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A pH-dependent SERS study of thiophene-2-carboxylic acid adsorbed on Ag-sols
Chemical Physics Letters 374 (2003) 341–347 www.elsevier.com/locate/cplett
A pH-dependent SERS study of thiophene-2-carboxylic acid adsorbed on Ag-sols Uttam K Sarkar
*
Department of Physics, Malda College, Malda 732101, WB, India Received 22 February 2003; in final form 30 April 2003
Abstract SERS is observed for thiophene-2-carboxylic acid (TCA) adsorbed on Ag-sols. The enhancement is maximum when TCA concentration, in Ag-sol, is 5 103 M. The SER spectra of TCA are different at different pH values, which suggest that the adsorption geometry is pH dependent. This is further supported by the absorption spectra at different pH values. It is inferred that the chemisorption takes place through both the S-atom and COOH group for pH 6 7 and through the p-electronic system for pH ¼ 10.5. Quick desulphurization at pH ¼ 2 and stable perpendicular and parallel surface bonding for pH ¼ 7 and 10.5, respectively, is revealed. Ó 2003 Published by Elsevier Science B.V.
1. Introduction Surface enhanced Raman Spectroscopy (SERS) is a useful technique for the study of chemical as well as physical transformation of molecules adsorbed on metal surfaces [1–4]. Two mechanisms, mainly, have been shown to contribute to SERS. One is long-range classical electromagnetic field effect and the other is short-range charge transfer effect [3]. The classical field effect is due to surface plasmon resonance by the incident electromagnetic field and is operative at larger distances. For isolated metal sphere it has been shown that the plasmon resonance occurs if the frequency of excitation matches with that at which the extinction *
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by the metal particles is maximum [5]. The shortrange effect, on the other hand, is because of exchange of electrons between the molecule and the metal. This latter effect is dependent on the concentration of the adsorbate and is maximum when monomolecular layer is formed on the surface [6]. According to the image dipole theory, Raman scattering from an adsorbed molecule arises from both a dipole induced in the molecules and from the image dipole of the molecules within the metal surface. Vibrational modes which have dipolar components perpendicular to the surface will show enhancement and the magnitude of the enhancement will be proportional to the projection of the dipole moment on the perpendicular to the surface. Thus, SERS can provide information about orientation of the molecule on the metal surface [7,8].
0009-2614/03/$ - see front matter Ó 2003 Published by Elsevier Science B.V. doi:10.1016/S0009-2614(03)00728-0
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In this Letter, we present the concentration dependence and pH dependence of SERS of thiophene-2-carboxylic acid (TCA) adsorbed on silver surface. Ag sol has been used for this purpose because of a number of advantages reported elsewhere [9]. Polythiophene possesses high electrical conductive properties and is, therefore, of great interest in different fields in recent times. For better understanding of the mechanisms involved in different electronic and optical properties of polythiophene the oligothiophenes and substituted thiophenes need more attention. The charge transfer effect and the orientation of different oligothiophenes in the adsorbed state have been reported in our earlier works [6,7]. SERS of TCA shows significant change of intensity patterns in the adsorbed state at different pH values. In addition, the short-range chemical effect is found to be predominant at an optimum concentration of TCA. Possible charge transfer interaction between the molecule and the metal surface has been discussed.
3. Results and discussion Raman spectra of TCA, in the solid phase and solution (1 M), are shown in Figs. 1a and b, respectively. Surface enhanced Raman (SER) spectra of TCA are shown in Fig. 1c. Assignments and apparent enhancement factor (AEF) of different Raman modes are shown in Table 1. The AEF has been calculated according to the relation AEF ¼ r1 C2 /r2 C1 where suffices 1 and 2 refer to SERS and RS, respectively; r and C are the intensity of a Raman mode and concentration of the molecule, respectively. The real enhancement factor will be a few orders of magnitudes higher owing to the fact that all the molecules may not get adsorbed on the
2. Experimental Silver sols were prepared by reducing aqueous solution of AgNO3 by NaBH4 according to the process described elsewhere [10]. The yellowish sol, thus prepared, showed a single extinction maximum at 392 nm, and was aged for several weeks before use for the present study. The TCA was purchased from Aldrich Chemical and the solutions were prepared with distilled and deionized water from Milli-Q-plus system of M/S Millipore Corporation, USA. Dilute HCl acid and NaOH solutions were used to adjust the sol pH and pH paper was used to measure the values. Raman spectra were recorded by a Spex Double Monochromator (model 1403) fitted with water-cooled photomultiplier tube (model R 928) from Hamamatsu Photonics (Japan). The samples were excited by the 514.5 nm line of the Arþ laser from Spectra Physics (model 2025-5). Spex datamate 1B was used for monochromator control, data acquisition and analysis. The electronic absorption spectra were recorded in a Shimadzu Spectrophotometer (model UV–Vis 2010 PC).
