Initial stages of palladium deposition on Au(h k l) Part III: Pd on Au(1 1 0)

Article abstract of DOI:10.1016/S0039-6028(01)01684-3

The deposition of palladium onto the unreconstructed Au(1 1 0) surface was studied by cyclic voltammetry and in situ scanning tunnelling microscopy. An ordered adlayer of [PdCl₄]^2- was imaged with atomic resolution on the bare Au(1 1 0) surface. Pd deposition starts at monoatomic high steps by forming a layer that grows onto the lower terrace. Coulometric data point towards the deposition of approximately three monolayer equivalents in the Pd underpotential region. This high coverage and the presence of holes in the Au(1 1 0) surface after the complete anodic dissolution of the Pd deposit are explained by surface alloy formation. Furthermore, the palladium overlayers on Au(1 1 0) appear to be rather rough, because there is no strict layer-by-layer growth. Important aspects of the initial stages of palladium deposition on the three low-index Au surfaces are summarised and the influence of the crystallographic orientation of the substrate as well as the effect of different Pd film thicknesses on the electrochemical properties are briefly discussed.

Full text of DOI:10.1016/S0039-6028(01)01684-3

Surface Science 498 (2002) 175–185
www.elsevier.com/locate/susc
Initial stages of palladium deposition on Au(hk l)
Part III: Pd on Au(110)
1
L.A. Kibler, M. Kleinert, V. Lazarescu , D.M. Kolb *
Department of Electrochemistry, University of Ulm, 89069 Ulm, Germany
Received 10 May 2001; accepted for publication 16 October 2001
Abstract
The deposition of palladium onto the unreconstructed Au(1 1 0) surface was studied by cyclic voltammetry and in situ
2
ꢀ
scanning tunnelling microscopy. An ordered adlayer of [PdCl
4
]
was imaged with atomicresolution on the bare
Au(1 1 0) surface. Pd deposition starts at monoatomic high steps by forming a layer that grows onto the lower terrace.
Coulometricdata point towards the deposition of approximately three monolayer equivalents in the Pd underpotential
region. This high coverage and the presence of holes in the Au(1 1 0) surface after the complete anodic dissolution of the
Pd deposit are explained by surface alloy formation. Furthermore, the palladium overlayers on Au(1 1 0) appear to be
rather rough, because there is no strict layer-by-layer growth. Important aspects of the initial stages of palladium
deposition on the three low-index Au surfaces are summarised and the influence of the crystallographic orientation of
the substrate as well as the effect of different Pd film thicknesses on the electrochemical properties are briefly dis-
cussed. Ó 2001 Published by Elsevier Science B.V.
Keywords: Alloys; Gold; Metal–electrolyte interfaces; Palladium; Scanning tunneling microscopy
1
. Introduction
sis [3], or producing thin overlayers of well-defined
structure.
Studies of the electrochemical deposition of
The deposition of palladium has been consid-
ered to be a promising case for basic investiga-
tions, because of the high catalytic activity [4].
Furthermore, the difficulties in preparing and
handling massive palladium single crystals were
tried to be circumvented by the use of ultrathin
palladium overlayers. Indeed, palladium deposi-
tion was found to be structure-sensitive: epitaxially
grown overlayers were obtained on gold and plat-
inum single crystal substrates [5,6]. In addition,
the electrochemical characterisation of palladium
overlayers on single crystal substrates was ex-
tremely useful for developing simple methods
metals on well-ordered single crystalline substrates
allow for a better understanding of the funda-
mental aspects of metal deposition [1,2]. Such
knowledge is of great interest for, e.g. deriving
structure–reactivity relationships in electrocataly-
*
Corresponding author. Tel.: +49-731-50-25400; fax: +49-
31-50-25409.
E-mail address: dieter.kolb@chemie.uni-ulm.de (D.M.
7
Kolb).
1
Present address: Institute of Physical Chemistry, I.G.
Murgulescu, 77208 Bucharest 6, Romania.
0
039-6028/01/$ - see front matter Ó 2001 Published by Elsevier Science B.V.
PII: S0039-6028(01)01684-3

176
L.A. Kibler et al. / Surface Science 498 (2002) 175–185
to prepare high-quality palladium single crystals
7].
