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ARTICLES Growth of Single-Crystalline Ni and Co Nanowires via Electrochemical Deposition and Their Magnetic Properties Hui Pan,† Binghai Liu,‡ Jiabao Yi,‡ Cheekok Poh,† Sanhua Lim,† Jun Ding,‡ Yuanping Feng,† C. H. A. Huan,† and Jianyi Lin*,†,§ Departments of Physics and Material Science, National UniVersity of Singapore, 2 Science DriVe 3, Singapore 117542, and Institute of Chemical and Engineering Sciences, 1 Pesek Road, Jurong Island, Singapore 627833 ReceiVed: October 21, 2004; In Final Form: December 23, 2004
Single-crystalline Ni nanowires have been successfully fabricated with anodic aluminum oxide as template by electrodeposition. Structural characterization (X-ray diffraction, XRD, and high-resolution transmission electron microscopy, HRTEM) shows that the single-crystalline Ni nanowire has a preferred orientation along the [220] direction. The effects of electrochemical deposition conditions on the structure of Ni nanowires are systematically studied to investigate the growth mechanism. Possible reasons for the growth of the singlecrystalline Ni nanowires were discussed on the basis of electrochemistry and thermodynamics. These singlecrystalline Ni nanowires have exhibited excellent magnetic properties (large anisotropy, large coercivity, and high remanence). By a similar process, single-crystalline Co nanowires with hexagonal close-packed (hcp) structure were achieved, also having large anisotropy, large coercivity (1.8 kOe), and high remanence ratio (80.8%).
Introduction Nanostructured ferromagnetic materials are of great interest in theoretical physics, solid-state science, and practical technological applications, such as ultra-high-density magnetic recording and spintronics.1-6 Methods used to produce the metallic nanowires include lithographic patterning,7 which is comparatively cumbersome, expensive, and not suitable for large-scale production, and “template synthesis”,1,2,8 which involves electrochemically depositing metal into nanopores in a template. A commonly used template is anodic aluminum oxide (AAO).9 To date, most metallic nanowires have been produced on AAO templates, including low melting point metals, such as Au, Ag, and Zn nanowires,10-13 and high melting point metals, such as Fe, Co, and Ni nanowires.8 Successful growth of single-crystal nanowires of low melting point metals have been reported,13 but growth of single-crystal nanowires of high-melting-point metals was claimed to be very difficult if not impossible.13 Single-crystal Fe or Ni nanowires have been reported in the literature.8,14 But the single-crystalline size was rather small, around 40 nm along the wire axis, and there was no discussion on the mechanism of single-crystal growth and the effect of single crystallinity on their magnetic properties. In this work we demonstrated the successful AAO template growth of single-crystalline Ni and Co nanowires and the excellent magnetic properties of these nanowires. * Corresponding author: e-mail
[email protected] or
[email protected]. † Department of Physics, National University of Singapore. ‡ Department of Material Science, National University of Singapore. § Institute of Chemical and Engineering Sciences.
Experimental Section The AAO template was prepared following the two-step anodization procedure.9 Briefly, the pure Al foil was first annealed and electropolished in a mixture of perchloric acid and ethanol. The anodization was performed at 40 V in an oxalic acid solution of 3 wt % at room temperature. After removal of the alumina in a mixture of phosphoric acid and chromic acid, the ordered pore arrangement was achieved during the second anodization with the same conditions as in the first anodization. The remaining Al was removed in CuCl2 solution. The oxide layer at the bottoms of the pores was removed in acid. To facilitate electrodeposition, a Pt layer was sputtered to the back of the AAO as the electrode. A Ni sulfate electrolyte (240 g of NiSO4‚7H2O/45 g of NiCl2‚6H2O/40 g of H3BO3‚2H2O) with a pH value of 2.5 was used. DC electrodeposition was performed at various applied voltages ranging from 0.4 to 4.0 V and temperatures ranging from 25 to 60 °C to investigate their effects on the structures and magnetic properties of Ni nanowires. Five Ni samples were prepared under different electrodeposition conditions. They are sample 1 [an applied voltage of 0.4 V and room temperature (RT)], sample 2 (1.0 V, RT), sample 3 (4.0 V, RT), sample 4 (1.0 V, 40 °C), and sample 5 (1.0 V, 60 °C). To keep the length of nanowires equal in all the samples, long deposition time was applied at low electrodeposition voltages, and the AAO used were prepared under the same anodizing conditions so that they have the same pore structures. The morphology of the deposited Ni nanowires was observed by scanning electron microscopy (SEM, JEOL JSM-6700F). The structure of the nanowires was characterized by high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction (XRD) (Brucker AXS D8). The magnetic properties of
10.1021/jp0451997 CCC: $30.25 © 2005 American Chemical Society Published on Web 02/08/2005
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Figure 1. SEM image of (a) Ni nanowires embedded in AAO template with the AAO surface slightly removed and (b) Ni nanowires totally released from AAO template.
