Palladium electrodeposition on polyaniline films

Article abstract of DOI:10.1016/j.electacta.2007.07.050

The electrochemical nucleation and growth of palladium particles onto polyaniline (PAni) films have been investigated by chronoamperometry and topographic and phase-mode atomic force microscopy (AFM). The films were synthesized under different potentiodynamic conditions in order to obtain polymer layers with comparable electroactivity but distinctly different morphology/porosity. The analysis of the current transients obtained for the initial stages of the Pd deposition indicates a 3D nucleative formation regime. A detailed Pd electrodeposition study onto the polymer matrix, using different deposition times, suggests that a constant number of critical nuclei is formed in the superficial part of the polymer porous matrix in the time scale between ca. 5 and 15 s.

Full text of DOI:10.1016/j.electacta.2007.07.050

Electrochimica Acta 53 (2007) 664–672
Palladium electrodeposition on polyaniline films
a,b
a
b
a,∗
A. Mourato , J.P. Correia , H. Siegenthaler , L.M. Abrantes
a
Departamento de Qu ´ı mica e Bioqu ´ı mica da Faculdade de Ci eˆ ncias da Universidade de Lisboa,
Campo Grande, 1749-016 Lisboa, Portugal
Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3, CH-3012 Bern, Switzerland
b
Received 21 June 2007; received in revised form 13 July 2007; accepted 14 July 2007
Available online 28 July 2007
Abstract
The electrochemical nucleation and growth of palladium particles onto polyaniline (PAni) films have been investigated by chronoamperometry
and topographic and phase-mode atomic force microscopy (AFM). The films were synthesized under different potentiodynamic conditions in
order to obtain polymer layers with comparable electroactivity but distinctly different morphology/porosity. The analysis of the current transients
obtained for the initial stages of the Pd deposition indicates a 3D nucleative formation regime. A detailed Pd electrodeposition study onto the
polymer matrix, using different deposition times, suggests that a constant number of critical nuclei is formed in the superficial part of the polymer
porous matrix in the time scale between ca. 5 and 15 s.
©
2007 Elsevier Ltd. All rights reserved.
Keywords: Polyaniline; Palladium; Electrodeposition; AFM
1
. Introduction
particles in polyaniline (PAni) matrices in order to enhance
the electrocatalytic activity as compared with the bulk-form
metal electrodes [2,5,7,9–13]. As a conducting polymer sub-
strate, PAni is one of the most attractive polymers on account of
its high conductivity, stability in ambient conditions, easy poly-
merisation and for its multiple oxidation and protonation states
[14]. Another important feature is that PAni films present a high
surface area due to the high porosity of the films formed by
various methods [15,16]. The electrochemical polymerisation
provides the possibility of controlling the thickness, homogene-
ity and morphology of the polymer film on the electrode surface
[10,17]. There are different methods for the deposition of Pd par-
ticles on the polymer matrix; most published studies are related
to metal electrodeposition [18,19], co-deposition [18] and elec-
troless precipitation [20–22]. In order to synthesise well-defined
highly dispersed metal nanoparticles, control over the nucle-
ation and growth process is essential. A suitable electrochemical
method is the electrodeposition, since the growth conditions can
be directly monitored with the applied potential and deposition
time. Nevertheless, if the electrodeposition of noble metals is
carried out into/onto PAni layers, it should be performed in such
a way that no electroless precipitation of the metal particles
occurs in the initial stages. This process arises when spontaneous
polymer oxidation takes place with simultaneous reduction of
the noble metal ion, provided that the reduction potential is in
Metal micro/nanoparticles are intensely studied due to their
unique optical, electrical and catalytic properties. The spe-
cial properties of sub-microparticles are due to their large
surface area, which is related with their size, distribution,
exposed surface area structure and mode of preparation in the
dispersing medium [1–6]. To utilize and optimize the chem-
ical/physical properties of nanosized metal particles, a large
amount of research has focused on the control of the size and
shape.
Conducting polymers can be used as proper host matrices to
obtain highly dispersed metallic particles and have been sub-
ject of great scientific interest since different and sometimes
novel properties are obtained as compared with bulk materials
[
1,7,8]. In particular, metal microparticles dispersed in poly-
mer modified electrodes have been recognized to have potential
applications in electrocatalysis. Many research groups have
studied the incorporation of metal microparticles (e.g. Pt, Pd)
∗
Corresponding author at: Dep. Qu ´ı mica e Bioqu ´ı mica da FCUL, Edif ´ı cio
C8, Campus da FCUL, Campo Grande, 1749-016 Lisboa, Portugal.