Fig. 1. Raman and surface enhanced Raman spectra of TCA: (a) solid phase, (b) aqueous solution (1M), (c) adsorbed on Agsols (5 103 M).
Table 1 Raman bands along with AEF in different conditions and their tentative assignments Aqueous soln (1 M) (cm1 )
pH ¼ 2 SERS (cm1 )
– –
– – –
428(vw) 455(w) 519(ms) 635(w) 721(w) 741(s) 754(s) 860(w) 926(w) 1050(w) 1078(s) 1114(w) – 1280(w) – 1353(s) – 1412(vs) – 1527(w)
230 312 – –
235 327 – –
480 – 650 – 741 758 859 932 988 1078 1113 – – – 1355 – 1417 1461 1532
586 – 710 – 761 – 924 988 –
1637(w)
1658
583 679 713 – 770 – 913 – – 1126 – 1236 – 1360 1384 – – 1512 1587 1620
1188 – 1305 1345 – – – 1512 1587 1620
AEF
1.11 103 1.73 103
1.51 103
4.66 103
pH ¼ 4 SERS (cm1 )
AEF
pH ¼ 7 SERS (cm1 )
AEF
236 320 – –
5.96 104
1.9 104 2.5 104
3.66 104
2.62 104
2.52 104
578 675 712 – 753 – 914 – 1070 1126 – 1234 – 1350 1390 1428 – – 1587 1620
1.42 103
pH ¼ 10.5 SERS (cm1 )
AEF
– – – – – – – – –
Ag–S stretching Substituent sensitive C–COOH in plane bending In plane ring bending C@O in plane bending
4.38 103 5.52 103 1.28 103 8.28 105
Tentative assignments (Refs. [11–14])
OH in plane bending 860 930 986 1061 1126 1166 1247 –
1.05 104 1.98 104 1.74 104 1.01 104
Ring stretching + ring bending
CH in plane bending C–COOH stretching
6.44 105
C3 –C4 stretching – – – 1537 1588 1620
C@C symmetric stretching 3.2 104
U.K. Sarkar / Chemical Physics Letters 374 (2003) 341–347
Solid phase (cm1 )
C@C asymmetric stretching
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surface and that the intensity of the Raman excitation radiation is reduced due to absorption and scattering by Ag-particles. 3.1. Concentration dependence Fig. 2 shows the concentration dependence of the apparent enhancement of the C3 –C4 stretching mode, which appears at 1355 cm1 in solution and at 1350 cm1 in the adsorbed state. During the concentration dependence, the pH was maintained at 7. Maximum enhancement is observed at a concentration of 5 103 M which indicates that monomolecular layer of TCA is formed on the silver surface at this concentration and the Ôfirst layer effectÕ [6] contributes to this large enhancement. At higher concentrations bulk properties manifest whereas at lower concentrations the re-
Fig. 2. Concentration dependence of SERS of 1350 cm1 band of TCA (pH ¼ 7).
