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Langmuir 1989,5, 783-787
In Situ Infrared Spectroscopy of Multilayer Copper Thiocyanate Films on Copper Electrodes F. Guillaume and G. L. Griffin**+ Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455 Received September 9, 1988. I n Final Form: January 25,1989 We have studied the growth kinetics of multilayer CuSCN films on a Cu(ll1) surface in aqueous NaSCN solutions using polarization-modulated infrared reflection absorbance spectroscopy (PM-IRRAS) and conventional voltammetric techniques. Formation of the first layer of adsorbed SCN- species is indicated by the voltammetric results but cannot be resolved spectroscopically. As the potential is increased to -0.430 V (SCE) in 0.1 M NaSCN, a band appears in the PM-IRRAS spectrum at 2173 cm-I. The intensity of the band reaches a maximum at E = -0.400 V. The frequency of this band is independent of film thickness, electrode potential, and electrolyte composition, which leads us to assign it to the v(C-N) mode of a homogeneous multilayer CuSCN film. A t higher potentials, the intensity decreases to about two-thirds of its maximum value and remains stable. We attribute this decrease to a thinning of the CuSCN film brought about by a decrease in the flux of Cu cations into the film, which is caused by the formation of a second, underlying phase of different composition (e.g., a hydrous copper oxide layer).
Introduction The formation of precipitated salt films can have a significant effect on the passivation and dissolution behavior of metal surfaces.'J On the one hand, the fact that a precipitated film is formed indicates that one or more electrolyte species has sufficient affinity for forming a compound with the dissolving cation species that a new phase may be produced. The presence of this phase may provide a diffusional barrier that partially inhibits the regular dissolution or passivation process. On the other hand, the fact that these electrolyte species can bind to cation species more strongly might instead lead to the formation of soluble complexes and possibly increase the rate of metal oxidation and overall dissolution. The oxidation of Cu in the presence of SCN- anions has been studied by several Figueroa et aL5 have reported that the initial reaction during a potentiodynamic sweep is the formation of a Cu(SCN)(s) monolayer: Cu + SCNCu(SCN)(s) + e(1) As the potential is increased, monolayer formation is followed by growth of a porous three-dimensional CuSCN film: nCuSCN(s) (CuSCN), (2) At still higher potential, reactions 1and 2 are replaced by oxidation reactions that involve OH- anions from the solvent, e.g.: Cu + OHCu(OH)(s) + e(3) The latter leads to formation of a hydrous cuprous oxide layer underneath the CuSCN layer. In studies using 0.2 M KSCN in borate buffer, Figueroa et al.5 reported that reactions 1, 2, and 3 occur a t -0.500 (SCE), -0.320, and -0.280 V, respectively. In situ spectroscopic techniques should prove useful for monitoring the formation of specific components in this type of multilayer film system.' The v(C-N) stretching mode of the SCN- anion has a moderately intense absorption band? which should permit the deposited films to be observed spectroscopically. Previous authors have reported a band a t 2115-2105 cm-l for a monolayer of SCN- adsorbed on Ag.gJO A similar band is observed at
-
-
-
'Present address: Department of Chemical Engineering, Louisiana State University, Baton Rouge, LA 70803. 0743-7463/89/2405-0783$01.50/0
2120-2130 cm-' for SCN- adsorbed on Au." In both cases, this value of the v(C-N) stretching frequency is interpreted to mean that the SCN- anion is adsorbed "end-on" with the sulfur atom bound to the surface. In this paper, we describe the application of polarization-modified IR reflection absorbance spectroscopy (PM-IRRAS) to observe the multilayer CuSCN films that are produced when a Cu electrode is oxidized in an electrolyte that contains NaSCN. Methods Infrared reflection absorbance spectroscopy (IRRAS) is based on the absorption of p-polarizedlight reflected at grazing incidence from a metal surface due to interaction with the dipole derivative of adsorbates oriented perpendicular to the surface. Reviews of the principles of the technique and various applications are a ~ a i l a b l e . ~ JThe ~ J ~polarization modulation