Langmuir 1988,4, 127-132 overlayers in the hope of probing the metal/metal oxide interface.
Summary The cyclic voltammetry SHG data for the iron electrode in a deoxygenated 0.1 M NaOH solution with the laser beam incident at 4 5 O were shown not to differ from studies in an aerated s ~ l u t i o n .Such ~ a result suggests that dissolved oxygen does not play a major role in the oxidation/reduction of iron in alkaline solutions. The smaller electrode to laser beam area ratio in conjunctionwith these results also demonstrated that negligible laser-induced chemical reactions are occurring. A more careful investigation of the reduced potential region with SHG in conjunction with cyclic voltammetry and also constant-current oxidation/reduction demonstrated the slow response of the electrode to the final reduction to bare metal. SHG during constant-current oxidation was used to show the presence of an Fe(I1) phase during oxidation. This phase was not seen with SHG during oxidative potential sweeps due to overpotential considerations.
127
Changes in the second harmonic intensity during the oxidation/reduction of Fe in deaerated 0.1 M NaOH solution have been assigned on the basis of past work with iron in aerated alkaline and borate buffer solutions and present preliminary work with the electrode in deaerated borate buffer. The reduction of the passive film, Fe(III), occurs via two intermediates: first reduction to a mixed valence Fe(I1-111) compound and then an Fe(II) phase, before final reduction to the bare metal accompanied by the adsorption of cations. In any event, the reduction of iron in mildly basic borate buffers is the same for iron in more alkaline solutions, such as 0.1 M NaOH. It was also found that the second harmonic intensity from the reduced metal surface in 0.1 M NaOH is much more sensitive to the angle that the laser beam makes with the surface normal than was to be expected relative to signal levels from oxidized surfaces. Geometric contributions from crystallites with the same single-crystal face in the macroscopic surface plane have been postulated and are now under investigation. Registry No. Fe, 7439-89-6; NaOH, 1310-73-2.
Surface-Enhanced Raman Spectroscopy of Poly(2-vinylpyridine) Adsorbed on Silver Electrode Surfaces Joseph L. Lippert and E. Steven Brandt* Research Laboratories, Eastman Kodak Company, Rochester, New York 14650 Received February 23,1987. I n Final Form: August 10, 1987 Surface-enhanced Raman scattering (SERS) of partially protonated poly(2-vinylpyridine) (P2VPy) adsorbed onto an electrochemicallyroughened silver electrode is reported as a function of applied potential. The species on the electrode surface is predominantly pyridinium ion when the electrode potential is positive of the point of zero charge (E.) and predominantly neutral pyridine near E,. Orientation effects involving the polymer chain are visible in the fingerprint region of these spectra. The species vs potential dependence is different for the polymer compared with low molecular weight probe molecules of pyridine and 2methylpyridine. The presence of the polymer substantially improves the stability of the active sites responsible for SERS activity to cathodic excursions to E, in chloride electrolyte. ?e differences observed with the polymer are explained on the basis of its lower solubility and the "anchoring" of positive charges within the double-layer region.
Introduction The initial observations of surface-enhanced Raman scattering (SERS) of pyridipe (Py) at roughened silver electrodes1" showing intensities 4-6 orders of magnitude greater than normal Raman spectra have been followed by numerous experiments examining the interactions of small molecules and ions adsorbed on such surfaces as a function of solution pH and applied potential."12 These (1) Flekhmann, M.; Hendra, P. J.; McQuillan, k J. Chem.Phys. Lett. 1974, 26, 163. (2) Jeanmaire, D. L.; Van Duyne, R. P. J. Electroanal. Chem. 1977, 84, 1. (3) Albrecht, M. G.; Creighton, J. A. J.Am. Chem. SOC.1977,99,5216. (4) VanDuyne, R. P. In Chemical and Biochemical Applicatiom of Lasers, 4; Moore, C. B., Ed.; Academic: New York, 1979. (5) Surface Enhnced Raman Scattering; Chang, R. K.; Furtak, T. E., E&.; Plenum: New York, 1982.
