Dependence of the Potential of Zero Charge of Stepped Platinum (111

Nov 8, 1999 - The dependence of the potential of zero charge (pzc) for Pt(111) surfaces in acidic aqueous solution having increasing densities of orde...
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J. Phys. Chem. B 2000, 104, 597-605

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Dependence of the Potential of Zero Charge of Stepped Platinum (111) Electrodes on the Oriented Step-Edge Density: Electrochemical Implications and Comparison with Work Function Behavior Roberto Go´ mez, Vı´ctor Climent, Juan M. Feliu,* and Michael J. Weaver*,† Department de Quı´mica Fı´sica, UniVersitat d’ Alacant, E-03080 Alacant, Spain ReceiVed: August 12, 1999; In Final Form: NoVember 8, 1999

The dependence of the potential of zero charge (pzc) for Pt(111) surfaces in acidic aqueous solution having increasing densities of ordered monoatomic steps in the (111)-(110) and (111)-(100) zones is evaluated from CO “charge-displacement” measurements, with the objective of elucidating the influence of the electrochemical double layer on the large step-induced changes in surface potential known for the clean uncharged surfaces in ultrahigh vacuum (UHV). This experimental strategy, which involves evaluating the charge flowing at controlled potentials upon “quenching” the aqueous double layer with chemisorbed CO, t t yields pzc values referring to zero “total” electronic charge, Epzc . The Epzc values in both 0.1 M HClO4 and 0.5 M H2SO4 electrolytes decrease noticeably (by ca. 0.15 V) upon increasing the (110) step density, N, t whereas smaller effects are found for (100) steps. The location of the Epzc values within the so-called t “hydrogen” region, however, complicates interpretation of the Epzc -N dependences due to the presence of faradaic charge associated with potential-dependent hydrogen adsorption. Procedures are outlined by which t can be removed, yielding approximate estimates of pzc values referring instead to this influence upon Epzc f zero “free” electronic charge, Epzc , as a function of step density. The analysis followed involves extrapolating charge-potential data from higher potentials where hydrogen adsorption is essentially absent, or evaluating instead “potentials of constant (nonzero) free charge” in this “double-layer” region, achievable most readily f with the data in 0.1 M HClO4. The resulting Epzc -N plots exhibit substantial negative slopes for dilute (110) f stepped surfaces (N e 107 cm-1), ∆Epzc values approaching ca. -0.7 V being obtained, although the f dependence changes sign close to the densely stepped (110) limit. Significantly, the Epzc -N profile obtained M-N) behavior for the Pt(111)-(110) for (110) steps is comparable to the corresponding work function (Φ surfaces in UHV. This indicates that the remarkably (ca. 1 eV) lower “local” ΦM values for Pt steps relative to (110) terrace regions known to be present in the latter environment are not attenuated (or otherwise altered) greatly by the presence of the aqueous double layer. Interpretation is given in terms of current understanding f of interfacial solvation effects on Epzc , and some electrochemical implications are pointed out.

Introduction The potential of zero charge (pzc), denoting the electrode potential of a metal-solution interface for which the excess electronic charge density, q, is zero, is a centrally important parameter in electrochemical surface science. Its evaluation, however, is often far from straightforward, especially for transition metal surfaces which are of particular interest in catalytic electrochemistry. Moreover, even the definition of pzc for such systems can be problematic in the common circumstance where adsorption occurs along with charge transfer. In this situation, Frumkin et al. showed that one can usefully distinguish two types of pzc, associated either with zero “free” (or true) excess electronic charge or the “total” charge, the latter containing also the faradaic charge transferred upon reversible adsorption.1 We label these quantities pzfc and pztc, respectively. With the advent of straightforward yet reliable procedures by which well-ordered single-crystal electrodes can be prepared, and their in-situ electrochemical as well as microscopic-level properties scrutinized in remarkable detail,2 evaluating their pzc values has taken on a new importance. † Permanent address: Dept. of Chemistry, Purdue University, West Lafayette, IN 47907.

This laboratory has recently outlined an approach by which t , can be reliably evaluated for Pt-group values of pztc, Epzc monocrystalline electrodes.3-5 The primary procedure involves measuring the charge displaced, qdis, at various electrode potentials, E, in a given electrolyte upon saturation CO chemisorption induced by controlled solution dosage.3 Plots of qdis versus E can readily be obtained by combining these data with cyclic voltammetric measurements within the region where the current-potential (i-E) features are reversible. Although the electrode potential for which qdis ) 0 as determined by this “CO charge displacement” procedure strictly equals instead the value, Eq)0, at which the charge-potential (q-E) curves for the aqueous double layer and the CO-saturated interfaces intersect, since the capacitances (q-E slopes) for the latter are t relatively small, usually to a good approximation Eq)0 ≈ Epzc (vide infra).6 Aside from evaluating pztc values and associated charge-potential relationships for transition metal-aqueous interfaces, the tactic also can yield the required double-layer corrections to the charge measured for chemisorbed CO electrooxidation, enabling accurate CO coverages to be evaluated.5 One intriguing issue which can be studied by means of the CO charge-displacement procedure concerns the sensitivity of

10.1021/jp992870c CCC: $19.00 © 2000 American Chemical Society Published on Web 12/31/1999

