electrochemical oxidation of pt(1 1 1) vicinal surfaces: effects of surface structure and specific...

Published: July 07, 2011

r 2011 American Chemical Society 15509 dx.doi.org/10.1021/jp204306k | J. Phys. Chem. C 2011, 115, 15509–15515

ARTICLE

pubs.acs.org/JPCC

Electrochemical Oxidation of Pt(1 1 1) Vicinal Surfaces: Effects ofSurface Structure and Specific Anion AdsorptionAlexander Bj€orling, Enrique Herrero, and Juan M. Feliu*

Instituto de Electroquímica, Universidad de Alicante, Apt 99, E-03080 Alicante, Spain

1. INTRODUCTION

The occurrence of underpotential deposition (UPD) phe-nomena is well known and extensively described for manyelectrochemical systems. Strictly, the term UPD refers to adsorp-tion of materials that form single adlayers at negative over-potential with respect to bulk deposition and that form thickerdeposits if sufficient driving force is applied,1 but adlayerspreceding gas evolution reactions are sometimes also describedas UPD layers. For example, adsorption of atomic hydrogenduring the early reduction of water over Pt is usually termedhydrogen UPD. The similarity is in the significant adsorbate�substrate interaction, which is stronger than the interactionswithin the bulk material deposit or within product gas molecules,respectively. Because these phenomena are strongly sensitive toatomic-scale structure, surfaces created by exposing differentcrystal planes of Pt crystals give very distinct voltammetricprofiles in the hydrogen adsorption region. For this reason,hydrogen UPD is an important tool in assessing the quality ofsingle-crystal electrode surfaces and for determining coverageand orientation of defects.2,3 Similarly, the opposite reaction, theearly oxidation of water over Pt, also involves specific andstructure-sensitive adsorption. This UPD process occurs outsidethe potential range that is usually explored in Pt single-crystalstudies, the upper limit of which is set by surface oxidation andthe associated surface disordering process observed after the veryfirst desorption of electrochemically adsorbed oxygen. Because ofthis, a quantitative analysis of oxygen adsorption effects atPt(111) and the subsequent surface disordering was only re-cently reported.3,4

The electrooxidation of (or oxygen UPD at) polycrystallinesurfaces of Pt and other noble metals, however, has beenthoroughly studied for the past 50 years. In early studies, amechanistic model was suggested where anodically depositedOH rearranges together with Pt surface atoms in a processtermed place-exchange.5 The “OHPt” phase thus formed from“PtOH” would then oxidize at higher potential, yielding a thinbut bulk-like PtO film. In the classic work by Conway andcoworkers,6�10 the various voltammetric peaks typical for Ptoxidation were considered to be due to the sequential OHsaturation of various surface sublattices. At certain coverage levels,new types of lateral OH�OH interactions would becomeimportant, increasing the adsorption energy enough to allowthe formation of a new peak at higher potential. The variation ingeometrical arrangement of Pt surface atoms was then notconsidered to influence the voltammetric profile. However, asrecently reported4 and will be further elaborated in the presentArticle, the peak multiplicity is likely due to the oxidation ofdifferent types of surface site. Moreover, there has been somedebate over whether OH is at all involved in the oxidationprocess or if adsorbed atomic oxygen is the only importantintermediate species.11,12 This question and others are still notcompletely resolved.

It appears that one limiting factor in this research has been alack of surface definition. The ambition of many studies has been

