Journal of Molecular Structure 516 (2000) 225–230 www.elsevier.nl/locate/molstruc
Surface-induced decomposition of 4,5-bis(benzoylthio)1,3-dithiol-2-thione on silver surface studied by surface-enhanced Raman spectroscopy V.T. Joy 1, T.K.K. Srinivasan* Regional Sophisticated Instrumentation Centre, Department of Chemistry, Indian Institute of Technology, Madras 600 036, India Received 14 December 1998; accepted 16 April 1999
Abstract Adsorption of 4,5-bis(benzoylthio)1,3-dithiol-2-thione (Bz2dmit) on silver surface has been studied by SERS technique. SERS spectrum of bz2dmit adsorbed on silver colloid obtained by using a dispersive Raman spectrometer with 514.5 nm excitation is found to contain only vibrational bands corresponding to the dmit group indicating that Bz2dmit undergoes decomposition on the silver surface to dmit 22 ion. FT-SERS spectra of Bz2dmit obtained with chemically deposited silver film substrates exposed to methanolic adsorbate solutions are found to vary with exposure times. Spectra obtained for shorter exposure times contain vibrational bands associated with dmit as well as benzoyl groups: however intensities of bands corresponding to benzoyl groups are found to diminish with increasing exposure times. Decomposition of Bz2dmit is found to be solely due to the surface induced reaction of adsorbed Bz2dmit molecules with solvent molecules. q 2000 Elsevier Science B.V. All rights reserved. Keywords: SERS; Raman; Adsorption; Silver surface; Decomposition
Determination of the details of various chemical processes occurring at metal surfaces continues to be a great challenge. SERS has been established as a useful tool in surface science for the study of metal– adsorbate interactions. Information on the structure, orientation, reactivity, etc. of the adsorbed species can be obtained from SERS spectra [1–3]. Although the exact mechanism of SERS is still not very clear, it has been generally accepted that much of the enhancement arises from substantial amplification of the * Corresponding author. Tel.: 1 91-44-4420231; fax: 1 91-442350509. E-mail address:
[email protected] (T.K.K. Srinivasan). 1 Present address: Department of Chemistry, Christ College, Irinjalakuda, Kerala, India.
electromagnetic fields resulting from the resonance interaction of the incident light with the surface plasmons of the metal [4–6]. The presence of strong electromagnetic fields at surfaces can sometimes lead to enhanced photochemical reactions for the adsorbed species [7,8]. In this work adsorption of 4,5-bis(benzoylthio)1,3dithiol-2-thione (bz2dmit) on a silver surface has been studied by SERS technique. Both a conventional dispersive type Raman spectrometer with visible excitation (514.5 nm) as well as an FT-Raman spectrometer with near-infrared excitation (1064 nm) are employed. This molecule contains several potential sites such as sulfur or oxygen atoms, or the p-system of the dmit moiety or that of the benzene rings, which can compete for interaction with the silver surface.
0022-2860/00/$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S0022-286 0(99)00199-4
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Fig. 1. (a) Normal FT-Raman spectrum of Bz2dmit (powder) with 1064 nm excitation. (b) SERS spectrum of Bz2dmit (10 25 M) adsorbed on silver colloid with 514.5 nm excitation. (c) FT-SERS spectrum of Bz2dmit adsorbed on chemically deposited silver film from 10 25 M methanolic solution with 1064 nm excitation.