598 J. Phys. Chem. B, Vol. 104, No. 3, 2000 the pzc to the introduction of steps on a low-index surface, i.e., as one moves along a given crystallographic zone. It is known that the presence of oriented monoatomic steps on clean surfaces in UHV markedly lowers the work function, Φ, by up to ca. 0.6 eV in the case of Pt(111) stepped surfaces, with -∆Φ being proportional to the step density for longer terraces (i.e., dilute step densities).7,8 These findings, along with “local” work function information deduced by means of “photoemission from adsorbed xenon” (PAX) measurements,9 indicate the presence of a large positive-outward surface dipole at step edges. This dipole arises from a “smoothing” of the metal electron density relative to the sharp step-edge profile formed by the atomic cores.9 Given the broad-based relationships anticipated between Φ and Epzc,10 such electrostatic effects are anticipated to have important consequences for the double-layer structure at stepped metal electrodes.8,11 Indeed, systematic voltammetric studies for stepped Pt surfaces of the type n(111) × (111) and n(111) × (100) exhibit notable sensitivities of the current-potential morphology to the step density in both crystallographic zones.12-16 Prompted partly by the availability of UHV work function data along with the detailed voltammetric measurements, we t have evaluated Epzc by the CO charge-displacement procedure for a sequence of n(111) × (111) stepped Pt surfaces in 0.1 M HClO4 and 0.5 M H2SO4, and for n(111) × (100) surfaces in the latter electrolyte. The salient findings are reported herein; some preliminary results are available in a recent conference proceedings.17 We emphasize here the likely roles of hydrogen t values, and describe proceand anion adsorption on the Epzc dures by which estimates of the potential of zero “free” charge, f Epzc , for the stepped surface sequences can also be extracted (cf., refs 4b and 6). The step-density dependence of the latter, fundamentally more desirable, quantity shows an interesting parallel with UHV-based work function behavior, shedding light on the interplay between the electrochemical double layer and the surface electronic properties at stepped oriented transition metal electrodes. Experimental Section Single-crystal electrodes were prepared from small (2-3 mm diameter) Pt beads obtained by melting 0.5 mm diameter Pt wires (99.99%). The ensuing facets present on the bead were used to select the desired orientation within (3 min of arc. The samples were fixed, cut, and polished as described previously.18 Before each experiment, the electrodes were flame-annealed and quenched with water in equilibrium with a mixture of H2 + Ar. The procedure employed in early work for Pt[n(111) × (111)] surfaces involved cooling in air after the flame treatment.12b However, Clavilier et al. pointed out that cooling in a H2/Ar “reductive” atmosphere yields sharper step-sensitive voltammetric features.19 For the experiments reported here, we have therefore employed this latter protocol. In addition, after transfer to the electrochemical cell, we applied a constant potential of -0.06 V for 1 min. Sharper voltammetric profiles, suggesting a higher degree of order (and cleanliness), are indeed obtained, especially for (110) steps. Furthermore, it has recently been shown by means of scanning tunneling microscopy that this pretreatment procedure results in uniformly monoatomic stepped Pt surfaces.27 Solutions were prepared from concentrated perchloric acid, sulfuric acid (Merck Suprapur), and Millipore Milli-Q water. The solution purity, as well as the surface order and cleanliness, was tested by standard voltammetric procedures. The working solutions were deaerated by bubbling Ar for 15 min. Chargedisplacement experiments at constant potential were performed

Go´mez et al. following the usual procedure.3,4 After recording the initial voltammetric profile, CO was dosed at a constant potential in the range between 0.06 and 0.45 V and the ensuing currenttime transient was recorded. Next, the CO was removed anodically, and the voltammetric profile was then recorded. A waveform generator (EG&G PARC 175), potentiostat (Amel 551), and X-Y-t recorder (Phillips PM 8133) were arranged in the conventional way. The cell was a conventional two-compartment glass cell with an additional inlet for dosing CO gas. Particular care was taken to avoid the presence of atmospheric oxygen in the vicinity of the meniscus. A coiled Pt wire immersed in the working electrolyte was used as the counter electrode. Potentials were measured against and are quoted versus a reversible hydrogen reference electrode (RHE). All experiments were performed at room temperature. Results and Discussion Significance of Double-Layer Charge Displacement. As a prelude to discussing the experimental results, it is appropriate to review briefly the fundamental meaning of the CO charge displacement measurements themselves, in particular the relationship of the experimental “potential of zero displaced charge”, Eq)0 (where qdis ) 0), to the desired potential of zero charge t f and Epzc . As already mentioned, Eq)0 actually parameters Epzc corresponds to the intersection point of the pair of total chargepotential (qt-E) curves for the metal-solution interface in the absence and presence of a saturated CO adlayer.6 The key to the usefulness of the procedure for obtaining reliable estimates t of Epzc for the former interface lies in the ability of the CO adlayer to largely “quench” the overall (total) double-layer charge qt, thereby yielding small qt values for the CO-covered t surface in the vicinity of Eq)0 so that Eq)0 ≈ Epzc . The degree to which this approximation is valid clearly depends on the capacitance (i.e., qt-E slope) of the CO-saturated surface, Cs, in relation to those for the unmodified double layer, Cdl, t value for the former and latter intercombined with the Epzc t f (dl), respectively.6 faces, Epzc(CO) and Epzc The influence of these different parameters on the relation t (dl) is illustrated schematically in ref 6. between Eq)0 and Epzc A straightforward algebraic representation can also be given for the simplified case where Cs and Cdl are both essentially independent of the electrode potential, expressed as17 t t Epzc (dl) ) Eq)0 + (Cs/Cdl) [Epzc (CO) - Eq)0]

(-1)

t An estimate of Epzc (CO) for the Pt(111) interface has been made from UHV-based data, which is substantially (ca. 0.7 V) higher than Eq)0 for this system.6 While this large dissimilarity t (dl) on clearly enhances the difference between Eq)0 and Epzc the basis of eq 1, this is offset by the much (10-20 fold) higher values of Cdl relative to Cs. Consequently, the difference between t and Eq)0 estimated for the Pt(111) system is only small, Epzc ca. 25 mV.6 It should be noted, however, that this approximate concordance arises chiefly because of the potential-dependent formation of adsorbed hydrogen (from hydronium ions) occurs t for Pt(111) and other Pt-group surfaces; in the vicinity of Epzc t that is, the Epzc values tend to lie in the so-called “hydrogen region”.4,5 This circumstance yields large adsorption pseudocat values. pacitances (Cdl values) which tend to “buffer” the Epzc As we shall see below, a different situation is encountered with f the Epzc values, since these necessarily refer to the absence of such charge-transfer chemisorption.4b,6

Stepped Platinum(111) Electrodes

Figure 1. Total charge density (right-hand axis) plotted against electrode potential for Pt(111) and (110) in 0.1 M HClO4 and 0.5 M H2SO4, as indicated. The potential of zero total displaced charge (Eq)0) is indicated by an open circle. Also shown are corresponding voltammetric profiles (at 50 mV s-1) for positive-going potential sweeps (see right-hand axis for current scale).

