Resonance surface X-ray scattering technique to determine the structure of electrodeposited Pt ultrathin layers on Au(1 1 1) surface

Article abstract of DOI:10.1016/j.electacta.2010.05.020

It is demonstrated that resonance surface X-ray scattering (RSXS), in which incident X-ray energy close to the Pt L_III absorption edge (11.55 keV) is used, is very useful for the determination of the structure of electrodeposited Pt thin layers on a Au(1 1 1) surface. This technique was applied to characterize the structure of electrodeposited Pt layers on Au(1 1 1) substrates prepared under two extreme conditions, which are known to provide rough and atomically flat layers. Detailed structural information was obtained by RSXS measurements and it was confirmed that the structures of the Pt layers were as reported. Pt atoms of the atomically flat monolayer were found to be situated at the threefold hollow cubic closest packing (ccp) sites of the Au(1 1 1)-(1 × 1) surface.

Full text of DOI:10.1016/j.electacta.2010.05.020

Electrochimica Acta 55 (2010) 8302–8306
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Resonance surface X-ray scattering technique to determine the structure of
electrodeposited Pt ultrathin layers on Au(1 1 1) surface
a,∗
b
a
a
b
b
Toshihiro Kondo , Masayo Shibata , Naoko Hayashi , Hitoshi Fukumitsu , Takuya Masuda ,
b,c,∗
Satoru Takakusagi , Kohei Uosaki
a
Division of Chemistry, Graduate School of Humanities and Sciences, Ochanomizu University, Ohtsuka, Bunkyo-ku, Tokyo 112-8610, Japan
Physical Chemistry Laboratory, Division of Chemistry, Graduate School of Science, Hokkaido University, Sapporo 060-0810, Japan
International Center for Materials Nanoarchitectonics (MANA) Satellite, National Institute for Materials Science (NIMS), Sapporo 060-0810, Japan
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
It is demonstrated that resonance surface X-ray scattering (RSXS), in which incident X-ray energy close to
the Pt LIII absorption edge (11.55 keV) is used, is very useful for the determination of the structure of elec-
trodeposited Pt thin layers on a Au(1 1 1) surface. This technique was applied to characterize the structure
of electrodeposited Pt layers on Au(1 1 1) substrates prepared under two extreme conditions, which are
known to provide rough and atomically flat layers. Detailed structural information was obtained by RSXS
measurements and it was confirmed that the structures of the Pt layers were as reported. Pt atoms of the
atomically flat monolayer were found to be situated at the threefold hollow cubic closest packing (ccp)
sites of the Au(1 1 1)-(1 × 1) surface.
Received 28 November 2009
Received in revised form 26 April 2010
Accepted 2 May 2010
Available online 11 May 2010
Keywords:
Au(1 1 1) single crystal electrode
Electrochemical deposition of Pt
Resonance surface X-ray scattering (RSXS)
Surface atomic arrangement
Pseudomorphic layer
©
2010 Elsevier Ltd. All rights reserved.
1
. Introduction
The surface X-ray scattering (SXS) technique is known to be
one of the best methods to investigate the interfacial structure
at an atomic level as the structural information of not only the
overlayer, but also of the substrate can be obtained [25–30]. We
have already carried out the structural investigation of underpoten-
tially deposited (UPD) Pd ultrathin films on Au(1 1 1) and Au(1 0 0)
using in situ STM and SXS techniques and discussed the relationship
between the interfacial structure and the electrochemical proper-
ties [3,31–33]. We have also studied the structure [34] and stability
[35] of a UPD Ag ultrathin layer on a Au(1 1 1) surface using the in
situ SXS technique.
Pt ultrathin layers on foreign metal substrates are expected to be
a good electrocatalyst in many areas. Although the structural stud-
ies of electrochemically deposited Pt ultrathin layers on a Au(1 1 1)
single crystal electrode surface have been carried out by several
groups [9–12] using STM, the results were different from each
other. While Naohara et al. reported that Pt grows on an atomically
flat Au(1 1 1) surface in the Frank–van der-Merwe (FM; layer-by-
layer) mode [12], Waibel et al. [9] and Strbac et al. [10] reported
that Pt grows on the Au(1 1 1) surface in the Volmer-Weber (VW,
islanding) mode. A more precise structural investigation is required
to discuss the deposition mechanism.
Ultrathin metal layers on foreign metal substrates have been
attracting interests because of their unique physical and chemical
properties, particularly their high electro-catalytic activities [1,2].
