The Journal of
Physical Chemistry
0 Copyright 1994 by the American Chemical Society
VOLUME 98, NUMBER 38, SEPTEMBER 22,1994
LETTERS Potential Dependence of the Orientation of (CH3)4NS Adsorbed on a Silver Electrode. A SERS Investigation Zhongyi Dengt and Donald E. Irish' Guelph-Waterloo Centre for Graduate Work in Chemistry, Department of Chemistry, University of Waterloo, Waterloo, Ontario N2L 3G1, Canada Received: March 9, 1994; In Final Form: July 1, 1994@
The surface-enhanced Raman scattering (SERS) from tetramethylammonium ( ( C H 3 ) N ) cations adsorbed via bromide ions to silver electrode surfaces in LiAsFa-methyl acetate solutions has been measured. The ratio of the intensities of the antisymmetric to the symmetric N-C stretching modes changes markedly as the potential is changed through the potential of zero charge. This change is attributed to different orientations of (CH&N+ on the surface.
Introduction One of the fundamental properties of a molecule adsorbed to an electrode surface, or a metal surface in general, is the adsorbate orientation. One approach to obtain such information is to measure surface vibrational band intensities and apply "surface selection Another approach has been suggested by Pemberton et al?-7 They linked the intensity ratio of two bands of methanol, the antisymmetric and the symmetric C-H stretching vibrations, to the orientation of methanol on the s ~ r f a c e .They ~ found that the intensity ratio was sensitive to changes in the electrode potential near the potential of zero charge (pzc). They assumed that all of the enhancement arises from electromaganetic (EM) effects and that resonance Ramanlike processes did not contribute significantly to the measured intensities. They also applied the approach to other alcohol^.^-^ Although their argument is somewhat ambiguous, and its application is probably limited to only a few situations, the approach bears further consideration. We report here the change in ratio with potential of the antisymmetric to the symmetric N-C stretching mode intensities of (CH3)Y+. This system has advantage over the one used by Pemberton and co-workers f Present address: Energy and Environment Division, Lawrence Berkeley Laboratory, University of Califomia, Berkeley, CA 94720. Abstract published in Advance ACS Absrrucrs, September 1, 1994. @
0022-3654/94/2098-9371$04.50/0
in that the two bands of interest are well resolved and separated from other bands arising from the system. These results are for solutions where the solvent is methyl acetate (MA) and the electrode is polycrystalline silver.
Experimental Section Methyl acetate (Aldrich, anhydrous, 99+%) was passed slowly through a column packed with 4 8,molecular sieves (812 mesh beads, Fisher 514), previously activated by heating at 250 OC under vacuum for 24 h. The solvent was then passed slowly to distillation flasks which had been previously dried with an open flame when under a vacuum of Torr. The solvent (boiling point 56 "C) was distilled in an argon atmosphere. The middle 60% cut was collected in a dried flask with a Teflon stopcock. Methyl acetate prepared in this way was transferred to a glovebox without exposure to the atmosphere. A 684 KF coulometer (Metrohm Ltd.) was used for water analysis. This instrument permits measurement of water concentrations below 10 ppm. The amount of water in methyl acetate (Aldrich, anhydrous, 99+%) as received was 88 zk 2 ppm. After purification the amount of water dropped to 17 f 0.2 ppm. For water analysis, a special sealed syringe was used to transfer solvents or electrolytes from the drybox to the Karl Fisher titroprocessor. LiAsF6 (Lithco, elecrochemical grade) 0 1994 American Chemical Society
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9372 J. Phys. Chem., Vol. 98, No. 38, 1994 I
a 600
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900 1000 RAMAN SHIFT / cm-' Figure 1. Raman spectra (a) before an ORC and (b) after an ORC (-0.6 V) in the 560-1000 cm-I region. salt was dried under vacuum Torr) and a controlled temperature of 60 "C for 5 days. This transfer was done without contact with the laboratory atmosphere. Tetramethylammonium bromide, (CH3)aBr (Aldrich Chemical Co., 98%), was used without further purification. The salt was slowly added to the solvent in a drybox with an argon atmosphere. The three-electrode spectroelectrochemical cell, the electrochemical instruments, and the Dilor OMARS-89 Raman spectrometer were described previously.8 Spectra were excited by using the 514.5 nm line of a Coherent Innova 70 argon ion laser chopped at 2 s cycle. The incident laser beam (25 mW power, ca. 60" to the surface normal) was p-polarized.
