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Platinum electrodeposition on graphite: electrochemicalstudy and
STM imaging
F. Gloaguenb, *, J.M. Leger b, C. Lamyb, A. Marmanna, U.
Stimminga, 1,R. Vogel a
aInstitut fur Energieverfahrenstechnik (IEV), Forschungszentrum
Julich GmbH (KFA), 52425 Julich, GermanybEquipe Electrocatalyse,
Universite de Poitiers, UMR CNRS 6503, 40 Avenue du Recteur Pineau,
86022 Poitiers, France
Received 8 May 1998; received in revised form 15 July 1998
Abstract
The electrodeposition of Pt on a thermally oxidized HOPG surface
was performed by single potentialperturbation in dilute
chloroplatinic acid solutions. The quantitative analysis of the
current versus time transientresponses, on the time scale of
seconds, indicated a low saturation density of nuclei of 2 106 cm2.
Thecharacterization by STM revealed a heterogeneous distribution of
deposited Pt on the substrate surface. Most of the
deposits were composed of agglomerates of spherical
nanoparticles. Local particle densities exceeding 1010 cm2
were observed, which is several orders of magnitude higher than
the saturation coverage of nuclei evaluated fromthe analysis of the
transient at t>0.3 s. From the electrochemical and the
structural investigations, it appears that
Pt electrodeposition on graphite involves several faradaic steps
on the time scale. Firstly, at t< 0.3 s, a largenumber of
nanosized clusters are possibly formed. In the course of the
electrodeposition process, these very smallclusters may be mobile
and assemble in an accidental distribution of Pt agglomerates.
Secondly, at t>0.3 s, it is
very likely that the more rapid growth of a small number (2 106
cm2) of Pt particles gives a transient response,which is adequately
described by the model of Scharifker and Hills for a diusion
controlled growth. As a result, itseems that cluster diusion
contributes significantly to the structural evolution of platinum
electrodeposits ongraphite. # 1999 Elsevier Science Ltd. All rights
reserved.
Keywords: Pt electrodeposition; Oxidized HOPG surface;
Nucleation process; Scanning tunneling microscopy
1. Introduction
Carbon-supported platinum particles are widely used
as electrocatalysts, e.g. for fuel oxidation and oxygen
reduction in low-temperature fuel cells. Considerable
eorts were made in order to correlate the catalytic ac-
tivity of such electrodes with their morphology, usually
by characterizing the catalyst particles within rather
complex composite electrodes. The conclusions derived
from such investigations remained inconsistent [1, 2],
which may be to some extent due to the structural
complexity of the investigated systems. Recent
advances in the understanding of the catalytic proper-
ties of nm-scale particles were derived from investi-
gations of model electrodes with reduced structural
complexity [35]. These advances showed clearly that
the degree of structural definition of model electrodes
is a crucial point regarding the correlation of structure
and catalytic properties.
It has been shown that well-defined distributions of
nm-scale silver clusters can be prepared on graphite
substrates by electrochemical deposition [6, 7]. It has
been recently pointed out [8] that narrow particle size
distribution on graphite substrates can be obtained, by
Electrochimica Acta 44 (1999) 18051816
0013-4686/99/$ - see front matter # 1999 Elsevier Science Ltd.
All rights reserved.PII: S0013-4686(98 )00332-6
PERGAMON
* Corresponding author. Fax: +33-549-453-580; E-mail:
[email protected] Present address: TU Munchen,
Physik Departement E19,
85748 Garching, Germany.
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a short time potential pulse (t< 100 ms); the metalloading of
the resulting electrodes was however very
low: equivalent to less than 0.1 monolayer of Pt. Weinvestigated
the possibilities to prepare electrocatalyticmodel electrodes, with
a metal loading equivalent to
1.0 monolayer of Pt, at least, by potential pulse overlarger
times (t>1.0 s). The surface morphology wascharacterized with
scanning tunneling microscopy
(STM) and cyclic voltammetry (CV), i.e. by hydrogenadsorption
coulometry.The influence of deposition parameters on the sur-
face morphology can, in principle, be derived frommonitoring the
metal deposition reaction in-situ withan electrochemical STM. An
influence of the STM tipon the local kinetics of galvanic reactions
has, how-
ever, to be anticipated [912]. Since for platinum aswell as for
palladium deposition on graphite substratesthe tip influence has
been found to be strongly
inhibitive [1315], this route was not considered for thepresent
investigation. Instead, platinum deposits wereprepared in a
conventional electrochemical cell and the
deposit morphology was investigated afterwards underex-situ
conditions.The electrodeposition of Pt on highly oriented pyro-
lytic graphite (HOPG) at small overpotentials has beenstudied in
detail [16]. The authors concluded that theformation of platinum
nuclei occurs at defects on thegraphite surface. Similar results
were obtained for Pd
deposition on HOPG [15]. A comparative structuralex-situ
characterization of Ag-clusters on HOPG withSTM and non-contact
atomic force microscopy (NC-
AFM) [6] indicated, however, that electrodepositedclusters on
the HOPG basal plane may escape detec-tion with an STM and can only
be characterized with
NC-AFM. Nevertheless, a preferential deposition atsubstrate
defects was also supported by the lattertechnique [7].It was
reported that the defect density of HOPG
surfaces can be increased by thermal oxidation [17]and such
defective HOPG surfaces were used as sub-strates for the
investigation of lead electro-
deposition [10]. Similar graphite surfaces with defectdensities
about 2 orders of magnitude higher than thatof a freshly cleaved
HOPG surface were used as sub-
strates for platinum electrodeposition at a high over-potential.