Such high electro-catalytic activities are caused by the geometric
and electronic structures of the interfacial layers different from
those of the bulk [3], and therefore, it is very important to know
the interfacial structures with atomic dimensions. The interfacial
structures of electrochemically deposited ultrathin metal layers on
foreign metal substrates have been investigated using various tech-
niques, such as scanning probe microscopy (SPM) such as scanning
tunneling microscopy (STM) and atomic force microscopy (AFM)
[
4–20] and several optical techniques [16,20–24]. Although SPM
measurements provide structural information with an atomic res-
olution, only information of the outermost layer can be obtained,
while the structure at the interface between the electrodeposited
metal layer and substrate surface cannot be determined by SPM.
Information obtained by optical techniques contains contributions
not only from the electrode surface, but also from the bulk.
The SXS technique is ideal to determine the structure of the
metal deposits with atomic precision as mentioned above, but since
the atomic number and, therefore, the scattering parameters of Pt
∗ Corresponding author.
E-mail address: uosaki.kohei@nims.go.jp (K. Uosaki).
0
013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.05.020

T. Kondo et al. / Electrochimica Acta 55 (2010) 8302–8306
8303
2000 s was used for the RSXS study as Pt/Au(1 1 1) 2,2 which is
expected to be atomically flat according to a previous report [12].
are too close to those of Au, the interfacial structure of the Pt layer
on the Au substrate cannot be precisely determined from the (0 0)
rod data obtained using the incident X-ray energy of 11.271 keV,
which is usually used in our SXS studies [30–37].
2.3. RSXS measurements
We have now demonstrated that the resonance SXS (RSXS)
method, in which the incident X-ray energy close to the Pt LIII
absorption edge (11.55 keV) is used so that the anomalous scatter-
ing parameter effect is utilized [38–41], is very useful to precisely
determine the interfacial structure of the Pt layers deposited on the
Au(1 1 1) surface by applying this technique to the electrodeposited
Pt on a Au(1 1 1) surface prepared under two extreme conditions as
already reported; one to form a rough Pt layer [9,10] and the other
to form an atomically flat Pt layer [12].
The RSXS measurements were carried out at the bending-
magnet beamline BL4C at the Photon Factory. After the Pt
deposition, the Au(1 1 1) disk was rinsed with conc. H SO and
2
4
2−
4−
ultrapure water in order to remove the adsorbed PtCl4 or PtCl6
,
dried by blowing N gas, and then placed in the SXS cell [35,36]. The
2
SXS cell was mounted on a six-circle diffractometer (HUBER, 5020)
installed at the beamline. X-ray radiation was monochromated by a
Si(1 1 1) double-crystal system and its energy was calibrated using
the absorption edge energy of a Pt foil. The X-ray beam was focused
by a Rh-coated bending mirror. The beam size of the incident X-
ray, which can be adjusted by a slit placed in front of the cell, was
2
. Experimental
2.1. Materials
0
.1–0.2 mm (vertical) × 0.2–0.5 mm (horizontal). The beam size of
the scattered X-ray was 0.05–0.1 mm (vertical) × 0.1–0.2 mm (hori-
A Au(1 1 1) single crystal disk (diameter: 10 mm, thickness:
mm) was purchased from the Surface Preparation Laboratory
zontal) as adjusted by a slit, which was placed in front of a detector
5
(
NaI scintillation counter) to avoid any fluorescence from the Pt
(
The Netherlands) and used after the previously described pre-
treatments [30,35,42]. Pt wire and Pt foil were purchased from
Nilaco. Ultrapure reagent grade H SO4 and HClO , reagent grade
layers. The energy range of the incident X-ray was selected to be
between 11.40 and 11.75 keV in order to contain the Pt LIII absorp-
tion edge (11.55 keV). It was confirmed that the beam position
was not out of position during changing the incident X-ray energy
between 11.40 and 11.75 keV. The intensity of the incident X-ray
was measured by an ion chamber, which was placed in front of the
sample, to normalize the data. All the RSXS measurements were
carried out in such a mode that the incoming and outgoing angles
with respect to the sample surface were equal. At each measuring
point, a rocking scan of the ω-axis was performed for the back-
ground subtraction and for the integration over the mosaic spread
of the sample. Ultrapure N2 gas was flowed into the cell during the
RSXS measurements to avoid X-ray induced formation of active
species such as ozone and radicals [35].
2
4
K PtCl , and reagent grade H PtCl were purchased from Wako
2
4
2
6
Pure Chemicals, Aldrich, and Sigma, respectively, and were used
without further purification. Water was purified using a Milli-Q
system (Yamato, WQ-500). Ultrapure N (99.9995%) was purchased
2
from Tomoe Shokai.
