J . Phys. Chem. 1991, 95, 793-798
793
Fourier Transform Raman Spectra and the Structure of Adsorbed 2,2'-Cyanine D. L. Akins,* J. W. Macklin,+ and H.-R. Zhu Department of Chemistry, The City College of The City University of New York. New York, New York 10031 (Received: November 30, 1989; In Final Form: July 16, 1990)
Near-infrared excited FT Raman spectra of 2,2'-cyanine adsorbed onto a smooth silver electrode and of polycrystalline 2,2'-cyanine are compared with dispersive Raman spectra obtained by using visible excitation to determine adsorbate structural characteristics. Raman spectra are interpreted as indicating that 2,2'-cyanine exists as a structural composite of polycrystalline and two intermolecular arrangements of cyanine monomers forming the J-aggregate. This structural model is shown to explain Raman intensity variations that depend upon dye concentration, supporting electrolyte, pH, and electrode potential. Variations T Raman relative band intensities over a surface potential range in which the surface concentration increases indicate of the l that one of the structural conformers in the J-aggregate is thermodynamically favored at more negative potentials.
I. Introduction Substantial progress has been made in this laboratory in explaining and exploiting the enhanced Raman scattering by aggregated dye molecules on, principally, smooth silver electrodes. Studies have led to a Raman enhancement theory based on the existence of the exciton state:' a method for using excitation frequency dependency of Raman band intensities to determine dipoltdipole interaction energies between molecules when incident radiation is tuned through resonance with the exciton band;* a method for using the surface potential dependency of Raman band intensities to determine excited-state relative dipole moments when resonance tuning is effectuated by applied surface ~ t e n t i a land ;~ a model involving two conformers of 2,2'-cyanine (spec., 1,l'diethyl-2,2'-quinocyanine iodide, whose structure is shown in Figure 1) from the conflation of pH, excitation wavelength, and potential dependencies which enables us to postulate two structural arrangements within the J-aggregate.4 All of the above studies are impacted by current work in our laboratory in which FT Raman spectrometric measurements are applied to our systems: The FT Raman spectra that are reported here were obtained by using a CW Nd:YAG laser at 1.064 pm (9395 cm-') and a FTIR spectrometer. Earlier Raman scattering investigations utilized visible excitation which, perforce, imbued some level of resonance Raman response in the spectra. In the present study 2,2'-cyanine is the focus, and we find particular advantage, in relation to the interpretation of spectra, in the lack of absorption of the near-infrared laser frequency by the dye and the consequent low cross section for resonance Raman enhancement and fluorescence. The efficacy of the combination of a near-infrared laser for excitation and a FTIR spectrometer for acquisition of Raman spectra has been widely discussed in the literature.s-'o We do not rehash the pros and cons here. The technique, however, offers some distinct benefits in the study of adsorbed aggregated dyes. In particular, while resonance Raman and SERS enhancements are unlikely to occur, "aggregation enhancement" should still be operative,' leading to increased Raman band intensities, since nonresonance Raman theory involving summation over (unoccupied) excited which must include the unpopulated excitonic substates, and polarizability additivity associated with the size of the aggregate should still apply.' Thus, upon application of Herzbern-Teller couolinn of excited states and for nonresonant scattering,-one finds, is dlveloped in ref 1, that, in addition to an additivity factor proportional to the number of molecules comprising the aggregate, the polarizability of the aggregated structure would exhibit near-resonance components associated with the small energy spacings between the molecular aggregate state and other states. Two consequences of the combined effects of aggregation enhancement and absence of absorption (and the concomitant resonance Raman scattering) that are taken advantage of in this Department of Chemistry,University of Washington, Seattle, WA 98195.