technique used in this work was described originally by Golden et d.l4 The present system uses a mechanical chopper (Laser Precision CTX-534)to produce a quasi-sinusoidal source beam modulated at 1600 Hz. A photoelastic modulator (Hinds PEM 80) followed by a fixed grid polarizer is used to modulate the polarization of the beam at a second frequency of 74 KHz after it is reflected from the sample surface. The reflected light is focused through a filter wheel monochromator and onto a liquid-nitrogen-cooledphotoconductive HgCdTe detector (InfraredAssociates HCT 100). The (1)Beck, T.R.J. Electrochem. SOC.1983,129,2412. (2)Alkire, R.;Cangellari, A. J. Electrochem. SOC.1983,130, 1252. (3)Kazantsev, A. A.; Kuznetsov, V. A. Electrokhimiya 1984,20,934. (4)Wey, A. W.; Abramovitch, M.; D'Alkaine, C. V. J. Electroanal. Chem. 1985,165,147. (5)Figueroa, M. G.; Salvarezza, R. C.; Arvia, A. J. Electrochim. Acta 1986,31, 671. (6) Figueroa, M. G.; DeMele, M. F. L.; Salvarezza, R. C.; Arvia, A. J. Electrochim. Acta 1987,32,231. (7)Bewick, A.; Pons, S. In Aduances in ZR and Raman Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Wiley-Heyden: London, 1985;Vol. 12,p 1. (8)Bailey, R. A.; Kozak, S. L.; Michelson, T. W.; Mills, W. N. Coord. Chem. Reu. 1971,6,407. (9)Weaver, M. J.;Barz, F.; Gordon, J. G., II; Philpott, M. R. Surf. Sci. 1983,125,409. (10)Corrigan, D.S.;Weaver, M. J. J. Phys. Chem. 1986,90,5300. (11)Corigan, D. S.;Foley, J. K.; Gao, P.; Pons, S.; Weaver, M. J. Langmuir 1985,1, 616. (12)Hoffman, F. Surf. Sci. Rep. 1983,3,107. (13)Benziger, J. B.; Preston, R. E.; Schoofs, G. R. Appl. O p t . 1987, 26,343. (14)Golden, W. G.; Dunn, D. S.; Overend, J. J. Catal. 1981,71, 395.
0 1989 American Chemical Society
784 Langmuir, Vol. 5, No. 3, 1989
two frequency components of the doubly modulated signal are extracted by band-passfiltering (fourth-order Butterworth fiters) and double demodulation with lock-in amplifiers (Princeton Applied Research 186). The two components are then combined by using a ratiometer (Brookdeal 9047). The final ratiometer signal is digitized by a la-bit, 5 0 - acquisition ~ time A/D converter, which is interfaced to a microcomputer (Cromenco 111). The computer is also interfaced to the filter wheel monochromator and an X-Y plotter. Experiments were performed in a specially designed threeelectrode cell described previously.15 Electrochemical measurementswere done with an EEG 363 potentiostat and EEG 175 programmer. All potentials are reported relative to the saturated calomel electrode (SCE). The working electrodewas a 2-mm-thick Cu disk cut from a 10-mm-diameter Cu(ll1) single-crystal rod (AESAR Puratronic grade). An insulated wire was soldered at the back for electrical connection to the potentiostat. After being painted with a nonconductive lacquer, the sample was embedded in a cylindrical plug of epoxy (Jarrett 9101). The front face of the plug and disk assembly was polished mechanically to a mirror finish with 1200 grade sandpaper and then polished on a felt pad with successive CH30H slurries of 1-,0.3-, and 0.05-pm A1203 polishing powder. The surface was degreased with acetone, rinsed with distilled H20, and inserted in the plunger of the electrochemical cell. We performed experiments using both phosphate (pH 11)and borate buffers (pH 7.5 and 9), with NaSCN concentrationsof 0.1, 0.01, and 0.001 M in each case. The solutionswere bubbled with N2 before use. The working electrode was cleaned in NaSCN-free buffer by holding it at -1.0 V (SCE) for 50 s and then cycling it between -1.0 and +0.35 V at 10 mV/s. These two steps were repeated until the voltammograms displayed a stable base line with a residual current less than 1 pA at -1.0 V. The steps for obtaining the PM-IRRAS spectra are as follows: After the current has dropped to less than 1 PA at a potential of -1.0 V, the electrode is pushed against the CaF2window, and a background IRRAS spectrum is recorded over the range 2060-2225 cm-'. The electrode is pulled back into solution, and the potential is raised at 20 mV/s to a specified holding potential,
CSCN-3 =0.1M
2.0 1.0 -
-
0.0
1-
I
k
0.2
-
C S C N - I = 0.001M
V = 20 mV/s I
I
-1.0 -0.6
p H = II 1
-0.6
I
I
1
-0.2 0.0 POTENTIAL, V(SCE) -0.4
1
0.2
0
Figure 1. Cyclic voltammogramsfor Cu(ll1) surface in different concentrations of NaSCN. Phosphate buffer, pH 11.
