Modeling Electrochemical Interfaces in UltrahighVacuum: Influence of Progressive Cation and Surface

Solvation upon Charge-Potential Double-Layer Behavioron 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 thework-function changes,∆Φ, onPt(111) for continuously increasing solvent exposuresθs* and in the presence of various coverages of potassium, θK, in ultrahigh vacuum (UHV) at 90 K arereported with the objective of ascertaining how the surface charge-potential properties of such “UHVelectrochemical model” interfaces are altered by progressive solvation. The solventsswater, methanol,acetonitrile, acetone, and ammoniasspan a range of dipolar and other solvating properties and have beenutilized in related vibrational spectroscopic studies from this laboratory. Since potassium dosage yieldsinterfacial electron transfer to form K+ together with surface electronic charge, the corresponding ∆Φ-θKplots for various solvent dosages extracted from the above data provide surface charge-potential (σ-φ)curves for systematically varyingextents of interfacial solvation. In contrast to the large (1-3eV)monotonicsolvent-induced Φ decreases observed in the absence of ionic charge, the presence of predosed K+ yieldsinitial Φ increases, associated with cation solvation, followed by Φ decreases due primarily to the ensuingmetal surface solvation. Examination of the corresponding ∆Φ-θK traces obtained for these differentsolvent dosage regions shows that the basic charge-potential features characteristic of the solvated doublelayer require only ionic solvation, even though complete metal surface solvation modifies significantly theelectrostatic behavior. While surface solvation by the different species examined in the absence of chargeyield substantially dissimilar Φ values (i.e., differing “potentials of zero charge”), the charge-potentialcharacteristics are relatively insensitive to the solvent. This finding, comparable to that obtained forin-situ electrochemical interfaces, indicates that the effective “interfacial solvent dielectric constant” variesby 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+- surfaceseparations. Corresponding ∆Φ-θK data for CO-saturated Pt(111) indicates that the CO adlayer playsa role in dielectric screening. Effectively θK-independent ∆Φ responses were obtained with ammonia-solvated Pt(111), however, suggestive of the formation of solvated electrons. Specific comparisons aremade between the UHV-based charge-potential behavior with that for in-situ electrochemical interfacesand for ionizablehigh-nuclearityPt carbonyl clusters innonaqueousmedia. The latter systems, inparticular,exhibit closely similar surface charge-potential characteristics to the corresponding UHV-based Pt(111)interfaces.

IntroductionAn issue of obvious fundamental importance in elec-

trochemistry concerns the manner by which the double-layer electrostatic properties are controlled by interfacialsolvation. Traditional models of the double layer em-phasize the importance of the inner layer of solvent (i.e.,the layer juxtaposed to the metal surface), especiallypotential-dependentdipole orientation, indetermining theelectronic charge-potential (and hence capacitance) char-acteristics.1 More recently, the likely important role ofthemetal electronic properties has also received detailed,albeit belated, attention.2 The last few years have seenburgeoning interest in employingmoleculardynamicsandother simulation techniques to obtain a more coherentmicroscopic-level picture of the interfacial solvent struc-ture,3 in some cases including ionic solvation. Takentogether, thesedevelopments lead toapictureof thedouble

layer where the role of the solvent is distinctly moremultifaceted than is the case in traditional models.4Nonetheless, there remains a paucity of experimentalmolecular-level information regarding double-layer sol-vation.We have recently been pursuing measurements involv-

ing ordered metal surfaces in ultrahigh vacuum (UHV),with one objective being the experimental elucidation ofinterfacial solvation effects of relevance to electro-chemistry.7-13 The general tactics, involving the sequen-tial dosing onto the metal-vacuum interfaces of various

† Present address: Department ofChemistry,University ofNewMexico, Albuquerque, NM 87131.

X Abstract published in Advance ACS Abstracts, November 15,1997.

(1) For reviews, see for example: (a)Reeves,R.M. InModernAspectsof Electrochemistry; Conway, B. E., Bockris, J. O’M., Eds.; Plenum:New York, 1974; p 239. (b) Trasatti, S. In Modern Aspects ofElectrochemistry; Conway, B. E., Bockris, J. O’M., Eds.; Plenum: NewYork, 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.

(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.ACSSymp. Ser. 1988,No.378, 54. (b) Sass, J. K.; Bange, K.; Dohl, Piltz, E.; Unwin, R. Ber.Bunsenges Phys. Chem. 1984, 88, 354.

(6) For recent erudite reviews, see: (a) Wagner, F. T. In Structureof Electrified Interfaces; Lipkowski, J., Ross, P. N., Eds.; VCH Publish-ers: 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.

