J . Phys. Chem. 1990, 94, 2552-2556
2552
the estimated index elevation of the homogeneous phase would be about 40, which is approximately the upper limit of the range of these data. The phase rule allows the D-phase region to touch the Krafft discontinuity only at one composition, and only along this unique isopleth does the simple eutectoid phase reaction
L
+ X.2W
+T -7
D
occur. Along all other isopleths one reactant or the other is in excess, and only partial conversion of reactants to products will occur precisely at the temperature of the discontinuity. It was not possible to accurately define the D-phase composition at the Krafft discontinuity during this study. I t seems likely, however, that it lies close to 31.5%. This is based on (1) evidence (above) that the boundary at the dilute limit of this phase is nearly
vertical, ( 2 ) evidence from the DIT data that the concentrated D-phase boundary is strongly curved (Figure 4), and (3) the typical form of liquid crystal boundaries in this region for other surfactant-water systems.29
Acknowledgment. The assistance o f Dr. C. Marcott and the use of software for infrared spectral analysis provided by Prof. D. J. Moffatt (Canada National Research Council) is gratefully acknowledged. Drs. J. D. Oliver and T. J. Emge, and W. B. Broering, were of great assistance with respect to X-ray analyses. G. M. Bunke synthesized the samples used for this study. Assistance with modeling of the CTN data was provided by A. M. Marrer. (29) Ekwall, P. Advances in Liquid Crystals; Brown, G. H., Ed.; Academic Press: New York, 1975; pp 1-142.
Adsorption and Decomposition of Benzyl Phenyl Sulfide on Silver Surface Investigated by Raman Spectroscopy Yong Hyeon Yim, Kwan Kim,* and Myung So0 Kim* Department of Chemistry, College of Natural Sciences, Seoul National University, Seoul 151 - 742, Korea (Received: March 30, 1989; In Final Form: July 6, 1989)
The surface-enhanced Raman scattering (SERS) of benzyl phenyl sulfide adsorbed on silver surface has been investigated. Benzyl phenyl sulfide was found to adsorb on the surface via its sulfur atom. Photodecomposition of adsorbed benzyl phenyl sulfide produced the phenylthio moiety on the surface. Two different SER spectra were obtained depending on the presence of CI- or BH4- ions. These are interpreted as due to the orientational differences caused by the different surface potential and coadsorbed ions.
Introduction When a molecule adsorbs on metal surface, its Raman scattering may be enhanced tremendously. This surface enhancement of Raman ~catteringl-~ decreases very rapidly with the increase of distance from the surface. Hence, surface-enhanced Raman scattering (SERS) becomes a useful tool for the investigation of an adsorbate on the surface such as its structure, conformation, orientation, and binding mechani~m."'~ Also, photochemical reactions may be facilitated for the surface-adsorbate system due to the substantial amplification of the electromagnetic field on the s u r f a ~ e . ~ , ' ~ - ~ ~ Surface-enhanced Raman scattering of aromatic sulfides was first reported by Sandroff et aLzo It was proposed that the C S bonds of the adsorbed sulfides cleaved on silver surface under quite mild condition. Joo et a1.I0 carried out SERS investigation of aliphatic sulfides adsorbed on silver and concluded that their CS bonds did not cleave easily. In the SERS study of aromatic sulfides on silver surface by the same authors,21it was suggested that the decomposition of these molecules was due to surface photoreaction. In the present work, a rather detailed SERS investigation has been carried out for benzyl phenyl sulfide adsorbed on silver surface. The major purpose of the present investigation is to study further the factors affecting the SER spectral features for aromatic sulfides. Experimental Section The method for silver sol preparation and details of Raman scattering measurement were reported previously.22 Ethanolic solution of benzyl phenyl sulfide was added to the silver sol to the final concentration of M. When the aggregation of silver sol particles did not occur easily, a small amount of BaCI,
-
To whom all correspondence should be addressed.
