Platinization assisted by Keggin-type heteropolytungstates

Article abstract of DOI:10.1016/S0013-4686(03)00513-9

Chronoamperometry and cyclic voltammetry (CV) were used to address the platinum deposition kinetics on glassy carbon and polycrystalline Pt supports from H₂PtCl₆-containing solutions with and without additives of different Keggin-type heteropoly acids (HPAs) of tungsten. The effect of addition of these polyoxometallates manifested itself in the pronounced disturbance of instantaneous nucleation and it can be interpreted in terms of strong HPA adsorption on certain nucleation centers followed by the modification of their activity. HPA additives decreased the current efficiency for Pt deposition despite the fact that the reduction of HPA additives was thermodynamically forbidden at the potentials chosen for electrodeposition. The latter fact can be explained as selective inhibition of the intermediate Pt ^II reduction by tungstates. CV of Pt deposits in sulfuric acid solutions indicated a noticeable increase in the deposit roughness if P- and Si-containing HPA were added in the course of platinization. More complex behavior was found for B-dodecatungstate that most likely originated from its tendency to undergo codeposition with Pt. Ex situ scanning tunneling microscopy (STM) images demonstrate more pronounced globular structure and smaller crystal sizes for HPA-affected deposits as compared with usual platinized platinum. These features can be explained by HPA effects on deposition kinetics assumed on the basis of electrochemical data and demonstrate that HPA can act in template-like manner.

Full text of DOI:10.1016/S0013-4686(03)00513-9

Electrochimica Acta 48 (2003) 3797ꢀ3804
/
www.elsevier.com/locate/electacta
Platinization assisted by Keggin-type heteropolytungstatesꢀ
a,
Marina I. Borzenko *, Malgorzata Chojak , Pawel J. Kulesza ,
b
b,1
a,1
Galina A. Tsirlina , Oleg A. Petrii
a,1
a
Department of Electrochemistry, Moscow State University, Moscow 119992, Russia
b
Department of Chemistry, University of Warsaw, Pasteura 1, Warsaw 02-093, Poland
Received 4 October 2002; received in revised form 25 December 2002; accepted 5 February 2003
Abstract
Chronoamperometry and cyclic voltammetry (CV) were used to address the platinum deposition kinetics on glassy carbon and
polycrystalline Pt supports from H PtCl -containing solutions with and without additives of different Keggin-type heteropoly acids
2
6
(
HPAs) of tungsten. The effect of addition of these polyoxometallates manifested itself in the pronounced disturbance of
instantaneous nucleation and it can be interpreted in terms of strong HPA adsorption on certain nucleation centers followed by the
modification of their activity. HPA additives decreased the current efficiency for Pt deposition despite the fact that the reduction of
HPA additives was thermodynamically forbidden at the potentials chosen for electrodeposition. The latter fact can be explained as
II
selective inhibition of the intermediate Pt reduction by tungstates. CV of Pt deposits in sulfuric acid solutions indicated a noticeable
increase in the deposit roughness if P- and Si-containing HPA were added in the course of platinization. More complex behavior was
found for B-dodecatungstate that most likely originated from its tendency to undergo codeposition with Pt. Ex situ scanning
tunneling microscopy (STM) images demonstrate more pronounced globular structure and smaller crystal sizes for HPA-affected
deposits as compared with usual platinized platinum. These features can be explained by HPA effects on deposition kinetics
assumed on the basis of electrochemical data and demonstrate that HPA can act in template-like manner.
#
2003 Elsevier Ltd. All rights reserved.
Keywords: Platinum deposition; Polyoxotungstates; Adsorption; Nucleation; Scanning tunneling microscopy
1
. Introduction
carbon (GC), and highly oriented pyrolytic graphite
HOPG) (see [5ꢀ8] for example). This phenomenon
provides a simple tool for the modification of the
surfaces [9ꢀ18]. While the adsorption interactions of
HPAs with platinum electrodes are commonly accepted,
they have not been studied in detail until recently. For
example, it has been demonstrated for a smooth and
platinized platinum [19] that Keggin-type adsorbed
HPAs interact with hydrogen adatoms. This interaction
leads to the appearance of new highly reversible redox
features that should be attributed to mixed hydrogen/
tungstate adlayers and interpreted in terms of spillover
effect. Similar features were found for multilayers
formed by isopolytungstates on smooth Pt [20].
(
/
Heteropoly acids (HPAs) of tungsten containing
electrochemically inert hetero atoms exhibit the redox
/
properties attractive for electrocatalysis and nanotech-
nology [1ꢀ4]. A group of well-characterized Keggin-type
/
complexes is among the most studied HPAs because of
their unique stability at low pH and the possibility to
tune the potentials of subsequent electron transfer
processes in a wide range.