2.2. Electrochemical deposition of Pt on Au(1 1 1)
Before the Pt deposition, the Au(1 1 1) disk was annealed using a
Bunsen burner, cooled in a quartz vessel for a few minutes and then
quenched in ultrapure water. A Pt wire and Hg/Hg SO4 electrode
2
(
MSE, double junction) were used as the counter and reference elec-
A reciprocal coordinate system (H, K, L) with two components
(H and K) lying parallel to the surface and the other one (L) along
the surface normal was used in this study. Structures along the
direction normal to the surface were quantitatively determined
from the least-square fitting to the (0 0) rod, (0 1) rod, and energy
dependence data with a kinematic calculation based on a specific
interfacial model [30,31,33–37] consisting of three layers on top
of the Au(1 1 1)-(1 × 1) substrate. When the Pt layers were made
in contact with the electrolyte solution after the expose to the air
at the Pt–O reduction potential, no cathodic current was observed.
Thus, only Pt and Au were considered in the fitting. The in-plane
structures of the Pt layer on the Au(1 1 1)-(1 × 1) surface were also
determined by comparing the experimentally obtained (0 1) rod
data with those calculated for two different structures with the
position of the Pt atoms of the first layer at (1) the cubic closest
packing (ccp) sites, and (2) the hexagonal closest packing (hcp) sites
trodes, respectively. The electrode potential was controlled and the
current was recorded by a CompactStat (Ivium).
The Pt was electrochemically deposited on the Au(1 1 1) disk
under two extreme conditions so that smooth and rough deposits
were obtained as reported by previous reports.
In the first method, the deposition was carried out in a 0.1 M
HClO solution containing 0.5 mM K PtCl and the electrode poten-
tial was negatively scanned from the open circuit potential (OCP)
4
2
4
−
1
to a certain pre-determined potential at the scan rate of 5 mV s
9,10]. The Au(1 1 1) disk was removed from the electrochemical
[
cell as soon as the potential reached the pre-determined potential.
The sample obtained with the negative potential limit of −0.15 V
(
vs. MSE) was used for the RSXS study as Pt/Au(1 1 1) 1,1 which is
expected to be rough according to previous reports [9,10].
In the second method, the deposition was carried out in a 0.1 M
HClO4 solution containing 0.05 mM H PtCl . The electrode poten-
2
6
of the Au(1 1 1) substrate. Coverage in each layer is described using
ML as a unit where 1 ML corresponds to 1.39 × 1015 atoms cm
−2
.
tial was negatively scanned from OCP to −0.02 V at the scan rate of
−
1
2
mV s , kept at this potential for a certain pre-determined time
[
12] and then the Au(1 1 1) disk was removed from the electro-
3. Results and discussion
chemical cell. The sample obtained with the deposition time of
Fig. 1 shows the (0 0) rods of (a) Pt/Au(1 1 1) 1 and (b)
Pt/Au(1 1 1) 2 samples and Fig. 2 shows (a) (0 1) rods and (b) the
1
The Au(1 1 1) disk removed from the deposition cell was transferred to another
cell containing a 0.05 M H2SO4 solution after being rinsed with pure water and conc.
H2SO4. Cyclic voltammograms (CVs) were then recorded. The CVs showed that the
cathodic charge due to the Au oxide reduction decreased as the negative potential
limit became more negative, indicating that the free Au site decreased as Pt was
deposited, but no further decrease was observed even if the negative potential limit
was made more negative than −0.15 V. Thus, we chose the sample obtained with
the negative potential limit of −0.15 V for the RSXS study as Pt/Au(1 1 1) 1. During
the preparation of the Pt/Au(1 1 1) 1, ca. 1.7 ML of Pt was deposited on the Au(1 1 1)
surface based on the cathodic charge.
2
The Au(1 1 1) disk removed from the deposition cell was transferred to another
cell containing a 0.05 M H2SO4 solution after being rinsed with pure water and conc.
H2SO4. The CVs were then recorded. The CVs showed that the cathodic charge due
to the Au oxide reduction decreased as the deposition time increased, indicating a
decrease in the free Au site as already described in footnote 1, but no further decrease
was observed after a 2000 s deposition time. Thus, we chose the sample obtained
with the deposition time of 2000 s for the RSXS study as Pt/Au(1 1 1) 2.

8
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T. Kondo et al. / Electrochimica Acta 55 (2010) 8302–8306
Fig. 1. (0 0) rod profiles of (a) Pt/Au(1 1 1) 1 and (b) Pt/Au(1 1 1) 2 samples. The circles and the solid lines are the experimental data and the calculated curves fitted by the
least-squares method with a kinematic calculation, respectively. The standard error values of the scattered X-ray intensity are shown as error bars in the figure.
