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Potential dependence of vibrational frequencies of adsorbates on a silver electrode
J. Electroanal. Chem., 123 (1981) 335--344 Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
335
POTENTIAL DEPENDENCE OF VIBRATIONAL FREQUENCIES OF ADSORBATES ON A SILVER ELECTRODE
R. KOTZ * and E. Y E A G E R
Case Laboratories for Electrochemical Studies, Case Western Reserve University, Cleveland, OH 44106 (U.S.A.) (Received 21st November 1980)
ABSTRACT The frequencies of several vibrational bands of pyridine, pyrazine, p-nitroso dimethylaniline and cyanide adsorbed on a silver electrode have been investigated as a function of the electrode potential using in situ Raman spectroscopy. The frequencies of all the bands investigated were found to decrease linearly with cathodic potential. This observation is independent of the adsorbate and the anions of the supporting electrolyte. Several models explaining this effect are discussed.
INTRODUCTION
The intriguing opportunities offered by the surface-enhanced Raman effect in surface science and electrochemistry have stimulated a variety of experimental and theoretical investigations in order to explain the giant enhancement observed for many adsorbates on a Ag substrate [1--3]. The spectacular enhancement factors of up to 10 6 have prompted much attention to be paid to changes in Raman intensities with variations of such factors as electrode potential and the type of anions in the supporting electrolyte. On the other hand, much less is known about the dependence of the vibrational frequencies of adsorbates showing surface enhancement on the electrochemical parameters. Frequency shifts of vibrational modes during adsorption and coverage changes can give important information about the substrate adsorbate interaction, the nature of the adsorption site, lateral interactions and structural rearrangement of the adsorbate species [4]. Frequency shifts due to adsorption and coverage changes have been calculated for halogen ions on a Ag electrode by Nichols and Hexter [5]. In the early investigations by Fleischmann et al. [6] and Jeanmaire and van Duyne [7], a few frequency values for pyridine modes at different potentials are given, but no systematic investigation was performed. The results for the potential dependence of pyridine modes obtained by Pettinger and Wenning [8] exhibited no common trend and were not discussed further. During time* Present address: Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4--6, 1000 Berlin 33, G.F.R.
336 dependence studies of surface-enhanced Raman spectra using an optical multichannel analyzer, Dornhaus et al. [9] observed a shift of 30 cm -1 for the C - N stretch mode in cyanide adsorbed on Ag for potentials more positive than - 0 . 0 5 V vs. SCE. This effect, however, is accompanied by a faradaic reaction, as indicated by the voltammetry curve. In those potential regions where only double-layer charging occurs, no shifts were detected, probably due to the relatively low resolution of the system. The first systematic investigation of potential-dependent vibrational frequency shifts in surface-enhanced Raman spectroscopy has been performed b y S. Venkatesan et al. [10] for the Ag--N stretching modes of several nitrogen-containing c o m p o u n d s adsorbed on Ag electrodes. They found a c o m m o n shift of the Ag--N frequency to lower values with more negative potentials. This behavior was explained as a consequence of the discreteness of charge of the specifically adsorbed anions, which is a function of the electrode potential [10]. Investigating the potential dependence of the low-lying pyridine m o d e (~220 c m - ' ) in different electrolytes Dornhaus and Chang [11 ], however, demonstrated that the assignment of this mode to the Ag--halogen ion stretching vibration is more likely. These authors again found a frequency decrease of this mode with more negative electrode potential. The objective of the present work has been the systematic investigation of the potential dependence of several internal Raman modes for adsorbates involving CN bonds on silver. The choices of the adsorbate bands and potential ranges to be examined were guided b y the need for good signal-to-noise ratios, welldefined sharp peaks and freedom from faradaic reactions. The adsorbates involved in this study were pyridine, pyrazine, p-nitrosodimethyl aniline and cyanide. EXPERIMENTAL The Raman spectra were obtained with a Spex spectrometer (Ramalog) in combination with an argon ion laser (Coherent Radiation CR-8). The spectral resolution of the m o n o c h r o m a t o r was set to 2.5 cm -1 and the integration time of the photocounting system was typically 1 s. The polycrystalline Ag disk, serving as the working electrode, was illuminated b y the laser beam at a mean angle of incidence of 60 ° , and the typical power at the sample was 100 mW at 514.5 nm. A detailed description of the optical alignment and the electrochemical cell for in situ Raman measurements has been given previously [12]. Before each measurement, the silver electrode of 0.2 cm 2 surface area was polished with Al203 (1 pm) and rinsed carefully in distilled water. The potential of the working electrode was controlled using the standard three-electrode arrangement, a PARC 173 potentiostat and a PARC 175 waveform generator. A platinum plate served as a counter electrode. All potentials are quoted with respect to either the saturated calomel reference electrode or the Hg/HgO reference electrode which was used in the NaOH electrolytes. The electrolytes 1.0 and 0.1 M NaOH, 0.05 M Na2SO4 and 0.1 M KC1 were prepared from reagent grade chemicals and triply distilled water. The adsorbate was added to the electrolyte in the following concentrations: 0.02 M pyridine (Aldrich Chemical), 0.05 M pyrazine (Aldrich Chemical), 10 -4 M p-NDNA
337 (Eastman Kodak), and 0.01 M KCN (Matheson Coleman, reagent grade). The electrolyte was deaerated by bubbling N2 through the solution. RESULTS
Pyridine The potential dependence of three of the most pronounced vibrational bands in the Raman spectrum of pyridine adsorbed on a Ag electrode corresponding to ring modes is shown in Fig. 1. Changing the electrode potential to more negative values causes the frequencies of all three bands to decrease. Within the experimental reproducibility the frequencies shift linearly with electrode potential with the possible exception of the 1032--1036 cm -~ band which in chloride solution showed some tendency to bend off at more positive potentials. The slopes of the lines in Fig. 1 are equal for the three vibrations (6 cm -~
V-l).
