Ubiquitous strategy for probing ATR surface-enhanced infrared absorption At platinum group metal-electrolyte interfaces

Article abstract of DOI:10.1021/jp044085s

A versatile two-step wet process to fabricate Pt, Pd, Rh, and Ru nanoparticle films (simplified as nanofilms hereafter) for in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) study of electrochemical interfaces is presented, which incorporates an initial chemical deposition of a gold nanofilm on the basal plane of a silicon prism with the subsequent electrodepostion of desired platinum group metal overlayers. Galvanostatic electrodeposition of Pt, Rh, and Pd from phosphate or perchloric acid electrolytes, or potentiostatic electrodeposition of Ru from a sulfuric acid electrolyte, yields sufficiently pinhole-free overlayers as evidenced by electrochemical and spectroscopic characterizations. The Pt group metal nanofilms thus obtained exhibit strongly enhanced IR absorption. In contrast to the corresponding metal films electrochemically deposited directly on glassy carbon and bulk metal electrodes, the observed enhanced absorption for the probe molecule CO exhibits normal unipolar band shapes. Scanning tunneling microscopic (STM) images reveal that fine nanoparticles of Pt group metals are deposited around wavy and stepped bunches of Au nanoparticles of relatively large sizes. This ubiquitous strategy is expected to open a wide avenue for extending ATR surface-enhanced IR absorption spectroscopy to explore molecular adsorption and reactions on technologically important transition metals, as exemplified by successful real-time spectroscopic and electrochemical monitoring of the oxidation of CO at Pd and that of methanol at Pt nanofilm electrodes. The spectral features of free water molecules coadsorbed with CO on Pt, Pd, Rh, and Ru are also discussed. ? 2005 American Chemical Society.

Full text of DOI:10.1021/jp044085s

7900
J. Phys. Chem. B 2005, 109, 7900-7906
Ubiquitous Strategy for Probing ATR Surface-Enhanced Infrared Absorption at Platinum
Group Metal-Electrolyte Interfaces
Yan-Gang Yan,† Qiao-Xia Li,† Sheng-Juan Huo,† Min Ma,† Wen-Bin Cai,*,† and
Masatoshi Osawa‡
Shanghai Key Laboratory for Molecular Catalysis and InnoVatiVe Materials and Department of Chemistry,
Fudan UniVersity, Shanghai 200433, China, and Catalysis Research Center, Hokkaido UniVersity,
Sapporo 001-0021, Japan
ReceiVed: December 29, 2004; In Final Form: February 24, 2005
A versatile two-step wet process to fabricate Pt, Pd, Rh, and Ru nanoparticle films (simplified as nanofilms
hereafter) for in situ attenuated total reflection Fourier transform infrared (ATR-FTIR) study of electrochemical
interfaces is presented, which incorporates an initial chemical deposition of a gold nanofilm on the basal
plane of a silicon prism with the subsequent electrodepostion of desired platinum group metal overlayers.
Galvanostatic electrodeposition of Pt, Rh, and Pd from phosphate or perchloric acid electrolytes, or potentiostatic
electrodeposition of Ru from a sulfuric acid electrolyte, yields sufficiently “pinhole-free” overlayers as evidenced
by electrochemical and spectroscopic characterizations. The Pt group metal nanofilms thus obtained exhibit
strongly enhanced IR absorption. In contrast to the corresponding metal films electrochemically deposited
directly on glassy carbon and bulk metal electrodes, the observed enhanced absorption for the probe molecule
CO exhibits normal unipolar band shapes. Scanning tunneling microscopic (STM) images reveal that fine
nanoparticles of Pt group metals are deposited around wavy and stepped bunches of Au nanoparticles of
relatively large sizes. This ubiquitous strategy is expected to open a wide avenue for extending ATR surface-
enhanced IR absorption spectroscopy to explore molecular adsorption and reactions on technologically important
transition metals, as exemplified by successful real-time spectroscopic and electrochemical monitoring of the
oxidation of CO at Pd and that of methanol at Pt nanofilm electrodes. The spectral features of free water
molecules coadsorbed with CO on Pt, Pd, Rh, and Ru are also discussed.
Introduction
the second one is referred to wet processes, consisting of
electrochemical and electroless (i.e., chemical) depositions.
