J. Phys. Chem. 1984,88, 5935-5944
5935
Surface-Enhanced Resonance Raman Spectroscopy of Rhodamine 6G Adsorbed on Colloidal Silver Peter Hildebrandt and Manfred Stockburger* Max- Planck-Institut fur biophysikalische Chemie, Abt. Spektroskopie, 0-3400 Gottingen-Nikolausberg, Federal Republic of Germany (Received: January 5, 1984; In Final Form: May 2, 1984)
Surface-enhanced resonance Raman scattering (SERRS) of rhodamine 6G (R6G) adsorbed on colloidal silver was studied. Adsorption isotherms could be obtained from SERRS and fluorescent measurements. Two different kinds of adsorption sites were inferred from the isotherms. One kind is rather unspecific and shows a high surface coverage. The enhancement factor at such sites is 3000, which can be well explained by the classical electromagnetic theory of colloids. The second kind is only observed in the presence of anions (Cl-, I-, Br-, F,SO:-). Specific active sites are formed at an extremely low surface coverage. It was concluded from the isotherms that at such sites the molecules are chemisorbed. Overall enhancement factors up to lo6 were found for molecules at anion-activated sites. The additional enhancement factor is ascribed to a local mechanism of an R6G-adatom (or cluster)-anion surface complex. SERRS excitation profiles of active sites are closely related to the molecular resonance at 530 nm. SERRS spectra of R6G were recorded and analyzed in a wide frequency range. The two adsorption sites could be distinguished by characteristic vibrational features. It was demonstrated that SERRS is also a powerful analytical tool for dye molecules.
Introduction The discovery of the surface-enhanced Raman (SER) effect of pyridine adsorbed on a roughened silver electrode has opened a wide research field both in physics and chemistry of interfaces and in Raman spectro~copy.'-~ By now a large number of experimental data have been accumulated. However, a generally accepted and uniform theoretical explanation of this exciting phenomenon has not been established." Two different theoretical approaches are mainly d i s c ~ s s e d . ~ The first one uses the plasma resonance model, which is related to the optical properties of free-electron-like metals.@ Irradiation of a microscopically rough metal surface gives rise to local plasma modes. The electric field at such a surface becomes very large if the incident photon energy is in resonance with a normal mode of the conduction electrons in the metal. This leads to enhanced Raman scattering from those molecules which are close to the metal surface. The second approach is based on the concept of "active sites" at the metal surface.'w12 A specific interaction between the metal and the molecules occupying these sites induces enhanced Raman scattering. The nature of this interaction is controversial. Some investigators have suggested that charge-transfer processes are involved between the metal and the molecule via the active sites, which are supposed to be metal adatoms or clusters. This model implies that Raman enhancement is restricted to molecules immediately adjacent to the metal while the electromagnetic approach predicts an enhancement also for molecules at a distance of more than 10 8, from the surface. There is a great amount of experimental evidence for either the first or the second approach. Therefore, it has been argued (1) Van Duyne, R. P. In "Chemical and Biochemical Applications of Lasers"; Moore, C. B., Ed.;Academic Press: New York, 1979; Vol. 4, Chapter 5. (2) Chang, R. K., Furtak, T. E., Eds. "Surface Enhanced Raman Scattering"; Plenum Press: New York, 1982. (3) Cooney, R. P.; Mahoney, M. R.; McQuillan, A. J. In "Advances of Infrared and Raman Spectroscopy"; Clark, R. J. H., Hester, R. E., Eds.; Heyden: London, 1982; Vol. 9, p 188. (4) Furtak, T. E.; Reyes, J. Surf. Sci. 1980, 93, 351. (5) Furtak, T. E. J . Electroanal. Chem. Interfacial Electrochem. 1982, 150, 375. (6) Gersten, J.; Nitzan, A. J . Chem. Phys. 1980, 73, 3023. (7) Gersten, J.; Nitzan, A. J . Chem. Phys. 1981, 75, 1139. (8) Kerker. M.: Wane. D. S.: Chew. H. A d . O D ~1980. . 19. 4159. (9) Jha, S. S.; Kirtley;;. R.; Tsang, T. C. P'iys. R'ev. B: Cindens. Matter 1980. 22. 3973. (IO) Otto, A. Appl. Surf. Sci. 1980, 101, 374. (11) Furtak, T. E.; Trott, G.; Loo, B. H. Surf. Sci. 1980, 101, 374. (12) Seki, H. J . Chem. Phys. 1982, 76, 4412.