Tel.: +351 21 7500000x28342; fax: +351 21 7500088.
E-mail address: luisa.abrantes@fc.ul.pt (L.M. Abrantes).
0
013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2007.07.050

A. Mourato et al. / Electrochimica Acta 53 (2007) 664–672
665
the range where the conversion of the oxidised forms of PAni
occurs [10,11].
(for E = +0.150 V, E = +0.190 V), and for 1200 s and 1860 s (for
E = +0.190 V), the last ones corresponding to deposition charges
of ∼15 mC cm and ∼33 mC cm , respectively. Blank tran-
sients of the PAni films measured in Pd-free 0.5 M H2SO4
solutions were also performed.
−
2
−2
In an earlier work [10], the present authors have shown that it
is possible to obtain palladium nanoparticles on the polyaniline
layers by electroless precipitation. Compact films were obtained
with a slow electropolymerisation sweep rate, and the electro-
lessly deposited Pd clusters were of different sizes (80–200 nm
and ∼20 nm) and uniformly dispersed on the top of the poly-
mer layer. In contrast, heterogeneous and porous films were
obtained with a faster sweep rate of 60 mV/s. In this case, the
results obtained by AFM demonstrated that the PAni nodules
were apparently covered by a large amount of Pd particles of
approximately 15 nm diameter, and SEM data also showed the
presence of palladium clusters with diameters of 100–200 nm.
Given the current interest devoted to these modified elec-
trodes, the aim of the present work was to evaluate the role of
the polymer synthesis conditions, and ultimately of the struc-
tural properties of the PAni films, on the dispersion and size
of the potentiostatically deposited palladium on/in a polymeric
matrix.
The equipments used for the electrochemical experiments
were a set-up consisting of a home-made potentiostat, a Wave-
form Generator PP R1 (Hi-Tek Instruments, England) and a
Recorder X–Y Model 200 (The Recorder Company) for the
potentiodynamic growth of the polyaniline films and for the
potentiostatic deposition of palladium for 1200 and 1860 s
(E = +0.190 V), and EG&G Princeton Applied Research (Model
473A) used for the palladium electrodeposition when pulse
potentials of 30 and 60 s were applied.
The morphology of the modified electrodes was examined
by Atomic Force Microscopy in the Tapping Mode, using a
Nanoscope IIIa Multimode Atomic Force Microscope (Digital
Instruments) equipped with the Extended Electronics Module
for simultaneous topographic and phase imaging. Etched sil-
icon tips, with ∼300 kHz resonance frequency, were used as
AFM probes.
2
. Experimental
The estimation of the visible amount of electrodeposited Pd
was performed by the statistical analysis of the topographic and
phase AFM images, by counting the features associated with Pd
particles.
The PAni films were electropolymerised on polycrystalline
gold electrodes in a 0.1 M solution of aniline in 0.5 M H2SO4
as described previously [10], by cycling the potential between
−
0.200 V and +0.800 V versus SCE for the first five cycles, then
3. Results and discussion
lowering the anodic limit to 0.750 V for the next three cycles,
and to 0.70 V after eight cycles. The electrolyte solution was
prepared with Milli-Q water from sulfuric acid (100%, Merck,
for conductivity measurements) and from freshly distilled ani-
3.1. Electrodeposition of palladium at polyaniline films
Potential-step has been used to investigate the palladium elec-
trodeposition on/in the two previously described distinct types
of polymer films (17/10 and 200/60). Systematic studies (not
presented) were performed using a palladium electrode and
polyaniline films as substrates to establish and select the palla-
dium deposition potential in acidic solutions. The potential of a
bulk palladium electrode in the 0.001 M PdSO4 solution in 0.5 M
H2SO4 is Eoc = +0.550 V versus SCE, and the electrodeposition
of palladium on this substrate occurs at +0.300 V versus SCE.
Taking into account this potential and the requirement to apply
an overpotential, the electrodeposition of Pd in/on the polymer
films was performed by applying a step potential from +0.400 to
+0.190 V versus SCE, where PAni remains in the oxidised state
(dashed line in the cyclic voltammograms shown in Fig. 1).
