J. Phys. Chem. 1995,99, 7677-7688
7677
Infrared Spectroscopy of Model Electrochemical Interfaces in Ultrahigh Vacuum: Roles of Adsorbate and Cation Double-Layer Hydration in the Pt(ll1)-Carbon Monoxide Aqueous System Naushad Kizhakevariam, Ignacio Villegas, and Michael J. Weaver” Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 Received: August 15, 1994; In Final Form: December 16, 1994@
Infrared reflection-absorption spectroscopic (IRAS) measurements are reported for carbon monoxide and deuterated water codosed along with potassium atoms onto Pt( 111) at 95 K in ultrahigh vacuum (uhv) with the objective of elucidating the nature and roles of adsorbate and cation hydration on the electrode potentialdependent structure and bonding for the analogous Pt( 11l)/CO aqueous electrochemical interface. This uhvbased ternary coadsorption system was chosen in view of the availability of in-situ IRAS data for the electrodesolution interface at negative electronic charges, thereby enabling the validity of the “uhv electrochemical modeling approach” to be directly assessed. Varying the potassium dosage in the uhv system is analogous to charging the electrochemical double layer since adsorbed alkali cations are formed along with the metal electronic charge. Variations in the metal-uhv surface potential attending alterations in the interfacial composition were evaluated with a Kelvin probe: besides yielding additional insight into surface solvation, the measurements provide the required link to the in-situ electrode potential scale. Indeed, decreasing the surface potential by progressively increasing the K+ coverage at high water dosages yields potential-dependent C - 0 stretching (VCO) bands for adsorbed CO that closely mimic corresponding IRAS data for the in-situ electrochemical interface. Infrared spectra in the 0 - D stretching (VOD) region for D2O as well as the vco bands are reported as a function of CO, D20, and Kf coverage in order to explore how the combined and controlled presence of the ions and solvent acts to modify the structure and bonding of the electrostatic double layer and the chemisorbed CO. The solvent itself is found to exhibit a crucial effect upon the chemisorbate at low CO coverages, whereas the solvated cations exert the largest influence upon saturated CO adlayers. The addition of only small (ca. 0.1 ML) water dosages is observed to attenuate severely the otherwise dominant short-range K W O interactions as gleaned from the vco spectra. Such primary hydrated cations, with stoichiometries K+-(D20),, where n I5, exhibit characteristic VOD spectra, which in the presence of only low (or zero) CO coverages are consistent with a preferential water orientation with the hydrogens tilted toward (and H-bonding with) the metal surface.
Introduction One fundamental goal of modern interfacial electrochemistry involves developing a molecular-level understanding of the influence exerted by the double-layer environment on adsorbate structure and bonding. Generally speaking, such influences inevitably consist of distinct yet strongly coupled, and inadequately understood, contributions from the interfacial solvent and the excess ionic charge. A central issue in this regard concerns elucidating the often substantial effects upon the adsorbate of altering the applied electrode potential on the basis of solvent and ionic properties. The traditional description of such effects, couched in terms of dielectric models, reflected an emphasis on the macroscopic-level properties to which electrochemical double-layer measurements have heretofore been restricted.’ The recent emergence of an increasing array of microscopiclevel techniques applicable to electrochemical interfaces, however, has mandated as well as encouraged more molecular-level descriptions of electrostatic double-layer effects2 The development of in-situ spectroscopic and spatial microscopic methods, in particular, now provide atomic-/molecular-levelinformation for ordered metal-solution interfaces that is on a par with, as well as common to, that available for metal-ultrahigh vacuum (uhv) systems3 This situation is establishing multidimensional
* Abstract published in Advance ACS Abstracts, May 1, 1995. OO22-3654/95/2O99-7677$O9.OO/O
new links between the fledgling area of “electrochemical surface science” and the inherently related uhv-based discipline, encouraging the examination of electrode-adsorbate systems which offer direct comparisons with metal-uhv analogs. The archetypical adsorbate in this context is carbon monoxide, which has been the subject of numerous investigations in uhv environments as well as having a particular relevance to electrochemical systems. The additional applicability of adsorbed carbon monoxide toward infrared spectroscopic characterization, along with the well-documented sensitivity of the C-0 stretching frequency (YCO) to the local electrostatic environment, makes this adsorbate uniquely suitable for such a purpose. Such considerations have driven much of our recent experimental enquiries into in-situ electrochemical surface science, utilizing in particular infrared absorption-reflection spectroscopy (IRAS)4j5 and scanning tunneling microscopy (STM).6s7 This new-found availability of vibrational spectroscopic and atomic-level spatial structural information for electrochemical interfaces is in turn encouraging further comparisons with related metal-uhv interfaces. More specifically, we have recently initiated uhv-based IRAS measurements whereby the effects of the solvent and ionic double-layer environment can be assessed by sequential controlled dosing of the relevant components along with the “solute” adsorbate of interest onto the clean metal surface. This so-called “uhv electrochemical modeling” approach, pioneered by Jurgen Sass and co-workers,8-’0 offers 0 1995 American Chemical Society
Kizhakevariam et al.
