Extending in situ attenuated-total-reflection surface-enhanced infrared absorption spectroscopy to Ni electrodes

Article abstract of DOI:10.1021/jp0566966

Surface-enhanced infrared absorption spectroscopy (SEIRAS) in the attenuated-total-reflection configuration (ATR-SEIRAS) has been applied for the first time to Ni electrodes. SEIRA-active Ni electrodes were obtained through initial chemical deposition of

Full text of DOI:10.1021/jp0566966

4162
J. Phys. Chem. B 2006, 110, 4162-4169
Extending in Situ Attenuated-Total-Reflection Surface-Enhanced Infrared Absorption
Spectroscopy to Ni Electrodes
†
†
†
†
†
,†,‡
Sheng-Juan Huo, Xiao-Kang Xue, Yan-Gang Yan, Qiao-Xia Li, Min Ma, Wen-Bin Cai,*
‡
§
Qun-Jie Xu, and Masatoshi Osawa
Shanghai Key Laboratory for Molecular Catalysis and InnoVatiVe Materials and Department of Chemistry,
Fudan UniVersity, Shanghai 200433, China, Department of EnVironmental Engineering, Shanghai UniVersity of
Electric Power, Shanghai 200090, China, and Catalysis Research Center, Hokkaido UniVersity,
Sapporo 001-0021, Japan
ReceiVed: NoVember 18, 2005; In Final Form: January 5, 2006
Surface-enhanced infrared absorption spectroscopy (SEIRAS) in the attenuated-total-reflection configuration
(ATR-SEIRAS) has been applied for the first time to Ni electrodes. SEIRA-active Ni electrodes were obtained
through initial chemical deposition of a 60-nm-thick Au underlayer on the reflecting plane of an ATR Si
prism followed by potentiostatic electrodeposition of a 40-nm-thick Ni overlayer in a modified Watt’s
electrolyte. The Ni nanoparticle film thus obtained exhibited exceptionally enhanced IR absorption for the
surface probe molecule CO while maintaining unipolar and normally directed bands. With the advantages of
2
ATR-SEIRAS, free H O molecules coadsorbed with CO at the Ni electrode were revealed, and their role in
the electrooxidation of the CO adlayer at the Ni electrode is discussed. In addition, the conversion of bridge
to linearly bonded CO at Ni electrode in a neutral solution was clearly identified upon electrooxidation of the
CO adlayer. ATR-SEIRAS was also used to characterize the adsorption configuration of a pyridine adlayer
1 1
at the Ni electrode. Both A and B modes of adsorbed pyridine were detected with comparably large intensities,
essentially maintaining the spectral feature of pyridine molecules rather than that of “R-pyridyl species”,
which strongly suggests an “edge-tilted pyridine” configuration present at the Ni electrode, a configuration
intermediate between the “end-on pyridine” and “edge-on R-pyridyl” adsorption modes reported in the literature.
Introduction
evaporation or sputtering and electrodeposition.3,6a,9 Appropriate
fabrication of a conductive metallic nanofilm on an ATR IR
Owing to its high signal sensitivity, simple surface selection
rule, and negligible interference from bulk solution signals,
surface-enhanced infrared absorption spectroscopy with the
attenuated-total-reflection configuration (ATR-SEIRAS) is re-
garded as an important spectroscopic method for in situ
characterization of surface adsorption and reaction, providing
insight into the chemistry and physics of metal-electrolyte and
metal-ambient interfaces.1 So far, for in situ electrochemical
applications, ATR-SEIRAS has been limited to coinage and
platinum-group metal electrodes. Extension of this technique
to other transition metal electrodes is highly desirable for it to
become a general tool in surface electrochemistry. As an element
of the iron group, nickel is a metal of both fundamental and
practical interests in electrochemistry, surface science, and
catalysis. To our best knowledge, ATR-SEIRAS at Ni electrodes
has not been explored.
window is essential for in situ ATR-SEIRAS. The most widely
used ATR IR window is an undoped Si prism, which exhibits
high stability in acidic and neutral electrolytes. The metallic
films should be sufficiently conductive and have electrochemical
features comparable to those of their bulk counterparts for use
in electrochemistry. Moreover, they should be SEIRA-active
without severely distorted or inverted bands to facilitate data
analysis and explanation.
-8
Very recently, SEIRA-active Pt-group metal nanofilms were
obtained in our laboratory with a two-step wet process that
incorporates an initial chemical deposition of a Au nanofilm
on the basal plane of a Si prism with the subsequent elec-
trodeposition of the desired Pt-group metal overlayers.8 This
technique is somewhat similar to the sample preparation method
used for surface-enhanced Raman spectroscopy (SERS) on
transition metals. In the latter, the underlying Au layer functions
as an amplifier of the Raman scattering of adsorbates on the
transition metal, whereas it functions as a stable template for
preparing SEIRA-active Pt-group metals in our case. The
strategy circumvented several disadvantages associated with
conventional dry processes (vacuum evaporation or sputtering),
such as high-cost facilities, time-consuming procedures, poor
enhancement reproducibility, bipolar band distortion, and sample
contamination by organic species. We believe that this strategy
can also be extended further to Ni electrodes for the following
three reasons: First, theoretical calculations predict that nearly
c,d
SEIRA originates from the interactions of IR photons with
the metal atoms and adsorbed molecules; thin metal films
consisting of nanoparticles particularly facilitate the SEIRA
2b,d,h
effect.
