Irregular Electrodeposition of Cu-Sn Alloy Coatings in [EMIM]Cl Outside the Glove Box with Large Layer Thickness

Abstract

Thick Cu−Sn alloy layers were produced in an [EMIM]Cl ionic-liquid solution from CuCl2 and SnCl2 in different ratios. All work, including the electrodeposition, took place outside the glovebox with a continuous argon stream over the electrolyte at 95 °C. The layer composition and layer thickness can be adjusted by the variation of the metal-salts content in the electrolyte. A layer with a thickness of up to 15 µm and a copper content of up to ωCu = 0.86 was obtained. The phase composition was characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and X-ray fluorescence (XRF). Furthermore, it was found that the relationship between the alloy composition and the concentration of the ions in the electrolyte is described as an irregular alloy system as according to Brenner. Brenner described such systems only for aqueous electrolytes containing complexing agents such as cyanide. In this work, it was confirmed that irregular alloy depositions also occur in [EMIM]Cl.

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coatings
Article
Irregular Electrodeposition of Cu-Sn Alloy Coatings in
[EMIM]Cl Outside the Glove Box with Large Layer Thickness
Lars Lehmann * , Dominik Höhlich , Thomas Mehner and Thomas Lampke


Citation: Lehmann, L.; Höhlich, D.;
Mehner, T.; Lampke, T. Irregular
Electrodeposition of Cu-Sn Alloy
Coatings in [EMIM]Cl Outside the
Glove Box with Large Layer
Thickness. Coatings 2021,11, 310.
https://doi.org/10.3390/
coatings11030310
Academic Editor: Paweł Nowak
Received: 5 February 2021
Accepted: 5 March 2021
Published: 9 March 2021
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Copyright: © 2021 by the authors.
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distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
Materials and Surface Engineering Group, Institute of Materials Science and Engineering, Faculty of Mechanical
Engineering, Chemnitz University of Technology, Erfenschlager Straße 73, D-09125 Chemnitz, Germany;
dominik.hoehlich@mb.tu-chemnitz.de (D.H.); thomas.mehner@mb.tu-chemnitz.de (T.M.);
thomas.lampke@mb.tu-chemnitz.de (T.L.)
*Correspondence: lars.lehmann@mb.tu-chemnitz.de; Tel.: +49-371-531-31910
Abstract:
Thick Cu
−
Sn alloy layers were produced in an [EMIM]Cl ionic-liquid solution from
CuCl
2
and SnCl
2
in different ratios. All work, including the electrodeposition, took place outside
the glovebox with a continuous argon stream over the electrolyte at 95
◦
C. The layer composition
and layer thickness can be adjusted by the variation of the metal-salts content in the electrolyte.
A layer with a thickness of up to 15
µ
m and a copper content of up to
ωCu
= 0.86 was obtained.
The phase composition was characterized by scanning electron microscopy (SEM), X-ray diffraction
(XRD), and X-ray fluorescence (XRF). Furthermore, it was found that the relationship between the
alloy composition and the concentration of the ions in the electrolyte is described as an irregular
alloy system as according to Brenner. Brenner described such systems only for aqueous electrolytes
containing complexing agents such as cyanide. In this work, it was confirmed that irregular alloy
depositions also occur in [EMIM]Cl.
Keywords: Cu-Sn alloy; ionic liquids; aprotic solvents; [EMIM]Cl; electrodeposition
1. Introduction
While pure copper is relatively soft, bronze has high strength and hardness due to
the alloy component tin. In addition, this system is highly resistant to corrosion (e.g., in
seawater) and wear. The conductivity for electricity and heat is good with low tin contents,
although not as high as that of pure copper. As the tin content increases, the conductivity
decreases, while strength and hardness increase. It also offers good spring and sliding
properties and has excellent fatigue strength. Thus, copper-tin layers could be used where
the replacement of nickel is required. Nickel coatings are among the most frequently
used galvanic coatings because of their good corrosion and wear properties, as well
as their appealing appearance. Their disadvantage is their allergenicity. Since nickel
layers release nickel over time, they are regulated by the REACH Regulation, Annex XVII,
entry 27, which applies to the European union. Bronze coatings have a long history of
being used as a replacement for nickel. In recent years, there has been much research on
improving aqueous electrolytes [
1
]. However, hydrogen embrittlement remains a challenge
for the deposition of such layers in these electrolytes. The standard potentials of the
metal ions are approximately 0.5 V apart. If too high a current density is used, significant
hydrogen development occurs, especially in acidic baths [
2
]. In addition, the solubility
of the metal salts is severely limited, and the stability of the bath in air is a challenge.