In order to prove whether the specifically adsorbing C1- anion of the KC1 electrolyte influences the potential dependence of the vibration frequencies, the Raman spectrum of pyridine adsorbed on Ag was investigated in 1 M NaOH. The potential dependence of three ring frequencies is essentially the same in both 1 M NaOH and 0.1 M KC1 (see Fig. 1). Although the enhancement factor in 1 M NaOH is about 50 times smaller than for pyridine in 0.1 M KC1, the peak frequencies could still be reliably determined. The potential range in which the Raman signal of pyridine on Ag can be studied is extended by 0.2 V to more negative values with 1 M NaOH as the electrolyte instead of KC1. The results presented by Fleischmann [6] and van Duyne [7] for a few potential values are in good agreement with the data in Fig. 1, whereas the potential dependence measured by Pettinger and Wenning [8] does not match with our curves. The potential dependence of the Raman intensity for the symmetric ring breathing mode at 1005 cm -~ is plotted in Fig. 2 in 1 M NaOH. The intensity vs. potential plot exhibits a symmetric maximum around --0.75 V vs. Hg/HgO. In the KC1 electrolyte the corresponding plot is asymmetric, most likely due to the specifically adsorbed chloride ions, as has been pointed out by van Duyne [1,7]. The change in anion, however, does not affect the potential dependence of the vibrational frequencies significantly, as is evident in Fig. 1.
Pyrazine Dornhaus et al. [9] have shown recently that the surface-enhanced Raman effect for pyrazine on Ag in KC1 electrolyte is almost as strong as that for pyridine. In order to find out whether the adsorption mode of the molecule has some influence on the potential dependence of the vibration modes, the symmetric ring breathing mode of pyridine at 1008 cm -1 should be compared to the equivalent mode in pyrazine at 1018 cm -1. In contrast to pyridine, which is believed to adsorb end-on [ 1 ] via the nitrogen atom, pyrazine has a second nitrogen atom in the para position and is believed to adsorb with its plane parallel to the surface [13]. Figure 3 demonstrates that the potential depen-
338 P O T E N T I A L / V vs HG/HGO -[6 -I.2 -O,S -0.4 0.0 ¸1216
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dences of the vibration frequency for the symmetric ring breathing mode in pyridine and in pyrazine are similar. The frequency for the pyrazine mode decreases from 1020 to 1012 cm-' if the potential is changed from +0.2 to --1.1 V vs. SCE or 7 cm -1 V -1 as compared with 6 cm -~ V -~ for pyridine. Again, the anion in the electrolyte (CI-, SO~-) has no significant influence on the frequency shift.
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339 Pyrazine adsorbed on Ag exhibits a rather strong low-lying Raman m o d e at 236 cm -~ which has been attributed to the Ag--N stretching m o d e [10]. The potential dependence of this m o d e in KC1 electrolyte is also shown in Fig. 3 and it has a slope of 26 cm -~ V -1. Similar results have been reported recently for the Ag--N vibrational modes of several N-containing organic molecules on Ag by Venkatesan et al. [10]. These authors also found a much stronger potential dependence for the external Ag--N modes compared to the internal modes. The Ag--N m o d e for pyrazine was also observed in the Na2SO4 electrolyte in the present work, b u t it was t o o weak for reliable frequency measurements.
p-Nitrosodimethyl aniline (p-NDMA ) Brazdil and Yeager [14] have shown that p-NDMA can be used as a probe molecule in Raman spectroscopy at the solid vacuum interface to probe the Lewis acid-base nature of the adsorption sites on insulators and semiconductors. This molecule exhibits a strong internal resonance-enhanced Raman signal and can be examined as an adsorbate without the need for large surface enhancement. The phenylnitroso stretch mode, at ~ 1 1 6 5 cm -1, shifts to higher wave numbers u p o n polarization of the molecule on an acid site. In addition, the N--O stretch vibration, at ~ 1 3 3 0 cm -1, shifts to lower frequencies if the molecule is polarized. These effects are caused b y a shift in electron density away from the ring and the amino group. Like that of pyridine the Raman spectrum of p-NDMA should yield information a b o u t the nature of the adsorption site. Hagen et al. [15] demonstrated that p-NDMA shows a surface-enhanced Raman effect on Ag, and they were able to detect the Raman spectrum of p-NDMA even on a Pt electrode because of the large intrinsic resonance enhancement. However, they did n o t investigate the potential dependence of the vibrational frequencies. In Fig. 4 the frequencies of the phenyl--nitroso stretch, the N--O stretch and the phenylamino stretch modes for the p-NDMA molecule adsorbed on a Ag electrode are plotted as functions of t h e electrode potential in 0.05 M Na2SO4. All bands exhibit decreasing frequencies with more negative potentials, although the slopes are different. For the phenyl-nitroso and the N--O mode the observed slope is 12 cm -1 V -1, whereas the phenylamino mode shifts with only 4 cm -~ V -~. The results obtained in different electrolytes such as 0.1 M NaOH (pH 13) and 0.05 M H~S.O4 (pH 2) do n o t deviate significantly from those obtained in 0.05 M Na2SO4 (pH 6) even though the intrinsic resonance enhancement in the solution phase species is greatly suppressed in the acid solution.