Received: May 9, 2011Revised: July 6, 2011

ABSTRACT:The initial oxidation of Pt surfaces is an important processthat could determine the reactivity of catalysts in a wide range ofreactions, from electrocatalytic oxidation of organics to oxygen reduc-tion, but the understanding of electrochemical Pt oxidation has beenhindered by a lack of surface-structural definition. We have investigatedthe process at surfaces vicinal to Pt(111) and show that these oxidize insuccessive stages depending on site geometry as well as the adsorptionbehavior of the electrolyte anion. Step sites of {100} orientation slowlyoxidize at low potential (0.7 V vs RHE) in a region overlapping that ofthe “butterfly” peak seen at Pt(111) in the absence of specific electrolyte anion adsorption. Almost regardless of the latter, both{110} and {100} steps also oxidize between 0.9 and 1.2 V, causing voltammetric peaks with shapes that are characteristic of steporientation. The complex oxidation behavior of Pt(111) in perchloric acid, ranging from 0.6 V to the onset ofO2 evolution at 1.5 V, ismostly suppressed in sulfuric acid. However, if steps are introduced, then the oxidation at lower potential is again facilitated,probably by breaking the ordered protective sulfate adlayer along the steps. It was found that {100} steps oxidize to the extent of oneelectron per step atom, whereas {110} steps show two consecutive oxidations, amounting to a total of two electrons per step atom.At the onset of O2 evolution, Pt(111) terraces are oxidized with two electrons per surface atom.

15510 dx.doi.org/10.1021/jp204306k |J. Phys. Chem. C 2011, 115, 15509–15515

The Journal of Physical Chemistry C ARTICLE

to establish a detailed surface�electrochemical reaction mechan-ism, apparently independently of local surface structure, based onthe behavior of polycrystalline Pt surfaces. To ensure reprodu-cibility and cleanliness, these electrodes are subjected to exten-sive potential-cycling prior to all measurements, meaning thatsuch studies are always conducted at completely disorderedsurfaces.3,13,14 Our previous results and those reported hereshow that surface oxidation is a highly structure-sensitive processthat also depends strongly on the specific adsorption of anionsfrom the electrolyte. Mechanistic properties such as the roles ofdifferent oxygen-containing species can then be expected to varywith these parameters. Therefore, it seems that a detailed under-standing of these processes can result only from carefully study-ing the interplay between surface geometry and anionadsorption. As part of our research along these lines, this Articlepresents results concerning the oxidation and reduction of well-defined platinum surfaces vicinal to the (111) plane. We willshow that the surface oxidation reactivity of these is primarilydetermined by the occurrence and orientation of steps on thesurface and by the adsorption behavior of the electrolyte anion.

2. EXPERIMENTAL SECTION

Electrodes with surfaces vicinal to the (111) plane wereprepared from small Pt beads, approximately 2 to 3 mm indiameter, by the method reported by Clavilier.15 These surfacescan be described as close-packed terraces of (111) orientation,separated at regular intervals by well-defined monatomic steps.Two types of steps are possible, forming {110} or {100}microfacets. Steps of {110} orientation can also be seen as{111} steps, but the {110} notation is preferred because theyshow hydrogen adsorption resembling that at the Pt(110) basalplane. The surfaces used here are therefore represented asPt(s)[(n � 1)(111) � (110)] � Pt(s)[n(111) � (111)] orPt(s)[n(111) � (100)].

Experiments were carried out in a two-compartment, three-electrode all-glass cell cleaned first with acidic KMnO4 solution,then with H2O2 and further by repeated and extensive boiling inultrapure water (Elga). Prior to each experiment, the electrodeswere flame-annealed, cooled in a hydrogen/argon atmosphere,and transferred to the cell protected by a drop of ultrapure watersaturated with these gases. Suprapure sulfuric and perchloricacids (Merck) were used to prepare solutions in ultrapure water.Potentials were measured against the reversible hydrogen elec-trode (RHE) with no liquid junctions, except during the fast scanmeasurements (with v > 50 mV s�1), where a hydrogen-chargedPd wire in the same cell compartment as the working electrodewas used for lower noise levels. The latter electrode wasreferenced to an actual RHE in the same solution between eachrun. The counter electrode was a coiled Pt wire, flamed to ensurecleanliness. All voltammetric scans were collected at freshlyannealed surfaces, cycled first between 0.06 and 0.85 or 0.9 Vto verify their quality as well as the cleanliness of the solution.

3. RESULTS AND DISCUSSION

An overview of the oxidation behavior of Pt(111) and itsvicinal surfaces with steps of {110} and {100} orientations, in thepresence and absence of weak specific electrolyte anion adsorp-tion, is given in Figure 1. Moderate anion adsorption is includedby using 0.5 M H2SO4, as shown on the right-hand side of theFigure, and compared with the case of 0.1 M HClO4, where nospecific adsorption occurs, shown on the left. The plots at the top

show the effect of introducing {110} steps, whereas the two at thebottom show that of {100} steps. The geometries of the steps arealso illustrated in the Figure.

