Surface-enhanced Raman spectroscopic observation of two kinds of adsorbed OH− ions at copper electrode

Electrochimica Acta 45 (2000) 3507 – 3519 www.elsevier.nl/locate/electacta

Surface-enhanced Raman spectroscopic observation of two kinds of adsorbed OH− ions at copper electrode Gediminas Niaura * Institute of Chemistry, Gosˇtauto 9, LT-2600 Vilnius, Lithuania Received 27 September 1999; received in revised form 26 January 2000

Abstract The surface of a polycrystalline roughened Cu electrode in 1 M NaOH solution, has been studied in situ using surface-enhanced Raman spectroscopy (SERS). Cu2O, adsorbed OH− ions, and water molecules have been detected as the electrode potential was varied from open circuit value to −1.20 V versus SHE. The vibrational spectrum of Cu2O consisted of three main peaks located at 150, 528, and 623 cm − 1. It was found that the intense and narrow feature at 150 cm − 1 is highly characteristic, and could be used for SER monitoring of Cu2O. Two different states of adsorbed OH− ions, giving CuOH vibrations around 450 – 470 cm − 1 and 540 – 580 cm − 1, have been detected. The distinct nature of the bands has been shown by opposite isotopic frequency shifts changing the solvent from H2O to D2O. The frequency of the first band decreased by 12 cm − 1, while the frequency of the second band increased by  35 cm − 1 in D2O solutions. These differences have been explained in terms of distinct surface ligation and the formation of strong hydrogen bonds between water molecules and the second type of adsorbed OH− ion. Water molecules were observed at the interface at an applied potential −1.20 V. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Surface enhanced Raman spectroscopy (SERS); Copper electrode; Adsorption; Hydroxide ions; Cu2O

1. Introduction The nature of the interface between copper electrodes and aqueous electrolyte solutions remains puzzling despite considerable research efforts [1]. This can be illustrated by the fact that one of the most important properties of the interface, the potential of the zero charge (pzc), varies by as much as 0.7 V depending upon the method of measurement and experimental conditions [2–11]. High reactivity of the Cu surface towards the solution components is responsible for such behavior. It has been suggested [6] that the tendency to 

Based on a presentation made at the 2nd Baltic Conference on Electrochemistry, 1999. * Fax: + 370-2-617018. E-mail address: [email protected] (G. Niaura).

form a surface oxide accounts for the more positive pzc values, obtained earlier mainly by differential capacitance measurements [2 – 4] compared with values obtained by methods using fresh electrode surfaces [5 – 7]. Indeed, the rich and interesting surface chemistry of Cu with oxygen species in aqueous solutions has been recently demonstrated. The complex investigation of the Cu/electrolyte interface at pH 12 using scanning force microscopy, quartz crystal microbalance, Fourier transform infrared spectroscopy, and glancing/grazing incidence X-ray diffractometry [12] revealed a complete reorganization of the surface morphology without any detectable dissolution of Cu after cyclic voltammetric scanning in the potential region between 0.76 V (passive oxygen region) and − 0.74 V versus SHE (hydrogen evolution region). The reversible exchange of H2O and OH− ions between the solution and oxide film was

0013-4686/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 0 0 ) 0 0 4 3 4 - 5

3508

G. Niaura / Electrochimica Acta 45 (2000) 3507–3519

observed. Only Cu2O and Cu(OH)2 species were detected. No experimental evidence for the existence of CuO was obtained. The specific adsorption of OH− ions at the Cu electrode was postulated based on the dependence of the pzc of a scratched electrode upon solution pH [7]. Materlik and co-workers observed oxygen incorporated below the outmost Cu(111) surface layer by X-ray standing way spectroscopy [13]. Presence of incorporated oxygen at a Cu(111) surface in NaF solutions was consistent with the irreversible reduction/oxidation process observed by shifted cathodic and anodic capacitance maxima [14]. It has been proposed that such oxygen acts as the adsorption site for Cl− ions [14], in the same way as it acts as the binding site for underpotentially deposited Tl adatoms [13]. Recently, the electrosorption of oxygen species has been detected by linear scan voltammetry and ac impedance measurements on a Cu(111) electrode in solutions containing F− and SO24 − ions at potentials more negative than the dissolution potential of copper [10]. The strong interaction of Cu electrodes with water has been pointed out from the chronocoulometric and electrochemical impedance studies, positioning the Cu electrode in the first place of the hydrophilicity sequence: Cu \Ag \Au [11]. New information about the molecular interactions at the surface can be provided by vibrational spectroscopies [15–18]. One of them, surface-enhanced Raman spectroscopy (SERS), is especially suitable for the analysis of metal–adsorbate bonding by the direct monitoring of the vibration of surface bond, because the low frequency region (100–400 cm − 1) is easily accessible [19–23]. Another attractive SERS feature is its extremely high sensitivity [24–26], permitting studies at low surface coverage or species with low Raman scattering cross section. SERS investigations of the adsorption of anions can shed some light on the pcz problem of Cu electrodes, due to the close relationship between these two phenomena. The detection of SER spectra ions [27], highly charged from physisorbed ClO− 4 chemisorbed sulfate [27–29] and even phosphate anions [30] at rather negative potential values supports the earlier proposed negative values of the pzc (in the vicinity of −0.3 to −0.8 V vs. SHE) obtained with fresh metal surface preparation techniques [5–7]. The power of Raman spectroscopy in Cu surface oxidation and OH− adsorption studies has been demonstrated by several groups [31–35]. The anodically formed films at Cu electrodes have been studied by Raman spectroscopy with 488 nm excitation [31–33]. At this conditions the surface Raman spectrum is enhanced due to the resonant Raman effect [32,36]. Interesting recent SERS work [35] on the formation of oxygen species on Cu electrodes in a wide pH range provided evidence for the presence of an oxygenated layer (oxygen and hydroxide film, adsorbed OH− ions, and adsorbed oxygen

atoms) on the Cu surface over a wide potential window — even in acidic solutions. Hydroxide ions adsorbed on a roughened Cu(111) electrode have been recently detected by SERS [34]. The unusually high CuOH vibrational frequency, around 700 – 740 cm − 1, was observed in 0.5 M NaOH solution over a potential range of − 0.5 to −1.3 V versus SCE. Moreover, it was also observed that there was an increase in frequency of  25 cm − 1 when solution H2O was exchanged by D2O. These results contrast the case of polycrystalline Au [20,23] and Cu electrodes [30,35], where a considerably lower (400 – 490 cm − 1) frequency of the vibration of AuOH and CuOH bonds were detected. This band also displayed red shift upon solution H2O exchange to D2O [20,23,30]. Clearly, the origin of these bands is different and the interactions of OH− ions with electrode surfaces require further vibrational spectroscopic studies. In this report we present SERS studies of a polycrystalline roughened Cu electrode immersed in 1 M NaOH solution over a wide potential range. A solution containing a high concentration of OH− ions was chosen in order to increase the intensity of the bands associated with adsorbed hydroxide ions. Two kinds of adsorbed OH− ions have been observed. In addition, the appearance of a very narrow low frequency band from Cu2O at open circuit potential and SER bands from surface water molecules at extremely negative potentials are discussed.