0022-365419 112095-0793$02.50/0
work are (a) intensities can be more readily related to amounts of various constituents of a mixture and (b) many bands that have weak intensities in resonance Raman enhanced measurements have greater relative intensities with near-infrared excitation. This latter consequence, the greater relative intensity of some of the bands in the FT Raman spectra, principally due to the absence of resonance, may also stem, in part, from both the scattering geometry used (spec., backscattering in the case of FT Raman, as opposed to a perpendicular configuration for scanning Raman) and the greater reflectivity of the smooth substrate at red wavelengths.'~" This paper describes the use of near-infrared excited FT Raman spectra in combination with Raman spectra obtained with visible radiation to gain enhanced detail about the structural character of 2,2'-cyanine adsorbed onto a silver electrode from solutions of high and low pH. The structural suggestions are then further supported by the explicit accounting for other previous as well as current observations from this laboratory. Finally, the structural model and proposed makeup of the adsorbate are applied to explain the effects of electrode potential on relative band intensities in FT Raman spectra. 11. Experimental System and Procedures
Both scanning Raman and FT Raman instruments have played a prominent role in the present study. The scanning Raman instrumentation consisted of a SPEX 1404 double monochromator with photomultiplier detection and has been thoroughly described in earlier publications from this laboratory.3J4 The near-infrared excited Raman measurements were conducted with a Bomem DA 3.16 FT Raman instrument. In these latter measurements, Raman scattering was collected in a backscattering configuration in which an on-axis ellipsoidal mirror (supplied with a hole allowing passage of incident excitation) reflected and focused the scattered radiation in a 90° direction into the entrance aperture of the interferometer. The incident, unpolarized near-infrared radiation at 1.064 bm was (1) Akins, D. L. J . Phys. Chem. 1986, 90, 1530. (2) Akins, D. L.; Lombardi, J. R. Chem. Phys. Lett. 1987, 136, 495. (3) Akins, D. L.; Akpabli, C. K.; Li, X. J . Phys. Chem. 1989, 93, 1977. (4) Akins, D. L.; Macklin, J. W. J . Phys. Chem. 1989, 93, 5999. (5) Hirschfeld, T.; Chase, E. Appl. Spectrosc. 1986, 40, 137. (6) Zimba, C. G.; Hallmark, V. M.; Swalen. J. D.; Rabolt, J. F. Appl. SDectrosc. 1987. 41. 721. ' (7) Parker, S.F.;Williams, K. P. J.; Hendra, P. J.; Turner, A. J. Appl. Soectrosc. 1988. 42. 196. ' (8) Angel, S.'M.; Katz, L. F.; Archibald, D. D.; Lin, L. T.; Honigs. D. E. Appl. Spectrosc. 1988,42, 1327. (9) Lewis, E. N.; Kalasinsky, V. F.; Levin, I. W. Appl. Spectrosc. 1988. 42, 1188. (10) Chase, D. B. J . Am. Chem. Soc. 1986, 108, 7485. (1 1) Albrecht, A. C. J . Chem. Phys. 1961, 34, 1476. (12) Moskovits, M. J . Chem. Phys. 1982, 77,4408. (13) Creighton, A. J. Advances in Spectroscopy: Spectroscopy of Surfaces; Clark, R. J. H., Hester, R. E., Us.John ; Wiley & Sons: New York. 1988; Vol. 16, pp 37-89. (14) Gu, B.; Akins, D. L. Chem. Phys. Lett. 1984, 105, 263.
0 1991 American Chemical Society
794 The Journal of Physical Chemistry, Vol. 95, No. 2, 1991
Figure 1. Structure of 2,2'-cyanine. Carbon atoms are represented by dark balls, nitrogen by spotted balls, and hydrogen atoms by white balls. The nitrogen showing four bonds has a net positive charge. The quinoline groups are twisted with respect to one another about the methine carbon, thus minimizing steric overlap of hydrogens below the methine carbon. The iodide counterion is not shown.
supplied by a TEM,,,, CW 4-W, Quantronix Model 114 Nd:YAG laser: usually about 1 W of power was focused through the ellipsoidal mirror onto the sample. A CaF, beam splitter was used in the interferometer stage, and a liquid N 2 cooled indium-galliumarsenide (InGaAs) photodiode was used as the detector. Two notch filters at the input, and two more placed in front of the detector, served to attenuate the laser fundamental. For both visible Raman and near-infrared FT Raman studies, spectra were acquired from cyanine dye ions adsorbed onto the working electrode of a three-electrode electrochemical cell. The cell consisted of a silver working electrode, Pt counter electrode, and saturated calomel electrode (SCE). The working electrode was polished with alumina, rinsed with distilled water, and sonicated for about l min, resulting in a visually polished electrode. Only polished electrodes, not pretreated as customary for surface-enhanced Raman scattering (SERS) studies, were used in the present investigation. FT Raman spectra presented here have been background corrected by computer interactive stripping of scattering profiles from systems composed of all components, at appropriate concentrations, save the adsorbed dye, or by using computer-generated line segments. The nominal resolution of spectral bands in FTRaman spectra, in most studies, was set at ca. 4 cm-I, while the precision of the frequencies is somewhat greater since the interferometric instrument utilizes the frequency of the helium-neon laser to determine moving mirror position.I0 Raman bands obtained by using visible excitation have a resolution of ca. 2 cm-' and a frequency uncertainty of at least f 4 cm-I. Solutions and chemicals were prepared and purchased as indicated earlier,I4 the supporting electrolyte, when used, was 0.1 M KCI, and NaOH and HCI were used to adjust pH. It is to be noted that no electrochemical reactivity of the dye occurred over the full surface potential range used in these studies. 111. Results and Discussion The Raman spectrum of polycrystalline 2,2'-cyanine is difficult to obtain by using visible radiation due to fluorescence and photodecomposition of the dye. However, we have easily acquired spectra of polycrystalline samples, as shown in Figure 2A, using the Bomem FT Raman instrument. We have also measured the FT Raman spectrum of 2,2'-cyanine adsorbed onto a smooth silver electrode at a potential of -0.6 V vs SCE from a 5 X lo4 M, pH = 12 solution of the dye. This spectrum is shown in Figure 2B. While the relative intensities of bands of the polycrystalline sample in Figure 2A differ considerably from those of the adsorbed dye in Figure 2B, indicating that the scattering species are different, the very good match of frequencies, as shown in Table I, points out general structural similarity. Moreover, the similarity in relative intensities of bands in Figure 2A given by the polycrystalline sample, such as those at 607, 1242, 1388, and 1438 cm-I, compared with those a t 606, 1241, 1384, and 1440 cm-I in Figure 2B for the electrochemical sample is suggestive that part of the electrochemical sample has the structure of crystalline
Akins et al.
