Electrodeposition of palladium-indium from 1-ethyl-3-methylimidazolium chloride tetrafluoroborate ionic liquid

Article abstract of DOI:10.1016/j.electacta.2005.07.042

Voltammetry at a glassy carbon electrode was used to study the electrochemical co-deposition of Pd-In from a chloride-rich 1-ethyl-3- methylimidazolium chloride/tetrafluoroborate air-stable room temperature ionic liquid at 120 °C. Deposition of Pd alone occurs prior to the overpotential deposition (OPD) of bulk In. However, underpotential deposition (UPD) of In on the deposited Pd was observed at the potential same as the deposition of Pd. The UPD of In on Pd was, however, limited by a slow charge transfer rate. Samples of Pd-In alloy coatings were prepared on nickel substrates and characterized by energy dispersive spectroscope (EDS), scanning electron microscope (SEM) and X-ray powder diffraction (XRD). The electrodeposited alloy composition was relatively independent on the deposition potential within the In UPD range. At more negative potentials where the OPD of Pd-In has reached mass-transport limited region, the alloy composition corresponds to the Pd(II)/In(III) composition in the plating bath. The Pd-In alloy coatings obtained by direct deposition of Pd and UPD of In on the deposited Pd appeared to be superior to the Pd-In alloys that were obtained via the co-deposition of Pd and bulk In at OPD potentials.

Full text of DOI:10.1016/j.electacta.2005.07.042

Electrochimica Acta 51 (2006) 2607–2613
Electrodeposition of palladium–indium from
1-ethyl-3-methylimidazolium chloride tetrafluoroborate ionic liquid
Shu-I Hsiu, Chia-Cheng Tai, I-Wen Sun∗
Department of Chemistry, National Cheng Kung University, Tainan, Taiwan, ROC
Received 6 April 2005; received in revised form 27 July 2005; accepted 30 July 2005
Available online 6 September 2005
Abstract
Voltammetry at a glassy carbon electrode was used to study the electrochemical co-deposition of Pd–In from a chloride-rich 1-ethyl-3-
methylimidazolium chloride/tetrafluoroborate air-stable room temperature ionic liquid at 120 ◦C. Deposition of Pd alone occurs prior to the
overpotential deposition (OPD) of bulk In. However, underpotential deposition (UPD) of In on the deposited Pd was observed at the potential
same as the deposition of Pd. The UPD of In on Pd was, however, limited by a slow charge transfer rate. Samples of Pd–In alloy coatings were
prepared on nickel substrates and characterized by energy dispersive spectroscope (EDS), scanning electron microscope (SEM) and X-ray
powder diffraction (XRD). The electrodeposited alloy composition was relatively independent on the deposition potential within the In UPD
range. At more negative potentials where the OPD of Pd–In has reached mass-transport limited region, the alloy composition corresponds to
the Pd(II)/In(III) composition in the plating bath. The Pd–In alloy coatings obtained by direct deposition of Pd and UPD of In on the deposited
Pd appeared to be superior to the Pd–In alloys that were obtained via the co-deposition of Pd and bulk In at OPD potentials.
© 2005 Elsevier Ltd. All rights reserved.
Keywords: Electrodeposition; Ionic liquid; Pd–In; Molten salt
1. Introduction
Over the past 2 decades, room temperature ionic liquids
(ILs) have been drawing considerable interest as media in the
area of electrochemical processes [7–14], organic synthesis
[15–18], catalysis [19–21] and chemical separation [22–24].
For electrochemical applications, ILs exhibit several advan-
tages over the conventional molecular solvent including
good electrical conductivity, wide electrochemical window,
low vapor pressure and good thermal stability. Moreover, ILs
are in most cases aprotic so that complications associated
with hydrogen evolution that occur in aqueous electrolytes
are excluded. Thus, ionic liquids have been employed in the
electrodeposition of pure metals and alloys, especially those
that are difficult to prepare in aqueous electrolytes. Generally,
ILs can be classified into water/air sensitive and water/air
stable. The water/air stable ILs are more versatile to work
with. Until now, studies of the electrodeposition of some
metals, metal alloys and semiconductors in the water/air
stable ILs have been reported [25–30], but the electrochem-
ical investigation of palladium and its alloys in ILs are very
limited [31–33]. In view of the difference between aqueous
Electrodeposition of palladium–indium alloys is of inter-
est [1] because the incorporation of indium into palladium
provides higher microhardness and wear-resistance than pure
palladiumwithoutsacrificingthelowcontactresistanceofthe
palladium. Thus, the Pd–In coatings can be used as replace-
ment for pure gold, silver and palladium coatings in vari-
ous applications. Many examples of the electrodeposition of
Pd–In in aqueous baths are available nowadays [2–6]. The
general difficulties associated with the electrodeposition of
Pd–In from aqueous baths are the considerable difference
between the deposition potentials of palladium and indium,
the low hydrogen overvoltage and the large hydrogen sol-
ubility in the Pd metal. Therefore, efforts have been made
to minimize these problems. One approach is to replace the
aqueous baths with aprotic solvents.
∗
Corresponding author.
E-mail address: iwsun@mail.ncku.edu.tw (I.-W. Sun).
0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2005.07.042