The prepared deposits were characterizedwith electrochemical
techniques and scanning tunneling
microscopy.
2. Experimental
Square shaped HOPG samples of 1 cm2 geometricarea were used as
substrates. The surfaces were pre-pared by cleaving with an
adhesive tape. Prior to ex-
periments, the freshly cleaved surface was treated with
a Bunsen flame for 2 s.
HClO4 (Suprapur Merck) and H2PtCl6 (Alfa) were
used as received. The solutions were prepared with
ultra pure water (Milli-Q system). Electrochemical
equipment included a potentiostat (PAR 362, EG&G),
a waveform generator (PAR 175, EG&G), a numerical
oscilloscope (Nicolet 2090-III) and an XY recorder
(HP).
The electrochemical experiments were carried out at
room temperature in a standard three-compartment
glass cell. The contact between the graphite working
electrode (WE) and the electrolyte was performed by
the hanging meniscus technique. The counter electrode
(CE) was a Pt wire. The CE and WE compartments
were separated by a glass frit. A reversible hydrogen
electrode (RHE), consisting of a Pt wire immersed in
H2-saturated 0.1 M HClO4, was connected to the WE
compartment by a Luggin capillary. All potentials are
quoted to the RHE scale.
Before each deposition experiment, several voltam-
mograms (0.05 to 1.2 V at 20 mV s1) were recordedin Ar purged
0.1 M HClO4, both to ensure that the
HOPG surface is free of Pt and to control the reprodu-
cibility of the surface area of the electrode. The plati-
num electrodeposition was performed by a potential
step in Ar purged and unstirred 0.1 M HClO4+x mM
H2PtCl6 (x= 0.2, 2, 10). The potential perturbation
(0.75 to 0.1 V) was applied immediately (i.e. at most
30 s after that the graphite surface was exposed to the
platinum plating solution) or after over-night contact
with H2PtCl6 solution (the so-called pre-impregnation
process). It has been recently pointed out [8], that
platinum particles are spontaneously formed at
freshly cleaved HOPG surface exposed to solutions
containing PtCl62 ions; electroless deposition of Pt is
however dramatically reduced on an oxidized graphite
surface [8], as used in this work. The amount of depos-
ited platinum, i.e. the loading W (mg cm2), was esti-mated from
the integrated charge measured on the It
transient response. After deposition, the electrodes
were removed from the deposition solution, thoroughly
rinsed with ultra pure water and transferred to a simi-
lar cell containing pure 0.1 M HClO4. The real surface
area, Ar (cm2), of deposited platinum was estimated
using the H adsorptiondesorption coulometry. All
current and charge densities are quoted in terms of mAcm2 and mC
cm2 of apparent geometric area, Ag, ofthe substrate electrode.
The rinsed sample was finally transferred to the
STM chamber. The structural characterization was
performed either with a Burleigh Instructional STM in
air, or with a modified Delta Phi STM in 0.1 M
HClO4.
F. Gloaguen et al. / Electrochimica Acta 44 (1998)
180518161806
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3. Results
3.1. Substrate characterization
STM images of dierently pretreated HOPG sur-
faces are presented in Fig. 1. Fig. 1(a) represents thefreshly
cleaved surface. Typically 1 monoatomic step isfound per m2. Fig.
1(b) to (d) show HOPG surfaces
after various times of oxidation in a Bunsen burnerflame. At
very short oxidation time (
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the oxidation process itself; the goal being to
obtainreproducible graphite surfaces.
Cyclic voltammetry in 0.1 M HClO4 was applied tomonitor the eect
of the surface oxidation on thedouble layer characteristics of the
HOPG electrodes.
