Specific adsorption of carbonate ions
Chapter 3
Specific adsorption of carbonate ions on single
crystal gold electrode surfaces
3.1 Introduction
Describing a more detailed picture of the electrical double layer is one of the
important issues in electrochemistry. Since electron transfer processes should be
drastically affected by adsorbed species, elucidation of adsorption states of ions and
molecules has been a subject of considerable interest [1,2]. Specifically adsorbed
anions alter the charge distribution at the interface and the electronic structure of the
substrate, and thus influence the electrochemical processes at the electrode surfaces.
One of the instances is under potential deposition processes such as copper deposition
on gold, where co-adsorbed anions affect both reaction kinetics and growth
mechanisms [3,4]. Also, some anions serve as mild corrosion inhibitors in metal/alloy
corrosion processes. Oxidation rates of methanol on Pt(111) vary by an order of
magnitude between perchloric and sulfuric acid solutions [5]. Therefore, a better
understanding of interactions between anions and metal electrodes and their
interfacial structure is imperative in making further inroads in interfacial
electrochemistry as well as in the field of electrocatalysis and corrosion processes.
Remarkable advances in the surface electrochemistry in recent years owe to
the development of both in situ and ex situ techniques to investigate the electrode
surfaces. These include low-energy electron diffraction (LEED), Auger electron
spectroscopy (AES), electron energy loss spectroscopy (EELS), X-ray photoelectron
spectroscopy (XPS), infrared reflection absorption spectroscopy (IRAS), scanning
probe microscopy (SPM), and so on. In situ IRAS provides information not only on
the electrode interface but also on the electrochemical interphase, i.e. the structure of
the electrical double layer. Recently, Kitamura et al. [6-8] reported in situ IR spectra
58
Specific adsorption of carbonate ions
of the water molecules present in the vicinity of a polycrystalline gold electrode in
alkali halide and alkali perchlorate electrolyte solutions. Though the IR spectra
showed reproducible and meaningful features, some unexpected bands were detected
simultaneously at ca. 1400 cm –1 . They can be ascribed to carbonate and/or
bicarbonate ions, which are inevitably present in neutral and alkaline aqueous
solutions because of the equilibrium with carbon dioxide that comes from the
atmosphere [9,10]. Adsorption of these ions contaminates the electrode | electrolyte
interface [6-8] and probably affects electron transfer processes as mentioned above.
Therefore, it has been desired to clarify the interfacial behavior of carbonate and/or
bicarbonate ions.
Carbonate ion is a planer molecule that belongs to the D 3h symmetry group
[11-13]. The carbonato groups have been known to form unidentate and bidentate
complexes with metals. Based on the molecular symmetry of carbonate ions, probable
orientations of the carbonates adsorbed on a substrate are unidentate, bidentate and
flat as shown in Fig. 1. Iwasita et al. measured the IR spectra for adsorbed carbonate
and bicarbonate ions on the Pt (111) electrode surface [2,14-16]. They concluded that
the adsorbed carbonate ions re-coordinate from one-fold to two-fold state and the
co-adsorbed bicarbonate ions are retained at the interface by the formation of H-bonds
with the adsorbed water. However, the adsorption behavior of carbonate and/or
bicarbonate ions on a gold electrode surface has not been investigated by means of
IRAS.
Since electrochemical activity of electrodes is largely affected by the surface
atomic arrangement of substrates, it is essential to investigate the process at
atomically well-ordered electrode surfaces [1]. The adsorption behavior of polyatomic
oxyanions, such as (bi)sulfate [14,17-25], nitrate [26], phosphate [27,28], carbonate
ions [2,14-16] and perchlorate ions [29,30], at various metal electrodes has been
widely investigated by means of in situ IRAS. Consideration in view of the
coordinational symmetry between an adsorbate and a substrate is important to
understand the adsorption conformation of these ions. In this respect, it is very
interesting to investigate the adsorption behavior of planer molecules such as
carbonate ion at low-index single crystal gold electrodes and to compare with the
results of other steric anions.
59
Specific adsorption of carbonate ions
This chapter describes the specific adsorption of carbonate ions at gold
electrodes of a single crystal, e.g. the (111), (100) and (110) faces. In situ IRAS
measurements at Au(111) in aqueous Na 2 CO 3 and NaHCO 3 solutions showed that the
adsorbed species was carbonate ion irrespective of the solution composition. The
coordinational state and molecular orientation of the adsorbed carbonates are
presented.
The
adsorption
behavior
of
carbonate
ions
depending
on
the
crystallographic orientation is discussed on the basis of IR spectra obtained at
Au(111), Au(100) and Au(110) electrodes in Na 2 CO 3 solutions. Spectra obtained in a
positive potential region where a surface oxide layer is formed are also presented. The
adsorption state of carbonate ions on the single crystal gold electrode surfaces is
compared with those of sulfate and phosphate ions.
3.2 Experimental
The working electrodes were Au (111), Au(100) and Au(110) single crystal
disks (MaTeck) with a diameter of ca. 8 mm. The electrode surfaces were
mechanically polished to a mirror finish and then annealed at 900 °C in a furnace for
several hours. Prior to every measurement, the crystal disks were annealed in a
hydrogen flame and quenched in ultrapure water deaerated by Ar gas. They were
transferred to electrochemical or spectro-electrochemical cell under the protection
with a droplet of water. The surface identification of the single crystal gold electrodes
was carried out by cyclic voltammetry (CV) in a perchloric acid solution. The CVs
obtained were in good agreement with those reported earlier [31]. A platinum wire
was used as the auxiliary electrode. Ag | AgCl | KCl(sat) was used as the reference
electrode and all the potentials presented here were quoted against this electrode. CVs
were recorded on a computer controlled electrochemical system (CS-1090, Cypress
System).
Aqueous solutions of 0.1 M Na 2 CO 3 and 0.1 M NaHCO 3 were prepared using
Na 2 CO 3 (Fluka, BioChemika), NaHCO 3 (MERCK, pro analysi) and water purified by
a Milli-Q system (Millipore Japan Ltd). NaF (Fluka, BioChemika) was used as a base
60
Specific adsorption of carbonate ions
electrolyte. Deuterium oxide, D 2 O (99.8 %, MERCK, Uvasol ® ) was used as received
to prepare 0.1 M Na 2 CO 3 / D 2 O and 0.1 M NaHCO 3 / D 2 O solutions. All the solutions
were thoroughly deareated by bubbling Ar gas (99.999 %).
IR transmission spectra were obtained on an FTS-135 Fourier transform
infrared spectrometer (Bio-Rad Laboratories) with a liquid cell comprised of CaF 2
windows. In total, 64 scans were co-added and averaged.
