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Template electrodeposition to cobalt-based alloys nanotube arrays
Materials Letters 63 (2009) 578–580
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
Template electrodeposition to cobalt-based alloys nanotube arrays Xiang-Zi Li a,b, Xian-Wen Wei a,⁎, Yin Ye a a College of Chemistry and Materials Science, Anhui Key Laboratory of Functional Molecular Solids, Anhui Key Laboratory of Molecule-based Materials, Anhui Normal University, Wuhu 241000, China b Department of Chemistry, WanNan Medical College, Wuhu 241000, China
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 4 November 2008 Accepted 2 December 2008 Available online 7 December 2008
Parallel uniform arrays of amorphous ferromagnetic Co81Ni19 and Co37Fe63 alloy nanotubes with outer diameter around 325–365 nm, wall thickness of 30–60 nm and length of over 40 μm were prepared by a direct current electrodeposition with mercury cathode using porous anodic aluminum oxide membrane as template. The morphology, structure, composition and magnetic property were studied. The results showed that mercury cathode is the key factor to form amorphous alloy nanotubes, and the as-prepared nanotube arrays exhibit obvious uniaxial magnetic anisotropy and the easy magnetization direction is perpendicular to the nanotubes axis. The mechanism of formation of Co based alloy nanotubes was also discussed. © 2008 Elsevier B.V. All rights reserved.
Keywords: Metals and alloys Nanomaterials Magnetic materials Electrodeposition
1. Introduction Since the discovery of carbon nanotubes, tubular nanostructures have attracted much more interest from researchers for their intriguing electronic, catalytic, optical, and magnetic properties [1– 10]. Among the synthesis methods, template-based synthesis, pioneered by Martin's group [2], is an important synthetic strategy to fabricate nanotube arrays. Magnetic metallic nanotubes, such as Fe [3,4], Co [5,6] and Ni [7–9], and Co–Cu alloy [10] nanotubes have been fabricated by electrodeposition in anodic aluminum oxide (AAO) template, but these methods commonly need some strategies of AAO channels modification, multistep template replication and high current. It is still a challenge to prepare alloy nanotube arrays by an easy one-step way. Herein we report a novel way to fabricate highfilling, large-area, and uniform amorphous ferromagnetic Co81Ni19 and Co37Fe63 nanotube arrays within AAO membrane by using mercury cathode during DC electrodeposition. Instead of Au or Ag electrode that is usually gained by vacuum spurting or painting and is difficult to be removed from and hardly reduces the metals with low potentials, Mercury has been used as the work electrode that has a good conductivity, fluidity and is dynamic under the electric current, which resulted in the formation of amorphous alloys tubular structures.
separates the two cells, then a little mercury (caution: mercury vapor is toxic) and the electrolyte consisted of (1) 120 g/L CoSO4∙7H2O, 107 g/ L NiSO4∙7H2O and 45 g/L H3BO3; (2) 4.0 g/L CoSO4∙7H2O, 8.0 g/L FeSO4∙7H2O, 30 g/L H3BO3 and 1.5 g/L ascorbic acid were injected into each cell, respectively. The pH was adjusted to 2.5–3.0 and all the reagents are AR purity. The AAO template clung mercury was served as working electrode, platinum plate was counter-electrode and standard saturated calomel electrode was reference electrode. Direct current (DC) electrodeposition was conducted at −1.050 V for Co–Ni (−1.150 V for Co–Fe). After electrodeposition, the mercury clinging to AAO membrane was swept with a brush, and the as-synthesized AAO membrane was polished by sand paper, and then washed several times with distilled water and absolute alcohol for further analysis. Structure, morphology and composition of the arrays of cobaltbased nanostructures have been characterized by X-ray diffraction (XRD, Shimadzu, XRD-6000), selected area electron diffraction (SAED), transmission electron microscopy (TEM, Hitachi H 800), scanning electron microscopy (SEM, LEO-1530VP), and energy dispersive X-ray spectroscopy (EDX, INCAx-Sight OXFORD) attached to SEM, respectively. Room temperature magnetic characterization of the alloy nanotubes embedded in the AAO membrane was performed by using a BHV-55 vibrating sample magnetometer (VSM). 3. Results and discussion
2. Experimental ®
AAO membrane (Anodise , Whatman Inc, 200 nm) was treated with phosphoric acid, then put into the self-made electrobath, which ⁎ Corresponding author. Fax: +86 553 3869303. E-mail address: [email protected] (X.-W. Wei). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.12.002
SEM and TEM images of the Co–Ni or Co–Fe alloy nanotubes are clearly shown in Fig. 1a–d. It can be seen from Fig. 1a and c that the high-filling, large-area, and uniform Co–Ni and Co–Fe nanotube arrays, with length up to 45 µm have been obtained, respectively. A typical top image of the Co–Ni nanotube arrays (Fig. 1b) shows the smooth, uniform and clean nanotubes with outer diameter of 325–360 nm which corresponds to the pore size of AAO membrane etched with phosphoric acid, and with the wall thickness of about 30 nm. Co–Fe nanotubes (Fig. 1d) have outer diameters in 330–365 nm and wall thickness of 60 nm.
