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Influence of Cobalt Nanoparticles’ Incorporation on the Magnetic Properties of the Nickel Nanofibers: Cobalt-Doped Nickel Nanofibers Prepared by Electrospinning Nasser A. M. Barakat,*,†,‡ Bongsoo Kim,§ Chuan Yi,‡ Younghun Jo,| Myung-Hwa Jung,⊥ Kong Hee Chu,# and Hak Yong Kim*,∇ Chemical Engineering Department, Faculty of Engineering, El-Minia UniVersity, El-Minia, Egypt, Center for Healthcare Technology DeVelopment, Chonbuk National UniVersity, Jeonju 561-756, Republic of Korea, Department of Chemistry, KAIST, Daejeon 305-701, Republic of Korea, Quantum Material Research Team, KBSI, Daejeon 305-333, Republic of Korea, Department of Physics, Sogang UniVersity, Seoul, Republic of Korea, Clean & science Co. Ltd., Samsung Domg, Kangnam Ku, Seoul, Republic of Korea, and Department of Textile Engineering, Chonbuk National UniVersity, Jeonju 561-756, Republic of Korea ReceiVed: June 16, 2009; ReVised Manuscript ReceiVed: September 14, 2009
Among the common ferromagnetic metals, cobalt has distinct magnetic properties, so incorporation of cobalt nanoparticles in the nickel nanofibers, which reveals better magnetic properties compared with the bulk, might have considerable impact. In this study, we are introducing cobalt-doped nickel nanofibers prepared by electrospinning. Electrospinning of a colloidal solution rather than a sol-gel (which is widely utilized in the conventional electrospinning technique) has been invoked as a novel strategy to prepare cobalt nanoparticles/ nickel acetate/poly(vinyl alcohol) nanofiber mats. Physiochemical characterizations indicated that calcination of the dried electrospun mats in argon atmosphere at 700 °C for 5 h leads to produce cobalt-doped nickel nanofibers. Overall, magnetic properties studied pointed to improvement of the magnetic parameters of the synthesized cobalt-doped nickel nanofibers compared with the pristine ones. Introduction Materials science has created magnetic materials far more powerful than those available only a few decades ago, resulting in a tremendous impact on modern technology. For instance, ferromagnetic metal nanostructures reveal physical and chemical properties that are characteristic of neither the atom nor the bulk counterparts.1 Quantum size effects and the large surface area of magnetic nanoparticles dramatically change some of the magnetic properties and exhibit superparamagnetic phenomena and quantum tunnelling of magnetization because each particle might be considered as a single magnetic domain.2 Consequently, some metal nanoparticles such as Fe, Co, and Ni have been given much attention to be utilized in various applications such as electronic, optical, and mechanic devices, magnetic recoding media, catalysis, superconductors, ferrofluids, magnetic refrigeration systems, and contrast enhancement in magnetic resonance imaging carriers for drugs and targeting.3-9 Many literatures have been reported, confirming that the magnetic properties of those materials are highly dependent on the particle shape.10-14 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.15,16 Practically, attempts have been done in this regard; for instance nanoscale * To whom correspondence should be addressed. Tel: +82 63 270 2351. Fax: +82 63 270 2348. E-mail:
[email protected] (H.Y.K.), nasbarakat@ yahoo.com (N.A.M.B). † Faculty of Engineering, El-Minia University. ‡ Center for Healthcare Technology Development, Chonbuk National University. § Department of Chemistry, KAIST. | Quantum Material Research Team, KBSI. ⊥ Department of Physics, Sogang University. # Clean & science Co. Ltd., Samsung Domg, Kangnam Ku. ∇ Department of Textile Engineering, Chonbuk National University.
