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Enhanced Raman characterization of adsorbed water at the electrochemical double layer on silver
556
Surface Science 122 (1982) 556-568 North-Holland Publishing Company
ENHANCED RAMAN CHARACTERIZATION OF ADSORBED WATER AT THE ELECTROCHEMICAL DOUBLE LAYER ON SILVER S.H. MACOMBER Department
and T.E. FURTAK
of Physics, Rensselaer Polytechnic Institute,
Troy, New York 12181, USA
and T.M. DEVINE Research USA
and Development
Center, General Electric Corporation,
Received
28 June 1982; accepted
for publication
16 August
Schenectady,
New York 12301,
1982
Surface enhanced Raman scattering (SERS) from molecular water adsorbed on silver in an aqueous electrolyte is reported. The SERS active water is most probably associated with active sites on the metal which are stabilized by the double layer structure. The line shapes of the primary vibrations associated with the O-H stretch and the H-O-H bend are strong functions of the applied voltage, the electrolyte pH, and the bulk concentration of cations. At least two types of adsorbed water are observed. These data directly demonstrate that the structure of the first layer of water is significantly influenced by interaction with adjacent layers.
1. Introduction Arguments persist concerning the structure of the first layer of water in the double layer [ 1,2]. Its influence on the traditional electrochemical experiments such as differential capacitance has been debated for many years [l-3]. Models have been proposed involving various degrees of sophistication and detail [ 1,4,5]. It has been pointed out that many of these fall short of what is expected when the water molecules are subjected to the influence of the large electric fields which exist in the double layer [6]. Some models suggest that the first layer is established quite independently of the adjacent layers [ 1,7]. There is no other molecule which is of more importance in solid-electrolyte interfacial science than water. However, surprisingly little direct information exists about the details of adsorbed water. During the last two years electron loss spectroscopy [8,9], thermal desorption experiments [lo], low energy electron diffraction [ 111, and photoemission spectroscopy [ 10,12,13] have been employed to study single and multi-layer water adsorbed onto Pt, Cu, and Ag 0039-6028/82/0000-0/$02.75
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at low temperature. These vacuum spectroscopies have demonstrated that water adsorbs molecularly unless perturbed by co-adsorption impurities such as oxygen [lo]. In some cases, room temperature adsorption of water does not occur. However, coadsorbed halogens induce adsorption under the same conditions [ 131. At low temperature (100 K) it was also learned that water forms structures which resemble bulk ice as succeeding layers build up [g]. While this information is direct and firmly established, it does not take the place of in situ diagnosis of interfacial water. Surface enhanced Raman scattering (SERS) is ideally suited or such a study. By association with a metal surface, most commonly Ag, molecules demonstrate an anomalously large interaction cross-section in the Raman process [14]. This makes it possible to observe the vibrational characteristics of the molecules near the metal surface even when there are large numbers of similar molecules in the adjacent, transparent medium, in this case the aqueous electrolyte solution. While SERS has been under intense invesigation for several years, it has been applied only recently to the interfacial water problem [ 15-211. Part of the resaon for this is associated with the elusiveness of the enhancement phenomenon itself. Approximately two years ago, although contrary evidence existed at that time [22], the overwhelming consensus concerning the mechanics for SERS rested with the geometrically defined surface optical resonance moels [23-261. These theories exploited the large optical fields which exist at the surface of free-electron-like metals which have been made rough on the scale of 5 to 50 nm. Since the origin of the enhancement, in this picture, is through an effect dealing with the characteristics of the metal and its boundary alone, there is no reason why all material in the vicinity of the metal surface should not demonstrate enhancement. In contrast to this prediction, interfacial water was strikingly absent from the earlier SERS data recorded in electrochemical environments [27-291. This fact, along with other evidence [30-321, has led to another school of thought, which while admitting to the importance of field enhancement, invoked the operation of a second, chemically specific, mechanism. This second, as yet controversial, mechanism is required in order that the highest levels of SERS be achieved. Since water is a relatively weak Raman scatterer by comparison to pyridine it is necessary to arrange conditions such that both mechanisms are operative so as to provide a signal which can be detected above the limit allowed by modern instrumentation. Since the nature of the chemically specific mechanism was poorly understood at that time it was only by chance that the conditions were achieved such that interfacial water was finally observed using SERS. This was first accomplished by Flieschmann et al. [ 151 who used a heavy electrochemical oxidation-reduction treatment to roughen a Ag electrode in molar KCl. They observed structure in the OH stretch region which, by its lineshape, its independence of the bulk water and its displacement upon addition of D,O was identified as water associated with the metal surface.
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Subs~uently, Pettinger, Philpott and Gordon [16] showed that the features observed under similar conditions in IO M NaBr were associated with the halide ions in solution. In a later report [19], Fleischmann and Hill demonstrated that the vibrational energies have an unusually large dependence on the applied voltage, p~ticularly when cyanide is present 1211, and that the OH lineshape was sensitive to the exchange of K+ for Naf in the electrolyte. They further concluded that the water is associated with the halide ions by the direct correlation of the intensity of the water modes and the Ag-halide stretch vibration. Recently, the identity of the spectra as due to interfacial water was unambiguously established by the observation of the HDO bending mode [20]. The model which has emerged from these works is that of a water-halide complex which is bonded to the metal surface through an “active site”. The character of the “active site” has been suggested to be a Ag adatom. Electronic communication between the Ag and the water molecule is presumed to be more efficient at the adatom. This leads to hybrid intermediate states which provide an avenue for what is essentially the conventional resonant Raman effect. Although there is considerably controversy surrounding the nature of the chemically sensitive mechanism of enhancement it is now clear that, under the proper conditions, interfacial water in the electrochemical situation can be studied with SERS. In the present study we sought to quantify the conditions under which water could be observed with SERS. We were also interested in establishing the role of the environment of the Raman active water on the observed spectra. Specifically, we wanted to establish the details of the applied voltage dependence of the vibrational energies and their associated Raman intensities. In doing so, we have answered the question of whether the water molecules observed with SERS are typical of those present in the first layer or whether the SERS active water is unique. We present evidence that the latter is the case. However, because of its extreme sensitivity to its environment, the SERS active water, may serve as a key to the structure of the double layer.
