Potential-Dependent Reorientation of Water Molecules at an Electrode

The interfacial water molecules are weakly hydrogen-bonded at potentials below the .... Scanning tunneling .... 80 cm. -1 indicates the presence of no...
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10664

J. Phys. Chem. 1996, 100, 10664-10672

Potential-Dependent Reorientation of Water Molecules at an Electrode/Electrolyte Interface Studied by Surface-Enhanced Infrared Absorption Spectroscopy Ken-ichi Ataka,† Takao Yotsuyanagi,† and Masatoshi Osawa*,‡ Department of Molecular Chemistry and Engineering, Faculty of Engineering, Tohoku UniVersity, Sendai 980-77, Japan, and Catalysis Research Center, Hokkaido UniVersity, Sapporo 060, Japan ReceiVed: December 7, 1995; In Final Form: April 1, 1996X

The structure and orientation of water molecules at a highly ordered Au(111) electrode surface in perchloric acid have been investigated in-situ as a function of applied potential by means of surface-enhanced infrared absorption spectroscopy. This newly developed infrared spectroscopy technique enables the observation of the electrode/electrolyte interface at a very high sensitivity without interference from the bulk solution. The spectrum of the interfacial water significantly differs from that of bulk water and drastically changes in peak frequencies and band widths around the potential of zero charge (pzc) of the electrode and at about 0.3 V positive from the pzc. The interfacial water molecules are weakly hydrogen-bonded at potentials below the pzc and form a strongly hydrogen-bonded ice-like structure at potentials slightly above the pzc. The ice-like structure is broken at more positive potentials due to the specific adsorption of perchlorate ion, where one OH moiety of water is non-hydrogen-bonded and the other OH moiety is hydrogen-bonded to another water molecule. The intensities of the fundamental modes of water are also a strong function of applied potential. They are very weak around the pzc and increase as the potential changes in both positive and negative directions. These results are explained in terms of the potential-dependent reorientation of water molecules from oxygenup to oxygen-down as the surface charge changes from negative to positive. The adsorption of hydronium and perchlorate ions on gold is also discussed.

Introduction Understanding the composition and structure of the electric double-layer at solid/liquid interfaces constitutes one of the major objectives in electrochemistry.1,2 In particular, detailed knowledge of water molecules at the interfaces is an essential prerequisite to understanding electrocatalytic reactions. The adsorption of water on metal surfaces has been studied extensively in ultrahigh vacuum (UHV) by means of modern surface analytical tools.3,4 Since there exists a very strong electric field on the order of 107 V/cm at electrode/electrolyte interfaces, however, the behavior of water at these interfaces is believed to be greatly different from those at solid/vacuum interfaces.1,2 Classical models of the electrochemical interfaces1,5-8 assume that water molecules are ordered at the interface due to the strong electric field and reorient from “oxygen-up” to “oxygen-down” as the electrode charge (or, equivalently, potential) changes from negative to positive. These models explain differential capacitance and other macroscopic measurements of interfacial properties. However, these macroscopic data do not give molecular level information. In the past decade, Monte Carlo9,10 and molecular dynamics11-16 simulations have been employed to investigate the structure and dynamics of water at electrified metal surfaces. These computer simulations suggest that the molecules become arranged in several layers from the surface and the molecules in the first layer reorient depending on the surface charge. It is also suggested that water molecules form an ice-like structure on uncharged and positively charged metal surfaces. Recently, the arrangement of water molecules at the Ag(111) electrode surface has been experimentally determined by Toney * Author to whom correspondence should be addressed. † Tohoku University. ‡ Catalysis Research Center, Hokkaido University. X Abstract published in AdVance ACS Abstracts, May 15, 1996.

S0022-3654(95)03636-7 CCC: $12.00

et al.17 by means of in-situ X-ray scattering. They also found that the spacing between the electrode surface and the first water layer changes depending on applied potential. This was attributed to the reorientation of water. However, the in-situ X-ray measurements did not provide evidence to support formation of known ice-like structure.17 In-situ surface vibrational spectroscopies also should be able to provide detailed molecular level information on the interface. Water on Pt, Au, Rh, and Ag electrode surfaces has been investigated by infrared reflection-absorption spectroscopy (IRRAS).18-25 All the data show that the characteristics of the interfacial water differ from those of bulk water. Some data have been interpreted as consistent with the current model of electric double-layers,19,20,24 but others, as inconstant with this.25 Surface-enhanced Raman scattering (SERS) has also been used for the study of water on electrochemically roughened Ag and Cu electrodes.26-28 However, the SERS data must be discussed separately from other spectroscopic data because the water molecule observed by SERS is believed to be that incorporated in a surface complex formed during the surface-roughening process.28 No evidence of potential-dependent reorientation of water has been obtained by SERS. Very recently, Du et al.29 applied visible-infrared sum frequency generation to the study of a quartz/liquid interface and demonstrated that the water spectrum changes significantly as the surface charge is changed by changing solution pH. They attributed the spectral changes to the reorientation of interfacial water. Despite these efforts, however, very little is known about the structural details of electrochemical interfaces. In this paper, we report a detailed infrared study of potentialdependent reorientation of water molecules at an Au electrode surface in 0.5 M perchloric acid. Gold was chosen as the electrode material because it has a wide double-layer potential region and, therefore, the reorientation of water can be investigated over a wide potential range without changing © 1996 American Chemical Society

Potential-Dependent Reorientation of Water Molecules

J. Phys. Chem., Vol. 100, No. 25, 1996 10665

Figure 1. Spectroelectrochemical cell used in the present study. A 20 nm thick Au film vacuum evaporated on the Si prism is used as the working electrode. The infrared radiation totally reflected at the metal/ solution interface is analyzed.

