SERS application to some electroorganic reactions

SERS application to some electroorganic reactions

307 Surface Science 158 (1985) 3077313 North-Holland, Amsterdam SERS APPLICATION Machiko TAKAHASHI, TO SOME Masato FUJITA Depurtment of Chemis...

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307

Surface Science 158 (1985) 3077313 North-Holland, Amsterdam

SERS

APPLICATION

Machiko

TAKAHASHI,

TO SOME Masato

FUJITA

Depurtment of Chemistv, Fucul~v of Science Kohoku - ku, Yokohama 223, Japan Received

4 September

1984; accepted

ELECTROORGANIC and Masatoki

and TechnoloK):

for publication

REACTIONS IT0

Kero Urucemty,

24 November

Hboshi

3 14 -I,

1984

SERS from four organic sulfides adsorbed on Ag electrodes are studied. 2-Mercaptopyridine (MPy) adsorbed on the electrode surface takes the enol form. The SER spectrum from 2.2’-dipyridyl disulfide (DPyDS) is exactly the same as that of MPy, which indicates the cleavage of the S-S bond of DPyDS. DPhDS also cleaves on a Ag electrode. as DPhDS and PhSH give identical SER spectra. All of these organic sulfides thus chemisorb dissociatively on the electrode surface. taking sticking-up configurations through S atoms.

1. Introduction

The development of the surface-enhanced Raman scattering (SERS) technique [l] has enabled us to observe the vibrational spectra of molecules adsorbed on metal surfaces. As SERS gives information on monolayer or submonolayer coverages in solutions, it provides a valuable in situ means for investigating the reactions in electrochemical systems. We have reported that the orientation of adsorbed species can be found from the spectral dependences on the applied potentials and solution concentrations [2]. On the basis of these results, here we report the SERS of 2-mercaptopyridine (MPy), 2,2’-dipyridyl disulfide (DPyDS), thiophenol (PhSH) and diphenyl disulfide (DPhDS) on Ag electrode surfaces. We are interested in these molecules because of their selective reactivity in electroorganic reactions. It is well known that PhSH dimerizes electrochemically to DPhDS [3], and DPyDS is easily cleaved and changes to MPy after reduction. We could observe SERS from these molecules, and here we discuss intermediates in these reactions on the Ag electrode surface.

2. Experimental The solvents were prepared with distilled water and spectra grade methanol of the composition of 1 : 1 and/or 1 : 3, and the pH of the solutions was 0039-6028/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

B.V.

adjusted by the addition of diluted HCI or sodium hydroxide aqueous solution. A polycrystalline silver electrode was mechanically polished with alumina, and then subjected to an oxidation-reduction cycle (ORC) from - 0.7 V to + 0.2 V and back to -0.7 V (versus SCE). Raman spectra were recorded by means of the apparatus described in previous papers [2].

3. The SERS of 2-mercaptopyridine Fig. 1 shows Raman spectra of 2-mercaptopyridine (MPy) in the region from 400 to 1600 cm- ‘_ The normal Raman spectra (NRS) in three pH conditions (figs. la-lc) are significantly different from one another, which reflects the keto-enol tautomerism. The six-membered aromatic ring system is readily identified by the presence of a strong ring-breathing Raman band near 1000 cm-’ which is greater in intensity than any other bands below 2000 cm- ‘. In basic solution, the most intense Raman band is located at 995 cm ‘. and therefore, MPy is truly aromatic, having the thiolate ion form analogous to

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Fig. 1. Raman spectra of 2.mercaptopyridine (MPy) in different pH solutions. (a. b. c) Normal Raman spectra of MPy solutions: (a) pH = 14, aqueous solution; (h) pH = 7. MeOH solution; (c) pH = 1. in mixed solvent (MeOH/H,O = 3: 1). (d, e, f) SER spectra of MPy adaorhed on Ag electrode in mixed solvent (MeOH/H,O = 3 : 1) of lo-’ M MPy concentration: (d) pH = 14; (e) pH = 7; (f) pH = 1. * indicates the signal from MeOH.

A4. Tukahashi et al. / SERS

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Fig. 2. Keto-enol tautomarism form in acidic solution.