Cyanide The Raman spectra of cyanide adsorbed on an Ag surface have been investigated extensively at the metal--electrolyte interface as well as at the solid vacuum interface [16]. Although Dornhaus et al. [9] detected a shift of the C - N vibration m o d e with electrode potential immediately after completion of
340 POTENTIAL/V vs. HG/HGO -I.4 I
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Fig. 4. Potential dependence for the phenyl--amino stretch, the N--O stretch and the phenyl--nitroso stretch vibration (from top to b o t t o m ) for p-NDMA adsorbed on an Ag electrode in: (+) 0.05 M H2804; (©) 0.05 M Na2804; (o) 0.1 M NaOH; 100 mW at the sample. Fig. 5. Potential dependence for the C-=N stretch (top) and the Ag--C stretch vibration for cyanide adsorbed on an Ag electrode in 0.1 M NaOH (©) and 0.05 M Na2SO4 (o) +10 -2 M KC1; k0 = 514.5 nm, 50 mW at the sample.
the oxidation--reduction cycle, attention was paid to the potential dependence of the vibration frequencies only in the potential range where a faradaic reaction occurs. The vibration frequencies of the Ag--C and the CN modes at 226 and 2114 TABLE 1 Frequency shift AV with electrode potential for the investigated vibrational modes of some adsorbates on an Ag electrode Adsorbate
Assignment
v/cm-1 a
Pyridine
Ring (vl) Ring (v12) Ring (V9a)
1006 1035 1214
6 6 6
Pyrazine
Ring (vl)
1016
7
226
26
Ag--N stretch (Ag--C1)
Ap/cm-1 v ~ l
p-NDMA
Phenyl--nitroso stretch N--O stretch Phenyl--amino stretch
1160 1326 1367
12 12 4
Cyanide
C--N stretch Ag--C stretch
2112 224
26 26
a The data for v were obtained at --0.4 V vs. SCE with the exception of CN- where the potential was --0.8 V vs. SCE.
341 cm -1 respectively, are shown in Fig. 5 as a function of the electrode potential. The potential dependence of the CN m o d e has been investigated in 0.05 M Na2SO4 as well as in 0.1 M NaOH. The results show no significant influence of the different anions in both electrolytes. Between --1.4 and --0.7 V vs. SCE the frequencies of b o t h vibrations increase b y 26 cm -~ V -~. At more positive potentials the frequency of the CN vibration decreases again until it jumps to a b o u t 2145 cm -~ at --0.2 V. However, in this potential range ( - 0 . 7 to - 0 . 2 V) the current--potential curves (see for example ref. 16) show a large oxidation current, indicating a faradaic reaction involving a bulk species in the solution. The frequency shift of the CN m o d e to 2145 cm -1 at - 0 . 2 V is also reflected in the behavior of the Ag--C mode, the frequency of which shifts to ~ 2 5 0 cm -~ a t - - 0 . 1 V. Table 1 sumarizes the results for the four different adsorbates on a Ag electrode. The Raman spectra of these systems are given in the following references, for example: pyridine [1,6,7,9], pyrazine [ 9 ] , p - N D M A [14,15] and cyanide [16,17]. DISCUSSION AND CONCLUSIONS Pyridine is one of the most frequently used probe molecules for the study of the surface structure of oxides at the solid vacuum interface. The symmetric ring breathing mode which occurs at 991 cm -I for physisorbed pyridine shifts to 1000 cm -I if the molecule is hydrogen bonded, and is found at 1010 cm -I in the pyridinium ion (Br~nsted pyridine). For coordinately b o u n d pyridine this vibration m o d e has a frequency of ~ 1 0 2 5 cm -~ (Lewis pyridine) [18]. On the basis of the sensitivity of the symmetric ring breathing m o d e to the acidity of the adsorption site, Fleischmann et al. [6] concluded that the observed shift of this m o d e from 1008 cm -I at 0.0 V to 1005 cm=' at --1.0 V vs. SCE for pyridine adsorbed on an Ag electrode is due to a change in the acidity of the H atoms of the water molecules in the inner Helmholtz layer. However, the potential dependence of the trigonal ring breathing m o d e does not support this interpretation [6]. The shifting of the potential to negative values is equivalent to increasing the Lewis base properties of the metal surface sites. An alternative w a y of viewing this Situation is that the strong electric field at the surface produces, u p o n negative shifts of the potential, a polarization on the adsorbed molecule equivalent to that of a strong Lewis base site. Therefore, the above interpretation requires a shift of this band to higher frequencies when the electrode potential is changed from 0.0 to --1.0 V. However, the frequencies observed experimer~t~Uy shift to lower values (see Fig. 1). The results of our studies demonstrate that the shift of the internal vibrational frequencies to lower values with more negative electrode potential is a general trend in pyridine and other molecules showing surface enhancement on Ag. Thus, the potential dependence of the two ring breathing modes under discussion in the Fleischmann model is n o t contradictory. However, the assignment of these shifts to changes in the acidity of the adsorption site was premature. A similar situation is encountered with p-NDMA adsorbed on a Ag electrode. Polarization of the p-NDMA molecule owing to the adsorption on an acidic site decreases the double-bond character of the N=O bond and simultaneously
342 increases the bond order of the phenyl--nitroso frequency (see Fig. 2 in ref. 14). The shift of the phenyl--nitroso frequency from 1158 cm -1 at --0.6 V vs. SCE to 1169 cm -1 at +0.3 V could be rationalized in the same way as has been done by Fleischmann for pyridine. At more positive potentials the p-NDMA could bind through the oxygen of the nitroso group to a more acidic H atom of the adsorbed water than at cathodic potentials. Alternatively, the p-NDMA may be coordinated directly to the metal through the O atoms of the nitroso group. The change in polarization of the molecule produced by a shift towards positive electrode potentials should result in a decrease in the NO stretching frequency and an increase in the phenyl--nitroso frequency. This is contrary to observation;both increase experimentally. Thus, polarization of the adsorbed molecule at the Ag electrolyte interface due to a change in acidity of the adsorption sites or to the strong electric field at the interface appears n o t to be a proper explanation for the observed potential-dependent frequency shift. An additional interpretation of the frequency shifts with electrode potential for pyridine was given by van Duyne [ 1 ]. He suggested that the different coverages are responsible for the shift. At low coverage the next neighbor to a pyridine molecule will be a water molecule, whereas at monolayer coverage each pyridine molecule will be surrounded by other pyridine molecules. Studies of the frequency position of the symmetric and trigonal ring breathing m o d e in aqueous pyridine solutions with different concentrations show a shift of both lines to higher frequencies with dilution [1 ]. Therefore, one should expect lower vibrational frequencies at the potential of zero charge (pzc) where organic molecules are believed to be adsorbed strongly on the electrode surface, and higher frequencies at potentials anodic and cathodic to the pzc where the coverage is decreased owing to the replacement of the organic molecules by adsorbed water molecules. The intensity--potential curve for the Raman vibration of pyridine at 1006 cm jl in 1 M NaOH (Fig. 2) probably reflects the expected adsorption isotherm. The m a x i m u m occurs at potentials which appear reasonable in terms of published pzc data for Ag [19]. Within the scatter of the points, however, the experimental frequency results (Fig. 1) show no evidence for a minimum at a potential near the m a x i m u m in Fig. 2. This makes the interpretation of van Duyne questionable. For pyrazine, which is believed to be adsorbed with the plane of the molecule parallel to the electrode surface [ 13 ], the environment effect should be much less pronounced. Independent of the coverage, the molecule is always surrounded by the Ag surface on one side and by water molecules on the other. However, the potential dependence of the symmetric ring breathing m o d e of pyrazine (7 cm -~ V -~) is close to that of pyridine (6 cm -~ V-~). Thus, either the interpretation of the frequency shift due to changes in the environment is inadequate, or the assumption of a parallel adsorbed pyrazine molecule is wrong. The model recently suggested by Venkatesan et al. [10], involving the discreteness of charge of adsorbed anions yields the potential dependence of Ag--N vibration modes for some nitrogen-containing compounds adsorbed on Ag. From their t r e a t m e n t of the model these authors found a linear relation between the square o f the Ag--N frequency and the electrode potential. How-
343 ever, owing to the limited potential range and the relatively small frequency shift (~30 cm -1 V -1) a plot of the actual frequency vs. the electrode potential would also result in a linear relation. Thus, the potential dependence of the Ag--N m o d e observed for pyrazine (Fig. 3) is in agreement with the results obtained by Venkatesan et al. [10]. However, applying the model suggested for the external Ag--N modes to the internal vibrational mode of the adsorbed molecules leads to contradictions. If the potential dependence of the vibrational frequencies is due to the changes in the discreteness of the charge of the specifically adsorbed anions, the effect should disappear at the pzc and more negative potentials because the anions desorb (see eqn. 15 in ref. 10). The experimental results in Figs. 1 and 3, however, do n o t confirm this conclusion. The vibrational frequencies of the symmetric ring breathing m o d e of pyridine and pyrazine continue to shift to lower values even at potentials negative of the pzc. The above discussion has shown that existing models are insufficient in explaining the potential dependence of vibrational bands of adsorbates on an Ag electrode. For the Ag cyanide system, Anderson, together with the authors has examined a model explaining the potential dependence of the C - N stretching frequency [17]. A molecular orbital theory combining t w o - b o d y atomic repulsion and one-electron molecular orbital delocalization energies [20] is used to calculate the force constant. The Ag surface is simulated b y an Ag4 or Ag~ cluster with a CN- molecule adsorbed on a one-fold site. Charging of the electrode is introduced into the calculations b y moving the energies of the electronic levels of the Ag cluster up or down, relative to those in the CN adsorbed on the surface. The calculations show that negative charging of the electrode results in a lowering of the C - N force constant due to a decrease in ap donation. A detailed description of the model and the theoretical results for the Ag cyanide system will be subject of a future publication [17]. The question exists whether the same types of interactions m a y explain the potential dependence of the modes for other adsorbed species. This remains to be established, b u t m a y well be the situation. The experimental results presented in this study demonstrate that the decrease of the vibrational frequencies of adsorbed species on an Ag electrode is a c o m m o n feature for internal and external modes. Models based on the polarization of the adsorbate due to the different acidity of the adsorption site, on environmental effects due to the coverage change or on the influence of specifically adsorbed anions are n o t able to explain all of the observed experimental features. We propose as a possibility a decrease of the force constants of the vibrational m o d e of the adsorbed molecule, with negative charging of the electrode surface due to a shift in the electronic levels o f the electrode relative to those of the adsorbates causing a decrease in ap donation in the case of an adsorbate such as CN- adsorbed on Ag. ACKNOWLEDGEMENT Stimulating discussions with A.B. Anderson are gratefully acknowledged. This research has been supported b y the U.S. Department of Energy. One of us (R.K.) is grateful to the Deutsche Forschungsgemeinschaft for supplementary financial support.
344
REFERENCES
1 R.P. van D u y n e in C.B. Moore (Ed.), Chemical and Biochemical Applications of Lasers,Vol. 4, Academic Press,N e w York, 1978, Ch. 4. 2 T.E. Furtak and J. Reyes, Surf. Sci.,93 (1980) 351. 3 A. Otto, Proc. 6th Solid-Vacuum Interface Conference, Delft, The Netherlands, M a y 1980. 4 W.N. Delgass, G.L. Hailer, R. Kellermann and J.H. Lunsford (Eds.),in Spectroscopy in Heterogeneous Catalysis, Academic Press,N e w York, 1979, Chs. 2 and 3. 5 H. Nichols and R.M. Hexter, J. Chem. Phys., to be published. 6 M. Fleischmann, p.J. Hendra and A.J. McQuillan, Chem. Phys. Left., 26 (1974) 163. 7 D.L. Jeanmaire and R.P. van Duyne, J. Electroanal. Chem., 84 (1977) 1. 8 B. Pettinger and U. Wenning, Chem. Phys. Lett., 56 (1978) 253. 9 R. Dornhaus, M.B. Long, R.E. Benner and R.K. Chang, Surf. Sci.,93 (1980) 240. 10 S. Venkatesan, G. Erdheim, J.R. Lombardi and R.L. Birke, Surf. Sci.,to be published. 11 R. Dornhaus and R.K. Chang, Solid State C o m m u n . , 34 (1980) 811. 12 R. KStz and E. Yeager, J. Electroanal. Chem., 113 (1980) 113. 13 B.E. Conway, J.G. Mathieson and D.P. Dhar, J. Phys. Chem., 78 (1974) 1226. 14 J.F. Brazdil and E. Yeager, J. Phys. Chem., in press. 15 G. Hagen, B. Simic-Glavaski and E. Yeager, J. E1ectroanal.Chem., 88 (1978) 269. 16 J. Billmann, G. Kovacs a n d A. O t t o , Surf, Sci., 9 2 ( 1 9 8 0 ) 1 5 3 ; A. O t t o , Surf. Sci., 75 ( 1 9 7 8 ) L 3 9 2 . 17 A.B. A n d e r s o n , R. KStz a n d E. Yeager, t o be p u b l i s h e d . 18 P.J. H e n d r a , J.R. H o r d e r a n d E.J. L o a d e r , J. C h e m . Soc. (A), 1 9 7 1 , p. 1 7 6 6 . 19 S. Trasatti, J. E l e c t r o a n a l . Chem., 3 3 ( 1 9 7 1 ) 351. 2 0 A.B. A n d e r s o n , J. Am. C h e m . Soc., 1 0 0 ( 1 9 7 8 ) 1 1 5 3 .
