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J. Phys. Chem. 1996, 100, 19502-19511
Infrared Spectroscopy of Model Electrochemical Interfaces in Ultrahigh Vacuum: Evidence for Coupled Cation-Anion Hydration in the Pt(111)/K+,Cl- System Ignacio Villegas and Michael J. Weaver* Department of Chemistry, Purdue UniVersity, West Lafayette, Indiana 47907 ReceiVed: June 4, 1996; In Final Form: September 29, 1996X
Infrared reflection-absorption spectroscopy (IRAS) measurements along with work-function (Φ) and temperature-programmed desorption (TPD) data are reported for deuterated water dosed onto Pt(111) in ultrahigh vacuum (uhv) in the presence of adsorbed chlorine and potassium in order to assess the nature and extent of hydration of a “chemisorbed” (or specifically adsorbed) anion in the absence and presence of a cation “countercharge”, and vice versa. As studied previously (ref 6a), potassium constitutes an ideal “cation” for this purpose since it induces large Φ decreases even for small K coverages (θK e 0.08) that are modified substantially by hydration; this primary solvation also yields downshifted O-D stretching (νOD) vibrations (ca. 2300-2500 cm-1) that are diagnostic of cation-induced water reorientation. Adsorption of atomic chlorine (dosed as Cl2) yields milder yet nonmonotonic Φ changes, suggestive of only partial charge transfer to form Clδ-. The presence of adsorbed Cl attenuates the large (ca. 1.2 eV) Φ decreases produced by water dosing onto clean Pt(111), yet in largely featureless fashion. An absence of strong Cl hydration, suggested by these data and from TPD measurements, is also indicated by lack of solute-induced νOD bands in the corresponding infrared spectra, except at high Cl coverages (θCl) and submonolayer D2O dosages. Markedly different behavior, however, is observed in the presence of both Cl and K adsorbates. Addition of roughly stoichiometric Cl coverages to adsorbed K in the absence of water yields substantial (ca. 1.5 eV) increases followed by only minor changes for higher θCl values, suggesting intimate electrostatic K/Cl interactions involving K+‚‚‚e-/ Cl-‚‚‚e+ dipole polarization. Furthermore, water addition to K/Cl mixtures where θCl g θK triggers only minor Φ changes, in contrast to the situation for the absence of Cl. This evidence that the interfacial hydration of both “ionic” solutes is being modified in their combined presence is further indicated by corresponding νOD infrared fingerprints that differ substantially from corresponding spectra obtained for each solute separately. Thus, the K+ “hydration shell” νOD signature is largely removed in the presence of Cl, and a new feature (at 2620 cm-1) appears; the latter is most likely associated with solvation of Clδ-/K+ pairs. Further evidence for the coupled nature of this hydration is also gleaned from the marked cooperative influences of Cl and/or K adsorption upon the water desorption temperatures. The qualitative implications of these findings to the conventional picture of solvation in the electrochemical double layer are also noted.
Introduction Improvements in our fundamental microscopic-level understanding of the electrode-solution interface as a unique chemical environment require in part an ability to probe solvation-type interactions in regions immediately adjacent to the metal surface. While a variety of powerful in-situ techniques have been developed recently that yield microscopic structural information, detailed insight into interfacial solvation factors has remained quite elusive.1 A further complicating feature is that the solvation of interfacial ions may well be influenced strongly by the metal surface, and possibly each other, so that a means of delineating these potentially coupled effects, preferably in stepwise fashion, would clearly be invaluable. One experimental approach which can offer substantial insight into these and related issues entails probing the physical properties of interfaces formed by sequential co-dosing in ultrahigh vacuum (uhv) of the various “double-layer” components present at electrochemical interfaces. This tactic, pioneered by Sass and co-workers in Berlin during the 1980’s,2 is usefully termed “uhv electrochemical modeling”. The approach not only provides a multifaceted experimental link between the physicochemical properties of in-situ electrochemical and X
Abstract published in AdVance ACS Abstracts, November 15, 1996.
S0022-3654(96)01627-9 CCC: $12.00
conventional (single-adsorbate) uhv-based systems but also enables the roles of the additional chemical components (solvent, ions, etc.) inevitably present at the former interface to be explored in a uniquely incremental fashion.2,3 We have recently been utilizing this approach, primarily in conjunction with infrared reflection-absorption spectroscopy (IRAS) and work-function measurements, in order to explore the roles of interfacial solvation on the vibrational and electrostatic properties of chemisorbed neutral and ionic species on Pt(111).4-7 The former technique not only yields quantitative information on solvent and other intermolecular adsorbate interactions but also provides a direct link to in-situ electrochemical systems, since IRAS is also applicable (albeit with more restrictions) in this environment.8 The work-function data supply invaluable information on the surface-potential changes induced by adsorbate dosing, thereby furnishing surfacepotential profiles and providing the necessary bridge between the uhv-based and electrochemical potential scales.3 Our initial studies of this type entailed delineating separate and combined effects of solvent and cationic charge on the vibrational properties of a chemisorbed probe molecule, carbon monoxide, on Pt(111).4 This choice arose partly in view of the extensive in-situ IRAS data available for the electrode potential-dependent vibrational properties of CO at mono© 1996 American Chemical Society
Electrochemical Interfaces in Ultrahigh Vacuum crystalline metal-solution interfaces, enabling detailed comparisons to be made between the uhv-based and in-situ behavior. Another chemisorbate probe, nitric oxide, was subject to a more cursory examination along these lines.5 Most recently, we have focused on exploring by means of IRAS and work-function measurements the interfacial solvation of a model cation, potassium, on Pt(111) by a variety of hydrogen-bonded and aprotic media as a function of both the alkali metal and solvent coverages.6,7 These studies have demonstrated the oftenimportant coupled nature of ion-solvent and surface-solvent interactions in a system which should mimic the behavior of some metal-solution interfaces at electrode potentials below the so-called “potential of zero charge” (Epzc), i.e., where the metal surface carries a negative electronic charge and where cations predominate in the ionic double layer. While this circumstance represents a relatively simple as well as important one in electrochemistry, a more common situation involves the