An increase of the double layer capacity from 12 to 18F cm2,
estimated from linear sweep voltammetry at0.1 V s1, was observed to
result from the flame treat-ment, but the voltammetric curve
remained as feature-less as for a freshly pealed surface. This
indicates thatno electroactive surface groups are formed by the
oxi-
dation process.
3.2. Detailed analysis of the I versus t transient
responses
Typical current versus time transient responses
recorded for a potential step from the open circuitpotential
(0.75 V) to 0.1 V are shown in Fig. 2. Thedeposition was performed
from Ar purged and
unstirred 0.1 M HClO4+x mM H2PtCl6 (x= 2, 10).In both cases, the
shapes of the curves are rather
similar. At very short time (t< 0.3 s), a sharp peak
isobserved. In the succeeding part of the transient, the
current increases with time and goes through a maxi-mum value,
IM, at the time tM. After the fall of thecurrent, a plateau is
finally reached.
The quantity of electricity corresponding to theinitial portion
of the transient is about 900 mC cm2,which is roughly equivalent to
the charge required for
the deposition of 1.0 monolayer of Pt from Pt(IV)ions. This does
not necessarily mean that one mono-layer of Pt is actually formed.
This charge is however
far in excess of that required to recharge the doublelayer of
the electrode. A similar observation was madein the case of Pt
deposition on glassy carbon [18] froma K2PtCl4 solution.
At t>0.3 s, the transient responses have the shapeexpected
for a 3D nucleation process with diusioncontrol. According to the
theory of Scharifker and
Hills [18], the rise in current corresponds to an increasein the
electroactive area. This increase in area, limitedby spherical
diusion around each nucleus, is due to
(i) an increase in the nucleus size and/or (ii) an increasein
the number of nuclei. The spherical diusion zonesthen overlap and
the mass transfer becomes linear to aplanar surface. This change in
diusion regime leads
to a decrease of the current with time.In multiple nucleation,
two limiting cases have to be
considered: high or low nucleation rate [18]. At a high
nucleation rate, all the nuclei are immediately createdand their
number remains constant during the growthprocess, i.e.
instantaneous nucleation occurs. On the
other hand, at low nucleation rate, new nuclei are con-tinuously
formed during the whole deposition process,i.e. progressive
nucleation occurs. In order to charac-
terize the nucleation process, the transient responses
are plotted in reduced variables [18], (I/IM)2 versus
t/tM (Fig. 3). For a rather dilute solution, i.e. 2 mM
H2PtCl6, a good correlation with the theoretical curve
for progressive nucleation is obtained (Fig. 3a). For a
more concentrated solution, i.e. 10 mM H2PtCl6, the
nucleation seems to become instantaneous (Fig. 3b).
When instantaneous nucleation occurs, the density
of nuclei, N (cm2), is estimated from [18]:
IM 0:6382zFDckN 1=2 1
Fig. 2. Current versus time transient responses recorded at
HOPG electrodes (Ag=1 cm2). The potential is stepped from
0.75 to 0.1 V in Ar purged and unstirred 0.1 M HClO4+2
mM H2PtCl6 (a) and 0.1 M HClO4+10 mM H2PtCl6 (b).
F. Gloaguen et al. / Electrochimica Acta 44 (1998)
180518161808
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where z= 4, F is the Faraday constant (96500 Cmol1), c (mol cm3)
is the PtCl6
2 concentration in sol-ution, D is the diusion coecient and k is
a material
constant. The material constant k for Pt is calculated
from
k 8pcM=r1=2 2where M= 195.1 g mol1 is the molecular mass andr=
21.4 g cm3 is the density of Pt. The diusioncoecient, D (cm2 s1),
is estimated according to
I 2MtM 0:1629DzFc2 3
Since c= 105 mol cm3, IM=2.42 103 A cm2and tM=1.55 s, the
calculation yields N= 1.4 106cm2 and D= 3.74 106 cm2 s1. On the
otherhand, when progressive nucleation occurs, the product
aN0 is calculated by
IM 0:4615zFcD3=4k 0aN01=2 4where the parameter a is the rate
constant of nuclea-tion at steady state and No the density of
active sites.
