Electrodeposition of Al in 1-butyl-1-methylpyrrolidinium Bis(trifluoromethylsulfonyl)amide and 1-Ethyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)amide ionic liquids: In situ STM and EQCM studies

Article abstract of DOI:10.1021/jp0670687

In the present paper, the electrodeposition of Al on flame-annealed Au(111) and polycrystalline Au substrates in two air- and water-stable ionic liquids namely, 1-butyl-1-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)-amide, [Py_1,4]Tf₂N, and 1-ethyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)amide, [EMIm]Tf₂N, has been investigated by in situ scanning tunneling microscopy (STM), electrochemical quartz crystal microbalance (EQCM), and cyclic voltammetry. The cyclic voltammogram of aluminum deposition and stripping on Au(111) in the upper phase of the biphasic mixture of AlCl₃/[EMIm]Tf₂N at room temperature (25°C) shows that the electrodeposition process is completely reversible as also evidenced by in situ STM and EQCM studies. Additionally, a cathodic peak at an electrode potential of about 0.55 V vs Al/Al(III) is correlated to the aluminum UPD process that was evidenced by in situ STM. A surface alloying of Al with Au at the early stage of deposition occurs. It has been found that the Au(111) surface is subject to a restructuring/reconstruction in the upper phase of the biphasic mixture of AlCl₃/[Py_1,4]Tf₂N at room temperature (25°C) and that the deposition is not fully reversible. Furthermore, the underpotential deposition of Al in [Py_1,4]Tf ₂N is not as clear as in [EMIm]Tf₂N. The frequency shift in the EQCM experiments in [Py_1,4]Tf₂N shows a surprising result as an increase in frequency and a decrease in damping with bulk aluminum deposition at potentials more negative than -1.8 V was observed at room temperature. However, at 100°C there is a frequency decrease with ongoing Al deposition. At -2.0 V vs Al/Al(III), a bulk aluminum deposition sets in.

Full text of DOI:10.1021/jp0670687

J. Phys. Chem. B 2007, 111, 4693-4704
4693
Electrodeposition of Al in 1-Butyl-1-methylpyrrolidinium Bis(trifluoromethylsulfonyl)amide
and 1-Ethyl-3-methylimidazolium Bis(trifluoromethylsulfonyl)amide Ionic Liquids: In Situ
STM and EQCM Studies†
E. M. Moustafa,‡,§ S. Zein El Abedin,‡,§ A. Shkurankov,‡ E. Zschippang,| A. Y. Saad,‡,§
A. Bund,| and F. Endres*,‡
Chair of electrochemistry, Clausthal UniVersity of Technology, Robert-Koch-Strasse 42,
38678 Clausthal-Zellerfeld, Germany and Dresden UniVersity of Technology, 01062 Dresden, Germany
ReceiVed: October 27, 2006; In Final Form: January 29, 2007
In the present paper, the electrodeposition of Al on flame-annealed Au(111) and polycrystalline Au substrates
in two air- and water-stable ionic liquids namely, 1-butyl-1-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)-
amide, [Py1,4]Tf2N, and 1-ethyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl)amide, [EMIm]Tf2N, has
been investigated by in situ scanning tunneling microscopy (STM), electrochemical quartz crystal microbalance
(EQCM), and cyclic voltammetry. The cyclic voltammogram of aluminum deposition and stripping on Au-
(111) in the upper phase of the biphasic mixture of AlCl3/[EMIm]Tf2N at room temperature (25 °C) shows
that the electrodeposition process is completely reversible as also evidenced by in situ STM and EQCM
studies. Additionally, a cathodic peak at an electrode potential of about 0.55 V vs Al/Al(III) is correlated to
the aluminum UPD process that was evidenced by in situ STM. A surface alloying of Al with Au at the early
stage of deposition occurs. It has been found that the Au(111) surface is subject to a restructuring/reconstruction
in the upper phase of the biphasic mixture of AlCl3/[Py1,4]Tf2N at room temperature (25 °C) and that the
deposition is not fully reversible. Furthermore, the underpotential deposition of Al in [Py1,4]Tf2N is not as
clear as in [EMIm]Tf2N. The frequency shift in the EQCM experiments in [Py1,4]Tf2N shows a surprising
result as an increase in frequency and a decrease in damping with bulk aluminum deposition at potentials
more negative than -1.8 V was observed at room temperature. However, at 100 °C there is a frequency
decrease with ongoing Al deposition. At -2.0 V vs Al/Al(III), a bulk aluminum deposition sets in.
