668
Langmuir 1992,8, 668-672
Adsorption and Electrooxidative Pathways for Sulfide on Gold As Probed by Real-Time Surface-Enhanced Raman Spectroscopy Xiaoping Gao, Yun Zhang, and Michael J. Weaver* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 Received July 1, 1991 The adsorption and electrooxidation of sulfide on gold in aqueous alkaline and acidic media have been examined by means of cyclic voltammetry coupled with sequences of surface-enhanced Raman (SER) spectra, the latter providing molecular-level informationon the adsorbed species involved. The required real-time acquisition of SER spectra was facilitated by the use of charge-coupleddevice (CCD) detection. At the most negative potentials, a single SER band at ca. 270 cm-1 is obtained, ascribed to the surfacemonoatomic sulfur stretch. Sweeping the potential positive, in the region below solution-phase sulfide electrooxidation, yielded the onset of a band at significantly higher frequencies, ca. 310 cm-1, coinciding with a reversible voltammetric prepeak. The spectra are consistent with an earlier suggestion that the voltammetric feature arises from adlayer deprotonation. The voltammetric onset of solution sulfide electrooxidation yields several new spectral features. For sulfide concentrations above ca. 1mM in alkaline solution, bands at 150,220,and about 450 cm-l appear; these are diagnostic of the formation of Ssrings. The additional formation of open-chain polymers and polysulfides is indicated by the breadth of the 450-cm-l feature; these adsorbates appear to dominate for reactant concentrations below 1mM from the observed absence of the 150- and 220-cm-l bands. The monomeric sulfur layer is re-formed during the cathodic portion of the voltammogram. Some mechanistic implications of these findings are discussed. Introduction Applications of in situ vibrational, specifically infrared and surface-enhanced Raman (SER), spectroscopies to the examination of molecular phenomena at metalsolution interfaces have become increasingly diverse as well as common in the last decade. (See refs 1and 2 for a representative pair of recent reviews.) Most recently, FTIR spectroscopy has received greater attention for this purpose, encouraged in part by its applicability to a wider range of metal surfaces, including ordered single crystal^.^ Nevertheless, SERS still retains the unique advantages of selectivity as well as sensitivity to interfacial species along with ready detection over wide vibrational frequency ranges, merits which are especially valuable when examining electrochemical systems. Our interest in SERS for such applications has centered on gold surface^.^ Of the SERS-active “coinage”metals, gold displays several desirable properties, including the availability of wide potential ranges (especially in the positive direction), and its suitability for chemical modification by deposition of both metallic and nonmetallic materials. The latter tactic enables the applicability of SERS to be expanded in suitable cases to encompass underpotential deposited (upd) metal^,^ transition metals: and metal oxides.’ A limitation of gold substrates for such studies, however, is that observation of the SERS phe(1)Christensen, P. A.; Hamnett, A. In Comprehensive Chemical Kinetics; Compton,R. G., Hamnett, A., Eds.; Elsevier: Amsterdam, 1989; Vol. 29,Chapter 1. (2)Hester, R.E. In ref 1, Chapter 2. (3)For an overview, see: Chang, S.-C.; Weaver, M. J. J. Phys. Chem. 1991,95,5391. (4)See for example: (a) Weaver, M. J.; Corrigan, D. S.; Gao, P.; Gosztola, D.; Leung, L.-W. H. J . Electron Spectrosc. Relat. Phenom. 1987, 45,291. (b) Weaver, M.J.; Corrigan, D. S.; Gao, P.; Gosztola, D.; Leung, L.-W. H. ACS Symp. Ser. 1988,378,303. (5) (a)Leung, L.-W. H.; Weaver,M. J. J.Electroanal. Chem. Interfacial Electrochem. 1987,217,367.(b) Leung, L.-W. H.; Gosztola, D.; Weaver, M. J. Langmuir 1987,3,45. (6)(a) Leung, L.-W. H.; Weaver, M. J. J. Am. Chem. SOC. 1987,109, 5113. (b) Leung, L.-W. H.; Weaver, M. J. Langmuir 1988,4,1076. (7)(a) Desilvestro, J.; Corrigan, D. A.; Weaver, M. J. J. Phys. Chem. 1986,90,6408. (b) Gosztola, D.; Weaver, M. J. Langmuir 1989,5,776.
