Electrochemical deposition of silver from 1-ethyl-3-methylimidazolium trifluoromethanesulfonate

Article abstract of DOI:10.1016/j.electacta.2011.04.060

Silver was successfully electrodeposited from the ionic liquid 1-ethyl-3-methylimidazolium trifluoromethanesulfonate ([EMIm][TfO]) containing silver ions in various concentrations. The electrochemical windows of the electrolytes were determined by cyclic

Full text of DOI:10.1016/j.electacta.2011.04.060

Electrochimica Acta 56 (2011) 10332–10339
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Electrochimica Acta
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Electrochemical deposition of silver from 1-ethyl-3-methylimidazolium
trifluoromethanesulfonate
Adriana Ispasa,∗, Manuel Pöllethb, Khanh Hoa Tran Bab, Andreas Bunda, Jürgen Janekb
a Fachgebiet Elektrochemie und Galvanotechnik, Technische Universität Ilmenau, 98693 Ilmenau, Germany
b Physikalisch-Chemisches Institut, Justus-Liebig-Universität Gießen, 35392 Gießen, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Silver was successfully electrodeposited from the ionic liquid 1-ethyl-3-methylimidazolium trifluo-
romethanesulfonate ([EMIm][TfO]) containing silver ions in various concentrations. The electrochemical
windows of the electrolytes were determined by cyclic voltammetry. Moreover, the conductivity and the
viscosity–density product were studied with electrochemical impedance spectroscopy (EIS) and elec-
trochemical quartz crystal microbalance (EQCM). Potentiodynamic and potentiostatic depositions were
performed at different temperatures. The morphology of the deposited Ag layers depends on temperature
and concentration. Thus, by choosing the right experimental conditions one can optimize the properties
of the deposited layers. The thus obtained layers can be used in different surface finishing processes, such
as in decorative plating, catalysis or for electrical contacts.
Received 8 December 2010
Received in revised form 15 April 2011
Accepted 18 April 2011
Available online 23 April 2011
Keywords:
Ag electrodeposition
[EMIm][TfO]
EQCM
© 2011 Elsevier Ltd. All rights reserved.
Physicochemical properties
1. Introduction
Pt, Cu and Au substrates [16], in order to monitor in situ the
electrodeposition process. The solutions used in these studies
Thin silver layers find various applications because of their spe-
cial properties, such as good corrosion resistance, shiny surface
(decorative purposes), antibacterial property and high electrical
conductivity [1,2].
were silver bis(trifluoromethylsulfonyl)amide (AgTFSA) in 1-butyl-
1methylpyrrolidinium bis(trifluoromethylsulfonyl)amide in [13]
and AgNO3, AgCl, AgSO4, and Ag(CH3COO) in choline chloride based
ILs (with ethylene glycol or with urea) in [16].
Silver electrodeposition from ionic liquids (ILs) presents an envi-
ronmentally friendly alternative to the classical aqueous cyanide
baths [1] or plating baths containing many organic and inorganic
additives for suppressing the dendritic growth [3].
A recently published review [4] discusses some of the tech-
niques and the ILs used so far for the electrodeposition of Ag from
non-aqueous media. Further reports on Ag deposition from various
ILs, with different Ag precursors as well as the deposit morpholo-
gies (nanowires, nanoparticles, thin layers) can be found in the
literature [5–19].
In a previous study, Zein El Abedin and Endres studied the
electrodeposition of thin Ag layers and Ag nanowires from a
solution of AgTfO in [EMIm][TfO] onto Au, glassy carbon, and
Au substrates evaporated on a polycarbonate membrane [6]. The
deposits were prepared by cyclic voltammetry (CV) and chronoam-
perometric techniques, and they were characterized by SEM
and XRD. The EQCM (electrochemical quartz crystal microbal-
ance) technique was used by Serizawa et al. [13] and by Abbot
et al. [16] when electrodepositing thin Ag layers on Au [13] or
In the present study we investigated the electrodeposition of
Ag from 1-ethyl-3-methylimidazolium trifluoromethanesulfonate
([EMIm][TfO]) with different concentrations of Ag(I) ions, using
in situ microgravimetry (electrochemical quartz crystal microbal-
ance, EQCM). Our study extends previous reports [6,13,16] that also
applied the EQCM technique or that used [EMIm][TfO] containing
AgTfO for Ag electrodeposition. In our case, all the measurements
were performed in a controlled atmosphere (inside a glove box),
while previously reported results were obtained in ambient atmo-
sphere, under 50% humidity [6]. It is well known that even small
traces of moisture and oxygen can have a strong influence of the
electrochemical behaviour of an IL. We also investigated other com-
binations of IL and silver precursor by EQCM than in [13,16]. The
comparison of the results obtained in this study with the previ-
ously published data should give a deeper understanding of the
silver deposition process in ILs.
