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 Rodrı´ guez-Lopez,†,‡ Paulino Tu~non,‡ Juan M. Feliu,§ Antonio Aldaz,*,§ and Arnaldo 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 compared at Pt(111) single-crystal surfaces (i) under stagnant hanging meniscus (HM) configuration and (ii) under hydrodynamic conditions imposed by combining the HM configuration with the rotating disk electrode (RDE) that merge in the socalled HMRDE technique. For the three cases studied, the results suggest that reductive desorption of the adlayers can be accomplished in aqueous 0.5 M H2SO4 solutions within the time frame of a single cathodic scan, i.e. the first half of a single CV experiment. The results highlight the simplicity of exploiting the hydrodynamic conditions imposed by RDE as a convenient electroanalytical strategy to elucidate controversies regarding whether desorption takes place or not during electrode processes studied under the HM configuration.
1. Introduction A current technological trend is the use of aromatic organic molecules for the construction of molecular electronic devices.1 Envisioned molecular electronic devices2 range from field effect transistors3 to solar cells.4 The theoretical description of fundamental aspects of these technologies relies extensively on electrochemical concepts, i.e. from the proposed mechanisms of charge transport through aromatic molecules to the injection of charge across aromatic/metal electrical contacts in the devices. A more detailed understanding of electron-transfer reactions at aromatic/ metal interfaces could help achieve advances in the design of such molecular electronic devices. A related field, electrochemical surface science (ESS),5 merges the inherent surface sensitivity of classical electrochemistry methods6 and surface science techniques and concepts7 with the aim of developing a comprehensive, atomic-level understanding of electron-transfer processes and reactions at electrode-electrolyte interfaces. At basal M(hkl) single-crystal surfaces, an enduring theme has been the structural and electrochemical characterization of adlayers which result from the adsorption of unsaturated molecules possessing varying aromatic character. Transfer of knowledge between the two fields seems desirable and possible, hence, the need for a detailed *Corresponding authors. A.C.: Department of Chemistry, University of Puerto Rico, Mayag€uez, PR, 00681-9019; e-mail, arnaldo.carrasquil@upr. edu and
[email protected]; tel, 787-832-4040 x2386; fax, 787-265-3849. A. A.: Departamento de Quı´ mica Fı´ sica, Instituto Universitario de Electroquı´ mica, Universidad de Alicante, Apartado 99, E03080, Alicante, Spain; e-mail,
[email protected]; 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.
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molecular-level understanding of differences in charge injection mechanisms. In ESS well-ordered Pt(hkl) single crystal electrodes, prepared by the methods pioneered by Clavilier,8 have been broadly adopted as model systems to elucidate the controlling role played by surface structure and composition over electrochemical reactivity. Studies of aniline,9 benzene (C6H6),10 parabanic acid,11 hydroquinone (p-H2Q)12,13 and catechol (o-H2Q)14 adsorption have been reported at bead-type Pt(111) single-crystal electrode surfaces. The spontaneous formation of compact adlayers derived, in acidic aqueous media, from such molecules have been reported. Wellordered benzene-derived and hydroquinone-derived adlayers were proven to form at Pt(111) facets by Itaya using in situ scanning tunneling microscopy (STM).10,15,16 A common trend, established by the cited studies, is that redox processes ascribed to oxidation/ reduction at the aromatic/metal interface have been observed during cyclic voltammetry experiments at potentials near the hydrogen evolution region, at well-ordered Pt(111) single crystal electrodes, in acidic media (vide infra). Discrepancies still exist regarding the exact chemical nature of those redox processes. One standing hypothesis17 suggests that the redox process observed for benzene at Pt(111) could be due to absorption of (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.