It is also appropriate to comment briefly on the thermodynamic significance of the Eq)0 values themselves. Given that Eq)0 is by definition the intersection point of the chargepotential curves of the CO-free and the CO-covered interfaces, by thermodynamic reasoning this value formally equals the potential of maximum “film pressure lowering” or, equivalently, the point where the free energy of CO chemisorption in the aqueous double layer, ∆Goad, reaches a maximum negative value. (See ref 22 for a discussion of this point.) In principle, then, shifting the potential to values sufficiently positive or negative of Eq)0 could eventually yield sufficiently diminished values of ∆Goad to engender subsaturated CO coverages. The occurrence of such partial CO desorption would clearly obfuscate severely the primary value of the “CO charge displacement” procedure.24 In practice, however, CO is sufficiently strongly chemisorbed at Pt-group electrodes, even from very dilute CO solutions, to maintain saturation adsorption (more specifically, hexagonal close-packed adlayers5,23) over the entire polarizable potential region lying between the onset of cathodic hydrogen evolution and CO (and metal) electrooxidation. Consequently, then, this potential complication to the above analysis can be discounted. Potential of Zero Displaced Charge: Step-Density Dependence. Figure 1 (upper and lower left-hand segments) contains plots of the charge displaced upon saturation CO chemisorption, qdis (µC cm-2), versus the electrode potential, E (vs RHE), for the “parent” ordered Pt(111) surface in 0.1 M HClO4 and 0.5 M H2SO4, respectively. These two electrolytes were chosen primarily on the basis of previous measurements.3,4,12-17 Perchloric acid exhibits only weak anion-specific adsorption, thereby minimizing complications to the double-layer analysis; while the sulfuric acid medium yields extensive sulfate adsorption at positive electrode charges, it is free of complications from trace halide adsorption and reduction. The qdis measurements were made at selected potentials between 0.1 and 0.4 V,

J. Phys. Chem. B, Vol. 104, No. 3, 2000 599

Figure 2. Similarly to Figure 1, but for four [n(111) × (111)] Pt surfaces, as indicated, in 0.1 M HClO4.

below the onset of CO electrooxidation, yielding the qdis-E plots shown over the range ca. 0-1.0 V by combining with the corresponding reversible voltammograms, also included (on a common potential axis) in Figure 1. These data yield the desired Eq)0 values (for which qdis ) 0) for Pt(111) of 0.33 and 0.335 V in 0.1 M HClO4 and 0.5 M H2SO4, respectively, marked as open circles in Figure 1. The close concordance in the Edis values suggests that the influence of anion adsorption is relatively small at this point, although marked differences in the qdis-E curves as well as the voltammograms themselves are seen toward higher potentials, as expected since anion adsorption is increasingly prevalent. The right-hand portion of Figure 1 shows corresponding data obtained on Pt(110). Since this basal plane can be regarded as 2(111) × (111), the Pt(111) and (110) faces form the two limiting surfaces in the n(111) × (111) zone, of primary interest here. In contrast to Pt(111), the qdis-E plots as well as the voltammetric profiles for Pt(110) are markedly different in the two electrolytes even at low potentials, in the vicinity of Eq)0. Indeed, the Eq)0 values, 0.24 and 0.15 V in 0.1 M HClO4 and 0.5 M H2SO4, respectively, are notably dissimilar on Pt(110). These differences undoubtedly reflect a substantial overlap between the occurrence of hydrogen and anion adsorption. The charges contained under the low-potential voltammetric features are also large, 175 and 220 µC cm-2 in 0.1 M HClO4 and 0.5 M H2SO4, respectively, after correction for the “double-layer” contribution. These values can be compared with that, 147 µC cm-2, anticipated if the voltammetric features arose purely from monolayer H adsorption, presuming an unreconstructed surface. The additional charges are probably due to anion adsorption, expected to be extensive for sulfate, possibly together with oxygenated species (OH, etc.) formed by water dissociation. Corresponding measurements were undertaken for a series of seven stepped Pt surfaces in the (111)-(110) zone, i.e., for [n(111) × (111)], with n ) 20, 14, 10, 7, 5, 4, and 3. Plots of qdis versus E, again together with the corresponding voltammograms, are shown for four stepped surfaces in 0.1 M HClO4 and 0.5 M H2SO4 in Figures 2 and 3, respectively. These faces,

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Go´mez et al.

Figure 5. Plots of the potential of zero displaced charge (essentially t equal to Epcc as a function of the Pt surface step density. Circles, n(111) × (111); triangles, n(111) × (100); filled circles, 0.1 M HClO4; open circles and triangles, 0.5 M H2SO4. Figure 3. Similar to Figure 1, but for four [n(111) × (111)] Pt surfaces, as indicated, in 0.5 M H2SO4 (cf., Figure 2).

Figure 4. Cyclic voltammograms (at 50 mV s-1) for Pt(775) in 0.1 M HClO4 before (solid trace) and after the sequential addition of 10-3 M (dashed trace) and 10-2 M (dashed-dotted trace) K2SO4.