The new material constant k 0 for Pt is
k 0 4k=3 5while the diusion coecient, D, is now estimated
from
I 2MtM 0:2598DzFc2 6Since c= 2 106 mol cm3, IM=3.34 104 A cm2and
tM=4.75 s, calculations yield aN0=6.87 105 s1cm2 with D= 3.40 106
cm2 s1. The density ofnuclei at saturation, Ns (cm
2), is finally given by:
Ns aN0=2k 0D1=2 1:9 106 cm2 7In both cases, instantaneous and
progressive nuclea-
tion, the calculated values for the diusion coecientare similar
and in good agreement with values reportedin literature, i.e. D=
4.5 106 cm2 s1, measured inthe course of PtCl6
2 reduction on glassy carbon [19].The density of nuclei, N,
calculated in the case of in-stantaneous nucleation (Eq. (1)), is
close to the density
at saturation, Ns, calculated in the case of
progressivenucleation (Eq. (7)), as expected.
3.3. Evaluation of the Pt deposit characteristics from
electrochemical measurements
The charge density, QPt (mC cm2), was calculated
from integration of the current versus time transientresponse.
Assuming that this quantity of charge ismainly due to the Faradaic
reaction
PtCl26 4e 4Pt 6Cl 8the Pt loading, W (mg cm2), is
W QPtM =4F 9where M= 195.1 g mol1 is the atomic weight of Pt.The
contribution of parasitic processes, such as doublelayer charging,
partial reduction of Pt4+ to Pt3+ or
Pt2+ and hydrogen evolution at the deposited Ptparticles, are
considered negligible under the givenexperimental conditions.
The real surface area of the dispersed platinum, Ar(cm2), was
estimated from cyclic voltammetry in Arpurged 0.1 M HClO4 solution.
On the voltammograms
Fig. 3. (I/IM)2 versus t/tM analysis of transient responses
shown in Fig. 2. The solid and dotted lines correspond to
data calculated for progressive and instantaneous
nucleation,
respectively [18].
F. Gloaguen et al. / Electrochimica Acta 44 (1998) 18051816
1809
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(Fig. 4), integration in a potential range from 0.05 to
0.45 V, and correction for the double layer charging,
allow estimation of the quantity of electricity, QH(mC), due to
the hydrogen adsorption or desorption.On the other hand, according
to the reaction
Had H e 10and assuming one Had per Pt surface atom, a
theoreti-
cal quantity of electricity QH0 =210 mC per real cm2 of
Pt is obtained [20]. Accordingly, the Pt surface area,
Ar (cm2), is estimated from
Ar QH=Q0H 11Since both the Pt loading and the Pt surface area
are
available from electrochemical data, the specific cata-
lyst area, S (m2 g1), is finally calculated according to:
S 100Ar=AgW 12The Pt loading W and the surface area Ar are
depen-
dent on PtCl62 concentration as well as on the length
of time, ts (s), during which the potential was stepped.
The results are summarized in Table 1. The code num-
bers appearing in the first row are used to identify the
deposits in mentioned below.
Under the same concentration, length of deposition
time and potential step amplitude (cf. samples #2 and
#4 in Table 1), previous pre-impregnation increases the
loading by a factor 1.5 and the specific area by a factor
2. This has already been observed [19] and may be due
to a slow chemisorption of the PtCl62 anions on the
carbon surface. This can also explain a dierent shape
for the transient response (not shown here).
For a more concentrated solution (cf. sample #6 inTable 1) an
increase in both the loading and the sur-
face area is consistent with a change in the nucleationprocess
from progressive to instantaneous.
3.4. Average particle size and distribution. Comparisonwith STM
measurements
The specific surface area is a usual macroscopicquantity for the
characterization of dispersed catalysts,
however it is not very illustrative for the respective sur-face
morphology. Assuming simple structural modelsand correlating them
with the specific surface areaallows to derive expectations for the
surface mor-
phology, which can be compared to the structuralcharacterization
of the respective electrode surfaces.A maximal density of nuclei,
Ns12 106 cm2, was
derived from evaluation of the electrodeposition transi-ents.
Hence it is a rather straightforward model toassume, that Ns equals
the number of particles on the
electrode. A characteristic interparticle distance around7 m
should then be expected. For loadings from 4.7 to17 mg cm2 the
typical mass of a single particle isbetween 2.4 and 8.5 1012 g for
the dierent samples.Assuming a hemispherical overall shape for the
par-ticles, typical radii in the range 0.37 to 0.58 mm are
cal-culated. The surface area of a single hemispherical
particle is then between 0.9 and 2.1 mm2, leading to1.7 102 to
4.2 102 cm2 total area of the depositwhen the particles are assumed
to be smooth. Since the
measured surface area of the deposit is between 0.6and 2 cm2,
typical roughness factors between 35 and120 are accordingly derived
for the particles.