Fig. 2displaysthecurrenttransientsobtainedforthepotentio-
staticPddepositiononbothPAnifilms. Attheinitialpotentialthe
PAni films are in the polyemeraldine form (i.e., equal numbers
of repetitive benzenoid and quinoid units) which corresponds
to a maximum of conductivity, and when the potential step is
applied, e.g. to 0.190 V versus SCE, the polymer matrix is also
slightly reduced (Fig. 1, dashed line). Hence, under these condi-
tions it should be expected, as reported in the literature [23–28],
that the metal deposition occurs mainly on the top of the film
surface since the polymer matrix is mostly in the conducting
state. However, it cannot be excluded that some nucleation and
growth of the metallic nuclei also takes place across the porous
matrix and at the polymer film and metal substrate interface.
line stored under N2 atmosphere. Polycrystalline gold disks
2
(A = 0.50 cm , purity 99.99%) were used as working electrodes
and hand-polished in an aqueous suspension of successively
finer grades of alumina (down to 0.05 ␮m) until a fresh mirror-
finish surface was generated. A saturated calomel electrode
(
SCE) and a Pt foil were used as reference and counter elec-
trodes, respectively, in a three-compartment cell.
Two different polyaniline film types were prepared by
applying 17 polymerisation cycles at a potential sweep rate
v = 0.01 V/s (17/10 films) and 200 polymerisation cycles at
−1
v = 0.06 s
(200/60 films). Prior to all measurements, the
solutions were deaerated with N2 for 15 min. The films were
washed with Milli-Q water and electrochemically charac-
terised in the electrolyte solution (monomer free solution) by
cycling the potential between −0.200 and +0.400 V versus SCE
at v = 0.05 V/s. The electrode potential was initially kept at
−
0.200 V for 10 min in order to completely discharge the film.
The synthesised PAni modified electrodes were used as sub-
strates for the palladium electrodeposition.The polymer film,
in its oxidised state, was transferred in the shortest lapse of
time to the solution of 0.001 M PdSO4 (98%, Aldrich) in 0.5 M
H2SO4 where it was kept at +0.400 V versus SCE for 1 s. This
procedure was done in order to avoid the electroless precipi-
tation of palladium in/on the polymer film. The potential was
then stepped to +0.120, +0.150 V or +0.190 V versus SCE, and
the current transients recorded for 30 s (for E = +0.120 V), 60 s

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A. Mourato et al. / Electrochimica Acta 53 (2007) 664–672
Fig. 1. Cyclic Voltammograms of the 17/10 and 200/60 polyaniline films in
monomer free solution (0.5 M H2SO4) at v = 0.05 mV/s. The dashed line indi-
cates the potential used for the electrodeposition of palladium.
The thickness of the 17/10 films and 200/60 films was mea-
sured by in situ AFM in the oxidised state (+0.400 V versus SCE)
and typical values obtained were 107 and 605 nm, respectively.
The chronoamperograms in Fig. 2 are characterized by an
initial current decay followed by an ascending part; where a
maximum is clearly visible. As expected, the equivalent cur-
rent transients collected for PAni films in Pd free 0.5 M H2SO4
solution (black lines in Fig. 2) only reveal the charging of the
polymer matrix.
Fig. 2. Chronoamperograms recorded during potential step polarisation of
polyaniline films in presence of Pd (a) 17/10 and (b) 200/60. Applied potential
2+
The main difference observed between the transients of the
7/10 and of the 200/60 films is the position tm of the current
step +0.400 to +0.190 V vs. SCE. Black curves: chronoamperograms recorded
in 0.5 M H2SO4.
1
maxima on the time scale, whereby the current maximum is
reachedafteralongertimeintervalinthecaseofthe200/60poly-
mer film. Considering the generally established current–time
correlations for 3D nucleation [29–31] the observed increase
of tm may be due to a decrease of the maximum density of
nucleation sites and/or a decrease of the growth rate of the indi-
vidual nuclei. Both properties may be affected by differences
in the morphology and thickness of the 17/10 and the 200/60
films. Pronounced morphological differences between the two
film types are clearly visible in the previously published SEM
images [10] and in the AFM images of Fig. 3: whereas the 17/10
films (Fig. 3a) show a rather compact morphology consisting
of small globular features with typical diameters between ca.
