Langmuir 1987,3, 777-785 The effect of added NaCl on &,/I,,, in the EPE system is small compared to the effects observed in the SDS and CgPhE, systems and is consistent with a higher solubility of Py-S in the EPE micelles.
Conclusions The effects of pressure, temperature, and added salt on the micellar solubilization of the Py-S probe are dependent on micelle type. In anionic micelles such as SDS, pressure and added salt cause an increase in probe solubility in the micellar phase, whereas temperature has a weaker and opposite effect. In nonionic micelles of the CgPhE, systems, solubilization depends on n, and two effects occur: an increase in micelle hydrophilicity and solubility of Py-S with increasing n and a decrease in aggregation numbers with increasing n. For a typical member of the CSPhE,
117
family, increased pressure and added salt causes an increase in probe solubility in the micellar phase whereas increased temperature causes the opposite effect. For the EPE micelles, the probe solubility is not sensitive to added salt but exhibits unusual pressure and temperature behavior, compared to SDS and CgPhE, micelles. The latter behavior suggests a significant change in micellar structure with pressure and temperature for the EPE micellar systems.
Acknowledgment. We thank the Army Office of Research and the 3M Corp. for their generous support of this research. Registry No. Py-S, 59323-54-5; SDS, 151-21-3; EPE, 106392-12-5; CBPhE,, 9016-45-9; C9PhE6,26264-02-8; NaCl, 7647-14-5.
Interpretation of SERS Intensity-Lead Coverage Profiles for Pyridine and Chloride Ion in Terms of Electromagnetic Enhancement and Charge-Transfer Excitation Anita L. Guy and Jeanne E. Pemberton" Department of Chemistry, University of Arizona, Tucson, Arizona 85721 Received August 8, 1986. I n Final Form: April 7, 1987 The excitation behavior of the quenching of SERS for pyridine and C1- at Ag electrodes in the presence of monolayer and submonolayer amounts of underpotentially deposited Pb is presented in this report. Intensity-coverageprofiies are presented for the ring-breathingmode of pyridine at 1013 cm-l and v(Ag-Cl) at 235 cm-' with laser excitation at 4579,4880,5145, and 6226 A. Electromagnetic contributions to the quenching behavior are evaluated by comparison of the experimental quenching profiles with theoretical profiles. Theoretical profiles are calculated from a model described by Murray which predids electromagnetic enhancement at overlayer-covered metal ellipsoids. The experimental and theoretical profiles are in poor agreement, especially for excitation in the red-wavelengthregion. A chemical model based on photoassisted charge transfer is proposed to account for the experimental behavior at other excitation wavelengths.
Introduction Recent experimental investigations into the phenomenon of surface-enhanced Raman scattering (SERS) have been directed toward unraveling the complex contributions to the overall surface enhancement made by classical electromagnetic (EM) and so-called chemical mechanisms; Elucidating the relative contribution of each has proven to be a difficult task. This is due, in part, to the difficulty in varying only one experimental parameter of the system in a systematic fashion while leaving others unchanged. One approach to the systematic variation of surface properties that has received considerable attention is the use of ultrathin metal films deposited onto foreign metal substrates. In electrochemical environments, this is readily accomplished through the use of underpotential metal deposition (UPD). This technique allows submonolayer amounts of the film metal to be deposited in a highly reproducible and quantifiable fashion. The electronic properties of the surface are known to be altered by small amounts with each submonolayer increment of the film deposited. When this metal deposition process is carried out in the presence of SERS-active adsorbates, the relative change in SERS intensity of the probed species can be *Author to whom correspondence should be addressed.
