In situ STM study of nanosized Ru and Os islands spontaneously deposited on Pt(111) and Au(111) electrodes

Full text of DOI:10.1016/j.susc.2004.04.060

Surface Science 573 (2004) 80–99
www.elsevier.com/locate/susc
In situ STM study of nanosized Ru and Os islands
spontaneously deposited on Pt(111) and Au(111) electrodes
a
S. Strbac , C.M. Johnston , G.Q. Lu , A. Crown , A. Wieckowski
b
b
b
b,
*
a
ICTM-Institute of Electrochemistry, University of Belgrade, 11001 Belgrade, Serbia and Montenegro
b
Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
Received 20 August 2003; accepted for publication 20 April 2004
Available online 2 August 2004
Abstract
We provide an overview of structure and reactivity of selected bimetallic single crystal electrodes obtained by the
method of spontaneous deposition. The surfaces that are described and compared are the following: Au(111)/Ru,
Pt(111)/Ru and Pt(111)/Os. Detailed morphological information is presented and the significance of this work in cur-
rent and further study of nanoisland covered surfaces in the catalytic and spectroscopic perspective is highlighted. All
surfaces were investigated by in situ STM and by electroanalytical techniques. The results confirm our previous data
that nanosized Ru islands are formed with specific and distinctive structural features, and that the Ru growth pattern
is different for Au(111) and Pt(111). For Au(111), Ru is preferentially deposited on steps, while a random and rela-
tively sparse distribution of Ru islands is observed on terraces. In contrast, for Ru deposited on Pt(111), a homogene-
ous deposition over all the Pt(111) surface was found. Os is also deposited homogeneously, and at a much higher rate
than Ru, and even within a single deposition it forms a large proportion of multilayer islands. On Au(111), the Ru
islands on both steps and terraces reach the saturation coverage within a short deposition time, and the Ru islands grow
to multilayer heights and assume hexagonal shapes. On Pt(111), the Ru saturation coverage is reached relatively fast,
but when a single deposition is applied, Ru nanoislands of mainly monoatomic height are formed, with the Ru coverage
not exceeding 0.2 ML. For Ru deposits on Pt(111), we demonstrate that larger and multilayer islands obtained in two
consecutive depositions can be reduced in size––both in height and width––by oxidizing the Ru islands and then by
reducing them back to a metallic state. A clear increase in the Ru island dispersion is then obtained. However, methanol
oxidation chronoamperometry shows that the surface with such a higher dispersion is less active to methanol oxidation
than the initial surface. A preliminary interpretation of this effect is provided. Finally, we studied CO stripping reaction
on Pt(111)/Ru, Au(111)/Ru and on Pt(111)/Os. We relate CO oxidation differences observed between Pt(111)/Ru and
*
Corresponding author. Fax: +1 217 244 8068.
E-mail address: andrzej@scs.uiuc.edu (A. Wieckowski).
0039-6028/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.susc.2004.04.060

S. Strbac et al. / Surface Science 573 (2004) 80–99
81
Pt(111)/Os to the difference in the oxophilicity of the two admetals. In turn, the difference in the CO stripping reaction
on Pt(111)/Ru and Au(111)/Ru with respect to the Ru islands is linked to the effect of the substrate on the bond
strength and/or adlayer structure of CO and OHads species.
ꢀ 2004 Elsevier B.V. All rights reserved.
Keywords: Scanning tunneling microscopy; Electrochemical methods; Catalysis; Osmium; Ruthenium; Platinum; Gold; Low index
single crystal surfaces
1. Introduction
The use of the voltammetry as the preparative
component of this method is simply a routine
practice of our laboratory, which yields the same
surface morphology as obtained by the potentio-
static treatment.
Catalysts for many chemical reactions are often
composed of two or more noble metals yielding
catalytic activity higher than that of the pure com-
ponents, and this enhancement has been extensively
explored for nanoparticle and bulk polycrystalline
[1–4], as well as for single crystal [3–5] electrodes.
Mixed-metal, single crystal catalytic surfaces of
interest to this report can be generated by evapora-
tion of the deposit onto the substrate in UHV [6–9],
by electrodeposition [6,10–16], electroless deposi-
tion [17], and spontaneous deposition [18–24] (see
also Ref. [6]).
Spontaneous deposition procedure involves
three major experimental steps. First, the electrode
made of a substrate material (Pt and Au in this
study) is immersed in an admetal-containing solu-
tion (usually, ionic forms of Ru and Os). Second,
the electrode is removed from the deposition bath,
rinsed (in water) and transferred to an electro-
chemical cell filled up with supporting electrolyte.
Third, while in an electrochemical cell, the elec-
trode is treated by a few negative- and positive-
going scans by cyclic voltammetry or by holding
the surface at ca. 50 mV (vs. RHE) in the support-
ing electrolyte, where the precursor is reduced to
metallic ruthenium (or osmium). This procedure
leads to formation of the substrate surface ‘‘deco-
rated’’ by metallic nanosized islands of adjustable
coverage and height, depending on the details of
the deposition, such as the concentration of the
electrolytic bath, duration of chemisorption and
a follow up electrochemical treatment such as de-
scribed in Sections 3.2.4 of this report. Notably,
holding the potential in the hydrogen adsorption
range (near the hydrogen evolution edge) is suffi-
cient to produce the metallic state of the deposit,
as was confirmed by XPS measurements [25].
The spontaneous deposition method encodes
specific and unique functions to the support mate-
rial and facilitates preparation of structurally het-
erogeneous bimetallic electrode catalysts. The
method is simple, does not involve electrochemical
instrumentation for producing the decorated sur-
face, and is immune to IR drop constrains during
deposition, which is of importance for high surface
area catalysts [26,27]. Notably, after a few minutes
of the first deposition, only Ru coverage at ca. 0.2
ML is achieved, and the deposition stops. Higher
deposit coverage can be obtained by using multiple
spontaneous depositions; i.e., by repeating the
exposure of the electrode obtained after first depo-
sition/reduction cycle to the depositing metal ion
containing solution, and again by stabilizing the
new deposit by electrochemical reduction [24,27,
28]. The fact that the ruthenium saturation cover-
age at the first deposition run is low indicates that
the Ru–Pt interactions leading to formation of the
Ru precursor chemisorbate [29] are no longer
operative at the Ru pretreated surface. This may
as well be due to the steric reasons: the bulky (oxi-
dized [25,29]) Ru chemisorbate keeps the Ru ions
at the interface away from active Pt sites to pre-
vent further chemisorption to continue. Because
the repeated deposition yields the Ru coverage
above the 0.2 ML level, we conclude that the
excess (above 0.2 ML) deposition may begin and/
or continue only on the surface of a metal, either
platinum or ruthenium. Since the Ru islands grow
mainly in height, the existing Ru islands are pre-
ferred sites for Ru deposition in the second (and
multiple) exposure to the Ru solution.

