Electrodeposition of nanocrystalline silver films and nanowires from the ionic liquid 1-ethyl-3-methylimidazolium trifluoromethylsulfonate

Article abstract of DOI:10.1016/j.electacta.2009.05.005

We report in this paper on the electrodeposition of nanocrystalline silver films and nanowires in the air and water stable ionic liquid 1-ethyl-3-methylimidazolium trifluoromethylsulfonate [EMIm]TfO containing Ag(TfO) as a source of silver. The study was

Full text of DOI:10.1016/j.electacta.2009.05.005

Electrochimica Acta 54 (2009) 5673–5677
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electrodeposition of nanocrystalline silver films and nanowires from the ionic
liquid 1-ethyl-3-methylimidazolium trifluoromethylsulfonate
Sherif Zein El Abedina,b, F. Endresa,∗
a Institute of Particle Technology, Clausthal University of Technology, Robert-Koch-Strasse 42, D-38678 Clausthal-Zellerfeld, Germany
b Electrochemistry and Corrosion Laboratory, National Research Centre, Dokki, Cairo, Egypt
a r t i c l e i n f o
a b s t r a c t
Article history:
We report in this paper on the electrodeposition of nanocrystalline silver films and nanowires in the
air and water stable ionic liquid 1-ethyl-3-methylimidazolium trifluoromethylsulfonate [EMIm]TfO con-
taining Ag(TfO) as a source of silver. The study was performed by means of cyclic voltammetry and
chronoamperometry, and the electrodeposits were characterized by SEM-EDX and XRD. The cyclic voltam-
metry behaviour showed typical reduction and oxidation peaks corresponding to the deposition and
stripping of silver in the employed electrolyte. XRD patterns of the electrodeposited silver layers revealed
the characteristic peaks of crystalline silver with crystallites in the nanosize regime. Silver nanowires with
average diameters and lengths of about 200 nm and 3 ␮m, respectively, were prepared by potentiostatic
deposition within a commercial nuclear track-etched polycarbonate template.
Received 11 March 2009
Received in revised form 28 April 2009
Accepted 1 May 2009
Available online 13 May 2009
Keywords:
Electrodeposition
Silver
Nanowires
Template
© 2009 Elsevier Ltd. All rights reserved.
Ionic liquids
1. Introduction
Despite the unique properties of ionic liquids as solvents for
electrodeposition some challenges have to be considered such as,
Nowadays, electrodeposition from ionic liquids has received
extensive attention due to their unique properties. The recent
efforts on the electrodeposition from ionic liquids, challenges and
prospects have been described and discussed in [1]. In principle,
most of the metals and alloys that can be electrodeposited from
aqueous solutions can also be electrodeposited from ionic liquids.
Since many of the ionic liquids are environmentally friendly, they
are considered as suitable alternatives for many poisonous electro-
plating baths. Furthermore, as ionic liquids have very low vapour
pressures (often between 10−11 and 10−10 mbar at or near room
temperature, at 100 ◦C the vapour pressure is – depending on the
liquid – in the region of 10−6 to 10−4 mbar), thus, they can be used in
open galvanic baths at variable temperatures without releasing of
harmful vapours which reduces the amount of volatile organic com-
pounds released into the atmosphere. Another advantage of using
ionic liquids instead of aqueous baths is that the thermal stability
of ionic liquids makes it easier to electrodeposit crystalline metals
and semiconductors through direct electrodeposition at elevated
temperatures, above 100 ◦C, without subsequent annealing. More-
over, there is no hydrogen embrittlement due to the hydrophobic
nature of many ionic liquids, that only slowly absorb water under
air.
impurities, cation/anion effects or viscositiy/conductivity. Using
pure ionic liquids in the electrodeposition is recommended as the
impurities would influence the quality of the deposit. Further-
more awareness of the cation/anion effects is necessary in order to
avoid unexpected influence resulting from employing of unsuitable
cation/anion. Compared with aqueous electrolytes the viscosity of
ionic liquids is high and the specific conductivity is low which lead
to low deposition rate. This could be improved just by heating as at
about 100 ◦C the viscosity of many ionic liquids approaches that of
water.