2. Experiment The details of the experimental plan have been presented before [33], however they bear repeating in view of the importance of the procedure on obtaining the optimum signals. Conventional electrochemical control of the polycrystalline Ag samples were employed (Pyrex sample chamber with three electrode control through isolated Pt counter electrode and saturated calomel reference electrode). The solutions were prepared from reagent grade chemicals with triple distilled water and thoroughly purged with zero grade nitrogen in the sample chamber prior to
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any experiments. The incident 5 14.5 nm p-polarized radiation was focused to a line which was parallel to the entrance slit of the monochromator and perpendicular to the scattering plane. This beam met the sample at an angle of 45” and was limited to a power of approximately 150 mW at the sample. The illuminated area on the sample was approximately 0.03 cm2. The slits of the ISA-JY Ramanor 2000 were set so as to allow a bandpass of about 4 cm-‘. The detected intensity was provided by a thermoelectrically cooled RCA 31034 photomultiplier and NIM photon counting electronics. The data were acquired with an HP-85 computer which also provided the capability for signal averaging. The latter feature was important in the determination of the dependence of peak energies on the applied voltage. In this process we used the computer to linearly subtract out the sloping background due to the weaker broad interference from the bulk water, and then identified the peak position using a least squares quadratic data fit of the lineshape in the immediate vicinity of the peak energy. Before each data run of this type the monochromator was calibrated with the emission lines of a low pressure neon discharge lamp. The Ag was mounted at the end of a glass tube using Torrseal epoxy (Varian) and Teflon. Prior to each test the Ag was mechanically polished in a wet slurry of 1 pm alpha alumina on a Beuhler microcloth. The sample was then rinsed in triple distilled water and placed in the sample chamber, which already contained the solution, under voltage control with the surface of the sample very close (about 200 pm) from the glass wall of the chamber. For each of the reported trials the sample was subjected to a single oxidation-reduction treatment consisting of a triangular voltage program starting at -0.4 V (with respect to the saturated calomel electrode, SCE) with a slope of 0.3 V/min and a positive limit of 0.1 V (SCE). In this procedure approximately 250 mC/cm’ of charge were passed, corresponding to 1000 layers of Ag being transformed into AgCl and back again to metallic Ag. These electrochemical conditions were the optimum among a wide variety of trial parameters. However, even more striking was the realization on our part that the most critical factor in achieving large signal intensities was related to a photon stimulated process during the oxidation-reduction sequence. An independent study of this phenomenon has been published elsewhere [34]. In that report we describe how the AgCl layer is sensitive to photon irradiation immediately preceding the reduction step. The role of the photons is to create electron-hole pairs via impurity assisted non-direct excitations. The excited electrons combine with Ag+ ions in the AgCl to produce centers which are similar to those created in the photographic process. Electrochemical reduction which is catalyzed by these centers gives rise to a metallic surface whose topography is very different from the surface which is reduced in the absence of light. Preliminary scanning microscopic evaluation of the photoelectrochemically reduced surfaces shows that the more favorable surfaces have a much higher density of macro-
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scopic roughness features (200 nm boulders). The surface areas of these samples are certainly larger. There may be additional surface optical resonance as well which would lead to a larger enhancement. The end result is that the photoelectrochemically reduced surfaces show up to a factor of 10 larger SERS signals for all adsorbed species, including water. The photoelectrochemical treatment does not explicitly produce a special complex which requires the photons for its formation. This we have checked by performing the optimum surface preparation in the absence of a surface active molecule such as pyridine. After adding the pyridine we are able to observe the increase in signal over a similar experiment with the oxidation-reduction treatment performed in the dark. It was this realization along with a correlated discovery which made it possible for us to routinely observe high intensity signals from water. The second factor has to do with the environmental sensitivity of the water itself. While investigating the types of solution which support the’ largest photoelectrochemical effect we discovered that there were at least two types of adsorbed water depending on the nature of the cation and that the SERS intensity was significantly larger for one than the other.
3. Results Fig. 1 shows the SERS spectrum of the OH stretch vibration of adsorbed water recorded in aqueous molar KBr under the conditions described above. The lineshape, as revealed by Fleischmann et al. [ 151 is much narrower than that which is observed in association with the bulk water. The gradual sloping background which is present in each curve is, in fact, due to the weak signal coming from the bulk water in the thin layer. The sharpness of the main feature and its energy have led to the conclusion that it is due to the symmetric stretch vibration of water which is not strongly hydrogen bonded with its neighbors. There is no evidence of a lower energy peak such as is characteristic of bulk water. The latter structure has been attributed to hydrogen bonded water [35] or to the overtone of the scissors vibration [36]. There is, however, a well defined higher energy shoulder which is particularly prominent at the most positive applied voltage. This is the same feature identified by Fleischmann et al. [ 151. Note how the shoulder appears to move into the main peak as the applied voltage is made more negative. It is worth noting that we expect the zero charge condition to be close to that which applies to the top curve [37]. This means that the halide ions are more strongly attracted to the Ag for the conditions which existed during acquisition of the bottom curves. To clarify the nature of the shoulder in fig. 1 we show the influence of the solution pH in fig. 2. These spectra were recorded on the same surface which was first exposed to the acid electrolyte containing molar NaBr and then made
S.H. Macomber et al. / Enhanced Raman characterization
PKITON
ENERGY
SHIFT
of water on Ag
561
(cdl
Fig. 1. SERS spectra of adsorbed water in the O-H stretching region at several applied voltages. The sloping back~ound is caused by weak Raman scattering from solution water. The dominant feature is at - 3500 cm-’ with a shoulder at - 3550 cm-‘.