1. Introduction
Aluminum plays an important role in modern industries as a
lightweight material in automotives and planes as well as for
decorative purposes, for example. Furthermore, the high cor-
rosion resistance of Al against chemical and atmospheric attack
makes it an interesting coating material for steel. The Hall-
Heroult process of aluminum electrowinning is not suitable to
coat other metals by aluminum because the electrolysis is carried
out at 1000 °C, a temperature at which cryolite (Na3AlF6),
alumina (Al2O3), and aluminum itself are in the liquid state.1,2
Currently, there are several methods available for low-temper-
ature aluminum plating such as thermal spray coating, cladding,
roll binding, physical and chemical vapor deposition, and
electroplating. All of these methods except electroplating are
quite expensive and technically demanding. Due to its reactivity
(-1.7 V vs NHE), aluminum cannot be electrodeposited in an
Figure 1. Typical cyclic voltammogram for aluminum deposition and
stripping on Au(111) in the upper phase of the biphasic mixture of
AlCl3/[EMIm]Tf2N at room temperature (25 °C). Scan rate: 10 mV/s.
aqueous bath. Thus, the electrodeposition process is restricted
to nonaqueous aprotic electrolytes such as organic solvents.3-5
Two commercial processes are the REAL (room-temperature
electroplated aluminum)6,7 and SIGAL (Siemens-Galvano-
of chemists and engineers as a new and novel generation of
Aluminum) processes.8,9 A main disadvantage of both processes
solvents that exhibit many attractive properties, including low
is the flammability of the Al species and/or organic solvents.
melting points, low vapor pressures, good chemical and thermal
During the past 6 years, ionic liquids have attracted the interest
stabilities, high intrinsic conductivities, and large electrochemical
windows. These properties give ionic liquids a certain potential
to play a vital role in a large number of modern applications.
The first low-temperature molten salt, ethylammonium nitrate,
with a melting point of 12 °C was synthesized by Walden in
1914.10 Hurley and Wier11 in 1951 developed the first low
melting salts with chloroaluminate ions for low-temperature
† Part of the special issue “Physical Chemistry of Ionic Liquids”.
* To whom correspondence should be addressed. Fax: +49-5323-
722460. E-mail: frank.endres@tu-clausthal.de.
‡ Clausthal University of Technology.
§ Permanent address: Electrochemistry and Corrosion Laboratory,
National Research Centre, Dokki, Cairo, Egypt.
| Dresden University of Technology.
10.1021/jp0670687 CCC: $37.00 © 2007 American Chemical Society
Published on Web 03/14/2007

4694 J. Phys. Chem. B, Vol. 111, No. 18, 2007
Moustafa et al.
Figure 2. Sequence of STM pictures recorded at 23 ( 1 °C in situ during the aluminum UPD process on Au(111) from the upper phase of the
biphasic mixture of AlCl3/[EMIm]Tf2N. Deposition of the first aluminum monolayer onto Au(111), Ebias ) 0.3 V, Itun) 2 nA, and a 2 Hz scan rate:
(a) monoatomic terraces on the Au(111) surface acquired at E ) 0.7 V vs Al/Al(III), (b) Al islands formed at E ) 0.6 V, (c) 500 × 500 nm2 area
from Figure 2b, (d) E ) 0.5 V, (e) E ) 0.4 V, and (f) E ) 0.3 V.
aluminum electroplating. Popular examples of ionic liquids
include the chloroaluminates, which are prepared by combining
anhydrous AlCl3 with a suitable organic chloride such as
N-butylpyridinium chloride (N-BPC) and 1-ethyl-3-methylimi-
dazolium chloride (EMIm Cl). These ionic liquids, which are
based on AlCl3, are considered today as the first generation of
ionic liquids. First-generation ionic liquids are the simplest
systems from which aluminum can be easily electrodeposited.
A lot of work has been done using chloroaluminate ionic liquids
to get high-quality aluminum and aluminum alloy deposits by
several authors.12-15 A shortcoming is that the chloroaluminate
ionic liquids can only be handled under inert-gas atmosphere
due to the hygroscopic nature of AlCl3. For this reason, synthesis
of air- and water-stable ionic liquids, which are considered as
the second generation of ionic liquids, has attracted much interest
for their use in various fields. Wilkes and Zaworotko introduced
the first more or less air- and water-stable ionic liquids in 199216
with either tetrafluoroborate or hexafluorophosphate as anions.
Later it was found that exposure to moisture for a long time
can cause decomposition of the anions liberating HF, as we
also observed upon drying wet PF6--based ionic liquids.17
Therefore, ionic liquids based on more hydrophobic and
stable anions such as, e.g., bis(trifluoromethylsulfonyl)amide
[(CF3SO2)2N-] and many others, have been developed.18-20
These ionic liquids can be considered as the third generation
of ionic liquids, although sometimes the task-specific ionic
liquids introduced by Jim Davis21 are considered as the third
generation. For details on the electrochemical properties we refer
to the book of Ohno.22 Recently, we presented a detailed
experimental study on the electrodeposition of aluminum in three
different air- and water-stable ionic liquids, namely, 1-butyl-
1-methylpyrrolidinium-bis(trifluoromethylsulfonyl)amide([Py1,4]-
Tf2N),23,24 1-ethyl-3-methyl-imidazolium-bis(trifluoromethyl-
sulfonyl)amide ([EMIm]Tf2N), and trihexyl-tetradecyl phosphon-
ium bis(trifluoromethylsulfonyl)amide (P14,6,6,6Tf2N).24 Our
original motivation was to dissolve AlCl3 in easy to dry
hydrophobic ionic liquids and thus eliminate the problems raised
by water in the first-generation ionic liquids. Interestingly, we
observed some surprising effects with these liquids. The two
ionic liquids [Py1,4]Tf2N and [EMIm]Tf2N exhibit biphasic
behavior in the AlCl3 concentration range from 1.6 to 2.5 and
2.5 to 5 mol/L, respectively. In [Py1,4]Tf2N, nanocrystalline
aluminum was obtained; however, in [EMIm]Tf2N, microcrys-
talline aluminum was deposited. It is unlikely that this observa-
tion is due to viscosity effects alone. In accordance to the (not
yet fully understood) role of brighteners and grain refiners in
aqueous electrochemistry, we interpreted these results on the
basis of cation adsorption on growing Al nuclei, which seems
to play a role in the liquid with the pyrrolidinium cation. In
order to shed more light on this pretty complicated aluminum
electrochemistry, we investigated the Al deposition in [Py1,4]-
Tf2N and [EMIm]Tf2N by both in situ STM and in situ
electrochemical quartz crystal microbalance (EQCM) in the
present paper. With the in situ STM we probe the initial surface

Electrodeposition of Al
J. Phys. Chem. B, Vol. 111, No. 18, 2007 4695
Figure 3. Series of STM images of Al deposition recorded at 23 ( 1 °C on the first aluminum monolayer with Itun) 2 nA, Ebias) 0.3 V, and a
2 Hz scan rate. Formation of a second and third aluminum layer followed by formation of 3D crystallites: (a) at E ) 0.3 V vs Al/Al(III), the
number and size of 2D aluminum islands increase with time (25 min between Figure 2f and 3a); (b) at E ) 0.2 V, a complete second aluminum
layer with some vacancy islands is formed, and a third aluminum layer starts to grow before completion of the second layer; (c) at E ) 0.1 V; (d)
aluminum deposit exhibits a granular structure at E ) - 0.1 V, and the crystallites have an average diameter of about 15 nm as shown from the
given height profile.