0743-7463/87/2403-0777$01.50/0
readily followed as a function of surface coverage. This approach has been successfully applied to the investigation of many systems.'-12 In all of the studies involving the deposition of a nonenhancing metal onto an enhancing metal, the SERS intensities of the probe adsorbate are quenched to undetectable levels upon deposition of monolayer amounts of the nonenhancing metal. Moreover, the quenching of SERS for some adsorbates at Ag has been shown to qualitatively correlate with an increase in the imaginary part of the surface dielectric function or surface absorptivity for fractional Pb coverages between 0% and ca. 40% of a monolayer.' This has been (1)Loo, B. H.; Furtak, T. E. Chem. Phys. Lett. 1980,71,68. (2)Pettinger, B.;Moerl, L. J. EEectron. Spectrosc. Relat. Phenom. 1983,29,383. (3) Moerl, L.; Pettinger, B. Solid State Commun. 1982,43,315. (4)Watanabe, T.;Yanagihara, H.; Honda, K.; Pettinger, B.; Moerl, L. Chem. Phys. Lett. 1983,78,7466. (5)Kester, J. J. J. Chem. Phys. 1983,78, 7466. (6)Pemberton, J. E.J. Electroanal. Chem. 1984,167,317. (7)Guy, A. L.;Bergami, B.; Pemberton, J. E. Surf. Sci. 1985,150,226. (8)Guy, A. L.;Pemberton, J. E. Langmuir 1985,1 , 518. (9)Guy, A. L.;Pemberton, J. E. Langmuir 1987,3,125. (10)Kellogg, D.S.;Pemberton, J. E. J.Phys. Chem. 1987,91,1127. (11)Coria-Garcia, J.; Pemberton, J. E.; Sobocinski, R. L. J. Electroanal. Chem. 1987,219,291. (12)Pemberton, J. E.;Coria-Garcia, J.; Hoff, R. L.Langmuir 1987,3, 150.
0 1987 American Chemical Society
778 Langmuir, Vol. 3, No. 5, 1987 interpreted as evidence for the importance of surface electronic properties in the quenching of SERS in these systems. The lack of correlation at all Pb coverages suggests that other factors may also contribute to the observed response. In fact, several other interrelated aspects of the SERS phenomenon are known to influence the measured SERS intensities. These aspects include changes in surface roughness, changes in adsorbate coverage, and chemical contributions to surface enhancement arising from differences in the chemical nature of activated complexes formed at the metal surface. Previous work in this laboratory has shown that the deposition of monolayer and submonolayer amounts of Pb results in the destruction of atomic-scale roughness (ASR) and the SERS intensity associated with these This has been shown to occur for large P b coverages (6 > 0.7) and at very long times at low P b coverages. Moreover, differences in the intensity-coverage profiles for SCN-, Br-, and C1- at 5145 8, previously have been explained in terms of an adsorbateto-metal charge-transfer (CT) model for surface enhancement.12 These previous studies have documented the involvement of changes in surface roughness, changes in adsorbate coverage, and differences in the chemical nature of the adsorbate in the overall quenching of the SERS response in the presence of underpotentially deposited nonenhancing thin metal f i s . However, no attempt has been made to systematically evaluate the excitation dependence of SERS intensity-coverage profiles for two chemically different adsorbates in the same solution environment in terms of EM enhancements or chemical contributions to SERS. This approach is especially attractive, because only the excitation wavelength is varied in these studies without the possibility of other chemical differences, such as oxidation-reduction cycle (ORC) conditions, contributing to the measured response. Therefore, this type of analysis is ideal for elucidating various contributions to the SERS response. Toward that end, SERS intensity-Pb coverage profiles were obtained for pyridine and C1- with laser excitation at 4759,4880,5145,and 6226 8, and are reported here. The excitation dependence has been evaluated for contributions from EM enhancement by using a model proposed by Murray13for EM enhancement at overlayer-covered metal ellipsoids. Contributions from a chemical mechanism were evaluated based on a photoassisted charge-transfer model for SERS. The results of these analyses are presented in this report.
Experimental Section The laser Raman system used for these studies has been described in detail previously.'~ The spectra at 4579,4880, and 5145 8, were obtained with excitation from an Ar+ laser. For excitation at 6226 A, the Ar+ laser was used t o pump a Coherent CR-599 dye laser containing a Rhodamine GG/ethylene glycol solution. Laser power measured a t the cell was 100 mW. Spectra in the v(Ag-Cl) region were acquired at 0.5-cm-' increments over a 0.5-s integration period between 150 and 300 cm-'. These spectra were obtained as single scans except at 4579 A where two scans were required t o obtain adequate signal-to-noise discrimination. Spectra for the ring-breathing mode of pyridine were acquired between 980 and 1050 cm-' by using the same data acquisition parameters. All spectra in this region were acquired as single scans. Electrochemical equipment and cells used for these investigations have also been described previously.'Ss Polycrystalline Ag (Johnson Matthey, 99.9%) electrodes were mechanically ~
(13) Murray, C. A. J . O p t . SOC.An.B: Opt. Phys. 1985, 2, 1330.