The deposition of Pd on Au(1 1 1) and Au(1 0 0)
always deposited at potentials, where gold surfaces
are unreconstructed. Structural changes upon the
transition from freshly prepared reconstructed to
ð1 Â 1Þ unreconstructed phases have an influence
on the initial stages of metal deposition, since de-
fects that arise from such lifting of the recon-
struction can act as nucleation centres. However,
compared to Au(1 1 1) or Au(1 0 0), there is little
information in the literature about the electro-
chemical behaviour of Au(1 1 0), probably because
it is still difficult to prepare large and well-ordered
terraces for this relatively open surface. The exis-
tence of an order–disorder transition at about 650
K [16] restricts the annealing to lower tempera-
tures. With the conventional flame-annealing and
quenching technique, Au(1 1 0) surfaces are ob-
tained with rather small domains and an inho-
mogeneous structure [17]. For this reason, we have
developed a method [18], which allows to prepare
Au(1 1 0) surfaces of high quality. Terraces of up
to 100 nm width were obtained and it was possible
to use in situ STM as a tool to study the electro-
chemical Pd deposition on Au(1 1 0) at an atomic
level. The characterisation of a well-ordered
Au(1 1 0) electrode in aqueous sulfuric acid solu-
tion by cyclic voltammetry (CV) and in situ STM
with regard to reconstruction phenomena will be
described elsewhere [18].
[
has recently been studied by our group, especially
with regard to the initial stages [5,8,9]. The oxi-
dation of small organicmole cu les on the ele ct ro-
chemically grown ultrathin palladium films on
Au(hk l) was found to depend significantly on the
crystallographic orientation of the substrate and
on the film thickness [10,11]. In order to under-
stand such relations it is essential to have available
detailed information about the palladium surface
morphology. In this respect, recent progress has
been made by the use of in situ scanning tunnelling
microscopy (STM) [5,8,9,12,13] and surface X-ray
scattering [14].
In general, Pd is deposited onto Au from
aqueous solutions of its chlorine compounds, e.g.
PdCl
to palladium deposition, [PdCl
2 2 4
or K PdCl . From such electrolytes, prior
2ꢀ
4
]
adsorbs on the
bare Au(1 1 1) and Au(1 0 0) surfaces and forms
ordered adlayers, the structures of which have
been studied by in situ STM [8,9,12,13]. This
complex anion has also been found to adsorb on
the first Pd monolayer on Au(1 1 1) [8], whereas
it is displaced by chloride in the case of Pd on
Au(1 0 0) [9]. On these unreconstructed Au(1 1 1)
and Au(1 0 0) surfaces, palladium nucleates first at
surface defects like monoatomic high steps, and
at higher overpotentials also on terraces [8,9].
In the underpotential deposition (upd) region,
a pseudomorphicPd monolayer is formed on
Au(1 1 1) [8,14]. No alloy formation was observed
for this system [8], while a Pd/Au surface alloy is
formed when Pd is deposited onto Au(1 0 0) [9].
Deviations in the electrochemical properties of
thin Pd films on Au(1 0 0) from those of well-
ordered Pd(1 0 0) surfaces [10] are most probably
caused by this alloying process.
In this communication, the initial stages of pal-
ladium deposition onto unreconstructed Au(1 1 0)
are investigated and general trends of palladium
deposition onto the three low-index Au surfaces
are presented. These surfaces after flame-annealing
are known to be reconstructed, even in an elec-
trochemical environment under certain conditions,
e.g. when specific adsorption of anions is avoided
2. Experimental
The experimental procedure is similar to that
described in the previous publications [8,9]. The
Au(1 1 0) single crystals were purchased by Ma-
TecK (J u€ lich, Germany), where they had been
polished down to 0.03 lm and oriented to better
than 1°. The electrode used for CV was 3 mm in
diameter, with a gold wire attached at its rear for
better handling. Before each experiment, it was
annealed for 1 min in a propane flame at dim red
heat, cooled down for 1 min in air just above a
small beaker filled with Milli-Q water, into which
it was subsequently dipped and finally, protected
by a drop of water, transferred to the electro-
chemical cell. For in situ STM, a larger single
crystal was used (12 mm in diameter). It was an-
nealed at about 200 °C for several hours (usually
[
15]. For the present three-part study, Pd was

L.A. Kibler et al. / Surface Science 498 (2002) 175–185
177
over night) in a drying locker. The surface was
3. Results
then cleaned by a short high-temperature anneal-
ing of a few seconds in a hydrogen flame, cooled
down for several minutes under a nitrogen stream
and mounted in the STM cell.
