Effects of acetonitrile on electrodeposition of Ni from a hydrophobic ionic liquid

Article abstract of DOI:10.1016/j.electacta.2010.07.097

The effects of addition of acetonitrile (ACN) on electrodeposition of nickel were investigated in a hydrophobic room-temperature ionic liquid, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA) containing Ni(TFSA)₂. Addition of ACN resulted in the change of the color of the ionic liquid. The UV-vis and FT-IR spectra of the electrolyte showed the coordination environment of Ni(II) changed gradually from [Ni(TFSA)₃]^- to [Ni(ACN)₆]²⁺ with an increase in the concentration of ACN. The diffusion coefficient of Ni(II) in BMPTFSA was increased and the reduction potential of Ni(II) shifted to the more positive side in the presence of ACN. The nucleation/growth process of Ni was not affected by the change in the coordination environment of Ni(II) from the chronoamperometric results although the more nuclei formed on the electrode surface. SEM showed smoother deposit was obtained in Ni(TFSA) ₂/BMPTFSA with ACN.

Full text of DOI:10.1016/j.electacta.2010.07.097

Electrochimica Acta 55 (2010) 9019–9023
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Effects of acetonitrile on electrodeposition of Ni from a hydrophobic ionic liquid
Yan-Li Zhu, Yasushi Katayama ,1, Takashi Miura
∗
Department of Applied Chemistry, Faculty of Science and Technology, Keio University, Hiyoshi, 3-14-1, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 May 2010
Received in revised form 30 July 2010
Accepted 30 July 2010
Available online 18 August 2010
The effects of addition of acetonitrile (ACN) on electrodeposition of nickel were inves-
tigated in
a
hydrophobic room-temperature ionic liquid, 1-butyl-1-methylpyrrolidinium
bis(trifluoromethylsulfonyl)amide (BMPTFSA) containing Ni(TFSA)2. Addition of ACN resulted in
the change of the color of the ionic liquid. The UV–vis and FT-IR spectra of the electrolyte showed
−
2+
the coordination environment of Ni(II) changed gradually from [Ni(TFSA)3] to [Ni(ACN)6] with an
increase in the concentration of ACN. The diffusion coefficient of Ni(II) in BMPTFSA was increased
and the reduction potential of Ni(II) shifted to the more positive side in the presence of ACN. The
nucleation/growth process of Ni was not affected by the change in the coordination environment of
Ni(II) from the chronoamperometric results although the more nuclei formed on the electrode surface.
SEM showed smoother deposit was obtained in Ni(TFSA)2/BMPTFSA with ACN.
Keywords:
Electrodeposition
Nickel
Acetonitrile
Ionic liquid
1
-butyl-1-methylpyrrolidinium
© 2010 Elsevier Ltd. All rights reserved.
Bis(trifluoromethylsulfonyl)amide
1
. Introduction
Much interest in the electrodeposition of nickel and nickel-
and then change their related properties. The additives have been
studied extensively in aqueous solutions [6,7,9–17]. Some of them
may reduce pitting and internal stress, leading to improvement
of the property of the deposit. The activity of an organic additive
mainly depends on the presence of certain types of unsaturations
among atoms of carbon, nitrogen, and oxygen (or certain sulfur-
containing radicals) [18]. On the other hand, a few studies have
been reported in room-temperature ionic liquids. Abbott and his
co-workers have reported the effects of ethylenediamine (en) and
acetylacetonate (acac) on the Ni electrodeposition in either a urea
or ethylene glycol/choline chloride based ionic liquid [19]. The
morphologies of Ni deposits can be changed after addition these
two additives. Both the additives suppressed the under potential
deposition of Ni on a platinum electrode and led to give finer-
grained deposits. Wang and co-workers also found the similar
behaviors for Co electrodeposition after adding 2-butyne-1.4-diol
into 1-ethyl-3-methylimidazolium (EMI) and ethylene glycol con-
taining cobalt chloride [20]. 2-Butyne-1.4-diol could also suppress
the under potential deposition of Co and smaller grain size of Co
deposits could be obtained in the presence of the additive. Wallace
and co-workers used pyrrol as an additive to induce deposition of
titanium species from EMIMTFSA containing TiCl4 [21]. However,
there are still a few reports about the effects of additives on the
electrodepostion of metals from ionic liquids up to now.
based alloys is due to their variety of commercial applications,
such as decoration, corrosion and heat resistance, magnetic
recording devices, etc [1–3]. Technological developments in
recent years like microelectromechanical systems require Ni or
its alloy, which exhibit optimal performance with a smooth
surface morphology since the morphology is one of the impor-
tant characteristics and influences the material properties
[
4–6].
