Electrochemistry of tin in the 1-ethyl-3-methylimidazolium dicyanamide room temperature ionic liquid

Article abstract of DOI:10.1016/j.electacta.2011.02.022

The electrochemistry of Sn(II) was studied in the room temperature ionic liquid 1-ethyl-3-methylimidazolium dicyanamide (EMI-DCA) on a glassy carbon (GC) and a polycrystalline Pt electrode at 40 °C. The Sn(II) species was introduced into the ionic liquid by either dissolution of SnCl₂ or anodizing a Sn wire. The reduction potential of the Sn(II)/Sn couple produced in these two solutions was found to be different, indicating that different Sn(II) species may be present. The order of the reduction potential of the two Sn(II) species indicates that the Gutmann donor ability of the anions is likely to be DCA^- > Cl^-. Cyclic voltammetry indicates the stripping efficiency is >90% on the Pt but only 40% on the GC electrode. Analysis of the chronoamperometric transient behavior during electrodeposition suggests that the deposition of Sn on the GC electrode involves a three dimensional progressive nucleation on a finite number of active sites. The diffusion coefficient of SnCl₂ dissolved in the EMI-DCA was found to be 9.8 × 10^-7 cm² s^-1 which is in the same order of magnitude as those reported for SnCl₂ in several other ionic liquids. Depending on the deposition potentials, potentiostatic electrolysis produced Sn deposits with various unusual morphologies such as hexagonal tubes, spiral nanowires, and dendrite.

Full text of DOI:10.1016/j.electacta.2011.02.022

Electrochimica Acta 56 (2011) 3941–3946
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electrochemistry of tin in the 1-ethyl-3-methylimidazolium dicyanamide room
temperature ionic liquid
Tin-Iao Leong, Yi-Ting Hsieh, I-Wen Sun∗,1
Department of Chemistry, National Cheng Kung University, Tainan, Taiwan
a r t i c l e i n f o
a b s t r a c t
Article history:
The electrochemistry of Sn(II) was studied in the room temperature ionic liquid 1-ethyl-3-
methylimidazolium dicyanamide (EMI-DCA) on a glassy carbon (GC) and a polycrystalline Pt electrode at
40 ◦C. The Sn(II) species was introduced into the ionic liquid by either dissolution of SnCl2 or anodizing
a Sn wire. The reduction potential of the Sn(II)/Sn couple produced in these two solutions was found to
be different, indicating that different Sn(II) species may be present. The order of the reduction potential
of the two Sn(II) species indicates that the Gutmann donor ability of the anions is likely to be DCA− > Cl−.
Cyclic voltammetry indicates the stripping efficiency is >90% on the Pt but only 40% on the GC electrode.
Analysis of the chronoamperometric transient behavior during electrodeposition suggests that the depo-
sition of Sn on the GC electrode involves a three dimensional progressive nucleation on a finite number of
active sites. The diffusion coefficient of SnCl2 dissolved in the EMI-DCA was found to be 9.8 × 10−7 cm2 s−1
which is in the same order of magnitude as those reported for SnCl2 in several other ionic liquids. Depend-
ing on the deposition potentials, potentiostatic electrolysis produced Sn deposits with various unusual
morphologies such as hexagonal tubes, spiral nanowires, and dendrite.
Received 21 September 2010
Received in revised form 1 February 2011
Accepted 4 February 2011
Available online 2 March 2011
Keywords:
Ionic liquid
Sn
Electrodeposition
Template-free
Hexagonal tube
Dicyanamide
© 2011 Elsevier Ltd. All rights reserved.
1. Introduction
Recently, it has been demonstrated that such solubility difficulty
can be solved by using ILs made with DCA− anion [14].