duction of enhancement is due to sub monolayer coverage. In the SER spectra of TCA, a band appears at 320 cm1 , which is related to the COOH substituent group [11–13]. It suggests that the adsorption is through the COOH group. Ag–S stretching has been reported to lie within 150–250 cm1 [14] and in SERS the involvement of heteroatom in the adsorption process is well established. In the present study the 236 cm1 band in the SER spectra of TCA may be tentatively assigned to the Ag–S stretching mode. Participation of both the S heteroatom and the substituent COOH group is thus inferred for the adsorption of TCA on Ag-surfaces. 3.2. pH dependence SER spectra of TCA have been taken for different pH values (Fig. 3) with TCA concentration of 5 103 M that is optimum for enhancement. Significant change in intensities of different Raman bands is observed with variation of pH. Both bare and chlorinated silver surfaces are present in the sol when dilute HCl is added. Thus, two types of adsorption sites are available for the TCA molecules and their relative concentration depend on the pH value. As the pH value is decreased the chlorinated surfaces are more in number than the bare surface. The spectra in Fig. 3 are, thus, combination of contributions by TCA molecules adsorbed on these two types of sites. At pH ¼ 7 the contribution is mainly from TCA molecules adsorbed on the bare silver surface whereas at lower pH main contribution is due to the chlorinated surface. At pH > 7 the presence of excess OH plays important role and there is remarkable change in the intensity pattern in SERS of TCA at pH ¼ 10.5 compared to that at pH ¼ 7. This hypothesis of presence of different adsorption sites resulting in the variation of intensity pattern [15] will be adopted for the interpretation of SERS at different pH values. Overall intensity reduction is observed with lowering of pH values from 7 to 2. The t(Ag–S) band at 230 cm1 is very strong at pH ¼ 2. However, its intensity decreases significantly with increase in pH and it appears at 236 cm1 at pH ¼ 7. On the other hand the substituent sensitive band at
U.K. Sarkar / Chemical Physics Letters 374 (2003) 341–347
Fig. 3. SER spectra of TCA at different pH values: (a) pH ¼ 2, (b) pH ¼ 4, (c) pH ¼ 7 and (d) pH ¼ 10.5 (concentration of TCA in Ag-sol ¼ 5 103 M).
320 cm1 is strong at neutral pH but it appears at 312 cm1 at pH ¼ 2 with reduction in intensity. The in plane ring bending mode at 578 cm1 does not undergo significant change in intensity when pH is varied from 7 to 2 but shifts to higher frequency and appears at 586 cm1 at pH ¼ 2. All these bands, along with the C@O in plane bending and OH in plane bending at 650 and 741 cm1 , respectively, disappear in the SER spectra at pH ¼ 10.5. The striking feature is that the intensity pattern is different when the pH is 10.5. The ringbreathing mode at 986 cm1 is enhanced most at this pH. There is reversal of intensity between 1247
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and 1350 cm1 band with change of pH from 7 to 10.5. The asymmetric C@C stretching mode at 1537 cm1 is also largely enhanced at pH ¼ 10.5. Different surface bonding modes of thiophene have been proposed on different metal surfaces. On molybdenum surface, the parallel and perpendicular adsorption geometries are suggested for low and saturation surface coverages, respectively [16– 18]. Both parallel and inclined adsorption of thiophene is inferred on other metal surfaces [19–24]. In the present investigation of SERS of TCA in Agsol, the intensity pattern remains unaltered in a wide range of concentration. It suggests that the adsorption geometry of TCA is independent of concentration. However, the orientation of the molecules is different for different pH values. For neutral pH, a perpendicular adsorption (i.e., the plane of the ring is perpendicular to the surface) through both S and COOH group seems operative as evidenced from the appearance of t(Ag–S) at 236 cm1 and the substituent sensitive band at 320 cm1 . As the pH is lowered, there is a significant intensity reversal of 236 and 320 cm1 bands along with an overall reduction of intensity. There are evidences of S-elimination, by metal surfaces, from thiophene and other S-containing molecules of the crude oil in the industrially important hydrodesulphurization [16–18,25–27]. In TCA elimination of S-atom may be inferred at pH ¼ 2. The SER spectra, in that case, are contributed by the molecules at a distance from the surface plus the decomposed molecules from which the S-atom has been eliminated. The SER spectra at pH ¼ 10.5 reveal that the adsorption might not have been through the S-atom as there is no t(Ag–S) band. This may be attributed to the fact that the hydrogen bonding in the COOH group of TCA [28] is stronger at higher pH value and that is why the adsorption through S-atom or the COOH group is inhibited at pH ¼ 10.5. The large enhancement of the ring-breathing mode at 986 cm1 suggests parallel adsorption geometry (i.e., the plane of the ring is parallel to the surface) at this pH value. 3.3. Absorption spectroscopy Fig. 4 shows the absorption spectra of TCA in the adsorbed state at different pH. Two bands are
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Fig. 4. Absorption spectra of TCA adsorbed on Ag-sols at different pH values: (a) pH ¼ 2, (b) pH ¼ 4, (c) pH ¼ 7 and (d) pH ¼ 10.5.
