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DOI: 10.1039/C5RA03750F (Communication) RSC Adv., 2015, 5, 39620-39624

Electrodeposition of SmCo alloy nanowires with a large length-diameter ratio from SmCl3–CoCl2–1-ethyl-3-methylimidazolium chloride ionic liquid without template

Yuqi Chen, Heng Wang and Bing Li*
East China University of Science and Technology, 130 Meilong Road, Shanghai, 200237, China Web: drlibing@163.com

Received 3rd March 2015 , Accepted 23rd April 2015

First published on 24th April 2015

Abstract

SmCo alloy nanowires were first electrodeposited from SmCl3–CoCl2–1-ethyl-3-methylimidazolium chloride (EMIC) ionic liquid at constant potential without template. The effects of SmCl3 concentration, potential, temperature and electrodeposition time on the diameter and morphology were examined. The SmCo alloy nanowires have shown an adjustable mole ratio of Sm and Co by changing the potential and ionic liquid composition. The smallest diameter was about 50–60 nm, far smaller than the diameter of Co nanowires electrodeposited from CoCl2–EMIC, and the crystallinity of SmCo nanowires was improved by increasing SmCl3 concentration. This finding has provided a meaningful method to electrodeposit nanowires with a smaller diameter and larger length-diameter ratio.

1. Introduction

SmCo alloys have important applications in microelectronic mechanical systems and high density recording media for high temperature application, ultrahigh magnetic anisotropy and high saturation magnetization.1–4 They are usually synthesized by physical methods such as pulsed laser deposition and sputtering.5–7 Compared with the above methods, electrodeposition is subject to controlling the morphology and ingredient of the deposits by a simple equipment and facile operation.8 However, electrodeposition of Sm has been restricted in nonaqueous electrolytes such as high temperature and room temperature molten salts. Room temperature molten salts, also named as ionic liquids (ILs), have been regarded as the promising electrolyte to electrodeposit SmCo alloys.9,10

Compared to the general magnetic films, the ordered arrays of nanowires have shown a higher area recording density and attracted much attention for the application of ultrahigh density perpendicular magnetic recording.11,12 Some magnetic nanowires, such as FeCo and SmCo alloys have been electrodeposited from ILs by using aluminum oxide nanoporous templates,13,14 however, the templates have to be prepared before the electrodeposition and dissolved after the electrodeposition. On the contrary, template-free method is a simple and time-saving process. For example, Al, CuSn, FeCoZn and amorphous Co nanowires have been electrodeposited from ILs without templates,15–18 though the formation mechanism for nanowires has remained unclear.

Inspired by the template-free method, we synthesized a new kind of IL (SmCl3–CoCl2–EMIC) and reported the direct electrodeposition of SmCo alloy nanowires without template. The influences of electrochemical parameters on the morphology, composition and crystallinity were researched.

2. Experimental

The ILs were prepared by mixing proper amounts of EMIC (ACROS, 97%), CoCl2 and SmCl3 (both from Alfa Aesar, 99.9%) in a beaker and heated to the required temperature in a Ar filled glove box. During the electrochemical measurements, three-electrode cell was used, in which a W wire (99%) was used as the working electrode for voltammetric study and a W foil for electrodeposition, a Co foil (Aladdin, 99.5%) as the counter electrode, the reference electrode was prepared by immersing a Co wire (Aladdin, 99.5%) into a 40[thin space (1/6-em)]:[thin space (1/6-em)]60 mol% CoCl2–EMIC contained in a fritted glass tube. All the electrodes were polished with emery papers to a mirror finish, washed with acetone and distilled water, then dried in a vacuum oven. All the electrochemical measurements were performed using a PAR-STAT2273 (PAR-Ametek Co, Ltd) with a PowerSuite software package. After the constant potential electrodeposition, the working electrode was washed with acetonitrile and acetone, then dried in air.19,20 Differential scanning calorimetry (DSC, DMA2980/DS) was used to measure melting points. Scanning electron microscopy (SEM, JEOLJSM-6360LV), transmission electron microscope and high-resolution transmission electron microscopy (TEM/HRTEM, JEM-2100, 200 kV) were employed to characterize the morphology, an energy dispersive X-ray spectroscopy (EDS, Falcon) was used to monitor the chemical composition, X-ray diffraction (XRD, D/Max2550), selected area electron diffraction and Fourier transformation (SAED/FFT, JEM-2100, 200 kV) were employed to analyze the structure. All the experiments were carried out inside a Ar filled glove box (O2 and H2O < 1 ppm).

