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Langmuir 1987, 3, 150-159
Articles Effect of Underpotentially Deposited Lead on the SERS of Chloride, Bromide, and Thiocyanate Adsorbed at Silver Electrodes: Evidence for Chemical Contributions to SERS Jeanne E. Pemberton," Jos6 C. Coria-Garcia, and Rebecca L. Hoff Department of Chemistry, University of Arizona, Tucson, Arizona 85721 Received March 17, 1986. I n Final Form: September 16, 1986 A study of the effect of underpotentially deposited Pb on the SERS of C1-, Br-, and SCN- adsorbed at Ag electrodes is presented in this report. The quenching behavior of the SERS of these adsorbates by the deposited Pb is different for each adsorbate. The SERS of SCN- is quenched rapidly by very small amounts of underpotentially deposited Pb, while the quenching for Br- and C1- is less drastic. These results are interpreted in terms of a charge-transfer model for SERS that is sensitive to the changes in work function at microscopic sites upon Pb deposition. The charge-transfer process is envisioned to occur at sites of atomic scale roughness as has been previously postulated. Evidence for the importance of atomic scale roughness in these studies comes from the lack of reversibility of the SERS intensities after the deposition and quantitative stripping of greater than 60-70% of a Pb monolayer on the Ag surface. In total, these results suggest that chemical contributions to SERS are significant.
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 and so-called chemical mechanisms. Elucidating the relative contributions made by each has proven to be a formidable task. This is due in large part to the difficulty in varying only one parameter of the system in a systematic fashion while leaving other parameters unchanged. It is a well-known fact that SERS can only be observed at a limited number of metal surfaces. Most notable of these metals are Ag, Cu, and Au. This limitation has been especially apparent in electrochemical systems. The discrete set of metal electrodes at which SERS is observable suggests that the electronic properties of the metal surface play a critical role in the enhancement process. One approach to the systematic variation of surface electronic properties that has received considerable recent 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) in which typically the first monolayer of a metal is deposited onto a foreign metal electrode at potentials positive of the reversible Nernst potential for the metal couple. This technique enables 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 probe species can be easily followed as a function of surface coverage of the film. This approach has been successfully applied to the investigation of many systems. Loo and Furtak were the first
* Author
t o whom correspondence should be addressed.
0743-7463/87/2403-0150$01.50/0
to report this method in a study of pyridine adsorbed at Au electrodes in the presence of submonolayer amounts of underpotentially deposited Ag.' Pettinger and Moerl have reported the damping of SERS of pyridine at Ag in the presence of very small amounts of electrochemically deposited Cu and Cd at0ms.~8 Watanabe and co-workers studied the effects of T1 and P b deposited at underpotential on Ag on the SERS response of pyridine in pH 8.2 ~ o l u t i o n . ~Kester has studied the effects of UPD T1 on the SERS behavior of benzotriazole at Ag.5 Pemberton has reported the SERS of pyridine at Pt electrodes in the presence of small amounts of electrochemically deposited Ag.6 The effect of UPD P b thin films on the SERS response of pyridine and C1- adsorbed at roughened Ag electrodes in slightly acidic media has been detailed in three previous reports from this l a b ~ r a t o r y . ~ -Other ~ adsorbates studied in this laboratory in the presence of UPD P b include HCN,'O hydroxolead(I1) and hydroxolead(I1) halide complexes,'l and 3,6-dihydro~ypyridazine.~ In all of the studies involving the deposition of a nonenhancing metal onto an enhancing metal, the SERS intensities of probe adsorbates are quenched to undetectable levels upon deposition of monolayer amounts of the nonenhancing metal. Moreover, the quenching of SERS for adsorbates at Ag in the presence of UPD P b has been shown to qualitatively correlate with an increase in the imaginary part of the surface dielectric function or (1) Loo, B. H.; Furtak, T. E. Chem. Phys. Lett. 1980,71, 68. (2) Pettinger, B.; Moerl, L. J . Electron Spectrosc. Relat. Phenom. 1983,29,383. (3) Moerl, L.; Pettinger, B. Solid State Commun., 1982,43, 315. (4)Watanabe, T.; Yanigahara, N.; Honda, K.; Pettinger, B.; Moerl, L. Chem. Phys. Lett. 1983,96,649. (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 1985,2, 518. (9) Guy, A. L.; Pemberton, J. E. Langmuir, in press. (10) Kellogg, D. S.; Pemberton, J. E. J . Phys. Chem., in press. (11) Pemberton, J. E.; Coria-Garcia, J. C.; Sobocinski, R. L. J . Elecfroanal. Chem., in press.