2. Experimental

2.1. Materials The solutions were made with chemically pure NaOH and triply distilled water. The second stage of distillation was carried out in the presence of KMnO4. Both the second and the third stages were performed by heating a 0.15 m length tube prior to the condenser to approximately 160°C. The D2O was doubly distilled.

2.2. Spectroelectrochemical experiment Surface-enhanced Raman measurements were performed in a cylindrical closed three-electrode cell. The equipment allowed the exchange of the cell solution without removal of the electrode from the cell. Prior to use, all solutions were deoxygenated by bubbling ultra pure Ar for about 60 min. The counter electrode was a platinum wire. Its compartment was separated from the working electrode compartment by glass frits. The potential of the working electrode was measured versus saturated Ag/AgCl electrode supplied via a Luggin capillary, which was placed within  1 mm from the electrode surface. In order to minimize the influence of

G. Niaura / Electrochimica Acta 45 (2000) 3507–3519

3509

Cl− ions, the reference electrode was placed in a separate cell and connected to the main cell by a salt bridge filled with concentrated Na2SO4 solution. All potentials are given versus the standard hydrogen electrode (SHE). The working electrode was placed 5 mm from the cell window and 632.8 nm of HeNe laser beam with power restricted to 10 mW at the sample was incident to the surface at 60° and focussed to a line of area  1 mm2. The experiments were carried out in 90° geometry. The laser plasma lines were attenuated by means of an interference filter. The Raman scattering light was analyzed with an f/5.3 double monochromator with 1200 lines/mm gratings and detected by a photomultiplier (cooled to 10°C) and a photon counting system. The cut of filter was put in front of the entrance slit of the monochromator to eliminate Rayleigh scattering from the electrode. The overlapping bands were deconvoluted into a sum of Gaussian and Lorentzian shapes. In the quantitative investigations, the Raman intensities were obtained from four to six experiments.

approximated by three components), and estimated frequencies are indicated in the Fig. 1. In order to discriminate species containing the OH group (adsorbed OH− ions or Cu(OH)2 compounds) from copper oxides (Cu2O or CuO), an identical experiment with solution prepared from D2O instead of H2O was performed (Fig. 1). In this case it was expected to observe a shift in frequency for vibrations containing motion of OH group. Table 1 summarizes the vibrational frequencies reported in the literature for cupric and cuprous oxides [18,31,32,35,37– 48], hydroxides [18,32,35], and adsorbed oxygen containing species [34,35,42,49 – 51]. The results obtained from SER experiments in this work in alkaline H2O and D2O solutions together with tentative assignments of the bands are reported in Table 2. As can be seen, several surface species were identified by SERS at open circuit potential. First, we will consider spectroscopic evidence for the presence of Cu2O. Cuprous oxide crystallizes in a cubic lattice with the space group symmetry O 4h [45]. The optical phonons of Cu2O are distributed by the symmetry as follows [39,43 – 45]:

2.3. Cu electrode preparation

(1) − (2) + − − Gop = G − 15 (IR) +G 15 (IR) +G 25(R) +G 25 + G 2

Polycrystalline Pt disk (0.1 cm2 geometrical area) inserted in a Teflon sheath was used as a working electrode. The preparation and activation of the Cu electrode was performed by the following three steps [22]: (a) Electrodeposition of 10 mm Cu layer from an acid copper plating bath (0.5 M H2SO4 +0.5 M CuSO4) at 20°C; (b) electrodeposition of an additional Cu layer from 0.02 M CuSO4 (pH 4.5) solution at constant E= −0.30 V, for 120 s, at 20°C; (c) exposure of the electrode in 0.5 M H2SO4 +0.5 M Na2SO4 solution at open circuit potential for 90 min at 20°C. After such procedures, the electrode was rinsed with deoxygenated water and transferred to the spectroelectrochemical cell containing 1 M NaOH in H2O or D2O. The Raman spectra were recorded starting from the open circuit potential to more negative potential values.

+ G− 12

3. Results and discussion

3.1. Surface oxide Cu2O The SER spectra from Cu electrode immersed in 1 M NaOH solution at open circuit potential are displayed in Fig. 1. There are two main features in the spectra: (a) low frequency narrow band at 150 cm − 1; and (b) broad feature in the frequency range from 300 to 700 cm − 1, consisting of four overlapped broad bands. A shoulder at 222 cm − 1 was also detected. In order to estimate the frequencies and widths of the bands, the original contour between 300 and 700 cm − 1 was approximated by four Gaussian components (the contour can not be

Fig. 1. SER spectra from Cu electrode in 1 M NaOH solution prepared from H2O or D2O at open circuit potential. The calculated Gaussian components of decomposed experimental spectra are also shown. Spectroscopic conditions: slit width, 4 cm − 1; integration time, 2 s; scan speed, 30 cm − 1 min − 1.

3510

Table 1 Vibrational frequencies for copper oxides, hydroxides and adsorbed oxygen species at Cu surfacesa Sample

Technique

Cu2O

Pressed powder Thin film Anodic film Crystal Anodic film Single-crystal Anodic film Oxidized Cu plate, 673 K Cu(110)/O, 300 K

RR RR RR R SERS IR IR IRES

CuO

Cu(OH)2

Pressed powder Pressed powder Oxidized Cu plate, 673 K Matrix isolated molecules Monoclinic crystals

Frequency/cm−1 297w 411m 154s 145s

HREELS Calc. Assignm.

153m 143 G−(1) 15

R R IRES

250w

198 2G− 2

308 G− 12

300s 298s

347w 345m

IR

410m

Cu(111)/O Cu(100)/O, 300 K Cu(110)/O, 300 K Electrochemical cell

EELS EELS HREELS SERS

238 345 (low u) 403 (low u) 625

Cu(100)OHad

Cu(100)/O, H2O

HREELS

CuOHad

Electrochemical E= −1.1 V vs. Electrochemical E = −1.1 V vs. Electrochemical

SERS

430–440 (nCuOH) 698s (nCuOH)

SERS

725s (nCuOD)

SERS

460 (nCuOH)

a

633s 786w 635s,br 644s

525

625 613 630vs 640

550 G+ 25

IR

Cu(111)Oad Cu(100)Oad Cu(110)Oad CuOad

CuOHad

492m 515w

292w 418vs

580 (f. transl)

500s

790w 1110w

[32] [37] [31] [38,39] [35] [40] [18] [41]

653s 608 G−(2) 15

[42] [43,44] [39,43–45]

635m 632m

[32] [46] [41]

540m

227m

CuODad

218s 214s 220

146

Pressed powder R Anodic film SERS Powder in Nujol mull IR

cell, SCE cell, SCE cell

Ref.