The STM images were recorded with a To-
pometrix TMX Discoverer 2010 and are all taken
with the scan direction from top to bottom. An
electrochemically etched Pt–Ir wire was used as
STM tip. It was coated with an electrophoretic
paint to reduce the Faradaic current at the tip–
electrolyte interface below 50 pA.
Pt wires served as counter electrodes in both
cases, but for STM measurements a platinum wire
served also as a quasi-reference electrode. Other-
wise a saturated calomel electrode (SCE) was used,
and in the following, all potentials are quoted
against SCE. As mentioned in previous work [8],
the Pt reference electrode allows for highly clean
conditions, but suffers somewhat from instabilities
and hence, the potential values for STM mea-
surements are less precise than those obtained with
an SCE. In addition, shielding effects due to the
thin-layer configuration of tip and sample may
also cause small potential shifts. The solutions
3.1. Cyclic voltammetry
The bulk deposition of palladium on the un-
reconstructed Au(1 1 0) surface from 0.1 M
H
2
SO
þ 1 mM PdCl
, which starts at 0.52 V, is
2
4
preceded by two distinct deposition steps (upd) as
indicated by both the cyclic voltammogram and
the charge isotherm (Fig. 1). Each data point for
the charge isotherm (Fig. 1b) represents the total
charge, recorded during the potential scan with 1
mV/s from 0.75 V to selected final potentials be-
tween 0.62 and 0.52 V, and holding the electrode
potential there for 20 min (Fig. 2). Obviously, the
current drops to zero as long as the potentials are
confined to the upd region. The formation of bulk
palladium starts around 0.52 V, where the depo-
sition current is seen to increase after the initial
decline brought about the potential stop (Fig. 2,
broken line).
The two upd peaks in Fig. 1a overlap and thus
are not well resolved. Nevertheless, there is a clear
change in the deposition mechanism around 0.56
V as indicated by the two cathodic peaks in
the cyclic voltammogram (Fig. 1a) and the de-
pendence of total cathodic charge on electrode
potential (Fig. 1b). The charges consumed in
the first step and in the whole upd region were
were prepared from H
PdCl (Merck, zur Synthese), PdSO
8%), HCl (Merck, suprapur) and Milli-Q water
18.2 MX cm, 3 ppb total organic carbon).
2
SO
4
(Merck, suprapur),
2
4
(Aldrich,
9
(
Fig. 1. Pd deposition on Au(1 1 0) from 0.1 M H
2
SO
4
þ 1 mM PdCl
2
: (a) cyclic voltammograms with different negative potential limits
for Pd upd on Au(1 1 0) (scan rate: 1 mV/s) and (b) maximum charge densities as derived from the current–time curves in Fig. 2.

178
L.A. Kibler et al. / Surface Science 498 (2002) 175–185
Fig. 2. Current–time curves for Pd deposition on Au(1 1 0). The potential was scanned with 1 mV/s from 0.75 V to various potentials in
the Pd upd regime and held for 20 min (the potential program is drawn schematically as inset). The broken line indicates the onset of
Pd bulk deposition at 0.52 V.
2
found to be about 350 and 800 lC/cm , respec-
tively (Fig. 1b).
chloride concentration up to 0.1 M did not change
significantly the shape of the cyclic voltammo-
grams. Contrary to the highly irreversible Pd
deposition on Au(1 1 1) and Au(1 0 0) from chlo-
ride-free solution, palladium deposited from
Although the first cathodic peak is relatively
sharp, its anodic co unterpart is far from looking
alike (Fig. 1a, dotted curve). The stripping of the
palladium overlayer, which has been formed in the
first deposition process, is spread out over a rather
wide potential range. The anodicbran hc reveals
two very small humps located around 0.65 and
4
PdSO —at least for low coverage—on Au(1 1 0)
can be stripped in the positive-going potential
sweep (see Fig. 4). At higher coverage, Pd disso-
lution in sulfate solution is much slower.