Fig. 2. (a) (0 1) rod profile of Pt/Au(1 1 1) 2 and (b) energy dependence of scattered X-ray intensity at (0 0 0.5) of Pt/Au(1 1 1) 2. The circles represent the experimental data
and the solid line is the calculated curve fitted by the least-squares method with a kinematic calculation for the Pt adsorption on the ccp sites. The dotted line is the calculated
curve for the Pt adsorption on the hcp site. The standard errors of the scattered X-ray intensity are shown as error bars in the figure. A Pt absorption spectrum, which was
used as an energy reference, is also shown.
energy dependence of the scattered X-ray intensity at (0 0 0.5)
of Pt/Au(1 1 1) 2. The structural parameters obtained from the
least-square fitting with a kinematical calculation are listed in
Table 1
Tables 1 and 2.
Structural parameters obtained from the analyses of the (0 0) rod
profile at Pt/Au(1 1 1) 1 as a three-layer model on Au(1 1 1)-(1 × 1).
At Pt/Au(1 1 1) 1, the best fit data from the (0 0) rod profile
(
(
Fig. 1(a)) show that three Pt layers are present on the Au(1 1 1)-
1 × 1) surface (Fig. 3(a)). The layer distance between the first Pt
a
and the outermost Au layers is 0.230 nm, which is slightly larger
than that calculated for the Pt layer with Pt atoms situated at the
threefold hollow site on the Au(1 1 1)-(1 × 1) surface, 0.227 nm, but
less than those calculated for the Pt layers with Pt atoms situated at
the on-top, 0.283 nm, and bridge sites, 0.243 nm, on the Au(1 1 1)-
Distance, zAu–Pt(1)/nm
Distance, zPt(1)–Pt(2)/nm
Distance, zPt(2)–Pt(3)/nm
Coverage, ꢀPt(1)/ML
Coverage, ꢀPt(2)/ML
Coverage, ꢀPt(3)/ML
RMS, ꢁPt(1)/nm
0.230 ± 0.008
0.228 ± 0.010
0.227 ± 0.009
0.93 ± 0.06
0.71 ± 0.09
0.26 ± 0.13
(
1 × 1) surface, showing that most of the Pt atoms in the first layer
0.032 ± 0.003
0.086 ± 0.007
0.141 ± 0.040
RMS, ꢁPt(2)/nm
RMS, ꢁPt(3)/nm
are at the threefold hollow site of the underlying Au(1 1 1)-(1 × 1)
surface. The layer distances between the first and second and the
second and third Pt layers are 0.227 and 0.228 nm, respectively,
which are larger than that calculated for the Pt layer with Pt atoms
situated at the threefold hollow site on the epitaxially formed Pt
layer on the Au(1 1 1)-(1 × 1) surface, 0.221 nm, but less than those
calculated for the Pt layers with Pt atoms situated at the on-top,
a
The subscripts of Au, Pt(1), Pt(2), and Pt(3) represent the outer-
most Au layer, the first Pt layer, second Pt layer, and third Pt layer,
respectively. Distances of zAu–Pt(1), zPt(1)–Pt(2), and zPt(2)–Pt(3) represent
atomic layer distances between the Au substrate and first Pt layer,
the first and second Pt layers, and the second and third Pt layers,
respectively.

T. Kondo et al. / Electrochimica Acta 55 (2010) 8302–8306
8305
Fig. 3. Schematic side views of (a) Pt/Au(1 1 1) 1 and (b) Pt/Au(1 1 1) 2 based on the RSXS results.
0
.277 nm, and bridge sites, 0.236 nm, on the epitaxially formed Pt
hollow sites on the Au(1 1 1)-(1 × 1) surface, i.e., the hcp site and the
ccp site. While the (0 0) rod profile contains the information only of
the structure normal to the surface, the (0 1) rod profile can be used
to obtain information about the in-plane structure. In Fig. 2(a), the
solid and dotted lines show the calculated curves fitted by the least-
squares method with a kinematic calculation for the Pt adsorption
on the ccp and hcp sites, respectively. It is clear that the fitting for
the Pt atoms on the ccp site gives the best fit to the experimental
data, indicating that the deposited Pt monolayer maintains the fcc
stacking sequence of the Au(1 1 1) substrate. This behavior is the
same as those in the case of the electrochemical deposition of Pd
[30–32] and Ag [33,34] on the Au(1 1 1) surface.
layer on the Au(1 1 1)-(1 × 1) surface. These results indicated that
the Pt atoms in the second and third Pt layers randomly situated on
the first and second Pt layers, respectively. Coverage of the first,
second, and third Pt layers are 0.93, 0.71, and 0.26 ML, respec-
tively, indicating that the surface of Pt/Au(1 1 1) 1 is very rough.
