Surface Science 597 (2005) 127–132

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Investigations of the stability of palladium clusterssupported on gold

J. Meier, H. Kleine, U. Stimming *

Department of Physics E19, TU-Munchen, James-Franck-Str. 1, D-85748 Garching, Germany

Available online 1 August 2005

Abstract

The stability of gold supported palladium clusters, generated with an electrochemical scanning tunnelling micro-scope, has been investigated in metal-free electrolyte. The clusters are found to be more stable, if the tip is moved fur-ther towards the electrode surface during the generation process. Only a weak dependence of the potential on thedissolution is observed, but a strong increase of the dissolution current density with decreasing particle size. Dissolutionof the clusters occurs from the edges rather than layer by layer.� 2005 Elsevier B.V. All rights reserved.

Keywords: Palladium cluster; Supported cluster; Tip-induced; STM; Stability

1. Introduction

The fundamental knowledge and understandingof the physical and chemical properties of nano-scale metal particles is of considerable scientificinterest as well as of technological relevance. Theproperties of nanoparticles are of general impor-tance in surface science, heterogeneous catalysis,electrochemistry and nanotechnology. An accuratecontrol of the particle size is of paramount impor-tance if one wants to investigate physical and

0039-6028/$ - see front matter � 2005 Elsevier B.V. All rights reservdoi:10.1016/j.susc.2004.07.058

* Corresponding author. Tel.: +49 89 289 12531; fax: +49 89289 12530.

E-mail address: stimming@ph.tum.de (U. Stimming).

chemical properties of supported particles. Typi-cally, a large number of particles is investigatedin order to obtain sufficiently large signals. Suchmeasurements have been performed using differ-ently prepared surfaces with a varying density ofstatistically arranged particles on the surface.Although a rather small variation in particle sizeis achievable by different preparation methods(e.g. vapor deposition, electrochemical depositionand adsorption from colloidal solutions) usuallya broader size distribution results on the surfacedue to agglomeration [1–6]. For these reasons,the measurement of the properties of one singleor a small number of structurally defined particle,is preferable.

ed.

128 J. Meier et al. / Surface Science 597 (2005) 127–132

In the last decade, several techniques were devel-oped using an electrochemical scanning tunnellingmicroscope as a tool for the nanostructuring of elec-trode surfaces [7–12]. In this work, we use anelectrochemical scanning tunnelling microscope(EC-STM) in order to investigate the stability ofsmall palladium clusters on a Au(111) electrode,which were deposited by a so-called tip-inducedmetal deposition. In a method developed in the groupof Kolb [8,13] the metal is first electrochemicallydeposited on the tip of an EC-STM, then the tip isbrought into contact with the electrode surface,and upon withdrawal a cluster is left behind. Thisparticular system was chosen because palladium isa good catalyst for a number of electrochemicalreactions. Previously we have investigated the cata-lytic activity of such single supported Pd cluster onAu(111) [14]. Therefore the technique first intro-duced by Kolb and co-workers [8] was modified tosuit our requirements of having the particles in anmetal-free electrolyte; palladium is electrolyticallydeposited onto the tip of a STM in a separate elec-trochemical cell and then the tip is transferred tothe EC-STM cell to avoid palladium contaminationof the solution in contact with Au(111) surface.Using a controlled tip approach, palladium isdeposited from the tip to the surface in a metal-freesolution. It is shown that the specific reactivity ofsuch Pd clusters towards electrochemical protonreduction is enhanced by two orders of magnitudeas the cluster height decrease from 10 atomic layersheight to 2 layers [14,15]. The cluster generated withdifferent approach parameters that were used in thisstudy varied in height between 0.3 nm and 1.8 nmand their initial diameter were found to lie between5 nm and 20 nm. We had discussed the effect of theapproach parameters on the cluster size in detailelsewhere [16]. It was found that the cluster size in-creasesmonotonously with increasingmovement ofthe tip towards the surface. Here we show studieswhich were performed on clusters generated at asample potential of 250 mV and investigated thestability measurements of such clusters. The stabi-lity of the tip-induced Pd cluster was investigatedby monitoring morphology changes of the clusterswith the aid of EC-STM at various electrode poten-tials by measuring their height and diameter as afunction of time. An effect of tip-shielding can be

excluded, because Kolb et al. [13] have shown thatthere is no influence of the tip distance on the disso-lution behavior of copper cluster on gold. Anyway,in all experiments the bias was 50 mV in order tokeep the influence of the tip on the results constant.

