J. Phys. Chem. C 2010, 114, 15589–15593
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CoNi Bimetallic Nanofibers by Electrospinning: Nickel-Based Soft Magnetic Material with Improved Magnetic Properties Nasser A. M. Barakat,*,†,| Khalil A. Khalil,§ Ibrahim H. Mahmoud,‡ Muzafar A. Kanjwal,| Faheem A. Sheikh,| and Hak Yong Kim* Chemical Engineering Department, Faculty of Engineering, El-Minia UniVersity, El-Minia, Egypt, Center for Healthcare Technology DeVelopment, and Department of Textile Engineering, Chonbuk National UniVersity, Jeonju 561-756, Republic of Korea, and Mechanical Engineering Department, Faculty of Engineering, King Saud UniVersity, Riyadh, Saudi Arabia ReceiVed: May 5, 2010; ReVised Manuscript ReceiVed: July 29, 2010
Recently, bimetallic nanostructures have received special interest due to their promising chemical and physical properties. In the literature, various complicated processes have been reported for the preparation of several bimetallic materials in a nanoparticulate shape. In this study, nanofibers, rather than nanoparticles, composed of cobalt and nickel are introduced; these nanofibers have been prepared by a facile technique, electrospinning. Typically, calcination of electrospun mats originating from nickel acetate, cobalt acetate, and poly(vinyl alcohol) in argon atmosphere led to complete elimination of the utilized polymer and abnormal decomposition of the metallic acetates to produce CoNi nanofibers. Physiochemical characterization indicated that both Ni and Co are uniformly distributed along the obtained nanofibers in the same profile which indicates that Ni and Co have been combined at the crystalline level. The prepared CoNi nanofibers revealed better magnetic properties compared with those of Co-doped Ni and pristine Ni nanofibers and have potential for use as nickel-based soft magnetic materials. 1. Introduction Nanostructures have physical and chemical properties that are characteristic of neither the atom nor the bulk counterparts.1 In particular, magnetic nanoparticles exhibit features that are different from those of the corresponding bulk magnets, including modifications of the intrinsic properties,2,3 superparamagnetism,3-5 enhanced coercitivity,3 shifts of the hysteresis loops,6,7 or the absence of magnetic saturation at high fields.7 Also, nanoparticles embedded in dielectric matrixes enhance the magnetoresistance8,9 and the magnetooptical features.10 Accordingly, metallic nanoparticles have wide and various application fields ranging from magnetic data storage to probes and vectors for the biomedical sciences.11-13 Concerning magnetic storage, high storage density requires high magnetic anisotropy to overcome thermal effects and prevent superparamagnetic behavior. Magnetic anisotropy strongly affects the shape of the hysteresis loops and controls the coercivity and remanence. There are several kinds of magnetic anisotropies, including magnetocrystalline anisotropy (depending on the crystallographic orientation of the sample in the magnetic field14-16), exchange anisotropy (due to antiferromagnetic and ferromagnetic materials interaction17,18), stress anisotropy (a uniaxial stress can produce a unique, easy axis of magnetization if the stress is sufficient to overcome all other anisotropies), and shape anisotropy of one-dimensional (1D) nanoparticles such as rods, wire, and nanofibers.19 Specifically, metallic * Corresponding authors: (H.Y.K.) Tel: +82 63 270 2351. Fax: +82 63 270 2348. E-mail:
[email protected]. (N.A.M.B.) E-mail: nasbarakat@ yahoo.com. † El-Minia University. ‡ Center for Healthcare Technology Development, Chonbuk National University. § King Saud University. | Department of Textile Engineering, Chonbuk National University.
