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Langmuir 1997, 13, 6836-6844
Modeling Electrochemical Interfaces in Ultrahigh Vacuum: Influence of Progressive Cation and Surface Solvation upon Charge-Potential Double-Layer Behavior on Pt(111) Michael J. Weaver* and Ignacio Villegas† Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-1393 Received July 21, 1997. In Final Form: September 29, 1997X Measurements of the work-function changes, ∆Φ, on Pt(111) for continuously increasing solvent exposures θs* and in the presence of various coverages of potassium, θK, in ultrahigh vacuum (UHV) at 90 K are reported with the objective of ascertaining how the surface charge-potential properties of such “UHV electrochemical model” interfaces are altered by progressive solvation. The solventsswater, methanol, acetonitrile, acetone, and ammoniasspan a range of dipolar and other solvating properties and have been utilized in related vibrational spectroscopic studies from this laboratory. Since potassium dosage yields interfacial electron transfer to form K+ together with surface electronic charge, the corresponding ∆Φ-θK plots for various solvent dosages extracted from the above data provide surface charge-potential (σ-φ) curves for systematically varying extents of interfacial solvation. In contrast to the large (1-3 eV) monotonic solvent-induced Φ decreases observed in the absence of ionic charge, the presence of predosed K+ yields initial Φ increases, associated with cation solvation, followed by Φ decreases due primarily to the ensuing metal surface solvation. Examination of the corresponding ∆Φ-θK traces obtained for these different solvent dosage regions shows that the basic charge-potential features characteristic of the solvated double layer require only ionic solvation, even though complete metal surface solvation modifies significantly the electrostatic behavior. While surface solvation by the different species examined in the absence of charge yield substantially dissimilar Φ values (i.e., differing “potentials of zero charge”), the charge-potential characteristics are relatively insensitive to the solvent. This finding, comparable to that obtained for in-situ electrochemical interfaces, indicates that the effective “interfacial solvent dielectric constant” varies by only 2-fold or less. ∆Φ-θK data obtained by K dosing after solvent addition yielded larger -∆Φ values (i.e., smaller capacitances), consistent with more complete K+ solvation and/or larger K+- surface separations. Corresponding ∆Φ-θK data for CO-saturated Pt(111) indicates that the CO adlayer plays a role in dielectric screening. Effectively θK-independent ∆Φ responses were obtained with ammoniasolvated Pt(111), however, suggestive of the formation of solvated electrons. Specific comparisons are made between the UHV-based charge-potential behavior with that for in-situ electrochemical interfaces and for ionizable high-nuclearity Pt carbonyl clusters in nonaqueous media. The latter systems, in particular, exhibit closely similar surface charge-potential characteristics to the corresponding UHV-based Pt(111) interfaces.
Introduction An issue of obvious fundamental importance in electrochemistry concerns the manner by which the doublelayer electrostatic properties are controlled by interfacial solvation. Traditional models of the double layer emphasize the importance of the inner layer of solvent (i.e., the layer juxtaposed to the metal surface), especially potential-dependent dipole orientation, in determining the electronic charge-potential (and hence capacitance) characteristics.1 More recently, the likely important role of the metal electronic properties has also received detailed, albeit belated, attention.2 The last few years have seen burgeoning interest in employing molecular dynamics and other simulation techniques to obtain a more coherent microscopic-level picture of the interfacial solvent structure,3 in some cases including ionic solvation. Taken together, these developments lead to a picture of the double † Present address: Department of Chemistry, University of New Mexico, Albuquerque, NM 87131. X Abstract published in Advance ACS Abstracts, November 15, 1997.
(1) For reviews, see for example: (a) Reeves, R. M. In Modern Aspects of Electrochemistry; Conway, B. E., Bockris, J. O’M., Eds.; Plenum: New York, 1974; p 239. (b) Trasatti, S. In Modern Aspects of Electrochemistry; Conway, B. E., Bockris, J. O’M., Eds.; Plenum: New York, 1979; Vol. 13, Chapter 2. (2) For a recent review, see: Schmickler, W. Chem. Rev. 1996, 96, 3177. (3) For example, see: (a) Xia, X.; Perera, L.; Essmann, U.; Berkowitz, M. L. Surf. Sci. 1995, 335, 401. (b) Spohr, E. ACS Symp. Ser. 1997, No. 656, 31.
S0743-7463(97)00820-2 CCC: $14.00
layer where the role of the solvent is distinctly more multifaceted than is the case in traditional models.4 Nonetheless, there remains a paucity of experimental molecular-level information regarding double-layer solvation. We have recently been pursuing measurements involving ordered metal surfaces in ultrahigh vacuum (UHV), with one objective being the experimental elucidation of interfacial solvation effects of relevance to electrochemistry.7-13 The general tactics, involving the sequential dosing onto the metal-vacuum interfaces of various (4) (a) Philpott, M. R.; Glosli, J. N. ACS Symp. Ser. 1997, No. 656, 13. (b) Philpott, M. R.; Glosli, J. N.; Zhu, S.-B. Surf. Sci. 1995, 335, 422. (5) For example: (a) Sass, J. K.; Bange, K. ACS Symp. Ser. 1988, No. 378, 54. (b) Sass, J. K.; Bange, K.; Do¨hl, Piltz, E.; Unwin, R. Ber. Bunsenges Phys. Chem. 1984, 88, 354. (6) For recent erudite reviews, see: (a) Wagner, F. T. In Structure of Electrified Interfaces; Lipkowski, J., Ross, P. N., Eds.; VCH Publishers: New York, 1993; Chapter 9. (b) Stuve, E. M.; Kizhakevariam, N. J. Vac. Sci. Technol. 1993, A11, 2217. (7) (a) Kizhakevariam, N.; Jiang, X.; Weaver, M. J. J. Chem. Phys. 1994, 100, 6750. (b) Kizhakevariam, N.; Villegas, I.; Weaver, M. J. Langmuir 1995, 112, 777. (c) Kizhakevariam, N.; Villegas, I.; Weaver, M. J. Surf. Sci. 1995, 336, 37. (8) (a) Kizhakevariam, N.; Villegas, I.; Weaver, M. J. J. Phys. Chem. 1995, 99, 7677. (b) Villegas, I.; Kizhakvariam, N.; Weaver, M. J. Surf. Sci. 1995, 335, 300. (9) (a) Villegas, I.; Weaver, M. J. 1995, 103, 2295. (b) Villegas, I.; Weaver, M. J. J. Am. Chem. Soc. 1996, 118, 458. (c) Villegas, I.; Weaver, M. J. Surf. Sci. 1996, 367, 162. (10) (a) Villegas, I.; Weaver, M. J. Electrochim. Acta. 1996, 41, 661. (b) Villegas, I.; Weaver, M. J. J. Phys. Chem. B 1997, 101, 10166. (11) Villegas, I.; Gomez, R.; Weaver, M. J. J. Phys. Chem. 1995, 99, 14832.