Figure 2. Cyclic voltammogram of a vacuum-evaporated Au thin film electrode in 0.5 M HClO4 at a sweep rate of 50 mV/s. The geometric area of the electrode is 1.54 cm2. The double-layer current is approximated by the fine dashed line.

chemical properties of the surface (namely, without the adsorption of hydrogen atom or surface oxidation). Perchloric acid is used as the supporting electrolyte in order to minimize the influence of specific adsorption of electrolyte anion. In the case of Au, it is not fluoride ion but perchlorate ion that is the least adsorbing among the supporting electrolyte anions generally used in electrochemistry.30,31 Nevertheless, perchlorate ion weakly adsorbs on Au at potentials more positive than the potential of zero charge (pzc).30-32 Therefore, the adsorption of the ion and its influence on the interfacial structure are also discussed. In the present study, surface-enhanced infrared absorption spectroscopy combined with attenuated-total-reflection technique (ATR-SEIRAS, cf., Figure 1)33,34 is used in acquiring the spectrum of water at the interface. A thin Au film evaporated on a prism is used as the working electrode, and infrared radiation totally reflected at the metal/solution interface is analyzed. The details of this newly developed spectroscopy have been described elsewhere.33-36 Briefly, this technique uses the phenomenon that infrared absorption of molecules in the very near vicinity of some kinds of metal surfaces is greatly enhanced. The enhanced infrared absorption arises from an electric field at the metal surface produced by the incident infrared radiation through the excitation of a localized plasmon of the metal.33,35,36 Since the enhancement is the largest at the metal surface and decays sharply within a distance of a few monolayers from the surface, the solid/liquid interface can be investigated selectively. In previous reports,33,34 we have demonstrated that monolayers at the interface can be detected at a very high sensitivity without interference from the bulk solution by this technique. Only the vibrational modes that have nonzero surface-normal dipole derivatives are observed.33,36 The absorption intensity is roughly proportional to cos2 θ, where θ represents the angle between an oscillating dipole and the surface normal. By using the surface selection rule, the orientation of water at the interface is discussed.

The working electrode was a 20 nm thick Au film evaporated on the flat plane of a Si hemicylindrical prism (1 cm in radius and 2.5 cm long). The metal-coated prism was attached to the electrochemical cell by sandwiching an O-ring. A copper foil was also inserted between the cell body and the edge of the metal film in order to make a connection between the electrode and a potentiostat (EG&G Princeton Applied Research, Model 263). A Pt foil and a trapped hydrogen electrode were used as the counter and reference electrodes, respectively. The reference electrode was prepared by evolving hydrogen prior to the experiment. The solution in the reference was the same as that in the cell. The potential of the reference electrode was stable during the experiment and was the same as that of the reversible hydrogen electrode (RHE) in the same solution. All the potentials in this paper are quoted against RHE. The evaporation of Au on the Si prism was performed in a vacuum of 5 × 10-5 Pa from a tungsten basket by thermal heating. The thickness of the metal film was measured with a quartz microbalance. The deposition rate was kept at 0.01 nm/s during the evaporation. The surface of the working electrode was cleaned in-situ in the test solution by cycling the potential repeatedly between 0 V (slightly positive of the hydrogen evolution) and 1.5 V (oxide formation region). The surface area of the electrode was determined by integrating the charge for oxide formation in the cyclic voltammogram of the electrode.30,31 It was typically about 4 times larger than the geometrical area. Infrared spectra were taken with a Bio-Rad FTS-60A/896 FTIR spectrometer. Unpolarized infrared radiation from a Globar source was focused at the electrode/electrolyte interface by passing through the prism (Figure 1). The incident angle was 60° from the surface normal. The radiation totally reflected at the interface was detected with a liquid-N2-cooled linearized narrow-band HgCdTe detector (Bio-Rad). The spectra were acquired at several potentials during one potential sweep and were plotted in absorbance units defined as A ) -log(I/I0), where I and I0 represent the intensities of the reflected radiation at the sample and reference potentials, respectively. The spectral resolution was 4 cm-1.

Experimental Section The electrolyte solution was prepared from super special grade perchloric acid (Wako Pure Chemical, Tokyo) and ultrapure Millipore water. The solution was deaerated with helium or argon prior to the beginning of the experiment. The electrochemical cell used in the present study was the glass one shown in Figure 1. Before use, the cell was cleaned in a hot HNO3 bath in order to minimize organic contaminants, followed by rinsing and soaking cycles with Millipore water.

Results and Discussion Voltammetry. Electrochemical characteristics of the vacuumevaporated Au electrode were investigated by cyclic voltammetry prior to the infrared measurements. The voltammogram of a thin film Au electrode in 0.5 M perchloric acid is shown in Figure 2. The current vs potential curve for the positive-

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Ataka et al.