Polyoxo acids, including HPAs, tend to undergo
spontaneous adsorption from aqueous solutions on
various electrode substrates, namely Hg, Au, Ag, glassy
ꢀ
It is plausible to expect that it is just the interaction of
2
immobilized tungstates with hydrogen adatoms (H-
From a lecture given at the Fourth International Conference on
‘
‘Electrocatalysis: From Theory to Industrial Applications’’, 23ꢀ
September 2002, Como, Italy.
Corresponding author. Fax: ꢁ
/25
*
/7-95-9328846.
2
E-mail address: borzenko@elch.chem.msu.ru (M.I. Borzenko).
1
ISE member.
Here and below we use notations H-upd and O-upd for the
corresponding regions of underpotential deposition.
0013-4686/03/$ - see front matter # 2003 Elsevier Ltd. All rights reserved.
doi:10.1016/S0013-4686(03)00513-9

3798
M.I. Borzenko et al. / Electrochimica Acta 48 (2003) 3797ꢀ3804
/
upd) (and/or fairly specific tungstate effect on oxygen
adatoms (O-upd) [2,21]) which is responsible for the
pronounced electrocatalytic activity of platinized films
tion (powdered glass) and washed before use. If to
compare the pretreatment mode with the modes applied
in Ref. [25], the surface of the present GC corresponds
to the less-oxidized GC.
of tungsten oxides, WO3×
/
nH O (1B
/
n B2), and Keggin-
/
2
Platinum was electrodeposited from 10ꢂ2
M
0.5 M H SO solution and in the presence
M HPAs at 0.25 V.
type heteropoly tungstic acids [22,23]. Consequently, the
activity of the resulting system should be sensitive to the
mutual location of tungstate and platinum nanosize
fragments in the mixed materials, and the latter feature
is expected to be sensitive to the choice of deposition
H PtCl ꢁ
of 10
/
2
6
ꢂ2
2
4
The solutions were deaerated thoroughly for 30 min
with pure argon. Experiments were carried out at room
temperature.
(
codeposition) mode.
Polyoxotungstate reduction at low pH values pro-
ceeds at potentials being hundreds of millivolts more
To obtain deposition current transients and to
fabricate the deposits, the electrode potential was first
maintained at 0.9 V (which corresponds to the potential
region in which chloroplatinate complex is not reduced,
IV
0
negative in comparison to the Pt /Pt formal potential.
Hence, contrary to polyoxomolybdate acids which
undergo multielectron reduction simultaneously with
0
provided that E values are equal to 0.726 and 0.758 V
II
IV
Pt and Pt complexes [24], a wide region of potentials
is available for the Pt cathodic deposition in the absence
of any parallel tungstate reduction. The purpose of this
work is to address the most general kinetic peculiarities
of Pt electrodeposition from polytungstate-containing
media in the absence of parallel reduction of the HPA
additive. We have also considered the effect of tungstate
adsorption in the course of deposition by using tradi-
tional approaches to the analysis of nucleation growth
phenomena in terms of current transients. Finally, we
compared deposit’s roughness as estimated from con-
ventional cyclic voltammetry (CV) with the structural
features imaged by means of ex situ scanning tunneling
microscopy (STM).
IV
II
II
0
for Pt /Pt and Pt /Pt , respectively [26]) for 1 min and
then switched to the electrodeposition potential of 0.25
V. In the preparative experiments on Pt supports, the
deposition charge was controlled and the process was
ꢂ2
interrupted after attaining 13 C cm . This charge
formally corresponded to the deposition of 3 mm of Pt
layers under assumption of the 100% current efficiency.
To check the real current efficiency, the supports were
weighted before and after deposition with the accuracy
of 0.2 mg.
To study H-upd and O-upd on resulting platinized
platinum samples, the potential was scanned in the
range 0.05ꢀ1.3 V in supporting electrolyte solution.
/
Prior to these measurements, the deposits were thor-
oughly washed by water. The real surface area S was
r
calculated from H-upd charge by normalizing it to 210
ꢂ2
2
. Experimental
mC cm value. As some difference of anodic and
cathodic H-upd charges was observed, we calculated
HPAs (H PW O (PW ), H SiW O (SiW12),
3
12 40
12
4
12 40
two S values (denoted below as ‘an’ and ‘cat’) for each
r
H BW O (BW )) and H SO were for analysis grade
4
5
12 40
12
2
deposit. The exact S value finds itself within the interval
r
quality (Merck). All solutions were prepared from
ultrapure water (Millipore). High-purity grade
limited by these values.