0743-7463/92/2408-0668$03.00/0
nomenon (or phenomena) on this metal requires the use of laser excitation at wavelengths above ca. 600 nm. Unfortunately, most conventional detectors, especiallythe photodiode arrays commonly used for time-resolved studies, exhibit poor quantum efficiencies a t the red-region wavelengths of the resulting Raman scattered light. This has restricted heretofore the applicability of time-resolved SERS in our laboratory, especially to the real-time examination of reactive electrochemicalsystems,since long (>lo s) spectral acquisition times are often required that are not readily compatible with the time scale of accompanying current-potential or other electrochemical measurements. This sensitivity limitation has obliged us in studies of this type to focus on reacting systemsthat display atypically intense SERS.8 Recently,however, two developmentshave occurredthat promise to alter the situation substantially. First, the feasibility of utilizing FT Raman spectroscopy on gold and other surfaces using near-IR excitation has been dem~nstrated.~ The second,and perhaps more significant, development involves the emergenceof commercial chargecoupled device (CCD)detectors suitable for Raman spectroscopic applications.1° These array detectors offer the major virtues of very high quantum efficiency even into the red spectral region (up to ca. 900 nm) of interest here, together with negligible dark currents. The CCD detectors exhibit sufficient sensitivity to enable surface spectra to be obtained on a “single shot” basis within ca. 1s even for adsorbate vibrational modes exhibiting only weak Raman scattering. This situation enables real-time sequences of SER spectra to readily be obtained for a (8) (a) Gao, P.; Gosztola, D.; Weaver, M. J. J. Phys. Chem. 1988,42, 7122. (b) Gao, P.; Gosztola, D.; Weaver, M. J. J. Phys. Chem. 1989,93, 3753. (c) Gao, P.; Gosztola, D.; Weaver, M. J. Anal. Chim. Acta 1988, 212,201. (9) (a) Chase, D. B.; Parkinson, B. A. Appl. Spectrosc. 1988,42,1186. (b) Angel, S.M.; Katz, L. F.; Archibald, D. D.; Lin, L. T.; Honigs, D. E. Appl. Spectrosc. 1988,42,1327. (10)See for example: (a) Epperson, P. M.; Sweedler, J. V.; Bilhorn, R. B.; Sims, G. R.; Denton, M. B. Anal. Chem. 1988,60,327A.(b) Pemberton, J. E.; Sobocinski, R. L.; Bryant, M. A.; Carter, D. A. Spectroscopy 1990,5, 26.
0 1992 American Chemical Society
Langmuir, Vol. 8, No. 2, 1992 669
Adsorption and Electrooxidative Pathways for Sulfide wide variety of reactive adsorbates acquired on a time scale commensurate with many conventional electrochemical measurements, such as cyclic voltammetry. We present here such real-time SER spectral data in conjunction with cyclic voltammetry on gold for such a reactive electrochemical system of mechanistic interest anticipated to involve multiple chemisorbed species: sulfide electrooxidation in acidic and basic aqueous media. While this system in basic solutions has been the subject of several investigations utilizing conventional electrochemistry," reflectance spectroscopy,12X-ray photoelectron spectroscopy,13 and SERS,14 the nature of the adsorbed species involved in the observed electrochemical transformations have yet to be discerned satisfactorily. The present results offer significant new insight in this regard.
