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J. Phys. Chem. C 2008, 112, 2256-2261
ARTICLES Highly Efficient Direct Electrodeposition of Co-Cu Alloy Nanotubes in an Anodic Alumina Template Lifeng Liu,* Weiya Zhou, Sishen Xie,* Li Song, Shudong Luo, Dongfang Liu, Jun Shen, Zengxing Zhang, Yanjuan Xiang, Wenjun Ma, Yan Ren, Chaoying Wang, and Gang Wang Beijing National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of Sciences, Beijing 100080, People’s Republic of China ReceiVed: August 12, 2007; In Final Form: NoVember 2, 2007
A highly efficient direct electrodeposition method was used to prepare Co-Cu alloy nanotubes in an anodic alumina template without modification. The morphology and structure of as-prepared Co-Cu nanotubes were examined by scanning electron microscopy, transmission electron microscopy, and X-ray diffraction. The formation mechanism of the tubular nanostructure is discussed. It was found that the template directed electrodeposition of Co-Cu at a large current density can result in the highly efficient growth of nanotubes and that the growth rate as well as the wall thickness of the nanotubes can be controlled via the current density of electrodeposition. Magnetic measurements of the Co-Cu nanotube array show that the nanotubes are ferromagnetic at room temperature and may find potential applications in the fields of biological separation and drug delivery.
Introduction Since the discovery of carbon nanotubes (NTs), tubular nanostructures have attracted much more interest from researchers. In addition to its quasi-one-dimensional feature that can give rise to some novel size-dependent physical and chemical properties, the hollow cylindrical structure of NTs also provides additional space for gas storage,1 ion transport,2,3 surface functionalization,4,5 catalytic reactions,6 drug delivery,7 etc. To date, a variety of NTs has been synthesized, and recent progress on semiconducting and organic NTs has been reviewed by Rao and Nath,8 Remskar,9 and Block et al10. However, the synthesis of aligned metallic NTs remains a challenge. Template-based synthesis is currently the commonly used approach to the fabrication of metallic NT arrays, although several solution-based syntheses have been demonstrated recently.11-13 In general, template synthesis of metallic NTs can be achieved through the following approaches. (1) Electrodeposition (ED) in the template with chemically modified nanopore walls:14 In this way, the nanopore walls of the template are usually first modified with “molecular anchors”, so that the electrodeposited metal atoms can be bound to the nanopore walls to form NTs. The molecular anchors, on one hand, are required to be able to attach to the nanopore walls; on the other hand, they should have a strong chemisorption to the deposited metal. To this end, organic silane molecules are generally chosen as the anchors. Although several metallic NTs have been synthesized by this method,15 the modification of nanopore walls with organic molecules will introduce organic impurities in the asprepared NTs because the silane sometimes forms not as a * To whom correspondence should be addressed. E-mail: ssxie@ aphy.iphy.ac.cn (S.S.X.) or
[email protected] (L.F.L.); fax: +8610-82640215; tel.: +86-10-82649081.
monolayer but as a thin polymer film.14 (2) Electroless deposition: Template electroless deposition is also a widely used method for the fabrication of metallic NTs. Before the deposition, the template is typically subjected to a sensitizationactivation process. Sn2+ and Pd2+ are the common sensitizer and activator, respectively. Using this method, Cu,16 Co,17 Ni,18 and Ni-P18,19 NTs have been synthesized. Similar to the ED method, however, Pd or Sn impurities are inevitably present in the NTs due to the sensitization-activation treatment. (3) Template wetting: Steinhart et al. developed a template wetting method to prepare metallic NTs.20-22 They first wetted the template with a polymer/metal precursor solution and then annealed the wetted template to form metal/polymer composite NTs or selectively removed the polymer to form metallic NTs. However, the polymer is difficult to remove completely in this manner so that one usually cannot obtain pure metallic NTs. (4) Multistep template replication: Mu et al. presented this method and prepared several NTs including Pt, Au, Bi, In, and Ni through it.23 Although it is not necessary to modify the nanopore walls chemically, this method is tedious and is hardly reproducible. (5) Atomic layer deposition (ALD): It has been demonstrated very recently that metallic NTs can also be synthesized through template-based ALD method.24 ALD offers a very precise control of the growth rate and a conformal coating on the nanopore walls of the template. Therefore, template ALD synthesis is a promising approach to study the influence of the tube thickness on the physical or chemical properties of NTs.24,25 (6) Direct ED method: Direct ED does not require a chemically modified template. The NTs can be directly electrodeposited in the nanopores as long as some conditions, which will be discussed thoroughly next, are satisfied. Yoo and Lee first discovered that some NTs were present when they tried to
10.1021/jp076477f CCC: $40.75 © 2008 American Chemical Society Published on Web 01/25/2008
Co-Cu Alloy Nanotubes
J. Phys. Chem. C, Vol. 112, No. 7, 2008 2257
Figure 1. (a) Overview SEM image of an as-prepared Co-Cu alloy nanotube array after the removal of the AAO template. (b) The magnified side-view SEM image of the rectangle area marked in panel a. (c) The top-view SEM image of the Co-Cu nanotube array. (d) The dispersed Co-Cu alloy NTs.
