Langmuir 2008, 24, 3551-3561
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Model System for the Study of 2D Phase Transitions and Supramolecular Interactions at Electrified Interfaces: Hydrogen-Assisted Reductive Desorption of Catechol-Derived Adlayers from Pt(111) Single-Crystal Electrodes Margarita Rodriguez-Lopez,†,§ Antonio Rodes,‡ Antonio Berna´,‡ Victor Climent,‡ Enrique Herrero,‡ Paulino Tun˜on,§ Juan M. Feliu,‡ Antonio Aldaz,*,‡ and Arnaldo Carrasquillo, Jr.*,| Department of Chemistry, Pontifical Catholic UniVersity of Puerto Rico, Ponce, Puerto Rico 00717, Departament de Quı´mica Fı´sica, UniVersitat d’Alacant, Apartat 99, E03080, Alacant, Spain, Department of Physical and Analytical Chemistry, UniVersity of OViedo, OViedo, Spain 33006, and Department of Chemistry, UniVersity of Puerto Rico, Mayagu¨ez, Puerto Rico 00681-9019 ReceiVed August 29, 2007. In Final Form: NoVember 20, 2007 Classical electroanalytical techniques and in situ FTIR are used to study the oxidative chemisorption of catechol (o-H2Q) and the hydrogen-assisted reductive desorption of catechol-derived adlayers (o-Q(ads)) at nearly defect-free Pt(111) single-crystal electrodes in 0.5 M H2SO4. At near equilibrium conditions (limυf0) the cyclic voltammetric response does not conform to the behavior expected from classical models of molecular adsorption at electrochemical interfaces. Instead, attractive interactions play a controlling role, i.e., hydrogen-assisted displacement of o-Q(ads) takes place as an electrochemically reversible two-dimensional (2D) phase transition controlled by collision-nucleationgrowth phenomena in the presence of 2 mM o-H2Q(aq). In contrast, different desorption dynamics are observed when the reductive desorption of the adlayers is carried out in clean (0 mM o-H2Q(aq)) supporting electrolyte. Donoracceptor (DA) interactions between the Pt(111)/o-Q(ads) surface adduct and o-H2Q(aq) are postulated as a possible intervening mechanism leading to the observed differences in the macroscopic electrochemical responses. The results also demonstrate that in aqueous solutions it is thermodynamically feasible to shift the formal oxidation potential of catechol-metal adducts to potentials near those of molecular hydrogen via chemically reversible, nondissociative interactions, taking place as a 2D phase transition.
1. Introduction Electrochemical surface science1 merges classical electrochemistry2 and surface science techniques3 to develop a comprehensive understanding of reactions at the electrodeelectrolyte interface. In fundamental studies, well-ordered Pt(hkl) single-crystal electrodes have received most of the attention, having been one of the most commonly used materials for the construction of solid electrodes. A fundamental understanding of molecular adsorption at such electrochemical interfaces could help achieve advances in the design and construction of molecular electronic circuits, fuel cells, sensors, electrocatalysts and nanometer-sized mechanical devices, where lubrication and adhesion could play a relevant role. Past studies have revealed a need to develop an atomic-level understanding of the nature of adsorbate/surface bonding (mode of attachment, relative strength, and stability) as a prerequisite to understand, control, and predict electrochemical reactivity at electrochemical interfaces. Chemists have traditionally sought to develop this needed atomic-level understanding of molecular adsorption * Author to whom correspondence should be addressed. (A.C.) E-mail:
[email protected]; tel: 787-832-2386; fax: 787-265-3849. (A.A.) E-mail:
[email protected]; tel: int+ 34 965 903 535; fax: int+ 34 965 903 537/3464. † Pontifical Catholic University of Puerto Rico. ‡ Universitat d’Alacant. § University of Oviedo. | University of Puerto Rico. (1) Interfacial Electrochemistry: Theory, Experiment and Applications; Wiekowski, A., Ed.; Marcel Dekker: New York, 1999. (2) Bard, A. J.; Faulkner L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; John Wiley and Sons, Inc.: New York, 2001. (3) Masel, R. I. Principles of Adsorption and Reaction on Solid Surfaces; 1st ed.; John Wiley and Sons, Inc: Toronto, Canada, 1996.
phenomena aided by the surface cluster analogy4 and/or the electrode surface coordination models.5 The chemically familiar concepts used in the coordination models are of a localized nature (i.e., they consider localized bonding and/or localized electronsharing) and are most effective for the description of electrochemical phenomena where the principal consideration is that of localized adsorbate-substrate interactions. Despite a growing ability to correlate macroscopic electrochemical responses with microscopic, atomic-level composition and structure, an improved ability to predict one from the other is still needed. Toward this end, a more detailed understanding of adsorbate-adsorbate interactions may be needed. The development of scanning tunneling microscopy (STM) for electrochemical studies,6 of the experimental protocols pioneered by Clavilier for working with single-crystal electrodes,7,8 as well as the proliferation of electrochemical studies combining other in situ surface sensitive techniques,1 have led to a remarkable increase in atomic-level structural information pertaining to molecular adsorption at the electrochemical interface. A growing trend arising from those studies is that adsorbateadsorbate interactions do play an important role and generally cannot be neglected during the interpretation of experimental results. In special instances, adsorbate-adsorbate interactions can play a controlling role over the final equilibrium structures (4) Yates, J. T., Jr. The Surface Scientist’s Guide to Organometallic Chemistry; American Chemical Society: Washington, D.C., 1987. (5) Soriaga, M. P. Chem. ReV. 1990, 90 (5), 771-793. (6) Itaya, K. Electrochemistry 2006, 74, 19-27. (7) Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J. Electroanal. Chem. 1980, 107 (1), 205-209. (8) Clavilier, J. J. Electroanal. Chem 1980, 107(1), 211-216.
