Electrodeposition of selenium, indium and copper in an air- and water-stable ionic liquid at variable temperatures

Article abstract of DOI:10.1016/j.electacta.2006.08.064

The electrochemical behaviour of Au(1 1 1) and highly oriented pyrolytic graphite (HOPG) substrates in the air- and water-stable ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([BMP]Tf₂N) was investigated using in situ scanning tunneling microscopy (STM). Furthermore, the electrodeposition of Se, In and Cu in the same ionic liquid was investigated. The high thermal stability as well as the large electrochemical window of this ionic liquid compared with aqueous electrolytes allow the direct electrodeposition of grey selenium, indium and copper at variable temperatures, as the first step in making CIS solar cells electrochemically, in a one pot reaction. The results show that grey selenium can be obtained at temperatures ≥100 °C. XRD patterns of the electrodeposit obtained at 100 °C show the characteristic peaks of crystalline grey selenium. Nanocrystalline indium with grain sizes between 100 and 200 nm was formed in the employed ionic liquid, containing 0.1 M InCl₃, at room temperature. It was also found that copper(I) species can be introduced into the ionic liquid [BMP]Tf₂N by anodic dissolution of a copper electrode and nanocrystalline copper with an average crystallite size of about 50 nm was obtained without additives in the resulting electrolyte.

Full text of DOI:10.1016/j.electacta.2006.08.064

Electrochimica Acta 52 (2007) 2746–2754
Electrodeposition of selenium, indium and copper in an air- and
water-stable ionic liquid at variable temperatures
1
1
2
∗
S. Zein El Abedin , A.Y. Saad , H.K. Farag , N. Borisenko, Q.X. Liu, F. Endres
Faculty of Natural and Materials Sciences, Clausthal University of Technology, Robert-Koch-Str. 42, D-38678 Clausthal-Zellerfeld, Germany
Received 14 June 2006; received in revised form 20 August 2006; accepted 22 August 2006
Available online 6 October 2006
Abstract
The electrochemical behaviour of Au(1 1 1) and highly oriented pyrolytic graphite (HOPG) substrates in the air- and water-stable ionic liquid
1
-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([BMP]Tf N) was investigated using in situ scanning tunneling microscopy
2
(
STM). Furthermore, the electrodeposition of Se, In and Cu in the same ionic liquid was investigated. The high thermal stability as well as the
large electrochemical window of this ionic liquid compared with aqueous electrolytes allow the direct electrodeposition of grey selenium, indium
and copper at variable temperatures, as the first step in making CIS solar cells electrochemically, in a one pot reaction. The results show that
◦
◦
grey selenium can be obtained at temperatures ≥100 C. XRD patterns of the electrodeposit obtained at 100 C show the characteristic peaks of
crystalline grey selenium. Nanocrystalline indium with grain sizes between 100 and 200 nm was formed in the employed ionic liquid, containing
0
3 2
.1 M InCl , at room temperature. It was also found that copper(I) species can be introduced into the ionic liquid [BMP]Tf N by anodic dissolution
of a copper electrode and nanocrystalline copper with an average crystallite size of about 50 nm was obtained without additives in the resulting
electrolyte.
©
2006 Elsevier Ltd. All rights reserved.
Keywords: Semiconductors; Electrodeposition; Ionic liquids; In situ STM; Selenium
1
. Introduction
nium in aqueous solutions is usually complicated by the forma-
tion of amorphous red selenium, which exhibits bad electronic
conductivity. Grey selenium can be deposited exclusively at ele-
Selenium exhibits both photovoltaic action, where light is
◦
converted directly into electricity, and photoconductive proper-
ties, where the conductivity increases with increased illumina-
tion. These properties make selenium useful in the production
of photocells and solar cells. Moreover, compound semiconduc-
tors containing selenium, such as InSe, CdSe or CuInSe2 (CIS),
have many optoelectric applications, including advanced solar
cells, IR detectors and solid-state lasers [1].
The electrodeposition of selenium in aqueous solutions has
been studied intensively, see for example [2–5]. However, the
exclusive electrodeposition of grey selenium has not yet been
successful in aqueous solutions. The electrodeposition of sele-
vated temperatures of more than 100 C, which is practically
impossible to be achieved in aqueous galvanic baths. It is known
that thermodynamically a phase transition from amorphous red
to crystalline grey selenium occurs at a temperature of about
◦
100 C. Therefore, the use of ionic liquids, especially the air-
and water-stable ones, in the electrodeposition of selenium is of
great benefit owing to their extraordinary physical properties.
