J. Phys. Chem. 1992,96,9940-9943
9940
Electrochemical Reductlon of Organlc Sulfldes Investlgated by Raman Spectroscopy Sang Bok Lee, Kwan Kim,* and Myuag So0 Kim* Department of Chemistry, College of Natural Sciences, Seoul National University, Seoul 151 -742, Korea (Received: June 1 I , 1992; In Final Form: August 17. 1992)
The electrochemical reduction of aromatic and aliphatic sulfides has been investigated at Ag electrodes by means of surface-enhanced Raman spectroscopy (SERS) and cyclic voltammetry. The SERS technique applied to a rotating electrode system was demonstrated to be very useful for microscopic study of the electrochemical process on metal surfaces. We have proven the validity of the earlier propition that addition of borohydride in aqueous Ag sol lowers the sol surface potential, enabling the electrochemical reduction of organic sulfides to thiolates. The effective sol potential measured by a silver wire electrode was found to be larger by 0.3 V than the actual potential.
Introduction The adsorption behavior of organic mono- and disulfides on the surfaces of metals such as Ag, Au, and Cu has been of great interest because of its technological importance, especially in boundary lubrication and catalysis.14 Investigations on disulfides with surface-enhanced Raman scattering (SERS) revealed that the molecules undergo surface reaction on silver, namely facile This finds its counterpart in the cleavage of the S-S electrochemical reduction of disulfides producing mercaptans or their salts. In the case of sulfides, Sandroff and Herschbach2 concluded from the SERS studies that C-S bonds in aromatic monosulfides were also easily cleaved on silver under quite mild conditions. Joo et al? carried out S E W investigation of aliphatic sulfides adsorbed on silver and concluded that their C-S bonds did not cleave easily. In the SERS study of aromatic sulfides on silver surface by the same it was suggested that the decompition of these molecules was due to surface photoreaction. In a later work on benzyl phenyl sulfide (BPS) in aqueous silver sol, its SER spectrum was observed to change depending on the concentration of borohydride ion in the sol solution. Even under conditions where surface photoreaction was not likely to occur, BPS decomposed, yielding the SER spectrum of adsorbed benzenethiolate, when an excess of NaBH4 was present in the sol solution. This was attributed to lowering of the surface potential by BH4-. It was reported that the C-S bond cleavage occurred at a potential more negative than -0.3 V vs saturated calomel electrode, measured with a silver electrode dipped in the sol solution. In view of the above observation, it is necessary to carry out detailed investigation under conditions where the surface potential is controlled reproducibly for better understanding of surface reactions of organic sulfides. In this paper, results of the investigation on the potential dependence of the SER spectral pattern for various sulfides, namely BPS, dibenzyl sulfide (DBS), and dimethyl sulfide (DMS), using the electrochemical system are reported.
Experimental Section The method of prepation of aqueous silver sol was described in detail previously.' Methanolic solutions of BPS, DBS, and DMS were added to Ag sol to give the desired concentrations. After the color of the sol solution changed from yellow to red or green, poly(vinylpyrro1idone) was added as a colloid stabilizer. The electrochemicalcell used in the SER experiment consisted of three electrodes, i.e., a Ag working electrode, a Pt counter electrode, and a saturated calomel reference electrode (SCE). A silver working electrode was freshly prepared in each experiment. After being polished with sandpaper and with 0.05-Mm alumina in distilled water, the silver electrode was made into a conic shape and then oxidized briefly with an equal volume mixture of 30% H202 and concentrated ammonia-water. The electrode was To whom all correspondence should be addressed.
roughened by performing sequential oxidation-reduction cycles (ORCs) in 0.1 M KCl. It was then transferred, after rinsing, to ~J~ the electrochemical cell containing the solution of i n t e r e s ~ ~The present roughening process prevents any undesirable electrochemical reactions of adsorbate from taking place during ORCs. A homemade potentiatat driven by a personal computer coupled with a homemade interface was used for potential control. All potentials are quoted versus SCE. Raman spectra were obtained with a Japan Spectroscopic Company Model R-300 laser Raman spectrophotometer. The 514.5-nm radiation from an argon ion laser (Spectra-Physics Model 164-06)was used for excitation. Raman scattering was observed with 90' geometry using a commercial photon counting system. In a typical experiment, the laser power was 100 mW at the sampling position and the spectral slit width was 10 cm-'. All chemicals were of reagent grade and triply distilled water was used for the preparation of solutions.
