J. Phys. Chem. 1981, 85,2746-2751
2746
and isomeric nitrocresols are found as products in addition to the expected benzaldehyde and cresols. The appearance of 3-nitrotoluene is readily accounted for by the elevated (=1ppm and greater) concentrations of NOz and high conversions used in the experiments. That the 2 and 4 isomers of nitrotoluene are observed is perplexing, because we have shown that OH attack meta to the methyl group (the reaction that would lead to the 2- and 4-nitrotoluenes) is of minor importance. Because ionic nitrations typically give predominately 2- and 4-substitution, and because we find27NO, to be a powerful nitrating agent in condensing our reaction mixtures, we suggest that the observance of
2- and 4-nitrotoluenes in smog-chamber studies is indicative of heterogeneous reactions that occur in the aerosol phase or during sampling of the products. Phenolic compounds are especially susceptible to heterogeneous nitration, and the origin of nitrophenols in smog-chamber experiments must be interpreted with extreme caution. Acknowledgment. We acknowledge Bosco Y. Lan for his technical assistance and the support of the Environmental Protection Agency under Research Grants R803846 and R-806093.
Surface-Enhanced Raman Spectroscopy of Metal-Halide Vibrations on Ag and Cu Electrodes B. Pettinger,+ Fritz-Haber-Institut der Max-Planck-Gesellschan, P 100 Berlin 33 (West), Federal Republic of Germany
M.
R. Phllpott,” and J.
G. Gordon, I1
IBM Research Laboratory, San Jose, California 95193 (Recelved: September 30, 1980; In Final Form: June 8, 1981)
Halide ions play an important role in the surface-enhanced Raman (SER) process in electrochemical environments. They are usually specifically adsorbed at Ag and Cu electrodes in the same potential range used in the SER experiments. This intimate contact with the metal surface is not sufficient to obtain an enhancement even when a rough metal is immersed in the electrolyte. After an oxidation-reduction cycle (ORC) is performed, however, an enormous enhancement appears when highly concentrated electrolytes are used. The Raman intensities observed for M-X vibrations are comparable to those found for the ring modes of pyridine. The irreversible potential dependence of the enhancement (vanishingwith sufficiently negative potential) indicates that particular silver (copper)halide structures are responsible for these effects. It is argued that these structures are different from simple specifically adsorbed halide ions.
Introduction In a recent paper Fleischmann, Hendra, Hill and Pemble’ reported the first observations of a surface-enhanced Raman (SER) spectrum for water at a roughened Ag electrode. In a second paper Fleischmann, Robinson, and Waser2reported an electrochemicalstudy of the adsorption of pyridine and chloride ions on Ag electrodes. Stimulated by these results, we have measured the SER spectra of water, D20, and halide ions on Ag and Cu electrodes under a variety of conditions, in particular at much higher halide concentration^^^^ than those used by Fleischmann and co-workers. The great utility of SER spectroscopy (SERS) for in situ investigation of the interfacial region of an electrode is amply justified by these and previous investigations.”’