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A SNIFTIRS study of the adsorption of pyridine at the Au(111) electrode–solution interface
Electrochimica Acta 45 (1999) 611 – 621 www.elsevier.nl/locate/electacta
A SNIFTIRS study of the adsorption of pyridine at the Au(111) electrode–solution interface Manjali Hoon-Khosla a, W. Ronald Fawcett a, Aicheng Chen b, Jacek Lipkowski b,*, Bruno Pettinger c a
Department of Chemistry, Uni6ersity of California, Da6is, CA 95616, USA Guelph-Waterloo Center for Graduate Work in Chemistry, Department of Chemistry and Biochemistry, Uni6ersity of Guelph, Guelph, Ont., Canada N1G 2W1 c Department of Physical Chemistry, Fritz-Haber-Institut, Max-Planck Gesellschaft, Faradayweg 4 – 6, D-14195 Berlin, Germany b
Received 18 February 1999; received in revised form 27 April 1999
Abstract Subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) has been applied to study coordination of the pyridine molecules to the Au(111) electrode surface. The IR spectra have been recorded using both p- and s-polarized radiation. The ratio of the integrated band intensities for the spectra recorded with pand s-polarized light was then used to study changes in the surface coordination of pyridine molecules. We have derived an expression describing the dependence of this ratio on the tilt angle. We have described the orientation of the adsorbed molecule in terms of angles formed between the surface, and: (i) C26 axis of the pyridine molecule, (ii) the direction in plane of the molecule and normal to the C26 axis. We were able to demonstrate that both angles increase by moving from negative to positive potentials. This result indicates that the pyridine molecule not only stands up at positive potentials but also its plane rotates somewhat with respect to the electrode surface. © 1999 Elsevier Science Ltd. All rights reserved. Keywords: SNIFTIRS; Pyridine; Au(111) electrode–solution interface
1. Introduction Pyridine is an ideal model compound for investigating surface coordination and orientation of molecules on surfaces and interfaces. It has a rigid ring-like structure closely related to benzene, with a large dipole moment (2 D) located along its C26 axis. Interactions of the pyridine molecule with the metal can involve both p and N lone-pair orbitals [1–6]. As a result, the pyridine molecule can assume both p (flat) and N-bonded (vertical, tilted and/or rotated) orientations at metal surfaces. Thermodynamics of pyridine adsorption at Au(hkl) has been thoroughly investigated in our laboratory with the * Corresponding author.
help of the chronocoulometric technique [7 – 13]. Pyridine adsorption at the Au(111) electrode surface displayed the most interesting behavior. For this plane of gold, the surface coverage data suggested that pyridine molecules undergo a potential controlled reorientation from a p-bonded state at negatively charged to a N-bonded state at a positively charged surface. The availability of precise surface coverage data stimulated a general interest in pyridine adsorption on the Au(111) single-crystal surface. Techniques, such as second harmonic generation [14], in situ difference frequency generation [15], scanning tunneling microscopy (STM) [16] and surface-enhanced infrared absorption spectroscopy (SEIRAS) [17], have been employed to investigate this system. These studies confirmed that the orientation of
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adsorbed pyridine molecules changes by moving from the negatively to the positively charged surface. In the present work, we have employed subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) [18,19] to investigate the adsorption of pyridine at the Au(111) electrode– solution interface. This technique has been previously used to study the orientation of several organic molecules adsorbed on metal surfaces, such as, benzonitrile [19,20] and 4-cyanopyridine [21,22] on Au(111), acetonitrile [23] and pyridine [24] on polycrystalline gold. The IR experiments described in the present paper, have been conducted using electrodes prepared and treated in the same way as that employed in our thermodynamic studies [11]. Consequently, we were able to compare our new spectroscopic measurements to the earlier thermodynamic data. In general, the results of IR studies confirm the previous interpretation of the affect of electrode potential on the orientation of pyridine molecules at the Au(111) electrode surface. However, we will demonstrate below that, at negative potentials, a fraction of the surface coverage, assigned earlier to the p-bonded state corresponds in fact to N-bonded pyridine adsorbed at defect sites.
ented with the help of the back Laue diffraction technique with the precision of 1°. The ratio of the step height to the terrace length on the image shown in Fig. 1 gives the miscut angle on the order of 0.8°
2. Experimental section
2.1. Reagents and electrodes The KClO4 (ACS Certified, Fisher) was purified as described elsewhere [25]. Pyridine (Silylation grade, Pierce Chemical Company) was used without further purification. All solutions were prepared from Deuterium Oxide, D2O (99.9%, Cambridge Isotope Laboratories). If not otherwise stated, all measurements were performed using a 0.003 M pyridine solution in D2O containing 0.05 M KClO4 as supporting electrolyte at 20 92°C. Most of the IR spectra were recorded in D2O as the solvent to avoid interference from the water bands in the spectral region of our interest. The working electrode was 1 cm2 singlecrystal rod oriented to expose either (111) or (210) plane. Both the Au(111) and Au(210) single-crystals, were grown, cut, and polished in our laboratory. The top layer perturbed by mechanical polishing was removed by electropolishing and the surface was smoothed by annealing [25]. The electrodes were flame annealed prior to each experiment. Fig. 1 shows the scanning tunneling microscopy image of the Au(111) surface recorded in air after electropolishing and flame annealing. The surface consists of relatively large (111) terraces separated by steps. The steps are produced by a miscut due to an error in the crystal orientation procedure. The crystals were ori-
Fig. 1. STM images of the flame annealed Au(111) surface in air; top panel image of 10 ×10 mm section of the surface showing surface topology after electrochemical polishing and flame annealing, middle panel 100 ×100 nm section showing long terraces smooth at the atomic level, bottom section, high resolution image of a section of the smooth terrace showing hexagonal packing of gold atoms on the surface.
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2.2. Apparatus
DR/R =(R(E2) −R(E1))/R(E1)
The SNIFTIRS experimental procedures and instrumentation were described in [19–21]. A syringe-type IR cell with a 60° CaF2 prism window was used in this study. The prism ensures an optimum angle of incidence for the IR reflection spectroscopy [26]. During the IR experiment, the working electrode was pushed against the flat part of the CaF2 window to form a thin layer. The thickness of the thin layer was estimated to be 6 mm. Its reproducibility was 10%; however, the uncertainty of the absolute value was at least 1–2 mm. A cylindrical platinum foil 3 cm2 was used as the counter electrode, and the reference electrode was Ag/ AgC1 (3 M KCl saturated with AgCl) electrode connected to the cell through a salt bridge. For the sake of comparison with previous electrochemical results, all potentials measured with respect to the Ag/AgCl electrode were converted to the saturated calomel electrode (SCE) scale. The electrolyte solution was deaerated by purging with pure argon for about 20 min before the measurements and argon was allowed to flow over the solution at all times during the experiment. A transmission spectrum was determined by squeezing the investigated solution between two flat ZnSe2 windows. The spectra were measured using a Nicolet 20SX/C FTIR apparatus equipped with an MCT-B detector cooled by liquid nitrogen with the resolution 4 cm − 1. The sample compartment of the FTIR apparatus was purged throughout the experiment using CO2- and H2O-free air provided by the Puregas Heatless Dryer. The electrode potential was controlled by a PAR 173 potentiostat.
where R(E1) and R(E2) are the electrode reflectivities at the reference and sample potentials E1 and E2, respectively. The reference potential was always equal to − 0.75 V (SCE) where pyridine molecules are totally desorbed from the electrode surface. Under these conditions, the measured changes of the reflectivity (DR/R) represent the difference between the absorption spectrum of G pyridine molecules that are desorbed from the surface and reside in the solution at potential E1 and the spectrum of G molecules adsorbed at the electrode surface at potential E2. The reflectivity is defined as R = IR/IO. In general the attenuation of light by reflection depends on absorption of radiation both by the metal which is weakly dependent on potential and by molecules present in the thin layer cell that depends strongly on potential. For a nonpolarized light, applying Beer’s law IR/IO = exp(− 2.3oG− const.)
(1)
(2)
when (DR/R) 1 the measured relative change of the electrode reflectivity can be expressed as: (DR/R)= 2.3G[o(E1) −o(E2)]
(3)
where o is the molar absorption coefficient and G is the surface concentration of the adsorbed species. According to Eq. (3) the changes in reflectivity can be expressed in absorbance units. The integrated band intensities were used for quantitative analysis. In that case:
&
&
(DR/R) dn= 2.3G [o(E1) −o(E2)] dn
(4)
2.3. Spectroscopic procedure The SNIFTIR spectra were determined using a multiple potential step (MPS) procedure in which the electrode potential was stepped m times between the reference and the sample potentials, E1 and E2 respectively. During each step, n interferograms were acquired at both potentials. Data acquisition was delayed for 10 s after each potential change to allow the interface to reach thermodynamic equilibrium at the imposed polarization. The change of the electrode potential was synchronized with the acquisition of the interferograms by connecting the external trigger port of the PAR 173 potentiostat to the communication port of the DX 486 computer of the FTIR instrument. This procedure was repeated m times until a total number of N=n×m interferograms was acquired for each of the two potentials. Typical values of n and m employed in this study were 100 and 20, respectively. The interferograms were added, Fourier transformed, and used to calculate a relative change of the electrode reflectivity, which is defined as:
For a linearly polarized light and a molecule adsorbed or in front of a reflecting metal surface the integrated absorption coefficient is proportional to the square of the dot product of the transition dipole m and the electric field of the photon E [27,28]:
&
o dn8 m·E 2 8 cos2 u m 2 B E 2 \
(5)
where u is the angle between directions of the electric field of the photon and the transition dipole in the molecule, m is the absolute value of the transition dipole moment and B E 2 \ is the mean square electric field of the photon. At the potential of total desorption E1, pyridine molecules are randomly oriented inside the thin layer of the electrolyte. In that case, the angle u has to be averaged over all possible orientations and the result is B cos2 u \ =1/3 [27]. Consequently, at E1 the integrated absorption coefficient is given by:
&
o(E1) dn8 (1/3) m 2 B E 2(z" 0\
(6)
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for both s and p-polarized radiation. Using s-polarized light, the electric field of the photon at the surface is nearly equal to zero [28,29] and hence o(E2) is close to zero as well. Eqs. (3) and (4) then, simplify to: (DR/R)s =2.3Go(E1)/3
(7)
&
(8)
and
&
(DR/R)s dn=2.3G o(E1) dn/3
In that case, the absorption band is positive and the band intensity is proportional to the product of the surface concentration of pyridine adsorbed at potential E2 and the integrated molar absorption coefficient of pyridine in the bulk of the thin layer cavity. When p-polarized radiation is employed both the adsorbed and solution species are IR active. In that case the (DR/R)p spectrum can display bipolar features, when band frequencies for the surface and solution species are different. The positive bands on this spectrum correspond to the absorption by molecules desorbed into the thin layer cavity at E1 and the negative bands to the absorption of the IR radiation by molecules adsorbed at the electrode surface at potential E2. In addition, the mean square field of the photon in the thin layer cell has a different value than in the incident beam. Consequently, in Eqs. (4)–(8), the molar absorption coefficients have a different value than the one measured in transmission.