processes during nucleation and grain growth; with EQCM we
get information about the mass balance during Al deposition.
Monitoring of small mass changes at the surface of an
electrode can be done by tracking the resonance frequency of
a quartz crystal (electrochemical quartz crystal microbalance
technique). This makes the EQCM a valuable method to analyze
crystallization during electrochemical processes. Characteriza-
tion of damping layers (such as viscoelastic or very rough layers)
can be done by measuring the electrical admittance, Y, of the
quartz crystal with a network analyzer or an impedance
bridge.25-28 The real part of Y vs frequency (usually called a
resonance curve) can be described with a Lorentzian function.
The center of the curve lies at the resonance frequency f, and
its full width at half-maximum (fwhm) w is proportional to the
damping of the quartz crystal. If a smooth rigid layer is deposited
on the quartz crystal (e.g., by vacuum evaporation) the resonance
curve shifts to lower frequencies but does not change its form.
If the layer dampens the oscillation of the quartz crystal (e.g.,
a liquid), the resonance curve shifts to lower frequencies and
at the same time broadens.
where ∆m/A is the change of the areal mass density, with ZQ
being the mechanical impedance of the quartz crystal (8.849 ×
106 kg m-2 s-1) and f0 the fundamental resonance frequency
of the unloaded quartz.
It is necessary to mention, that in the Sauerbrey case the
frequency shift is a purely real quantity, i.e., no damping change
(imaginary part of the frequency shift) occurs. This is a
consequence of the smoothness and rigidity of the layer.
However, electrochemically prepared layers often have a certain
surface roughness, which strongly depends on the preparation
conditions. The damping of the quartz gives a first indication
if the Sauerbrey equation can be applied.30-32 For metal
depositions an increase of the damping of the quartz crystal
usually indicates an increase of the surface roughness. Thus,
the EQCM with damping monitoring (which is applied in the
present paper) gives averaged qualitative information about the
morphology of the surface.
In the present study, we applied the EQCM technique to
investigate the electrochemical processes of aluminum deposi-
tion and redissolution in ionic liquids containing AlCl3. The
change of the damping, dw, is used as a diagnostic feature to
judge whether the Sauerbrey equation, eq 1, can be applied. As
a rule of thumb, as long as the absolute value of df is larger
than dw, df can be interpreted as a mass change.
For a smooth rigid layer with density F and thickness L, the
Sauerbrey equation was established to evaluate the mass change
from the change of resonance frequency29
2
2
2f0
2f0 ∆m
∆f ) -
FL ) -
(1)
ZQ
ZQ A

4696 J. Phys. Chem. B, Vol. 111, No. 18, 2007
Moustafa et al.
Figure 4. Sequence of STM images acquired during dissolution of bulk aluminum at 23 ( 1 °C with Itun) 2 nA, Ebias) 0.21 V, and a 2 Hz scan
rate: (a) E )0.3 V vs Al/Al (III), (b) E ) 0.5 V, (c) E ) 0.6 V, (d) E ) 0.7 V, and (e) E ) 0.82 V.
2. Experimental Section
a mixture of 50/50 vol % H2SO4/H2O2 followed by refluxing
in pyrogene-free water (aqua destillata ad iniectabilia).
The two employed ionic liquids for the present study 1-ethyl-
3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([EMIm]-
Tf2N) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethyl-
sulfonyl)amide ([Py1,4]Tf2N) were purchased in the highest
available (ultrapure) quality. They were dried under vacuum
for 12 h at a temperature of 100 °C to water contents below 3
ppm (by Karl-Fischer titration) and stored in a closed bottle
in an argon-filled glove box with water and oxygen levels below
1 ppm (OMNI-LAB from Vacuum-Atmospheres). Anhydrous
AlCl3 grains (Fluka, 99%) were used without further purification
as a source of aluminum.
