Langmuir 1991, 7, 1525-1528
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Adsorption and Decomposition of (CH3)4N+ on Ag Electrodes Observed with Surface-Enhanced Raman Spectroscopy Hitoshi Shindo,’ Masahiro Kaise, Chizuko Nishihara, and Hisakazu Nozoye National Chemical Laboratory for Industry, Tsukuba, Ibaraki 305,Japan Received August 16, 1990. I n Final Form: February 14,1991 Adsorption of tetramethylammonium ion (TMA+)on silver electrodeswas studied with surface-enhanced Raman spectroscopy (SERS) in the presence of various halide ions. TMA+ adsorbs Coulombically on halide ions which remain on the electrode surface. Compared to the solution spectra, some of the nontotally symmetric methyl rocking and deformation modes were markedly enhanced in the SERS spectra due to lowered symmetry in adsorption states. The difference between spectra observed in the presence of Br- and I- was explained as the difference in adsorption geometry of TMA+due to different populations of the halide ions at the electrode surface. A classical surface electromagnetic enhancement mechanism explainswell the above observations. As the halide ions desorbed toward cathodicpotentials, decomposition of TMA+ occurred, leaving trimethylamine at the surface. In the SER spectra of trimethylamine, almost all the bands observed were totally symmetric vibrations. C3N deformation and methyl rocking modes were especially very much enhanced, indicating a strong chemical effect in the process of surface enhancement. The nitrogen atom in the molecule is directly coordinated with the metal atom. The surface resonance Raman enhancement mechanism seems to prevail in this case.
Introduction Tetraalkylammonium salts have large solubilities in organic solvents and are often employed in electroorganic synthesis as supporting electrolytes. In some cases, like in reduction of acrylonitrile in aqueous solutions to form adiponitrile,lsz they are used in controlling reaction selectivitiesby forming aprotic layers on electrode surfaces. In order to study the molecular structure of the electrode/ solution interface of such systems, we have observed surface-enhanced Raman spectra of (CH3)4N+, a model compound, adsorbed on an Ag electrode. We have previously reported on adsorption of aniline in neutral and acidic solution^.^*^ In total, three different forms were identified by SER observation. The orientation of the benzene ring was determined by applying the classical electromagnetic selection rule of surface enhancement proposed by C r e i g h t ~ n . ~When ’ halide ions remain on the surface in a high concentration, anilinium ion (CaHsNH3+) “stands up” by electrostatic interaction of all three H atoms of the ammonium group with halide ions. When the halide concentration at the surface decreases toward more cathodic potentials, only one or two H atoms of the ammonium group can interact with the halide ions. The anilinium ion starts to lie down. With more cathodic potential, the ion loses a proton and adsorbs flat-on as aniline. Similar electrostatic interaction with electrode surfaces is expected for TMA+, since the H atoms of its methyl groups also bear positive charges. However, strong electronic interactions with the metals seems improbable because TMA+ does not have unsaturated bonds or lone pair electrons. Experimental Section Tetramethylammonium chloride (Nakarai Chemicals, SP grade,99%) was dissolved (0.01M) in water that had been deionized, distilled, and furtherpurified with the NANOpureI1system (1) Baizer, M. M. J. Electrochem. SOC.1964, 111, 215. (2) Janson, R. Chem. Eng. News 1984,62,43. (3) Shindo, H. J. Chem. SOC.,Faraday Tram. 1 1986,82,45. Faraday Trans. I 1988, (4) Shindo, H.; Nishihara, C. J. Chem. SOC., 84, 433. (5)Creighton, J. A. Surf. Sci. 1983, 124, 209. (6) Creighton, J. A. Surf. Sci. 1985, 158, 211. (7) Creighton, J. A. In Spectroscopy of Surfaces;Clark, R. J. H., Heater, R. E.,Eds.;John Wiley: New York, 1988; p 37.
(Barnstead). In preparation of a neutral solution, potassium halide (KX 0.1 or 0.2 M, X = C1, Br, I) (Wako Pure Chemical, 99.9%) was used as a supporting electrolyte. The electrochemical system used was described in our previous The electrode potential is quoted against a saturated calomel electrode (SCE)in this paper. Apolycrystalline Ag plate (Furu-UchiChemicals,99.99 7%)was used as the workingelectrode after being polished with alumina suspension (Baikalox, 0.05 pm) and ultrasonically cleaned. The electrodewas further cleaned in the sample solution of TMA+by generating Hz at -2.0 V. The rougheningof the electrodefor activation of surfaceenhancement was, then, performed by going through one or two cycles of oxidation (5s) and reduction treatment. The oxidation potential was chosen according to the halide ion used (+0.3, +0.1, and -0.1 V for C1-, Br-, and I-, respectively). Only weak Raman bands of TMA+ in the solution bulk were observed without the ORC treatment. This type of “in situ” roughening might give certain complexitiesto the system studied. However,it is an effectivemethod in studyingmoleculesthat are electrochemicallystable,but weak as adsorbates. With quickaccessto freshlyformed metal surfaces, those weak adsorbates can compete favorably with other adsorbates such as various impurities. A SPEX 1403 double spectrometer was used in Raman measurementwith a spectral resolution of 5 cm-l. For excitation, the 514.5-nm line of an Ar ion laser (Coherent,INNOVA 100-15) was used at a power of 100 mW at the cell entrance.