720s 628

[47]

610m

[48] [32] [35] [18]

488s 460 479s

515w

602m

290vs (high u) 331 (high u)

445m (high u) 500 (high u)

684m (high u)

880–940 (dCuOH)

3560 (nCuOH)

[51]

3465w (nCuOH) 2588w (nCuOD)

[34]

800 (dCuOH)

[49] [50] [42] [35]

[34] [35]

Abbreviations: R, Raman; IR, infrared; RR, resonance Raman; IRES, infrared emission spectroscopy; SERS, surface-enhanced Raman spectroscopy; EELS, electron energy loss spectroscopy; HREELS, high resolution electron energy loss spectroscopy; w, weak; m, middle; s, strong; u, oxygen coverage; calc., calculated; n, stretching; d, bending; f. transl., frustrated translation.

G. Niaura / Electrochimica Acta 45 (2000) 3507–3519

Compound

G. Niaura / Electrochimica Acta 45 (2000) 3507–3519

3511

Table 2 SERS vibrational data and assignments of surface species at Cu electrode in 1 M NaOH solutiona Frequency/cm−1 H2O

D2O

150 222 528 581 623 431 450 527

150 221 529 584 627 423 438 562

a b

D/cm−1

FWHM/cm−1

E/V (vs. SHE)

Assignmentb

Species

0 91 −193 194 3 95 494 −894 −129 4 359 5

119 2 – 71 9 4 40 9 5 59 9 5 98 9 5 839 6 939 6

Eoc Eoc Eoc Eoc Eoc Eoc −0.40 −0.60

G−(1) (TO) 15 2G− 12? G+ 25? – G−(2) (TO) 15 n(CuOH) n(CuOH) n(CuOH)

Cu2O Cu2O Cu2O Cu2O Cu2O OHad, A OHad, A OHad, B

Abbreviations: D = n(D2O)–n(H2O); FWHM, full width at half maximum; Eoc, open circuit potential; TO, transverse optical. Assignments for Cu2O are based on Refs. [39,43–45].

Because of the inversion symmetry, the mutual exclusion principle operates for IR and Raman activity of the vibrations. Analysis based on group theory predicts only one Raman active mode with G+ 25 symmetry in the first order non-resonant Raman spectra and two IR (1) (2) and G − modes [39,43–45]. Calculations active G − 15 15 + [43,44] predict the G 25 phonon frequency at 550 cm − 1 (1) (2) and vibrations of IR active modes G − and G − at 15 15 −1 143 and 608 cm , respectively (Table 1). As can be seen from the Table 1, usually more than one band is observed in the Raman experiments. Two main reasons could be responsible for such findings: (a) Appearance of overtones and combination bands due to the resonance Raman effect [39]; and (b) breakdown of the symmetry of the lattice due to the presence of defects, especially in the case of thin films grown electrochemically [31,37]. The electronic spectrum of Cu2O has been extensively studied in the past [36]. It consists of a broad absorption continuum and superimposed are several sharp lines in the yellow–blue spectral range. At low temperatures the absorption continuum starts at about 606 nm and extends to lower wavelengths exhibiting several sharp lines due to the creation of excitons [36]. The absorption continuum is associated with the phonon-assisted excitation of the 1s exciton [36,39]. It has been shown that G− 12 phonon is mainly involved in this absorption process [52]. The frequency of G− 12 phonon must be in the vicinity of 100–110 cm − 1 [39]. While one-phonon Raman scattering for the G− 12 mode is forbidden, the two-phonon scattering is allowed and −1 was observed as in fact the 2G− 12 band at  220 cm one of the most intense in the Raman spectra of Cu2O (Table 1). Intensity of this band gradually decreases at excitation wavelengths longer than 606 nm [39]. Other bands usually observed in the Raman spectra are IR (1) (2) and G − (Table 1); these appeared active modes G − 15 15 due to disorder in the film. Considering our SER experiment in this work, the excitation wavelength

(632.8 nm) is above the absorption edge of bulk Cu2O [36,39] thus, effects due to the resonance Raman process are less important. In this situation the disordered nature of the surface film [31,37] and orientational dependence of surface-enhanced Raman intensities (socalled SERS surface selection rules) [53] are responsible for the form of the observed spectrum. Firstly, only the bands displaying no clear defined frequency shift upon solution H2O exchange to D2O, can be assigned to surface Cu2O. Among them, the intense and narrow peak at 150 cm − 1 (Fig. 1) can be confidently assigned (1) to the G − (TO) phonon of Cu2O (Table 2). This 15 band cannot be misinterpreted as some metal – adsorbate stretching or deformation vibration usually observable in the low frequency range, because of characteristic narrow width (FWHM = 11 cm − 1, Table 2). This makes the 150 cm − 1 band the most useful one for the monitoring of Cu2O by SERS. To the best of (1) our knowledge, this is the first observation of the G − 15 (TO) mode from Cu2O in SER spectra. Another band, (2) which clearly can be identified as G − (TO) phonon of 15 Cu2O is a broad feature around 623 cm − 1 (Fig. 1). Both discussed modes are IR active and the appearance of these bands in the SER spectra is most likely associated with disorder in surface film [31,37]. The low intensity shoulder at 222 cm − 1 has been tentatively assigned to the two phonon 2G− 12 mode, based primarily on the close frequency correspondence to the earlier observed data (Table 1). The low intensity of this band indicates that the resonance Raman effect due to the phonon-assisted excitation of 1s excitons has the minor contribution in our SER experiments. The assignment of the 528 cm − 1 band is not obvious. The peak in the vicinity of 500 cm − 1 was observed in the IR spectra [41,48] of CuO (Table 1). Although in the Raman spectra of cupric oxide, a strong line at  300 cm − 1 is usually observed (Table 1) [32,46]. Taking this into account, the presence of bulk CuO must be ruled out.

3512

G. Niaura / Electrochimica Acta 45 (2000) 3507–3519

We tentatively assigned this band to the Raman active G+ 25 phonon of the Cu2O, enhanced through the SER mechanism. This band has been observed in earlier SERS experiments [30,35].

