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Magnetic chains of Co spheres synthesized by hydrothermal process under magnetic field
Materials Letters 62 (2008) 3431–3433
Contents lists available at ScienceDirect
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
Magnetic chains of Co spheres synthesized by hydrothermal process under magnetic field Jun Wang ⁎, Min Yao, Chuanhui Xu, Yuejin Zhu, Gaojie Xu, Ping Cui Faculty of Science, Ningbo University, Ningbo, 315211, China Ningbo Institute of Materials Technology and Engineering, Ningbo 315201, China
a r t i c l e
i n f o
Article history: Received 25 November 2007 Accepted 26 February 2008 Available online 8 March 2008 Keywords: Magnetic material Hydrothermal synthesis Magnetic field
a b s t r a c t Hydrothermal process under magnetic fields is successfully used to synthesize Co chains using reduction approach by carefully controlling the reaction conditions. The formation of the chain structure might be that magnetic fields drive the nanoscale crystals of Co to form chains. The Co sphere size and chain length are analyzed by X-ray diffraction (XRD) and scan electron microscopy (SEM). The result of the vibrating sample magnetometer (VSM) shows that the magnetic chains possess the saturation magnetization of 102 emu/g. The factors on the magnetic properties of the magnetic nanochains are discussed. © 2008 Elsevier B.V. All rights reserved.
1. Introduction During the past two decades, colloidal assemblies have been applied in various areas such as chemical and biological sensors [1], photonics [2], coating materials [3], and catalytic supports [4]. Considerable efforts have been directed toward controlling the selforganization of colloidal particles. For example, some progresses have been reported on colloidal self-assemblies in various physically or chemically confining geometries such as microchannels [5], porous membranes [6], capillaries [7], emulsion droplets [8], and patterned self-assembled monolayers [9]. However, these template-directed assemblies are not suitable for mass production because the preparation and subsequent removal of the sacrificial templates involve too much time-consuming processes. Recently, much attention has been paid to the effect of magnetic fields on the movement and self-assembly behavior of magnetic nanocrystallites. Indeed, it has been found that magnetic fields can significantly influence the movement of magnetic particles. For example, it has been demonstrated that the magnetic interactions can be used to generate ordered 3D self-assembled structures [10]. Magnetic fields have also been used to create 2D assemblies of magnetic nanoparticles [11], and 1D assembled chains or 2D rings on solid substrates [12]. In our previous experiment, we successfully utilized magnetic fields to synthesize single crystalline Fe3O4 nanowires [13]. Here we further report that ferromagnetic cobalt nanospheres can self-assemble into chains under magnetic fields. Furthermore, in contrast to the conventional
⁎ Corresponding author. Faculty of Science, Ningbo University, Ningbo, 315211, China. Tel.: +86 574 87600952; fax: +86 574 87600744. E-mail address: [email protected] (J. Wang). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.02.082
drying-mediated assemblies of magnetic nanoparticles on solid substrates under magnetic fields, these chains of nanoparticle can be directly formed by a hydrothermal process under magnetic fields. A magnetic field (0.3 T) is applied to the solution to induce the Co nanoparticles to form centimeter-long and nano-wide chains constrained to linearity along the direction of the applied magnetic field. The chains can keep after removal of the magnetic field. The long chain structures can be fragmented into short chain structures after more vigorous ultrasonic process and agitation. The factors on the magnetic properties of the magnetic chains are discussed. 2. Experiment The chains of Co spheres were synthesized by a hydrothermal redox process under magnetic fields. All chemicals were of analytical grade and were purchased from Shanghai Chemical Reagents Company. Assembly and synthesis of Co chains were performed by a one-step approach. In a typical procedure, an aqueous solution of 50 mL was first prepared by dissolving CoCl2 · 6H2O (50 mM), sodium tartrate (Na2C4H4O6, 0.75 M), NaOH (5 M), and sodium dodecyl benzenesulfonate (SDBS) (15 mM) in distilled water. After adding NaH2PO2 · H2O (0.4 M), the solution was stirred and then transferred into a stainless poly(tetrafluoroethylene) (Teflon)-lined autoclave with an induced magnetic field. The autoclave was maintained at 120 °C for 20 h and then was allowed to cool to room temperature. After the reaction was completed, a black fluffy solid product was deposited on the bottom of the Teflon cup. The final product was centrifuged, washed with distilled water and ethanol several times to remove any alkaline salt and surfactants that remained in the final products, and then dried in a vacuum oven at 50 °C for 4 h. The samples were characterized by XRD using an X-ray diffractometer with high-intensity Cu Kα radiation (λ = 1.5418 Å), recorded at
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J. Wang et al. / Materials Letters 62 (2008) 3431–3433
Fig. 1. XRD patterns for samples derived by a hydrothermal process under magnetic fields.