Published on Web 09/22/2009
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Figure 1. Schematic representation of sub-surface, benzene-promoted hydrogen absorption at Pt(111). Reprinted with permission from ref 17. Copyright 2005 American Chemical Society.
hydrogen into the Pt substrate occupying subsurface lattice sites. According to these authors, hydrogen absorption into the subsurface is promoted by the presence of the aromatic adlayer, which persists throughout the voltammetric experiment hence modifying the properties of the most stable, closed and dense Pt(hkl) crystallographic plane, i.e. Pt(111), as depicted in Figure 1. On the basis of their hypothesis, those authors17 have called for the re-examination of the seminal studies by Itaya et al.,10 which had established by in situ STM that the CV features were due to molecular desorption of the benzene-derived adlayers and to concomitant HUPD adsorption on surface sites. This report presents a simple electroanalytical strategy to elucidate this fundamental, but controversial, aspect of the redox reactivity of aromatic molecules at Pt(111) interfaces. All of the ESS studies cited above have made use of the hanging meniscus (HM) configuration. The HM configuration;depicted in the insets of Figures 2, 3 and 4;is ubiquitous in ESS because it permits selective wetting of the oriented and polished (hkl) crystallographic surface of interest. This prevents interference from other surfaces that may be present at the Pt metal sample and simplifies interpretation of the electrochemical measurements, which are notoriously sensitive to differences in surface structure and composition. In spite of these critical advantages, the HM configuration is not without caveats. For example, constituents (products, reactants, etc.) having relatively low density, low solubility, high volatility, among other properties, might exhibit mass-transport nuisances related to gravitational and/or solubility effects that are uniquely manifest under the HM configuration. In this manuscript such effects are avoided by use of the hanging meniscus rotating disk electrode (HMRDE) technique.18 Here the HM configuration is combined with the hydrodynamic conditions of the RDE technique to circumvent potential mass-transport nuisances. The redox behavior at Pt(111) electrodes for hydroquinone-derived, catechol-derived, and benzene-derived adlayers are compared under two different mass-transport conditions. Thus CV experiments under the stagnant mass-transport condition resulting from the HM configuration typically used in ESS studies, are compared vis-a-vis results obtained under the hydrodynamic conditions imposed using the HMRDE configuration. The aim of the comparison is to elucidate (i) if molecular desorption takes place during the CV excursions, hence permitting HUPD adsorption at Pt(111) surface atoms as proposed by Itaya et al.;10 or (ii) if, instead, aromatic molecules remain chemisorbed during electrode polarization, hence bringing about the new physicochemical properties postulated to exist by Jerkiewicz et al.17 at the noble metal/aromatic surface and/or subsurface. The hydroquinone
(18) Villullas, H. M.; Teijelo, M. L. J. Electroanal. Chem. 1995, 384(1-2), 25–30.
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Figure 2. (a) Cyclic voltammograms of well-ordered Pt(111) single crystal electrode collected in 0.5 M H2SO4 supporting electrolyte 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 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 supporting electrolyte 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 collected under HMRDE configuration. 600 rpm. Scan rate 50 mV s-1. Temp 25 °C.
and catechol molecule were selected for comparison purposes because their spectroelectrochemical reactivity at well-ordered Pt(111) electrodes has been studied and reported,12,14 thus serving as model, electrochemically active, aromatic molecular probes. For the three adlayers studied, the results demonstrate that reductive desorption of the molecules is accomplished within the time frame of the first cathodic scan, i.e. the first half of the CV cycle, supporting the description by Itaya et al.10 and suggesting that the hypothesized mechanism of adlayerpromoted hydrogen absorption cannot be responsible for the redox processes observed at these aromatic/Pt(111) electrochemical interfaces. More importantly, the results highlight the usefulness of HMRDE in elucidating similar electroanalytical controversies.