neutral by the addition of NaOH. As demonstrated here, the environment of the SERS active water has a direct influence on the lineshape. The peak around 3500 cm-’ is the same as the dominant feature in fig. 1. This peak is also observed in any solution which contains K+ ions, independent of the solution pH below 8. The peak around 3550 cm-’ is harder to contain without interference from the lower energy feature in IS” containing solutions. This is the dominant peak in K+ free bromide solutions which are neutral or basic. Under neutral conditions in NaBr we can independently study the applied voltage dependence of the 3550 cm-’ peak as shown in fig. 3. The maximum shifts to lower vibrational energy as the applied voltage is made more negative at a rate which is significantly larger than the shift of the 3500 cm-’ peak. If we compare fig. 3 with fig. 1 we can see that the shoulder in fig. 1 has become the main peak in fig. 3. It is quite possible that the origin of the large applied voltage dependence in fig. 3 is not a vibrational energy shift but rather is an exchange of intensity between the 3550 and the 3500 cm-’ modes. To further demonstrate the remarkable sensitivity of SERS active water we
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li
I PHOTON ENERGY SHIFT hi'l
of water on Ag
I
I
I
I
I
3400 5500 3600 PHOTON ENERGY SHIFT hi'1
Fig. 2. The influence of electrolyte pH on the O-H stretching mode of adsorbed water. (a) Scan taken after an oxidation-reduction cycle in 10m3 N H,SO, and 1 M NaBr. (b) Same surface after NaOH was added to the sample chamber to neutralize the acid. Both scans were taken at - 0.4 V (SCE). This shows the strong effect of the cation on the lineshape in the OH stretching region. Fig. 3. Spectra similar to fig. 1 except NaBr is used instead of KBr. The dominant feature is at - 3550 cm-‘. In fig. 1, this same structure is only a shoulder on the 3500 cm-’ peak,
have plotted the peak energies of the two main features as functions of the applied voltage in fig. 4. This time the comparison has been made between neutral solutions containing either K+ or Na+ to emphasize either the 3500 or the 3550 cm-’ peak, respectively. The shape of fig. 4b is complicated by the exchange between the two dominant surface forms. It is clear that curves 4a and 4b have different sloes indicating two unique surface water species. In the water scissors vibration region we detect a weaker structure shown in fig. 5. This feature is also slightly asymmetric, however, its applied voltage dependence is not as dramatic as that of the OH stretch vibration. In this same energy region, carbon contamination modes have been seen in association with SERS as reported previously. However, these are much broader and are dependent on the care with which the sample is handled prior to insertion into the controlled environment of the sample chamber. In fig. 6 we plot the peak energy versus the applied voltage for the scissors mode of the adsorbed water. We see a marked reversal of slope in the positive side of the graph. This is an
S.H. Macomber et al. / Enhanced Raman cha~acterjzation of water on Ag
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NaBr +
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+
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Ill
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-0.9
APPLIED
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I -0.7
VOLTAGE
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Fig. 4. The peak vibrational energy versus applied voltage taken from data similar to that in figs. t and 2: (a) for molar KBr; (b) for molar NaBr. Each data point is averaged from at least two measurements. The error bars represent average scatter. Fig. 5. SERS spectra of the H-O-H bending mode in molar KBr at three different applied voltages. Note the nonlinear dependence of the peak vibrational energy on applied voltage.
uncommon observation in SERS. Most of the prior reports of the applied voltage dependence of peak energies have revealed a positive slope. Strikingly absent from figs. 5 and 6 is any evidence that the SERS active water undergoes an orientation reversal, as might be expected on the basis of considerations of the electrostatics of the water dipole. The disappearance of the scissors mode for the most negative applied voltages was originally interpreted as evidence that the water was reversing its orientation near the zero charge condition. Also, if we compare the KBr curve in fig. 4 with the scissors vibration of fig. 6 we can see that there is the suggestion of a correlated inflection at -0.7 V. It is our view that this is insignificant compared to what would be expected from a molecule which is reversing its orientation from oxygen down to oxygen up as the applied voltage is made more negative. The magnitude of SERS from the scissors vibration becomes weak beyond - 0.9 V (SCE) so it is impossible to evaluate whether this vibrational energy is significantly shifted as the applied voltage passes from the positive to the negative side of the zero charge condition, More dramatic than the effects of
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KBr
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Fig. 6. The peak vibrational energy versus applied voltage of the H-O-H bending mode taken from data similar to that in fig. 5. The reversal in slope at the more positive applied voltages is an uncommon observation in SERS.
applied voltage are the environmental influences which the SERS water feels as is evident from fig. 2. It is important to comment on the stability of the water SERS signal. As has been discussed [38] before we also have verified that the SERS activity vanishes when the applied voltage is made more negative than about - 1.2 V (SCE). We are in the process of studying the temperature dependence of the loss of activity as well [39]. We expect that the surface does not restructure on the macroscopic level under these conditions. Rather, the loss of activity is associated with the irreversible breakdown of the “active site” which is quite likely a Ag adatom.
4. Discussion Primary among our conclusions is that the water which we are able to observe with SERS is not simply coordinated in a physisorbed state with the Ag. There is a dramatic dependence of the water SERS intensity on the anion and the cation types. Only when “active sites” are available on the surface and, at the same time, the surface is rough enough on the macroscopic level to support surface optical resonances do we have enough enhancing ability to produce a detectable signal due to adsorbed water. These particular conditions
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are established simultaneously when the Ag surface is subjected to an electrochemical oxidation-reduction treatment in the presence of halide ions and light. The form of water which we observe is not the same as that which has been studied by infra-red electrochemical modulation spectroscopy [40]. In those studies water dimers were the primary species observed on Pt. We expect, by contrast, that it is the monomer which is the most probable species which demonstrates SERS activity. The role of the halide ions here is threefold. (1) The structure breaking capability of the halide ions promotes the formation of water monomers which are not hydrogen bonded to adjacent water molecules. These monomers are more available to form weak bonds with the “active sites” on the Ag than are larger water clusters. (2) The halide is important in the formation of the Ag-halide film during the oxidation treatment. This film is an efficient matrix within which photoreduction can occur. The photoreduction produces a metallic surface which has a higher density of macroscopic roughness features. These features are important in the production of surface optical resonances required to observe the largest SERS signals. (3) The halide ions are expected to stabilize the “active site” itself. If the “active site” is a Ag adatom then, p~ticularly when the surface is positively chrged, we expect the surface to have a high coverage of free halide ions which could prevent the adatom from migrating to a surface step or kink thus losing its integrity [41]. It has been shown in a vacuum environment that the halogens induce the formation of adsorption bonds between water and Cu at temperatures higher than the desorption limit for water on clean Cu [ 131. It is entirely possible that the halide influences the metal adatom so as to make the adsorbed monomer more probable in our experiment as well. The adatom, which would be a slightly positively charged projection of the Ag surface under the condition of positive surface charge in our experiments, would present a particularly favorable site for orbital overlap with the oxygen lone pairs on the water. On the other hand, one does not have to hypothesize an explicit bond between the Ag and the water to conclude that the most probable orientation of the water monomer is with the oxygen end facing the metal. This will be the preferred configuration in view of the direction of the interfacial electric field. We expect the vibrational energy shift with applied voltage to be due to the perturbation introduced, by the eleotric field, into the interatomic potential of the water. Other authors have explained the voltage dependence of SERS vibrational energies by invoking variable charge exchange between the molecule and the metal [42]. This is not necessary. Significant vibrational energy shifts are expected in the absence of specific orbital overlap [43]. The influence of the surrounding water molecules and ions is an equally significant factor in determining the vibrational energy shift. This is particularly true with regard to
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Fig. 7. A proposed model of SERS active water. The SERS active water is associated adatom, with the oxygen end facing the metal. Surface halides stabilize the adatom. hydrated cations in the outer layer influence the SERS active water through hydrogen with their associated waters of solvation (represented here by dotted lines).