A2
- 0.9
- 0.5
-0.1
0.3
E/V vs SCE
Experimental Section Anhydrous NaC104 (G. Frederick Smith Chemical Co.) was used as received. The 1 M HC104stock solution was prepared from redistilled 70% HClOd (GFS Chemicals)and the 1 M NaOH stocksolution fromultrapure NaOH-H20(Alfa). The NazS.9H20 (Aldrich)was purified as follows: it was dissolved into a minimal amount of warm deionized water with constant stirring under a flowof nitrogen; the resulting homogeneous solution was filtered while warm and the filtrate collected. Large, colorless, transparent crystals formed upon cooling the solution to room temperature. The crystals were filtered and dried on a watch glass in a glovebag;they formed a white solid after 2 days. All solutions were prepared by using water, purified by a Milli-Qsystem (Millipore). The working electrode was a 4-mm-diameter gold disk sheathed in Teflon, which was mechanically polished with 0.3pm alumina on a microcloth (Buchler) and rinsed thoroughly with water prior to use. The gold surface was electrochemically roughened by multiple oxidation-reduction cycles (ORC) in 0.1 M KCl to yield SERS activity as described previously.16 The electrochemicalsurface Raman measurements employed a conventional two-compartment glass cell with the working electrode illuminated through ita base. The Raman excitation was a Spectra-Physics Model 165 Kr+laser operated at 647.1nm at a power of -20 mW on the sample. The Raman-scattered light was collected with a 50-mm-diameter camera lens (DO Industries Model DO-5095) and focused into a SPEX Model 1877 triplemate spectrometer. The spectrometer was equipped with a Photometrics PM 512 CCD detector cooled to a temperature of 163 K. The detector was operated by a Photometrics CC200camera controller interfaced with a Zenith 386 computer for data acquisition and storage. The spectrometer configuration utilized 600 g mm-l ruled gratings in both the filter and spectrograph stages. All measurements were made at room temperature, 23 f 1 "C. All potentials quoted in this work are with respect to the saturated calomel electrode (SCE).
Results and Discussion The basic experimental strategy employed here was to acquire sequences of SER spectra during cyclic voltammetric potential excursions selected so to encompass the desired surface redox transformations. Figures 1 and 2 display representative voltammograms at 20 mV s-l for 2 (11) (a) Wierse, D.G.; Lohrengel, M. M.; Schultze, J. W. J . Electroanul. Chem. Interfacial Electrochem. 1978, 92, 121. (b) Van Huong, C. N.;Parsons,R.; Marcus, P.; Montes, S.;Oudar, J. J. Electroanal. Chem. Interfacial Electrochem. 1981, 119, 137. (12) Lezna, R.0.; Tacconi, N. R.; de Ania, A. J. J.Electroanal. Chem. Interfacial Electrochem. 1990, 283, 319. (13) Buckley, A. N.; Hamilton, J. C.; Woods, R. J.Electroanal. Chem. Interfacial Electrochem. 1987, 216, 213. (14) Baltruschat, H.; Staud, N.; Heitbaum, J. J. Electroanal. Chem. Interfacial Electrochem. 1988, 239, 361. (16)Gao, P.; Gosztola, D.; Leung, L.-W. H.; Weaver, M. J. J. Electroanal. Chem. Interfacial Electrochem. 1987, 233, 211.
Figure 1. Anodic-cathodic cyclic voltammograms at 20 mV s-l for 2 mM Nafi in 0.1M NaC104+ 0.01M NaOH on gold electrode, for various upper reversal potentials. Electrode area is 0.125 cm2.
I
A
I
CP -0.6
-0.3
0
0.3
0.6
E / V vs SCE
Figure 2. As in Figure 1,but in 0.1 M NaClO, + 0.01 M HClO,.
mM Na2S in basic (0.1 M NaC104 + 0.01 M NaOH, pH 12) and acidic (0.1 M NaC104 + 0.01 M HC104, pH 2) media, respectively. The negative potential limits, -1.05 and -0.6 V, were chosen to allow the observation of the various sulfide electrooxidation steps, yet largely avoid hydrogen evolution. The family of voltammograms displayed in both Figures 1 and 2 were selected to show the effect of altering the positive potential limit. The data obtained in alkaline solution are similar to those reported in ref 12. Initial Electrooxidation of Adsorbed Sulfide. Several voltammetric peaks in Figure 1 and 2 signal the occurrence of sulfide electrooxidation and subsequent faradaic electrochemistry. The first noticeable feature is the anodic peak marked A1 in Figures 1and 2;reversing the sweep prior to -0.5 and -0.1 V in these basic and acidic media, respectively, yields a cathodic partner labeled C1. Several features of the A1/C1 wave are worthy of note. Variation of the sweep rate v,from 0.02 to 0.2 V s-l, yielded peak currents proportional t o v , with a coulombic charge (200 p C cm-2) that is independent of v in both the basic and acidic media. The waves were also unaffected by variation in the sulfide concentration at least over the range 0.5-5 mM. These findings indicate that the A1/C1 feature is associated with initially adsorbed, rather than solution-phase (diffusing),species. Theca. 0.45-V negative shift in the reversible potential of the A1/C1 wave from the lower to higher pH suggests that a protonation equilibrium is coupled to the redox process.