Results and Discussion Adsorption of (CH3)4N+. The infraredg-I2 and Ramanl2-I9spectra of TMA+ have been reported by many groups, and normal coordinate analysis was also performed.20 While the ion has Td symmetry as a free (8) Shindo, H. Chem. Phys. Lett. 1989, 159, 85. (9) Ebsworth, E. A. V.; Sheppard, N. Spectrochim. Acta 1959,13,261. (10)Bottger, G. L.; Geddes, A. L. Spectrochim. Acta 1965,21, 1701. (11) Harmon, K. M.; Gennick, I.; Madeira, S. L. J. Phys. Chem. 1974, 78, 2585. (12) Berg, R. W. Spectrochim. Acta, Part A 1978, 344 655. (13) Anhouse, S. J.;Tobin, M. C. Spectrochim. Acta,Part A 1972,28a, 2141. (14) Von der Ohe, W. J. Chem. Phys. 1975,62,3933. (15) Kabisch, G.; Klose, M. J . Raman Spectrosc. 1978, 7 , 311. (16) Kabisch, G. J. Raman Spectrosc. 1980, 9, 279. (17) Pal, M.; Raghuvanshi, G. S.; Bist, H. D. Chem. Phys. Lett. 1982,
92, 85. (18) Kabisch, G.; Mobius, G. Spectrochim. Acta, Part A 1982, %a, 1189,1195. (19) Pal, M.; Agarwal, A,; Patel, M. B.; Bist, H. D. J.Raman Spectrosc. 1984, 15, 211. (20) Silver, S. J. Chem. Phys. 1940,8, 919.
0743-7463/91/2407-1525$02.50/00 1991 American Chemical Society
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(b)
(a)
(C)
(d)
Figure 1. Proposed geometries of adsorption of tetramethylammonium ion (a-c) and trimethylamine (d). Form b has Ck symmetry while all others have Ca symmetry. Internal rotation of the methyl groups is neglected here. Table I. Correlation Table for the Symmetry Species of the Ta Group and G,and G , SubgroupsU* a1
4
a2
4
e
tl t2 a
a1 a2
4
a1 + a2
4
a2
4
+ bl + bz a1 + bl + bz
a1 02
e a2
+e
a1 + e
Raman inactive species are italicized.
molecule, the symmetry is lowered in adsorption states. Three possible geometries of the adsorbed ion are shown in Figure la-c. The internal rotation of the methyl groups is neglected here. Forms a and c have C& symmetry, while form b has C%sy"etry. Correlation of symmetry species of the Td group with those of the two subgroups is shown in Table LZ4 Raman inactive modes under Td symmetry, namely a2 and tl modes, may be observed under lower symmetry. Wetzel et aL25 reported that I- adsorbed stably on an Ag electrode even at -0.9 V, where no C1- or Br- would remain on the surface. We have also reported' the potential dependence of an Ag-C1- Raman band a t 240 cm-I. The band started losing intensity a t -0.3 V and became very weak at -0.6 V. On the other hand, the Ag-IRaman band at 116cm-l, which was observed as a shoulder on a strong background, remained strong even at -0.9 V. The curves in Figure 2 are the Raman spectra observed at various electrode potentials when KI (0.2 M) was used as the supporting electrolyte. The four curves were obtained in a series by first stepping the potential from -0.1 to -0.8 V, then by further stepping down to -1.0, -1.2, and -1.4 V. At -0.8 V the Raman bands denoted A were observed. A group of new bands (denoted B) appeared at -1.0 V, where partial desorption of I-occurs. A t -1.4 V only weak Raman bands of the solution were observed. There is no SERS active species remaining at the surface. The spectra observed in the C-Hstretching region are shown in Figure 3. Let us now compare the SER spectra observed a t -0.8 V with the spectra of TMA+ in solution phase shown in Figure 4a and Figure 5a. The vibrational assignment given in Figure 4a is according to Berg.I2 Overall resemblance between the SER and the solution spectra assures that all the bands denoted as A belong to TMA+. Some differences are noticed, however. In Figure 2a, the methyl rocking mode in t 2 symmetry species (triply degenerate) at 1283cm-' is markedly enhanced in the SER spectrum. Four peaks are observed for the methyl (21)Dellepiane, G.; Zerbi, G. J. Chem. Phys. 1968,48, 3573. (22) Clippard, P. H.; Taylor, R. C. J. Chem. Phys. 1969,50, 1472. (23) Edsall, J. T. J. Chem. Phys. 1937,5,225. (24) Wilson, E. B., Jr.;Decius, J. C.; Cross, P. C. Molecular Vibrations; Dover: New York, 1980. (25) Wetzel, H.; Gerischer, H.; Pettinger, B. Chem. Phys. Lett. 1981, 78, 392.
