2124 DOI: 10.1021/la902569z Langmuir 2010, 26(3), 2124–2129Published on Web 09/22/2009

pubs.acs.org/Langmuir

© 2009 American Chemical Society

Use of Model Pt(111) Single Crystal Electrodes under HMRDE

Configuration To Study the Redox Mechanism for Charge Injection

at Aromatic/Metal Interfaces

Margarita Rodrı́guez-L�opez,†,‡ Paulino Tu~n�on,‡ Juan M. Feliu,§ Antonio Aldaz,*,§ andArnaldo Carrasquillo, Jr.*, )

†Pontifical Catholic University of Puerto Rico, Ponce, Puerto Rico, ‡University of Oviedo, Oviedo, Spain,§University of Alicante, Alicante, Spain, and )University of Puerto Rico, Mayag€uez, Puerto Rico

Received July 14, 2009. Revised Manuscript Received August 24, 2009

The electrochemical reactivity of hydroquinone-derived, catechol-derived and benzene-derived adlayers is comparedat Pt(111) single-crystal surfaces (i) under stagnant hanging meniscus (HM) configuration and (ii) under hydrodynamicconditions imposed by combining the HM configuration with the rotating disk electrode (RDE) that merge in the so-called HMRDE technique. For the three cases studied, the results suggest that reductive desorption of the adlayers canbe accomplished in aqueous 0.5MH2SO4 solutions within the time frame of a single cathodic scan, i.e. the first half of asingle CV experiment. The results highlight the simplicity of exploiting the hydrodynamic conditions imposed by RDEas a convenient electroanalytical strategy to elucidate controversies regarding whether desorption takes place or notduring electrode processes studied under the HM configuration.

1. Introduction

A current technological trend is the use of aromatic organicmolecules for the construction of molecular electronic devices.1

Envisioned molecular electronic devices2 range from field effecttransistors3 to solar cells.4 The theoretical description of funda-mental aspects of these technologies relies extensively on electro-chemical concepts, i.e. from the proposed mechanisms of chargetransport through aromatic molecules to the injection of chargeacross aromatic/metal electrical contacts in the devices. A moredetailed understanding of electron-transfer reactions at aromatic/metal interfaces could help achieve advances in the design of suchmolecular electronic devices. A related field, electrochemicalsurface science (ESS),5 merges the inherent surface sensitivity ofclassical electrochemistry methods6 and surface science techni-ques and concepts7 with the aim of developing a comprehensive,atomic-level understanding of electron-transfer processes andreactions at electrode-electrolyte interfaces. At basal M(hkl)single-crystal surfaces, an enduring theme has been the structuraland electrochemical characterization of adlayers which resultfrom the adsorption of unsaturated molecules possessing varyingaromatic character. Transfer of knowledge between the two fieldsseems desirable and possible, hence, the need for a detailed

molecular-level understanding of differences in charge injectionmechanisms.

In ESS well-ordered Pt(hkl) single crystal electrodes, preparedby themethods pioneeredbyClavilier,8 have beenbroadly adoptedasmodel systems to elucidate the controlling role played by surfacestructure and composition over electrochemical reactivity. Studiesof aniline,9 benzene (C6H6),

10 parabanic acid,11 hydroquinone(p-H2Q)12,13 and catechol (o-H2Q)14 adsorption have been re-ported at bead-type Pt(111) single-crystal electrode surfaces. Thespontaneous formation of compact adlayers derived, in acidicaqueous media, from such molecules have been reported. Well-ordered benzene-derived and hydroquinone-derived adlayers wereproven to form at Pt(111) facets by Itaya using in situ scanningtunneling microscopy (STM).10,15,16 A common trend, establishedby the cited studies, is that redox processes ascribed to oxidation/reduction at the aromatic/metal interface have been observedduring cyclic voltammetry experiments at potentials near thehydrogen evolution region, at well-ordered Pt(111) single crystalelectrodes, in acidic media (vide infra). Discrepancies still existregarding the exact chemical nature of those redox processes.