Figure 2. Typical EDX spectrum of an as-prepared Co-Cu nanotube array.
electrodeposit Pt nanowires at a high current density.26 Subsequently, Cao et al. reported the template synthesis of Fe, Co, and Ni NTs at a high current density deposition mode.27 In contrast to other methods, direct ED is a simple, impurity-free, and highly efficient approach to obtain metallic tubular nanostructures. In this study, we demonstrated that alloy NTs can also be fabricated by utilizing the template direct ED method. We used an anodic aluminum oxide (AAO) membrane as a template to fabricate Co-Cu ferromagnetic-nonmagnetic (FM-NM) alloy NTs. We characterized the morphology and structure of the asprepared samples and discussed their formation mechanism. Finally, we determined the magnetic measurement results of Co34Cu66 alloy NTs. As we know, Co-Cu is the most widely studied FM-NM alloy system due to its tunable magnetism and extraordinary giant magnetoresistance (GMR) effect.28,29 Experimental Procedures The AAO templates used in our experiments were purchased from the Whatman Company (Anodisc 47, 100 nm nominal pore diameter and 60 µm thickness). The electrodeposition was carried out with the galvanostatic mode in a two-electrode electrochemical cell. Before the electrodeposition, a layer of Au was sputtered on one side of the AAO template via an ion sputtering system (Hitachi E1030, 20 mA, 1000 s). The Aucoated AAO template and a piece of platinum plate were used as the working and counter electrodes, respectively. The electrolyte consisted of 1 g/L CuSO4, 50 g/L CoSO4, and 30 g/L H3BO3, with g18 MΩ deionized water as the solvent. All chemicals were analytical grade. The electrodeposition was performed at room temperature, with a high current density in the range of 20 to ∼80 mA/cm2.
The morphology of the as-prepared Co-Cu alloy NTs was characterized by scanning electron microscopy (SEM, Hitachi S-5200) equipped with energy-dispersive X-ray spectroscopy (EDX, Thermoelectron), and their structure and microstructure were determined by X-ray diffraction (XRD, Rigaku D8) and transmission electron microscopy (TEM, JEOL 2010). The magnetic properties of Co34Cu66 NTs were measured using a superconducting quantum interference device (SQUID, MPMS7) with the temperature down to 5 K. For SEM observations, the AAO template with Co-Cu NTs was immersed in a 3 M NaOH solution for 2 h to etch away the alumina completely. Then, the as-obtained sample was washed with distilled water several times and subsequently was put on the sample stage for the investigation. For TEM characterization, the as-obtained sample was subjected to ultrasonic treatment for 2 min, and a drop of suspension was placed on the carbon-coated copper grid. For magnetic measurements, the AAO template embedded with the deposited Co-Cu NTs was cut into a small piece with a size of 5 mm × 5 mm. Then, the sample was fixed laterally or vertically onto the plugstick of SQUID, whose axis was parallel to the applied magnetic field. Results and Discussion Figure 1a shows an overview SEM image of the as-prepared Co-Cu alloy NT array after the removal of the AAO template. It can be seen that the NTs are well-aligned and that their average length is about 20 µm. Moreover, the NTs are dense, continuous, and uniform throughout the whole length. Figure 1b is a magnified side-view SEM image of the NT array, which was taken from the rectangle area denoted in Figure 1a. As is shown from the cracks in the array, the as-deposited nanostructures here are really hollow NTs. The top-view SEM image of
2258 J. Phys. Chem. C, Vol. 112, No. 7, 2008
Liu et al. the wavelength of the X-ray and the full width at half-maximum of the diffraction peaks, respectively. The calculated average crystallite sizes along the and directions are about 27 and 38 nm, respectively. Here, the texture coefficient (TC) is introduced to describe the preferential orientation of the as-prepared Co-Cu NTs31
TC(hikili) )
Figure 3. Representative XRD pattern of an as-prepared Co-Cu nanotube array.