Surface-enhanced infrared absorption spectroscopy (SEIRAS)
has received increasing attention in surface science and elec-
trochemistry because of its high signal sensitivity and a simple
surface selection rule.1-10 Although external IR reflection
absorption spectroscopy (IRAS) has been widely used in these
fields, it has several inherent problems in application to
electrochemistry, such as high solution resistance, inhomoge-
neous current distribution, and limited mass transport, which
are caused by pressing the working electrode against a cell
window to reduce the strong absorption by the electrolyte
solution.11 SEIRAS in an attenuated-total-reflection (ATR)
mode, ATR-SEIRAS, is free from these problems and has been
applied successfully to the real-time monitoring of adsorption
and reactions at electrode surfaces.9,10,12
A key issue for successfully implementing this technique is
the fabrication of nanoparticle metal films with well-tuned size,
shape, and proximity so that a strong SEIRA effect can be
obtained. For in situ surface electrochemistry application,
additional requirements for these nanofilms to serve as qualified
electrodes are conductivity and stability in solution. There are
generally two strategies for fabricating the nanofilms for in situ
electrochemical SEIRA spectroscopy. The first one is referred
to dry processes, involving vacuum evaporation and sputtering;
Although dry processes have predominantly been used in
depositing various metals and alloys, including Au, Ag, Pt, and
Ru as well as Pt-Fe and Pt-Ru alloy nanofilms on IR windows
as working electrodes in earlier ATR-SEIRAS measure-
ments,1-8,12,13 several disadvantages exist such as the require-
ment of a high-cost vacuum evaporator, time-consuming opera-
tion, poor enhancement reproducibility, and sample contami-
nation by organic species. What is more, the emergence of
severely distorted bipolar IR absorption bands seen frequently
for vacuum-deposited transition metals such as Pt, Ru, and Pt
alloy films complicates the analysis and explanation of spectral
data.8,12,13
Electrodeposition is a wet process applicable to the deposition
of many metals and has been used for external reflection
SEIRAS measurements on Pt group metals.14-17 Nevertheless,
many reported enhanced spectra therein are severely distorted
as in the case of dry processes. Specifically for Pt group metals
electrochemically deposited on a glassy carbon electrode, the
enhanced IR bands are observed with negative absorption and
tremendously broadened bandwidth. These spectral features were
named as abnormal infrared effects (AIREs) by Sun’s group.15
The origin of the distorted band shapes is believed to be specific
morphologies of the deposited metals (shape, size, and proximity
of the metal particles).14,18 It should also be noted that the
electrodeposition technique is not suitable for ATR-SEIRAS
measurements because nondoped (i.e., nonconductive) Si prisms
* Corresponding author. Phone: +86-21-55664050. Fax: +86-21-
65641740. E-mail: wbcai@fudan.edu.cn.
† Fudan University.
‡ Hokkaido University.
10.1021/jp044085s CCC: $30.25 © 2005 American Chemical Society
Published on Web 04/02/2005

SEIRAS at Pt Group Metal-Electrolyte Interfaces
J. Phys. Chem. B, Vol. 109, No. 16, 2005 7901
are generally used in the measurements due to high chemical
stability and high transmittance.
the deposition of Pt, Rh, and Pd overlayers according to the
recipes reported by Zou et al.20 Briefly, the plating solution
consists of 4 mM H2PtCl6‚6H2O and 0.7 M Na2HPO4 for Pt
overlayer, 5 mM PdCl2 and 0.1 M HClO4 for Pd, and 5 mM
RhCl3‚5H2O and 0.1 M HClO4 for Rh. Pd was electrodeposited
at 20 µA for 180 s, Rh at 100 µA for 100 s, and Pt at 400 µA
for 160 s. Potentiostatic mode was used for the deposition of
Ru multilayers as suggested by Strbac et al.23 Ru overlayer was
electrodeposited onto Au at -0.25 V for 600 s and then at -0.2
V for 900 s in a plating electrolyte containing 0.1 mM RuCl3
and 0.5 M H2SO4.
Inductively coupled plasma (ICP) atomic emission spectros-
copy (AES) was used to determine the concentrations of
dissolved metallic ions from the metal films (Au, Pt, Pd, and
Ru) on Si in hot aqua regia. With known masses of the deposits,
the average thicknesses of the deposits were estimated by
assuming the same densities as their bulk materials. For the Rh
nanofilm, the cathodic charge accumulated during electrodepo-
sition was used to estimate its average thickness.20 The
thicknesses of Au, Pd, Pt, Rh, and Ru nanofilms were around
65, 1.4, 6.1, 2.4, and 4.6 nm, respectively, by taking the
geometric area for working electrodes. The existence of pinholes
in the overlayers was examined by voltammetry and SEIRAS
using CO as a probe molecule.
SEIRA Spectroscopy, Electrochemistry, and Scanning
Tunneling Microscopy (STM). A Nicolet Magna-IR 760 FTIR
spectrometer equipped with a liquid nitrogen cooled HgCdTe
MCT detector was used for SEIRAS measurements and was
operated at a resolution of 4 (for potentiostatic multistep
measurements) or 8 cm-1 (for potentiodynamic kinetics mea-
surements). Spectra of the Pt group metal/solution interfaces
were acquired with the so-called Kretschmann attenuated-total-
reflection (ATR) configuration (prism/thin metal film/solution
geometry), details of which can be found elsewhere.2,24 Unpo-
larized infrared radiation from a ceramic source was focused at
the interface by being passed through the prism at the incident
angle of 70°, and the totally reflected radiation was detected.