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S. Strbac et al. / Surface Science 573 (2004) 80–99
In this present report, we briefly review our al-
geometry before a spectroscopic assay is per-
formed. The paper we present below responds to
these particular needs in fuel cell catalysis science,
and provides a comprehensive structural determi-
nation of the surfaces selected for the future spect-
roscopic efforts.
ready published STM data on Ru spontaneously
deposited on Au(111) [29] and present new data
on morphological rearrangements of Ru islands
on Pt(111)/Ru induced by an electrochemical treat-
ment. All work reported in this communication was
carried out by the use of in situ STM [30]. Cyclic
voltammetry and chronoamperometry are used
for electrochemical characterization and reactivity
measurements. We also present data, both previous
[29] and new, which allow us to compare the CO
stripping reaction on Pt(111)/Ru and on Pt(111)/
Os, and on Au(111)/Ru [31]. Mechanistic details
of CO poison removal process from Pt(111)/Ru
are a key topic in present fuel cell catalysis research.
We note that the chemical state of the Ru and
Os deposits on Pt(111) obtained spontaneously
has recently been studied by electrochemical XPS
and synchrotron grazing incidence fluorescence-
X-ray absorption spectroscopy (GIF-XAS) (GIF-
XAS from Pt(111)/Os) only [25,32]. Immediately
after the deposition of Ru on Pt(111) at open cir-
cuit, the data show that the deposit contains
mainly Ru4+ and Ru6+ [18,25]. Such an original
(oxidized) Ru precursor is reduced to the predom-
inantly metallic state via the electrochemical treat-
ment described above and reported in Ref. [25].
The work on Pt(111)/Os [32] also included collect-
ing electrochemical data on osmium oxidation/
electrodissolution. Apparently, the Os deposit re-
mains purely metallic up to 500 mV (vs. RHE)
while above 500 mV it begins to oxidize to Os(IV).
Electrodissolution of osmium oxides to dissolved
Os forms begins above 900 mV and occurs simul-
taneously with platinum oxidation. (Oxidation of
metallic ruthenium begins at more negative poten-
tial than osmium, approximately at 300 mV.)
It is clear nowadays that there is a need to de-
velop and understand the promoting catalytic ef-
fects of nanosized metal islands on selected
electrode surfaces for organic molecule oxidation
that can be used for fuel cells. Before such catalytic
effects are understood, a significant control of the
morphology of such bimetallic surfaces needs to
be achieved. Also, further work by electrochemical
spectroscopies with adisland decorated single crys-
tal electrodes [25,32] requires very detailed struc-
tural information about the electrode surface
2. Experimental
An Au(111) single crystal, 6 mm in diameter
(Metal Crystals and Oxides, Cambridge, England),
cut and oriented to better than 0.5ꢁ, was used as
substrate for the electrochemical measurements re-
ported below. A 10 mm diameter Au(111) single
crystal (Accumet Materials, Ossining, New York),
cut and oriented to better than 0.5ꢁ was used for the
in situ STM imaging. After mechanical and electr-
ochemical polishing, the Au crystal was annealed in
a hydrogen flame for several minutes, cooled down
in air, and either mounted into the electrochemical
cell of the STM or in the external electrochemical
cell for cyclic voltammetry (CV) characterization.
Electrochemical measurements were performed
using an EcoChemie Autolab PSTAT100.
Pt(111) single crystals, 10, 6, and 2.2 mm in
diameter (Accumet Materials, Ossining, New
York), cut and oriented to better than 0.5ꢁ were
used as substrates for the in situ STM imaging,
chronoamperometry, and voltammetry measure-
ments, respectively. After mechanical and electro-
chemical polishing, the Pt crystal was annealed in
hydrogen flame for several minutes, and cooled
down in the mixture of H2 and Ar gases. The crys-
tal, protected by a drop of ultrapure water, was then
transferred to an external electrochemical cell for
the voltammetric and chronoamperometric meas-
urements. For the in situ STM work, the Pt surface
was additionally processed by the I/CO treatment
[33] modified in this work, which will be described
in detail in Section 3. Multiple depositions were
implemented by repeating the deposition and
reduction procedures on the Pt(111) substrate con-
taining previously deposited metallic Ru [24,26,27].
In this study, we conducted up to two repetitive
depositions of Ru on the Pt(111) substrate.
All potentials in this paper (unless noted other-
wise) are reported vs. Ag/AgCl/3M NaCl. Solu-

S. Strbac et al. / Surface Science 573 (2004) 80–99
83
tions were prepared using RuCl3ÆxH2O and
OsCl3ÆxH2O (Alfa Aesar) salts, double distilled
H2SO4 (GFS chemicals) and HClO4 (GFS chemi-
cals) and Milli-Q water. The spontaneous deposi-
tion of ruthenium on Au(111) and Pt(111)
electrodes was performed at an open circuit poten-
tial (OCP) for the stated deposition time from an
aged (over two weeks) 1 mM RuCl3 + 0.1 M
HClO4 electrolyte. For generating Pt(111)/Os sur-
faces, a similarly aged 0.1 mM OsCl3 + 0.1 M
H2SO4 was used. Before and after the measure-
ments, the electrode potential was kept for a few
minutes at 50 mV (vs. RHE) in order to ensure
that the Ru or Os deposit was reduced to the
metallic phase [25,32,34].
Scanning tunneling microscopy measurements
were performed in situ using a Molecular Imaging
(MI) Pico STM. Pt–Ir and W tips used were elec-
trochemically etched and insulated with clear nail
polish prior to each experiment. STM images were
obtained in a constant current mode with the tun-
neling current between 1 and 27 nA. The coverage
on terrace areas was determined as the fraction of
the substrate area covered by Ru or Os islands in
the respective STM images, while the coverage
on steps was calculated as the fraction of the step
length covered by the deposited Ru islands. Data
analysis was achieved using the Visual SPM soft-
ware from Molecular Imaging.
Fig. 1. Cyclic voltammograms recorded in 0.1 M H2SO4 of the
clean Au(111) surface (solid line); after Ru was spontaneously
deposited from 1 mM RuCl3 in 0.1 M HClO4 for 30 s (dotted
line) and 3 min (dashed line) (sweep rate: 50 mV/s).
voltammetric properties in sulfuric acid [35–37].
The peak at 0.35 V is due to lifting Au(111) recon-
struction via sulfate adsorption, and the peak at
ca. 0.80 V is associated with the formation of an
ordered sulfate adlattice [38]. The clean Au(111)
electrode was exposed to the Ru-containing solu-
tion for 1 min, during which the OCP value rapidly
increased from 0.38 to 0.72 V [29]. The electrode
potential was then stepped to 0.00 V in order to re-
duce the high Ru valency deposit species (see Sec-
tion 1) to a Ru metallic state. Also in Fig. 1, CVs
obtained after different Ru deposition times:
tdep = 30 s (dotted line) and tdep = 3 min (dashed
line) are shown. The peak at ca. 0.8 V, character-
istic of formation of an ordered sulfate adlayer,
is increasingly suppressed with the added Ru, indi-
cating that the long-range ordering of the sulfate
adlayer is severely inhibited.
If STM measurements were made following the
electrochemical treatment, e.g. via cyclic voltam-
metry (see Section 1), the STM image was taken
at after the CV run was interrupted and the inter-
ruption potential was held constant. A reasonable
assumption was made that at such a holding
potential, the state of the deposit was ‘‘frozen’’
and available for STM measurements as a genuine
signature of the deposit structure.
3.1.2. In situ STM images
The clean Au(111) surface was first character-
ized by STM at OCP (Fig. 2). The initial unrecon-
structed Au(111) surface with large 100–200 nm
terraces containing no additional features is shown
in Fig. 2A. Also shown in Fig. 2B is a highly
stepped area with terrace widths of 2–10 nm
(Fig. 2B). STM images were next recorded follow-
ing different times of the deposition of Ru on
Au(111) to display the time effect on the deposit
3. Results and discussion
3.1. Au(111)/Ru surfaces [29]
3.1.1. Cyclic voltammetry
A cyclic voltammogram (CV) of the clean
Au(111) surface is shown as a solid line in Fig.
1, and is representative of clean Au(111) surface