Nickel or its alloys are generally electrodeposited from chlo-
ride, sulfate, sulfamate and Watts-type electrolytes [4,7]. However,
the electrochemical deposition from these aqueous systems often
involves the evolution of hydrogen, which deteriorates the qual-
ity of the deposits. Room-temperature ionic liquids composed of
−
bis(trifluoromethylsulfonyl)amide (TFSA ) have attracted much
attention in the electrochemical field because of its hydrophobicity.
In addition, they have high thermal stability and negligible vapor
pressure [8]. These merits make the ionic liquids be favorable to
the practical applications.
It is well-known that organic additives added at small con-
centrations to electroplating baths have various effects on the
morphology and structure of the electrodeposited Ni or its alloys
We have already reported the electrochemical behavior of
+
Ni(II)/Ni in BMPTFSA (BMP : 1-butyl-1-methylpyrrolidinium) con-
taining Ni(TFSA)2 as a Ni source, and the XRD showed that the
metallic Ni could be obtained from this ionic liquid successfully
[22]. In the present study, the effects of the organic additive of ace-
∗ Corresponding author. Tel.: +81 45 566 1561; fax: +81 45 566 1561.
E-mail address: katayama@applc.keio.ac.jp (Y. Katayama).
1
ISE Member.
0
013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.07.097

9
020
Y.-L. Zhu et al. / Electrochimica Acta 55 (2010) 9019–9023
tonitrile (ACN) on the electrodeposition of Ni were examined in this
ionic liquid.
2
. Experimental
All the electrochemical experiments and handling of hygro-
scopic reagents were performed in an argon-filled glove box with
a continuous gas purification apparatus (Miwa Seisakujyo Co. Ltd.,
DB0-1K-SH). The concentrations of both H O and O in the gas were
2
2
kept under 1 ppm.
BMPTFSA and Ni(TFSA)2 were prepared by the following reac-
tions:
1
-methylpyrrolidine + C H Br → BMPBr
(1)
(2)
4
9
BMPBr + LiTFSA → BMPTFSA + LiBr
4
HTFSA + NiCO ·Ni(OH) ·4H O → 2Ni(TFSA) + CO + 7H O
3
2
2
2
2
2
(3)
The details about the preparation procedures can be found else-
Fig. 1. UV–vis spectra of 0.05 M Ni(TFSA)2/BMPTFSA with various concentrations of
ACN. The spectrum of 0.05 M Ni(TFSA)2/ACN is also shown.
where [22]. The water content in BMPTFSA was below 10 ppm,
which was determined by Karl Fischer titration (Metrohm, 831
KF Coulometer). Acetonitrile (ACN, Wako, >99.0%, anhydrous) was
used as supplied.
Electrochemical measurements and electrodeposition exper-
iments were carried out using a standard three-electrode cell
with the aid of a potentio/galvanostat (Hokuto Denko, HABF-
Ni(II) in BMPTFSA can be changed by ACN, leading to the changes in
the electrodeposition process and the morphology of the deposits.
3.1. Solvation structure of Ni(II) in BMPTFSA with ACN
5
(
01). For electrochemical measurements, a platinum disk electrode
−
3
2
7.85 × 10 cm ) was employed as a working electrode after
The color of 0.05 M Ni(TFSA) /BMPTFSA changed from yellow
2
mirror polishing by 0.05 ␮m alumina (Baikowski), electrolytic
to blue gradually with an increase in the concentration of ACN.