0022-3654/84/2088-5935$01.50/0
that more than one mechanism might be responsible for the overall enhan~ement.'~J~ The present paper is a contribution to this debate. Stimulated by a brief report of Baranov and B o b ~ v i c h ,we ' ~ have examined the SER effect of the laser dye rhodamine 6G (R6G) adsorbed on colloidal silver (Ag sol). Ag sol shows SERS properties equivalent to those of silver electrodes16-22and has been proved in our experiments to be a convenient system. R6G is a strongly fluorescent xanthene derivative which shows a molecular resonance Raman (RR) effect when excited into its visible absorption band. There were two main reasons for the choice of this system. Firstly, R R scatterers were not widely investigated by SERS and the few results on dyes23-29or biologically important molecules3w34
(13) Lyon, S.A.; Worlock, J. M. Phys. Reu. Lett. 1983, 51, 593. (14) Tsang, J. C.; Kirtley, J. R.; Theis, T. N . J . Chem. Phys. 1982, 77, 641. (15) Baranov, A. V.; Bobovich, Ya. Opt. Spectrosc. (Engl. Trawl.) 1982, 52, 231. (16) Creighton, J. A,; Blatchford, C. G.;Albrecht, M. G. J . Chem. SOC., Faraday Trans. 2 1980, 790. (17) Lippitsch, M. Chem. Phys. Lett. 1980, 74, 125. (18) McQuillan, A. J.; Pope, C. G. Chem. Phys. Lett. 1980, 71, 349. (19) Wetzel, H.; Gerischer, H. Chem. Phys. Lett. 1980, 76, 460. (20) Wetzel, H.; Gerischer, H.; Pettinger, B. Chem. Phys. Lett. 1982, 85, 187. (21) Siiman, 0.; Bumm, L. A.; Callaghan, R. A.; Blatchford, C. G.; Kerker, M. J. Phys. Chem. 1983, 87, 1014. (22) Suh, J.; DiLella, D. P.; Moskovits, M. J . Phys. Chem. 1983,87, 1540. (23) Jeanmaire, D. L.; Van Duyne, R. P. J. Electroanal, Chem. Interfacial Electrochem. 1977, 84, 1. (24) Lippitsch, M. Chem. Phys. Lett. 1981, 79, 224. (25) Akins, D. L. J . Colloid Interface Sci. 1982, 90, 373. (26) Watanabe, T.; Pettinger, B. Chem. Phys. Lett. 1982, 89, 501. (27) Kneipp, K.; Hinzmann, G . ;Fassler, D. Chem. Phys. Lett. 1982, 89, 501. (28) Weitz, D. A.; Garoff, $.; Gersten, J.; Nitzan, A. J . Chem. Phys. 1983, 78, 5324. (29) Bachackashvilli, A.; Efrima, S.;Katz, B.; Priel, Z. Chem. Phys. Lett. 1983, 99, 503. (30) Cotton, T. M.; Schultz, S. G.; Van Duyne, R. P. J . Am. Chem. SOC. 1980, 102, 7960. (31) Cotton, T. M.; Timkovich, R.; Cork, M. S. FEBS Lett. 1981, 133, 39. (32) Cotton, T. M.; Schultz, S.G.; Van Duyne, R. P. J . Am. Chem. SOC. 1982, 104, 6528. (33) Cotton, T. M.; Van Duyne, R. P. FEBS Lett. 1982, 147, 81.