2. Experimental Section Aqueous 0.5 M H2SO4 solutions were used as support ing electrolyte throughout the voltammetric study. They were DOI: 10.1021/la902569z
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Figure 3. (a) Cyclic voltammograms of well-ordered Pt(111) single crystal electrode collected in 0.5 M H2SO4 supporting electrolyte solution: (i) for a clean Pt(111) electrode, dashed line (- - -); (ii) for a catechol-treated Pt(111) electrode, solid lines. The two consecutive 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 supporting electrolyte solution: (i) for a clean Pt(111) electrode, dashed line (- - -); (ii) for a catechol-treated Pt(111) electrode, solid lines. The two consecutive cyclic voltammograms, 1st (thin line) and 2nd (thick line), were collected under HMRDE configuration. 600 rpm. Scan rate 50 mV s-1. Temp 25 °C. prepared from concentrated sulfuric acid (Merck Suprapur or Aldrich Teflon grade) and Purelab Ultra (Elga-Vivendi) water (18 MΩ cm). This electrolyte is convenient because the adsorption states at Pt(hkl) surfaces have been thoroughly studied and are well-defined voltammetrically. Pt(111) single crystal electrode surfaces were prepared from single-crystal beads using the procedures developed by Clavilier et al.19 All Pt(111) single crystal surfaces have been cooled in an atmosphere of argon/hydrogen. Working electrode diameters were around 2 mm for the voltammetric experiments. Hanging meniscus configuration was used throughout. All experiments were conducted at room temperature, 25 °C ((2 °C). p-H2Q and o-H2Q were obtained from Aldrich and used as received. Benzene (99.9%) was obtained from Fluka and used as received. High purity gases (5N) were (19) Clavilier, J.; El Achi, K.; Petit, M.; Rodes, A.; Zamakhchari, M. A. J. Electroanal. Chem. 1990, 295, 333–356.
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Figure 4. (a) Cyclic voltammograms of well-ordered Pt(111) single crystal electrode collected in 0.5 M H2SO4 supporting electrolyte solution: (i) for a clean Pt(111) electrode, dashed line (- - -); (ii) for a benzene-treated Pt(111) electrode, solid lines. The two consecutive 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 supporting electrolyte solution: (i) for a clean Pt(111) electrode, dashed line (- - -); (ii) for a benzene-treated Pt(111) electrode, solid lines. The two consecutive cyclic voltammograms, 1st (thin line) and 2nd (thick line), were collected under HMRDE configuration. 600 rpm. Scan rate 50 mV s-1. Temp 25 °C. used. An EG&G PAR model 175 Universal Programmer, an AMEL 551 potentiostat, a Soltec XY recorder and an eCorder401 (eDAQ, Australia) were used in the voltammetric experiments for the Pt(hkl) electrodes. An EDI 101 RDE and a CTV 101 controller from Radiometer Analytical where used during the HMRDE experiments. Platinum counter electrodes were used, and all potentials were measured and are reported versus the reversible hydrogen electrode (RHE) with the same supporting electrolyte solution. These were contained in a separate compartment from the working electrode with contacts made through a Luggin capillary. Six independent experiments (Figures 2a, 2b, 3a, 3b, 4a and 4b) were performed, using freshly prepared aromatic molecular adlayers created by (i) equilibrating the clean Pt(111) electrodes during 5 min at 0.6 V in a freshly prepared 2 mM solution of the aromatic molecule which also contained 0.5 M H2SO4 as a supporting electrolyte, (ii) removing each Pt(111) aromaticcoated electrode from the 2 mM solution, (iii) rinsing the coated electrode with clean supporting electrolyte solution to prevent direct transfer of the organic molecules into the final test cell, and (iv) starting to scan in the cathodic direction from the initial potential, i.e. 0.6 V, in clean supporting electrolyte. Langmuir 2010, 26(3), 2124–2129