Both CV and current transients were measured with
BAS Model 100B Electrochemical Analyzer and EG&G
PAR Model 173 Potentiostat/Galvanostat equipped
with a Model 175 Universal Programmer and Model
179 Digital Coulometer.
H PtCl ×
/
H O was provided by both Merck and Re-
2
6 2
akhim (Russia).
A three-electrode cell was used for all electrochemical
measurements. Polycrystalline Pt foil (apparent area: 1
2
cm ) and a commercial GC electrode (Mineral, Warsaw,
Polarographic experiments were carried out on the
dropping mercury electrode, as it is described in detail in
Ref. [8].
Poland) with a diameter of 0.3 cm were used as the
supports. The auxiliary electrode was a Pt wire. During
CV experiments, the potentials were measured with
respect to reversible hydrogen electrode (RHE), consist-
ing of Pt/Pt wire immersed in hydrogen-saturated 0.5 M
H SO and connected with working electrode compart-
ment by Luggin capillary. Saturated calomel electrode
was used as a reference electrode in deposition experi-
ments. All potentials are reported below in RHE scale.
Before each deposition experiment, Pt support was
etched in hot aqua regia, washed with water, and
STM imaging was carried out ex situ with the use of
Litscan-2 device [27] with bias voltage of 0.2ꢀ
typical tunneling currents up to 0.2ꢀ0.3 nA. To obtain
representative images, we scanned 2ꢀ3 samples of each
type, approaching 3ꢀ5 macroscopic regions of each
sample (total size of samples was ca. 5ꢃ
each tip approach, about 10ꢀ20 images were registered,
/0.3 V and
/
2
4
/
/
2
/
10 mm ). For
/
both in mm- and nm-size scales. The sign of tunneling
current and scan direction did not affect the images. The
smallest size of particle which we were able to visualize
at available resolution was ca. 2 nm.
subsequently polarized by anodic and cathodic current
ꢂ2
(
electrode was thoroughly polished prior to each deposi-
current density: 20 mA cm ) in 0.5 M H SO . GC
2 4

M.I. Borzenko et al. / Electrochimica Acta 48 (2003) 3797ꢀ
/3804
3799
3
. Results and discussion
approximated by two straight lines with non-zero
intersects corresponding to the points appearing at low
t (Fig. 1b). As before [29], the change of the slope can be
ꢂ
3
.1. Current transients
explained by inhibitory effect of Cl anions generated
in the course of chloroplatinate reduction. This inhibi-
tory effect manifests itself at longer times in HPA-
containing solutions as compared with conventional
platinization electrolyte.
The choice of electrodeposition potential was based
on the previously reported data concerning platinization
from H PtCl -based electrolytes [26,28,29]. The deposits
obtained earlier at 0.25 V exhibited fairly high current
2
6
The transients obtained on Pt are rather featureless
due to the presence of a large number of nucleation
centers. Typical nucleation maxima, if any, are expected
to appear at millisecond-range time and they super-
impose a region of a sharp current decrease induced by
double-layer charging. It appeared to be possible to
obtain reliable non-monotonic transients for a slower
nucleation process taking place on GC (the example
involving PW12 is given in Fig. 2a). In the latter case, by
analogy to HPA-free solutions [25], the more detailed
analysis of nucleation kinetics can be carried out.
The current maxima preceding plateau currents
IV
0
efficiency for the Pt /Pt process (up to 90%), high
stability, and their cyclic voltammetric (CV) responses
were typical for conventional polycrystalline Pt. The
latter observation can be interpreted in terms of the
absence of any specific defectiveness, which arises when
one is depositing Pt at more cathodic potentials or, on
the contrary, at very low overvoltages. Additional
forcible argument involved the possibility of avoiding
the reduction of tungstate because, for even the most
easily reducible PW , the polyoxometallate reduction
1
2
starts only at potentials more negative than 0.25 V
0
VI
VI
V
(
E ꢄ0.15 V (SHE) for PW /PW W [30]). Electro-
/
12 11
appeared on i ꢀ/t curves recorded on GC in all deposi-
reduction of SiW12 and BW12 starts at even more
negative potentials [30].
tion solutions under study following tꢄ
/
8ꢀ20 s (Fig. 2a).
/
Their positions were affected by both reagent concen-
tration in the absence of additive (curves 1 and 2 in Fig.