These considerable roughness factors may be mod-elled by
assuming that the submicrometer scale par-ticles are composed of
spherical nm-scale Pt-clusters.The characteristic cluster diameter,
d (nm), is then
related to the specific area, S (m2 g1), by:
d 6000=Sr 13where r= 21.4 g cm3. The density of Pt particles, N
0
(cm2), is then calculated according to
N 0 1014Ar=Agpd 2 14The values of d and N 0 calculated for the
varioussamples are listed in Table 2. The characteristic
cluster
diameters for the investigated electrodes range from 10to 50
nm.The characterization of the prepared electrodeposits
with STM revealed a heterogeneous distribution of Pton the
substrate surface. Occasional feedback instabil-ities were detected
throughout the measurements. Such
instabilities are generally accepted to result fromadparticles
being swept away by the tip. The relevanceof this tip-sample
interference is discussed later on.
Fig. 4. Voltammograms recorded, at 20 mV s1, in Ar purged0.1 M
HClO4. The Pt loading is 10.8 mg cm
2 (sample #5).The current density is related to the geometric
area.
F. Gloaguen et al. / Electrochimica Acta 44 (1998)
180518161810
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For deposits obtained from 0.2 and 2 mM solutionsextended areas
were observed, which showed thecharacteristic surface morphology of
the oxidized
graphite surface without evidence for the presence
ofelectrodeposited platinum. On the other hand, regionswere
observed where the typical characteristics of the
substrate were absent. These regions were composed
ofagglomerates of spherical particles and are attributedto the
electrodeposited platinum.The agglomerates were rather rough and
attempts to
image them on the scale of several m resulted in irre-versible
changes of the deposit distribution as well asof the tip due to the
imaging. Reproducible imaging
was only possible on top of the agglomerates and on asmaller
scale. Fig. 5 shows examples of such agglomer-ates from dierent
samples. Their presence is consist-
ent with a rather low density of large and roughparticles on the
surface. The agglomerates consist ofhills on the scale of 100 to
200 nm, the hills being com-
posed of 5 to 30 nm diameter clusters. The size ofthese clusters
is in reasonable agreement with the pre-dictions of Eq. (14), which
resulted in characteristiccluster diameters of 12 and 16 nm for the
samples rep-
resented in Fig. 5(a) and (b), respectively. For Fig. 5(c)a
comparison with model expectations is not availablesince the
respective deposition was performed in the
regime of H2-evolution at 1 V, thus estimation of the
Pt loading from the transient charge was impossible. Itis
evident that larger clusters are formed under con-ditions, where
the reactant transport in the interfacial
region is improved due to the stirring eect of
gasevolution.Smaller agglomerates with less overall roughness
were also observed. Imaging of such agglomerates wasrather
reproducible, even on the mm-scale, but pro-longed imaging was
nevertheless found to alter themorphology. An example for such an
agglomerate on
sample 1 is shown in Fig. 6. In Fig. 6(a) three
dierentcharacteristic deposit morphologies are discerned,namely an
agglomerate of Pt clusters with sizes ranging
from 5 to 40 nm, individual clusters of 10 to 40 nm di-ameter
and a ring-like structure, which supposedlystems from step
decoration of a hole in the graphite
top layer. The larger scale image, Fig. 6(b), shows thatthe
surrounding of the agglomerate is mainly baregraphite. It should be
noted that for such large scale
images it is not always clear, whether a small featureon the
surface represents an island of the substrate ora deposit particle.
This can, in principle, be clarified byzooming in on the structure
of interest, as exemplified
by Fig. 7, but this procedure is time consuming andthus
inadequate for quantitative application. In ad-dition, the
structures were occasionally removed in the
course of zooming in.
Table 1
Electrochemical characteristics of Pt deposits on HOPG
substrates (1 cm2 in geometric area). The potential was stepped
from 0.75
V to Ed during ts seconds. Solutions were Ar purged and
unstirred 0.1 M HClO4+x mM H2PtCl6. Other symbols are defined
in
the text
Samplea
#1 #2 #3 #4b #5b #6
x (mM) 0.2 2.0 2.0 2.0 2.0 10
Ed (V) 0.1 0.1 0.1 0.1 0.1 0.1
ts (s) 430 46 250 42 238 11
W (mg cm2) 17.0 4.7 13.0 7.1 10.8 7.8Ar (cm
2) 1.0 0.6 1.5 1.7 1.9 1.7
S (m2 g1) 5.9 12.8 11.5 23.9 17.6 21.8
aExperimental code number.bPre-impregnated over-night.