© 1997 American Chemical Society
Modeling Electrochemical Interfaces in UHV
double-layer components, including solvent, owe much to the pioneering efforts of Sass and co-workers in Berlin during the last decade;5 they are often referred to as “UHV electrochemical modeling”.6 In particular, we have employed infrared reflection-absorption spectroscopy (IRAS) along with work-function measurements. The former technique affords detailed information on bonding and intermolecular interactions and also provides a link to in-situ electrochemical systems since IRAS is also applicable (albeit with more restrictions) in such liquid-phase environments. While less frequently discussed in the surface-science literature, responses of the work function (Φ) to progressive alterations in the surface composition can provide unique insight into the interfacial potential profile and related electrostatic issues, of central importance in electrochemistry. In the context of “UHV electrochemical modeling”, examining Φ responses to controlled additions of solvent and ions (or ionizable species) can indeed constitute a powerful means of exploring double-layer interactions.5 One aspect of these activities entails combined IRAS/ work-function studies of potassium cation solvation on Pt(111).8-10 The solvents examinedswater, methanol, acetonitrile, acetone, and ammoniaswere selected partly in view of their varying dipolar, vibrational, and overall solvating properties. The choice of potassium was prompted by its nonspecifically adsorbing properties in electrochemistry, together with the availability of an accurate coverage assay from the large Φ decreases induced on Pt(111) by K adsorption.14 The vibrational spectra provide insight into the microscopic nature of cation solvation, and in some cases can delineate between cation-solvent and surface-solvent interactions. In the present communication, we examine more closely work function-surface composition data for these systems, involving systematic variations in the solvent as well as cation coverages, with the particular aim of gaining insight into the separate (or possibly separable) electrostatic consequences of double-layer ionic and surface solvation. Since adsorbed K ionizes to form (essentially) K+ cations along with negative electronic charging of the Pt(111) surface, examining relationships between the Φ (and hence surface potential) changes induced by K adsorption for various solvent dosages can yield insight into the roles of solvation in surface charge-potential (i.e., double-layer) properties. As well as K+/solvent coadsorption onto clean Pt(111), we discuss more briefly corresponding workfunction behavior in the presence of predosed saturated CO adlayers, this chemisorbate being chosen partly to eliminate access of the solvent to the metal surface (cf. refs 7c and 13). While portions of these data have been reported previously in conjunction with IRAS measurements,7-10 it is felt that the present comparative (and more comprehensive) examination of the solvent-dependent work function behavior would be beneficial. The results delineate the manner in which increasing the solvent coverages from submonolayer to multilayer levels transform progressively the electrostatic characteristics of primary ionic and metal surface solvation into the familiar charge-potential properties of electrochemical interfaces. Experimental Section Experimental details have been described in several earlier reports.7-9 Measurements were made in a stainless-steel chamber maintained at a base pressure of 5 × 10-10 Torr. The (12) (a) Villegas, I.; Weaver, M. J. J. Phys. Chem. 1996, 100, 19502. (b) Villegas, I.; Weaver, M. J. J. Electroanal. Chem. 1997, 426, 51. (13) Villegas, I.; Weaver, M. J. J. Phys. Chem. B. 1997, 101, 5842. (14) (a) Pirug, G.; Bonzel, H. P. Surf. Sci. 1988, 194, 159. (b) Bonzel, H. P.; Pirug, G.; Ritke, C. Langmuir 1991, 7, 3066.
Langmuir, Vol. 13, No. 25, 1997 6837 Pt(111) surface was maintained clean and ordered (as evidenced by Auger electron spectroscopy and low-energy electron diffraction (LEED)) by sputtering with 1 kV Ar+ followed by annealing to 1200 K. Carbon surface impurities were removed before each set of experiments by exposure to 3 × 10-7 Torr O2 at 900 K. The surface was held at 90 K by liquid N2 cooling during the measurements described here. Changes in the work function (∆Φ) with respect to clean Pt(111) were measured using a Kelvin probe (Delta Phi Elektronik). The ∆Φ values measured during continuous exposure to solvent are accurate to 10 meV. However, for variable potassium dosing the ∆Φ values quoted are accurate to only (50 meV since repositioning of the surface in front of the Kelvin probe was usually necessary. As described earlier,8a the potassium was dosed from a commercial thermal getter source (SAES). Determination of the required potassium coverages (θK) relied upon the linear ∆Φ-θK behavior along with the θK calibration with LEED reported earlier.14 Saturated CO adlayers were produced by exposure of clean Pt(111) to 12 langmuirs of the gas (Airco) at 2 × 10-8 Torr. Water (D2O, Aldrich), methanol (Fisher), acetonitrile (Mallinckrodt), and acetone (Sigma-Aldrich) vapors were obtained from the pure liquids contained in a UHV-compatible glass ampule connected to the gas-dosing manifold. Gaseous impurities were removed from the liquids by means of repeated freezing, pumping, and thawing cycles. Ammonia gas (Matheson) was dosed as received. Line-of-sight exposure through a nozzle located in front of the sample enabled stepwise increases in the relative solvent coverages while the background pressure was maintained below 1 × 10-10 Torr. Continuous monitoring of the solvent-induced Φ changes required instead exposure to 1 × 10-8 Torr of the corresponding solvent. As usual in our studies of interfacial solvation,8-10 the reported solvent-exposure values were normalized with respect to that required to complete the first “chemisorbed” layer on clean Pt(111), termed a unit “equivalent monolayer” (EL), as deduced from the separate temperatureprogrammed desorption (TPD) peak discerned for each solvent under these conditions.7 As before, we will therefore refer to these solvent coverages here in EL units (θs* values), the asterisk denoting a distinction from coverages referenced instead to the metal surface atomic density (1.5 × 1015 atoms cm-2) rather than a “close-packed” solvent monolayer. (The latter coverage scale, however, can be used for CO (θCO) and potassium (θK), these values thereby being referred to here in monolayer (ML) units.) The single solvent exception is water, with coverages (θW) that can also be given in ML units since the water “bilayer” (1 EL), as identified from TPD measurements,8a is known to correspond to a θW value of 2/3 ML. (We employed the same θW scale earlier.8a) Calibration of the solvent dosages in EL units also relied more generally upon TPD measurements which, importantly, also indicated that the solvent coverage increases essentially linearly with exposure even in the presence of predosed K and/or CO. In the case of ammonia, however, the dosage calibration relied primarily upon work-function changes induced on clean Pt(111) as reported by Fisher15 since TPD detection of the chemisorbed layer is hampered by the broadness of the desorption temperature range and the limit of detection of our instrument (see ref 9c for details).