Gao et al.
670 Langmuir, Vol. 8,No. 2, 1992
-0.43 v -0.53 v
-0.61 V
ESH -0.30 v
-0.40 v
I
/
-0.50 V
I
\
I
700
600
500
300
400
200
vwenumbm (cm-1)
Figure 3. Potential-dependent sequence of SER spectra in 130-700-~m-~ region obtained during voltammogram at 20 mV 5-1 for 2 mM NazS in 0.1 M NaC104+ 0.01 M HC104,from -0.5 to-O).3V,returnto-O.GV, andfinally to-0.33V. Eachspectrum shown was acquired over 3.5 s; the potentials indicated are the values at the beginning of each spectral acquisition. Raman excitation was 647.1 nm, with 20-mW power on sample. Consistent with this last piece of information, the process responsible for the AdC1 wave has been written formally as12 (SH-),, - 2e-
Sad
+ H+
(1)
where the subscript ad refers to an adsorbed species. Given that such adsorbates are attached via a t least partially covalent bonds to the metal surface, it is doubtful if the two-electron designation in eq 1has quantitative, rather than merely formal, significance. However, the protonated “sulfide” form, (SH-),d, is anticipated to be associated with greater electron density than unprotonated adsorbed sulfur, sad, so that passage of “anodic” current should be triggered by deprotonation, accounting qualitatively for the A& wave.16 Evidence favoring this qualitative designation of the voltammetric feature can be discerned from the corresponding potential-dependent SER spectra, acquired simultaneously with the voltammograms in Figures 1and 2. An example of such a set, obtained in the acidic medium, is displayed in Figure 3. The frequency region covered in Figure 3,130-700 cm-l, encompasses that where m e t a l 4 and also S-S stretching and related vibrations are anticipated. The potential was swept a t 20 mV s-l from -0.50 to -9.3 V, and returned to -0.6 V before sweeping again to -0.3 V. Each spectrum in the upward-going temporal sequence in Figure 3 was obtained with 3.5 s acquisition time, each starting at the potential indicated along the (16) These comments are predicated on the basis of the inherent inseparability of faradaic and nonfaradaic charges for redox reactions involving such chemisorbed species. Thus, there is good reason to expect that (SH-),d as well as smd would engage in substantial, yet differing, electron sharing (covalent bonding) with the metal surface, so that it is probably inappropriate to associate eq 1strictly with an integral (two-) electron transfer.
side; only selected spectra are shown for clarity. At potentials negative of the A1/C1 wave, below ca. -0.45 V, a single vibrational band at 270 cm-’ is observed. Altering the potential positive of ca. -0.45 V yields a diminution of this band and the appearance of a nearby, yet distinct, feature centered a t 310 cm-l. These spectral changes are entirely reversible upon repeated excursions through this potential region (Figure 3). Similar, albeit less easily resolvable,spectral changesare also observedin the vicinity of the A1/C1 wave in the basic medium. Both the 270- and 310-cm-’ bands can reasonably be assigned to a metal-monoatomic sulfur stretch. The corresponding frequencies of the gold surface-halide stretching vibrations for chloride and bromide in the same potential region, ca. 250 and 180 cm-l, respectively, yield a predicted value for sulfur of ca. 265-285 cm-l if the bond force constant,f,-,, is presumed to be similar.” The higher observed frequenciesfor the metal-sulfur stretches on gold are unsurprising in view of the anticipated covalency of the sulfur compared with these halide bonds. The increase in the metal-sulfur frequency, from 270 to 310 cm-l, upon