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S. Strbac et al. / Surface Science 573 (2004) 80–99
Fig. 2. STM images of the initial Au(111) surface prior to
ruthenium deposition. (A) Smooth area of the Au(111) surface
(285 · 285 nm2); (B) Highly stepped surface area (140 · 140
nm2). Images are recorded in 0.5 M H2SO4 at an open circuit
potential.
morphology (Fig. 3). The in situ images were ob-
tained under electrode potential control after the
Ru-containing electrolyte was replaced with the
0.5 M H2SO4 supporting electrolyte. They were re-
corded and found invariant with electrode poten-
tial in the range of À0.1–0.1 V, where metallic
Ru is present on the surface [25]. The images reveal
a strong step preference for the Ru deposition,
indicative of heterogeneous nucleation. The island
density on the flat terraces increases at a much
slower rate than on the steps, as is demonstrated
by the STM images in Fig. 3A and B (obtained
Fig. 3. STM images of Ru modified Au(111) recorded at 0.1 V
in 0.5 M H2SO4 after Ru was spontaneously deposited from 1
mM RuCl3 in 0.1 M HClO4 for (A) 30 s (330 · 330 nm2); (B) 3
min (360 · 360 nm2); (C) 3 min (230 · 230 nm2), obtained on a
highly stepped area of Au(111).
after 30 s and 3 min Ru deposition, respectively).
The step coverage, 22 3%, is much higher than
the terrace coverage, 10 2%, after 30 s deposi-
tion. The total coverage is only 12 2% because
of the large size of the terraces. After 3 min depo-

S. Strbac et al. / Surface Science 573 (2004) 80–99
85
sition, the step coverage is high, 83 8%, so that
the Ru islands appear to have coalesced into the
strings of multilayer islands. The island density
on the terraces increased much less to only
18 3%. Mostly due to the high step activity, the
total Ru coverage increases almost threefold to
32 2%.
Given the pronounced role that reconstruction
plays in electrochemical deposition of Ru onto
Au(111) [13,34], we note that the spontaneous
deposition of Ru is performed at high OCP poten-
tials, and on the unreconstructed Au(111) surface.
Therefore, the surface reconstruction does not
play a major role in the nucleation stage of the
deposition process. When the electrode potential
was reduced to lower values, and the EC-STM
measurements were carried out in the region of
surface reconstruction, large Ru islands have
already been formed, and the diffusion of Ru
atoms from such stable islands outward is an unli-
kely event to consider. On the other hand, there
was a clear influence of the deposited islands on
the reconstruction process; the reconstruction
was clearly inhibited at higher Ru coverage. At a
saturation coverage (3 min), no surface reconstruc-
tion was found.
Fig. 4. The island height distribution for Ru islands spontane-
ously deposited on Au(111) from 1 mM RuCl3 in 0.1 M HClO4
for (A) 30 s; (B) 3 min.
To further demonstrate the ability of the sur-
face steps to increase the total Ru coverage, highly
stepped areas were chosen for imaging of the
Au(111) substrate. After 3 min deposition on a
highly stepped surface, shown in Fig. 3C, higher
density of islands is observed than on a similarly
sized area of Au(111) terraces. Due to the higher
coverage along steps than on flat terraces, the total
coverage reaches 52 5% on the highly stepped
surface as compared to the 32 4% obtained on
the terraces (see, e.g., Fig. 3B).
osition time of 30 s, the islands are predominantly
one layer high (50%), while 33% are two layers
high, 13% are three layers high and 7% consists
of four monolayers. With the increase of the dep-
osition time to 3 min, the fraction of the one-layer
high islands decreases to 17%, while the fraction of
two monolayer high islands increases to 50%, as
well as the fraction of three and four monolayer
high islands to 25% and 9%, respectively. The
appearance of a very small amount of even higher
islands was also noticed. The increase in the island
width on terraces can be attributed to the activity
of the borders between the deposited Ru and sub-
strate Au sites acting as new centers for the Ru
nucleation upon the RuOx reduction process.
The limiting factor here is the size of the island;
when the top of Ru islands becomes more active
For particular deposition conditions, the Ru is-
lands are uniform in size. With increasing deposi-
tion time, the size of deposited Ru islands
increases up to saturation coverage within 3 min.
Namely, the size of islands increases from 5
2
nm width after 30 s to 6 2 nm after 3 min depo-
sition. The same holds true for the island height,
which increases from 1.6 layers to ca. 2.5 layers
(on the average). The island height distributions
are presented in Fig. 4A and B, for the deposition
times of 30 s and 3 min, respectively. For the dep-

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S. Strbac et al. / Surface Science 573 (2004) 80–99
in the deposition than the Ru–Au borders, it leads
to further vertical growth of the deposited Ru.
Consequently, the transformation of the surface
two-dimensional islands to surface clusters is then
observed.
clear demonstration of the hexagonal shape and
uniform size of the islands. The step decoration
consists of more than a single string of islands
along the step: the islands are arrayed on the steps
in strings that are at least two islands wide. The is-
lands are most likely located on both sides of the
steps arranged one besides another. Similarly, Ru
islands are grouped together also on the terraces.
These groupings are attributed to density fluctua-
tions in the nucleation phase, which leads to for-
mation of closely spaced nuclei. Fig. 5B and C
demonstrates the multilayer nature of the Ru is-
lands, presenting a large central island (10 nm)
and two smaller islands (6 nm) at the bottom. Tak-
ing that the height of Ru monolayer is 0.22 nm, the
cross section analysis shows (Fig. 5C) that the cen-
tral island (ꢀ1.4 nm high) consists of six monolay-
ers, while the bottom two islands (ꢀ0.9 nm high,
the cross section not presented) consist of four
monolayers.
A more resolved STM image in Fig. 5A shows
the Ru island structure in more detail, including
The data presented above reveal that for a par-
ticular concentration of ruthenium in solution, the
density of steps is the main factor determining cov-
erage of Ru on Au(111). Using overall highly
stepped surfaces of Au results in an increased Ru
coverage. For Pt, which does not display high step
activity for Ru deposition, further active sites can
be generated by multiple spontaneous deposition,
as demonstrated below for the case of deposition
of Ru on Pt(111).
3.2. Pt(111)/Ru surfaces
3.2.1. Cyclic voltammetry
Cyclic voltammetry (CV) was employed to
characterize the electrode surfaces obtained by
spontaneous deposition of Ru on Pt(111). A typ-
ical CV profile of the clean Pt(111) surface in
clean sulfuric acid electrolyte (0.1 M H2SO4) is
shown in Fig. 6 (solid line). The clean Pt(111) sur-
face was exposed to Ru-containing solution for 3
min, and the OCP was observed to quickly stabi-
lize at ꢁ0.66 V. The curves obtained on the same
surface with deposited ruthenium are shown as
the dotted (one deposition) and dashed-dotted
(two depositions) lines. Notice that the characteris-
tic ‘‘butterfly’’ feature [39] is suppressed after one
deposition, indicating that obtaining a long-range
Fig. 5. Smaller scale STM images taken from Fig. 3B: (A)
(65 · 65 nm2); (B) (21 · 21 nm2); the cross sectional analysis of
the line through the islands.