0 1987 American Chemical Society
Langmuir, Vol. 3, No. 2, 1987 151
Effect of UPD Pb on SERS surface absorptivity for fractional Pb coverages between zero and 40% of a m ~ n o l a y e r .This ~ has been interpreted as evidence for the importance of surface electronic properties in the quenching of SERS in these systems. The lack of correlation a t 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 a t 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 and the SERS intensity associated with these site^.^,^ This has been shown to occur for large P b submonolayer amounts (0 > 0.7) and a t very long times a t low Pb submonolayer coverages. Additionally, Kester has interpreted the quenching of the SERS of benzotriazole at T1-modified Ag electrodes as displacement of the probe adsorbate from the surface as the T1 monolayer is deposited.5 The previous studies described above have documented the involvement of changes in surface electronic structure, changes in surface roughness, and changes in adsorbate coverage in the overall quenching of the SERS response for a variety of adsorbates in the presence of underpotentially deposited nonenhancing thin metal films. However, no attempt has been made to elucidate the role of chemical contributions in the quenching of SERS in these systems. The study reported here was undertaken with the intent of better defining the importance of chemical effects in the quenching of SERS a t Ag electrodes in the presence of underpotentially deposited Pb thin films. Toward that end, a series of three simple adsorbates known to remain adsorbed at Pb-modified Ag electrodes have been investigated. These adsorbates are C1-, Br-, and SCN-. These species have different strengths of adsorption at Ag electrodes and different chemical properties which can be exploited in SERS experiments designed to probe chemical effects such as charge transfer. The SERS intensity behavior of these three adsorbates as a function of fractional P b coverage between zero and one complete monolayer is presented in this report. Attempts to fit the experimental data to an electromagnetic model developed by Murray12for the effect of overlayers on SERS are shown to fail for these three adsorbates. Excitation data for C1and SCN- suggest that these data are best interpreted in terms of a charge-transfer model for SERS.
Experimental Section The laser Raman system used for these studies has been described in detail p r e v i o ~ s l y .All ~ ~ spectra ~ reported here were obtained with 5145- or 4579-A excitation from an Ar+ laser. Laser power measured at the sample was tyically 150-200 mW. Spectra of C1- and Br- were acquired at 0.5-cm-' increments over a 0.5-s integration period. Spectra of SCN- were acquired at 1.0-cm-' increments over a 0.5-s integration period. All spectra in the low-frequencyregion for C1- and Br- were acquired as single scans. The spectra for SCN- in the region of the u(CN) band were also acquired as single scans. However, the spectra for SCN- in the frequency regions in which the u(S-C) and the G(SCN)vibrations are observed were signal averaged 3 times to improve the signal-to-noiseratio. In the low-frequencyregion where the u(Ag-Cl) and u(Ag-Br) bands are observed, significant background intensity can obscure these vibrational features if they are weak in intensity. (12) Murray, C. A. J. O p t . SOC.A m . B 1985, 2, 1330.
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Figure 1. Cyclic voltammetry of Pb UPD on Ag in (a) 0.1 M KCl, (b) 0.1 M KBr, and (c) 0.01 M KSCN/O.l M KC1.