0022-3654/90/2094-2552$02.50/0
was added to induce the aggregation. Poly(vinylpyrro1idone)(MW 360000) was added to stabilize the silver sol. Glass capillaries and a spinning cell spun at 3000 rpm were used as sampling device^.*^^*^ To observe the SER spectrum and the sol surface potential simultaneously, a liquid cell equipped with silver wire electrode and the saturated calomel electrode was used.24 ( I ) Chzng, R. K., Furtak, T. E., Eds. Surface-Enhanced Raman Scarrering; Plenum Press: New York, 1982. (2) Moskovits, M. Rev. Mod. Phys. 1985, 57, 783. (3) Moskovits, M. J . Chem. Phys. 1978, 69, 4159. (4) Adrian, F. J. J . Chem. Phys. 1982, 77, 5305. (5) Wokaun, A. Mol. Phys. 1985, 56, 1. (6) Sandroff, C. J.; Garoff, S.; Leung, K. P. Chem. Phys. Lett. 1983, 96, 547. (7) Joo, T. H.; Kim, K.; Kim, M. S. J . Phys. Chem. 1986, 90, 5816. (8) Joo, T. H.; Kim, K.; Kim, M. S. J . Mol. Struct. 1987, 158, 265. (9) Joo, T. H.; Kim, M. S.; Kim, K. J . Mol. Strucr. 1987, 160, 81. (IO) Joo, T. H.; Kim, K.; Kim, M. S. J . Mol. Struct. 1987, 162, 191. (11) Kwon, C. K.; Kim, M. S.; Kim, K. J . Mol. Struct. 1987, 162, 201. (12) Kwon, C. K.; Kim, K.; Kim, M. S. J . Mol. Struci., in press. (13) Gao, P.; Weaver, M. J. J . Phys. Chem. 1985, 89, 5040. (14) Loo, B. H.; Lee, Y.G.; Frazier, D. 0. J . Phys. Chem. 1985,89,4612. (15) Takahashi, M.; Furukawa, H.; Fujita, M.; Ito, M. J . Phys. Chem. 1987, 91, 5940. (16) Gersten, J. I.; Nitzan, A. Surf. Sci. 1985, 158, 165. (17) Goncher, G. M.; Harris, C. B. J . Chem. Phys. 1982, 77, 3767 (18) Chen, C. J.; Osgood, R. M. Phys. Reu. Lett. 1983, 50, 1705. (19) Goncher, G. M.; Parsons, C. A.; Harris, C. B. J. Phys. Chem. 1984, 88, 4200. (20) Sandroff, C. J.; Herschbach, D. R. J . Phys. Chem. 1982.86, 3277. (21) Joo, T. H.; Yim, Y . H.; Kim, K.; Kim, M. S.J . Phys. Chem. 1989, 93, 1422. (22) Joo, T. H.; Kim, K.; Kim, M. S. Chem. Phys. Lett. 1984, 112, 65. (23) Boo, D. W.; Oh, W. S.; Kim, M. S.; Kim, K.; Lee, H. C. Chem. Phys. Lett. 1985, 120, 301. (24) Blatchford, C . G.; Siiman, 0.; Kerker, M. J . Phys. Chem. 1983,87, 2503.
0 1990 American Chemical Society
Decomposition of Benzyl Phenyl Sulfide on Ag Surface
The Journal of Physical Chemistry, Vol. 94, No. 6,1990 2553 TABLE I: Vibrational Assignments for Benzyl Phenyl Sulfide neat
SERS type I
SERS type I1
215 230 280 303 343 410 471 569 618 674 698 709 (sh)b 768 780 806 816 1002 1027 1089 1118 1157 1181 1 I99 1238 1271 1583 1601
290
assignments
Ag-S stretching Ag-CI stretching
{ FSrLnding
766
698 763
7a (P)" 6a (B),16b (P) 16b (B) 6b (B,P) C-S stretching (B) 6a (P) 4 (B) 1 1 (B)
806
805
CH2 rocking (B)
1000
1000
1023 1069 1108 1156 1 I83 1201 1241
1022 1071 1112 1 I54 1180 1 I98 1238
1473
1469 1491 1570 1600
414 470 564 615 656 689
407 472 563 614 655 (sh)
1
1571 1600
(B)
Vibrational mode due to benzylthio (B) and phenylthio (P) moiety, respectively. bShoulder peak.