&V (m.3 F i p e 2. Near-infrared excited FT Raman spectra of 2,2'-cyanine. Full scale relative intensities are 300:l:l for (A), (B),and (C), respectively. (A) IT Raman spectrum of polycrystalline dye in a melting point ca-
pillary. Instrument resolution was set at 1 cm-l, laser power was 1 w, IT scan range was set from 10 to 3500 cm-I, and 400 scans were accumulated (taking ca. 13 min). (B) Raman spectrum of dye adsorbed onto a smooth silver electrode from a solution of 5 X 10-4M dye, 0.1 M KCI, and 0.01 M NaOH, at an electrode potential of -0.6 V vs SCE. Spectrum was excited by using 1 W of 1.064-pm radiation, has a nominal resolution of 4 cm-l, and was acquired in 400 scans. (C) FT Raman spectrum of dye adsorbed onto a smooth silver electrode from a solution of 5 X lo4 M dye, 0.1 M KCI,and 1 M HCI, at an electrode potential of -0.8 V vs SCE. Spectrum was excited by using 1 W of 1.064-pm radiation, has a nominal resolution of 4 cm-l, and was acquired in 400 scans. material. On the other hand, the presence of several bands in the spectrum of the electrochemical sample, for example at 670,976, 1084, 1173, 1286, 1364, and 1470 cm-I, which are absent or only weakly present in the polycrystalline spectrum, is taken as indicative that another adsorbed scattering entity besides the crystalline material has a significant concentration on the electrode. A number of experimental observations, including Raman and UV/vis measurements, indicate that J-aggregated 2,2'-cyanine is a major adsorbate constituent.'"16 Raman spectral evidence, supporting the presence of J-aggregated dye on the electrode, is obtained from comparison of the FT Raman spectra in Figure 2A,B with the ET-Raman spectrum shown in Figure 2C for the adsorbate deposited onto a silver electrode from a pH = 0 solution. This latter spectrum is expected to exhibit increased relative intensity of J-aggregate bands when compared with Figure 2B since the spectrum of protonated molecules is not resonantly enhanced by near-infrared excitation, and J-aggregate is apparently more difficult to protonate than other structural forms of the dye.17 Those bands noted above as not being prominent in Figure 2A, but present in Figure 2B spectrum, show significant intensity in Figure 2C, implicating J-aggregated dye (as suggested above) as the other structural species that gives rise to bands in Figure 2B.'34J7918 The weak appearance of some of these same bands in the spectrum of polycrystal is taken to indicate the inclusion of a small amount of J-aggregated dye on the polycrystalline grains, though the possibility of splitting due to crystal symmetry, especially for bands between 1300 and 1400 cm-', cannot be ignored. In order to further bolster the evidence for J-aggregate dye adsorbed onto the electrode, we acquired Raman spectra of 2,2'-cyanine adsorbed onto a silver electrode at a potential of 4 . 6 V vs SCE from a 5 X 10" M solution, using visible excitation and the scanning instrumentation mentioned earlier, for com( 1 5 ) Marchetti, A. P.; Salzberg, C. D.; Walker, E. I. P. J . Chem. Phys. 1976,64,4693. (16) Marchetti, A. P.;Salzberg, C. D.; Walker, E. I. P. Phorogr.Sci. Eng. 1976. 20, 107. (17) Herz, A. H.Adu. Colloid. Interfacial Sci. 1977, 8, 237. (18) Kasha, M. Radiar. Res. 1963, 20, 5 5 .