6836 Langmuir 1997, 13, 6836-6844

S0743-7463(97)00820-2 CCC: $14.00 © 1997 American Chemical Society

double-layer components, including solvent, owemuch tothe pioneering efforts of Sass and co-workers in Berlinduring the last decade;5 theyare often referred to as “UHVelectrochemical modeling”.6 In particular, we have em-ployed infraredreflection-absorptionspectroscopy (IRAS)along with work-function measurements. The formertechnique affords detailed information on bonding andintermolecular interactions and also provides a link toin-situ electrochemical systems since IRAS is also ap-plicable (albeitwithmorerestrictions) in such liquid-phaseenvironments. While less frequently discussed in thesurface-science literature, responses of thework function(Φ) to progressive alterations in the surface compositioncan provide unique insight into the interfacial potentialprofile and related electrostatic issues, of central impor-tance in electrochemistry. In the context of “UHVelectrochemical modeling”, examining Φ responses tocontrolled additions of solvent and ions (or ionizablespecies) can indeed constitute a powerful means ofexploring double-layer interactions.5One aspect of these activities entails combined IRAS/

work-function studies of potassium cation solvation onPt(111).8-10 The solvents examinedswater, methanol,acetonitrile, acetone, and ammoniaswere selected partlyin view of their varying dipolar, vibrational, and overallsolvating properties. The choice of potassium wasprompted by its nonspecifically adsorbing properties inelectrochemistry, together with the availability of anaccurate coverage assay from the large Φ decreasesinduced on Pt(111) by K adsorption.14 The vibrationalspectra provide insight into the microscopic nature ofcation solvation, and in some cases can delineate betweencation-solvent and surface-solvent interactions.In thepresent communication,weexaminemore closely

work function-surface compositiondata for thesesystems,involving systematic variations in the solvent as well ascationcoverages,with theparticularaimofgaining insightinto the separate (or possibly separable) electrostaticconsequences of double-layer ionic and surface solvation.Since adsorbed K ionizes to form (essentially) K+ cationsalong with negative electronic charging of the Pt(111)surface, examiningrelationshipsbetweentheΦ (andhencesurface potential) changes induced by K adsorption forvarious solvent dosages can yield insight into the roles ofsolvation in surface charge-potential (i.e., double-layer)properties. Aswell asK+/solvent coadsorption onto cleanPt(111), we discuss more briefly corresponding work-function behavior in the presence of predosed saturatedCO adlayers, this chemisorbate being chosen partly toeliminate access of the solvent to the metal surface (cf.refs 7c and 13). While portions of these data have beenreported previously in conjunction with IRAS mea-surements,7-10 it is felt that the present comparative (andmore comprehensive) examination of the solvent-depend-ent work function behavior would be beneficial. Theresults delineate the manner in which increasing thesolvent coverages fromsubmonolayer tomultilayer levelstransform progressively the electrostatic characteristicsof primary ionic and metal surface solvation into thefamiliar charge-potential properties of electrochemicalinterfaces.

Experimental SectionExperimental details have been described in several earlier

reports.7-9 Measurements were made in a stainless-steelchamber maintained at a base pressure of 5 × 10-10 Torr. The

Pt(111) surfacewasmaintained clean and ordered (as evidencedby Auger electron spectroscopy and low-energy electron diffrac-tion (LEED)) by sputtering with 1 kV Ar+ followed by annealingto 1200K. Carbon surface impurities were removed before eachset of experiments by exposure to 3× 10-7 Torr O2 at 900 K. Thesurface was held at 90 K by liquid N2 cooling during themeasurements described here. Changes in the work function(∆Φ) with respect to cleanPt(111)weremeasuredusing aKelvinprobe (Delta Phi Elektronik). The ∆Φ values measured duringcontinuousexposure to solventareaccurate to10meV. However,for variable potassiumdosing the∆Φ values quoted are accurateto only(50meV since repositioning of the surface in front of theKelvin probe was usually necessary.As described earlier,8a the potassium was dosed from a

commercial thermal getter source (SAES). Determination of therequired potassium coverages (θK) relied upon the linear∆Φ-θKbehavior along with the θK calibration with LEED reportedearlier.14 Saturated CO adlayers were produced by exposure ofclean Pt(111) to 12 langmuirs of the gas (Airco) at 2× 10-8 Torr.Water (D2O, Aldrich), methanol (Fisher), acetonitrile (Mallinck-rodt), and acetone (Sigma-Aldrich) vapors were obtained fromthe pure liquids contained in a UHV-compatible glass ampuleconnected to the gas-dosingmanifold. Gaseous impurities wereremoved fromthe liquidsbymeansof repeated freezing,pumping,and thawing cycles. Ammonia gas (Matheson) was dosed asreceived. Line-of-sight exposure throughanozzle located in frontof the sample enabled stepwise increases in the relative solventcoverageswhile the background pressurewasmaintained below1 × 10-10 Torr. Continuous monitoring of the solvent-inducedΦ changes required instead exposure to 1 × 10-8 Torr of thecorresponding solvent. As usual in our studies of interfacialsolvation,8-10 the reported solvent-exposure values were normal-ized with respect to that required to complete the first “chemi-sorbed” layer on clean Pt(111), termed a unit “equivalentmonolayer” (EL), as deduced from the separate temperature-programmed desorption (TPD) peak discerned for each solventunder these conditions.7 As before, we will therefore refer tothese solvent coverageshere inELunits (θs* values), the asteriskdenoting a distinction from coverages referenced instead to themetal surface atomic density (1.5×1015 atoms cm-2) rather thana “close-packed” solvent monolayer. (The latter coverage scale,however, can be used for CO (θCO) and potassium (θK), thesevalues thereby being referred to here in monolayer (ML) units.)The single solvent exception is water, with coverages (θW) thatcan also be given in ML units since the water “bilayer” (1 EL),as identified fromTPDmeasurements,8a is known to correspondtoaθWvalueof 2/3ML. (Weemployed the sameθWscale earlier.8a)Calibration of the solvent dosages in EL units also relied moregenerally upon TPD measurements which, importantly, alsoindicated that the solvent coverage increases essentially linearlywith exposure even in the presence of predosed K and/or CO. Inthe case of ammonia, however, the dosage calibration reliedprimarily upon work-function changes induced on clean Pt(111)as reported by Fisher15 since TPD detection of the chemisorbedlayer ishamperedby thebroadnessof thedesorption temperaturerange and the limit of detection of our instrument (see ref 9c fordetails).

