American Journal of Analytical Chemistry, 2013, 4, 642-646
Published Online November 2013 (http://www.scirp.org/journal/ajac)
http://dx.doi.org/10.4236/ajac.2013.411076
Open Access AJAC
Electrodeposition of Palladium Coatings from
Iminodiacetate Electrolyte
Valeriy S. Kublanovsky*, Vasiliy N. Nikitenko, Kostiantyn P. Rudenko
V. I. Vernadskii Institute of General and Inorganic Chemistry NAS Ukraine, Kyiv, Ukraine
Received August 18, 2013; revised September 25, 2013; accepted October 8, 2013
Copyright © 2013 Valeriy S. Kublanovsky et al. This is an open access article distributed under the Creative Commons Attribution
License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
The ionic composition of an iminodiacetate electrolyte as a function of solution composition and pH has been deter-
mined. The kinetic parameters (exchange currents and apparent transfer coefficients) of the electroreduction of a palla-
dium(II) bis-iminodiacetate complex from an electrolyte containing excess ligand have been calculated. It has been
shown that the rate of the electrode process is controlled by the diffusion of reduced ions to the electrode surface and by
the electron-transfer reaction. The possibility of using iminodiacetate electrolyte for palladium plating for the deposition
of fine-crystalline adherent and ductile palladium coatings has been examined.
Keywords: Palladium; Iminodiacetate; Complex Compounds; Electrodeposition; Palladium Coatings
1. Introduction
The high catalytic activity of palladium, the unique
physicochemical and functional properties of its coatings
(corrosion resistance in aggressive media, mechanical
and electrical erosion wear resistance, high reflectivity,
low specific and junction resistance, etc.) [1,2] make
them practically indispensable in many industries.
Taking into account the high cost of palladium, the re-
placement of the monolithic (compact) metal by its func-
tional plated coatings is very expedient.
In microelectronics, electrolytes based on complex-
ones are widely used for the deposition of functional
nanocrystalline palladium coatings since they are non-
toxic, stable and easily recoverable. Complexones, which
are polydentate ligands of acidic type, have a pronounced
ability for compatibility with other ligands in the same
coordination sphere of mixed-ligand complex and for the
retardation of the electrode process or its individual
stages; this makes it possible to control the structure and
hence the properties of deposited coatings by means of
appropriate electrode stages [3].
The practical use of complexonate electrolytes for
palladium plating is impossible without reliable informa-
tion on the ionic composition of the electrolyte, the com-
position of electrochemically active complexes, the na-
ture of rate-determining steps, the kinetics and mecha-
nism of the process.
The aim of this work is to examine the possibility of
using the complexonate system [PdL2]2-H2L-NaClO4-
H2O, whether L2 is anion iminodiacetic acid, in microe-
lectronics to obtain fine-crystalline, adherent, ductile
palladium coatings.
2. Experimental Procedure
Palladium (II) iminodiacetate complexes were synthe-
sized from palladium(II) chloride by the procedure re-
ported in [4] and identified by IR spectroscopy and X-
ray phase analysis [5].
The IR spectra of solid samples of the palladium (II)
iminodiacetate complexes [Pd(H2O)2L] and [PdL2]2
were recorded in the range 4000 - 250 cm1 on a SPE-
CORD-80 M (UR-20) device. The X-ray phase analysis
(XPA) was performed on a DRON-2 UM setup with
CuK radiation. The recording was performed at the
voltage U = 30 kV and the current I = 20 mA in the dis-
crete mode. After each 3 s, the sample was rotated by the
angle 2 = 0.04 and irradiated.
The palladium plating electrolyte was prepared by
dissolving a synthesized palladium(II) bis-iminodiacetate
complex in a 1 M NaClO4 solution in the presence of
100-fold complex one excess. As a result, a palladium
plating electrolyte of the composition (mol·l1): [PdL2]2
(5.11 × 104), [H2L] (5.11 × 102), NaClO4 (1.0) has been
*Corresponding author.

V. S. KYBLANOVSKY ET AL. 643
obtained.
