Characterization of Water Structure on Silver Electrode Surfaces by

Jan 26, 2010 - Camilo A. Angelucci , Renato C. Ambrosio , and Andrew A. Gewirth ... T. Y. Becky Leung , Dennis P. Butcher , Jr. , and Andrew A. Gewirt...
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Anal. Chem. 2010, 82, 1305–1310

Characterization of Water Structure on Silver Electrode Surfaces by SERS with Two-Dimensional Correlation Spectroscopy Renato C. Ambrosio† and Andrew A. Gewirth*,‡ Departamento de Quı´mica, Universidade Federal do Sergipe, Sa˜o Cristo´va˜o, SE, Brazil, and Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 Surface-enhanced Raman scattering (SERS) spectra were collected continuously during cyclic voltammetric measurements on silver electrodes in alkaline aqueous solution at room temperature. Three water librational modes as well as the bending mode peak were observed in cathodic potential range. The 2D-COS analysis of spectra collected in LiOH and KOH solutions showed that the librational bands appear prior to the bending band on the cathodic scan, while in CsOH solution the potential dependence of these bands was identical. A comparison of librational band frequencies revealed that the water molecules around Cs+ cations arranged on the electrode surface were poorly hydrogen bonded in contrast to Li+ and K+. The water bending band of spectra collected in LiOH solution was found to be the convolution of two contributions, consistent with a two state model of water arranged on an electrode surface. Understanding the interactions of water molecules at the electrode-electrolyte interface is one of the most important topics to improve fundamental microscopic-level knowledge about the solid-liquid interface.1-4 Such interactions affect directly electrode-electrolyte solution interfacial reactions.5-7 Water adsorption on charged metallic surfaces has been intensively investigated by means of a variety of in situ techniques which provided information about the microscopic structure of the electrochemical interface.8-15 Surface-enhanced Raman scattering (SERS) is a * To whom correspondence should be addressed. Phone: 217-333-8329. Fax: 217-244-3186. E-mail: [email protected]. † Universidade Federal do Sergipe. ‡ University of Illinois at Urbana-Champaign. (1) Ye, S.; Kondo, T.; Hoshi, N.; Inukai, J.; Yoshimoto, S.; Osawa, M.; Itaya, K. Electrochemistry 2009, 77, 2–20. (2) Bard, A. J.; Abruna, H. D.; Chidsey, C. E.; Faulkner, L. R.; Feldberg, S. W.; Itaya, K.; Majda, M.; Melroy, O.; Murray, R. W. J. Phys. Chem. 1993, 97, 7147–7173. (3) Thiel, P. A.; Madey, T. E. Surf. Sci. Rep. 1987, 7, 21. (4) Schmickler, W. In Structure of Electrified Interfaces; Lipkowski, J., Ross, P. N., Eds.; VCH-Publishers: New York, 1993; p 1. (5) Parsons, R. Chem. Rev. (Washington, DC, U. S.) 1990, 90, 813–826. (6) Savinova, E. R.; Zemlyanov, D. Y.; Scheybal, A.; Schlogl, R.; Doblhofer, E. Langmuir 1999, 15, 6552–6556. (7) Menzel, D. Science (Washington, DC, U. S.) 2002, 295, 58–59. (8) Schultz, Z. D.; Shaw, S. K.; Gewirth, A. A. J. Am. Chem. Soc. 2005, 127, 15916–15922. (9) Toney, M. F.; Howard, J. N.; Richer, J.; Borges, G. L.; Gordon, J. G.; Melroy, O. R.; Wiesler, D. G.; Yee, D.; Sorensen, L. B. Nature 1994, 368, 444– 446. 10.1021/ac902299u  2010 American Chemical Society Published on Web 01/26/2010