3. Results and discussion
3.1. Re6iew of electrochemical data To facilitate interpretation of the spectroscopic data, we will briefly review results of earlier electrochemical studies of pyridine adsorption at the Au(111) electrode from 0.01 M KClO4 and 0.1 M KClO4 +0.003 M pyridine aqueous solution [11]. Fig. 2 shows how the differential capacity, charge density and the surface concentration of pyridine change as a function of the electrode potential. Differential capacity and charge density curves for the pyridine solution merge with the curves for the supporting electrolyte and the surface concentration drops to zero at EB −0.7 V. Pyridine is apparently desorbed from the Au(111) electrode surface at these negative potentials and is adsorbed when E \ − 0.7 V. The surface concentration plot displays two steps. The first, with the limiting concentration 1.6× 10 − 10 mol cm − 2 is seen between −0.7 V and 0 V. It is observed at a negatively charged surface and was assigned previously to the flat p-bonded state of adsorbed pyridine. This conclusion was confirmed recently by Cai et al. [17] who showed STM images of p-bonded pyridine in this region. The second step is seen at E \0
Fig. 2. (a) Differential capacity curves recorded at the sweep rate of 5 mV s − 1 and an ac modulation frequency of 25 Hz for the Au(111) electrode in the presence of 0.01 M KClO4 (dotted line) and 0.1 M KClO4 +0.003 M pyridine aqueous solution (solid line), (b) charge density plots for 0.1 M KClO4 (dotted line) and 0.1 M KClO4 +0.003 M pyridine aqueous solution (solid line), and (c) an adsorption isotherm (G vs. E plot) recorded for the Au(111) electrode in 0.1 M KClO4 + 0.003 M pyridine aqueous solution.
V where the gold surface is positively charged. It was assigned to the N-bonded state of adsorbed pyridine. The differential capacity in this region is quite low and this feature suggests that pyridine molecules form a condensed monolayer. At these potentials, STM images by Cai et al. [17] show that adsorbed pyridine molecules form chain-like aggregates similar in appearance to rolls of coins. The data in Fig. 2 suggest that the transition from the p-bonded to the N-bonded state occurs around the potential of zero charge of the Au(111) electrode, measured in the presence of pyridine in the solution.
3.2. Description of IR spectra We will now employ FTIR spectroscopy to study the potential induced changes of the coordination of pyridine to the Au(111) surface and to verify conclusions of the electrochemical experiments. Fig. 3 shows: transmission spectra of neat pyridine and 0.4 M pyridine solution in D2O and two SNIFTIRS spectra
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recorded using either s- or p-polarized light, for pyridine adsorbed at E2 =0.362 V. An isolated pyridine molecule has a C26 symmetry. In the spectral range used in our experiments, the transmission spectrum of neat pyridine displays three bands. Two bands at 1582 and 1483 cm − 1 have a1 symmetry and the third band at 1439 cm − 1 has b1 symmetry [30]. They all are assigned to the in-plane ring deformations [31]. The a1 bands correspond to changes of the dipole moment along the C26 axis of the molecule and the b1 band is due to the change of the dipole moment in the direction perpendicular to the C26 axis. The transmission spectrum of pyridine in D2O has only two bands. The weak a1 band at 1483 cm − 1 is not visible in this spectrum. The two other bands appear at 1594 and 1445 cm − 1, respectively. They are blue shifted with respect to the bands in the spectrum of neat pyridine. This shift is chiefly due to the effect of the solvent. As explained earlier, the SNIFTIRS spectra were recorded using the reference potential E = −0.75 V at which pyridine molecules are desorbed from the electrode surface. Consequently, they represent a difference of the absorption spectrum for pyridine molecules in solution and the spectrum of molecules adsorbed at the Au(111) surface at potential E2. For s-polarized light the molecules adsorbed at the electrode surface are optically inactive and then SNIFTIRS measures the
Fig. 3. Comparison of the transmission and SNIFTIRS spectra for pyridine. 1 and 2 display the transmission spectrum for neat pyridine, and for a 0.4 M pyridine solution in D2O, respectively. 3 and 4 show SNIFTIRS spectrum obtained from s- and p-polarized light for a 0.003 M pyridine + 0.05 M KClO4 solution acquired using E1 = − 0.75 and E2 = 0.262 V.
615
Fig. 4. Comparison of the potential dependence of integrated absorptivity of the b1 band at 1445 cm − 1 and the surface concentration of pyridine determined from electrochemical data. The integrated absorptivities were multiplied by a constant factor to give the best fit to the surface concentration data.
spectrum of pyridine molecules in solution, inside the cavity of the thin layer cell. Indeed, in Fig. 3, the SNIFTIRS spectrum measured with s-polarized radiation is a mirror image of the transmission spectrum for the pyridine solution in D2O. According to Eq. (7), the integrated intensity of these bands should be proportional to the surface concentration of adsorbed pyridine. In Fig. 4 the potential dependence of the integrated intensities of the tallest band at 1445 cm − 1 is compared to the change of the surface concentration of pyridine determined by chronocoulometry. At negative potentials, the integrated intensities are somewhat lower than the surface concentrations. However, as predicted, the agreement between the two sets of data is good. For p-polarized light the shape of the spectrum for adsorbed molecules can be predicted from the surface selection rules. For a molecule adsorbed at a metal, surface selection rules state that the vibration will be allowed if a change in the dipole moment has a nonzero component in the direction normal to the surface [28]. When pyridine molecule assumes a flat, p-bonded coordination, the change of the dipole moment in the direction normal to the surface has a zero component for both a1 and b1 modes. In the spectral region shown
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in Fig. 3, the p-bonded molecule should be optically inactive. For this surface orientation, the SNIFTIRS should plot only the spectrum of molecules desorbed into the solution at the reference potential, which should resemble the spectrum recorded for s-polarized photons. For the N-bonded pyridine molecule, the change of the dipole in the direction normal to the surface is very strong for a1 modes while for b1 modes the change of the dipole moment is either small or zero. In addition the field strength of the p-polarized photon is significantly enhanced at the metal surface [28,29]. Consequently, the a1 bands for N-bonded pyridine are significantly stronger than the corresponding bands for the molecule desorbed into the solution at the reference potential. In the SNIFTIRS spectrum the b1 mode should appear as a positive band of the solution species while a1 bands should be either bipolar or negative. The a1 bands would be bipolar if the center of the absorption band for the N-bonded molecule is shifted with respect to the band of desorbed pyridine. When the band shift is insignificant only the negative features could be observed. In that case, the band for solution species is much weaker than the band for adsorbed molecule and when the two bands are subtracted the band for adsorbed molecules dominates the spectrum. In Fig. 3 the SNIFTIRS spectrum for p-polarized light illustrates that behavior. Consistently with the Nbonded state of adsorbed pyridine the b1 mode is positive while the two a1 modes are negative. A noteworthy feature in this spectrum is the appearance of the a1 band at 1475 cm − 1 which was absent in either the transmission or SNIFTIRS spectra for molecules in D2O solution. The presence of this band may be due to a combined effect of the (weak) surface enhancement and the solvent effect. At positive potentials the adsorbed pyridine forms a condensed monolayer and hence its molecule resides in an environment different from that in D2O solution.
1400 cm − 1, respectively. The b1 band at 1445 cm − 1 is positive for all potentials. The small a1 band at 1475 cm − 1 is absent when E B 0 V and appears as a negative feature at E\ 0 V. This band is expected to appear at potentials where pyridine is N-bonded and forms a condensed monolayer. Likewise, the a1 band at 1594 cm − 1 appears as a negative feature when E\0. However, in contrast to expectations, the spectra recorded for EB0 V show a new band at 1599 cm − 1. It is predominantly negative although it displays some positive features indicating that it has a weak bipolar character. The positive lob of this band is centered at 1594 cm − 1; however, it is impossible to tell whether it represents the expected positive band of p-bonded pyridine or a positive lob of the new band at 1599 cm − 1. At the reference potential E1, both the molecules that at the sample potential E2 absorbed IR radiation at 1599 cm − 1 and the molecules that were p-bonded and optically inactive are desorbed and produce a positive band at 1594 cm − 1. The amplitude of the new band initially increases with potential and then it attains a limiting value at E: − 138 V. The presence of the negative band at 1599 cm − 1 for EB0 V is inconsistent with the suggested p-bonded coordination of pyridine molecules at the negatively charged surface. Its origin has to be carefully examined. We note at this point that the band at 1599 cm − 1 was not observed at
3.3. Surface coordination of pyridine 3.3.1. Qualitati6e analysis At this point we will look at the shape of the subtractively normalized IR spectra, acquired with ppolarized IR light at EB0 V. The electrochemical data shown in Fig. 2 suggest that pyridine is p-bonded at these potentials. If the SNIFTIRS spectra are to be consistent with that interpretation of the electrochemical results the spectra for p- and s-polarized light should be similar and should consist of two positive bands at 1445 and 1594 cm − 1. Fig. 5 shows a series of spectra obtained by changing the sample potential (E2) from − 0.338 to +0.362 V (SCE). For the benefit of further discussion the spectra are divided into two regions, corresponding to 1650–1550 cm − 1 and 1500–
Fig. 5. A series of SNIFTIR spectra recorded for pyridine adsorbed at the Au(111) electrode acquired using p-polarized light. The solution of D2O contains a concentration of 0.003 M pyridine+ 0.05 M KClO4. For each spectrum, the reference potential E1 = −0.75 V and the value of E2 is indicated in the figure. All potentials are referred to the SCE scale.