a scanning rate of 0.05° per second with the 2θ range from 20° to 80°. TEM images were taken with a Hitachi model H-800 transmission electron microscope, using an accelerating voltage of 200 kV. Their magnetic properties were measured on a BHV-55 vibrating sample magnetometer at room temperature. 3. Results and discussion The redox reaction of hydrothermal synthesis of Co during the heating process can be expressed as follows: 2− CoðC4 H2 O6 Þ2− þ H2 PO−2 þ OH− ⇔Co þ C4 H4 O2− 6 þ HPO3
The XRD pattern (Fig. 1) shows the crystal structure of the product. As shown in Fig. 1, the recorded diffraction peaks are well assigned to the structure of Co, indicating the formation of Co metals according to the standard JCPDS (Card No. 5-727). Broadening of the peaks exhibited the nanocrystalline nature of the sample. No CoO peak was observed, showing the high stability of the Co nanospheres. The sharp and narrow reflection peaks indicate the well-crystallinity of the magnetic nanoparticles. The morphology of the sample was observed by SEM, revealing that they have chain-like structure composed by spherical nanoparticles. From Fig. 2A, one can see that the averaged diameter of spheres is about 1.5 µm, showing a relatively narrow size distribution. The centimeter-long chains formed in the process are parallel to each other. Note that the yield of the assembled spheres is as high as 100%. The enlarged image of some typical chains is presented in Fig. 2B, which further indicates that the chains are assembled by spheres. The chains persisted very stably after removal of the applied magnetic field. After the produce resuspended by strong ultrasonication, the SEM image of Fig. 2C shows that the long and array direction of the chains have been destroyed. The lengths of the chains vary from centimeter-long to micro-long and the directions of the chains vary from parallel to each other to arbitrary directions. The mechanism involved for the formation of the chain structures is still unknown. It seems that applied magnetic fields play an important role in driving the nanoscale crystals of Co to form chains. We propose that one possible function of SDBS is to kinetically control the growth rates through the interaction in the adsorption and desorption processes. Subsequently, the Co nanocrystals assemble into chain
Fig. 3. Magnetization hysteresis curves of applied field parallel and perpendicular to the lengthwise direction of the chains.
architectures in the presence of magnetic force. As is well-known, ferromagnetic spherical particles magnetizes one another by dipolar interaction in arbitrary directions. When an external magnetic field is applied, the spherical particles tend to align along the magnetic line of force and favor the formation of linear chains. Magnetization makes all magnetic particles orientate along the magnetic line of force, as a result, dipole-directed self-assembly through dipolar interaction along the magnetic line of force could occur, leading to the formation of linear chains. The chains were nearly parallel, which could be the result of magnetic attraction, resulting in the wires aligning parallel to the magnetic line of force. The magnetization hysteresis curves of applied field parallel and perpendicular to the lengthwise direction of the chain are shown in Fig. 3. From the Fig. 3, we see an obvious uniaxial anisotropy originated from shape in our aligned chains. The easy axis is along the length of chain and the hard axis was perpendicular to it. When the direction of the magnetic field was along easy axis, the coercive force (Hc) was lowest. It increased when the magnetic field was rotated away from the easy axis and reached a maximum value in perpendicular. Moreover, The sample exhibits a saturation magnetization (Ms) of ~ 102 emu/g at room temperature, lower than the corresponding values of bulk Co (168 emu/g) [14,15]. As well-known, the saturation magnetization of magnetic nanoparticles was found to be much smaller than the bulk value and decreased with decreasing particle size. The phenomena have been observed in several magnetic material systems [16] and were explained by a magnetically dead layer on the surface of the particles [17]. The existence of the nonmagnetic layer might be caused by the canting of the surface spins [18], a high anisotropy layer, or the loss of the long range order in the surface layer [19] or some other reasons. However, the hysteresis loop at room temperature shows that the sample has a ferromagnetic character.