2. Experiments

The electrochemical scanning tunnelling micro-scope (EC-STM), which was used both for gener-ating and imaging the Pd nanoparticles, consistsof a PicoSPM scanning unit and a PicoStat poten-tiostat (both Molecular Imaging) driven by aNanoscope E control unit (Digital Instruments).The control electronics was modified in order toallow voltage pulses to be applied to the z-piezoof the STM scanning unit with which the tip-in-duced deposition of nanoparticles was performed.The tip current and z-voltage can be monitoredwith high time resolution and accuracy by theuse of a digital storage oscilloscope (TDS 620B,Tektronix). All images were obtained in the con-stant-current mode. STM tips were etched fromPt/Ir wire (atomic ratio 90:10, 0.25 mm in diame-ter) and insulated with an electrophoreticallydeposited lacquer [17]. The free tip area was deter-mined by chronoamperometry in 30 mM potas-sium hexacyanoferrate and it lie between5.3 ± 0.6 · 10�3 lm2 for fresh prepared tips and2.1 ± 0.3 · 10�1 lm2 for used tips. All experi-ments were performed in an inert gas atmosphere(N2). Gold films, evaporated on tempax glass(Metallhandel Schroer GmbH) were used as sub-strates. Preparation of the substrates was doneby flame annealing and cooling to ambient tem-perature in a N2 stream. This resulted in large(up to several hundred nm) terraces, which areatomically flat and (111)-oriented. An oxidizedgold wire and a gold spiral were used as referenceand counter electrodes, respectively. All potentialsare given versus reversible hydrogen electrode po-tential (RHE). The electrolytes (0.1 M H2SO4 and0.1 mM PdSO4) were prepared from Milli-Q water(18.2 MX cm and 3 ppb TOC) and Merck supra-pure chemicals.

Palladium was deposited onto the tip in a sepa-rate electrochemical cell containing a 0.1 mM pal-

J. Meier et al. / Surface Science 597 (2005) 127–132 129

ladium sulphate solution in 0.1 M sulphuric acid ata potential of 650 mV for 5 s. The generation ofthe clusters and stability measurements were thenperformed in pure metal-free 0.1 M sulphuric acid.This procedure allows us to work at sample poten-tials at which otherwise electrodeposition of palla-dium onto the substrate would occur.

3. Results and discussion

Before we will consider the stability let us havea look at the different morphology of the particles.The upper part of Fig. 1 shows an STM image offour Pd particles generated over a period of 4 min.The particles A to C are of the same height but dif-

Fig. 1. Top: STM image of Pd particles on Au(111) in 0.1 MH2SO4 immediately after the generation. Itip = 1.1 nA, Ubias =170 mV. Bottom: change of the height of the particles with timeat 330 mV vs. RHE.

fer in diameter, while particle D is bigger, both inheight and width. The tip movement during thegeneration processes was increased in 0.1 nm steps,which leads to an increased particle size goingfrom cluster A to cluster D. The lower part ofthe figure shows the change of the height withtime. The potential was kept at 330 mV vs. RHE,at which a slow dissolution of palladium into thePd2+-free electrolyte occurs (standard potentialPd/Pd2+ U0 = 0.915 V vs. NHE [18]). The heightof the particles (determined in their center) doesnot change continuously with time, but rather insteps from one plateau to another. The plateausare indicated by dashed lines. The lines are equi-distant with a interval of 0.26 nm, which doesnot correspond to any known lattice distance inpalladium (a = 3.89 A). In any case, there is a fairscatter of data about the average plateau height, sothe dashed lines should be considered as guide forthe eye. It can also be seen that for particles largerin diameter the height stays at a constant value fora larger period of time before it decays. The disso-lution of the particles seems to occur from the rimof the layers rather than from the top, i.e. stepatoms with a lower coordination number dissolvemore easily than atoms from within the layer. InFig. 2 it is seen, that the diameter on the basis ofthe cluster and the volume decrease continuouslywhile the height falls stepwise (see Fig. 1). Eachparticle maintains at larger heights for longer than