7678 J. Phys. Chem., Vol. 99, No. 19, 1995 intriguing opportunities for examining the coupled effects of the various double-layer components on surface structure and bonding. Although not previously utilized in this fashion, such uhv-based IRAS measurements enable vibrational information on the solvent as well as the adsorbed solute species, along with the dosage-dependent intermolecular interactions, to be extracted at a level of comprehension that is well beyond, yet complementary to, the infrared spectral information attainable for insitu electrochemical interfaces. Our initial experimental efforts in this area have focused on the influence of both aqueousll and n o n a q u e ~ u ssolvent ~~~~~ coadsorption on the infrared spectral properties of adsorbed CO and the solvent on Pt( 111). An additional component of these experiments involves dosage-dependent work function measurements by means of a Kelvin probe. Besides providing further insight into surface- and adsorbate-solvent interactions, the surface potential information thus obtained provides the necessary link to the electrode potential-dependent spectral properties of the corresponding in-situ electrochemical interface. The desirability of such comparisons between the “uhv electrochemical model” and the actual electrode-solution interfaces is heightened by the need to employ markedly lower temperatures (usually below 170-200 K) in the former experiment so to avoid solvent evaporation. While our initial studies have uncovered substantial effects of solvent coadsorption on the surface potential profile as well as on adsorbate structure and it is clearly desirable to explore the combined effects of ionic as well as dipolar charge. The requirement of including the former charge component in uhv electrochemical modeling is obvious when one recalls that alterations in the electrode potential in electrochemical systems are necessarily achieved by corresponding variations in the excess ionic and electronic charges. Given the estimated high values of the potential of zero charge of COcovered Pt(ll1) in aqueous as well as nonaqueous media, Epzc 0.8V vs normal hydrogen electrode (NHE),14 the electrode surface will usually carry negative electronic charges. The interface will therefore contain a corresponding (and substantial) cationic surface excess and small or negligible anion surface concentrations. As a consequence, the electrostatic double layer should be modeled satisfactorily by dosing variable coverages of cations or ionizable atoms, most conveniently alkali metals, along with the solvent. We describe here uhv-based experiments that examine the infrared spectral and surface potential properties of Pt( 111) dosed with carbon monoxide, water, and potassium, with the central aim of exploring the various electrostatic and other environmental factors that influence the structure and bonding of both carbon monoxide and water at such electrochemical interfaces. This hydrated cation system was chosen for our initial foray into foray into uhv electrochemical modeling of complete (mixed ion/solvent/adsorbate) interfaces partly in view of the detailed earlier investigations of Pirug and Bonze1 of the binary Pt( 11l)/water, K+ system, especially with regard to the onset of water dissociation. l53l6 The results, described herein, allow one to discern separately and together the inherently interwoven roles of the solvent and excess ionic charge on the interfacial molecular structure and bonding.
Experimental Section The experiments were performed using a three-level stainless steel uhv chamber with facilities for IRAS, work function measurements, temperature-programmed desorption (TPD), and low-energy electron diffraction (LEED). The base pressure, 1 x lo-* Pa, was achieved by a combination of turbomolecular
and titanium sublimation pumps. Additional details can be found in refs 11 and 12. The infrared spectra were obtained with a Fourier transform infrared spectrometer (RS-1000, Mattson Instruments) equipped with a Globar light source and an MCT narrow-band detector. The p-polarized infrared beam was incident on the surface at near-grazing incidence through KBr windows. A reference spectrum was obtained on the clean Pt(ll1) crystal, thereby subtracting out the bulk-phase spectral features present in the optical path. The work function changes, A@, induced by adsorbate dosing were measured with a Kelvin probe (Delta Phi Elektronik, Jiilich, Germany). The A@ values were measured continuously during carbon monoxide and deuterated water dosing and were generally reproducible within k 1 0 meV. The sample had to be repositioned after potassium dosing, and in this case the A@ measurements were generally reproducible only to within 50.1 eV. The Pt( 111) surface was cleaned and characterized as previously described.’ Tantalum wire (0.5 mm) spot-welded to the back of the crystal provided support as well as resistive heating and cooling by conduction to a liquid nitrogen reservoir. The temperature was measured with a type K thermocouple. All measurements were made at 95 K unless otherwise indicated. Potassium was dosed at 95 K from a carefully outgassed SAES getter source located approximately 6 cm from the sample. Carbon monoxide (Matteson Gas Products) was used as supplied. Deuterated water (Cambridge Isotopes) was thoroughly degassed by repeated freeze-thaw cycles. Relative adsorbate coverages were obtained from the integrated areas under the TPD traces. All coverages are quoted here in monolayers (ML), referenced to the Pt(ll1) surface atomic density (1.5 x 1015 atoms cm-2). Potassium coverages, OK, were calibrated with work function measurements in accordance with data given in ref 16b. The absolute CO coverages, 8c0, were calibrated with respect to the saturation value of ca. 0.65 obtained on clean Pt( 111) at 100 K and also from the c(4 x 2 ) and (43 x 2/3)R30° LEED patterns observed for 8co values of 0.5 and 0.33, re~pectively.’~-’~ The absolute water coverages, Ow, were calibrated with reference to the higher-temperature pair of TPD peaks observed on Pt( 11l)(vide infra). This dosage is presumed to correspond to completion of the “bilayer” water adlayer, having a coverage of 2/3 ML.20*21(Note that this coverage scale, again referenced to the metal surface atomic density, differs from that employed in our earlier article, ref 11, where unit coverage, labeled 8*w,refers to the completion of a water bilayer. Consequently, the numerical coverage values on the scale used herein are two-thirds of those on the “bilayer scale”.)