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3. Results and Discussion 3.1. Desorption of Hydroquinone-Derived Adlayers (Q(ads)) at Model Pt(111) Single Crystals: CV Characterization Using a Hanging Meniscus under Stagnant MassTransport Conditions and under Hydrodynamic Conditions Imposed by Rotation of the Electrode under HMRDE Conditions. Figure 2a shows three CV experiments performed under stagnant HM conditions. They were obtained using a wellordered Pt(111) single crystal electrode immersed in 0.5 M H2SO4 supporting-electrolyte solutions. The dashed line in Figure 2a was collected first. It shows the now familiar shape of the CV for a clean, well-ordered Pt(111) single crystal electrode surface in clean 0.5 M H2SO4. The near defect-free nature of the Pt(111) electrode was verified using this CV, which serves as the starting point for the experiments. As reported previously by Clavilier,19 the CV should (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 onto more negative potentials with (iv) a sharp spike at ca. 0.45 V, which overlaps (v) the (bi)sulfate adsorption contribution from ca. 0.3 to 0.5 V. The onset of the hydrogen evolution reaction (HER) takes place at less positive potentials, from ca. 0.08 V toward smaller values. After characterization, the clean, well-ordered Pt(111) single crystal electrode was transferred into a 2 mM H2Q(aq) and treated at 0.6 V, to produce a Pt(111) electrode fully coated with Q(ads). At this potential, the oxidative chemisorption of hydroquinone is known to take place.12,13 The formation of such Q(ads) adlayers may be reverted, at well-ordered Pt(111) surfaces, by handling of an externally controlled parameter, i.e. applied electrode potential, in a manner consistent with the redox process depicted in eq 1: QðadsÞ Ptð111Þ zHðadsÞ þ ðz þ 2Þe - þ ðz þ 2ÞHþ s þ H2 QðaqÞ ð1Þ Ptð111Þ Ptð111Þ where z represents the number of hydrogens adsorbed on the Pt(111) surface domains when Q(ads) is replaced, and Q(ads) is believed to be the corresponding surface-coordinated quinone (vide infra). In Figure 2a, the CV curves shown as solid lines were collected in clean 0.5 M H2SO4 supporting electrolyte after formation of the Q(ads) adlayer. The redox pair at ca. 0.06 V is ascribed to hydrogen-assisted, reductive-desorption of Q(ads), a process described in eq 1 and described in detail elsewhere.12 Briefly, three aspects in the first cycle, thin solid line, reveal the formation 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 characteristic hydrogen UPD at clean Pt(111). The second is the appearance of the new redox process centered at ca. 0.06 V due to hydrogenassisted reductive desorption of Q(ads) and to the oxidative readsorption of the H2Q(aq) product created in the vicinity of the electrode, according to eq 1. These peaks are observed at 0.064 V during the negative-going scan and at 0.092 V during the positivegoing scan, respectively. The third aspect, suggesting formation of a compact arrangement of molecules in the adlayer, is the width of these redox peaks. The full width at half-maximum (fwhm) for the cathodic process is in the order of 22 mV. For an ideal nernstian reaction under Langmuir isotherm conditions the fwhm is expected to be (90.6/n) mV.20 In classic molecular adsorption (20) Bard, A. J.; Faulkner L. R. Electroactive Layers and Modified Electrodes. In Electrochemical Methods: Fundamentals and Applications, 2nd ed.; John Wiley and Sons, Inc.: New York, 2001; p 591. (21) Srinivasan, S.; Gileadi, E. Electrochim. Acta 1966, 11(3), 321–335.