20–80 nm, the 200/60 films (Fig. 3b) consist of larger size glob-
ules. If the Pd nucleation sites are predominantly located in the
contact zones and pores between different globuli of the poly-
mer matrix, a higher site density would indeed be expected for
the 17/10 films, and higher pore depths with a slower diffusional
supply of Pd2 can be anticipated at the 200/60 films.
In Table 1 the charge associated with the reduction of PAni
(QPAni), was subtracted from the value obtained from the pal-
ladium deposition current transient (Qmist), resulting in the
deposition charge (Qf Pd deposition). The duration of the poten-
tiostatic Pd deposition was chosen such that the amounts of
+
electrodeposited Pd, quantified by the charge Qf Pd deposition
,
Fig. 3. AFM- tapping mode images of polyaniline films prepared under different polymerisation conditions: (a) 17/10 film, (b) 200/60 film.

A. Mourato et al. / Electrochimica Acta 53 (2007) 664–672
667
Table 1
Deposition time and charges involved in palladium electrodeposition
−
2
−2
Qf Pd deposition Q(mist) − Q(polymer) mC cm−2
Film
Deposition time s
QPAni mC cm
Qmist mC cm
1
1
2
2
7/10
7/10
00/60
00/60
1200
1860
1200
1860
1.43
2.28
1.15
1.91
16.53
35.52
16.42
35.19
15.10
33.24
15.27
33.27
Applied potential step: +0.400 to +0.190 V vs. SCE
Fig. 4. Ex situ tapping mode AFM images after electrodeposition of Pd onto 17/10 polyaniline films from a solution of 0.001 M PdSO4 in 0.5 M H2SO4 by potential
step chronoamperometry. Pd deposition by step polarisation from +0.400 to +0.190 V vs. SCE, using deposition times of (a) 1200 s and (b) 1860 s.
equal approximately the amounts of Pd deposited in previous
work [10] by electroless deposition at 17/10 and 200/60 films:
The morphological characterization of the 17/10 films after
electrodepositon during 1200 and 1800 s is presented in Fig. 4,
where 3D processed topographic images are shown. Spherical
palladium particles are observed and are distributed randomly
on the top of the polymer compact layer. Their diameters range
−
2
1
5 mC cm
17/10/Pd 30 film) and 33 mC cm for electroless deposition
during 60 min (17/10/Pd60 film).
for electroless deposition during 30 min
−
2
(
Fig. 5. Ex situ tapping mode AFM images after electrodeposition of Pd onto 17/10 films from a solution of 0.001 M PdSO4 in 0.5 M H2SO4 by potential step
ꢀ
ꢀ
chronoamperometry. Pd deposition by step polarisation from +0.400 to +0.190 V vs. SCE, using deposition times of (a,a ) 1200 s and (b,b ) 1860 s. (a,b) Topographic
images; (a ,b ) phase images. Window size 0.5 ␮m × 0.5 ␮m.
ꢀ
ꢀ

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A. Mourato et al. / Electrochimica Acta 53 (2007) 664–672
Fig. 6. Ex situ tapping mode topographic AFM images after electrodeposition of Pd onto 200/60 polyaniline films from a solution of 0.001 M PdSO4 in 0.5 M H2SO4
by potential step chronoamperometry. Pd deposition by step polarisation from +0.400 to +0.190 V vs. SCE, using deposition times of (a) 1200 s and (b) 1860 s.
from ca. 50 to ca. 90 nm after 1200 s deposition time (Fig. 4a),
increasing to ca. 100 nm with time of deposition (Fig. 4b). In
Fig. 5 higher magnification topographic and phase images are
displayed, where the palladium clusters are clearly recognised.
Fig. 6 shows 3D processed ex situ topographic AFM images
of 200/60 polyaniline films after potential step deposition of Pd
during 1200 s (Fig. 6a) and 1860 s (Fig. 6b). The observed pal-
ladium clusters are distributed on the surface of the polyaniline
film and upon extension of the deposition time, additional small
cluster features appear in the phase-mode image, and the num-
ber density appears to increase compared to the value obtained
with 1200 s. A possible explanation for this observation is the
visualization of additional particles that were previously formed
in micropores inside the film matrix [1,2], or even at the polyani-
line/Au interface: these can only be visualized as the surface film
domains by the (surface-confined) phase-mode AFM technique
after growing into those layers during the extended deposition
times. Fig. 7 shows enlarged topographic and phase images
(0.5 ␮m × 0.5 ␮m) of the Pd modified polymer, where it is pos-
sible to distinguish Pd particles with diameters of approximately
10 nm which are part of the Pd clusters observed.