Results and Discussion Water/K+ Coadsorption on F’t(ll1). As a prelude to considering the infrared spectral properties of adsorbed CO in the presence of interfacial solvent and/or cations, it is appropriate to present work function and infrared data for water/K+ coadsorption on Pt( 111) in the absence as well as presence of CO. Figure 1 shows plots of the work function for clean Pt( 111) (dotted trace) and Pt( 111) predosed with either 0.04 ML (dashed traces) or 0.08 ML potassium (solid trace) as a continuous function of the postdosed D20 coverage. For curves labeled 4 and 7, a saturated CO adlayer (8co = 0.65) was added prior to water dosage, whereas curves 3 and 6 refer to the presence of low CO coverages (0.08 and 0.04, respectively). Note that the onset of water dissociation on Pt( 111) requires higher potassium coverages ( S O . 1).16.22While only changes in work function induced by adsorbates are actually monitored by the Kelvin
Model Electrochemical Interfaces in Ultrahigh Vacuum
J. Phys. Chem., Vol. 99,No. 19, 1995 7679 0
CO/D20/K
0 IUCOID~O D20/K
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K COVERAGE (ML) Figure 2. Work function of Pt( 111) surface versus the potassium coverage for surfaces also containing saturated CO adlayer and/or 2ML DzO,as indicated. Legend indicates order of adsorbate dosage (also
see text). (triangles) of coadsorbed D20 and/or saturated CO. The water dosage chosen, 2 ML, is suffient so that larger dosages yield no further CP changes (Figure 1). Included in Figure 2 are @-OK plots for both postdosed and predosed potassium along with water (filled and open squares, respectively) and for corresponding CO/D2O-containing interfaces (filled and open circles, respectively). Inspection of Figure 2 shows that the presence of interfacial water and/or CO yields markedly (ca. 2-fold) less negative @-OK slopes than are observed in the absence of coadsorbate. This difference can again readily be understood on the basis of dielectric screening. Given that the adsorbed potassium should be essentially completely ionized via surface electron transfer, one can consider such @-OK plots to be analogous to metal charge-electrode potential relationships for electrochemical interfaces. The effective capacitance for the uhv-based interfaces can be extracted from the relation Chv = (neOK/&), where n is the Pt(ll1) atomic density (1.5 x 1015 atoms cm-2), e is the electronic charge, and C#I is the surface potential (such that eC#I = a). The cuhv values extracted for the D20- and COcontaining surfaces from Figure 2, ca. 15 p F cm-2, are indeed comparable to those extracted for related electrochemical interfaces.26 Another feature of Figure 2 is the significant difference between the @-OK behavior of D20-containing interfaces formed by potassium predosing and postdosing. This finding indicates that equilibrium for the low-temperature interface is not completely attained. Another relevant property of the present coadsorbed D20/ Kf system is the temperature-programmed desorption (TPD) behavior. Figure 3 shows TPD data at 5 K s-l for three initial D2O coverages, Ow,as indicated, from both clean Pt(111) (solid traces) and Pt( 111)predosed with 0.08 ML of potassium (dashed traces). The former traces show a pair of closely spaced TPD peaks at 180/190 K, apparently associated with the so-called water bilayer s t r ~ c t u r e . ~ The ~ ~attainment ~ ~ , ~ ~ of the maximum area under these peaks is therefore presumed to correspond to Ow = 2/3 (vide supra). An additional “multilayer” desorption
7680 J. Phys. Chem., Vol. 99, No. 19, 1995
Kizhakevariam et al.
,170K
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T(K) Figure 3. Temperature-programmeddesorption (TPD)spectra at 5 K-I s-’ from 95 K for water (DzO, mass 20) from either clean (solid traces) or K-predosed Pt(ll1) (0, = 0.08, dashed traces) for initial D20
0.66 YL CO t I YL D20
T=&K
coverages as indicated. peak at 170 K is also present for OW 5 0.5 (top trace, Figure 3). The presence of adsorbed K+ is seen to yield a broad highertemperature TPD peak, presumably associated with cation hydration. This feature is seen to saturate for water dosages around 0.3, suggesting an effective “hydration number” of about 4. At higher water dosages, a sharp TPD peak at 170 K emerges (see top dotted trace), similar to that seen for the desorption of multilayer water. However, no “bilayer” peaks are obtained in the presence of adsorbed K+, suggesting that the adsorbed cations “break up” the water bilayer structure. Infrared Spectral Effects of Water Dosing onto CO Adlayers. As already mentioned, we have recently studied in some detail the infrared spectral consequences of dosing deuterated water onto CO adlayers, monitoring the CO coordination geometry and local environment from the characteristic C-0 stretching (VCO) bands and also the interfacial water structure from the 0-D (YOD) bands.” While these earlier findings were verified in the present study, an interesting spectral sensitivity to the precise temperature conditions came to light which is appropriate to note here prior to describing the mixed K+/D20/CO results. Figure 4A shows a sequence of infrared spectra in the vco frequency region for a saturated CO adlayer (8co 0.65) postdosed at 95 K with a sequence of D20 coverages as indicated. Careful inspection shows that the atop (i.e., terminal CO) YCO band at 2105 cm-I is replaced progressively by a broader “hydrated atop CO’ feature at 2089 cm-’, the replacement being essentially complete by about 2 ML of D20. This “titration-like’’ response, indicating the formation of water islands which spread to encompass the entire surface by Ow 2 ML, is similar to that shown in Figure 3 of ref 11 but with one significant difference: the latter data show the hydrated CO feature at 2077 cm-’, i.e., downshifted by almost twice as much as that seen in the present Figure 4A. Another vco spectral set, obtained under essentially the same conditions as Figure 4A, but at a higher temperature, 125 K, is shown in Figure 5A. While a qualitatively similar “titration-like” response
-
I
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Frequency (cm-1) Figure 4. Effect of progressively larger D20 dosages (as indicated) on infrared spectra in (A, top) vco and (B, bottom) YOD region for a saturated CO adlayer on Pt(l11) at 95 K.