Although the electrodeposition of Ag from ILs has been reported
before by other groups, very few attempts have been done in the
literature to correlate the physicochemical properties of the plat-
ing solutions with the properties of the deposited layers. With
this study we intended to fill this gap that exists in the liter-
ature: thus, we determined the electrical conductivity and the
viscosity–density product of the electrolytes and we investigated
∗
Corresponding author. Tel.: +49 367 7693 144; fax: +49 367 7693 104.
E-mail address: adriana.ispas@tu-ilmenau.de (A. Ispas).
0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2011.04.060

A. Ispas et al. / Electrochimica Acta 56 (2011) 10332–10339
10333
The dissolution of Ag was carried out galvanostatically. There-
fore, the silver content in [EMIm][TfO] can be directly calculated
using Faraday’s law. Another method to assess the silver content
was the determination of the mass loss of the silver electrode. In
the following all the silver concentration will be given in terms of
molality (mmol Ag(I) per kg IL). The stock solution was diluted to
a concentration of 202.8 mmol/kg Ag(I). This solution was further
diluted to 101.4 mmol/kg, 50.7 mmol/kg, and 25.3 mmol/kg. The
underlying assumption in calculating the silver content was that
no silver ions were lost due to cation migration towards the cath-
ode. Since all diluted solutions were prepared from the same stock
solution, the concentration ratios are correct even if the absolute
concentration could be slightly different.
The water content of the ILs was determined by Karl Fisher titra-
tion, with a KF Coulometer from Metrohm, model 831. The accuracy
of this device is 5 mg/kg.
2.2. Conductivity of the electrolytes
Fig. 1. Schematic representation of the set-up used for Ag dissolution.
An immersion conductivity cell from Metrohm (theoretical cell
constant 0.80 cm−1) was used to determine the conductivity of the
ILs. The conductivity cell consisted of two Pt electrodes embedded
in glass. The electrode fitted neatly into a temperature-controlled
double jacket glass cell. The exact value of the cell constant
was obtained from measurements at 25 ◦C in 0.1 M KCl Centripur
electrolyte from Merck (specific conductivity, 12.8 mS/cm). These
calibration measurements were the only ones performed outside
the glove box.
the effects produced by the temperature and the concentration
of Ag(I) on the physical properties of the electrolytes and on the
electrodeposited layers. Moreover, using the EQCM technique, we
studied in situ the electrodeposition process and we got informa-
tion on the current efficiency and on the deposited masses, beside
the viscosity–density product of the electrolytes.
2. Experimental
The conductivity of the ILs was determined in the tempera-
ture range between 20 ◦C and 100 ◦C in the double jacket glass cell
within an Ar filled glove box (MBraun, Germany). The temperature
of the IL was measured with a thermocouple with an accuracy of
0.1 ◦C. The oxygen and moisture contents inside the glove box
were always below 2.0 cm3/m3.
The impedance of the conductivity cell was determined with an
IM6 device from Zahner Elektrik (Germany), in the frequency range
from 10 kHz down to 1 Hz, at an amplitude of 50 mV.
2.1. Preparation of AgTfO in [EMIm][TfO]
Several [EMIm][TfO] samples with different Ag(I) concentra-
tions were prepared for the electrodeposition experiments. A stock
solution was obtained by anodic dissolution of a Ag metal foil in the
IL. This stock solution was then diluted to prepare the desired Ag(I)
concentrations. [EMIm][TfO], purchased from Iolitec with a purity
of 99 wt.%, a water content below 10 mg/kg, and an halide content
below 100 mg/kg, was always handled under an argon atmosphere
in a glove box. Two beakers connected via a U-shaped glass tube,
which served as a salt bridge, were used to prepare the solu-
tions (Fig. 1). One beaker contained the working electrode, which
was an electrochemically activated Ag plate (99.995 wt.%, thickness
0.5 mm, approximate area of 1 cm2 per side), and the reference
electrode (Ag wire, 99.995 wt.%, 0.25 mm diameter). The second
beaker housed the counter electrode, which was again a silver plate
similar to the working electrode. All Ag electrodes were electro-
chemically activated prior to the experiments by anodic dissolution
(at 800 ␮A for ca. 10 min) to remove impurities from the surface.