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negative potentials, in recent (SEIRAS) studies of pyridine adsorption at the Au(111) surface [17]. Perhaps, this may be due to the differently prepared electrode (a vapor deposited gold on silicon was used in Ref. [17]). It is also possible that the pyridine band was hidden in the stronger band of water, since water was used as the solvent in Ref. [17]. Pyridine adsorption at gold is known to be strongly dependent on the surface crystallography [1,12,13]. The images of the electrode surface shown in Fig. 1 indicate that, in addition to (111) terraces, the surface contains many defect sites. Adsorption of pyridine is known to be stronger at the high index surfaces and hence also at defect sites than on the (111) terraces [1,12,13]. Therefore, it is quite plausible that the first step on the G versus E plot in Fig. 2 corresponds to pyridine adsorption on the defect sites rather than to the p-bonded pyridine at the (111) terraces. To test this hypothesis we have performed additional SNIFTIRS experiments at the Au(210) electrode. The Au(210) surface is the most open single-crystal plane with the highest number of nearest-neighbor broken bonds per square of the lattice parameter [32]. Therefore, adsorption of pyridine at this crystallographic orientation should best mimic its coordination onto the defect sites present at the Au(111) surface. Fig. 6a compares the SNIFTIRS bands for the Au(111) and Au(210) surfaces in the spectral region 1620–1575 cm − 1. For the Au(210) surface only one negative band at 1599 cm − 1 was observed in this region, at all sample potentials investigated. Fig. 6a shows that the position of the a1 band at the Au(210) surface corresponds well to the negative band observed at the Au(111) surface at EB0 V. It also explains well the presence of the small a1 band at 1599 cm − 1 for the Au(111) surface at E\0 V. This result strongly suggests that at the Au(111) surface the small negative band at 1599 cm − 1 corresponds to pyridine N-bonded to defect sites while the band at 1593 cm − 1 corresponds to the N-bonded pyridine at the (111) terraces. In Fig. 6b, for Au(111) and Au(210) surfaces, the frequencies of the 1599 cm − 1 band are plotted against the rational potential. The data for two surfaces fit one linear relationship. They show that the a1 band frequency depends weakly on the electrode potential. The presence of pyridine adsorbed at defect sites may also explain the shape of the a1 band at 1475 cm − 1. Fig. 7 compares spectra for the Au(111) and Au(210) surfaces in the region centered around 1480 cm − 1. The band for the Au(111) electrode is asymmetric and has a shoulder at the high frequency side. The position of this shoulder corresponds quite well to position of the a1 band observed for the Au(210) surface. The shoulder may therefore, be assigned to pyridine adsorbed at defect sites. In conclusion, the qualitative analysis of SNIFTIRS spectra provides no evidence that pyridine
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Fig. 6. (a) Selective SNIFTIRS spectra of pyridine adsorbed at Au(111) and Au(210) electrode surfaces acquired using p-polarized light. The dotted line is the spectrum of Au(210) at −100 mV; the solid lines are the spectra of Au(111) at − 138 (top) and 2 mV (bottom), respectively. For each spectrum, the reference potential E1 was equal to −0.75 V. (b) A plot of the shift in frequency of the 1601 cm − 1 band vs. potential with respect to the potential of zero charge (pzc) with adsorbed pyridine. Open circles and solid lines for Au(111) and closed squares and dotted lines for Au(210).
is p-bonded to the (111) terraces at EB 0 V and suggests that at least a fraction of the first step on the G versus E plot corresponds to pyridine N-bonded to defect sites.
3.3.2. Quantitati6e analysis Further information concerning the orientation of adsorbed molecules could be obtained from the analysis of the integrated band intensities. For that purpose, it is convenient to calculate the ratio of the integrated intensities measured for p- and s-polarized radiation. Using Eqs. (4) – (8), the ratio of the integrated intensities may be expressed as:
&
(DR/R)p E 2p(z) E 2p(z= 0) dn = 2 − 3 cos2 u E s (z) E 2s (z) (DR/R)s
(9)
where; BE 2p,s(z) \ is the time and z averaged mean square field within the thin layer cell and BE 2p(z= 0) \ is the time averaged mean square field at the
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Fig. 7. Comparison of the spectrum acquired using p-polarized light in the region 1525 –1455 cm − 1 for Au(111) at 112 mV and Au(210) at −100 mV. The crystallographic orientation of gold is indicated in the figure.
electrode surface, z varies between 0 and d, where d is the thickness of the thin layer. The angle u in Eq. (9), is the angle between the direction of the transition dipole for the adsorbed molecule and the direction of the field of the p-polarized photon on the surface. The p-polarized photon on the surface has a predominant component in the z-direction (direction normal to the surface). Consequently, to a good approximation the angle u is a measure of the tilt angle for the adsorbed molecule (the tilt angle is 90° − u). Fig. 8 shows how, for the a1 band at 1600 cm − 1 and the b1 band at 1445 cm − 1, the ratio of the inte-
Fig. 8. Ratio of the integrated absorptivities for the SNIFTIRS bands acquired using p- and s-polarized radiation. Curve 1, b1 band at 1445 cm − 1; curve 2, a1 band in the 1600 cm − 1 spectral region.
grated band intensities depends on the electrode potential. The integrated intensities of the b1 band change from a value 1.32 at negative potentials to a value 1.02 at positive potentials. High positive values of the ratio indicate that the component of the transition dipole in the direction of the field of the photon is small and that the dipole is almost parallel to the surface. However, the small change of this ratio suggests that the direction of the transition dipole changes somewhat with potential. According to Eq. (9), the smaller value of the ratio at positive potentials indicates that the angle between the b1 axis of the molecule and the normal to the electrode surface is smaller. This behavior suggests that by moving from negative to positive potentials the tilt angle between the b1 axis and the electrode surface increases. For the a1 band, the absolute value of the ratio of integrated band intensities is quite large and its sign is negative. This behavior indicates that, for this band, the component of the transition dipole in the direction of the photon is quite large and suggests that pyridine molecules assume a tilted, N-bonded orientation. The absolute value of the ratio increases markedly by moving from negative to positive potentials indicating that the tilt angle increases when potential crosses the zero value. This result is consistent with thermodynamic studies. It suggests that in addition to the Nbonded pyridine at defect sites there are also molecules that are p-bonded to the (111) terraces. In order to determine the values of the tilt angle, the average fields of the photon have to be calculated. The fields BE 2p,s(z) \ and BE 2p(z= 0)\ may be calculated from the model of light propagation through a three-layer system consisting of a semi-infinite phase of CaF2, thin layer of electrolyte and semi-infinite metal phase. For this model, explicit formulas for computing the optical fields within the thin layer cell were derived by Seki et al. [33]. Figs. 9a and b show the changes of the mean square fields as a function of the distance from the electrode surface, calculated using optical constants given in Table 1. Due to the interference within the cavity of the thin layer cell, the fields vary as a function of the distance in an oscillatory manner. This is an unwanted behavior, since the strong dependence of the fields on the distance requires a precise knowledge of the thickness of the thin layer cavity in order to calculate the tilt angle for the adsorbed molecules. Unfortunately, only the approximate value of the thickness is known. The data shown in Fig. 9 were subsequently used to calculate the ratios B E 2p \ / B E 2s \ and B E 2p(z= 0)\ /B E 2s \ for frequencies corresponding to a1 and b1 bands. The result is plotted in Figs. 10a and b. The photon fields BE 2p \ and B E 2s \ plotted in this figure were averaged over the distance from the electrode surface. We can now make an attempt to use Fig. 10 to calculate the tilt angle. We will look first at the b1 band.
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Fig. 9. Time averaged mean squared electric fields of the photon plotted as a function of a distance from the electrode surface. (a) for 1445 cm − 1 and (b) for 1600 cm − 1.
For this mode and negative potentials, we may assume that the second term in Eq. (9) is negligible and that the ratio of the integrated intensities (equal to 1.3) is essentially equal to BE 2p \/BE 2s \. This is equivalent to the assumption that the transition dipole of this band is parallel to the electrode surface. In Fig. 10, the values of BE 2p \/B E 2s \, that are close to 1.3, are observed for the thin layer thickness equal to either 3 or 8.5 mm. The first value of the thickness is too low. The Table 1 Optical constants used to calculate the average field strength of the photon in the thin layer cell a n (cm−1)
Complex refractive index CaF2
1600 1445
D2O
Au
1.38137+0i
1.32213
3.06746
1.3707+0i
+ 0.0108363i 1.31653
+ 45.37i 3.71042
+ 0.00862582i
+49.9349i
a Optical constants for CaF2, Refs. [34,35], optical constants for D2O, Ref. [36] and optical constants for Au, Ref. [37].