Results and Discussion K+/Solvent Adsorption on Clean Pt(111). The primary experimental tactics employed here involve monitoring the changes in work function, ∆Φ (eV), brought about by increasing solvent dosages, θs*, onto Pt(111) modified by various potassium coverages, θK. Since the potassium atoms should be largely ionized to form adsorbed cations,18 at least when surrounded by polar solvent molecules, increasing θK acts to charge the metal (15) Fisher, G. B. Chem. Phys. Lett. 1981, 79, 452. (16) Bonzel, H. P.; Pirug, G.; Mu¨ller, J. E. Phys. Rev. Lett. 1987, 58, 2138. (17) Rotemund, H. H.; Jabubith, S.; Kubala, S.; van Oertzen, A.; Ertl, G. J. Electron. Spect. Related. Phenom. 1990, 52, 811. (18) For example: (a) Bonzel, H. P.; Pirug, G.; Mu¨ller, J. E. Phys. Rev. Lett. 1987, 58, 2138. (b) Mu¨ller, J. E. In Physics and Chemistry of Alkali Metal Adsorption; Bonzel, H. P., Bradshaw, A. M., Ertl, G., Eds. Elsevier: Amsterdam; 1989; p 271.
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Figure 1. Changes in the Pt(111) work function, ∆Φ, induced by continuously increasing solvent dosages, expressed in “equivalent monolayers” EL (see text) for the predosed potassium coverages indicated, for (A) deuterated water, (B) methanol, (C) acetonitrile, (D) acetone, and (E) ammonia.
surface negative, K+ thereby effectively constituting the ionic double-layer countercharge. Consequently, the resulting Φ-θK plots are analogous to the electrode potential-charge (E-σ) data familiar in electrochemistry, referring to negative electrode charges σ and therefore E values increasingly below the so-called potential of zero charge, Epzc (vide infra). Parts A-E of Figure 1 show ∆Φ-θs* data for water, methanol, acetonitrile, acetone, and ammonia, respectively, for several potassium coverages as indicated. The range of θK values selected, e0.08 ML, encompasses the typical span of negative electrode charges encountered in electrochemistry, given that a θK value of 0.1 corresponds to a charge density of 24 µC cm-2. (In addition, the onset of K-induced water dissociation on Pt(111) occurs only at higher θK values, >0.1 ML.14,16) While only ∆Φ values are obtainable by means of the Kelvin probe, referenced to the clean surface, these quantities can readily be converted into absolute work functions, Φ, given the Φ value for clean Pt(111), is ca. 5.9 eV.17 Several facets of the data in Figure 1 provide insight into the roles of solvation in determining double-layer properties. Since the last solvent, ammonia, displays rather unusual ∆Φ-θs* characteristics (Figure 1E), we consider first chiefly the behavior with the other four solvents. Dosing each solvent onto clean Pt(111) yields large (1-3 eV) work-function decreases. While most of
these Φ changes occur within the first 2-3 equivalent layers (EL), strictly θs*-independent ∆Φ values are not attained for most solvents until high solvent dosages, θs* ∼ 4 to 6 EL, are reached (Figure 1A-D). This suggests that propagation of solvent “structuring”, presumably involving net dipole orientation, extends to several layers from the metal surface. As discussed earlier,7c however, the bulk of these substantial solvent-induced Φ decreases, at least in the first monolayer or so, probably arise from factors other than solvent-dipole orientation. Most likely, an important contributor to the large -∆Φ values is an adsorbate-induced redistribution of charge in the metal surface,7c,19 involving a suppression of the surface electron “spillover” which is partly responsible for the high work functions of clean Pt(111) and other transition metals.7c Markedly different ∆Φ-θs* responses are seen, however, when the surface is predosed with potassium, especially for higher θK values (g0.04 ML). Distinctly non-monotonic ∆Φ-θs* traces are uniformly obtained under these conditions, the work function initially increasing up to θs* ∼ 1 to 2 EL before decreasing in a parallel fashion as for solvent dosing onto clean Pt(111) (Figure 1). The initial Φ increases, which are largest for water (19) (a) Baetzold, R. C. J. Phys. Chem. 1983, 87, 3858. (b) Shustorovich, E. J. Phys. Chem. 1982, 86, 114. (c) Baetzold, R. C.; Apai, G.; Stustorovich, E. Appl. Surf. Sci. 1984, 19, 135.