In general, ionic liquids can be regarded as a potential replace-
ment for many aqueous electroplating baths, especially the toxic
ones like cyanide baths for silver electroplating. The electrodeposi-
tion of silver from chloroaluminate ionic liquids is straightforward
[2]. However, these ionic liquids are extremely hygroscopic and
they must be prepared and handled under inert gas atmosphere.
Katayama et al. [3] reported that the – more or less – water
insensitive ionic liquid 1-ethyl-3-methylimidazolium tetrafluorob-
orate ([EMIm]BF4) is applicable as an alternative electroplating
bath for silver. Electrodeposition of silver in the ionic liquids
1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm]BF4) and
1-butyl-3-methylimidazolium hexafluorophosphate ([BMIm]PF6)
was reported [4]. Bardi and coworkers [5] showed the capabil-
ity of depositing very thin (∼0.3 ␮m), adherent silver layers on
commercial grade copper substrates in the ionic liquid [BMIm]BF4
at variable temperatures, up to 200 ◦C. Furthermore, templated
∗
Corresponding author. Tel.: +49 5323 72 3141; fax: +49 5323 722460.
E-mail address: frank.endres@tu-clausthal.de (F. Endres).
0013-4686/$ – see front matter © 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2009.05.005

5674
S. Zein El Abedin, F. Endres / Electrochimica Acta 54 (2009) 5673–5677
electrodeposition of silver nanowires in [BMIm]PF6 was reported
[6]. Although these ionic liquids are often regarded as water
insensitive, the exposure to moisture for a long time sometimes
causes some changes in their physical and chemical proper-
ties. This leads to a partial decomposition of the ionic liquids
generating HF which is pretty aggressive. Thus, ionic liquids
based on more stable anions such as trifluoromethylsulfonate
(CF3SO3−), bis(trifluoromethylsulfonyl) amide [(CF3SO2)2N−] and
tris(trifluoromethylsulfonyl) methide [(CF3SO2)3C−] might be quite
promising as new electroplating baths. Recently, we showed for
the first time that isolated, dispersed silver nanoparticles with
an average diameter of 20 nm can be deposited on the surface
of the ionic liquid 1-butyl-3-methylimidazolium trifluoromethyl-
sulfonate ([BMIm]TfO) by electrochemical reduction with free
electrons from a low temperature plasma [7,8].
In the present paper we report on the electrodeposition of
nanocrystalline films and nanowires of silver in the air and water
stable ionic liquid 1-ethyl-3-methylimidazolium trifluoromethyl-
sulfonate ([EMIm]TfO) using Ag(TfO) as a source of silver. Silver
nanowires were prepared by potentiostatic deposition within a
commercial nuclear track-etched polycarbonate template.
Fig. 1. Cyclic voltammograms of the ionic liquid [EMIm]TfO on gold with differ-
ent reversal potentials. Inset: cyclic voltammogram of the ionic liquid [EMIm]TfO
containing 50 mmol/l ferrocene on gold. The scan rate was 10 mV s−1
.
2. Experimental
3. Results and discussion
3.1. Electrodeposition of silver films
3.1.1. Cyclic voltammetry
The electrodeposition of silver was studied in the water and air
stable ionic liquid 1-ethyl-3-methylimidazolium trifluoromethyl-
sulfonate [EMIm]TfO with high purity (Merck). The ionic liquid was
used as received without further purification or drying. The vis-
cosity and conductivity of the employed ionic liquid at 20 ◦C were
reported to be 45 mPa s and 8.6 mS cm−1, respectively [9]. Silver tri-
fluoromethylsulfonate (Alfa, 99%) was used as a source of silver due
to its high solubility in the employed ionic liquid.