valuable tool to investigate the vibrational characteristics of interfacial species due to the extraordinary enhancement of the Raman signal of surface associated molecules and the relatively small contribution from the bulk water.14,15 However, previous work utilizing SERS to examine water and other species at the buried electrochemical interface suffers from broad bands and an inability to discriminate between oscillators in different environments.14-21 Spectral insight can be substantially augmented through the application of two-dimensional correlation analysis (2D-COS) methods.22,23 However, while correlation approaches have been applied to in situ IR spectroelectrochemical measurements,11,24 they have not yet been applied to SERS electrochemical problems. We report here the results of in situ spectroelectrochemical SERS experiments examining the water arrangement on electrode electrolyte interface in alkaline solutions. The sequential order of vibrational mode activation at cathodic potentials are elucidated by the analysis of SERS spectra with 2D-COS formalisms. Previous SERS studies of water-electrode interactions were performed in a stationary fashion in which the system is allowed to equilibrate at each potential before spectral acquisition.14,15,17-21 The present study differs from these through the use of continuous potential scanning during in situ spectral collection. Additionally, the experimental data are examined by using Generalized (10) Morgenstern, K.; Nieminen, J. Phys. Rev. Lett. 2002, 88, 066102/066101– 066102/066104. (11) Ataka, K.; Osawa, M. Langmuir 1998, 14, 951–959. (12) Ataka, K.; Yotsuyanagi, T.; Osawa, M. J. Phys. Chem. 1996, 100, 10664– 10672. (13) Magnussen, O. M.; Hagebock, J.; Hotlos, J.; Behm, R. J. Faraday Discuss. 1992, 94, 399–400. (14) Jiang, Y. X.; Li, J. F.; Wu, D. Y.; Yang, Z. L.; Ren, B.; Hu, J. W.; Chow, Y. L.; Tian, Z. Q. Chem. Commun. 2007, 4608–4610. (15) Iwasaki, N.; Sasaki, Y.; Nishina, Y. Surf. Sci. 1988, 198, 524–540. (16) Savinova, E. R.; Kraft, P.; Pettinger, B.; Doblhofer, K. J. Electroanal. Chem. 1997, 430, 47–56. (17) Itoh, T.; Sasaki, Y.; Maeda, T.; Horie, C. Surf. Sci. 1997, 389, 212–222. (18) Tian, Z.-Q.; Ren, B.; Chen, Y.-X.; Zou, S.-Z.; Mao, B.-W. J. Chem. Soc., Faraday Trans. 1996, 92, 3829–3838. (19) Tian, Z. Q.; Chen, Y. X.; Mao, B. W.; Li, C. Z.; Wang, J.; Liu, Z. F. Chem. Phys. Lett. 1995, 240, 224–229. (20) Chen, Y. X.; Zou, S. Z.; Huang, K. Q.; Tian, Z. Q. J. Raman Spectrosc. 1998, 29, 749–756. (21) Chen, Y. X.; Otto, A. J. Raman Spectrosc. 2005, 36, 736–747. (22) Ozaki, Y.; Ojima, S.; Noda, I. Vib. Spectrosc. 2004, 36, 141–142. (23) Noda, I. Vib. Spectrosc. 2004, 36, 143–165. (24) Osawa, M.; Yoshii, K.; Hibino, Y.-i.; Nakano, T.; Noda, I. J. Electroanal. Chem. 1997, 426, 11–16.

