3932
Langmuir 1998, 14, 3932-3936
Potentials of Zero Charge for Platinum(111)-Aqueous Interfaces: A Combined Assessment from In-Situ and Ultrahigh-Vacuum Measurements Michael J. Weaver Department of Chemistry, Purdue University, West Lafayette, Indiana 47907-1393 Received January 27, 1998 The nature and magnitude of corrections to published estimates of the potentials of zero charge Epzc for Pt(111)-aqueous interfaces obtained by the “CO charge-displacement” strategy are assessed by comparing such in-situ electrochemical data with surface charge-potential information extracted from work functionsurface composition data for related interfaces in ultrahigh vacuum (UHV). The corrections involve estimating the excess metal charge densities remaining upon displacement of the aqueous inner layer by chemisorbed CO. While the resulting adjustment to the so-called potential of zero total charge Etpzc for the Pt(111)-aqueous 0.1 M HClO4 interface is only small (ca. 25 mV), a larger correction to the corresponding f ≈ 0.2 V versus SHE. A closely concordant estimate of “free-charge” value Efpzc is deduced, yielding Epzc f Epzc is also extracted from UHV-based work function data. The substantial discrepancies in such Epzc values for the Pt(111)-acidic aqueous interface with those extracted by means of an ex-situ/electrode immersion procedure are also briefly considered in the light of the above analysis.
Introduction Evaluating the potentials of zero charge Epzc of transition metal-aqueous electrochemical interfaces has long been recognized as a key requirement for understanding the double-layer properties of these chemically important systems.1 As is well documented,1a however, the experimental evaluation and even the meaning of Epzc for such reactive interfaces are complicated by the occurrence of potential-dependent chemisorption, most characteristically involving the formation of adsorbed hydrogen and hydroxyl/oxide species. The now firmly established availability of well-ordered monocrystalline electrode surfaces of Pt group metals as prepared by flame-annealing tactics2 offers a rich spectrum of opportunities for the detailed exploration of their double-layer properties, involving both macroscopic and microscopic level techniques. As a consequence, the reliable elucidation of Epzc values for such structurally well-defined interfaces has taken on a greater importance. The recent literature contains several estimates of Epzc for Pt(111)-aqueous and related interfaces.3-7 These (1) (a) Frumkin, A. N.; Petri, 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. (b) Frumkin, A.; Petry, O.; Damaskin, B. J. Electroanal. Chem. 1970, 27, 81. (2) For example: (a) Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J. Electroanal. Chem. 1980, 107, 205. (b) Clavilier, J.; Armand, D.; Sun, S. G.; Petit, M. J. Electroanal. Chem. 1986, 205, 267. (c) Clavilier, J.; Wasberg, M.; Petit, M.; Klein, L. H. J. Electroanal. Chem. 1994, 374, 123. (3) Climent, V.; Go´mez, R.; Orts, J. M.; Aldaz, A.; Feliu, J. M. Electrochemical Society Proceedings; Korzeniewski, C., Conway, B. E., Eds.; The Electrochemical Society (pub.): Pennington, NJ, 1997; Vol. 97-17, p 222. (4) (a) Clavilier, J.; Albalat, R.; Go´mez, R.; Orts, J. M.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1992, 330, 489. (b) Feliu, J. M.; Orts, J. M.; Go´mez, R.; Aldaz, A.; Clavilier, J. J. Electroanal. Chem. 1994, 372, 265. (c) Clavilier, J.; Albalat, R.; Go´mez, R.; Orts, J. M.; Feliu, J. M. J. Electroanal. Chem. 1993, 360, 325. (d) Herrero, E.; Feliu, J. M.; Wieckowski, A.; Clavilier, J. Surf. Sci. 1995, 325, 131. (5) Hamm, U. W.; Kramer, D.; Zhai, R. S.; Kolb, D. M. J. Electroanal. Chem. 1996, 414, 85. (6) Attard, G. A.; Ahmadi, A. J. Electroanal. Chem. 1995, 389, 175. (7) Iwasita, T.; Xia, X. J. Electroanal. Chem. 1996, 411, 95.