combined presence of “specifically adsorbed” (or chemisorbed) anions along with a sufficient (and commonly large) surface excess of cations so to provide overall electroneutrality. This situation is especially prevalent on transitionmetal and other strongly chemisorbing metals, where anions are often strongly chemisorbed even at potentials negative of Epzc.9 Several fundamental questions arise in this context, not the least of which is the state of solvation of the adsorbed anions, together with the possible influence of the interfacial cations upon the solvation (and electrostatic state) of the anions, and vice versa. While there are several previous studies of the solvation (specifically hydration) of anions at metal surfaces by means of uhv electrochemical modeling (vide infra),3,10,11 the possible coupled nature of cation and chemisorbed anion solvation when both are present in substantial quantities at a given interface has remained largely unaddressed. We present here examination by means of uhv-based IRAS, work-function measurements, and temperature-programmed desorption (TPD) measurements of the hydration of chlorine atoms on Pt(111), both separately and in the presence of interfacial potassium cations. This system was chosen partly in view of the expectation that the chemisorbed chlorine would carry at least a partial negative charge under some conditions (i.e., be formally Clδ- rather than Cl), following our previous study of K+ interfacial hydration on Pt(111).6 In addition, the availability of vibrational spectroscopic information for progressive hydration of the chloride anion in the gas phase12 offers an interesting opportunity to assess directly the influence of the metal surface upon ionic hydration, similarly to some of our earlier studies involving interfacial cation solvation.6,7a,c The TPD measurements described here provide useful insight into the influence of the ionic hydration on the interfacial water stability. Such tactics have previously been utilized to excellent effect in this context by Sass et al.,2 Stuve et al.,10 and Wagner.11 Taken together, the following results provide multifaceted evidence of the strongly synergetic nature of adsorbed anionic and cationic hydration at Pt(111) electrodes. A related study concerning the coadsorption of K and Cl on Pt(111) in the presence of acetone, focusing on the effects upon solvent chemisorption, as deduced by IRAS, will be described elsewhere.7e Experimental Section The uhv instrument used in these experiments consists of a stainless steel chamber maintained at a base pressure of 5 × 10-10 Torr by means of turbo-, ion-, and titanium sublimation pumps. Available surface-analysis facilities include a singlepass cylindrical mirror analyzer with a concentric electron gun (Perkin-Elmer, 15-155) for Auger electron spectroscopy (AES), a reverse-view screen (Princeton Research Instruments, RVL
J. Phys. Chem., Vol. 100, No. 50, 1996 19503 6-120) for low-energy electron diffraction (LEED), and a quadrupole mass spectrometer (UTI, 100C) for temperatureprogrammed desorption (TPD) and trace gas analysis. The IRAS spectra were obtained with a Fourier transform spectrometer (ATI/Mattson, RS-1000) equipped with a Globar light source and a narrow-band MCT detector. The p-polarized light beam was reflected from the sample at a near-grazing angle through KBr windows. The spectra are reported here in a difference format using single-beam spectra of the solvent-free Pt(111) as a reference. Surface work-function changes (∆Φ) were measured as a function of the various adsorbate coverages using a Kelvin probe (Delta Phi Elektronik). In some cases, the ∆Φ values were recorded in a continuous fashion during exposure to the gas and are accurate to (10 eV. In other cases, however, repositioning of the sample in front of the probe between exposures was required, and the accuracy of those measurements is no better than (50 eV. The platinum surface was maintained clean and ordered by periodic sputtering with 1 kV Ar+ and subsequent annealing to 1200 K. Routine cleaning involved heating to 1200 K followed by exposure to 3 × 10-7 Torr of oxygen at 900 K. Surface cleanliness and order were confirmed using AES and LEED. Sample heating to temperatures up to 1200 K was achieved resistively using a tantalum wire spot-welded on the back of the crystal. The temperature of the sample was kept near 90 K during all the experiments by maintaining direct contact with a liquid nitrogen reservoir via a uhv-rated copper feedthrough and the tantalum wire. Deuterated water (Aldrich) was dosed from a glass ampule attached to a uhv-compatible gas dosing manifold. (D2O was used here rather than H2O so to avoid spectral interferences from the latter within the optical path.) Gaseous impurities in the liquid were removed by repeated cycles of freezing, pumping, and thawing, and the purity of the vapor was confirmed by means of its mass spectrum. Line-of-sight exposure via a nozzle located in front of the sample enabled the stepwise increase of the solvent coverage while maintaining the background pressure below 1 × 10-10 Torr. Exposure at 1 × 10-8 Torr was required during continuous monitoring of the solvent-induced work-function changes. As outlined earlier,6 the solvent coverages were calibrated using TPD, knowing that (1) the chemisorbed water “bilayer” with an absolute coverage (θW) of 2/3 monolayers (ML) desorbs at slightly higher temperatures (180-90 K) than the subsequent condensed multilayers (170 K) and (2) the sticking coefficient for D2O on Pt(111) at 90 K is constant and close to unity in both the submonolayer and multilayer regimes. Potassium was dosed from a thoroughly outgassed, commercially available getter (SAES). As outlined in ref 7, the calibration of the alkali coverages (θK) relied primarily on the substantial Φ decreases induced on the Pt(111) together with experimental results published by Pirug and Bonzel which indicate that, below 0.1 ML, the surface work function decreases linearly with K+ coverage as known from LEED and other means.13 Chlorine dosages were achieved by dissociative adsorption upon controlledtime exposures to 2 × 10-8 Torr of Cl2 gas. However, determination of the required chlorine atom coverages, θCl, was routinely based on the measured Φ changes rather than the exposure value itself since the former could be more accurately reproduced than the latter. Linking the ∆Φ values (and hence Cl2 exposures) to the θCl values was less straightforward than for K dosages since the Cl-induced Φ changes are smaller and nonmonotonic, and the exposure-θCl correlation is nonlinear. This procedure, involving AES and LEED data, is outlined in the next section. All coverages are reported in monolayers (ML), as usual normalized to the atomic density of the Pt(111)
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Figure 2. Suggested real-space adlattice structure for a chlorine adlayer on Pt(111) displaying a (3 × 3) unit cell and a coverage of 1/3.