0
.75 V. However, when the deposition time was
longer, like in the potential-arrest experiments
Fig. 2), only a single dissolution peak is observed,
(
3.2. Scanning tunnelling microscopy experiments
which is shifting continuously towards more neg-
ative potentials with lower deposition potential
Fig. 5 shows a high-resolution STM image of a
well-prepared Au(1 1 0) electrode in 0.1 M H SO
(
Fig. 3a and b). On the other hand, the stripping
of the palladium deposited in the second step
deposition < 0:55 V) takes place in a much nar-
2
4
at 0.55 V. At such a positive potential, the (1 Â 2)
reconstruction is no longer stable and the surface
is in its unreconstructed (1 Â 1) state. Atomically
flat terraces are seen, separated by monoatomic
high steps, which show a pronounced frizziness
[18]. Close inspection of Fig. 5 shows even the
(E
rower potential region yielding a pronounced peak
closely located to the bulk phase dissolution (Figs.
1
a and 3a).
Decreasing the PdCl concentration from 1 to
.1 mM is seen to shift the potential of Pd depo-
2
0
individual surface atoms. When PdCl is added to
2
sition and dissolution (Fig. 3b) in positive direc-
tion. This is obviously due to the lower free
the supporting electrolyte, an ordered adlayer is
observed at positive potentials, where Pd deposi-
tion does not yet start (Fig. 6a). The STM image
shows square and rectangular spots arranged in
chloride concentration in 0.1 mM PdCl
for similar considerations, see Ref. [8]). How-
ever, it is worth mentioning that variation of the
2
solution
(
ꢀ
rows, which run parallel to the [1 1 0] direction.

L.A. Kibler et al. / Surface Science 498 (2002) 175–185
179
Fig. 4. Cyclic voltammograms for Au(1 1 0) in 0.1 M H
:1 mM PdCl (dotted line) and 0.1 M H SO
þ 0:1 mM PdSO
solid and broken line). In the latter case, the negative poten-
tial limit was shifted by ꢀ0.1 V in the second cycle (scan rate:
mV/s).
2
SO
4
þ
0
2
2
4
4
(
1
Fig. 3. (a) Series of current–potential curves for Pd dissolution
from Au(1 1 0) in 0.1 M H SO after holding the
2
4
þ 1 mM PdCl
2
potential for 30 min at different values Edeposition in the upd re-
gion (the dotted line corresponds to bulk deposition region,
already, scan rate: 10 mV/s). (b) Plot of dissolution peak po-
tentials Edissolution from (a) and respective data for 0.1 mM PdCl
against the deposition potential Edeposition
2
.
The distance of the spots along the row is 8:7 Æ 0:9
ꢁ
A. Details of this structure will be given in a forth-
coming publication [19].
In Fig. 6, a sequence of STM images for the
initial stages of Pd deposition on Au(1 1 0) is
shown. Fig. 6a represents the gold surface, covered
by the ordered adlayer before Pd deposition and
Fig. 6j shows the electrode covered by about a 7
ML equivalent of Pd bulk deposit. In between
Fig. 5. STM image (32 nm  32 nm) for an unreconstructed
Au(1 1 0) surface in 0.1 M H SO at 0.55 V (I
¼ 2 nA).
2
4
T
(
Fig. 6b–i), the potential of the Au(1 1 0) electrode
is confined to the upd regime. We reemphasise that
the potential values obtained by the Pt quasi-ref-
erence electrode in the STM cell differ slightly but
erratically from those measured with the SCE.
This is reflected by the observation that 0.49 V still
refers to the upd range (Fig. 6e–i). As expected,
surface defects were found to play an important
role in the nucleation of the Pd on Au(1 1 0). The
palladium deposition process starts by a decora-
tion of the monoatomichigh steps, the deposit

180
L.A. Kibler et al. / Surface Science 498 (2002) 175–185
Fig. 6. Sequence of 10 STM images, (a–i) 30 nm  30 nm, (j) 500 nm  500 nm, showing the initial stages of Pd deposition onto
Au(1 1 0) from 0.1 M H SO þ 0:6 mM HCl. Deposition potentials as indicated in the figures. The location of one
þ 0:1 mM PdCl
monoatomichigh step in (a) is represented by the white line in (b) and ( c) as guide for the eye ( I
¼ 2 nA).
2
4
2
T
growing onto the lower terrace (Fig. 6b). The de-
position process then continues by formation of
two-dimensional islands on the flat terraces (Fig.