Spectra were acquired under potentiostatic or potentiodynamic
conditions. All the spectra in this paper are shown in the
absorbance units defined as -log(I/I0), where I and I0 represent
the intensities of the reflected radiation at the sample and
reference potentials, respectively. The adsorption and oxidation
of CO on the above-mentioned four nanofilms and the oxidation
of methanol on a Pt nanofilm were taken as the prototype
electrocatalytic reactions to check the validity of our strategy
in electrochemical ATR-SEIRAS.
A CHI 630B electrochemistry workstation was employed for
potential/current control and to record the cyclic voltammo-
grams. All electrode potentials are cited with a reversible
hydrogen electrode (RHE), unless specifically addressed.
Ex situ STM observation of the nanofilms was carried out
with a Molecular Imaging Pico-SPM. Tunneling tips were
prepared by electrochemical etching of a tungsten wire (0.25
mm in diameter). All STM images shown in this paper were
acquired in the constant-current mode with Vb ) 0.1 V and It
) 5 nA.
Very recently, electroless deposition, an alternative wet
process, has been introduced to make SEIRA-active Au, Pt, Ag,
and Cu nanofilms on Si and Ge windows.9,10,18,19 This simple
wet process can overcome the disadvantages of the dry processes
and yield a stronger SEIRA effect. In addition, the chemically
deposited metal films are more stable in solutions than vacuum-
evaporated ones.9,10,18,19 Unfortunately, so far the success of the
electroless deposition method (tentatively called the one-step
wet process) has been restricted to Au, Pt, Ag, and Cu. In
addition, a patented commercial electrolyte with unknown
additives was used to produce Pt nanofilms.9
The main purpose of the current work is to develop a
ubiquitous two-step wet-process strategy for in situ electro-
chemical ATR-SEIRAS application. This strategy incorporates
the electroless deposition of an SEIRA-active Au nanofilm on
Si with subsequent electrodeposition of a desired metal over-
layer. Similar tactics have been utilized by Weaver’s group to
obtain surface-enhanced Raman scattering (SERS) on Pt, Pd,
Rh, and Ir surfaces that show in nature far weaker SERS activity
than Ag, Au, and Cu surfaces.20,21 In their work, a Au bulk
surface was initially roughened by an oxidation-reduction cycle
(ORC) pretreatment in KCl to yield SERS effect and then was
covered by electrodeposition of an ultrathin overlayer of a Pt
group metal. With selected deposition conditions, they obtained
virtually pinhole-free SERS-active Pt, Pd, Rh, and Ir surfaces.
The basic idea of the SERS experiment is the use of the SERS
activity of the underlying Au surface to probe the surface of
the overlayer. Since SERS is a relatively short-ranged effect,
the thickness of the overlayers should be confined to a few
monolayers. Therefore, the preparation of pinhole-free over-
layers is not easy. The basic idea of the two-step wet process
for ATR-SEIRAS proposed here is slightly different and 2-fold.
First is the extension of metals available for ATR-SEIRAS by
using an electrochemical deposition technique. By using a
conducting Au underlayer, electrodeposition becomes possible.
Second is to solve the band distortion problem by using an
SEIRA-active Au film as a template. Different from SERS, a
significant SEIRA effect is not limited to coinage metals,2 and
thus thicker transition metal overlayers can be used, which
facilitates the preparation of pinhole-free overlayers. We will
demonstrate herein the feasibility of our two-step wet process
by applying it to Pt group metals (including Pt, Pd, Rh, and
Ru). The electrochemical and IR results reveal that sufficiently
pinhole-free Pt, Rh, Pd, and Ru overlayers that exhibit significant
SEIRA effects and normal electrochemical properties can be
prepared by this simple method.
Experimental Section
Nanofilm Preparation. A hemicylindrical Si prism was
polished and then cleaned with the RCA method:22 i.e., soaking
it in a 1:1:5 (v/v) solution of NH3, H2O2, and H2O, and afterward
in a 1:1:5 (v/v) solution of HCl, H2O2, and H2O at about 80
°C, followed by thorough rinsing with water. Then the total-
reflecting plane was immersed in 40% NH4F solution for 1.5
min to terminate the Si surface with hydrogen. The chemical
deposition of a Au film on Si was performed according to the
procedures and recipes reported by Miyake et al.19
The electrodeposition of Pt, Pd, Rh, and Ru overlayers was
carried out in selected plating electrolytes with the Au film, a
saturated calomel electrode (SCE), and a Pt mesh serving as
the working (working area 1.54 cm2), reference, and counter
electrodes, respectively. Galvanostatic mode was employed for
Results and Discussion
Electrochemical Properties. Cyclic or linear sweep voltam-
metry was used to evaluate the electrochemical properties of
deposited overlayers. Voltammograms of electrodeposited Pd,
Pt, and Rh overlayers on Au/Si in 0.1 M HClO4 and CO-
saturated 0.1 M HClO4 at 50 mV s-1 are shown in Figure 1A-C
(A, Pd; B, Pt; C, Rh). Dashed traces 1 in Figure 1A-C show

7902 J. Phys. Chem. B, Vol. 109, No. 16, 2005
Yan et al.