Figure 3. Series of infrared spectra of a highly ordered Au(111) electrode surface in HClO4 as a function of applied potential. The spectra were acquired during a potential sweep from 0.1 to 1.3 V vs RHE at a rate of 5 mV/s. The reference potential is 0.1 V.

Figure 4. Series of infrared spectra of a highly ordered Au(111) electrode surface in HClO4 as a function of applied potential. Same as in Figure 3, but the spectrum at 0.62 V was used as the reference.

going potential sweep is divided into two regions: the so-called double-layer region (1.3 V). The voltammogram resembles that of the Au(111) singlecrystal surface rather than that of the polycrystalline Au electrode.30-32 The preferential (111) orientation of the electrode surface was confirmed more clearly for underpotential deposition of copper on the surface.37 Scanning tunneling microscopic observations revealed that the Au film consists of small (50-100 nm in size) crystallites with highly ordered (111) surfaces. The results agree with the low-energy electron diffraction (LEED) and reflection high-energy electron diffraction (RHEED) studies of Au films evaporated on glass.38 Despite the island structure, the conductivity of the electrode was good enough and no problems were found in electrochemistry, as shown in Figure 2. Although a flat and well-ordered Au(111) surface can be obtained if evaporation is performed at elevated temperatures (e.g., 400 °C),38,39 the evaporation was done at room temperature in the present investigation because the island structure of the metal film plays a very important role in SEIRA.33,36,40 A close inspection of the double-layer region of the voltammogram reveals a weak reversible wave centered around 0.85 V (dashed trace). This reversible wave has been attributed to charge transfer (pseudocapacitance) associated with the specific adsorption/desorption of perchlorate ion.30-32 The charge due to the specific adsorption, estimated by integrating the weak wave after double-layer current assumed by the fine dashed line in the figure was subtracted, was about 11 µC cm-2. If it is assumed that one perchlorate anion occupies three surface atoms and discharges completely,30,31 the saturated coverage of the anion is calculated to be about 0.15. This value is almost the same as that determined by electrochemical quartz crystal microbalance (EQCM).41 Spectra of Water Molecules at the Interface. Infrared measurements were carried out in the double-layer potential region from 0.1 to 1.3 V. Particular attention was paid to spectral changes around the pzc of Au(111), about 0.55 V vs RHE,30-32 where reorientation of interfacial water is expected. The electrode surface is charged negatively and positively at potentials below and above the pzc, respectively. Figure 3 shows a series of infrared spectra of the Au/HClO4 interface acquired continuously during a positive-going potential sweep at a rate of 5 mV/s. Each spectrum was obtained by coadding

100 interferograms to improve the signal-to-noise ratio of the spectrum. The acquisition time was about 15 s per spectrum. Therefore, the potentials shown in the figure are the averages of every 75 mV interval. The single-beam spectrum taken at 0.1 V was used as the reference, chosen in view of the negligible anion adsorption at this potential.30-32 The spectral region below 1000 cm-1 could not be observed due to the strong absorption of the Si prism. The weak bands at 2920 and 2850 cm-1 were always observed and are assigned to the CH stretching modes of hydrocarbon contaminants.35 Since these bands did not change their intensities during the oxidationreduction cleaning of the surface, the contaminants are believed to be present at the metal/prism interface. The positive-going band at 1090-1130 cm-1, which monotonically increases in intensity with increasing potential, is assigned to perchlorate ion specifically adsorbed on the electrode surface. The last issue will be discussed in detail in the final section. The OH stretching (νOH) and HOH bending (or scissor, δHOH) modes of water are observed at 3600-3000 and 16501610 cm-1, respectively. Although an isolated water molecule has two OH stretching modes (symmetric and antisymmetric), the distinction between them breaks down for hydrogen-bonded water molecules due to intermolecular coupling.42,43 Rather, a spectrum has to be discussed in terms of two individual OH oscillators.43 Figure 3 clearly shows that the vibrational properties of water change around the pzc. At potentials below 0.6 V, the water vibrational modes are observed as negativegoing bands at 3507 and 1612 cm-1. On the other hand, positive-going bands appear at 3600, ∼3400, ∼3200, and 1645 cm-1 around 0.6 V and increase in intensity as potential increases. The negative (positive) sign indicates that the intensity decreases (increases) with increasing potential. Note that the intensity of the negative-going δHOH band at 1612 cm-1 remains constant at potentials above 0.6 V despite the growth of the positive-going band at 1645 cm-1. This suggests that the intensity of the 1612 cm-1 band (and also the corresponding νOH band at 3507 cm-1) decreases to zero around 0.6 V. The νOH region of the spectra shown in Figure 3 is complicated by the superposition of the positive- and negativegoing bands, which makes the interpretation of the spectra difficult. The superposition can be removed by using the spectrum at 0.62 V as the reference, as shown in Figure 4, where all the vibrational modes of water are observed as positivegoing bands. This reference potential is chosen somewhat

Potential-Dependent Reorientation of Water Molecules

Figure 5. Comparison of the interfacial water spectra at 0.12 (a), 0.77 (b), and 1.22 V (c) in Figure 4. Spectrum d is the transmission spectrum of 0.5 M HClO4. All the spectra are shown by scaling the strongest bands to be equal.