AES data. Unfortunately, however, the assignment that θCl ) 3 ML is not unique to the (3 × 3) unit cell, but rather any multiple of 1/9. An ordered Cl adlayer with 1/3 ML coverage, yet having (x3 × x3)R30° symmetry, is formed instead on the (111) faces of other transition metals at 300 K, specifically for Ag and Cu,15 Ni,16 and Rh.17 Calculation of absolute Cl coverages on the basis of LEED and AES data is then very straightforward as the only possible θCl value corresponding to an adlayer with (x3 × x3)R30° symmetry is 1/3 ML. Saturation coverages between 0.4 and 0.5 ML were deduced in those cases.15-17 Consequently, then, a reasonable (although not unique) coverage calibration for the present Pt(111)/Cl system can be based on the assumption that the saturated coverage is close to 0.4-0.5 ML. The AES and LEED data indicate that the Cl coverage associated with the (3 × 3) LEED pattern is ca. 20% lower than that for saturation at 300 K (cf. Figure 1). On this basis, saturation coverages of 0.41 ML at 300 K and 0.46 ML at 90 K are deduced by presuming that the (3 × 3) adlayer does indeed correspond to θCl ) 1/3 ML. A 4/9 ML coverage for the (3 × 3) adlayer would imply a saturation coverage of ca. 0.55 ML at 300 K, while a lower coverage of 2/ ML for the ordered adlayer would result in a saturation 9 coverage of only 0.27 ML at that temperature, both of which appear to be less reasonable. Consequently, then, the chlorine coverage calibration employed below presumes a saturation value of 0.5 ML at 90 K. It should be recognized, however, that this calibration choice is uncertain. Thus, the inferred “saturation coverage” of 0.45 ML is decidedly lower than the value, ca. 0.6, deduced on Pt(111) for hexagonal close packing of Cl- or Cl: presumably, such a coverage is difficult to attain for Cl2 dissociative chemisorption. Indeed, a higher θCl value, ca. 0.7, was deduced from TPD measurements for HCl adsorption on Pt(111) by presuming that the (ostensibly similar) (3 × 3) LEED pattern found for the system corresponds instead to a lower θCl value, 2/ .11 This argument therefore provides some (albeit not 9 persuasive) basis for asserting that the actual Cl coverages considered here are actually 1/3 smaller than the values quoted below. Fortunately, however, this uncertainty (seemingly not uncommon in surface science!) has little critical impact on the data interpretations presented below. The reasons why chlorine forms (3 × 3) adlayers at θCl ) 1/ ML on the Pt(111) substrate rather than the more sterically 3 uniform (x3 × x3)R30° also remain unclear. Nonetheless, the complete absence of a (x3 × x3)R30° LEED pattern over the whole range of possible Cl coverages supports our choice of the real-space structure in Figure 2A with the alternate (3 × 3) symmetry at θI ) 1/3 ML. For comparison, iodine also forms an ordered adlayer with (3 × 3) symmetry on Pt(111), but at the significantly higher coverage of 4/9 ML.18 The expected (x3 × x3)R30° LEED pattern was observed at 1/3 ML for that system. Work-function changes measured during continuous exposure of Pt(111) to 2 × 10-8 Torr of Cl2 are also plotted in Figure 1 1/
Figure 1. Chlorine (181 eV) versus platinum (231 eV) Auger signal ratios measured for various Cl2 exposures on Pt(111) at 300 K (open circles) and 90 K (filled circles) and the corresponding work-function changes, ∆Φ, measured in a continuous fashion during exposure to 2 × 10-8 Torr of Cl2 at those temperatures, as indicated.