The current-potential E  j curves of palladium(II)
electroreduction from an iminodiacetate electrolyte were
measured using a PI-50.1.1 potentiostat and a PR-8 pro-
grammer at a potential scan rate of 1 - 20 mV·s1 and
recorded with an N 307/1 X-Y potentiometer. The ex-
periments were made in a YaSE-2 thermostatted elec-
trolytic cell under argon in a temperature range of (26 -
60) ± 0.1˚C. The working electrode was a palladium
plate of 2.64 cm2 area. A platinum wire sealed in glass
was used as the auxiliary electrode. All measurements of
potentials were made with respect to a silver-chloride
reference electrode.
Before electrodepositing palladium coatings, the sur-
face of the samples on which palladium was deposited
was subjected to commonly used preparation. The sam-
ples were degreased with soda and Vienna lime, etched
for 1 - 2 s in a hydrochloric acid solution (150 g·l1),
washed with distilled water, activated for 1 - 2 s in a sul-
furic acid solution (50 g·l1) and washed again with dis-
tilled water.
The palladium coatings were tested for ductility by re-
versed bending with fracture (bending of specimen
through 180˚ sequentially on two sides with pressing
down of the bend and subsequent smoothing out till the
appearance of crack) [6]. The appearance of crack was
monitored by means of an XJL-17 AT metallographic
microscope with 300× magnification.
The morphology of palladium deposits was studied on
a JEOL SUPERPROBE 733 scanning electron micro-
scope with X-ray microanalyzer at an accelerating volt-
age of 25 kV at 3000 magnification.
3. Results and Discussion
The IR spectra of solid samples and the results of an
X-ray phase analysis (XPA) of the synthesized palla-
dium(II) iminodiacetate complexes are shown in Figures
1 and 2 respectively.
Based on possible equilibria occurring in iminodiace-
tate electrolyte (Table 1) and their constants [7], where k
is the stepwise formation constant, and K is the overall
formation constant of palladium(II) iminodiacetate com-
plexes and protonated forms of ligand, the distribution of
ionic forms of palladium(II) as a function of [L]2 equi-
librium concentration and solution pH has been calcu-
lated with allowance for material balance on palladium(II)
and ligand ions [5].
A diagram of distribution of ionic forms of palladium
(II) and ligand in an iminodiacetate electrolyte as a func-
tion of the logarithm of ligand equilibrium concentration
and pH in the bulk solution is shown in Figure 2.
As is seen from Figure 2(b), the main forms of exis-
tence of palladium(II) and ligand ions in slightly acid
iminodiacetate electrolyte (pH 3.8) are the complexes
Figure 1. (a): IR spectra of iminodiacetic acid (H2L, (a)), the
complexes [Pd(H2O)2L] (b) and [PdL2]2 (c) synthesized. (b):
X-ray diagrams of the complexes [Pd(H2O)2L] (a) and
[PdL2]2 (b) synthesized.
[Pd(H2O)2L] and [PdL2]2 and the protoated form of
ligand [HL] respectively [5].
To study the kinetics of the electroreduction of palla-
dium(II) from iminodiacetate electrolyte, stationary (1
mV· s 1) and nonstationary (2 - 20 mV·s1) ΔE  j curves
have been measured (Figure 3).
An analysis of nonstationary ΔE – j curves (Figure
3(a)), constructed in the jp - ν½ coordinates [8], showed
that as follows from Figure 4, the plot of jp (ν½) is a
straight line and is extrapolated to the origin of coordi-
nates, indicating the limiting current of palladium(II)
reduction from iminodiacetate electrolyte to be of diffu-
sion nature [5].
The kinetic parameters of palladium(II) electroreduc-
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V. S. KYBLANOVSKY ET AL.