S. Strbac et al. / Surface Science 573 (2004) 80–99
87
forming a protective iodine adlayer against plati-
num oxidation and contamination. Subsequently,
the I-protected crystal is safely mounted in the
STM cell, all operations (air transfer and mounting
in STM) carried out in air. A CO saturated sulfuric
acid solution is then admitted to the electrochemi-
cal STM cell and the potential is held at À0.1 V for
10 min to displace the iodide with carbon monox-
ide [33]. The CO saturated solution is replaced with
clean sulfuric acid while still under electrode poten-
tial control, and subsequently the surface is
scanned to 0.75 V in order to strip the surface of
CO without causing surface disorder. A cyclic vol-
tammetric (CV) curve is demonstrated in Fig. 7A,
where a regular CO stripping peak from the
Pt(111) electrode is observed [45–47]. The CV ob-
tained after the CO stripping (Fig. 7B) depicts the
character of a clean, well-ordered Pt(111) sub-
strate, with the relatively ‘‘flat’’ hydrogen adsorp-
tion/desorption regions, a well-defined and sharp
peak at ca. 0.22 V corresponding to sulfate adsorp-
tion on the (111) surface sites, and a small oxida-
tion and reduction peak at 0.48 and 0.44 V,
respectively. All these voltammetric features indi-
cate that the Pt surface maintains the proper purity
and crystallographic order. After mounting the
Pt(111) crystal in the STM cell, the surface was
characterized by STM at 0.1 V in 0.1 M H2SO4
(Fig. 7C). A typical STM image of the Pt(111) sur-
face, exhibits large terraces (ranging from 50 to 150
nm width) and monoatomic steps. These images
demonstrate the absence of surface islands, which
are later observable with Ru covered surfaces (see
below), which is characteristic of the lack of detect-
able contaminations and oxide(s).
Fig. 6. Cyclic voltammograms recorded in 0.1 M H2SO4 of the
clean Pt(111) surface (solid line), after 3 min of a single
spontaneous Ru deposition (dotted line), and after 2 · 3 min of
multiple deposition (dash-dot-dashed) line (2 · 3 min = two
spontaneous depositions, 3 min each).
order of the sulfate adlayer, demonstrated on
Pt(111) [40,41] is no longer possible on the
Pt(111)/Ru surface [42]. Additionally, the surface
exhibits a noticeable increase in the double layer
capacity.
3.2.2. Preparation of the Pt(111) surface for in situ
STM measurements
The integrity of the platinum single crystal order
is highly sensitive to oxygen chemisorption and con-
comitant surface oxidation (Ref. [43] and papers
cited therein). Therefore, the use of STM creates
an experimental difficulty due to the time it takes
to mount the crystal in the STM cell after the flame
annealing procedure [44], and the surface may dis-
order rapidly before the STM images are taken.
To prevent such disordering, as well as to eliminate
other possible platinum contamination reactions,
we have modified in this project a procedure [33]
that enables us to mount the crystal in the STM cell
while still maintaining the crystallographic long-
range order and cleanness of the crystal surface.
This procedure, based upon previous work [33],
is executed as follows. First, the crystal is annealed
in a hydrogen flame and cooled in hydrogen–argon
atmosphere [44]. The crystal is subsequently pro-
tected by a drop of deaerated water and transferred
to a 1 mM KI solution, where IÀ adsorbs on Pt
We notice that the modification involved
replacement of the gas phase iodine [33] as the sur-
face protecting factor with solution iodide, which
resulted in a major facilitation of the experiment.
However, the original idea to keep the protective
film of iodine on the surface as long as needed,
and then replace it by CO easily removable by
electrochemistry (without surface disorder) re-
mains the same [33].
3.2.3. A single spontaneous deposition
Following the clean Pt(111) examination by
in situ STM, ruthenium was spontaneously

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S. Strbac et al. / Surface Science 573 (2004) 80–99
Fig. 8. STM image (40 · 40) nm2 of Pt(111) in 0.1 M H2SO4
after Ru was spontaneously deposited for (A) 3 min, recorded
at 0.1 V, coverage = 18 3%; (B) the corresponding island
height distribution.
formed (the latter consecutively) for 3 min each,
and STM images from such Pt(111)/Ru surfaces
were obtained in 0.1 M H2SO4 and under the elec-
trode potential control. A representative in situ
STM image obtained after the first deposition at
saturation coverage of 18 3% reveals that ruthe-
nium islands nucleate homogeneously over the
Pt(111) surface (Fig. 8A). This is in agreement
with the results of previous ex situ STM investiga-
tions of the iodine protected Pt(111)/Ru surfaces
[24,42]. The average diameter of the mainly two-
dimensional Ru islands is in the range of 1–3
nm. As shown in Fig. 8B, most of the islands have
Fig. 7. (A) CO stripping after replacement of surface iodine by
CO on Pt(111) in 0.1 M H2SO4 at 50 mV/s (see text); (B) the
Pt(111) cyclic voltammogram in 0.1 M H2SO4 at 50 mV/s
obtained after the CO stripping (the next scan after the
stripping); (C) STM image (100 · 100) nm2 of the initial flat
Pt(111) surface prior to ruthenium deposition; the image
recorded in 0.1 M H2SO4 at 0.1 V.
deposited as described before [24] and in Section 1
to this report. One or two depositions were per-