magnetic logic junctions have recently been fabricated with ferromagnetic nanowires as building blocks;17 magneto-optical switches have been prepared using suspensions of ferromagnetic nanowires.18 Among the 1D nano shapes, nanofibers have considerable importance because of the longest axial ratio characteristic. Therefore, nanofibers are the best candidate for nanodevices and nanomembranes. Electrospinning is the most popular technique utilized in production of functional nanofibers because of its simplicity, low cost, and high yield.19 Metal base nanofibers are produced by electrospinning of a sol-gel composed of a metal precursor and an accordant polymer. In the field of pure metal nanofibers, the electrospinning process has been exploited to synthesize Co, Cu, Fe, and Ni in a nanofibrous shape by calcination of the electrospun metal precursor/polymer nanofiber mats in a hydrogen atmosphere.20-22 However, we have recently introduced calcination in an argon atmosphere as a safe, economically preferable, and more effective alternative strategy to produce silver, nickel, and cobalt nanofibers.23-25 It is noteworthy mentioning that, in the case of nickel metal, the nanofibrous shape strongly affects the magnetic properties. For instance, the coercivity at room temperature for the nickel nanofibers was about 100 times the magnitude of the bulk material.21,24 Soft magnetic materials basically consist of nickel and another ferromagnetic metal (Fe or Co). High magnetic properties soft magnetic materials are strongly required, as these materials are extensively used in power electronic circuits, as voltage and current transformers, saturable reactors, magnetic amplifiers, inductors, and chokes. Therefore, producing Ni/Co nanofibers was the main goal of this study to exploit the distinct advantage of this marvelous nanoshape for the improvement of the magnetic properties. Generally, the electrospun solution is either polymer(s) dissolved in a proper solvent or metallic precursor/polymer
10.1021/jp905667s CCC: $40.75 2009 American Chemical Society Published on Web 10/19/2009
Cobalt-Doped Nickel Nanofibers
J. Phys. Chem. C, Vol. 113, No. 45, 2009 19453
Figure 1. SEM images for the Co/Ni(AOc)/PVA electrospun nanofiber mats in low (A) and high (B) magnifications.
solution. The distinct feature of these solutions is that they have to be completely miscible. In other words, in a case of exploiting a metallic precursor, it should be soluble in a suitable solvent because it has to hydrolyze and polycondensate in the final precursor/polymer mixture to form the gel network. In this study, we have utilized electrospinning of a colloidal solution as a novel strategy to produce cobalt-doped nickel nanofibers as a proposal to further improve the magnetic properties of the nickel metal. Briefly, cobalt nanopowder/nickel acetate/poly(vinyl alcohol) colloid has been electrospun, and calcination of the dried nanofiber mats in an argon atmosphere resulted in the production of nickel nanofibers embedding cobalt nanoparticles. The synthesized cobalt-doped nickel nanofibers revealed better magnetic properties compared with the pristine ones. 2. Experimental Details 2.1. Materials. Nickel(II) acetate tetrahydrate (Ni(AOc), 98%) and poly(vinyl alcohol) (PVA, molecular weight (MW) ) 65 000 g/mol) were obtained from Showa Co., Japan and Aldrich Co., USA, respectively. Cobalt nanoparticles (99.9% purity, average particle size ∼28 nm) have been obtained from NaBond Tech. Co. Ltd., Shenzhen P.R. China. These materials were utilized without any further treatments. Distilled water was used as solvent. 