In order to improve the sensitivity of these measurements, the spectra acquired in this region before an oxidation-reduction cycle (ORC) pretreatment were digitally subtracted from the spectra acquired after the ORC. The resulting spectra demonstrate a much smaller influence from the background and allow accurate quantitation of peak areas even at low intensity. Electrochemical equipment and cells used for these investigations have also been described previ~usly.~,~ Polycrystalline Ag (Johnson Matthey, 99.9%) electrodes were mechanically polished with successively finer grades of alumina (Buehler) down to 0.05 wm, rinsed with distilled HzO,and sonicated in distilled HzO before use. All potentials were measured and are reported vs. a saturated calomel reference electrode (SCE). The solutions used to perform the experiments in C1- and Brconsisted of 0.1 M KC1 and 0.1 M KBr, respectively. The solutions in which the SCN- data were obtained consisted of 0.01 M KSCN in 0.1 M KCl. Pb2+was added as Pb(N0J2 at a concentration M to all solutions for the spectra acquired in the of 1 X presence of Pb. In order to maintain solubility of the Pb2+,the solutions were made acidic by the addition of HC1 or HBr as appropriate. All chemicals were reagent grade and used as received. All solutions were prepared from triply distilled, deionized HzO, the last distillation being from basic permanganate. All solutions were deaerated by bubbling with Nz prior to use. The Ag electrodes were subjected to ORCs before 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 at 5 or 10 mV s-l from an initial potential of -0.30 V until ca. 30 mC cm-2 of total anodic charge was passed. The electrodes were polished between the ORCs such that the Ag surface is oxidized and reduced only once before SERS spectra were acquired. The fractional Pb monolayer coverages were calculated by comparing the charge under the stripping wave obtained at different potentials with that obtained for a full monolayer on a given electrode surface. Dickertmann, Koppitz, and Schultze have determined that the deposition of one full monolayer of Pb corresponds to 300 gC cm-2.13 Areas under the stripping curves were measured with a planimeter.
Results and Discussion UPD Behavior in C1-, Br-, and SCN-/Cl- Electrolytes. The potential regions within which the underpotential deposition of Pb occurs in the electrolyte environments of interest were investigated by cyclic voltammetry. Parts a, b, and c of Figure 1 show the cyclic voltammetric responses measured a t a Ag electrode in 0.1 M KC1, 0.1 M KBr, and 0.01 M KSCN in 0.1 M KC1, respectively, containing 1 X M Pb2+in the UPD potential region. The potential region within which the first monolayer of P b is deposited is sensitive to the nature of supporting electrolyte anion. In C1- media, P b UPD occurs between -0.40 and -0.48 V. In Br- media, P b UPD is observed (13) Dickertmann, D.; Koppitz, F. D.; Schultze, J. W. Electrochim. Acta 1976, 21, 967.
152 Langmuir, Vol. 3, No. 2, 1987 between -0.43 and -0.52 V. The UPD of P b in the SCN-/Cl- medium is observed between -0.40 and -0.52 V. Kolb has summarized what is known about the effect of anions on the UPD behavior of Pb.14 Halides and pseudohalides affect the electrochemical response in two ways, resulting in distinct changes in the cyclic voltammetric beha~i0r.I~First, the halide and pseudo-halide ions interact with the Ag substrate decreasing the magnitude of the underpotential shift. Second, the area under the monolayer peak is increased, because the metal atom adsorption is accompanied by halide ion desorption which both contribute to the charge in the same direction. For the deposition of P b on Ag,the effect was found to increase for the series C104-, F < S042-< C1- < Br- < SCN- < I-. The data shown in Figure 1 for C1- and Br- are consistent with these previous observations. For the mixed SCN-/Clelectrolyte, the presence of the SCN- is seen to simply broaden the UPD peak relative to that observed in C1media only. The values of potential within which the UPD of P b occurs are very similar in all three electrolyte environments, however. Therefore, no significant differences in the SERS responses of these systems would be expected to result from drastic potential differences in the UPD behavior. Despite the similarity of the UPD processes in the presence of these three anions on a macroscopic level, it must