4. Conclusions This paper demonstrates that Co chains composed of Co spheres can be synthesized via reduction approach by carefully controlling the reaction conditions under magnetic fields. The formation of the chain structures might be that magnetic fields drive the nanoscale crystals of Co to form chains. The strategy of assembly is to use magnetic forces between ferromagnetic spheres to organize and stabilize the chains.
Fig. 2. SEM micrograph of a sample derived from a hydrothermal process under magnetic fields (A), (B) the enlarged image of some typical chains and (C) the SEM image of the produce resuspended by ultrasonication.
J. Wang et al. / Materials Letters 62 (2008) 3431–3433
The materials possessed a saturation magnetization of 102 emu/g at room temperature. The results of SEM discover that the centimeterlong and nano-wide chains are parallel to each other and the yield is as high as 100%. The material is expected to have promising applications for catalysts and other related nanodevices. The synthetic route can be extended to various types of magnetic nanoparticles. Acknowledgments This work was supported by the Natural Science Foundation of Ningbo (2007A610023), the Natural Science Foundation of China, Zhe Jiang (Y407267), China Postdoctoral Science Foundation (20070420675), the National Science Foundation of China (10774079) and K. C. Wong Magna Foundation in Ningbo University. References [1] J.H. Holtz, S.A. Asher, Nature 389 (1997) 829.
[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
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J.E. Wijnhoven, W.L. Vos, Science 281 (1998) 802. Y. Zhao, J. Wang, G. Mao, Opt. Lett. 30 (2005) 1885. M.A. Daous, A. Stein, Chem. Mater. 15 (2003) 2638. G.R. Yi, T. Thorsen, V.N. Manoharan, M.J. Hwang, S.J. Jeon, D.J. Pine, S.R. Quake, S.M. Yang, Adv. Mater. 15 (2003) 1300. J. Huang, T. Kunitake, Chem. Commun. 21 (2005) 2680. T.D. Clark, R. Ferrigno, J. Tien, K.E. Paul, G.M. Whitesides, J. Am. Chem. Soc. 124 (2002) 5419. V.N. Manoharan, M.T. Elsesser, D.J. Pine, Science 25 (2003) 483. H. Zheng, I. Lee, M.F. Rubner, P.T. Hammond, Adv. Mater. 14 (2002) 569. J.C. Love, A.R. Urbach, M.G. Prentiss, G.M. Whitesides, J. Am. Chem. Soc. 125 (2003) 12696. B.A. Grzybowski, H.A. Stone, G.M. Whitesides, Nature 405 (2000) 1033. U. Wiedwald, M. Spasova, M. Farle, M. Hilgendorff, M. Giersig, J. Vasc. Sci. Technol. A 19 (2001) 1773. J. Wang, Q.W. Chen, C. Zeng, B.Y. Hou, Adv. Mater. 16 (2004) 137. G. Dumpich, T.P. Krome, B. Hausmans, J. Magn. Magn. Mater. 248 (2002) 241. D.F. Wan, X.L. Ma, Magnetic Physics, Publishing House of Electronics Industry, Beijing, 1999 361 pp. T. Sato, T. Iijima, M. Seki, N. Inagaki, J. Magn. Magn. Mater. 65 (1987) 252. A.E. Berkowitz, W.J. Schuele, P.J. Flanders, J. Appl. Phys. 39 (1987) 1261. J.M.D. Coey, Phys. Rev. Lett. 27 (1971) 1140. A.E. Berkowitz, J.L. Walter, Mater. Sci. Eng. 55 (1982) 275.