0743-7463/88/2404-0127$01.50/0
experiments have demonstrated that SERS is a potentially powerful tool for examining the chemical and physical phenomena associated with adsorption and the electrode/solution interface. By contrast, relatively few experiments on macromolecules adsorbed to silver surfaces have been reported.13 (6) Allen, C. S.;VanDuyne, R. P. Chem. Phys. Lett. 1979, 63, 455. (7) Bunding, K. A.; Lombardi, J. R.; Birke, R. L. Chem. Phys. 1980,
49, 53.
(8) Dornhaus, R.; Chang, R. K. Solid State Commun. 1980, 34, 811.
(9) Dornhaus, R.; Long,M. B.; Brenner, R. E.; Chang, R. K. Surf. Sci. 1980, 93, 240. (10) Chang, H.; Hwang, K. C. J. Am. Chem. SOC.1984, 106, 6586. (11) Lombardi, J. R.; Birke, R. L.; Sanchez, L. A.; Bernard, I,;Sun, S. C. Chem. Phys. Lett. 1984,104, 240. (12) Rodgers, D. J.; Luck, S. D.; Irish,D. E.; Guzonae, D. A.; Atkinson, G. F. J. Electroanal. Chem. 1984, 167, 237. (13) Kerker, M. Pure Appl. Chem. 1984,56, 1429.
0 1988 American Chemical Society
128 Langmuir, Vol. 4, No. 1, 1988 Among these are the elegant experiments of Murray et al.,14-16*42 who used spun-cast polymer films on UHV-deposited silver island films to examine the dependence of the SERS phenomenon on distance from the silver surface. In addition, SERS has been reported for a number of polymeric stabilizers adsorbed on gold and silver hydroso1s,17i18 for y-irradiated DNA on a silver electrode, and for polyriboadenylic acid adsorbed to silver colloid^.'^ We have found no reports in which SERS has been used to examine the structure and orientation of polymers adsorbed onto an electrode surface as a function of applied potential. Such experiments are of fundamental and practical interest due to the chemical modification of electrode surfaces by polyelectrolytes, such as protonated or quaternized poly(viny1pyridine) (PVPy),zo~l and the use of polymer coatings for corrosion control. While thick (i.e., 21 pm) PVPy films have been used as matrices for incorporating electroactive groups at electrode surfaces,22*23 it is the first monomolecular layer of the polymer adsorbed to the electrode surface that often determinea the stability of the film to potential cycling.21 In addition, it is wellknown that adsorbed monolayers of polymers can, themselves, dramatically alter electrocatalytic properties of electrode surfaces. We report here the first observation of SERS from partially protonated poly(2-vinylpyridine) (P2VPy) adsorbed onto an electrochemicallyroughened silver electrode in the presence of a chloride supporting electrolyte. The spectra of the adsorbed polymer species show changes in band intensities and positions which occur at potentials different from those correspondingly observed with a monomeric analogue, 2-methylpyridine (2MePy). SERS intensities obtained in the presence of P2VPy are also substantially more stable and reversible than those obtained from adsorbed 2MePy or Py in a chloride electrolyte. The results are interpreted in terms of the specific adsorption of the charged polymer and coadsorbed chloride as a function of electrode charge.