* Initially SER scattering was found only with some organic molecules (such as pyridine and other amines,’ and carboxylic specieslg) adsorbed on Ag electrodes. However, in the last few years it has been reported for many neutral molecules and ions adsorbed on Ag e1e~trodes.l-l~There has also been an increase in the variety of substrates from which strongly enhanced Raman emission have been observed. In addition to Ag, SER effects have been observed from Cu and Au electrodes.lSz2 Raman spectra have also been reported for adsorbates on Pt electrode^.^^ Not all observations refer to polycrystalline electrodes. Enhanced Raman spectra have been reported from singel-crystal Ag electrodes with (111)and t IBM
World Trade Postdoctoral Fellow, 1980. 0022-3654/8 1/2085-2746$01.25/0
(100) surfaces after very weak electrochemical activation.24 There are also observations made in UHV of the Raman (1)M. Fleischmann, P. J. Hendra, I. R. Hill, and M. E. Pemble, J. Electroanal. Chem., 117,243 (1981). (2)M. Fleischmann, J. Robinson, and R. Waser, J. EZectroanaL Chem., 117,257 (1981). (3)B. Pettinger, M. R. Philpott, and J. G. Gordon, 11, J. Chem. Phys., 74, 934 (1981). (4)B.Pettinger, M. R. Philpott, and J. G. Gordon, 11, Surf. Sci., 105, 469 (1981). (5) M. Fleischmann, P. J. Hendra, and A. J. McQuillan, Chem. Phys. Lett., 26,163 (1974). (6)R. P. Van Duyne in “Chemical and Biochemical Applications of Lasers”, Vol. IV, C. B. Moore, Ed., Academic Press, New York, 1979. (7)R. P. Van Duyne, J. Phys. (Paris),38 (C5), 239 (1976). (8)D. L.Jeanmaire and R. P. Van Duyne, J. Electroanal. Chem., 84, l(1977). (9)E.Burstein, C. Y. Chen, and S. Lundquist, “Light Scattering in Solids. Proceedings of Joint US-USSR Symposium on the Theory of Light Scattering in Condensed Matter”, J. L. Birman, H. Z. Cummins, and K. K. Rebane, Eds., Plenum Press, New York, 1979,p 479. (10)T. E. Furtak and J. Reyes, Surf. Sci., 93,351(1980). (11)F. W. King and G. C. Schatz, Chem. Phys., 70, 1939 (1979). (12)J. Billmann, G. Kovacs, and A. Otto, Surf. Sci. 92,153 (1980). (13)M. Moskovits, J. Chem. Phys. 69,4159 (1978). (14)R. M. Hexter and M. G. Albrecht, Spectrochim. Acta, Part A, 35, 233 (1979). (15)J. C. Tsang, J. K. Kirtley, and J. A. Bradley, Phys. Reu. Lett., 43, 772 (1979). (16)J. R. Kirtly, S. S. Jha, and J. C. Tsang, Solid State Commun., 35, 509 (1980). (17)S.Efrima and H. Metiu, J. Chem. Phys., 70, 1602 (1979). (18)M. R. Philpott, J. Chem. Phys. 62,1812 (1975). (19)M. Fleischmann, P. J. Hendra, A. J. McQuillan, R. L. Paul, and E. S.Reid, J. Raman Spectrosc., 4,269 (1976).
0 1981 American Chemical Society
SERS of Metal-Halide Vibrations on Electrodes
spectra of adsorbates on the surfaces of single crystals.25 In a number of experiments, the role of coadsorbates has been studied in more detai1.26-2sJ-7 Thus, Van Duyne reported a strong influence of halide ions on the SER intensity for ~ y r i d i n e and , ~ Dornhaus et a1.26observed frequency shifts for silver-halide vibrations which corresponded to their atomic-mass differences. There is good evidence that Ag-X (X = C1, Br, I) vibrations are also enhanced. It is important to point out that (i) these Ag-X vibrations were only observed in the presence of pyridine and (ii) the presence of halide ions in the electrolyte increases the SER intensities of pyridine vibration^.^.