The SEIRA activity depends greatly on the morphol-
ogy of the metal nanoparticle film (simplified as nanofilm
hereafter). Severely distorted bipolar or even inverted IR bands
are often observed for transition metal nanofilms prepared by
*
Corresponding author. Phone: +86-21-55664050. Fax: +86-21-
6
5641740. E-mail: wbcai@fudan.edu.cn.
†
Fudan University.
Shanghai University of Electric Power.
Hokkaido University.
2
d,h
‡
§
all metals can yield the SEIRA effect per se,
without the
need of borrowing the SEIRA effect from the underlying Au
1
0.1021/jp0566966 CCC: $33.50 © 2006 American Chemical Society
Published on Web 02/03/2006

Extending ATR-SEIRAS to Ni Electrodes
J. Phys. Chem. B, Vol. 110, No. 9, 2006 4163
layer. Unlike SERS, in which a virtually pinhole-free metal layer
with a thickness of a few atomic layers is required, the longer
wavelengths of IR radiation allow for the deposition of a much
thicker Ni overlayer carrying SEIRA activity. Second, elec-
trodeposition of very thick Ni multilayers on Au from a modified
Watt’s solution is a quite mature technique, ensuring sufficiently
CHART 1. Related Adsorption Configurations for Py
on Metal Surfaces
a
“
pinhole-free” Ni overlayers.10 Third, Ni is easily passivated
in neutral and alkaline electrolytes at high potentials without
continuous active dissolution. This passivated layer can be
electrochemically reduced back to nominal Ni. Thus, it can be
expected that a very thick Ni overlayer (few tens of nanometers)
on a Au underlayer should resemble bulk Ni in electrochemical
behavior.
a
(
I) Flat-lying Py mode [e.g., Py at a Au(111) electrode at potentials
8a,22
sufficiently negative of the pzc ], (II) end-on Py mode [e.g., Py at
a Au(111) electrode at potentials positive of the pzc8 ], (III) edge-
tilted Py mode [e.g., Py at a Pt(111) surface in UHV at low
temperature21 and Py at a Ni nanofilm electrode (see text vide infra)],
a,22
In the present work, CO was used as the probe molecule to
examine the surface enhancement of Ni electrodes for the
following reasons. External infrared reflection-absorption
spectroscopy (IRAS) measurements of CO on Ni at solid-
24a
24b
and (IV) edge-on R-pyridyl mode [e.g., Py at Ni(100), Ni(111),
Pt(111),21 and W(011)29 surfaces in UHV around room temperature].
1
1-13
14a,15-17,18a,b
reported,8a,21,26 one of which is also shown as configuration III
in Chart 1. The orientations and configurations depend on the
electronic structure of the metal, the temperature, the coverage,
the medium, and specifically the potential in the electrochemical
gas
and solid-electrolyte
interfaces have been
reported. In particular, Cuesta and Guti e´ rrez systematically
investigated pH effects on the CO adsorption sites and oxidation
mechanism.1 As a result, these electrochemical and spectro-
scopic data can be used as references for the current study.
Nevertheless, the reported CO bands in the literature are rather
5b
8
a,21,29
system.
Although HREELS, XPS, and ARUPS measurements pro-
vided evidence of R-pyridyl species formed on Ni surfaces in
UHV at room temperature or higher,
functional theory calculation results for Py adsorbed at a Ni
electrode appeared to support the end-on adsorption geometry.31
Because the SERS selection rule is rather complicated, however,
the configuration of Py at a Ni electrode is still open for
discussion according to our latest ATR-SEIRAS measurements.
-4
-3
weak (around 10 -10 relative reflectance units, |∆R/R|), and
the signal-to-noise (S/N) ratio is poor, which, to some extent,
prevented a detailed study of the dynamic oxidation process of
CO at Ni electrodes such as the identification of coadsorbed
2
4,30
SERS and density
water molecules.1
4a,15-17
Recently, Wang et al.
18a,b
reported
interesting abnormal IR effects (denoted as AIREs, i.e., inverted
absorption and broadened bandwidth) in the external reflection
configuration for Ni nanofilms electrodeposited on glassy carbon
substrates via cycling potential control. The maximum absolute
Experimental Section
-
3
intensity of around 8 × 10 (∆R/R) was obtained for their
inverted major COB band. However, the controversy over the
origin of AIREs can complicate spectral analysis.2a,18c,19 The
normally directed COB band was also detected under certain
Ni Nanofilm Preparation and Characterization. The prism
used was an undoped Si hemicylinder (PASTEC, Osaka, Japan).