Results Figure 1 shows three cyclic voltammograms recorded in different concentrations of NaSCN (i.e., 0.1, 0.01, and 0.001 M). The supporting electrolyte is pH 11phosphate buffer, and the sweep rate is 20 mV/s. The upper limit of +0.35 V was chosen to avoid entering the passivation region and the film pitting that occurs there. The first feature in the anodic sweeps is a small oxidation peak that we assign to the thiocyanate-assisted oxidation of the first monolayer of Cu atoms (cf. reaction 1). The peak shifts to more negative potential with increasing SCN- concentration, e.g., from E, = -0.410 V for [NaSCN] = 0.01 M to E, = -0.500 V for [NaSCN] = 0.1 M. The latter agrees with the potential of the peak assigned to reaction 1by Figueroa et aL6 The area under the peak (i.e., the integrated charge density) is 42 5 pC/cm2 and is independent of [NaSCN]. This is somewhat smaller than the value of 200 &/cm2 reported as the maximum charge needed to produce a Cu(SCN)(s) monolayer on a Cu amalgam.16 Separate experiments show that the peak
potential is independent of the sweep rate over the range 10-250 mV/s, which suggests that the reaction is equilibrated during the potential sweep. All of these results support the assignment of this peak to the thiocyanateassisted oxidation of the first Cu monolayer. The next feature in the anodic sweeps is a much larger peak, corresponding to the oxidation of multiple layers of Cu atoms. The onset of this process shifts to more negative potential with increasing [NaSCN], e.g., from Eo = -0.295 V for [NaSCN] = 0.01 M to Eo = -0.376 V for [NaSCN] = 0.1 M. Thus the onset of multilayer oxidation occurs about 120 mV above the monolayer formation potential. The area under the multilayer oxidation peak is much larger for [NaSCN] = 0.1 and 0.01 M than for [NaSCN] = 0.001 M. The latter voltammogram is quite similar to results obtained in NaSCN-free borate buffer solution. Another multilayer oxidation peak is observed in the range -0.1 V < E < 0.0 V for the experiments where [SCN-] = 0.01 M and [SCN-] = 0.001 M. Figueroa et al.6 have assigned peaks a t this potential to the electroformation of hydrated Cu20, CuO, or C U ( O H ) ~X-ray photoemission and ion scattering spectroscopy experiments have shown that the oxidized layer produced in the absence of SCN- anions consists of an inner Cu20 layer and an outer CuO or CU(OH)~ layer." The cathodic return sweeps at all [NaSCN] values show a reduction peak at -0.6 V. For [SCN-] = 0.001 M, there is also a second peak at -0.5 V. Figueroa et al. also observed similar behavior.6 They assigned the more anodic peak to the reduction of Cu(I1) species to Cu(I), and the more cathodic peak to the reduction of Cu(1) to Cu(0). They concluded that the presence of SCN- anions stabi-
(15) Tornquist, W.J.;Guillaume, F.; Griffin, G. L. Langmuir 1987,3, 477.
(16) Bilewicz, R.; Kublik, Z. Electroanal. Chem. 1985, 195, 137. (17) Strehblow, H.H.; Titze, B. Electrochim. Acta 1980,25, 339.
Elr
After 180 s, the electrode is pushed back against the window, and the new spectrum is recorded at the potential Eb. The background spectrum is subtracted and the result displayed on the X-Y recorder and stored on disk. This procedure can take up to 120 s. If desired, the electrode is pulled back and another period of polarization at Ehis repeated. Up to five spectra are recorded at times of 180, 360, 540, 720, and 900 s. The potential is returned to -1.0 V, and the entire process is repeated for a new value of Eh.