X.-Z. Li et al. / Materials Letters 63 (2009) 578–580
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Fig. 1. SEM images of (a,b)Co–Ni and (c,d) Co–Fe alloy nanotube arrays; TEM images of (e) Co–Ni and (f) Co–Fe alloy nanotubes. Inserts are SAED patterns.
TEM images of the Co–Ni and Co–Fe alloy nanotubes shown in Fig. 1e–f indicated that the outer diameters of nanotubes vary from 280 to 340 nm, which are the same as that from SEM. It is difficult to see the hollow cavity in the TEM image (Fig. 1f) because the tube wall of Co–Fe is very thick (about 60 nm). In addition, the SAED patterns (inserts in Fig. 1e, f) illustrated that both Co–Ni and Co–Fe nanotubes are amorphous in structure, which are further confirmed by the XRD patterns. The composition of the as-prepared alloys (Co 81 at.% and Ni 19 at.%; Co 37 at.% and Fe 63 at.%, respectively) have been investigated by EDX, and no peaks for element Hg can be found from EDX spectrum, which indicate that the alloy nanotubes are pure and no amalgam exists.
It has been demonstrated [8,10] that the base electrode is important for the formation of tubular nanostructures. It is found here that mercury cathode is the key factor to form amorphous alloy nanotubes. Fig. 2 is a proposed growth mechanism for the alloy nanotubes. Mercury contacts with AAO membrane tightly and a little mercury can filter into the holes of the AAO membrane from one end. The metal ions such as Co2+, Ni2+, Fe2+ infiltrated in the AAO membrane holes from another end can be reduced on mercury cathode convex surface under DC. Since mercury is dynamic under electrifying, the metal atoms reduced couldn't stay at the surface of mercury, while the bottom edge of the template pore can serves as a preferential site for the first deposition of metals [3]. Subsequently, the reduced metals can
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X.-Z. Li et al. / Materials Letters 63 (2009) 578–580
Fig. 2. Schematic diagram of the growth mechanism of cobalt-based alloy nanotubes via the template-based electrodeposition with mercury cathode.