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Two-Dimensional Correlation Analysis.25 The method provides increased resolution enhancement of underlying overlapped bands which are deconvoluted effectively by spreading the peaks along the second spectral dimension, allowing the establishment of unambiguous assignments through correlation of bands and probing the specific sequential order of spectral intensity changes taking place in the system subjected to an external perturbation - in our case, the electrode potential. EXPERIMENTAL SECTION All solutions were prepared from ultrapure water (Milli-Q UVplus, Millipore, Inc.; 18.2 MΩ cm). Reagent-grade LiOH, KOH, and CsOH (Aldrich, 99.98%) were used for preparing solutions. Polycrystalline silver was used as a working electrode. The working surface of each electrode was hand-polished to optical flatness using a variety of diamond pastes, from 10 to 0.25 µm grit size (Buehler). Following the mechanical polishing step, the silver electrodes were polished chemically before each experiment to remove silver oxide and contaminants resulting from the exposure of the surface to ambient atmosphere. The chemical polishing was based upon a procedure described previously.26 Electrochemical roughness was induced on the electrode by applying 10 oxidation-reduction cycles ORCs (in the range of -0.3 to 0.3 V vs Ag AgCl/Ag) at a sweep rate of 5 mV s-1 in the presence of 3 mol L-1 KCl electrolyte. The cycling was finished at -0.3 V, then the applied potential was changed to -0.4 V, and the silver electrode was kept for 5 min at this potential to ensure that the silver surface is fully reduced and free of halide; after that the working electrode was removed and very carefully rinsed with water. SERS spectra were obtained from the roughened Ag polycrystal immersed in Ar-saturated 0.1 mol L-1 LiOH, KOH, and CsOH solutions, using instrumentation described previously and a leak free 0.1 mol L-1 KCl AgCl/Ag electrode (Cypress Systems) as the reference.27 Spectra were continuously obtained with an acquisition rate of 1 spectra per second, during a voltammetric scan at 5 mV s-1 at potentials between -1.7 and -0.8 V vs Ag/AgCl. This potential range lies well below the potential of zero charge (pzc) which is at -0.26 V vs Ag/AgCl in 0.1 M NaOH;28 it is expected that the collected spectra should be dependent on the identity of the cation as has been observed elsewhere.29 Voltammetry was consistent with that reported in the literature.15,17 SERS measurements were performed in triplicate to ensure reproducibility. The raw SERS data evince a strong potential dependent background which was corrected by fitting a polynomial curve to the spectra. The resulting curved polynomial lines were then subtracted from the corresponding spectra to be corrected. Finally, the background corrected SERS were spectra arranged in the columns of data matrix. Prior to calculation of 2 D correlation spectra, noise was removed from (25) Noda, I.; Ozaki, Y. Two-Dimensional Correlation Spectroscopy - Applications in Vibrational and Optical Spectroscopy; John Wiley and Sons: New York, 2004. (26) Smolinski, S.; Zelenay, P.; Sobkowski, J. J. Electroanal. Chem. 1998, 442, 41–47. (27) Bae, S.-E.; Stewart, K. L.; Gewirth, A. A. J. Am. Chem. Soc. 2007, 129, 10171–10180. (28) Maurice, V.; Klein, L. H.; Strehblow, H. H.; Marcus, P. J. Phys. Chem. C 2007, 111, 16351–16361. (29) Yamakata, A.; Osawa, M. J. Am. Chem. Soc. 2009, 131, 6892.