Fig. 1 shows the electrochemical window of the ionic liquid 1-
ethyl-3-methylimidazolium trifluoromethylsulfonate [EMIm]TfO
on gold under air. As seen, the employed ionic liquid exhibits
an electrochemical window of about 4.3 V, extending from −2.0
to +2.3 V vs. Ag quasi-reference. The cathodic and anodic limit-
ing potentials were evaluated when the reduction and oxidation
currents reached 100 ␮A cm−2. This current value is called cut-
off current density. Usually the cut-off current density is selected
between 0.1 and 1.0 mA cm−2 [10]. However, it is more common to
select the cut-off current density below 0.1 mA cm−2. The cathodic
and anodic limits of the cyclic voltammogram are corresponding to
the reduction of [EMIm]+ cation and the oxidation of gold and/or
anodic decomposition of TFO− anion, respectively (Fig. 1, curve
b). The increase in anodic current starting at a potential of about
−0.1 V is attributable to the oxidation of the reduction products
of [EMIm]+ formed on the preceding cathodic scan. If the poten-
tial is reversed before the cathodic decomposition of [EMIm]+ sets
in, no pronounced anodic current was recorded up to a potential
of +2.2 vs. Ag confirming the former interpretation (Fig. 1, curve
a). As all potentials in this study were measured vs. Ag reference
electrode the potential of the redox couple feerocene/ferrocinium
was measured in the employed ionic liquid to give a known ref-
erence electrode potential, see inset of Fig. 1. The redox couple
ferrocene/ferrocinium has a value of +320 mV vs. Ag.
The cyclic voltammetry behaviour of the ionic liquid [EMIm]TFO
containing 0.2 M Ag(TfO) on gold is displayed in Fig. 2a. It is clearly
seen that the cyclic voltammogram exhibits a clear redox couple.
The observed reduction peak is due to the bulk deposition of Ag and
the anodic peak is due to stripping of the deposited silver. The ratio
of the anodic to cathodic charges (Qa/Qc) determined from the inte-
grated peak areas in the voltammogram was close to unity. Beyond
the recorded anodic peak, the current reaches zero indicating the
complete stripping of the deposited silver obtained in the forward
scan.
All electrochemical measurements were performed using a Ver-
stat 263A Potentiostat/Galvanostat (Princeton Applied Research)
controlled by PowerCV and PowerStep software. The experiments
were carried out under air with a relative humidity of about 50% at
20 ◦C. In cyclic voltammograms measured on gold there is no evi-
dence for electrochemically active water on a timescale of about 1 h
under air. Gold substrates from Arrandee (gold films of 200–300 nm
thickness deposited on chromium-covered borosilicate glass) and
glassy carbon substrates (Alfa) were used as working electrodes,
respectively. Directly before use, the gold substrates were very
carefully heated in a hydrogen flame to red glow. Glassy carbon
substrates were cleaned for 10 min in an ultrasonic bath in acetone
followed by refluxing in isopropanol. For Ag-nanowires synthesis,
nuclear track-etched polycarbonate membranes (Millipore) with
pore diameters of 100 nm were used as templates. The back side
of the membrane was sputtered by a roughly 100 nm thick gold
film to serve as a working electrode. The Au-coated side was placed
onto a copper plate connected to the potentiostat. The electrochem-
ical cell made of polytetrafluoroethylene (Teflon) was clamped over
a Teflon covered Viton o-ring onto the other side of the mem-
brane, thus yielding an exposed surface area of 0.6 cm2. A platinum
wire (Alfa, 99.99%) and a silver wire (Alfa, 99.99%) were used
as counter and reference electrodes, respectively. The electrode
potential of the reference electrode was calibrated in several exper-
iments vs. ferrocene/ferrocinium. Its electrode potential exhibits
sufficient stability throughout the experiments. The redox couple
ferrocene/ferrocinium has a value of +320 mV vs. Ag reference.
A high resolution field emission scanning electron microscope
(Carl Zeiss DSM 982 Gemini) was utilized to investigate the sur-
face morphology of the deposited films and energy dispersive X-ray
analysis was used to determine the film composition. The X-ray
diffractograms of the deposited silver samples were recorded using
a Siemens D-5000 diffractometer with Co K␣ radiation.
The cyclic voltammetry behaviour of the employed ionic liquid
containing 0.2 mol/l Ag(TfO) was also recorded on glassy carbon
(Fig. 2b). As seen, the cyclic voltammogram shows the reduction
and oxidation peaks corresponding to the deposition and stripping

S. Zein El Abedin, F. Endres / Electrochimica Acta 54 (2009) 5673–5677
5675
Fig. 2. Cyclic voltammograms of the ionic liquid [EMIm]TfO containing 0.2 mol/l
Ag(TfO) recorded on (a) gold and (b) glassy carbon at a scan rate of 10 mV s−1
.
of silver. The ratio of the anodic to cathodic charges of the anodic
and cathodic peaks, respectively, was close to unity, indicating the
complete stripping of the electrodeposited silver. In the reverse
scan, a crossover loop was observed, indicating a nucleation pro-
cess. This means that the electrodeposition of Ag on glassy carbon
requires an overpotential to initiate nucleation and growth of Ag
bulk deposition.