These solutions, in which the electrodes were dissolved anodically
for cleaning, were disposed afterwards. Other cleaning procedures,
such as polishing the electrodes with abrasive paper and a sub-
sequent rinsing with doubly distilled water, ethanol, and acetone
led to unsatisfactory results. Such pre-treatments introduced more
impurities than they removed.
The setup for the dissolution experiment is presented in Fig. 1.
On the left hand side the cation of the IL is reduced, forming rad-
ical intermediate products, which can react further to unknown
coloured decomposition products. If these products would reach
the beaker on the right side, the Ag(I)/[EMIm][TfO] solution would
be contaminated. Hence, the reaction must be stopped before. The
pure product IL solution and the contaminated IL have different
shadings. The opening of the salt bridge, marked by an asterisk (*),
was connected to a syringe via a small flexible tube, with which
a small vacuum slight underpressure was applied to fill the salt
bridge with the IL.
2.3. Electrodeposition of silver
The electrodeposition experiments were performed in a three
electrode arrangement in a PTFE home-made electrochemical cell.
The working electrode was an Au layer evaporated on a quartz
crystal, the counter electrode was a Pt wire and the reference elec-
trode was an Ag wire. The active area of the working electrode
was ca. 0.22 cm2. The quartz crystals with a resonance frequency of
10 MHz (AT cut) with Au electrodes on both sides were purchased
from Vectron International, Germany. The counter and the refer-
ence electrodes were polished with abrasive paper, washed with
doubly distilled water and dried under a N2 flow. Afterwards, the
counter electrode was heated in a butane flame to red glow.
A potentiostat from EG&G (Model 263) was used for electro-
chemical measurements. A network analyser (Agilent E5100) was
used to monitor the shift of the resonance frequency and the dis-
sipated energy of the quartz crystal. The latter was determined via
the full width at half maximum of the resonance curve. From the
shift of the resonance frequency one can calculate the deposited
mass using the Sauerbrey equation [20].
The electrodeposition measurements were performed at dif-
ferent temperatures, from room temperature up to 100 ◦C. The
temperature inside the electrochemical cell was controlled with an
accuracy of 1 ◦C by a Wema temperature controller (Lüdenscheid,
Germany). All electrochemical measurements were performed
inside an Ar filled glove box.
The morphology of the deposited layers was studied by scan-
ning electron microscopy (SEM model DSM 982 Gemini from Zeiss

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A. Ispas et al. / Electrochimica Acta 56 (2011) 10332–10339
20
15
10
5
Oberkochen, Germany). Information on the composition of the Ag
layers was obtained with an energy dispersive X-ray detector (EDX
model Voyager III 3200 from SUS Pioneer), which was incorporated
in the SEM device.
3. Results and discussion
0
3.1. Characterization of the pure ionic liquid
-5
The ILs can contain traces of water that can strongly affect
their physicochemical properties or their electrochemical window
[21–24]. Usually, the conductivity of an IL increases and its viscos-
ity decreases in the presence of water impurities [22,24], while the
electrochemical window shrinks substantially [23,24]. The mani-
fold effects of water on the conductivity and on the viscosity are
often ignored. This is one of the reasons why data reported in the
literature can differ significantly, especially when conductivities
and viscosities of ILs are concerned [22 and references within].
Beside water impurities, further organic or inorganic impurities
can be found in ILs, often remains from the synthesis [25]. Special
attention was given so far to lithium impurities [25]. The group of
Endres has impressively shown that with the in situ-STM one can
detect the UPD of Li and that this can be mistaken for the deposi-
tion of another metal. Halide impurities (e.g., chloride) can generate
unexpected side reactions at the counter electrode [25]. Moreover,
it was shown that small amounts of chloride ions decrease the
conductivity of the ILs [26].
In our case, two different production lots of [EMIm][TfO] were
investigated (of 98 wt.% and 99 wt.% purity). The water content of
the first lot was initially ca. 247 mg/kg, while the other lot con-
tained initially just 2.1 mg/kg of water. Both types of ILs were dried
prior to the experiments for 20 h under vacuum at 80 ◦C. The water
content of the first lot decreased to 3 mg/kg after ca. 200 min and
to 2.1 mg/kg after 20 h, while the water content of the second lot
reached 1.7 mg/kg after 20 h of drying. Thus, with sufficient dry-
ing time one can reach practically the same water content in all
lots, independent of their initial water content or of the presence
of other impurities. The latter reveal themselves as a hue of the
IL, ranging from light yellowish to dark brownish. It is difficult
to identify the organic impurities, and this still remains an unre-
solved research issue. However, it seems that a minimum amount
of water (ca. 2 mg/kg) has to be accepted with the drying procedure
employed here. Furthermore, it is difficult to assess the influence
of heating under vacuum on the organic impurities.