The oxidative behavior of Pt(111), shown in thick black curvesin Figure 1, is similar to that found in the literature4,16,17 but withmuch lower levels of surface defects, as evident from the flatresponse around 0.9 V (HClO4) and below 1.2 V (H2SO4). Forelectrodes in contact with perchloric acid, the so-called “butter-fly” peak around 0.6 to 0.8 V is very reversible and is normallyassociated with the anodic adsorption of OH fromwater.18�21 Itscomplex shape is interpreted either as random adsorption,followed by a disorder�order phase transition in the adlayer,22

or as the adsorption of OH originating from two types of water.18

Alternatively, as suggested from carefully comparing voltamme-try to theoretical results based on the ab initio atomisticthermodynamics method,23 the sharp part of the butterfly couldcorrespond to the formation of an ordered p(2� 2) overlayer ofatomic oxygen. On the basis of the conventional OH interpreta-tion of the butterfly, the peak at ∼1.05 V would be due tooxidation of the OH adlayer. If the sharp spike in the butterflyregion corresponded to atomic oxygen, then the 1.05 V peakwould presumably be due to the further adsorption of oxygeninto the p(2 � 2) structure.

It is interesting to note that the total charge, if corrected bysubtraction for the apparent charging of the diffuse double-layer,reaches 241 μC cm�2, that is, one electron per Pt(111) surfaceatom, just positive of the 1.05 V peak. This is shown in Figure 2,where an oxidative sweep to high potentials is integrated from theminimum at 0.51 V with the current at that potential as thebaseline. For atomic oxygen, this charge would correspond to acoverage of half a monolayer, which, on the basis of vacuumstudies, could, for example, be a p(2� 1) arrangement of oxygenatoms.24,25 For an electrochemical interface, however, such asimple adlayer structure does not seem probable in light of thecomplex voltammetric behavior of Pt. It has been shown, both for

Figure 1. Positive-going voltammetric scans at Pt(111)-vicinal surfaces.Graphs on the left- and right-hand sides show responses in 0.1MHClO4

and 0.5 M H2SO4, respectively. Pt(111) is represented by thick blackcurves, whereas red (top graphs) show Pt(s)[(n � 1)(111) � (110)]and blue (bottom graphs) show Pt(s)[n(111) � (100)] surfaces.Numbers indicate terrace widths, and arrows show effects of increasingstep density. Hard-sphere models illustrate step geometries. Scan ratev = 50 mV s�1.



polycrystalline surfaces in a large number of studies7�10 andmore recently for well-defined surfaces,3,4,26 that place-exchangeis an important process in Pt surface oxidation. Indeed, whenreversing a scan such as that in Figure 2 just positive of the 1.05 Vpeak, the voltammetry is far from reversible. As an example ofthis, Figure 2 shows a negative-going scan obtained after holdingthe potential at 1.25 V (red curve, reduction peak). It is thus clearthat the oxidation peak at∼1.05 V leaves the surface oxidized tothe extent of one electron per Pt atom, in a geometricallyreorganized or otherwise stabilized state. The stabilization thatleads to voltammetric irreversibility of the 1.05 V peak does notinvolve permanent modifications of the surface. If the scan isreversed just positive of the peak or even held at this upperpotential limit (1.10 V) for several minutes, then no change isobserved in the potential region of hydrogen adsorption insubsequent scans (not shown). It is known that the voltammetryin this region is very sensitive to defects present on the surface,27

and any defects created at high potentials would be detected.Thus, although oxidation and reduction of Pt(111) to the extentof one electron per surface atom (i.e., up to ∼1.1 V) is notvoltammetrically reversible on the time scale employed here, itdoes not lead to permanent surface defects. It should be recalledthat irreversibility was considered to be a consequence of place-exchange on polycrystalline electrodes,10 but this is apparentlynot always the case when well-defined electrodes are used.