Experimental Section Isotactic poly(2-vinylpyridine) (PBVPy), MW 600000, was obtained from Pressure Chemical Co. 2-Methylpyridme (2MePy) was obtained from Aldrich Chemical Co. Pyridine (Py) was spectroscopic grade from Kodak Laboratory Chemicals. All chemicals were used as received without further purification. All solutions were prepared from water purified by a Continental Water Systems Milli-Q system. In all experiments, the supporting electrolyte was 0.1 M KCl, which had been deoxygenated with argon prior to introduction into the spectroelectrochemical cell.% P2VPy was dissolved in a solution containing 0.1 M KCl and enough HCl to protonate 30% of the pyridine side chains (pH 4.9). (P2VPy is insoluble in aqueous solutions unless it is 225% protonated.) An EG&G/PAR Model 173 potentiostat with a Model 176 dgtal coulometerplug-in and a Model 175 universal programmer (14) Murray, C. A,; Allara, D. L. J. Chem. Phys. 1982, 76, 1290. (15) Murray, C. A.; Allara, D. L.; Hebard, A. F.; Padden, F. J., Jr. Surf. Sci. 1982, 119, 449. (16) A h a , D. L.;Murray, C. A.; Bodoff, S. In Physicochemical Aspects of Polymer Surfaces, 1; Mittal, K. L., Ed.; Plenum: New York, 1983. (17) Lee, P. C.; Meisel, D. Chem. Phys. Lett. 1983, 99, 262. Lepp, A.; Kerker, M. Chem. Phys. Lett. 1983,100,163. (18) Siiman, 0.; (19) Koglin, E.; Sequarie, J. M. J. Phys., Colloq. 1983, 44, '210-487. (20) Anson, F. C. Acc. Chem. Res. 1976, 8, 400. (21) Murray, R. W. In Electroanulytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1984; pp 191-368. (22) Oyama, N.; Anson, F. C. J. Electrochem. SOC.1980, 127, 247. (23) Sharp, M.; Montgomery, D. D.; Anson, F. C. J. Electroanal. Chem. 1986,194,247. (24) Brandt, K. S. Anal. Chem. 1986,57, 1276.
Lippert and Brandt
i I
Figure 1. Cyclic voltammogram of oxidation and reduction of
a silver electrode in 0.1 M KC1 (solid line) and in 0.1 M KCl + 50 mM (in repeat units) P2VPy (broken line). Curves recorded in spectroelectrochemical cell described in ref 24, using a 2 X 4 mm planar electrode, v = 20 mV/s.
were used to control the potential of the working electrode and to monitor the charge paased during the oxidation/reductioncycle (ORC) necessary to produce the SERS enhancement of the adsorbate molecules (seebelow). The working electrode was a silver bar sealed in a TFE Teflon matrix exposing only a planar, 2 X 4 mm rectangular ~urface.2~ All potentials are reported vs the Ag/AgCl (saturated KCl) reference electrode. Raman spectra were obtained with a Spex Industries Model 1403double monochromator equipped with a Model 1459sample illuminator and a thermoelectrically cooled RCA C31034A photomultiplier. Data acquisition and display were controlled by a dedicated Spex Datamate computer. The standard Spex illuminator was modified to provide front-surfaceillumination of the electrode surface. The incident laser beam was focused to a line with a cylindrical lens and then directed at the electrode surface ca. 60° off the collection axis, which was 90" with respect to this surface. Spectra were obtained with either the 647-nm line of a Spectra Physics Model 165 krypton laser or the 680-nm line of a Coherent Radiation Model 599 dye laser containing DCM and pumped with an SP 165argon ion laser. For both sources, incident power was typically 20 mW at the plane of the sample surface. Silver electrodes were electrochemically roughened in 0.1 M KC1by using a double-potential-step ORC from -0.2 V to +0.15 V and back to -0.2 V, during which 25 mC/cm2 of charge was passed. Charge recovery was typically 198%. After the ORC, the electrolyte was exchanged for one containing 50 mM of the adsorbate of interest in 0.1 M KCl, while the electrode potential was maintained at -0.2 V. For experimentswith PBVPy, a second 25 mC/cm2 ORC was also necessary to obtain the SERS spectra of the polymer. If P2VPy was present during the first ORC, the electrode surface remained shiny and no enhancement was observed. However, exposure of a silver surface roughened in chloride only to P2VPy, but without the second ORC, produced barely detectable signals. This behavior is in marked contrast to that observed with Py or PMePy, both of which showed strong SERS spectra regardless of whether an ORC was performed with the organic species present (in situ) or introduced into the spectrwlectrochemicalcell after an ORC in chloride only (ex situ). SERS spectra obtained with 2MePy and Py under the same conditions as those required to observe measurableSERS signals from P2VPy produced spectra that were substantially (3-5X) more intense than observed with either in situ or ex situ ORC procedures alone, but these spectra did not differ substantially in relative band intensities or positions.