m

application to electroorganic reactions

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W

of MPy: (I) thiolate

ion form; (II) keto form: (III), (IV) assumed

fig. 2 structure (I). In neutral solution, the spectrum shows enormous changes in relative intensities. The largest band exists at 1260 cm-‘, and the band at 995 cm-’ reduces in scattering intensity. Therefore, MPy in neutral solution takes mainly the keto form as given by fig. 2, structure (II). However, the relative intensities of the bands at 1260 and 995 cm-’ and also the absence of a S-H stretching band in the NRS show that in acidic solution MPy has intermediate structures (III) and (IV). The corresponding SER spectra from these solutions are given in figs. Id-lf. The applied potential was - 0.7 V versus SCE. In the spectra given in figs. Id and le, the largest bands are located at 1000 cm-’ and the frequencies and relative intensities of the Raman bands closely resemble those of the NRS in basic solution. In addition to these bands, the S-H stretching vibration is completely missing from these SER spectra, which is also analogous to the NRS in basic solution. Therefore, it is suggested that, in basic and neutral solutions, adsorbed MPy takes the thiolate ion form without an S-H bond. It is interesting to note that the thiolate ion form MPy dominates on the electrode in the neutral solution in spite of the fact that the thiolate form is a rather minor component of MPy in the neutral solution as mentioned before. So, there are two possible sources for the origin of adsorbed thiolate species; one is the enolic tautomer and the other is the keto species coexisting in the solution. We found that the SERS of MPy is detectable in solutions ranging in concentration from 10-j to 10mh M, but, in lo-’ M solution, only a small signal at 1000 cm-’ can be observed. In lo-’ M solution, SERS is not detectable at all. This occurs in both neutral and basic solutions. Accordingly, it is concluded that the keto tautomer existing in the solution phase isomerizes to the enol form on the Ag electrode surface following adsorption. The SER spectrum in acidic solution is given in fig. If. The largest band again is located at 1000 cm-‘. The general features of the SER spectrum resemble those in neutral and basic solutions. Therefore, even in acidic solution, adsorbed MPy is aromatic and exists on the surface as the thiolate tautomer as in the case of the other pH conditions. The conspicuous spectral change, however, is the appearance of a new band at 1607 cm-’ with relatively large intensity. This is exactly the same as the behaviour seen in the SERS of pyridine in acidic solution, which reflects the protonation on the N atom. Consequently, the adsorbed species is N-protonated MPy. After recording the

SER spectrum of fig. If in the solution at pH = 1, enough NaOH was added to make the solution basic. We observed that the SER spectrum changed to fig. le without ORC treatment. When the solution was acidified successively, the band at 1607 cm-’ reappeared in the SER spectrum. These SER spectral changes indicate the occurrence of reversible protonation of adsorbed MPy. and, therefore, adsorption through the lone pair of the N atom is not possible. Pyridinium ion is known to be adsorbed through its T electrons [4] and gives a SER spectrum quite similar to that of H+MPy in the region around 1600 cm ‘. However, the SERS of pyridinium ion is observed at -0.05 V versus SCE, which is quite different from the potential at which the SERS of neutral pyridine is observed, i.e. ~ 0.6 V. Furthermore, the SER spectrum of pyridinium ion is not affected by the reduction of the H+ concentration unless an ORC treatment was performed [5]. This behaviour found in the SER spectrum of the pyridinium ion is completely different from that of H ’ MPy. Therefore, a 71 orientation of MPy is not probable. Thus, the adsorption of N-protonated MPy occurs through neither the lone pair of the N atom nor the 7 electrons. The result that the properties of the SERS of N-protonated MPy. i.e. the dependence on applied voltage, the spectral intensities, and the concentration dependence, are almost the same as that of the adsorbed species in neutral or basic solutions, are indicative of adsorption through the S atom. The adsorbed species was found to be very stable and not to be affected by protonation on the N atom. The occurrence of this strong chemical bond between S and Ag atoms is the reason why the adsorbed MPy persists in the thiolate form. Further evidence is discussed later.

4. S-S

cleavage of DPyDS

and DPhDS

and their adsorbed intermediates

Fig. 3 shows the Raman spectra of DPyDS in bulk powder and on a Ag electrode. The Raman spectra of MPy are also given for comparison. We notice the surprising coincidence of the DPyDS and MPy SER spectra. Moreover, the S-S stretching vibrational Raman line located at 550 cm ’ in the spectrum of bulk DPyDS is completely absent from the SER spectrum. These results indicate that DPyDS cleaves on the Ag electrode surface and the fragment is the PyS radical. A similar phenomenon was observed between DPhDS and PhSH in the Ag electrode system. Fig. 4 shows the Raman spectra of these compounds in the bulk phase and on Ag electrode surfaces. The two SER spectra completely coincide with each other, and the SS stretching vibration located at 542 cm~ ’ in the NRS of DPhDS is absent from these SER spectra. Sandroff and Herschbach (61 studied the SERS of these molecules on silver island films and observed the facile cleavage of the SS bond. Their result is in fair agreement with ours in the electrode system. In addition, we could observe the same

M. Tukahashi et al. / SERS application to electroorganic reactIons

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Raman Shift km-‘) Fig. 3. Raman spectra of dipyridyl disulfide (DPyDS) and 2-mercaptopyridine. (Ia) Powder sample of DPyDS. (Ib) MPy solution at Ph = 14. (Ha) DPyDS adsorbed on Ag electrode from the solution (MeOH:H,O = 3:l) of lo-’ M concentration. (Ilb) MPy adsorbed on Ag electrode from pH = 14 solution. * indicates the signal from MeOH.