3.2. Adsorbed OH− species Adsorbed OH− species can be clearly differentiated by the observable frequency shift upon isotopic exchange of solution H2O to D2O. We will discuss later that not only the mass effect, but also changes in strength of hydrogen bonding can be responsible for the observed differences in SER spectra when OH− at the interface is substituted by OD−. As can be seen from Fig. 1, one band located at 431 cm − 1 clearly shifts to 423 cm − 1 when the experiment was performed in D2O instead of H2O, under identical conditions. Other bands do not shift in the limit of experimental error. The experimental frequency ratio n(CuOH)/ n(CuOD) is 1.019. Based only upon mass considerations and assuming that Cu atom mass is equal to the atomic one, the calculated stretching frequencies ratio n(CuOH)/n(CuOD) is 1.023, which is in rather good agreement with the experiment. This band can be assigned either to vibration of bulk Cu(OH)2 or to the vibration of the CuOH bond from adsorbed OH− ions (Table 1). We tentatively assigned this band to the vibration of adsorbed OH− ions based on the following grounds: (i) Potential of the Cu2O formation is more negative compared with Cu(OH)2 formation. The most positive potential reached in this study is at open circuit (Eoc  −0.35 V vs. SHE). The equilibrium potential for the Cu/Cu2O and Cu2O/Cu(OH)2 couples at pH 14 is −0.356 and −0.08 V versus SHE, respectively [54]. Thus formation of bulk Cu(OH)2 was thermodynamically unexpected at experimental conditions; (ii) a similar band was observed at more negative potentials, after reduction of Cu2O. An interesting aspect is that this band is observed together with the surface Cu2O. The mechanism of Cu2O formation has been studied both in UHV experiments at dry conditions [42] and in NaOH solutions [55,56]. Direct monitoring of Cu(111) and Cu(100) surface by AFM, indicated a very rapid formation of Cu2O at potentials more positive than voltammetric oxidation peak, suggesting a lateral growth mechanism of Cu2O [55]. The growth of Cu2O by the formation of islands on the Cu(110) surface at dry conditions has been suggested from HREELS and LEED measurements [42]. Electroreflectance investigations of the Cu electrode in neutral and alkaline solutions also suggest that the first stage in surface oxide development is the formation of Cu2O islands [56]. Our results imply that the Cu2O film in NaOH solutions at open circuit potential is not compact, but has regions with chemisorbed OH− ions on Cu surface atoms.

The potential dependence of SER spectra is displayed in Fig. 2. As the potential is made more negative, to around − 0.40 V, the characteristic bands assigned to Cu2O (Table 2) disappeared. This is in agreement with the reversible potential value of the Cu/Cu2O couple which at pH 14 is known to be around − 0.356 V versus SHE [54]. Inspection of Fig. 2 shows that the strong and broad feature in the vicinity of 450 cm − 1 remained after the surface oxide reduction at − 0.40 V. In D2O solutions this band appeared at 438 cm − 1, yielding the frequency ratio of the vibrations 450 cm − 1/ 438 cm − 1 = 1.027. This frequency ratio is very close to the calculated value of 1.023 for the CuOH/CuOD vibrations, when only mass effect is accounted, and provide evidence that the origin of the 450 cm − 1 band is the same as the 431 cm − 1 one observed at open circuit potential in the presence of Cu2O. Thus it is clear from the spectroscopic data that at open circuit potential and at more negative potentials just after surface oxide reduction the OH− ions chemisorb at the Cu surface. As the potential was switched to more negative values to around − 0.50 V, the frequency of the 450 cm − 1 band slightly increased and its intensity decreased (Fig. 2). Although, around this potential the

Fig. 2. Dependence of the SER spectra from Cu electrode upon the potential (vs. SHE) in 1 M NaOH solution prepared from H2O ( —) or D2O (···). Spectroscopic conditions: slit width, 4 cm − 1; integration time, 2 s; scan speed, 30 cm − 1 min − 1.

G. Niaura / Electrochimica Acta 45 (2000) 3507–3519

new band with higher frequency started to develop. This tendency could be more clearly visible in spectra obtained in D2O solutions (Fig. 2). The intensity of the higher frequency feature increased at a more negative potential ( −0.60 V). Again, the nature of the band was investigated by substitution of solution H2O to D2O. The results are unexpected; instead of red shift, this band exhibits a clear blue shift by 35 cm − 1 (Fig. 2). When the electrode potential was brought to a rather negative value ( −1.20 V), the discussed feature disappeared (Fig. 2). The spectroscopic parameters of the two low frequency bands are presented in the Table 2. The first peak, which was observed at open circuit potential and just after the surface oxide reduction we denoted as a peak associated with type A adsorbed OH− ions (type A peak). The second peak observable at a more negative potential and at higher frequencies we denoted as peak associated with type B adsorbed OH− anions (type B peak). Both peaks are sensitive to the solution isotopic H2O/D2O exchange and thus belong to adsorbed OH− species. Although, the bonding and local environment of the adsorbed species are different, as clearly visible from the opposite frequency shift upon isotopic H2O/D2O exchange (Table 2). This matter will be further discussed later. The potential dependence of the low frequency bands is more clearly apparent in the potential difference spectra presented at selected sampling potentials in Fig. 3 and more quantitatively from the integrated intensity dependence in Fig. 4. From these figures one can see that the two low frequency bands showed different potential dependence. Peak A had the highest intensity just after reduction of the surface oxide and sharply decreased as potential became more negative (Fig. 4). This behavior is consistent with the general electrochemical view that adsorbed anions tend to desorb as potential become more negative [57]. At this point we must to recall the pzc problem in the case of the Cu electrode. The pzc data for polycrystalline Cu electrode are extremely inconsistent and two sets of data can be found in the literature, positive pzc values in the range from 0.025 to − 0.030 V versus SHE [3,8], and negative pzc values in the range from −0.31 to −0.64 V versus SHE [5–7] (for fluoride, perchlorate, sulfate and chloride solutions). Methods, where renewed Cu surfaces were used, always displayed more negative pzc [5,7]. It has been suggested [6] that surface oxidation might be responsible for more positive pzc values obtained earlier mainly by differential capacitance method. The importance of electrochemisorption of oxygen containing species in interpreting the electric double layer at Cu electrodes also have been emphasized based on linear scan voltammetric and impedance measurements [10]. Analysis of the reduction reaction of S2O28 − at Cu electrodes and comparison with Hg, Au and Pt metals suggested that the pzc value for Cu electrode was as negative as

3513

Fig. 3. Potential Difference SER spectra from Cu electrode in 1 M NaOH solution prepared from H2O at selected electrode potentials (vs. SHE). Potential Difference spectra were obtained by subtracting the reference spectrum at − 1.20 V from the SER spectra at indicated potentials.