The transients measured on Pt foil in all deposition
solutions (Fig. 1a) exhibit sharp current growth after
application of the deposition potential followed by
current decrease, which is more and more monotonous
with time. Transients flatten at sufficiently long times
2
a) and by the presence of HPA (curves 1? and 2? in Fig.
(
another in solutions containing HPA additives and seem
ꢀ30 s). The initial current values are very close to one
/
to be slightly higher for PW and SiW12 and lower for
1
2
ꢂ2
BW12 than those in the 10 M H PtCl ꢁ0.5 M H SO
/
2
6
2 4
solution studied here for comparison (curve 1 in Fig.
a). The transients in the presence of HPAs most likely
1
reflect the sum of several current transients correspond-
ing to various nucleation processes, and this fact can be
explained in terms of Pt nucleation at the centers of
various activities (or with various induction periods).
The transients replotted as i vs. tꢂ1/2 (i.e. according to
the model of diffusion to the planar electrode surface
rather realistic for supports under study) might be
Fig. 2. (a) Current transients for Pt deposition on GC at 0.25 V from
ꢂ3
ꢂ2
0
.5 M H
2
SO
and also (1?, 2?) 10 M H
) vs. t/t plots for progressive (1) and instantaneous (2) nucleation
as compared with the transients of Pt deposition on GC from 10 M
4
solutions containing (1, 1?) 10
M and (2, 2?) 10
M
ꢂ2
H
2
PtCl
6
3
PW12O40; (b) non-dimensional (I/
2
Fig. 1. (a) Current transients for Pt deposition at 0.25 V at Pt foil from
I
m
m
ꢂ2
ꢂ3
solutions: (1) 10 M H
2
PtCl
6
ꢁ
O
/
0.5 M H
40 in the presence of 10 M HPAs; (b)
transient responses for Pt deposition at 0.25
2
SO
4
, and (2) H
3
ꢂ2
PW12
O
40, (3)
H
4
SiW12
the analysis of i vs. t
V at Pt foil from solutions: (1) 10 M H
O
40, and (4) H
5
BW12
H
2
PtCl
6
ꢁ
/
0.5 M H
2
SO
4
solutions without additive (3, 4) and in the
PW12 40 (5); curve corresponds to
PW12 40-containing solution
under open circuit with subsequent washing.
ꢂ1/2
ꢂ2
presence of 10
M
H
3
O
4
ꢂ2
2
PtCl
6
ꢁ
/
0.5 M H
2
SO
4
and (2)
preliminary treatment of GC with H
3
O
ꢂ2
the same in the presence of 10
M H
3
40
PW12O .

3800
M.I. Borzenko et al. / Electrochimica Acta 48 (2003) 3797ꢀ3804
/
2
a). Most pronounced difference was observed in the
estimated according to
vicinity of maxima, manifesting the preferential HPA
effect on primary nucleation. It should be mentioned
that deposition current in the absence of HPA is not
exactly proportional to reagent concentration, and the
shift of maximum with concentration does not follow
usual tendencies, as it shifts towards higher time value.
This behavior demonstrates self-inhibition of reaction in
more concentrated solution.
2
2
i t ꢄ0:2598D(zFc)
(2)
m m
For 10ꢂ2 M H
PtCl
2
solution, the calculated value of
2
6
ꢂ1
D (4.25ꢃ
/
10ꢂ6 cm s ) from Eq. (1) is in satisfactory
ꢂ6
2
ꢂ1
agreement with the value Dꢄ s
reported earlier [33], but it is slightly higher than the
/
4.5ꢃ
/
10
cm
effective D value obtained for the deposition on HOPG
ꢂ3
ꢂ6
2
ꢂ1
(3.74ꢃ
/10
cm
s
[32]). For 10
M H PtCl
2 6
In accordance with the theoretical models of the mass
transport controlled nucleation and growth of hemi-
spherical clusters, we carried out the analysis of current
transients plotted in non-dimensional coordinates (I/
solution the choice of Eq. (1) or Eq. (2) for D
calculation looks ambiguous because of pronounced
deviations of the transient from both theoretical curves.
Under rough approximation we calculated formal D
values using both Eqs. (1) and (2) and found that in both
2
I ) vs. t/t (Fig. 2b), where tm is the time at current
m
m
ꢂ5
ꢂ5
maximum I , in order to recognize qualitatively the
m
cases these values are rather high (1ꢃ
/
10
ꢀ
/
1.5ꢃ10
/
2
cm s ). The same very high D value calculated for the
ꢂ1
progressive or instantaneous nucleation (corresponding
theoretical curves (solid) are numbered 1 and 2,
respectively) [31]. In the absence of HPA (curve 3 in
progressive nucleation (Eq. (2)) corresponds to deposi-
ꢂ3
tion from 10 M H PtCl solution with PW12 additive
2 6
ꢂ3
ꢂ5
2
ꢂ1
ꢂ2
Fig. 2), the reduced transient for more diluted 10
M
H PtCl solution finds itself between theoretical curves
(1.6ꢃ
solution a reasonable value of 4.7ꢃ
obtained.