RAMAN S H I F T , cm-' Figure 1. (a) Ordinary Raman spectrum of liquid benzyl phenyl sulfide (BPS). (b) S E R spectrum of BPS obtained using a spinning cell. (c) SER spectrum of BPS obtained using a sealed glass capillary. (d) S E R spectrum of benzenethiol. (e) SER spectrum of benzyl mercaptan. Laser, 514.5 nm; power, 100-200 mW; spectral slit width, 5-10 cm-'.
Results and Discussion
The ordinary Raman spectrum of neat benzyl phenyl sulfide (BPS) and its SER spectrum obtained by using a spinning cell are shown in Figure 1, a and b, respectively. To assign the bands appearing in the ordinary Raman spectrum, vibrational assignments for various benzene derivativesZShave been referred to. Especially, the vibrational spectra of benzyl chloride are very similar to those of benzyl mercaptan and dibenzyl sulfide. Hence, in the ordinary Raman spectrum of BPS the bands in close proximity to those in the benzyl chloride spectrum were identified as the bands due to benzylthio moiety in BPS.25*z6 Vibrations due to the phenylthio moiety were indentified by referring to the vibrational assignments for ben~enethiol.~' The CSC bending mode of aliphatic and aromatic sulfides usually appear at around 300 cm-l. In this region of ordinary Raman spectrum of BPS, two bands appear distinctly, namely at 280 and 303 cm-'. Since the v 1 5mode for benzyl mercaptan was reported to appear in the same spectral region, the assignments for the above two bands are not certain. Vibrational assignments for the Raman spectrum of BPS are summarized in Table I. Table I shows SER spectra obtained under two different conditions which will be discussed later. It is sufficient to mention (25)Varsanyi, G.Assignmentsfor Vibrational Spectra of Seuen Hundred Benzene Deriuatiues; Wiley: New York, 1974. (26)Verdonock, L.;van der Kelen, G. P. Spectrochim. Acta, Part A 1972, 28A, 51. (27)Joo, T.H.;Kim,M . S.;Kim,K. J . Raman Spectrosc. 1987.18, 57.
at the moment that the SER spectrum designated type I is essentially the same as that in Figure 1b. Even though some of the bands in the S E R spectrum are shifted noticeably from the corresponding bands in the ordinary Raman spectrum, overall correlation between the two spectra is very decent, indicating that the chemical species responsible for the SER spectrum is adsorbed BPS. This is in agreement with the cases for other aromatic sulfides investigated previously, namely dibenzyl sulfide, benzyl methyl sulfide, and methyl phenyl sulfide.21 There are three groups in BPS through which the molecule may adsorb to the silver surface, Le., two benzene rings and the sulfur atom. Adsorption of the benzene derivatives on the metal surfaces through the aromatic a systems has been investigated previously.289z9 Upon R adsorption, benzene ring modes involving C-C stretching move to lower frequencies, due to the anticipated back-donation of electron density from metal surface to the R* antibonding orbitals of the benzene ring.28 In particular, Gao and Weaver reportedz8that the frequencies of v12 and vlga ring modes decreased by 10-15 cm-l upon adsorption of alkylbenzenes through their R systems. On the other hand, shifts by 4-5 cm-' for the corresponding modes of the halobenzenes were interpreted as evidences of no direct interaction between the system and metal surface. In addition, significant band broadening for the benzene ring modes was reported in the cases of a adsorption, caused by interaction between the benzene ring and metal surface.28 In the case of BPS, no significant band broadening for the ring modes was observed in the SER spectrum. Also the peak shifts for the ylz and vibrations upon surface adsorption were only 2-5 cm-l, indicating that neither of the benzene rings was in strong interaction with metal surface. Some of the ring modes show substantial peak shifts upon surface adsorption. For example, the v1 and v8, vibrations for the benzene ring of the phenylthio group moved to lower frequencies by around 20 and 12 cm-I, respectively, upon surface adsorption. Similar amounts of shifts for the corresponding bands were observed in the S E R spectrum of ben~enethiol.~'Since the v1 vibration for monosubstituted benzene (28) Gao, P.; Weaver, M. J. J . Phys. Chem. 1985,89, 5040. (29) Fritz, H . P.Ado. Organomet. Chem. 1984,160, 321.