The results described above and in [10] clearly show that
the deposition method for embedding palladium particles on
polyaniline films, plays an important role on the amount and
Fig. 7. Ex situ tapping mode AFM images after electrodeposition of Pd onto 200/60 polyaniline films from a solution of 0.001 M PdSO4 in 0.5 M H2SO4 by
ꢀ
ꢀ
potential step chronoamperometry. Pd deposition by step polarisation from +0.400 to +0.190 V vs. SCE, using deposition times of (a,a ) 1200 s and (b,b ) 1860 s.
(
ꢀ ꢀ
a,b) Topographic images; (a ,b ) phase images.

A. Mourato et al. / Electrochimica Acta 53 (2007) 664–672
669
distribution of the metallic clusters. In general the electroless
precipitation of palladium on the 17/10 film leads to the for-
mation of particles distributed in a non-uniform arrangement
on the top of the polymer layer whose diameter ranges from
diameters of the particles, but also in a considerable increase of
the particle number density. This is an indication that, in the
deposition range between 5 and 15 s, still additional particles
are formed, in contrast to the limiting model of instantaneous
nucleation. Upon further extension of the deposition time from
15 to 30 s, the observed particle number density remains prac-
tically constant, but the particles grow to average sizes around
40 nm, where they are close to mutual contact. The substan-
tial increase of the deposition time to 1200 s results in a rather
compact appearance of the Pd coverage (Fig. 6a) combined
with a considerable decrease of the number density (Table 2).
These observations are most likely due to a substantial amount
of coalescence of individual particles by the extended growth
into larger clusters. For this reason, the number density value
is in brackets in Table 2, as it should not be considered a real
quantity.
2
1
0–30 nm and from 80 to 290 nm, if the process occurred for
800 and 3600 s, respectively. Also the amount of metallic par-
ticles increased with the time deposition. In the 200/60 film the
palladium clusters could not be clearly distinguished from the
polymer features in the topographic AFM images, although the
globules exhibit smaller clusters (diameter 15–20 nm) covering
their surface and were much more clearly visualized in a smaller
scale phase image. As additional criterion for the discrimination
between polymer globules and Pd particles in the AFM images,
increasing Pd deposition time could be associated on both films
with a slight enlargement of clustered features that obviously
represent Pd deposits. For example, after deposition of palla-
dium for 1200 s the 17/10 film presents clusters with diameters
ranging from 50 to 90 nm, whereas after 1860 s the diameters
increase to 100 nm. On the 200/60/Pd modified electrode, a simi-
lar behaviour was observed, where an increase of the diameter of
the palladium clusters from 100 to 120 nm was recorded. More-
over, the number density of the metallic particles synthesised
by this deposition method is much higher than that observed by
electroless precipitation.
This would also imply that the particles observed in Fig. 8
after short deposition times, are essentially particles at the sur-
face or in the superficial layer of the polymer matrix.
3.3. Nucleation and growth mechanism of the Pd particles
on the 200/60 films
A first appraisal on the nucleation and growth mechanism
of the Pd particles on the PAni films was performed by com-
parison of the experimental cathodic current transients of Pd
electrodeposition with the theoretical plots for different types
of nucleation. Taking into account the requirement of a well
defined nucleation peak at the initial moments of deposition, for
this study, the electrodeposition of Pd in/on the polymer films
was studied in the potential range between 0.120 and 0.190 V
versus SCE (Fig. 9). Also, if the transients recorded for the PAni
film in absence of Pd deposition (Fig. 9, inset) are subtracted
from the transients obtained during Pd deposition, the resulting
difference transients (Fig. 10) with a current maximum exhibit
the typical features of a metal deposition process controlled by
nucleation and growth [7–9], in particular when the potential
was stepped to +0.120 V.