nanofibers reveal distinct shape anisotropy features.20 Besides the shape anisotropy, 1D magnetic nanomaterials are expected to have interesting properties, as the geometrical dimensions of the material become comparable to key magnetic length scales, such as the exchange length or the domain wall width. The bimetallic structure of the magnetic nanoparticles further endows them with desirable properties. In particular, nanoparticles composed of ferrometallic alloys constitute an extremely appealing class of materials, since their magnetic properties can be tuned by combining size effect and alloy composition.21 CoNi nanoparticles are of particular interest for their potential to overcome the superparamagnetic limit and to enhance contrast in magnetic resonance imaging.21 There are several reported routes for producing CoNi nanoparticles including sol-gel for preparation of CoNi nanoclusters hosted in silica gel,22 an alginate-mediated growth of CoNi nanoparticles,23 a cobalt-nickel reduction in liquid polyol,24,25 a “guest-host” strategy using layered double hydroxides as a host,26 and an apoferritin biotemplate process.11 However, in the field of 1D nanostructures, CoNi nanowires have been prepared by a nucleation in liquid polyol technique27 and electrochemical deposition in a porous anodic aluminum oxide template.28 To the best of our knowledge, CoNi nanofibers have not yet been reported. The aforementioned techniques for conventional preparation of bimetallic nanoparticles or clusters, however, lack sufficient control of both particle stoichiometry and morphology.29 In this study, CoNi nanofibers are introduced using a facile technique, electrospinning. The technique is characterized by the simplicity of the electrospinning process, the diversity of the electrospinnable materials, and the unique features of the obtained electrospun nanofibers. In the field of metallic nanofibers, the electrospinning process has been used to produce several pristine metallic nanofibers.20,30-34 Moreover, Co-doped Ni nanofibers produced by electrospinning have been recently introduced.35
10.1021/jp1041074 2010 American Chemical Society Published on Web 08/31/2010
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In this study, the electrospinning technique has been utilized to produce nanofiber mats composed of poly(vinyl alcohol), nickel acetate tetrahydrate, and cobalt acetate tetrahydrate. Physiochemical characterization concluded that calcination of the electrospun mats in argon atmosphere leads to CoNi nanofibers with FCC single crystal structure. The prepared CoNi nanofibers revealed better magnetic properties compared to the pristine Ni and Co-doped Ni nanofibers. 2. Experimental Details 2.1. Materials. Cobalt(II) acetate tetrahydrate (CoAc) 98% assay (Junsei Chemical Co., Japan), nickel(II) acetate tetrahydrate (NiAc, 98%, Aldrich Co., Milwaukee, WI), and poly(vinyl alcohol) (PVA) with a molecular weight (MW) ) 65 000 g/mol (DC Chemical Co., South Korea) were utilized without any further modifications. Distilled water was used as a solvent. 2.2. Experimental Work. NiAc and CoAc aqueous solutions were first prepared and then mixed with PVA/H2O solution (10 wt %). The final mixture had a Ni:Co mole ratio of 1:1 and polymer content of 7.5 wt %. The mixture was vigorously stirred at 50 °C for 5 h. The electrospinning process was carried out according to the following procedure. The obtained solution was placed in a plastic capillary. A copper pin connected to a highvoltage generator was inserted in the solution; the solution was kept in the capillary by adjusting the inclination angle. A ground iron drum covered by a polyethylene sheet served as a counterelectrode. A voltage of 20 kV was applied to this solution. The formed nanofiber mats were initially dried for 24 h at 80 °C under vacuum and then calcined at 800 °C for 5 h in argon atmosphere with a heating rate of 2.3 °C/min. Actually, to keep the evolved gas in the reaction media, the argon gas flow rate was very small. 2.3. Characterization. Surface morphology was studied with a scanning electron microscope (SEM, JEOL JSM-5900, Japan) and field-emission scanning electron microscope (FESEM, Hitachi S-7400, Japan). Information about the phase and crystallinity was obtained by using a Rigaku X-ray diffractometer (XRD, Rigaku, Japan) with Cu KR (λ ) 1.540 Å) radiation over a Bragg angle ranging from 10° to 100°. Thermal properties were studied with a thermal gravimetric analyzer (TGA, Pyris1, PerkinElmer Inc.). High resolution images were obtained with a transmission electron microscope (TEM, JEOL JEM-2010, Japan) operated at 200 kV equipped with EDX analysis. Magnetic properties of the nanofibers were evaluated using a superconducting quantum interference device (SQUID) magnetometer. The nanofibers were weighed (6.29 mg) and then transferred into capsules in an inert gas environment. After this, the capsules were sealed with paraffin wax to protect the nanofibers from air oxidation. 