The electrodeposition of metals and alloys in aqueous solu-
tions can be limited by the narrow electrochemical window and
the hydrogen evolution. Therefore, aprotic media with large elec-
trochemical window is of great interest. In recent years, aprotic
ionic liquids (ILs) have become popular electrolytes for various
electrochemical applications including electrodeposition, modified
electrodes, dye sensitized solar cells, lithium batteries, and capac-
itors [1–7]. The use of aprotic ILs not only removes the problem of
hydrogen evolution but also makes the deposition of active metals
such as Mg [8], Na and Li [9–11] that can not be achieved in aque-
ous baths possible. The low vapor pressure, high thermal stability
and non-flammability of ILs provide extra benefits. The type of both
cations and anions of the ILs has profound effects on the physical
and chemical properties of ILs, which in turn may influence the
solubility, deposition potential and mechanism of the electrodepo-
sition and stripping process. For example, Endres and coworkers
showed that changing the cation has profound effect on the elec-
trodeposition [12,13]. Furthermore, metal halides exhibit very low
Studies of the electrodeposition of Sn in aqueous media indi-
cate that Sn(II) is used in acidic solutions whereas Sn(IV) is
used in alkaline solutions, and additives including surfactants
and oxidation inhibitors are required to suppress problems with
hydrogen evolution and formation of tin hydroxide precipitates
[15,16]. Alternatively, such problems are avoided by using organic
bath such as dimethyl sulfoxide (DMSO) [17]. However, the use
of organic media involves safety concern due to their volatil-
ity and flammability. The use of ILs would be beneficial in this
regard. Xu and Hussey [18] investigated the electrochemistry of
Sn(II) and Sn(IV) with voltammetry and chronoamperometry at
several different electrodes in the aluminum chloride-1-ethyl-3-
methylimidazolium chloride (AlCl3-EMIC) ionic liquids at 40 ◦C.
They found that the overpotential and nucleation process for
electrodeposition of Sn was substrate material dependent. They
also reported that the voltammetric oxidation of Sn(II) to Sn(IV)
was hindered by the weak adsorption of Sn(II). Similar behav-
iors were observed in
a later study performed in the zinc
−
solubility in ILs made with anions such as BF4−, PF6−, and NTf2
.
chloride-1-ethyl-3-methylimidazolium chloride (ZnCl2-EMIC) IL
[19]. Tachikawa et al. [20] obtained a smooth dense faceted
Sn deposits from a hydrophobic N-butyl-N-methylpyrrolidinium
bis(trifluoromethylsulfonyl)imide (BMP-NTf2) IL containing Sn(II)
prepared by anodic dissolution of a Sn wire in the IL whereas
macroporous Sn formed with interweaving Sn nanowires was pro-
duced from electrodeposition from 1-ethyl-3-methylimidazolium
∗
Corresponding author at: Department of Chemistry, National Cheng Kung Uni-
versity, No.1, Ta-Hsueh Road, Tainan, Taiwan.
E-mail address: iwsun@mail.ncku.edu.tw (I.-W. Sun).
ISE member.
1
0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.02.022

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T.-I. Leong et al. / Electrochimica Acta 56 (2011) 3941–3946
dicyanamide (EMI-DCA)IL containing SnCl2 [21]. Abbott et al. [22]
studied the electrodeposition of Zn–Sn alloy from urea and ethylene
glycol/choline chloride based ILs. They showed that the deposition
kinetics and thermodynamics differ from the aqueous processes
and that qualitatively different morphologies are obtained for
the metal coatings in different IL systems. Recently, Martin-
dale et al. [23] compared the microelectrode voltammetry of the
Sn(II)/Sn couple in BMP-NTf2 and N-butyl-N-methylpyrrolidinium
dicyanamide (BMP-DCA) ILs using solutions prepared by disso-
lution of SnCl2 and Sn(CF3SO3)2. They found that the diffusion
coefficient of the Sn(II) species varied with the ILs as well as
the Sn(II) salts used, reflecting that different Sn(II) species were
present. Furthermore, while a large difference in the voltammetry
of the different Sn(II) salts was seen in BMP-NTf2, the mechanism
for the Sn stripping process is largely similar for both Sn(II) salts in
BMP-DCA, implying that the morphology of the Sn deposit could be
similar.
Although DCA-based ILs are potentially useful for electro-
chemical studies [14], there is a paucity of information about
the electrochemistry of metals in these ILs. As a continuation of
this research effort, the electrochemical behavior of Sn(II)/Sn on
glassy carbon (GC) and platinum electrodes in the EMI-DCA IL
at 40 ◦C was investigated in this study using cyclic voltamme-
try and chronoamperometry. The diffusion coefficient of SnCl2
dissolved in the EMI-DCA was estimated. Potentiostatic electrol-
ysis produced Sn deposits with various unusual morphologies
were obtained and characterized. Of particular interest is the hol-
low tube structure that has not been obtained for Sn from other
media.