The families of chronoamperograms in Fig. 9 are charac-
terised by an initial current decay followed by an ascending part
and for steps to 0.150 and 0.120 V a maximum is clearly visi-
ble. As expected, the equivalent current transients collected for
PAni films in Pd free 0.5 M H2SO4 solution (inset in Fig. 9)
only reveal the charging of the polymer matrix. Following
early theoretical concepts for electrochemical nucleation [30],
the theoretical treatment of potential step chronoamperome-
try for multiple nucleation with diffusion-controlled growth
has been the subject of detailed studies by various groups
3
.2. Influence of the potential step duration on size and
distribution of Pd particles electrodeposited at the 200/60
polyaniline films
Further potential step polarization experiments (stepping
from +0.400 to +0.190 V versus SCE) were performed at 200/60
films using different step durations of 5, 15 and 30 s, followed by
ex situ AFM investigations of the resulting Pd/polyaniline com-
posite films, as illustrated by Fig. 8. The Pd nanoparticles are
visible in the phase-mode AFM images as small patterns with
enhanced brightness due to their different viscoelastic properties
with respect to the polyaniline film matrix. For short deposition
times, i.e. up to 30 s, the average Pd particle diameters and num-
ber densities were evaluated by counting the number of visible
clusters at different scan window sizes and are listed in Table 2.
As can be seen in Fig. 8 and from the data in Table 2, an exten-
sion of the deposition time from 5 to 15 s not only results in larger
Table 2
Diameters and number densities of Pd particles electrodeposited on 200/60
polyaniline films
Deposition
time s
N particles/cm2
Particle diameter (nm)
Average diameter
[31–36].
Diameter range
According to Scharifker et al. [31,32], dimensionless equa-
6
5
5
0
200
860
1.80 × 10
8.6
18.0
39.1
116.1a
109.8a
6.4–10.7
12.8–23.7
15.6–62.5
47.5–184.6a
44.7–174.9a
tions can be used for metallic deposits formed by nucleation
and growth in three-dimensional matrixes (e.g. polymers) of
limited thickness [15], to compare the experimentally observed
nucleation process with the two limiting cases of instantaneous
and progressive nucleation, and to discriminate between two- or
three-dimensional growth. While in the model of Scharifker et
al. the diffusion layer thickness used for the calculation of the
6
1
3
4.29 × 10
6
4.22 × 10
a
a
6 a
1
1
(3.8 × 10 )
6 a
(8.7 × 10 )
The values are determined from AFM images
See restrictions mentioned in the preceding text for the significance of the
values for the deposition times 1200 and 1860 s.
a

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A. Mourato et al. / Electrochimica Acta 53 (2007) 664–672
Fig. 8. Electrodeposition of Pd onto 200/60 polyaniline films from a solution of 0.001M PdSO4 in 0.5 M H2SO4 by potential step chronoamperometry. Pd deposition
by step polarisation from +0.400 to 0.190 V vs. SCE, using short deposition times of 5 s (a), 15 s (b) and 30 s (c). The tapping mode AFM chracterization of the Pd
ꢀ
ꢀ
ꢀ
ꢀ
deposits was carried out ex situ in the topographic mode and in the phase mode. (a, a , b, b , c) Topographic images; (a1, a 1, b1,b 1, c1) phase images.

A. Mourato et al. / Electrochimica Acta 53 (2007) 664–672
671
Fig. 9. Chronoamperograms recorded during potential step polarisation of polyaniline films in presence of Pd2+ in 0.5 M H2SO4 + 0.001 M PdSO4. (a) 17/10 films
and (b) 200/60 films. Applied potential step +0.400 to +0.120, +0.150 and +0.190 V vs. SCE. Insets: chronoamperograms recorded in 0.5 M H2SO4.
Fig. 10. Corrected potential step chronoamperograms obtained in presence of Pd2+ in 0.5 M H2SO4 +0.001 M PdSO4. The chronoamperograms have been corrected
by subtraction of the corresponding chronoamperograms recorded in Pd-free 0.5 M H2SO4. (a) 17/10 films and (b) 200/60 films. Applied potential steps +0.400 to
+
0.120, +0.150 and +0.190 V vs. SCE.
current densities only depends on the time, a modified model
has been presented by Heerman and Tarallo [35,36], where the
diffusion layer thickness also depends on the nucleation rate
constant.
on the two different PAni films; also shown are the theoreti-
cal curves corresponding to the limiting cases for instantaneous
and progressive nucleation for a 3D growth. It has been shown
[35,36] that these theoretical curves for instantaneous and pro-
gressive nucleation coincide for the two models by Scharifker
et al. and by Heerman and Tarallo.