10.1021/la702654v CCC: $40.75 © 2008 American Chemical Society Published on Web 02/27/2008
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observed at electrochemical interfaces.6,9-11 Adsorbate-adsorbate interactions can also influence the dynamics of molecular adsorption, hence defining the macroscopic electrochemical response that is observed during the course of molecular adsorption reactions at electrified interfaces, as in the limiting case of two-dimensional (2D) phase transitions.9-12 In addition to controlling the dynamics observed during electrochemical experiments, adsorbate-adsorbate interactions can influence the long-term chemical stability and reactivity of adlayers. Supramolecular interactions, exemplified by molecular conductors,13 extend beyond the intramolecular realm of localized-bonds and electron-sharing described by classical chemical coordination models. Fundamental understanding of molecular interactions at model 2D electrochemical interfaces can help achieve an improved understanding of supramolecular interactions, selfand directed-assembly, molecular electronics, and phase transition phenomena in systems of any dimensionality.14-17 Quinonoid molecules are known to form organometallic coordination compounds with transition metals; both molecular18-21 and supramolecular complexes13,22-27 are known. In addition to well-documented electrochemical reactivity, they also exhibit well-understood interactions with photons28-31 and wellunderstood magnetic properties.13,25,32 From an analytical standpoint, this documented and diverse physicochemical behavior makes them promising probes of chemical reactivity at electrode solution interfaces. Like CO, that served as a model chemisorbate in the study of gas-solid heterogeneous interfaces, quinonoid moieties continue to serve as prototypical organic chemisorbates in what is now an increasing body of work at solid-liquid (9) Wandlowski, T. Encyclopedia of Electrochemistry; Bard, A. J., Stratmann, M., Eds.; Wiley-VCH Verlag GmbH & Co. KGaA: Weinheim, Germany, 2003; pp 383-467. (10) Blum, L.; Huckaby, D. A.; Marzari, N.; Car, R. J. Electroanal. Chem. 2002, 537 (1-2), 7-19. (11) Blum, L.; Marzari, N.; Car, R. J. Phys. Chem. B 2004, 108 (51), 1967019680. (12) De Levie, R. Chem. ReV. 1988, 88 (4), 599-609. (13) Coronado, E.; Day, P. Chem. ReV. 2004, 104 (11), 5419-5448. (14) Pincus, P. Science 2000, 290 (5495), 1307-1308. (15) Dubi, Y.; Meir, Y.; Avishai, Y. Phys. ReV. Lett. 2005, 94 (15), 156406-4. (16) Pushpalatha, K.; Sangaranarayanan, M. V. J. Electroanal. Chem 1997, 425 (1-2), 39-48. (17) Pushpalatha, K.; Sangaranarayanan, M. V. Chem. Phys. Lett. 1998, 288 (2-4), 216-224. (18) Adams, R. D.; Miao, S. J. Am. Chem. Soc. 2004, 126 (16), 5056-5057. (19) Le, Bras, J.; Amouri, H.; Vaissermann, J. Organometallics 1998, 17 (6), 1116-1121. (20) Le Bras, J.; Amouri, H.; Vaissermann, J. J. Organomet. Chem. 1998, 553 (1-2), 483-485. (21) Kurihara, M.; Nishihara, H. Coord. Chem. ReV. 2002, 226 (1-2), 125135. (22) Kim, K. S.; Suh, S. B.; Kim, J. C.; Hong, B. H.; Lee, E. C.; Yun, S.; Tarakeshwar, P.; Lee, J. Y.; Kim, Y.; Ihm, H.; Kim, H. G.; Lee, J. W.; Kim, J. K.; Lee, H. M.; Kim, D.; Cui, C.; Youn, S. J.; Chung, H. Y.; Choi, H. S.; Lee, C. W.; Cho, S. J.; Jeong, S.; Cho, J. H. J. Am. Chem. Soc. 2002, 124 (47), 14268-14279. (23) Oh, M.; Carpenter G. B.; Sweigart D. A. Macromol. Symp. 2003, 196 (1), 101-112. (24) Kitagawa, S.; Kawata, S. Coord. Chem. ReV. 2002, 224 (1-2), 11-34. (25) Sato, O.; Tao, J.; Zhang, Y.-Z. Angew. Chem., Int. Ed. 2007, 46 (13), 2152-2187. (26) Horiuchi, S.; Okimoto, Y.; Kumai, R.; Tokura, Y. Science 2003, 299 (5604), 229-232. (27) Mitani, T.; Saito, G.; Urayama, H. Phys. ReV. Lett. 1988, 60 (22), 2299. (28) Hubig, S. M.; Rathore, R.; Kochi, J. K. J. Am. Chem. Soc. 1999, 121 (4), 617-626. (29) Weber, J.; Malsch, K.; Hohlneicher, G. Chem. Phys. 2001, 264 (3), 275318. (30) Zhao, X.; Imahori, H.; Zhan, C. G.; Sakata, Y.; Iwata, S.; Kitagawa, T. J. Phys. Chem. A 1997, 101 (4), 622-631. (31) Kjaergaard, H. G.; Howard, D. L.; Schofield, D. P.; Robinson, T. W.; Ishiuchi, S.; Fujii, M. J. Phys. Chem. A 2002, 106 (2), 258-266. (32) Pedersen, J. A. Spectrochim. Acta Pt. A: Mol. Biomol. Spectrosc. 2002, 58 (6), 1257-1270.