Without additives, it is difficult to maintain the composition of the bath due to the potential
difference [3]. That is why the first copper-tin electrolytes employed the use of poisonous
cyanides and later thio compounds [
4
]. Ionic liquids (ILs) offer a real alternative to aqueous
electrolytes. They have become more and more popular in recent years. They have a much
larger potential window without side reactions, and they can build up a coordination
to metal ions, which leads to good solubility of many metal ions. In addition, they are
Coatings 2021,11, 310. https://doi.org/10.3390/coatings11030310 https://www.mdpi.com/journal/coatings

Coatings 2021,11, 310 2 of 9
mostly non-toxic and therefore do require no treatment of the bath waste. As opposed
to many other organic solvents such as dichloromethane, they are electrochemically and
temperature stable in addition to being less flammable [
5
,
6
]. For these reasons, many
electrochemical properties of metals in ionic liquids have already been studied [
7
–
10
].
There have been experiments with Cu–Sn layers for deposition from ionic liquids. Murase
and co-workers electrodeposited Cu–Sn alloys and discussed the formation mechanism of
the intermetallic phases and their reduction potential [
11
,
12
]. Hsieh performed a deposition
of Cu–Sn alloy in 1-ethyl-3-methylimidazolium dicyanamide ([EMIM-DCA]), showing
that the formed nanobrush-shaped alloys have a phase composition of Cu
3
Sn and Sn [
13
].
Akira and Murase et al. coated non-conducting polymer structures with Cu-Sn layers.
First, they coated the substrate chemically with copper in 1-ethyl-3-methylimidazolium
bis(trifluoromethanesulfonyl)imide ([EMI-Tf
2
N]), and an electrochemical deposition in
the same ionic liquid was subsequently carried out. They obtained a layer consisting of
Cu
6
Sn
5
and/or Cu
3
Sn phases. Unfortunately, they do not give any information about the
thickness or the deposition rate [
14
,
15
]. Ghosh conducted investigations on the influence
of the concentrations of the metal salts on one another in choline chlorides. They were
able to achieve a deposition rate of up to (0.040
±
0.004)
µ
m/min on a steel substrate [
16
].
Jie et al. coated Cu–Sn alloys in 1-butyl-3-methylimidazolium chloride ([BMIM]Cl). The
deposition took place during cyclic voltammetry and the crystallization mechanisms was
studied with chronoamperometry [
17
]. These works illustrate an increasing interest in the
development of a suitable ionic liquid electrolyte.
In general aqueous media, Brenner described the deposition of copper-tin alloys in
cyanide baths as an irregular alloy system [
3
]. Such processes are characterized by the
fact that diffusion phenomena and thus deposition conditions play a subordinate role
during the deposition. Moreover, the individual potentials of the metal ions have a greater
influence on the layer composition. This is most common for electrolytes in which the
metal ions are complexly bound, which affects their potential, e.g., cyanide copper and
zinc. The irregular type of deposition occurs mainly in metals whose static potential are
close to one another. The nobler metal is deposited first. The proportion of the nobler
component in the layer increases logarithmically with the proportion of the metal ions
present in the electrolyte and asymptotically approaches a limit. To determine whether
this behaviour is also correct for the system [EMIM]Cl ionic liquid, CuCl
2
and SnCl
2
are
investigated in this study. [EMIM]Cl was chosen as a solvent based on previous promising
results, such as in the deposition of Al-W alloys from [EMIM]Cl [
18
] and Ag-W alloys from
[EMIM]Cl/AlCl
3
[
12
]. In addition, it is important for the further development of ionic
liquids as electrolytes to improve the deposition performance and better understand the
interrelations between different bath parameters such as the metal-salt concentration and
the obtained layer.
2. Materials and Methods
The electrolyte consists of Sn(II)-chloride (anhydrous) SnCl
2
(99.9%, abcr GmbH,
Germany), Cu(II)-chloride (anhydrous) CuCl
2
(99.9%, abcr GmbH, Karlsruhe, Germany),
and 1-Ethyl-3-methylimidazolium chloride [EMIM]Cl (>98%, (Ionic Liquids Technologies
GmbH, Heilbronn, Germany) ionic liquid (IL). The use of the ionic liquid outside the glove
box necessitates a special approach. Water contact with the electrolyte must be avoided.