with the Strongly bonding
the role of halide ions which are expected to be capable of coming closer to the SERS active water than the cations. As the surface is made more positively charged the higher concentration of halide ions in the vicinity of the SERS active water increases the probability for interaction between the hydrogens of the water and the halide. The strength of this interaction is appreciable and the complex is expected to be stable [44]. We assume that this is the major cause of the reversal in the vibrational energy versus applied voltage relationship for the scissors vibration at more positive applied voltage (fig. 6). The halide probably plays a role in the positive applied voltage behavior of the OH stretch vibrational energy in fig. 4 as well. With this information we propose the model of SERS active water shown in fig. 7. This is similar to other conclusions reached from SERS water studies [ 16,191. The “active site” is an individual adatom of Ag which is stabilized by halide ions. The SERS active water is associated with the adatom with the oxygen end facing the metal. The additional halide ions directly influence the SERS active water under conditions of high positive surface charge as shown. The effect of the cations is less clear, however we propose the following model: Since cations are more strongly hydrated [45], even in the double layer, we expect that they will interact with the SERS active water through hydrogen bonds with their associated waters of solvation. The details of how these interactions modify the vibrational energies is as yet unknown. However, based upon the significant effects which hydrogen bonding has on bulk water in the OH stretch region it is not surprising that the exchange of Na+ for K+ in the layer adjacent to the SERS active water gives rise to two different OH vibrational energies. These two ions have different hydration numbers and the structure of the second water layer is expected to be different in each case. Similarly, the effect of the solution pH in Na-halide solutions is through the second water layer. There is no question that, contrary to several theoretical
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models of the double layer [ 1,7], there are significant interactions between the first water layer and adjacent water layers. Since the applied voltage dependencies of the vibrational energies for water are quite significant compared to many other SERS active molecules, and the effect of neighboring molecules on the SERS active water is quite significant, this suggests that SERS active water itself is a sensitive probe of the conditions which persist in the double layer.
Acknowledgment The portion of this work performed at R.P.I. was supported by the United States Department of Energy through a grant from the Office of Basic Energy Sciences. We thank R.K. Chang, M.R. Philpott, and M. Fleischmann for sharing data prior to their publication. T.M.D. is grateful to Richard P. Messmer for several helpful discussions.
References [l] W.R. Fawcett, S. Levine, R.M. de Nobriga and A.C. McDonald, J. Electroanal. Chem. 111 (1980) 163. [2] I.L. Cooper and J.A. Harrison, J. Electroanal. Chem. 86 (1978) 425. [3] J.G’M. Bockris and M. Habib, Electrochim. Acta 22 (1973) 41. [4] B.B. Damaskin and A.N. Frumkin, Electrochim. Acta 22 (1977) 41. [S] R. Parsons, Electroanal. Chem. 59 (1975) 229. [6] S.H. Lui, Surface Sci. 101 (1980) 49. [7] R. Parsons and R.M. Reeves, J. Electroanal. Chem. 123 (1981) 141. [S] H. Ibach and S. Lehwald, Surface Sci. 91 (1980) 187. [9] B. Sexton, Surface Sci. 94 (1980) 435. [lo] E.M. Stuve and R.J. Madix, Surface Sci. 111 (1981) 11. [l l] R.M. Ishikawa and A.T. Hubbard, J. Electroanal. Chem. 69 (1976) 317. [ 121 G. Fisher and J. Gland, Surface Sci. 94 (1980) 446. [13] J.K. Sass, N.V. Richardson, H. Neff and P.K. Roe, Chem. Phys. Letters 73 (1980) 209. [ 141 R.K. Chang and T.E. Furtak, Eds., Surface Enhanced Raman Scattering (Plenum, New York, 1982). [IS] M. Fleischmann, P.J. Hendra, I.R. Hill and M.E. Pemble, J. Electroanal. Chem. 117 (1981) 243. [16] B. Pettinger, M. Philpott and J. Gordon II, J. Chem. Phys. 74 (1981) 934. [17] B. Pettinger and H. Wetzel, Ber. Bunsenges. Physik. Chem. 85 (1981) 473. [IS] B. Pettinger, M. Philpott and J. Gordon II, Surface Sci. 105 (1981) 469. [ 191 M. Fleischmann and I.R. Hill, in ref. [14]. [20] T. Chen, J. Owen and R.K. Chang, Chem. Phys. Letters, to be published. [21] M. Fleischmann and I.R. Hill, to be published. [22] T.E. Furtak, G. Trott and B.H. Loo, Surface Sci. 101 (1980) 374. [23] E. Burstein, Y.J. Chen, C.Y. Chen, S. Lundquist and E. Tosatti, Solid State Commun. 29 (1979) 567.
568 [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37]
[38] [39] [40] [41] [42] [43] [44] [45]
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S.S. Jha, J.R. Kirtley and J.C. Tsang, Phys. Rev. B22 (1980) 3973. J.I. Gersten and A. Nitzan, J. Chem. Phys. 73 (1980) 3023. M. Kerker, D.S. Wang and H. Chew, Appl. Opt. 19 (1980) 4159. A. McQuillan, P. Hendra and M. Fleischmann, J. Electroanal. Chem. 65 (1975) 937. D.L. Jeanmaire and R.P. Van Duyne, J. Electroanal. Chem. 84 (1977) 1. T.E. Furtak, Solid State Commun. 28 (1978) 903. B. Pettinger, U. Wenning and D.M. Kolb, Ber. Bunsenges. Physik. Chem. 82 (1978) 1326. A. Otto, Appl. Surface Sci. 6 (1980) 309. S.G. Schultz, M. Janik-Czachor and R.P. Van Duyne, Surface Sci. 104 (1981) 419. S.H. Macomber and T.E. Furtak, Chem. Phys. Letters, to be published. S.H. Macomber, T.E. Furtak and T.M. Devine, Chem. Phys. Letters, to be published. G.E. Walrafen, in: Water, a Comprehensive Treatise, Vol. 1, Ed. F. Franks (Plenum, New York, 1972). J.R. Scherer, M.K. Go and S. Kim, J. Phys. Chem. 77 (1973) 2108. G. Valette, J. Electroanal. Chem. 132 (1982) 311. An accurate determination of the zero charge condition of Ag for a concentrated Br electrolyte has not been made, however, the tendency is for the zero charge condition to shift towards negative voltages as specific adsorption increases. B. Pettinger and H. Wetzer, in ref. [14]. S. Macomber and T.E. Furtak, to be published. A. Bewick, K. Kunimatsu, J. Robinson and J.W. Russell, J. Electroanal. Chem. 119 (1981) 175. A. Otto, I. Pockrand, J. Billmann and C. Pettenkofer, in ref. [14]. A.B. Anderson, R. Katz and E. Yeager, Chem. Phys. Letters 82 (1981) 130. A. Bennett, Surface Sci. 50 (1975) 77. H. Kistenmacher, H. Popkie and E. Clementi, J. Chem. Phys. 59 (1973) 5842. J.O’M. Bockris and A.K.N. Reddy, Modern Electrochemistry, Vol. 1 (Plenum, New York, 1973).