2554
The Journal of Physical Chemistry, Vol. 94, No. 6, 1990
is very sensitive to the substituent mass,3oits substantial downshift was attributed to the change in effective substituent mass upon surface adsorption. The vga vibration downshifted by around 10 cm-' not only in the SER spectrum of benzenethiol but also in the Raman spectrum of silver benzenethiolate salt even though the exact cause for such shift is not clear at the moment. If BPS does not adsorb to metal surface via its benzene rings, the only remaining possibility is its adsorption through the sulfur atom. As evidence for this adsorption mechanism, the S-CH2 stretching vibration at 674 cm-I in the ordinary Raman spectrum is shifted to 656 cm-' in the SER spectrum. The magnitude of the shift is in good agreement with the cases for aliphatic sulfides which adsorb to metal surface via their sulfur atoms.I0 Hence, it can be concluded that BPS adsorbs to metal surface through its sulfur atom and that the aromatic rings are not in strong interaction with the surface. In the SER spectrum shown in Figure 1 b, a very intense peak appears at 230 cm-l which has no obvious counterpart in the ordinary Raman spectrum. Such bands in the low-frequency region are usually attributed to metal-molecule stretching modes.31 However, it has been observed in the present case that the intensity of this band relative to other bands in the SER spectrum changed substantially from spectrum to spectrum. Also, it has been found that the quality of SER spectrum is very good only when this peak appears prominently. It is well established that the Ag-CI stretching vibration appears in this spectral region.ls3* To check this possibility, a minute amount of Br- ion was added to the sample solution exhibiting SER spectrum shown in Figure 1b. Then, the peak at 230 cm-I disappeared completely and a new peak due to the Ag-Br stretching vibration33appeared at around 160 cm-'. More importantly, other spectral features in the SER spectrum of BPS remained almost the same. Hence it can be concluded that the bromide ion which has stronger adsorption strength on silver surface than the chloride ion3, replaced the latter on the surface and that the presence of halide ions was needed to observe SER spectra of BPS with decent quality. Since CIion was not added intentionally to obtain SER spectrum in Figure Ib, this must have originated from reagents such as water. Rather, it seems that of the so many samples used in our attempt to obtain the SER spectrum of BPS only those few contaminated with CIion produced decent SER spectra. The effect of coadsorbed halide ions on the SER spectra will be discussed in detail later. Sandroff et aLzo reported that diphenyl sulfide and dibenzyl sulfide decomposed to the corresponding mercaptides on the silver surface. On the other hand, according to the SERS investigation on the similar systems by Joo et al., such decompositions were found not solely due to the catalytic effect of the surface but also due to photoreactions.2' To investigate the possibility of photoreaction for BPS adsorbed on silver, SERS signal from the sample contained in a small-bore (- 1 mm i.d.) capillary was observed for prolonged time. The SER spectral feature of BPS was observed to change rather rapidly, resulting in a completely different spectrum about 5 min after laser irradiation. Such a spectrum is shown in Figure IC. All the peaks characteristic of the benzylthio moiety are completely missing in the spectrum. This spectrum is, however, essentially the same as the SER spectrum of benzenethiol shown in Figure Id. Disappearance of bands due to benzylthio moiety in Figure I C can be evidenced further in comparison with the SER spectrum of benzyl mercaptan shown in Figure le. It is possible that even though the benzylthio group as well as the phenylthio group might have been produced by photodecomposition of BPS the band due to the latter group appeared preferentially in the SER spectrum because the phenylthio group adsorbed more favorably on the surface. To check this possibility SER spectrum was obtained for a 1: 1 mixture of benzenethiol and benzyl mercaptan. It was observed that the (30) Varsanyi, G. Vibrational Spectra of Benzene Derivatives; Academic Press: New York, 1969 (31) Loo. B. H. Chem. Phys. Lett. 1982, 89, 346. (32) Roy, D.; Furtak, T. E. J . Electroanal. Chem. 1987, 228, 229. (33) Wetzel. H.; Gerischer, H. Chem. Phys. Lett. 1980, 76, 460. (34) Weaver. M . J.: Huff, J. T.J . Electroanal. Chem. 1984, 160, 321.