determinations utilize diverse experimental strategies, including the potential-dependent charge measured upon “displacing” the double layer with an inert chemisorbate3,4 or upon immersing a Pt(111) surface prepared in ultrahigh vacuum (UHV) into electrolyte5 and by analyzing the kinetics of N2O electroreduction.6 The first two strategies are conceptually closely related in that they attempt to evaluate the change in surface charge ∆qm at a given electrode potential induced by suddenly removing and forming, respectively, the aqueous double layer, so that E ) Epzc when ∆qm ) 0. The former technique apparently achieves this objective (at least approximately) by chemisorption of CO,3,4 whereas the latter utilizes a supposedly clean, and therefore ostensibly uncharged, Pt(111) surface for electrolyte immersion.5 While both these strategies should therefore yield approximately correct, and therefore concordant, estimates of Epzc, the derived values are in substantial disagreement, by over 0.5 V in 0.1 M HClO4.3,5 The lower Epzc estimates obtained by the “CO chargedisplacement” approach, about 0.27 V versus standard hydrogen electrode (SHE) in 0.1 M HClO4,3 are in approximate agreement with those for Pt(111) and polycrystalline Pt extracted by most other methods.1a,6,7 Nevertheless, the much higher value (around 0.8 V versus SHE) indicated by the ex-situ/immersion tactic5 might be considered to cast doubt on the validity of the former Epzc estimates. The in-situ as well as apparently “direct” nature of the CO charge-displacement technique makes this approach a particularly enticing one, since it should be free from possible complications from surface contamination which can plague ex-situ transfer methods. Consequently, it is desirable to check the validity of the former approach by independent means. One such approach, pursued herein, involves comparing the electrode charge-potential information derived from the in-situ method with corresponding data obtained for UHV-based “model electrochemical interfaces” by dosing controlled amounts of solvent and electronic charge (via ionizable species) onto a clean metal surface. This “non-situ” strategy, pioneered
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by Sass and co-workers,8 provides an invaluable means of scrutinizing double-layer interactions at electrochemically relevant interfaces,9,10 even though some behavioral differences may result from the dissimilar temperatures involved. More specifically, the present communication utilizes UHV-based surface charge-potential data obtained recently in our laboratory for Pt(111) dosed with adsorbed water, chemisorbed CO, and variable electronic charge11 to assess the reliability of the “CO charge-displacement” strategy for estimating Epzc values for Pt(111) and other Pt-group electrochemical interfaces. The findings support the approximate validity of the latter in-situ approach, even though significant corrections are signaled in some cases, arising from the non-negligible charge-potential characteristics of the CO-modified interfaces as deduced from the non-situ data. Double-Layer Analysis It is appropriate to first consider carefully the essential nature of the CO charge displacement technique itself, as conceived and applied by the Alicante-CNRS group.4 Briefly, this involves evaluating the transient charge flowing at a controlled electrode potential E upon introducing CO into the electrolyte near the metal surface, so to replace the double layer of interest by a saturated chemisorbed adlayer. Provided that the residual charge density remaining on the metal surface upon forming the saturated CO adlayer can be neglected (vide infra), the reverse of the measured displacement charge qdis as a function of potential will yield the desired electronic charge-potential (qm-E) behavior. (Strictly speaking, it is not necessary to record more than a single qdis value, since the qm-E plot can be obtained by combining qdis with cyclic voltammetric data.4 Nevertheless, matching the E-dependent qdis values with the voltammogram provides a check on the former.4) The desired Epzc value can then be found from the point where qm ) 0. One should recall, however, that the potential measured in this fashion is strictly the value where no charge will flow in or out of the interface upon changing its area.1a This so-called “potential of zero total charge” Etpzc will equal the “potential of zero free charge” Efpzc, the latter referring to the point where the actual excess electronic charge density on the metal surface equals zero, only when no interfacial charge transfer occurs as a result of solute (or solvent) adsorption (i.e., an “ideally polarizable interface”).1a In the case of adsorption with charge transfer, as commonly anticipated for Pt group electrodes, the Epzc defined in this manner actually equals Etpzc rather than Efpzc.1a In other words, it refers to the potential where the “total” electrode charge density qtm, rather than the “free” (or actual) excess charge residing on the metal surface qfm, equals zero. Nevertheless, although the latter is experimentally more elusive, instructive estimates of Efpzc can be obtained in some cases (vide infra). The appeal of the CO charge displacement technique for evaluating Etpzc lies in its apparent ability to rapidly (8) For example, see: Sass, J. K.; Bange, K. ACS Symp. Ser. 1988, 378, 54. (9) Wagner, F. T. In Structure of Electrified Interfaces; Lipkowski, J., Ross, P. N., Eds.; VCH Publishers: New York, 1993; Chapter 9. (10) (a) Stuve, E. M.; Kizhakevariam, N. J. Vac. Sci. Technol. 1993, A11, 2217. (b) Villegas, I.; Weaver, M. J. J. Phys. Chem. B 1997, 101, 10166. (11) (a) Weaver, M. J.; Villegas, I. Langmuir 1997, 13, 6836. (b) Kizhakevariam, N.; Villegas, I.; Weaver, M. J. J. Phys. Chem. 1995, 99, 7677.