Modeling Electrochemical Interfaces in UHV
and methanol, have been shown by vibrational spectroscopy to be associated with cation-induced solvent reorientation, featuring K+-oxygen coordination along with surface-hydrogen bonding interactions for these two solvents.10a,11a,20 (Indeed, the degree of cation-induced water reorientation has been estimated from the initial Φ changes observed upon solvent dosing.5a, 23) More generally (or equivalently), these Φ increases can be considered to arise from solvent “dielectric screening” of the K+‚‚‚e- “surface dipole”, which is largely responsible for the very marked (up to 3 eV) K-induced Φ decreases seen on clean Pt(111). The notable attenuation of these Φ decreases brought about by water addition (Figure 1A) reflects the particularly efficient dielectric screening characteristic of this solvent. Examining the overall morphology of the ∆Φ-θs* traces (Figure 1) suggests that the two aforementioned effectss(a) the Φ decreases due to metal surface solvation and (b) the Φ increases accompanying cation solvationsact in a combined, yet partly separable fashion to yield the nonmonotonic overall ∆Φ-θs* behavior. (Although these two interfacial solvation contributions should not be completely separable given that the cations are also “adsorbed”, one can imagine type (a) solvation as involving solvent adsorption at sites not in immediate proximity to the K cations.) The initial stages of solvent addition should result preferentially, if not exclusively, in cation solvation (type b): this expectation, made on energetic grounds as well as from the Φ increases noted above, is supported both by IRAS and temperature-programmed desorption (TPD) data.8a,9a,b The progressive development of surface solvation (type a), which may well occur initially in parallel with (b), becomes predominant following the completion of the primary (and possibly also secondary) cation solvation shells, as evidenced by the roughly parallel ∆Φθs* traces in the presence and absence of potassium seen for θs* g 1-2 EL (Figure 1A-D). (Note that deducing the solvent/K+ stoichiometries corresponding to these solvent dosage regions is hampered by a lack of a reliable conversion between the EL and ML scales. Nevertheless, for water 1 EL ) 2/3 ML,8a and for methanol 1 EL ≈ 0.35 ML,9a so that the onset of parallel ∆Φ-θs* traces for different θK values clearly corresponds to solvent dosages well beyond that required for complete primary cation solvation.) However, regardless of such details these two solvation modes (a) and (b) acting in concert apparently lead to the markedly smaller vertical displacements observed between each trace (i.e., for varying predosed θK values at fixed θs*) for solvent dosages θs* g 2 EL (Figure 1). Scrutinizing such ∆Φ-θK traces for a series of increasing solvent dosages lends complementary insight into the effects of progressive solvation on double-layer properties. The ∆Φ-θK plots derived in this fashion for a series of increasing fixed θs* values (as indicated) for each solvent are shown in Figure 2. Also included in each case is the ∆Φ-θK trace for the solvent-free (θs* ) 0) limit (open circles). It is important to recognize that the ∆Φ-θK slopes are inversely proportional to the well-known “double-layer capacitance” Cd. Thus by converting the K coverages into effective electronic charge densities by again presuming (20) Comparable findings have also been obtained by means of electron energy loss spectroscopy (EELS) for the Pt(111)/K+, water, and related systems.21,22 (21) Baumann, P.; Pirug, G.; Reuter, D.; Bonzel, H. P. Surf. Sci. 1995, 335, 186. (22) Lackey, D.; Schott, J.; Straehler, B.; Sass, J. K. J. Chem. Phys. 1989, 91, 1365. (23) Pirug, G.; Bonzel, H. P. In Structure of Electrified Interfaces; Lipkowski, J., Ross, P. N., Eds.; VCH Pubishers: Deerfield Beach, FL, 1993; Chapter 5.
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complete ionization, the linear ∆Φ-θK trace observed on solvent-free Pt(111) is deduced to have a slope corresponding to Cd ) 6.0 µF cm-2. (This is deduced by noting that the Pt(111) atomic density is 1.5 × 1015 atom cm-2, so that a K+ (and hence e-) “monolayer” would have a charge density, σ, of 240 µC cm-2.) The presence of even low solvent dosages, however (θs* ∼ 0.5 EL), is seen to yield marked (2-3-fold) increases in Cd (Figure 2A-D). Indeed, the essential morphology of the ∆Φ-θK curves in the presence of solvent is largely developed in each case by θs* ) 1 EL, even though solvent dosages of 4 EL or so are required to develop fully the traces corresponding to the “high-dosage” limit (Figure 2A-D). This finding indicates that the double-layer capacitive properties are controlled to a disproportionately large extent by solvation in the vicinity of the ionic countercharges, although as already noted solvation of the entire Pt(111) surface by 1-2 equivalent monolayers is required to modify completely the electrostatic situation. It is interesting to note that the Φ values attained even in the high-θs* limit for the largest potassium coverages (θK ∼ 0.075) are not much higher than, or are even comparable to, those in the complete absence of solvent, with the exception of water. At first sight, this observation might suggest that the extent of solvent dielectric screening of the ionic/electronic charges is rather mild. However, this argument does not take into account the large (1-1.7 eV) Φ decreases induced by solvent adsorption on Pt(111), which according to the above discussion is probably present to a large extent even on the θK-containing surface. Consequently, these Φ decreases tend to offset the positive ∆Φ component associated with dielectric screening. This “compensation effect” can also rationalize the otherwise surprisingly small shifts in the ∆Φ-θK traces seen with increasing solvent doses at intermediate and high θK values (see especially Figure 2A,C). Other factors tending to enhance the Φ decreases is that solvation may well “lift” the potassium cations off the metal surface,23,24 along with engendering more complete K ionization. Comparisons between the ∆Φ-θK behavior for different solvents are also of central interest. Such plots are shown in Figure 3 for the case of limitingly high solvent dosages. While knowledge of the precise morphology of the ∆Φ-θK plots is limited by the uncertainties in ∆Φ (( 50 meV) for variable potassium dosages, some interesting rough trends are nonetheless evident. Perhaps surprisingly, the ∆ΦθK dependences for the various solvents are not especially different (other than ammonia, vide infra), although the trace for water is displaced toward higher Φ values due largely to the ca. 0.4-0.7 eV smaller solvent-induced Φ decrease seen in the absence of potassium (Figure 3). This finding suggests that the ability of the different solvent media to screen the double-layer charge does not vary greatly, the “effective dielectric constant” of the interfacial solvent varying by ca. 2-fold or less. A comparable insensitivity of the double-layer capacitance to the solvent at mercury electrodes is also evident from the literature,1 although solvent-dependent data at Pt(111) (or other solid surfaces) remain rare. Admittedly, larger ∆Φ-θK slopes (i.e., smaller Cd values) are seen here with acetone, reflecting the poorer solvating properties in comparison with methanol, and especially acetonitrile and water. Another interesting feature of Figure 3 is the increasing ∆Φ-θK slopes for a given solvent seen toward the highest θK values (i.e., largest negative electrode charges). In acetonitrile, a distinct slope minimum is also seen at intermediate charges. The former effect can be attributed (24) Bonzel, H. P.; Pirug, G.; Winkler, A. Chem. Phys. Lett. 1985, 116, 133.