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Fig. 10. (a) Ratio of the time and distance averaged mean squared electric fields for the s- and p-polarized photons; (b) ratio of the time averaged mean squared field of the p-polarized photon at the electrode surface to the mean square field of the s-polarized photon averaged over the thickness of the thin layer cell, plotted as a function of the thickness of the thin layer cavity.
second value is closer to our estimate of the thickness of the gap between the electrode surface and the CaF2 window. We will therefore assume that 8.5 mm represents the thickness of our thin layer cavity. For that thickness, the decrease of the ratio of integrated intensity to a value of approximately 1, observed at positive potentials, corresponds to an increase of the angle between the direction of the transition dipole of the b1 band and the electrode surface to about 13°. This is quite a reasonable result. At that point we may calculate the tilt angle from the ratio of integrated intensities for the a1 band at 1600 cm − 1. For EB 0 V, this ratio is equal to −0.2 and for E\ 0 it is approximately equal to − 1.0. Using Eq. (9) and the values of BE 2p \ /B E 2s \ and B E 2p(z= 0)\ / B E 2s \ shown in Fig. 10, we find that the tilt angle between the C26 axis of the pyridine molecule and the electrode surface is equal to 30° for negative potentials and to about 40° for positive potentials. The first number should be seen as an average over the tilt angle
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of N-bonded pyridine at defect sites and the tilt angle of p-bonded pyridine at the (111) terraces. The latter number seems to be too low. Recent STM images by Cai et al. [17] indicate that, at positive potentials, pyridine molecules form surface aggregates similar in appearance to a roll of coins. In such a stack of aromatic molecules the tilt angle is expected to be 60–70° [38]. The low value of the tilt angle points out to inadequacies of our model or experimental set up. We have made attempts to improve the model by introducing an additional layer corresponding to the film of adsorbed pyridine. However, the presence of this additional layer did not change the calculated photon fields. The weakest point of our experimental set up was a poor control of the thickness of the thin layer cavity. In our experiments a massive gold electrode was simply pressed against the CaF2 window. We had no possibility to adjust the thickness of the gap between the electrode surface and the IR window. The cell design described by Roe et al. [39] would be more suitable for quantitative data analysis described in the present paper. Such a cell will be employed in our future work.
4. Summary and conclusions We have employed SNIFTIRS to study coordination of pyridine to the Au(111) electrode surface. Previous thermodynamic studies of pyridine adsorption at this gold electrode suggested that pyridine assumes a pbonded (flat) coordination at negative potentials and a N-bonded (vertical or tilted) geometry at positive potentials [11]. In contrast to these predictions, we have observed that the character of the IR spectra recorded at negative potentials is dominated by the N-bonded pyridine. We have demonstrated that this is the Nbonded state of molecules adsorbed at defect sites rather than at the (111) terraces. The p-bonded pyridine is optically inactive, however in the SNIFTIRS spectrum, the presence of the p-bonded state could be seen in the form of positive bands corresponding to IR absorption by molecules desorbed into the solution at the reference potential In the presence of pyridine adsorbed at defect sites, we were unable to distinguish whether the positive bands in the spectra corresponded to pyridine adsorbed on the defect sites or the p-bonded molecule at the (111) terraces. We have used the ratio of the integrated band intensity for the spectra recorded with p- and s-polarized light to study changes in the surface coordination of pyridine, quantitatively. We have derived an expression for the dependence of the ratio of the integrated intensities on the tilt angle. We have described the orientation of the adsorbed molecule in terms of angles formed between the surface and; (i) C26 axis of the pyridine
molecule, (ii) the direction in plane of the molecule and normal to the C26 axis. We were able to demonstrate that both angles increase by moving from negative to positive potentials. This result indicates that the pyridine molecule not only stands up at positive potentials but also its plane rotates somewhat with respect to the electrode surface. Our result is in accord with recent STM and FTIR studies by Cai et al. [17] and particularly with the difference frequency generation experiments by Hebert et al. [15], that have demonstrated that the plane of the pyridine molecule is rotated. We have made attempt to determine numerical values of the two angles. For this purpose, we had to calculate the average field strength of the photon in the thin layer cavity. The three layer model of the thin layer cell, originally proposed by Seki et al. [33] was used in these calculations. The numerical values are reasonable, however the angle between the surface and the C26 axis of the pyridine molecule appears to be lower than the expected value. This may be due to the oversimplifications of the model. The model has to be tested on a larger set of experimental data in order to gain confidence in the numerical values that are calculated.
Acknowledgements We thank V. Zamlynmy for preparing the Au(210) single-crystal electrode. This work was supported by the grant from the Natural Sciences and Engineering Research Council of Canada.
References [1] J. Lipkowski, L. Stolberg, D.-F. Yang, B. Pettinger, S. Mirwald, F. Henglein, D.M. Kolb, Electrochim. Acta 39 (1994) 1057. [2] S. Haq, D.A. King, J. Phys. Chem. 100 (1998) 16957. [3] J.E. Demuth, K. Christmann, P.N. Sanda, Chem. Phys. Lett. 76 (1980) 202. [4] N.J. DiNardo, P.h. Avouris, J.E. Demuth, J. Chem. Phys. 84 (1984) 2169. [5] B.J. Bandy, D.R. Lloyd, N. Richardson, Surf. Sci. 89 (1979) 344. [6] M. Bader, J. Haase, K.H. Frank, A. Puschmann, A. Otto, Phys. Rev. Lett. 56 (1986) 1921. [7] L. Stolberg, J. Richer, J. Lipkowski, D.E. Irish, J. Electroanal. Chem. 207 (1986) 213. [8] L. Stolberg, J. Lipkowski, D.E. Irish, J. Electroanal. Chem. 238 (1987) 333. [9] L. Stolberg, J. Lipkowski, D.E. Irish, J. Electroanal. Chem. 296 (1990) 171. [10] L. Stolberg, J. Lipkowski, D.E. Irish, J. Electroanal. Chem. 300 (1991) 563. [11] L. Stolberg, S. Morin, J. Lipkowski, D.E. Irish, J. Electroanal. Chem. 307 (1991) 241.
M. Hoon-Khosla et al. / Electrochimica Acta 45 (1999) 611–621 [12] D.F. Yang, L. Stolberg, J. Lipkowski, J. Electroanal. Chem. 329 (1992) 259. [13] J. Lipkowski, L. Stolberg, in: J. Lipkowski, P.N. Ross (Eds.), Adsorption of Molecules at Metal Electrodes, VCH, New York, 1992. [14] B. Pettinger, S. Mirwald, J. Lipkowski, J. Electroanal. Chem. 329 (1992) 289. [15] P. Hebert, A. Le Reille, W.Q. Zheng, A. Tadjeddine, J. Electroanal. Chem. 447 (1998) 5. [16] G. Andreasen, M.E. Vela, R.C. Salvarezza, A.J. Arvia, Langmuir 13 (1997) 1317. [17] W.-B. Cai, L.-J. Wan, H. Noda, Y.-I. Hibino, K.-I. Ataka, M. Osawa, Langmuir 14 (1998) 6992. [18] S. Pons, J. Electroanal. Chem. 150 (1983) 495. [19] A. Bewick, S. Pons, in: R.J.H. Clark, R.E. Hester (Eds.), Advances in Infrared and Raman Spectroscopy, vol. 12, Heyden and Son, New York, 1985 (Chapter 1). [20] A. Chen, J. Richer, S.G. Roscoe, J. Lipkowski, Langmuir 13 (1997) 4747. [21] A. Chen, S.G. Sun, D.F. Yang, B. Pettinger, J. Lipkowski, Can. J. Chem. 74 (1996) 2321. [22] A. Chen, D.-F. Yang, J. Lipkowski, J. Electroanal. Chem., in print. [23] P.W. Faguy, W.R. Fawcett, G. Liu, A.J. Motheo, J. Electroanal. Chem. 339 (1992) 339. [24] Y. Ikezawa, T. Sawatari, T.K.H. Goto, K. Toriba, Electrochim. Acta 43 (1998) 3297.
.
621
[25] J. Richer, J. Lipkowski, J. Electrochem. Soc. 133 (1986) 121. [26] P.W. Faguy, W.R. Fawcett, Appl. Spectrosc. 44 (1990) 1309. [27] W. Liptay, Angew. Chem. 81 (1969) 195. [28] M. Moskovits, J. Chem. Phys. 77 (1982) 4408. [29] R.G. Greenler, J. Chem. Phys. 44 (1966) 310. [30] L. Corrsin, B.J. Fax, R.C. Lord, J. Chem. Phys. 21 (1953) 1170. [31] D.A. Long, E.L. Thomas, Trans. Faraday Soc. 59 (1963) 783. [32] A. Hamelin, in: B.E. Conway, R.E. White, J.O’M. Bockris (Eds.), Modern Aspects of Electrochemistry, vol. 16, Plenum, New York, 1985 (Chapter 1). [33] H. Seki, K. Kunimatsu, W.G. Golden, Appl. Spectrosc. 39 (1985) 437. [34] W.W. Coblentz, J. Opt. Soc. Am. 4 (1920) 441. [35] R.A. Smith, F.E. Jones, R.P. Chasmar, The Detection and Measurement of Infrared Radiation, Oxford University Press, London, 1927, p. 342. [36] J.E. Bertie, M.K. Ahmed, H.H. Eysel, J. Phys. Chem. 93 (1989) 2210. [37] P.B. Johnson, R.W. Christy, Phys. Rev. B 6 (1972) 4370. [38] T.h. Dreschkow, D. Lampner, T.h. Wandlowski, J. Electroanal. Chem. 458 (1998) 121. [39] D.K. Roe, J.K. Sass, D.S. Bethune, A.C. Luntz, J. Electroanal. Chem. 216 (1987) 293.