All electrochemical and in situ STM measurements were
performed at room temperature under inert gas. The electro-
chemical measurements were performed using a PARSTAT
2263 Potentiostat/Galvanostat (Princeton Applied Research)
controlled by Power CV software. Au(111) (gold on mica)
substrates, purchased from Molecular Imaging, were used as
working electrodes for electrochemical and in situ STM
measurements. Prior to each experiment, the working surface
of Au(111) was subject to a careful heat treatment in a hydrogen
flame for several minutes to remove surface contaminants. Al
wires (Alfa, 99.999%) of 0.5 mm diameter were used as
reference and counter electrodes. Prior to use they were
mechanically polished to remove the oxide film, rinsed with
acetone, and finally dried under vacuum. The electrochemical
cell was made of polytetrafluoroethylene (PTFE) and clamped
over a PTFE-covered Viton O ring onto the substrate, thus
yielding a geometric surface area of 0.3 cm2. Prior to use, all
parts in contact with the electrolyte were thoroughly cleaned in
The STM experiments were performed with in-house-built
STM heads and scanners under inert gas conditions with a
Molecular Imaging PicoScan 2500 STM controller in feedback
mode. Assembling of the STM head and filling of the
electrochemical cell were performed in an argon-filled glovebox.
The STM head was placed inside a vacuum-tight stainless steel
vessel to ensure that the STM experiment was performed in a
clean and an inert gas atmosphere, transferred to the air-
conditioned laboratory with a constant temperature of 23 ( 1
°C, and placed onto a vibration-damped table from IDE-GmbH,
Germany. The STM tips were prepared by electrochemical
etching of 0.25 mm 90:10 Pt/Ir wires in 4 M NaCN solution
followed by electrophoretic coating with an electropaint from
BASF (ZQ 84-3225 0201) and subsequently heated to 100 and
200 °C for 1 h and 10 min, respectively. This coating was
sufficiently resistant against the used ionic liquids without
showing any detectable degradation. During the STM experi-
ments at 23 ( 1 °C the electrode potential of the working
electrode was controlled by the PicoStat from Molecular
Imaging. All STM pictures were obtained by scanning from
bottom to top with a low scan rate of 2 Hz and a resolution of
512 pixels per line.
The gold-coated quartz crystals (10 MHz AT) were provided
by Vectron International (Neckarbischofsheim, Germany). The
electrochemical cell was cleaned in a mixture of concentrated
sulfuric acid/30% aqueous hydrogen peroxide and subsequently
refluxed in aqua destillata ad iniectabilia. The quartz crystal
was cleaned with isopropyl alcohol and dried under air at

Electrodeposition of Al
J. Phys. Chem. B, Vol. 111, No. 18, 2007 4697
Figure 6. Typical cyclic voltammogram for aluminum deposition and
stripping on Au(111) in the upper phase of the biphasic mixture of
AlCl3/[Py1,4]Tf2N at room temperature (25 °C). Scan rate: 10 mV/s.
overpotential attributed to nucleation was found for Al bulk
deposition (process C) with a sharp corresponding stripping peak
of bulk aluminum, C′, at about 0.05 V vs Al/Al(III). The two
additional cathodic waves (A and B) at 0.55 and -0.02 V vs
Al/Al(III) are correlated to aluminum UPD and 3D aluminum
cluster formation in the beginning of the OPD regime, respec-
tively, as evidenced by in situ STM results presented below.
On the reverse scan, the dissolution peak C′ for aluminum
bulk deposition is followed by a broad wave A′ at a potential
of about 0.55 V which can be assigned to dissolution of
aluminum deposited in the UPD regime.
Figure 5. EQCM in [EMIm]Tf2N ionic liquid containing 5 M AlCl3
at room temperature (25 °C), upper phase: (a) cyclic voltammogram,
scan rate ) 10 mV‚s-1; (b) change in frequency df; and (c) change in
damping dw.
3.1.2. In Situ STM Measurements. In this section we present
in situ STM results on the initial stages of aluminum elec-
trodeposition in the underpotential (UPD) and overpotential
(OPD) regime. It has been reported that UPD refers to a
phenomenon by which an element electrodeposits on another
one at a potential positive to its Nernst equilibrium potential.33
UPD results from the difference in energetics of inter- vs
intraelemental bonding; that is, UPD can be thought of as
formation of a surface compound with the accompanying heat
of formation. As a consequence, UPD stops when the surface
is covered; thus, it is a surface-limited reaction, providing atomic
layer control. For systems where the elements show a good
miscibility, interaction of the adsorbate with the substrate may
not stop with a single atomic layer, and a surface alloy can be
formed as in our case.
The initial stages of the aluminum deposition process were
investigated by stepwise decreasing the potential from 0.93 V
vs Al/Al(III) until the bulk deposition started. For potentials
between 0.93 and 0.7 V vs Al/Al(III) we clearly observe the
typical terrace-like structure of Au(111). Figure 2 shows a
sequence of STM images recorded in situ during the aluminum
UPD process on Au(111) from the upper phase of the biphasic
mixture of AlCl3/[EMIm]Tf2N. Images a-f show the sequence
of deposition of the first aluminum monolayer onto Au(111).