^^ These observations also provide more evidence for the importance of surface complexes in the SER process on e l e ~ t r o d e s . ~ ~A- detailed ~~ study in the low-frequency region (20-300 cm-l) revealed at least five vibrational modes which could be assigned to surface compounds consisting of Ag adatom, pyridine, and halide These vibrations can be thought of as representing the “skeletal” modes of the complex which is generated at the surface during the oxidation-reduction cycle (ORC). In the investigation of water and deuterium oxide at Ag and Cu electrodes, it was found that the potential dependence of the enhanced water bands paralleled that of the silver-halide bands,lp3p4 I t is noteworthy that very intense silver-halide vibrations can be observed from the electrochemical cell. In contrast, bulk AgCl exhibits very weak Raman bands because only second-order Raman transitions are allowed in the crystal.35 The large SER signals found for metal halide surface compounds implies either that their structure is very different from that of crystalline AgCl (AgCl has a rock salt structure) or that selection rules for the SER processes are different from normal Raman scattering. In what follows, we present a detailed study of the potential dependence of the SER spectra for metal-halide vibrations for Ag and Cu electrodes. The results are interpreted in the last section in terms of complexes formed at the surface of the electrode. A definition of surface complex and a discussion of its involvement in SER processes can be found in that section. (20)B.Pettinger, U. Wenning, and H. Wetzel, Chem. Phys. Lett., 67, 192 (1979). (21)U.Wenning, B. Pettinger, and H. Wetzel, Chem. Phys. Lett., 70, 49 (1980). (22)B. Pettinger, U. Wenning, and H. Wetzel, Surf. Sci., 101, 409 (1980). (23)R. P. Cooney, P. J. Hendra, and M. Fleischmann, J. Raman Spectrosc., 6,244 (1977). (24)B.Pettinger and U. Wenning, Chem. Phys. Lett., 66,253(1978). (25)J. M.Stencel and E. B. Bradley, J. Raman Spectrosc., 8, 377 (1976);P. Sanda, J. M. Warlaumont, J. E. Demuth, J. C. Tsang, K. Christman, and J. A. Bradley, Phys. Reu. Lett., 45, 1519 (1980);M. Udagawa, Chih-Cong Chou, J. C. Hemminger, and S. Ushioda, submitted for publication in Phys. Reu. Lett. (26)R. Dornhaus and R. K. Chang, Solid State Commun.,34, 811 (1980). (27)B.Pettinger and H. Wetzel, Chem. Phys. Lett., 78, 398 (1981). (28)B.Pettinger, Chem. Phys. Lett., 78,404 (1981). (29)B.Pettinger, U. Wenning, and D. M. Kolb, Ber. Bunsenges. Phys. Chem.. 82,1326 (1978). (30)B.Pettinger, T. Tadjeddine, and D. M. Kolb, Chem. Phys. Lett., 66,544 (1979). (31)A. Girlando, J. G. Gordon, 11, D. Heitmann, M. R. Philpott, H. Seki, and J. D. Swalen, Surf. Sci., 101,417 (1981). (32)D. M. Kolb and T. Tadjeddine, J. Electroannl. Chem., 111,119 (1980). ‘ (33)H.R. Mahoney, M. W. Howard, and R. P. Cooney, Chem. Phys. Lett., 71,59 (1980). (34)R. E.Kunz, J. G. Gordon, 11,M. R. Philpott, and A. Girlando, J. Electroanal. Chem., 112,391 (1980). (35)S. Ushioda and M. J. Delaney, Solid State Commun.,32, 67 (1979).
The Journal of Physical Chemistry, Vol. 85, No. 19, 1981 2747
y
Ag Electrode in 1M NaCl PH 2
.-
BC
-
0 50
Raman Shift (cm-’ )
L 300
Figure 1. SER spectrum for the Ag/CI-/H20 Interface before (curve a) and after applying an ORC from -0.3 to X to -0.3 V, where X = 0.05, 0.075, 0.100 (curve b) and 0.125 V (curve c), respectively, at 20 mV/s. Electrolyte: 1 M NaCi, pH 2. E = -0.3 V vs. SCE Ag/AgCi. Curves for X = 0.05 and 0.075 V are not shown because their differences from curve a are too small to be depicted clearly with the Intensity scale used In the figure.