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spectra by wavelet transformation.30 Two Dimensional Correlation Analysis was performed by using in-house designed software. Prior to the calculations, the average (or mean) spectrum of the spectra in the data set was subtracted from each of the spectra to obtain a set of “dynamic” spectra. Synchronous and asynchronous correlation spectra were then calculated from these dynamic spectra using the formalisms described by Noda.25 2D-COS is a well established data analysis technique in many fields of spectroscopy, and further description of the method will not be given here. RESULTS Figure 1 presents the potential-dependent SERS spectral intensities observed during the cyclic voltammetry on Ag electrode in LiOH, KOH, and CsOH solutions. The peak at ∼1615 cm-1 is consistent with the intramolecular bending mode vibration of adsorbed water molecules with hydrogen atoms oriented toward the electrode surface.12 The low frequency of the bending mode of adsorbed water at potentials below the pzc has been explained as arising as a consequence of the tilted oxygen-up orientation of the molecule, which prevents hydrogen bonding among water molecules of the first layer and allows the interaction of one of the oxygen lone pairs with the surface.31 The signal between 400 cm-1 and 900 cm-1 is attributed to the intermolecular librational modes.20 Significant changes in the spectra occur at potentials where the voltammograms exhibit an increase in the magnitude of negative current due to hydrogen evolution (Supporting Information). Hydrogen evolution will result in production of OH- ions in the neighborhood of the surface. However, in contrast to spectra obtained at potentials positive of the pzc,16 there is no evidence that these OH- ions associate with the Ag surface. In particular, the strong observed bending and librational signals indicate that water molecules rather than OH- are predominant on the surface due to the very strong repulsive electrostatic field of the negatively charged electrode.20 The most interesting aspect in Figure 1 is the potential dependence of the band intensities. In the anodic scan, the bending mode band vanishes at more negative potentials relative to the librational bands observed in LiOH and KOH electrolytes. The opposite sequence can be observed on the cathodic scan, that is, the librational band appears prior to the bending band. Additionally, the three librational modes present distinct potential dependence. On the other hand, in CsOH electrolytes the potential dependence of the bending and libration modes are identical. Additionally, the signal-to-noise ratio is always inferior in CsOH solutions probably due to the diminished reflectivity of the Ag electrode in this solution.17 The librational envelope between 500 and 800 cm-1 shows three prominent contributions. These are sharper and more visible than that observed by Raman spectroscopy in pure water,32 presumably due to orientation disorder in pure water which makes the identification of the number of bands and their frequencies much more difficult relative to water molecules arranged at an electrode surface. In LiOH solutions, (30) Chau, F.-T.; Liang, Y.-Z.; Gao, J.; Shao, X.-G. Chemometrics - From Basics to Wavelet Transform; Wiley: New York, 2004. (31) Yamakata, A.; Osawa, M. J. Phys. Chem. C 2008, 112, 11427–11432. (32) Carey, D. M.; Korenowski, G. M. J. Chem. Phys. 1998, 108, 2669–2675.

Figure 2. Asynchronous plots calculated with spectra recorded between -1.0 and -1.4 V vs Ag/AgCl on a cathodic scan in (a) LiOH and (b) KOH solution. Blue hues show negative peaks.

Figure 1. SERS spectral intensities observed during cyclic voltammetry experiments at 5 mV s-1 on (a) 0.1 M LiOH, (b) 0.1 M KOH, and (c) 0.1 M CsOH. The bottom of each plot represents the cathodic scan.

frustrated rotation of water molecules appear at 535 cm-1 as wagging, at 642 cm-1 as twisting and at 751 cm-1 rocking librational modes.33,34 Figure 1 shows that the spectra collected in LiOH solution exhibit more pronounced potential dependence of bending mode band relative to the same band measured in the other two electrolytes. However, only with 2D-COS it is possible to determine whether this dependence relates to a frequency shift or intensity changes in highly overlapped bands. The synchronous (33) Abe, K.; Miasa, T.; Ohtake, Y.; Nakano, K.; Nakajima, M.; Yamamoto, H.; Shigenari, T. J. Korean Phys. Soc. 2005, 46, 300–302. (34) Itoh, H.; Kawamura, K.; Hondoh, T.; Mae, S. J. Chem. Phys. 1998, 109, 4894–4899.

and asynchronous 2D correlation plots were calculated with the cathodic and anodic branch of the spectroelectrochemcial data collected in LiOH and KOH solutions. The asynchronous intensities calculated from the spectra collected in LiOH and KOH solutions in the cathodic scan are plotted in Figure 2. The synchronous plots (Supporting Information) present essentially two autopeaks: one related to the bending mode and another related to the contribution from the librational modes. The corresponding cross peaks are positive since all the intensities change in the same direction, that is decrease on the anodic scan and increase on the cathodic scan. While the calculation of the synchronous correlation intensity is reliable for all data sets, the calculation of the asynchronous correlation intensities was not reliable for the data set collected in CsOH due to lower signalto-noise ratio and baseline fluctuations implying nonmonotonical behavior of spectral intensities changes resulting in a very noisy asynchronous correlation map. The asynchronous correlation plot shows both negative and positive cross peaks. The asynchronous spectrum is asymmetric with respect to the diagonal line and develops cross peaks only if the intensities of two spectral features change out of phase with Analytical Chemistry, Vol. 82, No. 4, February 15, 2010