surface (1.51 × 1015 atoms/cm2). All IRAS and work-function data presented here were obtained at 90 K unless stated otherwise. Results Cl Adsorption: Coverage Calibration. As just mentioned, it was necessary to determine the Cl2 exposure-θCl relationship by means of AES and LEED, along with ∆Φ measurements. The following preamble is concerned primarily with this objective. Figure 1 contains a plot of the Cl/Pt Auger signal intensity ratio recorded for various exposures of the clean Pt(111) surface maintained either at 90 K (filled circles) or 300 K (open circles) to a constant Cl2 pressure of 2 × 10-8 Torr. The Cl Auger peak intensity at 181 eV was normalized with respect to that of the Pt peak at 237 eV, both measured as peakto-peak heights in the differential Auger spectra. The Cl/Pt Auger signal ratio increased linearly with exposure up to about 5-6 langmuirs and saturated completely near 15 langmuirs. The LEED data indicated that an ordered Cl adlayer emerged as the coverage was increased, the observed (3 × 3) pattern being the sharpest at exposures near 5.5 langmuirs at both 90 and 300 K. No other LEED patterns were detected over the whole range of Cl dosages explored. Our AES results are qualitatively consistent with those of an earlier study by Erley in which a linear increase in the Cl Auger signal as a function of Cl2 exposure at 300 K was reported.14 Also in harmony with Erley’s observations is the symmetry of the ordered adlayer which displayed the sharpest (3 × 3) pattern when the Cl AES signal was about 75% of that reached upon saturation in that case. However, a discrepancy regarding the exposure required to achieve saturation of the Cl adlayer exists between our data and this earlier study, in that Erley reported that saturation was achieved after only 2 langmuirs at 1 × 10-8 Torr of Cl2. The reasons for the discrepancies between the results reported here and those reported by Erley are uncertain but are most likely due to morphological differences in the state of the surface and consequent Cl2 dissociation. The (3 × 3) adlayer structure as proposed by Erley,14 having θCl ) 1/3 ML, is illustrated in Figure 2A. [Note that there is no evidence supporting 2-fold bridge sites as preferential binding geometries for Cl on Pt(111).] The actual coverage corresponding to the (3 × 3) structure is clearly important for the present purposes since it would provide an absolute calibration of the
Electrochemical Interfaces in Ultrahigh Vacuum for two different substrate temperatures, 90 K (solid line) and 300 K (broken line). Note that a linear increase in the Cl Auger signal, and hence θCl, results in nonmonotonic changes in the Pt(111) work function (vide infra). The 0.8 eV difference in the saturation ∆Φ value at the two temperatures, 0.56 eV at 90 K and 0.48 eV at 300 K, is suggestive of a smaller saturation coverage at the higher temperature, as also indicated by the AES results shown in the same figure. Once again, the ∆Φ versus Cl2 exposure behavior reported here is only roughly consistent with that reported by Erley, which indicates that a significantly smaller ∆Φ value, 0.34-0.35 eV, was achieved upon saturation of the Cl adlayer.14 Work-Function Responses to Surface Hydration. Similar to our earlier studies,4-7 the primary experimental tactics employed here entail examining the changes in work function (∆Φ) and the form of intramolecular solvent vibrational features arising from systematic alterations in the surface composition. Since the dissociative Cl2 chemisorption required to produce the Cl adlayers is probably sensitive to the chemical state of the surface, the desired Cl coverages were usually formed first by using the above procedure, followed by incremental dosage of potassium and/or water. We first consider ∆Φ data gathered along these lines, followed by a more detailed presentation of the IRAS results. Work-function changes recorded during the continuous exposure of Pt(111) predosed with various Cl coverages, as noted, to 1 × 10-8 Torr of D2O are shown in Figure 3A. As reported earlier,4a,6 surface hydration in the absence of chlorine results in a sharp decrease in the Pt(111) work function (curve 1). Although most of the Φ decrease occurs prior to completion of the so-called “water bilayer” (θW ) 2/3 ML), the asymptotic ∆Φ value (1.2 eV) is reached after adsorption of ca. 2.5 ML.4a,6 Increasing Cl precoverages have a dampening effect upon the solvent-induced Φ changes. For example, the water-induced Φ decrease observed in the presence of a saturated Cl adlayer (θCl ) 0.46 ML) is only ca. 0.3 eV (Figure 3A, curve 5). Note that the effect of increasing Cl coverages at a given θW value is consistently to raise the work function. Figure 3B contains a similar set of data as in Figure 3A, but now the surface was predosed with 0.075 ML of potassium as well as the variable Cl coverages indicated prior to water addition. This particular θK value was chosen as a “typical electrochemically relevant” concentration, comparable to that emphasized in our earlier studies.6,7 For curve 1, obtained in the absence of chlorine, the work function increases markedly for low water dosages (θW e 0.7 ML) before maximizing and then reaching a plateau for θW > 1.5 ML. This nonmonotonic behavior has been interpreted in terms of K+-induced water reorientation.6 Interestingly, however, the presence of predosed θCl values that are comparable to or larger than θK removes largely, or entirely, the initial Φ increase upon water exposure, relatively small (albeit nonmonotonic) Φ changes occurring throughout the complete θW range (Figure 3B). Given the multicomponent nature of the present system, it is of obvious interest to examine the Φ responses to variations in θCl. Figure 4 shows some representative plots in the absence and presence of potassium, for both anhydrous and fully hydrated (θW ) 2.4 ML) interfaces. As can also be gleaned from Figure 3A, Cl dosage onto a full hydrated surface (open circles) yields Φ values that are typically 0.8-1 eV lower than obtained in the absence of the solvent (solid trace). Of particular interest is the corresponding Φ-θCl behavior in the presence of predosed (0.075 ML) K+.19 The presence of increasing Cl coverages on an anhydrous surface dosed with K+ (squares) yields dramatic (ca. 1.5 eV) Φ increases even for substoichiometric θCl values (ca. 0.05 ML), although the work function
J. Phys. Chem., Vol. 100, No. 50, 1996 19505
A
B
Figure 3. (A) Changes in the work function, ∆Φ, induced by increasing D2O coverages onto Pt(111) predosed with various Cl coverages as indicated. (B) Changes in the Pt(111) work function, ∆Φ, obtained as in (A), but now for increasing D2O coverages in the presence of various Cl coverages, as indicated, and 0.075 ML of K+. (Note that Cl was always dosed first.)
remains substantially (1-1.5 eV) below the corresponding K+free values except for near-saturation θCl values. A key finding concerns the comparison between this Φ-θCl behavior (squares) and the corresponding Φ-θCl dependence for the hydrated surface (upright triangles). Intriguingly, while the presence of water for θCl ) 0 (i.e., x-axis origin in Figure 4) yields a large (ca. 1.3 eV) Φ increase, this hydration effect almost disappears for θCl g 0.05 ML in that near-coincident Φ values are obtained at a given θCl value in the presence or absence of water. Significantly different Φ-θW behavior is nonetheless obtained when the K+ is added following chlorine and water dosage (inverted triangles) rather than prior to surface hydration (upright triangles) in that the former condition yields typically 0.3-0.5 eV lower Φ values for a given θCl than the latter. (As already mentioned, the latter dosing sequence was chiefly employed in the present study; cf. ref 6a). Infrared Spectroscopy. Similarly to our earlier study of K+ hydration alone,6 of central interest here is the behavior of the water vibrational bands in both the absence and presence of
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A
Figure 4. Changes in the Pt(111) work function, ∆Φ, induced by increasing Cl precoverages under various anhydrous and fully solvated (2.4 ML of D2O) conditions in the absence and presence of K+. The adsorbate dosing order is indicated in the legend.