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solutions and ILs, it is meaningful to accumulate fundamen-
tal data of the electrochemical investigation of Pd–In alloy in
the ionic liquids. In this study, the electrodeposition of Pd–In
was investigated in a hydrophilic water-stable Lewis basic
3. Results and discussion
3.1. Voltammetric behavior
1-ethyl-3-methylimidazolium
chloride-tetrafluoroborate
Both PdCl2 and InCl3 dissolve readily in the basic
[EMIm]Cl-BF4 ILs. Previous literature indicated that Pd(II)
and In(III) exist as the [PdCl4]2− and [InCl5]2− complex
anions, respectively, in chloride-rich chloroaluminate ILs
[31,35]. A typical cyclic voltammogram recorded at a glassy
carbon electrode for 10 mM Pd(II) in the basic [EMIm]Cl-
BF4 IL at 120 ◦C is shown in Fig. 1a. The reduction of Pd(II)
occurs at the cathodic wave c1 and the oxidation of the elec-
trodeposited Pd occurs at the anodic wave a1. A typical cyclic
voltammogram of 20 mM In(III) at a glassy carbon elec-
trode in the basic [EMIm]Cl-BF4 IL at 120 ◦C is presented
in Fig. 1b. This cyclic voltammogram reveals that the elec-
trodeposition of of In (wave c2) occurs at a potential far more
negative than the deposition of the Pd. The considerable dif-
ference between the deposition potentials of the individual
component implies that electrodeposition of compact Pd–In
alloys may be difficult. On the anodic scan, Fig. 1b shows
a sharp wave, a2, due to the stripping of the bulk indium
deposits and a small broad anodic wave at about −0.3 V
which may be due to the stripping of the In deposits having
stronger interactions with the GC substrate. Fig. 1c shows
the cyclic voltammogram of a solution containing 10 mM
Pd(II) and 20 mM In(III) in the basic [EMIm]Cl-BF4 IL at
120 ◦C. As can be seen in this voltammogram, the deposition
of In and Pd both shift negatively in comparison to the depo-
sition of individual metals shown in Fig. 1a and b. On the
([EMIm]Cl-BF4) IL that contained excess amounts of
chloride ions. The electrochemical reduction of Pd and In
was studied with cyclic voltammetry at 120 ◦C. Thin films of
Pd–In alloys were electrodeposited with constant potential
electrolysis at nickel substrates and characterized. The
effects of deposition potential and plating concentrations on
the composition and morphology of the electrodeposits were
examined.
2. Experimental
2.1. Apparatus
All electrochemical experiments were conducted in a
N2-filled glove-box system (Vacuum Atmosphere Co.) in
which the moisture and oxygen contents were kept below
1 ppm. The electrochemical experiments were accomplished
by an EG&G Princeton Applied Research Corporation
(PAR) model 273A potentiostat controlled by EG&G PAR
model 270 software. A three-electrode cell was used for the
electrochemical experiments. For static voltammetry, the
working electrode was a glassy carbon (GC) disk electrode
(A = 0.07 cm2). The counter electrode was an aluminum
(Aldrich, 99.99%) spiral immersed in the pure [EMIm]Cl-
BF4 IL contained in a glass tube having a fine porosity tip.
The reference electrode was an aluminum wire immersed
in a 60–40 mol% AlCl3-[EMIm]Cl ionic liquid contained in
the same type of glass tube as the counter electrode. A Pine
model AFMSR electrode rotator was employed for rotating
disk electrode voltammetry. Electrodeposition experiments
were conducted on nickel plates (Aldrich, 99.99%). A
Hitachi S-4200 field emission scanning electron microscope
(SEM) with an energy dispersive spectroscope (EDS) work-
ing at 15 kV was used to examine the surface morphology
of the electrodeposits. The crystalline phases of the deposits
were studied with a Shimadzu XD-D1 X-ray diffractometer
(XRD).
2.2. Chemicals
1-Ethyl-3-methylimidazolium chloride was prepared and
purifiedaccordingtoliterature[34]. Sodiumtetrafluoroborate
(Aldrich, 98%), anhydrous PdCl2 (Aldrich, 99.9%), anhy-
drous InCl3 (Stream, 99.99%) were used as received. The
[EMIm]Cl-BF4 ionic liquid was prepared by direct reaction
of proper amounts of [EMIm]Cl and NaBF4 in acetone as
described previously [25]. The ionic liquid used in this study
contains 0.80 M chloride, which was introduced by dissolu-
tion of [EMIm]Cl.
Fig. 1. Cyclic voltammograms of (a) 10 mM Pd(II), (b) 20 mM In(III) and
(c) 10 mM Pd(II) + 20 mM In(III) in a [EMIm]Cl-BF4 ionic liquid at a GC
electrode at 120 ◦C. Scan rate = 100 mV s−1
.