Results and Discussion The cyclic voltammograms of a Ag electrode in 0.1 M KC1 with and without 50 mM P2VPy are shown in Figure
Langmuir, Vol. 4,No.1, 1988 129
SERS of Poly(2-vinylpyridine) aD
Table I. Observed Frequencies (cm-l) and Intensities of Poly(2-vinylpyridine) and 2-Methylpyridine in Normal and Surface-EnhancedRaman Spectra"
8 r
normal Raman PVP PVPH+Clpowder aqueous 1629 (11) 1596 (33) sh sh 1572 (26) 1475 (6) 1480 (7) 1452 (13) 1451 (11) 1438 (11) 1329 (12) 1303 (11) 1315 (14) 1258 (17) 1219 (22) 1224 (51) 1154 (11) 1180 (12) sh 1106 (3) 1088 (16) 1086 (30) 1056 (55) 1054 (68) 998 (100) 1010 (100) sh sh
SERS
~
820 760 632 488 456 414 206
(13) (8) (21) (5) (8) (14) (32)
812 752 632 488 440 418
(15) (26) (33) (20) (33) (25)
PVP at -0.4 V 1624 (45) 1600 (19) 1570 (12) 1479 (8) 1444 (14)
-1.0 V 1624 (11) 1600 (50) 1570 (31) 1479 (11)
2-methylppidine at -0.6 V 1600 (17) 1572 (4) 1479 (4)
1440 (7) 1293 (12) 1252 (14) 1219 (23) 1158 (9) 1106 (13) 1084 (15) 1084 (5) 1057 (75) 1057 (45) 1008 (100) 1008 (100) 984 (17) 984 (11) 882 (8) 886 (8) 808 (17) 810 (12) 762 (40) 756 (37) 643 (27) 638 (43) 499 (36) 486 (14) 454 (47) 446 (20) 418 (43) 418 (23) 230 (100)
1258 (sh) 1222 (44)
1298 (6) 1242 (60) 1158 (8) 1110 (9) 1057 (69) 1013 (100) 984 (11) 806 (95) 644 486 454 418 230
(21) (4) (5) (6) (40)
a Numbers in parentheses are intensities relative to the ca. 1000-cm-' band. Laser wavelength 647 nm.
I
1
200
600
1000
1400
1800
WAVENUMBERS
Figure 2. SERS from Ag/O.l M KC1/0.015 M HC1/0.05 M (in
repeat units) P2VPy at different potentials. Laser wavelength 647 nm.
1. The influence of the polymer on both the formation and reduction of the AgCl layer is clearly indicated by the appearance of multiple waves and a general broadening of the regions associated with the cathodic and anodic processes. This behavior reflects the effect of P2VPy on the roughening procedure (cf. Experimental Section) and can be interpreted in terms of the polymer's ability to mediate acceas of chloride at the Ag surface by competitive adsorption and the presence of positively charged pyridinium groups. No such effect is observed with 2MePy or P Y ,both ~ ~ of which produce SERS active surfaces if present during an in situ ORC step. Figure 2 shows the SERS spectra of 30% protonated P2VPy at four potentials between -0.4 and -1.0 V vs Ag/AgCl. Spectra taken at more negative potentials are similar to those a t -1.0 V, up to the hydrogen evolution potential (HER) of ca. -1.2 Y, where the overall intensity begins to decrease rapidly with time. The most striking differences among these spectra are in the 1550-1650-~m-~ double-bond stretching region and the low-frequency 220-250-cm-' region associated with metal-ligand, vML, vibrations. A t potentials positive of the point of zero charge (E,),which occurs at ca. -0.95 V in 0.1 M chloride electrolyte, peaks at 230 and 1624 cm-' are prominent, but (25) Owen, J. F.;Chen, T. T.; Chang, R. K.; Laube, B. L. Swf. Sci. 1983,131,195.
