Fabrication of cobalt nanowires from mixture of 1-ethyl-3-methylimidazolium chloride ionic liquid and ethylene glycol using porous anodic alumina template

Article abstract of DOI:10.1016/j.electacta.2008.06.064

Porous anodic alumina template is synthesized by electrochemical anodization of aluminum and used to grow cobalt nanowires. The cobalt nanowires produced by direct current electrodeposition are characterized by field emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction and physical property measurement system. Test results indicate that the average diameter of cobalt nanowires is about 45 nm, which is generally the same as the pore diameter of porous anodic alumina template, and the cobalt nanowires electrodeposited from mixture of 1-ethyl-3-methylimidazolium chloride ionic liquid and ethylene glycol have a smoother surface and better magnetic properties than cobalt nanowires electrodeposited from aqueous solution, and they show a better squareness. Therefore it can be concluded that the cobalt nanowires electrodeposited from mixture of 1-ethyl-3-methylimidazolium chloride ionic liquid and ethylene glycol using porous anodic alumina template can be used as a perpendicular magnetic recording film.

Full text of DOI:10.1016/j.electacta.2008.06.064

Electrochimica Acta 54 (2008) 763–767
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Fabrication of cobalt nanowires from mixture of 1-ethyl-3-methylimidazolium
chloride ionic liquid and ethylene glycol using porous anodic alumina template
a,b,∗
b
b
a
Peixia Yang
, Maozhong An , Caina Su , Fuping Wang
a
Research Station on Material Science and Engineering for Postdoctoral Fellows, Harbin Institute of Technology, Harbin 150001, China
Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Porous anodic alumina template is synthesized by electrochemical anodization of aluminum and used to
grow cobalt nanowires. The cobalt nanowires produced by direct current electrodeposition are character-
ized by field emission scanning electron microscopy, transmission electron microscopy, X-ray diffraction
and physical property measurement system. Test results indicate that the average diameter of cobalt
nanowires is about 45 nm, which is generally the same as the pore diameter of porous anodic alumina
template, and the cobalt nanowires electrodeposited from mixture of 1-ethyl-3-methylimidazolium chlo-
ride ionic liquid and ethylene glycol have a smoother surface and better magnetic properties than cobalt
nanowires electrodeposited from aqueous solution, and they show a better squareness. Therefore it can
be concluded that the cobalt nanowires electrodeposited from mixture of 1-ethyl-3-methylimidazolium
chloride ionic liquid and ethylene glycol using porous anodic alumina template can be used as a perpen-
dicular magnetic recording film.
Received 7 March 2008
Received in revised form 17 June 2008
Accepted 28 June 2008
Available online 6 July 2008
Keywords:
Cobalt nanowires
Porous anodic alumina template
1
-Ethyl-3-methylimidazolium chloride
ionic liquid
Ethylene glycol
Electrodeposition
© 2008 Elsevier Ltd. All rights reserved.
1. Introduction
and no steam press [13]. To sum up, ionic liquid is a promising
electrolyte for electrodeposition of various metals. Ionic liquid is
One of the traditional methods used to fabricate nanowires
and nanotubes is based on electrodeposition of various materials
in porous anodic alumina (PAA) template [1–4]. PAA template is
now a key material for fabrication of devices on nanoscale, such as
ultrahigh-density magnetic recording devices, and electronic and
optoelectronic devices [5–8].
normally an aprotic medium, and so, it does not have the prob-
lem of hydrogen evolution, which frequently occurs in aqueous
solutions. Cobalt has become very attractive for its special mag-
netic properties. It was reported that metal cobalt coating has been
electrodeposited from ionic liquids. Carlin et al. [14] found that
the electrochemical reduction of cobalt(II) in 1.5:1.0 AlCl :EMIC
3
It was reported in recent years that different magnetic
nanowires have been fabricated using PAA template from aque-
ous solution by electrodeposition [9–12]. Hydrogen evolution is
an issue, when metals are electrodeposited from aqueous solu-
tion. It is difficult to discharge hydrogen bubbles which are held
up inside nanopores. The presence of these hydrogen bubbles has
its adverse effect on the uniformity and the performance of nano-
material. It is a new development trend in recent years to use room
temperature ionic liquids for electrodeposition of various metals.