Results and Discmion In the previous SERS studies, the silver sol solution was circulated through the sampling cell or a spinning cell was used to minimize the surface photoreaction of adsorbed moltcules.*lJs When electrochemical systems are used, the same goal may be better realized by spinning the electrode. Namely, at a high rotating frequency a surface-induced photoreaction may be s u p pressed sufficiently. Before getting into a detailed investigation of the surface reactions of sulfides, it was thought necessary to carry out the following tests on the performance of the homomade rotating electrode built for the suppression of surface photoreaction. The first requirement for a rotating electrode useful for SERS investigation is that rotation of the electrode does not affect the pattern and quality of SER spectra noticeably. For this end, the SER spectrum of pyridine was obtained with and without rotating the electrode. In this measurement, experimental conditions were set such that surface photoreaction was not feasible regardlessof the electrode rotation. Typical SER s p t m obtained under such a condition are shown in Figure 1. The spectra are observed to be insensitive to the rotation of electrode. Hence, in the case of negligible surface photoreaction, the SER s e r a l pattern was concluded not to be affected by the electrode rotation. To test the effectiveness of electrode rotation for the suppression of surface photoreaction, the SER spectrum was obtained for BPS with and without electrode rotation. Figure 2a shows the SER spectrum of BPS obtained at -0.2 V vs SCE without rotating the electrode. This spectrum is essentially the same as the SER spectrum of benzenethiol'6 which chemisorbs dissociatively on silver by rupture of its S-H bond. Considering that benzyl mercaptide adsorbs on silver more strongly than benzenethiolate, the species responsible for Figure 2a,namely benzenethidate, must have been produced by the cleavage of the C H z S bond in BPS on the silver surface. In fact, this is consistent with what has been observed previously in the sol system." When the electrode was subjected to rotation, a somewhat different SER spectral feature was observed. An intrinsic SER spectrum of BPS seemed to be
0022-36S4/92/2096-9940$03.00/0 Ca 1992 American Chemical Society
The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 9941
Electrochemical Reduction of Organic Sulfides 1w8
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Figure 3. SER spectra of lo-' M BPS on a Ag electrode surface in 0.1 M Na2S04with rotation of the electrode at 3000 rpm. The Ag electrode potential was initially set at (a) -0.2 V, lowered to (b) -0.4, (c) -0.5. (d) -0.6, and (e) -1.0 V, and then raised to (fj -0.2 and (g) 0.2 V vs SCE.
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M pyridine on a Ag electrode surface at F i i 1. SER spectra of -0.6 V vs SCE both (a) with and (b) without rotating the electrode at 3000 rpm. The supporting electrolyte was 0.1 M Na2S04 in water. 1076 999
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Figure 2. SER spectra of lo-' M BPS on a Ag electrode surface at -0.2 V vs SCE both (a) without and (b) with rotating the electrode at 3000 rpm. The supporting electrolyte was 0.1 M Na,S04 in a 1:l volume mixture of H 2 0 and CH30H.
obtainableeven under a substantial laser irradiation on the sampling electrode surface. Figure 2b shows the SER spectrum of BPS obtained after 1 h of exposure to laser with the silver electrode
kept at -0.2 V vs SCE being spun at 3000 rpm. Comparing the two spectra (a and b) shown in Figure 2, it can be seen immediately that the peaks at 654, 1238, and 1597 cm-' in the latter spectrum do not have their counterparts in the former spectrum. According to the previous vibrational assignments for BPS,I' these peaks correspond to the CHz-S stretch, the CHz twist, and the 8a mode of benzyl moiety, respectively. In addition, all other peaks in Figure 2b find their counterparts in the ordinary Raman spectrum of BPS. The fact that the SER spectral feature obtaiued with the rotation of the electrode can be explained by adsorbed BPS and that sulfide is a much weaker adsorber than thiolate indicate that the surface photoreaction hardly occurs when the electrode is spun at 3000 rpm. On the basis of the above discussions, the changes in the SER spectral feature with the variation of the potential of the rotating electrode will be attributed to the potential effect rather than to the radiation effect. The SER spectra of BPS obtained at several different electrode potentials with electrode rotation are shown in Figure 3. When the