The reflecting plane of the prism was polished with alumina
slurries with decreasing particle sizes down to 0.05 µm, followed
by sonication in acetone and then water. Then, the Si hemicyl-
-4
deposition conditions, but the band intensity was 4 × 10 (∆R/
R), only twice that of the polished bulk Ni electrode. By contrast,
current ATR-SEIRAS studies on Ni electrodes yield normally
directed and enhanced CO bands, with intensities as high as
8c
inder was cleaned with the RCA method. Afterward, the
reflecting surface was covered with 40% NH4F for 90 s to
terminate the Si surface with hydrogen and was placed in the
plating solution and 2% HF (2:1 by volume) for 3 min. Chemical
deposition of Au was performed at 60 °C. The composition of
the plating solution was 0.03 M AuCl3‚HCl‚H2O (adjusted to
-
2
3
.4 × 10 (∆R/R). The excellent S/N ratio for the surface
signals of ATR-SEIRAS has two advantages: First, it enables
simultaneous spectroscopic and electrochemical monitoring of
the irreversible oxidation of CO adlayer at Ni electrode with a
much better time resolution as compared to the previous linear
potential scan (LPS) FT-IR method.1 Second, new spectral
features can be revealed such as bands for coadsorbed free water
molecules at the Ni electrode, providing insight into the
oxidation mechanism.
pH 8 with NaOH) + 0.3 M Na2SO3 + 0.1 M Na2S2O3‚5H2O
2i,8c,d
+
0.1 M NH4Cl.
The Au-coated Si prism was rinsed with
5
Milli-Q water (>18 MΩ cm) and assembled into a custom-
made spectroelectrochemical cell by sandwiching an O-ring. Pt
gauze and a saturated calomel electrode (SCE) were used as
the counter and reference electrodes, respectively. The Au
Significant enhancement of surface IR signals for the as-
deposited Ni electrodes also allows for clarification of the
adsorption configuration of pyridine (Py) at the Ni electrode.
The orientations of Py at various metal surfaces in UHV and
electrochemical environments have been extensively investigated
surface was then cleaned in 0.1 M HClO by repeating potential
scans between 0.0 and 1.45 V.
Deposition of a Ni overlayer on the as-deposited clean Au
4
underlayer was conducted by electrolysis at -1.0 V for 1800 s
-
3
-2
in a modified Watt’s electrolyte (i.e., 10 M NiSO + 10
4
M
20-22
8a,23
-4
10
by a variety of techniques, such as IRAS,
STM,
SFG, and isotope label-
in addition to electrochemical measurements.8a,20,22
H BO + 10 M HCl). Then, the spectroelectrochemical cell
was thoroughly rinsed with copious amounts of Milli-Q water
3
3
24
24
25,26
27
HREELS, ARUPS, SERS,
ing,
2
0,28
and immediately thereafter injected with 0.1 M K HPO + 0.1
2
4
Three extreme modes of Py coordination to metal surfaces have
been proposed (Chart 1): (1) “flat-lying Py” adsorption via π
electrons of the aromatic ring (I, Chart 1), (2) N-coordinated
vertical “end-on Py” adsorption (II, Chart 1), and (3) “edge-on
R-pyridyl” adsorption with nearly vertical orientation through
the N and C(2) atoms (IV, Chart 1). In addition, intermediate
configurations of the above critical modes have also been
2 4
M KH PO (phosphate buffer solution, pH 6.9). The potential
was held at -1.20 V for 10 min to reduce the native oxides on
the Ni surfaces. The geometrical surface area of the Ni working
-2
electrode was 1.33 cm . All electrolyte solutions were deaerated
with high-purity Ar prior to measurements.
The double-layer capacitance of the Ni nanofilm electrode
was obtained by measuring its differential capacitance curve in

4164 J. Phys. Chem. B, Vol. 110, No. 9, 2006
Huo et al.
a freshly prepared phosphate buffer solution by ac voltammetry
in which a 100-Hz sinusoidal signal with an amplitude of 5
mV was superposed on a linear-scan dc potential at 10 mV s-
from -1.0 to 0.2 V and the ac current component was extracted
and analyzed with a lock-in amplifier built in a CHI 660 B
electrochemistry workstation (CH Instruments, Inc., Austin, TX).
The CHI 660B electrochemistry workstation was employed
for all potential control in the electrodeposition of Ni nanofilms
and subsequent (spectro)electrochemical measurements.
Inductively coupled plasma (ICP) atomic emission spectros-
copy (AES) was used to determine the concentrations of
dissolved Ni and Au ions from the metal films on Si in hot
aqua regia of known volume. With known amounts of Au and
Ni species, the thicknesses of the Au and Ni nanofilms were
estimated to be around 60 and 40 nm, respectively, by assuming
the same density as the bulk metals and using the geometric
area for the calculations.
1
Atomic force microscopy (AFM) images of a Au nanofilm
chemically deposited on Si before and after electrodeposition
of a Ni overlayer were acquired in tapping mode under ambient
conditions with a Pico-SPM (Molecular Imaging, Tempe, AZ).
Si cantilevers having spring constants of between 1.2 and 5.5
-
1
N m were used at resonance frequencies between 60 and 90
kHz.
XPS spectra of Ni-coated Au/Si samples were recorded on a
Perkin-Elmer PHI-5000C ESCA system equipped with a dual
X-ray source, using the Al KR radiation (hν ) 1486.6 eV) anode
and a hemispherical energy analyzer. The background pressure
-
9
during data acquisition was maintained at 10 Pa. Measure-
ments were performed at a pass energy of 93.90 eV. A survey
XPS spectrum (0-1200 eV) and narrow spectra of all elements
at high resolution were both recorded. Data were analyzed using
the PHI-MATLAB software provided by PHI Corporation.