They exhibit good electrical conductivity, low vapour pressure,
wide electrochemical windows and high thermal stability. For
further information we would like to refer to a recent review
from us [6]. Consequently, the direct electrodeposition of grey
selenium and its compounds is possible in ionic liquids since
the deposition process can be efficiently performed at elevated
temperatures due to the very low vapour pressure of most of the
liquids.
∗
Corresponding author. Fax: +49 5323 722460.
E-mail address: frank.endres@tu-clausthal.de (F. Endres).
Permanent address: Electrochemistry and Corrosion Laboratory, National
1
Copper and indium are regarded as important precursors for
the production of compound semiconductors with selenium.
The electrodeposition of copper in chloroaluminate ionic liq-
Research Centre, Dokki, Cairo, Egypt.
Permanent address: Inorganic Chemistry Department, National Research
Centre, Dokki, Cairo, Egypt.
2
0
013-4686/$ – see front matter © 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2006.08.064

S. Zein El Abedin et al. / Electrochimica Acta 52 (2007) 2746–2754
2747
uids has been widely investigated [7–10]. It has been found
that the reduction of Cu2 to metallic Cu occurs in two one-
electron steps: in the first step Cu is formed, in the second
mica, purchased from Molecular Imaging), highly oriented
pyrolytic graphite (HOPG), glassy carbon substrates (Alfa) and
platinum sheets of thickness 0.5 mm (Alfa, 99.99%) were used
as working electrodes, respectively. Directly before use, the gold
substrates were very carefully heated in a hydrogen flame to red
glow, HOPG substrates were freshly cleaved, Pt-substrates were
cleaned for 10 min in an ultrasonic bath in acetone then heated in
a hydrogen flame to red glow for a few minutes. Glassy carbon
substrates were cleaned by refluxing in isopropanol followed
by drying under vacuum. Pt-wires (Alfa, 99.99%) were used as
quasi-reference and counter electrodes, respectively. Currently
Pt-quasi reference electrodes are only a compromise as well
defined reference electrodes in ionic liquids, especially for in
situ STM, are still missing. A quartz round bottom flask was
used as the electrochemical cell. Prior to use, all parts in con-
tact with the solution were thoroughly cleaned in a mixture of
50/50 vol.% H2SO4/H2O2 followed by refluxing in bidistilled
water.
+
+
step the metal is deposited [7,8]. At high overvoltages for the
deposition, alloying with Al sets in [9]. Using in situ STM,
Endres and Schweizer [10] have shown that the bulk deposi-
tion of copper from acidic chloroaluminate liquids on Au(1 1 1)
is preceded by three underpotential processes. Furthermore, the
+
2+
electrode potential for the redox process Cu /Cu is more pos-
itive than the surface oxidation of Au(1 1 1) in that liquid. The
electrodeposition of copper in basic chloraluminate ionic liquids
has also been reported [11]. As aluminium can only be deposited
from the acidic liquids, the complexity of Cu–Al alloy forma-
tion can be avoided in basic liquids. The electrodeposition of
Cu from a basic 1-ethyl-3-methylimidazolium tetrafluoroborate
and a Lewis acidic ZnCl2-1-ethyl-3-methylimidazolium chlo-
ride room temperature ionic liquid has been investigated [12,13].
Furthermore, the electrodeposition of Pd–In [14] and InSb [15]
were investigated in ionic liquids.
A high-resolution field emission scanning electron micro-
scope (Carl Zeiss DSM 982 Gemini) was utilized to inves-
tigate the surface morphology of the deposited layers and
energy dispersive X-ray analysis was used to determine the
film composition. The X-ray diffractograms of the deposits
were acquired by a Siemens D-5000 diffractometer with Co K␣
radiation.
In the first section of this paper, in situ STM results
concerning the electrochemical behaviour of Au(1 1 1) and
of HOPG substrates in the air- and water-stable ionic liq-
uid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)
amide ([BMP]Tf2N) are presented. This was to explore the influ-
ence of the ionic liquid on the substrates, to better understand
the deposition processes in the employed ionic liquid. We have
showninarecentpaperthatthesurfaceofAu(1 1 1)issubjecttoa
restructuring/reconstruction in the ionic liquid [BMP]Tf2N [16].
