixed with the ferric nitrate saturated solution together (with volume ratio of 35:65) by stirring for 1 h, and the electrospinning solution was obtained. The electrospinning solution was loaded into a plastic syringe equipped with a stainless steel needle of which the internal diameter is 0.42 mm. A positive voltage of 25 kV was applied to the needle while the aluminum frame collector was grounded, and the distance between the spinneret and collector was 12 cm. The nanocomposite precursor was calcined in the tubular furnace in air at 550 °C for 3 h with a rising rate of 10 °C/min− 1. This process is for the degradation of PVA and conversion of ferric nitrate to ferric oxide. After it had cooled down to room temperature, the specimen was reheated at 750 °C for 3 h in a hydrogen atmosphere to convert the Fe2O3 to Fe. After cooling to room temperature, pure iron nanoﬁbers were formed. 2.2. Characterization
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(X. Zhang). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.03.049
The prepared as-spun PVA/Fe(NO3)3 composite nanoﬁbers were characterized by TG–DSC (WTG Thermal Balance and Mettler Toledo DSC 821e) at a heating rate of 10 °C/min in air from 25 °C to 500 °C. Ferric oxide nanoﬁbers after calcination at 550 °C and iron nanoﬁbers
H. Shao et al. / Materials Letters 65 (2011) 1775–1777
obtained by deoxidizing ferric oxide nanoﬁbers at 750 °C were investigated by XRD (40 kV 100 mA, D/MAX2500VL/PC) using Cu Kα radiation with wavelength of λ = 0.1540 nm at a scanning rate of 5°/min. Surface morphologies and ﬁber diameters were measured by Sirion200 scanning electron microscopy (SEM) and JEOL-2010 transmission electron microscopy (TEM). The molecular structure of nanoﬁbers was characterized by Fourier transform infrared (FT-IR, Nicolet 6700) at room temperature.
3. Results and discussion
b The composite nanoﬁbers fabricated by electrospinning consist of ferric nitrate, solvent and a mass of PVA. In order to obtain the pure iron nanoﬁbers, the composite ﬁbers have to be heat treated and the variations of the content and structure should be investigated. In Fig. 1, the TG–DSC curves of the PVA/Fe(NO3)3 composite nanoﬁbers show solvent evaporation, conversion of ferric nitrate and PVA decomposition behavior. The sharp peak of DSC curve at 73 °C can be ascribed to the loss of water from the composite nanoﬁbers and decomposition of ferric nitrate, which is corresponding to the TG curve. The melting temperature at which hydrolysis of ferric nitrate begins is 60 °C  and the thermal transformation of ferric nitrate ends at about 310 °C. The TG curve around 180 °C shows the dehydration of ferric nitrate to Fe hydroxide, and then Fe hydroxide translates to Fe2O3 from 200 °C to 310 °C. The whole ferric nitrate's transform process is shown as follows : FeðNO3 Þ3·9H2 O→FeðOHÞðNO3 Þ2 →FeðOHÞ2 NO3 →FeOOH→Fe2 O3 The peak of DSC curve at about 95 °C can be ascribed to glass transition of PVA . The TG curve from 200 °C to 460 °C indicates PVA decomposition. Based on the TG–DSC curves, PVA and ferric nitrate were almost completely eliminated at about 460 °C and the total weight loss is about 83.2%. Therefore, 550 °C was chosen as the calcination temperature for obtaining pure Fe oxide nanoﬁbers. Fig. 2(a) shows the FTIR spectra of the as-spun composite nanoﬁbers. The broad peak at about 3300 cm− 1 corresponds to H–OH stretch and the peaks are almost in the range of 900–1800 cm− 1 that are in accord with the bending and stretching vibrations of the PVA . The peaks of mineral do not appear in this sample due to low contents of the mineral. As shown in Fig. 2(b), the characteristic peaks of PVA have been completely disappeared, which indicates the entirely decomposition of PVA after calcination at 550 °C. Besides, there are two peaks appearing at about 457 and 534 cm− 1, which belong to the Fe–O vibration of the Fe2O3 nanoﬁbers respectively. It means that the well-crystallined Fe2O3 nanoﬁbers have been formed.
Wavenumber(cm-1) Fig. 2. FT-IR spectra of nanoﬁbers: (a) as-spun PVA/Fe(NO3)3 composite nanoﬁbers and (b) ferric oxide nanoﬁbers after calcined at 550 °C for 3 h.
