Surface enhanced raman spectroscopy of pyridine on Ag electrodes. Surface complex formation

SURFACE

1 March 1981

CHEMICAL PHYSICS LETTERS

Volume 78, number 2

ENHANCED

l?AiiAN

SPECTROSCOBY OF PYRIDINE

ON Ag ELECTRODES.

SURFACE COMPLEX FORMATION B. PETTINGER Fritz-Haber-lnstitut

and H_ WETZEL der ,~lax-Pianck-GeseIIsc;laft, D-1000

Berlin 33. FRG

Received 16 September 1980; in final form 8 December 1980

Experimental evidence is given for surface complexes consisting of metal-adatom, pyndme enhanced Raman process can occur. A large part of the enhanced continuum turns position of numerous extremely weak SER hnes attributed to these complexes. the surface

l_ Introduction

ported [3] a much larger SER intensity for pyridine with electrolytes containing halide ions than with

Initiated by the pioneering works of Fleischmann et al. [I ] and of van Duyne [2 J , surface (plasmon) enhanced Raman spectroscopy (SERS [3] or SPERS [4] ) is a new and promising approach to surface studies. Its power as we11 as its present-day Iimitations have been emphasized by several authors [514]_ Whereas initially SERS was found only with some orgamc molecules (pyridine and other arnines [3], and carboxylic species [13]) adsorbed on Ag electrodes, it is now reported for a variety of molecules, and organic and inorganic ions [ 14- 16]_ Re-

cently, strongly enhanced Raman processes have been found for single-crystalline sliver (111) and (100) interfaces [17], and also for other metal substrates such as Au and Cu [ 18,191, Pt [20] and single-crystalline nickel surfaces [ZI] _ Of most interest is evidence that the SER mechanism can combine with other Raman scattering enhancement processes such as preresonance or resonance Raman effects [S ,221 or with dedicated surface plasmon (SP) excitations [4] (correlated with a large increase of the surface light intensity [23 J )_ As a result, the total enhancement can rise up to 108-1010 [8,22]. So far, only interactions of the desired adsorbed species with metal surfaces have been studied_ But there are strong indications of a very important contribution of coadsorbates, in particular of the halide ions in the SER mechanism [15] . Van Duyne re398

and halide ions with which out to result from a super-

other ions such as CIO;

and NO;

[3] (we found dif-

ferences in intensities up to two orders of magnitude). Moreover, there are mutual influences of ad- and toad-sorbates on the Raman intensity of both species_ As reported by Dornhaus et al. [15], frequency

shifts of Ag-halide vibrations occur accordmg to their atomic mass differences, which is unquestionable evidence for Ag-X, X = Cl, Br, I motions participating in the enhancement process. These vibrations are only observed in the presence of pyridine, and the presence of halide ions in the electrolyte increases the SER intensities of pyridine vibrations [3,15] _ These observations forced us to take up our earlier idea of surface complexes 1241 formed at (electrochemically) pretreated surfaces_ According to the IR and Raman spectroscopic investigations of metalhalide-pyridine complexes (see e.g. refs. [25,26]), one would expect in the low-frequency region (20300 cm-l) not only the symmetrical Cl-Ag-CI stretch of the surface compounds, but also several other vibrational modes. The observation of these would give clear evidence

for the formation

of a sur-

In addition, carefully recorded spectra with sufficient signal-to-noise ratio should also solve the question to what extent the so-caIIed enhanced background [27] is a continuum as expected in the electron-hole pair enhancement model [5,1 l] . face complex_

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2. Experimental The experimental equipment to perform in situ Raman spectroscopic investigations has been described [ 18,24]_ To obtain very large Raman intensities required for a low signal-to-noise relationshrp, the electrode was roughened to a large extent with strong srrodizations (30 atomic layers), and additionally the intense laser beam was focused on the polycrystalline Ag electrode with a cylindrical lens, thus permitting a much larger mcrdent intensrty of the green argon laser line (5 145 A, 1 W) without destroying the surface compound_ All chemicals were of analytical grade. The solutions were made with distilled Hz0 and D20 using standard salt concentrations (0.1 M KCl) and 0.05 M pyridine.