Close D values for solutions with and without
additive of one and the same H PtCl concentration
/
10 cm s ), when for 10 M H
2
PtCl
6
ꢁ
/
PW12
2
ꢂ1
/
10ꢂ6 cm s
is
2
6
for progressive and instantaneous nucleation (curves 1
and 2 in Fig. 2b) in the region of deposition times
shorter than 50 s; further deviations can be explained by
2
6
secondary nucleation [25]. The reduced transient re-
ꢂ2
allow us to conclude that the degree of conformity in
respect to a simple models of mass transport controlled
nucleation is not affected by the additive. For more
concentrated solution, this degree of conformity is
evidently higher than for more diluted solutions, as for
the latter the D values are overestimated. However,
under these circumstances qualitative comparison of
curves with and without additive looks correct, even if
the real mechanism is more complex and probably
corresponds to comparable rates of nucleation and
growth or to the mixed control [34]. Detailed elabora-
tion of this mechanism is beyond the statement of this
study. What we demonstrate here is the qualitative HPA
effect, namely the fact that the presence of HPAs in the
deposition solutions results in higher heterogeneity of
nucleation centers. In the limiting case it results in the
transfer from instantaneous to progressive nucleation,
but in general case it is more correct to state the decrease
of nucleation rate at certain centers induced by adsorbed
tungstate.
corded in 10
model curve for instantaneous nucleation in the same
M H PtCl solution coincides with the
2 6
time interval. Similar instantaneous behavior was ob-
ꢂ2
served for Pt nucleation on HOPG from 10
H PtCl solution [32].
M
2
6
After preliminary contact of GC with HPA-contain-
ing solution under open-circuit conditions (i.e. before
the actual measurements) the reduced transient looks
much closer to progressive behavior (curve 4 in Fig. 2b).
The nucleation in the presence of PW12 (curve 5 in Fig.
2
progressive nucleation in a relatively wide time interval.
b) follows most precisely the theoretical transient for
ꢂ3
ꢂ2
These observations for both 10 and 10 M H PtCl
2
6
solutions are in agreement with the general idea of the
existence of various types of active centers on HPA-
modified surface. Deviations of transients observed at
longer deposition times are of the same type as in HPA-
free solution but they start at higher times. It can be
concluded that the secondary nucleation is slightly
slower in the presence of HPAs.
Let us stress that close shape of curves 4 and 5 which
both differ from curve 3 in HPA-free solution confirms
strong bonding of adsorbed HPA and impossibility to
delete it by washing the electrode under open circuit.
For instantaneous nucleation, the diffusion coefficient
3.2. Current efficiency
When the Pt electrodeposition was performed in
presence of HPAs additives, the mass (m) of deposit
corresponding to constant deposition charge appeared
to be lower in comparison to the situation where
deposition was achieved from tungstate-free solution.
In both cases, the electric charges consumed were equal
2
ꢂ1
D (cm s ) can be estimated [32] from the following
equation:
2
2
i t ꢄ0:1629D(zFc)
(1)
m m
(Table 1). The current efficiency values (w) somewhat
differ for Pt/Pt electrodes that were deposited from
where zꢄ
/
4, F is Faraday’s constant (96 500 C molꢂ1),
ꢂ3
2ꢂ
and c (in mol cm ) is the PtCl6 concentration in
solution. When progressive nucleation occurs, D can be
H PtCl solutions with PW and SiW12 additives. It
2 6
12
was found that w was higher in the case of deposition

M.I. Borzenko et al. / Electrochimica Acta 48 (2003) 3797ꢀ
/3804
3801
Table 1
Survey of data related to Pt deposition from 10 M H
measurements in 0.5 M H
ꢂ2
with and without 10ꢂ2 M HPAs additives (Qꢄ
13 C cmꢂ2) and to CV
2
PtCl
6
ꢁ
/
0.5 M H
2
SO
4
/
2 4
SO
2
(cm )
S
m
(M gꢂ1)
2
Deposition solution
m (mg)
S
r
R
w (%)
H
2
H
2
H
2
H
2
H
2
H
2
H
2
H
2
PtCl
PtCl
PtCl
PtCl
PtCl
PtCl
PtCl
PtCl
6
6
6
6
6
6
6
6
(an)
(cat)
5.3
5.3
4.0
4.0
3.9
3.9
2.0
2.0
690
622
645
581
640
581
180
252
690
622
645
581
640
581
180
252
80
80
61
61
59
59
30
30
13.0
11.7
16.1
14.3
16.4
14.9
9
ꢁPW12 (an)
/
ꢁ
/
PW12 (cat)
SiW12 (an)
SiW12 (cat)
BW12 (an)
BW12 (cat)
ꢁ
/
ꢁ
/
ꢁ
/
ꢁ/
12.6
from HPAs-free solution and it was lower for BW -
1
To verify indirectly the hypothesis about the more
2
II
pronounced inhibition of the Pt /Pt reduction in
0
containing solution (Table 1).