− 0.46 V versus SHE [10]. Recently the pzc problem of Cu(110) in acidic perchlorate solutions was analyzed by means of chronocoulometry and electrochemical impedance and a rather negative pzc value of −0.69 V versus SHE was determined [11]. Radioisotopic studies 2− clearly demonstrated the adsorption of HSO− 4 /SO4 anions in acidic solutions at submilimolar concentrations at potentials down to −0.30 V versus SHE on the Cu electrode and provided non-direct evidence of the rather negative pzc value of Cu electrode [58,59]. Finally, vibrational spectroscopic studies of adsorption of 2− such anions as ClO− [27 – 29], Cl− [21,22], 4 [27], SO4 HPO24 − [30], and PO34 − [30] at Cu electrodes and comparison with Ag and Au [30], suggest that the pcz value of Cu electrodes must be in the negative potential range between roughly − 0.30 and −0.8 V versus SHE. If we assume, based upon the above discussion, the negative pcz value for Cu electrode, then the sharp decrease in integrated intensity can be understood as desorption of OH− ions, or transformation of the adsorption site when the charge of the Cu surface becomes negative. Thus type A adsorbed anions behave as usual specifically adsorbed anions, retaining some negative charge upon adsorption. These type of anions

3514

G. Niaura / Electrochimica Acta 45 (2000) 3507–3519

cannot be observed at rather negative surface charges and the surface coverage decreases at more negative potentials. The isotopic frequency shift for CuOH motion for A type anions can be immediately understood as associated with mass effects. The same type of adsorbed OH− ions were observed earlier at Au electrodes in alkaline solutions [20]. As can be seen from Fig. 4, different potential dependence is characteristic for the type B adsorbed OH− species. Most unusual is the increase in integrated B band intensity as the potential became progressively negative until approximate −0.70 V versus SHE, indicating that other, not charge interactions between the surface and the anion are dominant for the adsorption of this type OH− species. Let us discuss now the nature of the adsorbed OH− ions at Cu electrode. The most striking aspect observed in this work is the presence of two different states, type A and B adsorbed OH− ions. The following experimental evidence indicate that the nature of the binding site among two states is different: 1. Opposite frequency shift upon isotopic solution H2O exchange to D2O; type A band showed red shift by 12 cm − 1, while type B band showed blue shift by 35 cm − 1 (Table 2). 2. Considerable difference in frequency; type B band has a higher frequency by approximate 70 cm − 1 compared to type A band in H2O solutions (Table 2). 3. Different intensity dependence on potential; intensity of type A band always decreased at more nega-

Fig. 4. Comparison of the dependence of the integrated SER intensities of the low frequency type A ( ) and type B () bands upon Cu electrode potential (vs. SHE) in 1 M NaOH solution prepared with H2O.

tive potentials, while where is a potential region where the intensity of B band increased as potential became more negative until certain negative potential value (Fig. 4). To the best of our knowledge, this is the first work where two different types of adsorbed OH− ions, depending on potential were observed. In a recent paper, Hartinger and co-workers [34] reported on unusually high frequency SER band from adsorbed hydroxide anions at Cu(111) electrode in alkaline solutions. This band was detected at 700 and 725 cm − 1 in H2O and D2O solutions, respectively at −0.86 V versus SHE. Such a blue frequency shift is consistent with our isotopic experiments for type B band, and indicates that the origin of these bands is the same. The feature was assigned by authors to the CuOH− vibration in Cu3(OH)− complex with local C26 symmetry, where an oxygen atom is sitting at the threefold hollow sites of Cu(111). The blue frequency shift upon isotopic H2O exchange to D2O was explained as the result of vibrational coupling in surface complex. While this interpretation might be valid, we would like to point out on additional factor important in anions bonding at interfaces, namely, hydrogen bonding effects. The differences in the hydrogen and deuterium bonds are well documented [60]. The adequate model must explain all three experimentally observed findings. The presence of two types of adsorbed OH− ions at Cu electrode we see as the result of competition between surface atoms and water molecules for the lone pair electrons of oxygen atom in OH−. The electrode potential controls this competition. The scenario proposed by us is the following: at potentials in the vicinity − 0.40 to −0.50 V versus SHE, the Cu electrode surface is positively charged and OH− ions strongly chemisorbs with considerable charge transfer from the anion to the metal. At these conditions the oxygen atom tends to occupy the sites with higher coordination number (type A adsorbed ions) (Fig. 5a). Threefold bridge or twofold bridge coordination through the lone pair electrons of oxygen atom is preferred over on-top coordination under these conditions. An analogy can be found in UHV experiments for oxygen adsorption on Cu at low surface coverage [50]. We were unable to observe sufficiently, resolved band associated with stretching OH vibration in the high frequency region at conditions where low frequency CuOH bands from adsorbed anions were detected. Based on these findings, we displayed twofold bridge coordination in Fig. 5a, since at this surface arrangement the OH bond forms an angle with surface normal and should not be observed as an intense band in SER spectra, due to the orientational dependence of surface-enhanced Raman intensities [53], in accordance with experiment. As the potential became progressively more negative, the electrode charge becomes progressively less positive and interaction of

G. Niaura / Electrochimica Acta 45 (2000) 3507–3519

Fig. 5. Schematic representation of the two types of adsorbed OH− ions at Cu electrode: (a) Type A adsorbed ions at positive surface charge, assuming twofold bridge coordination with Cu. The frequency of CuOH vibration is given at E= − 0.4 V versus SHE; (b) type B adsorbed ions at potential close to pzc, assuming on-top coordination with Cu. Frequency of CuOH vibration is given at E= −0.6 V. The strong hydrogen bond between OH− ions and water molecules (– – – ) is shown.

OH− ions with surface weakens. This causes changes in coordination from many bonded in an on-top coordination (Fig. 5b). As a result, the new higher frequency band (band type B) centered at 527 cm − 1 appeared at − 0.60 V versus SHE (Fig. 2). It is well documented that the metal–oxygen frequency increases as coordination number decreases [61]. On the other hand, the change in the interaction of the hydroxide ions with Cu surface as potential became progressively more negative resulted in the changes of the hydrogen bonding properties of the adsorbed OH− ions. Hydroxide ion is considered as a weak proton donor, although its proton accepting power is high [62,63], placing this anion in the first place of the sequence of proton acceptors [63]: OH− \F− \PO34 − \SO24 − \Cl− \H2O\Br− \I− − \NO− 3 \ClO4 . Thus formation of ther hydrogen bond can be expected primarily as the formation of surface complex − HOad···HOH displayed schematically in Fig. 5b. Indeed, a remarkably strong hydrogen bond between an oxygen atom of the hydroxide anion and the hydrogen atom of a water molecule was evidenced in hydroxide monohydate [64]. The acceptor strength of adsorbed OH− ions should increase at less positive surface charges or at more negative potentials. The effect on hydrogen bonding properties induced by the decrease in positive surface charge can be compared to the changes from Li+ to Cs+ in the vibrational studies of alkali metal hydroxide monohydrates [65]. In both cases the interaction strength between the metal and OH− ion decreased, causing increased acceptor properties. As a result, a strong covalent hydrogen bond at interface occurs. Formation of such a bond in cesium and rubidium hydroxide monohydrates, in contrast to lithium and sodium compounds, has been indeed evidenced by IR spectra [65]. Moreover, hydrogen and deuterium bonding are very subtle in such systems. For example, it was shown that while unusually strong,