The electrochemical windows of the two dried ILs were approx-
imately 5.4 V at 25 ◦C (Fig. 2, black line). The electrochemical
window was determined by cyclic voltammetry at a scan rate of
20 mV s−1 with Pt plates and wires as working, counter and ref-
erence electrodes. The electrochemical window was considered to
be the potential window defined by the two intersection points
between the ‘zero’ line (which was defined by the potentials around
the open circuit potential value) and the tangents to the reduc-
tion and respectively, to the oxidation potential limits, where the
current increased strongly and continuously, indicating the decom-
position of the ILs.
The presence of chloride impurities in the neat ILs can be cru-
cial in our case (formation of AgCl). The precipitation of AgCl
would lower the concentration of silver ions in our electrolytes.
The EDX (energy dispersive X-ray) spectroscopic analysis of pre-
ceding experiments hinted that the precipitation of AgCl leads to
an even larger loss of Ag(I) due to adsorption of Ag(I) at the surface
of the AgCl precipitate, or to co-precipitation of Ag(I), or to inclu-
sion of Ag(I) in the pores of the AgCl precipitate formed. Therefore,
we checked for the presence of chloride in the neat ILs by cyclic
voltammetry. Small amounts of NaCl were added to the neat ILs
-10
-15
-4
-2
0
2
4
E vs Pt / V
Fig. 2. Cyclic voltammograms (20 mV s−1) of the neat IL [EMIm][TfO] (black, con-
tinuous line) and of the IL spiked with a very small amount of NaCl (red, dashed
line). Working, reference and counter electrodes were Pt plates and wires in these
experiments. (For interpretation of the references to colour in this figure legend, the
reader is referred to the web version of the article.)
50
40
30
40
20
10
30
0
0
20 40 60 80 100
T / °C
20
10
0
0
20
40
60
80
100
T / °C
Fig. 3. Specific electrical conductivity of IL containing different amounts of Ag(I) as
a function of temperature. (-ꢀ-) neat [EMIm][TfO], (-•-) 202.8 mmol/kg Ag(I), (-ꢁ-)
101.4 mmol/kg Ag(I), (-ꢂ-) 50.7 mmol/kg Ag(I) (-ꢃ-) 25.3 mmol/kg Ag(I). Inset in the
figure shows the electrical conductivity of the neat [EMIm][TfO].
and the CVs of the spiked ILs were compared to those of the neat
IL. We found new peaks in the spiked IL, which were attributed to
the added NaCl (Fig. 2). As these peaks were absent in the neat IL
we conclude that chlorine content of ILs is very low.
3.2. Physicochemical properties of the IL based silver electrolytes
3.2.1. Ionic conductivity
The ionic conductivity of the ILs was extracted from the
impedance spectra (raw data not shown). The spectra were fitted
with the commercial software package Origin. The equivalent cir-
cuit consisted of a series combination of a capacitance and a resistor.
The capacitance reflects the double layers of the two Pt electrodes.
The specific conductivity of the ILs can then be calculated from the
fit value of the resistor, R, and the cell constant, C, according to Eq.
(1) [26].
1
R
ꢀ =
C
(1)
The conductivities of the pure IL (inset in Fig. 3) and the electrolytes
containing various concentrations of Ag(I) (Fig. 3), were deter-
mined in the temperature range from 10 ◦C to 100 ◦C. The specific

A. Ispas et al. / Electrochimica Acta 56 (2011) 10332–10339
10335
conductivity increases more than 8 times at 100 ◦C compared to
100
80
60
40
20
0
10 ◦C (Fig. 3). Higher conductivities were found for the ILs con-
taining Ag ions. The IL with 25.3 mmol/kg Ag(I) had the highest
conductivity in the entire temperature range investigated here.
Above this concentration of Ag the conductivity decreases with
increasing Ag concentration (Fig. 3). The conductivity of the solu-
tion with 202.8 mmol/kg Ag(I) was just slightly higher than that of
the pure IL.
