18640
J. Phys. Chem. B 2004, 108, 18640-18649
Hydroxide Adsorption on Ag(110) Electrodes: An in Situ Second Harmonic Generation and ex Situ Electron Diffraction Study Sarah L. Horswell, Alexei L. N. Pinheiro, Elena R. Savinova,† Bruno Pettinger,* Mau-Scheng Zei,‡ and Gerhard Ertl Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, D-14195 Berlin, Germany ReceiVed: April 30, 2004; In Final Form: August 9, 2004
The adsorption of hydroxide on the Ag(110) surface has been studied with cyclic voltammetry, in situ second harmonic generation (SHG), and ex situ low energy electron diffraction (LEED) and reflection high energy electron diffraction (RHEED). OH- is found to be adsorbed on the Ag(110) surface at potentials negative of the potential of zero charge, forming small antiphase domains of c(2 × 6) symmetry. Further adsorption leads to longer-range order and the removal of antiphase domain boundaries and is associated with a current peak in the cyclic voltammogram (CV). Concurrently, a change in symmetry patterns is observed in SHG. A c(2 × 2) pattern gradually replaces the c(2 × 6) pattern as the potential, and the OH- coverage, is increased. At the beginning of the second current wave, another symmetry change takes place which is accompanied by a sharp change in the LEED pattern from a c(2 × 2) pattern to a (1 × 1) pattern with strong background, indicating a disordered adlayer. However, RHEED results show that some patches of c(2 × 2) structure remain on the surface. The correlation between SHG and diffraction measurements and comparison of the information obtained from each technique allow us to develop a detailed picture of the structures and electronic effects at the Ag(110)|alkaline electrolyte interface.
1. Introduction The adsorption of anions on metal electrodes is one of the most fundamental issues in interfacial electrochemistry. A detailed understanding of the metal-ion and ion-ion interactions is required to elucidate the effects of specifically adsorbed ions on the reactivity of metal surfaces. Hydroxide adsorption attracts particular interest because it gives rise to a variety of overlayers depending on the electrode potential, from chemisorbed OH to metal oxide. These species can drastically change the reactivity of the electrode surface, as has been proposed for the reduction of oxygen and hydrogen peroxide on gold,1 platinum,2 and silver3 electrodes. The electrochemical behavior of Ag surfaces in alkaline electrolytes has been extensively investigated in the potential region of oxide formation with a variety of techniques, including electrochemical measurements,4 Raman spectroscopy,5 ellipsometry,6 electrochemical quartz crystal microbalance,7 and extended X-ray absorption fine structure spectroscopy.8 It has been proposed that a monolayer of Ag2O is formed,7 which then grows into a multilayer oxide film. Ag2O formation has been suggested to follow the dissolution of Ag (probably in the form of a Ag(OH)2- complex)4c,f or the formation of a film of AgOH.4g Studies in the Ag underpotential oxidation region, however, are much fewer. Cyclic voltammograms (CVs) of polycrystalline4a,9 and single crystalline10 Ag electrodes exhibit anodic peaks in alkaline electrolytes far below the reversible * Corresponding author. E-mail:
[email protected]. † Present address: Technische Universitaet Muenchen, Physik-Department E19, James-Franck Str. 1, D-85748 Garching, Germany. Permanent address: Boreskov Institute of Catalysis of the Siberian Branch of the Russian Academy of Sciences, Prospekt Akademika Lavrentieva, 5, Novosibirsk 630090, Russian Federation. ‡ Present address: Department of Physics, National Central University, Jungli, Taiwan 32054.