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Table 1. Peak Positions and Peak Signs Determined for the Asynchronous 2D Correlation Plots Including Spectra LiOH

KOH

location

peak sign

location

peak sign

1680-1615 cm-1 1615-650 cm-1 1615-550 cm-1 650-550 cm-1 760-650 cm-1

positive negative negative negative positive

1680-1615 cm-1 1615-640 cm-1 1615-540 cm-1 640-540 cm-1 750-640 cm-1

absent absent negative negative absent

each other (i.e., delayed or accelerated). The sign of an asynchronous cross-peak is positive if the intensity change at wavenumber ν1 (with ν1 > ν2) occurs prior to the change of intensity at ν2 in the sequence of spectra.25 However, if the sign of the corresponding synchronous cross-peak intensity is negative, this rule is reversed. The positions as well as the signs of the cross-peaks indicating the correlation of the spectral changes at the respective positions are compiled in Table 1. Figure 2 indicates that there are four asynchronous correlation peaks between the five spectral features that are changing out of phase in the spectra collected in LiOH solution. The negative correlation between twist and wag and between rock and wag librations suggests that the wag libration appears prior to twist and rock modes. The positive correlation between rock and twist librational modes suggests that the twist mode is activated prior to the rock mode, that is the sequence of librational mode appearance on the cathodic scan in LiOH electrolyte is wag prior to rock and rock prior to twist. The negative correlation between the bending and librational modes confirms that the bending band appears after all librational modes. The corresponding asynchronous correlation plots calculated with the spectra collected in LiOH and KOH solutions during the anodic scan confirmed that the bending band vanishes prior to the libration bands. Another interesting feature observed only in the asynchronous plot calculated with spectra collected in LiOH solution (Figure 2a) is the presence of a correlation between peaks located at 1615 cm-1 and 1660 cm-1 suggesting that the peak of the bending vibration in Figure 1a arises from the convolution of intensities resulting from changes of two bending contributions. This shows that there are five peaks (three libration peaks and two bending peaks) deconvolved from the spectra collected in LiOH solution. These findings are very important since previous analyses of SERS spectroelectrochemical data did not reveal the three librational contributions nor the phase difference between librational and bending modes.15,17-21 It is therefore clear now that there are subtle changes in all of the characteristic spectral features, and the generalized 2D correlation analysis is helpful in identifying these changes. DISCUSSION The SERS spectra presented above provide new insight into the structure of water at the Ag/electrolyte interface in basic electrolyte during cathodic potential excursions. The 2D-COS analysis reveals the potential dependence of bands is not identical in Li- and K-containing solutions. Some insights on water arrangement on the electrode surface can be extracted from the frequency analysis of librational bands (Table 2). Table 2 shows that the librational frequencies decrease slightly from LiOH to KOH 1308

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Table 2. Water Molecular Librational SERS Peak Centers As a Function of the Electrolyte Solution mode wag twist rock