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electrochemical interfaces5,33-41 and at well-defined solid-gas interfaces.42,43 This manuscript reports the direct electrochemical observation of the oxidative chemisorption of catechol (o-H2Q) and the reductive desorption of catechol-derived adlayers (o-Q(ads)) at nearly defect-free Pt(111) single-crystal electrodes in 0.5 M H2SO4. Classical electroanalytical techniques are combined with in situ spectroelectrochemical measurements to demonstrate that the overall adsorption-desorption process at the Pt(111) electrode (eq 1) can be described as a surface ligand exchange or displacement:
Q(ads) + Pt(111) zH(ads) Pt(111) + (z + 2)e + (z + 2)H 98 Pt(111) + H2Q(aq) (1) where z represents the number of hydrogen adsorbed on the Pt(111) surface when a o-Q(ads) molecule is replaced. The scanrate (υ) dependence of the process is also reported. Under quasistatic conditions (limυf0) the cyclic voltammetric response does not conform to the behavior expected from classical models of molecular adsorption at electrochemical interfaces. Instead, the adsorbate-adsorbate interactions seem to play a controlling role, i.e., hydrogen-assisted displacement of o-Q(ads) in the presence of 2 mM o-H2Q(aq) takes place as an electrochemically reversible 2D phase transition controlled by collision-nucleationgrowth phenomena44 at highly ordered Pt(111) electrode surfaces. In clean supporting electrolyte, where o-H2Q(aq) is not originally present, the phase transition is not observed. Donor-acceptor (DA) interactions between the Pt(111)o-Q(ads) (A) surface adduct and o-H2Q(aq) (D) may play an intervening role in the desorption mechanism leading to the observed macroscopic electrochemical response, suggesting the possible existence of long-range supramolecular interactions within the adlayer. Some potential applications are also discussed. 2. Experimental Section Pt(111) single-crystal electrode surfaces were prepared using the procedures developed by Clavilier et al.45 Electrode diameter was around 2 mm for the voltammetric experiments. All experiments were conducted at room temperature, 25 °C ((2 °C). Aqueous 0.5 M H2SO4 solutions were used as supporting electrolyte throughout the voltammetric study. They were prepared from concentrated sulfuric acid (Merck Suprapur or Aldrich Teflon Grade) and Purelab (33) Stern, D. A.; Salaita, G. N.; Lu, F.; McCargar, J. W.; Batina, N.; Frank, D. G.; Laguren-Davidson, L.; Lin, C. H.; Walton, N.; Gui, J. Y.; Hubbard, A. T. Langmuir 1988, 4 (3), 711-722. (34) Kahn, B. E.; Chaffins, S. A.; Gui, J. Y.; Lu, F.; Stern, D. A.; Hubbard, A. T. Chem. Phys. 1990, 141 (1), 21-39. (35) Ren, D.; Hubbard, A. T. J. Colloid Interface Sci. 1999, 209 (2), 435-441. (36) Krauskopf, E. K.; Wieckowski, A. J. Electroanal. Chem. 1990, 296 (1), 159-169. (37) Hubbard, A. T. Chem. ReV. 1988, 88 (4), 633-656. (38) Inukai, J.; Wakisaka, M.; Yamagishi, M.; Itaya, K. Langmuir 2004, 20 (18), 7507-7511. (39) Sanabria-Chinchilla, J.; Soriaga, M. P.; Bussar, R.; Baltruschat, H. J. Appl. Electrochem. 2006, 36 (11), 1253-1260. (40) Chia, V. K. F.; Stickney, J. L.; Soriaga, M. P.; Rosasco, S. D.; Salaita, G. N.; Hubbard, A. T.; Benziger, J. B.; Perter, Pang, K. W. J. Electroanal. Chem 1984, 163 (1-2), 407-413. (41) Rodriguez-Lopez, M.; Herrero, E.; Feliu, JM.; Tun˜o´n, P.; Aldaz, A.; Carrasquillo, A., Jr. J. Electroanal. Chem 2006, 594 (2), 143-151. (42) Pawin, G.; Wong, K. L.; Kwon, K. Y.; Bartels, L. A. Science 2006, 313 (5789), 961-962. (43) Wong, K. L.; Pawin, G.; Kwon, K. Y.; Lin, X.; Jiao, T.; Solanki, U.; Fawcett, R. H. J.; Bartels, L.; Stolbov, S.; Rahman, T. S. Science 2007, 315 (5817), 1391-1393. (44) Maestre, M. S.; Rodriguez-Amaro, R.; Mun˜oz, E.; Ruiz, J. J.; Camacho, L. J. Electroanal. Chem 1994, 373 (1-2), 31-37. (45) Clavilier, J.; Achi, K. E.; Petit, M.; Rodes, A.; Zamakhchari, M. A. J. Electroanal. Chem. 1990, 295 (1-2), 333-356.
Catechol 2D Phase Transition at Pt(111) Ultra (Elga-Vivendi) water (18 MΩ cm). o-H2Q was obtained from Aldrich and used as received. High purity Ar gas (5N) was 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. A platinum counter electrode was used, and all potentials were measured and are reported versus a reversible hydrogen electrode (RHE) with the same supporting electrolyte solution. Infrared spectroscopy experiments were carried out either with a Nicolet 8700 or with a Nicolet Magna 850 FTIR spectrometers equipped with MCT detectors. Spectra, collected at a resolution of 8 cm-1, are plotted as -log(R/R°) where R and R° stand for the sample and reference single beam spectra, respectively. Attenuated total reflectance (ATR) spectra of 0.2 M catechol + 0.1 M HClO4 solutions were collected with an ATR-IR cell provided with a semicylindrical ZnSe window. The spectra were obtained at an angle of incidence of 45° and results from the averaging of 100 interferograms. The single beam spectrum obtained for the catecholfree 0.1 M HClO4 solution was used as the reference spectrum. The spectroelectrochemical cell46,47 used in the in situ experiments was provided with a prismatic CaF2 window beveled at 60°. Sample and reference spectra, collected with either p- or s-polarized light, were obtained at the corresponding electrode potentials by using the socalled subtractively normalized interfacial Fourier transform infrared (SNIFTIR) technique. Ten sets of 100 interferograms were collected alternately at the sample and reference potential and then coadded. Positive- and negative-going absorbance bands in the resulting spectra correspond, respectively, to species produced and consumed at the sample potential. All the experiments were carried out at room temperature. Test solutions were 0.1 M HClO4, prepared from the concentrated acids (Merck Suprapur). Deuterium oxide (Aldrich 99.9 atom % D, glass distilled) was used in some spectroelectrochemical experiments as received. Solutions were deareated by argon bubbling. The Pt(111) electrode diameter was around 5 mm for the spectroelectrochemical experiments.