observed. One is representative of absorption by Ag-sol which appears at 392 nm for pure sol, at 393 nm at pH ¼ 2 and at 403 nm at other pH values. The intensity of this absorption band decreases with increase in pH. The other absorption band, representative of the TCA molecule, appears at 245 nm which is almost absent at pH ¼ 2. These observations substantiate the inference that the TCA molecules are decomposed by the process of S-elimination by silver at pH ¼ 2. At pH ¼ 7 intensity of the 245 nm band is maximum and a shoulder appears near 470 nm and it does not show any shift for several hours which indicates a stable chemisorption through charge transfer interaction [29] at pH ¼ 7. This is also supported by the appearance of 236 and 320 cm1 bands in the SER spectra (Fig. 3c). Fig. 4d shows the absorption spectrum of TCA in the adsorbed state at pH ¼ 10.5. At this pH value the absorption at 245 nm is reduced but is more than that at pH ¼ 2. This means the TCA molecule retains its identity at this pH value though the sol absorption is reduced significantly. 3.4. Time dependence The time evolution of SER spectra of TCA at pH ¼ 7 is shown in Fig. 5 which reveals that the
Fig. 5. Time evolution of SER spectra of TCA on Ag-sol: (a) 0 min, (b) 30 min, (c) 60 min, (d) 120 min and (e) 180 min.
surface bonding is stable at this pH. Similar spectra have been recorded at other pH values also. It is observed, for all pH values, that the spectra do not undergo any change in intensity pattern with time. However, some time (about 10 min) is elapsed between the moment at which the molecules are added to the Ag-sol and that at which the monochromator starts recording of Raman signal. In the present study t ¼ 0 instant is the later moment which means that the surface modification of the molecules, at pH ¼ 2, occurred within 10 min. This is suggestive of quick desulphurization of TCA at pH ¼ 2, stable perpendic-
U.K. Sarkar / Chemical Physics Letters 374 (2003) 341–347
ular surface adsorption at pH ¼ 7 and stable parallel surface adsorption at pH ¼ 10.5.
4. Conclusion Monomolecular layer of TCA is formed on the silver surface at a concentration of 5 103 M. The Ôfirst layer effectÕ contributes to the large enhancement at this concentration. In the process of surface adsorption, both the S-atom and the COOH group participate, with perpendicular adsorption geometry, at pH 6 7. On the other hand, at pH ¼ 10.5 the TCA molecule is chemisorbed, with parallel adsorption geometry, through its p-electronic system. A quick S-elimination from TCA takes place at pH ¼ 2 though the molecule is stable, in the adsorbed state, at higher pH values. References [1] A. Otto, in: M. Cardona, G. Guntherdot (Eds.), Light Scattering in Solids, vol. 4, Springer, Berlin, 1984. [2] D.P. DiLella, M. Moskovit, J. Phys. Chem. 85 (1981) 2042. [3] J.A. Creighton, in: R.J.H. Clark, R.E. Hester (Eds.), Spectroscopy of Surfaces, Wiley, New York, 1988. [4] Y. Furukawa, M. Akimoto, I. Harada, Synth. Met. 18 (1987) 151. [5] D.S. Wang, M. Kerker, Phys. Rev. B 24 (1982) 1777. [6] U.K. Sarkar, S. Chakrabarti, A.J. Pal, T.N. Misra, Chem. Phys. Lett. 190 (1992) 59. [7] U.K. Sarkar, S. Chakrabarti, A.J. Pal, T.N. Misra, Chem. Phys. Lett. 200 (1992) 55.