In situ IRAS was carried out in the subtractively normalized interfacial
Fourier transform infrared (SNIFTIR) mode [32]. All the spectra are shown in
reflectance unit defined as ∆R / R = (S – S 0 ) / S 0 , where S and S 0 represent single
beam spectra at the reference and sample potentials, respectively. Thus negative- and
positive-going bands indicate an increase and a decrease in the absorption intensity at
the sample potential, respectively. The spectra were obtained on the FTS-135
spectrometer equipped with a wide-band MCT detector cooled with liquid nitrogen. A
trapezoidal CaF 2 prism beveled at 60° or a flat CaF 2 prism (3 mm in thickness) was
used as a window, the latter of which enables to detect the absorption bands around
1000 cm –1 . The electrode surface was contacted against the window to form a thin
solution layer. Polarization of the infrared incident light was controlled with a wire
grid polarizer. Other details concerning the spectro-electrochemical cell have been
described elsewhere [8]. During measurements, electrode potential was controlled by
a potentiostat (Toho Technical Research, model PS-07) equipped with a homemade
potential controller that switched the applied potential between the sample and
reference potentials synchronized with spectral acquisition. In total, 640 or 1280
scans were co-added and averaged at each potential. The spectral resolution was 8
cm –1 . All the measurements were carried out at 20 ± 1 °C.
3.3 Results and discussion
3.3.1 Voltammetric studies at Au(111)
Fig. 2 shows CVs in the double layer region obtained at an Au(111) electrode
in (a) 0.1 M Na 2 CO 3 and (b) 0.1 M NaHCO 3 solutions. Sharp anodic peaks appeared
61
Specific adsorption of carbonate ions
at +203 mV and +259 mV in Na 2 CO 3 and NaHCO 3 solutions, respectively. In the
reverse sweep, only a broad wave was observed in each solution. Fig. 3 shows I-E
curves in the positive-going sweep obtained in NaHCO 3 solutions of different
concentrations. A CV obtained in a 0.1 M NaF solution, in which specific adsorption
of the ions on the electrode surface is negligible, serves as a background
voltammogram (Fig. 3d). With an increase in the NaHCO3 concentration, the potential
at which the curve in the double layer region deviated from the background shifted to
the negative from (c) ca. –100 mV to (a) ca. –300 mV. Based on the IRAS results
being shown later, one can concluded that carbonate ions started to adsorb on the
electrode surface at these potentials. On the other hand, it is clearly seen that the
sharp spikes shifted to the negative direction with the increase in the NaHCO 3
concentration. Similar sharp anodic peaks, which are ascribed to the structural
transition between the reconstructed and non-reconstructed surfaces accompanied by
the specific adsorption of anion, have been observed at Au(111) in HClO 4 , HNO 3 ,
H 2 SO 4 and H 3 PO 4 solutions [33,34]. Such a sharp peak was not observed on CVs of
polycrystalline gold in a 0.1 M Na 2 CO 3 solution [35], which supports the idea that the
present peaks are also ascribed to the surface reconstruction phenomena accompanied
by the adsorption of carbonate ions. Another explanation for the appearance of the
peaks, however, is given by the specific adsorption of OH – accompanied by the
surface reconstruction because the sample solution was weak alkaline. This point will
be discussed later in more detail in 3.3.5.
3.3.2 IR spectroscopic measurements at Au(111)
IR transmission spectra of Na 2 CO 3 and NaHCO 3 dissolved in H 2 O or D 2 O are
shown in Fig. 4, their band center positions are tabulated in Table 1 with their
assignment. These values are coincident with the previously reported ones [13,14,36].
Absorption bands at ca. 1400 cm–1 observed in both Na 2 CO 3 / H 2 O and Na 2 CO 3 / D 2 O
solutions (Fig. 4a and b) are due to the antisymmetric C-O stretching mode of
carbonate ions. Though the absorption band assignable to the antisymmetric C-O
stretching mode of bicarbonate ions (ca. 1650 cm–1 ) [13,14] is not discriminated due
to the superposition of the strong absorption by water molecules in the solution (Fig.
62
Specific adsorption of carbonate ions
4c), it can be clearly seen at 1629 cm –1 in the NaHCO 3 / D 2 O system (Fig. 4d) due to
the deuteration effect.
Fig. 5 shows a series of SNIFTIR spectra obtained at an Au(111) electrode in
a 0.1 M Na 2 CO 3 solution. Reference potential was –600 mV where adsorption of
anions scarcely occurred, and sample potentials are indicated in the figure. A
positive-going band at 1624 cm–1 can be ascribed to the H-O-H bending mode of
water molecules, which is confirmed by a frequency shift of this band down to 1195
cm –1 in D 2 O solutions as will be discussed later (Fig. 8(A)). Another positive-going
band at 1398 cm –1 is in accord with the frequency of carbonate ions in solution
observed in the IR transmission spectra (Fig. 4 and Table 1). A negative-going band
was observed around 1460 cm –1 , the center position of which shifted to the higher
frequency depending on the applied potential (1425-1499 cm –1 ), indicating that this
absorption band is ascribable to an adsorbed species on the electrode surface. In a
positive potential range from +200 to +400 mV, two additional negative-going bands
appeared at 1677 and 1301 cm–1 , which are due to bicarbonate ions. As indicated by
Bae et al. [37], local pH in the vicinity of a electrode surface changes depending on
the electrode potential, which gives rise to a shift in the equilibrium between
carbonate and bicarbonate ions.
Based on the surface selection rule of IRAS at highly reflective metal surfaces
[1,2], s-polarized light interacts only with species dissolved in the thin solution layer.
On the other hand, spectra obtained with p-polarized light contain information both on
dissolved and adsorbed species. Therefore, when the s-polarized spectrum (dotted
curves in Fig. 5 and 6) is properly subtracted from the corresponding p-polarized
spectrum (solid curves), only the absorption components due to the adsorbed species
will remain on the spectrum. The spectral subtraction was performed multiplying the
s-polarized spectrum by a factor so as to cancel out the absorption band due to the
dissolved carbonate ions at 1398 cm –1 . Fig. 7a shows the subtracted spectrum at 0 mV,
which retains only positive- and negative-going bands. These bands can successfully
be ascribed to the adsorbed species on the electrode surface and all the other bands in
Fig. 4 are due to the dissolved species. As the positive-going band at 1624 cm –1
represents a decrease in intensity at the sample potential, the water molecules depart
from the surface with an increase in the amount of the adsorbed species.