Ionic liquid can be conveniently referred to as a compound com-
posed entirely of ions, it has the merits of organic electrolytes and
high-temperature molten salts, such as wide working temperature
range, good electrical conductivity, wide electrochemical windows,
(1-ethyl-3-methylimidazolium chloride) room temperature ionic
liquid leads to cobalt metal at potentials positive of +0.4 V and to
CoAlx alloys at potentials negative of +0.4 V. Chen and Sun [15]
reported that the electrodeposition of cobalt and zinc–cobalt alloys
on the nickel electrode was conducted 40.0–60.0 mole percent
◦
(mol%) ZnCl –EMIC containing cobalt chloride at 80 C.
2
It was reported that Kazeminezhad et al. [16] recently fabri-
cated silver nanowires using a commercial nuclear track-etched
polycarbonate template from 1-butyl-3-methylimidazolium hex-
afluorophosphate ([bmim][PF ]) ionic liquid. However, the average
6
diameter of Ag nanowires they produced was much larger than
the nominal pore diameter of the template. To the best of our
knowledge, not much work has been done on the electrodeposi-
tion of magnetic nanowires using PAA template from ionic liquids.
So we synthesized PAA template by electrochemical anodiza-
tion of aluminum and used PAA template to grow Co nanowires.
The Co nanowires were fabricated from mixture of 1-ethyl-3-
methylimidazolium chloride ionic liquid and ethylene glycol and
∗ Corresponding author at: Department of Applied Chemistry, Harbin Institute of
Technology, Harbin 150001, China. Tel.: +86 451 86413721; fax: +86 451 86415527.
E-mail address: yangpeixia@hit.edu.cn (P. Yang).
0
013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2008.06.064

764
P. Yang et al. / Electrochimica Acta 54 (2008) 763–767
from aqueous solution by direct current (dc) electrodeposition,
respectively, and they were characterized by field emission scan-
ning electron microscopy, transmission electron microscopy, X-ray
diffraction and physical property measurement system. Test results
indicate that the average diameter of Co nanowires is about 45 nm,
which is generally the same as the pores diameter of PAA tem-
plate, and the Co nanowires electrodeposited from mixture of EMIC
ionic liquid and ethylene glycol have a smoother surface and better
magnetic properties than the Co nanowires electrodeposited from
aqueous solution, and they show a better squareness. Therefore it
can be concluded that cobalt nanowires electrodeposited from mix-
ture of EMIC ionic liquid and ethylene glycol using PAA template
could be used as a perpendicular magnetic recording film.
2. Experimental
2.1. Instruments
Fig. 1. FE-SEM image of surface of PAA template prepared in 1:1 sulfuric/oxalic acid
◦
The surface and cross-sectional morphology of PAA template
mixture at 26 V and 5 C.
and cobalt nanowires were characterized by field emission scan-
ning electron microscope (FE-SEM, Hitachi S4700). Co nanowires
were characterized by transmission electron microscopy (TEM,
JEM200CX) and selected area electron diffraction (SAED). X-ray
diffraction patterns were recorded using a D/Max-3B X-ray diffrac-
tometer (Rigaku). The magnetic properties of Co nanowires at room
temperature were determined using a physical property measure-
ment system (PPMS, Quantum Design).
were used as the counter and reference electrode, respectively.
Co nanowires were electrodeposited from the aqueous solution
−1
−1
containing 120 g L CoSO ·7H O and 45 g L H BO at room tem-
4
2
3
3
perature. Cobalt nanowires were fabricated by dc electrodeposition
at 1.0 V (SCE) for 2 h, the pH value of electrolyte was adjusted to
2
.0–3.0.