Figure 2. Cyclic voltammograms of Ru-coated Au nanofilm electrode
in 0.1 M HClO4 in the absence (solid line) and presence (dashed line)
of saturated CO (A), and in 0.1 M NaOH with varying positive limits
(B). Scan rate: 50 mV s-1
.
tion charge of around 559 µC cm-2 can be estimated for the
electrodeposited Ru overlayer.
Assuming 210 µC cm-2 for the hydrogen adsorption charge
at an ideal smooth electrode, the roughness factors are within
2-3 for the deposited Pd, Pt, Rh, and Ru overlayers, which are
slightly larger than polished bulk electrodes used in external
IRAS measurement (typically 1.5-2).
Figure 1. Cyclic voltammograms of Pd (A), Pt (B), and Rh (C) coated
Au nanofilm electrodes in 0.1 M HClO4 in the absence (traces 3) and
presence (traces 4) of saturated CO. Traces 1 represent cyclic
voltammograms for bare Au nanofilm electrodes, and traces 2 represent
linear voltammograms of Pd, Pt, or Rh coated Au nanofilms in 0.1 M
HClO4 recorded after holding the potential at 1.5-1.6 V for 5 s. Scan
The structures of the thin films prepared by electrodeposition
were characterized using ex situ scanning tunneling microscopy.
The images of a bare Au film and Pt group metal coated Au
films are shown in Figure 3. The bare Au film consists of
cascading slatelike crystallites, showing a face-centered-cubic
structure. The morphology of the Au film was totally changed
after the electrodeposition of Pd, Pt, Rh, and Ru. Basically,
rather small and compact irregular island crystallites of Pt, Pd,
Rh, and Ru grow on the Au film substrates with particle sizes
and substructures varying with metallic elements. It is noted
that the nanofilms thus obtained are quite different in particle
shapes from the corresponding ones formed by cyclic voltam-
metric deposition on a glassy carbon.15 In the latter, the
nanostructures with stacked layers were formed and the films
show an antiabsorption band feature (i.e., one of AIREs). The
specific nanostructures may be a possible origin of the antiab-
sorption band feature.
SEIRA Spectra of Adsorbed CO. To check further whether
the Au surface was sufficiently covered by the electrodeposited
overlayers, CO was used as the probe molecule since the C-O
stretching mode is sensitive to the adsorption sites. Previous
ATR-SEIRAS results19,26 indicate that CO adlayer on a bare
Au film on Si exhibits a major band ranging from 2110 to 2135
cm-1 with an intensity as high as 0.01 in absorbance units (or
0.022 in relative reflectivity units, ∆R/R ) 1 - I/I0) in CO-
saturated 0.1 M HClO4 electrolyte. This band was assigned to
linearly bonded CO molecules, COL, on Au. The bridge-bonded
CO (COB) is much weaker. The intensity of COL is 1 order of
magnitude more intense than that observed on bulk Au
electrodes by IRAS. Shown in Figure 4 are typical SEIRA
spectra of CO on metal-coated Au electrodes in CO-saturated
0.1 M HClO4 obtained at ca. 0.3 V (vs RHE), where CO can
stably exist on the surfaces of both Pt group metals and Au.
The spectra are obviously different from those for CO on Au.
The higher frequency bands at 2035-2076 cm-1 correspond to
COL, while bands at 1820-1959 cm-1 correspond to bridge-
rate: 50 mV s-1
.
cyclic voltammograms of unmodified Au films before depositing
overlayers of Pd, Pt, and Rh, respectively. The typical voltam-
metric features of polycrystalline Au can be found with the onset
of anodic oxide formation at high potentials (>1.3 V) and its
reduction peak at 1.15 V in the backward half-cycle. Trace 2
in Figure 1A represents the voltammogram of a Pd-coated Au
film for a linear potential sweep from 1.6 to 0.9 V after holding
the electrode potential at the positive limit for 5 s. This potential
range is high enough for Au oxidation, but the cathodic peak
corresponding to the reduction of Au oxide is entirely absent,
indicating that the Pd overlayer covers essentially the entire gold
surface, at least to block the Au oxide formation. Constant-
current deposition of 1.4-nm-thick Pd overlayer was sufficient
to remove the Au oxide features and replace them with
voltammetric characteristics similar to those for a polycrystalline
Pd bulk electrode. Similar results were obtained for Pt (Figure
1B) and Rh (Figure 1C) overlayers formed by galvanostatic
deposition, although thicker ones were needed for the Pt and
Rh to achieve the sufficiently pinhole-free property. The charges
of hydrogen adsorption obtained for Pd, Pt, and Rh are 623,
513, and 400 µC cm-2, respectively.