Ag|Ag2O potential. These have been assigned to OH- adsorption. X-ray photoelectron spectroscopy (XPS) investigations have indicated the presence of oxygen-containing species on polycrystalline Ag electrodes emersed from alkaline NaClO4,11 NaCl,12 and NaBr13 electrolytes. Ex situ XPS studies of the Ag(111)|alkaline NaF interface suggest that chemisorbed OHis discharged at positive potentials to form surface oxide of the Ag2O type, which is transformed into the bulk oxide at still more positive potentials.14 Little in situ information on hydroxide adsorption on Ag is available. Surface enhanced Raman spectroscopy (SERS) studies have revealed hydroxy and oxy adsorbates on polycrystalline Ag15 and Ag(111)10c in alkaline electrolytes. However, SERS has the disadvantage that roughened surfaces must be used. Recently, second harmonic generation (SHG) has been used to study hydroxide adsorption on Ag(111).14,16 SHG can be used to give in situ information on the electric field distribution at the electrode|electrolyte interface and on the symmetry of the substrate. It has the distinct advantage of being highly surface sensitive; only the topmost metal atoms contribute to the SH signal, unless molecules resonating with the fundamental or SH beams are present in the interfacial region. Thus, unlike many in situ spectroscopies, there are no background contributions from the electrolyte. SHG has been used to study Ag single crystal electrodes in the absence of adsorbates,17 in the presence of metal underpotential deposition (upd),17c in the presence of halides,18 and in alkaline fluoride electrolytes.14,16 In these latter studies, hydroxide was shown to retain its negative charge upon adsorption, which was inferred from the close correlation of the isotropic part of the SH signal and the charge density on the metal. To our knowledge, hydroxide adsorption on Ag(110) has not been studied yet. In this paper, it is investigated using in situ SHG and ex situ low energy electron diffraction (LEED) and
10.1021/jp0481198 CCC: $27.50 © 2004 American Chemical Society Published on Web 11/04/2004
Hydroxide Adsorption on Ag(110) Electrodes reflection high energy electron diffraction (RHEED). In situ SHG has already yielded important data for Ag electrodes,14,16-18 and LEED and RHEED have provided structural information for halides adsorbed on Ag(111) electrodes.19 The use of this combination of techniques enables us to obtain a detailed picture of the electronic effects and the structures formed by the adsorbates on the surface. 2. Experimental Section 2.1. Electrochemical and SHG Measurements. The Ag(110) crystal was produced in-house and was oriented to better than 0.5°. The surface was prepared for measurements by chemical etching with NaCN + H2O2 solutions (Caution: highly toxic), followed by hydrogen flame annealing under a gentle Ar stream, using a procedure similar to one described previously.14,17a The crystal was transferred with a drop of ultrapure water to the electrochemical cell and immersed into the Ar-purged electrolyte under potential control at a potential below the potential of zero charge (pzc). In this way, contact of the crystal with air was kept to a minimum. Cyclic voltammetry measurements were conducted in the hanging meniscus configuration in a Kel-F cell fitted with a Luggin capillary. SHG measurements were conducted in a Kel-F cell with an optical window at the bottom, which allows simultaneous optical and voltammetric measurements. All optical adjustments were made while the sample was in contact with the electrolyte and under potentiostatic control to ensure that the time between the preparation of the crystal and its immersion was minimal. The details of the setup have been published elsewhere.16a The cell allows rotation of the sample around its surface normal to measure the SHG anisotropy. In both cells, a Pt wire ring concentric to the working electrode was used as a counter electrode and the Hg|HgO|0.1 M NaOH electrode as a reference electrode. The potential of this reference electrode was +0.165 V versus the normal hydrogen electrode (NHE). The cells were cleaned with a 1:1 mixture of 25% NH3 and 30% H2O2 and thoroughly rinsed and soaked with ultrapure water. Before making electrochemical contact, the cells were purged with Ar. The 0.09 M NaF + 0.01 M NaOH electrolyte used for both cyclic voltammetry and SHG measurements was prepared from Suprapur grade chemicals (Merck, Germany) and Milli-Q water. This electrolyte has a pH value of 12. An in-house-built potentiostat (E-LAB, Fritz Haber Institut) was used to control the electrode potential and a computer to record the CV. For the optical measurements, a Nd:YAG laser was used to produce 5 ns laser pulses of ∼150 mJ of energy each at the fundamental wavelength 1064 nm and at a repetition rate of 11.5 Hz. The incident and SH beams were both polarized parallel to the reflection plane; that is, the pp configuration was used. The fundamental beam was blocked after reflection by a colored glass/interference filter setup, and the SH intensity was detected by a photomultiplier tube. The signal was normalized using a reference signal obtained with a photodiode. A Labview software program was used to control the experiment and to process the data. For the anisotropy measurements, the SH intensity was recorded as a function of the rotational angle; the rotation of the sample was controlled by a step motor. For the scan mode measurements, the SH intensity was recorded during a linear potential sweep for several fixed rotation angles. 