2.2. Experimental Work. In this study, three mixtures have been individually electrospun; Ni(AOc)/PVA, Co nanoparticles/ PVA, and Co nanoparticles/Ni(AOc)/PVA. A PVA aqueous solution (10 wt %) was utilized in all mixtures. Briefly, Ni(AOc)/ PVA sol-gel was prepared by mixing of aqueous Ni(AOc) solution (20 wt %) and the prepared PVA solution in a weight ratio of 1:3. Cobalt nanoparticles have been added to the prepared Ni(AOc)/PVA solution to produce Co/Ni(AOc)/PVA colloid containing 0.5 wt % cobalt. For the Co/PVA, the nanoparticles have been added to the polymer solution to get a final colloid containing 0.5 wt % solid material. These mixtures were vigorously stirred at 50 °C for 5 h. Later on, every mixture was placed in a plastic capillary. A copper pin connected to a high-voltage generator was inserted in the solution, and the solution was kept in the capillary by adjusting the inclination angle. A ground iron drum covered by a polyethylene sheet was serving as a counter-electrode. 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. Ni(AOc)/PVA and Co/ Ni(AOc)/PVA dried electrospun nanofiber mats were sintered at 700 °C for 5 h in an argon atmosphere with a heating rate of 2.3 °C/min. 2.3. Characterization. Surface morphology was studied by scanning electron microscope (SEM, JEOL JSM-5900, Japan) and field-emission scanning electron microscope (FESEM, Hitachi S-7400, Japan) equipped with energy dispersive X-ray (EDX). Thermal properties have been studied by thermal
gravimetric analyzer (TGA, Pyris1, PerkinElmer Inc., USA). Information about the phase and crystallinity was obtained by using Rigaku X-ray diffractometer (XRD, Rigaku, Japan) with Cu KR (λ ) 1.540 Å) radiation over the Bragg angle ranging from 30 to 100°. High-resolution images and selected area electron diffraction patterns were obtained with transmission electron microscope (TEM, JEOL JEM-2010, Japan) operated at 200 kV. Magnetic properties of the nanofibers were evaluated using commercial superconducting quantum interface device (SQUID) magnetometery. The nanofibers were weighed and then filled into capsules in an inert gas environment. After this, the capsules were sealed with paraffin wax to prevent the nanofibers from air oxidation. The weight of the pristine and cobalt-doped nickel nanofibers were 11.48 and 5.26 mg, respectively. 3. Results and Discussion Incorporation of metal nanoparticles in either polymeric or metallic nanofibers is a desirable demand because the metal nanoparticles enhance the physical and chemical properties and/ or provide the nanofibers with new characteristics. As the metals powders are physically insoluble in any solvent, the reported metallically doped electrospun nanofiber mats contain metal compounds rather than zero-oxidation state metal nanoparticles because soluble metallic compounds’ precursors have to be utilized in the conventional electrospinning technique. Obtaining metal nanoparticles from the utilized meal precursors needs strong reducing agents, which affect the final product morphology. According to our best knowledge, only some noble metals’ (e.g., Ag and Pt) nanoparticles could be successfully incorporated in polymeric or metallic nanofibers.26-29 Electrospinning of colloids might be an interesting solution for this dilemma as the metal nanopowder can be used. In this regard, Figure 1 represents the SEM for the electrospun nanofiber mats obtained from the Co nanoparticles/Ni(AOc)/PVA colloid. As shown in this figure, the obtained nanofibers are quite smooth and beadsfree. As the main peaks corresponding to nickel and cobalt in the energy dispersive X-ray (EDX) spectra are obtained at relatively close binding energy values, we have electrospun Co nanoparticles/PVA colloid to ensure that the cobalt nanoparticles were incorporated in the polymeric nanofibers. Figure 2 demonstrates the SEM images and EDX results for the obtained Co nanoparticles/PVA nanofibers. As shown in this figure, smooth nanofibers are obtained. Moreover, the EDX results affirmed incorporation of the cobalt nanoparticles in the polymeric nanofibers, as the peaks indicating the presence of cobalt are clearly apparent in the obtained spectra. Considering the small particle size of the utilized metal nanoparticles (∼28 nm) compared with the electrospun nanofibers (∼450 nm) of both of PVA and Ni(AOc)/PVA, we can say that the cobalt nanoparticles are imprisoned inside the polymeric nanofibers
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Figure 2. SEM images (up) and EDX results (down) for Co nanopowder/PVA nanofiber mats.