The reduction branch of Figure 2 also shows a correspondingnegative-going scan collected in 0.5 M H2SO4 (green curve).This curve is quite similar to that recorded in perchloric acid,illustrating that anion adsorption is less important for thereduction of oxidized Pt(111) and that the state of the rear-ranged, oxidized surface is similar in the presence and absence ofsulfate. At higher potential, the voltammetry of Figure 2 shows athird oxidation peak at 1.38 V for perchloric acid, just positive ofwhich the total adsorption charge reaches two electrons per Ptatom. So, at the onset of oxygen evolution, the process seen atand above 1.5 V, the surface is oxidized to a total extent of twoelectrons per original surface atom, and the oxygen UPD processcan be considered to be completed. This agrees with experiments

where the potential is scanned negatively after being heldconstant at high values (E g 1.30 V) for various lengths oftime,3 fromwhich the corresponding limiting reduction charge oftwo electrons per surface atom is found.

Returning to Figure 1, we now recall that the voltammetricfeatures just discussed are almost completely suppressed whenthe same surface is in contact with a 0.5 M H2SO4 solution. Alarge oxidation peak is observed at higher potential,14 the base ofwhich is seen in the Figure at E > 1.3 V. It is well known thatsulfate (or bisulfate) adsorbs specifically at Pt(111), giving rise toa characteristic voltammetric profile in the potential rangenormally explored in electrochemical studies (0 < E < 0.9 V).As illustrated in Figure 1, this adsorption also has a strong effecton the high-potential behavior of that surface. The effect is clearlyprotective because at the present scan rate of 50 mV s�1 thesurface is not notably oxidized below 1.20 V and the oxidationprocess does not reach significant rates until around 1.30 to1.40 V.

When steps are systematically introduced by using electrodesprepared around the (111) pole, so-called vicinal surfaces, similarchanges are seen in the two electrolytes. As previously reported,4

in 0.5 M H2SO4, surfaces with {110} steps separated by wide(111) terraces display two strongly overlapping oxidation peaksin the potential range 1.0 to 1.2 V (upper right part of Figure 1).In the same solution, {100} steps instead oxidize in a single peakat ∼1.0 V, as shown in the lower right part of Figure 1. Whereasthe molecular origin of the different appearances of these peaks isnot yet understood, it is interesting to compare this behavior withits counterpart in the absence of sulfate adsorption. On the left ofFigure 1, voltammetric scans at stepped (111)-vicinal surfaces in0.1 M HClO4 are shown. There too, {110} steps give rise tostrongly overlapping peaks, whereas {100} steps essentially onlydisplay one clearly discernible peak. Because of the growth ofthese peaks with increasing step density, they are assumed to bedue to the local oxidation of step sites. Compared with thesulfuric acid case, they are shifted in perchloric acid to lowerpotentials by around 50�100 mV, suggesting that sulfate ad-sorption also protects steps to some degree, but the effect ofanion adsorption is clearly much more important for the Pt(111)basal plane, where the shape of the oxidative scans is dramaticallychanged. It can be speculated that sulfate adsorption on (111)terraces also protects adjacent steps and that the 50�100 mVshift of step oxidation is likely due to such indirect effects ratherthan to direct interactions of sulfate with step sites. It should alsobe noted that the peak for hydrogen adsorption on the {110}steps is not affected by the presence of sulfate in solution, whereasthat corresponding to the {100} step is only slightly shiftedca. 10�20 mV toward negative potentials.28 These facts alsosuggest that the adsorption of sulfate on the step is much weakerthan that on the terrace.

For the perchloric acid case, increasing the step density asshown in Figure 1 has the expected effects on the voltammetricfeatures of the (111) terraces. Both the butterfly feature between0.6 and 0.8 V and the characteristic peak at ∼1.05 V decreasewith increasing step density, and the former shifts to slightlyhigher potentials. These changes are uniform except for a smallanomaly seen in the behavior around the 0.8 V spike for {100}steps (as will be discussed below). Well-defined isopotentialpoints are observed in the region where step oxidation and the1.05 V terrace oxidation peak overlap.