is again clearly evident, the complete removal of the “anhydrous CO’ feature now requires a markedly higher D20 dosage, above 8 ML. A further inkling into the water structural differences between the data sets in Figures 4A and 5A is obtained by examining the corresponding VOD spectral region, shown in Figures 4B and 5B, respectively. Both spectral sets show the progressive development of an intense broad VOD feature centered at about 2500 cm-’, indicative of a strongly hydrogen-bonded water layer (cf. ref 11). However, the VOD bands obtained at 125 K (Figure 5B) display significant “structure” toward lower frequencies, reminiscent of polycrystalline ice.28 This behavior supports the notion that multilayer ice “chunks” are formed at 125 K, but not (to any extent) at 95 K, thereby requiring a markedly larger D20 dosage to completely cover the surface and herein remove entirely the “anhydrous YC-” feature. These differences are
Model Electrochemical Interfaces in Ultrahigh Vacuum 0.85 YL co t I YL 020
I
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I
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~ e q w (a-1)
Figure 5. (A, top; B, bottom) As for Figure 4A,B, but at 125 K.
consistent with the observed transition between the formation of amorphous and polycrystalline ice films for deposition temperatures around 120-130 K.z8 It remains to account for the spectral differences between Figures 4A (and 5A) shown here and the earlier vco data in ref 11. This is probably due to the use of a less efficient liquid Nz sample cooling assembly in ref 11 than in the present setup. Consequently, the initial water dosages in Figure 3a of ref 11 were added in part while the Pt( 111) surface was cooling back to the steady-state temperature (1 10 K), apparently altering the structure of the deposited ice layer. Support for this notion comes from the observation that closely similar water dosagedependent vco spectra to those in ref 11 can be generated by gently heating the surface (by 20-50 K) during initial D2O dosing. Moreover, flash annealing the surface following DzO dosage onto a saturated CO adlayer to progressively higher temperatures up to the onset of water desorption at ca. 165 K
J. Phys. Chem., Vol. 99, No. 19, 1995 7681 yields marked alterations in the vco and VOD spectral features, consistent with the occurrence of increased water crystallinity. These unexpected temperature-dependent effects for water adsorption on CO adlayers will be discussed in detail elsewhere. Fortunately, however, such complexities are less prevalent for water layers dosed either onto clean Pt( 111) or at intermediate CO coverages and in the ternary CO/water/K+ system, which is described next. Infrared Spectral Effects of CatiodWater Coadsorption on Saturated CO Adlayers. Our earlier studies have shown that the presence of interfacial water or other solvents can yield infrared spectra for pt( 111)KO adlayers that mimic closely some aspects of the actual in-situ electrochemical As mentioned at the outset, however, the effects of altering the applied electrode potential can only be properly modeled by appropriate variations in the excess interfacial ionic charge and the corresponding metal (electronic) charge, as mimicked by the addition of ionizable potassium atoms. The influence of adsorbed alkali metal (i.e., alkali cations) on the structure of CO adlayers in the absence of coadsorbed solvent has been the subject of numerous recent investigations (see, for example, ref 15b). A recent detailed IRAS study of coadsorbed potassium/ CO adlayers on Pt(11l)z9 is particularly pertinent here. Even at the low K+ coverages (
2200
zioo
2doo 1900
iioo
i.iO0 id00
Frequency (”1)
Figure 11. Infrared YCO spectra for a low CO coverage (0.05 ML) on clean Pt(ll1) (1) and then modified sequentially by 0.02 ML K (2) and then also 0.15 and 1.3 ML DzO (3, 4), as indicated.