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models,21 the observation of peaks narrower than (90.6/n) mV is usually ascribed to attractive interaction within the adlayer.22 The fwhm suggests the presence of attractive adsorbate-adsorbate interactions within the Q(ads) layer.23 Quasi-linear surface aggregates have been reported to form at low surface coverage for H2Qderived adlayers resulting from gas-phase dosing of H2Q at Pt(111) using ultrahigh vacuum (UHV) and scanning tunneling microscopy (STM) techniques.15 An incommensurate (2.56 2.56)R16° adlayer structure was reported by the same group16 at full coverages for adlayers formed both from solution and from vacuum. Note that the formation of surface aggregates and of incommensurate adlayers may imply the presence of attractive lateral interactions within the organic adlayer, which would be consistent with the observed fwhm. However, small values of fwhm could also arise, in the absence of attractive lateral interactions, from other considerations24 such as competitive chemisorption processes. The arrows in Figure 2a highlight the trend between the first and second cycles.12,13 A decrease in the process at ca. 0.06 V and a concomitant, albeit limited, reappearance of hydrogen UPD and (bi)sulfate adsorption features, characteristic of the unperturbed Pt(111) surface, can be observed. Under the mass-transport condition of this CV experiment, i.e. stagnant HM configuration, the reductive desorption process seems gradual because the reaction product, H2Q(aq), can be readsorbed oxidatively during the positive going scan giving rise to the oxidation peak in the CV traces collected under stagnant mass-transport conditions. The trend, after continuous cycling (not shown), has been reported in the past12 and demonstrates that removal of the adlayer does take place during the excursions to negative potentials 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-transport conditions using the HMRDE configuration. As before (i) a clean, well-ordered Pt(111) single crystal electrode (see dashed CV for reference) was treated to produce a fully Q(ads)-coated Pt(111) electrode. The thin solid-line CV experiment in Figure 2b was performed in 0.5 M H2SO4 clean supporting electrolyte immediately after (ii) removing the quinone-coated Pt(111) electrode from the 2 mM H2Q(aq), (iii) rinsing the electrode with clean supporting electrolyte solution, (iv) imposing a rotation rate of 600 rpm on the Pt(111) electrode, and (v) starting to scan in the cathodic direction. Note that the cathodic branch in the first CV (thin solid line) in Figure 2b, using the HMRDE configuration, is nearly identical to the first cathodic branch (thin solid line) in Figure 2a, collected using stagnant HM conditions. Up to this point, no differences can be appreciated between the experiments in Figure 2a and Figure 2b, in spite of the differences in the hydrodynamic conditions, and the peak at 0.061 V is present. However, several dramatic differences are noted after reversal of the initial scan direction. The first difference is the complete disappearance of the oxidation process, when the experiment is performed under the hydrodynamic HMRDE conditions of Figure 2b. The oxidation process was previously observed, under the stagnant HM conditions of Figure 2a, at ca. 0.092 V during the positive-going scan. It had been ascribed to oxidative readsorption of the aqueous hydroquinone product, created in the vicinity of the electrode during the negative-going scan, according to eq 1. The second (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 characteristic of Pt(111) surfaces, when the experiment is performed under the hydrodynamic HMRDE conditions of Figure 2b. Third, when the experiment is performed under HMRDE hydrodynamic conditions, as in Figure 2b, the reductive desorption process is not observed during the second cycle. The reduction peak, observed during the first cathodic scan at 0.064 V, has disappeared in the second scan of Figure 2b. Instead, the recovery of the (bi)sulfate adsorption and of the hydrogen UPD features is apparent in the second cathodic scan of Figure 2b. In Figure 2b, the abrupt disappearance of the redox process at ca. 0.06 V and the concomitant reappearance of the hydrogen UPD and (bi)sulfate adsorption features characteristic of the Pt(111) surface can be explained by considering the hydrodynamic mass-transport condition imposed under the HMRDE configuration. Under the hydrodynamic mass-transport condition, HMRDE, the reductive desorption product, H2Q(aq), is carried away from the vicinity of the electrode by the forced convective stream of clean supporting electrolyte impinging on the 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 oxidative chemisorption process from Figure 2b and the disappearance of the reductive desorption process from the second cycle. The interpretation implies that reductive removal of the adlayer is nearly completed within the time frame of the initial cathodic excursion, without significant disordering of the Pt(111) substrate, as implied in eq 1. Previous reports demonstrate that Pt(111) surface order is preserved and can be presumed within this range of potentials.12,13 These results and their interpretation are in good agreement with the above-cited studies13 which used the so-called subtractively normalized interfacial Fourier transform infrared spectroscopy (SNIFTIRS) technique25 to achieve in situ characterization of the reaction products. At well-ordered Pt(111) domains, the presence of vertically adsorbed quinone molecules within the Q(ads) adlayer was deduced from the spectroelectrochemical SNIFTIRS measurements. The reductive desorption product was determined to be hydroquinone in solution, H2Q (aq). The reductive desorption of the Q(ads) layers and their full oxidative readsorption was determined to take place, even in the presence of 8 mM H2Q (aq), within the time frame of the potential steps used in those experiments. 3.2. Desorption of Catechol-Derived Adlayers at Model Pt(111) Single Crystals. Another aromatic/Pt(111) system that has been studied by in situ spectroelectrochemical SNIFTIRS measurements is the reductive desorption and oxidative chemisorption of catechol-derived adlayers (o-Q(ads)).14 In that case, the adlayer formation process, at the Pt(111) electrode surface, was reported to take place via a compositional 2D phase transition in 2 mM solutions containing 0.5 M H2SO4. In spite the difference in the electrodynamics of adlayer formation, the reaction could also be described using a reaction analogous to eq 1, with the distinction that the hydrogen-assisted, reductive-desorption product was reported to be catechol and the oxidative-chemisorption product was tentatively postulated to be surface-coordinated orthoquinone. These assignments were made on the basis of the in situ spectroscopic characterization afforded by the SNIFTIRS technique.