degreasing in an alkaline solution for 1 min, washing with 10 vol%
Fig. 1 shows the UV–vis spectra of 0.05 M Ni(TFSA) /BMPTFSA
2
H SO and distilled water, and dried in the air. Platinum wire was
2
4
with various concentrations of ACN at room-temperature. Two
used as the auxiliary electrode. Silver wire immersed in BMPTFSA
3
3
3
3
absorption bands around 430 ( T ( P) ← A ( F)) and 815 nm
1g
2g
−3
containing 0.1 M (mol dm ) AgCF SO3 (Aldrich, >99.0%) was used
3
3
3
3
3
( T ( F) ← A ( F)) can be assigned to the octahedrally coordi-
1g
2g
as a reference electrode. The AgCF SO /BMPTFSA solution was
2+
−
3
3
nated complexes of Ni and TFSA . Both maxima of these bands
shifted to the shorter wavelengths with an increase in the concen-
tration of ACN. When 0.3 M ACN was added into the ionic liquid,
the molar ratio of ACN to Ni (II) was 6. The absorption bands at
around 360 and 588 nm are consistent with those observed in
separated from main electrolyte by porous Vycor glass. For the
−
2
2
electrodeposition, a copper disk electrode (7.06 × 10 cm ) was
used as a substrate. The pretreatment was the same as the platinum
disk. Ni (Nilaco) wire was employed as an auxiliary electrode. The
reference electrode was as the same as described above.
0
.05 M Ni(TFSA) /ACN. Thus, this absorption spectrum indicates the
2
The absorption spectra of BMPTFSA containing Ni(TFSA)2 with
and without the additive were measured by UV–vis spectrometer
JASCO, V-530) and Fourier transform infrared spectrophotome-
2+
formation of the octahedrally coordinated species of [Ni(ACN)6]
[
25,26]. The change in the coordination environment of Ni(II) can
(
be attributed to the difference in the donor property of the lig-
ands. Since the donor number of ACN (about 14) is larger than
ter (FT-IR, SHIMADZU, Prestige-21). UV–vis spectra were obtained
using an air-tight quartz cell with a light path length of 0.1 cm.
The FT-IR spectra of the ionic liquids were measured using an
attenuated total reflectance attachment (ATR, ZnSe prism) with
−
2+
that of TFSA (about 7), [Ni(ACN) ] is expected to be more stable
6
−
−
than [Ni(TFSA)3] [24]. The absorbance for [Ni(TFSA) ] decreased
and that for [Ni(ACN)6] increased with an increase in the ACN
concentration, indicating TFSA coordinating Ni is replaced with
ACN.
3
2+
−1
an optical resolution of 4 cm . The electrodeposits washed with
acetone (Wako, >99.8%) and dried in the air were characterized
by a scanning electron microscopy (SEM, KEYENCE VE-9800) and
energy-dispersive X-ray analysis (EDX, EDAX Phoenix).
−
2+
Fig. 2 shows FT-IR spectra of BMPTFSA containing various con-
centrations of Ni(TFSA) . It has been known that there are two
2
−
conformational states in the structure of TFSA : a cisoid form of C1
3
. Results and discussion
symmetry and a transoid form of C2 symmetry with respect to the
S-N-S plane [27]. The intense bands appearing at 1330 and 1348 nm
are ascribed to the S–O stretching vibration of C -ꢀa(SO ) and C -
In our previous study, the UV–vis spectra indicated the Ni(II) in
1
2
2
−
Ni(TFSA) /BMPTFSA was octahedrally coordinated by TFSA [22].
ꢀa(SO ), respectively [28]. As shown in Fig. 2, the absorbance of
2
2
−
[
Ni(TFSA)3] was considered to be formed in this ionic liquid, in
C -ꢀa(SO ) was decreased and C -ꢀa(SO ) increased with increas-
2
2
1
2
−
−
which the TFSA acts as a bidentate ligand [23]. The diffusion coef-
ing the concentration of Ni(II) probably due to the TFSA bound to
the Ni [23]. Thus, it is possible to estimate the concentration of
[Ni(TFSA)3] from the ratio of the absorbances of C -ꢀa(SO ) and
−
8
2
−1
2+
ficient of this Ni species was estimated to be 9.3 × 10 cm s by
chronoamperometry, indicating its mobility is rather low [22]. The
reason for this is probably due to the formation of the bulky com-
plexes [24]. In order to change the dissolved Ni(II) species, ACN,
−
1
2
C -ꢀa(SO ).