Surface Science 122 (1982) 556-568 North-Holland Publishing Company
ENHANCED RAMAN CHARACTERIZATION OF ADSORBED WATER AT THE ELECTROCHEMICAL DOUBLE LAYER ON SILVER S.H. MACOMBER Department
and T.E. FURTAK
of Physics, Rensselaer Polytechnic Institute,
Troy, New York 12181, USA
and T.M. DEVINE Research USA
and Development
Center, General Electric Corporation,
Received
28 June 1982; accepted
for publication
16 August
Schenectady,
New York 12301,
1982
Surface enhanced Raman scattering (SERS) from molecular water adsorbed on silver in an aqueous electrolyte is reported. The SERS active water is most probably associated with active sites on the metal which are stabilized by the double layer structure. The line shapes of the primary vibrations associated with the O-H stretch and the H-O-H bend are strong functions of the applied voltage, the electrolyte pH, and the bulk concentration of cations. At least two types of adsorbed water are observed. These data directly demonstrate that the structure of the first layer of water is significantly influenced by interaction with adjacent layers.
1. Introduction Arguments persist concerning the structure of the first layer of water in the double layer [ 1,2]. Its influence on the traditional electrochemical experiments such as differential capacitance has been debated for many years [l-3]. Models have been proposed involving various degrees of sophistication and detail [ 1,4,5]. It has been pointed out that many of these fall short of what is expected when the water molecules are subjected to the influence of the large electric fields which exist in the double layer [6]. Some models suggest that the first layer is established quite independently of the adjacent layers [ 1,7]. There is no other molecule which is of more importance in solid-electrolyte interfacial science than water. However, surprisingly little direct information exists about the details of adsorbed water. During the last two years electron loss spectroscopy [8,9], thermal desorption experiments [lo], low energy electron diffraction [ 111, and photoemission spectroscopy [ 10,12,13] have been employed to study single and multi-layer water adsorbed onto Pt, Cu, and Ag 0039-6028/82/0000-0/$02.75
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at low temperature. These vacuum spectroscopies have demonstrated that water adsorbs molecularly unless perturbed by co-adsorption impurities such as oxygen [lo]. In some cases, room temperature adsorption of water does not occur. However, coadsorbed halogens induce adsorption under the same conditions [ 131. At low temperature (100 K) it was also learned that water forms structures which resemble bulk ice as succeeding layers build up [g]. While this information is direct and firmly established, it does not take the place of in situ diagnosis of interfacial water. Surface enhanced Raman scattering (SERS) is ideally suited or such a study. By association with a metal surface, most commonly Ag, molecules demonstrate an anomalously large interaction cross-section in the Raman process [14]. This makes it possible to observe the vibrational characteristics of the molecules near the metal surface even when there are large numbers of similar molecules in the adjacent, transparent medium, in this case the aqueous electrolyte solution. While SERS has been under intense invesigation for several years, it has been applied only recently to the interfacial water problem [ 15-211. Part of the resaon for this is associated with the elusiveness of the enhancement phenomenon itself. Approximately two years ago, although contrary evidence existed at that time [22], the overwhelming consensus concerning the mechanics for SERS rested with the geometrically defined surface optical resonance moels [23-261. These theories exploited the large optical fields which exist at the surface of free-electron-like metals which have been made rough on the scale of 5 to 50 nm. Since the origin of the enhancement, in this picture, is through an effect dealing with the characteristics of the metal and its boundary alone, there is no reason why all material in the vicinity of the metal surface should not demonstrate enhancement. In contrast to this prediction, interfacial water was strikingly absent from the earlier SERS data recorded in electrochemical environments [27-291. This fact, along with other evidence [30-321, has led to another school of thought, which while admitting to the importance of field enhancement, invoked the operation of a second, chemically specific, mechanism. This second, as yet controversial, mechanism is required in order that the highest levels of SERS be achieved. Since water is a relatively weak Raman scatterer by comparison to pyridine it is necessary to arrange conditions such that both mechanisms are operative so as to provide a signal which can be detected above the limit allowed by modern instrumentation. Since the nature of the chemically specific mechanism was poorly understood at that time it was only by chance that the conditions were achieved such that interfacial water was finally observed using SERS. This was first accomplished by Flieschmann et al. [ 151 who used a heavy electrochemical oxidation-reduction treatment to roughen a Ag electrode in molar KCl. They observed structure in the OH stretch region which, by its lineshape, its independence of the bulk water and its displacement upon addition of D,O was identified as water associated with the metal surface.
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Subs~uently, Pettinger, Philpott and Gordon [16] showed that the features observed under similar conditions in IO M NaBr were associated with the halide ions in solution. In a later report [19], Fleischmann and Hill demonstrated that the vibrational energies have an unusually large dependence on the applied voltage, p~ticularly when cyanide is present 1211, and that the OH lineshape was sensitive to the exchange of K+ for Naf in the electrolyte. They further concluded that the water is associated with the halide ions by the direct correlation of the intensity of the water modes and the Ag-halide stretch vibration. Recently, the identity of the spectra as due to interfacial water was unambiguously established by the observation of the HDO bending mode [20]. The model which has emerged from these works is that of a water-halide complex which is bonded to the metal surface through an “active site”. The character of the “active site” has been suggested to be a Ag adatom. Electronic communication between the Ag and the water molecule is presumed to be more efficient at the adatom. This leads to hybrid intermediate states which provide an avenue for what is essentially the conventional resonant Raman effect. Although there is considerably controversy surrounding the nature of the chemically sensitive mechanism of enhancement it is now clear that, under the proper conditions, interfacial water in the electrochemical situation can be studied with SERS. In the present study we sought to quantify the conditions under which water could be observed with SERS. We were also interested in establishing the role of the environment of the Raman active water on the observed spectra. Specifically, we wanted to establish the details of the applied voltage dependence of the vibrational energies and their associated Raman intensities. In doing so, we have answered the question of whether the water molecules observed with SERS are typical of those present in the first layer or whether the SERS active water is unique. We present evidence that the latter is the case. However, because of its extreme sensitivity to its environment, the SERS active water, may serve as a key to the structure of the double layer.