regrow along the inner walls of AAO automatically due to the high surface area of nanochannel and the deposition stability on the direction of parallel to the AAO holes, and forms the nanotubes finally. In addition the dynamic surface of mercury can't present a steady interface to form the crystalline alloy via electrodeposition, which leads to the amorphous alloys. In order to investigate the magnetic property of the amorphous Co81Ni19 and Co37Fe63 nanotube arrays, hysteresis loops shown in Fig. 3 with the external field perpendicular (⊥) and parallel (//) to the nanotubes' long axis were obtained at room temperature. When the magnetic field is applied parallel (dashed) and perpendicular (solid line) to the axis of nanotube arrays, the coercivity of Co81Ni19 (Co37Fe63) are HC// =272.6 Oe, HC⊥ =260.8 Oe (HC// = 258.3 Oe, HC⊥ = 212.2 Oe). Both of the nanotube arrays exhibit enhanced coercivities which are larger than those of bulk Ni or Co (around 0.7 Oe for bulk Ni and 10 Oe for bulk Co) [11]. Similar to Co and Ni nanotube arrays [6–9], lower magnetic squarenesses ratio (Mr/Ms) on parallel direction are shown in both M–H loops, which indicated that the amorphous Cobased nanotube arrays exhibit uniaxial magnetic anisotropy and the easy magnetization axes are perpendicular to the nanotubes, which are different to that of the crystalline Ni nanotubes with the easy magnetization axes being parallel to nanotubes [12]. Herein all curves are highly sheared, indicating strong interactions between the nanotubes, which are expected since the alloy nanotubes in the array are very close to each other. Due to the amorphous structure, the magnetic crystalline anisotropy is hardly to occur and magnetic behavior of alloy nanotube arrays is mainly decided on the competition among the shape anisotropy [13], the dipolar between the nanotubes [14] and the structure (hollow and aspect ratio) of nanotubes.
4. Conclusions In this work, magnetic amorphous Co81Ni19 and Co37Fe63 nanotube arrays have been fabricated firstly using DC electrodeposition in AAO membrane by mercury cathode at room temperature. The results indicate that mercury cathode is the key factor to form the amorphous structure and tubular shape during the electrodeposition process. The approach is a promising technique to fabricate alloy nanotube arrays for a variety of applications and may be extended to other alloy tubular structure. The magnetic properties have revealed that these Co-based alloy nanotube arrays exhibit uniaxial magnetic anisotropy and easy magnetization axes are perpendicular to the nanotubes. Acknowledgements This work was supported by Science and Technological Fund of Anhui Province for Outstanding Youth (No. 08040106906), the National Natural Science Foundation (Nos. 20671002, 20490217) and the State Education Ministry (EYTP, SRF for ROCS, SRFDP 20070370001) of PR China, and the Education Department (No. 2006KJ006TD) of Anhui Province. References [1] Umeda R, Awaji H, Nakahodo T, Fujihara H. J Am Chem Soc 2008;130:3240–1. [2] Martin CR. Science 1994;266:1961–5. [3] Cao HQ, Wang LD, Qiu Y, Wu QZ, Wang GZ, Zhang L, et al. Chem Phys Chem 2006;7:1500–4. [4] Xu XJ, Yu SF, Lau SP, Li L, Zhao BC. J Phys Chem C 2008;112:4168–74. [5] Narayanan TN, Shaijumon MM, Ajayan PM, Anantharaman MR. J Phys Chem C 2008;112:14281–5. [6] Li DD, Thompson RS, Bergmann G, Lu JG. Adv Mater 2008;20:1–4. [7] Tao FF, Guan MY, Jiang Y, Zhu JM, Xu Z, Xue ZL. Adv Mater 2006;18:2161–4. [8] Bao JC, Tie CY, Xu Z, Zhou QF, Shen D, Ma Q. Adv Mater 2001;13:1631–3. [9] Wang QT, Wang GZ, Han XH, Wang XP, Hou JG. J Phys Chem B 2005;109:23326–9. [10] Liu LF, Zhou WY, Xie SS, Song L, Luo SD, Liu DF, et al. J Phys Chem C 2008;112:2256–61. [11] Chikazumi S. Physics of magnetism. New York: John Wiley & Sons; 1964. [12] Wang XW, Yuan ZH, Suna SQ, Duan YQ, Bie LJ. Mater Chem Phys 2008;112:329–32. [13] Sellmyer DJ, Zheng M, Skomski R. J Phys Condens Mat 2001;13:R433–460. [14] Zhan QF, Gao JH, Liang YQ, Di NL, Cheng ZH. Phys Rev B 2005;72:024428.
Fig. 3. The M–H loops of the nanotube arrays of (a) Co81Ni19 and (b) Co37Fe63.