6c–e). The second layer starts to grow before the

L.A. Kibler et al. / Surface Science 498 (2002) 175–185
181
Fig. 6 (continued)
first layer is completed (Fig. 6f and g) as the ob-
3.3. Electrochemical behaviour
served overlapping of the peaks in the cyclic vol-
tammogram (Fig. 1a) also suggests. An adlayer
similar to, although significantly less well-ordered
than the one in Fig. 6a is observed on top of
the grown islands. After about 15 min, deposition
at 0.49 V ceased and high-resolution images
at more negative potentials did not yield any fur-
ther information. Large scale images for bulk
Pd reveal a rather rough and scale-like surface
The electrochemical properties of thin palla-
dium films on Au(1 1 0), which were determined in
2 4
0.1 M H SO after transfer to another electro-
chemical cell, were found to be strongly dependent
on the Pd coverage. The latter was estimated from
the charge flown during the deposition reaction,
and proved to be decisive for the voltammetric
profile. This is demonstrated in Fig. 8. The cyclic
voltammograms show two characteristic potential
regions: The region of surface oxidation and re-
duction for E > 0:4 V and the hydrogen adsorp-
tion and desorption region between 0.1 V and ꢀ0:2
V. The latter process can be ideally studied with
thin Pd films, because hydrogen absorption is
shifted towards more negative potentials as com-
pared to massive Pd single crystals [5].
(
Fig. 6j).
Complete dissolution of such a rather thick Pd
overlayer at 0.75 V led to the very same adlayer
structure on gold as before Pd deposition (Fig. 7a).
However, the Au(1 1 0) surface is now full of
monoatomicdeep holes of different size, whihc
suggests that surface alloying had taken place
during Pd deposition.

182
L.A. Kibler et al. / Surface Science 498 (2002) 175–185
Fig. 7. STM images of Au(1 1 0) in 0.1 M H
]
2
SO
adlayer on a surface full of holes: (a) 30 nm  30 nm, (b) 105 nm  105 nm (I
4
þ 0:1 mM PdCl
2
þ 0:6 mM HCl after dissolution of several layers of palladium at
¼ 2 nA).
2ꢀ
0
.75 V, showing the [PdCl
4
T
2 4
Fig. 8. Cyclic voltammograms for Pd overlayers of four different coverages on Au(1 1 0) in 0.1 M H SO (scan rate: 10 mV/s).
For the case (c)—3 ML Pd on Au(1 1 0)—the
positive potentials, since the anodic charge exceeds
the cathodic charge by far. (Redeposition from a
solution containing no Pd salt would be practically
nil.) The oxidation peak at 0.47 V, which resem-
bles the one of massive Pd(1 1 0), appears to be
present only for average overlayer thickness of 2
ML and more (see case b), while it is completely
absent at lower coverage. For monolayer and sub-
monolayer deposits, Pd dissolution at positive
potentials is predominant, as inferred from the
charge imbalance seen in case (a) and from chan-
voltammetricshape in the oxidation region is very
similar to that reported for a massive Pd(1 1 0)
electrode [20], but the peak around 0.5 V is not as
sharp. For 0.6 V as positive potential limit (broken
line in Fig. 8c), there is a perfect balance between
2
anodic and cathodic charge (about 150 lC/cm )
for this peak, indicating that the current is due to
ꢀ
oxide formation and reduction (or OH adsorp-
tion and desorption) rather than Pd dissolution.
The latter process becomes important for more

L.A. Kibler et al. / Surface Science 498 (2002) 175–185
183
ges in the voltammogram in the following cycles. It
appears that the anodicpeak at 0.7 V is related to
partial dissolution of Pd in the first or/and second
layer, because it is absent for higher coverage.
Hydrogen adsorption is seen to occur in two
overlapping steps, resulting in current peaks at
about ꢀ0:06 and ꢀ0:18 V (Fig. 8c). The total
charge after correction for double layer charging
reflected in the ordered structure of the adsorbate,
ꢀ
forming parallel chains along the [1 1 0] direction
of the substrate. Details of the adlayer structure
and its dependence on the electrode potential will
be described elsewhere [19].
2ꢀ
Thus, well-ordered adlayers of [PdCl
4
]
were
found for all three low-index Au single crystal
faces. From inspection of STM images like in Fig.