Guy and Pemberton polished with successively finer grades of alumina (Buehler) down to 0.05 wm, rinsed with triply distilled, deionized water, and sonicated before use. All potentials were measured and are reported vs. a Ag/AgCl reference electrode. The solutions used to perform these experiments consisted of 0.1 M KC1/0.05 M pyridine/l X M Pb(NO&, pH 5.5. All chemicals were reagent grade and used as received. All solutions were prepared from triply distilled, deionized water, the last distillation being from basic permanganate. All solutions were deaerated by bubbling with N2 prior to use. The Ag electrodes were subjected to ORCs prior to the acquisition of SERS spectra. These pretreatments were performed by using a potential sweep method in which the potential was ramped in a linear fashion a t 10 mV from an initial potential of -0.200 V until ca. 25 mC cm-2 of total anodic charge was passed. The electrodes were polished between ORCs such that the Ag surface was oxidized and reduced only once before SERS spectra were recorded. Deposition isotherms of UPD P b monolayer and submonolayer deposition were obtained by measuring the area under UPD P b stripping waves after maintaining the applied potential for 100 s. The fractional P b coverages were calculated by comparing the charge under the stripping wave obtained a t different potentials with that obtained for a full monolayer on a given electrode surface. Areas under the stripping waves were measured with a planimeter.
Results and Discussion Excitation Profiles. Typical spectral results obtained for pyridine and v(Ag-C1) with excitation at 4579 A are shown in Figure la. The fractional Pb coverage value associated with each spectrum is indicated. The spectral features in the v(Ag-C1) region between 150 and 300 cm-' are very weak and often required two scans of the spectral region to obtain adequate signal-to-noise ratios. Intensity-coverage profiles are shown in Figure 2a for the pyridine and the v(Ag-C1) band. Integrated intensities of the pyridine band above a straight-line background were calculated from the spectra after subtraction of the contribution from solution pyridine. In the low-frequency region, tailing from the Rayleigh line can yield erroneous values for the integrated band intensities without prior background subtraction. It is convenient in these studies to use the spectral results obtained in the frequency region at full monolayer coverage as the background response. This approach is valid, because the intensity of the v(AgC1) band is completely quenched at Pb coverages of 0.7-0.8 of a monolayer. Integrated intensities of the v(Ag-Cl) band were accurately determined by this subtraction procedure to yield the profiles in Figure 2. These profiles are normalized to unity at zero Pb coverage. The error bars reflect the scatter in SERS intensity between two experimental runs for each vibrational feature. The intensity-coverage profiles are very similar for both vibrational features with 4579 8,excitation except at low Pb coverages (less than 10-20% of a monolayer). In this coverage region, the integrated band intensity for v(Ag-C1) increases slightly with respect to that measured at the potential corresponding to zero P b coverage. As a result, the normalized intensity values are greater than 1.0 within this region. Examples of the spectral results obtained with excitation at 4880 8, are shown in Figure lb, and the corresponding intensity-coverage data are presented in Figure 2b. Good agreement is observed in the intensity-coverage profiles for pyridine and C1- except in the region between ca. 40% and 70% of a P b monolayer. In this region, the intensity of v(Ag-Cl) decreases more rapidly with increasing Pb coverage. The spectral results obtained at 5145 8, are shown in Figure IC,and the corresponding intensity-coverage pro-
SERS Intensity-Pb Coverage Profiles
Langmuir, Vol. 3, No. 5, 1987 779
b
a
I
4579
i
h
I
4880
i
_.. .
Figure 1. SERS spectra of the v(Ag-C1) and pyridine vibrations as a function of Pb coverage with laser excitation at (a) 4579, (b) 4880, (e) 5145, and (d) 6226 8,.
0 10
Pb Coveroge
P b Coverage
Figure 3. Comparisan of the excitation dependence of the in-
tensity-coverage profiles for: (a) pyridine at 4579 ( O ) ,5145 (A), and 6226 8, (0); (b) v(Ag-C1) at 4579 (O),5145 (A), and 6226 8, (0).