3. Results and Discussion According to our pervious studies of NiAc and CoAc, these metallic salts individually revealed good electrospun nanofiber morphology when a PVA polymer is utilized to prepare the electrospun solution.33,34 Figure 1 shows SEM and FE SEM images of an electrospun nanofiber mat resulting from the electrospinning of the prepared CoAc/NiAc/PVA sol-gel. As concluded from the figure, mixing of the metal acetates did not affect the quality of the electrospun nanofibers since nanofibers with smooth and good morphology were obtained without any observed beads. The average size of the electrospun nanofibers is ∼450 nm. Figure 2 demonstrates the morphology of the obtained powder after calcination of the electrospun nanofiber mats in argon
Figure 1. Scanning electron microscope (SEM) and field emission scanning electron microscope (FE SEM) images for the electrospun NiAc/CoAc/PVA nanofiber mats.
Figure 2. Field emission scanning electron microscope (FE SEM) images at different magnifications for the obtained powder after calcination of the electrospun NiAc/CoAc/PVA nanofiber mats in argon atmosphere at 800 °C for 5 h.
atmosphere. As shown in this figure, nanofibers with good morphology were acquired after the calcination process. However, the average diameter dramatically decreased to ∼140 nm. Decrease in the diameters can be explained as decomposition of the utilized acetate anions and elimination of the used polymer. Often cobalt and nickel have the same crystal structure. They have an FCC crystal lattice with space group class (S.G.) of Fm3m(225). Moreover, the cell parameters are very close, 3.544 and 3.523 Å for the cobalt and nickel, respectively (JCDPS 150806, Co; 04-0850, Ni). Therefore, XRD analysis cannot be utilized to distinguish between these two metals since their peaks appear at almost the same diffraction angles. Figure 3 shows the XRD analysis results for the sintered nanofibers. The strong diffraction peaks at 2θ values of 44.30°, 51.55°, 76.05°, and 92.55° corresponding to (111), (200), (220), and (311) crystal planes, respectively, indicate the formation of pure nickel or pure cobalt or both. However, as the utilized amount of the two metal precursors in the original electrospun solution are considerable and equal, and due to the high melting point of Co and Ni (1495 °C and 1453 °C, respectively), we cannot expect vaporization of any metal during the calcination process. Accordingly, one can confirm that the XRD results affirm formation of both cobalt and nickel metals with FCC crystal structure. Our studies and also those of others about NiAc and CoAc concluded that heating of these salts in an inert atmosphere leads to abnormal decomposition of the acetate anion to form reducing gases (namely, CO and H2) which results in pure metal rather than metal oxides.33,34,36,37 Briefly, formation of pure nickel is explained by the following reactions:
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Ni(CH3COO)2 · 4H2O f 0.86Ni(CH3COO)2 · 0.14Ni(OH)2 + 0.28CH3COOH + 3.72H2O
(1) 0.86Ni(CH3COO)2 · 0.14Ni(OH)2 f NiCO3 + NiO + CH3COCH3 + H2O
(2)
NiCO3 f NiO + CO2
(3)
NiO + CO f Ni + CO2
(4)
By the same fashion, cobalt acetate is decomposed as follows:
Co(CH3COO)2 · 4H2O 98 Co(OH)(CH3COO) + 3H2O + CH3COOH (5) Co(OH)(CH3COO) 98 0.5CoO + 0.5CoCO3 + 0.5H2O + 0.5CH3COCH3
(6)
CoCO3 f CoO + CO2
(7)
CoO + CO f Co + CO2
(8)
Carbon monoxide in eqs 4 and 8 comes from decomposition of the resultant acetic acid.36 The corresponding changes in the phase in both cases were supported by thermogravimetric studies (Figure 3 in the Supporting Information). To investigate the distribution of cobalt and nickel along the produced nanofibers, linear analysis TEM EDX was utilized. As shown in Figure 4, cobalt and nickel are found along the selected line. Interestingly, both metals have the same distribution curve which means that the two metals are mixed at the crystalline level. To affirm this finding, different samples were analyzed; Figure 4 in the Supporting Information shows the obtained results. As shown in the investigated samples, nickel
Figure 3. XRD analysis for the calcined nanofibers. The assigned peaks represent both nickel (FCC, S.G. Fm3m (225), cell parameter 0.3523 nm, PDF # 04-0850) and cobalt (FCC, S.G. Fm3m (225), cell parameter 0.3544 nm, PDF # 15-0806) metals.