A SNIFTIRS study of the adsorption of pyridine at the Au(111) electrode–solution interface Manjali Hoon-Khosla a, W. Ronald Fawcett a, Aicheng Chen b, Jacek Lipkowski b,*, Bruno Pettinger c a
Department of Chemistry, Uni6ersity of California, Da6is, CA 95616, USA Guelph-Waterloo Center for Graduate Work in Chemistry, Department of Chemistry and Biochemistry, Uni6ersity of Guelph, Guelph, Ont., Canada N1G 2W1 c Department of Physical Chemistry, Fritz-Haber-Institut, Max-Planck Gesellschaft, Faradayweg 4 – 6, D-14195 Berlin, Germany b
Received 18 February 1999; received in revised form 27 April 1999
Abstract Subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) has been applied to study coordination of the pyridine molecules to the Au(111) electrode surface. The IR spectra have been recorded using both p- and s-polarized radiation. The ratio of the integrated band intensities for the spectra recorded with pand s-polarized light was then used to study changes in the surface coordination of pyridine molecules. We have derived an expression describing the dependence of this ratio on the tilt angle. We have described the orientation of the adsorbed molecule in terms of angles formed between the surface, and: (i) C26 axis of the pyridine molecule, (ii) the direction in plane of the molecule and normal to the C26 axis. We were able to demonstrate that both angles increase by moving from negative to positive potentials. This result indicates that the pyridine molecule not only stands up at positive potentials but also its plane rotates somewhat with respect to the electrode surface. © 1999 Elsevier Science Ltd. All rights reserved. Keywords: SNIFTIRS; Pyridine; Au(111) electrode–solution interface
1. Introduction Pyridine is an ideal model compound for investigating surface coordination and orientation of molecules on surfaces and interfaces. It has a rigid ring-like structure closely related to benzene, with a large dipole moment (2 D) located along its C26 axis. Interactions of the pyridine molecule with the metal can involve both p and N lone-pair orbitals [1–6]. As a result, the pyridine molecule can assume both p (flat) and N-bonded (vertical, tilted and/or rotated) orientations at metal surfaces. Thermodynamics of pyridine adsorption at Au(hkl) has been thoroughly investigated in our laboratory with the * Corresponding author.
help of the chronocoulometric technique [7 – 13]. Pyridine adsorption at the Au(111) electrode surface displayed the most interesting behavior. For this plane of gold, the surface coverage data suggested that pyridine molecules undergo a potential controlled reorientation from a p-bonded state at negatively charged to a N-bonded state at a positively charged surface. The availability of precise surface coverage data stimulated a general interest in pyridine adsorption on the Au(111) single-crystal surface. Techniques, such as second harmonic generation [14], in situ difference frequency generation [15], scanning tunneling microscopy (STM) [16] and surface-enhanced infrared absorption spectroscopy (SEIRAS) [17], have been employed to investigate this system. These studies confirmed that the orientation of
0013-4686/99/$ - see front matter © 1999 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 9 9 ) 0 0 2 3 9 - X
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adsorbed pyridine molecules changes by moving from the negatively to the positively charged surface. In the present work, we have employed subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) [18,19] to investigate the adsorption of pyridine at the Au(111) electrode– solution interface. This technique has been previously used to study the orientation of several organic molecules adsorbed on metal surfaces, such as, benzonitrile [19,20] and 4-cyanopyridine [21,22] on Au(111), acetonitrile [23] and pyridine [24] on polycrystalline gold. The IR experiments described in the present paper, have been conducted using electrodes prepared and treated in the same way as that employed in our thermodynamic studies [11]. Consequently, we were able to compare our new spectroscopic measurements to the earlier thermodynamic data. In general, the results of IR studies confirm the previous interpretation of the affect of electrode potential on the orientation of pyridine molecules at the Au(111) electrode surface. However, we will demonstrate below that, at negative potentials, a fraction of the surface coverage, assigned earlier to the p-bonded state corresponds in fact to N-bonded pyridine adsorbed at defect sites.
ented with the help of the back Laue diffraction technique with the precision of 1°. The ratio of the step height to the terrace length on the image shown in Fig. 1 gives the miscut angle on the order of 0.8°
2. Experimental section
2.1. Reagents and electrodes The KClO4 (ACS Certified, Fisher) was purified as described elsewhere [25]. Pyridine (Silylation grade, Pierce Chemical Company) was used without further purification. All solutions were prepared from Deuterium Oxide, D2O (99.9%, Cambridge Isotope Laboratories). If not otherwise stated, all measurements were performed using a 0.003 M pyridine solution in D2O containing 0.05 M KClO4 as supporting electrolyte at 20 92°C. Most of the IR spectra were recorded in D2O as the solvent to avoid interference from the water bands in the spectral region of our interest. The working electrode was 1 cm2 singlecrystal rod oriented to expose either (111) or (210) plane. Both the Au(111) and Au(210) single-crystals, were grown, cut, and polished in our laboratory. The top layer perturbed by mechanical polishing was removed by electropolishing and the surface was smoothed by annealing [25]. The electrodes were flame annealed prior to each experiment. Fig. 1 shows the scanning tunneling microscopy image of the Au(111) surface recorded in air after electropolishing and flame annealing. The surface consists of relatively large (111) terraces separated by steps. The steps are produced by a miscut due to an error in the crystal orientation procedure. The crystals were ori-
Fig. 1. STM images of the flame annealed Au(111) surface in air; top panel image of 10 ×10 mm section of the surface showing surface topology after electrochemical polishing and flame annealing, middle panel 100 ×100 nm section showing long terraces smooth at the atomic level, bottom section, high resolution image of a section of the smooth terrace showing hexagonal packing of gold atoms on the surface.
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2.2. Apparatus
DR/R =(R(E2) −R(E1))/R(E1)
The SNIFTIRS experimental procedures and instrumentation were described in [19–21]. A syringe-type IR cell with a 60° CaF2 prism window was used in this study. The prism ensures an optimum angle of incidence for the IR reflection spectroscopy [26]. During the IR experiment, the working electrode was pushed against the flat part of the CaF2 window to form a thin layer. The thickness of the thin layer was estimated to be 6 mm. Its reproducibility was 10%; however, the uncertainty of the absolute value was at least 1–2 mm. A cylindrical platinum foil 3 cm2 was used as the counter electrode, and the reference electrode was Ag/ AgC1 (3 M KCl saturated with AgCl) electrode connected to the cell through a salt bridge. For the sake of comparison with previous electrochemical results, all potentials measured with respect to the Ag/AgCl electrode were converted to the saturated calomel electrode (SCE) scale. The electrolyte solution was deaerated by purging with pure argon for about 20 min before the measurements and argon was allowed to flow over the solution at all times during the experiment. A transmission spectrum was determined by squeezing the investigated solution between two flat ZnSe2 windows. The spectra were measured using a Nicolet 20SX/C FTIR apparatus equipped with an MCT-B detector cooled by liquid nitrogen with the resolution 4 cm − 1. The sample compartment of the FTIR apparatus was purged throughout the experiment using CO2- and H2O-free air provided by the Puregas Heatless Dryer. The electrode potential was controlled by a PAR 173 potentiostat.
where R(E1) and R(E2) are the electrode reflectivities at the reference and sample potentials E1 and E2, respectively. The reference potential was always equal to − 0.75 V (SCE) where pyridine molecules are totally desorbed from the electrode surface. Under these conditions, the measured changes of the reflectivity (DR/R) represent the difference between the absorption spectrum of G pyridine molecules that are desorbed from the surface and reside in the solution at potential E1 and the spectrum of G molecules adsorbed at the electrode surface at potential E2. The reflectivity is defined as R = IR/IO. In general the attenuation of light by reflection depends on absorption of radiation both by the metal which is weakly dependent on potential and by molecules present in the thin layer cell that depends strongly on potential. For a nonpolarized light, applying Beer’s law IR/IO = exp(− 2.3oG− const.)
(1)
(2)
when (DR/R) 1 the measured relative change of the electrode reflectivity can be expressed as: (DR/R)= 2.3G[o(E1) −o(E2)]
(3)
where o is the molar absorption coefficient and G is the surface concentration of the adsorbed species. According to Eq. (3) the changes in reflectivity can be expressed in absorbance units. The integrated band intensities were used for quantitative analysis. In that case:
&
&
(DR/R) dn= 2.3G [o(E1) −o(E2)] dn
(4)
2.3. Spectroscopic procedure The SNIFTIR spectra were determined using a multiple potential step (MPS) procedure in which the electrode potential was stepped m times between the reference and the sample potentials, E1 and E2 respectively. During each step, n interferograms were acquired at both potentials. Data acquisition was delayed for 10 s after each potential change to allow the interface to reach thermodynamic equilibrium at the imposed polarization. The change of the electrode potential was synchronized with the acquisition of the interferograms by connecting the external trigger port of the PAR 173 potentiostat to the communication port of the DX 486 computer of the FTIR instrument. This procedure was repeated m times until a total number of N=n×m interferograms was acquired for each of the two potentials. Typical values of n and m employed in this study were 100 and 20, respectively. The interferograms were added, Fourier transformed, and used to calculate a relative change of the electrode reflectivity, which is defined as:
For a linearly polarized light and a molecule adsorbed or in front of a reflecting metal surface the integrated absorption coefficient is proportional to the square of the dot product of the transition dipole m and the electric field of the photon E [27,28]:
&
o dn8 m·E 2 8 cos2 u m 2 B E 2 \
(5)
where u is the angle between directions of the electric field of the photon and the transition dipole in the molecule, m is the absolute value of the transition dipole moment and B E 2 \ is the mean square electric field of the photon. At the potential of total desorption E1, pyridine molecules are randomly oriented inside the thin layer of the electrolyte. In that case, the angle u has to be averaged over all possible orientations and the result is B cos2 u \ =1/3 [27]. Consequently, at E1 the integrated absorption coefficient is given by:
&
o(E1) dn8 (1/3) m 2 B E 2(z" 0\
(6)
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for both s and p-polarized radiation. Using s-polarized light, the electric field of the photon at the surface is nearly equal to zero [28,29] and hence o(E2) is close to zero as well. Eqs. (3) and (4) then, simplify to: (DR/R)s =2.3Go(E1)/3
(7)
&
(8)
and
&
(DR/R)s dn=2.3G o(E1) dn/3
In that case, the absorption band is positive and the band intensity is proportional to the product of the surface concentration of pyridine adsorbed at potential E2 and the integrated molar absorption coefficient of pyridine in the bulk of the thin layer cavity. When p-polarized radiation is employed both the adsorbed and solution species are IR active. In that case the (DR/R)p spectrum can display bipolar features, when band frequencies for the surface and solution species are different. The positive bands on this spectrum correspond to the absorption by molecules desorbed into the thin layer cavity at E1 and the negative bands to the absorption of the IR radiation by molecules adsorbed at the electrode surface at potential E2. In addition, the mean square field of the photon in the thin layer cell has a different value than in the incident beam. Consequently, in Eqs. (4)–(8), the molar absorption coefficients have a different value than the one measured in transmission.