Therefore, all flasks must be meticulously flushed with Ar. In addition, all tools must be
dried. The substances are hygroscopic and/or release HCl on contact with atmospheric
moisture. The preparation of 40 mL electrolyte took place in a round-bottom flask under
Ar. Before the experiments were conducted, the IL was heated up to 95
◦
C and dried for
4 h under vacuum at 3 mbar. Afterwards, the flask was put back under the protective Ar
atmosphere. Subsequently, the CuCl
2
and the SnCl
2
were added. It was then dried again
for 4 h under vacuum (3 mbar) at 95
◦
C until the metal salts were completely dissolved
and no more bubbles formed in the IL. For the galvanostatic deposition, the electrolyte
was put into another Ar-purged flask. An industrial Zn-covered hull-cell steel plate with

Coatings 2021,11, 310 3 of 9
a defined surface of 10
×
10 mm
2
was used as the cathode. The Zn layer was removed
by HCl (6 M), and the hull-cell plate was subsequently cleansed with ethanol. The anode,
which was a Sn pin (d= 6 mm), was positioned at a distance of approximately 40 mm
from the cathode. The electrochemical deposition was carried out at 95
◦
C under constant
stirring (100 rpm) with a magnetic stirring bar and Ar counterflow for 3 h. These conditions
had to be chosen because the necessary viscosity of the ionic liquid required it. If lower
stirring speeds or temperatures would have been selected, this would have resulted in the
solidification of the electrolyte, impairing the deposition. The plating baths used to deposit
the different alloy compositions contained different concentrations, as shown in Table 1,
ranging from 0 to 0.29 M SnCl
2
and 0.01 to 0.30 M CuCl
2
, so that the resultant electrolyte
ratios cCuCl2/(cCuCl2+cSnCl2were 0.03, 0.17, 0.50, 0.67, 0.83, or 1.00.
Table 1. Electrolyte composition.
cCuCl2
(cCuCl2+cSnCl2)
cSnCl2
(mol/L)
cCuCl2
(mol/L)
0.03 0.29 0.01
0.17 0.24 0.05
0.50 0.15 0.15
0.67 0.10 0.20
0.83 0.05 0.25
1.00 0.00 0.30
A potentiostat (Autolab PGSTAT204, Metrohm, Herisau, Switzerland) was used in the
galvanostatic mode. The potential values for the different depositions were not measured.
After the deposition, the cathode was removed from the electrolyte and rinsed first with
ethanol, then water. To avoid possible contamination by side reactions during the process,
every deposition was carried out in freshly prepared electrolyte. The deposit’s morphology
and composition were determined with a scanning electron microscope (NEON40EsB,
Zeiss, Oberkochen, Germany), which had an energy-dispersive X-ray spectroscopy (EDX)
system and a secondary-electron (SE) detector with an excitation voltage of 25 kV, and with
optical emission spectrometry (OES, Optima 8300, Perkin Elmer, Waltham, MA, USA). For
X-ray diffraction (XRD, D8 Discover, Bruker AXS, Karlsruhe, Germany) a diffractometer
with Co K
α
radiation (tube parameters: point focus, 40 kV, 40 mA), polycap optics, a
pinhole aperture with a diameter of 0.5 mm, and a 1D detector LYNXEYE XE with a step
size of 0.02
◦
and 17.8 s/step were used. The phase assignment was done using the PDF-2
2014 database. The potentiostat was also used for the measurement of cyclovoltammograms
with a sweep rate of 0.01 V/s. A three-electrode array was used. The arrangement was
chosen in the same way as in the practical deposition with the steel sheet to be coated as
working electrode, the tin pin as counter electrode, and a platinum sheet (5
×
10 mm
2
) as
reference electrode.
3. Results and Discussion
The coatings were deposited with different concentration ratios of CuCl
2
and SnCl
2
,
and the resulting effect on the coating was investigated. Figure 1shows how the Cu content
of the deposited layer changes with the ratio of Cu
2+
ions to Sn
2+
ions in the electrolyte.
The different ratios of the electrolyte concentrations are shown on the x-axis and the
resulting alloy compositions on the y-axis. A logarithmic course can be seen. The curve is
asymptotically approaching 0.88% mass fraction, which represents the maximum value of
copper in the layer. In addition, the layer appears to become thicker with an increasing
ratio of copper to tin ions. However, the results are too scattered to make a conclusive
statement on the relationship between layer thickness and metal salt concentration in the
electrolyte. The relationship between layer thickness and metal salt concentration is shown
in Figure 1b. The alloy compositions shown here were determined by XRF, but the accuracy

Coatings 2021,11, 310 4 of 9
of these values was randomly checked by OES. The deviation between the two methods
was less than 5%, demonstrating that the XRF results are accurate.
Coatings 2021, 11, x FOR PEER REVIEW 4 of 9
shown in Figure 1b. The alloy compositions shown here were determined by XRF, but the
accuracy of these values was randomly checked by OES. The deviation between the two
methods was less than 5%, demonstrating that the XRF results are accurate.
Figure 1. Illustration of the relationship between electrolyte composition and resulting alloy layer.
Amount of copper in the layer (a) and the obtained layer thickness (b). Deposition of Cu−Sn in
[EMIM]Cl with different Cu and Sn concentration ratios (0.5 A/dm2, 95 °C, 100 rpm, 10,800 s) outside
the glove box.