FT-Raman Spectra of Adsorbed 2,2’-Cyanine
The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 795 TABLE I: Ilr-
B8f&
Due to Cry~trluwmd J - A m a d
f2’Cydw
do
‘
& I ’
i
’
Id00
‘
ldw
’
14oo~leoo
AV (an.’)
Figwe 3. Visible radiation excited dispersive spectra of 2,Y-cyanine. Full scale relative intensities are 1:15:7 for (A), (B), and (C), respectively. (A) 647.1-nm krypton ion laser excitation of Raman spectrum of dye adsorbed from a solution of 5 X lod M dye, 0.1 M KCI, and 1 M HCI, at an electrode potential of -0.6 V vs SCE. Spectral resolution was ca. 2 cm-l. (B) 488.0-nm argon ion laser excitation of the same sample as used in (A). (C) 647.1-nm excitation of the dye adsorbed onto a silver electrode from a solution of 5 X IOd M dye, 0.1 M KCI, and 0.01 M NaOH, at an electrode potential of -0.6 V vs SCE.
parison with the FT Raman spectra in Figure 2. The spectrum shown in Figure 3A is of the dye adsorbed from a solution of pH = 0 and with 647.1-nm excitation. This excitation is expected to project out J-aggregate bands, as though other species were absent, because of preresonance Raman enhancement of J-aggregate bands, the absence of resonance enhancement by protonated species since there is no absorption near 647.1 nm, and the difficulty of protonating the J-aggregate (vide supra). The spectrum in Figure 3A, though relative band intensities are significantly modified by resonance scattering, compares favorably with the FT Raman spectra of Figure 2B,C, in that most of the bands listed above as indicative of the presence of J-aggregate are present, further supporting the inclusion of the J-aggregate in the electrode adsorbate. Figure 3B is a spectrum taken at the same spot on the same sample as that in Figure 3A, but with 488-nm excitation. It is included to emphasize the observation that the spectrum seen clearly with 488-nm excitation and attributed to the protonated ion is not strongly apparent in the Figure 3A spectrum obtained by using 647.1-nm excitation. Also, comparison of these two latter spectra points out the significant spectral change that occurs upon protonation. Finally, Figure 3C, the spectrum of 2.2’-cyanine adsorbed onto a silver electrode from a solution of pH = 12 and with 647.1-nm radiation for excitation, is provided for comparison with the Figure 3A spectrum at pH = 0. This spectrum is expected to, and dues, exhibit J-aggregate as well as polycrystalline dye modes since at high pH polycrystalline dye is a major constituent of the adsorbate. This spectrum is also quite similar to that of the FT Raman spectrum shown in Figure 2B that was taken under the same conditions when it is recognized that some of the bands in Figure 3C are probably r m n a n c t enhanced. It is concluded from the above observations that the deposit on a silver electrode is a structural composite of principally polycrystalline and J-aggregated 2,2’-cyanine. This conclusion is somewhat more specific than than indicated in an earlier report from this laboratory which stated that the adsorbate was a mixture of effectively monomeric (e.g., weakly aggregated or aggregate terminating) and J-aggregated 2,2’-cyanine molecules whose composition depended on concentration of the dye in solution, the nature of the supporting electrolyte, the solution pH, and the electrode potential.’ We are in essence, in this paper, acknowledging that the more easily protonated species that gives rise to the 1384-cm-1 band is the polycrystalline species. The greater
J-sggrcpd crystallinea J-aggngateb crystalline int bv, cm- int Au. cm-’ int Av, cm-l int Ar, cm-l 462 0.1 995 0.1 996 0.5 1041 4 0.5 0.2 1039 493 498 0.1 1042 0.4 she 1061 Ish 1064 0.1 506 0.2 1086 0.1 1083 1 529 0.2 540 0.7 1126 0.8sh 1126 2 571 0.5 1133 0.9 581 0.1 1148 0.3 I152 0.3 607 0.4 606 0.6 1167 0.3 644 0.2 sh 1173 2 0.1 649 0.1 1186 670 0.5 1224 3.2 1224 3 709 0.5 707 4 1242 1.8 1241 2 729 0.1 734 0.8 1292 0.1 sh 1286 3 0.2 1350 6 1348 9 742 1357 0.1 sh 748 0.1 0.3 1364 10 758 0.2 1368 788 0.2 786 0.3 1388 10 1.8 1440 1 815 0.2 1438 1470 1 844 1 846 6 0.6 1515 3 865 0.4 1514 892 0.1 889 0.2 1554 0.2 1556 0.4 946 0.4 1568 0.6 1567 3 952 0.1 1607 0.9 1607 5 976 0.4 1625 0.8 984 0.1 a Wavenumber values from the FT Raman polycrystalline spcctrum measured at 1-cm-’ resolution. Bands with relative intensities less than 0.05 are not included in table. * Wavenumber values from FT Raman measure ments. Spectral resolution set at 4 cm-I. CShoulder. (A) ci, m
~ 1561m ai’
Figure 4. (A) Cis conformer of 2,2’-cyanine involved in dimeric interaction with another cis conformer. (B) Trans conformer of 2,Z’cyanine involved in dimeric interaction with another trans conformer. Unsaturated nature of 2,2‘-cyanine is not shown in the figure. J-aggregate is formed by linear continuation of the dimeric interactions. Note that the transverse relationship between ethyl groups in overlapping quinoline groups is suggested from crystallographic studies (see ref 19).