In sulfuric acid, however, the behavior is not immediatelyclear. In the absence of steps, as discussed above, Pt(111) terraces

Figure 2. Positive-going, v-normalized voltammetric scan at Pt(111) in0.1 M HClO4 (red, left y axis) and integral of that curve after double-layer charging correction, expressed as the number of electrons ex-changed per Pt surface atom (black, right y axis). Cathodic curves shownegative-going scans after holding the potential for 90 s at 1.25 V in 0.1M HClO4 (red) and 0.5 M H2SO4 (green). Scan rate v = 50 mV s�1.



do not exhibit a significant voltammetric response in the poten-tial region above 0.8 V., but when steps are introduced, the peaksassociated with local step oxidation are superimposed on a slowlyand monotonously varying background which, below ∼1.4 V,also increases with step density. This slowly varying featurecould, in principle, be due to further local step oxidation, perhapsthrough inward oxygen transport by place-exchange. It could alsobe caused by oxidation of (111) terraces, somehow facilitated bythe presence of steps. Figure 3A shows the oxidative voltam-metric profile of Pt(775), a surface containing {110} steps, atsulfuric acid concentrations ranging from 5 mM to 0.5 M. As thesulfuric acid concentration decreases, the broad, monotonousfeature just positive of the step oxidation peak begins to takeshape and form a peak. At low sulfate concentration, this peakapproaches the shape and position of the (111) terrace oxidationpeak observed in perchloric acid at 1.05 V, shown for comparisonin Figure 3A for the same stepped surface (thick red curve). Onthe basis of this, it is concluded that the broad, slowly varyingfeature observed together with the peaks of step oxidation in 0.5Msulfuric acid corresponds to terrace oxidation. At this high con-centration, the terrace oxidation peak is apparently kineticallyhindered, loses its shape, and becomes displaced toward higherpotentials, forming the monotonous feature. Presumably, suchoxidation is possible in the presence of steps because these breakup and weaken the protective sulfate adlayer.29

In our previous report,4 we proposed that step oxidation in 0.5MH2SO4 amounts to one electron per step atom, independentlyof step orientation. This was concluded from integration of thecharacteristic peaks (right-hand side of Figure 1) with reasonablychosen baselines. Figure 3A shows that the peaks for stepoxidation and the slowly varying background become clearer andbetter separated as the electrolyte concentration is decreased.The voltammetric response of Pt(775) shown in that Figure cantherefore be decomposed into separate contributions, repre-sented by Gaussian basis functions, as shown in Figure 4. Twodecompositions of the data were performed, where the terrace-related response positive of the step oxidation peaks was treated

in different ways as shown. Following the peak assignments justderived from the H2SO4 concentration series, the first two peaks(shaded in Figure 4) are considered to be due to step oxidation.Their combined charges, projected on the (111) plane, can becompared with the theoretical density of step sites on thePt(775) surface. From this comparison, the extent of stepoxidation for {110} steps is close to n = 2 electrons per stepatom (1.8 and 2.1 for the two decompositions of Figure 4).Because the areas under the two contributions are quite similar,it can be concluded that the oxidation of {110} steps occurs intwo consecutive reactions, each involving one electron perstep atom.

The behavior of Pt(544), a surface containing {100} steps,when varying the H2SO4 concentration is shown in Figure 3B.For this step type, the step oxidation peak also shifts to slightlylower potential as the H2SO4 concentration decreases, but unlikethe response of {110} steps, the {100} step oxidation peak doesnot change its shape or become sharper. The peak becomesbetter separated from the monotonous background, but directintegration still yields a charge very close to the previousexpectation based on n = 1 electron per step site. Because thecharacteristics of step oxidation do not change dramatically withanion adsorption (as seen in the concentration series of Figure 3and generally in Figure 1), it is reasonable to conclude that in thepotential region 0.90 to 1.15 V steps oxidize to these extentsregardless of the electrolyte anion. This is also in line withGaussian decomposition of step oxidation in perchloric acid andthe theoretical results presented in ref 23. We can speculate thatthe differences in the number of electrons transferred betweenthe {100} and {110} steps may come from a different coveragevalue of the adsorbed species in the step sites. For the {100}, thecoverage would be half of that measured at the end of theoxidation process of the {110} steps, and for that reason only,one peak is observed. The second process is absent eitherbecause the adsorption is thermodynamically not favorable orbecause it is a very slow process. It should be highlighted that the{110} step sites appear at the intersection of two (111) planesand in such site the adsorption at the upper and lower part of thestep is possible.

