Seeded growth fabrication of Cu-on-Si electrodes for in situ ATR-SEIRAS applications

Article abstract of DOI:10.1016/j.electacta.2007.03.042

A seeded-growth approach has been developed to fabricate a Cu nanoparticle film (simplified hereafter with nanofilm) on Si for electrochemical ATR surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS). The approach comprises an initial activation of the reflecting plane of hemicylindrical Si prism by introducing a Cu seed layer in a CuSO₄-HF solution and the subsequent electroless deposition of the Cu nanofilms from an electroless Cu plating bath. The as-deposited Cu nanofilm exhibited strong SEIRA effect for the CO probe and interfacial free H₂O. ATR-SEIRAS was also applied to characterize the adsorbed geometries of pyridine at the Cu/electrolyte interface. Only vibrational bands assignable to the A₁ symmetry modes were detected in the entire potential window investigated, suggestive of an end-on adsorption via the ring N-atom on a Cu electrode.

Full text of DOI:10.1016/j.electacta.2007.03.042

Electrochimica Acta 52 (2007) 5950–5957
Seeded growth fabrication of Cu-on-Si electrodes for
in situ ATR-SEIRAS applications
Hui-Feng Wanga, Yan-Gang Yana, Sheng-Juan Huoa, Wen-Bin Caia,b,∗
,
Qun-Jie Xub, Masatoshi Osawac
a Shanghai Key Laboratory for Molecular Catalysis and Innovative Materials and Department of Chemistry,
Fudan University, Shanghai 200433, China
b Department of Enviromental Engineering, Shanghai University of Electric Power, Shanghai 200090, China
c Catalysis Research Center, Hokkaido University, Sapporo 001-0021, Japan
Received 23 December 2006; received in revised form 14 March 2007; accepted 16 March 2007
Available online 20 March 2007
Abstract
A seeded-growth approach has been developed to fabricate a Cu nanoparticle film (simplified hereafter with nanofilm) on Si for electrochemical
ATR surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS). The approach comprises an initial activation of the reflecting plane of
hemicylindrical Si prism by introducing a Cu seed layer in a CuSO4-HF solution and the subsequent electroless deposition of the Cu nanofilms
from an electroless Cu plating bath. The as-deposited Cu nanofilm exhibited strong SEIRA effect for the CO probe and interfacial free H2O. ATR-
SEIRAS was also applied to characterize the adsorbed geometries of pyridine at the Cu/electrolyte interface. Only vibrational bands assignable to
the A1 symmetry modes were detected in the entire potential window investigated, suggestive of an end-on adsorption via the ring N-atom on a Cu
electrode.
© 2007 Elsevier Ltd. All rights reserved.
Keywords: ATR-SEIRAS; Cu nanofilm electrode; Si; Seeded-growth; Carbon monoxide; Pyridine
1. Introduction
probing the interface at the molecule-level, with merits of high
sensitivity, capability of detecting low frequency vibrations, and
far less interference from bulk water signal, as compared to con-
ventional infrared absorption reflection spectroscopy (IRAS).
Nevertheless, the complexity of the enhancement mechanisms
ofSERS, aswellastherequirementofoxidation–reductioncycle
(ORC) pretreatment in a chloride solution may compromise its
versatility and strength [20].
Surface-enhanced infrared absorption spectroscopy
(SEIRAS) with attenuated-total reflection (ATR) configu-
ration is also an attractive analytical tool for probing electro-
chemical interfaces because of its high signal sensitivity and a
simple surface selection rule [21–23]. A key issue for success-
fully implementing this technique is to fabricate appropriate
nanoparticle metal films that can produce strong SEIRA effect
while maintain essentially the same electrochemical properties
as their bulk counterparts. Recently triggered by the effort in
Osawa’s group [24], simple wet fabrication techniques [25–40]
(including chemical deposition and electrochemical deposition
of metallic nanoparticle films on an IR window such as Si and
Because of its excellent electrical and thermal conductivity,
mechanical workability, copper has found extensive applica-
tions varying from conventional pipelines for condensed water
to advanced interconnects for ULSI chips [1–4]. The interfacial
electrochemistry of a Cu electrode has gained great interest due
to its practical importance in corrosion and inhibition [5–7], and
electrocatalysis [8–10]. Electrochemical measurement [11,12]
and surface-enhanced Raman spectroscopy (SERS) [13–19]
have been employed chiefly for the study of adsorption and
reaction at Cu electrodes. It is well known that electrochemical
measurement provides the current-potential-time information
about the electrochemical interface. SERS is very powerful in
∗
Corresponding author at: Shanghai Key Laboratory for Molecular Cataly-
sis and Innovative Materials and Department of Chemistry, Fudan University,
Shanghai 200433, China. Tel.: +86 21 55664050; fax: +86 21 65641740.
