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Langmuir 1987, 3,125-132
Contributions of Atomic-Scale Roughness and Adsorbate Coverage to the Quenching of the SERS Response at Lead-Modified Silver Electrodes Anita L. Guy and Jeanne E. Pemberton* Department of Chemistry, University of Arizona, Tucson, Arizona 85721 Received October 19, 1985 This report details a series of experiments and calculations designed to provide a more complete analysis of the factors which play contribute to the quenching of the SERS response at Pb-modified Ag electrodes. SERS intensity-Pb coverage profiles are presented for the pyridine ring-breathingvibration after the majority of atomic-scale roughness featuresare destroyed by Pb monolayer deposition. Comparison of the morphology of the intensity-coverage profiles obtained during and after the loss of atomic-scaleroughness indicates that loas of the majority of atomic-scale roughness during Pb deposition cannot fully account for the observed quenching of the SERS response. The SERS response of 3,6-dihydroxypyridazine (DHPZN) is examined in order to evaluate the impact of adsorbate coverage on the observed quenching behavior. X-ray photoelectron spectroscopic studies of DHPZN adsorbed at Ag and Pb-modified Ag electrodes indicate that changes in the adsorbate surface coverage during Pb deposition do not contribute significantly to the quenching of the SERS response. Theoretical SERS enhancement-Pb coverage profiles based on two different electromagneticpredictions and previously calculated optical constants for Pbmodified Ag surfaces are presented. The functional form of the linear approximation expression used for evaluation of surface optical constants from differential reflectivity data is examined in order to provide further insight into the role of surface electronic properties in the SERS mechanism(s). Introduction The effect of underpotentially deposited Pb thin films on the SERS response of pyridine and C1- adsorbed at roughened Ag electrodes has been detailed in two previous reports from this laboratory.'J The aim of this work is to systematically evaluate the role of surface electronic properties in the SERS mechanism(s) through the use of a series of surfaces fabricated by electrochemicaldeposition of foreign metal films on SEW-active electrodes. Previous work has shown that the underpotential deposition of between zero and 70% of a P b monolayer results in the quenching of the SERS intensity of vibrational bands associated with adsorbed Cl- and pyridine-l The decrease in SERS intensity as a function of fractional P b coverage between zero and 40% of a monolayer has been correlated with an increase in the imaginary part of the surface dielectric function or surface absorptivity. These results are consistent with those obtained for Ag/Au alloys3and are qualitatively in agreement with predictions based on electromagnetic models for surface enhancement. The correlation between observed SERS intensity and surface absorptivity at low P b coverages suggests that changes in surface optical/electronic properties may be responsible for the quenching of the SERS response. The lack of correlation at higher P b coverages (8> 40%) suggests that other factors may also contribute to the observed response. In fact, several other interrelated aspects of the electrochemical interface are known to influence the measured SERS intensity. Therefore, these may also contribute to the quenching of the SERS response at Pb-modified Ag electrodes. These interrelated aspects inclucde, but are not limited to, changes in surface roughness upon P b deposition and changes in the adsorbate surface coverage at Pb-modified Ag surfaces. This report details a series of experiments and calculations designed to provide a more complete interpretation of the morphology of the SERS intensity-Pb coverage profiles. Theoretical SERS enhancement-Pb coverage profiles based on two electromagnetic predictions and
* Author to whom correspondence
should be addressed.