335
POTENTIAL DEPENDENCE OF VIBRATIONAL FREQUENCIES OF ADSORBATES ON A SILVER ELECTRODE
R. KOTZ * and E. Y E A G E R
Case Laboratories for Electrochemical Studies, Case Western Reserve University, Cleveland, OH 44106 (U.S.A.) (Received 21st November 1980)
ABSTRACT The frequencies of several vibrational bands of pyridine, pyrazine, p-nitroso dimethylaniline and cyanide adsorbed on a silver electrode have been investigated as a function of the electrode potential using in situ Raman spectroscopy. The frequencies of all the bands investigated were found to decrease linearly with cathodic potential. This observation is independent of the adsorbate and the anions of the supporting electrolyte. Several models explaining this effect are discussed.
INTRODUCTION
The intriguing opportunities offered by the surface-enhanced Raman effect in surface science and electrochemistry have stimulated a variety of experimental and theoretical investigations in order to explain the giant enhancement observed for many adsorbates on a Ag substrate [1--3]. The spectacular enhancement factors of up to 10 6 have prompted much attention to be paid to changes in Raman intensities with variations of such factors as electrode potential and the type of anions in the supporting electrolyte. On the other hand, much less is known about the dependence of the vibrational frequencies of adsorbates showing surface enhancement on the electrochemical parameters. Frequency shifts of vibrational modes during adsorption and coverage changes can give important information about the substrate adsorbate interaction, the nature of the adsorption site, lateral interactions and structural rearrangement of the adsorbate species [4]. Frequency shifts due to adsorption and coverage changes have been calculated for halogen ions on a Ag electrode by Nichols and Hexter [5]. In the early investigations by Fleischmann et al. [6] and Jeanmaire and van Duyne [7], a few frequency values for pyridine modes at different potentials are given, but no systematic investigation was performed. The results for the potential dependence of pyridine modes obtained by Pettinger and Wenning [8] exhibited no common trend and were not discussed further. During time* Present address: Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4--6, 1000 Berlin 33, G.F.R.
336 dependence studies of surface-enhanced Raman spectra using an optical multichannel analyzer, Dornhaus et al. [9] observed a shift of 30 cm -1 for the C - N stretch mode in cyanide adsorbed on Ag for potentials more positive than - 0 . 0 5 V vs. SCE. This effect, however, is accompanied by a faradaic reaction, as indicated by the voltammetry curve. In those potential regions where only double-layer charging occurs, no shifts were detected, probably due to the relatively low resolution of the system. The first systematic investigation of potential-dependent vibrational frequency shifts in surface-enhanced Raman spectroscopy has been performed b y S. Venkatesan et al. [10] for the Ag--N stretching modes of several nitrogen-containing c o m p o u n d s adsorbed on Ag electrodes. They found a c o m m o n shift of the Ag--N frequency to lower values with more negative potentials. This behavior was explained as a consequence of the discreteness of charge of the specifically adsorbed anions, which is a function of the electrode potential [10]. Investigating the potential dependence of the low-lying pyridine m o d e (~220 c m - ' ) in different electrolytes Dornhaus and Chang [11 ], however, demonstrated that the assignment of this mode to the Ag--halogen ion stretching vibration is more likely. These authors again found a frequency decrease of this mode with more negative electrode potential. The objective of the present work has been the systematic investigation of the potential dependence of several internal Raman modes for adsorbates involving CN bonds on silver. The choices of the adsorbate bands and potential ranges to be examined were guided b y the need for good signal-to-noise ratios, welldefined sharp peaks and freedom from faradaic reactions. The adsorbates involved in this study were pyridine, pyrazine, p-nitrosodimethyl aniline and cyanide. EXPERIMENTAL The Raman spectra were obtained with a Spex spectrometer (Ramalog) in combination with an argon ion laser (Coherent Radiation CR-8). The spectral resolution of the m o n o c h r o m a t o r was set to 2.5 cm -1 and the integration time of the photocounting system was typically 1 s. The polycrystalline Ag disk, serving as the working electrode, was illuminated b y the laser beam at a mean angle of incidence of 60 ° , and the typical power at the sample was 100 mW at 514.5 nm. A detailed description of the optical alignment and the electrochemical cell for in situ Raman measurements has been given previously [12]. Before each measurement, the silver electrode of 0.2 cm 2 surface area was polished with Al203 (1 pm) and rinsed carefully in distilled water. The potential of the working electrode was controlled using the standard three-electrode arrangement, a PARC 173 potentiostat and a PARC 175 waveform generator. A platinum plate served as a counter electrode. All potentials are quoted with respect to either the saturated calomel reference electrode or the Hg/HgO reference electrode which was used in the NaOH electrolytes. The electrolytes 1.0 and 0.1 M NaOH, 0.05 M Na2SO4 and 0.1 M KC1 were prepared from reagent grade chemicals and triply distilled water. The adsorbate was added to the electrolyte in the following concentrations: 0.02 M pyridine (Aldrich Chemical), 0.05 M pyrazine (Aldrich Chemical), 10 -4 M p-NDNA
337 (Eastman Kodak), and 0.01 M KCN (Matheson Coleman, reagent grade). The electrolyte was deaerated by bubbling N2 through the solution. RESULTS
Pyridine The potential dependence of three of the most pronounced vibrational bands in the Raman spectrum of pyridine adsorbed on a Ag electrode corresponding to ring modes is shown in Fig. 1. Changing the electrode potential to more negative values causes the frequencies of all three bands to decrease. Within the experimental reproducibility the frequencies shift linearly with electrode potential with the possible exception of the 1032--1036 cm -~ band which in chloride solution showed some tendency to bend off at more positive potentials. The slopes of the lines in Fig. 1 are equal for the three vibrations (6 cm -~
V-l).