predosed interfacial solute. Detailed interpretation of the data is again deferred until after the results have been summarized. Figure 5A shows a sequence of infrared spectra for progressively increasing D2O coverages on clean Pt(111) at 90 K. The band near 2530 cm-1 which dominates the spectra at high water exposures can readily be attributed to the O-D stretching (νOD) vibration of condensed D2O (cf. refs 6 and 20). As expected for hydrogen-bonded water, this band is broader and downshifted by over 200 cm-1 with respect to the νOD mode of the gas-phase monomer. The sharper, weaker band at 2730 cm-1 has been assigned to the νOD mode of “free” O-D bonds which protrudes from the water adlayer into the vacuum.20 Note that the predominant spectral features observed for D2O coverages below 2/3 ML, where only adsorbed water is present, are two weak bands at 2470 and 2560 cm-1. (Bands associated with the CtO stretching vibration of some linearly bonded CO contamination can be observed near 2100 cm-1 in the spectra shown in Figure 5A. The CO coverage, however, was estimated to be less than 0.01.) Predosing low or intermediate Cl coverages yield relatively mild changes in the θW-dependent spectral response, except at low water coverages (e0.5 ML) where the adsorbed water bands are largely attenuated (compare Figure 5B to 5A). High coverages of adsorbed Cl (i.e., θCl g 0.4) yield, however, two clearly distinguishable spectral changes of the adsorbed D2O layer: a new νOD band appears at 2635 cm-1 while the feature near 2500 cm-1 becomes markedly broader, as shown in Figure 5C. The effect of increasing Cl coverages upon the infrared spectral response of submonolayer D2O is illustrated by the spectral sequence shown in Figure 6 which refers to 0.30 ML of D2O in the presence of various Cl coverages. Note the complete disappearance of the νOD bands near 2470 and 2560 cm-1 upon the addition of 0.15 ML of predosed Cl. The Clinduced νOD band at 2635 cm-1 and the broad feature near 2500 cm-1 are only observed at Cl coverages near or above 0.4 ML. Prior to presenting the spectral effects of K+/Cl coadsorption on surface hydration, it is useful to recall the form of corresponding θW-dependent spectra in the presence of K+ alone, as discussed earlier.6 Figure 7A shows such a sequence in the same format as Figure 5A-C for 0.075 ML of predosed K+. Comparison of Figure 7A with Figure 5A shows the substantial effect of the cation at low water coverages (e0.5
B
C
Figure 5. (A) Sequence of infrared spectra in the O-D stretching region for increasing D2O coverages, as indicated, onto clean Pt(111). (B) Sequence of infrared spectra obtained as in (A), but now for increasing D2O coverages, as indicated, onto Pt(111) predosed with 0.15 ML of Cl. (C) Sequence of infrared spectra obtained as in (A), but now for increasing D2O coverages, as indicated, onto Pt(111) predosed with 0.40 ML of Cl.
Electrochemical Interfaces in Ultrahigh Vacuum
J. Phys. Chem., Vol. 100, No. 50, 1996 19507
A
Figure 6. Infrared spectra in the O-D stretching region for 0.30 ML of D2O in the presence of increasing Cl precoverages, as indicated, on Pt(111).
ML) in yielding multiple and relatively sharp νOD bands at substantially downshifted frequencies, ca. 2300-2430 cm-1. These features have been attributed to water molecules having oxygen coordinated to the cation along with the hydrogen interacting with the Pt(111), thereby softening the O-D vibration6 (vide infra). Having illustrated the effects of surface hydration in the presence of Cl and K+ separately, a key question concerns the extent to which spectral features attributable to “anionic” or “cationic” hydration under these conditions are maintained within adlayers containing both solute species. Figure 7B,C shows a pair of θW-dependent spectral sequences obtained in the same manner as Figure 7A, but in the presence of an intermediate (0.15 ML) or high (0.4 ML) chlorine coverage, respectively, in addition to 0.075 ML of potassium. The lower Cl coverage is seen to attenuate, if not eliminate, the sharp downshifted νOD bands appearing at low (e0.5 ML) water coverages (Figure 7B) even though the Cl coverage is in 2-fold excess over the θK value. Nevertheless, these spectral features are essentially eliminated for the higher (0.4 ML) θCl value (Figure 7C). It is also of obvious interest to compare these spectral data for mixed Cl/K adlayers (Figures 7B,C) with corresponding data in the absence of K+ (Figure 5B,C). The presence of K+ is seen to exert a significant and unexpected influence in that the ca. 2630 cm-1 feature discernible only in the presence of high Cl coverages (Figure 5C) is now clearly evident even at intermediate θCl values (albeit slightly downshifted to 2620 cm-1) (Figure 7B) and markedly intensified for high θCl values (Figure 7C). The addition of K+ also appears to upshift significantly, by ca. 35 cm-1, the large broad νOD band corresponding to multilayer water, θW > 2/3 ML (compare Figures 7B and 5B). The spectral data considered so far nevertheless show that the most discernible K+/Cl-induced νOD spectral changes occur for “submonolayer” water dosages (θW < 2/3 ML), as might be expected given the likelihood of preferential “ionic” hydration under these conditions. Consequently, then, it is profitable to examine further spectral data profiles for this condition, but now for sequences of solute rather than solvent coverages. The K+induced changes in the spectral response of submonolayer water in the presence of Cl are clearly illustrated in the sequence shown in Figure 8, which refers to 0.30 ML of D2O along with
B
C
Figure 7. (A) Sequence of infrared spectra in the O-D stretching region for increasing D2O coverages, as indicated, onto Pt(111) precovered with 0.075 ML of K+. (B) Sequence of infrared spectra obtained as in (A), but now for increasing D2O coverages, as indicated, onto Pt(111) predosed with 0.15 ML of Cl and 0.075 ML of K+. (Note that Cl was always dosed first.) C) Sequence of infrared spectra obtained as in (A), but now for increasing D2O coverages, as indicated, onto Pt(111) predosed with 0.40 ML of Cl and 0.075 ML of K+. (Note that Cl was always dosed first.)