Figure 3. Positive-going voltammetry of Pt(775) (A) and Pt(544) (B)at total sulfuric acid concentrations of 5, 16, 50, 160, and 500 mM, asindicated. The curves are offset for clarity. Also shown is the response ofPt(111) in 0.5 M H2SO4 (thick black) and of the stepped surface in 0.1M HClO4 (thick red). Scan rate v = 50 mV s�1.

Figure 4. Decomposition of positive-going voltammetric response ofPt(775) in 5 mM H2SO4 into separate Gaussian contributions. Data(identical to that shown in Figure 3A) are represented by squares, theseparate Gaussians are represented by black lines, and their sums arerepresented by thicker lines. Shaded peaks are associated with stepoxidation. The two decompositions (bottom and top) were performedrepresenting the part at potentials positive of step oxidation by one ortwo Gaussians. The fitting was performed in the potential ranges wheredata is plotted. The obtained numbers n of electrons exchanged per stepatom were similar and are given in the Figure. Scan rate v = 50 mV s�1.



As mentioned above, the changes in the perchloric acidbutterfly peak upon introducing {100} steps deserve someattention. A complication can be seen in the bottom left quadrantof Figure 1, where, around 0.8 V, the current associated with thePt(111) spike does not decrease monotonously with increasingstep density as would be expected. The feature was furtherinvestigated because any surface oxidation in that region maybe important for electrocatalytic reactions, for example, oxidationof CO and organic molecules and the reduction of O2, whichoften depends on step sites for high reactivity. Figure 5 shows theabsolute magnitudes of the cyclic voltammetric currents in theregion for surfaces with various densities of the two step types. InFigure 5A, the effect of {110} step density on the butterflyvoltammetry is illustrated. The sharp spike quickly decreases withstep density, and there is a slow change in the broad feature, butthe forward and backward scans remain very similar and symme-trical. A small difference appears at 0.8 V, but in most of thepotential region, the positive- and negative-going scans areidentical in magnitude. So, for {110} steps, the adsorptionprocesses of the butterfly peak (irrespective of their details)remain reversible, and only the equilibrium coverage-potentialrelation changes with step density. When steps have {100}orientation, however, the effect is more dramatic. Figure 5Bshows that as the density of these steps is increased, anasymmetry between positive- and negative-going scans appears.Looking closely at the curves of that Figure, the irreversiblebehavior can be discerned even from the lowest step density(with terraces nominally 29 atoms wide). From these results, it isclear that there is a distinct difference in the reactivity of {110}and {100} steps in the butterfly region, a region where Pt(111) isbeing covered with OH or some other oxygen-containingspecies.

To investigate further the irreversible feature just discussed,we conducted a scan rate study at a Pt(544) electrode surface. InFigure 6, the current response normalized by the scan rate v isshown. At high v, the voltammetric profile is almost reversibleand closely resembles that of Pt(554), a surface with a verysimilar step density but on which the steps have {110} symmetry.The latter surface was found to display a v-normalized butterfly

peak that is independent of v in the whole range of sweep ratesstudied (not shown). Interestingly, the behavior shown inFigure 5B, where {100} steps give rise to a new peak at potentialslower than those at which the remaining terrace-associated spikeappears, is observed only at lower scan rates. This new peakgrows (on the v-normalized scale) with decreasing sweep rateuntil it dominates the voltammetric profile of the region. Thevoltammetric changes brought about by introducing {100} stepsmay be caused by the oxidation of step sites. Such oxidationwould then be better described as formation of a local oxiderather than as simple adsorption of an oxygen-containing species.For instance, place-exchange betweenmetal and oxygen, or someother rearrangement of the local structure at the step, couldexplain the observed irreversibility. As discussed below, the newpeak could also be due to the oxidation of some contaminantpresent in the solution at low concentration, as such a processwould also become more important at low scan rates.