2. Experiment The details of the experimental plan have been presented before [33], however they bear repeating in view of the importance of the procedure on obtaining the optimum signals. Conventional electrochemical control of the polycrystalline Ag samples were employed (Pyrex sample chamber with three electrode control through isolated Pt counter electrode and saturated calomel reference electrode). The solutions were prepared from reagent grade chemicals with triple distilled water and thoroughly purged with zero grade nitrogen in the sample chamber prior to
S.H. Macomber et al. / Enhanced Roman characterization
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any experiments. The incident 5 14.5 nm p-polarized radiation was focused to a line which was parallel to the entrance slit of the monochromator and perpendicular to the scattering plane. This beam met the sample at an angle of 45” and was limited to a power of approximately 150 mW at the sample. The illuminated area on the sample was approximately 0.03 cm2. The slits of the ISA-JY Ramanor 2000 were set so as to allow a bandpass of about 4 cm-‘. The detected intensity was provided by a thermoelectrically cooled RCA 31034 photomultiplier and NIM photon counting electronics. The data were acquired with an HP-85 computer which also provided the capability for signal averaging. The latter feature was important in the determination of the dependence of peak energies on the applied voltage. In this process we used the computer to linearly subtract out the sloping background due to the weaker broad interference from the bulk water, and then identified the peak position using a least squares quadratic data fit of the lineshape in the immediate vicinity of the peak energy. Before each data run of this type the monochromator was calibrated with the emission lines of a low pressure neon discharge lamp. The Ag was mounted at the end of a glass tube using Torrseal epoxy (Varian) and Teflon. Prior to each test the Ag was mechanically polished in a wet slurry of 1 pm alpha alumina on a Beuhler microcloth. The sample was then rinsed in triple distilled water and placed in the sample chamber, which already contained the solution, under voltage control with the surface of the sample very close (about 200 pm) from the glass wall of the chamber. For each of the reported trials the sample was subjected to a single oxidation-reduction treatment consisting of a triangular voltage program starting at -0.4 V (with respect to the saturated calomel electrode, SCE) with a slope of 0.3 V/min and a positive limit of 0.1 V (SCE). In this procedure approximately 250 mC/cm’ of charge were passed, corresponding to 1000 layers of Ag being transformed into AgCl and back again to metallic Ag. These electrochemical conditions were the optimum among a wide variety of trial parameters. However, even more striking was the realization on our part that the most critical factor in achieving large signal intensities was related to a photon stimulated process during the oxidation-reduction sequence. An independent study of this phenomenon has been published elsewhere [34]. In that report we describe how the AgCl layer is sensitive to photon irradiation immediately preceding the reduction step. The role of the photons is to create electron-hole pairs via impurity assisted non-direct excitations. The excited electrons combine with Ag+ ions in the AgCl to produce centers which are similar to those created in the photographic process. Electrochemical reduction which is catalyzed by these centers gives rise to a metallic surface whose topography is very different from the surface which is reduced in the absence of light. Preliminary scanning microscopic evaluation of the photoelectrochemically reduced surfaces shows that the more favorable surfaces have a much higher density of macro-
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scopic roughness features (200 nm boulders). The surface areas of these samples are certainly larger. There may be additional surface optical resonance as well which would lead to a larger enhancement. The end result is that the photoelectrochemically reduced surfaces show up to a factor of 10 larger SERS signals for all adsorbed species, including water. The photoelectrochemical treatment does not explicitly produce a special complex which requires the photons for its formation. This we have checked by performing the optimum surface preparation in the absence of a surface active molecule such as pyridine. After adding the pyridine we are able to observe the increase in signal over a similar experiment with the oxidation-reduction treatment performed in the dark. It was this realization along with a correlated discovery which made it possible for us to routinely observe high intensity signals from water. The second factor has to do with the environmental sensitivity of the water itself. While investigating the types of solution which support the’ largest photoelectrochemical effect we discovered that there were at least two types of adsorbed water depending on the nature of the cation and that the SERS intensity was significantly larger for one than the other.
3. Results Fig. 1 shows the SERS spectrum of the OH stretch vibration of adsorbed water recorded in aqueous molar KBr under the conditions described above. The lineshape, as revealed by Fleischmann et al. [ 151 is much narrower than that which is observed in association with the bulk water. The gradual sloping background which is present in each curve is, in fact, due to the weak signal coming from the bulk water in the thin layer. The sharpness of the main feature and its energy have led to the conclusion that it is due to the symmetric stretch vibration of water which is not strongly hydrogen bonded with its neighbors. There is no evidence of a lower energy peak such as is characteristic of bulk water. The latter structure has been attributed to hydrogen bonded water [35] or to the overtone of the scissors vibration [36]. There is, however, a well defined higher energy shoulder which is particularly prominent at the most positive applied voltage. This is the same feature identified by Fleischmann et al. [ 151. Note how the shoulder appears to move into the main peak as the applied voltage is made more negative. It is worth noting that we expect the zero charge condition to be close to that which applies to the top curve [37]. This means that the halide ions are more strongly attracted to the Ag for the conditions which existed during acquisition of the bottom curves. To clarify the nature of the shoulder in fig. 1 we show the influence of the solution pH in fig. 2. These spectra were recorded on the same surface which was first exposed to the acid electrolyte containing molar NaBr and then made
S.H. Macomber et al. / Enhanced Raman characterization
PKITON
ENERGY
SHIFT
of water on Ag
561
(cdl
Fig. 1. SERS spectra of adsorbed water in the O-H stretching region at several applied voltages. The sloping back~ound is caused by weak Raman scattering from solution water. The dominant feature is at - 3500 cm-’ with a shoulder at - 3550 cm-‘.