500
900
1300
R A M A N SHIFT cm" Figure 2. Raman spectra of Ag surface at various potentials observed for an aqueous solution of (CHs)dN+C1- (10 mM) with KI (0.2 M)addedasa supportingelectrolyte. The bands denoted A come from adsorbed (CHa)rN+,while those denoted B are assigned to adsorbed (CH&N. The bands with asterisks are impurity bands.
2700
2900
3100
RAMAN SHIFT / c m - l
Figure 3. Raman spectra of Ag surface in the C-H stretching region observed for the same sample as with Figure 2. The bands denoted B come from adsorbed (CHJsN, others from adsorbed (CHs)rN+. deformation modes of the adsorbate, while only two are observed in the solution spectrum. These differences are related to symmetry lowering caused by adsorption and selective surface enhancement. They will be discussed later in detail. All the polarized bands in Figure 5a are derived by Fermi resonance of a totally symmetric (a') methyl stretching mode with combination m ~ d e s . ~ ~Only J ~ the J ~ depolarized band at 3032 cm-I is considered as fundamental. All
(CH&N+ on Ag Electrodes
-
500
Langmuir, Vol. 7, No. 7, 1991 1527
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-
RAMAN SHIFT I cm-1
Figure 4. Raman spectra of (CH&N+ and (CH&N in solution
and adsorbed states. (a) (CH&N+ in solution phase observed (parallel polarization) for an aqueous solution of (CH&N+ C1(1.0 M).The vibrational assignmentgiven is accordingto Berg.12 (b) (CH&N in solution phase observed (parallel polarization) for an aqueousof (CH&N.HCl(O.5 M) added with KOH (1.0 M). The assignment is based on literature.21*22 (c)Surface-enhanced Raman spectra observed at -0.8 V in an aqueous solution of (CHs)rN+C1- (10 mM) with KBr (0.1 M). The bands denoted A belong to adsorbed (CH3)4N+while those denoted B belong to adsorbed (CH3)3N. The band with an asterisk is an impurity band. Note that most of the bands denoted B belong to a1 symmetry species of adsorbed (CH3)sN. 2834 2680 2808 5 A
2882
2800
AM
1
3000
When KC1was used as the supporting electrolyte instead of KI, Raman bands of adsorbed TMA+ were not observed unless the moleculewas used in much higher concentration (0.1 M). The dependence of adsorption on the choice of halide strongly indicates that TMA+ adsorbs by pairing with halide ions on silver surface. Such pair formation has been reported for adsorption of anilinium ion4 and pyridinium ion.28-29 The voltage and pH dependences observed for the latter are in good agreement with our present results. In the case of anilinium ion, pair formation with C1- was observed at the amine concentration of 5 mM. Compared to this, adsorption of TMA+ is relatively weak. It is probably because each of the 12 H atoms of TMA+ bears only a small part of the total positive charge of the ion. Formation of (CH3)sN. As shown in Figure 2b and Figure 3b, Raman bands denoted B appeared at -1.0 V when KI was used as the supporting electrolyte. When KBr (0.1 M) was used instead, they appeared even at -0.6 V. The chemical species B giving the bands are formed at the surface when partial desorption of the halide ions occurs. The spectrum in Figure 4c was observed at -0.8 V. In order to identify the species B, the spectra were compared with those of trimethylamine in aqueous solution, which are shown in Figure 4b and Figure 5b. The sample solution was prepared by addition of KOH (1.0 M) to an aqueous solution of (CH&N.HCl (0.5 M). Since (CH3)sN formed keeps evaporating out of the solution, its exact concentration in the solution was not known. The assignments given in Figure 4b are based on the literature.21122The shoulder band at 436 cm-l is polarized and should be assigned to the totally symmetric C3N deformation mode. The frequency is much higher than the values of 364 and 375 cm-I observed for gas phase and neat liquidF2 respectively. This large shift is probably due to hydrogen bonding of the nitrogen atom with a water molecule.23 Four polarized C-H stretching bands are observed in Figure 5b, while only two are expected under Cb symmetry. This will also be interpreted as Fermi resonance with overtones and combinations of methyl deformation modes.30 By comparing Figure 4b with Figure 4c and Figure 2b,c, we find out that all a1 modes of trimethylamine have their counterparts denoted B in the SERS spectra. The same relation is observed in comparison of Figure 5b with Figure 5c and Figure 3b,c. We conclude that the bands denoted B come from adsorbed trimethylamine as shown in Figure Id, which is formed by decomposition of TMA+ at the electrode surface. The assignment was confirmed by observing the intense methyl rocking mode a t 1209 cm-l in the reduction of adsorbed (CH&NH+ at the same electrode. The C3N deformation mode at 436 cm-' in Figure 4b is shifted further to higher frequencies for the adsorbate. This is due to coordination of the nitrogen lone pair with an Ag atom. The same mode for (CH&NH+, for instance, in aqueous solutions appears at 465 cm-l. Lombardi et aL31 studied surface Raman enhancement