potential of a freshly prepared electrode was maintained at -0.2 V or higher vs SCE, the SER spectrum (Figure 3a) was essentially the same as that for adsorbed BPS, Figure 2a. For example, the characteristic peaks of the benzyl moiety such as the CHz-S stretch at 654 cm-' and the CHz twist at 1238 cm-' appeared distinctly. These peaks lost intensity progressively, however, as the electrode potential was lowered as is shown in Figure 3 W . At -0.6 V vs SCE, the benzyl characteristic paah disappearedcompletely. The resulting SER spectrum was virtually the same as the one for benzenethiolate, namely Figure 2b. Further decrease in the electrode potential did not affect the SER spectrum noticeably (Figure 3e). Once the electrode potential was decreased to -0.6 V vs SCE, bringing it back again to -0.2 V or higher did not restore the SER spectrum of BPS. In fact, the minor variation in the SER spectrum with the increase of the electrode potential was virtually the same as for adsorbed benzenethiolate. Hence the electrochemical reaction of BPS at the silver electrode is clearly irreversible in the potential range between -1 .O and 0.2 V vs SCE. Namely, the reductive cleavage of the CH2-S bond is favorable but its reverse reaction to produce BPS is not feasible. This is probably due to the absence of the benzyl moiety on the surface which was produced by the electrochemical reduction of BPS. Electrochemical generation of diphenyl disulfide from adsorbed benzenetholate did not OCCUT kcawe the S-S bond has a strong tendency to dissociate on the silver surface under the experimental conditions. The present result is in general agreement with the previous observation in the aqueous silver sol system where electrochemid reduction of BPS to benzenethiolate occurred as the surface potential was lowered by the addition of BH4- ions." However, it
9942 The Journal of Physical Chemistry, Vol. 96, No. 24, 1992
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' 1500 '2700 3100 e m , cm-' F@e 5. SER sptctra of lo-) M DMS on a Ag electrode surface in 0.1 M Na804with rotation of the electrode at 3000 rpm. The Ag electrode potential was prognssively lowcred from (a) -0.2 V to (b) -0.4, (c) -0.5, (d), -0.6, and (e) -0.7 V and then brought back to (0 -0.2 V vs SCE. I
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Figwe 4. SER spectra of M DBS on a Ag electrode surface in 0.1 M Na$04 with rotation of the electrode at 3000 rpm. The Ag electrode potential was lowered from (a) -0.2 V to (b) -0.4 and (c) -0.6 V and then brought back to (d) -0.2 V vs SCE.
is to be mentioned that the SER spectrum which was entirely due to adsorbed benzenethiolate was observed when the surface potential as measured by the silver wire electrode dipped in the sol solution was as low as -0.3 V vs SCE. Namely, the same chemical process occurred at 0.3 V higher potential on the sol surface than on the electrode. It is thought that this difference arises due to the method employed for the " n e n t of the surface potential of the silver sol. Nonetheless, the fact that intensities of the peaks characteristic of the benzyl moiety decrease monotonically as the electrode potential is lowered from -0.2 to -0.6 V (Figure 3a-d) indicates that the electrochemical reduction of BPS begins to take place prcwnably at least at near -0.3 V vs SCE. This is a " t with the cyclic voltammetric study of the system which will be presented later. Similar electrochemical reduction was also observed for other aromatic sulfides. Figure 4a shows the SER spectrum of DBS obtained at -0.2 V vs SCE. In this case also, rotating the electrode supp.essed the surface photoreaction very effwtively. on the basis of the literature data,l0 the peaks at 303,659, and 713 cm-'in Figure 4a can be assigned to the CSC bending, C-S symmetric stretching, and c-s asymmetric stretching vibratim, respectively. Besides, a band characteristic of DBS is observed distinctly at 1255 cm-'. Hence, the species responsible for Figure 4a is DBS adsorbed on the silver surface. Also, the SER spectrum is virtually the same as the one observed in silver sol which was circulated through a glass capillary. As the electrode potential was lowered from -0.2 to -0.6 V vs SCE, the intensities of the peaks charactens * tic of DBS diminishad as can be seen in Figure 4a-c. The SER spectrum recorded at -0.6 V is essentially due to adsorbed benzyl mercaptide. The peak positions as well as the relative peak intensities are in good