As we know from Al deposition that pyrrolidinium cations in
contrast to imidazolium cations lead to nanocrystalline deposits,
it seemed of interest to shed further light on this subject, as we
observe here, that the deposits are also nanocrystalline.
In the second part, results concerning the direct electro-
deposition of crystalline grey selenium in 1-butyl-1-methyl-
pyrrolidinium bis(trifluoromethylsulfonyl) amide ([BMP]Tf2N)
containing SeCl4 at variable temperatures are reported. Further-
more, we present results on the electrodeposition of indium and
copper in the same liquid, as possible first steps in the formation
of InSe and CuInSe2 (CIS) semiconductor thin films.
The STM experiments were performed using self-built STM
heads and scanners under inert gas conditions (H2O and
O2 < 1 ppm)withaMolecularImagingPicoScan2500STMcon-
troller in feedback mode. The STM experiments were performed
◦
in an air-conditioned laboratory with ꢀT < ± 1 C. STM tips
were prepared by electrochemical etching of platinum–iridium
wires (0.25 mm diameter) and electrophoretically coated with an
electropaint (BASF ZQ 84-3225 0201). During the STM exper-
iments the electrode potential was controlled by the PicoStat
from Molecular Imaging.
3. Results and discussions
3.1. In situ STM in [BMP]Tf2N
2
. Experimental
In order to get information on the effect of the ionic liq-
uid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)
amide ([BMP]Tf2N) at the electrode/electrolyte interface, in
situ STM measurements were performed (also see [16]). Fig. 1
shows a set of STM images of Au(1 1 1) in the dry ionic liq-
uid [BMP]Tf2N, at different electrode potentials. As seen in the
STM image of Fig. 1a, at the open circuit potential (−0.4 V
The ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoro-
methylsulfonyl) amide ([BMP]Tf2N) was obtained from Merck
KGaA (EMD) in the highest available quality. The liquid was
dried under vacuum for 12 h at a temperature of 100 C, to a
water content below 3 ppm (by Karl–Fischer titration) and stored
in an argon filled glove box, with water and oxygen levels below
◦
+
versus Fc/Fc ), the gold surface was fairly rough, and only the
1
9
ppm (OMNI-LAB from Vacuum-Atmospheres). SeCl4 (Alfa,
9.5%) and InCl3 (Alfa, 99.999%) were used without further
Au(1 1 1) steps can be identified clearly. Such behaviour can be
subject to an adsorbed film at the electrode surface. By shifting
+
purification.
the potential to −0.7 V (versus Fc/Fc ), the Au(1 1 1) underwent
All liquid preparations as well as the electrochemical mea-
a restructuring, with the formation of vacancy islands over the
activesurface, Fig. 1b. Thesedefectsweremonoatomicallydeep.
When the potential is set to more negative values, these vacancy
islands disappear more and more (Fig. 1c), and at −1.6 V versus
TM
surements were performed in the glove box using a VersaStat
II Potentiostat/Galvanostat (Princeton Applied Research) con-
trolled by PowerCV and PowerStep software. Gold substrates
from Arrandee (gold films of 200–300 nm thickness deposited
on chromium-covered borosilicate glass), Au(1 1 1) (gold on
+
Fc/Fc the typical Au(1 1 1) surface with terraces of 250 pm in
height is obtained (Fig. 1d). Approaching the potential region

2
748
S. Zein El Abedin et al. / Electrochimica Acta 52 (2007) 2746–2754
Fig. 1. STM pictures of Au(1 1 1) in the ionic liquid [BMP]Tf2N at different potentials.
+
where the bulk reduction of the ionic liquid occurs (at E = −2.8
which indicate an adsorption, presumably [BMP] , at the gold
+
versus Fc/Fc , see Ref. [16]), the surface roughness increases
surface inducing a Au(1 1 1) surface restructuring. The deter-
mination of the potential of zero charge (pzc) might help to
investigate the role of cations and/or anions on the behaviour of
Au(1 1 1) in ionic liquids.
strongly and the gold terraces are completely concealed.
Based on these results, it was assumed that an adsorbed film
induced the surface restructuring/reconstruction of the Au(1 1 1)
on scanning the potential in the negative direction. Recent results
in this laboratory made this assumption reasonable, as this sur-
face reconstruction looks a bit different in the ionic liquid 1-
ethyl-3-methylimidazolium bis(trifluoromethylsufonyl) amide.
