Electroless versus electrodriven deposition of silver crystals in polyaniline: Role of silver anion complexes

Article abstract of DOI:10.1016/j.electacta.2005.03.040

Electrocrystallization of silver is studied at polyaniline-coated electrodes using silver cations and two silver anion complexes (silver thiosulfate and silver-EDTA) as reducing species. Use of the silver thiosulfate complex results in a significant shift of the silver deposition potential window in the negative direction and highly impeded metal crystallization. The silver cation and silver-EDTA plating solutions allow to perform both electrodriven and electroless metal deposition. Number and size of silver crystals obtained by the two deposition methods in the different plating solutions are compared. Electroless precipitation in the silver-EDTA solution results in the highest number (～10⁸ cm^-2) of small-sized crystals. This result is discussed in terms of the special role of the EDTA anions for the redox state of the polyaniline layers. It is demonstrated that factors such as polyaniline redox charge, concentration of reducing ions and dipping time allow effective control over the amount of electroless deposited metal.

Full text of DOI:10.1016/j.electacta.2005.03.040

Electrochimica Acta 50 (2005) 5616–5623
Electroless versus electrodriven deposition of silver crystals in polyaniline
Role of silver anion complexes
S. Ivanov, V. Tsakova∗
Institute of Physical Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria
Received 26 November 2004; received in revised form 7 March 2005; accepted 11 March 2005
Available online 26 April 2005
Abstract
Electrocrystallization of silver is studied at polyaniline-coated electrodes using silver cations and two silver anion complexes (silver
thiosulfate and silver–EDTA) as reducing species. Use of the silver thiosulfate complex results in a significant shift of the silver deposition
potential window in the negative direction and highly impeded metal crystallization. The silver cation and silver–EDTA plating solutions allow
to perform both electrodriven and electroless metal deposition. Number and size of silver crystals obtained by the two deposition methods in
the different plating solutions are compared. Electroless precipitation in the silver–EDTA solution results in the highest number (∼108 cm−2
)
of small-sized crystals. This result is discussed in terms of the special role of the EDTA anions for the redox state of the polyaniline layers.
It is demonstrated that factors such as polyaniline redox charge, concentration of reducing ions and dipping time allow effective control over
the amount of electroless deposited metal.
© 2005 Elsevier Ltd. All rights reserved.
Keywords: Silver; Silver anion complexes; Polyaniline; Electroless precipitation; Conjugated polymers
1. Introduction
ing species (cations and metal anions) on the metal deposi-
tion in the case of copper electrocrystallization in polyani-
line (PANI). Silver electrodeposition in polyaniline, which
occurs in a different potential window in comparison to cop-
per, was studied in the case of electrochemical reduction of
silver cations too [8,9]. The present work extends our pre-
vious studies to the use of silver anion complexes (instead
of silver cations) and to the special possibility to perform
electroless silver precipitation in reduced polyaniline coat-
ings.
Electroless metal precipitation in conducting polymer
layers, which occurs by coupling spontaneous polymer
oxidation and metal ions reduction, was initially suggested
as a way of recovery of precious metals from acid solutions
[10–12]. The specific, polymer-based electroless process
was exploited as a mean of producing metal deposit in CP
materials in few cases for obtaining palladium [13,14] and
gold [15] particles in polyaniline. Electroless deposition
was also used for deposition of gold on conducting polymer
powders and conducting polymer-coated latex particles
[16].
Since the early works of Tourillon and Garnier [1] and
Chandler and Pletcher [2] published in the mid 80s, metal
electrodeposition in conducting polymer (CP) materials has
been intensively studied. Although a great number of publi-
cations on this topic have appeared over the years, most of the
worksperformeddescribemetalelectrodepositioninpolymer
layers (mostly with ill specified characteristics) obtained un-
der particular conditions, not allowing for a comparison be-
tween results from different investigations. Therefore, it is
difficult to gain a general understanding on the possibilities
for influencing characteristics of the metal deposit such as
number, size and spatial distribution of the electrodeposited
metal crystals.
In a series of recent papers [3–7] we have studied in a
consistent way the influence of factors such as thickness,
initial redox state of the polymer coating and type of reduc-
∗
Corresponding author. Tel.: +359 2 9892586; fax: +359 2 9712688.
E-mail address: tsakova@ipchp.ipc.bas.bg (V. Tsakova).