insight provided by the combination of FT Raman and visible Raman measurements leads to this more detailed structural picture. On the basis of the above interpretation of the data in Figure 2, we can assign several bands solely to J-aggregate or polycrystalline material. Table I provides the band frequencies attributable to the two adsorbate constituents and indicates those bands that are not intense in the spectrum of the other constituent. Such information is useful for indicating the relative surface concentrations of the two scattering species. For example, relative intensities in the FT Raman of the adsorbed dye in Figure 2B suggest that the major constituents are present in nearly equal amounts. Most of the frequencies attributed in Table I to polycrystalline and J-aggregate material do not differ significantly, as is to be expected since the structure, shown in Figure 4, advanced from this laboratory for J-aggregated 2,2’-cyanine incorporates the face-teface r association between quinoline rings which has been
Akins et al.
796 The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 shown to be present in the ~ r y s t a l . ~ . A ' ~ second, more weakly overlapping A association found in the crystal is not expected to be favored in the linear, polymeric aggregate which manifests molecular exciton formation, thus strongly suggesting significant overlap of quinoline A orbitals. Figure 4 shows two arrangements of the ethyl groups on the quinoline moieties leading to the cis and trans conformers, whose existence has been advanced to explain the presence of various bands in the Raman ~ p e c t r u m . ~ The above more detailed conclusion about the composition of the adsorbate on the silver electrode must be consistent with known dependence of Raman band intensities upon chemical environment and electrode potential that have been reported from this laboratorye4 In ensuing discussions we will show that such measurements can be fully explained so as to strongly support the structural model. Dye Concentration Effect. Change of the dye concentration in the electrolyte solution leads to change in the relative intensities of several bands in the Raman spectrum of 2,2'-cyanine! Groups of bands increase or decrease in intensity relative to one another. Such changes are rationalized, through our adsorbate model, as due to a complex equilibrium involving two structural arrangements for adsorbed J-aggregated dye (described previ~usly)~ as well as involvement of the polycrystalline adsorbate. We infer that when a soluble halide is used as supporting electrolyte, the halide ions are attracted to the silver electrode for potentials more positive than -0.9 V vs SCE, and, in turn, attract the positive 2,2'-cyanine ions.*O At higher dye concentrations, the solubility product of the dye halide is more strongly exceeded and the proportion of crystalline deposit is high: J-aggregation is known to be a slow process with a reaction order of ca. 5,*l while crystal formation is likely diffusion controlled. Thus, crystal formation dominates at higher concentrations and the equilibrium with the J-aggregate is expected to be established at a slower rate, resulting in a gradual increase in the proportion of J-aggregate bands manifest in Raman spectra. At very low dye concentrations (lC7-I0-8 M), the solubility product at the electrode is not strongly exceeded and J-aggregation is not outpaced by crystallization. Supporting Halide Identity Effect. The amount of dye adsorbed onto the electrode (as measured by looking through the collection optics onto the electrode) is found to depend on the particular bulk halide counterion and its concentration (see also ref 14). The order observed for increasing quantity of adsorbate for a given concentration of supporting halide is that iodide gives more adsorbate than does bromide than does ch10ride.I~ This observation is understood in terms of solubility product driven crystallization in the order I- > Br- > CI-. pH Effect. Replacement of the high-pH supernatant from which 2,2'-cyanine has been adsorbed at -0.6 V vs SCE by a 1 .O M HCI, 0.1 M KCl solution, using visible excitation, leads to diminution or complete ablation of several Raman bands, while new bands appear at nearby frequencies. The intensity of the full spectrum is found to decrease by about an order of magnitude in a few hours. The resultant new spectrum is the same as that given by deposits from solutions at low pH. The bands that are rapidly and completely ablated at low pH are among those that were identified above as due to polycrystalline material. This result indicates that all components of the original adsorbate are affected by contact with the low-pH solution and that crystalline adsorbate is much more rapidly and completely removed from the electrode as a result of protonation. The overall intensities of the FT Raman measurements shown in Figure 5 , the surface potential dependency for the low-pH system, along with Figure 3A,B, also support the conclusion that less material is adsorbed from solution at low pH. It can be surmised from the low intensity of bands attributable to protonated dye in Figure 5A,B, as well as Figure 3A, that only a small amount of protonated material is adsorbed onto the electrode at potentials (19) Yoshioka, H.; Nakatsu, K. Chem. Phys. Lert. 1971, 1 1 , 255. (20) Otto, A. In Light Scattering in Solid IV, Topics in Applied Physics; Cardona, M., Guntherodt, G., Eds.; Springer: Berlin, 1984; Vol. 54. (21) Tanaka, T.;Saijo, H.; Iwasaki, M.; Hamazoe, K.; Fujiyama, H . J . SOC.Phofogr. Sci. Technol. Jpn. 1987, 50, 22.