The literature described the concentrations of the individual metals in the electrolyte
as not corresponding to the metal proportion in the obtained alloy deposits [17]. However,
this has been shown to not be valid for this electrolyte system, and the individual metal
concentrations correspond to their proportion in the deposit. The logarithmic course of
the different ratios of the electrolyte concentrations to the resulting alloy composition
shows a good agreement with the results from aqueous electrolytes that had been previ-
ously described by Brenner for aqueous cyanide Cu−Sn electrolytes ([3], pp. 82 and 512).
There, the curves denoting the percentage of tin in the deposit lay below an average value
at which the alloy composition is comparable to the conditions in the electrolyte, indicat-
ing that the percentage of tin in the deposit was lower than the metal percentage of tin in
the bath. Therefore, tin is less readily deposited than copper. The results (Figure 1) show
the same trend, confirming the statement that copper is preferentially deposited. In aque-
ous systems, the influence of the concentrations of complexing agents such as hydroxyl
and cyanide ions on the potentials of the metals and on their deposition efficiency play a
more important role than diffusion phenomena in the composition of the deposit. Similar
behaviour is exhibited in the depositions from the ionic liquid, even though no complex-
ing agents were added. This leads to the conclusion that the ionic liquid itself may act as
a complexing agent. The obtained Cu−Sn deposit is adhesive and compact over the entire
range as shown in Figure 2. The color of the layer ranges from gray with low Cu content
to the typical copper red. It was also found that a relatively higher copper-ion concentra-
tion in comparison to the tin-ion concentration in the bath creates a thicker layer regard-
less of the coating time; consequently, a higher growth rate is also found, as seen in the
comparison of the three-hour depositions shown here. It was possible to get layer thick-
nesses of up to 15 µm within the deposition time of 3 h, which corresponds to a mean
growth rate of 0.083 µm/min. Up until now, previous studies had only reached a layer
thickness of 10 µm in 4 h, a growth rate of 0.04 µm/min [5].
y = 0.1901ln(x) + 0.8873
0
0,25
0,5
0,75
1
0,0 0,2 0,4 0,6 0,8 1,0
w
Cu
c
Cu2+
/(c
Cu2+
+c
Sn2+
)
0
5
10
15
20
25
0,0 0,2 0,4 0,6 0,8 1,0
s/µm
c
Cu2+
/(c
Cu2+
+c
Sn2+
)
ab
0.2 0.4 0.6 0.8 1.0 0.2 0.4 0.6 0.8 1.0
0
0.25
0.50
0.75
1.0
0
10
15
20
25
5
Figure 1.
Illustration of the relationship between electrolyte composition and resulting alloy layer.
Amount of copper in the layer (
a
) and the obtained layer thickness (
b
). Deposition of Cu
−
Sn in
[EMIM]Cl with different Cu and Sn concentration ratios (0.5 A/dm
2
, 95
◦
C, 100 rpm, 10,800 s) outside
the glove box.
The literature described the concentrations of the individual metals in the electrolyte
as not corresponding to the metal proportion in the obtained alloy deposits [
17
]. However,
this has been shown to not be valid for this electrolyte system, and the individual metal
concentrations correspond to their proportion in the deposit. The logarithmic course of the
different ratios of the electrolyte concentrations to the resulting alloy composition shows
a good agreement with the results from aqueous electrolytes that had been previously
described by Brenner for aqueous cyanide Cu
−
Sn electrolytes ([
3
], pp. 82, 512). There, the
curves denoting the percentage of tin in the deposit lay below an average value at which
the alloy composition is comparable to the conditions in the electrolyte, indicating that
the percentage of tin in the deposit was lower than the metal percentage of tin in the bath.
Therefore, tin is less readily deposited than copper. The results (Figure 1) show the same
trend, confirming the statement that copper is preferentially deposited. In aqueous systems,
the influence of the concentrations of complexing agents such as hydroxyl and cyanide ions
on the potentials of the metals and on their deposition efficiency play a more important
role than diffusion phenomena in the composition of the deposit. Similar behaviour is
exhibited in the depositions from the ionic liquid, even though no complexing agents were
added. This leads to the conclusion that the ionic liquid itself may act as a complexing
agent. The obtained Cu
−
Sn deposit is adhesive and compact over the entire range as
shown in Figure 2. The color of the layer ranges from gray with low Cu content to the
typical copper red. It was also found that a relatively higher copper-ion concentration in
comparison to the tin-ion concentration in the bath creates a thicker layer regardless of the
coating time; consequently, a higher growth rate is also found, as seen in the comparison
of the three-hour depositions shown here. It was possible to get layer thicknesses of up
to 15
µ
m within the deposition time of 3 h, which corresponds to a mean growth rate of
0.083
µ
m/min. Up until now, previous studies had only reached a layer thickness of 10
µ
m
in 4 h, a growth rate of 0.04 µm/min [5].