arbitrary but because the intensities of all the water bands are almost zero around this potential. The most significant result obtained by using this reference potential is that very weak spectral features at 0.6-0.9 V become clear. Figure 4 shows three types of spectra: the spectra at potentials below 0.6 V, between 0.6 and 0.9 V, and above 0.9 V. To show the differences more clearly, the spectra at 0.14, 0.77, and 1.22 V in Figure 4 are compared in Figure 5 (spectrum a, b, and c, respectively) by scaling the strongest bands to be equal. The band intensities change greatly with potential, but no potentialdependent peak shifts are observed (within experimental accuracy) within the three potential regions. This suggests that at least three types of water molecules are present at the interface, and their relative populations change depending on applied potential. For comparison, the transmission spectrum of 0.5 M perchloric acid is also shown in Figure 5 (spectrum d). The νOH and δHOH modes are seen at 3400 and 1645 cm-1, respectively. The low-frequency shoulder of the νOH band, at about 3250 cm-1, is due to Fermi resonance between νOH and the binary overtone of δHOH.44 The spectra of water at the interface are apparently different from the bulk spectrum. This implies that the background absorption of the bulk solution is completely subtracted. At potentials below 0.6 V (spectrum a in Figure 5), the νOH and δHOH bands are shifted to higher (3507 cm-1) and lower (1612 cm-1) frequencies, respectively, compared with the bulk spectrum. In addition, the band shapes are apparently narrower (200 vs 400 cm-1 for νOH and 30 vs 80 cm-1 for δHOH in half-width). The shift and narrowing of the water bands are attributed to the decrease in hydrogen-bonding between water molecules.44 The decrease in Fermi resonance resulting from the upshift of νOH and the downshift of δHOH also contributes to the narrowing of the νOH band. The decrease in hydrogenbonding at the negatively charged surface is in good agreement with the molecular dynamics simulations of electric doublelayers.12-16 It is interesting to note that the frequency of the νOH mode (3507 cm-1) is much lower than those for the water monomer (3734 and 3638 cm-1) and the weakly hydrogen-bonded dimer (3724, 3710, 3634, and 3574 cm-1) in inert gas matrices.44

J. Phys. Chem., Vol. 100, No. 25, 1996 10667 Nevertheless, the δHOH mode (1612 cm-1) is in the very low frequency range of the water monomer (1595 cm-1) and dimer (1600 and 1620 cm-1).44 The unexpectedly low δHOH frequency is indicative of the interaction of water with the metal surface via an oxygen lone-pair orbital.3,45,46 A larger downward shift to 1560-1520 cm-1 has been observed for water adsorbed on Ru(001) in UHV.3,45,46 Since the relative downward shift of the δHOH band scales approximately with the strength of the metal-water bond unless intermolecular hydrogen-bonding is very strong,3,45,46 it is concluded that water adsorbs on Au more weakly than on Ru. This result is in good agreement with thermodynamic data for water-metal interactions obtained by thermal desorption spectroscopy,3 where the desorption peak of water is observed at 160 K on Au and at 220 K on Ru. In classical models of electric double-layers,1,5-8 water is assumed to adsorb on negatively charged surfaces via hydrogen atoms. However, no νOH bands characteristic for OH‚‚‚metal bonding are observed in the expected region around 2900 cm-1.3,45-47 In contrast with the low-potential region, water molecules are strongly hydrogen-bonded at potentials above 0.6 V, which is obvious from the very broad band features and low frequencies of the νOH modes (3460-3200 cm-1). Spectrum b in Figure 5 closely resembles the infrared spectrum of ice48 except for the presence of the very sharp band at 3612 cm-1, indicating that the interfacial water molecules form an ice-like structure at the positively charged electrode surface. This finding is in good agreement with the computer simulations of electric double-layers.9-16 Ice-like structures have also been observed on quartz surfaces in water29 and on metal surfaces in UHV.3,45-47,49,50 The ice-like structure formed on metal surfaces in UHV, as determined by LEED, electron-stimulated desorption ion angular distribution (ESDIAD), electron energy loss spectroscopy (EELS), and IR-RAS, is essentially equivalent to the basal plane of ice Ih in contact with the surface.3,45-47,49,50 The oxygen atoms form a network of edge-sharing puckered hexagonal rings similar to those of the chair form of cyclohexane. The sharp νOH band at 3612 cm-1 with half-width of about 80 cm-1 indicates the presence of non-hydrogen-bonded OH.3,45-47 This band is a minor component in the potential range 0.6-0.8 V and greatly increases in intensity at more positive potentials. In harmony with the growth of this sharp νOH band, a very broad νOH band, characteristic of strongly hydrogen-bonded OH, appears at 3460 cm-1 and grows with increasing potential. Because of the following reasons, the water molecule is believed to be asymmetrically hydrogen-bonded (that is, only one OH moiety is hydrogen-bonded to another water molecule). First, the ratio of the integrated intensities of the sharp and broad νOH bands is about 1:10. Since hydrogenbonding is known to increase the intensity of νOH by a factor about 10,51 the numbers of nearly free and hydrogen-bonded OH oscillators are comparable. Second, only one band is observed in the δHOH region (at 1648 cm-1). If the two νOH bands were attributed to water molecules in different states, two δHOH bands would be observed. Finally, asymmetrically hydrogen-bonded water molecules in organic solvents show sharp and broad νOH bands at about 3660 and 3450 cm-1, respectively.44 The growth of the bands of asymmetrically hydrogen-bonded water is well correlated with the specific adsorption of perchlorate ion, as shown in Figure 6, where the integrated band intensity (solid circles) and the charge due to the specific adsorption of perchlorate ion (QClO4, solid curve) determined by the first-order approximate approach described in the voltammery section are plotted as a function of potential. When