63
Specific adsorption of carbonate ions
On the other hand, the spectra obtained in a 0.1 M NaHCO 3 solution at an
Au(111) electrode were more complicated as shown in Fig. 6. As in the case of
Na 2 CO 3 , two positive-going bands at 1624 and 1401 cm –1 and a negative-going band
around 1480 cm –1 are assignable to the vibrational modes of the water molecules,
carbonate ions in solution and adsorbed species, respectively. Fig. 7b represents a
subtracted spectrum at +200 mV. Similarity of the spectral feature to that in Fig. 7a
indicates that the adsorbed species and their potential dependence are almost the same
in both electrolyte solutions irrespective of the solution compositions. In Fig. 6, three
negative-going bands appeared at 1673, 1350 and 1305 cm –1 in the entire potential
range investigated, and their spectral features were consistent with the IR
transmission spectrum of bicarbonate ions depicted in Fig. 4c. These bands
completely disappeared in the subtracted spectra (Fig. 7b), indicating that they are
due to the dissolved bicarbonate ions.
To identify the species adsorbed on the Au(111) electrode surface, i.e.
carbonate and/or bicarbonate ions, SNIFTIR spectra were measured in 0.1 M Na 2 CO 3
and 0.1 M NaHCO 3 dissolved in deuterium oxide and the results are shown in Fig. 8
(A) and (B), respectively. When the adsorbed species possesses an O-H segment
within its framework, frequencies of the vibrational modes concerning the hydrogen
segments are prospective to shift in the D 2 O solution due to the fast exchange reaction
of H for D (the isotope effect). At –200 mV, a positive-going band that was originally
observed at 1624 cm–1 in the light water (Fig. 5) completely disappeared, but it shifted
instead to appear at 1195 cm–1 in the D 2 O solution (Fig. 8(A)). The large shift toward
lower frequency due to the isotope effect can reasonably be explained by considering
that the band is ascribable to the bending mode of water molecules.
In a 0.1 M NaHCO 3 / D 2 O solution (Fig. 8(B)), an absorption band due to the
D-O-D bending mode of the heavy water also appeared to shift to a lower frequency
(1193 cm –1 ). A positive-going band (1407 cm –1 ) appeared at almost the same position
as that observed in the light water (1401 cm–1 ), which confirms that these bands are
assigned to the C-O antisymmetric stretching mode of the dissolved carbonate ions.
The slight difference of 6 cm–1 probably comes from the interaction with the hydrated
water molecules around carbonate ions. The spectral features of two negative-going
bands appeared at 1630 and 1362 cm –1 were in agreement with the IR transmission
64
Specific adsorption of carbonate ions
spectra obtained in the NaHCO 3 / D 2 O solution (Fig. 4d). These bands also
disappeared in the subtracted spectra (not shown). A negative-going intense band,
which was observed only in the p-polarized spectra and was ascribed to the adsorbed
species, appeared almost in the same frequency region of 1466-1512 cm–1 as well as
the case of the light water (1451-1511 cm –1 ). The band center position in the light and
heavy water solutions of 0.1 M NaHCO 3 is plotted as a function of the applied
potential in Fig. 9. It is clear that the center positions observed in the heavy water
solution agree excellently with those in the light water solution at each potential.
Almost the same tendency described above was observed in the case of 0.1 M Na 2 CO 3 .
Therefore, one can conclude that the adsorbed species on the Au (111) electrode
surface are carbonate ions irrespective of the solution compositions. This is surprising
because bicarbonate ions are the primary species in a 0.1 M NaHCO 3 solution
(pH~8.3) according to the equilibrium in the solution [9,10]. Similar results were also
reported for the adsorption of (bi)sulfate on gold single crystal electrodes in sulfuric
acid solutions [19,22], and they may partly arise from the extent of electronic
interactions between the gold surface and each molecule. The absorption frequencies
observed in a series of the SNIFTIR spectra (Fig. 5, 6 and 8) are summarized in Table
1 with their assignments based on the above discussion.
The center position and absorption intensity of the band due to the adsorbed
carbonate ions are plotted against the sample potential in Fig. 10(A) and (B). The
band shifted monotonically to the higher frequency with mean slopes of 85 and 77
cm –1 V –1 in 0.1 M Na 2 CO 3 (open circle) and 0.1 M NaHCO 3 (filled circle) solutions,
respectively. It is apparent that despite the solution composition, the absorption bands
appear at the particular frequencies depending on the electrode potential. The band
intensity also monotonically increases with the potential. The band intensity in a 0.1
M Na 2 CO 3 solution is larger than that observed in a 0.1 M NaHCO 3 solution in the
potential range investigated, indicating that the carbonate ions adsorb favorably on
the Au(111) electrode surface in the former electrolyte solution. It is apparent that the
band center position and the absorption intensity do not change significantly at the
peak potential observed in the CVs (Fig. 2).
65
Specific adsorption of carbonate ions
3.3.3 Coordination and orientation of adsorbed carbonates
The molecular coordination and orientation of the adsorbed carbonate species
should be considered. Fujita et al. [11] performed a normal mode analysis of Co(III)
carbonato complexes and compared the result with actual data of a number of
unidentate and bidentate Co(III) complexes. These carbonato complexes showed
inherent infrared absorption depending on their coordinational states. They stated that
lowering of symmetry of the carbonato group from D 3h (free ion) to C 2v (bidentate), or
to C s or C 2v (unidentate), splits the degenerate vibrations and activates a
infrared-inactive vibration of the free ion. Thereafter, Goldsmith and Ross [12]
carried out a similar normal coordinate analysis of the carbonato complex groups
(O*CO 2 and OCO 2 *, where O* means the bonded oxygen). Discussion of the
vibrational modes of the surface carbonate species in this chapter is mainly based on
their results. More recently, Seiferth et al. [38] summarized characteristic CO 3
vibrational frequencies of the surface carbonates in detail and they were tabulated in
Table 2.
Only one negative-going band responsible for the adsorbed species appeared
in the in-plane vibrational region, which distinctly indicates that the adsorbed
carbonate ions are not flat with respect to the electrode surface. Comparing its band
frequency of 1424-1493 cm –1 with those of the surface carbonates (Table 2), one can
conclude that it is assignable to the ν 1 mode of the surface carbonate in the unidentate
coordination (it appears in a frequency region of 1470-1530 cm –1 , see Table 2) rather
than the bidentate one (1530-1620 cm–1 ) [38]. The ν 1 mode corresponds to the
symmetric C-O 2 stretching of the two non-coordisnated oxygen, and thus its net
dipole moment change is directed along the C 2 symmetry axis. Based on the surface
selection rule of IRAS [1,2], molecular orientation with the symmetry axis nearly
perpendicular to the surface is necessary for the ν 1 band of the unidentate carbonate to
be observed. Provided that the perpendicular molecular orientation is retained, the ν 4
mode of the antisymmetric C-O 2 stretching cannot be observed because its net dipole
moment component should be parallel to the electrode surface. As expected, any
significant bands caused by the surface carbonates were not observed in Fig. 7 in the
ν 4 band region of 1300-1370 cm –1 .