Figure 2a shows the pure Au(111) surface with terraces of an
average step height of the typical 250 pm, i.e., the steps are
monoatomically high, E ) 0.7 V vs Al/Al(III). After switching
the potential to 0.6 V, aluminum starts to deposit by forming
2D islands as shown in Figure 2b. Magnification of the area
determined by the white rectangle in Figure 2b is shown in
Figure 2c. The height of these 2D islands is between 200 and
250 pm, which is within experimental error in agreement with
the monatomic height of aluminum as shown from a typical
100 °C. A cylindrical (home-built) PTFE cell was used for
EQCM measurements. The quartz crystal was placed at the
bottom of this cell and held between two O rings. On the ionic
liquid side it is essential to use a perfluoroelastomer such as
Kalrez (Dupont Dow Elastomers L.L.C., Newark, DE). Other-
wise swelling of the gasket can lead to considerable signal drifts.
A spiral-form electrical heater was mounted around the cell to
provide uniform heating inside the cell. The cell was operated
under argon in an inert-gas glove box (Vacuum Atmospheres
OMNILAB) in which deposition experiments were performed.
The potential of the Au electrode facing the ionic liquid was
controlled with a PAR Versastat II potentiostat; the other
electrode of the quartz crystal faced the inert-gas atmosphere
of the glove box. The potentials were measured versus an Al/
Al(III) reference electrode. An aluminum rod was used as the
counter electrode. The electrical admittance curve of the quartz
was measured in parallel to the electrochemical experiments
with a network analyzer (Agilent E5100A) and transferred to a
PC-compatible computer via a GPIB interface.
3. Results and Discussion
3.1. Electrodeposition of Aluminum in [EMIm]Tf2N. 3.1.1.
Cyclic Voltammetry. As shown in ref 24, Figure 1 shows a
typical cyclic voltammogram for aluminum deposition and
stripping on Au(111) in the upper phase of the biphasic mixture
of AlCl3/[EMIm]Tf2N at room temperature (25 °C). Here, the
UPD regime is depicted. Scans were initially swept cathodically
from the open-circuit potential (1.1 V vs Al/Al(III)) with a rate
of 10 mV/s. The voltammogram shows distinct features in both
the forward and the reverse scans. There is a direct correlation
between processes A and A′ and C and C′. A significant

4698 J. Phys. Chem. B, Vol. 111, No. 18, 2007
Moustafa et al.
Figure 7. Two STM images of Au(111) in the upper phase of the biphasic mixture of AlCl3/[Py1,4]Tf2N at 23 ( 1 °C showing surface restructuring
with tiny pits of height of about 250 pm as shown from the typical height profile: (a) At E ) 0.3 V vs Al/Al(III); (b) after 8 min at the same E
) 0.3 V vs Al/Al(III).
height profile of Figure 2c. There are no such islands at this
electrode potential in the pure ionic liquid. The number and
size of such islands increases with further aluminum deposition
by decreasing the potential to 0.5 V (Figure 2d). If the potential
is decreased further to 0.4 V the number of aluminum islands
still rises until, with the exception of few vacancies, one
aluminum monolayer is formed (Figure 2e). These vacancies
are still present even by further shifting the potential to 0.3 V,
at which a second aluminum layer starts to grow by forming
2D aluminum islands on top of the first monolayer (Figure 2f).
With ongoing time (about 25 min) the number and size of these
islands increase (see Figure 3a).
Figure 3 shows a series of STM images after the first
aluminum monolayer has been formed. The pictures show
formation of a second and third aluminum layer before 3D
crystallites start growing. As shown in Figure 3a, the number
and size of 2D aluminum islands on top of the first monolayer
increase with time (25 min between Figures 2f and 3a). By
switching the electrode potential to +0.2 V and with ongoing
time a complete second aluminum layer with some vacancies
forms. The first monatomic aluminum layer is not yet com-
pletely closed before a third aluminum layer starts to grow in
the UPD regime (see Figure 3b). When the electrode potential
is further reduced to 0.1 V, for several minutes, the number of
deposited crystallites rapidly increases and a 3D growth sets
in. Reducing the electrode potential to -0.1 V leads to crystal
growth, and the initial aluminum deposit exhibits a granular
structure as shown in Figure 3d. This picture shows that the
crystallites are homogeneously spread with an average diameter
of about 15 nm of crystallites in the height profile. Nevertheless,
when a bulk deposit is made in this liquid the obtained deposit
is clearly microcrystalline.24
The results in the UPD regime can be summarized as
follows: first, monoatomically high Al islands grow on the Au-
(111) terraces to form the first monatomic aluminum layer
followed by a second one in the UPD regime. On top of the
second Al monolayer crystallites start growing, which at the
onset of overpotential deposition get three dimensional.
