Pulse electrodeposition of Ni2Sb nanowires in polycarbonate template

Solid State Communications 166 (2013) 56–59

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Pulse electrodeposition of Ni2Sb nanowires in polycarbonate template Babak Jaleh n, Afshan Omidvar Dezfuli Physics Department, Bu-Ali Sina University, Hamedan, Iran

art ic l e i nf o

a b s t r a c t

Article history: Received 5 February 2013 Received in revised form 8 May 2013 Accepted 14 May 2013 by F. Peeters Available online 20 May 2013

Large aspect ratio nickel antimony (Ni2Sb) nanowire alloys were synthesized by a pulsed electrodeposition technique into ion track etched polycarbonate membranes. The morphological properties of the nanowires were studied by scanning electron microscopy (SEM) and the results showed that almost each nanowire had the same length of ≈4 μm and the diameter of 90 nm. X-ray diffraction pattern showed that new nanowire alloy of nickel antimony has been synthesized. The chemical composition was determined by examining of the energy dispersive X-ray (EDX) spectra. EDX analysis showed that the atomic ratio of Ni–Sb was very close to 2:1. & 2013 Elsevier Ltd. All rights reserved.

Keywords: A. Nanowire A. Nickel antimony alloy B. Pulse electrodeposition C. Ni2Sb

1. Introduction One-dimensional structures such as nanowires, nanorods, nanotubes, and nanobelts exhibit unusual behavior compared to their bulk counterparts; e.g. the manifestation of quantum phenomena in the electron transport of metallic nanowires [1], enhancement of the mechanical strength and hardness [2,3], the interesting magnetic properties including giant magnetic resonance of nanowires [4], and decay of wires into chains of nanospheres stimulated by Rayleigh instability [5,6]. In particular, nanowires are very suitable for investigating the dependency of physico-chemical properties on size reduction, and are tipped to play an important role both as interconnects and functional units in future electronic, optoelectronic, electrochemical, sensoric, and electromechanical devices with nanoscale dimensions [7,8]. Currently, carbon-based materials are the most commonly used anodes in commercial Li-ion batteries. A great deal of work and significant efforts have been dedicated to improving their performance and to finding new anode materials to enhance the Li-storage capacities and cycling stabilities [9,10]. Intermetallic compounds, such as antimony-based alloy materials, are regarded as alternative anode materials in Li-ion batteries due to their high Li-storage capacities and rate capabilities. It was found that, unlike Li, some binary antimonides, such as Co–Sb [11], Cu–Sb [12], Sn–Sb [13], and Ni–Sb [14], have large reversible electrochemical capacities and appropriate potentials. On the other hand, NixSby, as a n

Corresponding author. Tel.: +98 912 211 4707; fax: +98 837 1470. E-mail addresses: [email protected], [email protected] (B. Jaleh).

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kind of transition metal antimonide, has attracted tremendous research interest for its promising applications in magnetic, thermoelectric and electrochemical areas during the last decade [15–18]. Nanowires can be fabricated by diverse techniques including lithographic patterning [19,20] vapor transport techniques [21–24], template based-synthesis methods [8,25] and other synthesis methods [26–33]. The template-based synthesis involving electrodeposition is more promising owing to its specific advantages of low cost and control over the nanowire properties via changing the electrolyte composition, pH, temperature, and applied potential/current [34]. Possin was the first to report the electrodeposition of nanowires using a template-based method [35]. A comprehensive overview of the membrane/template based preparation method for a wide variety of nanowires has been covered in various reviews [36,37]. To the best of our knowledge, there are no prior reported studies focused on pulse electrodeposited Ni2Sb nanowires. Pulse electrodeposition is preferred over direct deposition because the latter does not result in uniform filling of the pores due to excessive cathodic side reactions. In addition, pulsing avoids excessive hydrogen evolution which may affect the deposition rate [38]. Pt counter electrode and a saturated calomel electrode (SCE) were used as the reference electrode. All the experiments were carried out with a polarograph instrument (Computrac polarography, Metrohm model 797 VA). Membranes were dissolved in methylene chloride (CH2Cl2) and sonicated for 30 min to release the nanowires. Thereafter, nanowires were separated from CH2Cl2 by centrifugation and dried in the oven at 110 1C for 12 h.