The diameter of the silver ion ranges between 134 pm and
256 pm for coordination numbers between 2 and 8 [27]. This leads
to ion volumes of 1.26 × 10−3 nm3 and 8.78 × 10−3 nm3, respec-
tively. The ion volume of the EMIm+ ion is 0.156 nm3 [28]. Due
to its smaller size one would expect that the silver ion has a much
higher mobility compared to the imidazolium cation, leading to
an increased conductivity with increasing concentration. How-
ever, this is only the case for the sample with a concentration of
25.3 mmol/kg. If the concentration of Ag ions in the IL is higher
than this value, it seems that the mobility of the ions is hindered.
This fact can be due to an association of the ions [26].
10
20
30
40
50
60
70
80
90 100
T / °C
Fig. 4. Viscosity-density product of the IL as a function of temperature. (-ꢀ-)
202.8 mmol/kg, (-᭹-) 101.4 mmol/kg, (-ꢁ-) 50.7 mmol/kg Ag(I), (-ꢀ-) 25.3 mmol/kg
Ag(I), (-ꢇ-) neat [EMIm][TfO].
3.2.2. Viscosity–density product
Based on the Stokes–Einstein relation, Di = RT/(6ꢁꢂri) (R is the
ideal gas constant, T the temperature, Di the diffusion coefficient,
ꢂ the dynamic viscosity, ri the radius of the solvated ion) and the
Nernst–Einstein relation, Di = RTꢃi/(|zi|F2) (ꢃi is the ionic equivalent
conductance, zi the number of proton charges carried by an ion, F
the Faraday constant), one can obtain a relationship between the
viscosity and conductivity. This approach works well for diluted
aqueous solutions [29]. Generally, the conductivity is proportional
to the diffusion coefficient and inversely proportional to the vis-
cosity, which means that if the conductivity of the liquids increases
with the temperature, their viscosity should decrease.
However, it seems that this approach cannot be applied to ILs
[30]. Some researchers argue that the charge transport in ILs can-
not be described properly by the Stokes–Einstein relationship [31].
Other groups showed that in some cases the Stokes–Einstein equa-
tion can be used for predicting the conductivity, or at least the
order of magnitude of the conductivity, for room temperature ILs
[32,33 and references within]. In molten salts (such as ILs) fluc-
tuating empty spaces exist, due to thermal motions [32–34 and
references within]. This model based on the conventional hole the-
ory was applied to estimate the conductivities and viscosities of
several ILs at ambient temperature [32,33].
Due to the lack of suitable theoretical models it is difficult to
predict how the conductivity or the viscosity of ILs varies with
the temperature [26]. Some authors use the Walden product, i.e.,
the product of the molar conductivity and the dynamic viscosity,
to describe the properties of the ILs [26]. Other groups showed
that the Walden product does not always describe properly the
physicochemical properties of ILs [35]. Therefore, we decided to
characterize separately the viscosity and conductivity of our ILs.
Conventional viscosimeters need a relatively large sample vol-
ume. We decided to determine the viscosity–density product of
our ILs with the EQCM technique. In this case we can work with
relatively small sample volumes (ca. 1 ml).
(2) can be successfully applied to determine the viscosity of a series
of ILs [37–39].
The product ꢅ·ꢆ of our electrolytes decreased with increas-
ing temperature (Fig. 4). At a given temperature, the product ꢅ·ꢆ
increases with increasing concentration of Ag ions in the ILs in the
range from 50.7 mmol/kg to 202.8 mmol/kg. The product ꢅ·ꢆ was
lowest at a relatively low concentration of Ag(I) (25.3 mmol/kg), at
room temperature, and it was the highest for the most concentrated
solution (202.8 mmol/kg) in the whole temperature range investi-
gated here. The values for the product ꢅ·ꢆ obtained at the lowest
concentration of Ag(I) (25.3 mmol/kg) at temperatures higher than
20 ◦C were practically the same as those obtained for the solution
containing 50.7 mmol/kg Ag(I). For the neat [EMIm][TfO] IL, the
product ꢅ·ꢆ was almost similar to that obtained in a 101.4 mmol/kg
Ag(I) for the temperature range from 20 ◦C to 100 ◦C. Thus, adding
small amounts of Ag(I) to the IL induced a decrease of the product
ꢅ·ꢆ for these solutions when compared to the neat IL, while further
increase of the Ag(I) concentration will induce higher ꢅ·ꢆ values.
These results show a nice correlation between the conductivities
and the ꢅ·ꢆ results of the ILs used, within the temperature range
from 20 ◦C to 100 ◦C. Thus, for the solution that presented a lower
conductivity (e.g., 202.8 mmol/kg Ag(I)) we obtained the highest
ꢅ·ꢆ values, while for a solution that presented a high conductivity
(e.g., 25.3 mmol/kg Ag(I)), we obtained low ꢅ·ꢆ values.