Fig. 3. (a) SEM micrograph of an about 4 ␮m Ag-layer electrodeposited on gold in
the ionic liquid [EMIm]TfO containing 0.2 mol/l Ag(TfO) at a potential of −0.3 V vs.
Ag for 2 h. Inset: SEM micrograph of higher magnification. (b) EDX analysis of the
area shown in the SEM micrograph of lower magnification.
of the deposited silver layer obtained on glassy carbon is shown in
Fig. 4a. Similar to the deposit obtained on gold, the deposited sil-
ver layer contains large crystallites formed by agglomeration of fine
crystallites. Furthermore, the deposit contains very fine crystallites
in the nanometer regime, see the inset of Fig. 4a. It is also seen that
the deposit is porous giving rise to high surface area which might
lead to increased catalytic activity of the deposited silver owing to
its high surface to volume ratio. The electrodeposit was analysed as
pure silver as revealed in the accompanied EDX profile of Fig. 4b. In
the light of the SEM results, it can be concluded that the electrode-
position of Ag involves first the formation of fine crystallites in the
nanometer regime followed by crystallites agglomeration.
The XRD patterns shown in Fig. 5 exhibit five pronounced
diffraction peaks at 2ꢀ = 44.56◦, 51.90◦, 76.48◦, 93.21◦ and 98.62◦
which agree well with the (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2)
diffractions of face centred cubic silver (JCPDS File No. 04-0783).
This confirms the metallic state of the coated silver. As seen in Fig. 5,
the recorded peaks are broad indicating the small crystallite size of
the electrodeposited silver film. The average crystallite size of Ag
was estimated according to Scherrer’s equation [11]:
3.1.2. Characterization of electrodeposited Ag films
Potentiostatic electrodeposition of silver was performed on
gold and on glassy carbon substrates at a potential of −0.3 V
for 2 h in [EMIm]TfO containing 0.2 mol/l Ag(TfO). The electrode-
posited silver layers were characterized by SEM-EDX and XRD to
explore surface morphology, composition and crystal structure,
respectively. Visually, a white silver deposit was obtained after
potentiostatic electrodeposition experiments. Following the depo-
sition experiments the deposits were washed thoroughly with
isopropanol and then cleaned with acetone in an ultrasonic bath
for about 3 min. This cleaning process is to remove the residual
ionic liquids. EDX analysis of the electrodeposited layers reveals
that only pure silver was deposited on the electrode surface. There
was no trace of any component of the ionic liquid in the deposits,
indicating the complete removal of the residual ionic liquid during
the rinsing process.
The SEM micrograph of Fig. 3a presents the surface morphol-
ogy of a deposited silver layer obtained on gold. The Ag deposit
appears to be nodular containing large agglomerated crystallites
in the order of 0.5–1 ␮m in size. Each of the silver crystallites con-
sists of small grains of sizes in the nanometer regime, as revealed in
the inset of Fig. 3a which shows a higher magnification SEM micro-
graph. The EDX analysis shows only silver (Fig. 3b). The morphology
d = 0.9ꢁ/ˇ1/2 cos ꢀ
(1)
where d is the crystallite size, ꢁ the x-ray wavelength (1.7889 Å for
Co K␣), ˇ1/2 the full-width half maximum (FWHM) and ꢀ is the
diffraction angle. The inset of Fig. 5 shows the estimation of the

5676
S. Zein El Abedin, F. Endres / Electrochimica Acta 54 (2009) 5673–5677
Fig. 4. (a) SEM micrograph of an about 4 ␮m Ag-layer electrodeposited on glassy
carbon in the ionic liquid [EMIm]TfO containing 0.2 mol/l Ag(TfO) at a potential of
−0.3 V vs. Ag for 2 h. Inset: SEM micrograph of higher magnification. (b) EDX analysis
of the area shown in the SEM micrograph of lower magnification.