2ꢀ
2
amounts to about 280 lC/cm , a value similar to
6i, [PdCl
4
]
appears to be also adsorbed on the Pd
overlayers on Au(1 1 0), as it has been observed for
that obtained for Pt(1 1 0) [21]. Because hydrogen
adsorption on platinum is well-known to be a
convenient tool for titration of surface atoms
2ꢀ
Pd on Au(1 1 1) [8]. In the latter case, [PdCl ]
4
seems to play a crucial role for the growth be-
haviour, since the formation of three-dimensional
clusters was observed for Pd deposition in the
(
e.g. for surface roughness determination), similar
considerations may be applied to palladium. If the
charge for hydrogen adsorption in Fig. 8c relates
to a Pd(1 1 0) surface, the clearly lower charge
2ꢀ
hydrogen adsorption region, i.e. with no [PdCl4]
on the surface [25], in contrast to two-dimensional
2
2ꢀ
values in Fig. 8a and b (80 and 190 lC/cm for 1
4
growth for deposition potentials, where [PdCl ]
and 2 ML equivalents of Pd, respectively) signal
incomplete coverage of the gold substrate, despite
ample Pd on the surface. This again supports the
hypothesis of massive alloying between Pd deposit
and Au(1 1 0) substrate.
is adsorbed. However, there might also be an effect
of the potential proper, i.e. different overvoltage.
4.2. Pd deposition on Au(110)
The cyclic voltammograms and the charge iso-
therms in Fig. 1 indicate that the Pd upd on
Au(1 1 0) proceeds in two steps. This is often re-
lated to the formation of superstructures, as found
for many upd systems [1,2]. However, the results
presented in Section 3, especially the STM images,
do not point in this direction. Discharge of one
4
. Discussion
2
ꢀ
4
.1. Adsorption of [PdCl4]
The adsorption of metal chloro complexes was
2þ
found to play a key role in electrochemical depo-
sition of Pt [22,23], Rh [24] and Pd [8,9,12,13,25]
on Au(1 1 1) and Au(1 0 0) surfaces. The square-
type maxima in the STM images in Fig. 6 are
very similar to those observed for [PdCl4] on
Au(1 0 0) in the same electrolyte [9], showing that
Me ion per unit cell on the unreconstructed
2
Au(1 1 0) surface corresponds to 272 lC/cm .
However, the (1 1 0) surface is only covered com-
pletely, when 2 ML of metal are deposited. Con-
2
2
ꢀ
sequently, 2 ML (corresponding to 544 lC/cm )
deposited in two steps would be expected to be
involved in the upd of palladium on the Au(1 1 0)
surface as previously found for Cu upd on the
unreconstructed (1 1 0) surfaces of Pt and Pd
[26,27].
2ꢀ
[
PdCl
4
]
planar [PdCl
is adsorbed on Au(1 1 0) too. So far, the
]
2ꢀ
4
molecule was observed to lie
down on the Au(1 1 1) and Au(1 0 0) surfaces when
being adsorbed. However, for the more open
(
1 1 0) surface, one could imagine, that the mole-
For Pd upd on Au(1 1 0), there is also a clear
change in mechanism after deposition of just more
than 1 ML (Fig. 1b), that gives rise to two upd
peaks in the voltammogram (Fig. 1a). However,
the measured total charge was in fact found to
cule may also be aligned perpendicular to the
surface in the substrate rows. Such a geometry
could be the origin of the thinner rows in the ad-
sorbate image (Fig. 6), although we cannot com-
pletely rule out that the latter may represent
coadsorbed chloride, which is assumed to be pre-
2
be around 800 lC/cm , which corresponds roughly
to the deposition of 3 (!) ML of Pd, assuming
2ꢀ
2þ
sent in the [PdCl adlayer on Au(1 0 0) [9]. In
any case, the anisotropy of the Au(1 1 0) surface is
4
]
that reduction of Pd is the main contribution
to this cathodic charge. Anion effects should be

184
L.A. Kibler et al. / Surface Science 498 (2002) 175–185
2
ꢀ
negligible, since [PdCl
4
]
is adsorbed both on the
4.3. Electrochemical behaviour of Pd overlayers on
Au(110)
Au(1 1 0) substrate and on the Pd adlayers as
mentioned above (see Section 4.1). The difference
in the current–potential characteristics for ad-
sorption and desorption (Fig. 1a) points towards
slow dissolution kinetics or towards some side re-
action, such as surface alloy formation as in the
case of Pd deposition on Au(1 0 0) [9]. There is
an obvious time effect on the desorption curves
Unlike for Au(1 1 1), a pseudomorphicPd
monolayer is not obtained on Au(1 1 0) by elec-
trochemical deposition. However, in the sub-
monolayer region and up to a coverage of about 2
ML, where both gold and palladium atoms are
present on the surface due to alloying, interesting
electrochemical and electrocatalytic properties are
expected. The thicker Pd films, which are rather
rough, are known to behave similar to a massive
Pd(1 1 0) single crystal surface [10]. This means
that Pd adlayers on Au(1 1 0) like the one shown
in Fig. 6j are indeed epitaxially grown. Thus, the
electrochemical and electrocatalytic properties of
electrochemically deposited Pd films on Au(1 1 0)
approach with increasing coverage the behaviour
of a massive Pd(1 1 0) electrode, as indicated by
literature data [10] and by our preliminary results.