On the other hand, since the ν 2 mode of the C-O* stretching possesses its net
66
Specific adsorption of carbonate ions
dipole moment component in a direction of the symmetry axis, a corresponding
absorption band is expected to appear in 1040-1080 cm–1 in the unidentate
coordination. Spectral noise originating from poor infrared transparency through the
trapezoidal CaF 2 window, however, obscures the ν 2 absorption band. Since a longer
optical path in a CaF 2 prism severely reduces the infrared energy in a frequency
region less than 1050 cm –1 , a thin flat CaF 2 window was utilized instead for this
purpose. Fig. 11 (A) shows a series of SNIFTIR spectra thus obtained at an Au (111)
electrode in a 0.1 M Na 2 CO 3 solution. Although some spectral noise was still
superimposed, two additional negative-going bands were clearly observed at 1058 and
1013 cm –1 (Fig. 11(B)). The latter band is assignable to the C-OH stretching mode of
bicarbonate ions in solution since the band shifted to 1038 cm–1 in a 0.1 M Na 2 CO 3 /
D 2 O solution (Fig. 11 (C)). The band frequencies well coincide with those observed in
the IR transmission spectra of HCO 3 – and DCO 3 – (1011 and 1038 cm –1 , respectively;
see Fig. 4 and Table 1). The 1058 cm –1 band, which indicated no frequency change in
the isotopic experiments, is assignable to the ν 2 absorption band of the C-O*
stretching mode in the unidentate coordination as expected (Table 2). If the adsorbed
carbonate ions reside in the bidentate coordination, the ν 2 band of the symmetric
C-O 2 * stretching is expected to appear in 1020-1030 cm –1 . Therefore, the observation
of the ν 2 band at 1058 cm –1 leads to the conclusion that carbonate ions singly
coordinate with the Au(111) surface in the perpendicular orientation (see Fig. 1).
3.3.4 Adsorption of carbonate ions on Au(100) and Au(110)
View of the symmetrical matching between molecules and substrate atomic
arrangements is important in a discussion of molecular adsorption states [1,2,22].
Thus similar experiments as mentioned above were performed using Au(100) and
Au(110) electrodes. Fig. 12 shows CVs in the double layer region obtained at (a) Au
(111), (b) Au(100) and (c) Au(110) electrodes in a 0.1 M Na 2 CO 3 solution. The dotted
lines represent background voltammograms in the absence of specifically adsorbed
anions, which were obtained in a 0.1 M NaF solution. The CV features were largely
influenced by the presence of carbonate ions in solution as well as the case of 0.1 M
NaHCO 3 (Fig. 3). The distinct deviation from the background voltammograms started
67
Specific adsorption of carbonate ions
to occur in a sufficiently negative potential region (–300-–400 mV) at every electrode
investigated. A sharp anodic peak appeared at +202 and +132 mV at Au(111) and
Au(100) electrodes, respectively. On the other hand, a broad wave was observed in the
reverse sweep to the negative direction. Similar sharp anodic peaks have been
reported at Au(111) and Au(100) electrodes in HClO 4 , HNO 3 , H 2 SO 4 and H 3 PO 4
solutions [33,34]. Such current peaks have been considered to originate from changes
in both the pzc of substrates and the amount of the specifically adsorbed anions,
which are concurrent with the lifting of substrates from the reconstructed to the
non-reconstructed structure [33,34]. The structural transitions from ( 3 × 22 ) to (1×1)
and from (5×20) to (1×1) occur at the peak potentials on the Au(111) and Au(100)
electrode surfaces, respectively. Such a sharp peak was not observed on CVs obtained
at a polycrystalline gold electrode in a 0.1 M Na 2 CO 3 solution [35], which supports
the idea that the peaks are associated with the surface atomic rearrangement. The
voltammetric feature observed at an Au(110) electrode was blunt, which can be
ascribed to a too small difference in the pzc values between the reconstructed and
non-reconstructed Au(110) surfaces to bring about a sudden current increase [33,34].
Figs. 13 and 14 show a series of SNIFTIR spectra obtained at Au(100) and
Au(110)
electrodes,
respectively,
measured
with
p-polarized
incident
light.
S-polarized spectra were also measured in order to distinguish absorption bands
caused by the adsorbed species from those by species dissolved in solution. Since the
spectral features were well consistent with those in Fig. 5, only the p-polarized
spectra are presented for clarity. The spectral features at both electrodes were almost
the same as those at Au(111) shown in Fig. 5, where an intense negative-going band of
the adsorbed carbonate ions was observed. Additional experiments using the flat CaF 2
window also provided the similar results as well as Fig. 11 with two negative-going
bands at 1058 and 1013 cm –1 (not shown). The ν 1 band center positions observed at
Au(111), Au(100) and Au(110) electrodes were plotted as a function of the applied
potential in Fig. 15. These bands appeared at almost the same position and shifted
monotonically to the higher frequency irrespective of the surface atomic arrangements.
Mean slopes of the shifts were 90, 92 and 103 cm –1 V –1 in a 0.1 M Na 2 CO 3 solution
for Au(111), Au(100) and Au(110), respectively.
The similarity in the spectral features observed at the three faces suggests that
68
Specific adsorption of carbonate ions
the adsorption state of carbonate ions is identical irrespective of the substrate
symmetry; the unidentate coordination in the perpendicular orientation is retained on
the Au(111), Au(100) and Au(110) electrode surfaces. Probable adsorption sites are
three- and four-fold hollow sites, bridge site and on-top site at the low-index single
crystal surfaces. The first two sites, however, might be excluded because both of the
ν 1 and ν 2 bands did not shift depending on the surface atomic arrangements. Suppose
that the carbonate ions adsorb at such hollow sites, the absorption band positions are
expected to appear in inherent frequencies corresponding to the three-fold site at
Au(111) and the four-fold site at Au(100). The consistent band frequencies suggest
that the adsorption site is the same among the three low-index faces. Therefore, the
bridge and/or on-top sites would be more probable candidates than others for the
adsorption site of carbonate ions.