E-mail address: wbcai@fudan.edu.cn (W.-B. Cai).
0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2007.03.042

H.-F. Wang et al. / Electrochimica Acta 52 (2007) 5950–5957
5951
Ge) are developing rapidly, with an expectation to replace the
traditional and expensive dry process fabrication (including
vacuum evaporation and sputtering). A SEIRA-active Cu film
chemically deposited on a Ge element has been reported [29].
However, Si is used more frequently than Ge as an IR element
in electrochemical ATR-SEIRAS applications owing to its
higher stability in acidic and neutral electrolytes over a much
wider potential window. In fact, the electrochemistry of the
desired metal over-films on Ge often suffers from abnormal
responses, probably due to the dissolution of Ge substrate as
well as the partial peeling of metal films. To our knowledge,
wet process fabrication of Cu nanofilms on IR transparent Si
for electrochemical ATR-SEIRAS has not been reported in the
literature.
Most recently, a seeded-growth tactics was proposed in our
group to fabricate the ATR-SEIRA-active Ag electrode on Si
without the introduction of a second undesired metal (mostly
Pd) as the catalytic seeds [40]. In this report we would like to
extend this tactics to fabricate a Cu nanofilm electrode on Si
for electrochemical ATR-SEIRAS application. Unlike chemical
deposition of a Cu nanofilm on Ge in which the growth of a
conductive and SEIRA-active Cu nanofilm occurs by the con-
tinuous dissolution of the Ge substrate via the displacement of
Ge with Cu [41], our current approach allows a Cu nanofilm
on Si to be deposited via initial activation with Cu seeds fol-
lowed by its further growth in a Cu chemical plating bath without
continuous dissolution of Si substrate. It should be emphasized
that our seeded-growth tactics is quite different from that aimed
for microelectronics application in which Pd [42] or Au [43]
seeds were introduced to Si wafer surfaces as the catalysts, thus
preventing possible contamination in the resultant Cu nanofilm
electrode by a second metal.
were formed by immersing the Si reflecting plane in the 0.625 M
HF solution containing 3.15 mM CuSO4·5H2O for 5–10 s. After
thoroughly being rinsed with copious amount of ultrapure
Milli-Q water, the electroless deposition of Cu nanofilms was
carried out in a plating solution (recipe: 5 g L−1 CuSO4·5H2O,
25 g L−1 C4H4O6KNa·4H2O (i.e., potassium sodium tartrate),
10 mL L−1 HCHO, 7 g L−1 NaOH, adjusted to pH 12.5∼13 with
HCl) at 25–35 ◦C for 15–25 min. For comparison, Cu nanofilm
on the reflecting plane of a Ge prism was prepared directly by
immersing the latter in the above-mentioned plating solution at
25 ◦C for 20–25 min.
Inductively coupled plasma (ICP) atomic emission spec-
troscopy (AES) was used to estimate the thickness of the
deposited Cu nanofilms by determining the concentrations of
dissolved Cu ions from the Cu films on Si in hot aqua regia of a
known volume. The average thickness of the Cu nanofilms was
estimated to be around 68 nm, by assuming the same density as
the bulk metals and using the geometric area for the calculations.
Atomic force microscopy (AFM) images of a Cu seed
layer and Cu nanofilms on Si 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 N m−1 were used at resonance frequencies between
60 and 90 kHz.