In order to prove whether the specifically adsorbing C1- anion of the KC1 electrolyte influences the potential dependence of the vibration frequencies, the Raman spectrum of pyridine adsorbed on Ag was investigated in 1 M NaOH. The potential dependence of three ring frequencies is essentially the same in both 1 M NaOH and 0.1 M KC1 (see Fig. 1). Although the enhancement factor in 1 M NaOH is about 50 times smaller than for pyridine in 0.1 M KC1, the peak frequencies could still be reliably determined. The potential range in which the Raman signal of pyridine on Ag can be studied is extended by 0.2 V to more negative values with 1 M NaOH as the electrolyte instead of KC1. The results presented by Fleischmann [6] and van Duyne [7] for a few potential values are in good agreement with the data in Fig. 1, whereas the potential dependence measured by Pettinger and Wenning [8] does not match with our curves. The potential dependence of the Raman intensity for the symmetric ring breathing mode at 1005 cm -~ is plotted in Fig. 2 in 1 M NaOH. The intensity vs. potential plot exhibits a symmetric maximum around --0.75 V vs. Hg/HgO. In the KC1 electrolyte the corresponding plot is asymmetric, most likely due to the specifically adsorbed chloride ions, as has been pointed out by van Duyne [1,7]. The change in anion, however, does not affect the potential dependence of the vibrational frequencies significantly, as is evident in Fig. 1.
Pyrazine Dornhaus et al. [9] have shown recently that the surface-enhanced Raman effect for pyrazine on Ag in KC1 electrolyte is almost as strong as that for pyridine. In order to find out whether the adsorption mode of the molecule has some influence on the potential dependence of the vibration modes, the symmetric ring breathing mode of pyridine at 1008 cm -1 should be compared to the equivalent mode in pyrazine at 1018 cm -1. In contrast to pyridine, which is believed to adsorb end-on [ 1 ] via the nitrogen atom, pyrazine has a second nitrogen atom in the para position and is believed to adsorb with its plane parallel to the surface [13]. Figure 3 demonstrates that the potential depen-
338 P O T E N T I A L / V vs HG/HGO -[6 -I.2 -O,S -0.4 0.0 ¸1216
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Fig. 1. P o t e n t i a l d e p e n d e n c e o f t h r e e ring v i b r a t i o n m o d e s o f p y r i d i n e a d s o r b e d o n a polycrystalline Ag electrode. E l e c t r o l y t e s : 2 X 10 -2 M p y r i d i n e in: (0) 1 M N a O H ; ($) 0.1 M KCl; ~, = 5 1 4 . 5 n m ; p o w e r o f t h e s a m p l e 100 mW. Fig. 2. R a m a n i n t e n s i t y o f t h e s y m m e t r i c ring b r e a t h i n g m o d e (Av = 1 0 0 5 c m -1) o f p y r i d i n e o n a p o l y c r y s t a l l i n e Ag e l e c t r o d e d u r i n g a p o t e n t i a l cycle f r o m -+0.0 V t o - - 1 . 4 V a n d b a c k t o -+0.0 V in 1 M N a O H +2 × 10 -2 M pyridine. Scan r a t e 10 m V s -1.
dences of the vibration frequency for the symmetric ring breathing mode in pyridine and in pyrazine are similar. The frequency for the pyrazine mode decreases from 1020 to 1012 cm-' if the potential is changed from +0.2 to --1.1 V vs. SCE or 7 cm -1 V -1 as compared with 6 cm -~ V -~ for pyridine. Again, the anion in the electrolyte (CI-, SO~-) has no significant influence on the frequency shift.
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339 Pyrazine adsorbed on Ag exhibits a rather strong low-lying Raman m o d e at 236 cm -~ which has been attributed to the Ag--N stretching m o d e [10]. The potential dependence of this m o d e in KC1 electrolyte is also shown in Fig. 3 and it has a slope of 26 cm -~ V -1. Similar results have been reported recently for the Ag--N vibrational modes of several N-containing organic molecules on Ag by Venkatesan et al. [10]. These authors also found a much stronger potential dependence for the external Ag--N modes compared to the internal modes. The Ag--N m o d e for pyrazine was also observed in the Na2SO4 electrolyte in the present work, b u t it was t o o weak for reliable frequency measurements.
p-Nitrosodimethyl aniline (p-NDMA ) Brazdil and Yeager [14] have shown that p-NDMA can be used as a probe molecule in Raman spectroscopy at the solid vacuum interface to probe the Lewis acid-base nature of the adsorption sites on insulators and semiconductors. This molecule exhibits a strong internal resonance-enhanced Raman signal and can be examined as an adsorbate without the need for large surface enhancement. The phenylnitroso stretch mode, at ~ 1 1 6 5 cm -1, shifts to higher wave numbers u p o n polarization of the molecule on an acid site. In addition, the N--O stretch vibration, at ~ 1 3 3 0 cm -1, shifts to lower frequencies if the molecule is polarized. These effects are caused b y a shift in electron density away from the ring and the amino group. Like that of pyridine the Raman spectrum of p-NDMA should yield information a b o u t the nature of the adsorption site. Hagen et al. [15] demonstrated that p-NDMA shows a surface-enhanced Raman effect on Ag, and they were able to detect the Raman spectrum of p-NDMA even on a Pt electrode because of the large intrinsic resonance enhancement. However, they did n o t investigate the potential dependence of the vibrational frequencies. In Fig. 4 the frequencies of the phenyl--nitroso stretch, the N--O stretch and the phenylamino stretch modes for the p-NDMA molecule adsorbed on a Ag electrode are plotted as functions of t h e electrode potential in 0.05 M Na2SO4. All bands exhibit decreasing frequencies with more negative potentials, although the slopes are different. For the phenyl-nitroso and the N--O mode the observed slope is 12 cm -1 V -1, whereas the phenylamino mode shifts with only 4 cm -~ V -~. The results obtained in different electrolytes such as 0.1 M NaOH (pH 13) and 0.05 M H~S.O4 (pH 2) do n o t deviate significantly from those obtained in 0.05 M Na2SO4 (pH 6) even though the intrinsic resonance enhancement in the solution phase species is greatly suppressed in the acid solution.