as the charge on the electrode becomes negative at potentials less positive than of E,, these bands disappear and the double-bond stretching regions become dominated by bands at 1570 and 1600 cm-'. We see no evidence for two sets of peaks in the 1000-1060-~m-~region at different potentials as has been observed for Py3J0912v26 and attributed to separate chemisorbed and physisorbed species. A comparison of the SERS spectra of P2VPy with the normal Raman spectra of a 5% aqueous solution of P2VPy completely protonated with HC1 (P2VPyH+Cl-) and of a solid powder of the unprotonated polymer (Table I) suggests that the potential-dependent changes in the 15501 6 5 0 - ~ m -region ~ are associated with the presence of charged, protonated species. The 1629-cm-I peak in P2VPyH+C1- has been reported2' and is assigned to the vg, ring mode of the pyridinium ion (PyH+) on the basis of similar bands in substitutedB and unsubstitutedB PyH+ salts. Likewise, the fingerprint bands at 1572 and 1596 cm-' in solid P2VPy are assigned to vsa and Vgb modes of the uncharged pyridine ring and are observed at similar frequencies in pyridine and substituted pyridine. (Observation of the NH stretching band, us, which occurs at ca. 2400 cm-' in solid PyH+C1-,28would provide direct evidence of the existence of P2VPyH+Cl- in both the bulk and SERS spectra. However, in aqueous solutions of PyH+Cl-, this band is weak and very broad, and was too weak to be observed in either the SERS or 5% solution spectra of the polymer.) Figure 3 is a comparison of the SERS spectra obtained positive of E,, at -0.4 V, with the normal Raman spectrum of aqueous P2VPyH+C1-; Figure 4 compares the SERS spectrum observed at -1.0 V, just negative of E,, with the normal Raman spectrum of P2VPy powder. It is clear (26) Fleischmann, M.; Hill, I. R.J. Electroanal. Chen. 1983,146,353. (27) Putterman, M.; Koenig, J. L.; Lando, J. B. J. Macromol. Sci., Phys. 1979, BI6, 89. (28) Clementa, R.; Wood, J. L. J . Mol. Struct. 1973, 17, 283. (29) Clementa, R.; Wood, J. L. J. Mol. Struct. 1973, 17, 265.
Lippert and Brandt
130 Langmuir, Vol. 4, No. 1, 1988
4
100
50 mM in 0.1M KCI
33% Protonated -0 4V at Ag Electrode
SERS
80
h
x
* .-
d I
60
ea
i
I
t
v
x
\ \ \ \ \
.".
0
5% Solution in 0. 1M KCI 100% Protonated Normal Raman
200
600
1000
1400
1800
WAVENUMBERS
Figure 3. Normal Raman spectrum of PZVPyH+Cl-(upper) and SERS spectrum of 50 mM PZVPy in 0.1 M KC1 at -0.4 V (lower). 33% Protonated
\I
- 1 . OV at Ag
Electrode
SERS
Powder - _.
Unprotonated Normal Raman
200
600
-.4
-.8
-1.2
-1.6
Potential (V vs. Ag/AgCI) Figure 5. SERS intensities of several fingerprint bands of 30% protonated P2VPy in 0.1 M KCl vs electrode potential.
50 mM in 0. 1M KCI b,
" -0
1000
1400
1800
WAVENUMBERS
Figure 4. Normal Raman spectrum of solid P2VPy (upper) and SERS spectrum of 50 mM P2VPy in 0.1 M KCl at -1.0 V (lower).
from these comparisons that the species present at the electrode surface near E, is predominantly neutral PBVPy, but at potentials positive of E,, where the electrode is positively charged, the adsorbed species is the positively charged P2VPyH+. The 230-cm-' band observed at potentials more positive than -0.8 V has been reported numerous times in the SERS spectra obtained from the Ag/Py/Cl- system.2*5J0,30~31 The intensity of this band is generally ob(30) Fbgis, A.; Dumas, P.; Corset, J. Chem. Phys. Lett. 1984,107, 502.