2.5mA cm-2
A
B
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
E vs (Fc+/Fc)/V
Fig. 1. Cyclic voltammogram of (A) a Pt electrode, and (B) a Sn wire in neat EMI-DCA
at 40 ◦C; scan rate: 50 mV s−1
.
2.5mA cm-2
B
A
2. Experimental
2.1. Chemicals
The EMI-DCA ionic liquid was prepared and purified by follow-
ing the previous literature [14,24]. The ionic liquid was washed
with acetone and dichloromethane sequentially and filtered to
remove NaCl precipitates, and dried at 393 K under vacuum.
Anhydrous SnCl2 (99.99%, Aldrich), Sn wire (99.99%, Aldrich), plat-
inum wire (99.95%, Alfa Aesar) and copper wires (area = 0.25 cm2)
(99.98%, Aldrich) were used as purchased.
0.2
0.0
-0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4
E vs (Fc+/Fc)/V
Fig. 2. Cyclic voltammogram of (A) 50 mM SnCl2, and (B) 50 mM Sn (II) by anodic
dissolution of Sn wire in EMI-DCA on Pt electrode at 40 ◦C; scan rate: 50 mV s−1
.
3. Results and discussion
2.2. Apparatus
3.1. Preparation of Sn(II) solution
All electrochemical experiments were performed under a puri-
fied nitrogen atmosphere in a glove box (Vacuum Atmospheres Co.).
The electrochemical experiments were conducted with an EG&G
PARC Model 273A potentiostat/galvanostat controlled by EG&G
Model 270 software. Electrochemical experiments were performed
in a conventional three-electrode cell. The working electrode was
either a Pt disk electrode (geometric surface area 0.02 cm2) or a GC
disk electrode (geometric surface area 0.07 cm2). The electrodes
were polished successively with increasingly finer grades of emery
paper followed with aqueous slurry of 0.05 ␮m alumina to a mirroe
finish, rinsed with distilled water, and dried under vacuum before
transfer into the glove box for use. A platinum wire immersed in
a ferrocene/ferrocenium (Fc/Fc+ = 50/50 mol%) BMP-TFSI solution
contained in a glass tube with porous Vycor tip (Bioanalytical Sys-
tems, MF-2042) was used as the reference electrode. The counter
electrode was a spiral Sn wire which was separated from the bulk
ionic liquid by a glass frit. Electrodeposition experiments were con-
ducted on 1 cm × 1 cm copper foils. The surface morphology of the
Sn nanosturctures were investigated with a Philip XL-40FEG field-
emission SEM. The crystal structure of the Sn coatings was analyzed
by XRD-7000.
Fig. 1 shows the cyclic voltammogram (CV) a Sn wire immersed
in a pure EMI-DCA at 40 ◦C. This CV was initially scanned posi-
tively from −2.0 V (versus the Fc+/Fc couple) with a scan rate of
50 mV s−1. It can be seen that the anodic dissolution of Sn initiats at
a potential of −0.84 V and proceeded rapidly until the potential
scan is reversed at −0.75 V, indicating that no significant passi-
vation occurs on the Sn surface during anodizing. Consequently,
solutions of EMI-DCA IL containing Sn(II) can be prepared by anodic
dissolution of Sn metal. Alternatively, Sn(II) was introduced into
the EMI-DCA IL by dissolution of SnCl2 which dissolves rapidly
and completely in the EMI-DCA in accordance with the fact that
DCA anions complex with the Sn(II) ions producing soluble com-
plex anions. It is natural to consider if the two different methods
would generate different Sn(II) species in the IL. Fig. 2 shows the
cyclic voltammograms recorded on a Pt electrode for solutions of
EMI-DCA IL containing 50 mM Sn(II) prepared by dissolution of
SnCl2 and by anodizing a Sn wire, respectively. For the solution
prepared by anodizing a Sn wire, the Sn(II) reduction current initi-
ates at −0.85 V, reaches a maximum value and then decays due to
the thickening of the diffusion layer. On the reverse scan, the cur-
rent initially stays at the cathodic side, indicating that reduction is

T.-I. Leong et al. / Electrochimica Acta 56 (2011) 3941–3946
3943
still occurring until the potential reaches −0.84 V. At −0.84 V the
c1
A
GC
anodic stripping overtakes the cathodic deposition and the cur-
rent crosses to the anodic side. The transition between deposition
and stripping occurs at a potential less negative than the potential