3.3.5 Co-adsorption of carbonates with surface oxides
Fig. 16 shows a series of SNIFTIR spectra obtained at an Au (111) electrode
in a 0.1 M Na 2 CO 3 solution at the sample potentials more positive than +500 mV,
where surface oxidation starts to take place. Similar spectral patterns were also
obtained at Au(100) and Au(110) electrodes (not shown). As the sample potential
became positive, the absorption bands caused by bicarbonate ions dissolved in
solution distinctly appeared at 1304 and 1655 cm–1 , which are assignable to the
symmetric and antisymmetric CO 2 stretching modes, respectively [13]. The C-OH
stretching mode could be observed at 1013 cm –1 in the measurements using the flat
CaF 2 window (see the spectra obtained at +500 and +600 mV in Fig. 11). The local pH
in the vicinity of a gold electrode surface changes depending on the electrode
potential because of the evolution of hydrogen ions accompanied with the surface
oxide formation [39,40]. As the surface oxidation proceeded, the initial pH of the
Na 2 CO 3 solution (~11.5) became lower and the amount of bicarbonate ions in solution
increased dramatically. A new band at 2343 cm–1 caused by CO 2 dissolved in the
solution appeared at potentials more positive than +800 mV, which indicates that the
solution pH in the vicinity of the electrode surface would be less than 8 based on the
equilibrium among H 2 CO 3 (~CO 2 ·aq), HCO 3 – and CO 3 2– [9,10].
69
Specific adsorption of carbonate ions
Fig. 17 plots the ν 1 band intensity of the adsorbed carbonate ions as a function
of the applied potential. It is clear that carbonate ions have start to adsorb at ca. –300
mV on every gold electrode surface investigated. The ν 1 band intensity monotonically
increased with the applied potential up to +400 mV, and then decreased with the
progress of the surface oxidation. An intensity change could originate from changes in
surface concentration and/or in molecular orientation. The orientational change of the
adsorbed carbonate ions should bring about an appearance of a new band in
1300-1370 cm –1 for the tilted unidentate state or in 1530-1620 cm –1 for the bidentate
state (Table 2). The absence of these bands indicates that the molecular orientation of
the adsorbed carbonate ions unchanged even in the co-presence of the surface oxide
layer. Some of the adsorbed carbonate ions were replaced by the surface oxides,
which leads to a decrease in the surface concentration of carbonate ions, i.e. the band
intensity.
The ν 1 band position shifted to the higher frequency with neither a sudden
change in peak position nor an appearance of a new band at a certain potential. Plots
of the ν 1 band center position up to +900 mV indicated the nonlinear potential
dependence as shown in Fig. 18. Similar tendency was observed for the bisulfate
adsorption on the Ru(0001) electrode surface accompanied with a formation of the
surface oxide layer [25]. Band shifts at the electrochemical interface have been
explained by the lateral dipole-dipole interaction, the Strak tuning effect and the
electronic interactions between an adsorbate and a substrate [1,2]. Since the band
position linearly shifted to the higher frequency accompanied by the increase in the
surface concentration of carbonate ions (Fig. 17 and 18), the dipole-dipole interaction
seems to be most predominant. However, the deviation from the linearity of the band
position in a potential region more positive than +400 mV could not be explained
solely by the dipole-dipole interaction. If the band positions are determined merely by
the dipole-dipole interaction among the adsorbed carbonate ions, the band should shift
to the lower frequency with the decrease in the surface concentration. Therefore, the
Stark tuning effect and/or the electronic interactions need to be considered. Koper et
al. [41] performed a theoretical study concerning the CO adsorption on platinum
surfaces and obtained the nonlinear dependence of the frequency shift of the C-O
stretching mode on the surface electric field. They pointed out that the absorption
70
Specific adsorption of carbonate ions
frequency was dominated by many factors such as the electronic interactions
(donation and back-donation), the steric repulsion and their adsorption geometries,
whose interaction energy were largely influenced by the outer electric field. Since it is
considerably difficult to evaluate separately each contribution to the experimentally
observed band shift at the electrode interface, a theoretical consideration should be
necessary to explain this phenomenon quantitatively.
The consistent molecular orientation of the carbonate ions adsorbed on the
gold electrode surfaces in the whole potential region is largely different from the case
of Pt(111). Iwasita et al. [2,14] performed in situ IRAS measurements at a Pt(111)
electrode in a 0.1 M HClO 4 solution saturated with gaseous CO 2 . The ν 1 band center
position initially shifted to the higher frequency with a slope of 190 cm–1 V –1 , and
then the slope suddenly changed to 25 cm –1 V –1 at ca. +500 mV concomitant with the
appearance of a new band at ca. 1540 cm–1 . Therefore, they stated that the singly
coordinated carbonate ions were present on the electrode surface in the less positive
potential region, and then the coordination changed to the two-fold state. They also
suggested that the bicarbonate ions were retained at the interface by forming hydrogen
bonds with the co-adsorbed water molecules.
The adsorption states of sulfate and phosphate ions at single crystal electrodes
have been extensively investigated by means of in situ IRAS [1,2,14,17-25,27,28].
Though the adsorption states of these ions are controversial, it has been recently
proposed that sulfate ions adsorb in the single coordinational state with the
perpendicular orientation at Au(111) and Au(100), and with the tilted orientation at
Au(110) in medium acidic solution [22]. Phosphate ions adsorb on the Au(111) and
Au(100) electrode surfaces in the two-fold coordinational state [2,28]. The
irrelevance of the adsorption states of carbonate, sulfate and phosphate ions to the
surface atomic symmetry is surprisingly different from the case of single crystal
platinum electrodes [2]. In situ IRAS measurements performed at Pt(111), Pt(110) and
Pt(100) electrode for the adsorption of carbonate ions provided largely different
absorption spectra [2,14-16]. Carbonate ions were found to adsorb strongly on Pt(111)
in the unidentate state, weekly on the Pt(110) in the bidentate state, and scarcely on
Pt(100). Such dependency of the adsorption states on the substrate arrangements
probably originates from a different matching between the bond length of the
71
Specific adsorption of carbonate ions
adsorbed species and the metal-metal distance, and/or a difference in the
molecule-metal interactions due to the different surface electronic state.
3.3.6 Lifting of substrates induced by specific adsorptions
The current peak observed in the double layer region at Au(111) and Au(100)
has been considered to originate from the lifting of the substrates from the
reconstructed to the non-reconstructed structure accompanied with the specific
adsorption of anions [33,34]. In situ IRAS experiments showed that the characteristic
absorption bands caused by the sulfate and phosphate ions adsorbed on Au(111) and
Au(100) started to appear at the peak potentials observed in their CVs [22,28]. This
confirms that the anion adsorption lowers the surface energy and motivates the lifting
of the substrates [33,34]. Although carbonate ions started to adsorb at ca. –300 mV
judging from the plots of the ν 1 band intensity (Fig. 17), no sharp peak was observed
in their CVs at this potential (Fig. 1 and 12). Thus one can conclude that the carbonate
adsorption cannot induce the lifting of the reconstructed structure. Since the plots of
the ν 1 band position and intensity did not change at the current peak potentials of
+202 mV and +132 mV at Au(111) and Au(100) (Fig. 12), respectively, these peaks
might have no correlation with the adsorbed carbonate ions.