Therefore the temperature of 550 °C is enough for the degradation of PVA and the conversion of ferric nitrate to ferric oxide. All of the XRD patterns of inorganic nanoﬁbers were analyzed to describe the phase and crystallinity. Fig. 3(a) and (b) show the XRD patterns of the calcined nanoﬁbers before and after reduction. As shown in Fig. 3(a), the PVA/Fe(NO3)3 composite nanoﬁbers that were calcined at 550 °C in air were in the Fe2O3 phase and the crystal face of each diffraction peak was characterized in chart (JCPDS card #33-0664). The XRD pattern shown in Fig. 3(b) belongs to Fe nanoﬁbers. The characteristic peaks of the nanoﬁbers were measured to be 2θ = 45° (d= 0.2012 nm), 65° (d= 0.1433 nm). Compared with the JCPDS card #06-0696, these data are in good agreement with that of iron and diffraction peaks separately correspond to crystal face of (110), (200). The strong and sharp diffraction peaks indicate a very high degree of crystallization. Conclusions are obtained: the treatment in a hydrogen atmosphere at 750 °C is enough to deoxidize Fe2O3 to iron. Fig. 4(a) show the SEM images of the as-electrospun composite nanoﬁbers. As shown in Fig. 4(a), these nanoﬁbers were collected in a random orientation because of the instability with the spinning jet. These nanoﬁbers have smooth and uniform surfaces due to the amorphous nature of PVA. The average diameter of the nanoﬁbers is about 350 nm. After the composite nanoﬁbers were calcined at 750 °C for 3 h in air resulting in degradation of PVA and further heating in a hydrogen atmosphere at 750 °C for 3 h resulted in conversion ferric oxide to iron, as shown in Fig. 4(b), the nanoﬁbers still remain as continuous structures with a relatively rough surface. The average
(200) (208) (1010) (220)
a 0 20
Temperature(oC) Fig. 1. TG–DSC curves of as-spun PVA/Fe(NO3)3.
2 Theta(degree) Fig. 3. X-ray diffraction patterns of nanoﬁbers treated at different conditions: (a) ferric oxide nanoﬁbers after calcined at 550 °C and (b) Fe nanoﬁbers obtained by deoxidizing ferric oxide nanoﬁbers at 750 °C.
H. Shao et al. / Materials Letters 65 (2011) 1775–1777
Fig. 4. (a) SEM image of electrospun PVA/Fe(NO3)3 composite nanoﬁbers; (b) SEM image of Fe nanoﬁbers; (c) SEM image of Fe nanoﬁbers; and (d) SAED pattern of Fe nanoﬁber.
diameter of the nanoﬁbers is about 180 nm. This diameter shrinkage can be accounted for the loss of PVA from the PVA/Fe(NO3)3 composite nanoﬁber and the crystallization of iron. Fig. 4(c) shows a typical TEM image of a single iron nanoﬁber, indicating that the nanoﬁber is formed of nanoparticles with a size range from about 5 to 20 nm, which is approximately consistent with the estimated crystallite sizes by the XRD. The diameter of the nanoﬁber is about 180 nm which is in agreement with the result of the SEM analysis. The SAED analysis is shown in Fig. 4(d), the diffraction rings can be indicated to the planes of (110), (200), (211) of iron and (012), (110) of Fe2O3. The SAED sample was placed in alcohol solution and ultrasonic dispersed for a long time, so the surface of iron have been oxidized that is why the diffraction rings of Fe2O3 appeared. The sample of XRD mentioned above was just taken out from the hydrogen atmosphere, so iron nanoﬁbers have not been oxidized, as shown in Fig. 3(b). The oxidation of iron ﬁber is a problem needed to be solved. 4. Conclusions In conclusion, iron nanoﬁbers were successfully prepared by electrospinning solution containing PVA and ferric nitrate, followed by calcinations in air and reduction in hydrogen atmosphere. The XRD patterns showed that the PVA/Fe(NO3)3 composite nanoﬁbers after being calcined in air at 550 °C for 3 h have changed into Fe2O3 nanoﬁbers, and the calcined sample after being deoxidized at 750 °C in hydrogen atmosphere for 3 h were Fe nanoﬁbers. The PVA/Fe(NO3)3 composite nanoﬁbers have a smooth and uniform surface and the average diameter is about 350 nm. After calcinations and reduction,
the nanoﬁbers still remain as continuous structures with a relatively rough surface. The average diameter of iron nanoﬁbers is approximately 180 nm. Acknowledgements This work was supported by the Major Research Program of the National Natural Science Foundation of China (Grant No. 91026018), the National Natural Science Foundation of China (Grant No. 60979017), and the Natural Science Foundation of Anhui Province of China (Grant No. 11040606M50). References               
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