Experimental Section The experimental arrangement has been described e l s e ~ h e r e .The ~ electrolyte solutions (pH 2) were made from suprapure salts and Baker HPLC (high-performance liquid chromatography) water. The electrodes consisted of metal rods 4-5 mm in diameter, one end of which was machined into a hemicylinder. Before the measurements, the electrodes were mechanically polished with 5-pm A1203 and rinsed with HPLC water. No chemical etching of the flat electrode surfaces were used. All potentials were measured against and are reported with respect to the Ag/AgCl electrode. Results SER Spectra from Ag Electrodes in Concentrated Halide Solution. In earlier papers, SER bands have been reported for water and deuterium oxide at Ag electrodes covered with specifically adsorbed halide ion^.^?^ The potential dependence of the Raman intensities closely followed that of halide adsorption on Ag or Cu electrodes. Consequently, the Raman bands were attributed to the presence of metal halide complexes at the surface. Since, with the particular cleaning treatment and ORC used, both water and silver-halide vibrations were strongly enhanced, this hypothesis can, in principle, be tested further by an analysis of the number and the potential dependence of modes in the silver-halide stretch region. This is one of the reasons for studying SER spectra between 50 and 500 cm-‘. The light scattered from the electrode surface before any electrochemical cycling of the cell is given by curve a in Figure 1. Near the laser line (Raman shifts 1300 cm-l) a smoothly decaying, potential-independent intensity was observed. This curve (curve a, Figure 1)probably results from the imperfect suppression of stray light and Rayleigh scattering. A sequential series of ORC’s were performed on the sample, and after each cycle the Raman spectrum was recorded. In each case, the initial and final potentials were the same, -0.3 V, and the ramp speed was 20 mV/s. The most positive potentials reached in the ramp were, successively, 0.05,0.075,0.100, and 0.125 V. The electrolyte was 1M NaCl in water, at pH 2 (adjusted with HC1). The fiist two ORC’s, to 0.05 and 0.075 V, were weak in the sense that the charge passed was equivalent to only submonolayer amounts of Ag. So it is not surprising that they resulted in only small, but detectable, increases in Raman intensity from the initial-state scattering (no ORC) shown by curve a in Figure 1. These changes were too small to be depicted clearly with the intensity scale used in the
2748
Pettinger et ai.
The Journal of Physical Chemistry, Vol. 85, No. 19, 198 1 b
-
Ag Electrode in 10M NaBr DH 2
.-’” C 3
.-x VI
m N
2
-
s
0
I
6. Potential Difference SDectra a. Difference for E, and E? b. Difference for E, and Eg
5
=
01 50
1
I 1
Raman Shift Icm-’)
300
Figure 2. (A) Dependence of the SER spectra on potential for the Ag/CT/H& interface after applying an ORC from -0.3 to 0.125 to -0.3 V: (a) El = -0.3 V, (b) E2 = -0.6 V, and (c) E3 = -0.9 V. Electrolyte: 1 M NaCI, pH 2. Each spectrum is the average of four separate scans. (B) Potential difference spectra (PDS) for the Ag/CI-/H20 interface generated from data displayed in Figure 2 A (a)difference between spectra at E , = -0.3 V and E2 = -0.6 V; (b) difference between spectra at El = -0.3 V and E, = -0.9 V. Electrolyte: 1 M NaCi, pH 2.