The underpotential deposition (UPD) of Pb monolayer on
the as-deposited Cu nanofilm electrode was carried out with
cyclic voltammetry in a deaerated 0.1 M HClO4 containing
1 mM Pb(ClO4)2·3H2O between −0.05 and −0.43 V (versus
SCE) at 10 mV s−1. The average charge required for UPD of a
Pb monolayer was used to estimate the surface roughness factor
of the Cu nanofilm electrode.
The SEIRA activity of the chemically deposited Cu film
on Si was examined by using CO as a probe molecule. Sig-
nificantly enhanced IR absorption for surface species on the
as-deposited Cu electrode enables to detect for the first time a
band possibly due to interfacial free H2O co-existed with CO
on the coinage metal. Meanwhile, as an initial move to extend
the as-deposited Cu electrode to electrochemical ATR-SEIRAS
application, the configuration of adsorbed pyridine (Py), a pro-
totype molecule, was examined as well. The conclusion derived
from present in situ SEIRAS is in accordance with that obtained
from previous in situ SERS studies [14,15] as well as previous
IRAS measurement in UHV [44] on Cu surfaces. However, the
so-called␣-pyridylspeciesonCusurfacesuponpyridineadsorp-
tion, deduced by ex situ SERS on Cu nanoparticles formed on
an aluminum foil [18], was not discerned.
2.2. In situ ATR-SEIRA spectroscopy
All the experiments were carried out at the room temperature.
After being rinsed with ultrapure water, the above Cu-deposited
Si prism was assembled to a spectroelectrochemical cell with
which in situ ATR-SEIRAS measurement was carried out. A
Pt gauze and a saturated calomel electrode (SCE) were used
as the counter and reference electrodes, respectively. The Cu
electrode was cleaned in 0.1 M KClO4 by cycling it in between
potentials of slight surface oxidation and hydrogen evolution fol-
lowedwithacathodicreductionat−1.1 Vfor2 min. Thespectral
measurements were performed with the Kretschmann ATR
configuration, experimental details of which were described
elsewhere [46]. Multi-step potential difference mode was used in
which single-beam spectrum was taken at each potential of inter-
est with respect to a suitable reference spectrum. For SEIRAS
measurement of CO on Cu, CO gas (with a purity of 99.9%) was
bubbled into the cell gently for 30 min to saturate 0.1 M KClO4
with CO. The reference spectrum was taken at −0.6 V (SCE) in
the same solution, at which the desorption of CO was virtually
completed. The SEIRAS measurement of Py on Cu was carried
out in a 0.1 M KClO4 solution containing 10 mM pyridine with
a reference spectrum taken in a 0.1 M KClO4 solution with-
out Py at each corresponding sample potential. All spectra are
shown in the absorbance units defined as A = −log(I/I0), where I
2. Experimental
2.1. Preparation and characterization of Cu nanofilm
The Cu nanofilm on Si was prepared as follows: the reflect-
ing plane of a hemicylindrical Si prism was polished and then
cleanedwiththe RCAmethod [45], then thetotal reflecting plane
was immersed in 40% NH4F and 40% HF (10:1, v/v) solution
for 2 min to terminate the Si surface with hydrogen. Cu seeds

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and I0 represent the intensities of the infrared radiation reflected
from the electrode at the sample potential and the reference one,
respectively.
3. Results and discussion
3.1. Electrochemical and AFM characterizations of a Cu
nanofilm
Fig. 1 shows the typical cyclic voltammograms in between
−1.0 and 0 V for a polished Cu bulk electrode (dotted trace), a
Cu nanofilm on Si electrode (dashed trace) and a Cu nanofilm
on Ge electrode (solid trace), respectively, in 0.1 M KClO4 at
50 mV s−1. The surface roughness of the Cu massive electrode
was not strictly controlled without being subjected to intensive
polishing. Nevertheless, this did not prevent us from a qualita-
tive comparison. It is revealed that the as-deposited Cu nanofilm
on Si is much alike to the Cu bulk, while the Cu nanofilm on Ge
suffers poor resemblance to the Cu bulk in terms of their electro-
chemical responses when the positive potential limit is relatively
high. The reason may be ascribed to the dissolution of the less
inactive Ge substrate (and thus possibly partial peeling of the Cu
film) which distorts the overall electrochemical response of Cu
film at least under the above conditions. Although there are some
cases (not used in a wider potential range and/or in strong acid
solutions) in which both Cu-on-Si and Cu-on-Ge electrodes may
share rather comparable electrochemical responses, the above
result suggests the necessity for developing ATR-SEIRA-active
Cu electrodes on Si as a good addition.