Results and Discussion In Figure 1 Raman spectra in the 560-1000 cm-' region from the silver electrode are shown before an ORC at open circuit (a) and after an ORC at -0.6 V (b) for 0.1 M LiAsF6 0.005 M (CH3)aBrMA. For the (a) spectrum, the laser (ppolarized) was reflected from the unanodized polished surface; it is identical to the spectrum of bulk solution. The following bands are visible: a band with a maximum at cu. 607 cm-' is ~24,the H3C-O out-of-plane bending vibration mode of MA, a very weak band; a band with a maximum at 638 cm-' is v15, the 0-C-0 bending mode of MA (this is the strongest band in the spectrum); a very weak band at ca. 676 cm-' is v1 (Alg) of As&- (the concentration of LiAsF6 is only 0.1 M and the exciting laser power is only 25 mW; this band became even weaker in SERS); a band with maximum at 842 cm-' is v14, the C-C stretching vibration of "free" methyl acetate (this is the second strongest band in the spectrum; in contrast, for an s-polarized laser beam8 this band is the strongest in this region). On careful examinatiom of the spectrum, a very weak, highfrequency shoulder of this band is observed. This is the -860 cm-' band from the C-C stretching of MA solvating the lithium ~ a t i o n .Another ~ weak band with a maximum at 976 cm-' is ~ 1 3 the , H3C-C rock of MA.* Shown in Figure l b is the SERS spectrum at -0.6 V. The spectrum is markedly different from Figure la. Some new bands occur in addition to all the bands in Figure la. A strong band at ca. 748 cm-' is v3, the C-N symmetric stretching mode of (CH3)4N+; the shoulder at cu. 737 cm-' is an overtone band, 2vg(E) of (CH3)4N+. Another band with a maximum at 942 cm-' is v18, the C-N antisymmetric stretching mode of (CH3)4N+?-13 The intensities of bands from (CH3)4N+ are very strong although the concentration of ( C H 3 ) m r is less than 0.005 M, and all these bands are invisible in the bulk solution
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PotentiaVV Figure 2. I(C-N)u&Z(C-N)sm ratio as a function of electrode
potential (V vs Ag/AgBr/Br-). spectrum (Figure la); the band positons are shifted to lower Raman frequencies compared to the spectrum of 1.O M (CH3)4NBr/H20. The band at ca. 860 cm-' is characteristic of methyl acetate solvating the lithium ~ a t i o n .This ~ band was a shoulder of the band at -840 cm-' for the whole potential range when the s-polarization exciting laser beam was used;8 here this band is better resolved from the 842 cm-' band. Very clearly we are detecting the solvated Li+ on the surface of the silver electrodes (more accurately it is coadsorbed with Br- ions on the Ag electrode), and the signal from this solvated lithium cation is greatly enhanced. The potential dependence of the spectra is similar to that illustrated as Figure 6 in ref 8. The intensities of both the 748 and 942 cm-' bands increased when the potential was changed from -0.3 to -1.1 V and then decreased (cf. Figure 9, ref 8). Qualitatively as more negative potentials are applied, the vsym(C-N) intensity loss is greater than the vaym(C-N) intensity loss.8 The Z(C-N)a,,,,JZ(C-N)sym ratio increases when the potential is stepped more negative than -1.1 V, suggesting a potential-dependent surface re~rientation.~-'The relationship between this ratio and the electrode potential for p-polarized 514.5 nm laser excitation is shown in Figure 2. (It is similar to the results which were obtaind for s-polarized 514.5 nm laser excitation.8) This plot suggests that orientations at potentials more negative than - 1.4 V and more positive than - 1 V are different. Between these potentials the Z(C-N)a,,,,JZ(C-N)sym ratio is sensitive to changes in electrode potential. Consequently, one may conclude that the orientation is similarly sensitive to potential in this region. The large spectroscopic changes in this potential region are consistent with the unique behavior often associated with potentials near the potential of zero charge.14 Therefore, it is assumed that the pzc is located in this region, around -1.2 V (consistent with the conclusion in ref 8 which viewed the ( C H 3 ) N as an uncharged organic molecule and placed the pzc at or near the surface coverage maximum of -1.1 V in this system), which is more negative than its value in aqueous media (-0.9 V).15 The potential dependence of the intensity of the symmetric and antisymmetic v(C-N) bands provides insight into the orientation of tetramethylammonium cations interacting directly with the Ag electrode. A model for the potential-dependent
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J. Phys. Chem., Vol. 98, No. 38, 1994 9373
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Figure 3. Proposed models of the potential-dependent interfacial (CH3)4N+ orientation.