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Figure 2. Plots of the work-function change, ∆Φ (referred to clean Pt(111)), as a function of the predosed potassium coverage θK (in monolayers, ML) for the various solvent dosages indicated. Extracted from data such as in Figure 1 (see text). Solvents in A-E as in Figure 1.
most simply to decreases in the efficiency of dielectric screening at higher double-layer charges because of greater solvent “dielectric saturation”. Such Cd decreases toward higher negative electrode charges are commonly seen for in-situ electrochemical interfaces (vide infra).1 While the experimental procedure of potassium dosing followed by solvent addition examined so far does enable the effects of progressive interfacial solvation to be followed in detail, it is also of interest to reverse the dosage order, that is, expose the surface to solvent followed by potassium addition. Representative results obtained by using the latter protocol are displayed for each solvent in Figure 4. The solvent dosages (about 3 EL in each case, see Figure caption) were chosen to correspond to the completion of the major ∆Φ-θs* responses seen in Figure 2. Comparison of the corresponding K “predosed” and “postdosed” ∆ΦθK plots in Figures 3 and 4, respectively, shows clearly that roughly 2-fold larger K-induced Φ decreases are seen in the latter case. Such larger -∆Φ values (and correspondingly smaller Cd values) can be understood simply (on the basis of Gauss’ law) from the larger surface-K+ separations occurring when the K atoms are “soft-landed” into the multilayer solvent films. These differences do highlight the admittedly nonequilibrium properties of these low-temperature “double-layers”, even though this limitation can be turned to advantage in altering the interfacial composition profile.13
K+/Solvent Adsorption on CO-Modified Pt(111). Given that the nature of the interfacial solvation, and hence the double-layer behavior, just described are undoubtedly influenced by metal surface-solvent interactions, it is of interest to examine corresponding data for Pt(111) with a saturated preadsorbed CO adlayer, so to deny direct access of the solvent to the metal surface. The choice of CO was also motivated in part by the opportunity to monitor changes in the local electrostatic field attending solvent and ion adsorption from alterations in the C-O infrared stretching frequency (the socalled Stark tuning effect), as exploited in some of our studies.7c,8,13 The three dotted traces in Figure 5 are examples of ∆Φ-θs* data for acetone addition on CO-precovered Pt(111), for a pair of θK values (0.056, 0.072 ML) as well as for θK ) 0. The three corresponding traces in the absence of CO (cf. Figure 1D) are also plotted (solid curves) for comparison in Figure 5. (Note that the y-intercept of the ∆Φ-θs* plot on CO-covered Pt(111) (trace 2 in Figure 5) falls essentially at ∆Φ ) 0, as in the absence of CO; this is because saturation CO adsorption on the K-free surface engenders an almost negligible net Φ change.) Comparison between each pair of the ∆Φ-θs* traces on CO-covered and unmodified Pt(111) reveals that the former yields smaller -∆Φ values for limitingly high solvent dosages (>4 EL), especially for θK ) 0. The latter difference is
Modeling Electrochemical Interfaces in UHV
Figure 3. Plots of the work-function change, ∆Φ, as a function of the predosed potassium coverage for limitingly high dosages of the various solvents indicated.
Figure 4. Plots of the work-function change, ∆Φ, as a function of the postdosed potassium coverage for the various solvents indicated. Key to solvent dosages: deuterated water, 3.2 EL; methanol, 3.2 EL; acetonitrile, 2.7 EL; acetone, 2.7 EL.
consistent with the expectation that CO adlayer blocks access of the solvent to the metal surface.7c The CO adlayer also alters the ∆Φ-θs* behavior in the presence of potassium (curves 4 and 6) in that monotonic solventinduced Φ decreases are observed rather than the peaked morphology seen in the absence of CO (curves 3, 5). This difference suggests that the CO adlayer modifies the K+ solvation-induced solvent orientation, although the CO itself clearly acts to screen the K+‚‚‚e- dipole charge, as seen from the larger Φ values for CO-saturated versus clean Pt(111) prior to solvent addition (Figure 5). Qualitatively similar effects of predosing CO on the ∆Φ-θs* responses are also observed for the other solvents examined here.13
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Figure 5. Changes in the work function, ∆Φ, induced by continuously increasing acetone dosages for Pt(111) precovered by the potassium coverages indicated and/or a saturated CO layer.
Figure 6. Plots of the work-function change, ∆Φ, as a function of the predosed potassium coverage in the presence of a saturated CO layer and limitingly high dosages of the various solvents indicated.