Figure 3. SEM images for the Ni(AOc)-PVA-Co electrospun nanofiber mats after calcination in Ar atmosphere at 700 °C, (A) and (B); FE SEM image for a single calcined nanofibers, (C); and (D) represents the FE SEM image of pure nickel nanofibers.
in both formulations. Therefore, the metallic nanoparticles could not be observed in either of Figure 1 or Figure 2. Calcination of the Co/Ni(AOc)/PVA electrospun nanofiber mats in an argon atmosphere resulted in keeping the general nanofibrous morphology with an observable decrease in the average diameter compared with the original ones. As shown in parts A and B of Figure 3, the sintered powder consists of nanofibers with an average diameter of ∼230 nm, which is a half of the average diameter of the electrospun nanofibers (∼450 nm). Part C of Figure 3 demonstrates the FESEM image of a single nanofiber, as shown in this figure, the surface is rough. However, the surface of the cobalt-free nanofibers that were obtained form calcination of Ni(AOc)/PVA is comparatively smooth, as shown in part D of Figure 3. According to our previous study,24 the fibers in part D of Figure 3 are composed of pure nickel. Elimination of the utilized polymer is the main reason for the decrease of the average diameter.24,25 Actually, the weight of the electrospun mat decreases greatly during the calcination process due to decomposition of the utilized Ni(AOc)
Figure 4. TGA in argon atmosphere for Co nanoparticles/Ni(AOc)/ PVA nanofiber mats.
and elimination of the polymer, which is the main content of the electrospun fibers. Figure 4 shows the thermal gravimetric analysis (TGA) results in an argon atmosphere. As shown in this figure, calcination of the electrospun nanofibers leads to the loss of more than 80% of the original weight. According to the JCPDS XRD database, cobalt and nickel do have the same crystal structure. They have 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 15-0806, Co; and 04-0850, Ni). Therefore, XRD analysis cannot be utilized to distinguish between these two metals because their peaks are obtained at nearly identical diffraction angles. Figure 5 shows the XRD analysis results for the sintered nanofibers. The strong diffraction peaks at 2θ values of 44.30, 51.55, 76.05, 92.55, and 98.15° corresponding to (111), (200), (220), (311), and (222) crystal planes respectively indicate the formation of pure nickel or pure cobalt or both. As we have explained in details in our previous study24 and also by other authors,30 heating of Ni(AOc) in an inert atmosphere results in the formation of pure nickel. However, the original electrospun nanofiber mats contain cobalt
Cobalt-Doped Nickel Nanofibers
J. Phys. Chem. C, Vol. 113, No. 45, 2009 19455 SCHEME 1: Conceptual Illustration for the Utilized Procedure to Produce Co-Doped Nickel Nanofibers
Figure 5. XRD results for the obtained calcined nanofibers.
nanoparticles, which have a high melting point (1495 °C) and cannot be vaporized at the utilized calcination temperature (700 °C), so one can say that the XRD results explain that the obtained nanofibers consist of a Co and Ni mixture. The most important finding is that no oxide formulation for both metals was detected. Also, obtaining sharp and high peaks reveals good crystallinity for the produced nanofibers. EDX analysis can be relatively utilized to distinguish between the two metals, as is shown in figure 6, which represents the EDX results for the calcined nanofibers, and both metals could be detected. Figure 7 shows the transmission electron microscope (TEM) image for the obtained nanofibers. As shown in the upper insert, which represents the HRTEM image of the marked area, a nanoparticle can be observed incorporated inside the nanofiber. Because nickel acetate was used as solution and was well mixed with the PVA polymer, we can say that the crystalline matrix shown in the HRTEM represents nickel; however, the nanoparticle can be assigned as cobalt because the utilized cobalt nanoparticles have not melted or reacted during the treatment process. Therefore, one can say that the final obtained product is cobalt-doped nickel nanofibers. A ring pattern SAED image has been investigated and is presented in the lower insert in Figure 7; the ring pattern indicates the marked area is relatively thick because it contains cobalt nanoparticle. The rings are clear because the two metals have the same crystal planes, and they have no dislocations or imperfections observed in the lattice planes, which indicate good crystallinity of the synthesized nanofibers. To make the proposed strategy more understandable, we have built Scheme 1 as a conceptual illustration to show the utilized procedure for the production of Co-doped Ni nanofibers. Figure 8 shows the hysteresis loops for both the cobalt-doped and the cobalt-free nickel nanofibers at 5 and 300 K. It can be
Figure 6. EDX results for the produced cobalt-doped nickel nanofibers.