corresponding to n ) 20, 10, 7, and 4, thereby span a range from relatively low to high step densities. A characteristic voltammetric feature in both perchloric and sulfuric acid electrolytes is a sharp peak at ca. 0.12 V. The charge density contained under this peak, which is proportional to the step density, corresponds to one e- per step edge atom, thereby suggesting its origin to be adsorbed hydrogen bound to such sites.12 The peak shape and location are very similar in the two electrolytes, indicating that anion adsorption plays only a minor role. Indeed, addition of sulfate ions to the 0.1 M HClO4 electrolyte does not induce any significant change in the voltammetric peak attributed to the steps. This is exemplified for Pt(775) in Figure 4, showing the effect of adding 10-3 and 10-2 M K2SO4 (dashed and dashed-dotted traces, respectively)

to 0.1 M HClO4. In addition, the shape of the surrounding broad voltammetric feature below ca. 0.4 V in 0.1 M HClO4 is relatively insensitive to the presence of dilute (n > 7) steps and is therefore attributable primarily to the formation of hydrogen adsorbed on the intervening terraces (Figure 2). For E > 0.4 V, waves presumably corresponding to the adsorption of oxygenated species (OH, etc.) and bi(sulfate) are observed for perchloric and sulfuric acid solutions, respectively (Figures 1-3). It is remarkable that the adsorption of (bi)sulfate on the terraces is more sensitive to the presence of steps than is the adsorption of oxygenated species in the former electrolyte. The values of Eq)0 as a function of the (111) step density are also comparable in the two electrolytes for dilute stepped surfaces. Careful inspection of Figures 2 and 3 shows that Eq)0 (marked again by the open circle) shifts further into the “hydrogen adsorption” region, as denoted by its location within the left-hand (steeper) qdis - E segment, with increasing step density. These values of Eq)0 are plotted versus step density, expressed in terms of number of steps per cm, N (cm-1), in Figure 5, the Eq)0 values being referred to those for the Pt(100) surfaces. The Eq)0-N plots for (111) steps in both 0.1 M HClO4 and 0.5 M H2SO4 (filled and open circles, respectively) exhibit a near-linear Eq)0 decrease with increasing N for dilute steps (n > 5, equivalent to N < 1 × 107 cm-1), reaching a plateau (or weak inversion region) for larger step densities. Data are also included for the Pt(110) surface (Figure 1) in Figure 5, recalling that this basal plane can be considered as the “limiting stepped” surface 2(111) × (111) (for which N ) 3.1 × 107 cm-1). Figure 5 also shows increasing deviations between the Eq)0-N plots in perchloric and sulfuric acid electrolytes for N > 7 × 106 cm-1. The progressively lower Eq)0 values obtained in the latter electrolyte for increasing N under these conditions can again be ascribed to the influence of sulfate adsorption. These behavioral differences in the two electrolytes for high step densities are also clearly evident by comparing the corresponding voltammograms for E > 0.35 V for Pt(221) (Figures 2 and 3) as well as for Pt(110) (Figure 1), discussed above. Corresponding data were also obtained for seven stepped Pt surfaces along the (111)-(100) zone with n ) 21, 14, 10, 6, 4, 3, and 2 in 0.5 M H2SO4. Voltammetric and qdis-E data for

Stepped Platinum(111) Electrodes

Figure 6. Similar to Figure 1, but for four [n(111) × (100)] Pt surfaces, as indicated, in 0.5 M H2SO4 (cf., Figures 2 and 3).

four selected faces, having n ) 21, 14, 6, and 4, are shown in Figure 6. The voltammetric behavior is characterized by a sharp spike at 0.26 V, the charge under which increases linearly with (100) step density and corresponds essentially to one e- per step-edge atom, again suggesting its origin to be hydrogen adsorbed on these sites.13 This adsorption potential is significantly (0.15 V) higher than that for hydrogen on the (110) steps (compare Figures 3 and 6). A major complication, caused by this higher adsorption potential for “step-edge hydrogen” on the (100) steps, is that the Eq)0 values converge toward the potential of this dominant voltammetric feature toward higher step densities (Figure 6) so that the former are insensitive to increasing N for N > 8 × 106 cm-1 (i.e., for n < 6), becoming “buffered” by the large faradaic charge located at E ≈ 0.26 V (vide infra). The resulting Eq)0-N plot for (100) symmetry steps is included (open triangles) in Figure 5. Extraction of Potentials of Zero Charge: Step-Density Dependence. Of central interest in the present work is the deduction from the foregoing data of values of the potential of zero charge as a function of the oriented step density, and their comparison with documented work function data for the clean surfaces in UHV. As already mentioned, the experimental Eq)0 values can be related most simply to the so-called potential of zero total charge of a given surface in contact with an aqueous t double layer, Epzc (dl), by means of eq 1. The analysis involves deducing the residual charge densities on the CO-saturated surface in the vicinity of Eq)0, qco, and thereby estimating the t , i.e., where correction to the Eq)0 value required to find Epzc the total electronic charge in contact with the aqueous double layer, qt , equals zero.6 As outlined in ref 6, for Pt(111) the qco value estimated from a combination of in-situ and UHV-based double-layer data is about -10 µC cm-2. (This qco value results from a combination of a considerably, ca. 0.7 V, more positive t Epzc value for the CO-covered surface versus that for the aqueous double layer, tempered by a relatively small, ca. 14 µF cm-2, capacitance for the former.6) While this double-layer charge density is quite significant in itself, the very large (ca.

J. Phys. Chem. B, Vol. 104, No. 3, 2000 601 400-800 µF cm-2) capacitances, i.e., qt-E slopes, that characterize the hydrogen adsorption region within which Eq)0 is located (Figure 1) give rise to a relatively small difference t (Epzc -Eq)0), about 25 mV.6 The estimation of corresponding corrections to Eq)0 for stepped platinum faces is hampered by the absence of the required work function values, Φ, for the CO-covered surfaces. Nevertheless, given that the Φ values for the clean surfaces are known to decrease substantially (by up to 0.6 eV) upon the introduction of steps (vide infra),7,8 together with the observation that the CO-covered stepped surfaces exhibit similarly small capacitances in aqueous media, ca. 14 µF cm-2, as for Pt(111), t -Eq)0) for the one can surmise that the differences (Epzc stepped Pt-aqueous double-layer interfaces are even smaller than that estimated for Pt(111), i.e., Cdlf. We consider the simplest case where the measured capacitance (including “hydrogen pseudocapacitance”), Cdlt, and the corresponding free-charge capacitance in the absence of H adsorption (or other chargetransfer process), Cdlf, are both independent of step density as well as electrode potential, and the increases in qt at a given electrode potential with increasing step density equal those in qf. (These assumptions are tantamount to presuming that the terrace double-layer properties are unaffected by the introduction of dilute steps, as suggested by the data obtained in 0.1 M f f , ∆Epzc , HClO4, noted above.) In this case, the changes in Epzc induced by altering N will be related to the corresponding f t changes in Epzc , ∆Epzc , straightforwardly by f t ∆Epzc ) ∆Epzc (Cdlt/Cdlf)