Since oxygen evolution is significant on Ru even at lower
potentials, the linear sweep voltammetric evaluation was not
applied for Ru-coated electrodes to avoid being damaged by
rigorous oxygen evolution. The resultant voltammograms are
close to those of a bulk polycrystalline Ru electrode reported
by Gutierezz et al.25 The irreversible redox behavior of the Ru-
coated electrode prevents the evaluation of the charge for
hydrogen adsorption. To solve this problem, cyclic voltammetry
was carried out in 0.1 M NaOH instead of the acidic solution,
the result for which is shown in Figure 2 (lower panel). The
peaks at 0-0.2 V have been attributed to hydrogen adsorption
and desorption.25 By integrating the peaks, a hydrogen adsorp-

SEIRAS at Pt Group Metal-Electrolyte Interfaces
J. Phys. Chem. B, Vol. 109, No. 16, 2005 7903
TABLE 1: Stark Tuning Rates of Major CO Bands for
Electrodeposited Pd, Pt, Rh, and Ru Overlayers for
Potentials Ranging from 0.1 to 0.6 V (RHE) in
CO-Saturated 0.1 M HClO4
metal
major band
Stark tuning rate (cm-1 V-1
)
Pd
Pt
Rh
Ru
COB
COL
COL
COL
∼37.4
∼30.0
∼34.5
∼45.0
rates of the potential-dependent peak shift, Stark tuning rates,
for major CO bands are listed in Table 1. The frequency
positions and relative intensities of COL and COB bands and
the Stark tuning rates for the electrodeposited overlayers are in
reasonable agreement with those for corresponding polycrys-
talline bulk electrodes.11,25 These IR results as well as cyclic
voltammograms clearly demonstrate that the Pt group metal
overlayers are almost pinhole-free and have electrochemical and
surface properties similar to those of the corresponding bulk
metals.
Three other important features can be found from Figure 4.
First, the peak intensities of major CO bands for the four metals
(i.e., COL bands for Pt, Rh, and Ru and COB band for Pd) range
from 0.03 to 0.11 in absorbance units, or ∆R/R ) 0.07 to 22.
According to Lamy and Beden,11 the maximum theoretical band
intensity for a CO adlayer on a smooth metal surface is around
0.003 in ∆R/R units in the external reflection mode with
unpolarized IR radiation. Hence the apparent surface enhance-
ment factors of around 20-70 can be estimated for the Pt group
metals from a simple comparison of peak intensities. The
enhancement is the most significant for the electrodeposited Pt
layer. The peak intensity of the COL band (0.12 absorbance
unit for unpolarized radiation) is twice that observed on a
chemically deposited Pt nanofilm on Si (0.06 absorbance unit
for unpolarized radiation and 0.3 absorbance unit for p-
polarization).9
Figure 3. STM images (700 nm × 700 nm) of Au (a), Pd (b), Pt (c),
Rh (d), and Ru (e) coated Au nanofilms. It ) 5 nA; Vb ) 0.1 V.
Second, the observed enhanced bands have unipolar band
shapes similar to those observed on bulk metal surfaces. This
is a marked difference from the enhanced bands observed on
electrochemically deposited Pt group metal films on glassy
carbon. The enhanced bands are inverted upside down and
broadened on the latter films.15 This significant advantage
ensures an undisputed analysis and explanation for spectroscopic
data obtained.
Third, the bands of water coadsorbed with CO molecules are
clearly observed on all four metals investigated. The sharp bands
at 3631-3658 cm-1 can be assigned to υ(OH) and those at
1625-1633 cm-1 to δ(HOH) of water molecules, the peak
positions of which depend on the substrates and potentials.7,9a,12a,24
The peak frequencies of the υ(OH) and δ(HOH) bands are
remarkably higher and lower, respectively, compared with those
of bulk water (∼3400 and 1645 cm-1, respectively), indicating
that the water molecules are free from hydrogen bonding. The
bands of free water molecules have been already reported on
Pt,9a Ru, and PtRu.12a According to the surface selection rule,2,12a
the water is oriented on the surface with its C2 axis perpendicular
to the surface. Miki et al.9a ascribed these bands to water
molecules embedded in the CO adlayer, while Watanabe et al.12a
ascribed them to water molecules directly adsorbed on the
surface. A further discussion on this issue will be given later.
CO Oxidation on Pd. The SEIRA-active nanofilms prepared
by the two-step wet process are favorable for real-time monitor-
ing and analysis of electrocatalytic reactions, as exemplified
by the oxidative removal of a CO adlayer on the electrodeposited
Pd overlayer. The upper panel of Figure 5 shows a series of
Figure 4. SEIRA spectra of CO on Pd (A), Pt (B), Rh (C), and Ru
(D) nanofilm electrodes in 0.1 M HClO4 saturated with CO at 0.3 V.
Reference spectra were taken at 1.3 V in (A), 1.3 V in (B), 1.25 V in
(C), and 1.0 V in (D), respectively.
bonded (COB) or multicoordinated CO (COM) adsorbed on the
Pt group metals. No bands of CO adsorbed on Au are found.
Assuming a detection sensitivity limit of 2 × 10-4 absorbance
or 5 × 10-4 relative reflectivity of SEIRAS for COL on
chemically deposited Au electrodes,19,26 at least 98% of Au sites
are covered by the electrodeposited overlayers.