The potential of the peak induced by the lifting is used as a probe for the order
of adsorbability of anions [33,34]. The lifting at a relatively negative potential
corresponds to stronger adsorption of anions, and the adsorbability orders have been
reported as Cl – < Br – < I – and HClO 4 < HNO 3 < H 2 SO 4 < H 3 PO 4 [33,34]. Since the
peak potentials observed in a 0.1 Na 2 CO 3 solution are more negative by ca. 40 and
110 mV than those in a 0.1 M H 2 SO 4 solution at Au(111) and Au(100) electrodes,
respectively [33], the adsorption of carbonate ions seems to be stronger than that of
sulfate ions. However, this is not plausible since the adsorbed carbonate ions showed
no relation to these peaks as mentioned above. Another candidate for the anion
responsible for the lifting is hydroxide ion because the solution is alkaline (pH ≈ 11.5
in a 0.1 M Na 2 CO 3 solution). The CV features observed at the three low-index gold
electrodes considerably resemble to those obtained in NaOH solutions [42,43] rather
than those in H 2 SO 4 and H 3 PO 4 solutions [33,34]. Hydroxide ions adsorb more
72
Specific adsorption of carbonate ions
strongly than sulfate ions on the Au(100) electrode surface [34]. Therefore, the
adsorption of hydroxide ions following the adsorption of carbonate ions at the more
positive potential may promote the lifting. However, no direct evidence for the
adsorption of hydroxide ions has been acquired since absorption bands expected for
the adsorbed hydroxide ions would be largely obscured by the absorption due to the
bulk water molecules.
3.4 Conclusion
In situ IRAS measurements showed that carbonate ions adsorbed on the
Au(111) electrode surface in the unidentate perpendicular orientation, which was
confirmed from the observation of ν 1 and ν 2 bands. Since almost the same spectra
were obtained at Au(100) and Au(110) electrodes, the molecular orientation of the
adsorbates proved to be independent of the symmetry of the surface atomic
arrangement, and the unidentate perpendicular orientation was retained. Although any
direct evidence as to the adsorption site has not been gained, it was predicted that the
adsorption site was the on-top site and/or bridge site based on the IRAS results at the
low-index single crystal gold electrode surfaces.
The current peaks observed in the CVs at Au(111) and Au(100) electrodes
were not induced by the carbonate adsorption since the position and intensity of the ν 1
band never changed at the peak potential. The ν 1 band started to appear at appreciably
less positive potential of ca. –300 mV. Therefore, it was concluded that the adsorption
of carbonate ions on the gold electrode surface could not induce the lifting of the
reconstructed structure. It can be preferably considered that the peaks are originated
from the hydroxide ions that start to adsorb at the more positive potentials.
73
Specific adsorption of carbonate ions
References
[1]
R.J. Nichols, in: J. Lipkowski, P.N. Ross (Eds.), Adsorption of Molecules at
Metal Electrodes, VCH, New York, 1992.
[2]
T. Iwasita, F.C. Nart, Prog. Surf. Sci. 55 (1997) 271.
[3]
C.H. Chen, S.M. Vesecky, A.A. Gewirth, J. Am. Chem. Soc. 114 (1992) 452.
[4]
Z. Shi, S. Wu, J. Lipkowski, I. Electrochim. Acta 40 (1995) 9.
[5]
E. Herrero, K. Franaszczuk, A. Wieckowski, J. Phys. Chem. 98 (1994) 5704.
[6]
F. Kitamura, T. Ohsaka, K. Tokuda, J. Electroanal. Chem. 412 (1996) 183.
[7]
F. Kitamura, T. Ohsaka, K. Tokuda, Electrochim. Acta 42 (1997) 1235.
[8]
F. Kitamura, N. Nanbu, T. Ohsaka, K. Tokuda, J. Electroanal. Chem. 452
(1998) 241.
[9]
D.M. Kern, J. Chem. Edu. 37 (1960) 14.
[10] W. Stumm, J.J. Morgan, Aquatic Chemistry, John Wiley & Sons, USA, 1970.
[11] J. Fujita, A.E. Martell, K. Nakamoto, J. Chem. Phys. 36 (1962) 339.
[12] J.A. Goldsmith, S.D. Ross, Spectrochim. Acta 24A (1968) 993.
[13] S.D. Ross, Inorganic Infrared and Raman Spectra, McGraw-Hill Book
Company, London, 1972.
[14] T. Iwasita, F.C. Nart, A. Rodes, E. Pastor, M. Weber, Electrochim. Acta 40
(1995) 53.
[15] A. Rodes, E. Pastor, T. Iwasita, J. Electroanal. Chem. 373 (1994) 167.
[16] T. Iwasita, A. Rodes, E. Pastor, J. Electroanal. Chem. 383 (1995) 181.
[17] P.W. Faguy, N. Marinković, R.R. Adžić, C.A. Fierro, E.B. Yeager, J.
Electroanal. Chem. 289 (1990) 245.
[18] Y. Sawatari, J. Inukai, M. Ito, J. Electron Spectrosc. Relat. Phenom. 64/65
(1993) 515.
[19] G.J. Edens, X. Gao, M. J. Weaver, J. Electroanal. Chem. 375 (1994) 357.
[20] P.W. Faguy, N.S. Marinković, R.R. Adžić, J. Electroanal. Chem. 407 (1996)
209.
[21] K. Ataka, M. Osawa, Langmuir 14 (1998) 951.
[22] I.R. de Moraes, R.C. Nart, J. Electroanal. Chem. 461 (1999) 110.
74
Specific adsorption of carbonate ions
[23] N.S. Marinković, J.S. Marinković R.R. Adžić, J. Electroanal. Chem. 467
(1999) 291.
[24] Y. Shingaya, M. Ito, J. Electroanal. Chem. 467 (1999) 299.
[25] N.S. Marinković, J.X. Wang, H. Zajonz, R.R. Adžić, J. Electroanal. Chem. 500
(2001) 388.
[26] N.S. Marinković, J.J. Calvente, A. Kloss, Z. Kováčová, W.R. Fawcett, J.
Electroanal. Chem. 467 (1999) 325.
[27] M. Weber, F.C. Nart, Electrochim. Acta 41 (1996) 653.
[28] M. Weber, I.R. de Moraes, A.J. Motheo, F.C. Nart, Coll. Sur. A: 134 (1998)
103.
[29] N.S. Marinković, J.J. Calvente, Z. Kováčová, W.R. Fawcett, J. Electrochem.
Soc. 143 (1996) L171.