(-2)

Since we expect > by factors of ca. 4-5 fold (vide infra), from the data in Figure 4 (bearing in mind that Eq)0 ≈ t ) one can surmise that substantially larger, say up to 0.5Epzc f t compared with Epzc are induced by the 1.0 V, decreases in Epzc presence of (110) steps. t Given that the Eq)0, and hence the Epzc , values obtained for the n(111) × (111) surfaces in both perchloric and sulfuric acid electrolytes reside on a roughly linear qt-E segment lying progressively deeper into the “terrace hydrogen adsorption” region (vide supra) with increasing N (Figures 2 and 3), eq 2 f may be employed to extract rough estimates of ∆Epzc provided f that Cdl can also be estimated. Here, however, we follow a related yet distinct approach which also enables absolute f estimates of Epzc to be obtained. This strategy has been outlined for Pt(111) in refs 4b and 6. It takes advantage of the presence of an apparent “double-layer” region in 0.1 M HClO4 electrolyte at ca. 0.4-0.6 V, i.e., at potentials above the onset of hydrogen adsorption where the Eq)0 values are located. Assuming that the minimum Cdl value in this region (corresponding to the inflection point on the qt-E curve) approximates the “true nonfaradaic” capacitance, Cdlf, and also at these potentials qt ≈ qf (i.e., reductive or oxidative chemisorption is largely absent), a linear extrapolation with this same Cdlf slope back to qt ) 0 should yield a potential for which qf ) 0, i.e., f 4b . The analysis presumes, of course, that Cdlf approximates Epzc is E-independent over this range, which cannot be verified. This procedure was applied in ref 4b to qdis-E data on Ptf ≈ 0.13 V. As noted in ref (111) in 0.1 M HClO4, yielding Epzc 6, correcting the q-E data for the residual charges on the COsaturated surface, so to extract qt-E curves, results in a f value for Pt(111), ca. 0.25 V. This significantly higher Epzc t , is more correction, similar to that noted above for Epzc f t f important when estimating Epzc since Cdl < Cdl, so that the latter Epzc values are less “buffered”. This same procedure was applied to the present data for the n(111) × (111) stepped surfaces in 0.1 M HClO4. The desired corrections to the qdis-E data for the residual charge on the CO-saturated layer cannot be applied as reliably as for the Pt(111) basal plane due to the unavailability of the UHV-based t work function data required to extract Epzc (CO) [eq 1, vide supra]. Nevertheless, we presumed that these values are lower than that for Pt(111), ca. 1.0 V,6 by the same numerical amounts as are the work functions, Φ, for the corresponding clean Cdlt

Cdlf

f Figure 7. Plot of the potential of zero free charge, Epzc , referred to that for Pt(111), (filled circles, left-hand axis) versus step density for Pt[n(111) × (111)] surfaces, obtained from data in 0.1 M HClO4. The diagonal and upright crosses (right-hand axis) are the corresponding work functions of the clean surfaces in UHV, also referenced to the value for Pt(111), taken from refs 7 and 8, respectively.

surfaces. (A justification for this assumption is evident below f -N and Φ-N plots; the from the parallel behavior of the Epzc f form of the resulting Epzc-N relation, however, is insensitive to this correction.) f versus the n(111) × (111) step The resulting plot of Epzc density, N, normalized to the Pt(111) value is shown (filled f value for the Pt(110) surface, circles) in Figure 7. Again, a Epzc given its formal designation as 2(111) × (111), is included in Figure 7, extracted from the qdis-E data in 0.1 M HClO4 in the same manner as for the other stepped faces. Shown for comparison in Figure 7 are corresponding values of the work function, Φ (eV), for the clean Pt stepped surfaces in UHV (diagonal, upright crosses), obtained from refs 7 and 8, respectively, again normalized to the Pt(111) value (5.95 eV),25 with Pt(110) (for which Φ ) 5.65 eV)25 also included. f -N and ∆Φ-N plots reveals Comparison between the ∆Epzc a notable similarity in the overall behavior, roughly comparable f and Φ being observed for n > 5, linear decreases in Epzc followed at higher step densities by a slope reversal, higher t and Φ for Pt(110) (n ) 2) being evident, even values of Epzc though the two traces are quite divergent at these high step densities. Before accepting the approximate validity of these findings, however, given the inevitable uncertainties in the f values, it extrapolation procedure employed to extract the Epzc is advisable to examine alternative data analyses. One such procedure is to select a fixed nonzero electrode charge, qt, chosen to correspond to the “double-layer” potential region on the stepped surfaces so that qt ≈ qf. Provided that the double-layer capacitance Cdlf within this region is roughly independent of the step density N, and, ideally, that anion specific adsorption is relatively weak, one anticipates that the dependence of this f , on N should mirror “potential of constant free charge”, Epzc f the corresponding changes in Epzc. To this end, Figure 8 shows qt-E plots for five n(111) × (111) surfaces as well as for Pt(111), as indicated, in 0.1 M HClO4. These were obtained from the corresponding qdis-E data by correcting for the residual potential-dependent charges on

Stepped Platinum(111) Electrodes

Figure 8. Plots of the total charge density versus the electrode potential for several Pt[n(111) × (111)] stepped surfaces, as indicated, in 0.1 M HClO4. The horizontal dashed line refers to qt ) 18 µC cm-2, selected for constructing Figure 9 (see text).