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Figure 1. (a) Cyclic voltammograms of well-ordered Pt(111) singlecrystal electrode collected in clean 0.5 M H2SO4 supporting electrolyte solution: (i) for a clean, well-ordered Pt(111) electrode, dotted line (‚‚‚‚); (ii) first CV for a well-ordered quinone-coated Pt(111) electrode, solid line (s). Scan rate 50 mV s-1. Temperature 25 °C. (b) Consecutive (second through tenth) cyclic voltammograms for the quinone-coated well-ordered Pt(111) electrode from part a. All other conditions as in part a.
equilibrated at 0.6 V before the next voltammogram (solid line in Figure 1a) was collected. On the basis of previous studies, a monolayer of oxidized catechol would be expected to form spontaneously under the selected conditions according to eq 2: Pt(111)
o-H2Q(aq) 798 o-Q(ads) + H2
(2)
3. Results and Discussion 3.1. Cyclic Voltammetry of Catechol-Coated Pt(111) Electrodes in Clean Supporting Electrolyte Solutions: Hydrogen-Assisted Reductive Desorption of o-Q(ads). The direct electrochemical observation of the reductive desorption of a catechol-derived adlayer at Pt(111) electrodes is documented in Figure 1. Figure 1a contrasts two cyclic voltammetry (CV) experiments performed at a nearly defect-free Pt(111) single-crystal electrode in clean 0.5 M H2SO4 supporting electrolyte. The dotted line was collected first and shows the now familiar shape of the CV for a clean, well-ordered Pt(111) single-crystal electrode surface. The near defect-free nature of the Pt(111) electrode surface was verified using the CV, i.e., a near featureless hydrogen UPD region, and a sharp preceding spike can be observed for fully ordered Pt(111) as reported by Clavilier.45 The clean, well-ordered Pt(111) single-crystal electrode was then (i) transferred at open circuit into a 0.5 M H2SO4 supporting electrolyte solution containing 2 mM o-H2Q(aq) for 5 min to produce a Pt(111) electrode coated with o-Q(ads). (ii) After the electrode was coated, excess o-H2Q(aq) was rinsed with clean supporting electrolyte solution at open circuit so that only irreversibly adsorbed species would remain at the electrode surface. The coated Pt(111) electrode was then (iii) returned to the test cell where the first voltammogram (dotted line in Figure 1a) had been collected. The electrode was exposed to the supporting electrolyte and (46) Iwasita, T.; Nart, F. C. AdVanced Electrochemical and Science Engineering; Gerischer, H., Tobias, C. W., Eds.; VCH Verlagsgesellschaft mbH: Weinheim, 1990; pp 123-216. (47) Rodes, A.; Pe´rez, J. M.; Aldaz, A. In Handbook of Fuel Cells Fundamentals, Technology and Applications, 1st ed.; Vielstich, W., Lamm, A., Gasteiger, H. A., Eds.; John Wiley & Sons, Ltd: Chichester, 2003; pp 191-219.
Only one group has reported structural data of catechol at well-defined Pt(hkl) electrodes.33,34 A well-ordered (3 × 3) structure was observed using low-energy electron diffraction (LEED) after emersion of Pt(111) electrodes from catecholcontaining solutions. The aromatic ring was reported to orient parallel to the Pt(111) surface due to experimental evidence of strong interactions, which affected the frequencies and the relative intensities of the high-resolution electron energy loss spectroscopy (HREELS) bands: weak CH stretching modes, a large CC stretching band (1600-1650 cm-1), and weak CH bending (700800 cm-1). OH stretching and bending were absent from the HREELS spectra as well. The observation was ascribed to dissociation of the hydroxyl hydrogen during adsorption of the molecule. The adsorbed monolayers resulting from similar treatment with p-hydroquinone have been studied more extensively in the past35-37,40,41 using STM, LEED, AES, HREELS, FTIR, and electrochemistry at Pt(hkl). In spite those efforts, the chemical identity of the resulting adlayer Q(ads) has been a subject of controversy.38 Hubbard35,37 has shown that upon adsorption at Pt(111) electrodes the hydrogen in the hydroxyl groups of p-hydroquinone (p-H2Q) also dissociates from the molecule. This suggests that the most likely product of adsorption reactions, such as eq 1, should yield the corresponding quinone as an oxidation product. We have recently documented the direct electrochemical observation of the reductive desorption of p-hydroquinone derived adlayers.41 Our own spectroelectrochemical studies (vide infra) tend to confirm that a similar hypothesis can be extended to the reactivity of catechol. For that reason, during the ensuing discussion, o-Q(ads) will be assumed to be the adsorbed quinone form of the molecule, but bare in mind that other surface adducts are also possible and have been
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postulated to form in the case of p-hydroquinone adsorption at poly Pt surface sites by Hubbard. Using that formalism, the solid line in Figure 1a corresponds to the cyclic voltammogram of the Pt(111) electrode coated with o-Q(ads). All other experimental conditions were kept constant between the two experiments. Two differences are noted in the negative-going, cathodic scan at the o-Q(ads) coated electrode; the appearance of two broad reduction peaks at 0.157 V and at 0.092 V are accompanied by the near disappearance of the adsorption states related to hydrogen and (bi)sulfate adsorption on the Pt(111) well-ordered domains. The presence of two reduction peaks suggests the existence of two different reductive desorption states. The two peaks are negative from 0.33 V and are also observed in perchloric supporting electrolyte (not shown); hence, they are ascribed to the reductive desorption of the adsorbed organic layer. In the positive-going scan, the appearance of a small oxidation peak is observed with maximum at 0.106 V. Also observed in the positive-going scan is a noticeable resurgence of the currents in the electrode potential window immediately above 0.3 V, where (bi)sulfate adsorption/desorption is usually observed. The behavior is analogous to that previously reported for adsorbed p-hydroquinone adlayers41 except that in the case of the p-hydroquinone, a single peak was observed during reductive desorption of the adlayers. The reduction peaks are related to the reductive desorption of a catechol-derived adlayer o-Q(ads) from Pt(111) according to eq 3a:33,34 Pt(111)
o-Q(ads) + 2e- + 2H+ 98 o-H2Q Pt(111)
o-H2Q 98 o-Q(ads) + 2e- + 2H+
(3a) (3b)
The onset of the hydrogen evolution reaction (HER) is also observed to take place in the more negative branch of the cathodic peak, as described in eq 4. This process begins at ∼0.08 V and increases toward more negative potentials: Pt(111)
2H+ + 2e- 798 H2
(4)
Figure 2. (a) Cyclic voltammograms of a quinone-coated, wellordered Pt(111) single-crystal electrode: (i) in contact with clean 0.5 M H2SO4 supporting electrolyte solution, dotted line (‚‚‚‚), and (ii) in contact with 2 mM o-H2Q in 0.5 M H2SO4 supporting electrolyte solution, solid line (s). All other conditions as in Figure 1a. (b) Cyclic voltammograms of a quinone-coated, well-ordered Pt(111) single-crystal electrode: (i) in contact with 2 mM o-H2Q in 0.5 M H2SO4 supporting electrolyte solution, thin solid line (s), and (ii) in contact with 0 mM o-H2Q in 0.5 M H2SO4 supporting electrolyte solution, thick solid line (s). All other conditions as in part a.