Figure 2. SERS intensity-Pb coverage profiles for v ( A g 4 1 ) (A) and pyridine ( 0 )vibrations at (a) 4579, (b) 4880, (e) 5145, and
(d) 6226 A.
files are shown in Figure 2c. The intensity-coverage profiles for pyridine and v(Ag-C1) show significant differences for P b coverages between 0% and 70-80% of a monolayer. Typical spectral results in the pyridine and v(Ag-C1) regions with laser excitation at 6226 A are shown in Figure Id. The reduced data are plotted in Figure 2d. The intensity-coverage profiles for the two adsorbates exhibit deviations for P b coverages up to 7040% of a monolayer. The similarities in intensity-coverage profiles observed for the pyridine and v(Ag-C1) vibrations with excitation at 4579 and 4880 A suggest that electromagnetic contributions may be an important contribution to the measured response in this wavelength region. Theoretical enhancement-coverage profiies discussed in the next section tend to support this conclusion.
Discrepancies between the intensity-coverage behavior for the two adsorbate bands with excitation at 5145 and 6226 A suggest the importance of additional contributions to the SERS response in this wavelength region. Another important aspect of these results is the fact that the intensity of the pyridine vibration is essentially independent of Pb coverage up to 6&70% of a monolayer for excitation of 6226 A. These results demonstrate that Pb-modified Ag surfaces are capable of supporting surface enhancement with judicious choice of the experimental conditions. This system represents one of only a few examples of a nonenhancing modifier on a SERS substrqte to exhibit significant SERS activity at higher modifier coverages. Finally, these results suggest that the decrease in SERS intensity is not due primarily to changes in adsorbate coverage at Pb-modified Ag relative to clean Ag. If changes in adsorbate coverage were the predominant contribution to the quenching of SERS,one would expect intensitycoverage profiles for pyridine and C1- to be only weakly dependent on excitation freq~ency.'~ The intensitycoverage data shown in Figure 2 suggest that this is not the case. Figure 3a shows a comparison of the measured SERS intensity-Pb coverage profiles for the pyridine vibration (14)Murray, C. A.; Bodoff, S.Phys. Rev. B. 1985, 32, 671.
Guy and Pemberton
780 Langmuir, Vol. 3, No. 5, 1987 at 1013 cm-l at three of the four excitation wavelengths examined. The SERS intensity decreases less rapidly as a function of increasing Pb coverage as the wavelength increases from 4579 to 6226 A. The corresponding behavior for the v(Ag-C1) vibration at 235 cm-' is shown in Figure 3b. As the incident wavelength increases, the SERS intensity for this feature decreases more rapidly as a function of increasing Pb coverage. These results are opposite to the intensity-coverage trends for pyridine shown in Figure 3a. This reversal in SERS responses may be related to the different chemical natures of the adsorbates. Theoretical Enhancement Profiles. Theoretical enhancement profiles were calculated by using an EM model for molecules adsorbed onto overlayers on metal particles developed by Murray.13 In this approach, the electric field enhancement at the tip of a pair of confocal ellipsoids is calculated where the inner shell is, in this case, Ag and the outer shell is Pb. By proper choice of the optical constants of the inner ellipsoid, the broadening effects of particleparticle dipolar interactions can be taken into account. In this way, the model can be used to predict the electromagnetic behavior of island films comprised of these confocal ellipsoids placed randomly in a two-dimensional lattice. This situation physically approximates that of roughened Ag electrodes covered with UPD Pb. In the first set of calculations, the shapes of the surface roughness features were taken to be ellipsoids with semimajor axes of 150 A and semiminor axes of 50 A. These values are the closest approximation to the large-scale surface roughness observed on electrochemicallyroughened Ag electrodes that can be used without exceeding the limits of the model. The model assumes that the outer ellipsoid is small compared to the wavelength of incident light so that the problem can be solved without retardation in the small particle limit.15J6 The real part of the dielectric constant of the Ag inner ellipsoid was taken from Johnson and Christy" for the wavelength of interest. The imaginary part of the Ag dielectric constant was set to 3.0 to approximate the interaction between roughness features after the manner of Murray.13 Use of this model requires values for the optical constants of the overlayer. For the majority of the results presented here, the values for a Pb/electrolyte interfacial phase were estimated by using the Bruggeman effective medium approximation (EMA).18 For some of the results, dielectric constants were calculated from measured differential reflectivity data reported by Kolb and co-worke r ~ by ' ~using the linear approximation method.20 The SERS enhancement, 7,is calculated from this model according to the expression letip(wo)121etip(ws)Iz
v=
leti,(wo)12
iE0i4
where is th_e intensity at the tip at the incident laser frequency wo, IEtip(wa)12is the intensity at the tip at the scattered frequency we, and IEo12is the incident applied field intensity at w,,. This quantity represents the classical electromagnetic enhancement of the Raman scattering of an isolated molecule located exactly at the tip of the outer ellipsoid. (15)Kerker, M.; Wang, D. S.; Chew, H. Appl. Opt. 1980, 19, 4159. (16) Barber, P.W.; Chang, R. K.; Massoudi, H. Phys. Reo. B. 1983,27, 7251.