One standing hypothesis17 suggests that the redox processobserved for benzene at Pt(111) could be due to absorption of

*Corresponding authors. A.C.: Department of Chemistry, University ofPuerto Rico, Mayag€uez, PR, 00681-9019; e-mail, arnaldo.carrasquil@upr.edu and c4rr4s4@gmail.com; tel, 787-832-4040 x2386; fax, 787-265-3849. A.A.: Departamento de Quı́mica Fı́sica, Instituto Universitario de Electroquı́-mica, Universidad de Alicante, Apartado 99, E03080, Alicante, Spain; e-mail,aldaz@ua.es; tel, int+ 34 965 903 535; fax, int+ 34 965 903 537/3464.(1) Forrest, S. R.; Thompson, M. E. Chem. Rev. 2007, 107(4), 923–925.(2) Ulgut, B.; Abru~na, H. D. Chem. Rev. 2008, 108(7), 2721–2736.(3) Bao, Z., Locklin, J., Eds. Organic Field-Effect Transistors; CRC Press Taylor

& Francis Group, LLC: Boca Raton, FL, 2007.(4) Pagliaro,M. ; Palmisano, G.; Ciriminna, R.Flexible Solar Cells;Wiley-VCH:

Weinheim, 2008.(5) Wieckowski, A., Ed. Interfacial Electrochemistry. Theory, Experiments and

Applications; Marcel Dekker: New York, 1999.(6) Bard, A. J.; Faulkner L. R. Electrochemical Methods: Fundamentals and

Applications; 2nd ed.; John Wiley and Sons, Inc: New York, 2001.(7) Masel, R. I. Principles of Adsorption and Reaction on Solid Surfaces.; John

Wiley & Sons: New York, 1996.

(8) Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J. Electroanal. Chem. 1979,107(1), 205–209.

(9) Albalat, R.; Claret, J.; Feliu, J. M.; Clavilier, J. J. Electroanal. Chem. 1990,288(1-2), 277–283.

(10) Yau, S. L.; Kim, Y. G.; Itaya, K. J. Am. Chem. Soc. 1996, 118(33), 7795–7803.

(11) Albalat, R.; Claret, J.; Rodes, A.; Feliu, J. M. J. Electroanal. Chem. 2003,550-551, 53–65.

(12) Rodriguez-Lopez, M.; Herrero, E.; Feliu, J. M.; Tunon, P.; Aldaz, A.;Carrasquillo, A. J. Electroanal. Chem. 2006, 594(2), 143–151.

(13) Rodriguez-Lopez, M.; Rodes, A.; Herrero, E.; Tunon, P.; Feliu, J. M.;Aldaz, A.; Carrasquillo, A. Langmuir. 2009, 25(17), 10337-10344.

(14) Rodriguez-Lopez,M.; Rodes,A.; Berna,A.; Climent,V.;Herrero, E.; Tunon,P.; Feliu, J. M.; Aldaz, A.; Carrasquillo, A. Langmuir 2008, 24(7), 3551–3561.

(15) Inukai, J.; Wakisaka, M.; Itaya, K. Chem. Phys. Lett. 2004, 399(4-6), 373–377.

(16) Inukai, J.; Wakisaka, M.; Yamagishi, M.; Itaya, K. Langmuir 2004, 20(18),7507–7511.

(17) Jerkiewicz, G.; DeBlois,M.; Radovic-Hrapovic, Z.; Tessier, J. P.; Perreault,F.; Lessard, J. Langmuir 2005, 21(8), 3511–3520.

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Rodrı́guez-L�opez et al. Article

hydrogen into the Pt substrate occupying subsurface latticesites. According to these authors, hydrogen absorption intothe subsurface is promoted by the presence of the aromaticadlayer, which persists throughout the voltammetric experimenthence modifying the properties of the most stable, closed anddense Pt(hkl) crystallographic plane, i.e. Pt(111), as depicted inFigure 1.

On the basis of their hypothesis, those authors17 have called forthe re-examination of the seminal studies by Itaya et al.,10 whichhad established by in situ STM that the CV features were due tomolecular desorption of the benzene-derived adlayers and toconcomitant HUPD adsorption on surface sites. This reportpresents a simple electroanalytical strategy to elucidate thisfundamental, but controversial, aspect of the redox reactivity ofaromatic molecules at Pt(111) interfaces.