0.2 and then saturating for Ow > 0.5. Similar IRAS bands are commonly seen for low-temperature DzO layers (see Figures 4B, 5B, and 6B) and have been attributed to a “free 0-D’ vibration, i.e., for “surface” water molecules not fully participating in the hydrogen-bound network.’ 1,40-42 Distinct VOD spectra associated with the progressive cation and further surface hydration can also be discerned clearly for water dosing onto Kf layers in the absence of CO, as exemplified in Figure 12A. In this case, a trio of low-frequency VOD bands is seen initially, giving way to the broad VOD envelope for Ow 2 0.5, Le., for a D20/Kf stoichiometry (“hydration number”) of ca. 6. Similarly downshifted VOD (and VOH) bands have also been observed recently under these conditions by means of electron energy loss spectroscopy (EELS)!3 The corresponding spectral sequence in the 0 - D bending (or “scissoring”) region, ~ O D is , displayed in Figure 12B. Note that the “cation hydration water” yields an 601, band at 1177 cm-’, close to the gas-phase freq~ency:~ which is replaced by a broadened and upshifted 601, envelope at 1200- 1250 cm-’, characteristic of hydrogen-bonded water.44 Variations in the predosed potassium coverage for a given small water dosage also provide an instructive view of interfacial cation hydration. Figure 13A shows a VOD spectral sequence of 0.1 ML D20 in the absence and presence of 0.01-0.08 ML of predosed K+, as indicated. These data show a striking increase in VOD band absorptivity as well as marked frequency and band shape changes triggered by K+-D20 association. Another clear feature is the frequency downshift in the “free 0-D stretch” band at ca 2700 cm-’ induced by cation hydration. (A comparable effect is also evident in Figure 12A.) The sequence of 6 0 spectra ~ corresponding to the VOD set in Figure 13A, shown in Figure 13B, shows that sharp 6 0 bands ~ also arise from cation hydration at such low Ow coverages. These large effects of coadsorbed K+ upon the Dz0 spectra are only evident, however, at low water dosages; hydrogen-bonding interactions apparently dominate even the hydration layer water for Ow 2 0.5, yielding broader and more featureless VOD spectra (Figure 12A). Implications for Double-Layer Hydration. At least in an approximate sense, the similarity in the potential-dependent
Kizhakevariam et al.
7686 J. Phys. Chem., Vol. 99, No. 19, 1995 x
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Frequency (cm-1) Figure 12. Effect of progressively larger DzO dosages on infrared spectra in (A, top) VOD and (B, bottom) 600 region for Pt(ll1) predosed
I 0 I350 1300 I250 1200 11‘50 1 00 Frequency (cm-1) Figure 13. Infrared spectra in (A, top), VOD and (B, bottom) ~
O region D for 0.1 ML DzO predosed with varying K coverages as indicated.
with 0.08 ML K. infrared spectral properties of Pt( 111)/CO adlayers in the uhvbased and in-situ electrochemical interfaces ( e g , Figure 8A,B) provides confidence in the validity of the “uhv electrochemical modeling” approach for elucidating electrostatic and other intermolecular interactions of relevance to the double layer. Despite the imperfections of the approach, then, the infrared spectral response to variations in the solvent andor cation dosage can yield invaluable insight concerning interfacial solvation. The most striking feature of the VOD spectra at zero or low CO coverages is the several low-frequency (2330-2430 cm-’) bands that appear for D20 dosages that are ca. 1-4 times the cation dosage (Figures 10B, 12A, and 13A). Such sharp VOD features are quite distinct from the broad VOD envelope centered at higher frequencies (ca. 2450-2550 cm-’) observed for larger water dosages, characteristic of an amorphous or polycrystalline
hydrogen-bound network. Interpretation of these spectra is pursued advantageously by comparison with vibrational data for hydrated cations in either bulk condensed or gas phases. Vibrational spectra for aqueous alkali metal electrolytes, however, are complicated by the presence of nonhydrating water and hydrogen-bonding interactions between the first and subsequent hydration shells.44 While comparison with spectra for stoichiometrically defined gas-phase hydrated cations would be most valuable, unfortunately no such data for alkali metalwater complexes are yet available. Nevertheless, one recent gas-phase study of NO+-(H20)n complexes is ~ertinent.4~ For n = 1-3, the water forms a simple (N-bound) primary hydration shell surrounding the NO+ cation. As anticipated from theoretical consideration^^^^^ a symmetric/ antisymmetric pair of VOH bands is observed with frequencies not greatly shifted (150-100 cm-’) from those for free HzO. For n = 4, however, an additional pair of broader VOH bands
Model Electrochemical Interfaces in Ultrahigh Vacuum
Pt Figure 14. Sketch of proposed water orientations for initial stage of K+ hydration on Pt(ll1).