at the vacated surface sites. However, not all surface sites are recovered. This is demonstrated from a comparison of the CV response between the clean Pt(111) in Figure 1a and the tenth CV in Figure 1b. Estimates based on hydrogen adsorption coulometry suggest a recovery of only 87% of the original surface sites. A comprehensive description of the DL structure and the composition of clean Pt(111) in dilute sulfuric acid media is available from the literature.10 In the potential window where reductive desorption of o-Q(ads) takes place, the predominant faradaic process expected at the vacated sites is the formation of adsorbed hydrogen. Although more complicated descriptions are possible,10,11 the process will be referred herein according to eq 5: Pt(111)
The small oxidation feature with maximum at 0.106 V, observed during the positive-going scan, is ascribed to the reaction in eq 3b, i.e., low coverage, oxidative readsorption of electrochemically generated o-H2Q(aq) from the vicinity of the electrode. The inset in Figure 1 (Figure 1b) shows several CVs (second through tenth CVs) obtained by successive cycling of the o-Q(ads)coated Pt(111) electrode in the same test electrolyte solution. The hanging meniscus and the argon purge were maintained throughout the experiment. The reductive desorption feature at 0.157 V is no longer observed. The existence of two cyclic voltammetry peaks, related to desorption of the catechol-derived adlayer, suggests that the process is either surface coverage dependent and/or concentration dependent. (The effect of increased solution concentration will be discussed further in Figure 2.) A gradual decrease is observed of the reductive desorption feature at 0.09 V, also ascribed to the reaction in eq 3. The systematic decrease in peak currents at 0.09 V could come as a consequence of diffusion of the desorbed o-H2Q(aq) species away from the electrode vicinity and into the bulk 0.5 M H2SO4 supporting electrolyte solution. A small, concomitant increase of the currents for hydrogen and (bi)sulfate adsorption is also observed. The existence of well-defined isopotential regions, e.g., 0.254 and 0.084 V, suggest that, upon desorption of o-Q(ads), the structure and composition at the electrochemical doublelayer (DL) of the clean Pt(111) surface does become reestablished
H+(aq) + 1e- 798 H(ads)
(5)
The isopotential regions in Figure 1b suggest that the processes in eq 3 and eq 5 occur concomitantly. The electrode surface coordination model has been used in the past to understand, predict, and explain the broad range of complex and interrelated phenomena that takes place at electrode-electrolyte interfaces, in terms of familiar molecular concepts derived from homogeneous coordination chemistry studies.5 Borrowing from those concepts, the overall electrode process can be described as a surface ligand exchange or displacement that takes place as in eq 1. Except for the noted differences, similar reactivity has been documented for adlayers derived from adsorption of p-hydroquinone at Pt(111) electrodes.41 3.2. CV Characterization of Pt(111) in 2 mM o-H2Q(aq) Solutions: Hydrogen-Assisted Reductive Desorption of oQ(ads) and Oxidative Chemisorption of o-H2Q(aq) as a 2D Phase Transition. Figure 2 shows the cyclic voltammogram (solid line) obtained for a well-ordered Pt(111) single-crystal electrode immersed in a 0.5 M H2SO4 supporting electrolyte solution containing 2 mM o-H2Q(aq). Two redox pairs can be observed. The redox pair at ca. 0.078 V is the surface adsorption/desorption process under investigation in this manuscript. The pair at ca. 0.8 V is ascribed to the well-known solution process (o-Q(aq)/ o-H2Q(aq)). During all other experiments described in this report,
Catechol 2D Phase Transition at Pt(111)
the electrodes were not exposed to potentials above 0.6 V, but it is done here for reference. Except for the presence of 2 mM o-H2Q(aq) in solution, all other experimental conditions are as in Figure 1. The Pt(111) electrode was equilibrated in 2 mM o-H2Q(aq) at 0.6 V and cycled 3 times (3×) from 0.60 to 0.04 V. The third cycle is shown. The only difference noticed during cycling was the height of the peak, which increases in the first and second scan and remains stable afterward. A striking difference is observed when catechol is present in solution. The inset (Figure 2b) shows for reference the first CV from Figure 1, which was collected from a catechol-coated Pt(111) electrode in catecol-free 0.5 M H2SO4 supporting electrolyte. A decrease in the double-layer charging currents (see Figure 2b) accompanies a noticeable shift toward more negative potentials in the 2 mM o-H2Q(aq) solution. The effect of the presence of catechol in the bulk solution is consistent with the assignment of the voltammetric peaks from Figure 1 to the process in eq 1. The potential window was opened to 1.13 V, a potential at which the surface order of Pt(111) is presumed, from studies in catechol-free 0.5 M H2SO4. This potential excursion was performed, only during this experiment, to show the voltammetry of the solution species for reference, i.e., the ensuing discussion pertains to the adsorption/ desorption process exclusively and not the solution species. A remarkable narrowing in the full width at half-maximum (FWHM) and a concomitant increase in the cathodic and anodic current densities is observed in the presence of 2 mM o-H2Q(aq) solution (Figure 2a). The FWHM are in the order of 5 mV for both the cathodic and anodic processes. The peaks are nonGaussian and exhibit noticeable kurtosis and a small extent of hysteresis, with a voltage difference between the anodic and the cathodic peaks (|Epeak anodic - Epeak cathodic| ) ∆Epeak) of ∼11 mV. The observation of very narrow and sharp CV peaks (or “spikes”) are traditionally attributed, in classic molecular adsorption models,48 to strong attractive interaction within the adlayer.49 The classic molecular adsorption models are traditionally classified according to the isotherm assumed in the model, Langmuir and Frumkin isotherms being the most prevalent. 