0013-4686/$ – see front matter © 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2005.03.040

S. Ivanov, V. Tsakova / Electrochimica Acta 50 (2005) 5616–5623
5617
As far as the silver anion complexes are concerned, to
our knowledge there is no previous experience on their use
for silver deposition in CP layers. Silver anion complexes,
mostly silver thiosulfate, silver cyanide, silver rodanide and
silver pyrophosphate, are in the rule introduced in galvanic
baths for improving the quality of the silver coatings [17].
However, it turned out that there are no investigations on the
initial stages of electrocrystallization performed in the pres-
ence of these complex anions at simple (not polymer-coated)
metal electrodes. Thus, our choice of a suitable silver anion
complex could not be bound to previous findings on the role
of silver anion complexes for the electrochemical nucleation
and growth kinetics. It was merely based on practical consid-
erations: (i) to keep the pH value of the silver plating solution
low for preserving the polyaniline layer electroactivity, (ii) to
use a simple anion complex with fixed ligand number in the
anionic shell and (iii) to use non-toxic chemical compounds.
For these reasons two silver complexes were chosen: silver
thiosulfate being of common use for silver plating (see, e.g.
[18]) and silver–EDTA [19] allowing to keep the pH at a
lower value.
from the negative potential limit (E = −0.2 V) in the positive
direction.
The silver thiosulfate plating solution (pH 6.4) was pre-
pared by mixing aqueous solutions of 0.01 M AgNO3,
and 0.04 M Na2S2O3 + 0.5 M KNO3. The silver thio-
sulfate complex—[Ag(S2O3)2]3− has a stability constant
β = 4.2 × 1013. The equilibrium potential of silver in this so-
lutionwasmeasuredtobeE = −0.59 V(versusMSE/K2SO4).
Silver–EDTA plating solutions (pH 4.1) with differ-
ent concentrations (10 mM, 2 mM and 0.4 mM) of the
silver–EDTA complex were prepared by mixing the cor-
responding aqueous solution of AgNO3 with aqueous
solution of 0.02 M Na2C10H14O8N2 (disodium salt of
ethylenediaminetetraacetic acid) + 0.5 M KNO3. The three-
valence silver–EDTA complex—[AgEDTA]3− is the dom-
inating species in these solutions and has a stability con-
stant β = 2.1 × 107. The equilibrium potential of silver in
the 10 mM [AgEDTA]3− solution was measured to be
E = −0.04 V (versus MSE/K2SO4).
Comparative measurements for silver deposition by elec-
trochemical reduction of silver cations were performed in
aqueous solution of 0.01 M AgClO4 and 0.5 M HClO4 (pH
0.3). The equilibrium potential of silver in this solution was
measured to be E = −0.002 V (versus MSE/K2SO4).
Imaging of the silver deposit at PANI-coated electrodes
was performed at a JSM 5300 (Jeol) scanning electron mi-
croscope. In this case platinum plates with surface area of
about 2 cm2 were used as working electrodes.
2. Experimental
All experiments were performed in three-electrode ar-
rangement at room temperature. The electrodes consisted of a
platinum plate counter electrode, a mercury/mercury sulfate
reference electrode and a single-crystal polyfaced platinum
bead, melted in a glass tube, used as working electrode. The
surfaceareaoftheworkingelectrodewasS = 2.3 × 10−3 cm2.
The measurements were performed by means of a com-
puterised potentiostat/galvanostat (AUTOLAB PGSTAT 30,
Ecochemie). Beforeexperimentationtheelectrolytesolutions
were de-aerated by bubbling of argon for at least 15 min. The
argon flux was kept over the solution surface in the course of
all electrochemical measurements.