Surface roughness factor of the as-deposited Cu nanofilm
was estimated based on the averaged depositing and stripping
charges for UPD of a Pb monolayer on Cu in 0.1 M HClO4 con-
taining 1 mM Pb(ClO4)2·3H2O as shown in Fig. 2, by assuming
a theoretical value of 300 ␮C cm−2 for an ideal Cu(1 1 1) elec-
trode as the reference [47,48]. The cyclic voltammetric feature
of the above Pb-UPD on the as-deposited Cu film electrode is
not identical to that on a polycrystalline Cu bulk electrode, rather
it is somehow in between that on Cu(1 1 1) electrode and that
Fig. 2. Cyclic voltammogram for the UPD of a Pb monolayer on the Cu nanofilm
electrode on Si in deaerated 0.1 M HClO4 containing 1 mM Pb(ClO4)2·3H2O,
the scan rate is 10 mV s−1. The slightly larger deposition charge than striping
one is probably due to the partial Cu–Pb alloying at the initial deposition stage.
on Cu(1 1 0) electrode, suggestive of preferred growth to some
extent of these two crystalline orientations [48]. As comparison,
the corresponding cyclic voltammogram for Pb-UPD on a bulk
polycrystalline Cu electrode is shown in Appendix A. The some-
what larger deposition charge than the stripping one is possibly
due to the partial Cu–Pb alloying at the initial deposition stage,
as also found in previous measurements on bulk Cu electrodes
[47]. The average charge calculated from Fig. 2 is ca. 780 ␮C
cm−2, yielding a roughness factor of ca. 2.6, which was slightly
smaller than estimated for the Cu nanofilm on Ge [29].
The mechanism of the seeding process is believed as follows,
which is associated with the dissolution of Si in the presence of
F− and a reduction of Cu2+, i.e.,
Cu2+ + 2e− → Cu, ϕ0 = 0.34 V
Si + 6F− → SiF62− + 4e−, ϕ0 = −1.20 V
Fig. 3(a) shows the AFM image of the initial Cu seed layer
on Si. It can be found that the Cu seed layer was composed of
loosely packed ellipsoidal particles with an average lateral size
of ca. 80 nm. These nanoparticles are not fully interconnected,
and thus no significant electrical conductivity was detected for
the entire film. Subsequent seeded growth of Cu particles occurs
according to the following simplified reactions, i.e.,
Cu2+ + 2e− → Cu, ϕ0 = 0.34 V
2HCHO + 4OH− → 2HCOO− + H2 + 2H2O + 2e−,
ϕ0 = −1.24 V (at pH 13)
The AFM image of the as-deposited Cu film on Si shown
in Fig. 3(b) reveals an average lateral diameter of particles of
ca.150 nm and the feature height of ca. 50 nm. On the other hand,
the mass equivalent thickness of the film estimated from ICP
measurement was ca. 68 nm by taking the bulk Cu density for
thecalculation(i.e., by assumingCu atomsbeingdenselypacked
as in Cu crystals). Since the island film contains voids, it can rea-
sonably be deduced that more than one layer of nanoparticles are
Fig. 1. Cyclic voltammograms for the chemically deposited Cu films on Ge
(solid trace) and Si (dashed trace), and bulk Cu electrode (dotted trace) in
deaerated neat 0.1 M KClO4. Scan rate was 50 mV s−1
.