LiOH -1

542 cm 650 cm-1 756 cm-1

KOH

CsOH -1

538 cm 644 cm-1 747 cm-1

479 cm-1 590 cm-1 712 cm-1

solutions. On the other hand, the librational band positions measured in CsOH electrolyte are red-shifted by around 60 cm-1 relative to KOH solution. The hydrogen bonds constitute the restraining force which converts the three rotations which free water molecules would have in the gas phase to the corresponding three librations in liquid phase. The librational frequency decrease occurs because the librational force constants decreases as intermolecular hydrogen bonding decreases.32 The librational band position in CsOH solution is consistent with less hindered rotations of water molecules resulting from water surrounding this cation being only slightly hydrogen bonded. This conclusion seems to be inconsistent with the chaotrope nature of Cs+ ions which suggests that the low Cs charge density should result in water molecules surrounding it being more strongly hydrogen bonded.35 On the other hand, it is important to emphasize that the kosmotrope and chaotrope concepts are related to the effects of ions at infinite dilution on the water structure, while our SERS measurements sample a few layers of solution near the electrode surface under the influence electrode potential.21,36 It is well-known that water attached to lithium ions is more tightly coordinated than that attached to cesium ions and that lithium has a well-defined first hydration shell consisting of four water molecules arranged with tetrahedral symmetry.37 Each of these four water molecules is bonded to three other water molecules in the second hydratation shell.37 The electrostrictive effect of the ionic field of Li+ causes the water in the hydratation shells to have a larger mean density than neat water and even CsOH solutions.35 This effect of ions on the structure of water results in a negative partial molar volume of Li+ i.e. the LiOH solutions assumes a high-density configuration. On the other hand, the CsOH solution can be in low-density configuration and, additionally, Cs+ ions contact-adsorb on the silver electrode.17 Evoking an electromagnetic enhancement mechanism for the SERS which can be ca. 1-2 nm in extent,38 we propose that the distance from the electrode sampled by SERS is approximately the same for LiOH and KOH solutions. However, it is known that the adsorption of Cs+ ions on silver electrode causes lower reflectivity of Ag electrode in CsOH solutions17 which should result in less water layers sampled by SERS. Despite the chaotropic nature of Cs+ ions, the lower-density of its solutions and its adsorption on the electrode results in poorly hydrogen bonded water sampled by SERS nearby the Ag surface. In (35) Hribar, B.; Southall, N. T.; Vlachy, V.; Dill, K. A. J. Am. Chem. Soc. 2002, 124, 12302–12311. (36) Marcus, Y. Chem. Rev. 2009, 109, 1346–1370. (37) Lyubartsev, A. P.; Laasonen, K.; Laaksonen, A. J. Chem. Phys. 2001, 114, 3120–3126. (38) Campion, A.; Kambhampati, P. Chem. Soc. Rev. 1998, 27, 241–250.

Figure 3. Bending band SERS spectral intensities observed during a cathodic scan in 0.1 M LiOH solution.

addition, specific adsorption of Cs+ at very negative potentials may result in decomposition of the Cs+ hydration shell, as suggested by Osawa for another system.29 This hydration shell decomposition may result in water less strongly associated with the cation and hence more disordered near the electrode surface. In addition to its high charge density which arranges Li+ cations as kosmotropes, the analysis of librational band frequency suggests that water molecules arranged on the electrode in LiOH solution are more hydrogen bonded than in CsOH solution, so we can suppose that the breaking hydrogen bonds on nearby water molecules should be restricted to the first hydration shell of Li+ ions. Additionally, the smaller atomic radius (i.e., high charge density) of Li+ ions results in a thinner thickness of the inner Helmnholtz plane, so we can suppose that more water layers should be sampled in this solution. Thus the second hydration shell, more hydrogen bonded, should have high influence on SERS librational signal measured in LiOH solutions. This hypothesis is corroborated with the positive correlation found between two bending contributions (1615 cm-1 and 1660 cm-1) in the spectra collected in LiOH solution. It should be emphasized that the 1660 cm-1 contribution appears at more anodic potentials on the cathodic sweep relative to the 1615 cm-1 contribution as shown in Figure 3. A decrease in the bend vibration band frequency and increase in band intensity are expected with decrease in H-bonding.39 In other words, the results obtained for LiOH solutions suggests a twostate model, where the two states represent weaker and stronger H-bonding resulting in intramolecular bend band positions at 1615 cm-1 and 1660 cm-1, respectively. This hypothesis is consistent with previous work and suggests that interfacial water molecules are weakly hydrogen-bonded at potentials below the pzc and form a strongly hydrogen-bonded icelike structure at potentials slightly above the pzc.12 We note calculations suggest that increased water association with the negatively charged metal surface should also result in enhancement of the OH bending mode.40 We have probed the sequential order of water inter- and intramolecular vibration bands spectral intensity changes in (39) Devlin, J. P.; Sadlej, J.; Buch, V. J. Phys. Chem. A 2001, 105, 974–983. (40) Wu, D.-Y.; Duan, S.; Liu, X.-M.; Xu, Y.-C.; Jiang, Y.-X.; Ren, B.; Xu, X.; Lin, S. H.; Tian, Z.-Q. J. Phys. Chem. A 2008, 112, 1313–1321.