2.2. UHV Transfer Measurements. These experiments were performed with a system comprising an ultrahigh vacuum (UHV) chamber (base pressure, 43 µC cm-2) and |A - C| is greater for -0.92 V < E < -0.37 V (6.1 µC cm-2 < σM < 43 µC cm-2). The potentials correspond to those where changes in the SHG patterns were observed (where cos ψ changes sign); the x terms dominate in regions I and III of the CV and the y terms dominate in region II. From this, one can conclude that the changes in the SHG patterns are caused by variations in the relative polarizabilites in the x and y directions on the crystal surface. These appear to be correlated with the same processes that are observed in the CV and thus to be caused by the adsorption and state of OH on the surface. Since the dependence of |A + C| and |A - C| seems to be intimately related to the structures formed on the metal surface, further discussion will be postponed to a later section where a comparison with the results from the LEED and RHEED experiments will be made. Let us now consider the dependence of the A and C terms on charge density (Figure 6). The A and C terms both increase linearly with the charge density in region II, and there is an inflection at the border with region III. Linear increase of the SH signal is in agreement with the parabolic model,17e,25 which predicts that the SH intensity, in the free-electron approximation, should be proportional to the square of the electrostatic field EDC perpendicular to the electrode surface. A very high static electric field at the electrode|electrolyte interface can give rise to nonlinear processes of third order, which contribute to the second order polarizability. Thus, the susceptibility tensor elements can be represented as a sum of (2) essentially three contributions: surface (χ(2) s ), bulk (χb ), and (3) the effective cubic susceptibility (χeff ), which is proportional to
Figure 6. A and C terms as a function of charge density on the electrode. The dotted lines are guides for the eye.
the static electric field at the interface: (2) (3) χ(2) s + χb + χeff δEDC
(6)
Here, δ is a proportionality coefficient. Guyot-Sionnest and Tadjeddine17e discussed two limiting (2) (2) cases. If (i) χ(3) eff . χs + χb , the SHG intensity is parabolic with a minimum at the pzc:
ISHG ) |aEDC + b|2
(7)
In this case, the (ISHG)1/2 versus charge plot is expected to show two straight lines intersecting in the vicinity of the pzc. However, (2) (2) if (ii) χ(3) eff , χs + χb , the nonlinear response is dominated by (2) the interference of the terms χ(2) s and χb and may not obey eq 7. In this case, the signal would be largely dominated by the surface term. Which of the two cases comes into play depends on the type of the metal, the wavelength of the incident light, the angle of incidence, and the presence of an adsorbate layer at the interface. Guyot-Sionnest and Tadjeddine17e showed that the behavior of Ag(111) in KClO4 electrolyte was consistent with the parabolic model in the potential interval below silver oxidation, when the excitation wavelength was 1064 nm. (The minimum of the (ISHG)1/2 plot was, however, shifted far positive from the pzc on excitation at 532 nm, which was ascribed to the smaller free-electron contribution in this energy range.) The A term displays an asymmetric minimum close to the pzc; this minimum is slightly deeper in the calculations from the scan mode data (see below). The parabolic model predicts a symmetric minimum in the absence of specific adsorption.17e,25 The C term displays a distinct minimum, and since the SH intensity is the absolute square of the sum of the two terms A and C, the overall SH intensity has a minimum near the pzc, in agreement with the parabolic model. According to Beltramo et al.,18 the relationship of the isotropic term and the charge density is noticeably influenced by the surface crystallography and by specific adsorption. For Ag(100) and Ag(110), the minimum was shifted positive, whereas, for Ag(111), it was shifted negative of the pzc. Of the three surfaces, the Ag(100) surface displays the most symmetric minimum, while those of the (111) and (110) planes are much more asymmetric. Specific adsorption lowers the amplitude of the A term and increases the asymmetry in the A-term-charge plot.18 Since OH- is specifically adsorbed,