H.-F. Wang et al. / Electrochimica Acta 52 (2007) 5950–5957
5953
Fig. 3. AFM images for Cu seed layer (a) and Cu nanofilm (b) grown on Si.
packed and interconnected. In fact, the Cu nanofilms were shiny
and quite homogeneous by eye-inspection and the conductivity
was sufficient for (spectro)-electrochemical measurement.
inhomogeneity of the Cu nanoparticles. The ␯CO band intensity
increased as the potential moved negatively to −1.3 V as shown
in Fig. 6, in good agreement with that obtained by Kalaji and co-
workers [49] and Miyake et al. [29]. This may be explained in
3.2. In situ SEIRAS on Cu-on-Si electrode
Fig. 4 shows the cyclic voltammogram of the Cu electrode in
Ar-saturated (solid trace) and CO-saturated (dotted trace) 0.1 M
KClO4 solution with the initial potential at −0.5 V, which is
less positive than the potential of Cu oxidation. The hydrogen
evolution current at potentials negative of −0.9 V is markedly
suppressed in the CO-saturated solution, suggestive of CO
adsorption on the Cu electrode. The current suppression is not
significant at potentials more positive than −0.9 V, the reason of
which can be understood by in situ ATR-SEIRAS measurement.
Shown in Fig. 5 are the potential-dependent SEIRA spectra
for CO adsorption at the Cu electrode in CO-saturated 0.1 M
KClO4 solution by taking the single-beam spectrum at −0.6 V
as the reference. Spectral collection was performed stepwise
from the initial potential at −0.6 V with a multi-step FTIR mode
toward −1.3 V. The band at 2036–2080 cm−1 can be assigned
to the linearly adsorbed CO on Cu. The asymmetric band shape
with a tail at the low frequency side may be ascribed to the
Fig. 4. Cyclic voltammograms for the chemically deposited Cu nanofilm in Ar-
saturated (solid trace) and CO-saturated (dashed trace) 0.1 M KClO4. Scan rate
was 50 mV s−1
.

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H.-F. Wang et al. / Electrochimica Acta 52 (2007) 5950–5957
was present only at −0.9 V and higher potentials where CO
adsorption was negligible, and upon significant CO adsorption
at lower potentials, that band disappeared. Hence, the assign-
ment of CuOHads for this band is suspicious. The second, which
is more likely, is the interfacial free H2O with its hydrogen-
bonding broken as a result of CO adsorption. The detailed
structure of these free H2O molecules at metal/electrolyte inter-
faces is still open for discussion [53], but free H2O co-existed
with CO on Cu electrode may be inferred by the concurrent
potential dependence of the ␯OH and ␯CO bands, as can be seen in
Fig. 6. Since the ␦HOH band of the free H2O was weak and hardly
separated from that of other H2O, it was submerged in the broad
band ca. 1640 cm−1 [31,54]. Similar ␯ bands were observed
OH
on previous ATR-SEIRAS on CO adsorbed on Pt [26,31,54–55],
Pd [30,31], Rh [31], Ru [31] and Ni [33] electrodes, although
the peak positions and intensities varied with metal and CO cov-
erage. Notably, such free H2O bands have not been detected on
a Au electrode at potentials with maximum CO adsorption in
an acid solution, possibly because of lower CO coverage, as
revealed by in situ STM measurement [56].
Fig. 5. Potential-dependent SEIRA spectra for CO adsorbed on the Cu electrode
in 0.1 M KClO4. Sample spectra were taken at potentials indicated, and the
reference spectrum at −0.6 V.
High SEIRA-activity of as-deposited Cu nanofilms on Si
enables to probe varieties of adsorbates at the electrochemical
interface. As an initial step along this line, the configuration
of adsorbed Py at a Cu electrode was examined. Compared to
SERS, SEIRAS has a straightforward surface selection rule and
does not require the ORC pretreatment in a chloride electrolyte
for activation. Several possible configurations have been sug-
gested for (sub) monolayer Py adsorption on different metal
surfaces, including flat-lying, N-end-on, edge-tilted, plane-tilted
and ␣-pyridyl ones [14–15,18,33,44,57–63]. Haq and King
determined a N-end-on configuration for Py on Cu(1 1 0) in a
UHV system based on external IRAS measurement [44], while
based on the SERS selection rule, Zuo and Jagodzinski [18]
proposed a predominant formation of ␣-pyridyl species on Cu
nanoparticles formed on a Al foil by a metathesis reaction. The
end-on configuration was also suggested previously on an ORC-
roughened Cu electrode by in situ SERS studies [14,15]. In
the following, we take the advantage of our SEIRA-active Cu
nanofilms to clarify these two extreme configurations.