S. Strbac et al. / Surface Science 573 (2004) 80–99
89
monolayer height (76 5%), but a clearly detecta-
ble amount of the multilayer growth appears as
well. Note that only the apex height is considered
in the counting process, and the islands are not
necessarily of homogeneous height across the
whole island. In contrast to spontaneous deposi-
tion of ruthenium on Au(111), no preferential
deposition along the Pt(111) steps is found (Fig.
8A). It therefore seems that the diffusion of Ru
adatoms over the Au(111) surface is much faster
than on Pt(111), enabling adatoms to diffuse to-
wards and along the steps forming the observed
step decoration morphology. The hexagonal shape
of growing islands in both cases indicates that ada-
toms diffuse along atomic rows as well as along the
edges of the growing Ru island i.e. there is no pref-
erential direction of diffusion.
3.2.4. Two spontaneous depositions
After imaging the deposit obtained from the
first deposition, the crystal was held at À0.1 V in
H2SO4 for a short time to ensure that metallic
Ru was present. H2SO4 solution was then replaced
by the ruthenium-containing solution (1 mM
RuCl3 in 0.1 M HClO4) at OCP and the second
deposition was performed for another 3 min. The
STM image of the resulting surface recorded at
0.1 V is presented in Fig. 9A. The coverage now
obtained is 22 3%, and the size of the islands in-
creased vs. data obtained during a single deposi-
tion (Fig. 8), with most islands having widths
between 2 and 5 nm. However, a significant in-
crease in the average island height is observed
(cf. Figs. 8B and 10A). Only 50 5% of the islands
are now of monolayer height, while the remaining
islands are made of two to four monolayers. The
data show high preference for ruthenium islands
to predominantly form on top of the previously
adsorbed islands, indicating the capability of
ruthenium to spontaneously deposit on the ruthe-
nium surface. Notice that the border sites of the
formerly deposited Ru islands of ruthenium also
act as nucleation centers for the subsequent depo-
sitions, as some increase in the island size is ob-
served (see above for Au(111)).
Fig. 9. STM images (40 · 40) nm2 of Pt(111) in 0.1 M H2SO4
after Ru was spontaneously deposited for 2 · 3 min (2 · 3
min = two spontaneous depositions, 3 min each): (A) recorded
at 0.1 V, coverage = 22 3%; (B) recorded at 0.35 V (partially
oxidized); (C) upon decreasing of the electrode potential back
to 0.1 V after surface oxidation at 0.7 V (see Fig. 11C). The
coverage = 25 3%.
Using the Pt(111) electrode covered by Ru via
two spontaneous depositions, we found that a
major factor determining the size of the ruthenium
islands was the ruthenium oxidation state, as it
depends on the electrode potential at which STM

90
S. Strbac et al. / Surface Science 573 (2004) 80–99
is given to illustrate the hexagonal shape of the
deposited islands. The island height can be envis-
aged from the cross-section analysis. With a fur-
ther increase in the electrode potential, a more
pronounced oxidation of the deposited Ru islands
takes place and, consequently, the morphology of
the deposited Ru changes. This is illustrated by the
STM image and the corresponding cross section
analysis obtained at the electrode potentials of
0.50 and 0.70 V, Fig. 11B and C, respectively. It
can also be seen that upon applying the potential
of 0.50 V, the disintegration of the Ru islands into
smaller islands begins. Breaking of the Ru islands
becomes even more pronounced with increasing
the potential to 0.70 V. Here, either one island is
close to another (like twin islands), or the islands
are fully separated.
3.2.5. The increase in Ru adisland dispersion;
consequences for methanol oxidation
Data in Figs. 9 and 11 show that when the oxi-
dized Pt(111)/Ru electrode at 0.7 V (Figs. 9B and
11) is polarized to 0.1 V a new STM image is ob-
tained (Fig. 9C). The inspection of these images
indicates that, upon reduction, the originally oxi-
dized and partially disintegrated islands (Fig. 11)
become smaller in size and more dispersed than
those at 0.7 V. They are also smaller than would
be expected if the surface were simply returning
to the original metallic state from before the oxida-
tion (compare Fig. 9A and C). Namely, while the
island width before the oxidation was 2–5 nm, it
is now reduced to 1–2 nm (Fig. 9C). At the same
time: (i) the island density is higher, (ii) the island
height decreases with 79 5% of the islands dis-
playing only monolayer height, Figs. 9C and
10B, and (iii) the Ru surface coverage increases
to 25 3%. We therefore conclude that a signifi-
cant change in surface morphology and an in-
crease in the adisland dispersion were obtained
after the redox sequence: 0.1–0.7 V and back to
0.1 V.
Fig. 10. The island height distribution for Ru islands sponta-
neously deposited on Pt(111) from 1 mM RuCl3 in 0.1 M
HClO4 for 2 · 3 min deposition (see Fig. 9): (A) at 0.1 V, initial
surface before oxidation; (B) after oxidation at 0.7 V and
subsequent reduction at 0.1 V (Fig. 9C and text).
imaging was performed. As shown in Fig. 9B,
increasing the electrode potential to 0.35 V leads
to an increase in the island lateral size and height.
These new features may be attributed to the trans-
formation of metallic Ru to Ru oxides, which al-
ready takes place at 0.35 V [25]. Notably, after
lowering the potential back to 0.10 V (Fig. 9C),
a new surface morphology was obtained which is
very much different from those shown in Fig. 9A
and B, but shows some resemblance the surface
presented in Fig. 8A, that is to the one obtained
by a single deposition, although at a higher Ru
coverage. Detailed comments on this image will
be provided in subsection below.
The increase in the island dispersion, as it in-
creases the ruthenium coverage, removes platinum
sites that were present on the bimetallic surface be-
fore the experiment reported above was executed.
In other words, the number of ‘‘ensembles’’ of Pt
sites [48,49] available, e.g., for chemisorption on
In Fig. 11A, an STM image obtained while
scanning over a small (20 · 20) nm2 surface area

S. Strbac et al. / Surface Science 573 (2004) 80–99
91
Fig. 11. STM images (20 · 20) nm2 in 0.1 M H2SO4, and corresponding cross sectional analysis of Pt(111) after Ru was spontaneously
deposited for 2 · 3 min (2 · 3 min = 2 spontaneous depositions, 3 min each), recorded: (A) at 0.35 V; (B) upon increasing the electrode
potential to 0.5 V; (C) upon increasing the electrode potential to 0.7 V.