traversing through the A1feature is qualitatively consistent with eq 1since deprotonation of the chemisorbed sulfur should yield greater metal-sulfur electron sharing, and hence a higher force constant. Indeed, significantly (60 cm-’) higher surface-sulfur vibrational frequencies have been observed for atomic sulfur compared with HpS on platinum in vacuum by means of electron energy loss spectroscopy.ls The sulfur adlayer on gold was also found to be irreversibly adsorbed at higher potentials. Specifically, emersing the electrode from sulfide-containing solutions at potentials positive of the A1/C1 wave and transferring to the corresponding supporting electrolyte in the absence of sulfide yielded the A& wave essentially as before. Maintaining the electrode at negative potentials close to the onset of hydrogen evolution, however, resulted in slow sulfide desorption. Nevertheless, altering the potential to such negative values in the presence of solution sulfide left the intensity of the 270-cm-l SERSfeature essentially unchanged, indicating that the “reduced” sulfur adlayer remains unaffected under these conditions. Further Electrooxidation. Formation of Sulfur Adlayers. Voltammetric sweeps continued to more positive potentials exhibit a broad yet pronounced anodic wave, Az, beginning at ca. -0.4 and 0 V in the alkaline and acidic media, respectively, employed here (Figures 1and 2). This feature has been assigned to the overall formation of solution polysulfides on the basis of conventional electrochemicaldata.lZJ3Thus, in basic media, where HSis the predominant species (as for pH 12), one can write (cf. ref 12) sad
-
+ XHS- + XOH- - 2(X - 1)e-
and
(s;il),d
+ XHzO
(2)
(3) During the reverse (negative-going) potential sweep in basic media, two voltammetric peaks are obtained (Cp and C3 in Figure l), the first of which does not shift in potential with varying pH.13 On the basis of this pH independence, Cp has been ascribed to the reverse of eq 3 and C3 to the reverse of eq 2.13 These two processes cannot be resolved voltammetrically in acidic media, only a single major cathodic wave being observed (CZin Figure 2). (sEi1)ad - 2e-
(Sz+l)ad
(17)Gao, P.; Weaver, M. J. J. Phys. Chem. 1986,90,4057. (18)Koestner, R. J.;Salmeron, M.; Kollin,E. B.; Gland, J. L. Surj.Sci. 1986, 173,668.
Adsorption and Electrooxidative Pathways for Sulfide
Langmuir, Vol. 8, No. 2, 1992 671
n
--450
"I
-0.38 V
-0.28
V
.. -0.25 V -0.35 V t
M -0.05
V
-0.10 v
-1.05 V I
I
,
700
800
500
400
so0
200
Frv?numba (cm-1)
700-0.10 v 800
500
100
300
200
hnnumben (mn-1)
Figure 4. Potential-dependentSER spectral sequenceobtained during voltammogram at 20 mV s-l for 4 mM NazS in 0.1 M NaClOd + 0.01 M NaOH, from -1.05 to +0.2 V and return. Other details are as in Figure 3.
Figure 5. Potential-dependentSER spectral sequence obtained during voltammogram at 20 mV s-* for 2 mM NaZS in 0.1 M NaC104 + 0.01 M HC104, from -0.4 V to +0.2 V and return. Other details are as in Figure 3.
Inspection of SER spectral sequences obtained during such voltammetric potential excursions provides insight into the likely surface species present, which serve as templates for these overall redox transformations of bulkphase species. Representative sets of SER spectra obtained in the alkaline and acidic media employed for the voltammograms in Figures 1and 2 are shown in Figures 4 and 5, respectively. The spectral data acquisition conditions for Figures 4 and 5 were as in Figure 3; the voltammetric sweep rate was again 20 mV s-l. Several marked spectral changes are observed, corresponding initially to the onset of