observed that both formulations reveal typical ferromagnetic behavior. The ferromagnetism of the prepared nanofibers is clearly shown by coercivity (Hc), saturation magnetizations (Ms), remanent magnetization (Mr), and saturation field (Hs) listed in Table 1. The saturation magnetization is the maximum induced magnetic moment that can be obtained in a magnetic field; beyond this field no further increase in magnetization occurs. High saturation magnetization magnetic materials are required for future high-density recording heads as well as high-frequency inductors. As shown in Figure 8 and Table 1, incorporation of cobalt resulted in an increase of the saturation magnetization into almost 40%. Theoretically, cobalt has much more saturation magnetization compared with nickel; 162.55 and 58.57 emu/g for cobalt and nickel, respectively.31 Nanostructures usually have saturation magnetization lower than that for bulk materials.32-34 A logic explanation of that can be drawn as: the high surface area enhances oxidation of the surface of magnetic nanofibers, which may create a magnetically dead layer. Moreover, the large specific area and the imperfection of the crystalline structure at the surface may also lead to a significant decrease in the nanofiber saturation magnetization.21 For instance, the saturation magnetization for cobalt and nickel nanofibers have been reported as 81.97 and 27.23 emu/g, respectively.21 Therefore, it was expected that incorporation of cobalt nanoparticles inside nickel nanofibers will lead to an increase in the magnetic properties. Coercivity is the reverse magnetic field required to reduce the net magnetization to zero. For magnetic materials, it is necessary to reduce coercivity as a way to control the energy losses. As shown in the second row in Table 1, the cobalt-doped nickel nanofibers have relatively lower coercivity compared that of the pristine at low and high temperatures, which can be considered as a further improvement of the magnetic properties. The decrease of the coercivity can probably be explained in terms of the effect of CoNi bilayers. Simply, remanent magnetization (Mr) can be defined as the remaining magnetic momentum after realizing the magnetic
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Figure 7. TEM image for the obtained cobalt-doped nickel nanofibers. The inserts represent the HR TEM (up) and selected area electron diffraction pattern (SAED) (down). Figure 9. FC and ZFC of pristine and cobalt-doped nickel nanofibers at 100 Oe. The insert represents high-magnification plot at the blocking temperature range for the ZFC curve for both formulations.
Figure 8. Magnetic properties of pristine and cobalt-doped nickel nanofibers at 5 and 300 K.
TABLE 1: Magnetic Parameters of the Synthesized Cobalt-Doped Nickel Nanofibers Compared with Pristine Nickel Nanofibers at 5 K and Room Temperature At 5 K Parameter saturation magnetization, Ms (emu/g) coercivity, Hc (Oe) remanent magnetization, Mr (emu/g) saturation field, Hs (emu/g)
Co/Ni
Room temperature Ni
Co/Ni
Ni
32.7
25.3
32.17
23.12
323.5 10.4
382.52 10.4
41.57 2.34
67.66 3.06
3400
3700
900
800
field. Low remanent magnetization materials are classified as magnetically clean materials. In some distinct fields, low remanent magnetization is highly desirable for instance in data
storage applications. As shown in the third row in Table 1, both cobalt-doped and pristine nickel nanofibers have acceptable remanent magnetization. At low temperature (5 K), incorporation of cobalt nanoparticles has no effect; however, an observable decrease in the remanent magnetization was detected at room temperature due to cobalt incorporation. In general, the magnetization moment depends on the magnetic field direction until a certain threshold called the saturation field; so the popular hysteresis loop is obtained. In other words, beyond the saturation field, the magnetization moment is independent of the field sign. Just beyond this threshold, small increase in magnetization can be achieved; later on also the field magnitude