[30] K. Ataka, T. Yotsuyanagi, M. Osawa, J. Phys. Chem. 100 (1996) 10664.
[31] A. Hamelin, J. Electroanal. Chem. 407 (1996) 1.
[32] S. Pons, J. Electroanal. Chem. 150 (1983) 495.
[33] F. Silva, A. Martins, Electrochim. Acta, 44 (1998) 919.
[34] D.M. Kolb, Prog. Surf. Sci. 51 (1996) 109.
[35] H. Angerstein-Kozlowska, B.E. Conway, B. Barnett, J. Mozota, J. Electroanal.
Chem. 100 (1979) 417.
[36] F.A. Miller, C.H. Wilkins, Anal. Chem. 24 (1952) 1253.
[37] I.T. Bae, D.A. Scherson, E.B. Yeager, Anal. Chem. 62 (1990) 45.
[38] O. Seiferth, K. Wolter, B. Dillmann, G. Klivenyi, H.-J. Freund, D. Scarano, A.
Zecchina, Surf. Sci. 421 (1999) 176.
[39] H. Angerstein-Kozlowska, B.E. Conway, A. Hamelin, L. Stoicoviviu, J.
Electroanal. Chem. 228 (1987) 429.
[40] B.E. Conway, Prog. Surf. Sci. 49 (1995) 331.
[41] S. A. Wasileski, M. J. Weaver, M. T. M. Kpper, J. Electroanal. Chem. 500
(2001) 344.
[42] A. Hamelin, M.J. Sottomayor, F. Silva, S. Chang, M.J. Weaver, J. Electroanal.
Chem. 295 (1990) 291.
[43] Š. Strbac, A. Hamelin, R.R. Adžić, J. Electroanal. Chem. 362 (1993) 47.
75
Specific adsorption of carbonate ions
Table 1
Frequencies of the absorption bands observed in the IR transmission and SNIFTIR spectra and their assignment.
Transmission spectra a / cm –1
SNIFTIR spectra / cm –1
Na 2 CO 3
Na 2 CO 3 b
NaHCO 3
(1629)
1397 (1405)
1362
1011
Assignment d
ν as (CO 2 )
NaHCO 3 c
1677
(1629)
1673
(1630)
H(D)CO 3 – sol
1624
(1195)
1624
(1193)
H(D) 2 O
1425-1499
(1432-1494)
1451-1511
(1466-1512)
CO 3 2– ads
1398
(1408)
1401
(1407)
CO 3 2– sol
(1363)
1304
Species
(1362)
1301
1350
(1362)
1305
ν as (CO 2 )
H(D)CO 3
–
H(D)CO 3
–
sol
ν s (CO 2 )
sol
ν s (CO 2 )
H(D)CO 3 – sol
(1038)
The values in parentheses were obtained in the electrolyte solutions using D 2 O as a solvent.
a: From Fig. 4.
b: From Fig. 5 and 8(A).
c: From Fig. 6 and 8(B).
d: From Ref. [12,13]
76
ν(C-OH)
Specific adsorption of carbonate ions
Table 2
Calculated absorption frequencies (ν calc / cm –1 ) of the unidentate and bidentate
carbonato groups and their assignments, the absorption frequencies of the surface
carbonate (ν surf / cm –1 ), and the band frequencies observed in this work (ν obs / cm –1 )
Species
a
Assignment and description
ν calc
a
a
ν surf
b
ν obs
c
(A) Unidentate carbonate
A1
B1
B2
ν1
ν s (C-O 2 )
1499
1470-1530
1420-1526
ν2
ν (C-O*)
1040
1040-1080
1058
ν3
δ (O-C-O)
754
ν4
ν a (C-O 2 )
1351
ν5
δ (O-C-O*)
694
ν6
Out-of-plane bending (γ)
864
1300-1370
(B) Bidentate carbonate
A1
B1
B2
ν1
ν (C-O)
1577
1530-1620
ν2
ν s (C-O* 2 )
1052
1020-1030
ν3
δ (O*-C-O*)
771
ν4
ν a (C-O* 2 )
1274
ν5
δ (O-C-O*)
671
ν6
Out-of-plane bending (γ)
859
a: From Ref. [12,13].
b: From Ref. [38].
c: This work.
77
1250-1270
Specific adsorption of carbonate ions
unidentate bidentate
flat
Fig. 1 Probable molecular orientations of the surface carbonate ions.
78
Specific adsorption of carbonate ions
20 µA cm
–2
(a)
(b)
–600
–400
–200
0
200
400
E / mV vs. Ag | AgCl | KCl (sat)
Fig. 2 CVs obtained at an Au(111) electrode in (a) 0.1 M Na 2 CO 3 and (b) 0.1 M
NaHCO 3 solutions. Scan rate was 50 mV s –1 .
(a)
(b)
20
i / µA cm
–2
(c)
10
(d)
0
–600
–400
–200
0
200
400
E / mV vs. Ag | AgCl | KCl (sat)
Fig. 3 I-E curves in the positive-going sweeps obtained at an Au(111) electrode in
NaHCO 3 solutions of different compositions: (a) 100 mM NaHCO 3 , (b) 10 mM
NaHCO 3 + 90 mM NaF, (c) 1 mM NaHCO 3 + 100 mM NaF and (d) 100 mM NaF
solutions. Scan rate was 50 mV s –1 .
79
Specific adsorption of carbonate ions
(a)
Absorption
(b)
(c)
(d)
1800
1600
1400
1200
1000
–1
Wavenumber / cm
Fig. 4 IR transmission spectra of concentrated Na 2 CO 3 and NaHCO 3 solutions: (a)
Na 2 CO 3 / H 2 O, (b) Na 2 CO 3 / D 2 O, (c) NaHCO 3 / H 2 O and (d) NaHCO 3 / D 2 O.
Absorption bands due to pure D 2 O was subtracted in the spectra (b) and (d).
80
2 × 10
∆R / R
∆R / R
Specific adsorption of carbonate ions
–4
5 × 10
–4
0 mV
400 mV
–100 mV
300 mV
–200 mV
200 mV
–300 mV
100 mV
–400 mV
–500 mV
1800
1600
1400
1200
1800
1600
Wavenumber / cm
1400
1200
–1
Fig. 5 A series of SNIFTIR spectra obtained at an Au (111) electrode in a 0.1 M
Na 2 CO 3 solution. The reference potential was –600 mV and the sample potentials
are indicated in the figure. Solid and dotted curves represent the spectra obtained
with p- and s-polarized incident light, respectively.