In Situ ATR-SEIRA Spectroscopy. The three-electrode
system used for in situ ATR-SEIRAS is the same as that
reported for the electrochemical measurements (vide supra). The
detailed structure of the spectroelectrochemical cell and the
Figure 1. AFM images of (a) a Au underlayer (ca. 60 nm thick)
chemically deposited on a Si wafer and (b) a Ni overlayer (ca. 40 nm
thick) subsequently added by potentiostatic electrodeposition in a
modified Watt’s solution. Preparation conditions are detailed in the
text.
1-8
ATR-SEIRAS measurements have been described elsewhere.
Briefly, unpolarized infrared radiation from a ceramic source
was focused at the interface by being passed through the
hemicylindrical Si prism at an incident angle of 70°, and the
totally reflected radiation was detected. Spectra were acquired
under either potentiostatic (multistep) or potentiodynamic
spectroelectrochemical measurements was characterized with
atomic force microscopy (AFM, Figure 1). For comparison, the
AFM image of a bare Au underlayer is also shown. The
thicknesses of the Au underlayer and Ni overlayer of the double-
layer film were estimated to be about 60 and 40 nm, respec-
tively, on average, based on the amount of each element
contained in the film determined by ICP-AES after the film
was dissolved in hot aqua regia. It can be seen that much finer
Ni nanoparticles with a diameter of around 15 nm closely pack
above the locus of the underlayer of larger Au nanoparticles.
By contrast, the Ni nanofilm electrodeposited on a glassy carbon
electrode by cycling potential deposition exhibited a layer
(
kinetics) mode. Infrared spectra were recorded on a Magna-
IR ESP System 760 Fourier transform infrared spectrometer
Nicolet) equipped with a liquid-nitrogen-cooled MCT detector.
The sample compartment of the FT-IR spectrometer was purged
(
-
1
with 20 L min of CO2- and H2O-free nitrogen. The spec-
trometer was operated at a resolution of 4 cm (for multistep)
-
1
-
1
or 8 cm (for kinetics). Two hundred fifty-six interferograms
were co-added to each single-beam spectrum in the multistep
mode, whereas in kinetics mode, only four interferograms were
collected for each spectrum. All spectra are shown in absorbance
units defined as A ) -log(R/R0), where R and R0 represent the
intensities of the infrared radiation reflected from the electrode
at the sample and the reference potential, respectively. Reference
spectra were measured at sufficiently positive potentials where
CO was electrooxidized or most of the Py was desorbed from
nominally reduced Ni surfaces.
18a,b
structure.
This difference in morphology between Ni nano-
films in the literature and those in the current work is probably
of structural origin based on the requirements for producing
2
d,3a,18a
AIREs and SEIRA effects.
The elemental composition of the surface layer of a Ni
overlayer electrodeposited on a Au nanofilm chemically de-
posited on a Si substrate can be observed from a survey XPS
spectrum, as shown in Figure 2. With the exception of the C
(1s) peak, all of the other peaks could be assigned to photo-
emission and Auger transitions of Ni and O as indicated. The
Ni (2p3/2) peaks at binding energies (BEs) of 852.4 and 854.5
Results and Discussion
AFM and XPS Characterization. The morphology of a Ni-
coated Au film prepared on a Si wafer plane by exactly the
same preparation process as for the electrodes used in the

Extending ATR-SEIRAS to Ni Electrodes
J. Phys. Chem. B, Vol. 110, No. 9, 2006 4165
The surface roughness factor of the Ni nanofilm electrodes
was estimated by taking the ratio of the measured double-layer
-
2
capacitance to 20 µF cm , a commonly used reference value
of the double-layer capacitance for a smooth metal electrode.
Shown in Figure 4 presents a typical differential capacitance
curve for a Ni nanofilm electrode as measured using ac
voltammetry. The peak at -0.34 V can be attributed to partial
electrodissolution of the Ni surface, leading to surface passi-
vation at potentials higher than -0.20 V. At potentials negative
of -0.70 V, electrosorption and even the evolution of hydrogen
can occur, contributing to the increased capacitance (i.e.,
pseudocapacitance). The practical double-layer region for a Ni
electrode is rather narrow, roughly from -0.70 to -0.40 V as
judged from Figure 4, which yields a double-layer capacitance
-
2
of ca. 87 µF cm , corresponding to a surface roughness factor
of ca. 4.3.
Figure 2. Survey XPS spectrum of an electrodeposited Ni overlayer
(40 nm) on a Au nanofilm (60 nm) chemically deposited on a Si
SEIRAS of CO at Ni Electrodes. Potential-dependent
SEIRA spectra of a CO-predosed Ni electrode obtained in
multistep mode are shown in Figure 5. The bands at 2000-
substrate and the narrow spectra of Au 4f and Ni 2p (insets).
eV in the right inset indicate that two types of Ni chemical
species, one reduced and one oxidized, were detected. The peak
at lower binding energy can be attributed to the metallic state,
-
1
2
050 cm can be assigned to linearly bonded CO (COL), and
-
1
those at 1865-1905 cm correspond to bridge-bonded CO
1
4a,15b
(COB).