previously calculated optical constants for Pb-modified Ag surfaces are presented. The functional form of the linear approximation expression,4used for evaluation of surface optical constants from differential reflectivity data for P b on Ag(111),5is examined in order to provide further insight into the role of optical properties in the SERS mechanism. Recent experimental work designed to evaluate the impact of surface roughness and adsorbate coverage on the observed quenching of the SERS response is also presented. Experimental Section The laser Raman system used for these studies has been described in detail elsewhere.' All spectra reported here were obtained with 514.5-nm excitation. Laser power measured at the sample was typically 160 mW. Spectra were acquired at 0.5-cm-' intervals with a 0.5-8integration period. All spectra were obtained as single scans. Electrochemical equipment used for these investigations has also been described previously.' The spectroelectrochemical cell used for the DHPZN studies is s i m i i to that used for the previous studies except that an isolated auxiliary electrode compartment was required. Polycrystalline Ag disk electrodes were mechanically polished to a mirror finish using successively finer grades of alumina (Buehler),rinsed with copious amounts of triply distilled, deionized water, and sonicated prior to an oxidation/reduction cycle (ORC)pretreatment. All potentials were measured and are reported vs. a Ag/AgCl reference electrode except where indicated otherwise. Solutions containing 0.1 M NaC10,/0.01 M DHPZN were prepared on a daily basis. DHPZN (Aldrich) was purified by recrystallization from an ethanol/water mixture. The electrolyte (NaC104)was obtained from Alfa (>99+%) and used as received. DHPZN was dissolved by sonication. Solution pH was unadjusted at 3.5. Solutions containing 1 X M Pb2+were prepared by M Pb(N03)2(pH 5.5) dilution of an aliquot of aqueous 2.5 X using a 0.1 M NaC104/0.01M DHPZN solution. All solutions (1)Guy, A. L.; Bergami, B.; Pemberton, J. E. Surf. Sci. 1985,150,226. ( 2 ) Guy, A. L.; Pemberton, J. E. Langmuir 1985, 1, 518. (3) Kester, J. J.; Furtak, T. E. Solid State Commun. 1982, 41, 457. (4) Kolb, D. M.; McIntyre, J. D. E. Surf. Sci. 1971, 28, 321. (5) Takayanagi, K.; Kolb, D. M.; Kambe, K.; Lehmpfuhl, G. G. Surf. Sci. 1980, 100, 407.
0743-7463/87/2403-0125$01.50/00 1987 American Chemical Society
Guy and Pemberton
126 Langmuir, Vol. 3, No. 1, 1987
, ..
* 980
1015
1050
cm-1
Figure 2. SEW spectra for pyridine band at 1013 em-' at rep resentative fractional Ph coverages after ASR destruction: (a) B = 0.00; (b) B = 0.13; (c) B = 0.29; (d) B = 0.49; (e) B = 0.69; (fl B = 1.00.
Figure 1. Scanning electron micrographs of roughened Ag electrodes: (a) hefore Ph monolayer deposition and stripping; (b) after Ph monolayer deposition and stripping. were prepared from triply distilled, deionized water, the last distillation being from permanganate. Total organic carbon levels in the triply distilled water are less than 70 ppb. All solutions were deaerated by bubbling with N, prior to use. X-ray photoelectron spectra were obtained with a Vacuum Generators ESCALAB MK I1 electron spectrometer with an AI Ko (1486.6 eV) anode. X-ray power on the samples was 300 W (20 mA, 15 kV). Vacuum levels during acquistion were typically torr. XPS samples were prepared by soldering Ag foils of 0.5." diameter to stainless steel mounts compatible with both the XPS sampling apparatus and electrochemical mounts. The Ag foil surfaces were prepared as described above. Following electrochemical preparation, the samples were transferred into a fast entry lock of the vacuum system. The samples were prepared by application of the appropriate potential in the absence or presence of PhZ+in solution for ea. 15 min. The electrode was removed from solution under potential control and rinsed with copious amounts of triply distilled, deionized water. The binding energies of the N Is signals are referenced to the C Is line taken as 284.6 eV.
Results a n d Discussion Contribution of Surface Roughness. Quenching of the SERS intensity at Pb-modified Ag surfaces could be the result of changes in surface roughness features brought about by the deposition process. Two classes of surface roughness have been implicated in the SERS response. Large-scale roughness features (>300 A) are thought necessary for surface enhancement from electromagnetic
considerations.6 Roughness features of atomic scale dimensions (