Av (cm.')
Figure 5. FT Raman spectra of 2,2'-cyanine adsorbcd onto a smooth silver electrode from a solution of 5 X IO4 M dye, 0.1 M KCI supporting electrolyte, and 1 M HCI, with electrolyte potentials -0.2, -0.4, -0.6, and -0.8 V vs SCE for (A)-(D), respectively. All spectra are plotted on the same ordinate intensity scale. Spectra were excited by using 1 W of 1.064-pm radiation, have a nominal resolution of 4 cm-I, and were acquired in 400 scans.
more positive than -0.6 V vs SCE. Apparently, since protonation leads to considerable change in the Raman band intensities, neither the J-aggregate nor the crystalline adsorbate is protonated to an extent that leads to strong intensity changes in the Raman spectra shown in Figures 3A and 5A,B. The above observations are surprising in view of the intense Raman spectra attributable to protonated species, when adsorbed from solutions with pH near 0 and with electrode potential ranging from ca. -0.2 to -0.6 V vs SCE, that are found when excitation at 457.9,488 (see Figure 3B for example), and 514.5 nm is used. In fact, the relative intensities of bands attributable to protonated dye increase markedly as the excitation wavelength decreases, most likely, signaling resonance Raman enhancement due to vibrations coupling with a higher energy state, or SERS. The latter explanation is unlikely on two scores: (i) our experiments involved smooth electrodes; and (ii) we observed strong enhancement of only a few bands, and these bands have resonance overtone and combination counterparts. Electrode Potential Effect. Examples of FT Raman band intensity changes as a function of increasingly negative electrode potential for solutions of pH = 0 and 12 are shown in Figures 5 and 6, respectively. Interpretation of spectra acquired by using visible excitation is complicated by resonance Raman enhancement compounded with potential-induced shifting of the excitonic states3 Consequently, such measurements have only been used to note the general intensity decrease at electrode potentials beyond the potential of zero charge (PZC).3J4On the other hand, such effects are far less important in FT Raman spectra, and intensity changes as a function of electrode potential are more clearly indicative of changes in the extent of various intermolecular associations within the adsorbate. We make use of this conjecture to analyze the FT Raman band intensity changes as a function of electrode potential. FT Raman spectra of the adsorbate from a solution with pH = 12, shown in Figure 6, display a general intensity increase with increasingly negative electrode potential to about -0.9 V vs SCE. However, some bands increase quite discernibly while others remain unchanged or increase only slightly. Prominent bands in the spectrum that clearly increase in intensity are at 707, 785, 1042, 1173, 1224, 1286, 1348, 1364, and 1567 cm-I. As the
The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 797
FT-Raman Spectra of Adsorbed 2,2'-Cyanine
TABLE III: Correlation of Band Intensity Variations with Intensities of 1348-, 1364- a d 1386-cm-I Bands 1348 1364 1384 1348 1364 1384
cm-'
cm-l
cm-'
493 540 527 581 606
670
707 734 758 786 846
789 844
865
4w
AT ( c m") Figure 6. FT Raman spectra of 2,2'-cyanine adsorbed onto a smooth silver electrode from a solution of 5 X lo-' M dye, 0.1 M KCI supporting electrolyte, and 0.01 M NaOH, with electrode potentials of -0.4, -0.6, 4.8, and -1.0 V vs SCE for (A)-(D), respectively. All spectra are plotted on the same ordinate intensity scale. Spectra were excited by using 1 W of 1.064-pm radiation, have a nominal resolution of 4 cm-I, and were acquired in 400 scans. TABLE II: F T Raman Band Intensity Increase (Relative to the 1384-cm-' B a d ) with Increasingly Negative Potential from -0.4 to -1.0 V vs SCE
increase 644 707 758 785 819 1042 1083 1 I73 1224 1286 1567
small to moderate increase
no increase
540 606 865 889 946 976 1348 1440 1470 1607
571 670 734 996 1061 1 I26 1152 1241 1364 1515 1625
decrease 846
potential is decreased to -1.0 and -1.2 V vs SCE (spectra not shown here), all bands decrease in intensity but not to the same extent: specifically, bands at 1 173, 1224, 1286, and 1567 cm-' display increased relative intensities while those at 846, 1 126, 1242, 1364, 1384, and 1625 cm-' exhibit much stronger intensity decrease. The independent relative intensity variations of the intense bands a t 1348, 1364, and 1384 cm-' indicate that each is due to the 2,2'-cyanine molecule in a different structural condition. This conclusion was drawn previously based upon intensity variations with solution c~ncentration.~ Slight frequency shifts (5-10 cm-I) with decreasing electrode potential are noted for bands at 644, 670,819,946,976,1083,1173,1346,1440,1470,1515, and 1567 cm-I. A complete listing of intensity changes with decreasing potential relative to the 1384-cm-l adsorbate band (attributed principally to crystalline species, as suggested in Table I) is given in Table 11. The overall intensity increase with decreasing potential can be attributed to an increased amount of adsorbate on the electrode. Comparison of Tables I and I1 leads to the interpretation that the bands that exhibit significant increase in intensity do so because of an increase in the proportion of material that is J-aggregated with decreased potential. Moreover, correlation of the relative intensity changes in the full spectrum with the relative intensity