However, large deviations from bulk behaviour
are observed for the thinner palladium films (up to
2 ML), where both gold and palladium atoms are
present on the surface. The potentials for palla-
dium electrodissolution, oxide formation (and CO
oxidation, to be complete) are strongly influenced
not only by the crystallographic orientation of
the Au substrate, but also by the surface mor-
phology and composition, which is determined
by the amount of Pd deposited. This is also true
for Pd on Au(1 0 0), but not in that extent for
Pd on Au(1 1 1). In the latter case, the electro-
chemical properties are determined by the presence
of pseudomorphicPd overlayers. In addition,
monoatomichigh steps on the Au(1 1 1) substrate
were found to play a decisive role for the oxidation
of the Pd films. This means that different poten-
tials were observed for oxidation of well-ordered
‘‘Pd(1 1 1)’’ terraces (ꢁ0.8 V) and of Pd defect sites
(ꢁ0.6 V). Similar effects were found for stepped
Pd(1 1 1) electrodes [29].
(
compare Fig. 1a with Fig. 3a) and, in addition,
the holes in the Au(1 1 0) surface, which are ob-
served in STM after dissolution of palladium (Fig.
7
), are a strong indication for an alloying process.
It might be possible, that in the course of this
surface alloy formation additional Pd is deposited
in the underpotential region besides the 2 ML,
which can be expected for a (1 1 0) surface. If a
surface Au atom and a Pd atom change their po-
sitions, one may understand that more Pd can be
deposited in the upd region due to the strong in-
teraction of the two metals. In this context it is
3
interesting to recall, that for a Au Pd(1 1 0) alloy, a
segregation of Au with a topmost layer con-
centration of 100 at.% Au was reported [28]. The
palladium deposition process on Au(1 1 0) was
seen in the STM images to start by a decoration of
monoatomichigh steps (Fig. 6b and c) , however,
in contrast to Pd deposition on Au(1 1 1) and on
Au(1 0 0), deposition on the upper terrace quickly
commences. Since the growth of palladium does
not proceed in a perfect layer-by-layer mode, exact
Pd coverage data, which could support the depo-
sition of 3 ML of Pd in the upd region, cannot be
obtained from the STM images.
So far, we have observed, that the tendency of
alloy formation during Pd deposition is strongly
influenced by the crystallographic orientation of
the Au substrate and is increasing in the order
Auð111Þ < Auð100Þ < Au(1 1 0) [8,9]. Intuitively,
this order is reasonable, since the surface atoms
are packed less and less densely. However, we are
still far from a mechanistic model for the alloying
process. Adatom diffusion that involves exchange
of substrate atoms could be related to surface al-
loying. If such exchange diffusion is absent for
close-packed surfaces, one might have an expla-
nation that Pd deposition on Au(1 1 1) does not
lead to surface alloy formation.
The situation changes when Pd is deposited on
Au(1 0 0) or Au(1 1 0). For these systems, there are
clear indications of surface alloy formation. This
means that for low coverages, surface structures
are formed, the properties of which do not re-
semble those of massive Pd(1 0 0) or Pd(1 1 0)
electrodes. Only for thicker Pd films, where Au

L.A. Kibler et al. / Surface Science 498 (2002) 175–185
185
atoms are no longer on the surface, the electro-
chemical properties are similar to the massive
electrodes.