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Figure 6. Plot of the integrated intensity of the 3612 cm-1 band in Figure 4 as a function of applied potential. The solid curve represents the charge due to the specific adsorption of perchlorate ion (QClO4) determined by the approximate method described in the voltammetry section.

a small amount of chloride ion (HCl, 1 mM) was added into the solution, the intensity vs potential profile shifted to the negative direction by about 0.6 V (not shown). The shift is explained by the stronger adsorption of chloride ion than perchlorate ion. Since perchlorate and chloride ions are poor hydrogen-bond acceptors,52,53 the coadsorbed anion is believed to disrupt the hydrogen-bonding between interfacial water molecules, thereby allowing the OH moieties adjacent to coadsorbed anion to oscillate freely at high frequency. To confirm this, we added sulfuric acid into the solution and found that the sharp νOH band is completely suppressed. Since sulfate (or bisulfate) ion is an effective hydrogen-bond acceptor,52,53 this result is interpreted that the sharp band is shifted to a lower frequency in the broad νOH band region due to the strong hydrogen-bonding with coadsorbed sulfate. The details of the effect of adsorbed sulfate on the interfacial structure will be reported elesewhere.37 On the basis of the results mentioned above, we conclude that coadsorbed anion breaks hydrogen-bonding of the ice-like structure and greatly changes the arrangement of the interfacial water molecules. The same conclusion has been obtained from coadsorption experiments of water and negative atoms (or ions) in UHV.3,54-57 Recently, Toney et al.17 studied water on the Ag(111) electrode surface in a NaF aqueous solution by in-situ X-ray scattering and found that the density of water at the interface is about twice as large as that for bulk water at +0.52 V from the pzc of the electrode. On the basis of this finding, they concluded that the interfacial water molecules cannot exist as the puckered hexagonal ice-like structure at this potential. Since fluoride ion weakly adsorbs on the Ag(111) surface at potentials more positive than the pzc58 and the ice-like structure will be broken by the adsorbed ion as described above, their conclusion seems to be consistent with ours. Comparison of Spectra Obtained by ATR-SEIRAS and RAS. Two IR-RAS studies of water on Au electrodes in acidic solutions have been reported in the literature.21,24 However, the reported spectra are different from those obtained in the present study. The differences are described in this section before discussing the orientation of water at the interface. Parry et al.24 studied water on an Au electrode surface in sulfuric acid by IR-RAS and observed an extremely broad (halfwidth ≈ 300 cm-1) δHOH band. On the other hand, the halfwidth of the δHOH band we observed in the present study is 30-70 cm-1. The discrepancy cannot be addressed to the difference in the supporting electrolyte anions (sulfate/bisulfate and perchlorate).37 Considering that the half-width of the δHOH band for bulk water is about 80 cm-1 and the interaction between

Ataka et al. water and Au is weak,3 the half-width of about 300 cm-1 is unbelievable. We think that the broad δHOH band observed by Parry et al.24 is an artifact caused by incomplete background subtraction. In IR-RAS studies, it always becomes a matter for argument whether the observed absorption bands are only of the species at the interface. This is because the absorption intensity of the molecules at the interface is several orders of magnitude smaller than that of bulk solution,33 and therefore, complete background subtraction is very difficult. To avoid this uncertainty, we used ATR-SEIRAS in the present study instead of IR-RAS. The solution background is completely canceled out in our spectra, which is evident from the fact that only the δHOH band of water directly interacting with the electrode surface (1612 cm-1 band in Figure 5a) is observed. The background absorption of the bulk solution is canceled out also in the IR-RAS study by Kunimatsu and Bewick.21 They observed three νOH bands at 3730, 3600-3510, and ∼3250 cm-1 in 1 M perchloric acid. The first two bands are sharp, and the last one very broad. Their spectra relatively resemble our ATR-SEIRA spectra except in the following three respects. First, we did not observe the 3730 cm-1 band. The observation of this band is surprising and very questionable because the frequency is in the highest range of νOH of free water molecules (monomer) observed only in the low-pressure gas phase43,44 and in inert gas matrices.44,51 Brooker at al.23 also observed a sharp νOH band at 3750 cm-1 in 0.1 M KOH but assigned it to hydroxy species because this band is observed only at potentials where AuOH is formed. Second, Kunimatsu and Bewick21 reported that the sharp band at 3510-3600 cm-1 shifts to higher wavenumbers with increasing potential, but we have clearly shown in Figures 3 and 4 that two νOH bands at 3507 and 3612 cm-1 are superposing and change in relative intensities with potential. Finally, the potential-dependent intensity variations of the water bands they reported are greatly different from those obtained in the present study. The reason for these discrepancies is not clear. However, it is noteworthy that it took 7 h to take one spectrum in the IRRAS measurements by Kunimatsu and Bewick.21 Therefore, there exists a possibility that the electrode surface is contaminated during the experiments, for example, by chloride ion from the Ag/AgCl reference electrode they used. We found in this study that the water spectrum changes greatly by the presence of surface contaminants. On the other hand, since the sensitivity of the ATR-SEIRAS is higher than that of IR-RAS,33,34 the 16 spectra shown in Figure 3 were obtained within only 4 min. In addition, we used a trapped hydrogen electrode as the reference to avoid contamination of the solution by chloride ion. Therefore, the possibility of surface contamination will be much less in our experiments. Orientation of Water. Classical models and recent computer simulations of electric double-layers predict that water molecules at the interface have an oxygen-up and oxygen-down average orientation on negatively and positively charged surfaces, respectively.1,2, 5-16 Some calculations also predict that the molecule lies nearly flat at an uncharged surface.12-15,59 Good evidence of the potential-dependent reorientation is shown in Figure 7, where the integrated intensity of the δHOH mode is plotted as a function of potential. The intensity is very small around 0.6 V (≈pzc) and increases as potential changes in both positive and negative directions. This is also the case for the νOH mode (Figure 4). The result is explained in terms of current models of electric double-layers12-16 and the surface selection rule mentioned in the Introduction as follows: The very small intensities of both the νOH and δHOH modes around the pzc indicate that water lies parallel to the surface at this

