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:

gloaguen@cri.univ-poitiers.fr1 Present address: TU Munchen, Physik Departement E19,

85748 Garching, Germany.

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

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 (

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

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

(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

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

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

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

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

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

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.

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