Hydroxide Adsorption on Ag(110) Electrodes the strong asymmetry observed in our results is in good agreement with the results reported by Beltramo et al.18 The remarkably close correlation of A with the charge density in the presence of specifically adsorbed hydroxide suggests that the interfacial field increases linearly with the charge on the metal and hence that the charge transfer between the adsorbate and the electrode is negligible in the potential region from -0.9 to -0.35 V. Similar behavior has been reported for the Ag(111) electrode in mixed F-/OH- electrolytes.14,16b Although Jovic et al. reported that hydroxide was discharged on Ag(111) and Ag(100) on the basis of a 60 mV shift in adsorption onset potential with pH,10b SHG data for the (111) surface14,16b and for Ag(110) (in the present work) indicate that this is not the case. In fact, the specific adsorption of anions would be expected to depend on the concentration even when not discharged.16b Additionally, retention of the negative charge by chemisorbed hydroxide is very much in agreement with the density functional theory (DFT) calculations carried out by Koper.26 A decrease of the A and C terms above ∼0 V is evidently due to oxide formation. Indeed, it has been reported17e,27 that oxide formation on Au and Ag surfaces decreases the SH intensity due to a significantly lower nonlinear polarizability of the oxide compared with a clean metal surface. Considering the potential interval where the decrease occurs (see the equilibrium Ag|Ag2O potential) and the total charge transferred (the deviation starts above ∼75 µC cm-2), it is clear that the oxidation process involves only the topmost layer of Ag atoms and that its coverage stays on the submonolayer level. Note that above 0 V |A + C| drops but |A - C| does not. This also indicates submonolayer oxidation, as bulk oxidation should result in a decrease of both quantities. Obviously, the polarizability changes due to oxidation are manifested differently in the x and y directions. Block et al. reported a substantial increase in the SH intensity obtained for a reconstructed oxygen-covered Ag(110) surface.28 This was attributed to a resonant interaction of the incident (1.17 eV) laser energy with an energy transition between the bulk bands and either surface states of y symmetry or oxygen py orbitals (calculated for the reconstructed surface). In those experiments, the adsorption of oxygen and the reconstruction were followed with LEED. To explain the intensity and pattern changes observed in our SHG measurements, experiments were conducted where the electrode was emersed from the electrolyte at various potentials and characterized with LEED and RHEED. To check if the SH anisotropy curves acquired via stepwise increase of the electrode potential can be directly related to the features observed in cyclic voltammograms, SHG experiments were also performed in the potential scan mode. The electrode was held at a fixed rotational angle, and the potential was swept linearly at 10 mV s-1 in order to increase the number of points collected and thus to reduce noise. Upon inspection of eq 1, assuming that the B and D terms are relatively small, it is obvious that only the isotropic term contributes when φ ) nπ/ 4, where n is an odd number, and ISHG ) |A|2. At φ ) nπ/2, ISHG ) |A - C|2 if n is an odd number and ISHG ) |A + C|2 if n is an even number. Therefore, to obtain representative values of the SHG intensity and to be able to use them for calculating the A and C terms, the sample was positioned such that a maximum in the signal occurred at 0°, and measurements were made at every 45° angle. Figure 7 represents the SH intensity scans at rotation angles set to 0, 45, and 90°. Although the scatter is rather high because of the short acquisition time, one can clearly see that the scan mode data agree very well with the anisotropy measurements (compare Figures 7 and 3). Note also
J. Phys. Chem. B, Vol. 108, No. 48, 2004 18645
Figure 7. SH intensity as a function of electrode potential recorded in the potential scan mode for three selected rotational angles.
Figure 8. A term calculated from the scan mode data as a function of total charge density. Solid line, positive-going sweep; dotted line, negative-going sweep.
the similarity between the 0° plot and the |A + C| plot (which corresponds to the square root of the 0° intensity) in Figure 5 and between the 90° plot and the |A - C| plot in Figure 5. This is a very important conclusion, which validates the comparison of SH anisotropy with cyclic voltammetry. The A and C terms were extracted from the scan mode measurements by substituting values of φ into the equations and simple manipulation of the equations. Calculations of the terms were performed on data that were first smoothed (using a Savitzgy-Golay second order smoothing algorithm). The A term calculated from the scan measurements is presented in Figure 8 and agrees very well with the one extracted from the anisotropy data. A smaller fall of the A term derived from the scan mode measurements may be discerned at potentials more positive than 0 V (region IV), possibly relating to a kinetic effect. However, the difference is not large. The SH intensity scan in the negative potential sweep matches exactly that of the positive sweep, indicating that any kinetic limitation is negligible. 3.3. LEED/RHEED Measurements. Figure 9 shows four LEED patterns, obtained (a) for the clean Ag(110) surface and (b-d) for the surface after emersion at three selected electrode potentials. The c(2 × 6) pattern reproduced in Figure 9b is representative for those obtained at negative potentials and can be interpreted as an overlayer of OH- on the surface, depicted
18646 J. Phys. Chem. B, Vol. 108, No. 48, 2004
Horswell et al.