Cyanide The Raman spectra of cyanide adsorbed on an Ag surface have been investigated extensively at the metal--electrolyte interface as well as at the solid vacuum interface [16]. Although Dornhaus et al. [9] detected a shift of the C - N vibration m o d e with electrode potential immediately after completion of
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Fig. 4. Potential dependence for the phenyl--amino stretch, the N--O stretch and the phenyl--nitroso stretch vibration (from top to b o t t o m ) for p-NDMA adsorbed on an Ag electrode in: (+) 0.05 M H2804; (©) 0.05 M Na2804; (o) 0.1 M NaOH; 100 mW at the sample. Fig. 5. Potential dependence for the C-=N stretch (top) and the Ag--C stretch vibration for cyanide adsorbed on an Ag electrode in 0.1 M NaOH (©) and 0.05 M Na2SO4 (o) +10 -2 M KC1; k0 = 514.5 nm, 50 mW at the sample.
the oxidation--reduction cycle, attention was paid to the potential dependence of the vibration frequencies only in the potential range where a faradaic reaction occurs. The vibration frequencies of the Ag--C and the CN modes at 226 and 2114 TABLE 1 Frequency shift AV with electrode potential for the investigated vibrational modes of some adsorbates on an Ag electrode Adsorbate
Assignment
v/cm-1 a
Pyridine
Ring (vl) Ring (v12) Ring (V9a)
1006 1035 1214
6 6 6
Pyrazine
Ring (vl)
1016
7
226
26
Ag--N stretch (Ag--C1)
Ap/cm-1 v ~ l
p-NDMA
Phenyl--nitroso stretch N--O stretch Phenyl--amino stretch
1160 1326 1367
12 12 4
Cyanide
C--N stretch Ag--C stretch
2112 224
26 26
a The data for v were obtained at --0.4 V vs. SCE with the exception of CN- where the potential was --0.8 V vs. SCE.
341 cm -1 respectively, are shown in Fig. 5 as a function of the electrode potential. The potential dependence of the CN m o d e has been investigated in 0.05 M Na2SO4 as well as in 0.1 M NaOH. The results show no significant influence of the different anions in both electrolytes. Between --1.4 and --0.7 V vs. SCE the frequencies of b o t h vibrations increase b y 26 cm -~ V -~. At more positive potentials the frequency of the CN vibration decreases again until it jumps to a b o u t 2145 cm -~ at --0.2 V. However, in this potential range ( - 0 . 7 to - 0 . 2 V) the current--potential curves (see for example ref. 16) show a large oxidation current, indicating a faradaic reaction involving a bulk species in the solution. The frequency shift of the CN m o d e to 2145 cm -1 at - 0 . 2 V is also reflected in the behavior of the Ag--C mode, the frequency of which shifts to ~ 2 5 0 cm -~ a t - - 0 . 1 V. Table 1 sumarizes the results for the four different adsorbates on a Ag electrode. The Raman spectra of these systems are given in the following references, for example: pyridine [1,6,7,9], pyrazine [ 9 ] , p - N D M A [14,15] and cyanide [16,17]. DISCUSSION AND CONCLUSIONS Pyridine is one of the most frequently used probe molecules for the study of the surface structure of oxides at the solid vacuum interface. The symmetric ring breathing mode which occurs at 991 cm -I for physisorbed pyridine shifts to 1000 cm -I if the molecule is hydrogen bonded, and is found at 1010 cm -I in the pyridinium ion (Br~nsted pyridine). For coordinately b o u n d pyridine this vibration m o d e has a frequency of ~ 1 0 2 5 cm -~ (Lewis pyridine) [18]. On the basis of the sensitivity of the symmetric ring breathing m o d e to the acidity of the adsorption site, Fleischmann et al. [6] concluded that the observed shift of this m o d e from 1008 cm -I at 0.0 V to 1005 cm=' at --1.0 V vs. SCE for pyridine adsorbed on an Ag electrode is due to a change in the acidity of the H atoms of the water molecules in the inner Helmholtz layer. However, the potential dependence of the trigonal ring breathing m o d e does not support this interpretation [6]. The shifting of the potential to negative values is equivalent to increasing the Lewis base properties of the metal surface sites. An alternative w a y of viewing this Situation is that the strong electric field at the surface produces, u p o n negative shifts of the potential, a polarization on the adsorbed molecule equivalent to that of a strong Lewis base site. Therefore, the above interpretation requires a shift of this band to higher frequencies when the electrode potential is changed from 0.0 to --1.0 V. However, the frequencies observed experimer~t~Uy shift to lower values (see Fig. 1). The results of our studies demonstrate that the shift of the internal vibrational frequencies to lower values with more negative electrode potential is a general trend in pyridine and other molecules showing surface enhancement on Ag. Thus, the potential dependence of the two ring breathing modes under discussion in the Fleischmann model is n o t contradictory. However, the assignment of these shifts to changes in the acidity of the adsorption site was premature. A similar situation is encountered with p-NDMA adsorbed on a Ag electrode. Polarization of the p-NDMA molecule owing to the adsorption on an acidic site decreases the double-bond character of the N=O bond and simultaneously
342 increases the bond order of the phenyl--nitroso frequency (see Fig. 2 in ref. 14). The shift of the phenyl--nitroso frequency from 1158 cm -1 at --0.6 V vs. SCE to 1169 cm -1 at +0.3 V could be rationalized in the same way as has been done by Fleischmann for pyridine. At more positive potentials the p-NDMA could bind through the oxygen of the nitroso group to a more acidic H atom of the adsorbed water than at cathodic potentials. Alternatively, the p-NDMA may be coordinated directly to the metal through the O atoms of the nitroso group. The change in polarization of the molecule produced by a shift towards positive electrode potentials should result in a decrease in the NO stretching frequency and an increase in the phenyl--nitroso frequency. This is contrary to observation;both increase experimentally. Thus, polarization of the adsorbed molecule at the Ag electrolyte interface due to a change in acidity of the adsorption sites or to the strong