Results and Discussion

K+/Solvent Adsorption on Clean Pt(111). Theprimary experimental tactics employed here involvemonitoring the changes inwork function,∆Φ (eV), broughtabout by increasing solvent dosages, θs*, onto Pt(111)modified by various potassium coverages, θK. Since thepotassium atoms should be largely ionized to formadsorbed cations,18 at least when surrounded by polarsolvent molecules, increasing θK acts to charge the metal

(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.

(15) Fisher, G. B. Chem. Phys. Lett. 1981, 79, 452.(16) Bonzel, H. P.; Pirug, G.; Muller, 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.; Muller, J. E. Phys.

Rev. Lett. 1987, 58, 2138. (b) Muller, J. E. In Physics and Chemistry ofAlkali Metal Adsorption; Bonzel, H. P., Bradshaw, A. M., Ertl, G., Eds.Elsevier: Amsterdam; 1989; p 271.

Modeling Electrochemical Interfaces in UHV Langmuir, Vol. 13, No. 25, 1997 6837

surface negative, K+ thereby effectively constituting theionic double-layer countercharge. Consequently, theresulting Φ-θK plots are analogous to the electrodepotential-charge (E-σ) data familiar in electrochemistry,referring to negative electrode charges σ and therefore Evalues increasingly below the so-called potential of zerocharge, Epzc (vide infra).Parts A-E of Figure 1 show ∆Φ-θs* data for water,

methanol, acetonitrile, acetone, and ammonia, respec-tively, for several potassium coverages as indicated. Therange of θK values selected, e0.08 ML, encompasses thetypical span of negative electrode charges encountered inelectrochemistry, given that a θK value of 0.1 correspondsto a charge density of 24 µC cm-2. (In addition, the onsetof K-induced water dissociation on Pt(111) occurs only athigher θK values, >0.1 ML.14,16) While only ∆Φ valuesare obtainable by means of the Kelvin probe, referencedto the clean surface, these quantities can readily beconverted into absolute work functions, Φ, given the Φvalue for clean Pt(111), is ca. 5.9 eV.17Several facets of the data in Figure 1 provide insight

into the roles of solvation in determining double-layerproperties. Since the last solvent, ammonia, displaysrather unusual ∆Φ-θs* characteristics (Figure 1E), weconsider first chiefly the behavior with the other foursolvents. Dosing each solvent onto clean Pt(111) yieldslarge (1-3 eV) work-function decreases. While most of

these Φ changes occur within the first 2-3 equivalentlayers (EL), strictly θs*-independent ∆Φ values are notattained for most solvents until high solvent dosages, θs*∼ 4 to 6 EL, are reached (Figure 1A-D). This suggeststhat propagation of solvent “structuring”, presumablyinvolving net dipole orientation, extends to several layersfrom the metal surface. As discussed earlier,7c however,thebulk of these substantial solvent-inducedΦdecreases,at least in the first monolayer or so, probably arise fromfactors other than solvent-dipole orientation. Most likely,an important contributor to the large -∆Φ values is anadsorbate-induced redistribution of charge in the metalsurface,7c,19 involvinga suppression of the surface electron“spillover” which is partly responsible for the high workfunctions of clean Pt(111) and other transition metals.7c

Markedly different ∆Φ-θs* responses are seen, how-ever, when the surface is predosed with potassium,especially for higher θK values (g0.04 ML). Distinctlynon-monotonic ∆Φ-θs* traces are uniformly obtainedunder these conditions, the work function initially in-creasingup toθs*∼1 to2ELbeforedecreasing inaparallelfashion as for solvent dosing onto clean Pt(111) (Figure1). The initial Φ increases, which are largest for water

(19) (a) Baetzold, R. C. J. Phys. Chem. 1983, 87, 3858. (b) Shustor-ovich, E. J. Phys. Chem. 1982, 86, 114. (c) Baetzold, R. C.; Apai, G.;Stustorovich, E. Appl. Surf. Sci. 1984, 19, 135.

Figure 1. Changes in the Pt(111) work function,∆Φ, induced by continuously increasing solvent dosages, expressed in “equivalentmonolayers” EL (see text) for the predosed potassium coverages indicated, for (A) deuterated water, (B) methanol, (C) acetonitrile,(D) acetone, and (E) ammonia.

6838 Langmuir, Vol. 13, No. 25, 1997 Weaver and Villegas

and methanol, have been shown by vibrational spectros-copy to be associated with cation-induced solvent reori-entation, featuring K+-oxygen coordination along withsurface-hydrogen bonding interactions for these twosolvents.10a,11a,20 (Indeed, the degree of cation-inducedwater reorientation has been estimated from the initialΦ changes observed upon solvent dosing.5a, 23) Moregenerally (or equivalently), these Φ increases can beconsidered to arise from solvent “dielectric screening” ofthe K+‚‚‚e- “surface dipole”, which is largely responsiblefor the very marked (up to 3 eV) K-induced Φ decreasesseen on clean Pt(111). The notable attenuation of theseΦ decreases brought about by water addition (Figure 1A)reflects the particularly efficient dielectric screeningcharacteristic of this solvent.Examining the overallmorphology of the∆Φ-θs* traces