3. Results and discussion 3.1. Low-freqzrency regime Raman spectra recorded in the low-frequency regime (25-300 cm-‘) show generally a large increase in intensity with decreasing Raman shift. This is partly the consequence of the Bose-Einstein term (&w/W - 1)-l in the prefactor of the Raman cross section_ As a consequence, only very pronounced peaks can be detected as bands or shoulders in this region. In the case of SER experiments an additional rise in total intensity with decreasing Raman shift occurs due to the necessanly imperfect suppression of stray light and Rayleigh scattenng next to the laser line, which is increased dramatically with rough metal surfaces_ To avoid these difficulties, Raman spectra have been recorded at different potentials (e.g. U = 0.0, -0.3, -0.6,0_05 V versus SCE), and then difference spectra were numerically calculated using the most positive (0.05 V) recorded spectrum as the actual background spectrum. Before the recording of the last spectrum at a.05 V versus SCE, the electrode was polarised to M-09 V for 2 s. This treatment reduces drastically the SER intensity by oxidizing and consequently destroying most of the surface compound without a regeneration at i-O.05 V versus SCE. The curve recorded at this potential resembles, there-

1 March 1981

fore, a true background spectrum. This difference technique is based on the fact that in the electrochemical cells the SER intensity (enhanced Raman bands and enhanced continuum) varies strongly with the applied potential, whereas the background (not suppressed stray and Rayleigh light) remains constant because the surface roughness does not change m the potential range used. By creating difference spectra, unwanted contributions due to background and bulk scattering in the liquid are suppressed_ The drfference spectrum, however, still shows a large increase of the SER intensity next to the laser line. According to Burns et al. [28], we applied the “reductron technique” on the difference spectra, which cuts down the steep increase in the very low frequency region with the normalization of the difference data to the Bose-Einstein function. After perfonnmg this procedure there appear several spectroscopic details (see fig. 1) which were not weII seen in the original curves. For each of the abovementroned potentrals several spectral peaks are found appearing clearly only in the reduced potential difference spectra (RPDS). AU these modes have to be attributed to vibrational modes of the surface compound(s)_ The most pronounced peak, shrfting in frequency between 219 and 23 1 cm-l with the applied potentral, has been assigned to Ag-Cl vibrations 1151. Its

800c

600 -

0

50

100

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250 ’

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Fig. l_ Reduced potential differencespectra (RPDS) of the surface compound AgCl pyridine. Electrolyte: 0.1 M KC1 f 0.05 M pyridiie,pH 7. (1) CT= 0.0 V, (2) Cl= -0.3 V, (3) U = -0.6 V versus SCE. Base spectrum at U = +0_05 V after 2 s at 0.09 v.

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potentiaI dependence indicates the variation of bonding forces with the electric field at the interface. The other (at least four) broad modes vary in intensity and frequency position with the apphed potential in a somewhat different way. It is noteworthy that there is no significant change in these frequencies when Hz0 is replaced by D20. Experrmental experience shows the existence of anaiogies between the SER and other Raman spectroscopies such as norma or resonance Raman spectroscopy_ Therefore, the comparison of SER spectra with bulk spectra can provide some important clues on cornpositron and structure of the Raman active entrties. AgCl crystals, for example, have a rock salt structure, where each ion is located on an inversion center with the consequence of forbidden Raman transitrons of first order [29] _ However, the high mobrlity of Ag ions mtroduces some disorder in the crystal, which may resuh in a breakdown of these selectron ruIes, thus permitting weak Raman transitions of first order [29-3 I J _The comparison of SER spectra given in fig. 1 with Raman spectra for Ag chloride crystals displaying weak first- and secondorder effects shows an obvious drfference in the number and frequency of the vibrational modes in the two phases under consideration. Even the simple superpositron of “pure” Ag-pyridine (one mode around 2 10 cm--l [S] ) and “pure” Ag-chloride spectra (with the set of bands similar to a silver chloride polycrystalline sampIe) cannot account for all the spectral characteristics seen in the RPDS (fig. 1). These spectroscopic data provide evidence for a more complex structure and composrtron of the Raman active surface center. This is not surprising, because the electrode interface may adsorb, coadsorb or chemisorb Ag-adatoms, Cl, pyridine, water and cations. In other words, it is very likely that surface complexes composed of pyridine, halide ions and silver (ad)atoms are created during the oxidation-reduction cycle. If these compIexes cannot be formed, for instance in the absence of halide ions, an enhancement is still found, but at remarkably lower level. These surface complexes have their correspondence with well known metal-halidepyridine complexes_ Relevant here are detailed IR and Raman spectroscopic studies in the low-frequency regime reported by Saito et al. [25,26] _These investigations clearly show the existence of several 400