In order to shed light on this suggested surface film, investi-
gations of the behaviour of highly oriented pyrolytic graphite
(HOPG) in the ionic liquid [BMP]Tf2N, were carried out
using in situ STM. HOPG is a well-defined surface, which
is not subject to restructuring. Fig. 2 shows a sequence of
STM images, obtained at different electrode potentials, before
cathodic decomposition of the ionic liquid set in. It is worth not-
ing that the quality of the STM images degraded at potentials
close to open circuit potential. This image quality degradation
(eveninultrapure[BMP]Tf2N)appearedsystematic. Thequality
of the images improved on scanning the potential in the nega-
tive direction. Poor image quality could be attributable to some
sort of adsorption at the electrode surface. The STM image
+
Therefore, it was assumed that the [BMP] cation was adsorbed
at the electrode surface at potentials close to the open circuit
potential. Neutralization of excess negative charge at the elec-
trode surface by adsorbed cations might influence the Au–Au
interaction facilitating gold atoms mobility on the surface,
which, in turn, gave rise to the gold restructuring. In that con-
+
text, it should be noted that [BMP] appears to act as a grain
refiner during electrodeposition of aluminium in the ionic liquid
[
BMP]Tf2N [17]. The aluminium deposit obtained in the ionic
+
liquid [BMP]Tf2N was nanocrystalline with an average grain
size around 30 nm, whereas in the ionic liquid [EMIm]Tf2N it
obtained at −2.1 V (versus Fc/Fc ), Fig. 2a, showed a more
or less typical HOPG surface with flat steps height about two
+
was microcrystalline [17]. Presumably [BMP] was adsorbed at
graphite monolayers. Shifting the potential to −2.5 V (versus
+
the surface of growing nuclei, thus hindering the further growth
of Al crystallites. This finding is in agreement with STM results
Fc/Fc ), an apparent etching of the step edges sets in as seen
in the STM picture of Fig. 2b. In our opinion this observation

S. Zein El Abedin et al. / Electrochimica Acta 52 (2007) 2746–2754
2749
Fig. 2. STM pictures of HOPG in the ionic liquid [BMP]Tf2N at different potentials, a, b, c and e. (d) Height profile along the white line shown in the STM picture
c).
(
signifies the starting breakdown or the reductive desorption of
the adsorbed film at the step edges. Breakdown/desorption con-
tinued at the flat terraces (Fig. 2c). A height profile (Fig. 2d)
indicated the height of the adsorbed film was in the range of
3.2.1. Electrodeposition of selenium
The cyclic voltammograms of [BMP]Tf2N, containing
0.1 mol/L SeCl4, on a Pt substrate at different temperatures,
are displayed in Fig. 3. The potential started at the open cir-
cuit potential and was initially swept in the negative direction
at a scan rate of 10 mV/s. In these experiments we had no
other choice but to employ a Pt quasi-reference. The peaks for
2
00–300 pm. On further STM scanning at the same potential
(
prior to the massive reduction of the organic cation of the
+
ionic liquid, −2.7 V versus Fc/Fc ) [18], the surface rough-
ness increased as a consequence of the reductive breakdown
of the adsorbed film, Fig. 2e. We would like to mention that
we have not yet succeeded in getting atomic resolution of
HOPG in [BMP]Tf2N, maybe a result of the suggested surface
film.
+
Fc/Fc were difficult to identify, and due to the complexity of
the voltammograms, it was decided to avoid any interference
fromferrocene. Fortunately, thePtquasi-referenceelectrodewas
fairly stable in the course of the experiment. At 25 C, the cyclic
◦
voltammogram was characterised by a main cathodic peak at
−
0.75 V (versus Pt), c3, and two shoulders, c2 and c1, preceding
3
.2. Electrodeposition in [BMP]Tf2N
c3. As a dark red deposit was obtained at −0.75 V after depo-
sition for 2 h, the main reduction peak, c3, might be attributed
to the reduction of Se(IV) to elemental Se, presumably the red
phase, and the two shoulders c2 and c1 might be associated with
two different under potential deposition processes. The anodic
peak a3 recorded in the reverse scan was ascribed to partial disso-
lution of the deposited selenium. The anodic counterparts to the
cathodic peaks c1 and c2 (a1 and a2) were recorded at potentials
Bearing in mind that the cation of [BMP]Tf2N seems to
adsorb on electrode surfaces and growing nuclei, in the fol-
lowing section, results concerning the electrodeposition of grey
selenium, indium and copper from the same ionic liquid were
studied as a function of temperature. The high thermal stability
of this ionic liquid (up to about 300 C) allowed direct electrode-
position of grey selenium at elevated temperatures.