Contents lists available at ScienceDirect
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
Magnetic chains of Co spheres synthesized by hydrothermal process under magnetic field Jun Wang ⁎, Min Yao, Chuanhui Xu, Yuejin Zhu, Gaojie Xu, Ping Cui Faculty of Science, Ningbo University, Ningbo, 315211, China Ningbo Institute of Materials Technology and Engineering, Ningbo 315201, China
a r t i c l e
i n f o
Article history: Received 25 November 2007 Accepted 26 February 2008 Available online 8 March 2008 Keywords: Magnetic material Hydrothermal synthesis Magnetic field
a b s t r a c t Hydrothermal process under magnetic fields is successfully used to synthesize Co chains using reduction approach by carefully controlling the reaction conditions. The formation of the chain structure might be that magnetic fields drive the nanoscale crystals of Co to form chains. The Co sphere size and chain length are analyzed by X-ray diffraction (XRD) and scan electron microscopy (SEM). The result of the vibrating sample magnetometer (VSM) shows that the magnetic chains possess the saturation magnetization of 102 emu/g. The factors on the magnetic properties of the magnetic nanochains are discussed. © 2008 Elsevier B.V. All rights reserved.
1. Introduction During the past two decades, colloidal assemblies have been applied in various areas such as chemical and biological sensors [1], photonics [2], coating materials [3], and catalytic supports [4]. Considerable efforts have been directed toward controlling the selforganization of colloidal particles. For example, some progresses have been reported on colloidal self-assemblies in various physically or chemically confining geometries such as microchannels [5], porous membranes [6], capillaries [7], emulsion droplets [8], and patterned self-assembled monolayers [9]. However, these template-directed assemblies are not suitable for mass production because the preparation and subsequent removal of the sacrificial templates involve too much time-consuming processes. Recently, much attention has been paid to the effect of magnetic fields on the movement and self-assembly behavior of magnetic nanocrystallites. Indeed, it has been found that magnetic fields can significantly influence the movement of magnetic particles. For example, it has been demonstrated that the magnetic interactions can be used to generate ordered 3D self-assembled structures [10]. Magnetic fields have also been used to create 2D assemblies of magnetic nanoparticles [11], and 1D assembled chains or 2D rings on solid substrates [12]. In our previous experiment, we successfully utilized magnetic fields to synthesize single crystalline Fe3O4 nanowires [13]. Here we further report that ferromagnetic cobalt nanospheres can self-assemble into chains under magnetic fields. Furthermore, in contrast to the conventional
⁎ Corresponding author. Faculty of Science, Ningbo University, Ningbo, 315211, China. Tel.: +86 574 87600952; fax: +86 574 87600744. E-mail address: [email protected] (J. Wang). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.02.082
drying-mediated assemblies of magnetic nanoparticles on solid substrates under magnetic fields, these chains of nanoparticle can be directly formed by a hydrothermal process under magnetic fields. A magnetic field (0.3 T) is applied to the solution to induce the Co nanoparticles to form centimeter-long and nano-wide chains constrained to linearity along the direction of the applied magnetic field. The chains can keep after removal of the magnetic field. The long chain structures can be fragmented into short chain structures after more vigorous ultrasonic process and agitation. The factors on the magnetic properties of the magnetic chains are discussed. 2. Experiment The chains of Co spheres were synthesized by a hydrothermal redox process under magnetic fields. All chemicals were of analytical grade and were purchased from Shanghai Chemical Reagents Company. Assembly and synthesis of Co chains were performed by a one-step approach. In a typical procedure, an aqueous solution of 50 mL was first prepared by dissolving CoCl2 · 6H2O (50 mM), sodium tartrate (Na2C4H4O6, 0.75 M), NaOH (5 M), and sodium dodecyl benzenesulfonate (SDBS) (15 mM) in distilled water. After adding NaH2PO2 · H2O (0.4 M), the solution was stirred and then transferred into a stainless poly(tetrafluoroethylene) (Teflon)-lined autoclave with an induced magnetic field. The autoclave was maintained at 120 °C for 20 h and then was allowed to cool to room temperature. After the reaction was completed, a black fluffy solid product was deposited on the bottom of the Teflon cup. The final product was centrifuged, washed with distilled water and ethanol several times to remove any alkaline salt and surfactants that remained in the final products, and then dried in a vacuum oven at 50 °C for 4 h. The samples were characterized by XRD using an X-ray diffractometer with high-intensity Cu Kα radiation (λ = 1.5418 Å), recorded at
3432
J. Wang et al. / Materials Letters 62 (2008) 3431–3433
Fig. 1. XRD patterns for samples derived by a hydrothermal process under magnetic fields.