Fig. 6. (a) SEM micrograph of Ag nanowires obtained in the ionic liquid [EMIm]TfO
containing 0.2 mol/l Ag(TfO) at a potential of −0.3 V vs. Ag for 4 h. (b) SEM micrograph
of higher magnification. (c) EDX analysis of the area shown in the SEM micrograph
of lower magnification.
full-width half maximum of the Ag(1 1 1) peak, as an example, of
XRD patterns of the electrodeposited Ag layer. The average size of
Ag crystallites was found to be 35 nm.
3.2. Electrodeposition of silver nanowires
Due to their potential applications in many areas, e.g. electron-
ics, sensors and catalysis, nanowires, especially metal nanowires,
have attracted extensive attention. Silver nanowires are of par-
Fig. 5. XRD patterns of an electrodeposited Ag layer obtained on glassy carbon in
the ionic liquid [EMIm]TfO containing 0.2 mol/l Ag(TfO) at a potential of −0.3 V vs.
Ag for 2 h. Inset: FWHM of Ag(1 1 1) peak of XRD patterns.

S. Zein El Abedin, F. Endres / Electrochimica Acta 54 (2009) 5673–5677
5677
ticular interest due to their excellent electrical and thermal
4. Conclusions
conductivities and high stability [12]. Furthermore, the high sur-
face area of silver nanowires gives rise to increased catalytic activity
when they are employed as a catalyst.
The present paper shows that air and water stable ionic liquids
can be regarded as promising baths to replace toxic cyanide baths
for silver electroplating. Furthermore, a simple synthesis approach
for Ag-nanowires was presented. In general the advantages of the
use of ionic liquids in electroplating baths are not only because they
offer environmentally benign conditions for electroplating but also
to avoid the problems associated with hydrogen ions in conven-
tional aqueous baths. For example, high quality deposits of metals
like Au, Pt and Pd and their alloys can be made in ionic liquids
without co-evolution of hydrogen which can alter the quality of
the deposits obtained in aqueous solutions.
It was found that silver films and nanowires can be
electrodeposited in the air and water stable ionic liquid 1-
ethyl-3-methylimidazolium trifluoromethylsulfonate [EMIm]TfO
containing Ag(TfO) as a source of silver under air. The cyclic voltam-
mograms measured on gold and glassy carbon substrates exhibit
a redox couple associated with deposition/stripping of silver in
the employed electrolyte. A crossover loop was observed in the
cyclic voltammogram recorded on glassy carbon, indicating a nucle-
ation process. XRD patterns of the electrodeposited silver layers
revealed the characteristics peaks of face centred cubic silver with
crystallites in the nanosize regime. Silver nanowires with average
diameters and lengths of about 200 nm and 3 ␮m, respectively,
were prepared by potentiostatic deposition within a commercial
nuclear track-etched polycarbonate template.
Template-assisted electrodeposition is an elegant and economic
approach for the synthesis of metal nanowires and it is cur-
rently widely used. The use of templates permits the synthesis
of well-defined nanowires with controllable size and shape. Com-
mercially available anodic aluminium oxide (AAO) and nuclear
track-etched polycarbonate (PC) membranes are commonly used as
nanoporous templates. Electrodeposition of silver nanowires using
(AAO) templates in aqueous solutions was reported [13–15]. Quite
recently synthesis of silver nanowires using PC templates in the
ionic liquid [EMIm]PF6 was reported [6]. Silver ions were intro-
duced into the solution by anodic dissolution of silver metal in
the employed ionic liquid [6]. It should be mentio−ned that the use
−
of ionic liquids based on anions like PF6 or BF4 as electrolytes
for electrodeposition is no longer recommended by the ionic liq-
uids community. Although these ionic liquids can be regarded as
water stable they partially decompose by long exposure to air
generating HF which is rather aggressive. Hence they cannot be
employed as electrolytes in open baths. Therefore, the ionic liq-
uid [EMIm]TfO was employed in our study as it is well suited for
use under air to make the synthesis process more versatile and
simple.