would have no impact on the magnetization (i.e., reachcing to the saturation magnetization status). Therefore, some researchers give a rough definition of the saturation field as the value at which all of the atomic magnetic dipole moments are aligned with the field, and the magnetic material in such a case is said to be saturated. As shown in Figure 8 and Table 1, doped and pristine nickel nanofibers have very small saturation fields at room temperature compared with the values obtained at low temperature (i.e., 5 K). Moreover, incorporation of cobalt in the nickel nanofibers has almost no influence on the saturation field at low and high temperatures. The zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves of the doped and primeval nickel nanofibers recorded under an applied magnetic field of 100 Oe (Figure 9) show irreversibility, which is typical of the blocking process for an assembly of superparamagnetic nanostructures.35 The temperature below which irreversibility in ZFC and FC magnetization occurs (Tirr) is found to be ∼365 K for both formulations. Moreover, the blocking temperature (TB) at which the ZFC magnetization peaks decreases as H is increased, as expected for a superparamagnet,36,37 was detected at almost the same temperature value (∼252 K) as shown in the lower insert in Figure 9. However, cobalt incorporation enhances the magnetization, as shown in Figure 9. In summary, considering the advantage of nickel metal, it is mainly utilized as a basic metal in the soft magnetic materials that do have wide applications, and the synthesized cobalt-doped nickel nanofibers might be of special interest. Although cobalt nanofibers have better magnetic properties when compared with the cobalt-doped and pristine nickel nanofibers, we believe that the cobalt-doped nickel might be more beneficial than these metal nanofibers because of high oxidation resistance and good
Cobalt-Doped Nickel Nanofibers magnetic properties when compared with cobalt and nickel nanofibers, respectively. Conclusions Electrospinning of a colloidal solution can be utilized to produce metal-doped electrospun nanofibers as a novel strategy to produce a new class of nanofibers that could not be obtained by the conventional electrospinning technique. The proposed strategy was successfully utilized to produce cobalt-doped nickel nanofibers. Incorporation of cobalt nanoparticles distinctly enhanced the magnetic properties of the nickel nanofibers. For instance, the saturation magnetization has been improved to be 40% greater than the pristine nickel nanofibers. The other magnetic parameters have been also relatively modified. Overall, as many metals nanofibers have been reported in the literature, this study can be considered as a new avenue for the researchers to produce metal-doped metallic nanofibers that will have wide applications according to the chosen metals. 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 and Mr. J. C. Lim, KBSI, Jeonju branch, and Mr. Jong-Gyun Kang, Centre for University Research Facility, for taking high-quality FESEM and TEM images, respectively. Supporting Information Available: Standard experimental setup and a photograph for the utilized electrospinning experiment. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Babes, L.; Denizot, B.; Tanguy, G.; Le Jeune, J. J.; Jallet, P. J. Colloid Interface Sci. 1999, 212, 474. (2) Goya, G. F.; Berquo, T. S.; Fonseca, F. C. J. Appl. Phys. 2003, 94, 3520. (3) Gangopadhyay, S.; Hadjipanayis, G. C.; Dale, B.; Sorensen, C. M.; Klabunde, K. J.; Papaefthymiou, V.; Kostikas, A. Phys. ReV. B 1992, 45, 9778. (4) Schmid, G.; Chi, L; F. AdV. Mater. 1998, 10, 515. (5) Zhang, D. E.; Ni, X. M.; Zheng, H. G.; Li, Y.; Zhang, X. J.; Yang, Z. P. Mater. Lett. 2005, 59, 2011. (6) Hyeon, T. Chem. Commun. 2003, 8, 927. (7) Shouheng, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989. (8) Mikulec, F. V.; Kuno, M.; Bennati, M.; Hall, D. A.; Griffin, R. G.; Bawendi, M. G. J. Am. Chem. Soc. 2000, 122, 2532.
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JP905667S