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A
Figure 8. Infrared spectra in the O-D stretching region for 0.30 ML of D2O postdosed onto a Pt(111) surface containing 0.25 ML of Cl and various K+ coverages as indicated. (The Cl was always dosed first.)
0.25 ML of predosed Cl and increasing amounts of K+. Note that, at this particular Cl coverage, the νOD band near 2620 cm-1 appears only in the presence of K+ and that all the bands in the νOD region gain intensity upon increasing θK. Anion-induced changes in the infrared spectral responses of water molecules involved in interfacial K+ solvation are illustrated by the spectral sequences shown in Figure 9A and B, referring to 0.15 and 0.30 ML of D2O, respectively, dosed onto Pt(111) containing various Cl coverages and 0.075 ML of K+. The series of bands appearing at 2330-2430 cm-1 in the Cl-free spectrum at the lower D2O coverage (Figure 9A, top) have been attributed to the νOD vibrations of D2O molecules occupying slightly nonequivalent sites within the primary cation solvation sheath (cf. Figure 7A).6 [Note that the water/K+ stoichiometry is 2 in this case; an earlier study indicates that there are at least 3-4 solvent molecules in the primary solvation layer of interfacial K+ on Pt(111).6] The addition of chlorine, to produce a Cl/K stoichiometry of two (θCl ) 0.15 ML), yields a marked attenuation and alteration of these features (Figure 9A), being eventually replaced at θCl ) 0.4 by the 2630 cm-1 νOD band. The effect of Cl coadsorption with K+ upon the water νOD spectra is also dramatic at slightly larger solvent coverages (0.30 ML), as illustrated by the sequence of spectra in Figure 9B. The two νOD bands observed near 2460 and 2535 cm-1 in the Cl-free spectrum (top) have been attributed to the νOD vibration of water molecules in the primary and secondary solvation layers around the K+.6 Addition of excess Cl (θCl ) 0.15) again attenuates these features, being followed at higher θCl values by a marked appearance of the ca. 2620 cm-1 feature (Figure 9B) that was noted above to be especially intense in the presence of K+/Cl coadsorption. Temperature-Programmed Desorption. There are two reasons for the inclusion of TPD data in the present study. First, as already mentioned, the effect of coadsorbates on the water desorption temperature can lend insight into the nature of ionsolvent interactions. An additional motivation concerns a desire to check the possibility that the interfacial water may undergo solute-induced decomposition, to form hydroxyl groups, etc. The occurrence of such a reaction in the presence of coadsorbed potassium on Pt(111) has been documented, although only at higher θK values (g0.1 ML) than are considered here.13 Nevertheless, the possibility arises that chlorine dosage may also trigger decomposition. Encouraging this chemistry are the relatively high work function values, up to 6 eV (vide infra), that can be attained for hydrated chlorine adlayers on Pt(111); in electrochemical terms such positive surface potentials favor the oxidative formation of adsorbed OD. As a check, TPD spectra were obtained for hydrated layers (θW ∼ 2-3 ML) containing various Cl and/or K coverages, monitoring the
B
Figure 9. (A) Infrared spectra in the O-D stretching region for 0.15 ML of D2O postdosed onto Pt(111) containing various Cl coverages, as indicated, and 0.075 ML of K+. (The Cl was always dosed first.) (B) Infrared spectra obtained as in (A), but now for 0.30 ML of D2O.
appearance of DCl (m/e ) 37) as well as D2O and OD (m/e ) 20 and 18). While the appearance and temperature (170-190 K) of the water TPD peak depended somewhat on the coadsorbate composition (vide infra), the quantity of the D2O desorbed (from the peak area) along with the OD peak (a daughter ion from D2O) was invariant (within approximately a few percent). Nevertheless, at high Cl coverages, θCl g 0.25 ML, small yet detectable DCl peaks were observed at 170180 K. While their magnitude (10-20-fold smaller than for D2O, OD) suggests that the formation of DCl, and presumably surface water dissociation, is only minor, the observation supports its occurrence, if not at 90 K, by the desorption temperature. Returning to the initial motivation for our TPD studies, Figure 10 shows TPD spectra obtained at 3 K/s for water desorption from Pt(111) in either the absence of (solid traces) or presence of 0.075 ML K+ (dashed traces) for the series of Cl coverages indicated. The initial water coverage, 0.12 ML, was chosen to be sufficiently small so that it should primarily hydrate the solute species. Comparison between the solid and dashed traces in the bottom (Cl-free) spectra in Figure 10 show clearly the stabilization afforded by cation hydration in that the TPD peak in the K+-containing case occurs at a markedly higher temper-
Electrochemical Interfaces in Ultrahigh Vacuum
Figure 10. Temperature-programmed desorption (TPD) mass spectra for 0.12 ML of D2O (m/e ) 20) postdosed onto Pt(111) containing increasing Cl coverages, as indicated, in the absence (solid traces) and presence of 0.075 ML of K+ (dashed traces). (The Cl was always dosed first.)