The CO bands shift to higher wavenumbers as the potential
increases as has been reported on many bulk electrodes. The

7904 J. Phys. Chem. B, Vol. 109, No. 16, 2005
Yan et al.
Figure 5. (upper) Series of time-resolved SEIRA spectra of CO-
predosed Pd nanofilm electrode in 0.1 M HClO4 collected sequentially
during a potential sweep cycling 0.05 V f 1.2 V f 0.05 V at 50 mV
s-1. The time resolution used and shown is 0.24 s. The Pd electrode
was predosed in CO-saturated 0.1 M HClO4 solution at 0.05 V RHE
for 15 min, and then the dissolved CO was purged with Ar bubbling
for 30 min. (lower) Cyclic voltammogram recorded at 50 mV s-1 during
in situ ATR-SEIRAS measurement along with selected potential-
dependent integrated intensities for IR bands of COB, COL, and
coadsorbed H2O.
Figure 6. Zoomed three- (upper) and two-dimensional (lower) plots
of potential-dependent SEIRA spectra for COB band on Pd nanofilm
electrode across the major CO oxidation peak in Figure 5. All the other
conditions are the same as in Figure 5. For each labeled curve is given
the median potential value at which the IR spectra were acquired during
the forward potential sweep: curve 1, 0.926 V; curve 2, 0.938 V; curve
3, 0.950 V; curve 4, 0.962 V; curve 5, 0.974 V; curve 6, 0.986 V; and
curve 7, 0.998 V.
lower potentials and COB is converted to COL with the increase
of potential.27 To see the CO oxidation process across the major
current peak in more detail, selected spectra in the corresponding
potential range in Figure 5 are replotted in Figure 6. It turned
out that, as the oxidative removal of adsorbed COB proceeds,
the band at 1966 cm-1 decreased and shifted to 1880-1900
cm-1, with the latter initially increasing and then decreasing in
intensity as the potential moved positively, suggestive of the
conversion of bridge to hollow bonding of CO species as its
surface coverage turned lower.28 It is stressed again that the
high sensitivity of ATR-SEIRAS is indispensable for monitoring
the irreversible conversion of adsorbed CO molecules occurring
within 1 s.
0.24 s time-resolved spectra of a Pd-coated Au electrode in 0.1
M HClO4 (first saturated with CO and then dissolved CO was
eliminated by Ar purging), which were collected sequentially
during a potential sweep from 0.05 to 1.2 V and then back to
0.05 V at a scan rate of 50 mV s-1. The reference spectrum
was taken at 1.2 V. The high sensitivity of ATR-SEIRAS
enables the quick spectral acquisition with such a good signal-
to-noise ratio. The lower panel of Figure 5 displays the plot of
the normalized IR band intensities of COL, COB, and υ(OH) of
the coadsorbed water against the potential together with the
corresponding cyclic voltammogram. The sharp major CO
oxidation peak locates at ca. 0.93 V in the voltammogram, with
a broad oxidation prewave ranging from ca. 0.65 to 0.9 V. The
minor COL band decreases continuously at the potentials positive
of 0.65 V, and disappears completely at ca. 0.95 V. The major
COB band and the free-water band decrease slightly in the
oxidation prewave region, due probably to preferential oxidation
of a fraction of weakly bound CO molecules at defect sites.27
These two CO bands decrease sharply, responding to the rapid
increase of the major CO oxidation current, together with the
coadsorbed water band. It can also be seen that COB can be
removed at higher potentials in comparison to COL. No
conversion of COL to COB (or vice versa) was detected in the
prewave region. These facts are in marked contrast with those
reported on a Pt electrode, where COB is more stable at relative
The bands of free water molecules coadsorbed with CO
mentioned above are observed also in Figures 5 and 6. The
frequency of the relatively weak δ(HOH) band is virtually
independent of potential scan, whereas that of the relatively
strong υ(OH) band shifts linearly with potential from 3629 to
3653 cm-1 at a rate of 25 cm-1 V-1 probably due to the Stark
effect. These water bands completely disappeared in association
with the oxidation of adsorbed CO, which may suggest an active
involvement of coadsorbed water molecules in the oxidation
of CO as an oxygen source.
It is worth noting that the sharp bands of free water molecules
disappear after the oxidation of CO and never reappear. In the

SEIRAS at Pt Group Metal-Electrolyte Interfaces
J. Phys. Chem. B, Vol. 109, No. 16, 2005 7905
metals for electrochemical ATR-SEIRAS application. The
strategy complements the one-step wet process for fabricating
SEIRA-active surfaces which is currently limited to Au and Pt
nanofilms on Si, and Ag and Cu nanofilms on Ge. With selected
recipes and conditions for electrodeposition, Pt, Pd, Rh, and
Ru overlayers with thicknesses ranging from several monolayers
to tens of monolayers can uniformly coat onto a chemically
deposited SEIRA-active Au nanofilm with almost no pinholes.