3.3. Electrochemical characterization
3.3.1. Cyclic voltammetry
Cyclic voltammograms started at the open circuit potential
(OCP) towards the cathodic regime at a scan rate of 20 mV s−1, and
were recorded in a temperature range from 20 ◦C to 100 ◦C. Simul-
taneously to the CVs experiments, the EQCM data were acquired.
Thus, we could follow in situ the deposition and the dissolution
process in the four Ag containing ILs.
A typical CV is shown in Fig. 5. The cathodic wave can be asso-
ciated with the deposition of Ag, while the anodic peak is due
to Ag dissolution. No crossover between the cathodic and anodic
branches was found. In other literature reports for other ILs systems
at temperature below 200 ◦C a crossover between the cathodic and
anodic branches was observed [10,17,18]. The peaks are relatively
broad in the ILs with higher Ag(I) concentrations (202.8 mmol/kg,
Fig. 5A). Sharp peaks were obtained usually in the anodic potential
regime in the ILs having lower Ag concentrations (25.3 mmol/kg),
while the cathodic peaks were typically broader (Fig. 5B).
Kanazawa and Gordon [36] showed that the shift of the reso-
nance frequency of a quartz crystal is proportional to the square
root of the viscosity–density product of the adjacent liquid layer,
Eq. (2).
ꢀ
nꢅꢆ
ꢄf = −f03/2
(2)
ꢁꢅqꢂq
In Eq. (2) n is the overtone number (n =1,3,5,. . .), ꢅ and ꢆ are the
density and the viscosity of the liquid, and ꢅq and ꢂq are the density
and shear modulus of the quartz crystal. It has been shown that Eq.

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A. Ispas et al. / Electrochimica Acta 56 (2011) 10332–10339
Table 1
8
6
4
2
0
Correlation between the current efficiencies calculated from the EQCM data
and the potential applied in the potentiodynamic experiments at 20 mV s−1 in
Ag(I)/[EMIm][TfO] solutions.
Ag concentration
(mmol/kg)
T (K)
Potential range
(V)
Current
efficiency (%)
202.8
202.8
202.8
202.8
101.4
101.4
101.4
101.4
50.7
293.15
323.15
353.15
373.15
293.15
323.15
353.15
373.15
293.15
323.15
353.15
373.15
293.15
323.15
353.15
373.15
−0.18 to −0.26
−0.37 to −0.55
−0.1 to −0.15
−0.2 to −0.6
51
96
82
100
100
99
100
93
63
100
79
90
92
91
95
74
74
100
82
93
78
91
89
92
77
98
76
84
80
88
82
93
−0.09 to −0.15
−0.2 to −0.6
-2
−0.07 to −0.1
−0.38 to −0.6
−0.08 to −0.15
−0.2 to −0.6
-4
-6
A
−0.1 to −0.13
−0.2 to −0.6
-0.6
-0.4
-0.2
0.0
0.2
0.4
−0.09 to −0.16
−0.4 to −0.6
E vs. Ag / V
−0.08 to −0.1
−0.3 to −0.6
−0.08 to −0.15
−0.2 to −0.6
5
50.7
−0.04 to −0.08
−0.3 to −0.6
4
3
2
1
0
50.7
−0.04 to −0.06
−0.17 to −0.6
−0.02 to −0.06
−0.25 to −0.6
−0.01 to −0.03
−0.2 to −0.6
50.7
25.3
25.3
−0.06 to −0.3
−0.5 to −0.6
25.3
−0.05 to −0.1
−0.45 to −0.6
−0.06 to −0.09
−0.16 to −0.26
-1
-2
25.3
B
-0.6
-0.4
-0.2
0.0
0.2
0.4
E vs. Ag / V
investigated here (Table 1). This indicates that other processes
occur in parallel to the Ag deposition. These processes could be
a partial decomposition of the cation of the IL (which could be elec-
trochemically induced), or/and the generation of another species
that can be adsorbed and desorbed very easily from the working
electrode surface, or/and the formation of a soluble species (see
also the EDX results, in Section 3.4).