◦
◦
higher than 0.0 V. From the experiments at 25 C it was con-

2
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S. Zein El Abedin et al. / Electrochimica Acta 52 (2007) 2746–2754
Fig. 3. Cyclic voltammograms of the ionic liquid 1-butyl-1-methyl pyrroli-
dinium bis(trifluoromethylsulfonyl)amide containing 0.1 M SeCl4 on platinum
at different temperatures. Scan rate 10 mV/s.
cluded that a thin film of X-ray amorphous red selenium was
obtained. This result is not at all surprising, given that thermo-
dynamically a phase transition from amorphous red selenium to
◦
crystalline grey selenium occurs only at temperatures ≥100 C.
Thus, the exclusive deposition of grey selenium should occur at
◦
temperatures above 100 C. Cyclic voltammetry measurements
◦
at 100 and 150 C were also performed, and are present as the
dashed curves in Fig. 3. Two new cathodic processes were evi-
dent in the cathodic branch, c4 and c , with corresponding anodic
5
counterparts, a4 and a , respectively. At c3, a red deposit was evi-
5
dent on the electrode surface with the naked eye, which turned
grey at c4. Thus, the peaks c3 and c4 are likely related to deposi-
tion of red and grey selenium, respectively. When the potential
was scanned further in the negative direction, after formation
Fig. 4. SEM micrographs of electrodeposited Se layers obtained potentiostati-
cally on Pt in the ionic liquid [BMP]Tf2N containing 0.1 M SeCl4 at a potential
of the grey selenium deposit, a further cathodic peak c was
5
◦
◦
of −1.1 V (vs. Pt) for 2 h at (a) 100 C and (b) 150 C. (c) EDAX profile of the
observed, with corresponding anodic peak a . This pair of peaks
5
area shown in the SEM micrograph (a).
might be associated with further reduction of the deposited sele-
nium to Se2 as the selenium film can disappear completely at
−
patterns of the electrodeposit showed the characteristic peaks of
the crystalline grey selenium, Fig. 5. At a deposition temper-
this potential. It should be mentioned that the reduction of sele-
nium to Se2− has been reported in aqueous solutions [19–22].
◦
ature of 150 C the morphology of the films changed slightly
Our results show that selenium electrochemistry in [BMP]Tf2N
ionic liquid is fairly difficult and requires a lot of fundamental
studies.
(Fig. 4b) but the XRD showed also only grey selenium. At this
temperature red or black selenium would be thermodynamically
excluded.
In order to perform a material analysis of selenium, the
◦
deposits were made at −1.1 V (versus Pt) for 2 h at 100 C on
Pt substrates and subsequently characterised by means of SEM-
EDAX and XRD to explore morphology and composition. After
the deposition experiments the deposits were washed by iso-
propanol and then cleaned with acetone in an ultrasonic bath for
3.2.2. Electrodeposition of Indium
Fig. 6 shows the cyclic voltammogram of the ionic liquid
[BMP]Tf2N containing 0.1 M InCl3, at a glassy carbon elec-
◦
trode, at 25 C. As seen, the cyclic voltammogram exhibits a
1
0 min. Visually, a dark grey, well adherent deposit was obtained
rapid increase of the cathodic current at a potential of about
after potentiostatic deposition. As seen in the SEM micrograph
shown in Fig. 4a, the deposit made at 100 C appeared dense
with a flower-like structure. The EDX profile of Fig. 4c revealed
only selenium in the deposit. There was neither chloride nor any
component of the ionic liquid in the deposited film. The XRD
−1.75 V (versus Pt) as a result of the bulk deposition of indium.
◦
3+
0
This signifies that the reduction of In to In occurs in one
step. In the reverse scan, an anodic peak at −2.0 V suggests
partial stripping of the electrodeposited indium. The increase
in anodic current at about 0.0 V might be the result of slower

S. Zein El Abedin et al. / Electrochimica Acta 52 (2007) 2746–2754
2751
Fig. 5. XRD patterns of an electrodeposited Se layer obtained potentiostatically
on Pt in ([BMP]Tf2N) containing 0.1 M SeCl4 at a potential of −1.1 V (vs. Pt)
◦
for 2 h at 100 C.
dissolution of larger In crystallites. After the cyclic voltamme-
try experiment, a metallic film of indium was clearly seen on
the electrode surface indicating the incomplete stripping of the
electrodeposited indium. As previously shown by this group,
with tantalum deposition, reoxidation of the deposits seems to
be hindered in [BMP]Tf2N [23].