appear, downshifted by ca. 300 cm-’ and assigned on this basis to primary-hydration water H-bonded to a second-shell H20.“ While the present 2330-2430 cm-’ “hydrated K+” features are similarly downshifted relative to free D20 (for which v, = 2671 cm-’ and v, = 2788 cm-’ 47), their narrow bandwidths are inconsistent with H-bonding to second-shell water. An alternative structural model of the hydrated K+ is suggested in part by comparison with infrared spectra for argon matrix-isolated alkali halide/water complexes?* Interestingly, such hydrated water yields a pair of sharp VOD (and VOH) vibrations, comparably downshifted to the present VOD bands, that were identified in ref 48 as monomeric water coordinated (via the oxygen) to the alkali cation and H-bonded to the counteranion. Transposed to the present charged interfacial environment, a pair of hydrated water orientations consistent on this basis with our VOD spectra are sketched in Figure 14. Structure I features both hydrogens tilted (so to H-bond) to the Pt( 111) surface, whereas structure 11has only one hydrogen so oriented, with another pointing away from the surface. The former structure is nicely consistent not only with the “hydrated K+” VOD spectra but also with the observed marked Q, increases attending initial water adsorption under these conditions (curves 2 and 5 in Figure 1). Indeed, such a hydrated water orientation for various alkali metal-water coadsorption systems was suggested earlier on the basis of similar work function-& profile^,^^.^^,^^ from ab initio cluster calculations,22and indeed from vibrational (EELS) data.24a The cluster calculations, undertaken for the Pt(11l)/K+/H20 system,22suggest that both hydrogens are pointing to the metal surface (Le., structure I). Although we cannot distinguish between structures I and 11from our IRAS data, the additional presence of the “free VOD” vibration at 2685 cm-’ (Figures 12A and 13A) suggests the occurrence of II. It is also instructive to contrast these VOD features for hydrated K+ at zero (or low) CO coverages with the corresponding behavior seen in the presence of a saturated CO adlayer. While characteristic VOD bands are also observed in the latter case, these occur at frequencies (ca. 2620-2650 cm-’, Figure 6B) approaching those expected for non-hydrogen-bonded D2O (vide supra). This behavior can readily be understood since the CO adlayer should impede access of the hydrated water hydrogens to the metal surface, preventing hydrogen bonding to the platinum. Also consistent with this picture is the observed absence of marked @-OW changes attending water dosage onto the K+/CO layer (curves 4 and 7 in Figure 1). The attenuation and eventual replacement of both types of “hydrated K+” VOD features at higher water dosages (Ow 2 0.5) by the usual broad VOD envelope associated with H-bonded water might be construed as indicating that the hydration structures in the absence of CO are altered in the presence of multiple solvent shells. While this may be the case, these VOD features will be altered drastically by H-bonding to additional water shells even if the primary hydration structures remain largely intact. An interesting aspect of the present findings is the efficient removal of the short-range K+/CO interactions as sensed by
J. Phys. Chem., Vol. 99,No. 19, I995 7687 the vco spectra for saturated as well as low CO coverages brought about by K+ hydration (Figures 6A, 10A, and 11). While the inferred preferential coordination of K+ by D2O rather than CO is unsurprising, the data in Figures 6A and 10A infer that the resultant dielectric screening requires only 2-3 waters per K+. On the basis of the present results, it is unclear whether the K+ ever becomes fully hydrated, and hence “lifted off’ the surface by the hydration sheath, although the primary hydration number of ca. 5 which is inferred for Pt( 11l)/K+ from the VOD spectra, TPD data (such as in Figure 3), and by other suggests that this may well happen. Regardless of such details, however, markedly larger water dosages, typically Ow 1-2 (i.e., 220-fold more water than K+), are required to fully hydrate the surface such that the vco fingerprint mimics that observed for the in-situ electrochemical interface. While this result is not surprising in itself, examining the vco spectral responses to additions of K+ and DzO, both separately and together, yields significant insight into the inherently synergetic roles of these free and dipolar charges in the electrochemical system. As before, it is useful to consider in turn the cases of saturated and low CO coverages. In the former case, while the effect of small water doses is to attenuate the short-range K+-CO interactions, larger water dosages yield in addition a markedly larger Stark shift as monitored in the atop vco frequency downshift than engendered separately by either K+ or D20. This point is clearly evident in Figures 4A and 6A: dosing either 0.08 ML K+ or 2 ML D20 alone yields atop vco downshifts of 33 and 16 cm-’, respectively, while their codosage yields a downshift of almost 60 cm-’. Nonetheless, the large additional Stark shift engendered by water postdosing is accompanied by small (0.3 eV) Q, increases (curve 7, Figure 1). This result can be understood most simply if the water removes the potassium from the chemisorbed layer, forming a solvated K+ layer at least partially above the CO adlayer. While the electrostatic screening engendered by the water dielectric will attenuate the short-range K+-CO interactions, the surface potential drop engendered by the K+-image charge separation yields a large as well as more uniform electrostatic field across the CO adlayer, accounting for the substantial observed vco downshift. It is worth emphasizing that the combined roles of the water dielectric in shrouding the cationic charges away from the metal surfaces (and chemisorbates) and screening lateral electrostatic interactions are critical factors in the success of simple “averaged potential” models of the double layer that abound in classical electrochemistry. A somewhat different situation is encountered for K+/water codosing at lower CO coverages, as exemplified in Figures 9A,B and IOA. Here both water and especially K+, dosed separately, yield substantial CO atop bridging site conversion as well as large vco Stark shifts. As already mentioned, the former effects can be understood in terms of short-range CO/water and C O W coadsorption. In the presence of excess water as well as K+, however, the influence of the latter component is severely attenuated, the resulting vco spectra having an appearance much closer to that obtained in the presence of water alone. This finding can again be rationalized in terms of K+ hydration. At least in the presence of excess water, the cations will be fully hydrated and efficiently screened as well as spatially separated from the CO adsorbate, which is also solvated by water. Consequently, then, the large “specific” (i.e., short-range) coadsorbate interactions that occur at low CO coverages will be dominated by water, with the cation exerting only an additional long-range Stark effect upon the chemisorbate in a similar fashion to that for high CO coverages. The dominant
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7688 J. Phys. Chem., Vol. 99, No. 19, 1995 role of cation hydration under these circumstances is further reinforced by recalling Figure 11, referring to very low CO and K+ coverages titrated with water. In the case, a sufficiency of open surface sites are available at small water dosages to yield effective local segregation of hydrated K+ and anhydrous CO, solvation of the latter commencing only at higher Ow.