20 25 30 35 40 45 5001

5

6

7

8

AB

d basi

s / n

m

t / min

0

2

4

6

8

10

12

14 V

/ nm

3

Fig. 2. Diameter on the basis of cluster A and B (see Fig. 1) andvolume of the particles as a function of time at 330 mV vs.RHE.

0.000

0.010

0.020

1

130 J. Meier et al. / Surface Science 597 (2005) 127–132

at smaller heights, e.g. the center of particle C hasa height of three layers for 90 min, but a height oftwo layers for only 30 min. This indicates that dis-solution at the rims of different layers occurs inparallel. In particular, the last two layers seem tovanish simultaneously. These results suggest thatthe nanoparticles exhibit a layered structure. Kolbet al. [19] observed a layer by layer dissolution ofCu particles on Au(111), which means that thediameter of the particles would be constant whilethe height decrease stepwise. This behavior wasnot seen in our case.

For electrodeposited Pd on Au(111) the growthof a pseudomorphic first layer with a latticestretched by approx. 5% was observed by Kibleret al. [20]. Pseudomorphic growth could be ex-pected considering the small lattice mismatch.The second and third layer grow layer by layer, be-fore in subsequent layers a three-dimensionalgrowth sets in (Stranski–Krastanov growth) [20].The spacing between the layers of the particles isexpend by approximately 7% as compared to thePd bulk.

The change of the volume with time was evalu-ated in detail for cluster A and B and the dissolu-tion current density of cluster A and B wascalculated from dV

dt in Fig. 3 divided by the particlesurface area. In the case of cluster A jjdis.,Ajincrease with decreasing volume from 1 ·10�8 A cm�2 to 5.9 · 10�7 A cm�2 (Cluster B

0 2 4 6 8 10 12 14

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

V / nm3

particle A particle B

dtdV /

nm3 s-1

Fig. 3. Absolute volume change j dVdt j

� �nm3

s

h i� �vs. volume of

the clusters A and B (see Fig. 1) 330 mV vs. RHE.

jjdis.,Bj rise from 9.6 · 10�8 A cm�2 to 4.2 · 10�7

A cm�2). Further analysis shows, that: jjdis.j /1

rparticle, with the particle radius rparticle.

The same experiments were also performed atmore positive electrode potentials. The bottompart of Fig. 4 shows the height of tree palladiumclusters on Au(111) as a function of time at an elec-trode potential of 750 mV vs. RHE. The clusterswere also generated at 250 mV vs. RHE and thenthe potential was changed to 750 mV vs. RHE. Inthe upper part of Fig. 4 the absolute volume changeis plotted as a function of time. It is seen, that thedissolution is four times faster as compared to anelectrode potential of 330 mV vs. RHE.

In Fig. 5 the dissolution current density as afunction of 1

r of different particles measured at acertain electrode potentials is shown. It is seen,that for particle A (h) and F (s) an almost linearincrease of the current density with increasingvalues of 1

r (decreasing particle size) is observed(correlation coefficient R2 = 0.89 and 0.88, respec-tively), whereas particle B and H shown a weakcorrelation. It is remarkable, that a weak depen-dence on the dissolution potential (approx. 790

12080 1004020 60

0.91.0

0.70.8

0.50.6

0.30.4

0.10.0

0.2

-0.040

-0.030

-0.020

-0.010

height volume E F G

parti

cleh

eigh

t / n

m

t / min

0

10

20

30

40

50

V /

nm3

volume changedtdV

E F G

/ nm

3 s-

0

Fig. 4. Bottom: height of the particles as a function at time at

750 mV vs. RHE. Top: absolute volume change j dVdt j

� �nm3

s

h i� �vs. time.