Potential-Dependent Reorientation of Water Molecules

Figure 7. Plot of the integrated intensities of δHOH bands in Figure 4 as a function of applied potential.

Figure 8. Possible orientations of water at an electrode/electrolyte interface (A-G) proposed in the previous theoretical and experimental studies.3,9-16,25 The solid lines represent the oxygen lone-pair orbitals interacting with the electrode surface, and the dashed lines hydrogenbonding (or lone-pair orbitals available for hydrogen-bonding). Model H is an ice-like structure proposed by Doering and Madey.50 The most plausible orientation deduced in the present study is A at potentials below the pzc, B around the pzc, C slightly above the pzc, and D at more positive potentials.

potential range. On the other hand, the oxygen-end of water becomes more oriented toward the surface (solution phase) at potentials below (above) the pzc, resulting in the increment of the band intensities. At potentials above 0.8 V, the intensities of all the water bands increase greatly. This suggests that water orients with its molecular plane nearly normal to the surface. The local density of interfacial water changes with applied potential; it is minimal around the pzc.17 Therefore, the change in the local density may contribute to some extent to the band intensity change shown in Figure 7. However, the density change is only a factor of 2,17 and it is much too small to produce the observed intensity change. The orientation of interfacial water molecules can be discussed in more detail from the spectral features. Possible orientations are shown in Figure 8A-G.3,9-16,25 However, water molecules hydrogen-bonded to the surface, orientations E and F, are ruled out because no νOH‚‚‚Au bands are observed at any of the potentials examined. Nagy and Heinzinger12-14 theoretically predicted orientation G at very positive potentials. However, the adsorption of electrolyte anion is not taken into account in the simulations. Since perchlorate ion adsorbs specifically at very positive potentials and greatly changes the water arrangement as described above, this orientation will also be ruled out. The flat-lying orientation B does not give infrared absorption, but its presence is apparent around the pzc as described above. The ice-like spectrum (Figure 5b) suggests that water orients as C at the potential range 0.6-0.9 V because the ice-like structure shown by the model H3,50 is formed from C by hydrogen-bonding with water molecules in the second layer. The dipole oscillations associated with the νOH and δHOH