All of the ESS studies cited above havemade use of the hangingmeniscus (HM) configuration. The HM configuration;depictedin the insets of Figures 2, 3 and 4;is ubiquitous in ESS becauseit permits selective wetting of the oriented and polished (hkl)crystallographic surface of interest. This prevents interferencefrom other surfaces that may be present at the Pt metal sampleand simplifies interpretation of the electrochemical mea-surements, which are notoriously sensitive to differences insurface structure and composition. In spite of these criticaladvantages, the HM configuration is not without caveats. Forexample, constituents (products, reactants, etc.) having relativelylow density, low solubility, high volatility, among other proper-ties, might exhibit mass-transport nuisances related to gravita-tional and/or solubility effects that are uniquely manifest undertheHMconfiguration. In thismanuscript such effects are avoidedby use of the hangingmeniscus rotating disk electrode (HMRDE)technique.18 Here the HM configuration is combined with thehydrodynamic conditions of the RDE technique to circumventpotential mass-transport nuisances. The redox behavior at Pt-(111) electrodes for hydroquinone-derived, catechol-derived,and benzene-derived adlayers are compared under two differentmass-transport conditions. Thus CV experiments under thestagnant mass-transport condition resulting from the HM con-figuration typically used in ESS studies, are compared vis-�a-visresults obtained under the hydrodynamic conditions imposedusing the HMRDE configuration. The aim of the comparisonis to elucidate (i) if molecular desorption takes place duringthe CV excursions, hence permitting HUPD adsorption atPt(111) surface atoms as proposed by Itaya et al.;10 or (ii) if,instead, aromaticmolecules remain chemisorbed during electrodepolarization, hence bringing about the new physicochemicalproperties postulated to exist by Jerkiewicz et al.17 at the noblemetal/aromatic surface and/or subsurface. The hydroquinone

and catechol molecule were selected for comparison purposesbecause their spectroelectrochemical reactivity at well-orderedPt(111) electrodes has been studied and reported,12,14 thusserving as model, electrochemically active, aromatic mole-cular probes. For the three adlayers studied, the results de-monstrate that reductive desorption of the molecules is accom-plishedwithin the time frameof the first cathodic scan, i.e. the firsthalf of the CV cycle, supporting the description by Itaya et al.10

and suggesting that the hypothesized mechanism of adlayer-promoted hydrogen absorption cannot be responsible for the redoxprocesses observed at these aromatic/Pt(111) electrochemicalinterfaces. More importantly, the results highlight the useful-ness of HMRDE in elucidating similar electroanalytical contro-versies.

2. Experimental Section

Aqueous 0.5 M H2SO4 solutions were used as supporting electrolyte throughout the voltammetric study. They were

Figure 1. Schematic representation of sub-surface, benzene-pro-moted hydrogen absorption at Pt(111). Reprinted with permissionfrom ref 17. Copyright 2005 American Chemical Society.

Figure 2. (a) Cyclic voltammograms of well-ordered Pt(111) sin-gle crystal electrode collected in 0.5 M H2SO4 supporting electro-lyte solution: (i) for a clean Pt(111) electrode, dashed line (- - -); (ii)for a hydroquinone-treated Pt(111) electrode, solid lines. The twoconsecutive cyclic voltammograms, 1st (thin line) and 2nd (thickline), were collected underHMconfiguration. Scan rate 50mVs-1.Temp 25 �C. (b) Cyclic voltammograms of well-ordered Pt(111)single crystal electrode collected in 0.5 M H2SO4 supportingelectrolyte solution: (i) for a clean Pt(111) electrode, dashed line(- - -); (ii) for a hydroquinone-treated Pt(111) electrode, solid lines.The two consecutive cyclic voltammograms, 1st (thin line) and 2nd(thick line),were collectedunderHMRDEconfiguration. 600 rpm.Scan rate 50 mV s-1. Temp 25 �C.

(18) Villullas, H.M.; Teijelo,M. L. J. Electroanal. Chem. 1995, 384(1-2), 25–30.

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Article Rodrı́guez-L�opez et al.

prepared from concentrated sulfuric acid (Merck Suprapur orAldrich Teflon grade) and Purelab Ultra (Elga-Vivendi) water(18MΩ cm). This electrolyte is convenient because the adsorptionstates at Pt(hkl) surfaces have been thoroughly studied and arewell-defined voltammetrically. Pt(111) single crystal electrodesurfaces were prepared from single-crystal beads using the pro-cedures developed by Clavilier et al.19 All Pt(111) single crystalsurfaces have been cooled in an atmosphere of argon/hydrogen.Working electrode diameters were around 2 mm for the voltam-metric experiments. Hanging meniscus configuration was usedthroughout. All experiments were conducted at room tempera-ture, 25 �C ((2 �C). p-H2Q and o-H2Q were obtained fromAldrich and used as received. Benzene (99.9%) was obtainedfrom Fluka and used as received. High purity gases (5N) were

used. An EG&G PAR model 175 Universal Programmer, anAMEL551potentiostat, a SoltecXYrecorder andan eCorder401(eDAQ,Australia) were used in the voltammetric experiments forthe Pt(hkl) electrodes. An EDI 101 RDE and a CTV 101controller from Radiometer Analytical where used during theHMRDE experiments. Platinum counter electrodes were used,and all potentials were measured and are reported versus thereversible hydrogen electrode (RHE) with the same supportingelectrolyte solution. These were contained in a separate compart-ment from the working electrode with contacts made through aLuggin capillary.