Fig. 5. Cyclic voltammograms (20 mV s−1
) at different temperatures in a
202.8 mmol/kg Ag(I) in [EMIm][TfO] (A) and in 25.3 mmol/kg Ag(I) in [EMIm][TfO]
(B). Black, continuous line shows the measurements at 20 ◦C, red dashed line is for
50 ◦C, green dotted line is for 80 ◦C and blue dashed-dot is for 100 ◦C. (For interpre-
tation of the references to colour in this figure legend, the reader is referred to the
web version of the article.)
A shoulder appears in the cathodic scan of the cyclic voltam-
mograms at ca. −0.3 V in the ILs containing a low amount of Ag(I)
(Fig. 5B). This shoulder is more clearly visible when the reduc-
tion of Ag(I) in [EMIm][TfO] was carried out at 100 ◦C (Fig. 5B).
The apparent molar masses associated with that second cathodic
wave were higher than those corresponding to the first peak. A
possible explanation is that two silver species are present in the
electrolytes, which are reduced at different potentials. However,
the exact mechanism of the reduction of these Ag species is not
clear yet. The EQCM data indicates however that the reduction of
the silver species that occurs at more anodic potential is a pro-
cess with a lower current efficiency than the reduction of the silver
species that occurs more cathodically.
For a given scan rate, the cathodic peak shifted towards more
anodic potential values, while the anodic peak shifted towards
more cathodic potential values with increasing temperature. The
spacing between the anodic and cathodic peak increases with
increasing scan rate. This indicates that the electrode reaction
follows quasireversible kinetics. The shift of the peak potential
results from the limiting charge transfer rate, which occurs together
with the mass transport of the electroactive species. The latter is
favoured by the decrease of the viscosity and the increase of the
conductivity of the electrolytes with increasing temperature. As
expected, higher currents were usually obtained at higher temper-
atures.
A typical mass vs. charge diagram is shown in Fig. 6. With
increasing temperature, more Ag was deposited due to the decrease
of the viscosity and the increased mass transport. During the anodic
sweep the deposited Ag is not dissolved completely. Such behaviour
is often observed in ILs [40].
One can determine the current efficiency from the EQCM data.
By multiplying the slope of a mass vs. charge plot with the Fara-
day constant, one can obtain the apparent molar mass of the
reacting species. In our case (Ag deposition), this apparent molar
mass should be 107.9 g/mol. The ratio between the apparent molar
mass calculated from the EQCM data and the theoretical molar
mass of Ag will give the current efficiency of the deposition
process.
3.3.2. Potentiostatic deposition of Ag layers from IL based
electrolytes
The apparent molar mass calculated from the EQCM data was
often lower than the theoretical value. Thus, the current effi-
ciencies of the Ag(I) electroreduction process varied generally
between 80 and 100% in the potential ranges and at temperatures
Potentiostatic depositions were carried out at −0.1 V, −0.2 V
and −0.3 V in all four electrolytes and in the temperature range
from 20 ◦C to 100 ◦C. The current efficiencies obtained at a depo-
sition potential of −0.1 V are summarized in Table 2. The current

A. Ispas et al. / Electrochimica Acta 56 (2011) 10332–10339
10337
Table 2
160
140
120
100
80
Current efficiencies (in %) obtained during the potentiostatic experiments at -0.1 V
for 1800 s as a function of temperature.
Ag(I) concentra-
tion/(mmol/kg)
20 ◦
C
50 ◦
C
80 ◦
C
100 ◦
C
202.8
101.4
50.7
79
96
98
95
100
99
98
89
97
96
79
92
86
90
98
25.3
100
60
40
High current efficiencies, with values between 80% and 100%,
were obtained in all the electrolytes used (Table 2) and for all three
deposition potentials used (−0.1 V, −0.2 V and −0.3 V) (not shown).
No significant trends of the current efficiency with the temperature
or with the deposition potential were observed.
20
0
-20
-0.12
-0.08
Q / (C cm-2)
-0.04
0.00
3.4. SEM characterization
Fig. 6. Mass-charge balance during potentiodynamic experiments performed at
20 mV s−1 and at different temperatures for 202.8 mmol/kg Ag(I) in [EMIm][TfO].
Black, continuous line shows the measurements at 20 ◦C, red dashed line is for 50 ◦C,
green dotted line is for 80 ◦C and blue dashed-dot is for 100 ◦C. (For interpretation
of the references to colour in this figure legend, the reader is referred to the web
version of the article.)