the CO-saturated surfaces, as outlined above. (The inclusion of this correction, however, has a relatively minor effect on the ensuing analysis.) The roughly parallel slopes (i.e., comparable Cdlf values, ca. 70 µF cm-2) obtained for the dilute stepped surfaces (especially n g 10) for qt ≈ 15-20 µC cm-2 suggest this region to be suitable for the analysis. [This notion is also supported by the similar Cdl values, 50-100 µF cm-2, obtained by work function measurements at low electrode charge densities for “model electrochemical double layers” by water cation dosing on Pt(111) in UHV at low temperatures,26 where complications from hydrogen adsorption and other faradaic processes are absent.] Shown in Figure 9 (filled circles) are the resulting values of the “potential of constant free charge”, Epcc, for these surfaces evaluated at a suitable fixed charge, qt ) 18 µC cm-2, again normalized to Pt(111), plotted against the (110) step density. A limitation with this choice of qt value is that the qt-E curvature at the highest step densities (Figure 8) is symptomatic of interference from the onset of hydrogen adsorption. A modified procedure was therefore also employed, correcting for H (or OH) adsorption by linearly extrapolating the near-linear “double-layer” segment so as to intersect the qt ) 18 µC cm-2 line. These “residual faradaic-corrected” values of Epcc are included as open circles in Figure 9 in cases where they deviate significantly from the uncorrected Epcc values (filled circles). As in Figure 7, we include in Figure 9 the corresponding work function values for the clean surfaces in UHV (crosses). Comparison between the Epcc-N and Φ-N plots in Figure 9 shows a similarly approximate concordance for the “dilutef -N data stepped” surfaces (n g10), as is evident from the Epzc in Figure 6, especially after correcting for the “residual faradaic” charge. While the step-induced Epcc decreases are somewhat f (ca. 20-30%) milder than the corresponding variations in Epzc , these differences are within the decidedly approximate nature of the double-layer analysis. Overall, then, it is apparent that the introduction of (110) steps on a (111) surface in contact with an aqueous double layer yields substantial decreases in

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Figure 9. Plots of the “potential of constant free charge”, Epcc, referred to value for Pt(111) (left-hand axis), as a function of the step density for Pt[n(111) × (111)] surfaces (filled, open circles), as indicated, obtained for qt ) 18 µC cm-2 (see Figure 8 and text). The open circles refer to “residual faradaic-corrected” Epcc values (see text). The diagonal and upright crosses (right-hand axis) are the corresponding work functions of the clean surfaces in UHV, also referenced to the value for Pt(111), taken from refs 7 and 8, respectively. f f (or Epcc ), which are roughly proportional to the step Epzc density and of comparable magnitude to the corresponding work function changes for the clean surface in UHV. While we have emphasized the influence of hydrogen adsorption in the above analysis, it is appropriate to comment briefly on the possible role of anion adsorption. The influence of sulfate adsorption on the stepped-Pt double-layer behavior in 0.5 M H2SO4 is clearly evident at positive qt values (i.e., E t ) in the marked divergence from the corresponding data > Epzc in 0.1 M HClO4. For this reason, of course, we selected instead the latter data for the analysis summarized in Figures 7 and 9. However, it is important to recall that the Eq)0-N (and hence t Epzc -N) plots for dilute (111) steps are essentially coincident in these two electrolytes (Figure 5), showing clearly the anioninsensitive nature of the double layer when qt ≈ 0, even though qfar > 0 under these conditions (vide supra). The lack of a major role of anion-specific adsorption, despite such net positive electronic charges (especially on the step sites), might be viewed as surprising, but may well arise in part from the presence of adsorbed hydrogen on adjacent terrace sites. This and related issues are considered further below. Fundamental Double-Layer Implications. A useful general framework for interpreting the influence of surface steps on f t and Epcc , following Trasatti and others, is given by10 Epzc

E ) ΦM/e + δχM + gs(dip) + gM s (ion) - Eabs(ref)

(-3)

Here ΦM is the work function of the clean metal surface, e is the electronic charge, δχM is the change in the surface electronic (“electron spillover”) contribution to ΦM caused by contact with solvent, gs(dip) is the surface potential component due to net solvent dipole orientation, gM s (ion) is the contribution arising from “free” charges, associated with excess electronic charge on the metal surface along with the ionic double-layer countercharge, and Eabs(ref) is the so-called “absolute” potential of f , the net free-charge the reference electrode. When E ) Epzc M component gs (ion) across the surface equals zero, at least in the absence of specific ionic adsorption. Consequently, the

604 J. Phys. Chem. B, Vol. 104, No. 3, 2000 f response of Epzc to the introduction of periodic steps in relation to the corresponding alterations in the work function for the clean surface, ΦM, can be interpreted in terms of the “surface solvation” potential components δχM + gs(dip) in eq 1.11 For dilute stepped surfaces, one can envisage ΦM to be composed of separate “local” contributions from the terrace and steps; indeed, the concept of local work functions for stepped clean surfaces as well as adsorbate-containing interfaces in UHV is well-known.9 The linear ΦM-N plots observed for dilute step densities in both the Pt(111)-(110) and (111)-(100) zones7 indicate the presence of a “local step” work function, Φsloc, that is markedly lower than for the (111) terrace, Φt. Direct estimates of Φsloc have been extracted from PAX measurements, yielding values at Pt(111) steps that almost 1 eV lower than for the terraces.9 These remarkably large differences (Φt - Φsloc) in work function, and hence surface potentials, can be understood in terms of the well-known “Smoluchowski smoothing” effect, whereby the usual negative-outward surface dipole associated with electron “spillover” on the close-packed terraces is replaced by a dipole of opposite sign at the steps, resulting from a less abrupt change in electron density along the surface than for the metal nuclei.9c Such marked changes in the surface potential profile close to (within 2-3 Å of) step sites can be anticipated to also be present at corresponding electrochemical interfaces, yet altered to some degree by the solvated double layer. However, the f f -N (and Epcc -N) slopes in observation of comparable Epzc M comparison with the Φ /e-N data for dilute (110) steps (Figures 6 and 8) indicate that the net modification exerted by the double layer on Φt - Φsloc is only modest; that is, the remarkably large differences in local surface potentials are largely maintained in the electrochemical environment. In other words, the changes in the surface potential in the vicinity of the steps brought about by the interfacial solvent, although probably substantial (vide infra), are not greatly different from those induced on the (111) terraces themselves. Essentially similar findings were reported earlier for various Au(111)-(110) and Au(111)-(100) stepped f -N slopes that are only about 20% surfaces, yielding Epzc larger and smaller, respectively, than the ΦM/e-N data.11 The latter analysis was facilitated by the polarizable nature of gold f electrochemical interfaces, enabling accurate Epzc values to be extracted from capacitance-potential data. While the present analysis is by comparison more complex and less quantitatively reliable, it should be borne in mind that the platinum surfaces yield markedly (2.5-fold) larger step-induced ΦM decreases, i.e., (Φt - Φsloc) values, than observed for gold.7 The observed modest double-layer influences on the stepped gold surfaces, just noted, have been interpreted in terms of water dipole orientation at the step sites.11 Although one might anticipate at first sight that the solvent dipoles would screen the surface electronic dipole responsible for the markedly lower Φsloc values, and hence yield attenuated step-induced changes f in Epzc relative to ΦM/e, the solvent molecular orientation is expected to be sensitive to the precise adsorption site and hence the step atomic structure.11 While this gs(dip) contribution is often regarded as constituting the major solvent influence on f Epzc , solvent-induced alterations to the interfacial electron density profile [i.e., the δχM term in eq 1] also provides an important or even dominant contribution.28,29 Consequently, the f differs extent to which the step density dependence of Epzc M from Φ /e for clean metal surfaces rest on the combined properties of the δχM and gs(dip) solvation terms at local step relative to the terrace sites.