R A M A N SHIFT I cm-1
Figure 5. Raman spectra observedin the C-H stretching region for the same samples as with Figure 4. (a) (CH&N+ in solution (parallelcomponent). The signs p and dp indicate polarized and depolarized bands, respectively. (b) (CH&,N in solution. (c) Adsorbates on Ag: A, (CHa)rN+;B, (CH3)aN.
the C-H stretching bands of the adsorbate in Figure 3a are shifted to lower frequencies compared to the free molecule.
(26) Regie, A.; Corset, J. Chem. Phys. Lett. 1980, 70, 305. (27) Fleischmann, M.; Hill, I. R. In Surface Enhanced Raman Scattering; Chang, R. K., Furtak, T. E., Eds.; Plenum: New York, 1982; p 275. (28) Birke, R. L.; Bernard, I.; Sanchez,L. A.; Lombardi, J. R. J . Electroanal. Chem. Interfacial Electrochem. 1983, 150, 447. (29) Sun,S. C.; Bernard, I.; Birke, R. L.; Lombardi, J. R. J. Electroanal. Chem. Interfacial Electrochem. 1985,196,359. (30) Goldfarb, T. D.; Khare, B. N. J. Chem. Phys. 1967,46, 3379. (31) Lombardi, J. R.; Birke, R. L.; Sanchez, L. A.; Bernard, I.; Sun, S. C.Chem. Phys. Lett. 1984, 104, 240.
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Four bands are expected for the methyl deformation of for saturated amines and concluded resonance enhanceTMA+ under Td symmetry, of which two are clearly ment accompanying the electron transfer from the amines observed in Figure 4a for the solution. Two additional to the metal. The same mechanism will be working for bands are observed in Figure 2a for the adsorbate. One trimethylamine adsorbed as in Figure Id. at 1479 cm-' belongs to a t z mode,12J5JsJs which is very From the fact that many cases of surface resonance weak in the solid or solution phases. The one at 1464 cm-l enhancement relate to totally symmetric modes, Creighton7 suggests that the A-term resonance m e c h a n i ~ mis~ ~ * ~might be the a1 mode missing in the reference spectra because of very low intensity.'8 However, there is no most likely to be involved in SERS. In the present study, guarantee on this point because internal rotation of the too, only a1 vibrations are observed for the adsorbed methyl groups might be giving different frequencies for (CHd3N. an e or t z mode. Anyway, there seems to be a mechanism Hirakawa and T s ~ b o i 3 ~noted 1 ~ ~ that vibrations encausing larger enhancement for the t z modes. hanced by the A-term resonance mechanism are the totally The situation is not the same, however, when bromide symmetric modes involving the normal coordinates along is used. The band at 1062 cm-l in Figure 4c is a tl methyl which the relaxation would occur following the electronic rocking mode, which is Raman inactive and was observed excitation. The marked enhancement of the totally only in crystal phases.16J9 The appearance of this band symmetric methyl rocking and C3N deformation modes means that TMA+ does not hold very high symmetry in observed at 1209 and 462 cm-l, respectively, in Figure 4c the adsorbed state. Another methyl rocking mode (e) a t indicates that the molecule is distorted in those directions 1176 cm-' has an intensity comparable to the tz mode at when the charge transfer occurs. This is reasonable for 1283 cm-l. the adsorption geometry shown in Figure Id. We consider that the difference between the Raman If the classical electromagnetic enhancement was the spectra of adsorbed TMA+ on the choice of the halide is only mechanism workin on the C3u molecule in that caused by different geometry of adsorption, just as in the geometry, some e modes would also be strongly enhanced case of the anilinium Using I- at -1.0 V means a since the mechanism allows the enhancement of some offmuch higher halide concentration at the surface than in diagonal components of the polarizability tensor in the using Br- at -0.8 V. When the halide concentration at the symmetry specie^."