a scanning rate of 0.05° per second with the 2θ range from 20° to 80°. TEM images were taken with a Hitachi model H-800 transmission electron microscope, using an accelerating voltage of 200 kV. Their magnetic properties were measured on a BHV-55 vibrating sample magnetometer at room temperature. 3. Results and discussion The redox reaction of hydrothermal synthesis of Co during the heating process can be expressed as follows: 2− CoðC4 H2 O6 Þ2− þ H2 PO−2 þ OH− ⇔Co þ C4 H4 O2− 6 þ HPO3
The XRD pattern (Fig. 1) shows the crystal structure of the product. As shown in Fig. 1, the recorded diffraction peaks are well assigned to the structure of Co, indicating the formation of Co metals according to the standard JCPDS (Card No. 5-727). Broadening of the peaks exhibited the nanocrystalline nature of the sample. No CoO peak was observed, showing the high stability of the Co nanospheres. The sharp and narrow reflection peaks indicate the well-crystallinity of the magnetic nanoparticles. The morphology of the sample was observed by SEM, revealing that they have chain-like structure composed by spherical nanoparticles. From Fig. 2A, one can see that the averaged diameter of spheres is about 1.5 µm, showing a relatively narrow size distribution. The centimeter-long chains formed in the process are parallel to each other. Note that the yield of the assembled spheres is as high as 100%. The enlarged image of some typical chains is presented in Fig. 2B, which further indicates that the chains are assembled by spheres. The chains persisted very stably after removal of the applied magnetic field. After the produce resuspended by strong ultrasonication, the SEM image of Fig. 2C shows that the long and array direction of the chains have been destroyed. The lengths of the chains vary from centimeter-long to micro-long and the directions of the chains vary from parallel to each other to arbitrary directions. The mechanism involved for the formation of the chain structures is still unknown. It seems that applied magnetic fields play an important role in driving the nanoscale crystals of Co to form chains. We propose that one possible function of SDBS is to kinetically control the growth rates through the interaction in the adsorption and desorption processes. Subsequently, the Co nanocrystals assemble into chain
Fig. 3. Magnetization hysteresis curves of applied field parallel and perpendicular to the lengthwise direction of the chains.
architectures in the presence of magnetic force. As is well-known, ferromagnetic spherical particles magnetizes one another by dipolar interaction in arbitrary directions. When an external magnetic field is applied, the spherical particles tend to align along the magnetic line of force and favor the formation of linear chains. Magnetization makes all magnetic particles orientate along the magnetic line of force, as a result, dipole-directed self-assembly through dipolar interaction along the magnetic line of force could occur, leading to the formation of linear chains. The chains were nearly parallel, which could be the result of magnetic attraction, resulting in the wires aligning parallel to the magnetic line of force. The magnetization hysteresis curves of applied field parallel and perpendicular to the lengthwise direction of the chain are shown in Fig. 3. From the Fig. 3, we see an obvious uniaxial anisotropy originated from shape in our aligned chains. The easy axis is along the length of chain and the hard axis was perpendicular to it. When the direction of the magnetic field was along easy axis, the coercive force (Hc) was lowest. It increased when the magnetic field was rotated away from the easy axis and reached a maximum value in perpendicular. Moreover, The sample exhibits a saturation magnetization (Ms) of ~ 102 emu/g at room temperature, lower than the corresponding values of bulk Co (168 emu/g) [14,15]. As well-known, the saturation magnetization of magnetic nanoparticles was found to be much smaller than the bulk value and decreased with decreasing particle size. The phenomena have been observed in several magnetic material systems [16] and were explained by a magnetically dead layer on the surface of the particles [17]. The existence of the nonmagnetic layer might be caused by the canting of the surface spins [18], a high anisotropy layer, or the loss of the long range order in the surface layer [19] or some other reasons. However, the hysteresis loop at room temperature shows that the sample has a ferromagnetic character.