J. Phys. Chem., Vol. 100, No. 25, 1996 10669 modes are tilted about 60° from the surface normal for this orientation, which explains the relatively weak intensity of the ice-like spectrum. The remaining two orientations, A and D, are very suitable for explaining the two different spectra shown in Figure 5a,c, respectively, as follows. Spectrum c implies that water is asymmetrically hydrogen-bonded at the potential region where perchlorate ion adsorbs on the surface. This cannot be explained by orientation A because two OH bonds are directing toward the surface and are free from hydrogen-bonding due to steric hindrance. Orientation A is rather favorable to explain the weak hydrogen-bonding at potentials below the pzc (Figure 5a). On the other hand, orientation D has a possibility to form asymmetric hydrogen-bonding due to the coadsorbed anion, as discussed above. In addition, it should be noted that the molecules oriented as A and D are hydrogen-acceptor and hydrogen-donor, respectively, in hydrogen-bonding with the second water layer. It has been well established from matrixisolation and molecular beam studies of water clusters that νOH bands of hydrogen-acceptors appear at higher frequencies than those of hydrogen-donors.43,44,51,60,61 The νOH bands of hydrogen-bonded OH are observed at 3507 and 3460 cm-1 in Figure 5a,c, respectively. In this point of view also, the attribution of spectra a and c to orientations A and D, respectively, is reasonable. The discussion above is summarized as follows. Water orients with hydrogen atoms closer to the surface than oxygen atoms (orientation A) at potentials below the pzc and reorients to flat-lying orientation B as the electrode potential approaches the pzc. The hydrogen-end of water becomes more oriented toward the solution phase above the pzc, as depicted by C (more exactly, the ice-like structure H). At very positive potentials where perchlorate ion adsorbs, water molecules further reorient to orientation D due to the coadsorption of anion. Since no potential-dependent peak shifts are observed for any of the water bands, the relative populations of water molecules in the four different states change with potential. The reorientation from A to B and to C with increasing potential deduced from the spectra is in good agreement with the molecular dynamics simulations by Heinzinger et al.12-14 and Glosli and Philpott.15 Since the adsorption of electrolyte anions is not taken into account, the simulations by Heinzinger et al.12-14 say nothing about the influence of the adsorbed ions. This was taken into account in the simulation by Glosli and Philpott,15 but the orientation of water in the presence of adsorbed ions was not discussed. Spectrum of Hydronium Ion. Figure 5a shows a very broad band centered at about 1710 cm-1, which decreases in intensity with increasing potential and disappears around 0.6 V. This band is too high in frequency to be assigned to the δHOH mode of water. The potential dependence of the band intensity suggests the adsorption of a cationic species. By comparison with the vibrational spectra of acid hydrates42 and water coadsorbed with HF, HCl, or hydrogen atom on Pt(111) and -(100) surfaces in UHV,4,62,63 we assign this band to the doubly degenerate asymmetric HOH bending (ν4a) mode of hydronium ion (H3O+). Hydronium ion has pyramidal C3V symmetry and has three additional vibrational modes: symmetric and doubly degenerate asymmetric OH stretching modes (ν1 and ν3a, respectively) and a symmetric HOH bending mode (ν2).42 The two OH stretching modes are difficult to identify due to the overlap of the strong νOH band of water. The ν2 mode is expected to be observed strongly around 1150 cm-1.4,42,62,63 However, this mode is also not clear in our spectra probably due to its weak intensity and

10670 J. Phys. Chem., Vol. 100, No. 25, 1996

Ataka et al.

Figure 9. Proposed orientation of hydronium ion at the Au(111) electrode surface.

due to the superposition of the perchlorate band (1090-1130 cm-1). Only curve fitting of the spectra suggested the presence of a very weak and broad band around 1170 cm-1. Since dipole oscillations associated with ν2 and ν4a modes are parallel and perpendicular to the C3 symmetry axis, respectively,42 the depletion of the ν2 mode indicates that the C3 axis is nearly parallel to the surface. If hydronium ion is strongly adsorbed on the electrode surface and its C3V symmetry is broken, the doubly degenerate ν4a band will be split into two components and the discussion above must be changed slightly. It is difficult to judge whether the band around 1710 cm-1 consists of two components due to its very broad band feature. Even if the symmetry of the adsorbed ion is lower than C3V, however, the same conclusion is reached. On the basis of the above consideration, we propose the hydronium orientation shown in Figure 9. It is noteworthy that water orients with one oxygen lone-pair directing toward the solution phase (Figure 8A) at potentials below the pzc, where the hydronium band is observed. Therefore, hydronium ion with the assumed orientation will be readily formed by adding the third proton to the lone-pair (Figure 9). The protonation of water and deprotonation of the hydronium ion do not disturb the interfacial structure. Since hydronium ion in solution and at the interface will be in equilibrium, this model seems to be also reasonable in view of thermodynamic considerations. Specific Adsorption of Perchlorate Ion on Au. Figure 3 shows the presence of a band centered at 1090-1130 cm-1, which grows with increasing potential. As is shown in Figure 10a, the potential-dependent intensity variation (solid circles) can be well correlated with the amount of adsorbed perchlorate ion represented by QClO4 (solid curve). The closely similar potential dependence of the band intensity and QClO4 argues strongly that this band arises from adsorbed perchlorate ion. This was further confirmed by adding a small amount of chloride ion ([HCl] ≈ 1 mM) into the solution, where no bands were observed in this spectral range. Since chloride ion specifically adsorbs on gold more strongly than perchlorate ion, the result is explained that the adsorbed perchlorate ion is displaced by chloride ion. The frequency of the perchlorate band shifts upward monotonically with increasing potential at a rate of about 55 cm-1/ V, as plotted in Figure 10b, suggesting that perchlorate ion is directly (or specifically) adsorbed on the electrode surface. The observed upshift is explained by the donation of oxygen lonepair electron to the electrode64 or back-donation of electron from the electrode to the Cl-O antibonding orbital.65 Since the Fermi level of the electrode is downshifted as potential increases with respect to the molecular levels, the electron donation (backdonation) becomes larger (smaller) with increasing potential, resulting in the peak shifting to higher frequencies. The frequency shift might be explained also by the Stark effect

Figure 10. Plot of the integrated intensity (a) and the frequency (b) of the perchlorate band in Figure 3. The solid curve in part a represents the charge passed due to the specific adsorption of perchlorate ion (QClO4).