The layer with the highest copper content as shown in Figure 3has a rough surface
comprised of spherical, well-distributed grains across the entire surface with few visible
defects. The formed spheres have an average diameter of approximately 20
µ
m, which is
shown in the top-surface SEM image. The dendritic morphology is usually associated with
coatings in which the deposition of copper ions is favored. For aqueous electrolytes, litera-
ture reported the morphology of Cu
−
Sn alloys in the form of spherical grains, needles, or
with a dendritic morphology [
19
–
22
]. A morphology that is similar to other IL depositions,

Coatings 2021,11, 310 5 of 9
e.g., for those of Pd and Pt, was observed [
23
]. Despite the sphere-like morphology, the
deposited layers are compact and adhesive.
Coatings 2021, 11, x FOR PEER REVIEW 5 of 9
Figure 2. Optical-microscopic grey scale image of the cross-section from a Cu−Sn layer (0.5 A/dm
2
,
95 °C, 100 rpm, 10,800 s) with 85% mass fraction of copper deposited outside the glove box.
The layer with the highest copper content as shown in Figure 3 has a rough surface
comprised of spherical, well-distributed grains across the entire surface with few visible
defects. The formed spheres have an average diameter of approximately 20 µm, which is
shown in the top-surface SEM image. The dendritic morphology is usually associated with
coatings in which the deposition of copper ions is favored. For aqueous electrolytes, liter-
ature reported the morphology of Cu−Sn alloys in the form of spherical grains, needles,
or with a dendritic morphology [19–22]. A morphology that is similar to other IL deposi-
tions, e.g., for those of Pd and Pt, was observed [23]. Despite the sphere-like morphology,
the deposited layers are compact and adhesive.
Figure 3. Deposition of a Cu−Sn alloy on steel substrate (a) and scanning electron microscopy (SEM)
images (b,c) with different magnifications of the surface of a Cu−Sn layer (0.5 A/dm
2
, 95 °C, 100 rpm,
10,800 s) with 85% mass fraction of copper deposited outside the glove box.
Cyclovoltammetric (CV) investigations in [EMIM]Cl with 0.075 mol/L SnCl
2
and
0.225 mol/L CuCl
2
are shown in Figure 4. These investigations were performed during the
regular electroplating process with a platinum sheet serving as the quasi-reference elec-
trode to the anode-cathode arrangement (steel cathode as the working electrode and the
tin anode as the counter electrode).
Figure 2.
Optical-microscopic grey scale image of the cross-section from a Cu
−
Sn layer (0.5 A/dm
2
,
95 ◦C, 100 rpm, 10,800 s) with 85% mass fraction of copper deposited outside the glove box.
Coatings 2021, 11, x FOR PEER REVIEW 5 of 9
Figure 2. Optical-microscopic grey scale image of the cross-section from a Cu−Sn layer (0.5 A/dm
2
,
95 °C, 100 rpm, 10,800 s) with 85% mass fraction of copper deposited outside the glove box.
The layer with the highest copper content as shown in Figure 3 has a rough surface
comprised of spherical, well-distributed grains across the entire surface with few visible
defects. The formed spheres have an average diameter of approximately 20 µm, which is
shown in the top-surface SEM image. The dendritic morphology is usually associated with
coatings in which the deposition of copper ions is favored. For aqueous electrolytes, liter-
ature reported the morphology of Cu−Sn alloys in the form of spherical grains, needles,
or with a dendritic morphology [19–22]. A morphology that is similar to other IL deposi-
tions, e.g., for those of Pd and Pt, was observed [23]. Despite the sphere-like morphology,
the deposited layers are compact and adhesive.
Figure 3. Deposition of a Cu−Sn alloy on steel substrate (a) and scanning electron microscopy (SEM)
images (b,c) with different magnifications of the surface of a Cu−Sn layer (0.5 A/dm
2
, 95 °C, 100 rpm,
10,800 s) with 85% mass fraction of copper deposited outside the glove box.
Cyclovoltammetric (CV) investigations in [EMIM]Cl with 0.075 mol/L SnCl
2
and
0.225 mol/L CuCl
2
are shown in Figure 4. These investigations were performed during the
regular electroplating process with a platinum sheet serving as the quasi-reference elec-
trode to the anode-cathode arrangement (steel cathode as the working electrode and the
tin anode as the counter electrode).
Figure 3.
Deposition of a Cu
−
Sn alloy on steel substrate (
a
) and scanning electron microscopy (SEM)
images (
b
,
c
) with different magnifications of the surface of a Cu
−
Sn layer (0.5 A/dm
2
, 95
◦
C, 100 rpm,
10,800 s) with 85% mass fraction of copper deposited outside the glove box.
Cyclovoltammetric (CV) investigations in [EMIM]Cl with 0.075 mol/L SnCl
2
and
0.225 mol/L CuCl
2
are shown in Figure 4. These investigations were performed during
the regular electroplating process with a platinum sheet serving as the quasi-reference
electrode to the anode-cathode arrangement (steel cathode as the working electrode and
the tin anode as the counter electrode).