2D phase transitions also lead to sharp CV peaks.9,12,44 The Frumkin isotherm predicts the occurrence of 2D phase transitions at electrode-solution interfaces50 but does not describe them analytically. The prediction of 2D phase transitions applies in the presence of strong attractive adlayer interactions, e.g., strong attractive adsorbate-adsorbate interactions. A distinguishing aspect of electrochemical 2D phase transitions is the existence of a singularity, typically an electrode potential, in which the system becomes nonanalytic in the sense that two competing structures (or phases) share the same free energy. This energetic degeneracy gives rise to the behavior associated to 2D phase transitions, i.e., abrupt changes in composition, structure, etc. The subject has been recently reviewed.9 2D phase transitions have been described, as (i) compositional phase transitions, i.e., new phase formation at the interface, such as in the underpotential deposition of a foreign metal on a metal electrode. (ii) Structural phase transitions have also been described, i.e., change in the spatial distribution of adsorbed species, such as in the “orientational” phase-transition of a molecular dipole under the influence of an electric field or like in “positional” phase-transitions such as order-disorder transitions of 2D adlayers. In addition to electrode surface structure and composition, additional crucial parameters known to influence the onset of 2D electrochemical phase transitions are temperature, adsorbate concentration and (48) Srinivasan, S.; Gileadi, E. Electrochim. Acta 1966, 11 (3), 321-335. (49) Laviron, E. J. Electroanal. Chem. 1974, 52 (3), 395-402. (50) Angerstein-Kozlowska, H.; Klinger, J.; Conway, B. E. J. Electroanal. Chem. 1977, 75, 45-60.
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Figure 3. (a) Electrode pseudocapacitance (C ˆ ) j/υ) of an o-quinonecoated, well-ordered Pt(111) single-crystal electrode in contact with 2 mM o-H2Q in 0.5 M H2SO4 supporting electrolyte solution at 0.001, 0.002, 0.005, 0.010, 0.020, 0.050, 0.100, 0.200, 0.500, and 1.000 V s-1 scan rate (υ). The arrow indicates the trend from 0.001 to 1.000 V s-1. All other conditions as in Figure 1a. (b) Total charge density (Qtotal) under the anodic (closed circles) and cathodic (open circles) peaks as a function of scan rate (υ) obtained during the CV experiments in part a. The linear regression (solid lines) used the last five data points and are shown for reference (no model is implied).
electrode potential.17 Note that a minimum concentration may exist above which new phase formation takes place as a 2D phase transition. The dramatic concentration effect observed in Figure 2 may originate from a compositional 2D phase transition requiring the presence of catechol in the supporting electrolyte solution. The ensuing spectroelectrochemical experiments suggest that this 2D phase transition involves new phase formation at the interface, i.e., a catechol-derived organic layer, o-Q(ads), is displaced by a hydronium-derived inorganic monolayer of H(ads) and vice versa. Because the 2D phase transition is induced at the well-ordered Pt(111) electrode by handle of the electric potential via a faradaic process, similar to that of eq 1, the dynamics of the displacement can be evaluated from the CV response.44 The CV response seems to be controlled by nucleation and growth phenomena, a prototypical behavior of 2D phase transitions at electrochemical interfaces. The 2D phase transition provides a possible method for estimating the surface coverage of o-Q(ads) at near-perfect Pt(111) single-crystal electrodes. The average charge density measured during the oxidative displacement process (eq 1) for experiments as in Figure 2 was 236 µC cm-2. The pseudocapacitive contribution of the faradaic process in eq 3b, i.e., oxidative chemisorption of o-H2Q(aq), may be estimated by substracting the pseudocapacitive contribution due to hydrogen adsorption/ desorption (eq 5), which is believed to contribute 160 µC cm-2 if the maximum coverage (2/3) is originally present. The estimate yields a pseudocapacitive contribution by the process in eq 3b of 76 µC cm-2. The result is larger than the charge expected (53 µC cm-2) if the Q(ads) adlayer formed an ordered (3 × 3) adlattice over the near-perfect Pt(111) substrate, the structure previously reported by Hubbard.33,34 Further in situ structural studies would be helpful in order to elucidate the quantitative aspects, in light of the 87% recovery noted previously. The sweep rate (υ) dependence of the electrochemical phase transition was studied. Figure 3 shows the electrode pseudocapacitance (C ˆ ) j/υ) as a function of electrode potential for 10 CV scans. All experimental conditions were as in Figure 2, except for the sweep rate which was varied. CV experiments at 1, 2,
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Figure 4. (a) Effect of the sweep rate on the anodic (closed circles) and cathodic (open circles) peak potential (Epeak) obtained during cyclic voltammetry of a well-ordered Pt(111) single-crystal electrode in contact with 2 mM o-H2Q in 0.5 M H2SO4 supporting electrolyte solution. The solid line shows the best fit through the experimental data points, no model is implied. All other conditions as in Figure 3a. (b) Effect of the sweep rate on the peak separation, ∆Epeak (Epeak anodic - Epeak cathodic), obtained during cyclic voltammetry of well-ordered Pt(111) single-crystal electrode in contact with 2 mM o-H2Q in 0.5 M H2SO4 supporting electrolyte solution. The solid line shows the best fit through the experimental data points, and no model is implied. All other conditions are as in Figure 3a. (c) Experimental log ∆Epeak as a function of log(υ) obtained during cyclic voltammetry of well-ordered Pt(111) single-crystal electrode in contact with 2 mM o-H2Q in 0.5 M H2SO4 supporting electrolyte solution. The solid line results from linear regression of experimental data collected at slow scan rates. The dotted line results from linear regression at faster scan rates. All other conditions are as in Figure 3a.