Figure 9. LEED patterns obtained for the Ag(110) surface (a) before electrochemical measurement, (b) after emersion at -1.34 V, (c) after emersion at -0.34 V, and (d) after emersion at +0.08 V. E = 44 eV.
Figure 10. Proposed structure for the OH- adlayer at negative potentials. White circles, top row Ag atoms; dotted circles, second row Ag atoms; hatched circles, adsorbed OH-.
schematically in Figure 10. For simplicity, the OH- is depicted in the figure as residing in the troughs along the [11h0] direction (although the binding sites of the OH- cannot be determined from the LEED patterns.) Hydroxyl species (formed from the reaction of (n × 1) atomic oxygen coadsorbed with water) produce (1 × n) LEED patterns, corresponding to the arrangement of OHads chains along the [11h0] direction.29 Electron stimulated desorption ion angular distribution (ESDIAD) results show that OHads chains reside within the troughs in (111)-like sites.29a Meanwhile, atomic oxygen adsorbed on Ag(110) adsorbs in bridging sites to form Ag-O-Ag chains in the [001] direction.30 These exhibit long-range order ranging from (7 × 1) to (2 × 1) overlayer structures, depending on the oxygen coverage. A c(2 × 6) LEED pattern has also been observed for adlayers of oxygen on Ru(101h0)31 and Rh(110)32 surfaces, which form a unit mesh similar to that of Ag(110). In the case of the Ru surface, the coverage of the adlayer, as determined by Auger measurements, was ∼0.8.31 The model for this adlayer structure was suggested to consist of a close-packed, noncommensurate
adlayer in coincidence with the substrate lattice at the c(2 × 6) positions.31,32 Although this model could also fit the results on Ag(110), the coverage required would be much higher than that suggested by the CV integration, where 1/6 is obtained at ∼ -0.77 V. This value was estimated from σM, assuming that (i) the diffuse double layer charge is zero, (ii) OH- retains its negative charge upon adsorption, and (iii) the atomic density of Ag(110) is equal to 8.5 × 1014 cm-2. The calculated charges transferred per surface atom on Ag(110) and Ag(111) are comparable throughout the CV, and oxygen coverages calculated for the latter from ex situ XPS measurements did not exceed 0.25.14 Therefore, we consider that the structure proposed in Figure 10 (θOH ) 1/6) is more consistent with the electrochemical measurements. The pattern in Figure 9b was observed at potentials as negative as -1.34 V, indicating that OH- is present on the surface at more negative potentials than that indicated by the cyclic voltammetry results. This suggests that the adsorption of OH- on the surface may be compensated in part by adsorption of the counterions, resulting in superequivalent OHadsorption. The c(2 × 6) LEED pattern was also observed when the electrode was emersed from a solution containing only NaOH, demonstrating that it is caused by OH- rather than Fadsorption. Although the c(2 × 6) structure was observed throughout regions I and II of the CV, systematic variations in the relative intensities of the spots could be discerned. First, at the most negative potentials, streaks between the fractional order spots along the [001] direction were observed, which vanished with increasing potential and, therefore, coverage. Second, the (1/2 1/ ) spot became gradually stronger than the other superstructure 2 spots. The presence of streaks at low coverages suggests that the overlayer is ordered along the [11h0] direction but that some distortion exists along the [001] direction; that is, the OH- ions are evenly spaced along/within the rails but not in the direction perpendicular to them. This is a consequence of the formation of relatively small, ordered domains. The domains of the
Hydroxide Adsorption on Ag(110) Electrodes
Figure 11. Intensity profiles obtained from the LEED patterns at different potentials, obtained for the middle row of spots in the unit cell [001] direction.