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2609
It is known that underpotential deposition (UPD) can
occur when the work function of the deposited metal, φM, is
fairly smaller than that of the substrate metal, φS. The under-
potential shift, ꢀEp, has been found to vary linearly with the
difference of work functions, ꢀφ, according to the empir-
ical expression: [36] ꢀEp = 0.5 ꢀφ. However, the presence
of specific interactions between the deposit and the substrate
can deviate this relationship. Considering the smaller work
function of indium (φIn = 4.12 eV) with respect to the work
function of palladium (φPd = 5.12 eV) [36], UPD of In on Pd
is expected to occur at about −0.5 V. To see if the UPD of In
on Pd indeed occurs, cyclic voltammograms of In(III) at the
Pd electrode were recorded. A typical cyclic voltammogram
of 20 mM In(III) recorded at a Pd wire electrode in the IL at
120 ◦C is shown in Fig. 3a. This figure reveals that, similar
to that was observed in Fig. 1c, the OPD of In (wave c2) at
the Pd electrode occurs at a potential more negative than that
was observed at a GC electrode (Fig. 1b), indicating a larger
overpotential is required for the OPD of In at Pd. Fig. 3a also
shows a small reduction wave c3 at ca. −0.5 V, which does
not occur at the GC electrode, prior to the bulk deposition of
In and two anodic waves, a3 and aꢀ3 in addition to wave aꢀ2
during the reverse scan. Because wave c3 occurs at a poten-
tial positive to the potential for bulk deposition of In, it must
be the UPD of In on Pd. To further study this behavior, the
potential scan was held at wave c3 for different periods of
time before scanning in the reverse direction. As shown in
Fig. 3b, prolonging the deposition time at wave c3 does not
increase significantly the anodic current (or the accumulated
charge) of wave a3, indicating that the deposition of In at
wave c3 is a surface confined process, which is typical for
a UPD behavior. Because the polycrystalline Pd electrode is
used, the In UPD wave c3 is broad. It is worthy to perform in
the future experiments on Pd single crystals in order to have
a more comprehensive understanding on the In UPD behav-
ior. Fig. 3c compares the cyclic voltammograms of a Pd(II)
solution and solutions containing fixed amount of Pd(II) and
various amounts of In(III) recorded at a GC electrode. As
can be seen, the presence of In(III) shifts the reduction of
Pd(II) and the stripping of the deposited Pd to more negative
potentials. The appearing of the alloy stripping waves (a3 and
aꢀ3) in addition to wave a1 indicates that wave c3 is resulting
from the deposition of Pd and the UPD of In. Furthermore,
increasing the In(III) concentration in the plating bath raises
the anodic waves a3 and a3ꢀ but suppresses wave a1. These
results also suggest the alloying of the deposited Pd with the
UPD In. Note that the anodic stripping wave aꢀ2 observed in
Fig. 2 is not seen in this figure, suggesting that wave aꢀ2 is
related to the alloys of Pd with OPD In whereas a3 and aꢀ3 are
related to the alloys of Pd with UPD In.
Fig. 2. Cyclic voltammograms of 10 mM Pd(II) and (a) 0, (b) 20, (c) 40
and (d) 80 mM In(III) in a [EMIm]Cl-BF4 ionic liquid at a GC electrode at
120 ◦C. Scan rate = 100 mV s−1
.
anodic potential scan, the anodic wave a2 for the stripping of
bulk In deposits decreases significantly while a new wave a2ꢀ
becomes apparent. Furthermore, the anodic wave a1 for the
stripping of bulk Pd shifts to a less positive potential and its
peak current is significantly larger than that is observed in
Fig. 1a. These changes in the voltammetric features strongly
indicate the formation of Pd–In alloy. The multiple stripping
peaks indicate the presence of different alloys phases. Based
on their potentials, one would expect that aꢀ2 alloy to have
a higher In concentration than the alloy stripped at a1. To
further study the co-deposition of Pd–In, cyclic voltammo-
grams were collected also for solutions containing 10 mM
of Pd(II) and different concentrations of In(III). As shown
in Fig. 2, wave a1 increases when In(III) concentration is
increased from 0 to 20 mM. Further increasing In(III) con-
centration, however, enhances waves c2, a2, aꢀ2 and a3 but
diminishes wave a1. Initially, at low In(III) concentration, the
incorporation of In deposits into the Pd deposit increases the
stripping current of wave a1 (Figs. 2b) because three electrons
are associated with each In atom and only two electrons are
associated with one Pd atom. Fig. 2c and d indicates that fur-
ther increasing the In(III) concentration increases the amount
of the indium in the deposits which favors the formation of
the alloy having higher In content and thus enhances wave
aꢀ2 and diminishes wave a1 which is due to the alloy hav-
ing lower In content. It is noted that wave a2 also increases
with increasing In(III) because more of the excess In deposits
remain as the bulk In deposits that do not interact effectively
with Pd.
In addition to the cyclic voltammetric experiments on
glassy carbon disc electrode, rotating disk electrode cyclic
voltammetry (RDECV) was employed to study the electrode-
position of Pd and In. Because the glassy carbon RDE used
in this study is stable only at temperatures below 80 ◦C, the
experiments were conducted at 70 ◦C. Some typical GCRDE