Figure 4. TEM image for a single calcined nanofiber along with the line TEM EDX analysis for Co and Ni: blue and red curves, respectively.
and cobalt are distributed in the same fashion which generalizes the finding for all the obtained nanofibers. Consider the following: (i) nickel and cobalt are neighbors in the periodic table, (ii) they have almost the same atomic weight (Ni ) 58.7 and Co ) 58.9) and thus almost the same atomic size, (iii) the XRD results affirm that both metals have FCC crystal structure with almost the same cell parameters (Co ) 3.544 Å and Ni ) 3.523 Å), and (iv) the two metals have the same valence; accordingly, these two metals can form a substitutional alloy. In other words, nickel atoms can replace cobalt atoms in the FCC cobalt crystal and vice versa. Therefore, the obtained nanofibers are composed of CoNi single crystals. PVA almost completely disappears due to calcination in argon atmosphere if there is no material to catalyze the graphitization reaction.31-33 However, in the case of calcination of CoAc/PVA electrospun mats in an argon atmosphere, the obtained free cobalt enhances formation of graphite34 which leads to an additional peak at 2θ ∼ 25° in the XRD spectra. As shown in Figure 3, no peak appears at the aforementioned diffraction angle. Among the most common ferromagnetic metals (Fe, Co, and Ni), nickel has the lowest magnetic properties. However, nickelbased metallic materials are widely used due to their distinct characteristics; they are known as soft magnetic materials. Actually, soft magnetic materials basically consist of nickel and another ferromagnetic metal (Fe or Co). High saturation magnetization soft magnetic materials are used extensively in power electronic circuits, as voltage and current transformers, in saturable reactors, and in magnetic amplifiers, inductors, and chokes. Therefore, considerable effort has been directed toward the production of nickel-based materials with high saturation magnetization. Figure 5 shows the hysteresis loops for pristine nickel, cobalt-doped nickel, and the produced CoNi nanofibers. In the cobalt-doped nickel nanofibers which have a Co content of 50%, cobalt metal has been introduced as nanoparticles in the electrospun solution; accordingly, the final product was Ni nanofibers containing Co nanoparticles. However, in the present study Co and Ni have been utilized as soluble salts (in acetate form); consequently and as aforementioned, these metals have been blended in the crystalline state to form an alloy. As shown
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Barakat et al. usually have saturation magnetization lower than that for bulk materials40-42 because the high surface area enhances oxidation of the surface of magnetic nanofibers, which may create a magnetically dead layer. Consequently, pristine and Co-doped Ni nanofibers have a lower saturation magnetization value compared with that of the bulk nickel as shown in Table 1. However, the difference between the saturation magnetization of the prepared CoNi nanofibers and the bulk is not large as shown in the first row of Table 1. The second row in Table 1 shows the average coercivities of the CoNi, Co-doped Ni and pristine Ni nanofibers, and the bulk nickel material. As shown in this table, the investigated metallic nanofibers exhibit good coercivity compared with that of the bulk material; for instance, the coercivity was duplicated 100 times in the case of pristine Ni nanofibers compared with that of the bulk material. Doping of cobalt nanoparticles decreased the coercivity compared with the pristine Ni nanofibers; however, in the case of CoNi, a considerable decrease was observed as shown in Table 1. The remaining magnetic properties are summarized in the third and fourth rows of Table 1. Cobalt-doped nickel nanofibers exhibit low remanent magnetization and saturation field values compared with both CoNi and pristine nanofibers. Nevertheless, such novel CoNi nanofibers are potentially useful for high-density information storage applications. Additionally, because the magnetic coercivities of the nanofibers are outstanding and the proposed method is safe and highly efficient, bulk amounts of the synthesized CoNi nanofibers could be used for manufacturing flexible magnets after blending different loads with polymers. 4. Conclusion
Figure 5. Magnetic properties for the obtained CoNi nanofibers compared with those for Co-doped Ni and pristine Ni nanofibers.