be kept in mind that significant differences may exist in the P b deposition process on a microscopic (i-e.,atomic or molecular) level that could result in significant differences in the SERS intensity-Pb coverage behavior for these anions. Although no means of adequately characterizing the existence of atomic scale differences in the Pb UPD process in these different environments currently exists, the possible impact of these effects on the SERS behavior must be kept in mind as a possible contributing source of differences in the observed behavior. SERS Behavior in the Absence of Pb. The SERS behavior of these three adsorbates was first characterized in the absence of Pb2+in solution in order to elucidate any potential-dependent effects which might be significant. SERS spectra obtained in 0.1 M KC1 exhibit a v(Ag-Cl) vibration at 238 cm-'. Spectra from 0.1 M KBr show the corresponding v(Ag-Br) vibration at 163 cm-l. In 0.01 M KSCN in 0.1 M KC1, the only bands observed arise from adsorbed SCN- at the Ag surface.lfi A peak at ca. 2120 cm-l is observed for the v(CN) vibration. This has been previously interpreted to indicate that SCN- adsorbs to the Ag electrode through the S at0m.l' Additional bands which are observed in SCN- media include the v(Ag-S) at 210 cm-l, the v(S-C) a t 740 cm-', and the G(SCN) a t 450 cm-'. The potential dependence of the v(Ag-C1) and v(Ag-Br) band intensities is plotted id Figure 2a. The potential dependence of the v(CN), v(S-C), and G(SCN) normalized band intensities is plotted in Figure 2b. The four stretching vibrations are observed to decrease in intensity as the potential is made more negative. The v(Ag-Cl) band is not detectable at potentials equal to or more negative than -0.45 V. The v(CN) and v(S-C) bands of adsorbed SCN- decrease in intensity with potential but remain detectable a t all potentials investigated here. The G(SCN) band has a different intensity-potential behavior than the other two bands of SCN-. As shown by (14) Kolb, D. M. Adu. Electrochem. Electrochem. Eng. 1978,11, 125. (15) Schmidt, E.; Gygax, H. R.; Bohlen, P. Helu. Chim. Acta 1966,49, 733. (16) Furtak, T. E.; Roy, D.Phys. Reu. Lett. 1983, 50, 1301. (17) Weaver, M. J.;Barz, F.; Gordon, J. G.; Philpott, M. R. Surf. Sci. 1983, 125, 409.
Pemberton et al.
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Figure 2. (a) Potential dependence of SERS intensity of v(Ag41) and u(Ag-Br) bands. (b) Potential dependence of SERS intensity of v(CN), u(S-C), and 6(SCN) bands.
the data in Figure 2b, the intensity of this band increases with potential until a maximum is reached at -0.45 V, after which the intensity drops. This observation has been made previously and attributed to reorientation of the adsorbed SCN- as the potential is made more negative to allow for additional interaction through the N atom." At the more negative potentials, the decrease in SERS intensity as a function of electrode potential has been attributed to the desorption of these anions from the Ag surface.ls Even though the intensity drops below a detectable level in C1- and Br- media and to very low levels in SCN- media, the desorption of these ions is not complete. Weaver and co-workers have shown using differential capacitance measurements that Cl- surface coverage is on the order of 60% at a potential of -0.20 V and decays to a coverage of 5% at a potential of -1.00 V. These workers also found that Br- coverages of 100% exist at the Ag electrode at a potential of -0.20 V and decay of 5% at a potential of -1.40 V.19 The differential capacitance measurements on the SCN- system have shown that SCNcoverages of 100% exist a t the Ag surface for potentials positive of -0.70 V in both C1- and C104- supporting electrolyte^.^' The relative ease with which these ions are desorbed from the surface as indicated by the potential a t which the SERS intensity decays to an undetectable level is indicative of the relative strength of adsorption of these anions at Ag. Thus, the SERS behavior correlates well with the electrochemical data in that the strength of adsorption is observed to be SCN- > Br- > C1-. Several aspects of these data are germane to the investigations reported here. First, these SERS data indicate (18) Kotz, R.; Yeager, E. J. Electroanal. Chem. 1981, 123, 335. (19) Weaver, M. J.; Hupp, J. T.; Barz, F.; Gordon, J. G.; Philpott, M. R. J . Electroanal, Chem. 1984, 160, 321.