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
Template electrodeposition to cobalt-based alloys nanotube arrays Xiang-Zi Li a,b, Xian-Wen Wei a,⁎, Yin Ye a a College of Chemistry and Materials Science, Anhui Key Laboratory of Functional Molecular Solids, Anhui Key Laboratory of Molecule-based Materials, Anhui Normal University, Wuhu 241000, China b Department of Chemistry, WanNan Medical College, Wuhu 241000, China
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 4 November 2008 Accepted 2 December 2008 Available online 7 December 2008
Parallel uniform arrays of amorphous ferromagnetic Co81Ni19 and Co37Fe63 alloy nanotubes with outer diameter around 325–365 nm, wall thickness of 30–60 nm and length of over 40 μm were prepared by a direct current electrodeposition with mercury cathode using porous anodic aluminum oxide membrane as template. The morphology, structure, composition and magnetic property were studied. The results showed that mercury cathode is the key factor to form amorphous alloy nanotubes, and the as-prepared nanotube arrays exhibit obvious uniaxial magnetic anisotropy and the easy magnetization direction is perpendicular to the nanotubes axis. The mechanism of formation of Co based alloy nanotubes was also discussed. © 2008 Elsevier B.V. All rights reserved.
Keywords: Metals and alloys Nanomaterials Magnetic materials Electrodeposition
1. Introduction Since the discovery of carbon nanotubes, tubular nanostructures have attracted much more interest from researchers for their intriguing electronic, catalytic, optical, and magnetic properties [1– 10]. Among the synthesis methods, template-based synthesis, pioneered by Martin's group [2], is an important synthetic strategy to fabricate nanotube arrays. Magnetic metallic nanotubes, such as Fe [3,4], Co [5,6] and Ni [7–9], and Co–Cu alloy [10] nanotubes have been fabricated by electrodeposition in anodic aluminum oxide (AAO) template, but these methods commonly need some strategies of AAO channels modification, multistep template replication and high current. It is still a challenge to prepare alloy nanotube arrays by an easy one-step way. Herein we report a novel way to fabricate highfilling, large-area, and uniform amorphous ferromagnetic Co81Ni19 and Co37Fe63 nanotube arrays within AAO membrane by using mercury cathode during DC electrodeposition. Instead of Au or Ag electrode that is usually gained by vacuum spurting or painting and is difficult to be removed from and hardly reduces the metals with low potentials, Mercury has been used as the work electrode that has a good conductivity, fluidity and is dynamic under the electric current, which resulted in the formation of amorphous alloys tubular structures.
separates the two cells, then a little mercury (caution: mercury vapor is toxic) and the electrolyte consisted of (1) 120 g/L CoSO4∙7H2O, 107 g/ L NiSO4∙7H2O and 45 g/L H3BO3; (2) 4.0 g/L CoSO4∙7H2O, 8.0 g/L FeSO4∙7H2O, 30 g/L H3BO3 and 1.5 g/L ascorbic acid were injected into each cell, respectively. The pH was adjusted to 2.5–3.0 and all the reagents are AR purity. The AAO template clung mercury was served as working electrode, platinum plate was counter-electrode and standard saturated calomel electrode was reference electrode. Direct current (DC) electrodeposition was conducted at −1.050 V for Co–Ni (−1.150 V for Co–Fe). After electrodeposition, the mercury clinging to AAO membrane was swept with a brush, and the as-synthesized AAO membrane was polished by sand paper, and then washed several times with distilled water and absolute alcohol for further analysis. Structure, morphology and composition of the arrays of cobaltbased nanostructures have been characterized by X-ray diffraction (XRD, Shimadzu, XRD-6000), selected area electron diffraction (SAED), transmission electron microscopy (TEM, Hitachi H 800), scanning electron microscopy (SEM, LEO-1530VP), and energy dispersive X-ray spectroscopy (EDX, INCAx-Sight OXFORD) attached to SEM, respectively. Room temperature magnetic characterization of the alloy nanotubes embedded in the AAO membrane was performed by using a BHV-55 vibrating sample magnetometer (VSM). 3. Results and discussion
2. Experimental ®
AAO membrane (Anodise , Whatman Inc, 200 nm) was treated with phosphoric acid, then put into the self-made electrobath, which ⁎ Corresponding author. Fax: +86 553 3869303. E-mail address: [email protected] (X.-W. Wei). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.12.002
SEM and TEM images of the Co–Ni or Co–Fe alloy nanotubes are clearly shown in Fig. 1a–d. It can be seen from Fig. 1a and c that the high-filling, large-area, and uniform Co–Ni and Co–Fe nanotube arrays, with length up to 45 µm have been obtained, respectively. A typical top image of the Co–Ni nanotube arrays (Fig. 1b) shows the smooth, uniform and clean nanotubes with outer diameter of 325–360 nm which corresponds to the pore size of AAO membrane etched with phosphoric acid, and with the wall thickness of about 30 nm. Co–Fe nanotubes (Fig. 1d) have outer diameters in 330–365 nm and wall thickness of 60 nm.