Figure 6 shows ∆Φ-θK plots in the presence of limitingly high dosages of each solvent on CO-saturated Pt(111), constructed from data such as in Figure 5. Comparison with the corresponding ∆Φ-θK data on unmodified Pt(111) (Figure 3) reveals several differences. The Cd value for solvent-free K+ dosing onto the saturated CO adlayer, 10.5 µF cm-2, is almost twice that on clean Pt(111), 6 µF cm-2 (i.e., the -∆Φ values induced in the former case are almost 2-fold smaller). This difference reflects the significant dielectric charge screening engendered by the CO adlayer. Indeed, the ∆Φ-θK slopes observed with the solvent-free CO adlayer are comparable to those obtained in the presence of methanol or acetone, the average Cd
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values being only 20-30% larger in the latter cases. These small effects induced by K+ solvation might suggest that the solvents are unable to screen the double-layer charge much more effectively than the nonpolar CO chemisorbate. However, an additional factor is the larger K+- surface separation likely to occur upon solvation, thereby tending to enhance the -∆Φ values (vide supra). Nevertheless, the more polar solvents water and acetonitrile engender noticeably milder θK-induced Φ decreases (i.e., yield larger average Cd values, around 20 µF cm-2) indicating a marked solvent-induced enhancement of the charge screening (Figure 6). Another overall difference between the ∆Φ-θK behavior on CO-covered and clean Pt(111) (Figures 6 and 3, respectively) is that the former traces are roughly linear with most solvents (i.e., the Cd values are roughly constant), rather than displaying the curved or even curvilinear responses seen for the latter double-layer environment. The former simpler behavior may well reflect the weaker electrostatic fields present within the solvent overlayer on CO-modified Pt(111). Special Case of Ammonia. As alluded to above, ammonia dosage yields distinctly unusual work-function responses. We briefly consider this behavior in comparative fashion; an earlier discussion is given in ref 9c. Inspection of Figure 1E shows that while the presence of predosed K+ lowers Φ considerably, the addition of more than about 2 EL of ammonia yields a Φ value, ca. 3.2 eV, which is essentially independent of the potassium as well as solvent dosage. Consequently, the corresponding ∆ΦθK traces in Figure 2E display virtually zero slopes; i.e., θK-independent Φ values for θs* values above 1-2 EL. At first sight, this behavior indicates that ammonia provides exceptionally efficient charge screening (i.e., a very high dielectric constant), so that K+ addition no longer exerts much influence on the surface potential. Some support for this notion might be gleaned from the nearflat ∆Φ-θK traces seen at intermediate θK values with the polar media water and acetonitrile at higher solvent dosages (Figure 2A,C), although substantial overall Φ decreases (0.5-1 eV) are observed in these cases by θK ∼ 0.07 ML. However, as outlined in ref 9c an alternative (we think more likely) explanation involves the additional formation of ammonia-solvated electrons, rather than interfacial electron transfer to Pt(111). Support for this explanation arises from the extremely low work functions, and hence surface potentials, Φ ∼ 3 eV, attained in the presence of multilayer ammonia. Such low “surface potentials”, Φ/e ) φ, are roughly comparable to the “reversible” electrode potential, φs, as deduced from electrochemical measurements for the equilibrium
e-m + solvent a e-s
(1)
where e-m is the electron residing in the metal Fermi level and e-s is the electron solvated in the ammonia phase, possibly paired with the K+ cation.9c Since eq 1 involves charge transfer across a substantial fraction of the potential drop within the metal-ammonia solvent boundary, as for any such “Nernstian” phenomena the position of equilibrium will be very sensitive to the surface potential when this falls in the vicinity of φs, lying increasingly to the right for decreasing φ values. When φ is lowered by ammonia adsorption close to φs or below, potassium dosage will therefore result in solvated electron formation rather than interfacial electron transfer to Pt(111). As a consequence, the surface potential and hence the work function will be “redox pinned” at values close to φs and therefore will not respond significantly to increasing θK, in harmony with the observations.
Also consistent with this picture is the ∆Φ-θK dependence observed with ammonia in the additional presence of a saturated CO adlayer (Figure 6). A significant ∆ΦθK response is observed, comparable to that seen for the similarly polar solvent water, contrasting the near-zero ∆Φ-θK slope obtained with ammonia in the absence of CO (Figure 3). This difference can be understood from the 0.5-1.2 eV higher work functions measured for K/ammonia adsorption in the presence of CO (compare Figure 6 with Figure 1E), so that K ionization involving interfacial electron transfer to form e-m should now be thermodynamically favored, as is the case for the other solvents. A related, yet distinct, approach to elucidating the distribution of charge in the K/NH3 layers on Pt(111) entails considering the ionization energetics of alkali metal-ammonia clusters, M(NH3)n. The gas-phase ionization energies, IE, of various Na- and Cs-ammonia clusters, with n ∼ 4-30, have been measured and fall in the range 2-3 eV, the IE values decreasing with increasing n.25 At least for larger n values, the clusters may be considered to consist of an alkali cation together with a nearby solvated electron.25b Providing these solvated assemblies possess similar thermodynamic properties to the present K-NH3 film, one would expect that ionization of such “K(NH3)n interfacial clusters”, involving electron transfer to Pt(111) and therefore formation of K(NH3)n+, would occur only at values of the surface work function that are larger than IE. Given that Φ for the Pt(111)/ K,NH3 interface is about 3 eV (vide supra), such electron transfer may well not occur, again suggesting (in similar fashion to above) that the interfacial potential is “redox pinned” by the presence of overall neutral K(NH3)n rather than charged K(NH3)n states. Interestingly, the occurrence of Φ values that are also essentially independent of the alkali metal coverage in the presence of multilayer solvent has been observed by Sass et al. for cesium coadsorption with water onto Cu(110).26 These authors interpreted their findings tentatively in terms of an unusually large dielectric constant of the interfacial water.26 However, the observations may also be rationalized on the basis of the ionization energetics of Cs(H2O)n clusters. The measured IE values for such gas-phase clusters have been measured; remarkably, they are independent of n for n > 3 (at least to n ) 21), and equal to 3.1 eV which is close to the “bulk-phase” electron affinity of water.25b (A Cs+-e-s pair is again surmised to be present for larger n values.25) Given that the work function for the Cu(110)/Cs,H2O interface of Sass et al. is estimated to be not much higher, 3.5 eV,26b it is again conceivable that Φ is sufficiently low so that net ionization of the Cs(H2O)n moieties (to yield Cs+ with surface electron transfer) will not occur, accounting for the observed insensitivity of Φ to the Cs coverage. Comparisons with Electrochemical and Metal Cluster Double-Layer Behavior. It is of obvious interest to compare more specifically the observed Φ-θK responses of these model UHV interfaces with the behavior of similar in-situ electrochemical systems. The relationship between UHV-based surface charge-potential (σφ) data, extracted from the θK-∆Φ measurements as noted above, and corresponding surface charge-electrode potential (σ-E) and corresponding capacitance-potential (Cd-E) data for electrochemical interfaces is in principle (25) (a) Hertel, I. V.; Hu¨glin, C.; Nitsch, C.; Schulz, C. P. Phys. Rev. Lett. 1991, 67, 1767. (b) Misaizu, F.; Tsukamoto, K.; Sanekata, M.; Fuke, K. Chem. Phys. Lett. 1992, 188, 241. (26) (a) Sass, J. K.; Schott, J.; Lackey, D. J. Electroanal. Chem. 1990, 283, 441. (b) Sass, J. K.; Lackey, D.; Schott, J. Electrochim. Acta 1991, 36, 1883.