Langmuir, Vol. 3, No. 2, 1987 153
Effect of UPD Pb on SERS
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intensity to be monitored as the Pb monolayer is deposited. The second aspect of these data which is significant is that the SERS intensities of these vibrational bands only decrease ca. 10-1596 within the small potential region in which the P b monolayer deposition occurs. Therefore, greater decreases in SERS intensity that may be observed during the UPD of P b cannot be fully explained on the W basis of potential-induced phenomena. SERS Behavior in the Presence of Pb. Representative SERS spectra a t 5145-A excitation for these adsorbates in solutions containing Pb2+in the potential region within which Pb is deposited are shown in parts a, b, and c of Figure 3 for the frequency regions corresponding to the u(Ag-Cl), v(Ag-Br), and u(CN) vibrations, respectively. 280 180 80 In the low-frequency region, the v(Ag-Cl) band is observed cm-1 at 238 cm-l, and the v(Ag-Br) band is observed at 163 cm-l. In addition to these vibrations, the spectra in this region exhibit an additional band at 135 cm-' in both electrolyte C -E(mV) media and a band at 110 cm-l in C1- media. These bands 2120 cm-' A 350 have been discussed in detail in two previous reports from n B 360 this laboratory and have been assigned to hydroxolead(I1) halide complexes which exist a t the interface before the onset of Pb underpotential deposition.l1rZ0Of importance G 500 >to this study is the observation that the intensities of the t v, v(Ag-Cl), u(Ag-Br), and v(CN) bands decrease as the P b Z coverage increases. The quenching of SERS by electroW ichemically deposited P b is consistent with the behavior z observed for all other adsorbates studied to date in this laboratory and o t h e r ~ . ~ * ~ - l l A more quantitative picture of the differences in the quenching behavior of SERS from these adsorbates comes from measuring the normalized peak areas for the different 1950 2100 2250 vibrational bands as a function of fractional Pb coverage. c m-1 Figure 4 shows plots of the normalized SERS intensities Figure 3. SERS spectra as a function of Pb UPD for (a) v(% ECT,Br- > EcT,c,-. Upon deposition of Pb, the Fermi level a t microscopic defect sites such as adatoms or adatom clusters increases in energy. For a given excitation energy, the impact of this energy change on the SERS will be a sensitive function of adsorbate. As shown in Figure 9, SERS for SCN- is very rapidly quenched with small P b coverages, because the charge transfer to the Fermi level is moved out of resonance with the exciting radiation. Similar quenching is observed for Br- and C1-. However, as a result of the higher energies of their filled levels participating in the charge transfer, more P b is required to move the Fermi level out of resonance with the exciting radiation. Thus, this effect of raising the Fermi energy by P b deposition is strictly analogous to the effect of imposing an increasingly negative potential. Moreover, the impact of both phenomena is to quench the SERS intensities. The excitation dependence shown in Figure 5 is also consistent with the model proposed in Figure 9. This model requires that as the excitation wavelength is decreased, the SERS intensities should be quenched less rapidly. This is, in fact, what is observed experimentally as shown in Figure 5. Further evidence for the importance of adatoms comes from investigation of the dependence of the continuum background intensity on Pb coverage. The presence of this continuum background has been previously interpreted in terms of strong photon electron-hole pair coupling a t atomic scale roughness.46 If the model in Figure 9 is accurate, one would expect a similar quenching effect of P b deposition on the continuum background intensity. Figure 10 shows the background intensity a t 2250, 280, and 300 cm-' for the SCN-, Br-, and C1- systems, respectively, as (40) Hulse, J.; Kuppers, J.; Wandelt, K.; Ertl, G. Appl. Surf. Sci. 1980, 6, 453. (41) Kuppers, J.; Wandelt, K.; Ertl, G. Phys. Reu. Lett. 1979,43,929. (42) Besocke, K.; Krahl-Urban, B.; Wagner, H. Surf. Sci. 1977,68,39. (43) Kuppers, J.; Michel, H.; Nitschke, F.; Wandelt, K.; Ertl, G. Surf. Sci. 1979, 89, 361. (44) Kuppers, K.; Nitschke, F.; Wandelt, K.; Ertl, G. Surf. Sci. 1979, 88, 1. (45) Wandelt, K. J. VUC.Sci. Technol. A 1984, 2, 802. (46) Otto, A. Appl. Surf. Sci. 1980,6, 309 and references cited therein.