In the performed cyclovoltamogramic measurements, there are four reduction peaks
located at 0.10 to
−
0.50 V (R1),
−
1.20 to
−
1.35 V (R2),
−
1.35 to
−
1.90 V (R3) and
−
1.90
to
−
2.40 V (R4). After the deposited layer was examined for its composition, the peaks
in the recorded voltammogram were able to be assigned to copper or tin. According to
the literature [
17
], there should be just three reduction peaks. This, combined with the
limitations of the conducted investigations, renders it impossible to say with certainty
which reduction reaction is responsible for which peak. R1 is likely caused by the reduc-
tion of Cu
2+ →
Cu
+
. The literature assigned R2 to the reduction peak of Cu
2+
and Sn
2+
codeposition [
17
]. The assignments of R3 and R4 are more ambiguous: one of them is

Coatings 2021,11, 310 6 of 9
probably caused by the reduction of Cu
+→
Cu
0
. The other could stem from the reduction
of Sn
2+ →
Sn
0
; perhaps in [EMIM]Cl, this reduction is better resolved, resulting in the
appearance of a new reduction peak. In the obtained CV, two oxidation peaks can be
observed: one between 0 V and 0.5 V and one in the region of 2 V. The first should belong
to the oxidation of Cu
0→
Cu
+
, while the latter, when considered in combination with
the background/blank measurement, is assumed to belong to the oxidative degradation
of the ionic liquid. According to the literature [
17
], however, there should be a total of
three oxidation peaks, the two already described as well as a third peak between them, a
peak not found in the [EMIM]Cl-based electrolyte. This absent peak would have shown
the oxidation of Cu
+
, Cu
−
Sn alloy, and water. One possible explanation for its absence
is that no further oxidation, i.e., dissolution, of Cu or Cu
−
Sn occurs in the system with
[EMIM]Cl. Since this reaction cannot be observed here, it also suggests that the electrolyte
is less susceptible to the absorption of water. The reduction and oxidation processes that
were associated with water (by Jie [
17
]) are also absent in the presented system, further sug-
gesting that, in contrast to these [BMIM]Cl deposits, the [EMIM]Cl-based electrolyte shown
here is less susceptible to water, which is beneficial for deposition outside the glove box.
This could enable the deposition of these layers under near-industrial conditions and avoid
time-consuming handling in a glove box. In addition, there are also less process-related,
oxidative resolutions, which allow for uniform and fast layer growth.
Coatings 2021, 11, x FOR PEER REVIEW 6 of 9
Figure 4. Cyclovoltammogram in pure [EMIM]Cl electrolyte as background ((a), red) and [EMIM]Cl
with 0.075 mol/L SnCl
2
and 0.225 mol/L CuCl
2
at a sweep rate of 0.01 V/s ((b), black).
In the performed cyclovoltamogramic measurements, there are four reduction peaks
located at 0.10 to −0.50 V (R1), −1.20 to −1.35 V (R2), −1.35 to −1.90 V (R3) and −1.90 to −2.40
V (R4). After the deposited layer was examined for its composition, the peaks in the rec-
orded voltammogram were able to be assigned to copper or tin. According to the literature
[17], there should be just three reduction peaks. This, combined with the limitations of the
conducted investigations, renders it impossible to say with certainty which reduction re-
action is responsible for which peak. R1 is likely caused by the reduction of Cu
2+
→ Cu
+
.
The literature assigned R2 to the reduction peak of Cu
2+
and Sn
2+
codeposition [17]. The
assignments of R3 and R4 are more ambiguous: one of them is probably caused by the
reduction of Cu
+
→ Cu
0
. The other could stem from the reduction of Sn
2+
→ Sn
0
; perhaps
in [EMIM]Cl, this reduction is better resolved, resulting in the appearance of a new reduc-
tion peak. In the obtained CV, two oxidation peaks can be observed: one between 0 V and
0.5 V and one in the region of 2 V. The first should belong to the oxidation of Cu
0
→ Cu
+
,
while the latter, when considered in combination with the background/blank measure-
ment, is assumed to belong to the oxidative degradation of the ionic liquid. According to
the literature [17], however, there should be a total of three oxidation peaks, the two al-
ready described as well as a third peak between them, a peak not found in the [EMIM]Cl-
based electrolyte. This absent peak would have shown the oxidation of Cu
+
, Cu−Sn alloy,
and water. One possible explanation for its absence is that no further oxidation, i.e., dis-
solution, of Cu or Cu−Sn occurs in the system with [EMIM]Cl. Since this reaction cannot
be observed here, it also suggests that the electrolyte is less susceptible to the absorption
of water. The reduction and oxidation processes that were associated with water (by Jie
[17]) are also absent in the presented system, further suggesting that, in contrast to these
[BMIM]Cl deposits, the [EMIM]Cl-based electrolyte shown here is less susceptible to wa-
ter, which is beneficial for deposition outside the glove box. This could enable the depo-
sition of these layers under near-industrial conditions and avoid time-consuming han-
dling in a glove box. In addition, there are also less process-related, oxidative resolutions,
which allow for uniform and fast layer growth.