Fig. 5. Dissolution current density vs. 1r of different particles

measured at a electrode potential of: (w) 200 mV, (j) and (h)330 mV, (m) 600 mV, and (s) 750 mV. The data were measuredin different experiments.

J. Meier et al. / Surface Science 597 (2005) 127–132 131

(±50) mV/decade) and at electrode potentials6250 mV vs. RHE no dissolution is observed(see w in Fig. 5) and particles larger than 10 nmalso not dissolve (e.g. Fig. 4), independent of po-tential. A large variation of the dissolution currentdensity is obvious. Further analysis of 14 particlesshows, that the current density as a function ofparticle size exhibits three different behaviors.First, the current density increases monotonouslywith decreasing particle radius (1r), e.g. Figs. 5, 3,and 4. Second, the current density increases step-wise with an stepwise decay of the particle heightgoing from 3 layers height to 2 layers height.Third, there is a considerable scatter of the data,i.e. a random behavior. The first point can be ex-plained by a negative shift of the Nernst potentialdue to an increased activity coefficient a(PdN) ofthe nanoparticles as compared to the bulk metal(see Eq. (1)) [21].

UN ¼ U 0 þ RT2F

lnaðPd2þÞaðPdNÞ

� �ð1Þ

with R—gas constant, T—temperature, F—Fara-day�s constant and a—activity coefficients. Wherea(PdN), the activity coefficient of the nanoparti-cles, depends on the Gibbs energy change (Eq. (2)).

aðPdNÞ ¼ exp�DRG

ex

RT

� �ð2Þ

with DRGex—excess free energy, which can be

made up from three different non-equilibriumcontributions.

DRGex ¼ DGex

GT þ DGexGW þ DGex

S ; ð3Þ

where both DGexGT (Gibbs–Thomson) and DGex

GW

(Gibbs–Wulff) are inversely proportional to theparticle radius [21]. Therefore a(PdN) shouldincrease with decreasing particle radius anda(PdN) > a(Pdbulk). The excess free energy of strainDGex

S is hardly determinable due to the generationprocess (tip-induced metal deposition). However,Lewis and Pearce [22] have shown a linear increaseof the surface free energy with increasing latticestrain. The second point can may be explained byan substrate induced change of the electronic struc-ture of the nanoparticle, which would lead to aheight dependence of the dissolution current den-sity. Such clusters also exhibit a height dependentreactivity regarding the reduction of proton whichwas explained by a change of the electronic struc-ture due to a mismatch of the lattice constants ofthe clusters and substrate, which decrease withincreasing cluster height [15]. Anyway, the thirdpoint and the two different dissolution behaviorsdiscussed are indications that the generation pro-cess leads to non-uniform clusters.

Roudgar and Groß [23] have shown by densityfunctional theoretical calculation a strong cou-pling of small Pd clusters (7 Pd atoms) to theAu(111) substrate, which leads to a metallic nat-ure of the cluster and therefore a continuous spec-trum of the d-band . This is also supported byMaupai et al. [24], who has concluded from disso-lution experiments that the stability of tip-inducedcopper clusters may be caused by a mixture be-tween copper and gold due to interfacial alloying.Alloy formation has been suggested to be a prere-quisite for the formation of stable clusters bymolecular dynamic simulations [25]. It was alsoshown, that the cluster height increases withincreasing tip movement during the generationprocess, which leads to an intermixing of palla-dium and gold [16]. Therefore alloy formationmay be play an important rule for the stability ofgold supported nanometer sized Pd clusters anda variation of the gold content of the cluster can

132 J. Meier et al. / Surface Science 597 (2005) 127–132

explain the non-uniform dissolution behavior ofthe tip-induced cluster on Au(111).

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