0 1984 American Chemical Society
Hildebrandt and Stockburger
5936 The Journal of Physical Chemistry, Vol. 88, No. 24, 1984 f
LOO
500
600
700
h/nm
Figure 1. Absorption spectrum of an aqueous Ag sol at a 2 X lo4 M concentration. were not comprehensively discussed on the background of theoretical models nor were they interpreted u n a m b i g ~ o u s l y .Sec~~ ondly, the combination of the molecular resonance and the surface enhancement (SERRS) results in an extremely high sensitivity which allows one to use very low R6G concentrations (picomole range) in the experiments. Thus, it was possible to study R 6 G A g interactions under conditions (submonolayer coverage) where interactions between the dye molecules can be ruled out. As the most important result of our work it turned out that two different mechanisms can contribute to the enhancement of Raman scattering. One mechanism is already effective when R6G molecules are spontaneously adsorbed on the particles of the Ag sol while the second requires an activation of the sol by a supporting electrolyte. In both cases Raman enhancement is accompanied by fluorescence quenching. By means of fluorescence and Raman measurements we were able to examine the adsorption behavior of the dye molecules on the silver particles. It turned out that the two mechanisms occur at different adsorption sites. The different interaction between the dye molecule and the metal at these two sites is reflected in the vibrational structure of the SERR spectra. On the basis of adsorption isotherms it was possible to evaluate the corresponding enhancement factors. In the activated sol SERRS was between lo4 and lo6 times stronger than R R scattering in solution. This factor depends on the added electrolyte and on the excitation wavelength. We took advantage of the huge enhancement factor in the activated sol to record high-quality Raman spectra of R6G. For the first time spectra in the overtone and low-frequency region could be obtained. Thus, we were able to demonstrate that SERRS is a powerful and simple technique to study the vibrational Raman spectra of R6G and related molecules. These spectra are of better quality than the vibrational spectra which were obtained by CARS, stimulated, inverse, and resonance Raman spectroscopy or by the Raman gain t e ~ h n i q u e . ~ ~ - ~ ~ Experimental Section Ag sols were prepared according to the description of Lee and Meisela40 A 90-mg sample of A g N 0 3 (Fluka puriss. p. a.) was suspended in 500 mL of quartz-distilled water, purged with pure N2, and heated to 100 OC; 10 mL of a 1% solution of sodium citrate (Fluka puriss. p. a.), purged with N2, was added dropwise to the boiling solution under vigorous stirring. The solution was kept boiling for 60-90 min. The absorption curve of the brownish suspension,which shows a maximum at 415 nm, is shown in Figure (34) Itabashi, M.; Kato, K.; Itoh, K. Chem. Phys. Lett. 1983, 97, 528. (35) Andrew, R. B.; Bobovich, Ya. S.;Bortkevich, A. V.; Volosov, V. D.; Tsenter, M. Ya. Opt. Spectrosc. (Engl. Transl.) 1976, 41, 462. (36) Neporent, B. S.;Spiro, A. G.; Shilov, V. B.; Fainberg, B. D. Opt. Spectrosc. (Engl. Transl.) 1980, 49, 606. (37) Werncke, W.; Lau, A.; Pfeiffer, M.; Weigmann, H. J.; Hunsalz, G.; Lenz, K. Opt. Commun. 1916,16, 128. (38) Carreira, L. A.; Gros, L. P. In "Springer Series in Chemical Physics"; Zewail, A. H., Ed.; Springer-Verlag: New York, 1978; Vol. 3, p 277. (39) Fabian, H.; Lau, A,; Werncke, W.; Pfeiffer, M.; Lenz, K.; Weigmann, H. J. Sou. J . Quantum Electron. (Engl. Transl.) 1979, 9, 40. (40) Lee, P. C.; Meisel, D. J. Phys. Chem. 1982, 86, 3391.