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644
(a) (b)
Figure 2. (a): Distribution of complex forms of palladium(II) in an iminodiacetate electrolyte as a function of the logarithm of
ligand equilibrium concentration: (♦—1) [Pd(H2O)4]2+, (●—2) [Pd(H2O)2L], (■—3) [PdL2]2. (b): Distribution of ionic forms
of palladium(II) and ligand in an iminodiacetate electrolyte as a function of solution pH at the ratio CPd
2+/CL 2 = 1:2. (1)
[PdL2]2, (2) [Pd(H2O)2L], (3) [HL], (4) [H2L], (5) [H3L]+, (6) [Pd (H2O)4]2+.
Table 1. Chemical equilibria occurring in an iminodiacetate electrolyte for palladium plating.
Equilibrium k

K

Pd(H2O)42+ + L2 ↔ Pd(H2O)2L (1) 3.16·1017 3.16·1017
Pd(H2O)2L + L2 ↔ PdL22 (2) 2.00·109 6.31·1026
L2 + H+ ↔ HL (3) 2.09·109 2.09·109
HL + H+ ↔ H2L (4) 4.37·102 9.12·1011
H2L + H+ ↔ H3L+ (5) 7.94·101 7.24·1013
k is step formation constants of palladium(II) glycinate complexes and protonated ligand species. K is overall formation constants of palladium(II) glycinate
complexes and protonated ligand species.
(a) (b)
Figure 3. (a): Polarization curves of palladium(II) reduction from an iminodiacetate electrolyte containing (mol·l1): [PdL2]2
(5.11 × 104), H2L (5.11 × 102), NaClO4 (1.0), pH 3.8 at 26˚C and potential scan rate (mV·s1): 1.0 (1), 5.0 (2), 10 (3), 20 (4). (b):
Polarization curves of palladium(II) reduction from an iminodiacetate electrolyte containing (mol·l1): [PdL2]2 (5.11 × 104),
H2L (5.11 × 102), NaClO4 (1.0), pH 3.8 at a potential scan rate of 1 mV·s1 and temperature (˚C): 26 (1), 35 (2), 40 (3), 50 (4),
60 (5).
The results of calculations show that the coordination
number of [PdL2]2-2n complex ions, which predominate in
the bulk of iminodiacetate electrolyte containing a 100-
fold excess of free ligand, at pH 3.8 is 2.
tion from an iminodiacetate electrolyte, determined from
stationary E  j curves (Figure 3(b)), constructed in the
ΔE  lg [(j·jd) / (jd  j)] coordinates, i.e. with allowance
for the effect of concentration polarization on electrode
kinetics, are listed in Table 2. It was shown earlier [5] that the electron-transfer reac-

V. S. KYBLANOVSKY ET AL. 645
Table 2. Kinetic parameters of palladium(II) electroreduction from an iminodiacetate electrolyte (CPd2+/CL2 = 1:100, pH 3.8).
T (˚C) lg jd
(mA·m2)
Di·105
(cm2·s1) bk (V) α' lg jowb()
(mA·cm2)
lg jowa()
(mA·cm2)
26 1.47 0.69 0.136 0.42 3.47 3.19
35 1.30 1.01 0.146 0.41 3.28 2.94
40 1.26 1.12 0.151 0.41 3.22 2.71
50 1.21 1.26 0.157 0.40 3.14 2.47
60 1.09 1.54 0.164 0.40 3.04 2.25
jowb()—exchange current density of barrierless palladium(II) discharge; jowa()—exchange current density of activationless palladium(II) discharge.
Figure 4. Dependence of limiting cathode current on square
root from sweep rate in an iminodiacetate electrolyte con-
taining (mol·l1): [PdL2]2 (5.11 × 104), H2L (5.11 × 102),
NaClO4 (1.0), pH 3.8, 26˚C.
tion in palladium(II) reduction from iminodiacetate elec-
trolyte containing a 100-fold excess of free ligand at pH
3.8 involves [PdL2]2 complexes. The mechanism of pal-
ladium(II) electroreduction from iminodiacetate electro-
lyte was proposed in Refs [4,5].
With allowance for ionic composition, mass transfer,
the kinetics and mechanism of palladium(II) electrore-
duction from iminodiacetate electrolyte and the nature of
the rate-determining step, the possibility of using this
complexonate electrolyte for the deposition of high-
quality fine-crystalline and adherent functional palladium
coatings has been examined.