3. Results and discussion
3.1. Re6iew of electrochemical data To facilitate interpretation of the spectroscopic data, we will briefly review results of earlier electrochemical studies of pyridine adsorption at the Au(111) electrode from 0.01 M KClO4 and 0.1 M KClO4 +0.003 M pyridine aqueous solution [11]. Fig. 2 shows how the differential capacity, charge density and the surface concentration of pyridine change as a function of the electrode potential. Differential capacity and charge density curves for the pyridine solution merge with the curves for the supporting electrolyte and the surface concentration drops to zero at EB −0.7 V. Pyridine is apparently desorbed from the Au(111) electrode surface at these negative potentials and is adsorbed when E \ − 0.7 V. The surface concentration plot displays two steps. The first, with the limiting concentration 1.6× 10 − 10 mol cm − 2 is seen between −0.7 V and 0 V. It is observed at a negatively charged surface and was assigned previously to the flat p-bonded state of adsorbed pyridine. This conclusion was confirmed recently by Cai et al. [17] who showed STM images of p-bonded pyridine in this region. The second step is seen at E \0
Fig. 2. (a) Differential capacity curves recorded at the sweep rate of 5 mV s − 1 and an ac modulation frequency of 25 Hz for the Au(111) electrode in the presence of 0.01 M KClO4 (dotted line) and 0.1 M KClO4 +0.003 M pyridine aqueous solution (solid line), (b) charge density plots for 0.1 M KClO4 (dotted line) and 0.1 M KClO4 +0.003 M pyridine aqueous solution (solid line), and (c) an adsorption isotherm (G vs. E plot) recorded for the Au(111) electrode in 0.1 M KClO4 + 0.003 M pyridine aqueous solution.
V where the gold surface is positively charged. It was assigned to the N-bonded state of adsorbed pyridine. The differential capacity in this region is quite low and this feature suggests that pyridine molecules form a condensed monolayer. At these potentials, STM images by Cai et al. [17] show that adsorbed pyridine molecules form chain-like aggregates similar in appearance to rolls of coins. The data in Fig. 2 suggest that the transition from the p-bonded to the N-bonded state occurs around the potential of zero charge of the Au(111) electrode, measured in the presence of pyridine in the solution.
3.2. Description of IR spectra We will now employ FTIR spectroscopy to study the potential induced changes of the coordination of pyridine to the Au(111) surface and to verify conclusions of the electrochemical experiments. Fig. 3 shows: transmission spectra of neat pyridine and 0.4 M pyridine solution in D2O and two SNIFTIRS spectra
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recorded using either s- or p-polarized light, for pyridine adsorbed at E2 =0.362 V. An isolated pyridine molecule has a C26 symmetry. In the spectral range used in our experiments, the transmission spectrum of neat pyridine displays three bands. Two bands at 1582 and 1483 cm − 1 have a1 symmetry and the third band at 1439 cm − 1 has b1 symmetry [30]. They all are assigned to the in-plane ring deformations [31]. The a1 bands correspond to changes of the dipole moment along the C26 axis of the molecule and the b1 band is due to the change of the dipole moment in the direction perpendicular to the C26 axis. The transmission spectrum of pyridine in D2O has only two bands. The weak a1 band at 1483 cm − 1 is not visible in this spectrum. The two other bands appear at 1594 and 1445 cm − 1, respectively. They are blue shifted with respect to the bands in the spectrum of neat pyridine. This shift is chiefly due to the effect of the solvent. As explained earlier, the SNIFTIRS spectra were recorded using the reference potential E = −0.75 V at which pyridine molecules are desorbed from the electrode surface. Consequently, they represent a difference of the absorption spectrum for pyridine molecules in solution and the spectrum of molecules adsorbed at the Au(111) surface at potential E2. For s-polarized light the molecules adsorbed at the electrode surface are optically inactive and then SNIFTIRS measures the
Fig. 3. Comparison of the transmission and SNIFTIRS spectra for pyridine. 1 and 2 display the transmission spectrum for neat pyridine, and for a 0.4 M pyridine solution in D2O, respectively. 3 and 4 show SNIFTIRS spectrum obtained from s- and p-polarized light for a 0.003 M pyridine + 0.05 M KClO4 solution acquired using E1 = − 0.75 and E2 = 0.262 V.
615
Fig. 4. Comparison of the potential dependence of integrated absorptivity of the b1 band at 1445 cm − 1 and the surface concentration of pyridine determined from electrochemical data. The integrated absorptivities were multiplied by a constant factor to give the best fit to the surface concentration data.
spectrum of pyridine molecules in solution, inside the cavity of the thin layer cell. Indeed, in Fig. 3, the SNIFTIRS spectrum measured with s-polarized radiation is a mirror image of the transmission spectrum for the pyridine solution in D2O. According to Eq. (7), the integrated intensity of these bands should be proportional to the surface concentration of adsorbed pyridine. In Fig. 4 the potential dependence of the integrated intensities of the tallest band at 1445 cm − 1 is compared to the change of the surface concentration of pyridine determined by chronocoulometry. At negative potentials, the integrated intensities are somewhat lower than the surface concentrations. However, as predicted, the agreement between the two sets of data is good. For p-polarized light the shape of the spectrum for adsorbed molecules can be predicted from the surface selection rules. For a molecule adsorbed at a metal, surface selection rules state that the vibration will be allowed if a change in the dipole moment has a nonzero component in the direction normal to the surface [28]. When pyridine molecule assumes a flat, p-bonded coordination, the change of the dipole moment in the direction normal to the surface has a zero component for both a1 and b1 modes. In the spectral region shown
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in Fig. 3, the p-bonded molecule should be optically inactive. For this surface orientation, the SNIFTIRS should plot only the spectrum of molecules desorbed into the solution at the reference potential, which should resemble the spectrum recorded for s-polarized photons. For the N-bonded pyridine molecule, the change of the dipole in the direction normal to the surface is very strong for a1 modes while for b1 modes the change of the dipole moment is either small or zero. In addition the field strength of the p-polarized photon is significantly enhanced at the metal surface [28,29]. Consequently, the a1 bands for N-bonded pyridine are significantly stronger than the corresponding bands for the molecule desorbed into the solution at the reference potential. In the SNIFTIRS spectrum the b1 mode should appear as a positive band of the solution species while a1 bands should be either bipolar or negative. The a1 bands would be bipolar if the center of the absorption band for the N-bonded molecule is shifted with respect to the band of desorbed pyridine. When the band shift is insignificant only the negative features could be observed. In that case, the band for solution species is much weaker than the band for adsorbed molecule and when the two bands are subtracted the band for adsorbed molecules dominates the spectrum. In Fig. 3 the SNIFTIRS spectrum for p-polarized light illustrates that behavior. Consistently with the Nbonded state of adsorbed pyridine the b1 mode is positive while the two a1 modes are negative. A noteworthy feature in this spectrum is the appearance of the a1 band at 1475 cm − 1 which was absent in either the transmission or SNIFTIRS spectra for molecules in D2O solution. The presence of this band may be due to a combined effect of the (weak) surface enhancement and the solvent effect. At positive potentials the adsorbed pyridine forms a condensed monolayer and hence its molecule resides in an environment different from that in D2O solution.
1400 cm − 1, respectively. The b1 band at 1445 cm − 1 is positive for all potentials. The small a1 band at 1475 cm − 1 is absent when E B 0 V and appears as a negative feature at E\ 0 V. This band is expected to appear at potentials where pyridine is N-bonded and forms a condensed monolayer. Likewise, the a1 band at 1594 cm − 1 appears as a negative feature when E\0. However, in contrast to expectations, the spectra recorded for EB0 V show a new band at 1599 cm − 1. It is predominantly negative although it displays some positive features indicating that it has a weak bipolar character. The positive lob of this band is centered at 1594 cm − 1; however, it is impossible to tell whether it represents the expected positive band of p-bonded pyridine or a positive lob of the new band at 1599 cm − 1. At the reference potential E1, both the molecules that at the sample potential E2 absorbed IR radiation at 1599 cm − 1 and the molecules that were p-bonded and optically inactive are desorbed and produce a positive band at 1594 cm − 1. The amplitude of the new band initially increases with potential and then it attains a limiting value at E: − 138 V. The presence of the negative band at 1599 cm − 1 for EB0 V is inconsistent with the suggested p-bonded coordination of pyridine molecules at the negatively charged surface. Its origin has to be carefully examined. We note at this point that the band at 1599 cm − 1 was not observed at
3.3. Surface coordination of pyridine 3.3.1. Qualitati6e analysis At this point we will look at the shape of the subtractively normalized IR spectra, acquired with ppolarized IR light at EB0 V. The electrochemical data shown in Fig. 2 suggest that pyridine is p-bonded at these potentials. If the SNIFTIRS spectra are to be consistent with that interpretation of the electrochemical results the spectra for p- and s-polarized light should be similar and should consist of two positive bands at 1445 and 1594 cm − 1. Fig. 5 shows a series of spectra obtained by changing the sample potential (E2) from − 0.338 to +0.362 V (SCE). For the benefit of further discussion the spectra are divided into two regions, corresponding to 1650–1550 cm − 1 and 1500–
Fig. 5. A series of SNIFTIR spectra recorded for pyridine adsorbed at the Au(111) electrode acquired using p-polarized light. The solution of D2O contains a concentration of 0.003 M pyridine+ 0.05 M KClO4. For each spectrum, the reference potential E1 = −0.75 V and the value of E2 is indicated in the figure. All potentials are referred to the SCE scale.