3. Results and discussion
3.1. Electrodriven silver deposition—comparison
between the three silver plating solutions
The values of the equilibrium potential of silver measured
in the three silver plating solutions show that the use of the sil-
ver thiosulfate anion complex will shift the potential window
for silver electrodeposition by at least 0.6 V in the negative
direction. On the other hand, the equilibrium potentials of
silver in the silver cations and silver–EDTA anion solutions
differ by only 40 mV, and thus, silver deposition in these two
solutions is expected to occur in one and the same poten-
tial window. This effect is demonstrated in Fig. 1a showing
cyclic voltammetric curves registered in the three silver plat-
ing solutions at PANI layers with one and the same redox
charge. Apart from the significant shift of the silver deposi-
tion potential window in the silver thiosulfate case, another
effect becomes evident: whereas the electrodeposition from
silver cations starts at small overpotentials, the silver crys-
tallization in both silver anion complex solutions requires
significant overpotentials. It is also interesting to note that
the voltammetric curve registered in the [AgEDTA]3− so-
lution shows a shoulder in the cathodic current before the
onset of the silver electrocrystallization wave and a corre-
sponding anodic “peak” overlapped by the much larger silver
The anodic deposition of polyaniline layers occurred in
aqueous solution of 0.1 M aniline and 0.5 M H2SO4 by means
of a pulse potentiostatic procedure [20,21]. The pulse se-
quence consists of rectangular anodic and cathodic pulses
with amplitudes Ea = 1.0 V, Ec = 0.2 V and pulse durations
ta = 0.1 s and tc = 0.1 s. The amount of polymerized mate-
rial and thus the thickness of the PANI layer depend on the
number of applied pulse sequences. The amount of deposited
polyaniline was estimated by measuring the reduction charge
in a potentiodynamic sweep between 0 and 1 V (in 0.5 M
H2SO4 solution). Most of the experiments were performed
at PANI layers with redox charge, Qred of about 20 mC cm−2
.
In the case of electroless silver precipitation the PANI
layer were subjected to “complete” reduction by keeping
the electrodes in supporting electrolyte (0.5 M H2SO4) at
−0.62 V for at least 15 min. After disconnecting the elec-
trodes, they were immediatly dipped in the silver plating so-
lution. The amount of deposited silver was measured in the
same supporting electrolyte by slow oxidation scans starting

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S. Ivanov, V. Tsakova / Electrochimica Acta 50 (2005) 5616–5623
Fig. 2. Galvanostatic curves (i = −0.2 mA cm−2) measured at PANI-coated
electrodes in different silver ions containing solutions: (a) Ag+; (b)
[AgEDTA]3−; (c) [Ag(S2O3)2]3− (Qred = 18 mC cm−2).
not exceed −0.12 V, which means that the electrocrystalliza-
tion process occurs close to the silver equilibrium potential
(E = −0.002 V). The initial linear part of the potential tran-
sient below the equilibrium potential reflects a partial reduc-
tion of the PANI layer corresponding to the flat capacitive
region in the usual PANI voltammetric response [9], far be-
fore the onset of the emeraldine to leucoemeraldine reduc-
tion wave. Both so-far discussed galvanostatic curves (a) and
(c) show well-expressed maxima reflecting the occurrence of
silver nucleation and growth. In contrast, the potential tran-
sient (curve b), obtained in the [AgEDTA]3− plating solution,
shows a peculiar behaviour due very probably to the fact that
parallel (polyaniline and silver–EDTA) reduction reactions
consume the applied current and are both potential determin-
ing.
Fig. 1. (a) Potentiodynamic curves measured at PANI layers in 0.01 M
silver-containing solutions of different ions: (a) Ag+; (b) [AgEDTA]3−; (c)
[Ag(S2O3)2]3−. Scan rate: 5 mV s−1 (Qred = 17 mC cm−2). (b) Potentiody-
namic curves measured at PANI layers in supporting solutions: (a) 0.5 M
HClO4; (b) 0.02 M Na2EDTA and 0.5 M KNO3; (c) 0.02 M Na2S2O3 and
0.5 M KNO3. Scan rate: 5 mV s−1 (Qred = 17 mC cm−2).
The considerations performed so-far show that the three
different silver reducing species allow to deposit silver at
three different states of the polyaniline layers: (i) in fully re-
duced, low conducting state—for silver thiosulfate; (ii) in
the oxidized, high conducting PANI state—for the silver
cations; (iii) in the course of PANI layer reduction—for the
silver–EDTA complex. This means that in the first two cases
it is reasonable to study the electrodeposition under potentio-
static conditions, while in the third case potentiostatic current
transients will reflect two interrelated processes. In this latter
case quantitative interpretation of the current transients will
be impeded. As far as potentiostatic measurements in the case
of deposition from silver cations are already performed [9], in
the following we show detailed data on the silver deposition
from [Ag(S2O3)2]3− anions under potentiostatic conditions.
oxidation peak. By means of measurements in the corre-
sponding supporting solutions, in absence of silver anion
complexes (Fig. 1b), we could establish that these two fea-
tures are due to the redox behaviour of the PANI layer in this
medium. In the two other cases of silver reducing species
(Ag+ and [Ag(S2O3)2]3−) the PANI redox current does not
interfere with the silver electroreduction due to the location
of the silver deposition potential windows before (in the case
of Ag+ [9]) and after (in the case of [Ag(S2O3)2]3−) the main
PANI reduction wave.