It can also be seen that the COL bands are slightly
and that at higher energy indicates the presence of Ni species
in the form of Nix+ ions.32 However, no Au peaks were
sharper than the COB bands. The appearance of both COB and
COL bands for a Ni electrode in a neutral electrolyte is in
observed, as shown in the left inset. Because XPS detects the
elemental composition of surface layers with a skin depth of
around 3-5 nm, the presence of Au-Ni alloys in the surface
layers of our electrodeposited Ni nanofilm can be excluded,
which ensures that all of the following (spectro)electrochemical
results correspond to Ni electrodes.
1
4a,15b
agreement with previous IRAS measurements.
A small
-
1
band at 2112 cm at relatively positive potentials can be
attributed to CO chemisorbed on nickel oxide or on metallic
Ni sites perturbed by the oxidation of the neighboring Ni atoms
o
15a
(
this band is hereafter denoted as CONi ). This band cannot
be ascribed to linearly adsorbed CO on few (if any) exposed
sites of the underlayer Au for the following reasons: First, the
interaction of CO with Au is weak, and CO will nearly
Electrochemical Characterization. The as-deposited Ni
nanofilm electrode was subjected to cathodic polarization at
-
1.20 V for 10 min to remove the native nickel oxides. The
33
completely desorb when it is purged from solution. Second,
cyclic voltammogram of the Ni electrode in a pH 6.9 phosphate
buffer solution (solid trace in Figure 3a) shows an anodic peak
-
1
the linear CO band is located below 2090 cm in neutral
34a
-
2
solution. Third, this band was not observed at lower potentials.
at -0.34 V with a charge of 3.0 mC cm in the first forward
potential scan. The anodic peak leads to the passivation of Ni
at higher potentials. After a cyclic potential excursion, the
electrode was set at -1.20 V while CO was bubbled for 30
min to attain a saturated adsorption of CO. Subsequent cyclic
voltammograms (dashed and dotted traces in Figure 3a) show
that both hydrogen evolution and Ni electrooxidation were
inhibited. In the presence of CO in solution, the anodic peaks
were shifted positively to 0.027 V (dashed trace) and -0.017
V (dotted trace). The charge in the first positive scan was 5.9
The potential-dependent spectra obtained by multistep FT-
IR spectroscopy show that the Stark tuning rates are 61.5 and
-
1
-1
53.0 cm
V
for linear CO (COL) and bridge CO (COB),
respectively. The former is in good agreement with the value
15b
measured for polycrystalline bulk Ni electrodes, and the latter
is close to that obtained for Ni films electrodeposited on glassy
1
8b
15b
carbon. Cuesta et al. detected a COB band with an intensity
-
4
of 8 × 10 (∆R/R) for a bulk Ni electrode in pH 6.9 solution
1
8b
with IRAS. With AIREs, Wang et al. obtained a strong but
-
2
-3
mC cm , which was somewhat higher than that in the absence
of CO, suggesting the mixed simultaneous electrooxidation of
the Ni surface, chemisorbed CO, and dissolved CO.
abnormally inverted COB band with an intensity up to 6 × 10
for a Ni nanofilm electrodeposited on glassy carbon at pH 5.6.
Using the formulation proposed by Griffiths et al.,3a they
calculated the maximum surface enhancement factor to be 15.5.
The COB band intensity herein observed is 0.015 absorbance
unit or 0.034 ∆R/R, that is, 42.5 times stronger than that reported
for bulk Ni electrodes and 5.7 times stronger than that for
electrodeposited Ni films measured by AIREs. Precise calcula-
tion of the surface enhancement factor (G) for the current Ni
nanofilms is not easy. Nevertheless, if G is defined here as the
ratio of band intensities between a molecule adsorbed on a Ni
nanofilm and a molecule adsorbed on a smooth bulk Ni
Figure 3b shows the voltammetric responses in phosphate
buffer solution following a saturated dosage of chemisorbed CO,
obtained by bubbling in gaseous CO followed by Ar sparging.
(Note that the same CO predosing procedure for the Ni
electrodes was used in the subsequent SEIRAS measurements.)
The voltammogram in the first cycle (solid trace in Figure 3b)
is similar to that in the CO-saturated solution (dashed trace in
Figure 3a), which suggests that the adsorption of CO is strong.
The voltammogram in the second cycle (dotted trace in Figure
2
c
3b) is also similar to that of the Ni electrode in Ar-saturated
surface, after rational calibration of the surface coverage,
surface roughness factor, incidence angle, a nd polarized state
of IR radiation, G can be estimated to be larger than 29 (see
Supporting Information). In addition, the CO bands obtained
with ATR-SEIRAS are unipolar and normally directed in
accordance with the definition of absorbance (i.e., normal
absorption), facilitating the spectral analysis and interpretation.
The asymmetry in CO bands with an extended “tail” toward
phosphate buffer solution (solid trace in Figure 3a), and the
CO peak is no longer present, indicating that the chemisorbed
CO had been electrooxidized in the first potential cycle.
All of the above voltammetric features are close to those
reported for bulk polycrystalline Ni electrodes in the same elec-
trolyte, which is essential for extending ATR-SEIRAS to the
14b,c,15b
study of surface phenomena and processes at Ni electrodes.

4166 J. Phys. Chem. B, Vol. 110, No. 9, 2006
Huo et al.