ature than that for the clean surface (ca. 235 vs 175 K). The additional presence of adsorbed chlorine modifies substantially this behavior. Thus, the presence of an essentially equal Cl coverage, 0.07 ML, attenuates markedly the thermal stabilization of water afforded by 0.075 ML K+ in that the latter TPD peak is downshifted to ca. 190 K. Note that the presence of the same Cl coverage in the absence of K+ yields a TPD peak centered at 175 K, similar to that obtained in the absence of solutes. For larger Cl coverages (such as the upper three pairs of TPD curves, Figure 10), a net thermal stabilization of the water is seen in comparison with either the clean or K+-containing surface, even though both the slopes of the TPD peaks and the temperature shifts seen with increasing θCl are somewhat complex (Figure 10). Only a cursory examination of TPD data in the presence of Cl and/or K+ was undertaken here for higher water coverages. However, the presence of even smaller (ca. 0.07 ML) Cl coverages essentially eliminated the so-called “bilayer” TPD peak(s) seen at 180-190 K for adsorbed water on clean Pt(111) (vide supra), leaving the “multilayer” desorption peak at 170 K largely intact. Discussion As a starting point on interpreting the present findings, especially in terms of interfacial hydration, it is useful to consider the insight into double-layer potential profiles afforded by the work-function data (Figures 1, 3, and 4). The nonmonotonic ∆Φ-θCl response onto clean Pt(111), featuring an initial small (ca. 0.05 eV) Φ decrease followed by a larger (0.55 eV) increase toward higher θCl values (Figures 1 and 4), contrasts the much larger (ca. 3.5 eV) decreases observed for adsorbed K at low θK values (e0.1).21 The latter behavior can be most simply understood in terms of potassium ionization to yield an array of K+-e- image dipoles.22 If chlorine adsorption is partly anionic (i.e., to form Clδ-), one might expect roughly comparable Φ increases. Although the observed more complex ∆Φ-θCl behavior may arise from a coverage-dependent adsorbate ionicity, other factors may well be more critical, specifically electronic redistribution within the metal surface.14,23 The latter can account, in particular, for Φ decreases observed even for electronegative adsorbates such as chlorine.23
J. Phys. Chem., Vol. 100, No. 50, 1996 19509 The marked behavioral differences between the effects of preadsorbed K+ and Cl upon the water-induced ∆Φ changes, i.e., the ∆Φ-θW traces in Figures 3A,B, can nevertheless be rationalized partly in terms of adsorbate ionicity. The wellknown substantial, ca. 1.2 eV, Φ decreases seen upon water exposure on clean Pt(111) are reasonably attributed to metal surface electronic redistribution as well as the influence of preferential dipole orientation within the water “bilayer”.4c As already mentioned, the large (ca. 1.3 eV) Φ increase observed for submonolayer water dosages onto K+-predosed Pt(111) (Figure 3B, trace 1) can be attributed to water reorientation, featuring oxygen coordination to the cation along with hydrogen bonding to the metal surface.6,21,24 (Spectroscopic evidence along these lines is considered below.) Alternatively, or equivalently, one can regard the solvent as providing “dielectric screening” in attenuating substantially the effect of the cationimage dipole in lowering the Pt(111) work function.6a By contrast, the influence of preadsorbed chlorine on the ∆Φ-θW behavior is ostensibly much milder, the attenuating effect of increasing chlorine coverages on the water-induced Φ decreases being roughly consistent with a simple “surface-blocking” effect. Nonetheless, it is possible that the featureless ∆Φ-θW traces (Figure 3A) mask the occurrence of some Cl-induced water reorientation associated with “anion” hydration in addition to the Φ decreases associated with surface solvation. The most instructive aspect of the work-function data in the present context concerns the Φ responses obtained in the presence of Cl/K+ coadsorption (Figures 3B and 4), as perhaps most clearly discerned in the ∆Φ-θW profiles (Figure 4). The finding that small, roughly stoichiometric, coverages of chlorine (∼0.05 ML) substantially nullify the large K+-induced Φ decrease in the absence of water, together with the relatively small Φ increases occurring toward higher θCl values (Figure 4, squares), suggests strongly that the Cl/K coadsorbates interact closely. A plausible model envisages placing the Cl and K atoms in adjacent sites, the alkali atom ionization to form K+ (together with an electronic image charge) inducing at least a partial negative charge onto the juxtaposed Cl atom. The opposite polarity of the K+-e- and Cl--e+ dipoles will thereby tend to enhance each other. Such “local charge polarization” should be present decreasingly for chlorine coverages in excess of θK, thereby yielding additional Clδ- with a markedly smaller δ- charge, accounting qualitatively for the observed curvilinear ∆Φ-θW profile. This model gains credence upon considering the corresponding ∆Φ-θW trace obtained in the presence of surface hydration (Figure 4, upright triangles). Specifically, the small Φ differences seen between the anhydrous and hydrated surfaces when θCl > θK contrast markedly the substantial hydration effect observed in the absence of chlorine. This points to the likelihood that the primary role of screening the K+-e- dipole in the presence of chlorine and water is taken by the former chemisorbate, the electrostatic role of cation hydration being diminished substantially compared to the θCl ) 0 case. Some role of the ionic hydration in the interfacial potential profile is nonetheless discernible from the nonmonotonic ∆Φ-θW responses seen for the coadsorbed Cl/K+ cases in Figure 3B. Before leaving this issue, it should be recognized that the significantly different ∆Φ-θW responses obtained for Cl/K mixtures where the potassium was added following, rather than prior to, the multilayer water dosage (Figure 4) does point out the nonequilibrium nature of the present low-temperature dosed interfaces. Similar ∆Φ differences have been noted earlier6a and attributed to the occurrence of more extensive cation hydration when the K+ is formed by “soft landing” into the multilayer water film.
19510 J. Phys. Chem., Vol. 100, No. 50, 1996
Figure 11. Cartoons illustrating possible D2O structures formed during the initial stages of Cl solvation on the Pt(111) surface in both the absence (A, B, C) and presence (D) of coadsorbed K+. See text for details.