The extent of interference of the parallel, polymer-based
reduction process with the electroreduction of silver was
checked by measuring galvanostatic curves at one and the
same cathodic current in the three silver plating solutions
(Fig. 2). It is evident that in the case of the silver thio-
sulfate complex (curve c) polyaniline reduction (completed
at E = −0.7 V) and silver reduction (starting at E > −0.7 V)
are separated. Thus, in this particular case silver deposi-
tion occurs in PANI layers in their reduced, low conduct-
ing state. This situation is in fact similar to the electrodepo-
sition of copper using copper citrate complexes [6]. In the
case of silver cations the galvanostatic curve (curve a) does
3.2. Potentiostatic deposition of silver from silver
thiosulfate complexes
Fig. 3a shows a set of potentiostatic current transients ob-
tained in the silver thiosulfate solution at PANI layers with
constant redox charge. Although very high overpotentials

S. Ivanov, V. Tsakova / Electrochimica Acta 50 (2005) 5616–5623
5619
Fig. 4. SEM image of silver deposit obtained through potentiostatic de-
position (E = −1.1 V) at PANI-coated electrode in [Ag(S2O3)2]3− solution
(Qred = 31 mC cm−2).
count for a possible delay in the nucleation process due to
gradual establishment of concentration of depositing species
in the volume of a 3D layer and are strictly valid for simple
electron transfer, which is certainly not the case when using
metal complex anions.
A direct comparison between the potentiostatic cur-
rent transients obtained at one and the same overpotential
(η = 450 mV) from silver thiosulfate (curve a) and silver
cation (curve b) solutions is shown in Fig. 3b. The same
overpotential corresponds in both cases to very different
deposition potentials—E = −1.05 V for [Ag(S2O3)2]3− and
E = −0.45 V for Ag+ and thus to different redox states of the
PANI coating. As was shown before [9], in contrast to the
silver thiosulfate case, silver deposition from silver cation
solution occurs at oxidized PANI layers and allows for for-
mation and growth of silver crystals on top of the polymer
surface.
Microscopic observation of the silver deposit obtained
from the silver thiosulfate solution (Fig. 4) shows the pres-
ence of two type of crystals: (i) large silver crystal agglom-
erates (amounting to 2 × 105 cm−2), decorating most prob-
ably macrodefects of the PANI layer and (ii) a larger num-
ber of much smaller silver crystals (roughly amounting to
106 cm−2), which obviously have grown under completely
different conditions. The latter could result from impeded
growth through the compact reduced PANI structure.
Fig. 3. (a) Silver deposition current transients measured in [Ag(S2O3)2]3−
ion containing solution at constant reduction charge of PANI layers and
different potentials (Qred = 20 mC cm−2). (b) Potentiostatic current tran-
sients of silver deposition measured at constant overpotential (η = 450 mV)
and redox charge of PANI layers in different silver-containing media: (a)
[Ag(S2O3)2]3− and (b) Ag+ (Qred = 20 mC cm−2).
(exceeding 0.4 V) were needed for obtaining diffusion lim-
itedtransientswithmaxima, theyneverthelessalwaysshowed
marked delays of several seconds before the onset of sil-
ver crystallization. Such “delayed” transients were ob-
served in the course of palladium deposition in poly-3,4-
ethylenedioxythiophene [22] and attributed to diffusion of
metal ions inside the polymer layer and delayed establish-
ment of a threshold concentration of depositing species in
the polymer coating. The initiation of the nucleation process
was suggested to occur at the metal/polymer interface. This
could be the case of silver deposition from silver thiosulfate
solution, too, the reduced low-conducting PANI layer imped-
ing the electron transfer. Thus, silver nucleation occurs very
probably at the underlying metal electrode by diffusion of
the silver thiosulfate anions through the pores of the polymer
coating. In such a situation the electrocrystallization process
becomes limited by the morphology characteristics of the
polymer layer.