*
IR Study
Langmuir, Vol. 5, No. 3, 1989 785
of Multilayer CuSCN Films
[ N o S C N l = 0.1M
E,
POTENTIAL (VSCE)
- 430 mV
TIME
I-
z W a a
3
u
I
W I
0
IO0
I
I
200
300
(SI Figure 2. Transient current response following "ramp and hold" sequence to different holding potentials. [NaSCN]= 0.1 M, borate buffer, pH 9. TIME
lized the formation of oxidized Cu(1) species but inhibited the formation of Cu(I1). The current response during the "ramp-and-hold" potential sequence used for recording IR spectra is shown in Figure 2. Three curves are shown, corresponding to different values of the holding potential, Eh. The three values of Eh all lie within the range bounded by the monolayer oxidation peak and the onset of multilayer oxidation (cf. Figure 1). Solution conditions for these experiments are [NaSCN] = 0.1 M and pH 9, vs the value of pH 11 used for the results'in Figure 1. Separate voltammograms recorded at pH 9 show that the monolayer formation peak and the onset of multilayer formation are both shifted anodically by less than 10 mV, relative to their potentials a t pH 11. Thus the difference in pH does not have a large effect on the two experiments. The first part of each curve shows the current increase as the potential is ramped from -1.0 V to the holding potentid, Eh. At the lowest potential, Eh = -0.450 V, the current decays monotonically after the holding potential is reached. As shown in the PM-IRRAS spectra discussed below (cf. Figure 3), no multilayer CuSCN film is formed at this potential. For Eh = -0.425 V, the current initially decays for a few seconds and then increases to a maximum before decaying to reach a steady-state (non-zero) value. A band in the PM-IRRAS spectra assigned to multilayer CuSCN can be observed at this potential. For Eh = -0.395 V, the same qualitative behavior is observed, except that the initial decay is much shorter and the maximum current much larger. The PM-IRRAS band reaches its maximum intensity near this potential (vide infra). The area under the latter two current vs time curves as they pass through their current maxima indicates that multilayer oxidation is occurring. The shape of the curves suggest that the transformation from monolayer to multilayer coverage follows nucleation and growth kinetics, with an induction period at the beginning of multilayer growth and a satu-
2 200
I
I
2150
2100
WAVENUMBER ( c m - ' 1
Figure 3. PM-IRRAS spectra of multilayer CuSCN films observed at different potentials. Same solution conditions as Figure 2.
rated, steady-state current at the end.ls Figure 3 shows IRRAS spectra recorded after 300 s at potentials between -0.430 and -0.400 V for the solution conditions used in Figure 2. No signal is observed for an electrode potential of -0.430 V, even though this is above the potential required for monolayer formation. Thus we are unable to resolve any unique band corresponding to the first monolayer of adsorbed SCN-. At higher potentials, a band appears at 2173 em-'. The intensity of this band increases as the potential is increased, until reaching a maximum at around -0.400 V. The frequency of the band is unchanged over this potential range. This frequency lies between the value of 2170 cm-' reported for CuSCN by earlier workerslOJ1and the value of 2176 cm-' we measured for CuSCN in a Nujol mull. Therefore we assign the peak to the v(C-N) band of a multilayer CuSCN film on the surface of the electrode. If the intensity of the band is assumed to be proportional to the thickness of the film (vide infra), then these results show that the film reaches a limiting thickness at -0.400 V. Spectra measured with [NaSCN] = 0.01 and 0.001 M show that this maximum intensity decreases by about a factor of 2 for each decade decrease in NaSCN concentration. The upper curve in Figure 4 shows the peak height of the PM-IRRAS band as a function of potential. Between -0.430 and -0.400 V, the intensity increases with potential as described above. At higher potentials, the intensity decreases to about two-thirds of its maximum value and then remains relatively constant. This suggests that another process is interfering with the steady-state formation/dissolution kinetics of the CuSCN film. It is possible that this process is the formation of the underlying hydrous copper oxide film described by Figueroa et ala5We cannot confirm this spectroscopically, however, because the fre(18) Fletcher, S.; Thomson, N.; Tran, T. J.Electroanal. Chem. 1986, 199, 241.