(17) Johnson, P. B.; Christy, R. W. Phys. Reo. B. 1972, 6, 4370. (18)Bruggeman, D. A. C. Ann. Phys. 1935,24,636. (19) Kolb, D. M., In Surface Polaritons; Agranovich, V. M., Mill, D. L., Eds.; North Holland: Amsterdam, 1982;326. (20) Kolb, D. M.; McIntyre, J. D. E. Surf. Sci. 1971, 28, 321.
2500
1"'
"
"
'
'
"
'
" " " "
'
" "
'
"
00
N a v e l e r g t h (A]
Figure 4. Theoretical SERS enhancement, 7, vs. excitation wavelength for A ellipsoid of semimajor axis of 150 8, and semiminor axis of 50% for Pb coverages of 0.0, 0.3, 0.6, 1.0, 2.0, 5.0, and 10.0 monolayers.
Figure 4 shows the predictions of this model for a Ag ellipsoid of the above dimensions with P b coverages ranging from 0 to 10 monolayers as a function of incident laser frequency for ws = wo. In an ambient dielectric medium of e 1.77, the resonance for this particle is observed at 5900 A. As the P b coverage increases, this resonance broadens, shifts toward the red, and decreases in amplitude. These observations are in agreement with those of Murray for similar systems.13 Plots of normalized enhancement as a function of P b coverage are easily obtained from these data at any excitation frequency for w, = wo. The predictions of this model for P b coverages from zero to one monolayer are of particular interest in this investigation. Theoretical SERS enhancement values were calculated for these P b coverages at 4500-, 5150-, and 6250-A excitations for the two cases where w, corresponds to the 1013-cm-' pyridine ring vibration and the 235-cm-' v(Ag-Cl) vibration. The results for the pyridine band are shown in Figure 5a,b,c for the measured and estimated optical constants of the P b overlayer. The SERS enhancement values are normalized to unity at zero coverage to allow for comparison to the experimental data. The dashed circles in Figure 5 represent the results obtained by using the optical constants calculated from the measured differential reflectivity data, and the open circles represent the results obtained by using estimated optical constants. At the lower Pb coverages, the results obtained by using the measured and estimated optical constants are in reasonable agreement. Significant discrepancies exist between the two seta of results at higher P b coverages, especially with 4500- and 5150-A excitation. Regardless of which set of optical constants are used, the overall enhancement at 4500-A excitation is expected to be very small as shown in Figure 4. Therefore, the discrepancy between the two sets of results is of limited significance. In contrast, the overall enhancement is predicted to be large with 6250-A excitation. Thus, the discrepancy between the two sets of results is significant and requires further discussion. The results obtained by using the measured optical constants exhibit much greater quenching of the enhancement at higher P b coverages than do those obtained with the EMA. On the basis of the trends evident in Figure 4,this greater quenching implies that the measured optical constants of the thin P b films resemble those of thicker P b films than do the optical constants obtained with the EMA. For example, the optical constants from the measured differential reflectivity data at one monolayer coverage for 6250-A excitation resemble those obtained for
0
0 tll
0
0
0
C
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