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S. Strbac et al. / Surface Science 573 (2004) 80–99
the Pt sites of the Pt(111)/Ru surface is reduced. A
typical case where this development is important is
the process of dissociative chemisorption of meth-
anol, as it requires as many as three adjacent sites
for methanol dissociation (dehydrogenation) to
the chemisorbed CO [2,48,49]. Because methanol
oxidation to CO2 predominantly occurs via CO
formation process [2,5], the overall rate of metha-
nol oxidation may be affected by the increase in
ruthenium coverage at the expense of the number
of the collective Pt sites needed for methanol
decomposition to CO [48,49].
We also performed chronoamperometric meas-
urements in a methanol containing solution (0.6 M
CH3OH + 0.1 M H2SO4 solution) at a constant
electrode potential after using several Pt(111)/Ru
surfaces prepared as described above. The elec-
trode was immersed in the solution at À0.1 V for
1 min to ensure that the Ru deposit was initially
metallic, and a very slow methanol decomposition
[19]. Current–time decays were then collected at
0.16 V, which are representative of the methanol
oxidations kinetics at this electrode potential [19].
Fig. 12 displays four chronoamperometric
methanol oxidation curves. Curve A was obtained
with clean Pt(111), i.e. with no ruthenium on the
surface. The low current at the end of the decay
(after 30 min [19]) is due to Pt site-blocking by ad-
sorbed CO from methanol dehydrogenation [2].
For the Pt(111)/Ru surface obtained after the first
Ru deposition with 18 3% coverage of ruthe-
nium (see Fig. 8A), there is a much higher metha-
nol oxidation current (curve B), showing the high
catalytic enhancement of the Pt(111)/Ru electrode
versus clean Pt(111). The current measured for the
Pt(111)/Ru surface obtained after two consecutive
Ru depositions (Fig. 9A) is 1.6 times that obtained
after the single spontaneous deposition (curve C of
Fig. 12). However, after the surface has been per-
turbed by oxidizing the Ru islands at 0.70 V and
subsequently reducing them again at 0.1 V (see
Fig. 9C), the methanol oxidation current drops
to 1.3 times of that measured after the first Ru
deposition, as shown in curve D. This conforms
well to the expectations presented above, which
are in agreement with the bifunctional methanol
oxidation mechanism [50] that, to become optim-
ized, requires the appropriate balance of the Pt
and Ru sites on the surface [2,19,48,49].
3.3. Pt(111)/Os surface
3.3.1. Cyclic voltammetry
The CV obtained after exposing a Pt(111) elec-
trode to a 0.1 mM OsCl3 + 0.1 M H2SO4 solution
for 1 min is shown in Fig. 13. The feature at À75
mV going in the negative direction we attribute
to the reduction of Os oxides/hydroxides obtained
during spontaneous deposition [32]. After several
sweeps, a stable voltammogram is obtained; four
cycles are shown in Fig. 13. The characteristic
‘‘butterfly’’ peak at 0.24 V is only slightly sup-
pressed by Os deposition, although complete sup-
pression can be obtained with deposition from 1
mM Os solution for 5 min [32].
3.3.2. STM images
A single deposition was performed for 1 min
from the 0.1 mM OsCl3 + 0.1 M H2SO4 solution
at the open circuit potential, after which the solu-
tion was exchanged for 0.1 M H2SO4. Nine poten-
tial cycles from À0.2 to 0.6 V at 50 mV/s were then
used to stabilize the Os deposit, attaining a com-
plete reduction of the osmium precursor to the
Fig. 12. Current–time transients for curve (A) clean Pt(111);
curve (B) modified by 3 min of Ru deposition (to produce the
STM image shown in Fig. 8A); curve (C) 2 · 3 min Ru
deposition before methanol oxidation, (see caption to Fig. 9,
and Fig. 9A); curve (D) also 2 · 3 min Ru deposition but the
potential was increased to 0.7 V, and subsequently returned to
0.1 V (see Fig. 9C and text). The current–time transient was
recorded at 0.16 V in 0.6 M CH3OH + 0.1 M H2SO4.

S. Strbac et al. / Surface Science 573 (2004) 80–99
93
Fig. 13. Cyclic voltammograms recorded in 0.1 M H2SO4 of
the clean Pt(111) surface (solid line), and after exposing the
electrode to 0.1 mM OsCl3 + 0.1 M H2SO4 solution for 1 min
(dotted lines, four cycles).
metallic Os. The STM image was obtained at 0.1
V, that is, at a potential at which Os on Pt(111)
is metallic, as determined by XPS [32] (Fig. 14A).
The total coverage obtained from the STM image
is 22 3%. In these measurements, we have found
a significant multilayer growth of the Os islands, in
contrast to the images presented in the previous
study [32], undoubtedly because of the IÀ/CO
method of surface preparation used in this work.
As shown in Fig. 14B, for 1 min deposition from
0.1 mM Os solution, only 39 5% of the islands
are one Os layer high (0.22 nm), and there is a
significant multilayer growth even up to five
monolayers height. To determine the height distri-
bution, zoomed portions of images were analyzed
by counting the apex height of each island. In gene-
ral, the tallest islands are the widest islands as well,
and they seem to be composed of several smaller
islands fused together. One layer high islands typi-
cally are of only 1.5–2.5 nm width, whereas islands
2–4 layers high were 2.0–3.5 nm wide. The tallest
islands (five monolayers or higher) are even larger
at 3.0–5.5 nm wide.
Fig. 14. (A) STM image (185 · 185 nm2) of Pt(111) after
exposing the electrode to 0.1 mM OsCl3 + 0.1 M H2SO4
solution for 1 min and performing nine potential cycles from
À0.2 to 0.6 V to reduce the deposited Os (see text). Surface
potential was 0.1 V, and Os coverage = 22 3%; (B) the island
height distribution for the Os islands.
in the depositing solution is 10 times lower than
the Ru one and the deposition time is shorter, irre-
spective of the electrolyte composition (from per-
chloric to sulfuric acid medium). Multilayer
growth appears more significantly with Os, with
61 5% multilayer growth after only 1 min/0.1
mM deposition versus only 24 3% multilayer
growth on Pt(111)/Ru after 1 min/1 mM deposi-
tion. There is a wider distribution of the island
width and height for a single deposition of Os ver-
sus a single deposition of Ru on Pt(111), and the
The deposition of Os on Pt(111) occurs much
faster than Ru. Comparing the surfaces obtained
under the deposition conditions of 1 min/0.1 mM
Os solution and 3 min/1 mM Ru solution, a similar
Os coverage vs. the Ru coverage limit is reached by
the Os coverage even when the concentration of Os

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S. Strbac et al. / Surface Science 573 (2004) 80–99
islands are on average wider and taller. The islands
are in fact more comparable to those obtained
from two Ru depositions. Many of the larger Os
islands are composed of smaller, rounded units
of 1–2 nm width, whereas segmented growth was
not pronounced for Ru. When measuring the
height of the Os units defined in this way (counting
each segment), the number of Os units present
with multilayer height was still found to be
43 4%, which is still much greater than for
Pt(111)/Ru. Like with all Pt(111)/Ru surfaces
studied here, osmium is deposited homogeneously
without any preference for steps (see above).
3.4. Oxidation of surface CO on Au(111)/Ru
and Pt(111)/Me surfaces (Me = Ru and Os)
3.4.1. CO on Au(111)/Ru
We will next use CO chemisorption as a probe
of surface state of the studied electrodes in the
CO stripping voltammetry experiment. The first
characterized electrode is Au(111)/Ru. Because
CO desorbs from Au(111) to CO-free H2SO4 elec-
trolyte upon argon purging in the electrode poten-
tial range of interest (with possible exception of
adsorption on step sites), the observed CO strip-
ping voltammetry profiles can be attributed to
the catalytic properties (chemisorption/oxidative
desorption) of the Ru islands on the surface of
gold.
Fig. 15 depicts the CO stripping voltammetry
for Au(111)/Ru prepared by a 30 s spontaneous
deposition of Ru (Fig. 3A) and by 3 min deposi-
tion (Fig. 3B). Two main peaks associated with
CO stripping (solid line) are observed at 0.36 and
0.51 V for the 30 s case (ꢁ12% coverage), and at
0.37 and 0.56 V for the 3 min case (ꢁ32% cover-
age). The peak positions are significantly more
positive than the single peak at 0.30 V for CO oxi-
dation on bulk Ru or thick electrodeposited Ru
layers on Au(111) [34,51]. The peak is also more
positive than the CO stripping peak from
Ru(0001) at ca. 0.28 V, although for Ru(0001)
several sweeps are required to completely remove
CO, perhaps due to the formation of compact,
CO islands that are only reactive at the edges as
inferred from in situ FTIR studies [52]. The strip-
ping was complete in a single sweep here, indicat-
Fig. 15. CO-stripping voltammograms recorded in 0.1 M
H2SO4 of Au(111)/Ru in 0.1 M H2SO4 (A) after Ru was
spontaneously deposited from 1 mM RuCl3 in 0.1 M HClO4 for
30 s and (B) after Ru was deposited for 3 min. The solid lines
show the CO-stripping and the dotted lines show the following
sweep depicting a CO-free surface. In A and B, CO was dosed
for 5 min at À0.1 V in the CO saturated solution, followed by
20 min Ar purging to remove CO gas from solution. Scan rate
was 10 mV/s.
ing a different CO adlayer structure in this case
that should be investigated by spectroscopic tech-
niques. As previously noted by Strbac et al. [34]
using electrodeposited Ru adatoms (15% cover-
age), the shift of the CO stripping peak to more
positive potential reflects the stronger bond be-
tween CO and Ru due to the pseudomorphic
expansion of the Ru lattice relative to bulk
Ru(0001), based on DFT calculations of lattice
strain effects on CO bond strength on Ru(0001)
by Mavrikakis et al. [53].