nanowires embedded in the AAO template were measured by a vibrating sample magnetometer (Oxford Instruments) at room temperature.
a mechanism based on 2D-like nucleation. The crystalline nanowires will grow after the nucleus size exceeds the critical
Results and Discussion Figure 1a shows AAO template with filled Ni nanowires. It can be seen that Ni nanowires were nearly 100%, homogeneously filled into and embedded in the highly ordered porous alumina matrix. The diameter of nanowires is approximately 50 nm with a spacing of 100 nm. In Figure 1b the Ni nanowires that were liberated from the AAO template are aligned and bundled together. Figure 2 displays the XRD patterns of the above-mentioned five Ni samples. All Ni samples exhibited face-centered cubic (fcc) structure. However, Ni samples 2 and 3 (see Figure 2b,c) have a preferred orientation along the [220] direction, with little intensity at the (111) and (200) reflections. The strength at (111) exceeds that at (220) with an increase in deposition temperature from room temperature to 40-60 °C (see Figure 2d,e). Decreasing the applied voltage also makes the (111) peak stronger, although (220) remains dominant (see Figure 2a), while increasing the applied voltage suppresses peaks other than (220). The single- and poly-crystal structures were further confirmed by HRTEM and selected area electron diffraction (SAED). In Figure 3 the single-crystal structure of sample 3 (sample 2 gives a similar image and pattern) is clearly illustrated by SAED and HRTEM. As shown in Figure 3a, the diffraction pattern was taken from a selected area covering a segment of the wire as long as 2.0 µm, with uniform diameter of 50 nm, indicating the single crystallinity of samples 2 and 3. The HRTEM image with [110] zone axis in Figure 3b clearly shows lattice fringes of (111), (002), and (220) planes, suggesting single-crystalline structure of the nanowire. The spotty diffraction rings in Figure 3c and the dark contrast of the small crystals in Figure 3d (inset) show the polycrystalline nature of the Ni nanowires in samples 1, 4, and 5. The crystalline grain size in these samples is about 5-10 nm. Figure 3 clearly demonstrates that high-quality singlecrystal Ni nanowires can be fabricated by AAO template electrochemical deposition under controlled conditions. Three different growth modes have been proposed to understand the nanowire growth mechanism.13-15 Tian et al.13 had attributed the single-crystal growth of electrodeposited low melting point metallic nanowires, such as Au, Ag, and Cu, to
Figure 2. XRD patterns for samples prepared under different conditions: (a) Ni sample 1 (prepared with 0.4 V applied DC electrodeposition voltage and at room temperature); (b) Ni sample 2 (1.0 V, room temperature); (c) Ni sample 3 (4.0 V, room temperature); (d) Ni sample 4 (1.0 V, 40 °C); and (e) Ni sample 5 (1.0 V, 60 °C).
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Figure 3. TEM images and selective area electron diffraction patterns of Ni nanowires: (a) Ni nanowire in sample 3; the inset is the corresponding selective area ED, and the circle indicates that the size of selected area is as large as ∼1.5 µm. (b) HRTEM of Ni sample 3; the 0.203 nm interlayer spacing is characteristic of Ni (111) planes, and [220] is along the nanowire long axis. (c) Ni nanowires in sample 4. (d) HRTEM images of Ni sample 4.