3515

mainly covalent hydrogen bonds can be observed for the RbOH·H2O system, this was not the case for the RbOD·D2O system, where only the usual mainly electrostatic deuterium bond was evidenced [65], indicating that hydrogen bonding was stronger in such alkali metal hydroxide monohydrate systems compared to deuterium ones. Considering the stretching frequency n(CuOH) shift upon solution H2O exchange to D2O, three effects must be taken into account: (i) mass effects, always leading to the red frequency shift; (ii) coupling in vibrational modes of surface complex [34]; and (iii) difference in hydrogen and deuterium bond strength. For the strongly chemisorbed A type OH− ions at relatively positive surface charges, the mass effect is dominant. In this case the bonding with surface takes place through the three or two oxygen lone pair electrons, thus preventing the possibility of hydrogen bond formation. In contrast, at potentials close to the pzc the interaction with surface weakens and hydrogens from the water molecules at the interface (possibly from the salvation shell of cations) become capable to compete with the surface for the lone pair electrons of adsorbed OH− ions. As a result, the changes in adsorption geometry and formation of a strong hydrogen bond take place. The different strength of the hydrogen and deuterium bonds can be responsible for the blue shift observed for B type band upon isotopic solution H2O substitution to D2O. The effect of the different strength of hydrogen and deuterium bonds in SER spectra was recently demonstrated [30] in the case of chemisorbed PO34 − ions at Au, Ag and Cu electrodes. The asymmetric PO vibrational frequency decreased by 8 – 12 cm − 1 when solution H2O was substituted to D2O. In the case of strong hydrogen bonds, the CuOH vibrational frequency should depend upon the hydrogen bond strength. Thus for B type adsorbed OH− anions the (i) and (iii) effects determine the isotopic frequency shift upon solution H2O exchange to D2O. Consequently, the differences in frequency and opposite isotopic shift of the A and B bands can be explained assuming the change in the coordination of oxygen atom to surface and increase in the strength of hydrogen bonding between the adsorbed OH− and water molecules at interface.

3.3. Water molecules at interface At very negative potentials from − 1.00 to −1.20 versus SHE, the low frequency bands from adsorbed OH− ions virtually disappear (Figs. 2 and 4) and two other intense bands located at  1200 and  2500 cm − 1 develop (Fig. 6). These two bands could be clearly assigned to the deformation and stretching vibrations of surface D2O molecules by comparing the SER spectrum (Fig. 6a) with the solution Raman spec-

G. Niaura / Electrochimica Acta 45 (2000) 3507–3519

3516

Fig. 6. Raman spectra from Cu electrode in 1 M NaOH, D2O solution. (a) SER spectrum from roughened electrode at − 1.20 V versus SHE; (b) solution spectrum from deactivated electrode at − 0.50 V. The deactivation of the electrode was performed by holding the potential at −1.4 V for 5 min; (c) difference SER spectra (a)–(b). Spectroscopic conditions: slit width, 6 cm − 1; integration time, 0.5 s; scan speed, 159 cm − 1 min − 1.

trum from D2O (Fig. 6b) observed from deactivated Cu electrode at −0.50 V versus SHE. The deactivation of the electrode was performed by holding the potential at −1.4 V versus SHE for 5 min. This procedure considerably reduced the intensity of the SER spectrum from adsorbed OH− ions in the low frequency region at − 0.50 V versus SHE and is considered to produce Raman spectrum free from SER bands in the frequency

region of bending and stretching motions of D2O (Fig. 6b). The close spectrum was observed also from smooth Cu electrodes, although the intensity of bulk water bands was higher due to the increased surface reflectivity. As can be seen, the high frequency band at 2500 cm − 1 is overlapped with the solution band arising from the O – D stretching motions. Two surface bands are clearly visible in the difference spectra (Fig. 6c). The parameters of the bands obtained from the difference spectrum are presented in the Table 3. The main differences between surface and solution spectra of D2O can be summarized as following: 1. Selective enhancement of bending d(DOD) mode intensity compared to the stretching n(OD) mode intensity for surface molecules; 2. narrowing of both bending and stretching vibrational bands in surface spectra; 3. from the two stretching O – D vibrational bands located at  2400 and 2494 cm − 1 usually observed in solution Raman spectra (Fig. 6b) only the high frequency component is enhanced in SER spectra (Fig. 6c); 4. red frequency shift for surface water bending motions and blue shift for surface water stretching motions compared with solution water vibrations. The similar SERS features from water molecules at potentials more negative than the pzc was observed by several groups at Ag electrodes in various aqueous electrolytes [66 – 70] as well as in nonaqueous solvents [71,72]. Recently, Chen and Tian detected SERS from water also at Cu electrode in 1 M Na2SO4 solution at potentials where hydrogen evolution reaction takes place [73]. In the following, a possible explanation of the features observed in SER spectra of water at negative potentials will be discussed. The selective enhancement of the bending mode can be explained by considering the superposition of the several effects: (a) a particular orientation of DOD molecule; for example with one D atom facing the surface and the oxygen atom interacting with a cation [73]; (b) dependence of surface EM enhancement on vibrational frequency; the enhance-

Table 3 Comparison of vibrational data of surface D2O observed by SERS from Cu electrode at −1.20 V vs. SHE and solution D2O observed by normal Raman spectroscopya Vibration

Frequency/cm−1 R

n(OD)b d(DOD)

2499 1204

o/cm−1

SERS 2508 1186

9 −18

FWHM/cm−1 R

SERS

222 47

169 39

9/cm−1

Relative surface enhancement

−53 −8

1 9.8

a Abbreviations: R, normal Raman spectroscopy; n, stretching; d, deformation; o =n(SERS)−n(R); 9 =FWHM(SERS)− FWHM(R). b Only the high frequency component of solution Raman stretching modes is compared with SERS data.

G. Niaura / Electrochimica Acta 45 (2000) 3507–3519

ment decreases with increasing frequency and the effect can be as high as an order of magnitude [74]; (c) breaking of hydrogen bonds of water molecules at interface. The intensity of the bending vibration considerably increased (up to five times) compared with the stretching mode in aqueous solutions containing I− ions as was originally demonstrated by Schultz and Horning [75] and confirmed later [70]. The effect was explained by reduced hydrogen bonding ability and as a consequence, the lower polarity of water molecules interacting with I−; and (d) importance of the hydrogen evolution reaction. Chen and Tian suggested [73] that the interfacial water molecules involved in the hydrogen evolution reaction may show enhancement of intensity of bending vibrations. It should be also noted that extensive hydrogen evolution critically modified solution composition at the interface due to the increase concentration of the OH− ions and the presence of H2 bubbles. Narrowing of the surface water bands and the decreased intensity of the lower frequency component in the stretching O–D vibrational region can be explain by the restriction of deuterium bonding ability due to the (i) interaction with Na+ cations through the oxygen atom [66,76] and (ii) the presence of surface. Red frequency shift of bending mode is also consistent with the interaction of oxygen lone pair electrons with Na+ [76], while the blue frequency shift of n(OD) motion is an additional indication of the decreased strength of deuterium bonds [77]. We would also like to point out the importance of surface electromagnetic enhancement in the studies of water molecules at very negative potentials. Usually, relatively high electrolyte concentration ( ] 0.5 M) and potentials more negative than the pzc are employed in such studies. Although, it must be noted that those SERS bands of water are not always observed even if these two conditions are satisfied. For example, no water bands were detected at an activated Cu(111) electrode in 0.5 M NaOH at such negative potentials as −1.06 V versus SHE [34]. A possible explanation for these differences is the requirement for the surface to produce an unusually high electromagnetic enhancement factor for SERS of water at negative potentials. If we assume that the observed bending and stretching modes arose from the water molecules of the solvation shell of the cations forced closer to the surface at negative charges, there is no chemical interaction between water and surface metal atoms, and as a consequence there is no possibility for the ‘chemical’ enhancement in the SER spectra. It is known that SERS bands from physisorbed species are weaker by a factor of 10–100 compared with chemisorbed ones [15,78]. Ag surfaces with unusually high EM enhancement were prepared by Funtikov et al. [68]. Such surfaces enabled the authors to observe not only the