1 March 1981

skeletal vrbrations of these compounds_ As an example, nine such fundamentals have been observed and assigned for the Zn-halide-pyridine complex which belongs to the same molecular point group (C2v) as the organic l&and itself. The occurrence of severaI low-frequency vibrations in the SER spectra can be considered as reasonable evidence for similar skeletal vibrations of a surface complex According to the spectroscopic data, a particular bonding of the Cl- ions to silver (or Cu or Au as discussed below) must occur, which is different from the usual structural situation for specifically adsorbed Cl- ions. The strong mutual influence of the three entities, metal, pyridine and halide ions, leads to the Jssumption that the bonding of the Iigands occurs to the same metal atom. From spatial requirements, such a structure probably exists only with adatoms, which have certainly more metallic than Ionic properties. The coadsqrption of halides and pyridine to the same adatom results in the additional skeletal vibrations from the N-_4gCl,z, n = 1,2, ___,group. Note the sirmlarity of this structure with that for CN- on Ag suggested by Billman et al. 11 l] , where the ligands are replaced by three CN- ions bounded to the same adatom. Consequently one would expect similar low-frequency vibrations in the SER spectra for such a Ag(CN), surface complex as for the Aghalide-pyridine complex_ These surface complexes develop during and immediately after an oxidation-reduction cycle by “trapping” the just redeposited adatoms before they can be incorporated completely in the metal structure. When a relatively rough silver surface, free of contamination, is immersed in the electrolyte, a similar process may take place during the reconstruction of the rough surface with the help of ligands that form complexes such as pyridine and halides. In consequence a SER effect should appear, as found by van Duyne c3].

According to our observations, similar effects occur for pyridine on Cu and Au electrodes in the Iowfrequency regime_ For instance, a peak around 230 cm-l is found, which appears together with a large rise of the SER signal when the Raman shift is lowered_ The similar spectroscopic data with the three metals indicate an analog nature of the corresponding surface complexes which probably means that these complexes are formed more by electrostatic forces

Volume 78, number2

CHEMICAL PHYSICS LETTERS

than by chemical bonding. Therefore, a large part of the spectroscopic changes with applied potentiai can be considered to arise from alterations of these electrostatic forces as well as from variations in complex structure and composition. The idea of complex formation easily accounts for the need of some surface roughness or an oxidation-reduction cycle to create the enhancement at least in electrochemical environments_ It seems to be that only with particular surface structures can the metallic polarizability be modulated by the molecular vibrations of the adsorbate efficiently enough to “create” the large enhancement. In this context we note that the occurrence of adatoms has been postulated in theoretical models as well as on the basis of spectroscopic data [ 1 l] _ 3.2. The enhanced continmcrn The large increase in the SER intensity in the lowfrequency region down to very small Raman shifts is obviously the result of the superposition of several broad vrbrational bands, as shown in the reduced potentiai difference spectra in fig. 1. This is further evidence for additional SER lines for ad- or chemisorbed pyridine, not found for those molecules in solution. In contrast to these experimental observations_ van Duyne [8] and Hexter [9] expect a restnction in the number of observable fundamentals; their theoretical analysis only allows an enhancement for a1 modes. On the other hand, Efrima and Metiu [32] predict overtones in their model of surfaceinduced resonance Raman enhancement. It is important, therefore, to determine which fundamentals and what type of additional modes appear in the SER spectrum_ Obviously this question is directly correlated to the symmetry of the adsorbed species and the enhancement mechanism(s)_ To obtain this information, we recorded spectra from 50 to 4000 cm-1 for pyridine on Ag electrodes at -0.6 V with a very large signal-to-noise ratio. By using water and deuterium oxide, we were able to record weak lines in regions which are usually masked by the intense bands due to stretching vibrations (2150-2600 and 3000-3650 cm-l for D20 and H20, respectively)_ The comparison of the corresponding SER spectra show minor frequency shifts for the fundamental vibrations, whereas their relative