Fig. 11 presents experimental dimensionless plots of (I/Im)2
versus (t/tm) for the electrodeposition of palladium at +0.120 V
Fig. 11. Plot of the dimensionless quantity (I/Im)2 vs. (t/tm) [30,31] for the palladium electrodeposition at +0.120 V vs. SCE on the different polyaniline films.
Comparison of the experimental results (for potential step polarisation from +0.400 to +0.120 V vs. SCE) with the theoretical results for 3D instantaneous and
progressive nucleation and growth, (a) 17/10/Pd and (b) 200/60/Pd.

6
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A. Mourato et al. / Electrochimica Acta 53 (2007) 664–672
On both films, the data of Fig. 11 show clearly that the exper-
Acknowledgments
imental transients neither coincide with the progressive nor with
the instantaneous nucleation scheme, but fall between these two
limiting cases. These results differ from a previous study of
Pd electrodeposition on polyaniline films by Leone et al. [19],
where the experimental results closely coincide with the limit-
ing case of instantaneous nucleation. A possible explanation for
the different results obtained in our work might be due to the
distinct morphology of the polyaniline films since the polymers
reported in [19] were grown galvanostatically and the Pd depo-
sition was carried out onto polyaniline in its reduced state, i.e., at
potentials where the film is electronically insulating in contrast
with the experiments described in the present work where it is
reported the palladium deposition on PAni at potentials where it
is in the conducting state.
The authors acknowledge Dr. A. S. Viana for the AFM imag-
ing, Laborat o´ rio de SPM, Faculdade de Ci eˆ ncias, Universidade
de Lisboa.
This work was financially supported by Funda c¸ a˜ o para a
Ci eˆ ncia e a Tecnologia, through Ph.D. scholarship PRAXIS
XXI/BD 21424/99.
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2
43.
4
. Conclusions
[
[
6] A. Drelinkiewicz, M. Hasik, J. Mol. Catal. A: Chemical 177 (2001) 149.
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Applying a pulse potential from +0.400 to +0.190 V it is
possible to incorporate Pd particles on potentiodynamically syn-
thesized PAni. The different morphologies of the films affect
the current transient features, namely the position of the current
maxima on the time scale (longer lapse for 200/60).
[
8] J. Wang, K.G. Neoh, E.T. Kang, J. Colloid Interface Sci. 239 (2001) 78.
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[
[
[
[
[
[
A decrease of the maximum density of nucleation sites was
obtained for the film displaying larger sized globules (200/60)
as well as a slower diffusional supply of Pd2 with concomitant
decrease of the growth rate of individual nuclei.
+
4
A detailed study of potential step electrodeposition on the
more porous polymer matrix using different deposition times
in combination with ex situ phase-mode AFM visualization of
the resulting Pd/polyaniline composites suggests that a constant
number density of critical Pd nuclei is reached in the superficial
parts of the film matrix within ca. 15 s, indicating that the elec-
trodepositionofthePdparticlesdoesnotfollowaninstantaneous
nucleation mechanism, as is also confirmed by a quantitative
evaluation of the current density transients. The achieved con-
stant number density of the particles in the superficial parts of
the polymer may be imposed by the density of special nucleation
sites in the superficial polymer matrix, e.g. grain boundaries or
micropores between polymer globules. Additional Pd nuclei that
are considered to be formed initially also in the internal porous
domains of the polymer matrix or even at the polymer/Au inter-
face are presumably not imaged initially by the phase-mode
AFM technique, but are only detected after their growth into the
superficial parts of the polymer during more extended deposition
times. The results illustrate the high potential of the AFM phase-
mode technique for the visualization of hard nanometer-size
particles in the superficial part of a soft polymer matrix, but show
also the limitation of this method for the particle visualization
in the polymer bulk.
3
(
[
[
[
19] A. Leone, W. Marino, B.R. Sharifker, J. Electrochem. Soc. 139 (1992) 438.
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574.
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[
[
[
[
The electrodeposition method provides a better control of
the size of the Pd clusters than the electroless precipitation,
where different sized Pd nuclei were observed even in the same
modified electrode.
[
[
[