served to be strongest at potentials between -0.2 and -0.6 V and to drop off rapidly near EZ.31Although this band has not been assigned unequivocally, it appears to be associated with a Ag-Cl,b--Py species, since its intensity is related to the presence of both Py and chloride in the electrolyte. This assignment is consistent with our observation of P2WyH+ at the electrode surface at potentials positive of E,, which requires that a negative bridging ion be present between the positively charged Ag surface and a cationic adsorbate. Figure 5 shows the intensity vs potential profiles of several bands in the P2VP SERS spectra. SERS intensities have been shown to be roughly proportional to surface coverage at electrode surfaces for a variety of ads o r b a t e ~ , although ~ ~ - ~ ~ charge-transfer effects are known to shift the potential of maximum SERS intensity when different excitation wavelengths are used." If a chloride bridge is necessary for bridging of P2WyH+ through PyH+ groups, then the intensities of the 230-cm-' band and the 1624-cm-I peak of P2VPyH+ should have similar potential dependencies (Figure 5). Although the steep Rayleigh background in the vicinity of the 230-cm-' peak does not allow precise intensity measurements in this region, clear similarities exist in the potential-dependent behavior of these bands. By contrast, the intensities of the 1008- and 1600-cm-' peaks, which are associated with uncharged Py groups, vide supra, follow substantially different intensity vs potential profiles (Figure 5). I t should be mentioned that a band at 206 cm-l is observed in solid P2VPy (Figure 4), and it is possible that the 230-cm-l band could be interpreted as arising from a shift in this band, without evoking chloride as a coadsorbate. However, such an as(31) Owen, J. F.; Chen, T.T.;Chang, R. K.; Laube, B. L. Surf. Sci. 1983,125,679.
(32) Weaver, M. J.; Hupp, J. T.; Barz, F.; Gordon, J. G.; Philpott, M. R. J.Electroanal. Chem. 1984, 160, 321. (33) Farquharson, S.; Guyer, K. L.; Lay, P. A.; Magnuson, R. H.; Weaver, M. J. J. Am. Chem. SOC.1984, 106,5123. (34) Roy, D.; Furtak, T. E. Chem. Phys. Lett. 1986, 129, 501.
Langmuir, Vol. 4,No.1, 1988 131
SERS of Poly(2-vinylpyridine)
C I
$1 Z
A
1100
900
WF&"M
BER s
Figure 6. (a) Normal Raman spectrum of 0.75 M 75% protonated P2YP; (b) SERS of 50 mM 30% protonated P2VP in 0.1 M KC1 at -1.0 V; (c) normal Raman spectrum of 1.5 M 65% protonated 2MePy; (d) SERS of 50 mM 2MePy in 0.1 M NaCl at -1.0 V. Intensities are arbitrarily scaled.
signment is not consistent with the observed potential dependence of the SERS band nor in agreement with the relative intensity of this lower frequency band to other bands in the SERS spectrum. While interpretation of bands in the SERS spectra of P2VPyH+ associated with Py and PyH+ groups is aided by the extensive SERS investigations of pyridine at Ag surfaces, obtaining similar information about bands arising from the polymer backbone is less straightforward. Bands due to the hydrocarbon chain have been assigned for p ~ l y s t y r e n eand ~ ~ P4VPy3s and should be similar for P2VPy. On this basis, tentative assignments can be made for bands at 1440-1450 (6 CH2), 1293-1315 (6 CHJ, 1084-1088 (vc4), and 752-760 cm-l (y CH2). The 750800-cm-' region is sensitive to the tacticity of P2VPy in solution, and the maximum we observe at 760 cm-', in both the SERS and normal Raman spectra (Figures 3 and 4)) is consistent with an isotactic polymer.27 The 1084-1088-cm-1 band observed in the polymer is not observed in the SERS or normal Raman spectrum of 2MePy (Figure 6); therefore, it can be reasonably assigned to the chain stretching mode. Relative to the bands arising from the P y ring, this band is much weaker in the SERS spectra at all potentials than in the normal Raman spectra. At least three possible explanations for this behavior exist: (1)the chain is farther from the SERS surface site than the ring and, thus, not as strongly enhanced; (2) the orientation of the v c 4 vibration is unfavorable for SERS;or (3) some change in chain conformation has changed the intensity of this mode. There is no shift in the position (35)Snyder, R. W.;Painter, P. G. Polymer 1981,22,283. (36)Vorontaov, Y.D.;Panov, B. P. Vysokomol. Soedin., Ser. A 1976, A18, 2412.