The high RMS values also support the fact that the surface of the Pt
deposited layers was very rough. This is in agreement with previ-
ous results [9,10]. Although there is a possibility of the formation
of the small Pt clusters, in which Pt–Pt distances maybe smaller,
we avoided the discussion about this in the present study because
more detailed experimental data such as STM, X-ray absorption
spectroscopy (XAS), grazing incidence X-ray scattering (GIXS) are
required.
4. Conclusions
At Pt/Au(1 1 1) 2, a dip was observed between the Bragg points
in the (0 0) (Fig. 1(b)) and (0 1) (Fig. 2(a)) rods, suggesting that the Pt
monolayer was formed on the Au(1 1 1) surface. The scattered X-ray
intensity at (0 0 0.5) (Fig. 2(b)) was dependent on the incident X-ray
energy, indicating that the outermost surface atom should be plat-
inum. All the best fit data from the present results, i.e., the (0 0) and
We demonstrated that resonance surface X-ray scattering
(RSXS) is very useful to precisely determine the interfacial structure
of Pt layers deposited on a Au(1 1 1) surface. The structures of elec-
trodeposited Pt layers prepared on a Au(1 1 1) surface under two
conditions were investigated. It was confirmed that a rough Pt mul-
tilayer and an atomically flat Pt monolayer, whose structures had
already been reported by STM, were formed on the Au(1 1 1)-(1 × 1)
surface. The Pt atoms of the atomically flat monolayer were found
to be situated at the threefold hollow cubic closest packing (ccp)
sites of the Au(1 1 1)-(1 × 1) surface. The in situ RSXS investigation
during the Pt deposition is now under way.
(
0 1) rod profiles and energy dependence, showed the formation
of an atomically flat Pt monolayer (0.96–0.98 ML) on the Au(1 1 1)-
1 × 1) surface (Fig. 3(b)), confirming the previous results [12]. The
layer distance between the first Pt and the outermost Au layers is
.227–0.228 nm, which is in good agreement with that calculated
(
0
for the Pt layer with Pt atoms situated at the threefold hollow site
on the Au(1 1 1)-(1 × 1) surface, 0.227 nm. There are two threefold
Acknowledgements
Table 2
We wish to thank Dr. Tamura of the Japan Atomic Energy Agency
for his help with the CTR and energy dependence calculation and
the fitting program. This work was partially supported by Grants-
in-Aid for Scientific Research (A) (No. 18205016) to KU and (C)
Structural parameters obtained from the analyses of the (0 0) and (0 1) rod profiles
and energy dependence of (0 0 0.5) scattered intensity at Pt/Au(1 1 1) 2 as a three-
layer model on Au(1 1 1)-(1 × 1).
a
From (0 0) rod
From (0 1) rod
From energy
dependence
(
(
No. 20550009) to TK from Japan Society of Promotion of Science
JSPS) and World Premier International Research Center (WPI) Ini-
Distance, zAu(3)–Au(2)/nm
Distance, zAu(2)–Au(1)/nm
Distance, zAu(1)–Pt/nm
Coverage, ꢀAu(2)/ML
Coverage, ꢀAu(1)/ML
Coverage, ꢀPt/ML
RMS, ꢁAu(2)/nm
0.236b
0.236b
0.236b
tiative on Materials Nanoarchitectonics (MANA) from Ministry of
Education, Culture, Sports, Science, and Technology (MEXT), Japan.
The synchrotron radiation experiments were performed as projects
approved by the Photon Factory Program Advisory Committee (PAC
Nos. 2007G085, 2008G124, and 2009G038).
0.236 ± 0.001
0.236 ± 0.01
0.236 ± 0.001
0.227 ± 0.002
0.228 ± 0.03
0.228 ± 0.005
1.00b
1.00b
b
1.00
0.99 ± 0.06
1.00 ± 0.01
0.99 ± 0.06
0.96 ± 0.08
0.98 ± 0.08
0.98 ± 0.08
0.008 ± 0.003
0.009 ± 0.006
0.017 ± 0.004
0.007 ± 0.003
0.010 ± 0.008
0.017 ± 0.005
0.009 ± 0.002
0.009 ± 0.002
0.017 ± 0.003
RMS, ꢁAu(1)/nm
RMS, ꢁPt/nm
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The subscripts of Pt, Au(1), Au(2), and Au(3) represent the Pt layer, first
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