786 Langmuir, Vol. 5, No. 3, 1989
Guillaume and Griffin
0 I M N a SCN p H ~ 9 ( B0O R A T E )
-I
a
z
z
0)
a
J
w
a z
c I
z
W
v)
0 c
a
a a
w
+ W
z
0
0
c a a
00
300
500
400
TIME
600
700
(5)
Figure 6. Peak heights of PM-IRRAS spectra observed for multilayer CuSCN films as a function of time during growth at low potentials. Same solution conditions as Figure 5. I
I
- 04 -03 ELECTRODE POTENTIAL, V ( S C E )
1
-0 2
Figure 4. Peak heights of PM-IRRAS spectra observed for multilayer CuSCN films formed at different potentials. Same solution conditions as Figure 3.
-1
4
a z !? v)
a
w
c
: 0
c a a
01 IS0 J
a
I
34 0
1
500 TIME ( 6 )
I
660
i
720
Figure 7. Peak heights of PM-IRRAS spectra observed for multilayer CuSCN films as a function of time during growth at high potentials. Same solution conditions as Figure 5.
2
'3 cn
a W
+ W 5
0
ta
2200
2150 2100 WAVENUMBER ( c m - ' )
Figure 5. PM-IRRAS spectra of multilayer CuSCN films observed at fixed potential and different times. Eh = -0.356 V, [NaSCN] = 0.01, M, borate buffer, pH 7.5. quency range of the Cu-0 vibrations expected in such a film lies outside the half-wavelength phase retardation limit of our photoelastic modulator. The lower curve in Figure 4 shows the steady-state current measured at each potential. The current also passes through a maximum at -0.400 V and then decays to a relatively constant value a t higher potentials. This similarity in behavior establishes a correlation between film thickness (as measured by the PM-IRRAS peak height) and the steady-state dissolution current. In particular, both quantities decrease when the proposed hydrous oxide is formed. It is also possible to use IRRAS spectra to observe the growing CuSCN film as a function of time, povided that the potential is chosen so that the growth rate is slower than the time required to record each IRRAS spectrum. Figure 5 shows a series of IRRAS spectra recorded after
the electrode was held for different times at a fixed potential of -0.356 V and solution conditions of [NaSCN] = 0.01 M and pH 7.5. The spectra again consist of a single feature at 2173 cm-' that grows monotonically with time. The frequency is independent of film thickness, which again suggests that the IRRAS peak is due to a uniform CuSCN layer. In Figure 6 we plot the IR intensity vs time for three different potentials. For Eh = -0.365 V, the peak height begins to deviate below linearity at the longer times. For Eh = -0.381 and Eh = -0.377 v, the curves are linear over the interval shown. Thus the multilayer CuSCN film grows with constant rate at lower potentials. The slope of the lines increases with more positive potentials, which shows that the growth rate increases. In Figure 7 we show the peak height as a function of time at higher potentials. For Eh = -0.356 V (cf. Figure 5), the thickness increases linearly at first and then begins to level off. For potentials greater than Eh = -0.356 V, the PM-IRRAS peak heights reach most of their final value by the time the first spectrum can be recorded (ca.180 s). These final values decrease somewhat with increasing potential, as noted in Figure 4.
Discussion As demonstrated above, the PM-IRRAS technique can be used to monitor the formation of the multilayer CuSCN salt film that is produced during the oxidation of Cu electrode in NaSCN solution. We are not able to observe the formation of the first SCN monolayer spectroscopically. There are two possible explanations for this. Either the dipole moment derivative of adsorbed SCN- is too small to permit a measurable absorbance change with our spectrometer, or else the first monolayer of SCN- anions
Langrnuir 1989,5, 787-796 is oriented parallel to the Cu surface and therefore violates the optical selection rule for absorbance at metal surfaces.1Q An argument against the first explanation can be presented as follows: We have previously been able to detect submonolayer coverages of CO adsorbed on Pt electrodes by using our ~pectrometer.'~ The integrated intensity of CO adsorbed on reduced Pt catalysts is (30-35) X lo6 cm/mol.20 The integrated intensity of sulfur-bound SCNin the AU(SCN)~anion is 5 X lo6 cm/mol.8 Since the intensity of adsorbed SCN- is estimated to be only 5-6 times smaller than for adsorbed CO, we should be able to detect a saturated monolayer of SCN- anions if they were vertically oriented. Instead, we conclude that the first monolayer of SCN- anions is oriented parallel to the surface. Weaver et al? have reported that a fraction of the SCNanions adsorbed on Ag are oriented parallel to the surface a t a sufficiently negative potential. The fact that Cu is a more electropositive element than Ag may mean that a similar effect can occur at a less negative potential on Cu. The value of v(C-N) for the multilayer CuSCN film, 2173 cm-', indicates that the SCN- anions in bulk CuSCN are arranged in a chain bridging configuration between multiple Cu cations.21 We propose that the first monolayer of SCN- anions is oriented parallel to the surface, in an effort to achieve a chain bridging configuration between surface Cu cations similar to that achieved in the bulk compound. In presenting our results for the multilayer film, at several points we have assumed that the PM-IRRAS peak height is proportional to the CuSCN film thickness. This (19) Greenler, R. G. J. Chern. Phys. 1966,44, 310. (20) Seanor, D. A.; Amberg, C. H. J. Chern. Phy8. 1966, 42,2967. (21) Hunter, J. A.; Massie, W. H. S.; Meiklejohn, J.; Reid, J. Znorg. NucZ. Chern. Lett. 1969,5, 1.