XRD measurements were carried out and a typical diffractogram is shown in
Figure 5. The layer shows reflections at 40.4°, 50.0°, 73.4°, 94.0°, and 108.0°, which are at-
tributable to the crystalline Cu
41
Sn
11
phase (PDF 01-071-7876). Two cubic CuSn lattices
Figure 4.
Cyclovoltammogram in pure [EMIM]Cl electrolyte as background ((
a
), red) and [EMIM]Cl
with 0.075 mol/L SnCl2and 0.225 mol/L CuCl2at a sweep rate of 0.01 V/s ((b), black).
XRD measurements were carried out and a typical diffractogram is shown in Figure 5.
The layer shows reflections at 40.4
◦
, 50.0
◦
, 73.4
◦
, 94.0
◦
, and 108.0
◦
, which are attributable
to the crystalline Cu
41
Sn
11
phase (PDF 01-071-7876). Two cubic CuSn lattices with slightly
different lattice parameters are present-minor shoulders can be observed at the main
reflections. This indicates that the cubic phase was deposited with two slightly different
compositions. The reflection at 45
◦
is narrow because it corresponds to the Co K
β
reflection
of the {660} peak of Cu
41
Sn
11
. No matching phase from the PDF-2 database could be
assigned to the peak at 52◦.

Coatings 2021,11, 310 7 of 9
Coatings 2021, 11, x FOR PEER REVIEW 7 of 9
with slightly different lattice parameters are present-minor shoulders can be observed at
the main reflections. This indicates that the cubic phase was deposited with two slightly
different compositions. The reflection at 45° is narrow because it corresponds to the Co
Kβ reflection of the {660} peak of Cu
41
Sn
11
. No matching phase from the PDF-2 database
could be assigned to the peak at 52°.
Figure 5. XRD measurement of a Cu–Sn layer with 85% mass fraction of copper obtained from
[EMIM]Cl measured with Co Kα radiation. According to the PDF-2 database (PDF 01-071-7876), the
lattice parameter of the Cu
41
Sn
11
phase is 17.964 Å. For the {660} peak, the corresponding peak orig-
inating from Co Kβ radiation can be observed as well (at a diffraction angle of about 45°).
Typical EDX results of a Cu−Sn layer are shown in Figure 6. Cu and Sn can clearly be
detected in the layer. The spectrum was recorded up to 40 keV and shows peaks of Cu Lα
at 0.93 keV, Cu Kα at 8.04 keV, and Cu Kβ at 8.90 keV. For tin, five characteristic peaks
were obtained: 0.42 keV of Sn Mα, 3.47 keV of Sn Lα, 3.68 keV and 3.96 keV for SnLβ, and
4.18 keV for SnLγ.
Figure 6. EDX area measurement of a Cu−Sn layer (0.5 A/dm
2
, 95 °C, 100 rpm, 10,800 s) with 85%
mass fraction of copper.
20 30 40 50 60 70 80 90 100 110 120 130
0
100
200
300
1100
Intensity/counts
2
θ
/°
35 40 45 50 55 60 65
0
50
100
150
200
{660}
{
12 00
}
{
12 66
}
{
444
}
{
14 82
}
{
642
}
{
642
}
{
844
}
{
10 20
}
{
12 12 0
}
{
10 10 0
}
PDF 01-071-7876
Cu
41
Sn
11
(cubic)
a=17.92585 Å
Cu
41
Sn
11
(cubic)
a= 18.00956 Å
Figure 5.
XRD measurement of a Cu–Sn layer with 85% mass fraction of copper obtained from
[EMIM]Cl measured with Co K
α
radiation. According to the PDF-2 database (PDF 01-071-7876),
the lattice parameter of the Cu
41
Sn
11
phase is 17.964 Å. For the {660} peak, the corresponding peak
originating from Co Kβradiation can be observed as well (at a diffraction angle of about 45◦).
Typical EDX results of a Cu
−
Sn layer are shown in Figure 6. Cu and Sn can clearly
be detected in the layer. The spectrum was recorded up to 40 keV and shows peaks of Cu
L
α
at 0.93 keV, Cu K
α
at 8.04 keV, and Cu K
β
at 8.90 keV. For tin, five characteristic peaks
were obtained: 0.42 keV of Sn M
α
, 3.47 keV of Sn L
α
, 3.68 keV and 3.96 keV for SnL
β
, and
4.18 keV for SnLγ.