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Fig. 4. Rotating disc electrode linear scan voltammograms of: (a) 20 mM
In(III), (b)10 mM Pd(II), (c) 10 mM Pd(II) and 20 mM In(III) and (d) 10 mM
Pd(II) and 40 mM In(III) in a [EMIm]Cl-BF4 ionic liquid at a GCRDE at
70 ◦C. Scan rate = 5 mV s−1
.
was estimated that the diffusion coefficients of In(III) and
Pd(II) are 4.92 × 10−7 and 5.25 × 10−7 cm2/s, respectively.
Fig. 4c is the voltammogram of a IL containing 10 mM Pd(II)
and20 mMIn(III). Thisvoltammogramshowsthatbothofthe
Pd(II) and In(III) reductions shifted to more negative poten-
tials compared to Fig. 4a and b. Almost no anodic stripping
(a2) peak due to the OPD In deposits is seen in this voltam-
mogram, suggesting that all the deposited In has alloyed with
Pd. Fig. 4d shows that when the In(III) concentration was
increased to 40 mM, the limiting current due to the OPD bulk
In (wave c2) increased and its corresponding anodic stripping
wave a2 became apparent, indicating that the In deposits did
not alloy completely with Pd. It is also noted that increasing
the In(III) concentration does not enhance wave c3, which is
due to the bulk deposition of Pd and UPD of In, indicating
that the UPD of In must have proceeded with a fairly slower
rate in comparison to the deposition of Pd.
3.2. Bulk electrodeposition of Pd–In
Fig. 3. Cyclic voltammograms of: (a) 20 mM In(III) IL solution recorded on
a Pd electrode, (b) 20 mM In(III) IL solution recorded on a Pd electrode with
potential scan was held at −0.85 V for 0, 20, 60, 100 and 140 s before reverse
scan and (c) (- - -) 10 mM Pd(II) and 10 mM Pd plus (1) 40, (2) 60 and (3)
Pd–In electrodeposits were prepared using constant poten-
tial electrolysis on nickel foils (0.5 × 0.5 cm2) in [EMIm]Cl-
BF4 IL containing Pd(II) and In(III). Following each depo-
sition experiment, the Pd–In-coated foil was immersed in a
small volume of blank ionic liquid and transferred out of the
glove box to be rinsed with deionized water to remove any
residual ionic liquid. The electrodeposits were characterized
by SEM, EDS and XRD analysis.
80 mM In(III) recorded on a GC electrode at 120 ◦C. Scan rate = 100 mV s−1
.
cyclic voltammograms are shown in Fig. 4. Fig. 4a and b
are the voltammograms of a 20 mM In(III) solution and a
10 mM Pd(II) solution, respectively. From the concentrations
of In(III) and Pd(II) and their limiting currents it is realized
that the diffusion coefficient of Pd(II) is about the same as that
of In(III). From separate chronoamperometry experiments, it
The compositions of the electrodeposits that were pre-
pared at 120 ◦C are plotted in Fig. 5 as a function of deposition