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negative potentials, in recent (SEIRAS) studies of pyridine adsorption at the Au(111) surface [17]. Perhaps, this may be due to the differently prepared electrode (a vapor deposited gold on silicon was used in Ref. [17]). It is also possible that the pyridine band was hidden in the stronger band of water, since water was used as the solvent in Ref. [17]. Pyridine adsorption at gold is known to be strongly dependent on the surface crystallography [1,12,13]. The images of the electrode surface shown in Fig. 1 indicate that, in addition to (111) terraces, the surface contains many defect sites. Adsorption of pyridine is known to be stronger at the high index surfaces and hence also at defect sites than on the (111) terraces [1,12,13]. Therefore, it is quite plausible that the first step on the G versus E plot in Fig. 2 corresponds to pyridine adsorption on the defect sites rather than to the p-bonded pyridine at the (111) terraces. To test this hypothesis we have performed additional SNIFTIRS experiments at the Au(210) electrode. The Au(210) surface is the most open single-crystal plane with the highest number of nearest-neighbor broken bonds per square of the lattice parameter [32]. Therefore, adsorption of pyridine at this crystallographic orientation should best mimic its coordination onto the defect sites present at the Au(111) surface. Fig. 6a compares the SNIFTIRS bands for the Au(111) and Au(210) surfaces in the spectral region 1620–1575 cm − 1. For the Au(210) surface only one negative band at 1599 cm − 1 was observed in this region, at all sample potentials investigated. Fig. 6a shows that the position of the a1 band at the Au(210) surface corresponds well to the negative band observed at the Au(111) surface at EB0 V. It also explains well the presence of the small a1 band at 1599 cm − 1 for the Au(111) surface at E\0 V. This result strongly suggests that at the Au(111) surface the small negative band at 1599 cm − 1 corresponds to pyridine N-bonded to defect sites while the band at 1593 cm − 1 corresponds to the N-bonded pyridine at the (111) terraces. In Fig. 6b, for Au(111) and Au(210) surfaces, the frequencies of the 1599 cm − 1 band are plotted against the rational potential. The data for two surfaces fit one linear relationship. They show that the a1 band frequency depends weakly on the electrode potential. The presence of pyridine adsorbed at defect sites may also explain the shape of the a1 band at 1475 cm − 1. Fig. 7 compares spectra for the Au(111) and Au(210) surfaces in the region centered around 1480 cm − 1. The band for the Au(111) electrode is asymmetric and has a shoulder at the high frequency side. The position of this shoulder corresponds quite well to position of the a1 band observed for the Au(210) surface. The shoulder may therefore, be assigned to pyridine adsorbed at defect sites. In conclusion, the qualitative analysis of SNIFTIRS spectra provides no evidence that pyridine
617
Fig. 6. (a) Selective SNIFTIRS spectra of pyridine adsorbed at Au(111) and Au(210) electrode surfaces acquired using p-polarized light. The dotted line is the spectrum of Au(210) at −100 mV; the solid lines are the spectra of Au(111) at − 138 (top) and 2 mV (bottom), respectively. For each spectrum, the reference potential E1 was equal to −0.75 V. (b) A plot of the shift in frequency of the 1601 cm − 1 band vs. potential with respect to the potential of zero charge (pzc) with adsorbed pyridine. Open circles and solid lines for Au(111) and closed squares and dotted lines for Au(210).
is p-bonded to the (111) terraces at EB 0 V and suggests that at least a fraction of the first step on the G versus E plot corresponds to pyridine N-bonded to defect sites.
3.3.2. Quantitati6e analysis Further information concerning the orientation of adsorbed molecules could be obtained from the analysis of the integrated band intensities. For that purpose, it is convenient to calculate the ratio of the integrated intensities measured for p- and s-polarized radiation. Using Eqs. (4) – (8), the ratio of the integrated intensities may be expressed as:
&
(DR/R)p E 2p(z) E 2p(z= 0) dn = 2 − 3 cos2 u E s (z) E 2s (z) (DR/R)s
(9)
where; BE 2p,s(z) \ is the time and z averaged mean square field within the thin layer cell and BE 2p(z= 0) \ is the time averaged mean square field at the
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Fig. 7. Comparison of the spectrum acquired using p-polarized light in the region 1525 –1455 cm − 1 for Au(111) at 112 mV and Au(210) at −100 mV. The crystallographic orientation of gold is indicated in the figure.
electrode surface, z varies between 0 and d, where d is the thickness of the thin layer. The angle u in Eq. (9), is the angle between the direction of the transition dipole for the adsorbed molecule and the direction of the field of the p-polarized photon on the surface. The p-polarized photon on the surface has a predominant component in the z-direction (direction normal to the surface). Consequently, to a good approximation the angle u is a measure of the tilt angle for the adsorbed molecule (the tilt angle is 90° − u). Fig. 8 shows how, for the a1 band at 1600 cm − 1 and the b1 band at 1445 cm − 1, the ratio of the inte-
Fig. 8. Ratio of the integrated absorptivities for the SNIFTIRS bands acquired using p- and s-polarized radiation. Curve 1, b1 band at 1445 cm − 1; curve 2, a1 band in the 1600 cm − 1 spectral region.
grated band intensities depends on the electrode potential. The integrated intensities of the b1 band change from a value 1.32 at negative potentials to a value 1.02 at positive potentials. High positive values of the ratio indicate that the component of the transition dipole in the direction of the field of the photon is small and that the dipole is almost parallel to the surface. However, the small change of this ratio suggests that the direction of the transition dipole changes somewhat with potential. According to Eq. (9), the smaller value of the ratio at positive potentials indicates that the angle between the b1 axis of the molecule and the normal to the electrode surface is smaller. This behavior suggests that by moving from negative to positive potentials the tilt angle between the b1 axis and the electrode surface increases. For the a1 band, the absolute value of the ratio of integrated band intensities is quite large and its sign is negative. This behavior indicates that, for this band, the component of the transition dipole in the direction of the photon is quite large and suggests that pyridine molecules assume a tilted, N-bonded orientation. The absolute value of the ratio increases markedly by moving from negative to positive potentials indicating that the tilt angle increases when potential crosses the zero value. This result is consistent with thermodynamic studies. It suggests that in addition to the Nbonded pyridine at defect sites there are also molecules that are p-bonded to the (111) terraces. In order to determine the values of the tilt angle, the average fields of the photon have to be calculated. The fields BE 2p,s(z) \ and BE 2p(z= 0)\ may be calculated from the model of light propagation through a three-layer system consisting of a semi-infinite phase of CaF2, thin layer of electrolyte and semi-infinite metal phase. For this model, explicit formulas for computing the optical fields within the thin layer cell were derived by Seki et al. [33]. Figs. 9a and b show the changes of the mean square fields as a function of the distance from the electrode surface, calculated using optical constants given in Table 1. Due to the interference within the cavity of the thin layer cell, the fields vary as a function of the distance in an oscillatory manner. This is an unwanted behavior, since the strong dependence of the fields on the distance requires a precise knowledge of the thickness of the thin layer cavity in order to calculate the tilt angle for the adsorbed molecules. Unfortunately, only the approximate value of the thickness is known. The data shown in Fig. 9 were subsequently used to calculate the ratios B E 2p \ / B E 2s \ and B E 2p(z= 0)\ /B E 2s \ for frequencies corresponding to a1 and b1 bands. The result is plotted in Figs. 10a and b. The photon fields BE 2p \ and B E 2s \ plotted in this figure were averaged over the distance from the electrode surface. We can now make an attempt to use Fig. 10 to calculate the tilt angle. We will look first at the b1 band.
M. Hoon-Khosla et al. / Electrochimica Acta 45 (1999) 611–621
Fig. 9. Time averaged mean squared electric fields of the photon plotted as a function of a distance from the electrode surface. (a) for 1445 cm − 1 and (b) for 1600 cm − 1.
For this mode and negative potentials, we may assume that the second term in Eq. (9) is negligible and that the ratio of the integrated intensities (equal to 1.3) is essentially equal to BE 2p \/BE 2s \. This is equivalent to the assumption that the transition dipole of this band is parallel to the electrode surface. In Fig. 10, the values of BE 2p \/B E 2s \, that are close to 1.3, are observed for the thin layer thickness equal to either 3 or 8.5 mm. The first value of the thickness is too low. The Table 1 Optical constants used to calculate the average field strength of the photon in the thin layer cell a n (cm−1)
Complex refractive index CaF2
1600 1445
D2O
Au
1.38137+0i
1.32213
3.06746
1.3707+0i
+ 0.0108363i 1.31653
+ 45.37i 3.71042
+ 0.00862582i
+49.9349i
a Optical constants for CaF2, Refs. [34,35], optical constants for D2O, Ref. [36] and optical constants for Au, Ref. [37].