X.-Z. Li et al. / Materials Letters 63 (2009) 578–580
579
Fig. 1. SEM images of (a,b)Co–Ni and (c,d) Co–Fe alloy nanotube arrays; TEM images of (e) Co–Ni and (f) Co–Fe alloy nanotubes. Inserts are SAED patterns.
TEM images of the Co–Ni and Co–Fe alloy nanotubes shown in Fig. 1e–f indicated that the outer diameters of nanotubes vary from 280 to 340 nm, which are the same as that from SEM. It is difficult to see the hollow cavity in the TEM image (Fig. 1f) because the tube wall of Co–Fe is very thick (about 60 nm). In addition, the SAED patterns (inserts in Fig. 1e, f) illustrated that both Co–Ni and Co–Fe nanotubes are amorphous in structure, which are further confirmed by the XRD patterns. The composition of the as-prepared alloys (Co 81 at.% and Ni 19 at.%; Co 37 at.% and Fe 63 at.%, respectively) have been investigated by EDX, and no peaks for element Hg can be found from EDX spectrum, which indicate that the alloy nanotubes are pure and no amalgam exists.
It has been demonstrated [8,10] that the base electrode is important for the formation of tubular nanostructures. It is found here that mercury cathode is the key factor to form amorphous alloy nanotubes. Fig. 2 is a proposed growth mechanism for the alloy nanotubes. Mercury contacts with AAO membrane tightly and a little mercury can filter into the holes of the AAO membrane from one end. The metal ions such as Co2+, Ni2+, Fe2+ infiltrated in the AAO membrane holes from another end can be reduced on mercury cathode convex surface under DC. Since mercury is dynamic under electrifying, the metal atoms reduced couldn't stay at the surface of mercury, while the bottom edge of the template pore can serves as a preferential site for the first deposition of metals [3]. Subsequently, the reduced metals can
580
X.-Z. Li et al. / Materials Letters 63 (2009) 578–580
Fig. 2. Schematic diagram of the growth mechanism of cobalt-based alloy nanotubes via the template-based electrodeposition with mercury cathode.