3.3. Electroless silver precipitation
The metal (Me) electroless precipitation process in con-
ducting polymer materials is based on the specific CP redox
behaviour and couples the following reactions:
Bearing this complicated situation in mind, we renounced
from further quantitative interpretation of the current tran-
sients. Indeed, the existing theoretical models for progressive
nucleation and growth under diffusion limitations do not ac-
Men+ + ne− → Me0
CPm+ → CP(m+n)+ + ne−
(1)
(2)

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S. Ivanov, V. Tsakova / Electrochimica Acta 50 (2005) 5616–5623
Fig. 5. Time dependence of open circuit potential of initially reduced PANI
layers, measured in supporting electrolyte (grey line) and in 10 mM Ag+ (full
line) and 10 mM [AgEDTA]3− (dashed line) solutions (Qred = 24 mC cm−2).
Fig. 6. Silver oxidation curves obtained after electroless (—) (dipping
time—15 s) and galvanostatic (. . .) (deposition time −150 s) deposi-
tions carried out in 10 mM solutions of Ag+ (black lines) and 10 mM
[AgEDTA]3− (grey lines). Scan rate: 5 mV s−1. The electroless and galvano-
static deposition experiments were performed for 15 and 150 s, respectively
(Qred = 32 mC cm−2).
where n is the number of electrons exchanged for reducing a
single metal ion and m denotes the extent of initial oxidation
of the CP material.
In absence of metal ions, reaction (2) occurs sponta-
neously at the expense of other reducing species, i.e. oxy-
gen present in the solution. It is apparent that in presence
of metal ions the electroless precipitation process will be ef-
fectively driven if the equilibrium potential of the depositing
metal ions is much more positive than the initial (dipping)
potential of the CP layer. This is true for reduced polyaniline
layers dipped in both silver cation and silver–EDTA plating
solutions. (The open circuit potential of reduced PANI lay-
ers dipped in the silver thiosulfate plating solution remains
more positive than the equilibrium potential of these silver
complex anions and thus, excludes the possibility for elec-
troless driven process in this solution.) Fig. 5 shows the time
dependence of the open circuit potential (OCP) measured
directly after complete reduction of the PANI layers in sup-
porting electrolytes (grey line) and after transferring the re-
duced PANI layer-coated electrodes in the silver-containing
solutions (black lines). It is obvious that in absence of sil-
ver ions spontaneous PANI oxidation occurs very slowly,
whereas PANI oxidation occurs much faster in the silver
plating solutions. In both cases the potential drops quickly
to values close to the equilibrium potential of silver in the
corresponding solution. The latter is an indirect indication
for the presence of silver crystals at the PANI-coated sur-
face.
A direct evidence for the availability of silver was ob-
tained by measuring slow oxidation potentiodynamic scans
in supporting electrolyte (Fig. 6—full lines). Large silver ox-
idation peaks were obtained after dipping the reduced PANI
layers in both silver cation and silver–EDTA complex solu-
tions. The comparison of the silver oxidation waves obtained
after electroless and electrodriven (galvanostatic) deposition
(Fig. 6) shows, however, a marked shift of the silver oxida-
tion peaks in the positive direction in the case of an electro-
driven process (dotted lines in Fig. 6). Thus, galvanostati-
cally deposited silver seems to be more strongly bonded to
the PANI layer than the electroless silver deposit. A possi-
ble explanation of this effect relates to the different prefer-
ential location of the deposited silver crystals. The electro-
less silver precipitation process starts at polymer sites able to
be easily oxidised and silver crystals are exclusively located
at the polymer surface. A similar observation was made by
Abrantes et al. [12] in the study of silver recovery at PANI-
coated electrodes. On the other hand, the application of ex-
ternal current or potential initiates diffusion of silver ions
through the polymer layer and electrodeposition startsmost
easily at defects in the polymer layer, i.e. at sites providing
better access for the silver ions and having the lowest elec-
tronic conductivity. Thus, electrodriven metal deposition is
strongly dependent on the morphological inhomogeneity of
the polymer coating whereas the electroless process responds
to the redox properties of the CP material. These assump-
tions were supported by scanning electron observations of
the silver deposit. In the case of electrodriven silver depo-
sition performed in [AgEDTA]3− ion solution (Fig. 7a) big
silver crystals growing most probably at microdefects of the
polymer coating are found. The amount of the sphere-like
crystals (5×106 cm−2) obtained in this solution is compara-
ble to the crystal numbers obtained by potentiostatic deposi-
tion in the silver cation solution (Table 1). In contrast, a ho-
mogeneous distribution of equally sized silver crystals was
obtained by the electroless precipitation process (Fig. 7b).