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Fig. 10. (a) Ratio of the time and distance averaged mean squared electric fields for the s- and p-polarized photons; (b) ratio of the time averaged mean squared field of the p-polarized photon at the electrode surface to the mean square field of the s-polarized photon averaged over the thickness of the thin layer cell, plotted as a function of the thickness of the thin layer cavity.
second value is closer to our estimate of the thickness of the gap between the electrode surface and the CaF2 window. We will therefore assume that 8.5 mm represents the thickness of our thin layer cavity. For that thickness, the decrease of the ratio of integrated intensity to a value of approximately 1, observed at positive potentials, corresponds to an increase of the angle between the direction of the transition dipole of the b1 band and the electrode surface to about 13°. This is quite a reasonable result. At that point we may calculate the tilt angle from the ratio of integrated intensities for the a1 band at 1600 cm − 1. For EB 0 V, this ratio is equal to −0.2 and for E\ 0 it is approximately equal to − 1.0. Using Eq. (9) and the values of BE 2p \ /B E 2s \ and B E 2p(z= 0)\ / B E 2s \ shown in Fig. 10, we find that the tilt angle between the C26 axis of the pyridine molecule and the electrode surface is equal to 30° for negative potentials and to about 40° for positive potentials. The first number should be seen as an average over the tilt angle
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of N-bonded pyridine at defect sites and the tilt angle of p-bonded pyridine at the (111) terraces. The latter number seems to be too low. Recent STM images by Cai et al. [17] indicate that, at positive potentials, pyridine molecules form surface aggregates similar in appearance to a roll of coins. In such a stack of aromatic molecules the tilt angle is expected to be 60–70° [38]. The low value of the tilt angle points out to inadequacies of our model or experimental set up. We have made attempts to improve the model by introducing an additional layer corresponding to the film of adsorbed pyridine. However, the presence of this additional layer did not change the calculated photon fields. The weakest point of our experimental set up was a poor control of the thickness of the thin layer cavity. In our experiments a massive gold electrode was simply pressed against the CaF2 window. We had no possibility to adjust the thickness of the gap between the electrode surface and the IR window. The cell design described by Roe et al. [39] would be more suitable for quantitative data analysis described in the present paper. Such a cell will be employed in our future work.
4. Summary and conclusions We have employed SNIFTIRS to study coordination of pyridine to the Au(111) electrode surface. Previous thermodynamic studies of pyridine adsorption at this gold electrode suggested that pyridine assumes a pbonded (flat) coordination at negative potentials and a N-bonded (vertical or tilted) geometry at positive potentials [11]. In contrast to these predictions, we have observed that the character of the IR spectra recorded at negative potentials is dominated by the N-bonded pyridine. We have demonstrated that this is the Nbonded state of molecules adsorbed at defect sites rather than at the (111) terraces. The p-bonded pyridine is optically inactive, however in the SNIFTIRS spectrum, the presence of the p-bonded state could be seen in the form of positive bands corresponding to IR absorption by molecules desorbed into the solution at the reference potential In the presence of pyridine adsorbed at defect sites, we were unable to distinguish whether the positive bands in the spectra corresponded to pyridine adsorbed on the defect sites or the p-bonded molecule at the (111) terraces. We have used the ratio of the integrated band intensity for the spectra recorded with p- and s-polarized light to study changes in the surface coordination of pyridine, quantitatively. We have derived an expression for the dependence of the ratio of the integrated intensities on the tilt angle. We have described the orientation of the adsorbed molecule in terms of angles formed between the surface and; (i) C26 axis of the pyridine
molecule, (ii) the direction in plane of the molecule and normal to the C26 axis. We were able to demonstrate that both angles increase by moving from negative to positive potentials. This result indicates that the pyridine molecule not only stands up at positive potentials but also its plane rotates somewhat with respect to the electrode surface. Our result is in accord with recent STM and FTIR studies by Cai et al. [17] and particularly with the difference frequency generation experiments by Hebert et al. [15], that have demonstrated that the plane of the pyridine molecule is rotated. We have made attempt to determine numerical values of the two angles. For this purpose, we had to calculate the average field strength of the photon in the thin layer cavity. The three layer model of the thin layer cell, originally proposed by Seki et al. [33] was used in these calculations. The numerical values are reasonable, however the angle between the surface and the C26 axis of the pyridine molecule appears to be lower than the expected value. This may be due to the oversimplifications of the model. The model has to be tested on a larger set of experimental data in order to gain confidence in the numerical values that are calculated.
Acknowledgements We thank V. Zamlynmy for preparing the Au(210) single-crystal electrode. This work was supported by the grant from the Natural Sciences and Engineering Research Council of Canada.
References [1] J. Lipkowski, L. Stolberg, D.-F. Yang, B. Pettinger, S. Mirwald, F. Henglein, D.M. Kolb, Electrochim. Acta 39 (1994) 1057. [2] S. Haq, D.A. King, J. Phys. Chem. 100 (1998) 16957. [3] J.E. Demuth, K. Christmann, P.N. Sanda, Chem. Phys. Lett. 76 (1980) 202. [4] N.J. DiNardo, P.h. Avouris, J.E. Demuth, J. Chem. Phys. 84 (1984) 2169. [5] B.J. Bandy, D.R. Lloyd, N. Richardson, Surf. Sci. 89 (1979) 344. [6] M. Bader, J. Haase, K.H. Frank, A. Puschmann, A. Otto, Phys. Rev. Lett. 56 (1986) 1921. [7] L. Stolberg, J. Richer, J. Lipkowski, D.E. Irish, J. Electroanal. Chem. 207 (1986) 213. [8] L. Stolberg, J. Lipkowski, D.E. Irish, J. Electroanal. Chem. 238 (1987) 333. [9] L. Stolberg, J. Lipkowski, D.E. Irish, J. Electroanal. Chem. 296 (1990) 171. [10] L. Stolberg, J. Lipkowski, D.E. Irish, J. Electroanal. Chem. 300 (1991) 563. [11] L. Stolberg, S. Morin, J. Lipkowski, D.E. Irish, J. Electroanal. Chem. 307 (1991) 241.
M. Hoon-Khosla et al. / Electrochimica Acta 45 (1999) 611–621 [12] D.F. Yang, L. Stolberg, J. Lipkowski, J. Electroanal. Chem. 329 (1992) 259. [13] J. Lipkowski, L. Stolberg, in: J. Lipkowski, P.N. Ross (Eds.), Adsorption of Molecules at Metal Electrodes, VCH, New York, 1992. [14] B. Pettinger, S. Mirwald, J. Lipkowski, J. Electroanal. Chem. 329 (1992) 289. [15] P. Hebert, A. Le Reille, W.Q. Zheng, A. Tadjeddine, J. Electroanal. Chem. 447 (1998) 5. [16] G. Andreasen, M.E. Vela, R.C. Salvarezza, A.J. Arvia, Langmuir 13 (1997) 1317. [17] W.-B. Cai, L.-J. Wan, H. Noda, Y.-I. Hibino, K.-I. Ataka, M. Osawa, Langmuir 14 (1998) 6992. [18] S. Pons, J. Electroanal. Chem. 150 (1983) 495. [19] A. Bewick, S. Pons, in: R.J.H. Clark, R.E. Hester (Eds.), Advances in Infrared and Raman Spectroscopy, vol. 12, Heyden and Son, New York, 1985 (Chapter 1). [20] A. Chen, J. Richer, S.G. Roscoe, J. Lipkowski, Langmuir 13 (1997) 4747. [21] A. Chen, S.G. Sun, D.F. Yang, B. Pettinger, J. Lipkowski, Can. J. Chem. 74 (1996) 2321. [22] A. Chen, D.-F. Yang, J. Lipkowski, J. Electroanal. Chem., in print. [23] P.W. Faguy, W.R. Fawcett, G. Liu, A.J. Motheo, J. Electroanal. Chem. 339 (1992) 339. [24] Y. Ikezawa, T. Sawatari, T.K.H. Goto, K. Toriba, Electrochim. Acta 43 (1998) 3297.
.
621
[25] J. Richer, J. Lipkowski, J. Electrochem. Soc. 133 (1986) 121. [26] P.W. Faguy, W.R. Fawcett, Appl. Spectrosc. 44 (1990) 1309. [27] W. Liptay, Angew. Chem. 81 (1969) 195. [28] M. Moskovits, J. Chem. Phys. 77 (1982) 4408. [29] R.G. Greenler, J. Chem. Phys. 44 (1966) 310. [30] L. Corrsin, B.J. Fax, R.C. Lord, J. Chem. Phys. 21 (1953) 1170. [31] D.A. Long, E.L. Thomas, Trans. Faraday Soc. 59 (1963) 783. [32] A. Hamelin, in: B.E. Conway, R.E. White, J.O’M. Bockris (Eds.), Modern Aspects of Electrochemistry, vol. 16, Plenum, New York, 1985 (Chapter 1). [33] H. Seki, K. Kunimatsu, W.G. Golden, Appl. Spectrosc. 39 (1985) 437. [34] W.W. Coblentz, J. Opt. Soc. Am. 4 (1920) 441. [35] R.A. Smith, F.E. Jones, R.P. Chasmar, The Detection and Measurement of Infrared Radiation, Oxford University Press, London, 1927, p. 342. [36] J.E. Bertie, M.K. Ahmed, H.H. Eysel, J. Phys. Chem. 93 (1989) 2210. [37] P.B. Johnson, R.W. Christy, Phys. Rev. B 6 (1972) 4370. [38] T.h. Dreschkow, D. Lampner, T.h. Wandlowski, J. Electroanal. Chem. 458 (1998) 121. [39] D.K. Roe, J.K. Sass, D.S. Bethune, A.C. Luntz, J. Electroanal. Chem. 216 (1987) 293.