Table 2
Geometrical characteristics of the Pt deposits estimated from
the electrochemical measurements. Characteristic cluster diameter
d
and particle density N 0 are calculated assuming smooth
spherical clusters
Samplea
#1 #2 #3 #4b #5b #6
d (nm) 47.5 21.9 24.4 11.7 15.9 12.9
N 0 (1010 cm2) 1.4 4.0 8.0 39.6 23.9 32.5
F. Gloaguen et al. / Electrochimica Acta 44 (1998) 18051816
1811
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Fig. 5. Morphology of large Pt agglomerates formed under dierent
deposition conditions. (A) Deposition at 0.1 V for 42 s from 2
mM solution (sample #4), (B) deposition at 0.1 V for 238 s from
2 mM solution (sample #5) and (C) deposition at 1 V for 5 sfrom 10
mM solution. The height scale (black to white) corresponds to 16,
55 and 70 nm for A, B and C, respectively.
Fig. 6. Pt agglomerate formed by deposition at 0.1 V for 430 s
from 0.2 mM solution (sample #1), imaged at dierent scales.
Fig. 7. High resolution image of isolated platinum particle on
defective HOPG.
F. Gloaguen et al. / Electrochimica Acta 44 (1998)
180518161812
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Fig. 8 shows the deposit distribution on sample 4.
On a small scale, Fig. 8(a), a similar agglomerate as inFig.
6(a) is seen. Larger scale images, Fig. 8(b) and (c),reveal that
the distribution of agglomerates is more
homogeneous than on sample 1. Between 50 and 100particles per
mm2 can be distinguished. This gives anextrapolated particle
density between 5 109 and 1010per cm2, more than 3 orders of
magnitude above theevaluated density of nucleation
centers.Qualitatively dierent results were obtained on
sample 8. On this sample, in the regions between plati-num
agglomerates, a considerable density of small iso-lated clusters
was observed which were mainlyattached to small graphite islands.
Examples are
shown in Fig. 9. The diameter of the clusters wasaround 10 nm,
their extrapolated density is of theorder of 1010 cm2. From size
and density these iso-lated clusters are estimated to represent at
most 10%of the total deposit.
4. Discussion
STM is a nm-scale technique and the full exploita-tion of its
capabilities regarding the structural charac-
terization of adparticles on substrates is only possiblefor
samples, which satisfy several severe restrictions.Firstly, the
used substrate should be extremely flat inorder to allow a
unambiguous separation of adparti-
cles and substrate features. Secondly, the adparticlesshould be
in the 150 nm diameter range in order toallow their reasonable
imaging. Thirdly, the adparticles
should be uniformly distributed with densities between109 and
1011 cm2.The goal of the present work was, to prepare model
electrodes which satisfy these restrictions by electro-chemical
deposition of Pt onto HOPG substrates witha controlled defect
density. Low Pt4+-concentrations
and high deposition overpotentials were considered as
the most promising conditions. For Ag-electrodeposi-
tion it has been shown, that defined nm-scale cluster
distributions on HOPG can be obtained by a similar
approach [6, 7]. For Pt this was also achieved, but for
a metal loading which is equivalent to 0.1 monolayer
of Pt, at best [8].
Evaluation of the transients recorded in the course
of the deposition indicated a rather low saturation cov-
erage for nuclei formation of 2 106 cm2.Consequently the
monoatomic steps, which have a
high density on the oxidized substrate surface, are sup-
posed to have a low probability to act as nucleation
centers. From the low density of nuclei one can derive
the expectation that very large and rough particles are
formed during deposition with typical interparticle
distances of several mm. STM images can thus not beexpected to
reveal the deposit distribution in a-
representative way.
The structural characterization of the respective elec-
trodes with STM showed, that indeed large agglomer-
ates of platinum are present on the surface (Fig. 5),
aside from extended uncovered substrate areas (similar
to Fig. 1c). On the agglomerates clusters on the scale
of several nm are observed. Considering the high
roughness factors evaluated from the electrochemical
data one needs to assume that the agglomerates are
porous, e.g. they may rather be envisaged as stochasti-
cally packed piles of nm-scale clusters than as massive
crystallites with a rough surface. So far, a reasonable
qualitative agreement between the expectations derived
from the electrochemical data and the structural inves-
tigation with the STM is obtained. The characteriz-
ation of Pt electrodeposits on glassy carbon with
TEM [21] and SEM [22] gave similar results, except
that the observed particle densities were significantly
higher.
Fig. 8. Distribution of the Pt deposit after deposition at 0.1 V
for 42 s from 2 mM solution (sample #4), imaged on dierent
scales.