(Figure 1) suggests that the twoaforementioned effectss-(a) theΦ decreases due to metal surface solvation and (b)the Φ increases accompanying cation solvationsact in acombined, yet partly separable fashion to yield the non-monotonic overall∆Φ-θs* behavior. (Although these twointerfacial solvationcontributionsshouldnotbe completelyseparable given that the cations are also “adsorbed”, onecan imagine type (a) solvationas involving solvent adsorp-tion at sites not in immediate proximity to the K cations.)The initial stages of solvent addition should resultpreferentially, if not exclusively, in cation solvation (typeb): this expectation, made on energetic grounds as wellas from theΦ increases noted above, is supported both byIRAS and temperature-programmed desorption (TPD)data.8a,9a,b The progressive development of surface sol-vation (type a), which may well occur initially in parallelwith (b), becomes predominant following the completionof the primary (and possibly also secondary) cationsolvation shells, as evidencedby the roughlyparallel∆Φ-θs* traces in the presence and absence of potassium seenfor θs*g 1-2EL (Figure 1A-D). (Note that deducing thesolvent/K+ stoichiometries corresponding to these solventdosage regions is hampered by a lack of a reliableconversion between theELandMLscales. Nevertheless,for water 1 EL ) 2/3 ML,8a and for methanol 1 EL ≈ 0.35ML,9a so that the onset of parallel ∆Φ-θs* traces fordifferent θK values clearly corresponds to solvent dosageswell beyond that required for complete primary cationsolvation.) However, regardless of such details these twosolvation modes (a) and (b) acting in concert apparentlylead to the markedly smaller vertical displacementsobserved between each trace (i.e., for varying predosed θKvalues at fixed θs*) for solvent dosages θs*g 2 EL (Figure1).Scrutinizingsuch∆Φ-θK traces foraseriesof increasing

solvent dosages lends complementary insight into theeffects of progressive solvation ondouble-layer properties.The ∆Φ-θK plots derived in this fashion for a series ofincreasing fixed θs* values (as indicated) for each solventare shown in Figure 2. Also included in each case is the∆Φ-θK trace for the solvent-free (θs* ) 0) limit (opencircles). It is important to recognize that the∆Φ-θKslopesare inverselyproportional to thewell-known“double-layercapacitance”Cd. Thus by converting theK coverages intoeffective electronic charge densities by again presuming

complete ionization, the linear ∆Φ-θK trace observed onsolvent-free Pt(111) is deduced to have a slope corre-sponding to Cd ) 6.0 µF cm-2. (This is deduced by notingthat the Pt(111) atomic density is 1.5 × 1015 atom cm-2,so that a K+ (and hence e-) “monolayer” would have acharge density, σ, of 240 µC cm-2.) The presence of evenlow solvent dosages, however (θs* ∼ 0.5 EL), is seen toyield marked (2-3-fold) increases in Cd (Figure 2A-D).Indeed, the essential morphology of the∆Φ-θK curves inthe presence of solvent is largely developed in each caseby θs* ) 1 EL, even though solvent dosages of 4 EL or soare required to develop fully the traces corresponding tothe “high-dosage” limit (Figure 2A-D). This findingindicates that the double-layer capacitive properties arecontrolled toadisproportionately large extentby solvationin the vicinity of the ionic countercharges, although asalready noted solvation of the entire Pt(111) surface by1-2 equivalent monolayers is required to modify com-pletely the electrostatic situation.It is interesting to note that theΦ values attained even

in the high-θs* limit for the largest potassium coverages(θK ∼ 0.075) are not much higher than, or are evencomparable to, those in the complete absence of solvent,with theexceptionofwater. At first sight, this observationmight suggest that the extent of solvent dielectric screen-ingof the ionic/electronic charges is rathermild. However,this argument does not take into account the large (1-1.7eV)Φdecreases inducedby solvent adsorption onPt(111),whichaccording to theabovediscussion isprobablypresentto a large extent even on the θK-containing surface.Consequently, theseΦdecreases tend to offset thepositive∆Φ component associatedwith dielectric screening. This“compensation effect” can also rationalize the otherwisesurprisingly small shifts in the ∆Φ-θK traces seen withincreasing solvent doses at intermediate and high θKvalues (see especiallyFigure2A,C). Other factors tendingto enhance the Φ decreases is that solvation may well“lift” thepotassiumcations off themetal surface,23,24 alongwith engendering more complete K ionization.Comparisonsbetween the∆Φ-θKbehavior for different

solvents are also of central interest. Suchplots are shownin Figure 3 for the case of limitingly high solvent dosages.While knowledge of the precisemorphology of the∆Φ-θKplots is limited by the uncertainties in∆Φ (( 50meV) forvariablepotassiumdosages, some interestingroughtrendsare nonetheless evident. Perhaps surprisingly, the∆Φ-θK dependences for the various solvents are not especiallydifferent (other than ammonia, vide infra), although thetrace for water is displaced toward higher Φ values duelargely to the ca. 0.4-0.7 eV smaller solvent-induced Φdecrease seen in the absence of potassium (Figure 3). Thisfinding suggests that the ability of the different solventmedia to screen the double-layer charge does not varygreatly, the “effectivedielectric constant” of the interfacialsolvent varying by ca. 2-fold or less. A comparableinsensitivity of thedouble-layer capacitance to the solventatmercury electrodes is also evident from the literature,1although solvent-dependentdataatPt(111) (or other solidsurfaces) remain rare. Admittedly, larger ∆Φ-θK slopes(i.e., smaller Cd values) are seen here with acetone,reflecting the poorer solvating properties in comparisonwith methanol, and especially acetonitrile and water.Another interesting feature ofFigure3 is the increasing

∆Φ-θK slopes for a given solvent seen toward the highestθK values (i.e., largest negative electrode charges). Inacetonitrile, a distinct slope minimum is also seen atintermediate charges. The former effect canbeattributed

(20) Comparable findings have also been obtained by means ofelectron 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.