Similar to the electrodeposition process we investigated in
situ the dissolution of formerly deposited aluminum on Au-
(111). First, an aluminum layer was deposited on Au(111) in
the bulk deposition regime at a potential of -0.1 V. Then it is
dissolved by stepwise switching of the potential to the anodic
regime. Figure 4 shows STM images acquired during dissolution
of such a bulk aluminum layer. At a potential of 0.0 V the as-
deposited aluminum layer starts to dissolve. By further increas-
ing the potential to 0.3 V, the surface morphology (see Figure
4a) is completely different than the one observed before any
deposition (see Figure 2a). It can happen at this electrode
potential that again 2D Al islands deposit. When the electrode
potential is set to 0.5 V, still some aluminum islands are left
over on the surface; furthermore, some vacancy islands are
formed on Au(111) surface as shown in Figure 4b. When the
dissolution process is continued by switching the potential to
0.6 V, further pitting of the Au surface is observed as a result
of stripping of Al from the Au lattice.34,35 The number and size
of pits increase (Figure 4c-e), and the original surface of Au-
(111) becomes visible. However, the whole Au(111) surface
presented in Figure 4e shows a large number of vacancy islands.
This is indicative of surface alloying of Al with Au at the early
stage of deposition.
3.1.3. EQCM Measurements. The aluminum deposition
processes in the upper phase of 5 M AlCl3/[EMIm]Tf2N was
also studied by EQCM. Figure 5a shows a typical cyclic

Electrodeposition of Al
J. Phys. Chem. B, Vol. 111, No. 18, 2007 4699
Figure 8. Sequence of STM images of Au(111) at 23 ( 1 °C in the upper phase of the biphasic mixture of AlCl3/[Py1,4]Tf2N at E ) 0.0 V vs
Al/Al(III), Ebias ) 0.3 V, Itun) 2 nA, and a 2 Hz scan rate: (a) a rough surface film of gold, (b) after 6 min, (c) after 12 min, and (d) after 24 min,
the gold surface is reconstructed showing the typical Au(111) terraces.
voltammogram acquired on the polycrystalline Au surface at
25 °C in the potential regime between -0.3 and 2.3 V vs Al/
Al(III) reference electrode. The processes A/A′ corresponds to
the reversible deposition/dissolution of aluminum. In the
potential region from +700 to +20 mV the EQCM indicates
an equivalent molar mass m/z ) 4.2 ( 0.1 g/mol. It can be
assumed that at a polycrystalline Au electrode Al UPD occurs
in this region. This value of m/z can be interpreted as
replacement of some adsorbed species by Al atoms (m/z for Al
is 8.99 g/mol, assuming the transfer of z ) 3 electrons).
Immediately after the UPD region there is a narrow potential
region (from +20 to -90 mV) with m/z ) 8.7 ( 0.6 g/mol,
close to the expected value. This is the beginning of the bulk
Al deposition.
Below -100 mV an equivalent molar mass, m/z ) 12.6 (
0.2 g/mol, can be calculated from the EQCM. For the stripping
region (from -60 to 350 mV) m/z ) 13.0 ( 0.1 g/mol. The
m/z values for thicker Al layers are higher than the expected
value. This discrepancy can be rationalized from the damping
behavior of the quartz crystal (Figure 5c). During deposition
damping increases considerably; a maximum value of 7 kHz is
obtained. This means that the deposited layer is relatively rough.
As a consequence, the measured frequency shift contains a
roughness-induced contribution which will make the calculated
mass change systematically too high. Future work will aim at
development of a correction procedure for the frequency data.
As shown with the STM images (Figure 3a-d), the visible
OPD deposition starts already at -0.1 V, where crystallites 15
nm in diameter form on the single-crystalline electrode surface.
The STM results show that aluminum deposition/dissolution is
completely reversible and followed by formation of pits on the
gold surface at 0.6 V due to surface alloying. Obviously the
surface roughness effect due to pit formation is smaller than
the effect caused by the three-dimensional nucleation. The return
of frequency change df to zero values during reverse polarization
from -0.3 V to the positive direction proves complete dissolu-
tion of deposited aluminum already at 0.3 V. Accordingly, we
can conclude that the process of aluminum deposition in [EMIm]-
Tf2N/AlCl3 is completely reversible. The mass loss beyond 2.0
V is caused by dissolution of the Au substrate. As this causes
a considerable roughening of the surface it is accompanied by
a damping increase.
3.2. Electrodeposition of Aluminum in [Py1,4]Tf2N. 3.2.1.
Cyclic Voltammetry. Figure 6 shows a typical cyclic voltam-
mogram of Au(111) substrate in the upper phase of the biphasic
mixture of AlCl3/[Py1,4]Tf2N at room temperature. The electrode
potential was scanned from the open-circuit potential to more
negative values with a scan rate of 10 mV s-1. All electrode
potentials are referred to Al/Al(III). A comparison of Figures 1
and 6 shows that the electrochemical behavior of aluminum
deposition in this ionic liquid is completely different than that
observed in [EMIm]Tf2N mentioned before. From the cyclic
voltammogram there is a large nucleation overpotential for
deposition of Al on Au(111), and in situ STM does not show a
clear aluminum underpotential deposition. Moreover, this cyclic
voltammogram is characterized by the presence of five cathodic
waves A, B, C, D, and E at potentials of about 0.4, 0.0, -1.0,
-1.4, and -1.7 V, respectively. Without any information from
STM one could argue that the small cathodic processes A and
B might be attributed to underpotential deposition of aluminum.
In a preceding paper we attributed the two additional cathodic
waves C and D to deposition of Al from different aluminum
species. However, as we will show below, there is no bulk
deposition visible with the in situ STM. At a potential of -1.7

4700 J. Phys. Chem. B, Vol. 111, No. 18, 2007
Moustafa et al.