F. Gloaguen et al. / Electrochimica Acta 44 (1998) 18051816
1813
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This seems to indicate that each agglomerate is the
result of the growth of a single nucleus and that the
density of nuclei at saturation equals the area density
of Pt agglomerates on the substrate. Several obser-
vations are, however, inconsistent with this conclusion.
These are namely the overall shape of the agglomerates
(Fig. 6) and the observed local densities of small
particles (Figs. 8 and 9).
The dimensions of the elongated agglomerate shown
in Fig. 6 (height:width:length11:10:50), as anexample, are very
far from hemispherical. In addition,
several small particles are resolved in the vicinity of
the agglomerate, but without direct contact to it. Such
a morphology can not be imagined as resulting from
the growth of a single nucleus. Further on, the local
densities of particles of the order of 1010 cm2, exem-plified by
Figs. 8 and 9, is dicult to correlate with
the determined saturation density of nuclei. Thus it is
concluded that the evaluation of electrochemical data
and the structural investigation, although not totally
contradictive, are not compatible in detail.
Regarding the electrochemical data, a source ofuncertainty may
be the excess charge convoluted with
the double layer recharging. This charge is roughly
equivalent to that required for the deposition of 1.0
monolayer of Pt and was not considered for the
evaluation. Regarding the structural investigation, two
crucial questions need to be discussed. The first is, how
the ex-situ surface structure is related to the in-situ
deposit morphology or, in other words, how the
Fig. 9. Distribution of Pt clusters after deposition at 0.1 V
for 11 s from 10 mM solution (sample #8).
F. Gloaguen et al. / Electrochimica Acta 44 (1998)
180518161814
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process of emersion, rinsing and drying may aect the
deposit distribution. For electrodeposited silver it has
been shown, that this process leaves the deposit mor-
phology unchanged [6, 7], which is surprising in view
of the high exchange current density of silver. Thus
it may be considered unlikely that the situation is
dierent for platinum.
The second question is, how the imaging process
may aect the deposit distribution. It has already been
stated that occasional feedback instabilities indicated
that particles were swept away by the tip. Hence more
particles are present on the surface than are imaged
with the STM. The discrepancy between particle den-
sity and evaluated density of nuclei may thus be more
pronounced than evident from the STM data, but this
is of minor concern for the present discussion.
A possible explanation for the apparent discrepancy
between electrodeposition kinetics and surface mor-
phology is derived from recent investigations of cluster
formation on HOPG under UHV conditions [23, 24].
These made evident that the deposit distribution on
HOPG can only be rationalized by assuming a
significant mobility of clusters on the surface at room
temperature.
Considering these observations and a recently
reported work [8] it seems reasonable to conclude that
a large amount of very small (11 nm) platinum clus-ters is
formed during the initial spike of the electrode-
position transient, i.e. at t< 0.3 s. In the course of Pt
electrodeposition, these very small clusters may be
mobile and assemble in an accidental distribution of Pt
agglomerates. At t>0.3 s, the rapid growth of a small
number (2 106 cm2) of Pt particles give a transientresponse,
which is adequately described by the model
of Scharifker and Hills for a diusion controlled
growth.
An additional experimental result (Fig. 10) supports
the assumption of cluster mobility in an electrochemi-
cal environment: in the course of the imaging process
with contact mode AFM, no Pt clusters moved under
the tip, conversely to what was sometimes observed
with STM, even at very low tunneling currents (0.5
nA). This discrepancy between contact AFM and STM
may be explained by a poor electronic contact between
some Pt crystallites and the graphite surface yielding
Fig. 10. Contact mode AFM images of Pt clusters
electrochemically deposited onto HOPG. The Pt loading is ca. 155 mg
cm2.
F. Gloaguen et al. / Electrochimica Acta 44 (1998) 18051816
1815
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to a crash of the STM tip (the AFM technique is notsensitive to
variations of the electronic conduction of
the sample). A poor electronic contact between Pt andgraphite
implies that some Pt crystallites are notlocated at their
nucleation sites, where Pt electroreduc-
tion takes place. According to this, the final mor-phology of
the Pt electrodeposit may not depend onthe density of nucleation
sites only, but may result in
the surface diusion of Pt ad-particles formed in thecourse of
the electrodeposition process.A more systematic investigation,
including the eect
of the deposition overpotential and the Pt salt concen-tration,
as well as the influence of the substrate mor-phology and
structure, should, however, be performedin order to clarify the
above assumptions. This is
beyond the purpose of this paper.
5. Conclusions
The electrodeposition of platinum on graphite sub-
strates with a high density of monoatomic steps wasinvestigated
with electrochemical techniques and ex-situ STM. The quantitative
evaluation of the electrode-
position current versus time, at t>0.3 s, was carriedout
according to the nucleation-growth model ofScharifker and Hills.