The electrodeposition of indium on polycrystalline platinum
substrates was also investigated in this electrolyte. The cyclic
voltammograms recorded at Pt substrates with different reversal
potentials are shown in Fig. 7. Unlike in the case of glassy car-
bon, several cathodic processes, c1–c4, are recorded at platinum
substrate, prior to bulk deposition of indium, Fig. 7a. Visually,
Fig. 7. Cyclic voltammograms of the ionic liquid [BMP]Tf2N containing 0.1 M
InCl3 on platinum with different reversal potentials at 25 C. Scan rate 10 mV/s.
◦
more information on the different electrochemical processes,
cyclic voltammograms with varying reversal potentials were
performed, Fig. 7b and c. If the scan was reversed at a poten-
tial of −1.1 V, only one reduction peak, c1, was observed with
the corresponding anodic peak, a1, Fig. 7c. EDX analysis indi-
cated a very low concentration of indium on the electrode after
applying a constant potential of −1.1 V for 1 h. The presence of
indium at the electrode surface before bulk deposition suggests
that this peak corresponds to a UPD process. At a lower switch-
ing potential −1.6 V, two new cathodic peaks, c2 and c3, were
observed with corresponding anodic peaks, a2 and a3, respec-
bulk deposition was found to occur at c . As cathodic peaks
5
c1–c4 occur at potentials positive to the potential of bulk depo-
sition of In, they are presumably correlated with different UPD
processes for In on polycrystalline platinum. In order to obtain
ꢀ
tively (Fig. 7b). A further new anodic peak, a , was observed at
1
a potential more positive than the potential of the anodic peak
a1. This peak might result from alloying of In with Pt. As shown
in Fig. 7a, a further new cathodic process (c4) with its counter-
part (a4) was recorded prior to bulk deposition of In (c ). Those
5
results showed that electrodeposition of indium on glassy car-
bon was relatively straightforward, whereas on platinum there
was evidence for multiple UPD/alloying processes. This indi-
cates a need to investigate In deposition on platinum using in
situ scanning tunneling microscopy.
The surface morphology of In deposited on platinum in the
ionic liquid [BMP]Tf N, containing 0.1 M InCl , after poten-
2 3
Fig. 6. Cyclic voltammogram of the ionic liquid [BMP]Tf2N containing 0.1 M
InCl3 on glassy carbon at 25 C. Scan rate 10 mV/s.
◦

2
752
S. Zein El Abedin et al. / Electrochimica Acta 52 (2007) 2746–2754
Fig. 10. Voltammetric behaviour of copper electrode in the ionic liquid
BMP]Tf2N. Scan rate 10 mV/s.
[
Fig. 8. (a) SEM micrograph of an electrodeposited indium film obtained poten-
tiostatically on platinum in the ionic liquid [BMP]Tf2N containing 0.1 M InCl3
are significantly more complicated, and alloying with platinum
seems to be facilitated. Moreover, the deposits obtained showed
worse adherence. This was not surprising, as the melting point of
◦
at a potential of −2.2 V (vs. Pt) for 2 h at 25 C. (b) EDAX profile taken on a
big crystallite.
◦
indium is 156.6 C and hence the deposition of adherent metal-
tiostatic polarization at −2.2 V (versus Pt) for 2 h is shown in
Fig. 8a. The deposit consists of indium crystallites with sizes
of approximately 100–200 nm. The accompanying EDX profile
reveals the deposition of pure metallic indium, Fig. 8b.
lic indium might be difficult at temperatures close to its melting
point. Nevertheless, these results show that indium can be elec-
trodeposited in [BMP]Tf2N at temperatures above 100 C, at
◦
temperatures where grey selenium can be obtained, as well.