Six independent experiments (Figures 2a, 2b, 3a, 3b, 4a and 4b)were performed, using freshly prepared aromatic molecular ad-layers created by (i) equilibrating the clean Pt(111) electrodesduring 5 min at 0.6 V in a freshly prepared 2 mM solution of thearomatic molecule which also contained 0.5 M H2SO4 as asupporting electrolyte, (ii) removing each Pt(111) aromatic-coated electrode from the 2 mM solution, (iii) rinsing the coatedelectrode with clean supporting electrolyte solution to preventdirect transfer of the organic molecules into the final test cell, and(iv) starting to scan in the cathodic direction from the initialpotential, i.e. 0.6 V, in clean supporting electrolyte.

Figure 3. (a) Cyclic voltammograms of well-ordered Pt(111) sin-gle crystal electrode collected in 0.5 M H2SO4 supporting electro-lyte solution: (i) for a clean Pt(111) electrode, dashed line (- - -); (ii)for a catechol-treated Pt(111) electrode, solid lines. The two con-secutive cyclic voltammograms, 1st (thin line) and 2nd (thick line),were collected under HM configuration. Scan rate 50 mV s-1.Temp 25 �C. (b) Cyclic voltammograms of well-ordered Pt(111)single crystal electrode collected in 0.5 M H2SO4 supportingelectrolyte solution: (i) for a clean Pt(111) electrode, dashed line(- - -); (ii) for a catechol-treated Pt(111) electrode, solid lines. Thetwo consecutive cyclic voltammograms, 1st (thin line) and 2nd(thick line),were collected underHMRDEconfiguration. 600 rpm.Scan rate 50 mV s-1. Temp 25 �C.

Figure 4. (a) Cyclic voltammograms of well-ordered Pt(111) sin-gle crystal electrode collected in 0.5 M H2SO4 supporting electro-lyte solution: (i) for a clean Pt(111) electrode, dashed line (- - -); (ii)for a benzene-treated Pt(111) electrode, solid lines. The two con-secutive cyclic voltammograms, 1st (thin line) and 2nd (thick line),were collected under HM configuration. Scan rate 50 mV s-1.Temp 25 �C. (b) Cyclic voltammograms of well-ordered Pt(111)single crystal electrode collected in 0.5 M H2SO4 supportingelectrolyte solution: (i) for a clean Pt(111) electrode, dashed line(- - -); (ii) for a benzene-treated Pt(111) electrode, solid lines. Thetwo consecutive cyclic voltammograms, 1st (thin line) and 2nd(thick line),were collectedunderHMRDEconfiguration. 600 rpm.Scan rate 50 mV s-1. Temp 25 �C.

(19) Clavilier, J.; El Achi, K.; Petit, M.; Rodes, A.; Zamakhchari, M. A. J.Electroanal. Chem. 1990, 295, 333–356.

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3. Results and Discussion

3.1. Desorption of Hydroquinone-Derived Adlayers

(Q(ads)) at Model Pt(111) Single Crystals: CV Characteri-

zation Using a Hanging Meniscus under Stagnant Mass-

Transport Conditions and under Hydrodynamic Conditions

Imposed by Rotation of the Electrode under HMRDE

Conditions. Figure 2a shows three CV experiments performedunder stagnant HM conditions. They were obtained using a well-ordered Pt(111) single crystal electrode immersed in 0.5MH2SO4

supporting-electrolyte solutions. The dashed line in Figure 2awascollected first. It shows the now familiar shape of the CV for aclean,well-ordered Pt(111) single crystal electrode surface in clean0.5MH2SO4. The near defect-free nature of the Pt(111) electrodewas verified using this CV, which serves as the starting point forthe experiments. As reported previously by Clavilier,19 the CVshould (i) be reproducible and stable upon repetitive cycling and(ii) contain reversible anodic and cathodic features that exhibit(iii) a near featureless hydrogen UPD region from 0.3 V ontomore negative potentials with (iv) a sharp spike at ca. 0.45 V,which overlaps (v) the (bi)sulfate adsorption contribution fromca. 0.3 to 0.5 V. The onset of the hydrogen evolution reaction(HER) takes place at less positive potentials, from ca. 0.08 Vtoward smaller values.