where the deposition was initially started is indicative of the need
of an overpotential driven nucleation and growth process before
bulk deposition of Sn can proceed. The stripping current increases
rapidly as the anodic scan continues, reaches a peak at −0.59 V,
and then drops sharply to zero. The sharp drop is typical of a metal
stripping peak where the metal deposits are all consumed at the
same time. Similar cyclic voltammetry behavior is observed for the
solution prepared by dissolving SnCl2 into the IL, but the deposi-
tion/stripping process of Sn0 occurs at about −0.74 V which is less
negative than that observed in the solution prepared by anodiz-
ing the Sn wire. Furthermore, the stripping peak observed for the
SnCl2 solution appears to be slightly broader. These discrepancies
suggest that the Sn(II) species present in these two solutions are
likely to be in different forms. In the solution prepared by anodiz-
ing the Sn metal wire, Sn(II) is likely to coordinate with the DCA−
anions to give the [Sn(DCA)y](y−2)− complex anion whereas in the
a1
Pt
a2
-2
7.5mA cm
1.0
0.5
0.0
-0.5
-1.0
-1.5
E vs (Fc+/Fc)/V
c
c
1
B
2
GC
Pt
SnCl2 solution, Sn(II) is likely to coordinate with both Cl− and DCA−
(y−2)−
complex anion. In other
anions forming the [SnCl2(DCA)y−2
]
a
a
1
words, the Cl− in SnCl2 are not replaced by DCA− anions when SnCl2
dissolves in the EMI-DCA IL. Gutmann proposed that the poten-
tial of Mn+/M tends to be more negative with an increase in the
donor property of solvent [25]. Therefore, the order of the poten-
tials of Sn(II)/Sn described above indicates that the donor ability
of DCA− is likely to be slightly larger than Cl−. However, there
may be a kinetic limitation for DCA− to replace the Cl− in SnCl2,
otherwise [Sn(DCA)y](y−2)−, would also be the Sn(II) species pro-
duced in the solution by dissolution of SnCl2, and the anodic peak
due to the oxidation of the Cl− (100 mM) that would be released
from SnCl2 would be observed in the voltammogram. On the other
hand, it should be noted that DCA-based ILs usually contain a mod-
erate amount of Cl− and it is also possible that, in addition to
the [Sn(DCA)y](y−2)−, a small amount of the [Sn(DCA)xClz](x+z−2)−
species are also formed in the solution prepared by anodic dissolu-
tion of Sn wire. In view of that SnCl2 solutions are easier to prepare
they were employed in the subsequent studies.
2
-2
7.5mA cm
1.0
0.5
0.0
-0.5
-1.0
-1.5
E vs (Fc+/Fc)/V
Fig. 3. Cyclic voltammograms of 50 mM SnCl2 in EMI-DCA initially scanned to (A)
negative, and (B) positive potential on GC and Pt electrode at 40 ◦C; scan rate:
50 mV s−1
.
the GC electrode (Fig. 3B) reveals that the peak c1 is overlapped
with a small shoulder (peak c2) that is attributed to the reduction
of the Sn(IV) species that was generated during the initial anodic
sweep. Apparently, the reduction potential of Sn(IV)/Sn(II) couple
is very close to the Sn(II)/Sn0 couple. Meanwhile, the large peak
separation between peaks c2 and a2 indicates that the Sn(IV)/Sn(II)
couple is kinetically irreversible. It is also apparent from Fig. 3 that
the Sn0 stripping peak observed on the GC electrode is smaller than
that on the Pt electrode. To further study this phenomenon, cyclic
voltammograms with different Es were recorded on the GC and
Pt electrodes, and the stripping efficiency, (Qa/Qc, where Qc is the
charge passed during the deposition process and Qa is the charge
passed during the stripping process) integrated from these voltam-
mograms are collected in Table 1. This table indicates that the Qa/Qc
on the Pt electrode is higher than 0.87 and approaches to 1.0 as Es
gets more negative, confirming that the stripping of Sn0 is a two
electron transfer process to Sn(II). On the other hand, the Qa/Qc
observed on GC is fairly small. The small Qa/Qc observed on the GC
indicates that Sn deposits with poor adhesion were formed at the
GC electrode surface, and during the anodic scan, as the Sn close to
the GC is oxidized, the rest looses good contact with the electrode
and could not be oxidized, resulting in diminished anodic charge.