Silver nanowires were prepared in a PC membrane at a con-
stant potential of −0.3 V for 4 h in [EMIm]TfO containing 0.2 mol/l
Ag(TfO). The electrodeposition of the silver nanowires starts on the
gold cathode at the bottom of the pores and grow along the pores.
Fig. 6 shows typical SEM micrographs of silver nanowires after
removal of the PC membrane by dissolution in dichloromethane.
As shown in the SEM micrograph of Fig. 6a the electrodeposited
Ag nanowires are highly dense. The average diameters and lengths
of the synthesized nanowires are about 200 nm and 3 ␮m, respec-
tively (Fig. 6b). The measured diameter of the obtained nanowires
is significantly larger than the pore diameter quoted by the pro-
ducer (100 nm). This is not surprising as the employed commercial
PC membranes are mainly fabricated for use as filters, and hence
the pore diameter is neither optimized nor precisely defined.
Similar results were reported by several authors [16,6]. The mea-
sured diameters of Ni, Co, Cu and Au nanowires obtained in
aqueous solutions using commercial PC membranes with nomi-
nal pore diameters between 10 and 200 nm were higher by up
to a factor 3 than the quoted pore diameters [16]. Moreover,
it was reported [6] that the average diameter of Ag nanowires
obtained in the ionic liquid [EMIm]PF6 was about 80 nm although
the nominal pore diameter was 30 nm. The EDX profile of the
nanowires shown in the SEM micrograph of Fig. 6b indicates
the deposition of pure silver nanowires (Fig. 6c). The sput-
tered gold film (working electrode) is also detected in the EDX
profile.
References
[1] F. Endres, D.R. MacFarlane, A.P. Abbott (Eds.), Electrodeposition from Ionic Liq-
uids, Wiley–VCH, Weinheim, 2008.
[2] X.-H. Xu, C.L. Hussey, J. Electrochem. Soc. 139 (1992) 1295.
[3] Y. Katayama, S. Dan, T. Miura, T. Kishi, J. Electrochem. Soc. 148 (2001) C102.
[4] P. He, H.T. Liu, Z.Y. Li, Y. Liu, X.D. Xu, J.H. Li, Langmuir 20 (2004) 10260.
[5] R. Bomparola, S. Caporali, A. Lavacchi, U. Bardi, Surf. Coat. Technol. 201 (2007)
9485.
[6] I. Kazeminezhad, A.C. Barnes, J.D. Holbrey, K.R. Seddon, W. Schwarzacher, Appl.
Phys. A 86 (2007) 373.
[7] S.A. Meiss, M. Rohnke, L. Kienle, S. Zein El Abedin, F. Endres, J. Janek,
ChemPhysChem 8 (2007) 50.
[8] S. Zein El Abedin, M. Pölleth, S.A. Meiss, J. Janek, F. Endres, Green Chem. 9 (2007)
549.
[9] P. Bonhôte, A. Dias, N. Papageorgiou, K. Kalyanasundaram, M. Grätzel, Inorg.
Chem. 35 (1996) 1168.
[10] H. Matsumoto, in: Y. Ohno (Ed.), Electrochemical Aspects of Ionic Liquids, John
Wiley & Sons, Inc., New Jersey, 2005, p. 35.
[11] P. Scherrer, Göttinger Nachrichten 2 (1918) 98.
[12] Y.G. Sun, Y.N. Xia, Adv. Mater. 14 (2002) 833.
[13] G. Sauer, G. Brehm, S. Schneider, K. Nielsch, R.B. Wehrspohn, J. Choi, H. Hofmeis-
ter, U. Gösele, J. Appl. Phys. 91 (2002) 3243.
[14] J. Zhang, X. Wang, X. Peng, L. Zhang, Appl. Phys. A 75 (2002) 485.
[15] G. Riveros, S. Green, A. Cortes, H. Gomez, R.E. Marotti, E.A. Dalchiele, Nanotech-
nology 17 (2006) 561.
[16] C. Schönenberger, B.M.I. van der Zande, L.G.J. Fokkink, M. Henny, C. Schmid, M.
Krüger, A. Bachtold, R. Huber, H. Birk, U. Staufer, J. Phys. Chem. B 101 (1997)
5497.