IV
II
Consumption of charge during any Faradaic reaction
with HPAs participation cannot be a reason for such a
strong w decrease because the reduction of HPAs from
the solution bulk at 0.25 V is impossible. Some redox
processes involving adsorption of HPAs [19] cannot give
so strong contribution to w decrease, at least for PW12
and SiW12 which are forming submonolayer adsorbates.
A possible explanation of w decrease may take into
comparison to the Pt /Pt preceding process by ad-
sorbed HPAs, we have applied polarography utilizing a
dropping mercury electrode. The advantage of this
electrode is the stable state of its surface during reaction
and the possibility of overcoming mass transport
limitations in a certain potential region. As it has been
2
ꢂ
already demonstrated [8], the PtCl
strongly inhibited by tungstates at positive free electrode
reduction is
4
II
account an additional accumulation of Pt intermediate
in the reaction layer in the presence of HPA.
charges. This inhibition appears during electroreduction
2
ꢂ
ꢂ
III
of many anions (e.g. S O , BrO , Co (EDTA) ).
ꢂ
2
8
4
2
ꢂ
2
6
ꢂ
The results obtained now for PtCl
6
imply some
The electroreduction of PtCl
to metallic Pt is
similarities between the reactions because the polaro-
graphic maximum is also suppressed and the currents at
positive charges are also lower than the limiting diffu-
sion current. However, the current growth that accom-
panies polytungstate charge-induced desorption (with
the potential shift towards more negative values) has
usually considered under the first approximation as a
two-step reaction [35,36] having comparable redox
potentials for two subsequent two-electron steps [26].
The observed splitting of the thermodynamically pre-
dicted united four-electron reduction wave has kinetic
origin ([29] and references therein). One of the widely
accepted explanations involves the inhibition of the first
and/or the second two-electron steps by Clꢂ anion (in
particular, by negligible amount of chloride generated in
the course of reduction of platinum complexes). It is also
known that the sensitivity of the second step to the
chloride adsorption is more pronounced in comparison
to the first one, thus resulting in the accumulation of
2
ꢂ
been found to be more pronounced for PtCl
ꢂ
rather
which is in agreement with our
6
2
4
than for PtCl
hypothesis about the stronger inhibition of the second
step at a given tungstate surface coverage. This result
cannot be extended automatically and quantitatively to
solid electrodes and to lower overvoltages, but it seems
to be at least qualitatively consistent with the above-
mentioned hypothesis. Further experiments are in pro-
gress and they should lead to final conclusions.
2
ꢂ
PtCl4 in solution and in corresponding decrease of the
current efficiency [29]. In the tungstate-free solutions,
the latter effect is potential-dependent and the rates of
IV
II
II
0
Pt /Pt and Pt /Pt reactions become comparable as
the overvoltage increases. Consequently, the inhibitory
3.3. Roughness of deposited platinum
ꢂ
ability of Cl anions at 0.25 V is not very high.
One can assume that adsorbed HPAs can act like
chlorides but in a wider potential range, since the
specific and strong adsorption of various polyoxome-
tallates on various electrode materials has been demon-
All deposits considered here were characterized by
similar H-upd and O-upd voltammetric responses,
which were separated by smooth double-layer region
(Fig. 3). For all deposits studied, no additional peaks
appeared in comparison to platinum deposited from the
HPA-free solution. For PW12 undergoing one-electron
reduction in the H-upd region, the result implies the
absence of the dissolved additive in the bulk of deposit
(for example, in the pores). In the case of the samples
deposited from BW -containing solutions, the anodic
strated [1ꢀ
/
18]. The adsorbed HPAs can form well-
41] and some
of them even have a tendency to undergo multilayer
ordered self-assembled monolayers [18,37ꢀ
/
adsorption (for example, the number of layers reaches
6
charges [7]).