3517

bending mode from physisorbed water molecules at negative potentials, but also the totally symmetric mode at 989 cm − 1 of physisorbed SO24 − ions at more positive potentials. Our roughening procedure for the Cu electrode also yielded high EM enhancement [22], and as a consequence, the water bands at negative potentials were observed. On the other hand, only slightly activated Cu(111) electrodes [34] produce excellent SER spectra from chemisorbed OH− species, but physisorbed water molecules were not detected at negative potentials, presumably due to the deficiency of EM enhancement. An interesting correlation between the SERS intensity of the bending mode of water and the current density of the hydrogen evolution reaction has been shown [73] and might be important in the understanding of the mechanisms of the enhancement of intensity of water bands at negative potentials. Clearly, future work is required in this fascinating SERS field.

4. Conclusions The interface of a roughened Cu electrode in 1 M NaOH solution was in situ characterized in an electrochemical cell by SERS. The results can be summarized as follows: 1. The surface Cu2O was identified by three main bands located at 150, 528, and 623 cm − 1. The narrow and intense feature at 150 cm − 1 was found to be very characteristic, and could be useful for the SER monitoring of cuprous oxide. Not only surface Cu2O, but also chemisorbed OH− ions were distinguished spectroscopically at open circuit potential, indicating that at these conditions the cuprous oxide film is not compact. 2. Two different states of chemisorbed OH− ions, dependent upon electrode potential, were evidenced by means of solvent isotopic H2O/D2O substitution effect on the CuOH vibration. The SER band associated with type A adsorbed OH− species was observed in the vicinity of 450 – 470 cm − 1 at positive surface charges, starting at open circuit potential and had a maximum intensity just after reduction of surface oxide. The frequency of this band shifted to lower wavenumbers by 12 cm − 1 upon solution H2O exchange to D2O. In contrast, the type B band was observed at considerably higher frequencies, in the vicinity 540 – 580 cm − 1 and had a maximum intensity at more negative potentials, close to the pzc (  − 0.7 V vs. SHE). This band showed an opposite isotopic frequency shift; the frequency increased by 35 cm − 1 when solution H2O was exchanged to D2O. The results were interpreted by assuming that two states are associated with different OH− ligation to the surface and the formation of a strong hydrogen bond at the interface between

3518

G. Niaura / Electrochimica Acta 45 (2000) 3507–3519

water molecules and B type of adsorbed OH− species. The different strength of hydrogen and deuterium bonds was proposed to be mainly responsible for the observed blue isotopic frequency shift when H2O was replaced to D2O in the case of B type adsorbed OH− ions. 3. Water molecules were detected at negative potential, − 1.20 V versus SHE with characteristic bending d(DOD) and stretching n(OD) frequencies at 1186 and 2508 cm − 1, respectively.

References [1] A. Vorotyntsev, in: J.O’M Bockris, B.E. Conway, R.E. White (Eds.), Modern Aspects of Electrochemistry, vol. 17, Plenum, New York, 1986 Chapter 2. [2] D. Armstrong, N.A. Hampson, R.J. Tatham, J. Electroanal. Chem. 23 (1969) 361. [3] I. Novoselski, N. Konevskich, L. Egorov, Elektrokhimiya 8 (1972) 1480. [4] R.O. Loutfy, Electrochim. Acta 18 (1973) 227. [5] G.J. Clark, T.N. Andersen, R.S. Valentine, H. Eyring, J. Electrochem. Soc. 121 (1974) 618. [6] H. Henning, V. Batrakov, Elektrokhimiya 15 (1979) 1833. [7] E. Lazarova, C. Nikolov, Elektrokhimiya 22 (1986) 1217. [8] M. Turowska, J. Baczynski, J. Sokolowski, Russ. J. Electrochem. 33 (1997) 1301. [9] T. Agladze, A. Podobayev, Electrochim. Acta 36 (1991) 859. [10] S. Ha¨rtinger, K. Doblhofer, J. Electroanal. Chem. 380 (1995) 185. [11] M.L. Foresti, G. Pezzatini, M. Innocenti, J. Electroanal. Chem. 434 (1997) 191. [12] W. Kautek, M. Geuß, M. Sahre, P. Zhao, S. Mirwald, Surf. Interf. Anal. 25 (1997) 548. [13] G. Materlik, M. Schma¨h, J. Zegenhagen, W. Uelhoff, Ber. Bunsenges. Phys. Chem. 91 (1987) 292. [14] M. Weidenauer, K.G. Weil, Ber. Bunsenges. Phys. Chem. 92 (1988) 1368. [15] R.L. Birke, T. Lu, J. Lombardi, in: R. Varma, J.R. Selman (Eds.), Techniques for Characterization of Electrodes and Electrochemical Processes, Wiley, New York, 1991 Chapter 5. [16] B. Pettinger, in: J. Lipkowski, P.N. Ross (Eds.), Adsorption of Molecules at Metal Electrodes, VCH, New York, 1992 Chapter 6. [17] R.J. Nichols, in: J. Lipkowski, P.N. Ross (Eds.), Adsorption of Molecules at Metal Electrodes, VCH, New York, 1992 Chapter 7. [18] C.A. Melendres, G.A. Bowmaker, J.M. Leger, B. Beden, J. Electroanal. Chem. 449 (1998) 215. [19] P. Gao, M.J. Weaver, J. Phys. Chem. 90 (1986) 4057. [20] J. Desilvestro, M.J. Weaver, J. Electroanal. Chem. 209 (1986) 377. [21] B. Pettinger, M.R. Philpott, J.G. Gordon II, J. Phys. Chem. 85 (1981) 2746. [22] G. Niaura, A. Malinauskas, Chem. Phys. Lett. 207 (1993) 455.