^ However, no C3N deformation or surface is higher, the geometry of adsorption will be closer stretching, nor methyl rocking modes of e symmetry in to Figure IC,while it will be closer to Figure l a for the Figure 4b were observed in Figure 2b,c. The charge lower concentration. transfer mechanism seems to be more important in this As mentioned before, the Td symmetry of free TMA+ case. (a spherical top molecule) will be lowered to CzUor CsUin For the free (CH&N molecule, two a1 modes and three the adsorbed states. Let us discuss now the difference e modes are expected for the methyl deformation. All between the surface enhancement of Raman bands for five have been observed in infrared although it the two different symmetries. is difficult to determine which two are the a1 modes. The With the choice of z direction as surface normal, the azz fact that only two peaks are detected for the adsorbed component of the polarizability tensor of an adsorbate molecule, at 1402 and 1440 cm-' in Figure 2c, seems to will enjoy the largest surface enhancement, because indicate that they are the a1 modes. electromagneticfields of both incident and scattered light The first step of the decomposition of TMA+is probably are enhan~ed."~The cyzz component belongs to a1 species the electron transfer to the nitrogen atom from the halide in both CzUand CJ,, symmetries. As shown in Table I, e ion which is electrostatically interactingwith TMA+.Methand t z species of Td symmetry give a1 species under CzU yl halide will desorb, then, leaving trimethylamine at the symmetry, while tz species only give a1 species under CaO surface. Electron transfer to TMA+ directly from the symmetry. The a1 vibrational modes thus derived from metal does not seem likely, since trimethylamine was not nontotally symmetric species for the adsorbed TMA+ will formed when KC1 was used as the supporting electrolyte. be markedly enhanced relative to those of the free Coadsorption of halide and TMA+ ions is necessary. molecule. That is what we have observed for the methyl rocking vibrations. Geometry of Adsorption. For the adsorption of The dependence on the choice of halide is interpreted (CH3)3N the geometry shown in Figure Id seems reasonas follows. When I- was used, TMA+ takes the form of CaU able. However, there are several possibilities for the symmetry as shown in Figure IC. Symmetric methyl adsorption of TMA+ as proposed in Figure la-c. Since rocking mode, which was originally a t z mode, is markedly TMA+ does not have lone pair electrons or unsaturated enhanced. When Br- was used, the adsorbed molecule is bonds, a strong electronic interaction with the metal seems in the form of CzUsymmetry as shown in Figure lb. In this improbable. Only electrostatic interaction via halide ions case large enhancementsoccur for the two symmetric methis likely. In this case a classical electromagnetic enhanceyl rocking modes, which were originally e and t 2 modes, ment mechanism will be playing a major role in the surface respectively. Raman enhancement. Let us now compare in detail the Raman spectra of Conclusions TMA+ in adsorption and solution states. When iodide Tetramethylammonium ion adsorbs electrostatically on was used as the supporting electrolyte, the methyl rocking halide ions remaining a t Ag electrodes. According to the mode (1283 cm-l in Figure 2a) of t z species is markedly choice of the halide, TMA+ took two different forms of enhanced at the surface, while the e mode (1171 cm-' in adsorption of CzUand CsUsymmetry. A classical surface Figure 4a) is not. The same thing is observed for C4N electromagnetic enhancement mechanism explains the deformation modes. The t z mode a t 456 cm-l in Figure difference between Raman spectra of adsorbates in 4a is more enhanced than the e mode at 372 cm-' for addifferent forms. sorbed TMA+ as seen in Figure 2a. Decomposition of the ion occurs toward cathodic potentials and trimethylamine is formed, which adsorbs (32) Albrecht, A. C.J. Chem. Phys. 1961,34, 1476. directly on the metal atom. Only a1 Raman bands are (33) Clark,R. J. H.; Dines, T. J. Angew. Chem., Int. Ed. Engl. 1986, observed with high intensities. A large contribution from 25, 131. surface resonance Raman enhancement mechanism is (34) Hirakawa, A. Y.; Tsuboi, M. Science 1975, 188, 359. suggested for this molecule. (35) Tsuboi, M.; Hirakawa, A. Y. J. Raman Spectrosc. 1976, 5, 75.