Figure 7A shows the positive-going, v-normalized voltamme-try of Figure 6 after subtracting the curve for v = 1 V s�1. If it isassumed that only terrace sites contribute to the current at thishigh scan rate, in accordance with the behavior of {110}-steppedsurfaces at any v, then the auxiliary variable Cstep = j(v)/v� j(v =1 V s�1) can be considered to be an estimate of the {100} stepresponse. Therefore, the charge qstep involved in the adsorptionof oxygenated species at {100} step sites can be obtained bydirect integration of the main Cstep peak. If the auxiliary quantityCstep is renormalized by v1/2 (that is, multiplied with v1/2 becauseit is already v-normalized), then the main peaks of Figure 7Aattain a constant height, as shown in Figure 7B. This means thatsuperimposed on the terrace response, which itself scales with v,the process causing the irreversibility contributes to a voltam-metric current that scales only with v1/2. Such a dependence isusually caused by diffusion control, either from transport of adissolved reactant to the surface30 or from the diffusion of anadsorbed species to active surface sites.31 Although the differencein the behavior between {110} and {100} steps may look strange,previous results on the electrocatalytic behavior of both stepsreveal that their effects on the reactions are different.32,33 For

Figure 5. Magnitude of the cyclic voltammetric current in the butterflyregion at Pt(111)-vicinal surfaces in 0.1 M HClO4. (A) Pt(s)[n(111)�(100)] and (B) Pt(s)[(n � 1)(111) � (110)] surfaces. Numbersindicate nominal terrace widths, and arrows show the scan directions.Black curves correspond to positive-going and red curves to negative-going scans. Scan rate v = 50 mV s�1.

Figure 6. Cyclic voltammetry of Pt(544) in the butterfly region at scanrates v = 2, 5, 10, 20, 50, 100, 200, 500, and 1000 mV s�1 and normalizedby v. Arrows indicate increasing v, electrolyte 0.1 M HClO4.



instance, the {110} step is able to break the C�C bond in theethanol oxidation reaction, whereas {100} is inactive.32 Thus, it ispossible that the oxidation of both steps may follow differentreaction kinetics. More work is in progress to understand theorigin of these differences.

Whereas the apparent diffusion control of the unknown peakrelated to {100} steps is consistent with a contamination, there isgood reason to conclude that the irreversible process is in factstep oxidation. First, the peak is entirely reproducible, andFigure 6 contains superimposed scans collected on differentdays with meticulous cleaning in between (green and pink curvesare doubled). In fact, when closely examining the blank voltam-metry of stepped (111)-vicinal surfaces in the literature, theirreversibility in the butterfly is always seen for {100} steps,whereas {110} steps give more symmetric peaks.29,34,35 Second,the behavior is entirely specific to {100} steps because no surfacecontaining {110} steps shows the asymmetry at 50 mV s�1

(Figure 5A) and also because the v-normalized voltammetry ofPt(554) is independent of v in this potential region, as discussedabove. An entirely site-specific reaction with a contaminantcannot be ruled out, but it seems very unlikely that a contaminantwould not at all affect (111) terraces or {110} steps. Therefore,we propose that steps of {100} orientation, at least whenseparating (111) terraces, oxidize in a particular structure-sensitive and rather slow process, in a potential region thatcoincides with the butterfly peak of Pt(111) in HClO4.

On the basis of this conclusion, we propose that the observeddiffusion behavior is caused by oxygen-containing adsorbatesslowly moving across (111) terraces and eventually arriving andreacting at {100} step sites. Once at the step, this terrace oxygenspecies, assumed to be OH in agreement with the conventionalinterpretation,18�21 would oxidize to step-bonded O