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Figure 11. Percent SERS intensity recovery as a function of fractional Pb coverage for (a) v(Ag-Cl) and (b) v(Ag-Br).
a function of P b coverage. In all three cases, the continuum background intensity is quenched at approximately the same rate as the adsorbate band intensities. This behavior is analogous to that observed for the quenching of pyridine and ethylene SERS and the background intensity associated with this SERS, by dissociated oxygen on Ag.34147 These results provide additional evidence for alteration of the energy levels of microscopic sites on the Ag surface by the deposition of Pb. Reversibility of the SERS of C1- and Br-. The above model relies on the existence of Ag adatoms or adatom clusters at the surface which have available electron levels for the charge transfer. Additional evidence for the existence of atomic scale roughness in these systems comes from studies of the reversibility of the SERS response after the deposition and quantitative stripping of varying amounts of UPD Pb from the Ag surface. Similar studies have shown that the SERS intensity is irreversibly quenched for other adsorbates at Ag electrodes only when the deposited P b coverage exceeds ca. 60% of a monol a ~ e r . These ~ , ~ data can be easily obtained by monitoring the intensity of the vibrational band of interest while scanning the potential of the electrode from an initial value a t which no P b is deposited to a potential corresponding to the desired fractional P b coverage and back at a slow sweep rate. The percent recovery values, % R , can be calculated by taking the ratio of the intensity of the band after deposition and stripping of the fractional P b layer to the intensity of the band before any Pb deposition. The reversibility behavior of SCN- could not be investigated here, because it has been previously shown that in the absence of Pb2+in solution, the SERS intensity is not reversible as a function of potential within the potential (47) Ertuck, U.; Pettenkofer, C.; Otto, A. J. Electron Spectrosc. Related Phenom. 1986, 38, 113.
158 Langmuir, Vol. 3, No. 2, 1987 region where P b UPD occurs. The % R as a function of potential for SCN- in the absence of Pb2+has dropped to ca. 70% by scanning to potentials corresponding to a complete P b monolayer were Pb2+present in solution.48 It is pointless to further investigate the reversibility behavior of SCN- in the presence of Pb2+,because it will be impossible to distinguish potential effects from UPD effects in this system. Therefore, the reversibility behavior of only C1- and Br- will be presented here. Parts a and b of Figure 11show the % R of the 238 and 163 cm-l bands as a function of P b coverage from zero to one P b monolayer, respectively. These data were obtained by monitoring the intensities of these bands as the POtential was scanned from -0.30 V to a final potential corresponding to the desired fractional P b coverage and back at 5 mV s-'. Each % R value was obtained on a fresh electrode surface. Fractional P b coverages were calculated by comparing the charge under the stripping wave with that obtained for a complete monolayer on any given electrode surface. The data in Figure 11 show three distinct 70R coverage regions. At P b coverages between 0.0 and 0.6, the % R values for the 238- and 163-cm-' bands are on the order of 90-100%. At P b coverages of 0.6-0.7, the % R values for both bands decrease to 6040%. For coverages above ca. 0.8, the 7 ' 0 R values are maintained at a value of 60-70%. The behavior observed in the course of this investigation is consistent with the reversibility behavior observed previously for adsorbates a t UPD-modified Ag. Previous results obtained in this laboratory have shown that the % R values of the SERS signals at 238 and 1013 cm-' in pyridine/C1-/Pb2+ media are on the order of 90-100% for coverages smaller than 0.6. '70R values of ca. 80% were found for the 238-cm-' band at coverages greater than 0.8, and 7'0 R values of ca. 40-50% were found for the 1013cm-' band at coverages greater than 0.8.7v8 Kester reported only a slight recovery of the original SERS intensity of benzotriazole at Ag as a function of T1 fractional ~overage.~ The quenching of SERS reversibility in these systems has been attributed to the destruction of atomic scale roughness by rearrangement of the ultrathin metal film to form its most energetically favorable monolayer configuration.8 Takayanagi and co-workers have shown that during the formation of the P b monolayer on Ag, the P b rearranges from random atoms at coverages less than 0.2 to a (d3Xv'3)R3Oo structure for coverages between 0.2 and 0.6. This structure then rearranges to the hexagonal-close-packed structure of the P b monolayer.21 In a study of the time dependence of the loss of SERS reversibility for pyridine and C1-, it was shown that the loss of reversibility occurs during the dynamical surface processes required to form the (d3Xv'3)R3Oo structure and continues through rearrangement to form the hexagonalclose-packed monolayer.8 It is important to note that the coverage region in which the reversibility of the SERS signals is lost in the data in Figure 11 exactly corresponds to the sharp drop in intensity of both adsorbates in the normalized SERS intensity-fractional P b coverage data shown in Figure 4. This correspondence suggests that atomic scale roughness is present on the electrode surface at fractional P b coverages less than ca. 0.6. These atomic scale roughness features are therefore available to participate in a charge-transfer mechanism as postulated above. In order to further assess the role of atomic scale roughness in the SERS intensity-Pb coverage data shown (48) Hoff, R. L. B. S. Honors Thesis, University of Arizona, 1985.