[2] E. Budevski, G. Staikov, W.J. Lorenz, Electrochemical
Phase Formation and Growth, VCH, Weinheim, 1996.
[3] R. Parsons, T. VanderNoot, J. Electroanal. Chem. 257
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(
[4] G.A. Somorjai, Introduction to Surface Chemistry and
Catalysis, Wiley, New York, 1994.
[
[
5] M. Baldauf, D.M. Kolb, Electrochim. Acta 38 (1993) 2145.
6] M.J. Llorca, J.M. Feliu, A. Aldaz, J. Clavilier, J. Electro-
anal. Chem. 351 (1993) 299.
5
. Conclusions
2ꢀ
(
1) [PdCl
4
]
forms ordered adlayers on all three
[7] A. Cuesta, L.A. Kibler, D.M. Kolb, J. Electroanal. Chem.
466 (1999) 165.
low-index faces of gold. On Au(1 0 0) and on
Au(1 1 0), chloride might be coadsorbed.
[
8] L.A. Kibler, M. Kleinert, R. Randler, D.M. Kolb, Surf.
Sci. 443 (1999) 19.
2ꢀ
(
2) [PdCl
4
]
is also adsorbed on electrochemi-
[
9] L.A. Kibler, M. Kleinert, D.M. Kolb, Surf. Sci. 461 (2000)
1
cally deposited Pd overlayers on Au(1 1 1) and on
Au(1 1 0), whereas it is replaced by chloride for Pd
on Au(1 0 0).
55.
[10] M. Baldauf, D.M. Kolb, J. Phys. Chem. 100 (1996) 11375.
[11] H. Naohara, S. Ye, K. Uosaki, 51st ISE Meeting 2000,
Abstracts
45-658.pdf.
12] H. Naohara, S. Ye, K. Uosaki, J. Phys. Chem. B 102
1998) 4366.
http://www.chem.uw.edu.pl/ise2000/ext_abs/
(
3) The deposition of Pd on Au(hk l) starts in
9
the upd region by the decoration of surface defects
and is followed by the formation of two-dimen-
sional islands.
[
(
[13] H. Naohara, S. Ye, K. Uosaki, J. Electroanal. Chem. 473
(1999) 2.
(
4) In the case of Pd deposition on Au(1 0 0) and
[
14] M. Takahasi, Y. Hayashi, J. Mizuki, K. Tamura, T.
Kondo, H. Naohara, K. Uosaki, Surf. Sci. 461 (2000) 213.
15] D.M. Kolb, Prog. Surf. Sci. 51 (1996) 109.
16] M. Sturmat, R. Koch, K.H. Rieder, Phys. Rev. Lett. 77
(1996) 5071.
Au(1 1 0), there are strong indications of alloy
formation, which may be explained by adatom
exchange diffusion.
[
[
(
5) PseudomorphicPd layers are only formed
on Au(1 1 1). Pd growth on Au(1 1 0) does not
follow a perfect layer-by-layer mechanism and no
pseudomorphicPd overlayers were seen in the
STM images. Pd films thicker than 3 ML exhibit a
potentiodynamicbehaviour typi ac l for Pd(1 1 0)
surface, proving an epitaxial film growth.
[17] Y. Gao, A. Hamelin, M.J. Weaver, Phys. Rev. B 44 (1991)
0983.
1
[18] M. Kleinert, V. Lazarescu, L.A. Kibler, D.M. Kolb, in
preparation.
[19] M. Kleinert, D.M. Kolb, in preparation.
[20] K. Sashikata, Y. Matsui, K. Itaya, M.P. Soriaga, J. Phys.
Chem. 100 (1996) 20027.
[
[
[
[
21] L.A. Kibler, A. Cuesta, M. Kleinert, D.M. Kolb, J.
Electroanal. Chem. 484 (2000) 73.
22] K. Uosaki, S. Ye, Y. Oda, T. Haba, K. Hamada,
Langmuir 13 (1997) 594.
Acknowledgements
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Kondo, J. Phys. Chem. B 101 (1997) 7566.
24] L.A. Kibler, M. Kleinert, D.M. Kolb, J. Electroanal.
Chem. 467 (1999) 249.
One of us (V.L.) acknowledges gratefully an
Alexander von Humboldt stipend as well as fi-
nancial assistance of ANSTI.
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01.
2
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