−
considering that the desorption of anions (OH and/or ClO4
)
facilitates the adsorption of CO at potentials more negative
than the pzc (ca. −0.8 V) [50,51]. It is noted that the inten-
sity of the ␯CO band at −1.3 V is 0.044 Abs, which far exceeded
those reported for IRAS measurement on bulk Cu electrodes
(0.001–0.002 Abs) [52], and was even somewhat larger than
obtained for ATR-SEIRAS on a Cu-on-Ge electrode (0.025)
[29]. After the appropriate calibration of the effects of surface
roughness factor, surface coverage, incident angles and polar-
ization states of IR radiation [33,40], the enhancement factor
was estimated to be larger than 28.
Interestingly, a sharp band around 3668 cm−1 was observed
which was imposed on a broad band centered at ca. 3500 cm−1
.
The latter together with the ␦HOH band at ca.1640 cm−1 could
be assigned to the ␯ of interfacial H2O strongly hydrogen-
OH
bonded. Two possible surface OH-containing species may be
ascribed to this band. The first is CuOHads as suggested by Kalaji
and co-workers [49]. However, they reported that this species
Fig. 7 shows cyclic voltammograms obtained from an as-
deposited Cu nanofilm electrode in 0.1 M KClO4 solution
without (solid trace) and with (dashed trace) 10 mM Py. The oxi-
dation of Cu initiated at potentials around −0.3 V in the forward
scan, and the hydrogen evolution initiated at potentials around
−1.0 V in the negative scan. Obviously, both are suppressed by
the presence of 10 mM Py.
Unlike on Au and Ag electrodes [40,62], Py was found to be
adsorbed on the Cu electrode over a very wide potential region;
it can survive at very negative potentials of strong H2 evolution.
The strong adsorption of Py on Cu prevented from finding a suit-
able reference potential in the Py-containing solution and thus
the corresponding spectra measured in 0.1 M KClO4 without Py
at the same potentials as for the sample spectra were taken as
the reference ones, respectively.
Fig. 8 shows a series of in situ ATR-SEIRA spectra for
a Cu nanofilm electrode in 0.1 M KClO4 solution containing
10 mM Py. Six bands characteristic of intact pyridine (rather
Fig. 6. The potential-dependent integrated intensities of IR bands of CO and
coadsorbed H2O, re-plotted based on Fig. 5.

H.-F. Wang et al. / Electrochimica Acta 52 (2007) 5950–5957
5955
Fig. 7. Cyclic voltammograms of a Cu nanofilm electrode in 0.1 M KClO4
solution in the absence (solid) and presence (dashed) of 10 mM Py recorded at
50 mV s−1
.
Fig. 9. Potential-dependent SEIRA spectra of pyridine (Py) adsorbed on the Cu
electrode chemically deposited on Ge in 0.1 M KClO4 containing 10 mM Py.
Reference spectra were measured in neat 0.1 M KClO4 at the same potentials as
their corresponding sample spectra.
than ␣-pyridyl) adsorbed on a Cu electrode were detected at
1600 (␯8a, C C and C N stretch, vs), 1482 (␯19a, C C and C N
stretch, s), 1444 (␯19b, C C and C N stretch, vw), 1068 (␯
,
18a
Cu-on-Ge electrode did not detect such a band either as seen
in Fig. 9 {note: the weak band at 1218 cm−1 is also a A1 mode
(␯9a)[64,65]}, suggesting that Py ring plane was nearly vertical
to the local surface and no Py multilayers were formed [44].