Pemberton et al. 10
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Figure 12. Normalized SERS intensity as a function of fractional Pb coverage before and after deposition and stripping of one Pb "hw for (a) 4Ag-Cl) and (b) u(Ag-Br).
in Figure 4,additional intensity-coverage profiles were obtained for the C1- and Br- bands after the majority of atomic scale roughness features were destroyed by the deposition and quantitative stripping of a P b monolayer from the Ag surface. This deactivation pretreatment consisted of a potential excursion to a potential corresponding to P b monolayer coverage where the potential was held for 60 s and back to the initial potential of -0.20 V. The absolute SERS intensities are considerably diminished after this deactivation. However, careful spectral acquisition yields spectra which can be quantitated with a precision only slightly worse than that before the deactivation. Figure 12, parts a and b, shows the normalized SERS intensities for Cl- and Br- both before and after the destruction of the majority of atomic scale roughness features with P b UPD. The intensity-coverage profiles for both adsorbates are not significantly altered by the destruction of the majority of atomic scale roughness features. These results are consistent with those obtained earlier for ~ y r i d i n e . ~ Several explanations can be tenured to account for this somewhat surprising result. One explanation is that atomic scale roughness cannot be completely destroyed by the deposition and stripping of monolayer amounts of Pb. A similar conclusion regarding the presence of atomic scale roughness after a similar deactivation treatment has recently been reported by F ~ r t a k . ~In~fact, it may be possible that atomic scale roughness is reformed during the stripping process as a result of the very strong interaction of the P b with the underlying Ag surface. This interaction is equivalent to the underpotential shift of ca. 0.12 V or 2.65 kcal mol-'. Regardless of whether the atomic scale roughness is either not completely destroyed or reformed during the stripping process, these results seem to suggest that chemical enhancement effects such as charge transfer dominate the response of electrochemical SERS systems.
Conclusions In summary, the SERS intensity-Pb coverage behavior of C1-, Br-, and SCN- has been reported for P b coverages between zero and one P b monolayer. The differences in the morphologies of this behavior for the different adsorbates has been proposed to be the result of differences in the effect of P b atoms on charge-transfer enhancement for these adsorbates. This charge transfer is envisioned to occur at sites of atomic scale roughness such as adatoms or adatom clusters as postulated previously by other researchers. Additional evidence for the existence of atomic scale roughness and its role in SERS in these systems comes from the reversibility behavior reported here and comparison of the intensity-coverage profiles obtained (49) Roy, D.; Furtak, T. E. Chem. Phys. Lett. 1986, 124, 299.
Langmuir 1987, 3, 159-163 before and after the destruction of the atomic scale roughness features by the deposition of a monolayer of Pb. It is not possible from the data reported here to unequivocally decipher the roles played by surface electronic properties or atomic scale roughness. However, it can be concluded that the SERS response of adsorbates in the presence of underpotentially deposited Pb appears to be sensitive t o the chemical nature of the adsorbate and the
159
extent of adsorbate-metal interaction.
Acknowledgment. We acknowledge support of this research by the National &+~~ce l k ~ ~ M (CHEo n 8309454). The experimental assistance of Anita GUY and Mark A. Bryant is greatfully acknowledged. Registry No. Pb, 7439-92-1; Ag, 7440-22-4; C1-, 16887-00-6; Br-, 24959-67-9; SCN-, 302-04-5.
LEED Study of Benzene and Naphthalene Monolayers Adsorbed on the Basal Plane of Graphite U. Bardi, S. Magnanelli, and G. Rovida" Dipartimento di Chimica, Universitci d i Firenze, 50121 Firenze, Italy Received M a y 21, 1986. I n Final Form: November 6, 1986 The adsorption at low temperature of benzene and naphthalene on the basal plane of graphite has been studied by low-energy electron diffraction. One ordered monolayer phase was found for benzene and two distinct monolayer phases were found for naphthalene with different surface coverages. Benzene forms a hexagonal lattice where all molecules are adsorbed on unique sites of the substrate. In the case of naphthalene, in the low-coverage phase molecules are also adsorbed on unique sites. In the high-coverage phase, a long-range coincidence with the substrate periodicity is present.