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[8] C.S. Allen, R.P. van Duyne, Chem. Phys. Lett. 63 (1979) 455. [9] U.K. Sarkar, T.N. Misra, Ind. J. Phys. 76B (2002) 567. [10] U.K. Sarkar, S. Chakrabarti, T.N. Misra, J. Raman Spectrosc. 24 (1993) 97. [11] N.B. Cothulp, L.H. Daly, S.E. Wiberley, Introduction to Infrared and Raman Spectroscopy, third ed., Academic Press, Boston, 1990. [12] R.M. Silverstein, C.C. Basseler, Spectrophotometric Identification of Organic Compounds, second ed., Wiley, New York, 1964. [13] S. Sanchez-Cortes, J.V. Garcia-Ramos, J. Raman Spectrosc. 21 (1990) 679. [14] G.A. Bowmaker, L.C. Tan, Aust. J. Chem. 32 (1979) 1443. [15] M. Takahasi, H. Idetsuki, M. Ito, Surf. Sci. 232 (1990) 346. [16] D.G. Kelly, M. Salmeron, G.A. Somorjai, Surf. Sci. 175 (1986) 465. [17] F. Zaera, E.B. Kollin, J.L. Gland, Surf. Sci. 184 (1987) 75. [18] J.T. Roberts, C.M. Friend, Surf. Sci. 186 (1987) 201. [19] F.P. Netzer, E. Bertel, A. Goldmann, Surf. Sci 201 (1988) 257. [20] G.R. Schoofs, R.E. Preston, J.B. Benziger, Langmuir 1 (1985) 313. [21] B.A. Sexton, Surf. Sci. 163 (1985) 99. [22] J. St€ ohr, J.L. Gland, E.B. Kollin, R.J. Koestner, A.L. Johnson, E.L. Muetterties, F. Sette, Phys. Rev. Lett. 53 (1984) 2161. [23] J.F. Lang, R.I. Masel, Surf. Sci. 183 (1987) 44. [24] N.V. Richardson, J.C. Campuzano, Vacuum 31 (1981) 449. [25] M. Salmeron, G.A. Somorjai, A. Wold, R. Chianellir, K.S. Liang, Chem. Phys. Lett. 90 (1982) 105. [26] A.J. Gellman, M.H. Farias, M. Salmeron, G.A. Somorjai, Surf. Sci. 136 (1984) 217. [27] M.C. Zonnevylle, R. Hoffmann, S. Haris, Surf. Sci. 199 (1988) 320. [28] Z. Popovic, Z. Soldin, J. Plavec, D. Vikic-Topic, Appl. Organometall. Chem. 14 (2000) 598. [29] H. Yamada, H. Nagata, K. Tabu, Y. Nakao, Surf. Sci. 182 (1987) 269.
A pH-dependent SERS study of thiophene-2-carboxylic acid adsorbed on Ag-sols Uttam K Sarkar
*
Department of Physics, Malda College, Malda 732101, WB, India Received 22 February 2003; in final form 30 April 2003
Abstract SERS is observed for thiophene-2-carboxylic acid (TCA) adsorbed on Ag-sols. The enhancement is maximum when TCA concentration, in Ag-sol, is 5 103 M. The SER spectra of TCA are different at different pH values, which suggest that the adsorption geometry is pH dependent. This is further supported by the absorption spectra at different pH values. It is inferred that the chemisorption takes place through both the S-atom and COOH group for pH 6 7 and through the p-electronic system for pH ¼ 10.5. Quick desulphurization at pH ¼ 2 and stable perpendicular and parallel surface bonding for pH ¼ 7 and 10.5, respectively, is revealed. Ó 2003 Published by Elsevier Science B.V.
1. Introduction Surface enhanced Raman Spectroscopy (SERS) is a useful technique for the study of chemical as well as physical transformation of molecules adsorbed on metal surfaces [1–4]. Two mechanisms, mainly, have been shown to contribute to SERS. One is long-range classical electromagnetic field effect and the other is short-range charge transfer effect [3]. The classical field effect is due to surface plasmon resonance by the incident electromagnetic field and is operative at larger distances. For isolated metal sphere it has been shown that the plasmon resonance occurs if the frequency of excitation matches with that at which the extinction *
Fax: +913512220516. E-mail address: [email protected].
by the metal particles is maximum [5]. The shortrange effect, on the other hand, is because of exchange of electrons between the molecule and the metal. This latter effect is dependent on the concentration of the adsorbate and is maximum when monomolecular layer is formed on the surface [6]. According to the image dipole theory, Raman scattering from an adsorbed molecule arises from both a dipole induced in the molecules and from the image dipole of the molecules within the metal surface. Vibrational modes which have dipolar components perpendicular to the surface will show enhancement and the magnitude of the enhancement will be proportional to the projection of the dipole moment on the perpendicular to the surface. Thus, SERS can provide information about orientation of the molecule on the metal surface [7,8].
0009-2614/03/$ - see front matter Ó 2003 Published by Elsevier Science B.V. doi:10.1016/S0009-2614(03)00728-0
342
U.K. Sarkar / Chemical Physics Letters 374 (2003) 341–347
In this Letter, we present the concentration dependence and pH dependence of SERS of thiophene-2-carboxylic acid (TCA) adsorbed on silver surface. Ag sol has been used for this purpose because of a number of advantages reported elsewhere [9]. Polythiophene possesses high electrical conductive properties and is, therefore, of great interest in different fields in recent times. For better understanding of the mechanisms involved in different electronic and optical properties of polythiophene the oligothiophenes and substituted thiophenes need more attention. The charge transfer effect and the orientation of different oligothiophenes in the adsorbed state have been reported in our earlier works [6,7]. SERS of TCA shows significant change of intensity patterns in the adsorbed state at different pH values. In addition, the short-range chemical effect is found to be predominant at an optimum concentration of TCA. Possible charge transfer interaction between the molecule and the metal surface has been discussed.