3. Results and discussion

Fig. 1(a) compares the cyclic voltammograms (CVs) recorded on a W electrode in various CoCl2–EMIC ILs. As the mole ratio is 26[thin space (1/6-em)]:[thin space (1/6-em)]60, no reduction and oxidation peaks appear. In the IL (40[thin space (1/6-em)]:[thin space (1/6-em)]60 mole ratio), a reduction peak A at −0.5 V and the corresponding oxidation peak at 0.2 V are observed, which are correlated to Co2+ ions reduction and subsequent oxidation. As it is 60[thin space (1/6-em)]:[thin space (1/6-em)]60, the reduction current density is much higher, demonstrating that there are more electrochemically active Co2+ ions in the IL.18 CVs in various SmCl3–CoCl2–EMIC ILs are shown in Fig. 1(b). In the SmCl3–EMIC IL (10[thin space (1/6-em)]:[thin space (1/6-em)]60 mole ratio) without CoCl2, no reduction and oxidation peaks appear in the range of 0.6 V to −1.3 V, indicating that Sm3+ can't be reduced alone within the potential range. But reduction peaks B, C, D appear in SmCl3–CoCl2–EMIC IL (10[thin space (1/6-em)]:[thin space (1/6-em)]40[thin space (1/6-em)]:[thin space (1/6-em)]60 mole ratio). The reduction peak B at −0.5 V is similar to peak A, only the onset reduction potential is 0.2 V more positive than that in CoCl2–EMIC, moreover the reduction current is far larger. In order to indentify the reasons for the differences, constant potential electrolysis in SmCl3–CoCl2–EMIC IL (10[thin space (1/6-em)]:[thin space (1/6-em)]40[thin space (1/6-em)]:[thin space (1/6-em)]60 mole ratio) at −0.30 V is carried out for 1200 s at 120 °C. The obtained deposit is composed of SmCo alloy with a mole ratio of Sm and Co of 1[thin space (1/6-em)]:[thin space (1/6-em)]123.9 by EDS analysis, implying that the main reduction species is Co2+ ions with very small amounts of Sm3+ ions. Obviously, the co-reduction of Sm3+ ions with Co2+ ions significantly improves the reduction current density due to the formation of SmCo alloy, which not only releases formation enthalpy but also presents more nucleus as electroactive intermediates because of a polynuclear complex containing Co2+ and Sm3+ in the electrolyte.21 The mole percentage of Sm in SmCo alloys gradually enhances as the applied potential is more negative. The SmCo alloy electrodeposited at −0.85 V has a mole ratio of Sm and Co of 1[thin space (1/6-em)]:[thin space (1/6-em)]0.61. So the two reduction peaks (C and D) at −0.85 V are attributed to the Co2+ induced bulk Sm3+ reduction. Three plateaus at −0.3 V, −0.8V and −1.5 V, respectively in chronopotentiograms in the SmCl3–CoCl2–EMIC in Fig. 1(c) are observed as the current density increases from −0.42 to −4.24 mA cm−2, which are associated to the co-electrodeposition of Co2+ ions with a modicum of Sm3+ ions, Co2+ with abundant Sm3+ ions, respectively, and EMI+ reduction. The chronoamperometric curves, at −0.90 V, −0.91 V and −0.92 V, respectively are used to explore the SmCo nucleation mechanism in the inset of Fig. 1(d). Fig. 1(d) has shown the theoretical and experimental plots of (I/Im) vs. (t′/t′m), which implies that SmCo nuclei on W electrode follow the 3D instantaneous nucleation growth process, in well agreement with the Co nucleation mechanism on the W electrode.18
image file: c5ra03750f-f1.tif
Fig. 1 (a) Cyclic voltammograms recorded on a W electrode in different mole ratio (60[thin space (1/6-em)]:[thin space (1/6-em)]60, 40[thin space (1/6-em)]:[thin space (1/6-em)]60, 26[thin space (1/6-em)]:[thin space (1/6-em)]60) CoCl2–EMIC ILs at 120 °C, scan rate = 50 mV s−1. (b) Cyclic voltammograms recorded on a W electrode in different mole ratio (10[thin space (1/6-em)]:[thin space (1/6-em)]40[thin space (1/6-em)]:[thin space (1/6-em)]60, 0[thin space (1/6-em)]:[thin space (1/6-em)]40[thin space (1/6-em)]:[thin space (1/6-em)]60, 10[thin space (1/6-em)]:[thin space (1/6-em)]0[thin space (1/6-em)]:[thin space (1/6-em)]60) SmCl3–CoCl2–EMIC ILs at 120 °C, scan rate = 50 mV s−1. (c) Chronopotentiograms recorded at different current densities. (d) Non-dimensional (I/Im)2–(t′/t′m) curves recorded at different potentials. Inset: current–time transients of the chronoamperometric experiments.