5, 10, 20, 50, 100, 200, 500, and 1000 mV/s were performed and are shown in Figure 3. Two limiting behaviors are observed: (i) At the slowest scan rate (i.e., 1 mV/s), very narrow sharp, tailless peaks are observed. Their morphology is similar to previously published voltammograms for 2D phase formation.51 In addition to having very narrow FWHM, 1 mV and 1.6 mV, for the anodic and cathodic processes, respectively, the corresponding peak capacitances are also very high, ca. 91 mF/cm2 for the anodic peak and 77 mF/cm2 for the cathodic peak. A nonzero value of ∆Epeak ∼ 3 mV is observed, which cannot be attributed to ohmic losses (iR drop) arising from uncompensated solution resistance. (ii) At the higher scan rates, progressively broader peaks are observed. The morphology and symmetry of the peaks is also changed, as if the underlying statistics of the process were different. The increases in FWHM are accompanied by a progressive increase in ∆Epeak values and by a concomitant decrease in peak pseudocapacitance (C ˆ peak). Throughout the (51) Peter, L. M.; Reid, J. D.; Scharifker, B. R. E. J. Electroanal. Chem. 1981, 119 (1), 73-91.
spectrum of behavior observed in Figure 3, nearly constant values of total charge are obtained from integration of the pseudocapacitance-voltage curves as can be observed in the inset, Figure 3b. The divergence in the cathodic charge in the limυf0 is ascribed to an increase in the relative contribution to the absolute cathodic currents from the hydrogen evolution reaction. The monotonic decrease in absolute charge observed as the sweep rate is increased is likely due to time-dependent nucleation and growth phenomena. Figures 4, 5, 6, and 7 present the sweep rate dependence of the voltammetric peak potential (Epeak), ∆Epeak, peak pseudocapacitance (C ˆ peak), the FWHM values, and the peak current densities (jpeak) obtained from the experiments in Figure 3. The figures allow a comparison of the experimental CV response with existent theoretical models, such as classical adsorption isothermal models,48,49 unusual quasireversible behavior,52 and 2D nucleation-growth-collision models for electrochemical phase (52) Feldberg, S. W.; Rubinstein, I. J. Electroanal. Chem. 1988, 240 (1-2), 1-15. (53) Lana-Villarreal, T.; Rodes, A.; Perez, J. M.; Gomez, R. J. Am. Chem. Soc. 2005, 127 (36), 12601-12611.
Catechol 2D Phase Transition at Pt(111)
Figure 5. (a) Anodic (closed circles) and cathodic (open circles) peak pseudocapacitance (C ˆ peak ) jpeak/υ) as a function of scan rate (υ) obtained during cyclic voltammetry of well-ordered Pt(111) single-crystal electrode in contact with 2 mM o-H2Q in 0.5 M H2SO4 supporting electrolyte solution. All other conditions are as in Figure 3a. The solid lines show the best fit through the experimental data points; no model is implied. (b) Experimental log|C ˆ peak| as a function of log(υ), where C ˆ peak is the absolute value of peak pseudocapacitance, for the anodic (closed circles) and cathodic (open circles) peaks during cyclic voltammetry of well-ordered Pt(111) single-crystal electrode in contact with 2 mM o-H2Q in 0.5 M H2SO4 supporting electrolyte solution. For clarity, values for the cathodic log|C ˆ peak| were scaled by minus one [x (-1)]. All other conditions are as in Figure 3a.
formation.44 Analytical criteria, that have been derived and proposed using the 2D nucleation, growth, and collision model,44 will be emphasized. Specifically, jpeak, FWHM, and ∆Epeak are predicted by those studies to scale with υx, υ1-x, υ1-x where x reaches a limiting value of x ≈ 0.6 in the limit of slow scan rates. A further assumption is that the minimum number (nc) of molecules forming critically sized nuclei approaches zero, and that the effect of molecular orientation during collision is also negligible. For cases where the number of molecules forming critically sized nuclei is higher than zero (nc > 0), x will tend toward higher values (x > 0.6). Many aspects of the experimental (54) Gerhards, M.; Perl, W.; Schumm, S.; Henrichs, U.; Jacoby, C.; Kleinermanns, K. J. Chem. Phys. 1996, 104 (23), 9362-9375. (55) Socrates G. Infrared and Raman Characteristic Group Frequencies; 3rd ed.; John Wiley & Sons: New York, 2001. (56) Tzeng, W. B.; Narayanan, K.; Hsieh, C. Y.; Tung, C. C. Spectrochim. Acta Pt. A: Mol. Biomol. Spectrosc. 1997, 53 (14), 2595-2604. (57) Venkata Ramana Rao, P.; Ramana, Rao, G. Spectrochim. Acta Pt. A: Mol. Biomol. Spectrosc. 2002, 58 (14), 3039-3065.