superstructure (e.g., c(2 × 6)) on the surface apparently cannot be connected with each other by multiples of the basis vectors of the surface structure; hence, antiphase domains are formed. The streaking of the fractional order spots may thus result from the superposition of the fractional order spots and the split spots due to the antiphase domains. The streaking is reduced upon completion of the first current peak, indicating that the peak represents completion of 1/6 monolayer (the amount necessary to form a complete c(2 × 6) adlayer) or collapse of the small islands to form a larger ordered domain. The relative increase in intensity of the (1/2 1/2) spot in region II of the CV can be rationalized as follows. The (1/2 1/2) spot occurs in both the c(2 × 6) and c(2 × 2) structures (see the inset of Figure 9). If the former structure were slowly replaced by the latter as the OH- coverage increased, the other spots would fade and the (1/2 1/2) spot would remain. At -0.34 V (the onset of the second current peak and close to the border between regions II and III), only a c(2 × 2) structure is observed, although the low charge density suggests that islands of this structure are formed. As the electrode potential was moved positive and entered region III, the c(2 × 2) adlayer structure disappeared abruptly
J. Phys. Chem. B, Vol. 108, No. 48, 2004 18647 and only a (1 × 1) pattern was observed. The background in potential regions III and IV was very strong, indicating the formation of a disordered adlayer. For further clarification, LEED intensity profiles were obtained from the photographs taken at each electrode potential. This procedure was carried out along lines in the [001] direction, passing through the integral order spots and through the fractional order spots, and along the diagonal (the [11h1] direction). The results for the fractional order spots are displayed in Figure 11. The intensities are normalized to that of the (01) spot. The LEED results provide a useful overview of the structures formed on the electrode surface. However, strong backgrounds, caused by disordered adsorbates and/or crystallites from the electrolyte, may sometimes mask faint features. The latter can be better observed with RHEED.33 RHEED was carried out at three characteristic azimuths, summarized in Figure 12 using example patterns obtained at -1.34 V. As seen in Figure 12, the central (1/2 1/2) spots are visible when the incident beam is in the [11h2] direction, as the Ewald sphere intercepts these half order lattice rods. The other fractional order spots (e.g., 1/2 1/6) are visible in the RHEED pattern between the 0- and 1-Laue zones when the beam is in the [11h4] direction. In this case, the Ewald sphere crosses the surface lattice close enough to these fractional order rods that they can still intersect the Ewald sphere, resulting in relatively bright streaks. No extra reflections are observed when the beam is aligned to the [001] direction because there is no interception of the Ewald sphere with reciprocal lattice rods of either the c(2 × 6) or c(2 × 2) superstructure. The 1/6 order reflections are relatively faint in Figure 12b because the lattice rods are not exactly intercepted by the Ewald sphere, whereas the 1/2 order rods are intercepted by the Ewald sphere in Figure 12a, resulting in a brighter reflection. The 1/6 order reflections remain until the potential reaches ∼ -0.38 V and the 1/2 order reflections are clearly visible up to -0.34 V, where the c(2 × 2) structure was also observed with LEED. Interestingly, these streaks are still present at more positive potentials, whereas they were not observed in the LEED patterns, due to the partial disordering, which diminishes the extension of the 2-D reciprocal lattice rods. This demonstrates the greater sensitivity of RHEED and shows that at least some patches of order remain on the surface, up until the current rise at the positive limit of the CV. 3.4. Comparison of LEED/RHEED and SHG Measurements. LEED and SHG both illustrate two main features of
Figure 12. RHEED patterns at -1.34 V taken along different azimuths: (a) [11h2]; (b) [11h4]; (c) [001].