The effect of increasing the temperature on the electrode-
position of indium on platinum substrates in the employed
electrolyte was also investigated, Fig. 9. The cyclic voltam-
3
.2.3. Electrodeposition of copper
Coppercanalsobedepositedfrom[BMP]Tf2N, atavarietyof
◦
mogram recorded at 50 C exhibits the same general features
temperatures. Unfortunately, most copper compounds tested had
only limited solubility in the ionic liquid [BMP]Tf2N, and the
best method for introducing copper cations into the ionic liquid
was anodic dissolution of a copper electrode. Fig. 10 shows the
◦
of the cyclic voltammogram measured at 25 C (see Fig. 7a).
◦
However, the cyclic voltammograms obtained at 100 and 150 C
Fig. 9. Cyclic voltammograms of the ionic liquid [BMP]Tf2N containing 0.1 M
InCl3 on platinum at different temperatures. Scan rate 10 mV/s.
Fig. 11. Cyclic voltammograms of the ionic liquid [BMP]Tf2N containing
60 mmol/L Cu(I) on platinum at different temperatures. Scan rate 10 mV/s.

S. Zein El Abedin et al. / Electrochimica Acta 52 (2007) 2746–2754
2753
electrode. It is unlikely that the Tf2N anion was decomposed at
this electrode potential.
Fig. 11 presents cyclic voltammograms obtained for Cu(I)
species on gold substrates at different temperatures, 25, 50,
◦
1
00 and 150 C. As shown, all cyclic voltammograms exhibit
the same general features: there is a pronounced cathodic peak
for copper deposition with its corresponding stripping peak.
Increasing the temperature resulted in the deposition potential
moving positive, as well as the current increasing. This was
ascribed both to an increase in the mobility of the electroactive
species towards the electrode surface and in a reduced nucleation
overvoltage at elevated temperatures.
The SEM micrograph of Fig. 12a shows the surface mor-
phology of an electrodeposited copper layer on gold substrate
obtained at a constant potential of −0.250 V (versus Pt) for 2 h
in the ionic liquid [BMP]Tf2N containing 60 mmol/L of Cu(I) at
◦
2
5 C. As seen, the deposit is dense and contains fine crystallites
with average sizes of about 50 nm. The deposit was analysed as
metallic copper as revealed by the corresponding EDX profile,
Fig. 12b. Interestingly, the deposited copper was nanocrystalline
without any additive, similar to the deposition of nanocrystalline
Al [17]. This finding is of some interest as nanocrystalline cop-
per has excellent mechanical and electronic properties, superior
to those of microcrystalline copper. The increase of temperature
had a dramatic effect on the crystallite sizes as revealed in the
SEM micrograph of Fig. 12c. The deposit made potentiostati-
◦
cally at −0.12 V (versus Pt) for 2 h at 100 C rather contained
microcrystalline copper particles.
4. Conclusions
In part 1 of the present paper, surface processes at
the interface of Au(1 1 1) and HOPG in the ionic liq-
uid 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)
amide ([BMP]Tf2N) were described. In situ STM showed that
Au(1 1 1) was initially restructured to a worm-like surface, and
to the typical Au(1 1 1) surface at more negative electrode poten-
tials. On HOPG, a surface film, which disappeared at elec-
trode potentials close to the cathodic decomposition of the
liquid, was observed. Presumably, the cation of the ionic liq-
uid was adsorbed, causing the restructuring/reconstruction of
the Au(1 1 1) surface.
Fig. 12. (a) SEM micrograph of nanocrystalline copper obtained potentiostat-
ically on Au in the ionic liquid [BMP]Tf2N containing 60 mmol/L Cu(I) at a
potential of −0.25 V (vs. Pt) for 2 h at room temperature. (b) EDAX profile of
the area shown in the SEM micrograph. (c) SEM micrograph of a deposited Cu
◦
layer obtained potentiostatically at −0.12 V (vs. Pt) for 2 h at 100 C.
cyclic voltammogram of copper in the employed ionic liquid.
The potential was scanned first in the negative direction down
to −1.12 V (versus Cu) then it was scanned back to +0.38 V
and finally stopped at the open circuit potential. It is clear that
anodic dissolution of copper starts at a potential of +0.05 V,
indicated by the rapid increase in anodic current. The reduction
peak observed at −0.2 V (versus Cu) is attributed to redeposition
ofdissolvedcopper. Basedonthisinformation, Cucationscanbe
introduced into the ionic liquid by applying a positive potential,
In part 2 the electrodeposition of grey selenium, indium and
copper in [BMP]Tf2N at variable temperatures was described.