Go´mez et al. Evidence for the importance of the δχM term for Pt(111) electrode surfaces has been obtained from a UHV-based study of solvation-induced ΦM changes at suitably low temperatures.29 The large (ca. 1 eV) ΦM decreases measured on Pt(111) in UHV upon dosing water can therefore be attributed to important contributions from both the gs(dip) and δχM terms.26,29 Comf value for Pt(111) extracted as parison of the absolute Epzc outlined above for the CO charge displacement data, ca. 0.2 V vs SHE, with the known ΦM value for clean Pt(111), 5.9 eV, by means of eq 1 also suggests the presence of a large negative, ca. -1 V, interfacial solvent contribution to the electrode surface potential.6 Unfortunately, deducing the likely magnitude of δχM as well as gs(dip) at step sites is hampered severely by a lack of work function data for solvent adsorption on stepped surfaces. f Nevertheless, the present Epzc -N data indicates clearly that M the water-induced Φ decreases are not greatly different at step and terrace sites. Consequently, the substantially (ca. 1 eV) lower Φloc values at Pt step versus terrace sites implied by the foregoing in the solvated electrochemical as well as clean surface UHV environments indicates that the electrochemical double layer on a stepped Pt(111) surface is profoundly different in comparison with that on Pt(111) or other electrodes featuring atomically f uniform terraces. Specifically, while the condition E ) Epzc in f the latter case corresponds uniformly to q ) 0, the periodic stepped surface will necessarily feature local segments having either positive and negative qf values, corresponding to step sites and terrace regions, respectively. Moreover, for dilute stepped surfaces, say for N < 107 cm-1, the distances between steps (>10 D) will be larger than the diffuse-layer thickness in concentrated (g0.1 M) electrolyte so that the electrochemical interface will be compared of effectively separate microscopic step and terrace “double-layer patches”.30 Under these conditions, the surface potential profile is anticipated to be markedly different in the electrochemical compared to the clean surface UHV environment. The latter surfaces will also feature electrondeficient steps, the excess charge being distributed over the surrounding terraces.8 However, the double-layer countercharge necessarily present in the electrochemical environments can be expected to facilitate such local electronic charge variations at stepped surfaces. The electrochemical interfaces should therefore feature even larger alterations in the “local” potential profile across step-terrace regions within the inner layer (i.e., within the first solvent monolayer), although these variations will decrease sharply to zero more than a few angstroms away from the surface in concentrated electrolytes, i.e., just outside the diffuse layer. A likely complication, at least in sulfuric acid electrolyte, is the occurrence of anion-specific adsorption at the steps, given the substantial positive electrode charges residing f 8 at these sites, even for electrode potentials negative of Epzc . t However, the closely similar Epzc-N plots for dilute Pt(111)(110) stepped surfaces obtained in perchloric and sulfuric acid electrolytes (Figure 4) argues against a dominant influence of t f , and by implication also Epzc since sulfate adsorption on Epzc f t Epzc < Epzc. Overall, then, while the double layer can certainly be expected to exert a major modifying influence on the local potential distributions near steps, the observation of comparable, and f possibly even larger, step-induced variations in Epzc in comM parison with Φ /e for the clean metal surfaces are not unreasonable, although reliable quantitative predictions require level understanding of solvent-induced changes on local surface potentials which is not yet forthcoming. The similarly nonf monotonic Epzc -N and ΦM-N profiles observed for more