Such behavior has been discussed by Hyde et al. [28]. Fig. 4 shows
the multi-scan cyclic voltammograms recorded on the GC and Pt
electrodes, respectively, for a 50 mM SnCl2 solution in EMI-DCA
IL at 40 ◦C between −0.20 V and 1.20 V. In all cases, the deposi-
tion in the first potential cycle starts at a potential more negative
than that in the subsequent cycles. During the first cycle, a larger
3.2. Voltammetry of Sn(II) and Sn(IV)
The cyclic voltammograms recorded on GC and Pt electrodes,
respectively, for a 50 mM SnCl2 solution in EMI-DCA IL at 40 ◦C
are shown in Fig. 3. In Fig. 3A, the potential was initially scanned
toward negative direction from the open circuit potential (OCP) and
reversed at a switching potential (Es). This figure clearly indicates
that in comparison with that on the Pt electrode, the deposition of
Sn0 (peak c1) on the GC electrode requires a larger overpotential
to initiate the nucleation and growth process. Nevertheless, the
stripping of the deposited Sn (peak a1) occurs at approximately
the same potential on both electrodes. Furthermore, the oxidation
peak of Sn(II) to Sn(IV) (peak a2) is clearly seen on the GC but barely
seen on Pt. Cyclic voltammograms of which the potential was ini-
tially scanned positively from OCP are presented in Fig. 3B. Again,
oxidation of Sn(II) to Sn(IV) proceeds nicely on GC but very slug-
gish on Pt. The absence of obvious Sn(II)/Sn(IV) couple on Pt has
also been reported in previous studies carried in aqueous solution
[26], AlCl3-EMIC [18], ZnCl2-EMIC [19], and BMP-NTf2 [20] ILs. Such
behavior has been suggested to be due to the strong adsorption of
Sn(II) species on the Pt surface which hinders the oxidation of Sn(II)
to Sn(IV). On the other hand, the prominent oxidation of Sn(II) to
Sn(IV) on the GC electrode may be due to the presence of some oxide
groups on GC electrode acting as docking sites which facilitate the
oxidation of certain species as has been pointed out by McDermott
et al. [27]. Careful examination of the voltammogram recorded on

3944
T.-I. Leong et al. / Electrochimica Acta 56 (2011) 3941–3946
Table 1
Summary of the charge passed during the deposition process Qa, the charge passed during the stripping process, Qc and Qa/Qc taken rom cyclic voltammograms with different
switching potential, Es, for 50 mM SnCl2 in EMI-DCA on GC and Pt electrodes at 40 ◦C; scan rate: 50 mV s−1
.
Electrode
GC
Es (E = V (vs Fc))
Qc (Q = mC cm−2
)
Qa (Q = mC cm−2
)
Qa/Qc (Q = mC cm−2
)
−0.94
−1.04
−1.09
−1.19
−1.49
18.06
28.59
32.67
40.61
55.87
4.84
6.94
12.12
19.77
22.46
27%
24%
37%
49%
40%
Pt
−0.8
2.97
9.92
15.34
22.29
2.57
8.79
13.15
20.35
87%
89%
86%
91%
−0.84
−0.89
−0.99
1.0
0.8
0.6
0.4
0.2
0.0
Instantaneous
1st
Pt
Progressive
-2
-0.814V
-0.813V
2.5 mA cm
5 mA cm-2
GC
1st
-0.812V
A
0
5
10
15
20
25
30
Time (sec)
0.0
0.5
1.0
1.5
2.0
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
Time/s
E vs (Fc+/Fc)/V
Fig. 5. Dimensionless experimental curves derived from current–time curves
shown in the inset with the theoretical models for 3D nucleation/growth process.
Inset: current–time transients of the chronoamperometric experiments recorded at
GC electrode for 50 mM SnCl2 in EMI-DCA at 40 ◦C.
Fig. 4. Multi-scan cyclic voltammograms of 50 mM SnCl2 in EMI-DCA on GC and Pt
electrode at 40 ◦C; scan rate: 50 mV s−1
.
overpotential is required for the formation of Sn nuclei on the for-
eign substrate materials (Pt and GC). However, on the subsequent
cycles, required deposition overpotential decreased significantly.