◦
A key result was that at temperatures above 100 C, grey sele-
◦
nium was deposited as a single phase, whereas at 25 C the
red amorphous phase was formed. Both indium and copper
were electrodeposited from [BMP]Tf2N at variable tempera-
tures. Interestingly, the grain sizes of electrochemically formed
In and Cu were in the nanosize regime. Based on former studies
on the electrodeposition of aluminium, and based on in situ STM
results both on Au(1 1 1) and on HOPG, it could be concluded
≥
+0.1 V. In order to determine the oxidation state of the copper
species, the weight loss of the copper electrode, as well as the
charge consumed during its anodic dissolution at a potential of
+
that the [BMP] cation was adsorbed on the substrates and on
growing nuclei, leading to nanosized deposits. It is unclear at
the moment to which extent the Tf2N anion is co-adsorbed. The
possibility of depositing grey selenium, indium and copper in
one ionic liquid, at variable temperatures, suggests that semicon-
+
0.3 V (versus Cu) were measured. The oxidation state of copper
species was found to be 1. This indicates that Cu(I), presumably
Cu(Tf2N), was the product of the anodic dissolution of copper

2
754
S. Zein El Abedin et al. / Electrochimica Acta 52 (2007) 2746–2754
ductor CuInSe2 (CIS) might be formed electrochemically using
such an ionic liquid.
[10] F. Endres, A. Schweizer, Phys. Chem. Chem. Phys. 2 (2000) 5455.
[
11] J.-J. Lee, B. Miller, X. Shi, R. Kalish, K.A. Wheeler, J. Electrochem. Soc.
48 (2001) C183.
1
[
[
[
[
12] P.-Y. Chen, I.-W. Sun, Electrochim. Acta 45 (1999) 441.
13] P.-Y. Chen, M.-C. Lin, I.-W. Sun, J. Electrochem. Soc. 147 (2000) 3350.
14] S.-I. Hsiu, C.-C. Tai, I.-W. Sun, Electrochim. Acta 51 (2006) 2607.
15] M.-H. Yang, M.-C. Yang, I.-W. Sun, J. Electrochem. Soc. 150 (2003)
C544.
References
[
[
[
1] H.W. Schock, Appl. Surf. Sci. 92 (1996) 606.
2] B.M. Huang, T.E. Lister, J.L. Stickney, Surf. Sci. 392 (1997) 27.
3] T.A. Sorenson, T.E. Lister, B.M. Huang, J.L. Stickney, J. Electrochem. Soc.
[
[
[
16] N. Borisenko, S. Zein El Abedin, F. Endres, J. Phys. Chem. B 110 (2006)
1
46 (1999) 1019.
4] M. Alanyalioglu, U. Demir, C. Shannon, J. Electroanal. Chem. 561 (2004)
1.
5] X.Y. Zhang, Y. Cai, J.Y. Miao, K.Y. Ng, Y.F. Chan, X.X. Zhang, N. Wang,
J. Cryst. Growth 276 (2005) 674.
6250.
[
[
17] S. Zein El Abedin, E. Moustafa, R. Hempelmann, H. Natter, F. Endres,
Chem. Phys. Chem. 7 (2006) 1535.
18] S. Zein El Abedin, N. Borisenko, F. Endres, Electrochem. Commun. 6
2
(
2004) 510.
[
[
6] F. Endres, S. Zein El Abedin, Phys. Chem. Chem. Phys. 8 (2006) 2101.
7] C.L. Hussey, L.A. King, R.A. Carpio, J. Electrochem. Soc. 126 (1979)
[19] M.C. Santos, S.A.S. Machado, J. Electroanal. Chem. 567 (2004) 203.
[20] N. Furuya, S. Motoo, J. Electroanal. Chem. 98 (1979) 189.
[21] R. Modolo, M. Traore, O. Vittori, Electrochim. Acta 31 (1986) 859.
[22] T.E. Lister, J.L. Stickney, J. Phys. Chem. 100 (1996) 19568.
[23] S. Zein El Abedin, H.K. Farag, E.M. Moustafa, U. Welz-Biermann, F.
Endres, Phys. Chem. Chem. Phys. 7 (2005) 2333.
1
029.
[
[
8] C. Nanjundiah, R.A. Osteryoung, J. Electrochem. Soc. 130 (1983) 1312.
9] B.J. Tierney, W.R. Pitner, J.A. Mitchell, C.L. Hussey, J. Electrochem. Soc.
1
45 (1998) 3110.