cm-l
996
996 1041 1061
892 976
1061 1083 1126 1152 1173
65 1 709 730 748
819 889 946 916
cm-l
1041 571
644 670
cm-I
1224 1286 (1 348) 1440 1470 1515 1552 1567 1607
1241 1286 (1 364) 1440 1470 1515 1552 1567
1126 1148 1167 1186 1225 1242 ( 1 384) 1438
1514 1552 1567 1607 1625
variations of the intense bands at 1348, 1364, and 1384 cm-' indicates bands in the overall spectrum that are most related to these bands. Spectral bands analyzed in this manner are listed in Table 111. This analysis leads to the suggestion that band intensity increases, with decreasing electrode potential, are associated with the same structural component in the J-aggregate as is the 1348-cm-' band. We have previously assigned this band to the more stable association of trans conformers in the J-aggregate. Thus, decreasing electrode potential appears to increase the number of trans-trans intermolecular associations in the J-aggregate (see Figure 4B). Even with the consideration that the 1348-cm-l band's intensity likely has a contribution due to polycrystallite, which appears to increase and then decrease with decreasing electrode potential, we ascertain the net increase in the intensity of the 1348-cm-' band is not as large as that of other bands in the spectrum, such as those in column 1 of Table 11, attributable to trans conformers. This observation may be due to the altered structure given by extended associations of trans conformers. Alternatively, as suggested earlier, various intensity increases may result from various resonances associated with surface potential lowering of the excitonic states or other electronic states resulting from adsorbate/surface interactions3 The variation of intensities in FT Raman spectra of 2,2'-cyanine with decreasing electrode potential at low solution pH includes the effect of protonation on the spectra (Figure 5 ) . When no potential is applied to the electrode in solution of pH near zero, no Raman spectrum is observed. The FT Raman spectrum weakly obtained at -0.2 V vs SCE at -0.4 V becomes intense and is somewhat similar to that obtained for adsorbates from high-pH solutions. The spectrum is more strongly altered by protonation at -0.6 and -0.8 V vs SCE. When the solution pH is 1.5, the effect of protonation on the spectrum is not seen before the voltage is lowered to ca. -0.8 V vs SCE. These observations are in contrast with those made by using blue and green excitation which show the dominance of protonated material in the Raman spectrum at -0.2 and -0.4 V vs SCE for solutions of pH = 0 and 1.5, respectively. Bands that are attributable to the polycrystalline component of the adsorbate from solution of pH near zero clearly decrease in intensity with decreasing electrode potential in the FT Raman measurements shown in Figure 5 . This observation supports the conclusion that principally J-aggregate is favored on the electrode under the imposed conditions. While there are no large shifts in frequencies of bands with increased protonation, except the broad feature between 745 and 765 cm-I, most of the spectral features assigned to J-aggregate are broadened and their relative intensities are considerably changed, supporting the conclusion that the adsorbate giving rise to the spectrum in Figure 5D is J-aggregate that is altered by partial protonation. The relative intensities of
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J. Phys. Chem. 1991, 95, 798-801
bands attributable to partially protonated J-aggregate in Figure 5D are much like those observed for J-aggregate at high pH and high negative electrode potential in Figure 6D,again suggesting that the resultant adsorbate structure is similar. Since the FT Raman spectrum attributed to J-aggregate is not drastically altered by partial protonation, also recognizing that J-aggregate must expand in volume upon protonation, and that J-aggregated ions apparently do not exchange rapidly with solution-phase ions (the mean enthalpy barrier for dissolution of monomers from J-aggregate is found to be 42 kJ/mol),20 it is likely that protonation occurs primarily at J-aggregate chain ends, as suggested in our earlier publication!