81
∆R / R
∆R / R
Specific adsorption of carbonate ions
–4
4 × 10
–3
1 × 10
600 mV
200 mV
100 mV
500 mV
0 mV
400 mV
–100 mV
–200 mV
–300 mV
300 mV
–400 mV
–500 mV
1800
1600
1400
1200
1800
1600
1400
1200
–1
Wavenumber / cm
Fig. 6 A series of SNIFTIR spectra obtained at an Au (111) electrode in a 0.1 M
NaHCO 3 solution. Other experimental details were the same as those indicated in
Fig. 5.
82
Specific adsorption of carbonate ions
(a)
∆R / R
(b)
–4
4 × 10
2000
1800
1600
1400
1200
–1
Wavenumber / cm
Fig. 7 Subtracted spectra in (a) 0.1 M Na 2 CO 3 and (b) 0.1 M NaHCO 3 solutions.
Subtractions were done using the p- and s-spectra in (a) Fig. 5 at 0 mV and (b) Fig.
6 at +200 mV.
(B)
1 × 10
∆R / R
∆R / R
(A)
–3
1 × 10
–3
600 mV
400 mV
400 mV
200 mV
200 mV
0 mV
0 mV
–200 mV
–200 mV
–400 mV
1800
–400 mV
1600
1400
1200
1800
1600
Wavenumber / cm
1400
1200
–1
Fig. 8 A series of SNIFTIR spectra obtained at an Au (111) electrode in (A) 0.1 M
Na 2 CO 3 / D 2 O and (B) 0.1 M NaHCO 3 / D 2 O solutions. Other experimental details
were the same as those indicated in Fig. 5.
83
Specific adsorption of carbonate ions
1510
1500
Wavenumber / cm
–1
1490
1480
1470
1460
1450
–200
0
200
400
600
E / mV vs. Ag | AgCl | KCl (sat)
Wavenumber / cm
–1
Fig. 9 Plots of the band center position as a function of the applied potential. Openand filled-triangles indicate the data obtained in 0.1 M NaHCO 3 / H 2 O (from Fig.
5) and 0.1 M NaHCO 3 / D 2 O solutions (from Fig. 7(B)), respectively.
1500
(A)
1450
1400
Intensity / a. u.
(B)
10
5
0
–600
–400
–200
0
200
400
600
E / mV vs. Ag | AgCl | KCl (sat)
Fig. 10 Plots of (A) center position and (B) intensity of the ν 1 band at ca. 1450
cm –1 as a function of the applied potential. Open- and filled-circles indicate the
data obtained in 0.1 M Na 2 CO 3 (form Fig. 5) and 0.1 M NaHCO 3 (form Fig. 6)
solutions, respectively.
84
(A)
∆R / R
Specific adsorption of carbonate ions
(C)
–4
1 × 10
∆R / R
600 mV
–4
5 × 10
500 mV
400 mV
1100
1050
1000
(B)
600 mV
600 mV
500 mV
500 mV
400 mV
400 mV
300 mV
300 mV
200 mV
100 mV
200 mV
0 mV
100 mV
1800
1600
1400
1200
1000 1100
–1
1050
1000
–1
Wavenumber / cm
Wavenumber / cm
Fig. 11 A series of SNIFTIR spectra obtained at an Au (111) electrode in (A) and
(B) 0.1 M Na 2 CO 3 / H 2 O, and (C) 0.1 M Na 2 CO 3 / D 2 O solutions measured with a
flat CaF 2 window. The reference potential was –600 mV and the sample potentials
are indicated in the figure.
85
Specific adsorption of carbonate ions
20 µA cm
–2
(a) Au(111)
(b) Au(100)
(c) Au(110)
–600
–400
–200
0
200
400
E / mV vs. Ag | AgCl | KCl (sat)
Fig. 12 CVs obtained at (a) Au (111), (b) Au(100) and (c) Au(110) electrodes in a
0.1 M Na 2 CO 3 solution. Scan rate was 50 mV s –1 . Dotted lines represents the
background voltammograms obtained in a 0.1 M NaF solution.
86
∆R / R
Specific adsorption of carbonate ions
5 × 10
–4
400 mV
300 mV
200 mV
100 mV
0 mV
–100 mV
–200 mV
–300 mV
–400 mV
1800
1600
1400
1200
–1
Wavenumber / cm
∆R / R
Fig. 13 A series of SNIFTIR spectra obtained at an Au (100) electrode in a 0.1 M
Na 2 CO 3 solution measured with a trapezoidal CaF 2 window. The reference
potential was –600 mV and the sample potentials are indicated in the figure.
5 × 10
–4
400 mV
300 mV
200 mV
100 mV
0 mV
–100 mV
–200 mV
–300 mV
–400 mV
1800
1600
1400
1200
–1
Wavenumber / cm
Fig. 14 A series of SNIFTIR spectra obtained at an Au (110) electrode in a 0.1 M
Na 2 CO 3 solution measured with a trapezoidal CaF 2 window. The reference
potential was –600 mV and the sample potentials are indicated in the figure.
87
Specific adsorption of carbonate ions
1500
Au(111):
Au(100):
Au(110):
Wavenumber / cm
–1
1480
1460
1440
1420
–200
0
200
400
E / mV vs. Ag | AgCl | KCl (sat)
Fig. 15 Plots of the ν 1 band center position as a function of the applied potential
for the adsorbed carbonate ions at the Au(111) (○), Au(100) (□) and Au(110) (△)
electrodes.
88
∆R / R
Specific adsorption of carbonate ions
2 × 10
–3
900 mV
800 mV
700 mV
600 mV
500 mV
2400
2200
2000
1800
1600
1400
1200
–1
Wavenumber / cm
Fig. 16 A series of SNIFTIR spectra obtained at an Au (111) electrode in a 0.1 M
Na 2 CO 3 solution. The reference potential was –600 mV and the sample potentials
are indicated in the figure.
89
Specific adsorption of carbonate ions
15
(A) Au(111)
(B) Au(100)
(C) Au(110)
0
10
–100
–2
i / µA cm
Intensity / a. u.
100
5
–200
0
–500
0
500
1000
–500
0
500
1000
–500
0
500
1000
E / mV vs. Ag | AgCl | KCl (sat)
Fig. 17 Plots of the ν 1 band intensity as a function of the applied potential. CVs
were obtained at (A) Au(111), (B) Au(100) and (C) Au(110) electrodes in a 0.1 M
Na 2 CO 3 solution at a scan rate of 50 mV s –1 .
1540
Wavenumber / cm
–1
1520
Au(111):
Au(100):
Au(110):
1500
1480
1460
1440
1420
0
500
1000
E / mV vs. Ag | AgCl | KCl (sat)
Fig. 18 Plots of the ν 1 band center position as a function of the applied potential
for the adsorbed carbonate ions at Au(111) (○), Au(100) (□) and Au(110) (△)
electrodes.
90