Recently we carried out a SERS study [9], on the adsorption of disodium 1,3-dithiol-2-thione-4,5dithiolate (Na2dmit) on silver surfaces. The presence of an intense electronic absorption around 500nm in this compound, arising from a p–p p electronic absorption, allowed the study of surface Raman spectra at extreme dilutions ( , 10 8 M), with an excitation frequency of 514.5 nm. The 1,3-dithiol-2thione-4,5-dithiolate ion was found to adsorb on the surface via its p-system assuming a parallel orientation with respect to the silver surface. The lowest electronic absorption band of Bz2dmit is at 430 nm and hence Bz2dmit is not expected to show appreciable resonance Raman enhancement when excited with 514.5 nm radiation. Thus the overall enhancement mechanisms for Bz2dmit and Na2dmit on the silver surface may be completely different when excited with 514.5 nm radiation, even when adsorption of the Bz2dmit molecule is through the dmit moiety. The frequency of near-infrared excitation
(1064 nm) used for obtaining FT-SERS spectra is far removed from the lowest electronic excitation frequency of Bz2dmit and hence its Raman spectrum on the surface or in the bulk phase will not have any contribution from molecular resonance Raman effects. All these factors make the SERS investigation of Bz2dmit especially interesting and worth investigating. Bz2dmit was prepared as reported by Steimecke [10]. Silver colloids were prepared according to the chemical reduction method adopted by Creighton et al. [11]. Such sols were aged for one week to allow for the decomposition of NaBH4. For SERS, samples of , 10 25 M concentration were prepared by mixing five volumes of colloid with one volume of methanolic solutions of adsorbates. On adding Bz2dmit, the color of the silver colloid was found to change from yellow to brown within 3 to 4 h and such colloidaladsorbate solutions were found to be stable for many days. SERS spectra were recorded using a Dilor Z24
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Raman spectrometer and the samples were circulated through a capillary tube at a rate of 100 ml per minute to avoid photodecomposition. The 514.5 nm exciting line of an Ar ion laser with a power of 80 mW at the sample was used for obtaining the Raman spectrum. Spectral resolution of 1 cm 21 was used. Chemically deposited silver films were prepared according to the procedure reported in the literature [12]. For FT-SERS spectra, silver deposited glass slides were dipped in methanolic solutions of adsorbates for about 30 min. The films were then taken out, washed with methanol to remove physisorbed molecules and then dried in air. The bulk phase Raman spectrum of a solid sample of Bz2dmit could not be obtained with 514.5 nm excitation due to the decomposition of the sample during exposure to this radiation. The Raman spectrum of an aqueous solution of Bz2dmit could not be obtained due to its very poor solubility. Fig. 1(a) shows the normal FT-Raman spectrum of solid Bz2dmit obtained with near-infrared excitation (1064 nm). The strong band at 1461 cm 21 in the FT-Raman spectrum of Bz2dmit has been assigned to CyC stretch of the dmit moiety by Dyer et al. [13] and this band is quite characteristic of 1,3-dithiol-2-thione and its related compounds. Silver colloids were found to be more ideal substrates for obtaining SERS spectrum using dispersive Raman spectrometer with visible excitation (514.5 nm), since the decomposition of the samples could be prevented by circulating the colloidal samples using a peristaltic pump. Good SERS spectra could be obtained only after there was a distinct color change of the colloidal solution from yellow to brown. Fig. 1(b) shows the SERS spectrum of Bz2dmit adsorbed on colloidal silver particles using 514.5 nm excitation. Since SERS spectra obtained with chemically deposited silver films and 514.5 nm excitation are found to very weak and noisy, they are not presented. It is seen that, in the SERS spectrum of Bz2dmit adsorbed on Ag colloid excited with visible radiation (514.5 nm) peaks corresponding to the vibrations of dmit moiety only are present and those associated with benzoyl group are completely absent. For example, the bands at 1670 and 1688 cm 21 due to CyO stretches and the band at 1593 cm 21 due to benzene ring vibration in the normal FT-Raman spectrum [10,13] are completely missing in the SERS spectra. The band due to CyC stretch of the dmit
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group is substantially downshifted to around 1406 cm 21 along with considerable broadening. The SERS spectrum of Bz2dmit is found to be remarkably similar to that of Na2dmit [9]. The lowest lying electronic transition of Bz2dmit (l max 430 nm) is a p–p p transition localized within the dmit framework of the molecule. Thus it is possible that the peaks due to the vibrations associated with dmit part of the molecule are enhanced by both surface and molecular resonance Raman enhancement mechanisms (SERRS) whereas peaks arising from the vibrations of benzoyl group are enhanced only by surface enhancement (SERS). Another possibility is that Bz2dmit has undergone decomposition on the silver surface producing dmit 22 ions and dmit 22 ions so produced get strongly adsorbed to the surface giving a surface Raman spectrum identical to that of Na2dmit. In this regard, it is pertinent to note that many organic sulfides have been reported to undergo facile surface-induced reactions on the silver surface. Sandroff and Herschbach [14] studied the adsorption of diphenyl sulfide and dibenzyl sulfide on silver surfaces and concluded that the cleavage was solely due to adsorption. Joo et al. [15] studied the adsorption of dialkyl sulfides and suggested that the cleavage was caused by the radiation used for exciting the Raman spectrum, since changes due to such cleavage were not observed in the SERS spectra when Ag colloid containing the adsorbate was circulated through a capillary tube while recording the Raman spectrum. In a SERS study of aromatic sulfides adsorbed on silver surfaces, by Joo et al. [16], it was suggested that the decomposition of these molecules was due to a surface-induced photoreaction. A more recent SERS study of 4(methylthio) benzonitrile (4MTB) on a silver surface by Yi et al. [17] has shown that, in the adsorbed state, 4MTB undergoes a surface-induced photoreaction to pcyanophenyl mercaptide. In the present case, since the SERS spectra of Bz2dmit were recorded by circulating the samples, it appears that the decomposition of Bz2dmit on silver colloidal particles may be solely caused by the surface. To ascertain which of the above factors is responsible for the anomalous features in the SERS spectrum of Bz2dmit obtained with visible radiation, we also studied the FT-SERS spectra of Bz2dmit. The advantage of using FT-Raman spectroscopy for the present system is that the wavelength of the exciting
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Fig. 2. FT-SERS spectra of Bz2dmit obtained from Ag films dipped in 10 25 M methanolic solution for (a) 5 (b) 10 (c) 30 (d) 60 min (1064 nm excitation).