(24) Bonzel, H. P.; Pirug, G.; Winkler, A. Chem. Phys. Lett. 1985,116, 133.

Modeling Electrochemical Interfaces in UHV Langmuir, Vol. 13, No. 25, 1997 6839

most simply to decreases in the efficiency of dielectricscreening at higher double-layer charges because ofgreater solvent “dielectric saturation”. SuchCd decreasestoward higher negative electrode charges are commonlyseen for in-situ electrochemical interfaces (vide infra).1While the experimental procedure of potassium dosing

followed by solvent addition examined so far does enabletheeffectsofprogressive interfacial solvation tobe followedin detail, it is also of interest to reverse the dosage order,that is, expose the surface to solvent followedbypotassiumaddition. Representative results obtained by using thelatter protocol are displayed for each solvent in Figure 4.The solvent dosages (about 3 EL in each case, see Figurecaption) were chosen to correspond to the completion ofthemajor∆Φ-θs* responsesseen inFigure2. Comparisonof the corresponding K “predosed” and “postdosed” ∆Φ-θK plots in Figures 3 and 4, respectively, shows clearlythat roughly 2-fold largerK-inducedΦdecreases are seenin the latter case. Such larger -∆Φ values (and cor-respondingly smallerCd values) canbeunderstood simply(on the basis of Gauss’ law) from the larger surface-K+

separations occurringwhen theKatomsare “soft-landed”into the multilayer solvent films. These differences dohighlight the admittedly nonequilibrium properties ofthese low-temperature “double-layers”, even though thislimitation can be turned to advantage in altering theinterfacial composition profile.13

K+/Solvent Adsorption on CO-Modified Pt(111).Given that the nature of the interfacial solvation, andhence the double-layer behavior, just described areundoubtedly influenced by metal surface-solvent inter-actions, it is of interest to examine corresponding datafor Pt(111) with a saturated preadsorbed CO adlayer, soto deny direct access of the solvent to the metal surface.The choice of CO was also motivated in part by theopportunity to monitor changes in the local electrostaticfield attending solvent and ion adsorption from altera-tions in the C-O infrared stretching frequency (the so-called Stark tuning effect), as exploited in some of ourstudies.7c,8,13The 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 asfor θK ) 0. The three corresponding traces in the absenceof CO (cf. Figure 1D) are also plotted (solid curves) forcomparison 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; thisis because saturationCOadsorption on theK-free surfaceengenders an almost negligible net Φ change.) Compari-sonbetweeneachpair of the∆Φ-θs* traces onCO-coveredand unmodified Pt(111) reveals that the former yieldssmaller -∆Φ values for limitingly high solvent dosages(>4 EL), especially for θK ) 0. The latter difference is

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-Eas in Figure 1.

6840 Langmuir, Vol. 13, No. 25, 1997 Weaver and Villegas

consistent with the expectation that CO adlayer blocksaccessof thesolvent to themetal surface.7c TheCOadlayeralso alters the ∆Φ-θs* behavior in the presence ofpotassium (curves 4 and 6) in that monotonic solvent-inducedΦ decreases are observed rather than the peakedmorphology seen in the absence of CO (curves 3, 5). Thisdifference suggests that the CO adlayer modifies the K+

solvation-induced solvent orientation, although the COitself clearly acts to screen the K+‚‚‚e- dipole charge, asseen from the larger Φ values for CO-saturated versusclean Pt(111) prior to solvent addition (Figure 5). Quali-tatively similar effects of predosing CO on the ∆Φ-θs*responses are also observed for the other solvents exam-ined here.13

Figure6shows∆Φ-θKplots in thepresenceof limitinglyhigh dosages of each solvent on CO-saturated Pt(111),constructed from data such as in Figure 5. Comparisonwith the corresponding ∆Φ-θK data on unmodified Pt-(111) (Figure 3) reveals several differences. TheCd valuefor solvent-free K+ dosing onto the saturated CO adlayer,10.5 µF cm-2, is almost twice that on clean Pt(111), 6 µFcm-2 (i.e., the -∆Φ values induced in the former case arealmost 2-fold smaller). This difference reflects the sig-nificant dielectric charge screening engenderedby theCOadlayer. Indeed, the ∆Φ-θK slopes observed with thesolvent-freeCO adlayer are comparable to those obtainedin the presence of methanol or acetone, the average Cd

Figure3. Plots of thework-function change,∆Φ, as a functionof the predosed potassium coverage for limitingly high dosagesof the various solvents indicated.

Figure4. Plots of thework-function change,∆Φ, as a functionof the postdosed potassium coverage for the various solventsindicated. Key to solvent dosages: deuterated water, 3.2 EL;methanol, 3.2 EL; acetonitrile, 2.7 EL; acetone, 2.7 EL.