By series of experiments, the Co nanowires obtained in CoCl2–EMIC (40[thin space (1/6-em)]:[thin space (1/6-em)]60 mole ratio) at −0.65 V for 1200 s at 120 °C show an average diameter of 530 nm in Fig. 2(a). As the mole ratio of CoCl2–EMIC is 60[thin space (1/6-em)]:[thin space (1/6-em)]60, the thinnest Co nanowires with an average diameter of 500 nm are obtained at −0.68 V for 1200 s at 90 °C in Fig. 2(b). The length of Co nanowires is about 6 μm.


image file: c5ra03750f-f2.tif
Fig. 2 SEM images of deposits obtained from CoCl2–EMIC, 1200 s, (a) 120 °C, −0.65 V 40[thin space (1/6-em)]:[thin space (1/6-em)]60 mol%; (b) 90 °C, −0.68 V, 60[thin space (1/6-em)]:[thin space (1/6-em)]60 mol%.

As a new kind of IL, SmCl3–CoCl2–EMIC is synthesized by the introduction of SmCl3 into CoCl2–EMIC IL. Table 1 gives the melting points of various SmCl3–CoCl2–EMIC ILs. Compared to CoCl2–EMIC, the melting points of SmCl3–CoCl2–EMIC decrease.

Table 1 Melting points of various ILs
  Composition of SmCl3–CoCl2–EMIC in mol%
0[thin space (1/6-em)]:[thin space (1/6-em)]40[thin space (1/6-em)]:[thin space (1/6-em)]60 1.25[thin space (1/6-em)]:[thin space (1/6-em)]40[thin space (1/6-em)]:[thin space (1/6-em)]60 1.67[thin space (1/6-em)]:[thin space (1/6-em)]40[thin space (1/6-em)]:[thin space (1/6-em)]60 2.5[thin space (1/6-em)]:[thin space (1/6-em)]40[thin space (1/6-em)]:[thin space (1/6-em)]60 5[thin space (1/6-em)]:[thin space (1/6-em)]40[thin space (1/6-em)]:[thin space (1/6-em)]60
T (°C) 79 68 67 63 71

As 1.25 mol% SmCl3 is added into the CoCl2–EMIC (40[thin space (1/6-em)]:[thin space (1/6-em)]60 mole ratio), the average diameter of the SmCo nanowires is reduced to 190 nm and the length increases from 6 μm to 25 μm in Fig. 3(a). The morphology has little change when the content of SmCl3 is 2.5 mol% in Fig. 3(b). Based on the previous results, the introduction of small amount of Sm3+ ions into the CoCl2–EMIC causes a higher current density and results in more nuclei formation, so the active species near the electrode are almost completely consumed and a concentration gradient between the bulk IL and the electrode surface is built. Further electrodeposition is subject to carrying out on the top of the nanowires which leads to an oriented growth along the nanowires and a large length-diameter ratio.22,23 However, as SmCl3 concentration increases to 5 mol%, the morphology transforms into films with a rough surface as shown in Fig. 3(c). This different morphology is caused by the decrease in the active species concentration gradient between the electrode and bulk electrolyte due to SmCl3 concentration change in the IL.