Concluding Remarks Despite the nuances or even major behavioral differences in the uhv-based and in-situ electrochemical interfaces engendered by the temperature disparity and related factors, the present results give us additional confidence in the fundamental validity of utilizing uhv IRAS-based tactics to explore intermolecular and electrostatic interactions of relevance to interfacial electrochemistry. An extension of these measurements to encompass a range of nonaqueous media will be described e1~ewhere.l~ While the net interactions triggered by the combined presence of chemisorbate, solvent, and ions (also electrons) may seem daunting (or even hopelessly complex!) to uhv surface scientists, the inherently synergetic effects exerted by iodsolvent coadsorption can now in principle be elucidated satisfactorily on a molecular level. The ability to “synthesize” in stepwise fashion the key ingredients of electrochemical interfaces and examine their mutual interactions by uhv-based IRAS can provide unique insight that is inherently overlapping with that obtained from in-situ vibrational spectra. In particular, the former approach enables the nature and roles of interfacial ionic solvation to be explored in a truly molecular-level fashion. The receipt of such vibrational spectral information as a function of iodsolvent stoichiometry also invites comparisons with corresponding data for gas-phase ion solvation, leading to a new appreciation of the role of surface interactions in the former case. More generally, the insight thus afforded into electrostatic interactions involving ionic-dipolar coadsorption will have a broader significance in helping to unify the formerly entirely disparate fields of uhv and electrochemical surface science.
Acknowledgment. This work is supported by the National Science Foundation. References and Notes (1) For example, see: Delahay, P. Double Layer and Electrode Kinetics; Interscience: New York, 1965. (2) For example, see: Lipkowski, J., Ross, P. N., Eds. Structure of Electr@ed Interfaces; VCH Publishers: New York, 1993. (3) Weaver, M. I.; Gao, X. Annu. Rev. Phys. Chem. 1993, 44, 459. (4) Chang, S.-C.; Weaver, M. J. Surf. Sci. 1990, 238, 142. ( 5 ) Chang, S.-C.; Weaver, M. J. J . Phys. Chem. 1991, 95, 5391. (6) Yau, S.-L.; Gao, X.; Chang, S.-C.; Schardt, B. C.; Weaver, M. J. J . Am. Chem. SOC. 1991, 113, 6049. (7) Villegas, I.; Weaver, M. J. J . Chem. Phys. 1994, 101, 1648. (8) For example, see: Sass, J. K.; Bange, K. ACS Symp. Ser. 1988, 378, 54. (9) For an erudite recent review, see: Wagner, F. T. In ref 2, Chapter 9. (10) Stuve, E. M.; Kizhakevariam, N. J . Vac. Sci. Technol. 1993, A l l , 2217. (1 1) Kizhakevariam, N.; Jiang, X.; Weaver, M. J. J. Chem. Phys. 1994, 100, 6750. (12) Kizhakevariam, N.; Villegas, I.; Jiang, X.; Weaver, M. J. Surf. Sci., in press. (13) Kizhakevariam, N.; Villegas, I.;Weaver, M. J. Langmuir, submitted. (b) Villegas, I.; Weaver, M. J. J. Chem. Phys., submitted; manuscripts in preparation. (14) Chang, S.-C.; Jiang, X.; Roth, J. D.; Weaver, M. J. J . Phys. Chem. 1991, 95, 5378. (15) For recent reviews, see: (a) Pirug, G . ; Bonzel, H. P. In ref 2, Chapter 5 . (b) Bonzel, H. P.; Pirug, G. In The Chemical Physics of Solid Surfaces; King, D. A., Woodruff, D. P.; Eds.; Elsevier: New York, 1993; Vol. 6, Chapter 3. (16) (a) Bonzel, H. P.; Pirug, G.; Ritke, C. Langmuir 1991, 7, 3066. (b) Pirug, G.; Bonzel, H. P. Surf. Sci. 1988, 194, 159.