dimension Nc.15 Nc is expressed as
Nc )
bs2 (zeη)2
(1a)
for 2D-like growth or
Nc )
8BVm2σ3 27(ze|η|)3
(1b)
for 3D-like growth, where s (Vm), (σ), z, and b (B) are the area (volume) occupied by one metallic atom on the surface of the nucleus, the edge (surface) energy, the effective electron number, and a constant (b ) π for circular), respectively; η is the overpotential. In our case, Ni nanowires are deposited on an amorphous Pt film that was sputtered on the back of AAO. An amorphous substrate is inert with respect to the growth process of the deposit and has no effect on the nanowire growth.16 In the initial stage of Ni growth, the orientation of Ni nuclei is random and a newly coalesced compact deposit has perfectly random orientation. The texture of thicker Ni deposits is a result of competitive growth between adjacent grains occurring in a stage of growth after the coalescence stage. Low-surface-energy grains grow faster than high-energy grains.17,18 The rapid growth of low-surfaceenergy at the expense of the high-energy grains results in an increase in grain size, favoring the formation of columnar grain. The confinement of the nanopore structure in the AAO template facilitates the formation of columnar grains and single-crystalline nanowires in the nanopores. Nevertheless, this process depends also on the deposition conditions such as electrodeposition
overpotential and temperature, etc. Under low deposition potentials (e.g., for sample 1 grown at 0.4 V at room temperature), polycrystals form, since low deposition potential leads to large Nc, which prevents the formation of columnar grains in the nanopores. The increase of deposition temperature at a constant deposition potential between 1 and 4 V leads to the appearance of polycrystals (e.g., samples 4 and 5) because the thermal energy agitates the growth and distorts the competition between adjacent grains. The strength of the (111) peak in XRD patterns increases with increasing deposition temperature because Ni(111) is the plane of the lowest surface energy. At 25 °C and higher electrodeposition voltages (1.0-4.0 V), Nc decreases (with increasing overpotential), which favors the coalescence of grains and the formation of columnar grains. Higher overpotential was reported to have two other effects. It favors the adsorption of H ions on the cathode, which stabilizes the (110) face,15 and it induces the thermodynamic to kinetic phase transition from [100] to [110] in the nucleation process.19 For these reasons the Ni(110) plane is energetically more stable than (100) and (111) under 1.0-4.0 V and room temperature conditions, and single-crystal Ni nanowires are preferredoriented along the [110] direction. The stabilization of the (110) face due to the adsorption of hydrogen prevents the formation of single-crystalline Ni nanowires with (111) orientation under low-pH conditions. The magnetic properties of Ni nanowires embedded in an AAO template are closely related to their physical properties and therefore to the growth conditions. It is well-known that reducing the nanowire diameter could improve the squareness of the magnetization hysteresis, and raise the coercivity of nanowires.10 Additionally, the coercivity of the nanowires can
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Figure 4. Magnetization curves of Ni nanowires embedded in the AAO template: (a) Ni sample 1, (b) Ni sample 2, (c) Ni sample 3. (s) Applied magnetic field parallel to the long axis; (---) perpendicular field.
TABLE 1: Coercivity Hc and Squareness of Ni and Co Nanowires sample
Hc (Oe) (⊥)
Hc (Oe) (|)
Mr/Ms (⊥)
Mr/Ms (|)
Ni sample 1 Ni sample 2 Ni sample 3 Ni sample 4 Ni sample 5 Co sample A Co sample B
182 193 101 159 147 353 775
746 1000 1000 764 857 1800 1680
0.094 0.066 0.045 0.091 0.148 0.056 0.117
0.369 0.887 0.937 0.453 0.639 0.808 0.678
be enhanced by increasing the nanowire length, which becomes saturated when the length exceeds a critical value at a constant diameter.2 Our work has demonstrated that the crystallinity of the nanowires can greatly affect their magnetic properties. The magnetization hysteresis loops in Figure 4 were measured for our Ni nanowires embedded in the AAO template at two different magnetic field directions, parallel (out-of-plane with the AAO template) or perpendicular to the nanowire long axis (in-plane with the AAO template). At the parallel direction, Ni sample 2, which was prepared at an applied voltage of 1.0 V and a temperature of 25 °C, has its coercivity Hc (Hc|) ) 1000 Oe and the remanent magnetization Mr (Mr|) 88.6% of the saturated data Ms; in other words, the squareness of the hysteresis of the Ni nanowires is about 0.886. The Hc and Mr values are both rather low at perpendicular direction, indicating an evident perpendicular anisotropy of the nanowires. When the single crystallinity is further improved in Ni sample 3 by increasing the applied electrodeposition voltage from 1.0 to 4.0 V while keeping the deposition at room temperature, the squareness of Ni sample 3 is enhanced to 93.7%. Note that the coercivity of sample 3 remains at 1000 Oe, while its Mr increases up to the value, 93.7%, of the saturated data. On the other hand, polycrystalline Ni samples 1, 4, and 5 have smaller Hc|, Mr|, and squareness of the hysteresis than sample 2 (see Table 1). The reduction of Hc| (1000 f 740 Oe) is not as much as that of Mr| and the squareness (94% f 36%). It is worthy to note that our sample 3 has much higher magnetic squareness compared to sample B in ref 10 (94% vs 86%). Sample B in ref 10 is smaller in diameter (40 nm), but our sample is 50 nm in diameter. So the better magnetic squareness must be due to better single crystallinity of our sample 3. All these results indicate that the crystalline structure and arrangements of Ni nanowires can greatly affect the magnetic properties. Hence, by controlling the growth conditions, we are able to prepare high-quality single-crystalline Ni nanowires with excellent magnetic properties. The magnetic anisotropy of the nanowires results from the interplay of a series of effects: the macroscopic demagnetization field (Hd ) -6πMsP,20 where P is the porosity of the template and Ms ) 500 emu/cm3 is the Ni bulk value of the saturation magnetization; for Ni nanowires with diameter 50 nm, P ) 22.6% and Hd ) -2130 Oe); the form effect of individual
Figure 5. XRD patterns for samples prepared under different conditions: (a) Co sample A (1.0 V, room temperature); (b) Co sample B (0.4 V, room temperature).