3. Results and discussion Raman spectra of TCA, in the solid phase and solution (1 M), are shown in Figs. 1a and b, respectively. Surface enhanced Raman (SER) spectra of TCA are shown in Fig. 1c. Assignments and apparent enhancement factor (AEF) of different Raman modes are shown in Table 1. The AEF has been calculated according to the relation AEF ¼ r1 C2 /r2 C1 where suffices 1 and 2 refer to SERS and RS, respectively; r and C are the intensity of a Raman mode and concentration of the molecule, respectively. The real enhancement factor will be a few orders of magnitudes higher owing to the fact that all the molecules may not get adsorbed on the
2. Experimental Silver sols were prepared by reducing aqueous solution of AgNO3 by NaBH4 according to the process described elsewhere [10]. The yellowish sol, thus prepared, showed a single extinction maximum at 392 nm, and was aged for several weeks before use for the present study. The TCA was purchased from Aldrich Chemical and the solutions were prepared with distilled and deionized water from Milli-Q-plus system of M/S Millipore Corporation, USA. Dilute HCl acid and NaOH solutions were used to adjust the sol pH and pH paper was used to measure the values. Raman spectra were recorded by a Spex Double Monochromator (model 1403) fitted with water-cooled photomultiplier tube (model R 928) from Hamamatsu Photonics (Japan). The samples were excited by the 514.5 nm line of the Arþ laser from Spectra Physics (model 2025-5). Spex datamate 1B was used for monochromator control, data acquisition and analysis. The electronic absorption spectra were recorded in a Shimadzu Spectrophotometer (model UV–Vis 2010 PC).
Fig. 1. Raman and surface enhanced Raman spectra of TCA: (a) solid phase, (b) aqueous solution (1M), (c) adsorbed on Agsols (5 103 M).
Table 1 Raman bands along with AEF in different conditions and their tentative assignments Aqueous soln (1 M) (cm1 )
pH ¼ 2 SERS (cm1 )
– –
– – –
428(vw) 455(w) 519(ms) 635(w) 721(w) 741(s) 754(s) 860(w) 926(w) 1050(w) 1078(s) 1114(w) – 1280(w) – 1353(s) – 1412(vs) – 1527(w)
230 312 – –
235 327 – –
480 – 650 – 741 758 859 932 988 1078 1113 – – – 1355 – 1417 1461 1532
586 – 710 – 761 – 924 988 –
1637(w)
1658
583 679 713 – 770 – 913 – – 1126 – 1236 – 1360 1384 – – 1512 1587 1620
1188 – 1305 1345 – – – 1512 1587 1620
AEF
1.11 103 1.73 103
1.51 103
4.66 103
pH ¼ 4 SERS (cm1 )
AEF
pH ¼ 7 SERS (cm1 )
AEF
236 320 – –
5.96 104
1.9 104 2.5 104
3.66 104
2.62 104
2.52 104
578 675 712 – 753 – 914 – 1070 1126 – 1234 – 1350 1390 1428 – – 1587 1620
1.42 103
pH ¼ 10.5 SERS (cm1 )
AEF
– – – – – – – – –
Ag–S stretching Substituent sensitive C–COOH in plane bending In plane ring bending C@O in plane bending
4.38 103 5.52 103 1.28 103 8.28 105
Tentative assignments (Refs. [11–14])
OH in plane bending 860 930 986 1061 1126 1166 1247 –
1.05 104 1.98 104 1.74 104 1.01 104
Ring stretching + ring bending
CH in plane bending C–COOH stretching
6.44 105
C3 –C4 stretching – – – 1537 1588 1620
C@C symmetric stretching 3.2 104
U.K. Sarkar / Chemical Physics Letters 374 (2003) 341–347
Solid phase (cm1 )
C@C asymmetric stretching
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surface and that the intensity of the Raman excitation radiation is reduced due to absorption and scattering by Ag-particles. 3.1. Concentration dependence Fig. 2 shows the concentration dependence of the apparent enhancement of the C3 –C4 stretching mode, which appears at 1355 cm1 in solution and at 1350 cm1 in the adsorbed state. During the concentration dependence, the pH was maintained at 7. Maximum enhancement is observed at a concentration of 5 103 M which indicates that monomolecular layer of TCA is formed on the silver surface at this concentration and the Ôfirst layer effectÕ [6] contributes to this large enhancement. At higher concentrations bulk properties manifest whereas at lower concentrations the re-
Fig. 2. Concentration dependence of SERS of 1350 cm1 band of TCA (pH ¼ 7).