Yim et ai.
\
I
T I M E , min
Figure 2. Electrochemical potential at Ag wire electrode vs SCE as a function of time after the addition of NaBH, to the silver sol. The initial concentrations of NaBH, in silver sol are ( 0 )1.0 X IO4 M, (0) 5.0 X 104 M, (A) 1.0 x 10-3 M, (A) 5.0 x 10-3 M, (x) 1.0 x io-* M.
bands due to benzyl mercaptan were stronger than those due to benzenethiol. This shows that the predominance of phenylthio bands in the SER spectrum of photoreaction products is not due to the favorable adsorption of phenylthio moiety. Hence, it can be concluded that the photodecomposition of adsorbed BPS produces phenylthio group on silver surface. As mentioned previously, two different types of SER spectra were observed depending on the sol condition. It has been found experimentally that this spectral difference was caused by difference in borohydride ion concentration in the sol. According to Wetzel et and Blatchford et aI.,*, the borohydride concentration in the sol affects the surface potential of the silver colloidal particles. Figure 2 shows the effect of NaBH, addition on the potential at the silver electrode dipped in the sol solution relative to a saturated calomel electrode (SCE). Even though the potential thus measured cannot be the same as the effective potential on the colloidal surface, Wetzel et al. have shown that it approximates the effective potential very closely. The SER spectrum of BPS was observed in a silver sol solution containing M borohydride ion. Around lov4M of BaCI2 was added to induce aggregation of the colloidal particles.36 The potential of the silver sol changed from around 0.0 V to -0.24 V vs SCE when this amount of NaBH, was added. The SER spectrum of BPS obtained from such a silver sol solution is shown in Figure 3b. For comparison, the SER spectrum of BPS obtained from a silver sol solution without additional NaBH, is shown in Figure 3a which is virtually the same as the spectrum shown in Figure 1b. The SER spectra in parts a and b of Figure 3 will be called type I and type I1 spectra, respectively. As mentioned above, type I1 SER spectrum was observed upon the addition of NaBH,. Then, as time lapsed, the decomposition of borohydride ionz4increased the sol potential as shown in Figure 2. At the potential around -0.03 to -0.01 V vs SCE, spectral features began to change, resulting in type I spectrum eventually. The interconversion between type I and type I1 spectra was found reversible. Namely, additional injection of NaBH, after the sol reached the condition showing type I spectrum restored the SER spectrum of type 11. The reversibility held down to the sol potential of around -0.3 V vs SCE. When the sol potential was made more negative than -0.3 V, BPS decomposed, yielding the SER spectrum of adsorbed benzenethiolate as shown in Figure 3c. Even though relative intensities for some bands were remarkably different between the type I and type I1 SER spectra, the peak positions and widths of the most of the bands were hardly different (35) Wetzel, H.; Gerischer, H.; Pettinger, B. Chem. Phys. Lett. 1982,85, 187. (36) Zhang, C.; Yu, F.; Zhang, G. J . Raman Spectrow., to be submitted.
Decomposition of Benzyl Phenyl Sulfide on Ag Surface
The Journal of Physical Chemistry, Vol. 94, No. 6, 1990 2555
Figure 4. Two possible orientations of adsorbed BPS.