[23] Y. Zhang, X. Gao, M.J. Weaver, J. Phys. Chem. 97 (1993) 8656. [24] P. Hildebrandt, M. Stockburger, J. Phys. Chem. 88 (1984) 5935. [25] G. Niaura, A. Malinauskas, Ber. Bunsenges. Phys. Chem. 99 (1995) 1563. [26] S. Nie, S.R. Emory, Science 275 (1997) 1102. [27] G. Niaura, A. Malinauskas, J. Chem. Soc. Faraday Trans. 94 (1998) 2205. [28] G.M. Brown, G.A. Hope, J. Electroanal. Chem. 382 (1995) 179. [29] S. Martusevicˇius, G. Niaura, Z. Talaikyte; , V. Razumas, Vibr. Spectrosc. 10 (1996) 271. [30] G. Niaura, A.K. Gaigalas, V.L. Vilker, J. Phys. Chem. B 101 (1997) 9250. [31] C.A. Melendres, S. Xu, B. Tani, J. Electroanal. Chem. 162 (1984) 343. [32] J.C. Hamilton, J.C. Farmer, R.J. Anderson, J. Electrochem. Soc. 133 (1986) 739. [33] D.T. Schwartz, R.H. Muller, Surf. Sci. 248 (1991) 349. [34] S. Ha¨rtinger, B. Pettinger, K. Doblhofer, J. Electroanal. Chem. 397 (1995) 335. [35] H.Y.H. Chan, C.G. Takoudis, M.J. Weaver, J. Phys. Chem. B 103 (1999) 357. [36] J.L. Deiss, A. Daunois, Surf. Sci. 37 (1973) 804. [37] I.P. Herman, Optical Diagnostics for Thin Films Processing, Academic, San Diego, 1996. [38] M. Balkanski, M.A. Nusimovici, J. Reydellet, Solid State Commun. 7 (1969) 815. [39] P.Y. Yu, Y.R. Shen, Phys. Rev. B 12 (1975) 1377. [40] E.C. Heltemes, Phys. Rev. 141 (1966) 803. [41] D.H. Sullivan, W.C. Conner, M.P. Harold, Appl. Spectrosc. 46 (1992) 811. [42] A.P. Baddorf, J.F. Wendelken, Surf. Sci. 256 (1991) 264. [43] C. Carabatos, Phys. Status Solidi 37 (1970) 773. [44] C. Carabatos, B. Prevot, Phys. Status Solidi B 44 (1971) 701. [45] K. Huang, Z. Phys. 171 (1963) 213. [46] J. Chrzanowski, J.C. Irwin, Solid State Commun. 70 (1989) 11. [47] D.E. Tevault, R.L. Mowery, R.A. De Marco, R.R. Smardzewski, J. Chem. Phys. 74 (1981) 4342. [48] N.T. McDevitt, W.L. Baun, Spectrochim. Acta 20 (1964) 799. [49] F. Besenbacher, J.K. Narskøv, Progr. Surf. Sci. 44 (1993) 5. [50] M. Wuttig, R. Franchy, H. Ibach, Surf. Sci. 213 (1989) 103. [51] T. Sueyoshi, T. Sasaki, Y. Iwasawa, J. Phys. Chem. B 101 (1997) 4648. [52] R.J. Elliott, Phys. Rev. 124 (1961) 340. [53] J.A. Creighton, in: R.J.H. Clark, R.E. Hester (Eds.), Spectroscopy of Surfaces, Wiley, New York, 1988 Chapter 2. [54] U. Bertocci, D.R. Turner, in: A.J. Bard (Ed.), Encyclopedia of Electrochemistry of the Elements, vol. 2, Marcel Dekker, New York, 1974, p. 383. [55] N. Ikemiya, T. Kubo, S. Hara, Surf. Sci. 323 (1995) 81. [56] A.G. Akimov, M.G. Astafjev, I.L. Rozenfeld, Elektrokhimiya 13 (1977) 1493.

G. Niaura / Electrochimica Acta 45 (2000) 3507–3519 [57] M.A. Habib, J.O’M. Bockris, in: J.O’M. Bockris, B.E. Conway, E. Yeager (Eds.), Comprehensive Treatise of Electrochemistry, Plenum, New York, 1980 Chapter 4. [58] G. Horanyi, E.M. Rizmayer, P. Joo, J. Electroanal. Chem. 154 (1983) 281. [59] L.M. Rice-Jackson, G. Horanyi, A. Wieckowski, Electrochim. Acta 36 (1991) 753. [60] A.D. Buckingham, L. Fan-Chen, Int. Rev. Phys. Chem. 1 (1981) 253. [61] H. Ibach, D.L. Mills, Electron Energy Loss Spectroscopy and Surface Vibrations, Academic, New York, 1982. [62] W.C. Hamilton, J.A. Iberis, Hydrogen Bonding in Solids, W.A. Benjamin, New York, 1968. [63] H.D. Lutz, K. Beckenkamp, H. Mo¨ller, J. Mol. Stuct. 322 (1994) 263. [64] K. Abu-Dari, K.N. Raymond, D.P. Freyberg, J. Am. Chem. Soc. 101 (1979) 3688. [65] K.M. Harmon, G.F. Avci, D. Duffy, M. Janos, J. Mol. Struct. 216 (1990) 63. [66] M. Fleischmann, I.R. Hill, J. Electroanal. Chem. 146 (1983) 367.

.

3519

[67] T.T. Chen, R.K. Chang, Surf. Sci. 158 (1985) 325. [68] A.M. Funtikov, S.K. Sigalaev, V.E. Kazarinov, J. Electroanal. Chem. 228 (1987) 197. [69] N. Iwasaki, Y. Sasaki, Y. Nishina, Surf. Sci. 198 (1988) 524. [70] Z.Q. Tian, B. Ren, Y.X. Chen, S.Z. Zou, B.W. Mao, J. Chem. Soc. Faraday Trans. 92 (1996) 3829. [71] D.E. Irish, I.R. Hill, P. Archambault, G.F. Atkinson, J. Solution Chem. 14 (1985) 221. [72] J.E. Pemberton, S.L. Joa, J. Electroanal. Chem. 378 (1994) 149. [73] Y.X. Chen, Z.Q. Tian, Chem. Phys. Lett. 281 (1997) 379. [74] C.G. Blatchford, M. Kerker, D.S. Wang, Chem. Phys. Lett. 100 (1983) 230. [75] J.W. Schultz, D.F. Hornig, J. Phys. Chem. 65 (1961) 2131. [76] A. Zecchina, F. Geobaldo, G. Spoto, S. Bordiga, G. Ricchiardi, R. Buzzoni, G. Petrini, J. Phys. Chem. 100 (1996) 16584. [77] C.I. Ratcliffe, D.E. Irish, J. Phys. Chem. 86 (1982) 4897. [78] X. Jiang, A. Campion, Chem. Phys. Lett. 140 (1987) 95.