The electrochemical behaviors of the Pt, Pd, Rh, and Ru
electrodes were almost identical to those of bulk metals. In
addition, these metal films exhibited extremely large SEIRA
effects. The resultant SEIRA intensity of the probe molecule
CO is 1 order of magnitude larger than the theoretical value
calculated for CO adsorption at a smooth polycrystalline Pt
electrode. For Pt electrodes prepared by the two-step method,
the enhancement is a few times larger than those prepared by
one-step wet process. Different from some earlier reports, the
enhanced infrared spectra did not show any bipolar or inverted
shapes, allowing for an easy and undisputed spectral analysis.
The high detection limit of SEIRAS and the ATR configuration
facilitated the simultaneous monitoring and analysis of molecular
adsorption and reactions at these Pt group metals. As demon-
strated, the two-step wet process is quite useful for in situ ATR-
SEIRAS studies of electrochemical reactions. Efforts in further
extension of metals are currently underway and will be reported
in due course.
Acknowledgment. The NSFC (Nos. 20333040, 20473025),
the SRFDP (No.20040246008), the NCET, and the SNPC (No.
0452nm064-2), China (W.-B.C.), and the MEXT (No. 14205121
and Priority Areas 417), Japan (M.O.), are gratefully acknowl-
edged for financial support.
Figure 7. (upper) Cyclic voltammogram of Pt nanofilm electrode in
0.1 M HClO4 + 0.5 M CH3OH during a potential sweep from 0.03 to
1.3 V and then back to 0.03 V (RHE) at a scan rate of 50 mV s-1
.
(lower) Series of corresponding time-resolved SEIRA spectra collected
sequentially during the cyclic potential sweep. The time resolution used
was 0.20 s. Reference spectrum was acquired at 1.3 V.
References and Notes
(1) Hartstein, A.; Kirtley, J. R.; Tsang, J. C. Phys. ReV. Lett. 1980, 45,
201.
(2) (a) Osawa, M. In Handbook of Vibrational Spectroscopy; Chalmers
J. M., Griffiths, P. R., Eds.; John Wiley & Sons: Chichester, UK, 2002;
Vol. 1, pp 785-799. (b) Osawa, M. Bull. Chem. Soc. Jpn. 1997, 70, 2861.
(3) Kellner, R.; Mizaikoff, B.; Jakusch, M.; Wanzenbock, H. D.;
Weissenbacher, N. Appl. Spectrosc. 1997, 51, 495.
(4) Sato, S.; Suzuki, T. Appl. Spectrosc. 1997, 51, 1170.
(5) (a) Cai, W.-B.; Wan, L.-J.; Noda, H.; Hibino, Y.; Ataka, K.; Osawa,
M. Langmuir 1998, 14, 6992. (b) Cai, W.-B.; Amano, M.; Osawa, M. J.
Electroanal. Chem. 2001, 500, 147.
(6) Maroun, F.; Ozanam, F.; Chazalviel, J. N.; Theiss, W. Vib.
Spectrosc. 1999, 19, 193.
(7) Wandlowski, T.; Ataka, K.; Pronkin, S.; Diesing, D. Electrochim.
Acta 2004, 49, 1233.
backward potential scan, a broad υ(OH) band and δ(HOH) band
appear at ∼3400 and 1605-1629 cm-1, respectively, instead
of the sharp bands after the Pd oxide layer was removed by
reduction. The result clearly indicates that free water molecules
can exist on the surface only when CO is adsorbed. Conse-
quently, the sharp water bands can be ascribed more reasonably
to those embedded in the CO adlayer9a than to those directly
adsorbed on the surface,12a although the slight differences in
peak positions on the four different metals seem to suggest some
interactions between free water and the surfaces.
(8) Bjerke, A. E.; Griffiths, P. R. Appl. Spectrosc. 2002, 56, 1275.
(9) (a) Miki, A.; Ye, S.; Osawa, M. Chem. Commun. 2003, 1500. (b)
Chen, Y. X.; Miki, A.; Ye, S.; Sakai, H.; Osawa, M. J. Am. Chem. Soc.
2003, 125, 3680. (c) Miki, A.; Ye, S.; Senzaki, T.; Osawa, M. J. Electroanal.
Chem. 2004, 563, 23.
(10) Rodes, A.; Orts, J. M.; Pe´rez, J. M.; Feliu, J. M.; Aldaz, A.
Electrochem. Commun. 2003, 5, 56.
(11) (a) Beden, B.; Lamy, C. In Spectroelectrochemistry: Theory and
Practice; Gale, R. J., Ed.; Plenum Press: New York, 1988; Chapter 5. (b)
Nichols, R. J. In Adsorption of Molecules at Metal Electrodes; Lipkowski,
J., Ross, P. N., Eds.; VCH: New York, 1992; Chapter 7.