While not forming an integral part of the present interpretations, it is worth noting that work-function data of the type in Figures 3 and 4 also provide a useful, and indeed often critical, link to electrode potentials, E, of central relevance to in-situ electrochemistry. Thus, bearing in mind that the work function of clean Pt(111) is about 5.9 eV25 and the effective electrode potential of a normal hydrogen electrode (NHE) is about 4.54.8 V on the vacuum scale,8b,26 such Φ-interfacial composition data can be readily transposed, at least approximately, into corresponding E-compositional data. Applications of such a link between uhv-based and electrochemical surface-potential data are pursued for some related systems in separate reports.4-7 The above notion that hydrated Cl/K+ adlayers involve intimate interaction between the solute coadsorbates is enriched upon considering the IRAS and TPD data. The lack of strong specific hydration of adsorbed chlorine in the absence of coadsorbed K+, suggested by the ∆Φ data, is further indicated by the absence of discernible νOD features upon water addition to Cl-containing surfaces, except at high θCl values and especially submonolayer D2O dosages where a band at 2635 cm-1 is clearly evident (Figures 5C and 6). Additionally, however, the removal of νOD features at 2400-2600 cm-1 for submonolayer water at lower Cl coverages (Figure 6) points to disruption of the hydrogen-bonded solvent structure by the solute. Nevertheless, the presence of a νOD band at high chlorine coverages at a frequency, 2635 cm-1, intermediate between that for the “free” νOD band at 2730 cm-1 and the major broad hydrogen-bonded feature centered at ca. 2530 cm-1 (Figure 5C), at least suggests the occurrence of some specific solute hydration. The frequency of this band (allowing for the D/H isotope shift) is markedly higher than that calculated for Cl-bonded νOD for a single bound H in gas phase Cl-(H2O), although roughly comparable to that in an alternative “bridging” structure featuring coordination of both water hydrogens to the anion.12,27 This latter concordance suggests the occurrence of the bridging configuration depicted in (A) of Figure 11, with (B) as a possible alternative; the former model is consistent with the need for higher Cl coverages to yield the 2635 cm-1 feature. Of particular interest here is the nature of the interfacial hydration in the presence of both Cl and K+. As already mentioned, the multiple low-frequency νOD features seen at ca. 2300-2500 cm-1 for submonolayer water dosages in the presence of K+ (Figure 7A) can be identified with water molecules in the primary cation solvation shell, having an
Villegas and Weaver orientation allowing the hydrogen(s) to interact with the metal surface.6 Significantly, the marked attenuation, alteration, and even elimination of these features in the additional presence of intermediate Cl coverages (compare Figure 7A,B and Figure 9A,B) indicates that the primary solvation of the cation is influenced strongly by the coadsorbed anion. This finding is entirely consistent with the above deduction from the workfunction data that the coadsorbed chlorine forms an interfacial Cl-/K+ “ion pair” with adjacent potassium ions, thereby modifying substantially the hydration sheath of the latter. Further insight into the mutual influence exerted upon interfacial hydration by these coadsorbate partners can be gleaned from the appearance of the 2620 cm-1 νOD band for predosed chlorine and submonolayer amounts of water, triggered by even small coverages (∼0.04 ML) of K+ (Figure 8; also see Figure 9B). Even though this νOD feature is similar in frequency to the band seen for the hydration of high Cl coverages, the markedly lower θCl values required for its appearance upon K+ addition suggest the occurrence of coupled anion-cation hydration. Two alternative structures for this hydration seem most plausible on the basis of the present evidence. The first, depicted in Figure 11D, has the water molecules in a bridging configuration between adjacent K+ and Cl- ions. Alternatively, the water molecules responsible for the ca. 2620 cm-1 feature may solvate only the Cl as in Figure 11C, the role of the adjacent K+ being then to induce additional negative charge and hence strengthen the Cl-D2O interaction. It remains to consider the information regarding such interadsorbate interactions provided by the TPD data in Figure 10. The lack of strong hydration of low coverages of adsorbed Cl alone in comparison with K+ alone is further evidenced by the marked (ca. 60 K) stabilization afforded by primary hydration of the latter, which is essentially absent for the former. This finding differs at first sight from that of Stuve and coworkers, who found from TPD data that water adsorbed on Ag(110) is stabilized significantly by the presence of even small Cl coverages, the desorption temperature increasing by 35-40 K.10 The extent of this effect, dubbed “adsorbate-induced hydrophilicity”, is such that a single Cl atom is capable of stabilizing 10 or more water molecules. In the present case, however, the extent of the water thermal stabilization as seen in ref 10 would be masked by the comparable stabilization afforded to submonolayer water by the chemisorbed “bilayer” on Pt(111) even in the absence of chlorine, essentially thwarting such a comparison of the Ag and Pt systems. Nevertheless, the upshift in the water TPD peak, to above 200 K, seen for intermediate and high Cl coverages (see top three solid traces in Figure 10) constitutes evidence of a Cl-induced stabilization of water under these more restrictive conditions. This observation supports the occurrence of specific Cl hydration at higher coverages indicated by the IRAS data already discussed. Of primary interest in the present context, however, are the changes wrought upon the water TPD peaks in the presence of both Cl and K+. Most significantly, the severe attenuation of the K+-induced thermal stabilization brought about by the additional presence of comparable Cl coverages provides further support to the model proposed above, involving intimate Cl-K+ interactions and hence drastic modifications to the interfacial hydration of both solutes. Not surprisingly on this basis, the differences between the water desorption temperatures in the absence and presence of 0.075 ML K+ diminishes as the Cl coverage increases into marked excess, although the involved nature of the residual dissimilarities attests further to the intimate nature of the interadsorbate interactions.
Electrochemical Interfaces in Ultrahigh Vacuum Concluding Remarks The central picture that emerges from the present study is straightforward: the interfacial hydration of an archetypical “noninteracting” cation is affected strongly and specifically by the presence of a typical “chemisorbed” anion, and vice versa, even when both are present at small fractional coverages (