neutral by the addition of NaOH. As demonstrated here, the environment of the SERS active water has a direct influence on the lineshape. The peak around 3500 cm-’ is the same as the dominant feature in fig. 1. This peak is also observed in any solution which contains K+ ions, independent of the solution pH below 8. The peak around 3550 cm-’ is harder to contain without interference from the lower energy feature in IS” containing solutions. This is the dominant peak in K+ free bromide solutions which are neutral or basic. Under neutral conditions in NaBr we can independently study the applied voltage dependence of the 3550 cm-’ peak as shown in fig. 3. The maximum shifts to lower vibrational energy as the applied voltage is made more negative at a rate which is significantly larger than the shift of the 3500 cm-’ peak. If we compare fig. 3 with fig. 1 we can see that the shoulder in fig. 1 has become the main peak in fig. 3. It is quite possible that the origin of the large applied voltage dependence in fig. 3 is not a vibrational energy shift but rather is an exchange of intensity between the 3550 and the 3500 cm-’ modes. To further demonstrate the remarkable sensitivity of SERS active water we
562
S.H. Macomber et al. / Enhanced Raman characterization
li
I PHOTON ENERGY SHIFT hi'l
of water on Ag
I
I
I
I
I
3400 5500 3600 PHOTON ENERGY SHIFT hi'1
Fig. 2. The influence of electrolyte pH on the O-H stretching mode of adsorbed water. (a) Scan taken after an oxidation-reduction cycle in 10m3 N H,SO, and 1 M NaBr. (b) Same surface after NaOH was added to the sample chamber to neutralize the acid. Both scans were taken at - 0.4 V (SCE). This shows the strong effect of the cation on the lineshape in the OH stretching region. Fig. 3. Spectra similar to fig. 1 except NaBr is used instead of KBr. The dominant feature is at - 3550 cm-‘. In fig. 1, this same structure is only a shoulder on the 3500 cm-’ peak,
have plotted the peak energies of the two main features as functions of the applied voltage in fig. 4. This time the comparison has been made between neutral solutions containing either K+ or Na+ to emphasize either the 3500 or the 3550 cm-’ peak, respectively. The shape of fig. 4b is complicated by the exchange between the two dominant surface forms. It is clear that curves 4a and 4b have different sloes indicating two unique surface water species. In the water scissors vibration region we detect a weaker structure shown in fig. 5. This feature is also slightly asymmetric, however, its applied voltage dependence is not as dramatic as that of the OH stretch vibration. In this same energy region, carbon contamination modes have been seen in association with SERS as reported previously. However, these are much broader and are dependent on the care with which the sample is handled prior to insertion into the controlled environment of the sample chamber. In fig. 6 we plot the peak energy versus the applied voltage for the scissors mode of the adsorbed water. We see a marked reversal of slope in the positive side of the graph. This is an
S.H. Macomber et al. / Enhanced Raman cha~acterjzation of water on Ag
563
++
NaBr +
b
+
+ *
Ill
I
I
-I I
-0.9
APPLIED
I
I -0.7
VOLTAGE
I
I II -0.5 -0.3 tvalls
YJ SCE)
I
I -0.1
I
I
I
1570 PHOTON
1590 ENERGY
I 1610 SHIFT (cme’)
I 1630
Fig. 4. The peak vibrational energy versus applied voltage taken from data similar to that in figs. t and 2: (a) for molar KBr; (b) for molar NaBr. Each data point is averaged from at least two measurements. The error bars represent average scatter. Fig. 5. SERS spectra of the H-O-H bending mode in molar KBr at three different applied voltages. Note the nonlinear dependence of the peak vibrational energy on applied voltage.
uncommon observation in SERS. Most of the prior reports of the applied voltage dependence of peak energies have revealed a positive slope. Strikingly absent from figs. 5 and 6 is any evidence that the SERS active water undergoes an orientation reversal, as might be expected on the basis of considerations of the electrostatics of the water dipole. The disappearance of the scissors mode for the most negative applied voltages was originally interpreted as evidence that the water was reversing its orientation near the zero charge condition. Also, if we compare the KBr curve in fig. 4 with the scissors vibration of fig. 6 we can see that there is the suggestion of a correlated inflection at -0.7 V. It is our view that this is insignificant compared to what would be expected from a molecule which is reversing its orientation from oxygen down to oxygen up as the applied voltage is made more negative. The magnitude of SERS from the scissors vibration becomes weak beyond - 0.9 V (SCE) so it is impossible to evaluate whether this vibrational energy is significantly shifted as the applied voltage passes from the positive to the negative side of the zero charge condition, More dramatic than the effects of
564
S. H. Macomber et al. / Enhanced Raman characterization
of water on Ag
KBr
_._I608 -
t
;E u
t
bk 2 &
t
tt
tt
t
1604 -
w" 5 5 s B
1600 -
t
t
II
I -1.1
I
I -0.9
I
I -0.7
APPLIED VOLTAGE
I I I -0.3 -0.5 (voltsvs SCEI I
I
I -0.1
I
Fig. 6. The peak vibrational energy versus applied voltage of the H-O-H bending mode taken from data similar to that in fig. 5. The reversal in slope at the more positive applied voltages is an uncommon observation in SERS.
applied voltage are the environmental influences which the SERS water feels as is evident from fig. 2. It is important to comment on the stability of the water SERS signal. As has been discussed [38] before we also have verified that the SERS activity vanishes when the applied voltage is made more negative than about - 1.2 V (SCE). We are in the process of studying the temperature dependence of the loss of activity as well [39]. We expect that the surface does not restructure on the macroscopic level under these conditions. Rather, the loss of activity is associated with the irreversible breakdown of the “active site” which is quite likely a Ag adatom.