Uniform layers with a shiny metallic visual appearance were
obtained by potentiostatic deposition. The morphology of some Ag
layers is shown in Fig. 7. The morphology of the layers deposited
at −0.1 V were very similar to those obtained at −0.2 V, for iden-
tical temperatures and Ag(I) concentrations (not shown). When
the deposition was carried out at 20 ◦C, the layer contained mainly
fine grains. Larger grains were obtained at 100 ◦C. This is probably
due to the improved mass transport. Ordered grains, that resem-
bled a hexagonal structure, were observed in the deposits prepared
at 100 ◦C in the most concentrated solution (202.8 mmol/kg)
efficiencies have been calculated as already mentioned above from
the ratio between the apparent molar mass obtained from the
EQCM data (obtained at its turn from the linear portions of the
mass vs. charge diagrams) and the theoretical molar mass of Ag.
Fig. 7. Scanning electron micrographs of Ag layers obtained by potentiostatic deposition (E = −0.2 V) at a deposition time of 1800 s. (A) T = 20 ◦C, 25.3 mmol/kg Ag(I); (B)
T = 100 ◦C, 25.3 mmol/kg Ag(I); (C) T = 20 ◦C, 50.7 mmol/kg Ag(I); (D) T = 100 ◦C, 50.7 mmol/kg Ag(I); (E) T = 20 ◦C, 101.4 mmol/kg Ag(I); (F) T = 100 ◦C, 101.4 mmol/kg Ag(I); (G)
T = 20 ◦C, 202.8 mmol/kg Ag(I); (H) T = 100 ◦C, 202.8 mmol/kg Ag(I).

10338
A. Ispas et al. / Electrochimica Acta 56 (2011) 10332–10339
Fig. 7. (Continued )
(Fig. 7H). Moreover, the grains of the deposits formed at 100 ◦C
in 202.8 mmol/kg were smaller than those obtained at 100 ◦C in
less concentrated solutions. The reason why the intermediate con-
centrations exhibit the extreme deposition morphologies could be
attributed to the interference of several competing influence fac-
tors. The intermediate concentrations exhibit high conductivities
and low viscosity–density products; this could be one significant
factor. One big unknown is the nuclei formation process. Are more
nuclei formed at low temperature as at higher temperature? Or do
the nuclei coalesce better at higher temperature, when more ions
reach the electrode due to better migration? Moreover, maybe the
nuclei themselves are more mobile at higher temperatures.
EDX analysis proved that the deposits consist of Ag (ca.
93–96 wt.%) and C (ca. 4–7 wt.%). The carbon signal is probably due
to rests of IL trapped between the grains.
Silver electrodeposition was successfully carried out in the
IL [EMIm][TfO]. The EQCM technique was used to monitor the
viscosity–density product of the electrolytes and to obtain in situ
information on the electrodeposited mass.
Different concentrations of Ag(I) were established via electro-
chemical dissolution of Ag and subsequent dilution. The electrical
conductivity of the electrolytes was determined by EIS measure-
ments. The conductivity increased with increasing temperature,
while the viscosity–density product decreased. The solution con-
taining 25.3 mmol/kg Ag(I) had the highest conductivity at a fixed
temperature in comparison to the solutions containing more Ag
ions or to the neat IL. Simultaneously, the viscosity-density prod-
uct of the solutions containing 25.3 mmol/kg and 50.7 mmol/kg
Ag(I) was the smallest from all the solutions investigated here. This
behaviour could be attributed to the specific interaction of Ag ions
with the ions of the IL. The cyclic voltammetry coupled with the
EQCM allowed us to identify the potential regions where elemen-
tal Ag is obtained and the potential regions where Ag dissolves.
The current efficiency was calculated in different potentiostatic and
potentiodynamic experiments, and at different temperatures. The
deposition conditions influenced the morphology of the layers. The
smallest grain sizes were obtained for the layers electrodeposited
from the 50.7 mmol/kg Ag(I) solutions at 20 ◦C and the biggest grain
sizes for the layers obtained from 101.4 mmol/kg Ag(I)solutions at
100 ◦C. The higher the temperature is, the bigger the particles grow.
This is most likely due to a facilitated mass transport towards the
electrode at higher temperatures.
The granular morphology of our Ag layers is similar to that
reported by other groups onto Au substrates, in the other ILs
[13,15,16], or in [EMIm][TfO] with 50% humidity [6], but completely
different from the dendritic or filiform morphology that one can
obtain from aqueous electrolytes [3].
4. Conclusions
This paper is dedicated to studying the electrodeposition of Ag
from [EMIm][TfO] and to correlate the physical properties of the
solution used with the properties of the deposited layers as a func-
tion of temperature and the concentration of Ag(I).

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10339
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