Figure 5. Changes in the work function, ∆Φ, induced bycontinuously increasingacetonedosages forPt(111) precoveredby the potassium coverages indicated and/or a saturated COlayer.

Figure6. Plots of thework-function change,∆Φ, as a functionof the predosed potassium coverage in the presence of asaturated CO layer and limitingly high dosages of the varioussolvents indicated.

Modeling Electrochemical Interfaces in UHV Langmuir, Vol. 13, No. 25, 1997 6841

values being only 20-30% larger in the latter cases. Thesesmall effects induced by K+ solvation might suggest thatthe solvents are unable to screen the double-layer chargemuchmoreeffectively thanthenonpolarCOchemisorbate.However, an additional factor is the larger K+- surfaceseparation likely to occur upon solvation, thereby tendingto enhance the -∆Φ values (vide supra). Nevertheless,the more polar solvents water and acetonitrile engendernoticeablymilderθK-inducedΦdecreases (i.e., yield largeraverageCd values, around20µFcm-2) indicatingamarkedsolvent-induced enhancement of the charge screening(Figure 6).Another overall differencebetween the∆Φ-θKbehavior

on CO-covered and clean Pt(111) (Figures 6 and 3,respectively) is that the former traces are roughly linearwith most solvents (i.e., the Cd values are roughlyconstant), rather than displaying the curved or evencurvilinear responses seen for the latter double-layerenvironment. The former simpler behavior may wellreflect the weaker electrostatic fields present within thesolvent overlayer on CO-modified Pt(111).Special Case of Ammonia. As alluded to above,

ammonia dosage yields distinctly unusual work-functionresponses. We briefly consider this behavior in compara-tive fashion; an earlier discussion is given in ref 9c.Inspection of Figure 1E shows that while the presence ofpredosed K+ lowers Φ considerably, the addition of morethan about 2 EL of ammonia yields a Φ value, ca. 3.2 eV,which is essentially independent of the potassium as wellas solvent dosage. Consequently, the corresponding∆Φ-θK 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., averyhighdielectric constant), so thatK+additionno longerexerts much influence on the surface potential. Somesupport for this notion might be gleaned from the near-flat ∆Φ-θK traces seen at intermediate θK values withthe polar media water and acetonitrile at higher solventdosages (Figure 2A,C), although substantial overall Φdecreases (0.5-1 eV) are observed in these cases by θK ∼0.07 ML. However, as outlined in ref 9c an alternative(we thinkmore likely) explanation involves theadditionalformation of ammonia-solvated electrons, rather thaninterfacial electron transfer to Pt(111). Support for thisexplanationarises fromthe extremely lowwork functions,and hence surface potentials, Φ ∼ 3 eV, attained in thepresence of multilayer ammonia. Such low “surfacepotentials”, Φ/e ) φ, are roughly comparable to the“reversible” electrode potential, φs, as deduced fromelectrochemical measurements for the equilibrium

where e-m is the electron residing in themetal Fermi level

and e-s is the electron solvated in the ammonia phase,

possibly paired with the K+ cation.9c Since eq 1 involvescharge transfer across a substantial fraction of thepotential drop within the metal-ammonia solvent bound-ary, as for any such “Nernstian” phenomena the positionof equilibriumwill beverysensitive to thesurfacepotentialwhen this falls in the vicinity of φs, lying increasingly tothe right for decreasing φ values. When φ is lowered byammoniaadsorption close toφs or below,potassiumdosagewill therefore result in solvated electron formation ratherthan interfacial electron transfer to Pt(111). As aconsequence, the surface potential and hence the workfunction will be “redox pinned” at values close to φs andtherefore will not respond significantly to increasing θK,in harmony with the observations.

Also consistent with this picture is the ∆Φ-θK depen-dence observed with ammonia in the additional presenceof a saturated CO adlayer (Figure 6). A significant ∆Φ-θK response is observed, comparable to that seen for thesimilarly polar solvent water, contrasting the near-zero∆Φ-θK slope obtained with ammonia in the absence ofCO (Figure 3). This difference can be understood fromthe 0.5-1.2 eV higher work functions measured forK/ammonia adsorption in the presence of CO (compareFigure 6 with Figure 1E), so that K ionization involvinginterfacial electron transfer to form e-

m should now bethermodynamically favored, as is the case for the othersolvents.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 alkalimetal-ammonia clusters, M(NH3)n. The gas-phase ion-ization energies, IE, of various Na- and Cs-ammoniaclusters, with n ∼ 4-30, have been measured and fall intherange2-3eV, the IEvaluesdecreasingwith increasingn.25 At least for larger n values, the clusters may beconsidered to consist of an alkali cation together with anearby solvated electron.25b Providing these solvatedassemblies possess similar thermodynamic properties tothe presentK-NH3 film, onewould expect that ionizationof such “K(NH3)n interfacial clusters”, involving electrontransfer to Pt(111) and therefore formation of K(NH3)n+,would occur only at values of the surface work functionthat are larger than IE. Given that Φ for the Pt(111)/K,NH3 interface is about 3 eV (vide supra), such electrontransfer may well not occur, again suggesting (in similarfashion to above) that the interfacial potential is “redoxpinned” by the presence of overall neutralK(NH3)n ratherthan charged K(NH3)n states.Interestingly, the occurrence of Φ values that are also