4. Discussion Primary among our conclusions is that the water which we are able to observe with SERS is not simply coordinated in a physisorbed state with the Ag. There is a dramatic dependence of the water SERS intensity on the anion and the cation types. Only when “active sites” are available on the surface and, at the same time, the surface is rough enough on the macroscopic level to support surface optical resonances do we have enough enhancing ability to produce a detectable signal due to adsorbed water. These particular conditions
S. H. Macomber et al. / Enhanced Raman characterization
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are established simultaneously when the Ag surface is subjected to an electrochemical oxidation-reduction treatment in the presence of halide ions and light. The form of water which we observe is not the same as that which has been studied by infra-red electrochemical modulation spectroscopy [40]. In those studies water dimers were the primary species observed on Pt. We expect, by contrast, that it is the monomer which is the most probable species which demonstrates SERS activity. The role of the halide ions here is threefold. (1) The structure breaking capability of the halide ions promotes the formation of water monomers which are not hydrogen bonded to adjacent water molecules. These monomers are more available to form weak bonds with the “active sites” on the Ag than are larger water clusters. (2) The halide is important in the formation of the Ag-halide film during the oxidation treatment. This film is an efficient matrix within which photoreduction can occur. The photoreduction produces a metallic surface which has a higher density of macroscopic roughness features. These features are important in the production of surface optical resonances required to observe the largest SERS signals. (3) The halide ions are expected to stabilize the “active site” itself. If the “active site” is a Ag adatom then, p~ticularly when the surface is positively chrged, we expect the surface to have a high coverage of free halide ions which could prevent the adatom from migrating to a surface step or kink thus losing its integrity [41]. It has been shown in a vacuum environment that the halogens induce the formation of adsorption bonds between water and Cu at temperatures higher than the desorption limit for water on clean Cu [ 131. It is entirely possible that the halide influences the metal adatom so as to make the adsorbed monomer more probable in our experiment as well. The adatom, which would be a slightly positively charged projection of the Ag surface under the condition of positive surface charge in our experiments, would present a particularly favorable site for orbital overlap with the oxygen lone pairs on the water. On the other hand, one does not have to hypothesize an explicit bond between the Ag and the water to conclude that the most probable orientation of the water monomer is with the oxygen end facing the metal. This will be the preferred configuration in view of the direction of the interfacial electric field. We expect the vibrational energy shift with applied voltage to be due to the perturbation introduced, by the eleotric field, into the interatomic potential of the water. Other authors have explained the voltage dependence of SERS vibrational energies by invoking variable charge exchange between the molecule and the metal [42]. This is not necessary. Significant vibrational energy shifts are expected in the absence of specific orbital overlap [43]. The influence of the surrounding water molecules and ions is an equally significant factor in determining the vibrational energy shift. This is particularly true with regard to
566
S.H. Macomber et al. / Enhanced Raman characterization
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Fig. 7. A proposed model of SERS active water. The SERS active water is associated adatom, with the oxygen end facing the metal. Surface halides stabilize the adatom. hydrated cations in the outer layer influence the SERS active water through hydrogen with their associated waters of solvation (represented here by dotted lines).
with the Strongly bonding
the role of halide ions which are expected to be capable of coming closer to the SERS active water than the cations. As the surface is made more positively charged the higher concentration of halide ions in the vicinity of the SERS active water increases the probability for interaction between the hydrogens of the water and the halide. The strength of this interaction is appreciable and the complex is expected to be stable [44]. We assume that this is the major cause of the reversal in the vibrational energy versus applied voltage relationship for the scissors vibration at more positive applied voltage (fig. 6). The halide probably plays a role in the positive applied voltage behavior of the OH stretch vibrational energy in fig. 4 as well. With this information we propose the model of SERS active water shown in fig. 7. This is similar to other conclusions reached from SERS water studies [ 16,191. The “active site” is an individual adatom of Ag which is stabilized by halide ions. The SERS active water is associated with the adatom with the oxygen end facing the metal. The additional halide ions directly influence the SERS active water under conditions of high positive surface charge as shown. The effect of the cations is less clear, however we propose the following model: Since cations are more strongly hydrated [45], even in the double layer, we expect that they will interact with the SERS active water through hydrogen bonds with their associated waters of solvation. The details of how these interactions modify the vibrational energies is as yet unknown. However, based upon the significant effects which hydrogen bonding has on bulk water in the OH stretch region it is not surprising that the exchange of Na+ for K+ in the layer adjacent to the SERS active water gives rise to two different OH vibrational energies. These two ions have different hydration numbers and the structure of the second water layer is expected to be different in each case. Similarly, the effect of the solution pH in Na-halide solutions is through the second water layer. There is no question that, contrary to several theoretical
S.H. Macomber et al. / Enhanced Raman characterization
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models of the double layer [ 1,7], there are significant interactions between the first water layer and adjacent water layers. Since the applied voltage dependencies of the vibrational energies for water are quite significant compared to many other SERS active molecules, and the effect of neighboring molecules on the SERS active water is quite significant, this suggests that SERS active water itself is a sensitive probe of the conditions which persist in the double layer.
Acknowledgment The portion of this work performed at R.P.I. was supported by the United States Department of Energy through a grant from the Office of Basic Energy Sciences. We thank R.K. Chang, M.R. Philpott, and M. Fleischmann for sharing data prior to their publication. T.M.D. is grateful to Richard P. Messmer for several helpful discussions.
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S.S. Jha, J.R. Kirtley and J.C. Tsang, Phys. Rev. B22 (1980) 3973. J.I. Gersten and A. Nitzan, J. Chem. Phys. 73 (1980) 3023. M. Kerker, D.S. Wang and H. Chew, Appl. Opt. 19 (1980) 4159. A. McQuillan, P. Hendra and M. Fleischmann, J. Electroanal. Chem. 65 (1975) 937. D.L. Jeanmaire and R.P. Van Duyne, J. Electroanal. Chem. 84 (1977) 1. T.E. Furtak, Solid State Commun. 28 (1978) 903. B. Pettinger, U. Wenning and D.M. Kolb, Ber. Bunsenges. Physik. Chem. 82 (1978) 1326. A. Otto, Appl. Surface Sci. 6 (1980) 309. S.G. Schultz, M. Janik-Czachor and R.P. Van Duyne, Surface Sci. 104 (1981) 419. S.H. Macomber and T.E. Furtak, Chem. Phys. Letters, to be published. S.H. Macomber, T.E. Furtak and T.M. Devine, Chem. Phys. Letters, to be published. G.E. Walrafen, in: Water, a Comprehensive Treatise, Vol. 1, Ed. F. Franks (Plenum, New York, 1972). J.R. Scherer, M.K. Go and S. Kim, J. Phys. Chem. 77 (1973) 2108. G. Valette, J. Electroanal. Chem. 132 (1982) 311. An accurate determination of the zero charge condition of Ag for a concentrated Br electrolyte has not been made, however, the tendency is for the zero charge condition to shift towards negative voltages as specific adsorption increases. B. Pettinger and H. Wetzer, in ref. [14]. S. Macomber and T.E. Furtak, to be published. A. Bewick, K. Kunimatsu, J. Robinson and J.W. Russell, J. Electroanal. Chem. 119 (1981) 175. A. Otto, I. Pockrand, J. Billmann and C. Pettenkofer, in ref. [14]. A.B. Anderson, R. Katz and E. Yeager, Chem. Phys. Letters 82 (1981) 130. A. Bennett, Surface Sci. 50 (1975) 77. H. Kistenmacher, H. Popkie and E. Clementi, J. Chem. Phys. 59 (1973) 5842. J.O’M. Bockris and A.K.N. Reddy, Modern Electrochemistry, Vol. 1 (Plenum, New York, 1973).