The electrolyte under investigation contained at the
bottom of the electrolytic cell excess palladium(II) com-
plex in order that constant palladium ion concentration
might be maintained during the experiment. The palla-
dium(II) ion concentration in the electrolyte for palla-
dium plating was not over 6 × 104 mol·l1 at the 50-fold
excess of iminodiacetate due to the low solubility of pal-
ladium(II) iminodiacetate complex.
Palladium coatings were deposited from a prepared
iminodiacetate electrolyte containing (mol·l1): [PdLn]2
(6.0 × 104), [H2L] (3.0 × 102), NaClO4 (1.0) at 23˚C, pH
4.2 - 4.3 and a current density of 0.03 A·dm2. Palladium
was deposited on one side of a 20 μm thick polyethylene
film chemically coated with a 1 - 2 μm thick nickel layer.
The nickel layer on the other side of the polyethylene
film was previously stripped with a concentrated hydro-
chloric acid solution. The electrolysis time was calcu-
lated by the Faraday law with allowance for the current
yield of metal, which was 75% - 99% depending on coat-
ing thickness, and monitored by cathode weight increase.
A platinum plate was used as the anode.
Micrographs of the structure of palladium coatings
electrodeposited from the iminodiacetate electrolyte un-
der investigation as a function of coating thickness (0.5
μm and 0.75 μm) are shown in Figure 5. On the basis of
morphological data (Figure 5), the dependence of the
particle size distribution of palladium coatings, deposited
from an iminodiacetate electrolyte, on an electroless-
nickel-coated polyethylene film was studied.
The dependence of crystallite size distribution is
shown in Figure 6, from which it is seen that in 0.5 μm
thick palladium coatings (Figure 6(a)), 0.08 - 0.58 μm
crystallites predominate, though 0.08 - 1.20 μm crystal-
lites are also present. Palladium coatings 0.75 μm in
thickness (Figure 6(b)) consist of 0.25 - 1.70 μm crystal-
lites, but 0.60 - 1.20 μm crystallites predominate.
It follows from Figure 6 that the formation of palla-
dium electrodeposits on a polyethylene film precoated
with electroless nickel begins with the formation on the
substrate surface of crystallizing nuclei, from which
crystallites of definite size grow during the cathodic de-
position of the metal, which enlarge with the growth of
the electrodeposit.
The ductility tests of palladium coatings deposited
from an iminodiacetate electrolyte were carried out for
thicknesses of 0.50 μm and 0.75 μm. The 0.50 and 0.75
μm thick palladium coatings electrodeposited from an
iminodiacetate electrolyte withstand 4 and 5 bendings
respectively.
4. Conclusions
Allowing for ionic composition, the kinetics and mecha-
nism of the electroreduction of a palladium(II) bis-imi-
nodiacetate complex and the nature of the rate-deter-
mining step of the process, it has been shown that imino-
diacetate electrolyte can be used for the deposition of
fine-crystalline adherent and very ductile palladium coat-
ings.
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V. S. KYBLANOVSKY ET AL. 646
Figure 5. Micrographs of palladium deposits 0.5 μm (a) and 0.75 μm (b) in thickness obtained from an iminodiacetate elec-
trolyte containing (mol·l1): [PdL2]2 (6.0 × 104), H2L (3.0 × 102), NaClO4 (1.0), pH 4.3 on a nickel-plated polyethylene film at
a current density of 0.03 A·dm2.
Figure 6. Size distribution of palladium particles deposited from an iminodiacetate electrolyte containing (mol·l1): [PdL2]2
(6.0 × 104), H2L (3.0 × 102), NaClO4 (1.0), pH 4.3 on a nickel-plated polyethylene film at a current density of 0.03 A·dm2;
deposit thickness 0.5 μm (a) and 0.75 μm (b).
The advantages of the proposed complexonate elec-
trolyte are its nontoxicity (environmental safety), stabil-
ity during operation, ease of recovery and relative chea-
pness in comparison with other palladium plating meth-
ods.
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