1. Introduction The adsorption of aromatic molecules on the graphite basal plane is a subject of interest in the field of physisorption. This system represents also a simplified experimental model of the interactions of the carbon surface with aromatic surfactants used to obtain stable aqueous carbon slurries. In order to obtain informations on this subject, we carried out a study on the adsorption at low temperature of benzene and naphthalene on the basal plane of graphite, using low-energy electron diffraction (LEED) to obtain data on the structural parameters of the ordered phases formed. Benzene adsorption on the graphite (0001) surface has been already the object of several studies. Information about the structure of the adsorbed layer could be inferred from adsorption NMR ~ t u d i e s ,and ~ , ~PIES studies? Neutron diffraction7y8and X-ray diffraction9 gave direct structural information. However, no experimental data relative to naphthalene adsorption on graphite are available in the literature. Theoretical calculations relative to benzene and naphthalenelOJ1 indicate that aromatic (1)Pierce, C. J. Phys. Chem. 1969, 73,813. (2)Isirikyan, A. A.;Kiselev, A. V. J. Phys. Chem. 1969,65,601. (3)Isirikyan, A. A.; Kiselev, A. V. J. Phys. Chem. 1962,66,205. (4)Boddenborg, B.;Moreno, J. A. Ber. Bunsenges. Phys. Chem. 1983, 87,83. (5)Tabony, J.; White, J. W.; Delachaume, J. C.; Coulon, M. Surf. Sci. 1980,95,L282. (6) Kubota, H.; Munakata, T.; Hirooka, T.; Kondow, T.; Kuchitsu, K.; Ohno, K.; Harada, Y. Chem. Phys 1984,87,399. (7)Monkenbusch, M.: Stockmever, R. Ber. Bunsenpes. Phvs. Chem. 1980,84,808. (8) Meehan. P.: Ravment. T.: Thomas. R. K. J . Chem. SOC..Faraday Trans. 1980, 76,2011: (9)Gameson, I.; Rayment, T. Chem. Phys. Lett. 1986, 123, 150. (10)Bondi, C.; Baglioni, P.; Taddei, G. Chem. Phys. 1986,96, 277. (11)Bondi, C., tesi di Laurea in Chimica, Universitl di Firenze, 1984.
molecules should be adsorbed on graphite with the ring plane parallel to the surface plane and that benzene and naphthalene should form an incommensurate cell with respect to the substrate. In a previous paper,12we reported the results of a LEED study of low-temperature benzene adsorption on graphite. In the present work we will review these results and report data relative to the adsorption of naphthalene on the same surface in the range 135-150 K. We found that one ordered adsorbed phase exists for benzene and two different ordered phases for naphthalene. 2. Experimental Section LEED measurements were carried out in an UHV chamber with torr range, equipped with three-grid base pressure in the LEED optics. The sample was a single-crystal graphite platelet oriented along the (OOO1) plane, cleaved in air before introduction in the vacuum chamber. The sample was mounted on a tantalum plate which was in contact with a copper surface, cooled by liquid nitrogen circulation. The sample could also be annealed by heating the tantalum plate by resistive effect. The purity of the sample surface was controlled by AES spectroscopy, using a grazing incidence electron gun and the LEED optics as a retarding field analyzer of the Auger electrons. Temperatures were measured by means of a copper/constantan thermocouple spot welded on the tantalum plate. Due to the position of the thermocouple, temperature, measurements can be expected to be accurate within about f 5 K. Benzene and Naphthalene vapors were introduced directly in the vacuum chamber by means of a leak valve. The purity of the gases was monitored by a quadrupole mass spectrometer.
3. Results After annealing in vacuum a t about 600 K, it was possible to find regions of the sample surface showing a sharp (12)Bardi, U.;Magnanelli, S.; Rovida, G. Surf. Sci. 1986, 165, L7.
0743-7463/87/2403-0159$01.50/0 0 1987 American Chemical Society