Modeling Electrochemical Interfaces in UHV
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straightforward since27
E ) Φ/e - Ek
(2)
where e is the electronic charge and Ek is the so-called “absolute” potential of the reference electrode used for the electrochemical measurements. Since the present θKinduced ∆Φ responses necessarily involve negatively charged surfaces, the corresponding σ-E data refer to electrode potentials below the potential of zero charge, Epzc. Capacitance-electrode potential (Cd-E) data for Pt(111) and Pt(100) in acetonitrile containing 10 mM Na+ and tetraalkylammonium cations have recently been reported.28 The Epzc values for these systems are uncertain since electrolyte concentration-dependent data were not included. However, the Cd-E data appear to refer roughly to E ∼ Epzc.29 After correction for diffuse-layer effects, the Cd values, ca. 15 µF cm-2, are comparable to those obtained here for the “postdosed” K case (Figure 4), although somewhat smaller than with predosed K (Figure 2C). Another (admittedly cursory) examination along these lines concerns Cd-E measurements for CO-covered Pt(111) in acetonitrile, which yielded Cd ≈ 6 ((1) µF cm-2 over the potential range 0 to -1.0 V vs ferroceniumferrocene (Fc+/0),33 corresponding to E < Epzc.34 The corresponding Cd value extracted from the average ∆ΦθK slope for the CO-saturated Pt(111)-acetonitrile system in UHV (Figure 6) is ca. 15 µF cm-2. The smaller in-situ Cd value can be ascribed partly to more complete cation solvation, yielding an effectively thicker inner layer (cf. comments regarding Figure 4 above). Capacitancepotential data for aqueous electrolytes over a range of temperatures, including the frozen state, indicate instead that Cd decreases toward lower temperatures.37 (Note, however, that the formation of frozen, rather than liquidphase, electrolyte exerts no significant effect on the Cd-E behavior.37 This indicates that the use of lower temperatures, involving “frozen solvent” layers, to model doublelayer behavior is not necessarily invalid.) (27) (a) Trasatti, S. J. Electroanal. Chem. 1983, 150, 1; 1982, 137, 1. (b) Trasatti, S. Electrochim. Acta. 1983, 28, 1083. (c) Trasatti, S. Electrochim. Acta 1991, 36, 1659. (d) Trasatti, S. Surf. Sci. 1995, 335, 1. (28) Marinkovic, N. S.; Hecht, M.; Loring, J. S.; Fawcett, L. R. Electrochim. Acta 1996, 41, 641. (29) The ferrocenium-ferrocene (Fc+/0) reference electrode employed in ref 28 has a potential in acetonitrile that is about 0.6 V positive of the aqueous standard hydrogen electrode (SHE). The work function, Φ, of the Pt(111)-acetonitrile UHV (i.e., uncharged) interface is about 4.5 eV. Estimates of Ek in eq 1 vary from about 4.45 to 4.85 V.6a,30 Presuming, nevertheless, that Ek ∼ 4.6 eV for the SHE leads via eq 1 to an estimate of Epzc for the Pt(111)-acetonitrile interface of about -0.7 V vs Fc+/0. However, this Epzc value may well be too negative since the solventinduced -∆Φ values are probably smaller at ambient-temperature electrochemical interfaces than in the (low-temperature) UHV environment,7c,27c,d and Ek for the SHE may well be below 4.6 V.6a,30 (30) For example, see: Hansen, W. N.; Hansen, G. J. ACS Symp. Ser. 1988, No. 378, 166. (31) Frumkin, A. N.; Petrii, O. A.; Damaskin, B. B. In Comprehensive Treatise of Electrochemistry; Bockris, J. O’M., Conway, B. E., Yeager, E., Eds.; Plenum: New York, 1980; Chapter 5. (32) (a) Clavilier, J.; Albalat, R.; Go´mez, R.; Orts, J. M.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1992, 330, 489. (b) Climent, V.; Go´mez, R.; Orts, J. M.; Aldaz, A.; Feliu, J. M. Proc. Electrochem. Soc., in press. (33) Roth, J. D.; Lewis, G. J.; Safford, L. K.; Jiang, X.; Dahl, L. F.; Weaver, M. J. J. Am. Chem. Soc. 1992, 114, 6159. (34) Following the line of reasoning in footnote 29 and given that Φ for the CO-saturated Pt(111)-acetonitrile interface is about 5.2 eV,7c we deduce an estimate of Epzc around 0 V vs Fc+/0. A substantially higher value, 0.7 V vs Fc+/0, has been extracted from in-situ infrared spectroscopy involving a cation-dependent analysis of C-O stretching frequency-potential data.35 However, the former (UHV) value is probably too low (see footnote 29), even though the latter analysis is rather indirect.7c (35) From ref 36, quoted in ref 33.