B. Jaleh, A. Omidvar Dezfuli / Solid State Communications 166 (2013) 56–59

2. Materials and methods The pulsed electrodeposition of Ni2Sb nanowires were performed in a computer-controlled, three-electrode electrochemical cell. Commercially available ion track-etched polycarbonate templates (PC) with pore-diameter of 100 nm and pore length of 4 μm (Millipore, USA) were used as a template material for growing nanowires. A gold layer of approximately 100 nm thick was sputtered at the backside of this membrane. The gold-sputtered membrane was used as the working electrode of the electrodeposition process, and a platinum wire was used as the counter electrode. The fabrication was performed at 298 K at a voltage from −2.0 to −3.0 V. The typical electrolyte for the electrodeposition of Ni2Sb nanowires contains 0.01 M K(SbO)C4H4O6
1/2H2O, 0.2 M NiCl2
6H2O, 0.18 M H3BO3. Moreover, the pH value of the final electrolyte was adjusted to 3.5. Both the pulse and delay times were 1 ms. Sample structure was studied using X-ray diffraction (Philips powder diffractometer type PW 1373 gonimeter). The XRD was equipped with a graphite monochromator crystal. The X-ray wavelength was 1.5405 Å and diffraction patterns were recorded in a 2θ range (10–801), with a scanning speed of 2 deg/min. To obtain the SEM images of electrodeposited nanowires, polycarbonate templates were dissolved in dichloromethane, the remains consisted of nanowires, and a gold layer was served as a sample for SEM observation. The sample morphology and images of nanowires were studied by VEGA TEScan SEM and by transmission electronic microscopy (TEM, Philips, CM120, 100 KV). Chemical composition of the prepared nanostructures was measured by EDX.

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affects the crystalline properties of the nanowire [41,42]. To obtain a high-filling and uniform Ni2Sb nanowire array in nanopores of the PC template, several factors should be considered. Firstly, the PC template should be ultrasonicated in water for a few minutes to remove impurities and air bubbles inside the nanopores before electrodeposition because ions will preferentially nucleate and grow with impurities, causing the heterogeneous nanowire growth. Air bubbles will further hinder the ion diffusion into the nanopores of the PC template, causing electrodeposition on the surface of the PC template. Secondly, a suitable deposition rate is critical to obtain dense, ordered, and uniform nanowires. To avoid the rapid nucleation and growth and hence inhomogeneous concentration gradients in the nanopores, the pulsed electrodeposition technique is employed, which allows better control over the deposition parameters (such as deposition rate and ion concentration at the deposition interface, as compared with the direct and alternating current deposition techniques) [43,44]. In this paper, the pulsed time in each pulse cycle was sufficiently short (1 ms) so that only a small number of metal ions at the interfaces were consumed, and the metal ions of consumption were recruited in the delayed time (1 ms) through diffusion. Because of this short pulse interval, the crystal growth was hindered, epitaxial growth was reduced and growth trends were changed, so it was difficult to obtain a thick crystal [44]. No evident concentration gradient near the reaction interface existed during the deposition, and the pulse time controls the atom-toatom deposition of the nanowires, which increases the crystallinity and compositional homogeneity of the nanowires. According to Eq. (3), the proper choice of the pH of the electrolyte is also important. 3.2. Crystal structure and morphology

3. Results and discussion 3.1. Fabrication and composition Obtaining appropriate stoichiometric compositions is the most important aspect of Ni2Sb nanowires fabricating. To fabricate the stoichiometric Ni2Sb nanowires, we fixed electrolysis temperature at 298 K, pH value of the electrolyte at 3.5 and with the concentration of Sb3+ ions in the electrolyte of 0.01 M. The electrodeposition process of nanowires basically involves diffusion, adsorption and reaction [39,40]. The alloying process for Ni2Sb nanowires could be described using three steps as follows: (i) Diffusion and the adsorption of [(SbO) C4H4O6]− and Ni2+ ions to the Au electrode surface by the electric field applied between two electrodes. (ii) The adsorbed [(SbO) C4H4O6]− and Ni2+ acquire electrons to form elemental Sb and Ni by the following reactions: (1)