Figure 3. (a) Cyclic voltammograms of a Ni nanofilm electrode in 0.1 M phosphate buffer solution (pH 6.9) in high-purity Ar atmosphere (solid
curve) and in CO-saturated solution (dashed and dotted curves for the first and second cycles, respectively). (b) Cyclic voltammograms of the
CO-predosed Ni nanofilm electrode in a phosphate buffer solution after purging CO with Ar: first cycle (solid curve), second cycle (dotted curve).
-
1
Scan rate ) 50 mV s
.
free water molecules embedded in the CO adlayer on a Ni
electrode surface (i.e., the hydrogen bonds between water
molecules were broken) are readily identified in the current
-
1
ATR-SEIRAS study. The sharp bands at 3592-3637 cm can
-
1
be assigned to υ(OH), and those at 1619-1626 cm can be
2
e,8c,35,36
assigned to δ(HOH) of free water molecules.
Specifi-
cally, the frequency of υ(OH) increases and that of δ(HOH)
decreases with the potential. These water bands decrease in
intensity with decreasing CO bands and become nearly invisible
as the adsorbed CO adlayers are completely oxidized (vide
infra). The decrease of coadsorbed free water bands ac-
companied by the oxidation of CO species has also been
6
b
observed at Pt-Ru and Ru electrodes by Watanabe’s group;
3
6
at Pt electrodes by Okada’s group; and at Pd, Pt, Ru, and Rh
8
c,8d
electrodes by our group
using in situ ATR-SEIRAS.
Figure 4. Differential capacitance curve for a Ni nanofilm electrode
in 0.1 M phosphate buffer solution obtained with ac voltammetry.
Experimental details can be found in the text.
Time-resolved measurements (kinetics mode) are more ac-
curate for tracing an irreversible process, as compared to the
above-mentioned multistep measurements. The ATR-SEIRA-
active Ni nanofilms are highly beneficial for real-time monitor-
ing and analysis of the electrooxidative removal of a CO adlayer
at a Ni electrode. Figure 6 shows a series of 0.27-s-resolved
spectra of a Ni-coated Au electrode predosed with CO in 0.1
M phosphate buffer solution, which were collected sequentially
during a linear potential sweep (LPS) from -1.20 to 0.60 V at
-
1
a scan rate 50 mV s (i.e., each successive spectrum was
acquired for a period of 0.27 s over a potential interval of 13.5
mV.). The reference spectrum was taken at 0.60 V. The spectra
at potentials positive of 0.05 V are not shown because of a large
background undulation, which might be caused by passivation
of the Ni surface. Figure 7 displays the plots of the normalized
o
integrated intensities of the COL, COB, CONi , ν(OH), and δ-
(
HOH) bands against the potential and the corresponding
voltammogram as well. The ν(OH) band of free H2O from
1.20 to -0.90 V in Figure 6 is not very distinct because it is
Figure 5. Potential-dependent multistep SEIRA spectra for a CO-
predosed Ni nanofilm electrode in 0.1 M phosphate buffer solution.
The reference potential is 0.60 V vs SCE, and the sample potentials
are as indicated. The Ni electrode was predosed in CO-saturated 0.1
M phosphate buffer solution at -1.20 V SCE for 30 min, and then the
dissolved CO was purged with Ar bubbling for 30 min.
-
buried in the relatively broad ν(OH) band of adjacent H2O
layers, so its intensity in this region is not plotted. We tentatively
attribute this result to the effect of hydrogen evolution at these
negative potentials, which might change the hydrogen-bonding
state of coadsorbed H2O molecules to some extent.36
lower wavenumbers can be partly ascribed to nanoparticle
polydispersity.3
4b,34c
The high surface sensitivity of ATR-SEIRAS provides insight
into the adsorption and oxidation of CO adlayers at Ni
electrodes. First, because the optical absorption coefficient of
Previous IRAS measurements, including AIREs studies, of
CO adsorption on Ni electrodes have not been able to detect
coadsorbed free water bands because of their lower surface
sensitivity and the interference of bulk water bands. By contrast,
15b
COB is a factor of about 2.5 lower than that of COL, the higher
COB band intensity indicates that COB species are dominant at

Extending ATR-SEIRAS to Ni Electrodes
J. Phys. Chem. B, Vol. 110, No. 9, 2006 4167
Figure 8. Cyclic voltammograms of a Ni nanofilm electrode in 0.1
M KClO
4
solution in the absence (solid curve) and presence (dotted
-
1
curve) of 10 mM Py recorded at 50 mV s
.
Figure 6. Series of time-resolved SEIRA spectra of a CO-predosed
Ni nanofilm electrode in 0.1 M phosphate buffer solution collected
sequentially during a potential sweep cycling from -1.20 to 0.05V at
-
1
5
0 mV s . The time resolution used was 0.27 s. The Ni nanofilm
electrode was predosed in CO-saturated phosphate buffer solution at
1.20 V for 30 min, and then the dissolved CO was purged with Ar
-
bubbling for 30 min.
Figure 9. ATR-SEIRA spectra of adsorbed Py at the Ni-solution
interface. The reference potential is 0.20 V vs SCE, and the sample
potentials are as indicated. The solution contained 0.1 M KClO + 10
4
mM Py. Spectra are an average of 256 interferometer scans at each
potential.