TABLE 1: Magnetic Parameters of the Synthesized CoNi Nanofibers Compared with Those of the Cobalt-Doped and Pristine Nickel Nanofibers and Bulk Nickel Materials at Room Temperature nanofibers parameter
Co/Ni
Co-doped Ni
Ni
bulk
saturation magnetization, Ms (emu/g) coercivity, Hc (Oe) remanent magnetization, Mr (emu/g) saturation field, Hs (Oe)
47.45
32.17
26.8
58.638
65.6 5.94
41.57 2.34
70 3.6
0.739 -
4000
900
6000
-
in Figure 5 all formulations reveal typical ferromagnetic behavior. The ferromagnetism of the utilized nanofibers is clearly shown by coercivity (Hc), saturation magnetization (Ms), remanent magnetization (Mr), and saturation field (Hs) as listed in Table 1. As shown in Figure 5 and Table 1, incorporation of cobalt nanoparticles in the nickel nanofibers relatively increased the saturation magnetization of the nickel nanofibers (20% increase). However, combination of cobalt and nickel in the crystalline state had more effect on the saturation magnetization, as the increase has reached to 77%. This increase in the saturation magnetization might be explained as the dual effect of magnetocrystalline and shape anisotropies. Nanostructures
Electrospinning of a sol-gel consisting of cobalt acetate/ nickel acetate/poly(vinyl alcohol) leads to smooth and beadfree electrospun nanofiber mats. Calcination of the obtained mats in argon atmosphere caused elimination of the utilized polymer and abnormal decomposition of the acetate anions which resulted in formation of nanofibers composed of pure nickel and cobalt metals. Due to the close atomic and crystalline properties of these two metals, they alloy and form CoNi single crystal nanofibers. The obtained CoNi nanofibers possess high saturation magnetization compared with that of Co-doped and pristine nickel nanofibers. Acknowledgment. This work was supported by a grant of the Korean Ministry of Education, Science and Technology (The Regional Core Research Program/Center for Healthcare Technology & Development, Chonbuk National University, Jeonju 561-756, Republic of Korea). We thank Mr. T. S. Bae, Mr. J. C. Lim, Dr. Lee Young-Boo, KBSI, Jeonju Branch, and Mr. JongGyun Kang, Centre for University Research Facility, for taking high-quality FESEM, TEM-EDX, and TEM images, respectively. Supporting Information Available: Additional information as noted in the text. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Singh, N. H., Ed. Handbook of Nanostructured Materials and Nanotechnology; Academic Press: San Diego, 2000. (2) Himpsel, F. J.; Ortega, J. E.; Mankey, G. J.; Willis, R. F. AdV. Phys. 1998, 47, 511. (3) Chien, C. L. Science and Technology of Nanostructures Magnetic Materials; Hadjipanayis, G., Prinz, G. A., Eds.; Plenum Press: New York, 1991; Vol. 259, pp 477-495. (4) Bean, C. P.; DeBlois, R. W.; Nesbitt, L. B. J. Appl. Phys. 1959, 30, 1976.
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