figure. A very intense SER signal appeared after the third ORC (upper limit 0.100 V, curve b, Figure 1). An even more intense spectrum appeared after the final ORC with upper potential 0.125 V (curve c, Figure 1). During these last two cycles the amount of charge, and hence, the number of silver atoms affected in the electrochemical process underwent a large increase (equivalent to several monolayers and some tens of monolayers, respectively) leading to the large rise in the SER intensities between curves a and b, and between b and c in Figure 1. It is important to note, however, that the first two anodizations (to 0.05- and 0.075-V upper limt, respectively) were relatively weak. Only about a monolayer of silver was disturbed. Even so, enhanced Raman scattering of low relative intensity was observed. In a separate experiment the dependence of SER scattering on electrode potential after one ORC was investigated. A typical result is shown in Figure 2. During the ORC the potential was ramped from -0.3 to 0.12 V and back to -0.3 V at 20 mV/s. As generally observed for SER spectroscopy at electrodes, there is a clear dependence on potential which is shown in Figure 2A with three curves recorded at El = -0.3 V, E2 = -0.6 V, and E3 = 0.9 V, (curves a-c, respectively). The electrolyte was 1 M aqueous NaC1. These three curves display large changes in frequency and intensity, as the applied potential is shifted further into negative region. The most pronounced band at 232 cm-l undergoes an apparent shift toward 216 cm-l and decreases in intensity. The question is, however, whether there is such a large shift in the frequency of a single band or a change in relative intensities of at least two adjacent overlapping bands. When the electrode is polarized at less negative values, for instance, to E = -0.7 V (not shown in the figure), a weak SER effect is still observable, consisting of only a more or less continuously decaying intensity, rather than a set of distinct bands. If the potential is increased to more positive potentials, the SER scattering also increases and distinct bands reappear. This is in contrast to the behavior observed when the electrode is made more negative than -0.9 V. Here, the SER effect is completely and irreversibly destroyed; a
0’ 50
100
150 Raman Shift (cm-’
200
250
Flgure 3. Potential dependence of the SER effect for the Ag/Br-/H20 Interface after applying an ORC from -0.3 to 0.0 to -0.3 V at 20 mV/s: 40-mC charge passed: highest curve = -0.3 V; lowest curve = -0.8 V; potential for each curve changed by 50 mV. Electrolyte: 10 M NaBr, pH 2.
subsequent change to more positive potentials does not regenerate the surface Raman scattering. To extract more information from the halide spectra, a potential difference technique was employed which relies on the already mentioned observation that the SER effect varies strongly with the applied potential, whereas the background due to stray light is constant as long the surface roughness does not change in the potential range used. The background may show an artificial potential dependence when the variation of the potential is accompanied by a dissolution or redepositon of metal atoms, because this affects directly the surface roughness. To avoid such microscopic changes of the surface structure, the electrode potential after the ORC was always more negative than -0.3 V. In the difference spectra, all unwanted contributions due to background and bulk scattering in the liquid are eliminated, revealing in detail the changes in the SER spectra due to the variation of the interfacial electric field alone. To answer the question posed earlier concerning the potential dependence of the frequencies of bands, we calculated potential difference spectra (PDS) by subtracting curves b and c in Figure 2A from curve a. The resulting PDS are shown in Figure 2B. The spectra are composed of several broad bands, which change in different ways when the applied potential is varied. The most pronounced peak now shows a more symmetric shape than that seen in the original spectrum (Figure 2A, curve a). Also the Raman shift is slightly increased to 234 and 236 cm-l, respectively, for the two curves. But there is a residual asymmetry, which is most simply interpreted in terms of a PDS composed of one strong band and several weak broad ones. Because the difference between spectra at -0.3 and -0.6 V does not show a “hole” at 216 cm-l, the two peaks at 216 and 234 cm-l are not of the same origin. To probe the properties of the surface halide vibrations further, we selected bromide as the electrolyte. From the mass difference between C1- and Br- one would expect a frequency shift similar to that reported for the halide vibrations in the case of the SER of pyridine on Ag electrodes.26 The electrolyte was a 10 M solution of NaBr at pH 2. Figure 3 shows that for bromide the strongest band occurs with a Raman shift of 150 cm-’ compared to 230 cm-l for chloride. Because all modes are “compressed” into a smaller frequency region, distinguishing between the individual modes is even more problematic than for the Ag-C1 vibrations. All bands exhibit a potential dependence, which is shown in Figure 3 with a set of curves each differing from the other by 50 mV. The highest curve is recorded at -0.3 V, the lowest at -0.8 V. Most striking,
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The Journal of Physical Chemistty, Vol. 85, No. 19, 198 1 2740
SERS of Metal-Halide Vibrations on Electrodes
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