After characterization, the clean, well-ordered Pt(111) singlecrystal electrode was transferred into a 2mMH2Q(aq) and treatedat 0.6V, to produce a Pt(111) electrode fully coatedwithQ(ads). Atthis potential, the oxidative chemisorption of hydroquinone isknown to take place.12,13 The formation of such Q(ads) adlayersmay be reverted, at well-ordered Pt(111) surfaces, by handling ofan externally controlled parameter, i.e. applied electrode poten-tial, in amanner consistentwith the redox process depicted in eq1:

QðadsÞPtð111Þ þ ðzþ 2Þe- þðzþ 2ÞHþ

sPtð111Þ zHðadsÞ

Ptð111Þ þH2QðaqÞ ð1Þ

where z represents the number of hydrogens adsorbed on thePt(111) surface domains when Q(ads) is replaced, and Q(ads) isbelieved to be the corresponding surface-coordinated quinone(vide infra). In Figure 2a, the CV curves shown as solid lines werecollected in clean 0.5 M H2SO4 supporting electrolyte afterformation of the Q(ads) adlayer. The redox pair at ca. 0.06 V isascribed to hydrogen-assisted, reductive-desorption of Q(ads), aprocess described in eq 1 and described in detail elsewhere.12

Briefly, three aspects in the first cycle, thin solid line, reveal theformation of a compact, full monolayer of Q(ads) on the Pt(111)electrode surface. First is the complete disappearance of the(bi)sulfate adsorption, of the spike and of the characteristichydrogen UPD at clean Pt(111). The second is the appearanceof the new redox process centered at ca. 0.06 V due to hydrogen-assisted reductive desorption of Q(ads) and to the oxidative read-sorption of the H2Q(aq) product created in the vicinity of theelectrode, according to eq 1. These peaks are observed at 0.064 Vduring the negative-going scan and at 0.092 V during the positive-going scan, respectively. The third aspect, suggesting formationof a compact arrangement ofmolecules in the adlayer, is thewidthof these redox peaks. The full width at half-maximum (fwhm) forthe cathodic process is in the order of 22 mV. For an idealnernstian reaction under Langmuir isotherm conditions the fwhmis expected to be (90.6/n) mV.20 In classic molecular adsorption

models,21 the observation of peaks narrower than (90.6/n) mV isusually ascribed to attractive interactionwithin the adlayer.22 Thefwhm suggests the presence of attractive adsorbate-adsorbateinteractions within the Q(ads) layer.

23 Quasi-linear surface aggre-gates have been reported to format low surface coverage forH2Q-derived adlayers resulting from gas-phase dosing of H2Q atPt(111) using ultrahigh vacuum (UHV) and scanning tunnelingmicroscopy (STM) techniques.15 An incommensurate (2.56 �2.56)R16� adlayer structure was reported by the same group16 atfull coverages for adlayers formed both from solution and fromvacuum. Note that the formation of surface aggregates and ofincommensurate adlayers may imply the presence of attractivelateral interactions within the organic adlayer, which would beconsistent with the observed fwhm. However, small values offwhm could also arise, in the absence of attractive lateral inter-actions, from other considerations24 such as competitive chemi-sorption processes.

The arrows in Figure 2a highlight the trend between the firstand second cycles.12,13 A decrease in the process at ca. 0.06 V anda concomitant, albeit limited, reappearance of hydrogen UPDand (bi)sulfate adsorption features, characteristic of the unper-turbed Pt(111) surface, can be observed. Under the mass-trans-port condition of this CV experiment, i.e. stagnant HM con-figuration, the reductive desorption process seems gradualbecause the reaction product, H2Q(aq), can be readsorbed oxida-tively during the positive going scan giving rise to the oxidationpeak in the CV traces collected under stagnant mass-transportconditions. The trend, after continuous cycling (not shown), hasbeen reported in the past12 and demonstrates that removal of theadlayer does take place during the excursions to negative poten-tials without disordering of the Pt(111) substrate.