Stepped Platinum(111) Electrodes densely stepped (111)-(110) surface indicate that the above notions also apply in this regime, although the electronic interactions between adjacent steps which is responsible for this linear behavior provide for a more complex situation. Most importantly, despite the quantitative uncertainties in the f (and Epcc) analyses, the present results clearly above Epzc indicate that the double layer does not attenuate greatly the remarkably large differences in Φloc observed between Pt step and terrace sites in UHV, comparable (or possibly even larger) effects being observed in the aqueous environment. This finding has important broad implications for interfacial electrochemistry; since step and other types of “defect” sites abound on polycrystalline metal surfaces, one has good reason to anticipate that their electrode kinetic and particularly electrocatalytic, in addition to equilibrium double-layer, properties could be greatly affected by such atomic morphologies. Systematic studies along these general lines would clearly be of substantial practical as well as fundamental interest. Acknowledgment. V.C. acknowledges the “Conselleria de Cultura, Educacio´ i Cie`ncia” of the “Generalitat Valenciana” for a doctoral grant. M.J.W. is grateful to the “Ministerio de Educacio´n y cultura” (Spain) for a Visiting Scientist Fellowship (“Programa de Estancias de Investigadores extranjeros en re´gimen de an˜o saba´tico en Espan˜a”). This research was also supported by the DGES through Project PB96-0409. References and Notes (1) (a) Frumkin, A. N.; Petrii, O. A. Electrochim. Acta 1975, 20, 347. (b) Frumkin, A. N.; Petrii, O. A.; Damaskin, B. B. In ComprehensiVe Treatise in Electrochemistry; Bockris, J’O. M., Conway, B. E., Yeager, E., Eds.; Plenum: New York, 1980; Vol. 1, p 221. (2) For example, see: Weaver, M. J. J. Phys. Chem. 1996, 100, 13079. (3) (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. (e) 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. (4) (a) Clavilier, J.; Orts, J. M.; Go´mez, R.; Feliu, J. M.; Aldaz, A. In Electrochemical Society Proceedings; Conway, B. E., Jerkiewicz, G., Eds.; The Electrochemical Society: Pennington, NJ, 1994; Vol. 94-21, p 167. (b) Climent, V.; Go´mez, R.; Orts, J. M.; Aldaz, A.; Feliu, O. M. In Electrochemical Society Proceedings; Korzeniewski, C., Conway, B. E., Eds.; The Electrochemical Society: Pennington, NJ, 1997; Vol. 97-17, p 222. (5) Go´mez, R.; Feliu, J. M.; Aldaz, A.; Weaver, M. J. Surf. Sci. 1998, 410, 48.

J. Phys. Chem. B, Vol. 104, No. 3, 2000 605 (6) Weaver, M. J. Langmuir 1998, 14, 3932. (7) Besocke, K.; Krahl-Urban, B.; Wagner, H. Surf. Sci. 1977, 68, 39. (8) Ross, P. N. J. Chim. Phys. 1991, 88, 1353. (9) For example: (a) Wandelt, K. In Chemistry and Physics of Solid Surfaces; Vanselow, R., Howe, R., Eds.; Springer Series in Surface Science, Vol. 8; Springer: Berlin, 1990; p 25. (b) Wandelt, K. Appl. Surf. Sci. 1997, 111 1. (c) Wandelt, K. In Thin Metal Films and Gas Chemisorption; Wissmann, P., Ed.; Studies in Surface Science and Catalysis, Vol. 32; Elsevier: Amsterdam, 1987; Chapter 7. (10) (a) Trasatti, S. J. Electroanal. Chem. 1983, 150, 1. (b) Trasatti, S. Electrochim. Acta 1991, 36, 1659. (c) Trasatti, S. Surf. Sci. 1995, 335, 1. (11) Lecoeur, J.; Andro, J.; Parsons, R. Surf. Sci. 1982, 114, 320. (12) (a) Clavilier, J.; El Achi, K.; Rodes, A. J. Electroanal. Chem. 1989, 272, 253. (b) Clavilier, J.; El Achi, K.; Rodes, A. Chem. Phys. 1990, 141, 1. (13) Rodes, A.; El Achi, K.; Zamakhchari, M. A.; Clavilier, J. J. Electroanal. Chem. 1990, 284, 245. (14) Rodes, A.; El Achi, K.; Zamakhchari, M. A.; Clavilier, J. In Fundamental Aspects of Heterogeneous Catalysis Studied by Particle Beams; Brongersma, H. H., van Santen, R. A., Eds.; Plenum: New York, 1991; p 75. (15) Clavilier, J.; Rodes, A.; El Achi, K.; Zamakhchari, M. A. J. Chim. Phys. 1991, 88, 1291. (16) Rodes, A. Tesis Doctoral, Universidad de Alicante, Spain, 1991. (17) Climent, V.; Go´mez, R.; Feliu, J. M. Electrochim Acta 1999, 45, 629. (18) Clavilier, J.; Armand, D.; Sun, S. G.; Petit, M. J. Electroanal. Chem. 1986, 205, 267. (19) Clavilier, J.; El Achi, K.; Petit, M.; Rodes, A.; Zamakhchari, M. A. J. Electroanal Chem. 1990, 295, 333. (20) Go´mez, R.; Clavilier, J. J. Electroanal. Chem. 1993, 354, 189. (21) Markovic, N. M.; Grgur, B. N.; Lucas, C. A.; Ross, P. N. Surf. Sci. 1997, 384, L805. (22) For example: Lipkowski, J.; Stolberg, L. In Adsorption of Molecules at Electrodes; Lipkowski, J., Ross, P. N., Eds.; VCH Publishers: New York, 1992; Chapter 4. (23) Villegas, I.; Weaver, M. J. J. Chem. Phys. 1994, 101, 1648. (24) We thank, J. Lipkowski for bringing this point to our attention. (25) Rotermund, H. H.; Jakubith, S.; Kubala, S.; Von Oertzen, A.; Ertl, G. J. Electron. Spectrosc. Relat. Phenom. 1990, 52, 811. (26) Weaver, M. J.; Villegas, I. Langmuir 1997, 13, 6836. (27) (a) Herrero, E.; Orts, J. M.; Feliu, J. M. 194th Meeting of the Electrochemical Society, Boston, 1998; Abstract 1059. (b) Herrero, E.; Orts, J. M.; Aldaz, A.; Feliu, J. M. Surf. Sci., in press. (28) (a) Baetzold, R. C. J. Phys. Chem 1983, 87, 3858. (b) Shustorovich, E. J. Phys. Chem 1982, 86, 3114. (c) Baetzold, R. C.; Apai, G.; Shustorovich, E. Appl. Surf. Sci. 1984, 19. 135. (29) (a) Kizhakevariam, N.; Villegas, I.; Weaver, M. J. Surf. Sci. 1995, 336, 37. (b) Kizhakevariam, N.; Villegas, I.; Weaver, M. J. J. Phys. Chem. 1995, 99, 7677 (c) Villegas, I.; Weaver, M. J. Electrochim. Acta 1996, 41, 661. (30) For example: Vorotyntsev, M. A. J. Electroanal. Chem. 1981, 123, 379.