electric field at the interface appears n o t to be a proper explanation for the observed potential-dependent frequency shift. An additional interpretation of the frequency shifts with electrode potential for pyridine was given by van Duyne [ 1 ]. He suggested that the different coverages are responsible for the shift. At low coverage the next neighbor to a pyridine molecule will be a water molecule, whereas at monolayer coverage each pyridine molecule will be surrounded by other pyridine molecules. Studies of the frequency position of the symmetric and trigonal ring breathing m o d e in aqueous pyridine solutions with different concentrations show a shift of both lines to higher frequencies with dilution [1 ]. Therefore, one should expect lower vibrational frequencies at the potential of zero charge (pzc) where organic molecules are believed to be adsorbed strongly on the electrode surface, and higher frequencies at potentials anodic and cathodic to the pzc where the coverage is decreased owing to the replacement of the organic molecules by adsorbed water molecules. The intensity--potential curve for the Raman vibration of pyridine at 1006 cm jl in 1 M NaOH (Fig. 2) probably reflects the expected adsorption isotherm. The m a x i m u m occurs at potentials which appear reasonable in terms of published pzc data for Ag [19]. Within the scatter of the points, however, the experimental frequency results (Fig. 1) show no evidence for a minimum at a potential near the m a x i m u m in Fig. 2. This makes the interpretation of van Duyne questionable. For pyrazine, which is believed to be adsorbed with the plane of the molecule parallel to the electrode surface [ 13 ], the environment effect should be much less pronounced. Independent of the coverage, the molecule is always surrounded by the Ag surface on one side and by water molecules on the other. However, the potential dependence of the symmetric ring breathing m o d e of pyrazine (7 cm -~ V -~) is close to that of pyridine (6 cm -~ V-~). Thus, either the interpretation of the frequency shift due to changes in the environment is inadequate, or the assumption of a parallel adsorbed pyrazine molecule is wrong. The model recently suggested by Venkatesan et al. [10], involving the discreteness of charge of adsorbed anions yields the potential dependence of Ag--N vibration modes for some nitrogen-containing compounds adsorbed on Ag. From their t r e a t m e n t of the model these authors found a linear relation between the square o f the Ag--N frequency and the electrode potential. How-
343 ever, owing to the limited potential range and the relatively small frequency shift (~30 cm -1 V -1) a plot of the actual frequency vs. the electrode potential would also result in a linear relation. Thus, the potential dependence of the Ag--N m o d e observed for pyrazine (Fig. 3) is in agreement with the results obtained by Venkatesan et al. [10]. However, applying the model suggested for the external Ag--N modes to the internal vibrational mode of the adsorbed molecules leads to contradictions. If the potential dependence of the vibrational frequencies is due to the changes in the discreteness of the charge of the specifically adsorbed anions, the effect should disappear at the pzc and more negative potentials because the anions desorb (see eqn. 15 in ref. 10). The experimental results in Figs. 1 and 3, however, do n o t confirm this conclusion. The vibrational frequencies of the symmetric ring breathing m o d e of pyridine and pyrazine continue to shift to lower values even at potentials negative of the pzc. The above discussion has shown that existing models are insufficient in explaining the potential dependence of vibrational bands of adsorbates on an Ag electrode. For the Ag cyanide system, Anderson, together with the authors has examined a model explaining the potential dependence of the C - N stretching frequency [17]. A molecular orbital theory combining t w o - b o d y atomic repulsion and one-electron molecular orbital delocalization energies [20] is used to calculate the force constant. The Ag surface is simulated b y an Ag4 or Ag~ cluster with a CN- molecule adsorbed on a one-fold site. Charging of the electrode is introduced into the calculations b y moving the energies of the electronic levels of the Ag cluster up or down, relative to those in the CN adsorbed on the surface. The calculations show that negative charging of the electrode results in a lowering of the C - N force constant due to a decrease in ap donation. A detailed description of the model and the theoretical results for the Ag cyanide system will be subject of a future publication [17]. The question exists whether the same types of interactions m a y explain the potential dependence of the modes for other adsorbed species. This remains to be established, b u t m a y well be the situation. The experimental results presented in this study demonstrate that the decrease of the vibrational frequencies of adsorbed species on an Ag electrode is a c o m m o n feature for internal and external modes. Models based on the polarization of the adsorbate due to the different acidity of the adsorption site, on environmental effects due to the coverage change or on the influence of specifically adsorbed anions are n o t able to explain all of the observed experimental features. We propose as a possibility a decrease of the force constants of the vibrational m o d e of the adsorbed molecule, with negative charging of the electrode surface due to a shift in the electronic levels o f the electrode relative to those of the adsorbates causing a decrease in ap donation in the case of an adsorbate such as CN- adsorbed on Ag. ACKNOWLEDGEMENT Stimulating discussions with A.B. Anderson are gratefully acknowledged. This research has been supported b y the U.S. Department of Energy. One of us (R.K.) is grateful to the Deutsche Forschungsgemeinschaft for supplementary financial support.
344
REFERENCES
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