essentially independent of the alkali metal coverage inthe presence of multilayer solvent has been observed bySass et al. for cesium coadsorption with water onto Cu-(110).26 These authors interpreted their findings tenta-tively in terms of an unusually large dielectric constantof the interfacialwater.26 However, the observationsmayalsobe rationalizedon thebasis of the ionizationenergeticsof Cs(H2O)n clusters. The measured IE values for suchgas-phase clustershavebeenmeasured; remarkably, theyare independent of n for n > 3 (at least to n ) 21), andequal to 3.1 eVwhich is close to the “bulk-phase” electronaffinity of water.25b (A Cs+-e-

s pair is again surmised tobe present for larger n values.25) Given that the workfunction for the Cu(110)/Cs,H2O interface of Sass et al. isestimated to be not much higher, 3.5 eV,26b it is againconceivable thatΦ is sufficiently lowso thatnet ionizationof theCs(H2O)nmoieties (to yieldCs+with surface electrontransfer) will not occur, accounting for the observedinsensitivity of Φ to the Cs coverage.Comparisons with Electrochemical and Metal

Cluster Double-Layer Behavior. It is of obviousinterest to compare more specifically the observed Φ-θKresponsesof thesemodelUHVinterfaceswith thebehaviorof similar in-situ electrochemical systems. The relation-ship between UHV-based surface charge-potential (σ-φ) data, extracted fromtheθK-∆Φmeasurementsasnotedabove, and corresponding surface charge-electrode po-tential (σ-E) and corresponding capacitance-potential(Cd-E) data for electrochemical interfaces is in principle

(25) (a) Hertel, I. V.; Huglin, 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.

e-m + solvent a e-

s (1)

6842 Langmuir, Vol. 13, No. 25, 1997 Weaver and Villegas

straightforward since27

where e is the electronic charge and Ek is the so-called“absolute” potential of the reference electrode used forthe electrochemicalmeasurements. Since thepresent θK-induced ∆Φ responses necessarily involve negativelycharged surfaces, the corresponding σ-E data refer toelectrode 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 beenreported.28 TheEpzc values for these systemsareuncertainsince electrolyte concentration-dependent data were notincluded. However, theCd-Edataappear to refer roughlytoE∼Epzc.29 After correction for diffuse-layer effects, theCd values, ca. 15µFcm-2, are comparable to those obtainedhere for the “postdosed” K case (Figure 4), althoughsomewhat 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 ferrocenium-ferrocene (Fc+/0),33 corresponding to E < Epzc.34 Thecorresponding Cd value extracted from the average ∆Φ-θK slope for theCO-saturatedPt(111)-acetonitrile systemin UHV (Figure 6) is ca. 15 µF cm-2. The smaller in-situCd value can be ascribed partly to more complete cationsolvation, yielding an effectively thicker inner layer (cf.comments regarding Figure 4 above). Capacitance-potential data for aqueous electrolytes over a range oftemperatures, including the frozen state, indicate insteadthat Cd decreases toward lower temperatures.37 (Note,however, that the formation of frozen, rather than liquid-phase, electrolyte exerts no significant effect on theCd-Ebehavior.37 This indicates that the use of lower temper-atures, involving “frozen solvent” layers, tomodel double-layer behavior is not necessarily invalid.)

Also worth mentioning in this context is the Epzc valuefor the Pt(111)-aqueous interface. Its experimentalestimation is besetwith complications, arisingpartly fromthe common occurrence of charge-transfer chemisorptionto form adsorbed hydrogen as well as anion specificadsorption.31 Suchconsiderationsgive rise toadistinctionbetween Epzc values referring to the zero point of “totalcharge” versus “free charge”, Epzc

t and Epzcf , respectively.

The formerquantity,which refers to thepotential atwhichzeronet charge is transferred toand fromthemetal surfaceincluding charge-transfer chemisorption, apparently dif-fers from the latter parameter forPt(111)-acidic aqueousinterfacesprimarily because of the occurrence ofhydrogenchemisorption.31,32

Estimates ofEpzct for Pt(111) in aqueous perchloric acid

electrolytes have been obtained recently by means of a“CO-charge displacement” strategy, involvingmeasuringpotential-dependent current transients induced by COchemisorption.32 This procedure leads not only to aEpzct value, around 0.3 V vs standard hydrogen electrode

(SHE) in 0.1 M HClO4, but also to an estimate of Epzcf ,

about 0.07 V vs SHE.32b This latter quantity is theappropriate Epzc value to compare with UHV-basedmeasurements, since water adsorption on Pt(111) yieldsno chemisorbed hydrogen. (Note that the Epzc

f valuewould be the measured Epzc if no charge-transfer chemi-sorption occurred.31) Interestingly, the above Epzc

f valueis close to that obtained from theUHVdata, 0.2 V vs SHEfor K+-free Pt(111) covered with water, by means of eq 2.The latter value is extracted from the measured workfunction forwater-modifiedPt(111), 4.8 eV (see Figure 3),by taking Ek ∼ 4.6 V (cf. footnote 29).Aside fromthekeyability to alter the interfacial solvent