regrow along the inner walls of AAO automatically due to the high surface area of nanochannel and the deposition stability on the direction of parallel to the AAO holes, and forms the nanotubes finally. In addition the dynamic surface of mercury can't present a steady interface to form the crystalline alloy via electrodeposition, which leads to the amorphous alloys. In order to investigate the magnetic property of the amorphous Co81Ni19 and Co37Fe63 nanotube arrays, hysteresis loops shown in Fig. 3 with the external field perpendicular (⊥) and parallel (//) to the nanotubes' long axis were obtained at room temperature. When the magnetic field is applied parallel (dashed) and perpendicular (solid line) to the axis of nanotube arrays, the coercivity of Co81Ni19 (Co37Fe63) are HC// =272.6 Oe, HC⊥ =260.8 Oe (HC// = 258.3 Oe, HC⊥ = 212.2 Oe). Both of the nanotube arrays exhibit enhanced coercivities which are larger than those of bulk Ni or Co (around 0.7 Oe for bulk Ni and 10 Oe for bulk Co) [11]. Similar to Co and Ni nanotube arrays [6–9], lower magnetic squarenesses ratio (Mr/Ms) on parallel direction are shown in both M–H loops, which indicated that the amorphous Cobased nanotube arrays exhibit uniaxial magnetic anisotropy and the easy magnetization axes are perpendicular to the nanotubes, which are different to that of the crystalline Ni nanotubes with the easy magnetization axes being parallel to nanotubes [12]. Herein all curves are highly sheared, indicating strong interactions between the nanotubes, which are expected since the alloy nanotubes in the array are very close to each other. Due to the amorphous structure, the magnetic crystalline anisotropy is hardly to occur and magnetic behavior of alloy nanotube arrays is mainly decided on the competition among the shape anisotropy [13], the dipolar between the nanotubes [14] and the structure (hollow and aspect ratio) of nanotubes.
4. Conclusions In this work, magnetic amorphous Co81Ni19 and Co37Fe63 nanotube arrays have been fabricated firstly using DC electrodeposition in AAO membrane by mercury cathode at room temperature. The results indicate that mercury cathode is the key factor to form the amorphous structure and tubular shape during the electrodeposition process. The approach is a promising technique to fabricate alloy nanotube arrays for a variety of applications and may be extended to other alloy tubular structure. The magnetic properties have revealed that these Co-based alloy nanotube arrays exhibit uniaxial magnetic anisotropy and easy magnetization axes are perpendicular to the nanotubes. Acknowledgements This work was supported by Science and Technological Fund of Anhui Province for Outstanding Youth (No. 08040106906), the National Natural Science Foundation (Nos. 20671002, 20490217) and the State Education Ministry (EYTP, SRF for ROCS, SRFDP 20070370001) of PR China, and the Education Department (No. 2006KJ006TD) of Anhui Province. References [1] Umeda R, Awaji H, Nakahodo T, Fujihara H. J Am Chem Soc 2008;130:3240–1. [2] Martin CR. Science 1994;266:1961–5. [3] Cao HQ, Wang LD, Qiu Y, Wu QZ, Wang GZ, Zhang L, et al. Chem Phys Chem 2006;7:1500–4. [4] Xu XJ, Yu SF, Lau SP, Li L, Zhao BC. J Phys Chem C 2008;112:4168–74. [5] Narayanan TN, Shaijumon MM, Ajayan PM, Anantharaman MR. J Phys Chem C 2008;112:14281–5. [6] Li DD, Thompson RS, Bergmann G, Lu JG. Adv Mater 2008;20:1–4. [7] Tao FF, Guan MY, Jiang Y, Zhu JM, Xu Z, Xue ZL. Adv Mater 2006;18:2161–4. [8] Bao JC, Tie CY, Xu Z, Zhou QF, Shen D, Ma Q. Adv Mater 2001;13:1631–3. [9] Wang QT, Wang GZ, Han XH, Wang XP, Hou JG. J Phys Chem B 2005;109:23326–9. [10] Liu LF, Zhou WY, Xie SS, Song L, Luo SD, Liu DF, et al. J Phys Chem C 2008;112:2256–61. [11] Chikazumi S. Physics of magnetism. New York: John Wiley & Sons; 1964. [12] Wang XW, Yuan ZH, Suna SQ, Duan YQ, Bie LJ. Mater Chem Phys 2008;112:329–32. [13] Sellmyer DJ, Zheng M, Skomski R. J Phys Condens Mat 2001;13:R433–460. [14] Zhan QF, Gao JH, Liang YQ, Di NL, Cheng ZH. Phys Rev B 2005;72:024428.
Fig. 3. The M–H loops of the nanotube arrays of (a) Co81Ni19 and (b) Co37Fe63.