Figure 9. Initial stages of aluminum electrodeposition in the upper phase of the biphasic mixture of AlCl3/[Py1,4]Tf2N on Au(111) at room temperature
(23 ( 1 °C). (a) A typical Au(111) surface is obtained at a potential of -0.9 V vs Al/Al(III), Ebias ) 0.59 V, Itun) 2 nA, and a 2 Hz scan rate. (b)
2D aluminum islands of 250 pm height are formed as shown from the typical height profile at E ) -1.1 V.
V the bulk aluminum deposition starts (process E). In the reverse
scan, the cathodic current continues to flow, forming a current
loop, which evidences nucleation. A very small anodic wave
(E′) is recorded on the reverse scan at a potential of about -0.17
V, which is correlated to partial stripping of the as-deposited
aluminum. From in situ STM and EQCM we will suggest an
explanation for the complex electrochemical behavior.
As already mentioned in ref 24, in this liquid at room
temperature the Al/Al(III) electrode potential is stable but
surprisingly high, so that oxidation apparently occurs at negative
values. At 50, 75, and 100 °C oxidation occurs at 0 V vs Al/
Al(III).24
ing/reconstruction of single-crystalline metal surfaces in ionic
liquids. Thus, it is currently an open question to which extent
cation/anion adsorption influences the surface electrochemistry
in ionic liquids. When the electrode potential is set to 0.0 V,
the roughness of the surface decreases remarkably (Figure 8a-
c). At 0.0 V we finally observe the terraced Au(111) surface
with monoatomically high steps (see Figure 8d and the
respective height profile). We would like to mention clearly
that in the potential regime from 0.8 to -1.1 V vs Al/Al (III)
we do not see clear evidence for deposition of Al islands. Island
formation, which we sometimes observed, was rather correlated
to a surface restructuring process.
Figure 9 shows the initial stages of aluminum electrodepo-
sition in the upper phase of the biphasic mixture of AlCl3/[Py1,4]-
Tf2N on Au(111) at room temperature (25 °C).
3.2.2. In Situ STM Measurements. Figure 7 shows STM
pictures of the Au(111) surface in the upper phase of the mixture
of AlCl3/[Py1,4]Tf2N at room temperature (25 °C). In the
beginning of the STM experiment a rough Au(111) surface with
monoatomically high steps was observed at a potential of 0.3
V vs Al/Al(III). The surface is characterized by a large number
of tiny pits with a measurable depth range from 230 to 250
pm, which is typical for monatomic defects. Quite similar to
what we observed for the pure liquid on Au(111),36 it can be
concluded that the Au(111) surface is as in the pure ionic liquid
restructured under the upper phase of mixture of AlCl3/[Py1,4]-
Tf2N. Such effects are known, and adsorption of ions can be
responsible for formation of vacancies and pits as a result of
their interaction with Au surface atoms.37-40 On the other hand,
the presence of Cl- ions can weaken the Au-Au bond at the
surface and consequently lead to an increased mobility of gold
atoms.41 There is very little information available on restructur-
Figure 9a shows a typical Au(111) surface at a potential of
-0.9 V. From the data obtained from the acquired STM pictures,
there are no clear hints for aluminum deposition down to a
potential of -1.1 V. At this electrode potential 2D aluminum
islands of 200-250 pm height were formed as shown in Figure
9b and in the respective height profile. We have to mention
that the gold surface at -1.1 V appears reproducibly noisy, and
this noise does not disappear before reaching an electrode
potential of -2.0 V, at which the bulk aluminum deposition
starts. It is likely that in this potential regime the irreversible
cathodic breakdown of the Tf2N anion is observed which is
known to occur in two to three steps (see refs 42 and 43). Thus,
peaks C and D in Figure 6 are likely to be due to Tf2N
breakdown, presumably interfering with or even preventing the

Electrodeposition of Al
J. Phys. Chem. B, Vol. 111, No. 18, 2007 4701
Figure 10. Sequence of STM images recorded at 23 ( 1 °C after complete dissolution of 2D aluminum film by switching the potential to E )
0.5 V vs Al/Al(III), Ebias ) 0.51 V, Itun) 2 nA, and a 2 Hz scan rate. (a) STM image shows gold terraces with monatomic steps. (b) The gold
surface is oxidized at the step edges (after 4 min). (c) After 17 min. This series of STM images shows the redeposition of gold by switching the
potential to the cathodic regime. (d) At E ) 0.0 V, typical monoatomically high gold terraces are observed. (e) At E ) -0.3 V, 2D gold islands
about 15 nm in diameter are formed together with vacancy islands. (f) The number of gold islands and vacancy islands has increased (about 4 min
between Figure 11e and 11f). (g) At E ) - 0.4 V, it can be seen that the step edges have grown laterally.

4702 J. Phys. Chem. B, Vol. 111, No. 18, 2007
Moustafa et al.
underpotential deposition of Al. It is unlikely that anion
breakdown is responsible for the bulk deposition of nanocrys-
talline Al in this liquid as at 100 °C the deposits are still
nanocrystalline without clear evidence for anion breakdown.