the anodic wave A2. We consider first SER spectra obtained for 4 mM Na2S in the basic electrolyte (Figure 4). Traversing the potential positive of ca. -0.4 V leads to the production of several new bands, most prominently a broad feature centered at 450 cm-l with sharper features at 220 and 150 cm-l. Each band grows in intensity as the potential is swept in the positive direction. The broad 450-cm-l feature is also sharpened somewhat, gaining higher frequency components, under these conditions. After reversing the potential sweep direction at 0.2 V, little change in the SER spectra occurs until about -0.25 V, where the 220- and 150-cm-l bands diminish in intensity and the ca. 450-cm-' feature broadens and shifts to lower frequency (Figure 4). These changes coincide with the onset of cathodic voltammetric current (Figure l),and are consistent with the notion that the initial reverse wave C2 is associated with oxidation of polymeric sulfur species (eq 3). At more negative potentials, ca. -0.85 V, the 450-cm-1band virtually disappears and the 300-cm-l feature returns. These spectral changes are repeatable upon further potential cycling. No spectral features were obtained for gold electrodes not subjected previously to ORC's so to engender SERS activity. This confirms that the potential-dependent Raman signals arise from SERS of adsorbed species
rather than from unenhanced (or resonantly enhanced) Raman scattering of bulk-phase electrogenerated species (cf. ref 19). The form of the Raman spectra observed in the anodic oxidation region is closely reminiscent of those obtained for elemental sulfur under conditions (rhombic crystalline, liquid state at ca. 120-200 "C)where predominantly S8 rings are present.20 Specifically, the 220- and 470-cm-l bands are attributable to a1 stretching modes, and the 150-cm-l feature to an e2 mode. When observed together, these features appear to be quite diagnostic of S8 in comparison with alternative ring structures and open chains.21 Nevertheless, a number of species, including short sulfur chains and polysulfides, exhibit stretching modes in the vicinity of 450 cm-I (A50 cm-I) as a chief spectral feature.22 Not surprisingly, then, in addition to Sa it appears likely that a mixture of other adsorbed sulfur species are formed by sulfide electrooxidation. This contention is further supported by voltammetric SERS measurements performed for varying NazS concentrations in 0.1 M NaC104 + 0.01 M NaOH. For sulfide concentrations below ca. 1 mM, only spectral features attributed to monomericadsorbed sulfur at about 300 cm-l were observed throughout the potential range -1.1 to +0.2 V and return. The band frequency is largely unaltered (510 cm-l) over this potentia1 range. At higher reactant (19)Tadayyoni, M.A.;Gao, P.; Weaver, M. J. J.ElectroanaL Chem. Interfacial Electrochem. 1986, 198, 125. (20)(a) Ward, A. T. J. Phys. Chem. 1968,72,744,4133.(b) Hattori, K.;Kawamura, H. J. Non-Cryst. Solids 1983,59/60,1063. (21)For example, (a) Steudel, R.; Relsch, M. J. Mol. Spectrosc. 1974, 51,189.(b)Meyer, B.; Stroyer-Hansen,T. J. Phys. Chem. 1972,76,3968. (c)Steudel, R.;Steidel, J.; Sandow, T.; Schuter, F. 2.Naturforsch. 1978, B33,1198.(d) Lenain, P.;Picquenard, E.; Lesne, J. L.; Corset, J. J.Mol. Struct. 1986,142,355. (22)For example, (a) Daly, F. P.; Brown, C. W. J.Phys. Chem. 1976, 80,480;1975,79,350.(b) Dubois, P.;Lelieur, J. P.; Lepontre, G. Znorg. Chem. 1988, 27,1883. (c) Janz, G.J.; Downey, Jr., J. R.; Roduner, E.; Waailczyk, G. J.; Coutts, J. W.; Eluard, A. Znorg. Chem. 1976,15, 1759 and preceding papers in same issue.