Their number amounts to 7×107 cm−2 and exceeds signif-
icantly the crystal numbers obtained by electrodriven depo-
sition in all three plating solutions (Table 1). On the other
hand, electroless precipitation from the silver cation solution
does not change the microscopic picture (not shown here) of
the silver deposit compared to the result obtained by poten-
tiostatic deposition [9]. Thus, it seems that the presence of
EDTA in the plating solution plays a special role for the silver
electroless precipitation. This point will be addressed further
below.

S. Ivanov, V. Tsakova / Electrochimica Acta 50 (2005) 5616–5623
5621
Fig. 8. Silver oxidation charge (QAg) in dependence on redox charge of
PANIlayers(Qred)obtainedafterelectrolessprecipitationatconstantdipping
time (15 s) and different concentrations of [AgEDTA]3−: 10 mM (ꢀ), 2 mM
(ꢁ), 0.4 mM (ꢂ). Circles (ꢀ) are obtained at c[AgEDTA] = 0.4 mM by
3−
increasing the dipping time from 15 to 300 s.
Q
red, ofthePANIlayer, (ii)theconcentrationof[AgEDTA]3−
and (iii) the dipping time in the silver plating solution. Af-
ter each silver deposition experiment the working electrode
was transferred in supporting electrolyte (0.5 M H2SO4) and
subjected to a slow oxidation scan (as shown in Fig. 6). The
silver oxidation charge QAg, corresponding to the electroless
deposited metal, was obtained by integrating the anodic peak
current.
Fig. 8 shows the dependence of the silver oxidation charge
(i.e. the amount of electroless deposited silver) on the re-
dox charge of the polymer layer measured at three different
concentrations of the [AgEDTA]3− solution. All experiments
(except the results marked by circles) were performed at fixed
dipping time—15 s. Keeping apart the data obtained at the
lowest concentration (0.4 mM), all other data for the silver
amount follow a similar parabolic increase with increasing
PANI redox charge. This means that the amount of the sil-
ver deposit can be easily controlled by monitoring the redox
charge of the polymer layer. This investigation, however, does
not show whether a bigger amount of silver corresponds to a
larger number of crystals or rather to the formation of bigger
silver aggregates. This question was approached by preparing
two specimens for SEM with different PANI redox charges
and dipping them for one and the same time in the 2 mM
Fig. 7. SEM images of silver deposit on PANI layers obtained in 10 mM
[AgEDTA]3− solution after galvanostatic (a) and electroless (b) silver de-
position (Qred = 37 mC cm−2).
3.4. Factors allowing control over the electroless
precipitation process
Theresultspresentedso-farshowthatelectrolessmetalde-
position offers an easy and effective procedure for depositing
silver crystals at the polymer surface. The question that arises
is to what extent this spontaneous process could be controlled
and whether it would allow for varying the silver amount and
crystal numbers.
To answer these questions studies of silver electroless pre-
cipitation in AgEDTA complex solution were performed by
varying the following three parameters: (i) the redox charge,
Table 1
Data for the characteristics of the silver deposit obtained in different silver ions containing solutions
Silver reducing
species
cAg ions
(mM/l)
Deposition
method
Number of
crystals (cm−2
Average crystal
size (␮m)
Crystal size distribution
crystal shape
)
Ag+ [9]
10
10
10
10
10
2
Potentiostatic
Electroless
Potentiostatic
Galvanostatic
Electroless
(1–8) × 106
0.5–4.0
1–2
7–10, <1.0
0.2–1.2
0.22
Broad agglomerates
Ag+
∼106
Narrow snow flake like
Broad—bimodal, large agglomerates
Broad large spheres
Monodisperse cubic
Monodisperse/narrow cubic
Narrow cubic
[Ag(S2O3)2]3−
[AgEDTA]3−
[AgEDTA]3−
[AgEDTA]3−
[AgEDTA]3−
∼106
5 × 106
7 × 107
(1–6) × 108
∼108
Electroless
Electroless
0.07–0.25
0.15
0.4

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S. Ivanov, V. Tsakova / Electrochimica Acta 50 (2005) 5616–5623
4. Conclusions
The investigations presented in this work demonstrate the
possibility to influence the characteristics (number, size dis-
tribution and shape) of silver crystals deposited at polyaniline
layers by varying both the type of silver reducing species and
thewayofmetaldeposition(electrolessorelectrodriven). The
best results in terms of number, shape and size distribution
of the crystals were obtained by combining the silver–EDTA
complex solution with electroless precipitation. Although sil-
verelectrolessprecipitationwasfoundtooccurinacontrolled
way in both silver cation and silver–EDTA plating solutions,
the deposit obtained in Ag+ solution consisted of a low num-
ber of large silver crystals (Table 1). Thus, the [AgEDTA]3−
solution seems to play a special role for the characteristics
of the polyaniline coating in the course of metal electroless
precipitation.