Figure 2b shows CV experiments, similar to those in Figure 2a,which have been performed under hydrodynamic mass-transportconditions using the HMRDE configuration. As before (i) aclean,well-ordered Pt(111) single crystal electrode (see dashedCVfor reference) was treated to produce a fully Q(ads)-coated Pt(111)electrode. The thin solid-line CV experiment in Figure 2b wasperformed in 0.5 M H2SO4 clean supporting electrolyte immedi-ately after (ii) removing the quinone-coated Pt(111) electrodefrom the 2 mM H2Q(aq), (iii) rinsing the electrode with cleansupporting electrolyte solution, (iv) imposing a rotation rate of600 rpm on the Pt(111) electrode, and (v) starting to scan in thecathodic direction.

Note that the cathodic branch in the first CV (thin solid line) inFigure 2b, using theHMRDE configuration, is nearly identical tothe first cathodic branch (thin solid line) in Figure 2a, collectedusing stagnant HM conditions. Up to this point, no differencescan be appreciated between the experiments in Figure 2a andFigure 2b, in spite of the differences in the hydrodynamicconditions, and the peak at 0.061 V is present. However, severaldramatic differences are noted after reversal of the initial scandirection. The first difference is the complete disappearance of theoxidation process, when the experiment is performed under thehydrodynamic HMRDE conditions of Figure 2b. The oxidationprocess was previously observed, under the stagnant HM condi-tions of Figure 2a, at ca. 0.092 V during the positive-going scan. Ithad been ascribed to oxidative readsorption of the aqueoushydroquinone product, created in the vicinity of the electrodeduring the negative-going scan, according to eq 1. The second

(20) Bard, A. J.; Faulkner L. R. Electroactive Layers and Modified Electrodes.In Electrochemical Methods: Fundamentals and Applications, 2nd ed.; John Wileyand Sons, Inc.: New York, 2001; p 591.(21) Srinivasan, S.; Gileadi, E. Electrochim. Acta 1966, 11(3), 321–335.

(22) Laviron, E. J. Electroanal. Chem. 1974, 52(3), 395–402.(23) Angerstein-Kozlowska, H.; Klinger, J.; Conway, B. E. J. Electroanal.

Chem. 1977, 75(1, Part 1), 45–60.(24) Garcia-Araez, N.; Lukkien, J. J.; Koper, M. T. M.; Feliu, J. M. J.

Electroanal. Chem. 2006, 588(1), 1–14.

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notable difference is a more pronounced recovery of the(bi)sulfate adsorption and of the hydrogen UPD features char-acteristic of Pt(111) surfaces, when the experiment is performedunder the hydrodynamic HMRDE conditions of Figure 2b.Third, when the experiment is performed under HMRDE hydro-dynamic conditions, as in Figure 2b, the reductive desorptionprocess is not observed during the second cycle. The reductionpeak, observed during the first cathodic scan at 0.064 V, hasdisappeared in the second scan of Figure 2b. Instead, the recoveryof the (bi)sulfate adsorption and of the hydrogenUPD features isapparent in the second cathodic scan of Figure 2b.

In Figure 2b, the abrupt disappearance of the redox process atca. 0.06 V and the concomitant reappearance of the hydrogenUPD and (bi)sulfate adsorption features characteristic of thePt(111) surface can be explained by considering the hydrody-namic mass-transport condition imposed under the HMRDEconfiguration. Under the hydrodynamic mass-transport condi-tion, HMRDE, the reductive desorption product, H2Q(aq), iscarried away from the vicinity of the electrode by the forcedconvective stream of clean supporting electrolyte impinging onthe electrode surface. The reductively desorbed product, H2Q(aq),cannot be readsorbed oxidatively during the positive going scan,because it is transported away from the vicinity of the electrode,hence explaining the abrupt disappearance of the oxidativechemisorption process from Figure 2b and the disappearance ofthe reductive desorption process from the second cycle. Theinterpretation implies that reductive removal of the adlayer isnearly completed within the time frame of the initial cathodicexcursion, without significant disordering of the Pt(111) sub-strate, as implied in eq 1. Previous reports demonstrate thatPt(111) surface order is preserved and can bepresumedwithin thisrange of potentials.12,13

These results and their interpretation are in good agreementwith the above-cited studies13 which used the so-called subtrac-tively normalized interfacial Fourier transform infrared spectros-copy (SNIFTIRS) technique25 to achieve in situ characterizationof the reaction products. At well-ordered Pt(111) domains, thepresence of vertically adsorbed quinone molecules within theQ(ads) adlayer was deduced from the spectroelectrochemicalSNIFTIRS measurements. The reductive desorption productwas determined to be hydroquinone in solution, H2Q (aq). Thereductive desorption of the Q(ads) layers and their full oxidativereadsorptionwas determined to take place, even in the presence of8mMH2Q (aq), within the time frame of the potential steps used inthose experiments.3.2. Desorption of Catechol-Derived Adlayers at Model

Pt(111) Single Crystals.Another aromatic/Pt(111) system thathas been studied by in situ spectroelectrochemical SNIFTIRSmeasurements is the reductive desorption and oxidative chemi-sorption of catechol-derived adlayers (o-Q(ads)).