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2611
concentration in the plating solution was kept constant. On
the other hand, when the concentration of In(III) in the plating
bath waskept constant, the In content lowered withincreasing
Pd(II) concentration. These results suggest that the In UPD
rate was slower than the deposition rate of Pd.
In the potential range (E < −0.7 V) where OPD of In
occurred, the In content in the deposits increased with
increasing In(III) concentration. When the potential reached
the mass-transport-limited region, the electrodeposits com-
position ratio approximated the Pd(II)/In(III) concentration
ratio in the plating solution in consistent with the fact that
the diffusion coefficients of Pd(II) and In(III) are about the
same.
The morphologies of the electrodeposits obtained at dif-
ferent deposition potentials are illustrated in Fig. 6. These
micrographs show that the electrodeposits prepared within
the potential region of the simultaneous deposition of Pd
and UPD of In consist uniformly distributed nodules which
grow in size with increasing deposition overpotential due to
the increased deposition rate. As the deposition potential is
changed to more negative value where OPD of bulk In also
occurs, dendrite morphologies become apparent, suggesting
that the deposits are not homogenous but mixtures of Pd,
In and Pd–In alloys. Minor cracks appear in some of the
deposits. These cracks may result from the entrained ionic
liquid as indicated by EDS analysis which showed the pres-
ence of traces Cl in these deposits. Typical XRD patterns of
Pd–In films prepared at 120 ◦C from [EMIm]Cl-BF4 contain-
Fig. 5. Variation of the Pd–In electrodeposit composition with deposition
potential. The deposits were prepared in a [EMIm]Cl-BF4 ionic liquid at
120 ◦C containing: (ꢁ) 10 mM Pd(II) and 10 mM In(III); (ꢀ) 10 mM Pd(II)
and 40 mM In(III); (♦) 40 mM Pd(II) and 10 mM In(III).
potential. In general, the In composition (a/o) in the deposit
increases slowly as the deposition potential changes from
−0.3 to −0.7 V, which is the potential region for the UPD of
In on Pd. As the deposition potential becomes more negative
than −0.7 V, where OPD of In takes place, the In content in
the deposits increased rapidly until the deposition potential
reached the region where OPD of In was mass-transport lim-
ited. When the deposition was conducted within the In UPD
potential range, the deposits composition did not change sig-
nificantly with increasing In(III) concentration if the Pd(II)
Fig. 6. SEM micrographs of Pd–In electrodeposits prepared on nickel substrates from a [EMIm]Cl-BF4 ionic liquid solution containing 10 mM Pd and 20 mM
In(III) at 120 ◦C. The depotential potential and the Pd content (a/o), respectively, in the deposits are: (a) −0.28 V, 73 a/o; (b) −0.5 V, 74 a/o; (c) −0.7 V, 67 a/o;
(d) −1.0 V, 36 a/o.

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4. Summary and conclusions
The electrochemical deposition of palladium and indium
in the [EMIm]Cl-BF4 ILs was studied with cyclic voltam-
metry at a GC electrode at 120 ◦C. The OPD of Pd occurs
at a potential far more positive than the OPD of In. Such
a big difference in deposition potential makes it difficult to
electrodeposit compact Pd–In alloys coating. Nevertheless,
the underpotential deposition of In occurs on the electrode-
posited Pd at a potential overlaps with the deposition of Pd.
This fact makes it possible to prepare smooth and compact
Pd–In coatings. Because the UPD rate of In on Pd is slow,
the In content in the deposited Pd–In alloys is lower than that
in the plating bath.
Acknowledgment
Fig. 7. XRD patterns (Cu K␣) of Pd–In sample prepared on nickel substrate
from[EMIm]Cl-BF4 ionicliquidcontaining10 mMPd(II)and20 mMIn(III)
at 120 ◦C.
Funding was provided by the National Science Council of
the Republic of China, Taiwan.
ing 10 mM In(III) and 20 mM Pd(II) at different potentials are
shown in Fig. 7. For comparison, XRD patterns of pure Pd
and In deposits are also presented in this figure.
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