2
2
The FT-IR spectra of 0.3 M Ni(TFSA) /BMPTFSA with various con-
2
−
which has a larger donor number than TFSA [24], was added to
centrationsofACN is shownin Fig. 3. Theabsorbanceofstretching of
the electrolyte. It is expected that the coordination environment of
C -ꢀa(SO ) became stronger and that of C -ꢀa(SO ) became weaker
2
2
1
2

Y.-L. Zhu et al. / Electrochimica Acta 55 (2010) 9019–9023
9021
Fig. 4. Cyclic voltammograms of a Pt electrode in Ni(TFSA)2/BMPTFSA without and
with ACN at 25 C. Scan rate: 100 mV s .
Fig. 2. FT-IR spectra of BMPTFSA with various concentrations of Ni(TFSA)2.
◦
−1
with an increase in the amount of ACN in the ionic liquid, indicat-
−
2+
ing TFSA coordinated to Ni were replaced with ACN. Moreover,
species. However, the cathodic peak potential shifted to the
more negative side with an increase in the concentration of
ACN among the ionic liquids containing ACN. When the con-
centration ratio of ACN to Ni2+ is less than 6, there may exist
the FT-IR spectra of 0.3 M Ni(TFSA) /BMPTFSA with 1.8 M ACN was
identical to that for neat BMPTFSA, as shown in Fig. 2. The same
spectra of these two electrolytes indicate all of the TFSA coordi-
nated to Ni(II) species was replaced by ACN and a new kind of Ni(II)
species, [Ni(ACN)6]2+ was formed, as anticipated from the UV–vis
spectra.
2
−
2−x
some mixed ligand complexes like [Ni(TFSA)x(ACN)6−2x]
, as
suggested by the UV–vis spectroscopy (Fig. 1). Since the ther-
modynamic stability of [Ni(ACN)6]2+ is expected to be higher
than those of the mixed ligand complexes, the reduction poten-
tial was considered changed with the concentration of ACN
3
.2. Cyclic voltammetry
[
29].
The positive shift of the cathodic current peak in the
Fig. 4 shows the cyclic voltammograms of a Pt electrode
in 0.05 M Ni(TFSA) /BMPTFSA with various concentrations of
2
−
cyclic voltammogram upon transformation from [Ni(TFSA) ] to
[
of Ni(II). Since the donor property of ACN is higher than TFSA ,
the thermodynamic stability of [Ni(ACN) ] is considered higher
than that of [Ni(TFSA) ] . Thus, the overpotential for reduction of
[
◦
3
ACN at 25 C. The potential of cathodic reduction of Ni(II)
Ni(ACN)6]2+ suggests a decrease in the overpotential for reduction
shifted to the more positive side in the presence of ACN due
to the change in the coordination environment of the Ni(II)
−
2+
6
−
3
Ni(ACN)6]2+ should be larger than that of [Ni(TFSA) ] . This incon-
−
3
sistency may be explained by the unique double layer structure
in the ionic liquid. The ionic liquid does not contain any neutral
molecule. Thus, the electrode surface is always in contact with the
ions of ionic liquid. When the electrode is polarized negatively,
the electrode surface will be charged negatively. Then, the popula-
tion of positively charged species at the electrode–liquid interface
must increase in order to compensate the negative charge on the
−
electrode. In the absence of ACN, negatively charged [Ni(TFSA)3]
can not participate in the compensation at the interface. Thus, the
+
electrode surface is covered dominantly with bulky BMP . There-
−
fore, the charge transfer between the electrode and [Ni(TFSA) ] is
3
expected to occur across the BMP+ layer (∼0.7 nm thick). In addi-
tion, adsorption of nickel atoms may be hindered by adsorption of
+
BMP on the electrode surface. Thus, the overpotential for reduction
−
of [Ni(TFSA)3] is expected to become very large. On the other hand,
2+
in case of addition of ACN, positively charged [Ni(ACN)6] can par-
ticipate in the compensation at the interface, so that [Ni(ACN)6]2+ is
considered accessible to the electrode surface. Since this situation
is very similar to that in conventional electrolytes, the overpoten-
2
+
tial for reduction of [Ni(ACN) ] is expected to be smaller than
6
−
that of [Ni(TFSA) ] . The similar effect has been already found in
3
electrodeposition of Co from BMPTFSA with addition of acetone
Fig. 3. FT-IR spectra of 0.3 M Ni (TFSA)2/BMPTFSA with various concentrations of
ACN.
[
8].