1. In samples prepared for Raman measurements the Ag concentration was about 7 X M , corresponding to an optical density of 4 at 415 nm. A comparison of several sol preparations showed no differences of optical properties and SERR activity. Transmission electron microscopy (TEM) performed on a JEM 100 B microscope showed a wide distribution in particle size. Spheres (80%) and rods (20%) are the dominating shapes. In only a few cases were aggregates found. While the mean diameter of the spheres was 30 nm, the average length of the rods was 60 nm. An average particle size can be approximated by spheres with a diameter of 35 nm. The Ag concentration which was used in the SERR experiments then corresponds to a silver particle density of 3.2 x 1014 L-1. Centrifugation of the sols at 13000g for 3 min leads to a black pellet and a colorless supernatant. Rhodamine 6G hydrochloride (Kodak Eastman) was used without further purification. All other reagents were of high purity or analytical grade (Merck, Fluka). Starting with R6G stock solutions of 10-3-104 M, we prepared concentrations down to lo-', M by successive dilution by factors of 10 or 100. Dye concentrations in samples from different stock solutions were monitored by their fluorescence intensity. It turned out that dilution errors were negligible. Sample solutions for Raman measurements were prepared at least 3 h before they were used in the experiment. Standard samples containing 10-9-10-8 M R6G and 7.5 X M C1- did not show a detectable influence on the absorption of the sol. Raman and fluorescence spectra were recorded by conventional equipment using an argon ion laser (Coherent CR 52), a Jarrell-Ash 1-m double monochromator (Model 25- 100) provided with a RKB control system (Model 125-610 polydrive) which allowed repetitive scanning, an ITT photomultiplier (FW 130), an Ortec detection system (Models 113 and 486), and an Elscint multichannel analyzer (promeda). For excitation by krypton laser lines (Coherent CR 2000 K) we used the equipment described in ref 41. The laser beam was focused into a rotating cell. In most cases the laser power was 15 mW. The scattered Raman and fluorescence radiation was monitored at a right angle to the exciting beam by using a spectral bandwidth of 5 cm-I or less. The spectral data were accumulated in the MCA and transferred to a PDP 11 computer. The repeatability of the spectrometer was fl cm-'. Results and Discussion Intensity Measurements at Very Low Concentrations. In this paper important conclusions were obtained from Raman and fluorescence intensity measurements of aqueous dye solutions at M. In this limit adsorption initial concentrations, c,, below and desorption processes at the cuvette walls are significant. For M the fraction example, at an initial R6G concentration of of adsorbed molecules was between 30% and 70% depending on the pretreatment of the cuvettes. Thus, great efforts had to be made to arrive at stable and reproducible signals. In order to obtain a stable concentration in the bulk solution the clean cuvettes were rinsed with portions of the sample solution several times until the equilibrium between the adsorbed and dissolved molecules was established. This was controlled by the fluorescence intensity of the solution. In this way the expected linear relationship between concentration and fluorescence intensity could be established in the range between lo-', and M. In suspensions of colloidal silver the dye molecules had a tendency to migrate to the silver particles. This is described by the equilibrium k
W-R6G
2 R6Gdi, k2
k4
R6G-Ag
(1)
where W stands for the cuvette wall. Obviously the desorption from the wall ( k , ) followed by the adsorption on the Ag sol ( k 3 ) dominates ( k 3 / k 4> k l / k , ) . Since already R6G concentrations (41) Alshuth, T.; Stockburger, M. Ber. Bunsenges. Phys. Chem. 1981,85, 484.