radiation (1064 nm) is far removed from the lowest electronic transition of Bz2dmit. Thus its Raman scattering on the silver surface can be enhanced only by surface factors, and not by molecular resonance Raman enhancement. Further, since the energy of the exciting radiation is much less, it not likely to cause any photochemical decomposition of the sample. Again the FT-SERS spectra can be obtained within a relatively shorter time compared with the conventional dispersive type Raman spectrometer. The FT-SERS spectra of Bz2dmit are found to be
very weak when adsorbed on silver sols. However, intense FT-SERS signals are observed for samples on chemically deposited silver films, a behavior which is similar to that observed with Na2dmit [9]. Fig. 1(c) shows the FT-SERS spectrum of Bz2dmit obtained using a silver film dipped in methanolic solution (10 5 M) of adsorbate for about 30 min. It is seen that bands due to the vibrations of benzoyl groups are very weak or absent in the FT-SERS spectrum also. This clearly shows that Bz2dmit undergoes decomposition on the surface producing dmit 22 ions and these
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229 22
Fig. 3. Structure of Bz2dmit.
ions get adsorbed to the surface in the same manner as that observed for Na2dmit on silver surfaces. Thus the SERS spectra of Bz2dmit arise from the adsorbed dmit 22 ions. However, the possibility that the decomposition of Bz2dmit may be induced by the exciting radiation itself cannot be ruled out completely, since the electromagnetic radiations are greatly amplified at the surfaces. We recorded the FT-SERS spectrum of Bz2dmit, using four different silver films, which had been exposed to a methanolic solution of Bz2dmit for 1, 5, 30, and 60 min (Fig. 2(a)–(d)). It is seen that in Fig. 2(a) the peaks due to the vibrations associated with benzoyl groups are also present, although their intensities are very weak. Intensities of these peaks decrease with increase in exposure times. To understand clearly, whether the decomposition of Bz2dmit is solely caused by the surface or by the exciting radiation, the silver film used for obtaining Fig. 2(a) was exposed to high power radiation (1064 m, 350 mW) for about 30 min and then FT-SERS spectrum was recorded again. The spectrum so obtained is found to be very similar to the previous spectrum (Fig. 2(a)). This clearly shows that the cleavage of Bz2dmit on the silver surface is not caused by the radiation used for exciting the Raman spectrum and that the films are very stable in air. Thus all the spectral observations suggest that the Bz2dmit molecule
decomposes on the silver film surface to dmit ion and this cleavage occurs solely by the interaction of adsorbed Bz2dmit molecule with solvent molecules (methanol). The strong adsorption via the p-system of dmit part of the Bz2dmit molecule weakens the SC(O) bonds, the carbonyl carbon thereby acquiring more positive charge. Such carbon atoms are easily attacked by solvent (methanol) molecules resulting in the cleavage of SC(O) bonds, and thereby dmit 22 ion and methyl benzoate are produced. The dmit 22 ions thus produced strongly adsorb to the surface through its p-electrons as observed in the case of Na2dmit. The complete absence of the bands corresponding to the benzoyl group vibrations in the SERS of Bz2dmit on silver colloids (Fig. 1(b)) can also thus be attributed to decomposition of this molecule on the colloidal surface. It may be noted, however, that the sample solutions for SERS with silver colloids as substrates were prepared by mixing methanolic solutions of Bz2dmit with aqueous silver colloids (Fig. 3). Hence it is not possible in this case to conclude whether the cleavage of Bz2dmit on silver colloidal particles occurs solely by interaction with methanol or with water or with both. Thus the decomposition of Bz2dmit on both silver films and colloids is due to the surface induced reaction with solvent molecules and is not induced by the exciting radiation.
Acknowledgements VTJ is grateful to CSIR for providing research fellowship.
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