of the vcc band between the S E W and the normal Raman spectra of the polymer as might be expected if a conformational change was responsible for the lowered relative intensity. In addition, if adsorption occurs involving either the ring nitrogen or the entire Py ring in a flat-on orientation, it is difficult tQ envision a conformationof adsorbed P2VPyH+ which does not bring the C-C backbone close to the Ag surface. Therefore, the second explanation seems to be the one that best supports the experimental observations; i.e., the polymer backbone is oriented such that the C-C stretch does not couple strongly with the SERS excitation. Several recent studies6J7* have suggested that vibrations that have tensor components predominantly parallel to the surface of .a metal particle will show smaller SERS enhancements than those that have tensor components perpendicular to the surface. The observation of a smaller enhancement of the C-C chain stretching vibration suggests that the polymer is partially oriented parallel to the Ag surface. Since SERS is only observed on a Ag surface that has both microscopic and atomic scale roughness features, this interpretation implies that it is the orientation of the adsorbate relative to the Ag surface sites that is responsible for SERS activity, rather than the orientation relative to the incident laser beam (assuming, of course, that a statistical distribution of Usitesnexists across the illuminated electrode surface). Unfortunately, insufficient work has been done in the area of SERS orientation effects to make interpretation of these results u n a m b i g u o ~ s . For ~ ~ example, while it is well-known that electromagnetic and resonance effects associated with Raman enhancement by a SERS-active surface can also influence the intensity vs potential profiles of adsorbed pyridine derivatives, these effects are as yet poorly understood and cannot easily be resolved from changes in intensity brought about by reorientation of the adsorbed species or modification of the SERS-active site^.^^.^ Future mechanistic SERS studies in this area, as well as studies dealing with polymers attached to electrode surfaces via "anchoring" groups,P1may be helpful in understanding these effects. As a low molecular weight analogue of the P2VPy polymer repeat unit, 2MePy is a good probe molecule with which to compare adsorption behavior of the macromolecule at a SERS-active surface. For a solution of 50 mM ZMePy, 30% protonated with HC1 in 0.1 M KC1, we see no evidence of the 1624-cm-' band of 2MePyH+Cl- at potentials as positive as -0.2 V. (Frequencies and relative intensities of the major bands observed at -0.6 V, where the maximum in the v m 230-cm-l band is observed, are listed for comparison in Table I.) By contrast, SERS from P2VPyHah+ is observed at potentials as negative as -0.8 V (Figure 5). Chang and Hwang, having investigated the SERS of Py and PyH+X- (where X = Cl-, Br-, and I-) at roughened Ag electrodes, found that even in completely protonated solution (pH 2.7 in HC1) PyHads+dominates the SERS spectrum only at potentials more positive than -0.3 V.O ' Neutral Pya& is uniquely observed at more negative potentials, in agreement with our results with 2MePy. They suggest that this behavior can be explained by the instability of the PyH+Clad, complex, which decomposes to Pya& and Haq+ at potentials where clad, is desorbed. However, in view of the accepted instability of SERS-active (37)Creighton, J. A. Surf. Sci. 1983,124,209; 1983,158,211. (38)Moskovita, M.;Suh, J. S. J. Phys. Chem. 1984,88,1292,5526. (39)Creighton, J. A. Surf. Sci. 1986,173,665. (40)Otto, A. J. Electron Spectrosc. Relat. Phenom. 1983,29, 329.