ꢀ
/
7 on Hg for some systems at high positive surface
1
2

3802
M.I. Borzenko et al. / Electrochimica Acta 48 (2003) 3797ꢀ3804
/
feasible. Charge used for the formation of such films
may be responsible for the abnormally low current
efficiency observed (Table 1). If it is operative, the true
Pt mass is expected to be lower in comparison to those
given in Table 1.
It should be stressed that all above-mentioned CV
data refer to stable deposit structures. The constant real
surface area observed in 10 subsequent potentiodynamic
cycles was taken as a criterion of stationarity. It took ca.
1
h to reach this state in the course of potentiodynamic
aging. Characterization of the aging processes of
deposits shall provide additional information on the
deposit structure and stability.
Potentiodynamic aging in the potential region from
.05 to 1.3 V resulted in the decrease of S for all
0
r
deposits under study. It was observed that Pt/Pt
electrodes deposited from HPA-free solutions were
slightly more stable against aging (the decrease of Sr
was ca. 12%, whereas it was ca. 14% for deposits
obtained in the presence of HPAs additive). On the
other hand, this difference does not seem to be
significant enough to make any definite conclusion.
Further, there are no special non-equilibrium features of
HPA-affected deposits that induce their instability.
The aging affects the shape of H-upd response. To
illustrate these results, we consider the h /h ratio (where
Fig. 3. Cyclic voltammograms recorded at 0.01 V sꢂ1 in the solution
of 0.5 M H SO on Pt/Pt deposited at 0.25 V (Qdep
13 C cmꢂ2) from
M H PtCl 0.5 M H SO , and (b) H PW12 40, (c)
2
4
ꢄ/
ꢂ2
(a) 10
2
6
ꢁ/
2
4
3
ꢂ2
O
H
4
SiW12O40, and (d) H BW12O40 in the presence of 10 M HPAs.
5
and cathodic charges were observed to be non-equiva-
lent (the latter being higher).
2
1
h and h are the heights of H-upd current peaks related
2
1
The real surface area of the deposits (S ) was
r
to strongly and weakly bound H adatoms, respectively).
The h /h value of 0.9190.01 was obtained for freshly
estimated from H-upd charges. Current was integrated
/
2
1
in the potential range 0.05ꢀ0.45 V and corrected for
/
deposited Pt/Pt and it exhibited no dependence on the
presence or absence of HPAs. During the aging process,
the parameter decreased and reached the values of 0.7
for Pt deposited from HPA-free solution (the result is
consistent with the ratio for smooth Pt [29]) and 0.8 for
the solutions with HPAs additives. Having in mind the
earlier study [29], in which interpretation based on the
preferential surface crystallographic orientation was
provided, the present data imply the possibility of
some specific structural features induced by tungstates.
double-layer charging by means of linear extrapolation.
The values of S are listed in Table 1. There are some
r
differences in the values obtained from anodic and
cathodic portions of H-upd responses. For BW12
cathodic charge is higher than corresponding anodic
value, when for all other deposits the anodic charge is
slightly higher. To clarify the reasons of these differ-
ences, the equilibrium charge values are necessary which
can be obtained in future from more slow measure-
ments. At this stage we limit ourselves by rough
comparison of S values for various deposits based on
r
3.4. STM measurements
the difference of only anodic or only cathodic H-upd
charges. A general qualitative tendency can be formu-
In order to get the direct information about the
surface morphology of electrodeposits under study, we
compared ex situ STM images of three types of aged
samples (Fig. 4): Pt/Pt (images a, d) and Pt deposited
from PW - and BW -containing solutions (images b, e
and c, f, respectively). Typical STM images of submi-
cron regions (see Section 2 for statistical details)
demonstrate the presence of globules consisting of nm-
lated as follows: the additives of PW12 and SiW12
increase the specific areas of deposits, whereas the
BW12 additive probably slightly decreases it in compar-
ison to the area of a usual deposit. The latter follows
only from comparison of anodic H-upd charges for pure
Pt/Pt and Pt deposited from BW -containing medium.
1
2
12
1
2
However, there are no doubts that BW12 is less effective
for Pt roughening than PW12 and SiW12 (compare both
anodic and cathodic values in Table 1).
It is noteworthy that the formation of rechargeable
tungstate multilayers on Pt surface cannot be excluded
size crystals (Fig. 4aꢀc). For Pt/Pt the globules are more
/
smooth and homogeneously distributed along the sur-
face; the difference of deposit height in the vicinity of
any globule does not exceed 100 nm and is comparable
[19]. Correspondingly, for this type of samples, the
presence of solid polytungstate in the deposit bulk is
with the linear size of globule (typically 80ꢀ
This fact means that the globules inside the deposit bulk
/
120 nm).