Hyde et al. [28] have indicated that incomplete anodic stripping
generally occurs so that not all the metal particles deposited in the
cathodic half-cycle are oxidized in the anodic-half cycle. Conse-
quently, in the second and subsequent cycles, the deposition of Sn
on the Sn that was remained from the previous cycle requires less
overpotential, making the deposition peak shifts to less negative
potential. On the Pt electrode, the subsequent cycles overlap with
each other in accordance with the fact that effectively most of the Sn
deposited in each cathodic half cycle are re-oxidized in each corre-
sponding anodic half cycle and the Sn(II) concentration profile near
the electrode surface does not alter much between cycles. On the
contrary, the reduction and oxidation peak currents observed on
the GC electrode diminish with increasing cycling number, indi-
cating that the reducible Sn(II) concentration near the electrode
surface keeps declining after each cycle. This is because that most
of the loosely adhered Sn deposits were lost from the electrode
and can not be re-oxidized back to Sn(II) to the solution during
each anodic half cycle, and results in declined Sn(II) concentra-
tion and lower peak current. The positive shift in reduction peak
observed in Fig. 4 has an important implication to the potentio-
static bulk deposition of Sn. For example, while the voltammogram
of the 1st cycle indicates that the electrodeposition at −0.85 V may
be under charge-transfer limited, the voltammograms of the subse-
quent cycles indicate that the deposition at −0.85 V is at a potential
beyond the peak potential and the growth of the deposits may actu-
ally be under mass-transfer limited once a thin layer of Sn nuclei is
formed. This means that the morphology of the deposited Sn film
obtained at −0.85 V would be dendritic as what is often obtained
for mass-transfer limited case rather than what is anticipated for
a charge-transfer controlled case. Consequently, for potentiostatic
deposition, the desired deposition potential should be selected not
based on the 1st cycle but on the subsequent cycles.
3.3. Nucleation study on GC
The cyclic voltammograms (Fig. 3) reveal that the deposition
of Sn at the GC electrode requires a substantial overpotential to
initiate the nucleation. The nucleation process was studied by
chronoamperometry experiments by stepping the potential of the
working electrode from where no reduction takes place to poten-
tials sufficiently negative to initiate the nucleation/growth process
after a short induction time, to. Typical current–time transients
resulting from these experiments are shown in the inset in Fig. 5.
The transients exhibited the classic shape for a nucleation process;
after the capacitive current decay, the current increased slowly
until a current maximum, im, was reached at time tm, and then
decreased with time as the diffusion layer became thicker. Note
that the current densities observed at longer time did not converge
together. Instead, the current increased with increasing applied
overpotential, indicating that the nucleation process did not follow
the ideal Cottrell equation. Two extreme nucleation mechanisms
have been classified as either “instantaneous” or “progressive”. The
instantaneous model refers to the situation in which all nucleation
sites are activated at the beginning of the potential step, whereas
the progressive model refers to the situation in which the nucle-
ation sites gradually become activated as the chronoamperometric
experiment proceeds. To distinguish between the two nucleation
models, the data taken from the chronoamperometric experiments
2
were compared to the theoretical dimensionless (i/im
)
vs (t/tm
)
curves developed by Sharifker et al. for three-dimensional (3D)
instantaneous and progressive nucleation, respectively.[29] Before
the fitting, the experimental t and tm were corrected to tꢀ (= t − to)

T.-I. Leong et al. / Electrochimica Acta 56 (2011) 3941–3946
3945
Table 2
Cu
Sn
Diffusion coefficient and Stokes–Einstein Product data for Sn(II) in various ionic
liquid.
Ionic liquid
107 Do (cm2 s−1
)
Temp. (◦C)
Ref.
EMI-DCAb
9.84
0.93
3.0
0.35
5.1
40
25
20
20
40
This work
20
23
23
18,17,11
(200)
(101)
BMP-NTf2a
BMP-DCAb
BMP-NTf2a
55.6–44.4 mol%
(211)
(220)
40
(301)
b
(112)
60
(321)
EMIC-AlCl3
(312)
80
75–25 mol%
4.61
90
19
b
EMIC-ZnCl2
a
Sn(II) introduced by anodizing Sn wire.
Sn(II) introduced by dissolution of SnCl2.
30
50
70
b
2 Theta (degree)
Fig. 6. XRD pattern of Sn-deposited Cu foil.