132 Langmuir, Vol. 4, No. 1, 1988
Lippert and Brandt
1.0
En/
7
1
b
1
&6
5min ,
Init
I
1
1
1
Time/min
Figure 7. Reversibiltiy of 1008-cm-' band for (a) 50 mM Py in 0.1 M KC1 (pH 8.3) and (b) 50 mM 30% protonated PPVPy in 0.1 M KCl (pH 4.5), as a function of potential cycling between -0.2 and -1.0 V (upper trace). Two double-potential-stepORCs from -0.2 V to +0.2 V and back to -0.2 V (tinit) were used (50 mC/cm2 total charge) as pretreatment. sites in the absence of Clad;, it is unclear from this mechanism why SERS is observed from the remaining Py* As previously mentioned, we did not observe a band similar to the 1026-cm-l band assigned by these authors and others3J0J2v26 to the PyHads+species in the spectra of 2MePyH+. Also, only a weak band near this frequency was discernible in some of the SERS spectra of 30% protonated P2VPy obtained at potentials more positive than -0.6 V. This band seems to be most intense in the SERS spectra of Py after pretreatments that involve at least 1 order of magnitude more anodic charge being passed during the ORC step than the 25 or 50 mC/cm2 used in these studies;26therefore, it was not followed as closely as other bands that were more reproducible under our electrode pretreatment conditions. The differences between SERS from P2VPy and lower molecular weight Py and 2MePy adsorbates also extend to the reversibility of the enhancement as the potential of the Ag electrode approaches E,. We observed that, at the pH necessary to protonate 30% of the P2VP (see Experimental Section), the HER was ca.200-300 mV more positive for SERS-active Ag electrodes in the presence of Py and 2MePy than for the polymer, suggesting that the polymer is more tenaciously adsorbed to the electrode than the low molecular weight species. To see to what extent the strength of adsorption influenced the stability of the SERS-producing sites, we cycled the potential of the electrode between -1.0 V (a potential negative of E,, but not sufficiently negative to rapidly evolve hydrogen at the pHs of the partially protonated polymer, or unbuffered, pH &8.5,50 mM solutions of Py and 2MePy in 0.1 M KC1)
and -0.2 V (just negative of the AgCl wave (cf. Figure 1)) while following the intensity of the vg Py ring-breathing mode at ca. 1006 cm-l, which was common to all spectra. Our results with Py and 2MePy (Figure 7a) are similar to those obtained by Owen and co-workers, who found an irreversible loss of SERS activity for PyA as the potential was scanned negative of EZ.25They attributed the loss of SERS enhancement to reincorporation of Ag "adatoms" into the bulk Ag lattice as Clads-and Pya&were desorbed by largely Coulombic repulsion with the negatively charged electrode as the electrode potential was scanned negatively through E,. By contrast, Ag electrodes in the presence of P2VPy show a significant recovery (>85%) of the SERS intensity as the potential is repeatedly cycled between -0.2 and -1.0 V (Figure 7b). The reversibility of SERS from the low molecular weight species was also found to be sensitive to the electrode pretreatment step (a phenomenon currently under more detailed study in this laboratory); however, in the presence of the polymer, reversibility (to the extent discussed above) was always observed under our pretreatment conditions (see Experimental Section). The potential-dependent behavior of the polymer can be explained by two factors which are not operative in the Py or 2MePy systems. The first is the solubility of the polymer. Even when 30% protonated, the polymer is sparingly soluble in 0.1 M C1-. Therefore, as the potential is scanned negatively to potentials where the lower molecular weight species desorb, P2VPy remains "clinging" to the electrode surface, mainly due to hydrophobic interactions, since the Ag surface should be free of oxides at potentials approaching The second factor contributing to the relative strength of adsorption and higher degree of SERS reversibility at negative potentials is the presence of PyH+ groups, along with uncharged Py groups, connected by a common backbone in the polymer molecule. These species, if present as individual ions and molecules, would desorb as clad[ is desorbed near E,, and SERS acitivity would be lost as the active sites become unstable. However, since the solubility of the polymer restricts the desorption process, a significant concentration of positively charged sites can remain within the double-layer region. These positive centers can stabilize active sites by restricting the diffusion of C1- away from the Ag surface or by increasing the overpotential for the adsorption of H+, which would replace any remaining positively charged Ag atoms or clusters at the electrode surface. Both the polymer solubility and the anchored positively charged surface sites could increase the stability of the SERS active sites and would be expected to affect the pretreatment step (Figure l),as well as the HER, as is observed. Differential capacitance measurements currently in progress are expected to provide further insight into this mechanism. Registry No. P2VPy, 25014-15-7; Ag, 7440-22-4; KC1, 744740-7; HCl, 7647-01-0; C1-, 16887-00-6; pyridine, 110-86-1; 2methylpyridine, 109-06-8. (41) Droog, J. M. M.; Huisman, F. J. Electround. Chem. 1980, 115,
211. (42) Suh, J. S.; Michaelian, K. H. J.Raman Spectrosc. 1987,18,409.