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agreement with those in the SER spectrum of benzyl mercaptan taken separately under the same experimental conditions." As was the case with BPS,the SER spectrum obtained after restoring the electrode potential back to -0.2 V from -0.6 V was due to adsorbed b m y l mercaptide. Namely, the electrochemical reaction of DBS was not reversible. The SER spectrum of benzyl mercaptide was obtained even with a freshly prepared silver electrode at -0.2 V when the electrode was not rotated. The observations described so far indicate that the two aromatic sulfides, BPS and DBS,have similar electrochemical and photochemid properties. The previous SERS investigation in aqueous silver sol showed that the C-S bonds of aliphatic sulfides were not cleaved by surface photoreaction? In the present work DMS was chosen as a prototype of aliphatic sulfide and its SERS has been investigated with the electrode system. Figure 5a shows its SER spectrum obtained at -0.2 V vs SCE of the electrode potential. This s p a " , which is in excellent agreement with that reported previously in the sol system, can be attributed to adsorbed DMS? For example, the CSC bending and the C-S asymmetric stretching bands appear distinctly at 290 and 730 an-',respectively. Moreover, the SER spectral feature did not change as the rotation of the electrode was stopped. Namely, the surface photoreaction of DMS does not occuf on the electrode in agreement with the sol study. The electrochemical reductionof DMS occumd however, evm though at lower electrode potential than that for aromatic sulfides. The SER spectra of DMS rccordcd at various electrode potentials arc shown in Figure 5a-e. As the potential was lowered, intensities of the bands characteristic of the CSC group decreased together with the intensities of the CH3deformation modes at 983,1036, 1329, and 1430 C d . F ' h d y 8t -0.7 v V8 SCE, tbt CharactSristiC featwe of DMS disappeared almat completely (Figure 5c). After that the spectral pattern was hardly dependent on the potential
Electrochemical Reduction of Organic Sulfides
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The Journal of Physical Chemistry, Vol. 96, No. 24, 1992 9943 Raman signals were much smaller for sulfidea than for thiolates. Alternatively, more favorable adsorption of thiolatesthan sulfides on the SERS active sites may explain the above observation. Namely, even though the SERS active sites are fully Oovmci with the thiolates below -0.6 V, sulfides and their electrochemical reductions may persist at the SERS inactive sites. One cannot judge definitely at the moment which is the proper explanation. As a test of larger surface enhancement for thiolates than sulfides, the SER spectrum was recorded for a 1:lOOO mixture of methanethiolate and dimethyl sulfide. Even though the concentration of dimethyl sulfide was much larger than that of methanethiolate, the SER spectrum obtained was essentially due to adsorbed methanethiolate. It is informative to note that the cathodic peaks in the cyclic voltammograms of F i i 6b,c appear at nearly the same potential, -0.37 V vs SCE. The cathodic currents are comparable atso. This indicates that the C H 2 Sbond energies in BPS and DBS are very similar. The cathodic peak in the DMS system appears, however, at a more negative potential, -0.45 V vs SCE. Besides, the cathodic current in this system is substantially smaller compared to those for the systems containing BPS and DBS. The fact that the C-S bond energy in DMS is larger by 40 W/mol than those in aromatic sulfides may explain the above discrepan~ies.'~The earlier observation that the SERS feature due to DMS disappears at near -0.7 V while that due to either BPS or DBS disappears at near -0.6 V vs SCE may be explained similarly. In summary, electrochemical reduction of aromatic sulfides, BPS and DBS, and an aliphatic sulfide, DMS, on a silver electrode has been investigated. Complication arising from surface photoreactions has been removed by rotating the electrode. The present investigation has proven the validity of the previous proposition that addition of B h - ions in aqueous silver sol lowers the surface potential and thus enabling electrochemicalreduction of organic suifides to thiolata. However, the effective sol surface potential measured by a silver wire electrode dipped in the sol solution has been found to be larger by 0.3 V than the actual potential. It has been found also that the potential dependence of the SER spectral pattern could be correlated with the cyclic voltammographic behavior. It has been demonstrated that the surfaceenhand Raman spectroscopy using a rotating electrode system provides a useful technique for microscopic investigation of electrochemical processes occurring on metal surfaces.