nanowire (Hf ) 2πMs; for Ni nanowires, Hf ) 3140 Oe); and magnetocrystalline anisotropy energy (Hm ) -2k1/µ0Ms, where k1 is the magnetocrystalline anisotropy coefficient; for Ni nanowires, Hm ) 140 Oe along the [111] direction). By neglecting Hm, the theoretical effective coercive field of the nanowires is given by Hc| ) Hd + Hf. For Ni nanowires with diameter 50 nm, Hc| ) 1010 Oe. The measured Hc| ()1000 Oe) is consistent with the theoretical result. Following the same principles, single-crystalline Co nanowires have been fabricated by use of a Co sulfate electrolyte (270 g of CoSO4‚7H2O/50 g of CoCl2‚6H2O/40 g of H3BO3‚ 2H2O) with a pH value of 2.5 at room temperature and 1.0 V (Co sample A). The crystalline structure of Co nanowires greatly depends on the pH value of the electrolyte.21,22 Co nanowires with fcc or hexagonal close-packed (hcp) structures can be produced by changing the pH value. In our experiments, Co nanowires have the hcp structure, as shown in Figure 5. The strong (100) peak in the XRD pattern (see Figure 5a) indicates that the Co nanowires are [100]-preferred-oriented. The magnetization hysteresis loops in Figure 6a were measured for Co sample A, that is, the Co nanowires embedded in the AAO template (50 nm diameter and 100 nm interpore spacing) at two different magnetic field directions, parallel or perpendicular to the nanowires’ long axis, respectively. For Co sample A the coercivity Hc (Hc|) ) 1800 Oe, and the remanent magnetization Mr (Mr|) equals 80.8% of the saturated data Ms. The Hc and Mr values are both rather low at perpendicular field direction. It is worthy to note that the coercivity and magnetic squareness of
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Figure 6. Magnetization curves of Co nanowires embedded in the AAO template: (a) Co sample A; (b) Co sample B. (s) Applied magnetic field parallel to the long axis; (---) perpendicular field.
sample A are comparable to the best sample (pH ) 6.4) in ref 21, although the diameter of our Co sample A (50 nm) is much larger than that of their sample in ref 21 (30 nm). The possible reason should attribute to the much better crystallinity in our samples. Co sample B was prepared under lower deposition voltage 0.4 V. As shown in Figure 5b, the (002) and (110) diffractions that are not observable in Figure 5a are now observed, though in very low intensity. For Co sample B, the coercivity Hc (Hc|) ) 1620 Oe and the remanent magnetization Mr (Mr|)/Ms ) 67.8% (Figure 6b). Both are lower than those of Co sample A due to the reduction of single crystallinity in Co sample B. The anisotropy of Co sample B is less large also. It should be noted that the easy magnetic axis of bulk Co is [001] so that the magnetic properties (remanence and coercivity) of Co nanowires would be further improved when [001]-oriented Co nanowires are fabricated. For Co nanowires with diameter 50 nm, Hd ) 5960 Oe (Ms ) 1400 emu/cm3, the Co bulk value of the saturation magnetization), Hf ) 8790 Oe, and average magnetocrystalline anisotropy energy Hm ) 6400 Oe. Physically, the theoretical effective coercive field of the Co nanowires is given by Hc| ) Hd + Hf. For Co nanowires with diameter 50 nm, Hc| ) 2830 Oe. The measured Hc| ()1800 Oe) is smaller than the theoretical result because of the strong dipolar interactions among the nanowires. Conclusions In summary, single-crystalline Ni and Co (high melting point metals) could be produced by use of AAO-template elec-
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