V, with nearly unchanged band position. It is also noted that
-1
the emergence of this 2112 cm band positively shifts by about
.10 V as compared to that in the multistep measurements (see
0
Figure 5), reflecting the advantage of time-resolved ATR-
SEIRAS. Finally, it can be judged from Figures 5 and 6 that
the δ(HOH) and ν(OH) bands for coadsorbed free H2O decrease
simultaneously in response to the sharp decrease of the COB
band, suggesting that adsorbed CO molecules are oxidized by
consuming the coadsorbed H2O on the Ni electrode. In this
regard, the active involvement of the coadsorbed H2O in the
oxidative removal of CO adlayers makes no difference for Ni-
and Pt-group metal electrodes. In other words, activated free
water molecules at these electrodes are likely oxygen donors
Figure 7. Cyclic voltammogram of a Ni-coated Au nanofilm electrode
recorded at 50 mV s-1 during in situ time-resolved ATR-SEIRA spectra
measurements, along with the potential-dependent integrated intensities
o
of the IR bands of CO
B
, CO
L
, CONi , and coadsorbed water.
relatively low potentials. Second, it can be seen that, at potentials
above -0.40 V, the intensity of the COB band decreases
monotonically and significantly as the oxidation of the CO
adlayer proceeds, whereas that of the COL band first increases
with decreasing COB and then starts to decrease as the
concentration of COB becomes negligibly small. These trends
strongly suggest that, in the oxidation process, the conversion
of some COB to COL occurs with decreasing CO surface
coverage. In other words, COL is more resistive than COB
against electrooxidation at the Ni electrode. The transition of
COB to COL at the Ni electrode with decreasing surface coverage
is in accordance with the behavior measured for Ni(100) in UHV
3
4d,36
for the electrooxidation of CO adlayers.
SEIRAS of Pyridine (Py) at Ni Electrodes. Significant
enhancement of the surface IR signals for the as-deposited Ni
electrodes also allows the examination of the adsorbed structure
of Py at the Ni electrode. As mentioned in the Introduction, the
formation of R-pyridyl species on Ni surfaces upon Py adsorp-
tion has been well characterized in UHV systems by a variety
of spectroscopic techniques. However, spectroscopic charac-
terization of the configuration of Py adsorbed at Ni electrodes
has been limited to only SERS. Tian’s group succeeded for the
first time in obtaining high-quality SER spectra of Py at a Ni
1
3
by FT-IRAS. This feature is quite different from that found
during oxidation of a CO adlayer at a Pd electrode, where COB
8
c
is also dominant. In the latter, COL is first oxidized, and part
of the COB is converted to CO adsorbed on hollow sites (COT),
but interconversion between of COL and COB does not occur.
2
5
electrode surface, and they proposed an end-on configuration
for the Py adlayer. One key piece of supporting evidence is the
-
1
Third, a new and rather weak band at 2112 cm attributable
o
-1
to CONi appears at -0.07 V and reaches a maximum at -0.03
band at 262 cm , which was assigned to the metal-N vibration.

4
168 J. Phys. Chem. B, Vol. 110, No. 9, 2006
Huo et al.
symmetry
TABLE 1. Wavenumbers and Assignments for Py at a Ni Electrodea
assignmentsb
Wilson number
8af
Py/Ni
electrode
Py/Au electrode,
Py/Pt(111), UHV,
R-pyridyl/Pt(111),
UHV, room Te
liquid Pyb
c
>pzc
d
low Te
1
1
1
1
592 s
583 m
482 s
1607 s
1599 vs
not detected
1479 m
1595 s
not detected
1470 m
1568 vs
1568 vs
1427 s
A1
B1
A1
B1
f
1575 vw
1490 m
1448 vs
8b
19a
19b
439 vs
1448 w
1440 vs
1408 m
a
Relative intensity: vs, very strong; s, strong; m, medium; w, weak; vw, very weak. b From ref 28. c Current result. d From ref 8a. e From ref
f
-1
2
1. When R-pyridyl forms, it is difficult to assign the 1568 cm band exactly to 8a or to 8b; cf. refs 21 and 29.
The other piece of evidence is that the bands are predominantly
assigned to the in-plane A1 mode. Nevertheless, even though
positive of the point of zero charge (pzc), the A1 mode was
predominant, and the B1 mode was extremely weak, suggesting
that a Py adlayer on the Au(111) electrode surface retains a
mirror plane perpendicular to the molecular plane, i.e., the mirror
plane containing the C2 axis of Py and the surface normal are
vertical with respect to the Py plane.
-
1
the 262 cm band can be ascribed to metal-N stretching, it
does not necessarily indicate that the C2 axis of Py is vertical
-
1
with respect to the surface. In addition, the 262 cm band is
-
1
also close to the calculated frequency (265 cm ) for the
26
R-pyridyl-metal2 symmetric stretching mode. As a result, the
As mentioned before, HREELS, ARUPS, and XPS charac-
terizations of Py adsorption at Ni(111) and Ni(100) in UHV
indicate that exposures at elevated or room temperature produce
an approximately vertically oriented species that can be at-
tributed to the chemisorbed R-pyridyl species. The question is
whether the R-pyridyl species is formed at Ni electrodes from
the end-on configuration. To address this issue, a straightforward
approach is to compare the spectral features of Py at a Ni
electrode with those of R-pyridyl species and edge-on Py species
on metal surfaces. Fortunately, IRAS data for Py adsorption at
-
1
2
62 cm band in the SERS spectra cannot be exclusively
assigned to the end-on configuration of adsorbed Py. On the
other hand, the strict dipole selection rule based on counteraction
of s-polarized light at metal surfaces in the “image-charge”
theory, which applies to IR radiation (SEIRAS), does not exactly
apply to SERS owing to the much lower electric conductivity
of metals in the visible region.3
7-40
Moreover, in SERS, one
must consider the polarizability tensor R, rather than the dipole
moment µ. Even simply based on an electromagnetic enhance-
ment mechanism, A1 modes have three Raman tensor compo-
nents, Rxx, Ryy, and Rzz. In other words, the surface selection
rule for SERS is not as straightforward as that for its counterpart
SEIRAS. On the basis of the current ATR-SEIRAS measure-
ments, we aim to clarify the adsorption geometry of Py at a Ni
electrode.