duction of enhancement is due to sub monolayer coverage. In the SER spectra of TCA, a band appears at 320 cm1 , which is related to the COOH substituent group [11–13]. It suggests that the adsorption is through the COOH group. Ag–S stretching has been reported to lie within 150–250 cm1 [14] and in SERS the involvement of heteroatom in the adsorption process is well established. In the present study the 236 cm1 band in the SER spectra of TCA may be tentatively assigned to the Ag–S stretching mode. Participation of both the S heteroatom and the substituent COOH group is thus inferred for the adsorption of TCA on Ag-surfaces. 3.2. pH dependence SER spectra of TCA have been taken for different pH values (Fig. 3) with TCA concentration of 5 103 M that is optimum for enhancement. Significant change in intensities of different Raman bands is observed with variation of pH. Both bare and chlorinated silver surfaces are present in the sol when dilute HCl is added. Thus, two types of adsorption sites are available for the TCA molecules and their relative concentration depend on the pH value. As the pH value is decreased the chlorinated surfaces are more in number than the bare surface. The spectra in Fig. 3 are, thus, combination of contributions by TCA molecules adsorbed on these two types of sites. At pH ¼ 7 the contribution is mainly from TCA molecules adsorbed on the bare silver surface whereas at lower pH main contribution is due to the chlorinated surface. At pH > 7 the presence of excess OH plays important role and there is remarkable change in the intensity pattern in SERS of TCA at pH ¼ 10.5 compared to that at pH ¼ 7. This hypothesis of presence of different adsorption sites resulting in the variation of intensity pattern [15] will be adopted for the interpretation of SERS at different pH values. Overall intensity reduction is observed with lowering of pH values from 7 to 2. The t(Ag–S) band at 230 cm1 is very strong at pH ¼ 2. However, its intensity decreases significantly with increase in pH and it appears at 236 cm1 at pH ¼ 7. On the other hand the substituent sensitive band at
U.K. Sarkar / Chemical Physics Letters 374 (2003) 341–347
Fig. 3. SER spectra of TCA at different pH values: (a) pH ¼ 2, (b) pH ¼ 4, (c) pH ¼ 7 and (d) pH ¼ 10.5 (concentration of TCA in Ag-sol ¼ 5 103 M).
320 cm1 is strong at neutral pH but it appears at 312 cm1 at pH ¼ 2 with reduction in intensity. The in plane ring bending mode at 578 cm1 does not undergo significant change in intensity when pH is varied from 7 to 2 but shifts to higher frequency and appears at 586 cm1 at pH ¼ 2. All these bands, along with the C@O in plane bending and OH in plane bending at 650 and 741 cm1 , respectively, disappear in the SER spectra at pH ¼ 10.5. The striking feature is that the intensity pattern is different when the pH is 10.5. The ringbreathing mode at 986 cm1 is enhanced most at this pH. There is reversal of intensity between 1247
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and 1350 cm1 band with change of pH from 7 to 10.5. The asymmetric C@C stretching mode at 1537 cm1 is also largely enhanced at pH ¼ 10.5. Different surface bonding modes of thiophene have been proposed on different metal surfaces. On molybdenum surface, the parallel and perpendicular adsorption geometries are suggested for low and saturation surface coverages, respectively [16– 18]. Both parallel and inclined adsorption of thiophene is inferred on other metal surfaces [19–24]. In the present investigation of SERS of TCA in Agsol, the intensity pattern remains unaltered in a wide range of concentration. It suggests that the adsorption geometry of TCA is independent of concentration. However, the orientation of the molecules is different for different pH values. For neutral pH, a perpendicular adsorption (i.e., the plane of the ring is perpendicular to the surface) through both S and COOH group seems operative as evidenced from the appearance of t(Ag–S) at 236 cm1 and the substituent sensitive band at 320 cm1 . As the pH is lowered, there is a significant intensity reversal of 236 and 320 cm1 bands along with an overall reduction of intensity. There are evidences of S-elimination, by metal surfaces, from thiophene and other S-containing molecules of the crude oil in the industrially important hydrodesulphurization [16–18,25–27]. In TCA elimination of S-atom may be inferred at pH ¼ 2. The SER spectra, in that case, are contributed by the molecules at a distance from the surface plus the decomposed molecules from which the S-atom has been eliminated. The SER spectra at pH ¼ 10.5 reveal that the adsorption might not have been through the S-atom as there is no t(Ag–S) band. This may be attributed to the fact that the hydrogen bonding in the COOH group of TCA [28] is stronger at higher pH value and that is why the adsorption through S-atom or the COOH group is inhibited at pH ¼ 10.5. The large enhancement of the ring-breathing mode at 986 cm1 suggests parallel adsorption geometry (i.e., the plane of the ring is parallel to the surface) at this pH value. 3.3. Absorption spectroscopy Fig. 4 shows the absorption spectra of TCA in the adsorbed state at different pH. Two bands are
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Fig. 4. Absorption spectra of TCA adsorbed on Ag-sols at different pH values: (a) pH ¼ 2, (b) pH ¼ 4, (c) pH ¼ 7 and (d) pH ¼ 10.5.