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672 Langmuir, Vol. 8, No. 2, 1992
concentrations, the SSfeatures become discernable and are prominent for concentrations above ca. 10 mM. A SER spectrum exhibiting similar spectral features as those in Figure 4 has been reported for sulfide in 1 M KOH a t gold held at ca. -0.1 V vs SCE.I4 The 150-cm-' band, however, was assigned incorrectly to a Au-S stretch; no potential-dependent spectral behavior was apparently examined in ref 14. Corresponding voltammetric SERS measurements obtained for sulfide (present as H2S) in 0.1 M NaC104 + 0.01 M HC104 yielded similar features as those obtained in alkaline solution,although the reactant concentration was limited to ca. 2 mM. A typical sequence of SER spectra obtained under these conditions is shown in Figure 5. As before, the initial appearance of the 450- and 200-cm-' features coincides with the onset of anodic current, at ca. 0 V, although the latter band (characteristic of Sa)is weak and the 150-cm-1feature is barely discernable. The likely survival of some monomeric adsorbed sulfur during the voltammetric potential excursion is indicated by the retention of a band at around 310 cm-l (Figure 5). Comparable potential-dependent SERS features are also obtained following electroreduction of sulfur dioxide at gold and platinum-coated gold in acidic media.23 These observations formed part of a larger SERS investigation of the oxidation and reduction of adsorbed SO2 in related gas-phase and electrochemical en~ironmenta.~3 At least at higher sulfide concentrations in alkaline solutions, then, the present results indicate that the sulfur adlayer undergoes substantial structural modification as the overall anodic oxidation proceeds. The reaction can be perceived as occurring on a chemisorbed sulfur layer, which acta to electrooxidize incoming sulfide ions by successive S-S bond formation followed by desorption. On this basis, the polymerization of the initially apparently monomeric chemisorbate is not entirely surprising. However, the close spectral similarities between 'liquid sulfur" and the adsorbate layer during electrooxidation conjures a picture of a polymerizedinterfacial region, through which efficient coupled electron-atom transfer can occur. It is not clear if the Sa species that apparently is a predominant component of this adlayer constitutes a reactive intermediate in itself. One can envisage a facile cleavage and re-formation of the Sa rings, with the open chains thus produced linking to incoming sulfide, or nearby polysulfide moieties. The absence of such sulfur polymerization at lower sulfide concentrations,below ca. 1mM, can be rationalized in terms of the lower electrooxidation rates with the consequent smaller probability of S-S bond formation occurring under these conditions. An earlier photoelectron spectroscopic study of gold electrodes emersed from alkaline sulfide solutions a t potentials where anodic oxidation proceeded gave no evidence for Sa f0rmati0n.l~
However,given that dilute (0.2 mM) sulfide solutions were employed in ref 13, the results are not inconsistent with those in the present work. Very recently, we have examined sulfide electrooxidation on Au(ll1) in aqueous acidic media by means of in situ scanning tunneling microscopy (STM)." At potentials below where the anodic oxidation commences,the atomicresolution STM images indicate the presence of a (d3Xd3)R30° adlayer, as expected for adsorbed monomeric sulfur. A t the onset of sulfide electrooxidation, arrays of rectangular close-packed structures are formed which consist predominantly of Sa rings.24 Further reaction yields multilayers of S, species, including s8 and larger rings, which exhibit structural disorder. It is appropriate to compare briefly the present findings with our earlier SERS study of iodide electrooxidation on gold.lg Similarly to sulfide, iodide yields an irreversibly adsorbed atomic adlayer, presumably featuring covalent bonding. Consistent with this notion is the observation for both sulfur and iodide adlayers of surfaceX stretching frequencies, vm-=, that remain almost invariant with electrode potential,E, in the absence of solution reactant.lg Adsorbates, such as chloride, that form less covalent (more ionic) surface bonds exhibit significant vm-x - E dependencies;" these arise at least partly from a first-order Stark effect on the oscillating dipole formed from the adsorbed ion and its metal image charge. As in the present sulfide case, the occurrence of extensive iodide electrooxidation brings about structural changes in the iodine adlayer as deduced from the SER spectra.lg These changesare milder for iodide than for sulfide electrooxidation,since the bond association in the former case is limited to the production of I2 and Is-. A qualitative similarity between sulfide and iodide electrooxidation, however, is that such adsorbed intermediates featuring bond association are prevalent in both cases. Overall, while the complexity of the sulfide electrooxidation process restricta the quantitative data interpretation, the present results provide a simple demonstration of the virtues of real-time SERS with CCD detection for providing additional mechanistic interpretation of voltammetric data for reactions involving chemisorbed intermediates. Other applications along these general lines are currently being pursued in our laboratory, involving in particular the catalytic electrooxidation of small organic molecules.
(23) Wilke, T.;Gao, X.; Takoudis, C. G.; Weaver, M. J. J.Catal. 1991, 130, 62.
(24) Gao, X.; Zhang, Y.; Weaver, M. J. Submitted for publication in Science.
Acknowledgment. This work is supported by the National Science Foundation. Registry No. S2-, 18496-25-8; Au, 7440-57-5; NazS,1313-822;NaClO4,7601-89-0;NaOH,1310-73-2;HClO4,7601-90-3;Sa, 10544-50-0;S,7704-34-9. ~