image file: c5ra03750f-f3.tif
Fig. 3 SEM images of deposits obtained from SmCl3–CoCl2–EMIC ILs at 120 °C for 1200 s, (a) 1.25[thin space (1/6-em)]:[thin space (1/6-em)]40[thin space (1/6-em)]:[thin space (1/6-em)]60 mol%, −0.65 V; (b) 2.5[thin space (1/6-em)]:[thin space (1/6-em)]40[thin space (1/6-em)]:[thin space (1/6-em)]60 mol%, −0.65 V; (c) 5[thin space (1/6-em)]:[thin space (1/6-em)]40[thin space (1/6-em)]:[thin space (1/6-em)]60 mol%, −0.65 V; (d) 1.25[thin space (1/6-em)]:[thin space (1/6-em)]40[thin space (1/6-em)]:[thin space (1/6-em)]60 mol%, −0.60 V; (e) 1.25[thin space (1/6-em)]:[thin space (1/6-em)]40[thin space (1/6-em)]:[thin space (1/6-em)]60 mol%, −0.70 V.

The nanowires electrodeposited at −0.60 V show a diameter of about 240 nm with lots of tiny branches in Fig. 3(d). At −0.70 V, the nanowires exhibit the biggest diameter of about 300 nm with a rather rough surface for the isotropic growth.16 Obviously, among the applied potentials, the nanowires obtained at −0.65 V have given the smallest diameter with smooth surface.

The nanowires' diameter and morphology are controlled by both ions mass transport rate (IMTR) and ions consumption rate (ICR) during electrodeposition. When they are in a balanced condition such as under an appropriately applied potential, the nanowires have shown a relatively smaller diameter and smoother surface. At a lower over-potential condition, the smaller ICR will result in less nanowires formation and isotropic growth.23 Under a larger over-potential, though the ICR is faster, the IMTR is also enhanced caused by the high temperature as a result of the Joule heating effects and more Sm3+ ions involvement in the electrodeposition, therefore, the active species concentration near the electrode is still in a higher concentration, the nanowires grow in all directions. Usually, the high current density is beneficial to the formation of nanowires with a large length-diameter ratio.22,24 The ILs near the electrode cannot supply enough active ions for reduction, leading to a fast growth toward the top of the nanowires. Finally, numerous nanowires with a smaller diameter are electrodeposited.

As shown in Fig. 4(a), the average diameter of the SmCo nanowires obtained in 1.67[thin space (1/6-em)]:[thin space (1/6-em)]60[thin space (1/6-em)]:[thin space (1/6-em)]60 mol% SmCl3–CoCl2–EMIC at −0.68 V for 1200 s at 90 °C is reduced to 80–120 nm and the length enlarges from 6 μm to 25 μm. At 100 °C, the diameter is 240 nm and tiny branches appear on the edge in Fig. 4(b), confirming that higher temperature facilitates to enlarge the diffusion rate of active species and enhance the concentration of the active species near the electrode, therefore, more nuclei are subject to formation in various directions. Fig. 4(c) shows the average diameter is reduced below 100 nm with a length of 10 μm as the electrodeposition time decreases to 400 s. The deposits are removed from the substrate and examined by TEM as shown in Fig. 4(d), in which the smallest nanowires' diameter is about 50–60 nm and the mole ratio of O on the nanowires' surface area is higher than that in the middle area. In summary, lower temperature and less electrodeposition time can reduce the nanowires' diameter in a certain degree.15


image file: c5ra03750f-f4.tif
Fig. 4 SEM images of deposits obtained from 1.67[thin space (1/6-em)]:[thin space (1/6-em)]60[thin space (1/6-em)]:[thin space (1/6-em)]60 mol% SmCl3–CoCl2–EMIC at −0.68 V, (a) 1200 s, 90 °C; (b) 1200 s, 100 °C; (c) 400 s, 90 °C; (d) TEM images of (c).