21
29
Pt(111) and W(110) are available, which might lend some
useful information for our judgment. For both edge-tilted Py
and edge-on R-pyridyl species, the original A1 and B1 modes
are dipole-active. Yet, significant differences are found between
the two surface species, as summarized in Table 1 (columns 4
and 5). First, the annealing of the substrate to room temperature
results in the significant red shifts of the major bands associated
with the conformation change from edge-tilted to R-pyridyl
species. Second, the conformation change also gives a significant
Shown in Figure 8 are cyclic voltammograms for a Ni-coated
Au electrode in 0.1 M KClO4 without (solid trace) and with
-
2
-1
(
dotted trace) 10 M Py recorded at a scan rate of 50 mV s .
It can be seen that the adsorption of Py significantly hinders
the anodic dissolution and passivation of the Ni electrode but
slightly promotes hydrogen evolution. Because of the high
chemsorption energy for Py on Ni surfaces,2 even at rather
negative potentials, Py does not desorb from the Ni surface, as
will be revealed by SEIRA spectra. At the reference potential
of 0.2 V, however, a fraction of Py molecules might still weakly
adsorb at the nickel hydroxide surface.
-1
change in relative intensities. The 19b mode (B1) at 1440 cm
-1
is the strongest for edge-tilted Py, whereas the 1568 cm band
(which corresponds to the original 8a and/or 8b modes for
4a
pyridine) is the strongest for R-pyridyl. Similar spectral features
29
can be found for Py on the W(110) surface. Our spectral
pattern matches reasonably well with that of Py adsorption on
Pt(111) at low temperature (see Table 1), suggesting that Py is
adsorbed on the Ni electrode surface through the edge-tilted
Py configuration without R-C-H bond cleavage. This edge-
tilted Py is likely a transitional configuration between the end-
on Py and the edge-on R-pyridyl species. For clarity, these three
adsorption configurations can be found in Chart 1, in which
the differentiation between plane-tilted and vertical orientations
is not intended. Again, we demonstrate that the simple surface
selection rule and high surface sensitivity of ATR-SEIRAS are
very useful for characterization and analysis of molecular
configurations at Ni electrodes, complementary to the widely
used SERS technique.
Figure 9 shows the potential-dependent ATR-SEIRA spectra
of Py adsorbed at a Ni electrode in 0.1 M KClO4 and 0.01 M
-1
Py. Four bands at 1607, 1575, 1490, and 1448 cm correspond
to ν8a(A1), ν8b(B1), ν19a(A1), and ν19b(B1), respectively, of
2
8
adsorbed Py. The relatively weak negative bands can be
ascribed to Py adsorbed at the nickel hydroxide surface. Here,
the A1 and B1 modes are in-plane vibrations yielding dipole
changes along and perpendicular to the C2 axis of Py,
respectively. These bands and their assignments are listed in
Table 1. For comparison, IR bands for liquid Py28 and SEIRA
bands for adsorbed Py at a Au(111) electrode8 are also
tabulated. Both A1 and B1 modes appear for Py at a Ni electrode.
a
Conclusion
-
1
In particular, the band at 1448 cm , a B1 mode, is the most
intense. According to the surface selection rule of SEIRAS, only
ATR-SEIRAS has been successfully extended for the first
time to Ni electrodes. A two-step wet process was employed to
fabricate SEIRA-active Ni electrodes, which incorporates an
initial chemical deposition of a 60-nm-thick Au underlayer on
the reflecting plane of an ATR Si prism and the subsequent
potentiostatic electrodeposition of a 40-nm-thick Ni overlayer
in a modified Watt’s electrolyte. Significant enhancement was
achieved without the emergence of bipolar or abnormally
inverted absorption bands. The surface enhancement factor for
those vibrations that cause dipole changes perpendicular to the
metal surface are SEIRA-active.2
b,d,h
The simultaneous appear-
ance of A1 and B1 modes suggests an edge-tilted Py adsorption
geometry at the Ni electrode surface, with tilting of the C2 axis
edgewise apart from the normal to the Ni surface. The
orientation and coordination of the adsorbed Py are nearly
unchanged in the potential range from -1.20 to -0.20 V. In
contrast, for Py adsorption on Au(111) electrodes at potentials

Extending ATR-SEIRAS to Ni Electrodes
J. Phys. Chem. B, Vol. 110, No. 9, 2006 4169
the Ni nanofilms thus obtained was evaluated to be more than
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