Figure 1. illustration of the nature and sign of corrections to the Epzc values as determined by the CO charge-displacement technique, resulting from residual charges on the Pt(111)/CO adlayer (see text).
“quench” the surface charge at a given (controlled) electrode potential by CO adsorption, thereby evaluating qtm by purely in-situ means. The Alicante-CNRS group assumed that the residual qtm value remaining upon CO chemisorption is negligible, so that qtm ≈ -qdis. In support of this presumption, they noted that the doublelayer capacitance of the CO-saturated Pt(111)-aqueous interface as measured by cyclic voltammetry is markedly smaller than that observed in the absence of CO within the “double-layer” region, between about 0.35 and 0.5 V, on Pt(111) in 0.1 M HClO4.3 Strictly speaking, however, this assumption will be valid only if the Etpzc value for the Pt(111)/CO-aqueous system Etpzc(CO) is not greatly different from the desired value for the Pt(111)-aqueous interface, so that the residual qtm value for the former surface at Etpzc is negligibly small. This point is illustrated schematically in Figure 1, where the chargepotential (qtm-E) curves AB and CD refer to the metalaqueous and metal/CO-aqueous interfaces, respectively. The desired (“corrected”) Etpzc value for the former interface Etpzc(corr) is seen to differ from the “apparent” value Etpzc(app) obtained from the CO charge displacement method by an amount that is dependent both on the difference between the actual Etpzc values in the absence and presence of the CO adlayer, Etpzc(corr) and Etpzc(CO), and the relative AB and CD slopes, that is, the capacitance values. While values of Etpzc(CO) for the Pt(111)/CO-aqueous (and related) interfaces are unknown, a useful estimate may be obtained from UHV-based work function measurements. Thus the work function Φ of clean Pt(111) is about 5.9 eV,12 which is decreased by 0.3 V by chemisorbing a saturated CO layer followed by water dosing.11b The resulting Φ value, 5.6 eV, can be considered to be a reasonable estimate for the uncharged Pt(111)/COaqueous interface.11a Conversion of the vacuum-based Φ scale to electrode potentials E can be achieved by noting that (12) Rotemund, H. H.; Jabibith, S.; Kubala, S.; van Oertzen, A.; Ertl, G. J. Electron Spectrosc. Relat. Phenom. 1990, 52, 811.
3934 Langmuir, Vol. 14, No. 14, 1998
E ) Φ/e - Ek
Weaver
(1)
where e is the electronic charge and Ek is the so-called “absolute” potential of the reference electrode.13 Unfortunately, the correct value of Ek for the aqueous SHE (and other reference electrodes) remains somewhat uncertain, estimates between about 4.4 and 4.8 V being obtained by various routes.9,13 Employing an “average” Ek estimate of 4.6 V, as in our earlier analyses (e.g., ref 11), yields a Etpzc value for the Pt(111)/CO-aqueous interface of 1.0 ((0.2) V. The uncertainties in Ek, along with the possibility that the (albeit small) contribution of water to the surface potential measured in UHV may differ from the in-situ case, renders this Etpzc(CO) estimate only approximate. It is nevertheless markedly (over 0.7 V) more positive than the value of Etpzc(app), 0.27 V versus SHE, evaluated for Pt(111) in 0.1 M HClO4 by means of the CO charge-displacement technique.3 The capacitance of the Pt(111)/CO-0.1 M HClO4 interface, Cd evaluated by cyclic voltammetry at potentials below where CO electrooxidation proceeds is about 14 µF cm-2.14 (Indeed, closely similar Cd values are obtained for the Pt(111)/ CO-water interface in UHV by electronic charging by means of increasing dosages of potassium.15) Consequently, then, the qmt values for the Pt(111)/CO surface in the vicinity of the above Etpzc(app) estimate are about 10 µC cm-2. Fortunately, the correction to Etpzc for the Pt(111)-0.1 M HClO4 interface resulting from this analysis is only small, shifting the value positively by about 25 mV, so that Etpzc(corr) ≈ 0.30 V versus SHE. [The sign and magnitude of the correction can readily be judged by inspecting the qtm-E plots extracted from the CO chargedisplacement technique, locating the E value where the apparent (i.e., uncorrected) charge qtm ≈ 10 µC cm-2.16] The minor extent of the correction to Etpzc in this case stems largely from its location, at least for the Pt(111)0.1 M HClO4 system, within the potential region