RAMAN SHIFT, cm-' Figure 3. SER spectra of BPS (5.0 X M) in silver sol: (a) without M NaBH,, and (c) with 1.0 X additional NaBH,, (b) with 1.0 X IO-* M NaBH,. All the samples contained 5.0 X lo4 M BaCI2. Laser, 514.5 nm; power, 100-200.mW; spectral slit width, ca. 10 cm-I.
in the two spectra. Other than the changes in relative intensities, the major difference between the two spectra is observed in the low-frequency region. Namely, the Ag-C1 stretching vibration at 230 cm-l in the type I spectrum is missing in the type I1 spectrum. Instead, two new bands appear at 215 and 290 cm-' in the type I1 spectrum. The band at 290 cm-I may be assigned either to the v I s vibration of benzylthio moiety or to the CSC bending vibration. However, the very broad nature of this band makes it more likely to assign this as a composite band of above two vibrations. The band at 215 cm-' in the type I1 spectrum may be attributed to the Ag-S stretching vibration which might have been buried under the Ag-C1 stretching band at 230 cm-I in the type I spectrum. As mentioned above, the peak positions for the corresponding bands are very similar in the two S E R spectra except in the low-frequency region. Hence, based on the same argument used in the interpretation of type I spectrum it may be concluded that type I1 spectrum is also due to adsorbed BPS. Some of the bands in type I1 spectrum show much stronger relative intensities than the corresponding bands in type I spectrum. These bands are the S-CH2 stretching band at 655 cm-', CH2 twisting at 1238 cm-I, and the v I 1 , ~ 1 3 ,and vs, bands of the benzylthio moiety at 763, 1198, and 1600 cm-', respectively. In addition, a composite band assignable to CSC bending vibration and v I 5 vibration of the benzylthio moiety appears prominently in the type I1 spectrum, as mentioned above. Changes in the relative intensities of SER bands may be explained by one or a combination of the following effects: (i) reaction on the surface, (ii) charge-transfer resonance, (iii) reorientation of adsorbates, (iv) effect of the coadsorbed species, and (v) effect of the surface potential. Since the type I1 spectrum correlated very well with the ordinary Raman spectrum of BPS and the interconversion between the two SER spectra was reversible, the first possibility may be safely ruled out. If the
charge-transfer resonance is responsible for the spectral change, the spectral pattern is expected to change when the wavelength of the incident light is varied.'^'' The experimental observation that the type I1 SER spectrum did not change when other lines of argon ion laser output were used seems to rule out the second possibility. The remaining three effects may not be independent but interrelated. For example, a change in the surface potential may change the surface concentration of the coadsorbed species which, in turn, may change the orientation of the adsorbates. It is well-known that the changes in the surface condition can induce the ~ r i e n t a t i o n a l or ' ~ c ~ n f o r m a t i o n a l ~ - ~change J ' J ~ of the adsorbates. We will attempt first to see if the observed spectral change can be explained by the reorientation of the adsorbates. According to the electromagnetic surface selection rule proposed by Moskovits and Suh3' and by Creight01-1,~~ a vibrational mode will show strong surface enhancement when it has a large polarizability component perpendicular to the surface. Two different orientations, designated a and 6, of BPS on the surface are shown in Figure 4. When BPS assumes a-type orientation on the surface, the S-CH, bond lies almost parallel to the surface. Then, the S-CH2 stretching vibration which is expected to have a large polarizability derivative component along the bond direction cannot be strongly enhanced compared to the other vibrations. By similar arguments, the twisting vibration of benzylic CH2, CSC bending vibration, and the v15 vibration of benzylthio group are not expected to be strongly enhanced for adsorbates with a-type orientation. This description is in agreement with the spectral features of type I spectrum. On the other hand, all the vibrational modes mentioned above can be more or less strongly enhanced when the adsorbate assumes 6-type orientation in agreement with type I1 spectrum. Even though the pictures presented here are qualitative only, the above arguments suggest the feasibility of interpreting the different features in the type I and I1 SER spectra as due to the difference in surface orientation of the adsorbates. The fact that the Ag-S stretching bands in the SER spectra of BPS are not so much intense as expected from the surface selection rule would be attributed to their very small intrinsic Raman cross sections. For instance, we could not observe any distinct Ag-S stretching bands in the ordinary Raman spectra of both the silver benzenethiolate and the silver benzyl mercaptide salts. Considering the fact that the type I spectrum is obtained only when the Ag-Cl stretching vibration is observable, it seems likely that the orientation of the adsorbate described above is determined by the presence or absence of coadsorbed chloride ions on the surface. The effect of the surface potential may then be to control the adsorption of chloride ions on the surface. As a matter of fact, the surface coverage of halide ions is known to decrease as the surface potential decreases. The surface concentrations of various halide ions as a function of electrode potential have been reported by Weaver et al.34 The adsorption strengths of halide ions decrease in the following sequence: I- > Br- > C1-. At -0.2 (37) Moskovits, M.; Suh, J. S. J . Phys. Chem. 1984, 88, 5526. (38) Creighton, J. A. Surf. Sci. 1983, 124, 209.