observed. One is representative of absorption by Ag-sol which appears at 392 nm for pure sol, at 393 nm at pH ¼ 2 and at 403 nm at other pH values. The intensity of this absorption band decreases with increase in pH. The other absorption band, representative of the TCA molecule, appears at 245 nm which is almost absent at pH ¼ 2. These observations substantiate the inference that the TCA molecules are decomposed by the process of S-elimination by silver at pH ¼ 2. At pH ¼ 7 intensity of the 245 nm band is maximum and a shoulder appears near 470 nm and it does not show any shift for several hours which indicates a stable chemisorption through charge transfer interaction [29] at pH ¼ 7. This is also supported by the appearance of 236 and 320 cm1 bands in the SER spectra (Fig. 3c). Fig. 4d shows the absorption spectrum of TCA in the adsorbed state at pH ¼ 10.5. At this pH value the absorption at 245 nm is reduced but is more than that at pH ¼ 2. This means the TCA molecule retains its identity at this pH value though the sol absorption is reduced significantly. 3.4. Time dependence The time evolution of SER spectra of TCA at pH ¼ 7 is shown in Fig. 5 which reveals that the
Fig. 5. Time evolution of SER spectra of TCA on Ag-sol: (a) 0 min, (b) 30 min, (c) 60 min, (d) 120 min and (e) 180 min.
surface bonding is stable at this pH. Similar spectra have been recorded at other pH values also. It is observed, for all pH values, that the spectra do not undergo any change in intensity pattern with time. However, some time (about 10 min) is elapsed between the moment at which the molecules are added to the Ag-sol and that at which the monochromator starts recording of Raman signal. In the present study t ¼ 0 instant is the later moment which means that the surface modification of the molecules, at pH ¼ 2, occurred within 10 min. This is suggestive of quick desulphurization of TCA at pH ¼ 2, stable perpendic-
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ular surface adsorption at pH ¼ 7 and stable parallel surface adsorption at pH ¼ 10.5.
4. Conclusion Monomolecular layer of TCA is formed on the silver surface at a concentration of 5 103 M. The Ôfirst layer effectÕ contributes to the large enhancement at this concentration. In the process of surface adsorption, both the S-atom and the COOH group participate, with perpendicular adsorption geometry, at pH 6 7. On the other hand, at pH ¼ 10.5 the TCA molecule is chemisorbed, with parallel adsorption geometry, through its p-electronic system. A quick S-elimination from TCA takes place at pH ¼ 2 though the molecule is stable, in the adsorbed state, at higher pH values. References [1] A. Otto, in: M. Cardona, G. Guntherdot (Eds.), Light Scattering in Solids, vol. 4, Springer, Berlin, 1984. [2] D.P. DiLella, M. Moskovit, J. Phys. Chem. 85 (1981) 2042. [3] J.A. Creighton, in: R.J.H. Clark, R.E. Hester (Eds.), Spectroscopy of Surfaces, Wiley, New York, 1988. [4] Y. Furukawa, M. Akimoto, I. Harada, Synth. Met. 18 (1987) 151. [5] D.S. Wang, M. Kerker, Phys. Rev. B 24 (1982) 1777. [6] U.K. Sarkar, S. Chakrabarti, A.J. Pal, T.N. Misra, Chem. Phys. Lett. 190 (1992) 59. [7] U.K. Sarkar, S. Chakrabarti, A.J. Pal, T.N. Misra, Chem. Phys. Lett. 200 (1992) 55.
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