(2) Ni2++2e−-Ni(s) (iii) The reduced Sb and Ni atoms react with each other to form Ni2Sb; the overall reaction can be expressed as follows: 2Ni2++[(SbO)

−

C4H4O6]−+2H++7e−-Ni2Sb(s)+H2O+C4H4O62 (3)

Then, the nucleated Ni2Sb species grows to form Ni2Sb nanowires in the nanopores of the PC. Using the electrodeposition conditions developed in this work, the chemical reaction shown in Eq. (3) proceeds without any other by products. Step (i) is dependent on the current density or potential. Steps (i) and (ii) determine the composition of the nanowires, and step (iii) mainly

300

Ni2Sb

250 Intensity (a.u.)

[(SbO)C4H4O6]−+2H++3e−-Sb(s)+H2O+C4H4O62−

Fig. 1 shows the XRD pattern of the synthesized nanowire. There is no JCPDS card referring to this pattern. Moreover, peaks of pure Ni [7] and Sb [45] are not observed at this XRD pattern. Fig. 2(a) shows typical surface and cross-sectional SEM images of the empty PC template. The PC template has an ordered pore array, with an average pore size of approximately 90 nm. The morphology of the Ni2Sb nanowire array after different etching times is represented in Fig. 2(b) and (c). Fig. 2(b) shows the crosssectional SEM micrograph of the Ni2Sb nanowire array after etching for 30 s, and the photograph indicates that the nanowires are high filling (approximately 98%), uniformly distributed, highly ordered, and parallel to each other. The results shown in Fig. 2(b) are ubiquitous, and when the SEM probe moves to the different areas on the sample, the same image is always observed. Exposed parts of the nanowires increases with increasing etching time.

200 150 PC

100

Ni2Sb Ni2Sb Ni2Sb

50 0

10

15

20

25

30

35

40

45

50

Fig. 1. (Color online) XRD pattern of Ni2Sb nanowire arrays.

55

60

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Fig. 2. SEM image of PC template and Ni2Sb nanowire arrays: (a) surface view of PC template, (b) cross-sectional view (after etching for 30 s), (c) top view (after etching for 2 min) and (d) cross-sectional view (after etching for 5 min) of Ni2Sb nanowire array.

deposition pulse time. This is because the deposition of the nanowires starts at the bottom pores of the Au cathode, and then the nanowires grow up to the top of the PC template along the pores. Notably, the Ni2Sb nanowires have the same lengths and diameters, implying that the electrodeposition process is well controlled and that all the nanowires grow at the same rate along the pores. A typical TEM image of the Ni2Sb nanowires is shown in Fig. 3. It is evident that the Ni2Sb nanowires have a smooth surface and a high aspect ratio. The diameters of the nanowires are uniform and equal to the pore size (90 nm) of the PC used, which is in agreement with the SEM result. The EDX spectrum for typical Ni2Sb alloy nanowires, which proves that the nanowires consist of only Ni and Sb, is shown in Fig. 4. The quantitative analysis of the EDX spectrum indicates that the atomic ratio of Ni to Sb is approximately 2:1 (Table 1).

4. Conclusion Fig. 3. TEM images of the Ni2Sb nanowires.

Fig. 2(d) shows that the length and diameter of the Ni2Sb nanowires are between about 4 mm and 90 nm, which correspond to the thickness and the pore diameter of the PC used, respectively. The SEM indicated that the length of the nanowires could be modulated by changing the thickness of the PC or the

In summary, high-filling and ordered Ni2Sb nanowire arrays, with uniform lengths and diameters, were successfully prepared by the pulsed electrodeposition technique. The alloying process will perfectly proceed unless the voltage value achieves −2.0 V, which causes the co-deposition of these two kinds of atom. This method displays a simple, quick and economic route. Furthermore, it confirms that the co-deposition parameters play an important role in the composition of the product. Further study on the investigation of its electrochemical properties served as a Li-ion battery anode material is underway.

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Fig. 4. (Color online) EDX of the Ni2Sb nanowires deposited in polycarbonate template.

Table 1 EDX elemental composition of the Ni2Sb nanowires. Element

Wt%

At%

Ni Sb Total

50.39 49.61 100.00

67.82 32.18 100.00

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