SERS on a Cu electrode using a 635.0 nm dye laser as the excitation source. Consequently, S-S bonds in DPyDS and DPhDS are easily broken in the process of adsorption on Ag and Cu metal surfaces and the resulting species are assumed to be PyS and PhS radicals, respectively. Sandroff and Herschbach suggested the flat orientation of these species, PhSH and DPhDS, from the frequency shifts of the SERS bands. As we reported previously [2], the vibrations which are perpendicular to the metal surface tend to have large Raman intensities in the SER spectrum. On that basis, if the PhS radical is adsorbed in a flat orientation, the out-of-plane vibrations should have detectable intensities in the SER spectrum. We could not find the signal assignable to out-of-plane vibrations, and hence, the possibility of a flat configuration may be excluded. The vibrational assignments of DPhDS have been given by Green [7] by reference to monosubstituted benzene, and the assumed vibrational modes for the moiety DPhDS are illustrated in fig. 5 [8]. If PhS adsorbs on the Ag surface through the S atom, we can adopt these vibrational modes for this species. The very small

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Fig. 4. Raman spectra of diphenyl disulfide (DPhDS) and thiophenol (PhSH). (I) Normal Raman spectra of powder DPhDS (a) and PhSH in MeOH solution (b). (II) SER spectra of DPhDS (a) and PhSH (b) on Ag electrode in MeOH/H,O (1 : 1) solution of 10-j M concentration. * indicates the signal from MeOH.

frequency changes support this assignment. Looking at figs. 4 and 5, the distinguishable bands at 421, 692, 1000, 1027, 1074 and 1576 cm-’ in SER spectrum are known to have a large vibrational component parallel to Ph-S bond. On the other hand, the Raman line at 611 cm- ’ in the bulk phase which has a small parallel vibrational component is completely missing in the SER spectrum. Therefore, the Ph-S bond is thought to be perpendicular to the metal surface and the PhS radical to be adsorbed through the S atom.

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Fig. 5. Assumed

vibrational

996

modes of PhS group. These modes are from the report by Whiffen

[81.

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applicatmn to elecrroorganic reactions

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Next we discuss the PyS species. By comparing the SER spectra of PhS and PyS, the overall features are quite similar to each other, and therefore, these two species seem to take the same adsorbed configurations, The only difference in the spectrum is the clear existence of the 635 cm-’ band in the PyS SERS. This Raman line corresponds to that at 611 cm-’ in the phenyl system. It is reasonable to consider that the hetero nitrogen atom in the ring might distort the vibrational mode and the mode at 635 cm-’ gains an appreciable vibrational component parallel to the ring-S stretching vibration. Therefore, the 635 cm-’ band displays considerable SERS intensity. No out-of-plane vibrations were detected in the PyS, as in the PhS spectra. Furthermore, if the six-membered ring is lying flat and the r electrons of pyridine interact with the metal directly, it is impossible to change the SER spectrum reversibly by varying the pH of the solution as previously described. Therefore, we conclude that PhS and PyS radicals are the adsorbed species and that they are adsorbed on the Ag surface through the S atom in sticking-up orientations.

5. Conclusions MPy yields the PyS radical on the Ag electrode and chemisorbs through the S atom on the surface forming a thiolate framework in basic, neutral and acidic solutions. DPyDH and DPhDS are easily cleaved at the Ag surface and turn out to be PyS and PhS radicals, respectively. Referring to the vibrational data, we indicate these species orient in the sticking-up conformation on the surface.

References [l] R.K. Chang and T.E. Furtak, Eds., Surface Enhanced Raman Scattering (Plenum, New York, 1982). [2] M. Takahashi and M. Ito, Chem. Phys. Letters 103 (1984) 512; M. Takahashi, M. Fujita and M. Ito, Chem. Phys. Letters 109 (1984) 122. [3] F. Magnv, G. Bontempelli and G. Pilloni, J. Electroanal. Chem. 30 (1971) 375. [4] M. Fleischmann and I.R. Hill, in: Surface Enhanced Raman Scattering, Eds. R.K. Chang and T.E. Furtak (Plenum, New York,1982) p. 275. [5] A. Regis and J. Corset, Chem. Phys. Letters 70 (1980) 305. [6] C.J. Sandroff and D.R. Herschbach, J. Phys. Chem. 86 (1982) 3277. [7] J.H.S. Green, Spectrochim. Acta A24 (1968) 1627. [8] D.H. Whiffen, J. Chem. Sot. (1956) 1350.