(CH3)fl+ orientation at the Ag electrode is shown in Figure 3 (cf. ref 16). At positive potentials, the dominant form of adsorbed tetramethylammonium cations on the silver electrode surface is assumed to be form I, which has C3” symmetry with its three methyl functional groups strongly bound to bromide anions. (A high Br- coverage exists on the electrode surface; see ref 8.) In the pzc region it is proposed that the adsorbate is as shown in Figure 3, form 11, which has CzVsymmetry; it is bonded to adsorbed Br- through two methyl functional groups; the fraction of molecules with this orientation would increase when the potential is stepped to more negative values because the surface bromide concentration would go down, and at a certain potential form I1 would become dominant. Compared to form I, the antisymmetric stretching mode of form I1 is less constrained; thus, the ratio Z(C-N)ayJZ(C-N)sym for form I1 is larger than that for form I. At very negative potentials (more negative than - 1.4 V), form 111, which also has C3, symmetry but only one methyl functional group bonded to Br-, would be the dominant surface species. The intensity of the C-N antisymmetric stretching band of form 111 should be the strongest; thus, the ratio Z(C-N)asym/Z(C-N)symshould be the largest. This model involves a transition from form I to form 111 as the bromide coverage diminishes. Because the intensity ratio at positive potentials is essentially equal to that from (CH3)4N+ in homogeneous solutions (isotropic distributions), one could argue that there is no preferred orientation at positive potentials. On sweeping to the pzc and negative potentials a
preferred orientation (forms II and 111) is induced. This is an alternative way of looking at form I.17 The intensity of the symmetric stretching band is usually much stronger than that of the antisymmetric stretching band in normal Raman scattering, but the reverse is true in the IR spectrum. It is very interesting to note that, under very negative electrode potentials, the Raman intensity of the C-N antisymmetric stretching band is comparable to or stronger than the intensity of the C-N symmetric stretching band. For aqueous and methanol electrolyte s o l ~ t i o n s ~the ~ J *antisymmetric stretching band intensity is weak, and we have not found it possible to obtain comparable accurate values of the intensity ratio for this adsorbate in these solvents.
Acknowledgment. This work was supported by grants from the Natural Sciences and Engineering Research Council of Canada. References and Notes (1) Moskovits, M. Rev. Mod. Phys. 1985, 57, 783. (2) Pettinger, B. In Adsoption of Molecules at Metal Electrodes; Lipkowski, J., Ross, P. N., Eds.; VCH: New York, 1992; p 285. (3) Otto, A,; Mrozek, I.; Grabhom, H.; Akemann, W. J . Phys.: Condens. Matter 1992, 4, 1143. (4) Creighton, J. A. In Spectroscopy of Surfaces; Clark, R. J. H., Hester, R. E. Eds.; John Wiley: New York, 1988; p 37. (5) Pemberton, J. E.; Bryant, M. K.; Sobocinski, R. L.; Joa, S. L. J. Phys. Chem. 1992, 96, 3116. (6) Joa, S. L.; Pemberton, J. E. Langmuir 1992, 8, 2301. (7) Pemberton, J. E.; Sobocinski, R. L. J. Electroanal. Chem. 1991, 318, 157. ( 8 ) Deng, Z.; Irish, D. E. Langmuir 1994, 10, 586. (9) Deng, Z.; Irish, D. E. Can. J . Chem. 1991, 69, 1766. (10) Kabisch, G.; Klose, M. J. Raman Spectrosc. 1978, 7, 311. (11) Berg, R. W. Spectrochim. Acta 1978, 34A, 655. (12) Mylrajan, M.; Srinivasan, T. K. K. J . Raman Spectrosc. 1991, 22, 53. (13) Deng, Z.; Irish, D. E. J . Phys. Chem., in press. (14) Anson, F. C. Acc. Chem. Res. 1975, 8, 400. (15) Larkin, D.; Guyer, K. L.; Hupp, J. T.; Weaver, M. J. J. Electroanal. Chem. 1982, 138, 401. (16) Shindo, H.; Kaise, M.; Nishihara, C.; Nozoye, H. Langmuir 1991, 7, 1525. (17) We are grateful to a reviewer for this opinion. (18) Deng, Z.; Irish, D. E. To be published.