SERRS of R6G on Colloidal Silver
f
Figure 2. Dependence of the relative intensity of the 1650-cm-’ SERR band of R6G on the C1- (triangles)and I- (circles) concentration. Dotted line: intensity of the same SERR band in the absence of anions. R6G: M; A,, = 514 nm. co = of a few picomoles can lead to distinct SERR signals, great care was necessary to clean those cuvettes which had contained more concentrated R6G solutions before. Even a repeated chemical cleaning (for example, treatment with concentrated HN03/HC1 or concentrated H202/NH3)was not sufficient to remove all the adsorbed R6G molecules. Thus, dye-free solutions which were used to fill such cuvettes showed a slow growing-in of characteristic R6G SERR bands up to an intensity equivalent to concentrations of 1-3 pmol. Only after the cuvettes were baked a t 350 O C for 12 h were the R6G bands removed almost completely. Most of the experiments were carried out at R6G concentrations of lo4 M. In this case the chemical cleaning procedure was sufficient. In order to avoid errors caused by accidentally introduced dyestuff each series of experiments (SERR and fluorescence) was started by monitoring an R6G-free sample. Although the tendency of R6G molecules to migrate from Ag sol adsorption sites to the cuvette walls was not significant, the cuvettes were rinsed with portions of the sample several times until the adsorption equilibrium was established. In order to obtain reliable data for Raman and fluorescence intensity for each concentration of R6G (10-12-10-4M) several measurements from independent samples were performed. Continuous illumination by the laser beam (15 mW, 514 nm) focused into the rotating sample caused the SERRS intensity to decrease by 5-10% during one scan (1 5 min). This photochemical lability of the R6G/Ag system42 was still enhanced when the rotation of the cuvette was stopped. A comparable effect was not observed in aqueous solutions of R6G under the same conditions. We therefore conclude that the decrease of SERRS intensity is due to photodecomposition of a surface adsorbatesilver complex or an enhanced photolability of R6G on the surface.43 Similar observations in different systems were reported by other^.^^,^^ Without laser irradiation and in the absence of electrolytes the R6G/Ag system was stable up to R6G concentrations between and 10” M. After addition of electrolytes the stability of the system strongly depended on the kind of anion, its concentration, and the R6G concentration. For each anion at a given concentration there was a R6G concentration limit above which no measurements were possible due to the fast aggregation and flocculation of the sol. Below these limits the SERRS signal remained unchanged for a period of several days. Activation ofthe Ag Sol. Mixing a lo4 M R6G solution with Ag sol leads to a drastic decrease of the fluorescence signal. This phenomenon was attributed to fluorescence quenching of R6G molecules which are adsorbed on the silver particles.46 In the case of R6G adsorbed on silver films it has been estimated that the fluorescence yield decreases to values between and 10-3.28*47Thus, the residual fluorescence should mainly result (42) Weitz, D. A.; Garoff, s.;~ ~ ~T. J.~ Opt. i k tlt . 1982, ~ , 7, 168. (43) Chen, C. J.; Osgood, R. M. ~ p p lPhys., . [Purr] A 1983, A31, 171. (44) Kerker, M.; Siiman, 0.; Bumm, L. A.; Wang, D. S.Appl. Opt. 1980, 19, 3253. (45) Fleischmann, M.;Hill, I. R. J . Electroanul. Chem. Interjaciul EIeo trochem. 1983, 146, 353. (46) Chance, R. R.; Prock, A.; Silbey, R. Ado. Chem. Phys. 1979,37, 1. (47) Ritchie, G.; Burstein, E. Phys. Reu. B: Condens. Mutter 1981, 24, 4843.
The Journal of Physical Chemistry, Vol. 88, No. 24, 1984 5937 TABLE I: SERR Activation of the Ag Sol by Different Anions anion c,“ I(~,,)f Nob c*,/cAgc Nsa,/NAgd F 10-3 46 1.3 1.8 x 106 c17.5 x 10-4 166 1.o 1.4 X lo6 Br240 13 1.8 x 107 15 x 10-3 10 6.7 9.1 X lo6 so42- - 5 x 10-3 9 -6.7 -9.1 X lo6 ‘Saturation concentration of anions in molar units. Intensity ratio of the 1650-cm-’ band (Aexc = 514 nm) at co(R6G) = lo-’ M in the presence and in the absence of anions. ‘Concentration ratio anion/ silver in molar units. ”Particle density of anions to silver spheres.
t
2-
11
lb
9
8
-lg [c,/MI
7
6
5
1
.