2.5. Electrochemical measurement
2.2. Preparation of porous anodic alumina template
Electrochemical experiments were carried out in
uum atmosphere glove-box system filled with dry nitrogen.
a vac-
Aluminum sheets (thickness: 0.5 mm; purity: >99.99%) were
obtained from Joinworld Co., Ltd. (Xinjiang, China). PAA template
was synthesized using a two-step anodization technique described
in Ref. [17]. The first step anodization and the second step anodiza-
tion were performed in 1:1 sulfuric/oxalic acid mixture at 26 V and
C for 3 h each. After the first step anodization, the formed alu-
EMIC–CoCl –ethylene glycol were poured into the self-made glass
2
electrolytic cell in the glove-box. The electrochemical measure-
ments were performed in a conventional three-electrode cell using
2
◦
a potentiostat/CHI630B. A platinum plate (0.5 cm ) was used as the
5
working electrode (WE), the counter electrode (CE) was a platinum
mina was etched off in aqueous solution containing 1.8% CrO (wt%)
3
2
◦
electrode (1.0 cm ), the reference electrode was a zinc (Aldrich,
and 6% H PO (wt%) at 60 C for 2 h. After the second anodization,
3
4
9
9.999%) wire immersed in a ZnCl /EMIC ionic liquid contained
2
the remaining aluminum substrate was removed by soaking in a
CuCl -based solution (100 ml of HCl (38%) + 100 ml of H O + 3.4 g of
in a glass tube having fine porosity frits at the tip. For the voltam-
metric experiments, the temperature was kept constant at 80 C.
2
2
◦
CuCl ·H O) at room temperature for about 10 min. The perforated
2
2
Before every measurement, the working electrode was immerged
in hydrochloric acid (1:1) for a few minutes, polished successively
with increasingly finer grades of emery paper, followed by sili-
con carbide grit, and finally to a mirror finish with aqueous slurry
of 0.15 ␮m alumina, rinsed with distilled water, and dried under
vacuum.
porous alumina template was prepared by removing the bottom
◦
part (barrier layer) of the template in 5 wt% H PO4 at 35 C. A plat-
3
inum layer of ∼100-nm thick was sputtered on one side of PAA
template, which served as the working electrode in the subsequent
metal electrodeposition process.
2.3. Preparation of cobalt nanowires from mixture of EMIC ionic
liquid and ethylene glycol
3. Results and discussion
The platinum layer on one side of PAA template and the cobalt
sheet with a purity of 99.99% were used as the cathode and the
anode, respectively. Co nanowires were fabricated by dc electrode-
Fig. 1 shows a FE-SEM image of surface morphology of PAA tem-
plate prepared in mixture acid electrolyte at 26 V.
It can be seen from Fig. 1 that the nanopores are uniform and
highly ordered, and have an average diameter of about 45 nm. The
space between nearest-neighboring pores is about 20 nm, slightly
smaller than the pore diameter, the interspace is about 65 nm, and
◦
position at 80 C for 1 h from the electrolyte containing EMIC,
anhydrous CoCl2 and ethylene glycol = 2:1:18 (by molar). After the
experiment, the PAA template embedded with cobalt nanowires
was washed in ethanol and deionized water, and then stored in the
desiccator.
the pore density is about 1010 pore cm
−2
.
A typical stationary staircase cyclic voltammogram recorded on
platinum electrode in the 2:1:18 (by molar) EMIC–CoCl –ethylene
2
2.4. Preparation of cobalt nanowires from aqueous solution
glycol is shown in Fig. 2.
It can be seen from Fig. 2 that the cathodic current begins to
appear at 170 mV (vs. ZnCl /Zn) in the EMIC–CoCl –ethylene gly-
Co nanowires were fabricated in a conventional three-electrode
2
2
electrochemical cell. The platinum layer on one side of PAA
template was used as the working electrode. A cobalt sheet
with a purity of 99.99% and a saturated calomel electrode (SCE)
col melt bath. There are a single reduction wave and a coupled
oxidation wave on the CV curve, the corresponding potentials of
reduction wave and oxidation wave are −150 and 395 mV (vs.