787
assumption is supported by the fact that the band frequency is independent of potential and intensity, which we interpret as evidence that the film exists as a homogeneous phase. In this case, the molar absorbance should be constant. The current vs time behavior for the formation of the multilayer CuSCN (cf. Figure 2) is interpreted in terms of a nucleation and growth process.18 We propose that the nucleation step is the formation of defect domains in the SCN--covered surface that are produced as subsurface Cu atoms start to be oxidized. As the size of the defect domains increases, the oxidation current increases and the CuSCN film grows more rapidly. Eventually, the growing CuSCN film inhibits the transport of SCN- anions to the surface, and the oxidation current decays. When the potential is made sufficiently anodic, OH- ions can also serve as counterions to assist the oxidation process (cf. reaction 3). We interpret the decrease in IR peak height seen at higher potentials as being due to a thinning of the CuSCN layer at the onset of this competitive process. This would be consistent with the model of an outer CUSCN layer with an underlying hydrous oxide layer." If the rate of solvation of Cu cations from the outer surface of the CuSCN layer is not a strong function of potential, then the thinning of the CuSCN film indicates that there is a decrease in the flux of Cu cations entering the film from the electrode side. This would imply that Cu cations are not supplied as readily from the hydrous oxide layer as from the metallic Cu interface. Acknowledgment. This work sponsored by the Corrosion Research Center at the University of Minnesota, supported by the US. Department of Energy, Grant DEFG02-84-ER45173. Registry No. Ca, 7440-50-8;SCN-, 302-04-5;CuSCN, 111167-7.
Surface-Enhanced Raman Scattering of the Protonated Forms of 1,4-Diazabicyclo[2.2.2loctane at a Silver Electrode David A. Guzonas, Donald E. Irish,* and George F. Atkinson Guelph- Waterloo Center for Graduate Work in Chemistry, Waterloo Campus, Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada Received August 8, 1988. In Final Form: January 3, 1989 The surface-enhanced Raman spectra of the aliphatic diamine 1,4-diazabicyclo[2.2.2]octanefrom a silver electrode have been measured as a function of the bulk solution pH. The SER spectrum of the diprotonated DABCO molecule at pH 1.8 is almost identical with the normal Raman spectrum of the bulk solution at the same pH, in contrast to the SER spectrum of the unprotonated DABCO molecule at pH 12, which shows large differences from the spectrum of the bulk solution at pH 12. It is proposed that the diprotonated DABCO molecule is coadsorbed with specifically adsorbed anions, e.g., C1-, and that ita spectrum is enhanced by the electromagnetic enhancement mechanism alone. At intermediate pH values, all three possible forms (di-, mono-, and unprotonated)of DABCO are observed, in amounts strongly dependent upon the electrode potential. Both band-fitting programs and factor analysis were used to quantify relative populations. Introduction The solution pH is an important variable in surfaceenhanced Raman (SER) studies of electrochemical systems, especially as the adsorbates most frequently studied by SERS are aliphatic or aromatic amines. The importance of pH as an experimental variable can be seen in the
studies of the pyridine/silver electrode system, in which the pyridinium ion adsorption was found to be important in understanding the SER spectra.' In other publica(1) Rogers, D. J.; Luck, S. D.; Irish, D. E.; Guzonas, D. A.; Atkinson, G . F.J.Electroanal. Chern. 1984, 167, 237.
0743-7463f 89f 2405-0787$01.50/0 0 1989 American Chemical Society