2
and tmꢀ (= tm − to). The experimental and theoretical plots of (i/im
)
vs (tꢀ/tm ) are shown in Fig. 5. This figure indicates that the elec-
trodeposition of Sn on the GC electrode is complicate because the
experimental data fall between the two theoretical curves. This
indicates that the nucleation of Sn at the GC electrode involves
three-dimensional progressive nucleation on a finite number of
active sites [29]. The nucleation behavior observed in this study
differs from those observed in other ILs. For example, Xu et al.
[18] and Huang et al. [19] suggested the instantaneous nucleation
on a GC electrode in AlCl3-EMICl and on W and GC electrodes in
ZnCl2-EMICl, respectively whereas Tachikawa and co-workers [20]
suggested the progressive nucleation on a Pt electrode in BMP-
NTf2. When chronoamperometry experiments are performed by
stepping the potential of the working electrode to potentials more
negative than −1.2 V, the deposition process occurs immediately
upon the applying of the potential step and the current–time tran-
sient follows the Cottrell behavior. The diffusion coefficient of the
dissolved Sn(II) species in this solution, calculated from data taken
from these current–time transient using Cottrell equation, was
ꢀ
found to be 9.8 × 10−7 cm2 s−1. As shown in Table 2, this value is
in the same order of magnitude as those reported for SnCl2 in BMP-
DCA, AlCl3-EMIC, and EMIC-BF4 but larger than the Sn(II) species in
BMP-NTf2 [20,23]. The difference in diffusion coefficients has been
related to the different viscosity and solvation environment of the
ILs [20,23]. Assuming the coordination spheres of the SnCl2 dis-
solved in EMI-DCA and BMP-DCA ILs are similar, the larger diffusion
coefficient of Sn(II) species in the EMI-DCA compared to BMP-DCA
is consistent with the lower viscosity of EMI-DCA (19.8 cP) with
respect to that of BMP-DCA (45.3 cP) [14].
3.4. Preparation and characterization of Sn electrodeposits
Bulk electrodeposition of Sn was performed on Cu substrate in
EMI-DCA containing 50 mM SnCl2 at 40 ◦C using constant poten-
tial electrolysis. Before each experiment, a potential of −1.05 V
was applied for 30 s to ensure the generation of enough Sn nuclei,
Fig. 7. SEM images of Sn structures electrodeposited at (A and B) −0.65 V, (C) −0.75 V, and (D) −0.84 V on Cu foils from 50 mM SnCl2 in EMI-DCA at 40 ◦C.

3946
T.-I. Leong et al. / Electrochimica Acta 56 (2011) 3941–3946
and the deposition was then carried out at the selected poten-
tial. Following each deposition experiment, the resulting deposit
was rinsed with deionized water to remove residual IL and then
dried. The diffraction peaks in the XRD pattern of a typical deposit
were sharp, as shown in Fig. 6, indicating the crystallinity to be
good. EDX reveals that in addition to Sn and Cu substrate, some
O also present in the samples, indicating the samples could be
contaminated when exposed to the ambient atmosphere during
the deionized water rinsing procedure after deposition. The SEM
micrographs of the surface morphologies of the Sn-deposited sam-
ples are shown in Fig. 7. As can be seen, Sn deposits with interesting
morphologies which varied with the deposition potential are pro-
duced. At −0.65 V where the deposition is most likely to proceed
in the charge-transfer controlled region the surface is covered by
hollow hexagonal tubes with diameter of about 500 nm. When the
deposition rate is raised by changing the deposition potential to
−0.75 V, a layer of sponge formed by entangled wires is apparent.
Further change the deposition potential to make the deposition
to be performed in the mass-transport limited region, −0.84 V,
dendrite structures are obtained. While most previous studies on
electrodeposition of metals from ILs focused on the deposition of
dense and compact coatings, the growth of hollow tubes, entan-
gled wires, and dendrites from ILs has not been intensively studied.
In addition, although dendrites are commonly obtained from elec-
trodeposition in aqueous electrolytes, the hollow tubes and wires
obtained in this study is rare. These unusual morphologies may
be related to the nature of both DCA− anion and tin. As has been
pointed out by Martindale et al. [23] the morphology of the Sn
deposit is dependent of the anion of the ILs.
magnitude as those reported for SnCl2 in several other ILs. Further-
more, Sn deposits with unusual morphologies especially hexagonal
hollow tubes are obtained by potentiostatic electrolysis at various
potentials.
Acknowledgement
This work was funded by the National Science Council of Repub-
lic of China, Taiwan (Grant no. NSC 99-2113-M-006-002-MY3)
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