1 March 1981

intensities can vary to a greater extent (e.g. v12). The coincidence of most of the SER lines for pyridine in a Hz0 and D,O electrolyte is evidence for the accuracy of recording_ For convenience, the spectra are plotted in two sections and normalized with respect to the strongest fundamental v1 _In addition, several frequency regions are shown at high intensity. The assignment of the fundamentals for pyridine on Ag (the possible change in symmetry 1s not taken into account) have been made according to Wrlmhorst et al. [33] _Nearly all (25) of the 27 fundamentals of pyridine appear in the SER spectra. Some is assumed to be coincident with others as reported for hquid pyridine. Most strrking is the appearance of ~50 addltronal weak SER bands covering most of the spectral region (tables listing their frequencies are given in a following paper, they can be attrrbuted to pyridme vibrations). The assrgnment of all those modes is impossrble at present as discussed below. Some of the lines observed may be attributed to molecular vibrations of impurities ad- or chemi-sorbed at the electrode surface which show large SERS effects. Due to the low concentration of such Impurities, their SER intensity is low and hence only the strongest fundamentals of these species may be detectable, if at all. However, there is evidence for the contribution of impurities to SER effects_ Mahoney et al. [34] found very broad bands around 1 lOO1700 cm-l attributed to amorphous carbon present at Ag-electrodes in 0-i M KF solution. These amorphous carbon bands seem to be very weak and may be dominated by weak pyndine lines. One has to consider other effects besides impurity bands which can result in an increase of the number of surface enhanced Raman lines for pyridine. An increase can be caused for instance by surface complex formation, evidence for which is given in the first part of this paper for pyridine on Ag. At least five additional vibrational modes in the low-frequency region have been detected experimentally. Questionable, however, is the possible influence in frequency regions above 300 cm-l _ Surprisingly, we found SER bands at 1018 cm-l and 1025 cm-l with intensities depending on the crystahographic orientation of the silver electrode [ 171. These observations might be due to different adsorption sites and/or surface complexes_ Some of the weak SER lines not belonging to the 401

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PHYSICS

uted to pyridine or impurity vibrations. In contrast to this observation other authors believe the “enhanced continuum” arises from surface plasmon luminescence [35,36] or from recombmation of electron-hole pairs created at the rough metal surfaces alone [l l] However, the above results are evidence for the importance of the adsorbed species and their vrbratrons even for the continuum-like inelastic light scattering.

,-

c?80

Hz0

------/G ,

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March 1981

In spite of numerous experimental results, we still lack an understanding of the physical basis for SERS. Obviously, specific interactions between the adsorbed species and the metal surfaces occur [4--141 which lead either to an increase of the molecular polarizability or to an inelastic light-scattering mechanism involving surface plasmons [9,10,18,19] or electronhole pair excitations [L5,1 l] or a combination of these three excitation possibilities [4,5,18,19] _These enhancement models seem to be quite different at first glance. But they take into account and emphasize only one aspect of the complex nature of the adsorbate-metallic-substrate interaction. No doubt there is a need to take into account the whole interaction spectrum of the metal (surface) and its adsorbates and coadsorbates wi*h electromagnetic radiation for a reliable description of the SER process.

many very weak SER lines which have to be attrib-

00

I

4. Conclusion

fundamentals may arise from overtones or combination modes. The appearance of such modes is well known in normal Raman spectroscopy. We believe that at least some (> 6) overtones and (> 12) combination modes are present in the SER spectra. Concerning the “enhanced continuum’-, the spectra shown in fig. 2 ciearly indicate that a large part of the continuum is formed by the superposition of

I

LE-ITERS

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zi I

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Energy

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-/

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40C

[crt-~-~ I

?g. 2. SER spectra for pyridine with DzO (upper part) and Hz0 Qower part) from 0 to 4000 cm-‘. Several amp’lifed sections regiven to show weak SER lines. Electrolyte- 0.1 M KC1 + 0.05 M pyridine in Hz0 or D20. U= -0.6 V versus SCE. Assignnent after ref. 133J .

CHEMICAL PHYSICS LETTERS

Volume 78, number 2 Acknowledgement We thank Professor

l-l. Gerischer

for his continu-

ing support and very stlzulating interest. We also thank Dr. Kolb, Dr. Rath and U. Wenning for helpful discussions and Miss Schaefer for valuable technical assistance.