18648 J. Phys. Chem. B, Vol. 108, No. 48, 2004 OH- adsorption on Ag(110): a difference can be observed between the [11h0] and [001] directions of the surface and a sharp transition occurs at a potential of ∼ -0.34 V. These features can be easily correlated with the CV. At low coverages, the LEED patterns (Figures 9b and 11) display regular spacing of overlayer spots along the [11h0] direction, indicating that the adsorbed OH- ions are evenly spaced along the troughs of the substrate surface. Along the [001] direction, however, the longrange order is distorted, due to the formation of relatively small antiphase domains (with [11h0] domain walls), evidenced by the streaking of the fractional order spots in the [001] direction. These observations may imply that the OH- is adsorbed preferentially in the troughs and that diffusion is easier along the troughs than between them. At the completion of the first current peak in the CV, the spots become sharp in the [001] direction, inferring that collapse of the islands takes place as more OH- is adsorbed. The SH intensities can be split into [11h0] and [001] components, which show very different behaviors due to the different densities of adsorbed OH- in each direction. As Figure 5 shows, the crossing of the curves corresponding to the two different directions gives rise to a change in the phase of the SH light which manifests itself as a change in the anisotropy patterns. It is important to note that the swapping of the peak positions in the anisotropy curves itself did not indicate the point at which a physical change took place but rather that it was a result of the changing of different parameters contributing to the SH signal. By resolving the SH components into the two symmetry directions of the surface, it can be concluded that a minimum in one of the curves coincides roughly with the current maximum. The LEED results show that, upon completion of the peak, a structure is formed which is regular in both surface symmetry directions. As the electrode potential is made more positive, further OHadsorption occurs, which is observed in the LEED patterns as a gradual filling in of the rows between the c(2 × 6) structure to form a c(2 × 2) structure. In the SHG results in Figure 5, the two components of the SH intensity increase with electrode potential (and hence charge) at different rates, as the “coverages” in each direction begin to converge. When they converge, a change in the positions of the anisotropy peaks is again observed. At approximately the same potential, the LEED pattern shows a dominance of the c(2 × 2) structure, which changes sharply to a (1 × 1) pattern. Examination of the dependence of the A term on potential (Figure 4) and charge (Figures 6 and 8) shows an inflection at this point. This potential (on the rational scale) corresponds to that at which charge transfer takes place on the Ag(111) electrode,14,16b and it is thus likely that the inflection and change in LEED pattern is a result of charge transfer, accompanied by a change in the adlayer structure. Loss of the ionic charge would remove the Coulombic repulsive interactions which keep the adlayer ordered. At more positive potentials, differences in the response to the laser light in the two directions persist, perhaps as a consequence of further adsorption and oxidation of OH-, resulting in the formation of a Ag(O)OH adlayer. RHEED shows that some regions of c(2 × 2) still exist. Finally, after the second current peak, LEED and RHEED both show an absence of ordered structure and the SH intensity corresponding to both directions declines. The decline in SH intensity is a result of the lower polarizability of the surface oxide compared with the metal. 4. Conclusions In situ SHG and ex situ diffraction measurements (LEED and RHEED) have been employed to study the Ag(110)|alkaline