Also worth mentioning in this context is the Epzc value for the Pt(111)-aqueous interface. Its experimental estimation is beset with complications, arising partly from the common occurrence of charge-transfer chemisorption to form adsorbed hydrogen as well as anion specific adsorption.31 Such considerations give rise to a distinction between Epzc values referring to the zero point of “total t f charge” versus “free charge”, Epzc and Epzc , respectively. The former quantity, which refers to the potential at which zero net charge is transferred to and from the metal surface including charge-transfer chemisorption, apparently differs from the latter parameter for Pt(111)-acidic aqueous interfaces primarily because of the occurrence of hydrogen chemisorption.31,32 t Estimates of Epzc for Pt(111) in aqueous perchloric acid electrolytes have been obtained recently by means of a “CO-charge displacement” strategy, involving measuring potential-dependent current transients induced by CO chemisorption.32 This procedure leads not only to a t Epzc value, around 0.3 V vs standard hydrogen electrode f (SHE) in 0.1 M HClO4, but also to an estimate of Epzc , about 0.07 V vs SHE.32b This latter quantity is the appropriate Epzc value to compare with UHV-based measurements, since water adsorption on Pt(111) yields f value no chemisorbed hydrogen. (Note that the Epzc would be the measured Epzc if no charge-transfer chemif sorption occurred.31) Interestingly, the above Epzc value is close to that obtained from the UHV data, 0.2 V vs SHE for K+-free Pt(111) covered with water, by means of eq 2. The latter value is extracted from the measured work function for water-modified Pt(111), 4.8 eV (see Figure 3), by taking Ek ∼ 4.6 V (cf. footnote 29). Aside from the key ability to alter the interfacial solvent as well as ionic composition, emphasized in the bulk of this paper, the “UHV double-layer modeling” approach enables charge-potential data to be extracted in more direct fashion than for electrochemical interfaces, given that Cd measurements and Epzc estimates are often problematical for the latter. Interestingly, this advantage is shared with some metal cluster solute systems, such as the high-nuclearity Pt carbonyl clusters described in ref 33. These systems, specifically [Pt24(CO)30]n, [Pt26(CO)32]n, and [Pt38(CO)44]n, can be charged reversibly to redox states, n, between 0 and -10, thereby effectively constituting “ionizable metal cluster” analogies of the negatively chargeable metal surfaces of concern here.33 (Indeed, the CO-covered Pt(111) microfacets present on these clusters render them close structural neighbors of the present COsaturated surface.33) Effective σ-E data for the clusters can be obtained directly from voltammetric redox potential data, yielding effective “molecular capacitance” values, Cdm, in addition to Epzc values, as outlined in ref 38. This analysis yielded an average Cdm value of about 15 µF cm-2 for the above clusters in several nonaqueous solvents, including acetonitrile, acetone, and methanol of interest here. This value is indeed comparable to (( 20%) the average Cd values extracted for the analogous COsaturated Pt(111)-nonaqueous interfaces in UHV (Figure 6). Admittedly, the higher Cdm values found for the clusters compared with the analogous in-situ electrochemical interface (vide supra) may be due partly to geometry-based electrostatic effects,38 but the concordance with the UHV-based data is still noteworthy. The Epzc value extracted for the clusters by extrapolated the σ-E data to σ ) 0 is about 0.5 ((0.3) V vs Fc+/0 in the nonaqueous solvents mentioned above.38 (Scatter in the (36) Chang, S.-C.; Jiang, X.; Roth, J. D.; Weaver, M. J. J. Phys. Chem. 1991, 95, 5378. (37) Borkowska, Z.; Stimming, U. In ref 6a, Chapter 8. (38) Weaver, M. J.; Gao, X. J. Phys. Chem. 1993, 97, 332.
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data precludes a delineation of Epzc values between the different solvents.) Converting this Epzc value, as before, to the vacuum scale given that Ek for the Fc+/0 reference electrode is roughly 5.0 V yields a corresponding Φ value of about 5.5 eV for the uncharged CO-saturated Pt(111) surface in these solvents. This value is indeed comparable to (within ca. 0.3 eV) the measured Φ values for COsaturated Pt(111) modified by the above nonaqueous solvents.7c Overall, then, the charge-potential properties of the present low-temperature UHV-based systems appear to provide, at least in some respects, a reasonable facsimile of related ambient-temperature chargeable interfaces, most directly the high-nuclearity metal clusters. As such, the overall conclusions of the above discussion regarding progressive double-layer solvation are deemed to be of direct electrochemical relevance. In particular, the deduction that solvation of the countercharge ions exerts a disproportionately large, if not dominant, influence on the charge-potential properties is believed to be significant. This notion bears a close resemblance to some recent observations of the effect of solvent addition upon the infrared spectral (C-O stretching, νCO) properties of CO adlayers on Pt(111) with varying K+ coverages.13 Specifically, only primary K+ solvation is required in order to
Weaver and Villegas
remove the νCO features associated with short-range K+CO interactions and generate the long-range electrostatic interactions characteristic of the well-known νCO frequency-Φ dependence (“Stark-tuning behavior”).13 However, complete solvation of the CO adlayer as well as the cation charges is necessary in order to reproduce the precise Stark-tuning behavior observed for the in-situ electrochemical systems, demonstrating (as above) the significant role of overall surface solvation in modifying the electrostatic situation.13 Consequently, such UHV electrochemical modeling tactics appear to prove useful for understanding multifaceted electrostatic aspects of double-layer phenomena. Indeed, this general theme is to be found in earlier related work by other groups.6 It would, however, clearly be interesting to pursue such systematic solvent dosage dependent charge-potential measurements for surfaces such as gold for which analogous in-situ capacitance data are more readily obtainable. Further studies along these lines are planned. Acknowledgment. This work is supported by the National Science Foundation. LA970820Y