The mole ratio of Sm and Co elements in the SmCo deposits is analyzed by EDS and listed in Table 2. By comparison, the Sm content in the SmCo alloy nanowires improves by applying a larger overpotential and higher SmCl3 concentration. Therefore, the mole ratio of Sm and Co is adjustable. In the XRD patterns, it is hardly seen any diffraction peaks of both Co and SmCo alloys because of the poor crystallinity (did not show here). The XRD patterns exhibit a summary statistic of the whole nanowires, the selected area electron diffraction (SAED) can observe the crystallinity of very small area. The SAED analysis for the Co nanowires obtained from 40[thin space (1/6-em)]:[thin space (1/6-em)]60 mol% CoCl2–EMIC shows an amorphous Co in Fig. 5(a), in well agreement with the FFT analysis result in the inset of Fig. 5(a). When 1.67 mol% is introduced into CoCl2–EMIC, blurry single crystal diffraction spots appear as shown in Fig. 5(b). The SmCo nanowires electrodeposited from 2.5[thin space (1/6-em)]:[thin space (1/6-em)]40[thin space (1/6-em)]:[thin space (1/6-em)]60 mol% SmCl3–CoCl2–EMIC have presented apparent single crystal diffraction spots in some areas caused by α-Co with zone axis of [001] in Fig. 5(c). It is hard to distinguish the interatomic spacing of the SmCo nanowires from the HRTEM images, and some areas in FFT also show amorphous state in the inset of Fig. 5(c). The above results indicate that the deposited SmCo nanowires have shown a mainly amorphous state, but the crystal quality is improved by increasing SmCl3 concentration.

Table 2 The EDS analysis of SmCo alloy nanowires
Sm[thin space (1/6-em)]:[thin space (1/6-em)]Co (mol%) Composition of SmCl3–CoCl2–EMIC in mol%
Applied potential 1.25[thin space (1/6-em)]:[thin space (1/6-em)]40[thin space (1/6-em)]:[thin space (1/6-em)]60 2.5[thin space (1/6-em)]:[thin space (1/6-em)]40[thin space (1/6-em)]:[thin space (1/6-em)]60 10[thin space (1/6-em)]:[thin space (1/6-em)]40[thin space (1/6-em)]:[thin space (1/6-em)]60
−0.60 V 1[thin space (1/6-em)]:[thin space (1/6-em)]35.2 — —
−0.65 V 1[thin space (1/6-em)]:[thin space (1/6-em)]14.99 1[thin space (1/6-em)]:[thin space (1/6-em)]11.12 1[thin space (1/6-em)]:[thin space (1/6-em)]6.12
−0.70 V 1[thin space (1/6-em)]:[thin space (1/6-em)]5.68 — 1[thin space (1/6-em)]:[thin space (1/6-em)]1.68


image file: c5ra03750f-f5.tif
Fig. 5 Deposits obtained from different SmCl3–CoCl2–EMIC ILs at −0.65 V for 1200 s at 120 °C. (a) 0[thin space (1/6-em)]:[thin space (1/6-em)]40[thin space (1/6-em)]:[thin space (1/6-em)]60 mol% SAED, inset: FFT; (b) 1.67[thin space (1/6-em)]:[thin space (1/6-em)]40[thin space (1/6-em)]:[thin space (1/6-em)]60 mol% SEAD, inset: FFT; (c) 2.5[thin space (1/6-em)]:[thin space (1/6-em)]40[thin space (1/6-em)]:[thin space (1/6-em)]60 mol% SEAD, inset: FFT; (d) 2.5[thin space (1/6-em)]:[thin space (1/6-em)]40[thin space (1/6-em)]:[thin space (1/6-em)]60 mol% HRTEM.

4. Conclusions

For the first time, SmCo alloy nanowires were successfully electrodeposited from SmCl3–CoCl2–EMIC IL. Without using a template, this approach greatly simplified the preparation. The addition of SmCl3 to the CoCl2–EMIC IL resulted in a 0.2 V more positive potential for Co2+ ions onset reduction potential and a higher current density. The smallest diameter of SmCo obtained in 1.67[thin space (1/6-em)]:[thin space (1/6-em)]60[thin space (1/6-em)]:[thin space (1/6-em)]60 mol% SmCl3–CoCl2–EMIC at −0.68 V for 400 s at 90 °C was 50–60 nm, far smaller than the diameter of the Co nanowires of 500 nm. The nanowires' diameter could be reduced by lower temperature and less electrodeposition time. The crystallinity of SmCo nanowires improved when more SmCl3 was added and the mole ratio of Sm and Co was adjustable by changing the potential and IL composition.

Acknowledgements

This work was financially supported by National Natural Science Foundation of China (no. 51474107).

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