OHt f Os þ Hþ þ e

This reaction would involve a small amount of charge, the scanrate dependence of which would give information about thediffusion process. At the lowest scan rate used, the estimated

charge qstep was 16 μC cm�2 (Figure 8), equivalent to 0.57oxygen coverage on the step. To establish a relationship betweenscan rate and step oxidation charge, the Cottrell equation30 forpotential-jump experiments can be considered to be a roughapproximation, even under the present potential-sweep condi-tions. Surface diffusion to a step line is essentially equivalent tobulk diffusion to a plane, and the Cottrell equation becomes

jd ¼ ðnFD1=2Γ�Þ=½ðπtÞ1=2d�whereD is the surface diffusion coefficient,Γ* is the initial surfaceconcentration on the terrace, d is the terrace length, and the othersymbols have their usual meanings. Integration gives the chargedensity

qd ¼ ð2nFD1=2Γ�Þt1=2=ðπ1=2dÞ

¼ fð2nFD1=2Γ�Þ=ðπ1=2dÞgðΔE=νÞ1=2

where on the right-hand side t has been replaced by the peakwidth ΔE divided by v to estimate the amount of time duringwhich the reaction proceeds for a given scan rate. Because thepeak width can be considered to be constant (Figure 7B), theequation predicts a linear plot of charge density with ν�1/2,which is experimentally observed, as shown in Figure 8. From theslope, it is possible to determine the diffusion coefficient forthe surface process, provided that the values of the magnitudes inthe brackets are known. If we assume that the initial surfaceconcentration of OH on the terrace is ∼50% of the saturationmonolayer, measured on Pt(111) (Figure 2), then Γ* = 3.4 �1014 molecules cm�2 if the whole charge is considered. However,if only the broad part of the feature, the so-called (OH)B,18 isconsidered because the peak appears in the lower potentialregion of the butterfly, then Γ* = 2.5 � 1014 molecules cm�2.Taking these values, the surface diffusion coefficient would beD =1.4 � 10�20 m2 s�1 or 2.6 � 10�20 m2 s�1, respectively. Theseapproximate values are reasonable, corresponding to root-mean-square displacements

√(2Dt) no greater than 8 or 11 Å (with t =

25 s, as given by the slowest scan rate used). These distances arecomparable to d = 21 Å, the terrace width of Pt(544).

4. CONCLUSIONS

In the present study, we have shown that Pt(111) and itsvicinal surfaces oxidize in several stages, depending on surfacegeometry and anion adsorption. At potentials as low as 0.7 V,overlapping with the so-called butterfly peak originating from(111) terrace sites, a hitherto unseen oxidation process occursexclusively at {100} steps in the absence of specific anion

Figure 8. Charge qstep involved in the slow oxidation process at {100}steps plotted against v�1/2.

Figure 7. (A) Auxiliary quantity Cstep obtained from the positive-goingscans of Figure 6 after subtracting that of v = 1 V s�1. (B) Cstep

renormalized by v1/2.



adsorption. This process is relatively slow on the conventionalvoltammetric time scale (say, 50 mV s�1) and is clearlyobserved only at lower scan rate. It involves a diffusion process,assumed to be the migration of oxygen-containing species tostep sites, and a simple diffusion model allows estimation of thediffusion coefficient. Almost irrespective of the electrolyteanion adsorption behavior, both {110} and {100} steps alsooxidize between 0.9 and 1.2 V. The shapes of the correspondingvoltammetric peaks are characteristic of the respective stepgeometries, and their positions are slightly shifted by thepresence of an adsorbing anion. In perchloric acid, Pt(111) iselectrochemically oxidized in a complex process, ranging fromthe butterfly at 0.6 V to the onset of O2 evolution at 1.5 V, butmost of this behavior is suppressed when in contact with asulfuric acid electrolyte. Interestingly, however, the introduc-tion of steps again allows oxidation at lower potentials undersulfuric acid conditions, probably because of the breaking oflong-range order in the protective sulfate adlayer. It was foundthat {100} steps oxidize to the extent of one electron per stepatom, whereas {110} steps show two consecutive oxidations,amounting to a total of two electrons per atom. At the onset ofO2 evolution, Pt(111) itself is oxidized with two electrons persurface atom.

’AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected].

’ACKNOWLEDGMENT

This study has been carried out in the framework of theEuropean Commission FP7 Initial Training Network “ELCAT”,grant agreement no. 214936-2.

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electrochemical oxidation of pt(1 1 1) vicinal surfaces: effects of surface structure and specific...

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