The upright N end-on adsorption configuration agrees with the
results observed by IRAS in UHV [44] and in situ electrochemi-
cal SERS [14,15]. We did not detect any spectral features leading
to the possible conclusion of ␣-pyridyl species formation on Cu
such as the appearance of intense bands with originally B1 sym-
metry. In particular, a strong band at ca. 1568 cm−1, a marker
of the predominant presence of ␣-pyridyl observed in previous
studies of Py on Pt(1 1 1) [44] and W(1 1 0) [68] in UHV, and
Py on a Pt electrode [63], did not show up at all. It is also noted
that the band at 1482 cm−1 gains intensity more rapidly than
other A1 modes as the potential is made more negative, similar
to that found in IRAS for Py adsorption on a Cu(1 1 0) in UHV
with increasing coverage [44]. The increase in Py coverage with
decreasing potential can be understood by the weakened compe-
tition from co-adsorbed anions. The reason why vibrations with
the same A1 symmetry were affected to different extents with
increasing coverage deserves further consideration in the future.
The broad band at ca.1225 cm−1 in Fig. 8 is commonly
observed for a metal-on-Si electrode especially in an un-acidic
solution [21,62–63], but it is absent for a metal-on-Ge electrode
(see Fig. 9). This band is much weaker in a strong acid solu-
tion like 0.1 M HClO4 or H2SO4 [31,36,69]. It’s tempting to
assign it to the O Si O stretching of the silicon oxides formed
(and/or grown) on the exposed Si sites [70], which is thermo-
dynamically facilitated with increasing local pH upon hydrogen
evolution. Another possible candidate species for this band is
the silicates formed at certain potentials [36].
C H in plane bend, m), 1042 (␯12, asymmetric ring breathing,
m) and 1013 cm−1 (␯1, symmetric ring breathing, m) [64,65].
All the bands observed can be classified into A1 symmetry except
the very weak ␯ band assignable to B1 symmetry mode (A1
19b
and B1 modes are defined here as in-plane vibrations yielding
dipole changes along and perpendicular to the C2-axis of Py,
respectively). It should be pointed out that the weakest ␯
19b
band observed in ATR-SEIRAS is the most intense IR band
of all for liquid pyridine. According to the surface selection rule
of SEIRAS, the selective enhancement of A1 modes of adsorbed
Py suggests the “end-on” adsorption geometry on Cu electrode
via lone pair of N atom without significant edge-tilting of its C2-
axis. The blue-shift of the totally symmetric ring vibration band
(␯1) further supports the end-on configuration [66,67]. Effort
to detect a band for CH out of plane bend (B2 mode) at ca.
750–850 cm−1 was hampered by the strong IR absorption of
the Si prism itself. Nevertheless, the ATR-SEIRAS of Py on a
4. Conclusions
Fig. 8. Potential-dependent SEIRA spectra of Py adsorbed on the Cu electrode
in 0.1 M KClO4 containing 10 mM Py. The reference spectra were the spectra
measured in neat 0.1 M KClO4 at the corresponding potentials as indicated.
Based on a seeded-growth tactics, chemical deposition of Cu
nanofilm electrodes on Si has been achieved to replace tradi-

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Fig. A.1. Cyclic voltammogram for the UPD of a Pb monolayer on the bulk Cu
electrode on Si in deaerated 0.1 M HClO4 containing 1 mM Pb(ClO4)2·3H2O,
the scan rate is 10 mV s−1
.
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With CO as the probe molecule, the unipolar surface signal is
ca. 30-fold stronger than those reported for a bulk Cu electrode
in IRAS, facilitating the detection of probable interfacial free
H2O species. In situ ATR-SEIRAS of Py adsorption on a Cu
electrode did not support the formation of ␣-pyridyl species. An
upright N-end-on configuration of Py on Cu was confirmed over
the potential range under investigation.
Acknowledgements
The NSFC (No. 20473025), the SRFDP (No. 20040246008),
the NCET (No. 04-0349), the SNPC (No. 0452nm064) and Inno-
vative Fund of Fudan University (CQH1615018), China (WBC),
and the MEXT (No. 18350038 and Priority Areas 417), Japan
(MO) are gratefully acknowledged for financial supports. We
also thank the State Key Laboratory for Physical Chemistry of
SolidSurfaces, XiamenUniversityforprovidingpartialfinancial
support through the Coordinator, Prof. Z. Q. Tian (No. 200303).
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Appendix A
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