Figure 3. Dependence of the intensity of the 1650-cm-’ SERR band of R6G on the R6G concentration, co, in the absence of anions (0)(A,,, = 457 nm)and in the presence of anions (hXc = 514 nm) at their saturation concentrations (I-: A; C1-: 0). from dissolved dye molecules. Indeed, after centrifugation of the sol, we found the same fluorescence intensity in the colorless supernatant as was monitored in the sol. At such low concentrations of the dye, Raman signals are very weak. However, addition of only a small amount of C1- ions to the sol causes a significant increase of Raman intensity. The dependence of the peak height of the 1650-cm-’ band on the chloride concentration is shown in Figure 2 (triangles) for a constant R6G concentration of M. A saturation limit is defined for an anion concentration at which the Raman signal reaches 90% of its final value. This is reached at 7.5 X M M no reliable data of C1-. At concentrations greater than could be obtained due to instability of the sol. We found that the Cl- saturation value does not depend on the R6G concentration. However, if the Ag concentration is lowered, the C1- saturation value shifts down proportionally. This implies that the SERR signal is proportional to the ratio Cl-/Ag, which in the saturation limit is about 1:l in molar units. According to our previous estimation of particle density this corresponds to 1.4 X lo6 C1- ions for one colloidal particle. Going to very low C1concentrations Raman intensity approaches the “anion-free” limit (Figure 2, dotted line) where SERR intensity is about 170 times smaller than at the C1- saturation value. Chloride can be replaced by other anions. In the case of iodide (Figure 2) the saturation value is shifted to higher concentrations by about 1 order of magnitude while the SERR intensity is about 17 times lower than in the presence of chloride. Characteristic parameters for various anions are presented in Table I. In the case of perchlorate, nitrate, and phosphate no additional enhancement of the Raman signal was observed. It was reported by others that cations do not influence the SERR intensitv.’~~~ This was confirmed bv our own exDeriments since it makes-no difference if chloride isadded as H k l , NaC1, or KCl. Therefore, the conclusion of Baranov and Bobovich15that SERRS of R6G in Ag sol is activated by acidification has to be modified. The activation by anions, e.g., the formation of “active sites”, as monitored by the SERR signal took more than 1 h. During (48) Garell, R. L.; Shaw, K. D.; Krimm, S . Surf. Sci. 1983, 124, 613.
5938
The Journal of Physical Chemistry, Vol. 88, No. 24, 1984
this process no absorption changes of the sol could be detected. N o evidence for an increasing number of aggregates of particles in the presence of C1- anions could be established on the basis of our TEM studies. This is in line with results obtained by Garell et al.48 So far the results show that in Ag sols the fluorescence of adsorbed R6G is quenched while Raman scattering is enhanced. In the presence of anions Raman enhancement is up to 2 orders of magnitude greater. Since the anion/Ag ratio and not the anion/R6G ratio is the parameter which governs SERR intensity, one can exclude a primary effect of anions on R6G, e.g., a direct anion-R6G interaction. Adsorption Isotherms and Quantitative Determination of Active Sites. In Figure 3 the SERR intensity is depicted as a function of the initial dye concentration, co, for the anion-free R6G/Ag = 457 nm) as well as in the presence of C1- and suspension (bXc I- (A,, = 514 nm). All data were corrected for attenuation of the exciting beam in the sample at the two different wavelengths as well as for the spectral response of the detection system. In the anion-free limit the SERR signal increases linearly with co until saturation is approached at values above M. In the presence of C1- (7.5 X lo4 M) and for very low values of co the SERR signal is about 2 orders of magnitude stronger. However, it approaches saturation already at dye concentrations as low as M. This behavior suggests that the physical and chemical forces which are responsible for the formation of the absorption complex in the absence and presence of anions are quite different in nature. The molecular adsorption parameters can be described by Langmuir’s adsorption k = kmaxadisw/(wadis + 1)
Hildebrandt and Stockburger
6-
/ E-
10 4
”1 Figure 4. Adsorption isotherms of R6G/Ag systems: (a) in the absence