Coatings 2021, 11, x FOR PEER REVIEW 7 of 9
with slightly different lattice parameters are present-minor shoulders can be observed at
the main reflections. This indicates that the cubic phase was deposited with two slightly
different compositions. The reflection at 45° is narrow because it corresponds to the Co
Kβ reflection of the {660} peak of Cu
41
Sn
11
. No matching phase from the PDF-2 database
could be assigned to the peak at 52°.
Figure 5. XRD measurement of a Cu–Sn layer with 85% mass fraction of copper obtained from
[EMIM]Cl measured with Co Kα radiation. According to the PDF-2 database (PDF 01-071-7876), the
lattice parameter of the Cu
41
Sn
11
phase is 17.964 Å. For the {660} peak, the corresponding peak orig-
inating from Co Kβ radiation can be observed as well (at a diffraction angle of about 45°).
Typical EDX results of a Cu−Sn layer are shown in Figure 6. Cu and Sn can clearly be
detected in the layer. The spectrum was recorded up to 40 keV and shows peaks of Cu Lα
at 0.93 keV, Cu Kα at 8.04 keV, and Cu Kβ at 8.90 keV. For tin, five characteristic peaks
were obtained: 0.42 keV of Sn Mα, 3.47 keV of Sn Lα, 3.68 keV and 3.96 keV for SnLβ, and
4.18 keV for SnLγ.
Figure 6. EDX area measurement of a Cu−Sn layer (0.5 A/dm
2
, 95 °C, 100 rpm, 10,800 s) with 85%
mass fraction of copper.
20 30 40 50 60 70 80 90 100 110 120 130
0
100
200
300
1100
Intensity/counts
2
θ
/°
35 40 45 50 55 60 65
0
50
100
150
200
{660}
{
12 00
}
{
12 66
}
{
444
}
{
14 82
}
{
642
}
{
642
}
{
844
}
{
10 20
}
{
12 12 0
}
{
10 10 0
}
PDF 01-071-7876
Cu
41
Sn
11
(cubic)
a=17.92585 Å
Cu
41
Sn
11
(cubic)
a= 18.00956 Å
Figure 6.
EDX area measurement of a Cu
−
Sn layer (0.5 A/dm
2
, 95
◦
C, 100 rpm, 10,800 s) with 85%
mass fraction of copper.
C and especially O can only be detected outside the layer, in the region of the embed-
ding agent of the cross-section. The oxygen content was measured in the same manner
and must be attributed to fast oxidation during the preparation process. As no further
signals were detected in the XRD diffractogram and EDX, the results suggest a clear alloy
deposition with no crystalline oxides or fragments of the IL incorporated into the layer,
demonstrating that it is a pure Sn–Cu system.

Coatings 2021,11, 310 8 of 9
4. Conclusions
The deposition was done in [EMIM]Cl ionic liquid for 3 h, at a current density of
0.5 A/dm
2
and at 95
◦
C. Thick compact layers with a thickness of 15
µ
m were obtained.
The layers were examined by optical and electron microscopy, and it was found that the
layer consisted of spherical grains. Differences in composition and thickness of the alloy
layer are affected by the variation in the metal-salt concentration in the electrolyte. The
results prove that an irregular alloy deposition occurs as earlier described by Brenner for
complexed electrolytes such as Cu
−
Sn with cyanides in water [
3
]. The absence of water has
been confirmed by cyclovoltammetry. The resulting layer thickness and alloy composition,
which was measured with XRF and OES, have also been confirmed microscopically, with
EDX, and XRD. Further tribological tests of these layers could determine whether the layers
would be suitable for practical applications. However, the work shows that it is possible to
produce thick electrodeposited bronze layers outside of the glove box using ionic liquids.
This is a further step towards replacing conventional aqueous electrolytes containing
cyanide with ionic liquids. As each deposition was performed in a fresh electrolyte, the
ageing behavior was not investigated in this work. This would have to be explored in
further work.
Author Contributions:
Conceptualization, L.L., D.H., T.M., and T.L.; methodology, L.L., D.H., and
T.M.; validation, L.L. and D.H.; formal analysis, L.L. and T.M.; investigation, L.L. and T.M.; writing—
original draft preparation, L.L. and D.H.; writing—review and editing, T.M. and T.L.; supervision,
T.M. and T.L.; project administration, L.L., D.H., and T.L. All authors have read and agreed to the
published version of the manuscript.
Funding: The publication of this article was funded by Chemnitz University of Technology.
Data Availability Statement:
The data presented in this study are available on request from the
corresponding author.
Acknowledgments:
The authors acknowledge the support from the Institute of materials science
and materials engineering at the Chemnitz University of Technology. We would like to thank Marc
Pügner for the XRD measurements, Steffen Clauß for the SEM measurements, and Morgan Uland for
English proofreading.
Conflicts of Interest: The authors declare no conflict of interest.
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