(25) Ashley, K.; Pons, S. Chem. Rev. 1988, 88(4), 673–695.
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Figure 3 shows the result of a set of experiments, performed at well-ordered Pt(111) coated with o-Q(ads). The experiments contrast the electrochemical reactivity of the o-Q(ads) adlayer under stagnant HM conditions (Figure 3a) and under hydrodynamic HMRDE conditions (Figure 3b). A full discussion under stagnant HM conditions may be found elsewhere.14 The CV experiments shown as solid lines in Figure 3 were performed immediately after analyzing the clean and well-ordered Pt(111) electrodes (dashed line CV) and treating them to ensure the presence of a fresh o-Q(ads) molecular adlayer. Initially, i.e. first cathodic scan in Figure 3a and in Figure 3b, the CV traces are identical (see thin solid lines). As before, differences emerge upon changing the scan direction. The small oxidative processes, 0.1 V, are not observed in Figure 3b. Note the disappearance of the reductive 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 be understood by considering that adlayer desorption takes place during the cathodic excursion, and that the reductive-desorption product, H2Q(aq), is transported away from the vicinity of the electrode as a result of the hydrodynamic conditions imposed under the HMRDE configuration used in Figure 3b. The consideration of mass-transport effects provides a simple explanation for the disappearance of the oxidative chemisorption and reductive desorption features from the CV in Figure 2b and in Figure 3b, as well as for the concomitant reappearance of the hydrogen UPD and (bi)sulfate adsorption features. The redox process associated with the organic adlayers does not disappear from Figure 2a and Figure 3a because, under stagnant masstransport conditions, the reaction product cannot efficiently diffuse away from the vicinity of the electrode, and hence can be oxidatively readsorbed onto the electrode surface. These results serve to confirm the possibility of using the HMRDE configuration as part of a simple electroanalytical scheme to detect if desorption takes place during the time frame of a CV experiment for benzene adlayers, as reported by Itaya et al.16 Irrespective of solubility or gravitational effects or of the relative density, volatility or diffusivity of the reaction products, the mass action of the supporting electrolyte impinging on the electrode surface should assist the transport of the reaction products away from the electrode vicinity, as demonstrated under the wellcharacterized 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 of experiments, performed at well-ordered Pt(111) electrode coated with benzene-derived adlayers. The experiments contrast the electrochemical reactivity of benzene-derived adlayers under stagnant HM conditions (Figure 4a) and under hydrodynamic HMRDE conditions (Figure 4b). Except for the use of benzene as an adsorbate, the experimental protocol used was identical to that discussed in Figure 2 and Figure 3. The CV experiments in Figure 4 were performed in clean 0.5 M H2SO4 supporting electrolyte after treatment of the clean Pt(111) electrode (dashed line) with freshly prepared 2 mM benzene. As expected, a redox process, cEpk = 0.085 V and aEpk = 0.099 V, can be observed. As before, the first cathodic scan is very similar for both benzenetreated electrodes in Figure 4a and Figure 4b. Contrary to the results under stagnant HM conditions (see Figure 4a), in Figure 4b an abrupt disappearance of the redox processes is noted after the initial cathodic scan. Instead of the anodic feature associated to the adlayer, features characteristic of the Pt(111) surface, i.e. hydrogen UPD and (bi)sulfate adsorption, are Langmuir 2010, 26(3), 2124–2129