associated with the strong electric field at the interface.66 Since no potential-dependent shifts are observed for water bands, this effect seems to be small also for the perchlorate band, although there still exists a possibility that adsorbed perchlorate ions feel a much stronger change in local field than water molecules due to the selective concentration of positive ions over the adsorbed anion.67 The electron transfer vs Stark effect argument is still open for discussion. No matter which mechanism is more responsible, however, the potential-dependent peak shift is expected only for specifically adsorbed ions. For specific adsorption, some of the hydration water molecules must be removed from the central ion. Dehydration of perchlorate ion is evident from the frequency of the OH moiety adjacent to the coadsorbed ion (a very sharp band at 3612 cm-1 in Figures 3, 4, and 5). This frequency is much higher than that of water in the first hydration shell of the ion in solution (3575 cm-1)52,53 and is close to that of non-hydrogen-bonded OH (about 3660 cm-1).44 Recently, Watanabe and Uchida41 demonstrated by using EQCM that perchlorate ion adsorbed on Au(111) surface is hydrated with two water molecules. Since the maximum hydration number of perchlorate ions in solution is about 5.5,68 this data suggest that the ion is partly dehydrated at the electrode surface. Considering that the spatial region observed by ATR-SEIRAS is almost one water layer distance from the surface, our conclusion does not conflict with the EQCM data if water molecules are hydrogen-bonded to the solution-end of the adsorbed ion. A strong argument exists that perchlorate ion absorbs on the Au(111) electrode surface via three oxygen atoms at a 3-fold hollow site.30,31 This is based on the fact that perchlorate ion adsorbs on the (111) surface and fully discharges, but very little or not at all on the (110) and (100) surfaces.30,31 In addition, the 3-fold geometry of the tetrahedral anion matches well with the symmetry of the arrangement of surface atoms on the (111)

Potential-Dependent Reorientation of Water Molecules surface.30 The coordination geometry of adsorbed perchlorate ion can, in principle, be deduced from vibrational spectra on the basis of the spectral patterns and group symmetry arguments. Sawatari et al.64 studied perchlorate adsorption on the Pt(111) electrode surface by IR-RAS and concluded that perchlorate is adsorbed on the surface via one or three oxygen atoms with C3V symmetry. In practice, however, this procedure is quite ambiguous in the case of perchlorate ion, as discussed below. When perchlorate ion adsorbs specifically on a metal surface, its symmetry is reduced from the Td of the free ion to C3V, C2V, or lower depending on the coordination geometry (i.e., orientation and adsorption site). Due to the symmetry reduction, the triply degenerate Cl-O stretching mode (ν3, 1100 cm-1 in the solution) splits into two for C3V symmetry (A1 and E modes) or three for C2V (A1, B1, and B2 modes) and lower symmetries.42,69 If the adsorbed ion has C3V or C2V symmetry, however, the surface selection rule36 indicates that only the A1 mode (that gives a dipole change normal to the surface) would be infrared active. Since Figure 3 shows only one band in the expected spectral range (1250-1000 cm-1),42,69 the adsorbed perchlorate ion on the Au(111) electrode surface has C3V or C2V symmetry, and the observed band at 1090-1130 cm-1 is assigned to the symmetric Cl-O stretching (A1) mode. Since this frequency is very close to those of the symmetric Cl-O stretching modes of perchlorate ions with monodentate and bidentate coordination to metal ions (1060-1030 cm-1),42,69 the symmetry consideration above is reasonable. Unfortunately, however, the distinction between C3V and C2V symmetries is impossible from the infrared spectra alone. Conclusion We have used surface-enhanced infrared spectroscopy to study the orientation of water molecules at the interface between the Au(111) electrode surface and perchloric acid. Four types of water molecules are deduced from the spectra and are attributed to different orientations. The relative populations of these orientations change with potential. At potentials below the pzc, water molecules adsorbed on the surface via an oxygen lone-pair and oriented with the two hydrogen atoms closer to the surface than the oxygen atom are dominant. Around the pzc, flat-lying water molecules become dominant. At potentials slightly above the pzc, the hydrogen-ends of the molecule are oriented more toward the solution phase and an ice-like structure is formed by hydrogen-bonding with the second water layer. The ice-like structure is broken at more positive potentials due to the adsorption of perchlorate ion. In this potential range, water is asymmetrically hydrogen-bonded, where one OH moiety is blocked from hydrogen-bonding due to a weak interaction with the adsorbed ion and the other OH moiety is hydrogen-bonded to another water molecule. It was also found that hydronium ion exists at the interface at potentials below the pzc, with its C3 axis nearly parallel to the electrode surface. Acknowledgment. This work was supported by The Ministry of Education, Science, and Culture of Japan, Grant-inAid for Scientific Research Nos. 06554033, 07640765, and 07242208. References and Notes (1) Bockris, J. O’M.; Khan, S. U. M. Surface Electrochemistry: A Molecular LeVel Approach; Plenum Press: New York, 1993; Chapter 2. (2) Lipkowski, J., Ross, P. H., Eds. Structure of Electrified Interfaces; VCH: New York, 1993. (3) Thiel, P. A.; Madey, T. E. Surf. Sci. Rep. 1987, 7, 211, and references therein. (4) Wagner, F. T. In Structure of Electrified Interfaces; Lipkowski, J., Ross, P. H., Eds.; VCH: New York, 1993; Chapter 9.

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