This analysis was found to be in-consistent with the structural
investigation of the elec-
trode surfaces. A very heterogeneous depositdistribution was
observed for all investigated samples,which cannot reasonably be
correlated with a distri-
bution of active nucleation sites as calculated from
thetransient analysis at t>0.3 s. As a trend it is observed,that
the density of particles increases with increasing
concentration of the electroactive species in solution.
Aqualitative understanding of the experimental results isobtained
under the following assumptions, (i) Pt clus-ters are formed at
t< 0.3 s and (ii) the diusion of Pt
clusters on the graphite surface is an important step inthe
course of the phase formation process.
Acknowledgements
This work was carried out with the support ofADEME, the French
Agency for Environment andEnergy Conservation, to which we are
greatly
indebted. We also acknowledge the support of theFrenchGerman
Exchange Program PROCOPEthrough the Deutsche Akademischer
Austauschdienstand the French Ministe`re des Aaires
Etrange`res.
References
[1] S. Mukerjee, J. Appl. Electrochem. 20 (1990) 537.
[2] M.S. Wilson, F.H. Garzon, K.E. Sickafus, S. Gottesfeld,
J. Electrochem. Soc. 140 (1993) 2872.
[3] Y. Takasu, N. Ohashi, X.-G. Zhang, Y. Murakami, H.
Minagawa, S. Sato, K. Yahikozawa, Electrochim. Acta
41 (1996) 2595.
[4] P.C. Biswas, Y. Nodasaka, M. Enyo, J. Appl.
Electrochem. 26 (1996) 30.
[5] G. Tamizhmani, J.P. Dodelet, D. Guay, J. Electrochem.
Soc. 143 (1996) 18.
[6] J.V. Zoval, R.B. Stiger, P.R. Biernacki, R.M. Penner, J.
Phys. Chem. 100 (1996) 837.
[7] J.V. Zoval, P.R. Biernacki, R.M. Penner, Anal. Chem.
68 (1996) 1585.
[8] J.V. Zoval, J. Lee, S. Gore, R.M. Penner, J. Phys. Chem.
B 102 (1998) 1166.
[9] R.J. Nichols, D.M. Kolb, R.J. Behm, J. Electroanal.
Chem. 313 (1991) 109.
[10] S.A. Hendricks, Y.-T. Kim, A.J. Bard, J. Electrochem.
Soc. 139 (1992) 2818.
[11] U. Stimming, R. Vogel, D.M. Kolb, T. Will, J. Power
Sources 43 (1993) 169.
[12] N. Breuer, U. Stimming, R. Vogel, Electrochim. Acta 40
(1995) 1401.
[13] N. Breuer, U. Stimming, R. Vogel, Surf. Coatings
Technol. 67 (1994) 145.
[14] N. Breuer, U. Stimming, R. Vogel, in: A.A. Gewirth, H.
Siegenthaler (Eds.), Nanoscale Probes of the Liquid
Solid Interface, vol. 288, Kluwer, Dordrecht, 1995, p.
121.
[15] X.Q. Tong, M. Aindow, J.P.G. Farr, J. Electroanal.
Chem. 395 (1995) 117.
[16] J.L. Zubimendi, L. Vazquez, P. Ocon, J.M. Vara, W.E.
Triaca, R.C. Salvarezza, A.J. Arvia, J. Phys. Chem. 97
(1993) 5095.
[17] H. Chang, A.J. Bard, J. Am. Chem. Soc. 112 (1990)
4598.
[18] J. Lin-Cai, D. Pletcher, J. Electroanal. Chem. 149
(1983)
237.
[19] B. Scharifker, G. Hills, Electrochim. Acta 28 (1983)
879.
[20] K. Shimazu, D. Weisshaar, T. Kuwana, J. Electroanal.
Chem. 223 (1987) 223.
[21] P. Allongue, E. Souteyrand, J. Electroanal. Chem. 286
(1990) 217.
[22] H. Shimazu, K. Uosaki, H. Kita, Y. Nodasaka, J.
Electroanal. Chem. 256 (1988) 481.
[23] G.M. Francis, I.M. Goldby, L. Kuipers, B.
VonIssendor, R.E. Palmer, J. Chem. Soc. Dalton
Trans. 0 (1996) 665.
[24] G.M. Francis, L. Kuipers, J.R.A. Cleaver, R.E. Palmer,
J. Appl. Phys. 79 (1996) 2942.
F. Gloaguen et al. / Electrochimica Acta 44 (1998)
180518161816