cathodic potentials and in LiOH and KOH solutions; the librational bands appear prior to the bending band. This sequential order coincides with that found in the work of Haq et al.41 which reported RAIRS results about the growth of intact water ice on Ru(0001) at low temperatures. The RAIRS results showed that low frequency libration modes associated with out of plane bend of hydrogen bonded water appear at low water coverage, with the water stretch and bending bands appearing only as the coverage is increased. To explain this results, based on DFT calculations, the authors proposed that, at low coverage, water lies flat, making stretch and bending dipole forbidden and, therefore, weak in the RAIRS spectrum. As the water coverage increases, the clusters grow larger and more water is forced to lose its planar geometry. As water buckles out the plane, the stretching and bending bands become dipole active, resulting in the increase in the intensity of these bands observed at high coverage. Although the observations of Haq et al. resembles our results, it is important to emphasize that our system is more complex due to the presence of the electrified interface, and, additionally, the selections rules of SERS and RAIRS are different. The particular importance of the librations is that the breaking of hydrogen bonds results from this rotational motion. Theory suggests that covalent stabilization between water molecules and a silver slab does not take place.42 The small adsorption energy computed for water was, therefore, attributed to electrostatic interactions, thus the hydrogen bond between water molecules and silver electrode can be ruled out.42 Excitation of the three librational modes of water molecules adsorbed on Ag(111) under the tip of a scanning tunneling microscope by the tunneling electrons has been observed.10 At HER potential range, the water molecules are not fully hydrogen bonded at inner Helmholtz plane but present a special order in which the nuclear motion evolves under the influence of the tunneling electrons from the electrode to OH moieties. It has been proposed21 that Raman scattering of water molecules during HER is observed from molecules to be dissociated rather than from molecules during dissociation although, up to now, there has been no information regarding along which normal coordinate the H2O molecule dissociates. The specific sequential order of inter- and intramolecular vibration bands spectral intensity changes probed by twodimensional correlation analysis can open new initiatives on the complicated structures evinced by interfacial water at cathodic potentials. CONCLUSIONS 2D-COS analysis of SERS spectra from Ag reveals a total of five distinct bands in the librational and water bending region in alkali hydroxide solutions. Three of these bands are associated with distinct librational modes, the analysis of which shows that the water molecules around Cs+ cations are poorly hydrogen bonded in contrast to those around Li+ and K+ at the Ag electrode surface. 2D-COS analysis of spectra collected in LiOH and KOH solutions on cathodic scan showed that the librational bands appear prior to the bending band, while in the CsOH solution the potential dependence of the two manifolds was (41) Haq, S.; Clay, C.; Darling, G. R.; Zimbitas, G.; Hodgson, A. Phys. Rev. B 2006, 73, 115414. (42) Izvekov, S.; Voth, G. A. J. Chem. Phys. 2001, 115, 7196–7206.

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identical. The water bending band of spectra collected in LiOH solution was found to be the convolution of two contributions. These features are consistent with a two state model of water arranged on the electrode surface.

SUPPORTING INFORMATION AVAILABLE Cyclic voltammetry of Ag in the different electrolyte solutions and synchronous 2D correlation plots. This material is available free of charge via the Internet at http://pubs.acs.org.

ACKNOWLEDGMENT R.C.A. acknowledges CNPq and UFRN-Brazil (grant number. 200961/2007-1) for financial support. A.A.G acknowledges the NSF for support of this research.

Received for review October 12, 2009. Accepted January 17, 2010.

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AC902299U