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Y.-L. Zhu et al. / Electrochimica Acta 55 (2010) 9019–9023
2
Fig. 6. Non-dimensional plots of (j/jm) vs. t/tm from 0.05 M Ni(TFSA)2/BMPTFSA
with 0.3 M ACN at 25 C.
Fig. 5. Chronoamperograms of a Pt electrode in 0.05 M Ni(TFSA)2/BMPTFSA without
and with 0.3 M ACN at 25 C.
◦
◦
3.3. Chronoamperogram
Fig. 5 shows the chronoamperograms of a Pt electrode in 0.05 M
Ni(TFSA) /BMPTFSA without and with 0.3 M ACN. In the presence of
2
0.3 M ACN, sharp current peaks were observed after the charging of
the electric double layer in the electrolyte and the current densities
converged with the time, indicating the electrodeposition of Ni was
controlled by the diffusion process, as observed in the ionic liquid
without ACN [22]. The increase in the peak current density with
ACN may suggest the increase in the number of nuclei formed on the
substrate probably due to the adsorption of the positively charged
species, [Ni(ACN)6]2+
.
The dependence of the viscosities of 0.05 M Ni(TFSA) /BMPTFSA
2
without and with 0.3 M ACN on temperature (not shown) indicates
that the viscosities of both ionic liquids decreased with elevating
temperature according to the Andrade’s formula (ꢁ = Aexp(Ea/RT),
A: constant). The activation energy for 0.05 M Ni(TFSA) /BMPTFSA
2
−
−
1
with ACN for viscosities was 27.7 kJ mol , which was slightly
1
smaller than that without ACN (28.7 kJ mol ). There was little dif-
ference in the viscosity and its activation energy probably because
no free ACN molecule existed in the ionic liquid with ACN.
According to the Cottrell’s equation for semi-infinite one-
dimensional diffusion [30], the diffusion coefficients of 0.05 M
◦
Ni(TFSA) /BMPTFSA without and with 0.3 M ACN at 25 C were esti-
2
−7
−7
2
−1
mated to be 1.0 × 10 and 1.3 × 10 cm s , respectively. Based
on the Stokes–Einstein relation, the diffusion of a charged species in
a diluted solution is mostly determined by the size of the diffusing
species and the viscosity of the solution. Since the viscosity of these
two kinds of electrolytes has little difference (as described above),
the difference in the diffusion coefficients between [Ni(ACN) ]
and [Ni(TFSA)3] is considered ascribed to that in the sizes of these
complexes. The molar volumes of ACN and TFSA are estimated
to be 53 and 159 cm mol , respectively [31]. The total volume
of coordinating ligands per mole in [Ni(TFSA) ] is 477 cm mol
while that of [Ni(ACN) ] is 318 cm mol . Since the size of
Ni(ACN)6] is smaller than that of Ni(TFSA) ] , the diffusion of
Ni(ACN)6] is expected to be faster than that of [Ni(TFSA) ] .
2+
6
−
−
3
−1
−
3
−1
3
−1
2+
3
6
2
+
−
[
[
3
2
+
−
3
Fig. 7. SEM images of the deposits obtained on
a
Cu substrate in 0.05 M
From the chronoamperometric results, the relationship
◦
Ni(TFSA)2/BMPTFSA (a) without and (b) with 0.3 M ACN at 50 C. Current density:
2
between the non-dimensional parameters, (j/jm) and t/tm, where
jm is the peak current density, t is the time and tm is the time at
jm, is shown in Fig. 6. Compared to the theoretical model reported
−0.05 mA cm−2. Charge density: 3.6 C cm−2.

Y.-L. Zhu et al. / Electrochimica Acta 55 (2010) 9019–9023
9023
by the Shaifker’s nucleation and growth model [32], the initial
Acknowledgment
stage of Ni deposition on a Pt electrode in the electrolyte with
ACN can be explained better by instantaneous nucleation rather
than progressive one. Thus, the nucleation mechanism was not
The authors gratefully acknowledge the financial support of the
Japanese Government (Monbukagakusho) Scholarship.
2+
affected by the change in the coordination environment of Ni
−
2+
from [Ni(TFSA)3] to [Ni(ACN)6] [22].
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2
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+
+
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[
[
[
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2
−
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[
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2
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in the aqueous solutions. However, it is necessary to elucidate
the detailed mechanism of the effects of these additives on the
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[
[
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