14 In that case, theadlayer formation process, at the Pt(111) electrode surface, wasreported to take place via a compositional 2D phase transition in2mMsolutions containing 0.5MH2SO4. In spite the difference inthe electrodynamics of adlayer formation, the reaction could alsobe described using a reaction analogous to eq 1, with thedistinction that the hydrogen-assisted, reductive-desorption pro-duct was reported to be catechol and the oxidative-chemisorptionproduct was tentatively postulated to be surface-coordinatedorthoquinone. These assignments were made on the basis of thein situ spectroscopic characterization afforded by the SNIFTIRStechnique.

Figure 3 shows the result of a set of experiments, performedat well-ordered Pt(111) coated with o-Q(ads). The experimentscontrast the electrochemical reactivity of the o-Q(ads) adlayerunder stagnant HM conditions (Figure 3a) and under hydro-dynamicHMRDEconditions (Figure 3b).A full discussionunderstagnant HM conditions may be found elsewhere.14 The CVexperiments shown as solid lines in Figure 3 were performedimmediately after analyzing the clean and well-ordered Pt(111)electrodes (dashed line CV) and treating them to ensure thepresence of a fresh o-Q(ads) molecular adlayer. Initially, i.e. firstcathodic scan in Figure 3a and in Figure 3b, the CV traces areidentical (see thin solid lines). As before, differences emerge uponchanging the scan direction. The small oxidative processes, 0.1 V,are not observed in Figure 3b. Note the disappearance of thereductive desorption process from the second cycle in Figure 3b.The concomitant reappearance of the hydrogen UPD and(bi)sulfate adsorption features characteristic of the clean Pt(111)surface is also observed. As before, these differences may beunderstood by considering that adlayer desorption takes placeduring the cathodic excursion, and that the reductive-desorptionproduct, H2Q(aq), is transported away from the vicinity of theelectrode as a result of the hydrodynamic conditions imposedunder the HMRDE configuration used in Figure 3b.

The consideration of mass-transport effects provides a simpleexplanation for the disappearance of the oxidative chemisorptionand reductive desorption features from theCV inFigure 2b and inFigure 3b, as well as for the concomitant reappearance of thehydrogen UPD and (bi)sulfate adsorption features. The redoxprocess associated with the organic adlayers does not disappearfrom Figure 2a and Figure 3a because, under stagnant mass-transport conditions, the reaction product cannot efficientlydiffuse away from the vicinity of the electrode, and hence canbe oxidatively readsorbed onto the electrode surface. Theseresults serve to confirm the possibility of using the HMRDEconfiguration as part of a simple electroanalytical scheme todetect if desorption takes place during the time frame of a CVexperiment for benzene adlayers, as reported by Itaya et al.16

Irrespective of solubility or gravitational effects or of the relativedensity, volatility or diffusivity of the reaction products, the massaction of the supporting electrolyte impinging on the electrodesurface should assist the transport of the reaction products awayfrom the electrode vicinity, as demonstrated under the well-characterized conditions afforded by hydroquinone and catechol,in Figure 2 and Figure 3.3.3. Desorption of Benzene-Derived Adlayers at Model

Pt(111) Single Crystals. Figure 4 shows the result of a set ofexperiments, performed at well-ordered Pt(111) electrode coatedwith benzene-derived adlayers. The experiments contrast theelectrochemical reactivity of benzene-derived adlayers understagnant HM conditions (Figure 4a) and under hydrodynamicHMRDE conditions (Figure 4b). Except for the use of benzene asan adsorbate, the experimental protocol usedwas identical to thatdiscussed in Figure 2 and Figure 3. The CV experiments inFigure 4 were performed in clean 0.5 M H2SO4 supportingelectrolyte after treatment of the clean Pt(111) electrode (dashedline) with freshly prepared 2 mM benzene. As expected, a redoxprocess, cEpk= 0.085 V and aEpk= 0.099 V, can be observed. Asbefore, the first cathodic scan is very similar for both benzene-treated electrodes in Figure 4a and Figure 4b. Contrary to theresults under stagnant HM conditions (see Figure 4a), inFigure 4b an abrupt disappearance of the redox processes isnoted after the initial cathodic scan. Instead of the anodic featureassociated to the adlayer, features characteristic of the Pt(111)surface, i.e. hydrogen UPD and (bi)sulfate adsorption, are(25) Ashley, K.; Pons, S. Chem. Rev. 1988, 88(4), 673–695.