2556
Yim et al.
The Journal of Physical Chemistry, Vol. 94, No. 6, 1990
I
I
, \
1500 RAMAN SHIFT, cm-' Figure 5. SER spectra of BPS (5.0 X lo-' M) in silver sol: (a) with 1.O X M NaBH,, (b) with 1.0 X lW3 M NaBH, and 1.0 X lo4 M KBr, (c) with 1.0 X M NaBH, and 1.0 X IO4 M KI, and (d) without NaBH4. All the samples contained 5.0 X IO4 M BaCI,. Laser, 514.5
500
IO00
nm; power, 100-200 mW; spectral slit width, ca. 10 cm-'.
V vs SCE, the surface coverage of C1- is about 50% while the same degree of coverage is achieved at around -0.8 V for Br-. In the case of iodide ion, the adsorption strength is so high that full monolayer coverage is achieved even at -0.9 V vs SCE. To investigate further the effect of coadsorbed halide ions on the SER spectral pattern, the SER spectra were obtained from the sol
solutions containing M of either bromide or iodide ions. Surface potential was maintained at around -0.2 V in each sol solutions by adding NaBH,. The SER spectra thus obtained are shown in Figure 5, b and c, respectively. For comparison, type I and type I1 SER spectra are shown in Figure 5, d and a, respectively. It is to be mentioned that all the spectra shown in Figure 5 were obtained from the sol solutions containing 5 X lo4 M CI-. As described previously, Figure 5d M BaCI,, Le., is the SER spectrum of BPS adsorbed on the sol surface covered with CI- ions. As the surface potential is reduced to -0.2 V vs SCE, 50% of C1- ions desorb from the surface and the type 11 spectrum shown in Figure 5a is observed. At this surface potential, bromide and iodide ions can adsorb on the surface efficiently. Indeed, the SER spectrum obtained at -0.2 V in the presence of Br- looks halfway between type I and type I1 spectra. The SER spectrum obtained in the presence of I-, Figure 5c, resembles type I spectrum (Figure 5d) more closely. Thus, it seems that the SER spectral features of BPS are mainly affected by coadsorption of halide ions and that the surface potential affects the spectral features indirectly through controlling halide ion adsorption. It is not clear at the moment how the coadsorbed chloride (halide) ions influence the orientation of BPS adsorbed on silver sol surface. One plausible explanation is the formation of specific adsorption sites on the surface when the chloride ions are present. Then, BPS molecules adsorbed on these specific sites assume a-type orientation. The formation of specific structure on silver surface by chloride ions has been indeed p r o p o ~ e d . ~At ~ , more ~~*~ negative potential, desorption of chloride ions may result in the collapse of these specific sites. Then, BPS may adsorb on different sites on the surface. Alternatively, changes in the adsorbate orientation may be due to the interaction among adsorbed BPS molecules. When the surface is densely populated with BPS, the @-typeorientation may be favored since it occupies less surface area than the a-type orientation. On the other hand, when most of the surface sites are occupied by CI-, interaction among adsorbed BPS molecules may become less important, enabling the flat stance on the surface as the a-type orientation. Nevertheless, it is thought that more detailed investigation in which surface potential and morphology can be controlled reproducibly is needed for better understanding of the SER spectra of BPS. A SERS study of BPS using silver electrode system is being attempted for this purpose.
Acknowledgment. This work was supported by Daewoo research fund administered by Seoul National University. D.Surf. Sci. 1985, 158, 126. (40) Roy, D.; Furtak, T. E. J . Chem. Phys. 1984, 81, 4168. (39) Furtak, T. E.: Roy,