of anions determined from fluorescence (+) and Raman data in Figure 3 (0); (b) in the presence of C1- (7.5 X M) from Raman data in Figure 3. In curve a k denotes the molarity of R6G adsorbed at “normal” adsorption sites. In curve b k represents active sites. cdi, is the R6G concentration in the bulk solution (a) or of those molecules not adsorbed on active sites (b). portional to k,. With k, = ISERR/A,where A is a constant, it follows from eq 2 that
(2)
ISERR = ks,maxwsA(co - Z S E R R / ~[ W ~ ( C O- ISERR/A)+ 11-’ (5)
where adisdenotes the activity of the dissolved R6G in the bulk solution which is approximated by the concentration cdis. k and ,k refer to the concentration of the absorbed molecules. w (M-I) is the adsorption coefficient, which is related to the adsorption energy Gad by
ISERR was set proportional to the peak height of the 1650-cm-’ band and was monitored in the range lo4 < co < lo4 M. A good fit of the data (squares in Figure 4a) to eq 5 was obtained with
(3)
This confirms what one expects intuitively, that fluorescence quenching and SERRS occur at identical molecular adsorption sites. From eq 3 one obtains, with w, = w4 = 4 X lo4 M-’, an adsorption energy Gad of 35.8 kJ/mol. This value is rather high to account for adsorptive forces of the van der Waals type,51suggesting that R6G is chemisorbed to the surface. It is very likely that a chemisorptive bond would be formed via the nitrogen atom of the ethylamino group. For geometrical reasons this would imply that the long molecular axis is not parallel to the surface but is more or less tilted. In this respect it is interesting to consider the value of k,, given in eq 4 for the anion-free limit. This corresponds to a density of adsorbed molecules of 6 X lo’* L-I. Since we have estimated a particle density of 3.2 X lOI4 L-I, a number of 1.8 X lo4molecules would cover one colloidal particle in the limiting case. For flat orientation of the dye molecules on the surface one estimates that more than 10 sandwich-type molecular layers would cover one silver particle (mean diameter of 35 nm). Such a configuration can be ruled out for thermodynamic reasons. Moreover, the linear relationship between Cdis and k, which, as is seen in Figure 4a, extends over several orders of magnitude, suggests that all molecules that are attached to the silver particle are exposed to the same enhancement process and therefore must occupy sites of the same quality, which requires that the molecules cover the surface as a monolayer. Since in this limit only 20 AZ would then be available for a single molecule, the molecular axis must be tilted significantly with respect to the surface to enable such a dense coverage. As outlined above the molecules are very likely linked to the surface via a N-Ag covalent bond. It should be noted that in Langmuir’s approach (eq 2) interactions between the adsorbed R6G molecules (e.g., dimerization
where cHIOis the molarity of water. We first consider under anion-free conditions those molecules which are adsorbed at the surface and whose fluorescence is quenched. Their concentration is denoted by k,. It was shown above that the residual fluorescence from a silver sol, Zn(sol), stems from the dissolved molecules only. This quantity, which was measured from the supernatant of the centrifuged sol, as well as the fluorescence from the pure aqueous R6G solution, In(aq), were monitored a t 560 nm in the range < co < M under otherwise identical experimental conditions (Aexc = 514 nm). The data were corrected for attenuation of the probe beam in the M) for sample, and a t higher R6G concentrations (c,, 2 fluorescence quenching due to d i m e r i z a t i ~ n .Thus, ~ ~ values for Cdi, and the concentration, k,, of the adsorbed molecules were obtained according to Cdia
= [Itl(sol)/Ifl(aq)lcO
kq
= CO - Cdis
The results averaged over different series of measurements are shown in Figure 4a (crosses). When the data are fitted by a least-squares procedure to eq 2, one obtains k,,,,, = 1.0 X
M
wq = 4.0 X lo4 M-’
(4)
Now we consider those molecules which are adsorbed at the silver under anion-free conditions and give rise to a SERR signal. They are labeled by the index s. Since predominantly the adsorbed molecules contribute to the recorded Raman signal, this is pro(49) Hiemenz, P. C. “Principles of Colloid and Surface Science”; Marcel Dekker: New York, 1977; p 287. (50) Levshin, V. L.; Baranova, E.G. Opt. Spectrosc. (Engl. Trunsl.) 1959,
6, 31.
ks,max = kq,max
(51) Newmiller, R. J.; Pontius, R.