Two factors may affect the properties of the PANI layer
in the silver–EDTA solutions. The first one is the higher
value of pH (pH 4.1) compared to the pH value in the sil-
ver cation solution (pH 0.3). It is known that with increas-
ing values of pH PANI looses its electroactivity and con-
ductivity. However, our measurements performed in EDTA-
containing supporting electrolyte at pH 4.1 show that the
polyaniline layer keeps partially its electroactivity. Besides,
the amounts of electroless precipitated silver at PANI lay-
ers with one and the same charge does not differ for both
plating solutions (see the silver oxidation peaks in Fig. 6).
Thus, spontaneous oxidation of the PANI layer in the EDTA-
containing medium may effectively occur. Nevertheless, it
could be suggested that the release of the silver ion from
the [AgEDTA]3− complex is impeded in comparison to the
direct reduction of the silver cation and this may be the rea-
son for slowing down the electroless precipitation. The sec-
ond factor, which might influence the characteristics of the
PANI layer, relates to the special role of the EDTA anions
present in the plating solution. It is known that the poly-
merization of aniline in presence of EDTA anions results
in an increase of the degree of cross-linking reactions in
the polymer and leads to less conducting layers [23]. In the
present experiments PANI was always synthesized in sul-
fate solutions and transferred in the EDTA-containing so-
lution after polymerization. Nevertheless, it could be ex-
pected that in the course of PANI oxidation the EDTA an-
ions, allow for an additional cross-linking of the polymer
chains, having the effect of a surface homogenization of
the polymer layer and redistribution of the polymer redox
centers. The combination of the two effects (slowed down
release of silver and redistribution of PANI redox centers)
favors most probably the formation of a large number of
small, equally sized crystals deposited in the silver–EDTA
solution.
Fig. 9. SEM images of electroless deposited silver in 2 mM
[AgEDTA]3− solution on PANI layers with different reduction charge: (a)
Q
red = 16 mC cm−2; (b) Qred = 42 mC cm−2
.
[AgEDTA]3− solution. The micrographs obtained at these
specimens (Fig. 9) show that by varying the PANI layer re-
dox charge not only the amount of silver but also the crystal
numbers and sizes are significantly influenced. The crystal
number varies from 1.4 × 108 cm−2 for the PANI layer with
lower redox charge to 5.7 × 108 cm−2 for the PANI layer
with the larger redox charge. Use of PANI layers with large
redox charges also results in a broader size distribution of the
deposited silver crystals.
The results obtained at the lowest concentration (0.4 mM)
of [AgEDTA]3− (open triangles in Fig. 8) show a differ-
ent trend. In this case, the electroless precipitation occurs
very slowly and the short dipping time used in the se-
ries of experiments is not enough for consuming the whole
redox capacity of the polymer layer. Increasing the dip-
ping time (circles in Fig. 8) allows increase in the amount
of deposited silver up to the values limited by the redox
charge of the PANI layer. Therefore, a proper choice of the
two parameters—[AgEDTA]3− concentration and dipping
time—allows to switch between redox charge-limited and
time-limited electroless metal deposition. Working at low
concentrations and short dipping times results in deposition
of one and the same amount of metal at PANI layers with
very different redox charge.
In contrast, the use of plating solutions with much higher
pH values (pH 6) and silver complex anions with large stabil-
ity constants (as was the case of silver thiosulfate) results in
highly impeded electrodriven metal deposition. Under these

S. Ivanov, V. Tsakova / Electrochimica Acta 50 (2005) 5616–5623
5623
conditions the PANI layer, dipped in the silver plating solu-
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