as well as ionic composition, emphasized in the bulk ofthis paper, the “UHV double-layer modeling” approachenables charge-potential data to be extracted in moredirect fashion than for electrochemical interfaces, giventhat Cd measurements and Epzc estimates are oftenproblematical for the latter. Interestingly, this advantageis sharedwith somemetal cluster solute systems, such asthe high-nuclearity Pt carbonyl clusters described in ref33. These systems, specifically [Pt24(CO)30]n, [Pt26(CO)32]n,and [Pt38(CO)44]n, canbechargedreversibly to redoxstates,n, between 0 and -10, thereby effectively constituting“ionizable metal cluster” analogies of the negativelychargeablemetal surfaces of concern here.33 (Indeed, theCO-covered Pt(111)microfacets present on these clustersrender themclose structural neighbors of the presentCO-saturated surface.33) Effective σ-E data for the clusterscanbeobtaineddirectly fromvoltammetric redoxpotentialdata, yielding effective “molecular capacitance” values,Cd

m, in addition to Epzc values, as outlined in ref 38.This analysis yielded an average Cd

m value of about 15µF cm-2 for the above clusters in several nonaqueoussolvents, including acetonitrile, acetone, andmethanol ofinterest here. This value is indeed comparable to ((20%)the average Cd values extracted for the analogous CO-saturatedPt(111)-nonaqueous interfaces inUHV(Figure6). Admittedly, the higher Cd

m values found for theclusters compared with the analogous in-situ electro-chemical interface (vide supra) may be due partly togeometry-basedelectrostatic effects,38 but the concordancewith the UHV-based data is still noteworthy.TheEpzc 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 thenonaqueous solvents mentioned above.38 (Scatter in the

(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 employedin ref 28 has a potential in acetonitrile that is about 0.6 V positive ofthe aqueous standard hydrogen electrode (SHE). Thework function,Φ,of the Pt(111)-acetonitrile UHV (i.e., uncharged) interface is about 4.5eV.Estimates ofEk in eq1vary fromabout4.45 to4.85V.6a,30Presuming,nevertheless, thatEk∼ 4.6 eV for the SHE leads via eq 1 to an estimateof 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 solvent-induced -∆Φ values are probably smaller at ambient-temperatureelectrochemical interfaces than in the (low-temperature) UHVenvironment,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.ACSSymp.Ser.1988, No. 378, 166.

(31) Frumkin,A.N.; Petrii, O.A.;Damaskin,B.B. InComprehensiveTreatise of Electrochemistry; Bockris, J. O’M., Conway, B. E., Yeager,E., Eds.; Plenum: New York, 1980; Chapter 5.

(32) (a) Clavilier, J.; Albalat, R.; Gomez, R.; Orts, J. M.; Feliu, J. M.;Aldaz, A. J. Electroanal. Chem. 1992, 330, 489. (b) Climent, V.; Gomez,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 Φfor the CO-saturated Pt(111)-acetonitrile interface is about 5.2 eV,7cwe deduce an estimate of Epzc around 0 V vs Fc+/0. A substantiallyhigher value, 0.7 V vs Fc+/0, has been extracted from in-situ infraredspectroscopy involving a cation-dependent analysis of C-O stretchingfrequency-potential data.35 However, the former (UHV) value isprobably too low (see footnote 29), even though the latter analysis israther indirect.7c

(35) From ref 36, quoted in ref 33.

(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.

E ) Φ/e - Ek (2)

Modeling Electrochemical Interfaces in UHV Langmuir, Vol. 13, No. 25, 1997 6843

data precludes a delineation of Epzc values between thedifferent solvents.) Converting this Epzc value, as before,to the vacuum scale given that Ek for the Fc+/0 referenceelectrode is roughly 5.0 V yields a corresponding Φ valueof about 5.5 eV for the uncharged CO-saturated Pt(111)surface in these solvents. This value is indeed comparableto (within ca. 0.3 eV) the measured Φ values for CO-saturated Pt(111) modified by the above nonaqueoussolvents.7cOverall, then, the charge-potential properties of the

present low-temperature UHV-based systems appear toprovide, at least in some respects, a reasonable facsimileof related ambient-temperature chargeable interfaces,most directly thehigh-nuclearitymetal clusters. As such,the overall conclusions of the above discussion regardingprogressive double-layer solvation are deemed to be ofdirect electrochemical relevance. In particular, the de-duction that solvation of the countercharge ions exerts adisproportionately large, if not dominant, influence onthe charge-potential properties is believed to be signifi-cant. Thisnotionbearsa close resemblance to somerecentobservations of the effect of solvent addition upon theinfrared spectral (C-O stretching, νCO) properties of COadlayers on Pt(111) with varying K+ coverages.13 Specif-ically, only primary K+ solvation is required in order to

remove the νCO features associated with short-range K+-CO interactions andgenerate the long-range electrostaticinteractions characteristic of the well-known νCO fre-quency-Φ dependence (“Stark-tuning behavior”).13 How-ever, complete solvation of the CO adlayer as well as thecationcharges isnecessary inorder to reproduce thepreciseStark-tuning behavior observed for the in-situ electro-chemical systems,demonstrating (asabove) thesignificantrole of overall surface solvation in modifying the electro-static situation.13

Consequently, such UHV electrochemical modelingtactics appear to prove useful for understanding multi-faceted electrostatic aspects of double-layer phenomena.Indeed, this general theme is to be found in earlier relatedwork by other groups.6 It would, however, clearly beinteresting to pursue such systematic solvent dosagedependent charge-potential measurements for surfacessuch as gold for which analogous in-situ capacitance dataaremore readily obtainable. Further studies along theselines are planned.

Acknowledgment. This work is supported by theNational Science Foundation.

LA970820Y

6844 Langmuir, Vol. 13, No. 25, 1997 Weaver and Villegas