ws = wq
P. J . Phys. Chem. 1960, 64, 584.
The Journal of Physical Chemistry, Vol. 88, No. 24, 1984 5939
SERRS of R6G on Colloidal Silver on the surface) are neglected. This assumption might not be valid for the high density of adsorbed molecules as we found in the anion-free limit. However, when Frumkin's isotherms2 was used, which takes account of molecular interactions, values for k, and w were obtained that do not substantially differ from those in eq 4. So far we have examined the pure R6G/Ag system. In order to investigate the adsorption behavior of R6G on the activated sol we had to confine ourselves to Raman measurements, since only the SERR signal is clearly correlated with the activated sol. The quantitative description shall be restricted to R6G/Ag sols which contain C1- anions at their saturation concentrations (7.5 X lo4 M). For concentrations of R6G below 6 X M it can be seen from the data in Figure 3 that the SERR intensity is at least 10 times higher in the presence of C1- anions. A concentration of active sites, k(Cl-), then can be defined which in this range is proportional to the SERR intensity. This does not mean that in the presence of C1- nonactive sites are not occupied. However, in the low concentration range which is considered the two kinds of sites would not interact. Thus, one can be sure that the SERR signal of the C1--activated sol represents the unperturbed active sites only. With the definitions k(C1) = ZSERR/AC~ kmax(C1-1 = @%R/AcI
H
H
Figure 5. Structural formula of R6G.
(6)
where Acl is a proportionality constant one obtains from eq 2 ISERR(C1) = @%R(cl) w(C1) [CO - ZSSERR(C1)/&I [W(cl) ( t o - ZSERR(C~)/ACJ + 11-' (7) When the data of Figure 3 are fitted to eq 7 by a least-squares procedure, one obtains = 5.5 x 109 M-1 (8) and according to eq 6
M (9) In Figure 4b k(C1) is depicted as a function of co - k(Cl), a quantity which reflects all molecules that are not adsorbed at active sites, e.g., dissolved molecules and those adsorbed at nonactivated sites. Additional measurements were performed on less concentrated Ag sols. As expected, k,, decreases with decreasing sol concentration. On the other hand, the same adsorption coefficient is obtained as given in eq 8. This also holds for different excitation wavelengths. The value obtained of k,(Cl) in eq 9 corresponds to a density of active sites of 1015L-l, which gives a 3.3:l ratio of active sites and silver particles. This is nearly 4 orders of magnitude smaller than in the anion-free limit, indicating that anion-induced SERRS is restricted to a small number of special adsorption sites. With the value of w(C1) in eq 8 one obtains an adsorption energy of 64.9 kJ/mol (eq 3), which shows that R6G is more tightly bound to these active sites than to adsorption sites in the anion-free limit, whose adsorption energy was only 35.8 kJ/M. In this context it is interesting to consider the surface potential of the sol particles. From the correlation between the intensity ratio of the v, and v2 S E R bands of pyridine and the surface potentia120we have found that the potential (vs. SCE) shifts from -0.1 1 to -0.40 V when C1- is raised to its saturation limit. The higher adsorption energy in the C1--activated sol could therefore at least partially be due to the increased negative potential. The adsorption capability of the silver sol is reflected by the slope, k,,w, of the linear part of an isotherm, which gives the ratio of adsorbed to nonadsorbed molecules. In the anion-free limit this ratio is 0.4 while in the R6G/Cl-/Ag system it is 10. This implies that in the linear part of the isotherms and for a given concentration of R6G the ratio of active site adsorbed molecules (in the presence of C1-) to molecules a t nonactive sites (in the absence kmax(Cl)= 1.8 X
(52) Damaskin, B. B.; Petrii, 0. A.; Batrakov, V. V. "Adsorption of Organic Compounds on Electrodes"; Plenum Press: New York, 1971; Chapter
3.
400
200
Av/cm'
Av/cm-'
Figure 6. Low-frequency spectra of R6G from (A) aqueous solution (RR, A,, = 457 nm, co = 8.35 X lo4 M) and from Ag sols (D) without anions (SERR, A,, = 457 nm,co = lod M) and containing different M): (B) C1-, (C) I-, (E) Br-, anions (SERR, A,,, = 514 nm, co = (F) F,and (G) SO>-.
of C1-) is 3.2. This, however, can only to a small part account for the increase of SERR intensity by a factor of 170 on addition of C1- (Figure 2). The main anion effect therefore must be ascribed, as already mentioned, to a particular enhancement mechanism. Spectroscopic Characterization of Surface Complexes. SERR spectra provide additional information about the nature of the interaction between R6G and the silver surface. In the frequency region