P. Yang et al. / Electrochimica Acta 54 (2008) 763–767
765
As shown in Fig. 3(a), the Co nanowires are uniformly dis-
tributed, highly ordered, and parallel to each other. As byproducts
of the etching process, some residual etched alumina flakes can be
seen on the top of the Co nanowires. As shown in Fig. 3(b), the well-
paralleled nanowires suggest alumina matrix is not completely
dissolved, and so, they remain parallel in the PAA template.
Shown in Fig. 4(a) and (b) are typical TEM images of Co
nanowires electrodeposited from mixture of EMIC ionic liquid and
ethylene glycol and from aqueous solution, respectively. Shown in
Fig. 4(c) is the SAED pattern of Co nanowire electrodeposited from
mixture of EMIC ionic liquid and ethylene glycol.
As shown in Fig. 4(a), the surface of Co nanowire is smooth, an
average diameter is about 45 nm and an average length is about
several micrometers. The average diameter of Co nanowires is gen-
erally the same as the pore diameter of PAA template. It can be seen
from Fig. 4(a) and (b), the surface of Co nanowires electrodeposited
from mixture of EMIC ionic liquid and ethylene glycol is relatively
smoother than that of Co nanowires from aqueous solution. The
SAED pattern of Co nanowire shown in Fig. 4(c) indicates that the
Co nanowire is of a polycrystalline structure.
Fig. 5 illustrates the X-ray diffraction patterns of Co nanowires
embedded within the PAA template from aqueous solution (a) and
from mixture of EMIC ionic liquid and ethylene glycol (b), respec-
tively.
Fig. 2. Cyclic voltammogram for EMIC–CoCl2–ethylene glycol (2:1:18) (by molar)
on Pt electrode at 80 C. Scan rate was 50 mV/s. Electrode area was 0.5 cm .
◦
2
ZnCl /Zn), respectively. There is only a pair of redox waves which
2
corresponds to the deposition and dissolution of Co.
Fig. 3 shows FE-SEM images of surface (a) and cross-sectional
The XRD patterns show the smooth amorphous broad peaks
(
b) morphology of Co nanowires after removing alumina partially
◦
◦
between 20 and 30 come from Al O3 film. We can see from
using 0.5 M NaOH solution.
2
Fig. 3. FE-SEM images of surface (a) and cross-sectional (b) morphology of Co nanowires with alumina partially removed using 0.5 M NaOH solution.
Fig. 4. TEM images of Co nanowires from the electrolyte containing EMIC, anhydrous CoCl2 and ethylene glycol = 2:1:18 (by molar) (a), from an aqueous electrolyte containing
−
1
−1
1
20 g L CoSO4·7H2O and 45 g L H3BO3 (b) and selected area electron diffraction of Co nanowire electrodeposited from mixture of EMIC ionic liquid and ethylene glycol
(
c).

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P. Yang et al. / Electrochimica Acta 54 (2008) 763–767
Fig. 5. X-ray diffraction patterns of Co nanowires from aqueous solution containing 120 g L CoSO4·7H2O and 45 g L−1 H3BO3 (a), from the electrolyte containing EMIC,
−1
anhydrous CoCl2 and ethylene glycol = 2:1:18 (by molar) (b).
Fig. 6. Hysteresis loops of Co nanowires electrodeposited from aqueous solution containing 120 g L CoSO4·7H2O and 45 g L−1 H3BO3 (a), from the electrolyte containing
−1
EMIC, anhydrous CoCl2 and ethylene glycol = 2:1:18 (by molar) (b).
these two X-ray diffraction patterns that the diffraction peaks of
Co nanowires electrodeposited in the nanopores of PAA template
are consistent with those of bulk Co and Co nanowires are of a
hexagonal close-packed (hcp) structure. As shown in Fig. 5(a), the
Co nanowires electrodeposited from aqueous solution only exhibit
three clear diffraction peaks, (0 0 2) diffraction peak is the high-
est, indicating that there is a preferred orientation along the (0 0 2)
direction. However, Co nanowires electrodeposited from mixture
of EMIC ionic liquid and ethylene glycol exhibit six clear diffraction
peaks, indicating that there is not a preferred orientation direction
nanowires inside PAA template show a strong perpendicular mag-
netic anisotropy, which can be used as a perpendicular magnetic
recording film.