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immediately recovered after reversing the scan direction in the HMRDE experiment (see Figure 4b). The results, in Figure 4b, are reminiscent of the results in Figure 2b and Figure 3b, and emerge from the hydrodynamic conditions imposed under the HMRDE configuration. It seems as if the effect of forced convection is highest for benzene. According to the electroanalytical strategy outlined previously, these results demonstrate that the benzene-derived adlayer in fact desorbs and is transported away from the vicinity of the Pt(111) electrode, during the first negative-going excursion, when the HMRDE hydrodynamic conditions are used (Figure 4b), i.e. nearly full desorption of the benzene-derived adlayer takes place within the time frame of a single voltammetric scan. In light of the near quantitative desorption implicit from the HMRDE experiment in Figure 4b, it would be difficult to interpret the results within the constraints imposed by the hypothesis of adlayer-promoted hydrogen absorption into the Pt(111) subsurface. It would seem that the strong adlayer-substrate interaction needed to modify Pt-Pt interactions within the Pt(111) subsurface could not be justified to exist at negative potentials. The HMRDE results, however, (i) are consistent with the model based on in situ STM measurements and constructed by Itaya et al.,10 i.e. claiming that CV features are due to molecular desorption of the benzene-derived adlayers and to the concomitant HUPD adsorption. The HMRDE results (ii) are also consistent with the interpretation proposed to explain the redox features of hydroquinone-derived and catechol-derived adlayers at Pt(111) electrodes, i.e. hydrogen-assisted reductive desorption, which is supported by SNIFTIRS in situ characterization of the desorption products,13,14 as depicted in eq 1. Finally (iii) these results highlight the simplicity of exploiting the forced convection hydrodynamic conditions imposed by the RDE as a convenient electroanalytical strategy to elucidate similar controversies, regarding whether desorption (26) Laredo, T.; Leitch, J.; Chen, M.; Burgess, I. J.; Dutcher, J. R.; Lipkowski, J. Langmuir 2007, 23(11), 6205–6211.
Langmuir 2010, 26(3), 2124–2129
Article
takes place or not at electrode processes studied under the HM configuration. The general picture that emerges from these experiments is that the surface controls adsorption, i.e. at high potentials the organic molecules are adsorbed and at low enough potentials desorption takes place. Upon desorption, hydrogen adsorption, which is a fast process, takes place concomitantly leading to a re-establishment of the conditions characteristic of the organic-free Pt(111) sites and to mass-transport of the organic molecules away from the vicinity of the electrode. As reported elsewhere, the extent of recovery of organic-free Pt(111) sites depends on both the identity of the molecules and the specific experimental conditions used. Experimentally observed recovery has varied (i) from nearquantitative recovery12 for hydroquinone (ii) to a recovery of only 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 be accompanied by hydrogen desorption. In the absence of the organic molecules, an equivalent amount of anion specific adsorption takes place at potentials higher than ca. 0.3 V. In the specific case of benzene at Pt(111), the charge balance analysis performed by Jerkiewicz et al.17 does suggest the need for a more complex model, i.e. involving additional capacitive26 or faradaic contributions, but the HMRDE results seem to point away from the proposed hypothesis17 of adlayer-promoted subsurface hydrogen absorption. Note Added after ASAP Publication. This article was published ASAP on September 22, 2009. Figure 3 has been modified. The correct version was published on September 25, 2009. Acknowledgment. A.C. and M.R.-L. gratefully acknowledge support from PCUPR, from UPRM, and from the Institute of Electrochemistry at University of Alicante through Project CTQ 2006-04071 (Ministerio de Ciencia y Tecnologı´ a).
DOI: 10.1021/la902569z
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