Furthermore, adsorption of the pyrrolidinium ion to the surface
might prevent an Al UPD. The 2D aluminum islands can be
completely removed by switching the potential to 0.5 V. Hence,
clear, flat, and monoatomically high gold terraces are obtained.
In the following we demonstrate that already at 0.5 V vs Al/
Al(III) the dissolution of the Au(111) surface begins in the upper
phase of AlCl3/[Py1,4]Tf2N. The STM images in Figure 10 were
successively recorded in a surface area exhibiting gold terraces
with monatomic steps. Figure 10a was recorded after the
potential was raised to a potential of 0.5 V to remove the as-
deposited 2D aluminum islands. At this potential, 0.5 V, the
gold surface topography changes continuously with time. As
can be seen in Figure 10a and the subsequently recorded image
in Figure 10b, a hole in the center of the picture is formed.
This hole continuously increases in size, and there is a clear
oxidation at the step edges. Hence, surface dissolution occurs
only at step edges.
In the following, the electrode potential was decreased again.
Figure 10d shows typical gold terraces at a potential of 0.0 V.
If the potential is held at this potential or set to -0.3 V,
monoatomically high islands of about 15 nm in diameter are
formed. Furthermore, some tiny pits or defects, marked with
the black arrow in Figure 10e, can be observed. In addition,
deposition at the step edges is obvious (see the white arrow).
The number of gold islands increases with ongoing time, see
Figure 10f (4 min between Figures 10e and 10f). By switching
the electrode potential to -0.4 V, pits with a width of about 6
nm and more islands were formed as shown in Figure 10g. As
quite obviously the step edges grow two dimensionally and there
is no such process when gold was not subject to oxidation, we
might conclude that formerly oxidized gold is redeposited. It is
not straightforward to understand why at the same time islands
and vacancy islands form on the surface. This might be a hint
for a surface film, which has been suggested several times for
metals in the presence of the Tf2N anion.
Figure 11. STM image shows the topography of an aluminum layer
on Au(111) at 23 ( 1 °C in the upper phase of the biphasic mixture of
AlCl3/[Py1,4]Tf2N at E ) -2.0 V vs Al/Al(III), Ebias ) 0.3 V, Itun) 2
nA, and a 2 Hz scan rate.
At -2.0 V bulk deposition of Al occurs, and under these
potentiostatic conditions close to equilibrium (slow growth rate)
a width of 50-100 nm for the crystallites is obtained (see Figure
11). However, if bulk deposits of Al are made, we clearly get
a nanocrystalline deposit.
3.2.3. EQCM Measurements. In comparison to the EQCM
studies in AlCl3/[EMIm]Tf2N, a different behavior of aluminum
deposition in [Py1,4]Tf2N is observed. A typical voltammogram
of Al deposition on polycrystalline gold surface acquired at 25
°C in the upper phase of AlCl3/[Py1,4]Tf2N electrolyte (Figure
12a) shows as on Au(111) that the process of aluminum bulk
deposition is, as expected, also not reversible (waves E, E′).
The reduction waves A and B from Figure 6 are not visible;
processes C and D, however, are also observed on the poly-
crystalline gold surface of the quartz crystal. The frequency shift
in the EQCM experiments (Figure 12b) shows a surprising result
as an increase in frequency and decrease in the damping with
bulk aluminum deposition at potentials negative of -1.8 V was
observed at room temperature. We interpret this finding as
follows: deposition of aluminum (frequency decrease) is
overlaid by a viscosity decrease (frequency increase and
damping decrease). Obviously, the viscosity effects completely
hide the frequency decrease caused by Al deposition. We
speculate that the viscosity decrease is related to decomposition
of the anion. It was found that during potentiostatic electrolysis
Figure 12. EQCM in [Py1,4]Tf2N ionic liquid containing 1.6 M AlCl3
at 25 °C: (a) cyclic voltammogram, scan rate ) 10 mV‚s-1; (b) change
in frequency df; (c) change in damping dw.
at -1.7 V the change in damping (dw) is higher than the
frequency change (df), which makes use of the Sauerbrey
equation impossible. It is known that the Sauerbrey equation
can only be applied without correction to situations where the
films are acoustically thin.44 At 100 °C the EQCM signal can
be readily interpreted in terms of mass changes. Now the
damping change is small compared to the frequency shifts

Electrodeposition of Al
J. Phys. Chem. B, Vol. 111, No. 18, 2007 4703
Au(111) in this liquid, and as one explanation, the breakdown
of the Tf2N anion interferes with Al UPD. The frequency shift
in the EQCM experiments in the overpotential deposition regime
shows a surprising result as an increase in resonance frequency
and a decrease in damping with bulk aluminum deposition at
electrode potentials more negative than -1.8 V was observed
at room temperature. However, at 100 °C there is a frequency
decrease with ongoing Al deposition as expected for metal
deposition. At -2.0 V vs Al/Al(III), a deposited aluminum layer
of about 100 nm thickness with aluminum nanocrystals is
formed. However, when the electrode potential is reversed to
more positive potentials the frequency does not return to the
initial value, thus proving the irreversibility of Al deposition in
that system. The missing UPD of Al and restructuring/
reconstruction of Au(111) in [Py1,4]Tf2N as well as the low
reversibility of the Al deposition are further hints that cations
and/or anions influence nucleation and crystallization in ionic
liquids.
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