Horswell et al. electrolyte interface. Adsorption of OH- occurs at potentials negative of the pzc and proceeds initially without charge transfer. Small ordered antiphase domains are formed which collapse to form larger regions of an ordered c(2 × 6) structure. The changes in structure are accompanied by changes in the SH response. Further adsorption of OH- leads to the formation of a c(2 × 2) structure, which changes rapidly to a (1 × 1) LEED pattern when the formation of surface oxide sets in, with a concurrent change in the SH anisotropy patterns. A disordered phase of surface oxide is formed, although RHEED shows that some small regions of order persist. Acknowledgment. Financial support from the European Union 5th Framework program (Contract No. HPMF-CT-200000955) (S.L.H.) and from the Max Planck Gesellschaft (S.L.H., A.L.N.P., and E.R.S.) is gratefully acknowledged. The authors also thank E. Santos for providing useful references and details of the Ag sample preparation and A. Scheybal for his advice on the UHV Ag sample preparation. References and Notes (1) (a) Strbac, S.; Adzˇic´, R. R. J. Electroanal. Chem. 1992, 337, 355. (b) Strbac, S.; Adzˇic´, R. R. J. Electroanal. Chem. 1996, 403, 169. (2) Markovic´, N. M.; Ross, P. N., Jr. In Interfacial Electrochemistry; Wieckowski, A., Ed.; Marcel Dekker: New York, 1999; p 821. (3) (a) Kicheev, A. G.; Sheblovinskii, V. M. Elektrokhimiya 1983, 19, 1071. (b) Honda, M.; Kodera, T.; Kita, H. Electrochim. Acta 1983, 28, 727. (c) Savinova, E. R.; Wasle, S.; Doblhofer, K. Electrochim. Acta 1998, 44, 1341. (4) (a) Shumilova, N. A.; Zhutaeva, G. V. In Encyclopedia of Electrochemistry of the Elements; Bard, A. J., Ed.; Marcel Dekker: New York, 1978; Vol. 8, p 1. (b) Dirske, T. P. J. Electrochem. Soc. 1959, 106, 920. (c) Giles, R. D.; Harrison, J. A.; Thirsk, H. R. J. Electroanal. Chem. 1969, 22, 375. (d) Droog, J. M. M.; Huisman, F. J. Electroanal. Chem. 1980, 115, 211. (e) Lopez Teijelo, M.; Vilche, J. R.; Arvia, A. J. J. Electroanal. Chem. 1982, 131, 331. (f) Hepel, M.; Tomkiewicz, M. J. Electrochem. Soc. 1984, 131, 1288. (g) Lopez Teijelo, M.; Vilche, J. R.; Arvia, A. J. J. Appl. Electrochem. 1988, 18, 691. (h) Alonso, C.; Salvarezza, R. C.; Vara, J. M.; Arvia, A. J. Electrochim. Acta 1990, 35, 489. (5) (a) Melendes, C. A.; Xu, S.; Tani, B. J. Electroanal. Chem. 1984, 162, 343. (b) Hamilton, J. C.; Farmer, J. C.; Anderson, R. J. J. Electrochem. Soc. 1986, 133, 739. (c) Temperini, M. L. A.; Lacconi, G. I.; Sala, O. J. Electroanal. Chem. 1987, 227, 21. (6) Droog, J. M. M. J. Electroanal. Chem. 1980, 115, 225. (7) (a) Chen, S.; Wu, B.; Cha, C. J. Electroanal. Chem. 1996, 416, 53. (b) Chen, S.; Wu, B.; Cha, C. J. Electroanal. Chem. 1997, 420, 111. (8) (a) Hecht, D.; Borthen, P.; Strehblow, H.-H. J. Electroanal. Chem. 1995, 381, 113. (b) Hecht, D.; Borthen, P.; Strehblow, H.-H. Surf. Sci. 1996, 365, 263. (9) Orozco, G.; Perez, C.; Rincon, A.; Gutierrez, C. Langmuir 1998, 14, 6297. (10) (a) Stevenson, K. J.; Gao, X.; Hatchett, D. W.; White, H. S. J. Electroanal. Chem. 1998, 447, 43-51. (b) Jovic, B. M.; Jovic, V. D.; Stafford, G. R. Electrochem. Commun. 1999, 1, 247-251. (c) Savinova, E. R.; Kraft, P.; Pettinger, B.; Doblhofer, K. J. Electroanal. Chem. 1997, 430, 47. (11) Lu¨tzenkirchen-Hecht, D.; Strehblow, H.-H. Electrochim. Acta 1998, 43, 2957. (12) Hecht, D.; Strehblow, H.-H. J. Electroanal. Chem. 1997, 440, 211. (13) Lu¨tzenkirchen-Hecht, D.; Strehblow, H.-H. Ber. Bunsen-Ges. Phys. Chem. 1998, 102, 826. (14) Savinova, E. R.; Scheybal, A.; Danckwerts, M.; Wild, U.; Pettinger, B.; Doblhofer, K.; Schlo¨gl, R.; Ertl, G. Faraday Discuss. 2002, 121, 181198. (15) Iwasaki, N.; Sasaki, Y.; Nishina, Y. Surf. Sci. 1988, 198, 524. (16) (a) Danckwerts, M.; Savinova, E. R.; Pettinger, B.; Doblhofer, K. Appl. Phys. B 2002, 74, 635. (b) Danckwerts, M.; Savinova, E. R.; Horswell, S. L.; Pettinger, B.; Doblhofer, K. Z. Phys. Chem. 2003, 217 (5), 557. (17) (a) Beltramo, G.; Santos, E.; Schmickler, W. J. Electroanal. Chem. 1998, 447, 71-80. (b) Shannon, V. L.; Koos, D. A.; Richmond, G. L. J. Chem. Phys. 1987, 87 1440. (c) Koos, D. A.; Shannon, V. L.; Richmond, G. L. J. Phys. Chem. 1990, 94, 2091. (d) Bradley, R. A.; Georgiadis, R.; Kevan, S. D.; Richmond, G. L. J. Chem. Phys. 1993, 99, 5535. (e) GuyotSionnest, P.; Tadjeddine, A. J. Chem. Phys. 1990, 92, 734. (f) Shannon, V. L.; Koos, D. A.; Richmond, G. L. J. Phys. Chem. 1987, 91, 5548. (18) Beltramo, G.; Santos, E.; Schmickler, W. Langmuir 2003, 19, 4723-4727.
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