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v m SCE. Figure 6. Cyclic voltammograms of lo-' M solutions of (b) BPS, (c) DBS,and (d) DMS and (a) their supporting electrolyte,0.1 M Na2S04 in a 1:l volume mixture of H 2 0 and CH30H, on a Ag electrode at 20 WTE".
mV/s scan rate.
variation as can be seen from Figure 5f. The SER spectrum of DMS obtained after reaching down to -0.7 V is virtually identical with the SER spectrum of methanethiolateobtained in the aqueous silver sol. The SERS investigation discussad so far shows that the electrochemicalproperty of aliphatic sulfides should be rather similar to thoseof aromatic d d e s even though the photochemical properties might differ. To obtain further information on electrochemical reduction of organic sulfides,'* a cyclic voltammetric study on these molecules has been carried out. The cyclic voltammograms obtained for three analytes, namely BPS, DBS, and DMS, are shown in Figure 6, b, c, and d, respectively. For comparison, the cyclic voltammogram of the supporting electrolyte, 0.1 M Na2S0, in a 1:l volume mixture of H 2 0 and CH,OH, is shown in Figure 6a. The concentrations of sulfides (1 .O X lo-' M) were the same as those used in the SERS measurement. The voltammographic feature in Figure 6a indicates that the electrolyte itself does not experience any significant electrochemical reaction in the potential region used in the SERS investigation. In the case of sulfide solutions, only cathodic peaks were observed, indicating that the product formed in the initial reduction is not rcoxidized at any rate. This is consistent with the observations in the SERS study. The cyclic voltammograms show that the electrochemical reduction begins at -0.25 V vs SCE for all the organic sulfides investigated, also in agreement with the SER spectroscopic study. Interestingly, the electrochemicalreduction of organic sulfides persists until the electrode potential reaches -1.0 V vs SCE. This is in contrast with the observation made in the SERS experiment. In the SER spectra, spectral features due to adsorbed sulfides were hardly detectable below the ekctrode potential of about -0.6 V. Namely, the SERS peaks due to sulfides disappeared at this potential, even though their presence was indicated in the cyclic voltammograms. Thii is most likely to arise because the surface enhancements of
Acknowledgment. This work was supported by the Korea Science and Engineering Foundation and by the Ministry of Education, Republic of Korea.
Refereaccs and Notes (1) Sandroff, C.J.; Herschbach, D. R. Bull. Am. Phys. Soc. 1982,3,27. (2) Sandroff, C.J.; Herschbach, D. R. J. Phys. Chem. 1982,86,3277. (3) (a) Troughton, E. E.;Bain, C. D.; Whitaides, G. M.;N u w , R. G.; AUara, D. L.; Porter, M. D. Longmuir 1988,I , 365. (b) Nuuo, R. G.; Zegarsky, B. R.; Dubois, L. H. J . Am. Chem. Soc. 1987,109,733. (4) Oh,S.T.; Kim, K.; Kim, M.S. J. Mol. Struct. 1991, 213, 307. (5) Sandroff, C.J.; Herschbach, D. R. J. Phys. Chem. 1981,85, 248. (6)Takahaahi, M.;Fujita, M.;Ito, M.Surf. Sci. 1985,158, 307. (7) Taniguchi, I.; Iseki, M.;Yamaguchi, H.; Yasukouchi, K. J. ElectmaM I . Chem. 1985,186,299. (8) Watanabe, T.;Maeda, H. J . Phys. Chem. 1989,93,3258. (9)Joo, T. H.; Kim, K.; Kim, M. S . J. Mol. Struct. 1987, 162, 191. (10)Joo, T.H.; Yim, Y. H.; Kim, K.; Kim, M.S . J. Phys. Chem. 1989, 93, 1422. (11) Yim, Y. H.;Kim, K.; Kim, M.S. J. Phys. Chem. 1990,91,2552. (12)Joo, T. H.; Kim, K.; Kim, M.S . Chem. Phys. Lett. 1984,112,65. (13) Gao, P.;Gosztola, D.; hung, L. H.; Weaver, M.J. J. Electroanal. Chem. 1987,233,211. (14)Park, H.;Lee,S.B.; Kim, K.; Kim, M.S . J. Phys. Chem. 1990,91, 7576. (15) Chun, H. A.; Yi, S. S.; Kim, M.S.;Kim, K . J. Ramon Spcctrasc. 1990,21,743. (16)Joo, T. H.; Kim, M. S.; Kim, K.J. Ramon Spcctrosc. 1987,18,57.
(17)Lee,T.G.;Kim,K.;Kim,M.S.J.RamanSpcctrarc.1991,22,339. (18) Bard, A.J.; Lund, H. Encyclopedia of Electrochemistry of the Elements; Marcel Dekker: New YorL, 1978;Vol. XII. (19) Benson, S.W. Chem. Rev. 1978, 78,23.