S. Strbac et al. / Surface Science 573 (2004) 80–99
95
As before [34], we attribute the appearance of
the two peaks to the existence of two different
types of Ru sites, in the island centers and at the
island edge. However, the position of the second
peak for 30 s deposition (ꢁ12% coverage) is not
as positive as that obtained from electrodeposited
Ru (15% coverage), 0.51 V versus 0.60 V [34]. The
Ru islands obtained by electrodeposition were
smaller, with monolayer height and ca. 1.5 nm
size. The peak shift is likely due to the larger,
multilayer islands obtained by spontaneous depo-
sition behaving more similarly to bulk Ru. For this
case, however, there is also a broad region of sig-
nificant CO oxidation occurring after the second
peak up to 0.9 V. This region is due to the hetero-
geneous morphology of surfaces prepared by
spontaneous deposition (Fig. 3A). In contrast,
electrodeposition resulted in a homogeneous dis-
tribution of deposits with no preferential step dec-
oration. Because the coverage values are similar,
the differences in CO stripping profiles are clearly
related to the specific surface morphologies.
3.4.2. CO on Pt(111)/Ru (comparison with
Au(111)/Ru), and Pt(111)/Os
CO stripping voltammetric measurements were
next carried out using the Pt(111)/Ru and
Pt(111)/Os electrodes prepared as described above
in this report. Fig. 16A shows a typical CV curve
for the CO stripping on Pt(111)/Ru, and the CO
stripping from Pt(111)/Os is shown in Fig. 16B.
While in both cases a clear split in the CO strip-
ping profile was observed, the separation is mark-
edly different, and the threshold for the CO
oxidation changes by 70 mV in the positive poten-
tial direction upon the Ru by Os replacement.
While it is well known that on Pt(111)/Ru the first
CO oxidation peak originates from the CO oxida-
tion on the Ru adisland [31], an equivalent evi-
dence for the CO oxidation on the Pt(111)/Os
surface has been missing, although it is very likely
to be the case. The shift in the CO oxidation
threshold is then indicative of the Pt(111)/Os-CO
phase being less active to CO oxidation and overall
to organic molecule oxidation (at low potentials,
see below) that undergoes oxidation via the surface
CO intermediate path. This can be linked to the
difference in oxophilicity of the two metals and
Fig. 16. (A) CO stripping from the Pt(111)/Ru electrode in 0.1
M H2SO4. The Pt(111) crystal was covered by Ru via two
spontaneous depositions in 1 mM RuCl3 + 0.1 M HClO4 for 2
min, and stabilized with three voltammetric cycles. (B) CO
stripping from Pt(111)/Os surface in 0.1 M H2SO4. The Os
deposit obtained in 0.5 mM OsCl3 + 0.1 M HClO4 for 30 s. In
A and B, CO was dosed for 5 min at 0.0 V in the CO saturated
solution, followed by 20 min Ar purging to remove CO gas
from solution. Scan rate was 50 mV/s.
their relative ability to provide oxygen-containing
species for CO oxidation.
Indeed, our previous XPS results demonstrate
that Os is less oxophilic than Ru on Pt(111): Os
is entirely metallic at potentials lower than 0.31
V whereas Ru shows oxide formation already at
0.12 V [25]. This is in accordance with the expecta-
tion based on the work function or electronegativ-
ity difference between Os and Ru. The oxophilicity
difference between Ru and Os may also account
for the observed better performance of Pt(111)/
Ru for methanol oxidation at potentials below

96
S. Strbac et al. / Surface Science 573 (2004) 80–99
0.21 V [18,20]. However, the Pt(111)/Os system is
more active for methanol oxidation than Pt(111)/
Ru at 0.27 V (and at higher potentials) [54]. Fur-
ther spectroscopic studies are however needed to
evaluate the contribution of anion adsorption ef-
fects to the unsatisfactory performance of
Pt(111)/Os towards CO oxidation and methanol
oxidation below 0.27 V [55].
kJ/mol on Ru(0001) [59] versus ꢁ230 kJ/mol on
Pt(111) [60,61]). The sharper CO stripping peak
appearing at lower potential for the Pt54Ru46 alloy
was explained by the reduction of the OHads bond
strength due to fewer Ru–Ru neighbors. Support-
ing Gasteiger et al., but not definitively, DFT stud-
ies by Koper et al. [62] show that the OH
adsorption energy on Ru(0001) at the atop site
(À3.09 eV) is slightly greater than that for a full
Ru monolayer on Pt(111) (À3.06 eV) and for Pt2
Ru alloy (À3.04 eV), and the adsorption energy
of OH at the hollow hcp site (the most stable site
in this calculation) was also slightly greater for
Ru(0001) (À3.49 eV) compared to the Ru mono-
layer on Pt(111) (À3.47 eV). However, DFT cal-
culations by Liu et al. [63] show that the
adsorption energies of OH on Ru(0001) and OH
on Pt50Ru50 (111) alloy are the same at 0.17 eV.
Thus a lower OHads bond strength for PtRu sur-
faces versus bulk Ru may play a part in the ob-
served CO stripping behavior, although the
results are not yet conclusive and the difference ap-
pears to be small. Turning to the structure of the
CO adlayer, the sharpness of CO stripping peak
corresponding to the Ru islands in our results
could perhaps be explained by the existence of a
looser CO adlayer structure that allows for more
interaction between OH and CO. This would be
analogous to the more loosely packed CO struc-
ture observed by Lin et al. on PtRu alloy surfaces
compared to the tightly packed islands they ob-
served on bulk Ru and Ru(0001) surfaces
[52,57]. Future spectroscopic analyses by in situ
FTIR and SFG and more theoretical studies spe-
cific to our particular system will help determine
the relative contributions of such effects.
Recalling data from above and from the Section
3.4.1, the comparison of the data between the CO
stripping from Au(111)/Ru (Fig. 15) and from
Pt(111)/Ru (Fig. 16A) demonstrates an interesting
feature. Namely, the CO stripping peak at 0.23 V
from Pt(111)/Ru, which we previously attributed
to oxidation of CO at Ru sites and adjacent Pt
sites [31], is shifted negatively with respect to the
peaks for Ru(0001) and bulk Ru, ca. 0.28 and
0.30 V respectively [51,52], in contrast to the posi-
tive shift observed for Au(111)/Ru (Fig. 15). A
similar observation was reported by Gasteiger et
al. for CO oxidation on well-characterized Pt–Ru
alloys versus bulk polycrystalline Ru [49]. As
noted above, the Au(111) substrate causes a de-
crease in the CO oxidation rate due to the
strengthening of the Ru–CO bond by the expan-
sion of the Ru lattice relative to bulk Ru [56]. Be-
cause Au (0.29 nm) and Pt (0.28 nm) are both
larger than Ru (0.27 nm), the peak shift based
on lattice expansion alone should be positive for
both [53]. In contrast, the modification of Ru by
the Pt(111) substrate increases the rate of CO oxi-
dation at lower potentials. This enhancement
could occur for a number of well-studied reasons,
likely in concert: (1) a weakening of the Ru–CO
bond by electronic interactions with Pt, (2) a de-
crease in the strength of the OHads bond, [49], or
(3) a more loosely packed CO adlattice that allows
for more nucleation of OHads [57], as well as other
possible reasons not considered here. The weaken-
ing of the Ru–CO bond by electronic effects was
observed by our NMR studies of Pt/Ru nanopar-
ticles [58], but it was reported to be rather small.
As for a decrease in the OHads bond strength,
Gasteiger et al. [49] attributed the slower CO strip-
ping rate on bulk Ru (after the onset of the reac-
tion) compared to bulk Pt to a stronger OHads
bond on Ru, deduced from UHV studies of OH
desorption and adsorption on the surfaces (ꢁ330
3.4.3. CO as a probe of Ru stability on Pt(111)/Ru
Finally, CO is also a good probe to test the sta-
bility [55,56] of the Ru deposits before going
through the electrochemical stabilization/reduc-
tion cycle. Such a behavior is of interest in order
to enhance understanding of the spontaneous dep-
osition reaction per se. Shown in Fig. 17, the
Pt(111)/Ru was first obtained after a single Ru
spontaneous deposition and a successive voltam-
metric stabilization (reduction) of the Ru deposit
(Section 3.2.3), and the solid line shows the CO