References [ 11 M. Fleischmann, P.J. Hendra and A J. McQuilIan, Chem. Phys. Letters 26 (1974) 163. [2] R.P. van Duyne, J. Phys. (Pars) 38 (1976) C5-239. [3] D.L. Jeanmaire and R.P. van Duyne, J. Electroanal. Chem. 84 (1977) l_ [4 ] B. Pettinger, U. Wenning and K. Wetzel, in Proceedings of the Conference on Non-Traditional Approaches to the Study of Solid-Electrolyte Interfaces, Snowmass, Colorado, September 1979, Surface Sci., 101 (1980) 409. [S] E Burstein, C.Y. Chen and S. Lundquist, Proceedings of the Joint US-USSR Symposium on the Theory of Light Scattering in Condensed Matter (Plenum Press, New York, 1979). [6] A. Otto, Lecture at the International Conference on Vibrations in Adsorbed Layers, Jiilich, June 1978. [7] T.E. Furtak and J. Reyes, Surface Sci. 93 (1980) 35 I_ [8 ] R P. van Duyne, in: Chemical and biochemical applieations of lasers, Vol. 4, ed. C.B. Moore (Academic Press, New York, 1979). [9] R-M. Hexter and M.G. Albrecht, Spectrochim. Acta 35A (1979) 233. [lo] M. Moskovlts, J. Chem. Phys. 69 (1978) 4159. Ill ] J. Billmann, G_ Kovacs and A. Otto, Surface Ser., to be published. [12] S. Efrima and H. Metiu, J. Chem. Phys. 70 (1979) 1602. 1131 M. Fleischmann, P.J. Hendra, A J. McQuillan, R.L Paui and-E.8 Reid, J. Raman Spectry. 4 (1976) 269. 1141 G.C. Schatz and R.P. van Duyne, in: Proceedings of the Conference on Non-Traditional Approaches to the Study

[15] [16] [ 17 ] [18] 1191 [20] [21] 1221 1231 [24] [25] [261 [27] [28] [29] [30] [31] [32] [33] [34] 1351 [36]

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of Solid-Electrolyte Interfaces, Snowmass, Colorado, September 1979, Surface Sci. 101 (1980) 425. R. Dornhaus and R K. Chang, Surface Sci., to be published. R. Domhaus, M-B. Long, R-E. Benner and R.K. Chang, Surface Sci., 93 (1980) 240. B. Pettinger and U. Wenning, Chem. Phys. Letters 56 (1978) 253. B. Pettinger, U. Wenrung and H. Wetzel, Chem. Phys. Letters 67 (1979) 192. U. Wenning, B. Pettlnger and H. Wetzel, to be pubhshed. R.P. Cooney, P.J. Hendra and M. Fleischmatm, J. Raman Spectry. 6 (1977) 244. J.M. Stencel and E.B. Bradley, J. Raman Spectry. 8 (1979) 377. B. Pettinger and T. Watanabe, to be pubhshed. B. Pettinger, T. Tadjeddine and D.M. Kolb, Chem. Phys. Letters 66 (1979) 544. B. Pettinger, U. Wenning and D.M. Kolb, Ber. Bunsenges. Physik. Chem. 82 (1978) 1326. Y. Saito, M. Cordes and K. Nakamoto, Spectrochnn. Acta 28A (1972) 1459. Y. Saito, C.W. Schlaepfer, M. Cordes and K. Nakamoto, Appl. Spectry. 27 (1973) 213. R.L. Birke, J.R. Lombardi and G-1. Gersten, Phys. Rev. Letters 43 (1979) 71. G. Burns, F. Dacol and R. Alben, Sobd State Commun. 32 (1979) 71. R. Alben and G. Burns, Phys. Rev. B16 (1977) 3746. B. Bootz, W. von der Osten and H. Uhle, Phys. Stat. Sol. 66 (1974) 169. S. Ushioda and M.J. Delaney, Solid State Commun. 32 (1979) 67. S. Efrima and H. hletiu, Surface Sci. 92 (1980) 417. J.K. Wiiurst and H.J. Bernstein, Can. J. Chem. 35 (1952) 1183. H.R. Mahoney, M.W. Howard and R_P_Cooney, Chem. Phys. Letters 71 (1980) 59. J.C. Tsang, J.C. Kiiley and J A. Bradley, Phys. Rev. Letters 43 (1979) 772. J-R. Kirtley, S.S. lha and J.C. Tsang, Solid State Commun., to be published_

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