The values of coercivity and squareness of Co nanowires are
summarized in Table 1.
As shown in Table 1, Co nanowires fabricated from mixture of
EMIC ionic liquid and ethylene glycol possess the higher squareness
of about 0.92, and Co nanowires fabricated from aqueous solution
have a lower squareness. Co nanowires fabricated from mixture of
EMIC ionic liquid and ethylene glycol have a strong perpendicular
magnetic anisotropy and a moderate coercivity.
(
1 0 0), (0 0 2) and (1 0 1) diffraction peaks interconnect with each
other. Therefore, it can be concluded that Co nanowires grow in a
microcrystal structure. All these results indicate that Co nanowires
electrodeposited from mixture of EMIC ionic liquid and ethylene
glycol are a mixed phase of crystalline and microcrystalline, which
display a polycrystalline structure. Co nanowires have a higher
stress, so the size of Co crystalline grain embedded inside PAA
template is not calculated by full-width half-maximum (FWHM),
possibly because of the confinement of Co nanowires within PAA
template.
The differences in coercivity and squareness are thought to be
related to such factors as the crystalline structure of nanowire, the
crystalline grain size, and the defects in nanowires. Our TEM results
given above indicate that over the entire length the surface of Co
nanowires electrodeposited from mixture of EMIC ionic liquid and
ethylene glycol is smoother than that of Co nanowires electrode-
posited from aqueous solution. The defects reduce the Ms of Co
nanowire. The difficulty of the magnetization reversal increases
with the perfection of crystallite and the deterioration of coercivity.
Moreover, small crystalline grain size leads to a low coercivity. All
these properties show that the perpendicular magnetic anisotropy
of Co nanowires electrodeposited from mixture of EMIC ionic liquid
and ethylene glycol is enhanced, i.e. a good squareness is obtained.
Fig. 6 shows the hysteresis loops of ordered Co nanowires elec-
trodeposited from mixture of EMIC ionic liquid and ethylene glycol
(
a) and from aqueous solution (b), respectively.
As shown in Fig. 6, the hysteresis loops exhibit a low coerciv-
ity (Hc) and squareness of hysteresis loop (Mr/Ms), when the field
is applied perpendicular to the Co nanowires. When the field is
applied parallel to the Co nanowires, the coercivity and square-
ness of Co nanowires are higher than those of bulk Co, i.e. the
preferred direction of magnetization is always along the long axis
of Co nanowire and the hard direction of magnetization is along
the short axis of Co nanowires. These results indicate that Co
Table 1
Magnetic properties of Co nanowires in PAA template
Electrolyte
Squareness (Mr/Ms)
Coercivity (Oe)
Aqueous
Ionic liquid
0.86
0.92
930
900

P. Yang et al. / Electrochimica Acta 54 (2008) 763–767
767
4
. Conclusions
Fund for the Doctoral Program of Higher Education of China
No. 20060213020).
(
It can be seen from the results and discussion above that
porous anodic alumina template is synthesized by electrochemical
anodization of aluminum. Co nanowires are fabricated using PAA
template by dc electrodeposition from mixture of EMIC ionic liquid
and ethylene glycol and from aqueous solution, respectively. The
Co nanowires produced are characterized by FE-SEM, TEM, XRD
and PPMS. Test results indicate that the average diameter of Co
nanowires is about 45 nm, which is generally equal to the pore
diameter of PAA template. Two X-ray diffraction patterns show
that Co nanowires fabricated from mixture of EMIC ionic liquid and
ethylene glycol are of a polycrystalline structure. The Co nanowires
electrodeposited from mixture of EMIC ionic liquid and ethylene
glycol have a smoother surface and better magnetic properties
than Co nanowires electrodeposited from aqueous solution, and
they show a better squareness, so that cobalt nanowires electrode-
posited from mixture of EMIC ionic liquid and ethylene glycol using
PAA template can be applied to a perpendicular magnetic recording
film.
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