the array is described in Figure 1c, illustrating the open-ended tubular morphology more clearly. Figure 1d gives the morphology of dispersed Co-Cu NTs. It can be seen that the outer diameter of the NTs is more than 200 nm, corresponding well to the pore diameter of the AAO template (the actual pore diameter of Whatman AAO is usually much larger than the nominal pore diameter). Again, the cracks demonstrate a hollow tubular structure. Figure 2 gives a typical EDX spectrum of as-prepared CoCu NTs. From Figure 2, one can see that the NTs consist of Co and Cu. The Au peak originates from the sputtered Au film, and the Al and O peaks are from the small amount of residual alumina after the sample was rinsed. Figure 3 shows a XRD pattern of the Co-Cu NTs prepared at 20 mA/cm2. For comparison, the standard powder diffraction pattern of the Co52Cu48 alloy (JCPDF No. 50-1452) is also given. It was found that the diffraction peaks centered at 44.1, 51.1, and 74.4° correspond well to the diffractions of the standard Co52Cu48 alloy sample and can be indexed as fcc Co-Cu (111), Co-Cu (200), and Co-Cu (220), respectively. The lattice constants of the Co-Cu and Co-Cu directions are 0.2050 and 0.1785 nm, respectively, which are consistent with previous reports.29,30 The other three peaks in this spectrum resulted from the diffractions of the sputtered Au film. We can roughly estimate the size of the crystallites consisting of the NTs via the Debye-Scherrer formula
D)
Kλ β cos θ
(1)
where K ) 0.89 is the Scherrer constant, and λ and β represent
[I(hikili)/I0(hikili)]
∑(I(hnknln)/I0(hnknln))]
[(1/N)
(2)
where I(hikili) and I0(hikili) stand for the diffraction intensities of the (hikili) lattice plane of the sample under investigation and of the standard diffraction pattern, respectively. If TC(hikili) > 1, it indicates that there is a preferred orientation of the nanocrystallites in the sample. According to eq 2, the TC values of Co-Cu (111), Co-Cu (200), and Co-Cu (220) are 1.35, 0.67, and 0.98, respectively, showing that the as-prepared CoCu NTs orientate along the Co-Cu direction. For further investigating the microstructure of the as-prepared Co-Cu NTs, TEM characterization was carried out. Figure 4a shows a representative TEM image of Co-Cu NTs prepared at 20 mA/cm2. As can be seen from Figure 4, the Co-Cu NTs are dense and continuous. The outer diameter of NTs is about 280 nm, consistent with the pore diameter of the template. The upper right of Figure 4a is a TEM image of a single NT, illustrating the tubular structure more clearly. From Figure 4, the thickness of the nanotube wall is nonhomogeneous, and the average thickness is nearly 50 nm. The ED pattern of the asprepared Co-Cu NTs is given in the inset of Figure 4a (lower left), which was taken from the circle area denoted in Figure 4a. The rings can be well-indexed as the diffractions of the CoCu alloy phase (JCPDF No. 50--1452), which is in agreement with XRD results. However, some diffraction spots resulting from the hcp Co phase were also observed, which suggests that phase separation may take place locally during the nonequilibrium electrodeposition process.29 The presence of a hcp Co phase in the alloy NTs was not detected by XRD, probably because the size and volume fraction of the Co nanoclusters were too small. Figure 4b represents a TEM image of thinwalled Co-Cu NTs, which were deposited at 80 mA/cm2. It is seen that the tube wall is as thin as