4. Conclusions This paper demonstrates that Co chains composed of Co spheres can be synthesized via reduction approach by carefully controlling the reaction conditions under magnetic fields. The formation of the chain structures might be that magnetic fields drive the nanoscale crystals of Co to form chains. The strategy of assembly is to use magnetic forces between ferromagnetic spheres to organize and stabilize the chains.
Fig. 2. SEM micrograph of a sample derived from a hydrothermal process under magnetic fields (A), (B) the enlarged image of some typical chains and (C) the SEM image of the produce resuspended by ultrasonication.
J. Wang et al. / Materials Letters 62 (2008) 3431–3433
The materials possessed a saturation magnetization of 102 emu/g at room temperature. The results of SEM discover that the centimeterlong and nano-wide chains are parallel to each other and the yield is as high as 100%. The material is expected to have promising applications for catalysts and other related nanodevices. The synthetic route can be extended to various types of magnetic nanoparticles. Acknowledgments This work was supported by the Natural Science Foundation of Ningbo (2007A610023), the Natural Science Foundation of China, Zhe Jiang (Y407267), China Postdoctoral Science Foundation (20070420675), the National Science Foundation of China (10774079) and K. C. Wong Magna Foundation in Ningbo University. References [1] J.H. Holtz, S.A. Asher, Nature 389 (1997) 829.
[2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
3433
J.E. Wijnhoven, W.L. Vos, Science 281 (1998) 802. Y. Zhao, J. Wang, G. Mao, Opt. Lett. 30 (2005) 1885. M.A. Daous, A. Stein, Chem. Mater. 15 (2003) 2638. G.R. Yi, T. Thorsen, V.N. Manoharan, M.J. Hwang, S.J. Jeon, D.J. Pine, S.R. Quake, S.M. Yang, Adv. Mater. 15 (2003) 1300. J. Huang, T. Kunitake, Chem. Commun. 21 (2005) 2680. T.D. Clark, R. Ferrigno, J. Tien, K.E. Paul, G.M. Whitesides, J. Am. Chem. Soc. 124 (2002) 5419. V.N. Manoharan, M.T. Elsesser, D.J. Pine, Science 25 (2003) 483. H. Zheng, I. Lee, M.F. Rubner, P.T. Hammond, Adv. Mater. 14 (2002) 569. J.C. Love, A.R. Urbach, M.G. Prentiss, G.M. Whitesides, J. Am. Chem. Soc. 125 (2003) 12696. B.A. Grzybowski, H.A. Stone, G.M. Whitesides, Nature 405 (2000) 1033. U. Wiedwald, M. Spasova, M. Farle, M. Hilgendorff, M. Giersig, J. Vasc. Sci. Technol. A 19 (2001) 1773. J. Wang, Q.W. Chen, C. Zeng, B.Y. Hou, Adv. Mater. 16 (2004) 137. G. Dumpich, T.P. Krome, B. Hausmans, J. Magn. Magn. Mater. 248 (2002) 241. D.F. Wan, X.L. Ma, Magnetic Physics, Publishing House of Electronics Industry, Beijing, 1999 361 pp. T. Sato, T. Iijima, M. Seki, N. Inagaki, J. Magn. Magn. Mater. 65 (1987) 252. A.E. Berkowitz, W.J. Schuele, P.J. Flanders, J. Appl. Phys. 39 (1987) 1261. J.M.D. Coey, Phys. Rev. Lett. 27 (1971) 1140. A.E. Berkowitz, J.L. Walter, Mater. Sci. Eng. 55 (1982) 275.