IV. Conclusion The merging of near-infrared FT Raman and visible Raman studies for 2,2’-cyanine adsorbed onto a smooth silver electrode in various chemical environments and under different excitation
frequency and applied surface conditions leads to the following conclusions: (1) Adsorption of 2,2’-cyanine from high-pH solution onto a silver electrode at negative potentials to -1.0 V vs SCE results in an adsorbate that is primarily polycrystalline and Jaggregated materials. (2) Lowering the pH of the supernatant to near 0 with an electrode potential ca. -0.8 V vs SCE leads to dissolution of most of the crystalline and part of the J-aggregated adsorbate, and the J-aggregate that remains on the electrode is partially protonated. (3) Variations in the Raman spectra of 2,2’-cyanine as a function of solution makeup and electrode potential can be explained in terms of adsorbate composition and partial protonation. (4) Increasingly negative electrode potential increases both J-aggregation and protonation.
Acknowledgment. Support for this research by the National Science Foundation (NSF) under Grants RII-8504995 and RII-8802964 is gratefully acknowledged.
Structure and Catalytic Activity of Alumina-Supported Pt-Co Bimetallic Catalysts. 1. Charactorlzation by X-ray Photoelectron Spectroscopy %Itin Zsoldos, TamPs Hoffer, and Liszlo C u d * Institute of Isotopes of the Hungarian Academy of Sciences, P.O.Box 77, H-1525 Budapest, Hungary (Received: January 30, 1990; In Final Form: June 25, 1990) A series of Ptl-,CoX/Al2O3 bimetallic catalysts has been studied by X-ray photoelectron spectroscopy following in situ hydrogen reduction of the samples previously calcined in oxygen at 570 K. Platinum was found to be in the zerovalent state for the entire composition range and its dispersion increased with decreasing platinum loading. When combined with platinum, cobalt stayed partly in a hardly reducible cobalt oxide surface phase (denoted by CSP) and partly in the metallic state, whereas without platinum COOwas the predominant form. At low cobalt concentration the variation of the XPS intensity changes of the cobalt 2pYz line in the different states was interpreted by assuming that the cobalt surface phase was being covered by platinum. On the other hand, at high cobalt contents saturation of the alumina surface by the cobalt surface phase was concluded from the Occurrence of a constant, high amount of cobalt in the surface phase. On the basis of the comparison of surface platinum and cobalt contents first platinum-like properties and then cobalt-like is predicted for these catalysts with increasing cobalt content.
Introduction Addition of second metal to a supported metallic catalyst may affect its catalytic behavior via various effects.’ Nowadays, bimetallic catalysts such as Pt-Re, Pt-Ir, Pt-Sn are being successfully utilized in some industrially feasible catalytic processes, e.g., in naphtha reforming, and Pd-Cu and Pd-Ag are used in selective hydrogenations. The bimetallic catalysts studied so far revealed different effects, such as improved stability or retarded deactivation. For instance, rhenium in Pt-Re/AI2O3 catalyst maintained its catalytic activity during naphtha reforming at lower pressure and temperature by hindering deactivation processes, whereby the catalyst lifetime increased.2 In other cases the catalytic activity was also enhanced3 or the selectivity was modified? Little is available on the Pt-Co bimetallic catalystss.6 which have high activity in methanol formation at certain compositions. Extensive studies on the surface characteristics of both (1) In ‘Metal-Support and Metal-Additive Effects in Catalysis”, Imelik,
and catalysts supported on alumina started only a few years ago. In the present work the main goal is to determine the surface composition and the possible structure of a series of Pt-Co/A1203 bimetallic catalysts by X-ray photoelectron spectroscopy and to obtain information about the chemical environment of the surface components (valence states, quantities). In the second part of this series the surface structures found here at different compositions will be related to the catalytic properties observed. Experimental Section The Ptl-xC~,/A1203samples were prepared by the incipient wetness method using y-alumina (Woelm). Appropriate amounts of H,PtCI, and C O ( N O ~with ) ~ 10 wt Q total metal loading were dissolved in water and used for impregnation. The atomic fractions of cobalt, X,,, were 0, 0.2, 0.4, 0.5, 0.67,0.85, and 1.0. After drying in air at room temperature the samples were calcined in flowing O2(40 cm3 m i d ) at 570 K for 1 h. The reduction was
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0 1991 American Chemical Society