M.I. Borzenko et al. / Electrochimica Acta 48 (2003) 3797ꢀ
/3804
3803
ꢂ2
Fig. 4. STM images of aged Pt/Pt deposited from (a, d) 10 M H
2
2
PtCl
6
ꢁ
/
0.5 M H
2
2
SO
4
, and (b, e) H
3
PW12
O
40 and (c, f) H
5
BW12
O40 in the presence
ꢂ2
of 10
M HPAs; image scales: (aꢀ
/
c) 240ꢃ
/
240 nm and (dꢀ
/
f) 20ꢃ20 nm .
/
are close-packed. Note that Pt/Pt images treated in Ref.
29] were obtained for slightly thinner deposits in which
Decrease of particle size and more pronounced
[
globular growth with ordered location of crystals in
the globules are the typical features of quasi-template
deposition [42]. Qualitatively, all these features can be
explained by adsorption of template molecule (HPA in
the systems under discussion) on the support and newly
formed nm-scale crystals. Adsorption-induced decrease
in the number and/or activity of primary nucleation
centers result in the start of secondary nucleation at
lower surface coverage by the primary nuclei. As the
latter appear to be more rarely distributed along the
surface, the secondary nucleation growth takes place not
only on top of the primary crystals but also at other
positions, and separated globules start to form instead
of iterative parallel layers. At a certain moment the
adsorbate is also formed on already deposited crystals,
affecting the growth rate and, probably, growth direc-
tion. As the size of HPA molecule is comparable with
the size of nuclei, this effect results in above-mentioned
structural difference not only for first and second but
also for subsequent layers.
globules were rarely imaged because of more smooth
morphology.
Images of samples deposited from PW - (Fig. 4b)
1
2
and BW12 (Fig. 4c)-containing solutions demonstrate
more pronounced surface heterogeneity. HPA-affected
deposits contain more separated globules with the
average diameter of 50ꢀ120 nm. In general, these
/
aglomerates are chaotically located; however, there are
the regions in which the globules look ordered, espe-
cially for BW -affected deposits (Fig. 4c). Differences
1
2
of deposit height in the vicinity of globule are in general
higher than for Pt/Pt and sometimes reach 350ꢀ400 nm.
/
There are, however, some closely packed globules which
look coalesced in BW -affected deposits.
1
2
The smallest imaged fragments of deposits are repre-
sented by nm-size particles (crystals) of elliptical shape,
with average diameter ranging from 7 to 9 nm for Pt/Pt
(
Fig. 4d). This observation is in agreement with the data
29] for aged Pt/Pt. Similarly, aged deposits prepared in
PW -containing solutions also consist of elliptical
[
By analogy with Pt deposition of Pt [29] and GC [25],
the lower concentration of primary active centers and
corresponding preference of secondary nucleation lead
to formation of smaller crystals and simultaneously
increase the degree of crystal coalescence inside globules.
The first feature should increase the specific surface
area, when the second one is expected to decrease it. The
interplay of both factors can finally lead to higher or
1
2
particles (Fig. 4e). However, their elongation is less
pronounced and their average diameter is noticeably
smaller (5ꢀ
containing medium the majority of particles look
hemispherical, with diameter of ca. 4ꢀ5 nm (Fig. 4f).
/
7 nm). For deposit prepared in BW -
12
/
The smaller the particles in the globules, the higher
pronounced is the order inside the globule. For BW -
1
2
affected deposits, this order is most pronounced, as the
globules consist of linear rows of particles.
lower S , so that STM data explain the effects discussed
r
in the previous section.

3804
M.I. Borzenko et al. / Electrochimica Acta 48 (2003) 3797ꢀ3804
/
[
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8] M.I. Borzenko, G.A. Tsirlina, O.A. Petrii, Mendeleev Commun.
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Polytungstate-assisted potentiostatic electrodeposi-
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containing HPAs additives. Structural features result
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governed by the number of corresponding nuclear
centers. This effect results in more pronounced globular
structure of deposits and smaller size of crystals. The
degree of crystal coalescence depends on the additive
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strong adsorption of HPA on Pt nucleation centers is
expected. This work has much in common with recent
concepts of Pt codeposition with oxometallates and
related species under galvanostatic and potentiodynamic
modes. Applications in the field of electrocatalysis and
analytical sensing can be envisioned. For example, it has
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NATO Linkage Collaborative Research Grant PST/
CLG 979085 is also acknowledged. P.J.K. and M.C.
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