Kizhakevariam et al. (17) Stenniger, H.; Lehwald, S.; Ibach, H. Surf. Sci. 1982, 123, 264. (18) Schweizer, E.; Persson, B. N. J.; Tiishaus, M.; Hoge, D.; Bradshaw, A. M. Surf. Sci. 1989, 213, 49 and references therein. (19) Ogletree, D. F.; Van Hove, M. A.; Somoqai, G. A. Surf. Sci. 1986, 173, 351. (20) (a) Fisher, G. B.; Gland, J. L. Surf. Sci. 1980, 94, 446. (b) Jo, S. K.; Kiss, J.; Polanco, J. A,; White, J. A. Surf. Sci. 1991, 253, 233. (21) (a) Doering, D. L.; Madey, T. E. Surf. Sci. 1982, 123, 305. (b) Thiel, P. A,; Madey, T. E. Surf. Sci. Rep. 1987, 7, 211. (22) Bonzel, H. P.; Pirug, G.; Miller, J. E. Phys. Rev. Lett. 1987, 58, 2138. (23) Rotemund, H. H.; Jakubith, S.; Kubala, S.; von Oertzen, A,; Ertl, G. J . Electron Spectrosc. Relat. Phenom. 1990, 52, 811. (24) (a) Lackey, D.; Schott, J.; Straehler, B.; Sass, J. K. J . Chem. Phys. 1989, 91, 1365. (b) Sass, J. K.; Schott, J.; Lackey, D. J . Electroanal. Chem. 1990, 283, 441. (25) For example, see: Aruga, T.; Murata, Y. Prog. Surf. Sci. 1989, 31, 61. (26) (a) The unambiguous measurement of double-layer capacitance, Cd, for Pt( 111)-aqueous interfaces is commonly thwarted by the occurrence of “pseudocapacitance” associated with adsorbate redox processes. Nevertheless, for CO-coated Pt(lll), for example, cdl values of 15-20 p F cm-* can be deduced from cyclic voltammetry (as in Figure 3 of ref 26b). (b) Clavilier, J.; Albalat, R.; Gomez, R.; Orts, J. M.; Feliu, I. M.; Aldaz, A. J . Electroanal. Chem. 1992, 330, 489. (27) (a) A referee suggested that this TPD peak splitting might be associated with the adsorption of impurity hydrogen. While we could not eliminate this possibility, a major effect from hydrogen is deemed unlikely. This assertion follows from the small or negligible hydrogen TPD peaks observed at ca. 320 K, together with the absence of atop-to-bridging CO site transfer that would be exDected at lower CO coverages in the Dresence of coadsorbed hydrogen.27b ib) Hoge, D.; Tiishaus, M., Bradshai, A. M. Surf. Sci. 1988, 207, L935. (28) Nuzzo, R. G.; Zegarski, B. R.; Korenic, E. M.; Dubois, L. H. J . Phys. Chem. 1992, 96, 1355. (29) Tiishaus, M.; Gardner, P.; Bradshaw, A. M. Surf. Sci. 1993, 286, 212. (30) (a) Muller, J. E. In The Physics and Chemistry of Alkali Metal Adsorption; Bonzel, H. P., Bradshaw, A. M., Ertl, G., Eds.; Elsevier: Amsterdam, 1989; p 271. (b) Muller, J. E. In ref 15b, Chapter 2. (31) Chang, S.-C.; Weaver, M. J. Surf. Sci. 1990, 238, 142. (32) As noted in the Experimental Section, the K coverages were calibrated in the present study by utilizing the work function-& data reported in ref 16a. The sensitivity of the YCO spectral features to the predosed K coverage, as reported in ref 29 and also examined here, allows the OK scales in refs 16a and 29 to be cross-checked. By following this procedure, however, the apparent OK values reported in ref 29 were consistently found to be about 2-fold higher than expected on the basis of the ref 16a data, in the range 0 OK 5 0.1. We suspect that this discrepancy arises from a miscalibration of the OK values in ref 29 from a LEED analysis. In our opinion, the OK values based on ref 16a are more likely to be reliable given the extensive LEED analysis undertaken by the KFA Jiilich group.33 (33) We are grateful to G. Pirug for a clarification of this point. (34) Chang, S.-C.; Weaver, M. J. J . Phys. chem. 1991, 95, 5391. (35) Kitamura, F.; Takahashi, M.; Ito, M. Surf. Sci. 1989, 223, 493. (36) Chang, S. C.; Weaver, M. J. J . Chem. Phys. 1990, 92, 4582. (37) Wagner, F. T.; Moylan, T. E.; Schmieg, S. J. Surf. Sci. 1988, 195, 403. (38) Leung, L.-W. H.; Goodman, D. W. Langmuir 1991, 7, 493. (39) (a) Ellis, T. H.; Kreus, E. J.; Wang, H. Surf. Sci. 1992, 273, 73. (b) Ellis, T. H.; Kreus, E. J.; Wang, H. J . Electron Spectrosc. Relat. Phenom. 1993, 64/65, 421. (40) Rowland, B.; Devlin, J. P. J . Chem. Phys. 1991, 94, 812. (41) (a) Callen, B. W.; Griffiths, K.; Norton, P. R. Surf. Sci. 1992,261, L44. (b) Callen, B. W.; Griffiths, K.; Kasza, R. V.; Jenson, M. B.; Thiel, P. A.; Norton, P. R. J . Chem. Phys. 1992, 97, 3760. (42) Schaff, J. E.; Roberts, J. T. J . Phys. Chem. 1994, 98, 6988. (43) Pirug, G. Personal communication. (44)Irish, D. E.; Brooker, M. H. In Advances in Infrared and Raman Spectroscopy; Clark, R. J. H., Hester, R. E., Eds.; Heyden: London, 1976; Vol. 2, Chapter 6. (45) Choi, J.-H.; Kuwata, K. T.; Haas, B.-M.; Gao, Y.; Johnson, M. S.; Okumura, M. J . Chem. Phys. 1994, 100, 7153. (46) Kistenmacher, H.; Popkie, H.; Clementi, E. J . Chem. Phys. 1973, 59, 5842. (47) (a) Benedict, W. S.; Gailer, N.; Plyler, K. J . Chem. Phys. 1956, 24, 1139. (b) Pinchas, S.; Halmaan, M. J. Chem. Phys. 1959, 31, 1692. (c) See also: Ayers, G. P.; Pullin, A. D. E. Chem. Phys. Lett. 1974, 29, 609. (48) Ault, B. S. J . Am. Chem. SOC. 1978, 100, 2426. (49) (a) Kiskinova, M.; Pirug, G.; Bonzel, H. P. Surf. Sci. 1985, 150, 319. (b) Stuve, E. M.; Dohl-Oelze, R.; Bange, K.; Sass, J. K. J . Vac. Sci. Technol. 1986, A4, 1307. JP9422 12M