S. Strbac et al. / Surface Science 573 (2004) 80–99
97
4. Summary and conclusions
Ru islands were deposited spontaneously on two
single crystal surfaces, Au(111) and Pt(111), and
characterized by in situ STM. Os was spontane-
ously deposited on Pt(111) for comparison. Many
new structural details of such nanosized islands
were revealed. From such STM data together
with methanol oxidation chronoamperometry and
CO stripping voltammetry results, the main
comparative conclusions of this study are the
following:
Fig. 17. CO stripping from the Pt(111)/Ru surface in 0.1 M
H2SO4 with (solid line) and without (dashed line) stabilizing of
the Ru deposit before CO dosing. The Pt(111) surface was
covered by Ru nano-islands via single spontaneous deposition
in 1 mM RuCl3 + 0.1 M HClO4 solution for 2 min. The
stabilized surface: three voltammetric cycles between À0.2 and
0.6 V in 0.1 M H2SO4. Without the stabilization: the potential
cycles were omitted. Other conditions as in Fig. 16A.
(1) The change in morphology of the already
reduced and stable Ru deposits on Pt(111)
obtained via two spontaneous depositions
can be induced by an electrochemical treat-
ment. The treatment involves oxidizing and
reducing the islands that leads to the islands
becoming smaller both in width and height.
The change in the island morphology affects
the observed methanol oxidation rates.
stripping reaction from this surface. Next, the
experiment was re-started, and the new Pt(111)
surface was exposed to the Ru containing solution,
rinsed and transferred to the electrochemical cell.
Such a freshly prepared ruthenized electrode was
exposed to CO without the electrochemical deposit
stabilization. The dashed line in Fig. 17 shows the
CO oxidation features obtained from this proce-
dure. The data clearly indicate that most of the
freshly prepared Ru precursor was displaced from
the electrode by CO chemisorption, similarly to
the CO displacement of some underpotentially
deposited, apparently unstable metals, as reported
by Markovic et al. [64,65]. This gives additional
credit to the idea that the electrochemical stabiliza-
tion is an essential step in the utilization of sponta-
neous deposition for stable bimetallic electrode
(Pt/Ru) preparation. Also, assuming that CO is
displacing, but not reacting the Ru precursors,
the data show that the amount of metallic ruthe-
nium in the deposit is very low, as only loosely
bound surface complexes of ruthenium [66] can
be displaced by CO. It conclusively documents
that the main mechanism for the deposit precursor
formation is the one involving chemisorption (see
Section 1) rather than disproportionation pro-
posed in Ref. [66].
(2) Spontaneous deposition of Ru on Au(111)
and Pt(111) occurs via the formation of nano-
sized Ru islands. On Au(111), deposition of
islands on steps dominates, whereas on
Pt(111), ruthenium deposits homogeneously.
The extent of deposition on the terraces is sim-
ilar for the respective saturation coverages, but
details concerning island topography are quite
different.
(3) For Pt(111), multiple spontaneous depositions
were implemented. The existing islands from
the first deposition are the preferred sites for
Ru deposition during the second (and subse-
quent) exposure as evidenced by the growth
of the islands mainly in height.
(4) Os on Pt(111), as Ru on Pt(111), is deposited
homogeneously. Compared to Pt(111)/Ru,
however, Os is deposited on Pt(111) at a much
higher rate and it forms a larger proportion of
multilayer islands within only
deposition.
a
single
(5) CO stripping voltammetry from Pt(111)/Ru
and Pt(111)/Os was also investigated and the
data were compared. The differences were
related to the difference in the oxophilicity of
the two metals.

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S. Strbac et al. / Surface Science 573 (2004) 80–99
(6) The shift in CV peaks associated with CO
stripping from Au(111)/Ru compared to that
from the bulk Ru indicates a stronger bond
between CO and Ru surface atoms from
Au(111)/Ru than between CO and surface
Ru atoms from bulk Ru.
References
[1] P.N. Ross Jr., in: J. Lipkowski, P.N. Ross (Eds.), Electro-
catalysis, Wiley-VCH, New York, 1998, p. 43.
[2] A. Hamnett, in: A. Wieckowski (Ed.), Interfacial Electr-
ochemistry: Experimental, Theory and Applications, Mar-
cel Dekker, New York, 1999, p. 843.
(7) CO stripping activity from the Ru islands also
depends on the substrate used, Pt(111) or
Au(111). Apparently, the effect of strain in
the pseudomorphic Ru lattice on Au(111)
increases the strength of the Ru–CO bond,
resulting in a positive shift in the CO stripping
peak. In contrast, the modification of Ru
by the Pt(111) substrate causes the peak to
shift negatively, which cannot be due to the
lattice expansion. Alternate explanations
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Acknowledgments
This material is based upon work supported by
the US Department of Energy, Division of
Materials Sciences under Award No. DEFG02-
91ER45439, through the Frederick Seitz Materi-
als Research Laboratory at the University of
Illinois at Urbana-Champaign. This work was
also supported by the National Science Founda-
tion grant number Grant CHE 03-4999. Svetlana
Strbac acknowledges support by the MSEP of
Republic of Serbia (project H-1796). Christina
Johnston acknowledges the National Science
Foundation for a fellowship and Alechia Crown
acknowledges the American Chemical Society
for a fellowship sponsored by the Pittsburgh Soci-
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