(12) (a) Yajima, T.; Uchida, H.; Watanabe, M. J. Phys. Chem. B 2004,
108, 2654. (b) Watanabe, M.; Zhu, Y. M.; Uchida, H. J. Phys. Chem. B
2000, 104, 1762. (c) Zhu, Y. M.; Uchida, H.; Watanabe, M. Langmuir 1999,
15, 875.
Electrocatalytic Oxidation of Methanol on Pt. The elec-
trodeposited Pt nanofilm electrode has also been applied to an
in situ study of the electrocatalytic oxidation of methanol. The
time-resolved spectra of the Pt nanofilm electrode in 0.1 M
HClO4 + 0.5 M CH3OH obtained during cyclic voltammetric
measurement are shown in Figure 7, which are exactly the same
as for a chemically deposited Pt nanofilm on Si.9b Briefly,
starting from the initial potential of 0.03 V, the intermediate
for the self-dissociation of CH3OH on Pt at lower potentials is
undoubtedly ascribed to adsorbed CO, as evidenced by the
presence of COL and COB bands. As the methanol oxidation
current starts to rise at ca. 0.7 V, a band at 1320 cm-1 appears
and is maximized around 1.0 V, suggestive of the adsorbed
formate as the reactive intermediate species.9b
(13) Loster, M.; Friedrich, K. A. Surf. Sci. 2003, 523, 287.
(14) Bjerke, A. E.; Griffiths, P. R.; Theiss, W. Anal. Chem. 1999, 71,
1967.
(15) (a) Lu, G.-Q.; Sun, S.-G.; Cai, L.-R.; Chen, S.-P.; Tian, Z.-W.;
Shiu, K.-K. Langmuir 2000, 16, 778. (b) Gong, H.; Sun, S.-G.; Li, J.-T.;
Chen, Y.-J.; Chen, S.-P. Electrochim. Acta 2003, 48, 2933. (c) Sun, S.-G.
In Catalysis and Electrocatalysis at Nanoparticle Surfaces; Wieckowski,
A., Savinova, E. R., Vayenas, C. G., Eds.; Marcel Dekker: New York,
2003; Chapter 21.
Conclusion
We presented herein a ubiquitous two-step wet process, an
economical and easy method to prepare nanofilms of Pt group

7906 J. Phys. Chem. B, Vol. 109, No. 16, 2005
Yan et al.
(16) Park, S.; Wiekowski, A.; Weaver, M. J. J. Am. Chem. Soc. 2003,
125, 2282.
(22) Kern, W.; Puotinen, D. A. RCA ReV. 1970 31, 187.
(23) Strbac, S.; Maroun, F.; Magnussen, O. M.; Behm, R. J. J.
Electroanal. Chem. 2001, 500, 479.
(17) (a) Pecharroma´n, C.; Cuesta, A.; Gutie´rrez, C. J. Electroanal. Chem.
2002, 529, 145. (b) Ortiz, R.; Cuesta, A.; Marquez, O. P.; Marquez, J.;
Mendez, J. A.; Cutierrez, C. J. Electroanal. Chem. 1999, 465, 234.
(18) Miyake, H.; Osawa, M. Chem. Lett. 2004, 33, 278.
(19) Miyake, H.; Ye, S.; Osawa, M. Electrochem. Commun. 2002, 4,
973.
(20) (a) Zou, S.; Weaver, M. J. Anal. Chem. 1998, 70, 2387. (b) Zou,
S.; Williams, C. T.; Chen, E. K.-Y.; Weaver, M. J. J. Am. Chem. Soc. 1998,
120, 3811.
(21) (a) Cai, W. B.; Ren, B.; Liu, F. M.; Li, X. Q.; She, C. X.; Cai, X.
W.; Tian, Z. Q. Surf. Sci. 1998, 406, 9. (b) Tian, Z. Q.; Ren, B.; Wu, D. Y.
J. Phys. Chem. B 2002, 106, 9463. (c) Gomez, R.; Perez, J. M.; Solla-
Gullon, J.; Montiel, V.; Aldaz, A. J. Phys. Chem. B 2004, 108, 9943.
(24) (a) Ataka, K.; Yotsuyanagi, T.; Osawa, M. J. Phys. Chem. 1996,
100, 10664. (b) Ataka, K.; Osawa, M. Langmuir 1998, 14, 951.
(25) Gutierrez, C.; Caram, J. A.; Beden, B. J. Electroanal. Chem. 1991,
305, 289.
(26) Sun, S.-G.; Cai, W.-B.; Wan, L.-J.; Osawa, M. J. Phys. Chem. B
1999, 103, 2460.
(27) (a) Lebedeva, N. P.; Koper, M. T. M.; Feliu, J. M.; van Santen, R.
A. J. Electroanal. Chem. 2002, 524, 242. (b) Markovic, N. M.; Schmidt,
T. J.; Grgur, B. N.; Gasteiger, H. A.; Behm, R. J.; Ross, P. N. J. Phys.
Chem. B 1999, 103, 8568.
(28) Zou, S.; Gomez, R.; Weaver, M. J. J. Electroanal. Chem. 1999,
474, 155.