DOI: 10.1021/la902569z 2129Langmuir 2010, 26(3), 2124–2129

Rodrı́guez-L�opez et al. Article

immediately recovered after reversing the scan direction in theHMRDE experiment (see Figure 4b). The results, in Figure 4b,are reminiscent of the results in Figure 2b and Figure 3b, andemerge from the hydrodynamic conditions imposed under theHMRDE configuration. It seems as if the effect of forcedconvection is highest for benzene. According to the electroanaly-tical strategy outlined previously, these results demonstrate thatthe benzene-derived adlayer in fact desorbs and is transportedaway from the vicinity of the Pt(111) electrode, during the firstnegative-going excursion, when the HMRDE hydrodynamicconditions are used (Figure 4b), i.e. nearly full desorption of thebenzene-derived adlayer takes place within the time frame of asingle voltammetric scan.

In light of the near quantitative desorption implicit from theHMRDE experiment in Figure 4b, it would be difficult tointerpret the results within the constraints imposed by thehypothesis of adlayer-promoted hydrogen absorption into thePt(111) subsurface. It would seem that the strong adlayer-sub-strate interaction needed to modify Pt-Pt interactions within thePt(111) subsurface could not be justified to exist at negativepotentials. The HMRDE results, however, (i) are consistentwith the model based on in situ STM measurements and con-structed by Itaya et al.,10 i.e. claiming that CV features are dueto molecular desorption of the benzene-derived adlayers andto the concomitant HUPD adsorption. The HMRDE results(ii) are also consistent with the interpretation proposed to explainthe redox features of hydroquinone-derived and catechol-derivedadlayers at Pt(111) electrodes, i.e. hydrogen-assisted reduc-tive desorption, which is supported by SNIFTIRS in situcharacterization of the desorption products,13,14 as depicted ineq 1. Finally (iii) these results highlight the simplicity of explo-iting the forced convection hydrodynamic conditions imposedby the RDE as a convenient electroanalytical strategy toelucidate similar controversies, regarding whether desorption

takes place or not at electrode processes studied under the HMconfiguration.

The general picture that emerges from these experiments is thatthe surface controls adsorption, i.e. at high potentials the organicmolecules are adsorbed and at low enough potentials desorptiontakes place. Upon desorption, hydrogen adsorption, which is afast process, takes place concomitantly leading to a re-establish-ment of the conditions characteristic of the organic-free Pt(111)sites and to mass-transport of the organic molecules away fromthe vicinity of the electrode. As reported elsewhere, the extent ofrecovery of organic-free Pt(111) sites depends on both the identityof the molecules and the specific experimental conditions used.Experimentally observed recovery has varied (i) from near-quantitative recovery12 for hydroquinone (ii) to a recovery ofonly 87% of the original surface sites14 in the case of catechol,reported after ten cycles. After reversing the scan direction,readsorption of the organic molecules is favored and will beaccompanied by hydrogen desorption. In the absence of theorganic molecules, an equivalent amount of anion specific ad-sorption takes place at potentials higher than ca. 0.3 V. In thespecific case of benzene at Pt(111), the charge balance analysisperformed by Jerkiewicz et al.17 does suggest the need for a morecomplex model, i.e. involving additional capacitive26 or faradaiccontributions, but the HMRDE results seem to point away fromthe proposed hypothesis17 of adlayer-promoted subsurface hy-drogen absorption.

Note Added after ASAP Publication. This article waspublished ASAP on September 22, 2009. Figure 3 has beenmodified. The correct version was published on September25, 2009.

Acknowledgment. A.C. and M.R.-L. gratefully acknowledgesupport from PCUPR, from UPRM, and from the Institute ofElectrochemistry at University of Alicante through Project CTQ2006-04071 (Ministerio de Ciencia y Tecnologı́a).

(26) Laredo, T.; Leitch, J.; Chen,M.; Burgess, I. J.; Dutcher, J. R.; Lipkowski, J.Langmuir 2007, 23(11), 6205–6211.