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CV response observed during catechol adsorption/desorption are consistent with the analytical criteria above, in line with expectations for a 2D phase transition. Figure 4 explores the relation between the peak potential (Epeak) and sweep rate (υ). Figure 4a shows Epeak as a function of log(υ). A continuous change in slope is observed throughout the window explored in the experiments (three decades). The behavior is observed even under quasistatic conditions (limυf0), where a horizontal section would be expected48 based on continuous isotherm models of molecular adsorption. The continuous change in slope is also observed at the higher sweep rates where a linear region would be predicted48 if heterogeneous electron-transfer kinetics controlled the overall dynamics (the Taffel approximation). Similar behavior is observed from both anodic and cathodic branches in Figure 4a. This symmetry leads to similar behavior for ∆Epeak as a function of log(υ) in Figure 4b. An important aspect derived from Figures 4a and 4b, is the presence of a small hysteresis, even at the slowest scan rates. Extrapolation to the limυf0 leads to a ∆Epeak value of 2.5 mV after ohmic drop is considered. Figure 4a also highlights that under quasistatic conditions the redox displacement process (eq 1) takes place in a very narrow window of potentials with a formal potential (E0′) of 0.078 V, i.e., E0′ for the displacement process (eq 1) coincides with the exact potential where maximum coverage (2/3) of adsorbed hydrogen is obtained experimentally at clean Pt(111) in 0.5 M H2SO4. These facts are consistent with the hypothesis that displacement takes place as a 2D phase transition involving replacement of a catechol-coated Pt(111) surface which is replaced, at E0′,by a hydrogen-coated Pt(111) surface. Figure 4c shows the experimental log ∆Epeak as a function of log(υ). In the low scan rate regime a linear behavior is observed, as expected from the 2D nucleation-growth model.44 The slope in that region, i.e., 0.35, is in good agreement with the proposed analytical criteria derived from the 2D nucleation-growth model, which predicts a slope of 0.4 or smaller. At faster scan rates, a deviation from linearity is observed. At fast scan rates this parameter (∆Epeak) should be considered judiciously because other complications can arise, due to heterogeneous electron-transfer kinetics as well as due to nucleation-growth phenomena, and a convolution of the effects may be observed. Figure 5a shows the relation between the peak pseudocapacitance and sweep rate. Figure 5a shows C ˆ peak as a function of υ. As noted previously in the limυf0, the peak pseudocapacitance has a trend toward exceptionally high values. Concomitantly, the FWHM of the peaks tends toward a very small limiting value, as demonstrated in Figure 6a. The combined consideration of these two aspects of the CV response is consistent with the hypothesis that the hydrogen-assisted displacement of o-Q(ads) in the presence of 2 mM o-H2Q(aq) takes place as an electrochemically reversible 2D phase transition at the Pt(111) electrode surface, i.e., under quasistatic conditions (nearequilibrium), all the charge-transfer tends to take place at the formal potential of the phase transition (E1/2), such that as FWHM f 0, then C ˆ peak f ∞. Figure 5b shows the experimental log|C ˆ peak| as a function of log(υ). The cathodic values have been scaled by (-1) for clarity. Near-linear behavior is observed throughout the full range. Absolute values of slope are 0.39 and 0.33 for the anodic and cathodic processes, respectively. If, as before, only the low scan rate regime is considered, the absolute values of slope decrease to 0.30 (R ) 0.9918) and 0.23 (R ) 0.9966), respectively. As analytical criteria for the determination of the phase transition phenomena in this study, the slope log|C ˆ peak| vs log(υ) seems to be less sensitive to convolution with other phenomena than the
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Figure 6. (a) Experimental FWHM as a function of scan rate (υ) for the anodic (closed circles) and cathodic (open circles) peaks obtained during cyclic voltammetry of well-ordered Pt(111) singlecrystal electrode in contact with 2 mM o-H2Q in 0.5 M H2SO4 supporting electrolyte solution. All other conditions as in Figure 3a. 6(b) Experimental log(FWHM) as a function of log(υ) for the anodic (closed circles) and cathodic (open circles) peaks obtained during cyclic voltammetry of well-ordered Pt(111) single-crystal electrode in contact with 2 mM o-H2Q in 0.5 M H2SO4 supporting electrolyte solution. The solid line results from linear regression of experimental data collected at slow scan rates. The dotted line results from linear regression at faster scan rates. All other conditions are as in Figure 3a.
log ∆Epeak vs log(υ) slope, i.e., showing a smaller deviation from linearity during transition from the low scan rate to the high scan rate regime. Figure 6b shows a plot of log(FWHM) as a function of log(υ). In the low scan rate regime of Figure 6b a near-linear behavior is observed, as predicted based on the 2D nucleation-growth model.44 The slope in that region is 0.35 and 0.27 for the anodic and cathodic processes, respectively, within reasonable agreement with the analytical criteria proposed from the 2D nucleationgrowth model. At faster scan rates, a second near-linear region is observed. Figure 7 shows the relation between the peak current densities and sweep rate. Figure 7a shows jpeak as a function of υ. Nonlinear behavior is observed (no model is implied by the solid lines connecting the experimental data). The observation cannot be explained by the molecular adsorption models described before,48,49 because those predict that jpeak should scale with the sweep rate, υ. Figure 7b shows the experimental log|jpeak| as a
Rodriguez-Lopez et al.
Figure 7. (a) Current densities for the anodic (closed circles) and cathodic (open circles) peaks as a function of scan rate (υ) obtained during cyclic voltammetry of well-ordered Pt(111) single-crystal electrode in contact with 2 mM o-H2Q in 0.5 M H2SO4 supporting electrolyte solution. The solid lines connect the experimental data points; no model is implied. All other conditions as in Figure 3a. 7(b) Linear fit between log of the absolute value of the peak current density (log|jpeak|) for the anodic (closed circles) and cathodic (open circles) peaks and log(υ) obtained during cyclic voltammetry of well-ordered Pt(111) single-crystal electrode in contact with 2 mM o-H2Q in 0.5 M H2SO4 supporting electrolyte solution. For clarity, cathodic values of log|jpeak| are scaled by minus one (x - 1)). All other conditions are as in Figure 3a.
function of log(υ). As in Figure 5b, the cathodic values have been scaled by (-1). Near-linear behavior is again observed throughout the full range. Absolute values of slope are 0.61 and 0.67 for the anodic and cathodic processes, respectively, in good agreement with the theoretical expectation for a 2D phase transition occurring through nucleation and growth phenomena. If, as before, only the low scan rate regime is considered the absolute values of slope increase to 0.70 (R ) 0.9984) and 0.77 (R ) 0.9997), respectively. The slope from log|jpeak| vs log(υ) plots also seems less sensitive, to convolution with other phenomena, than the slope derived from log ∆Epeak vs log (υ) plots and than the log(FWHM) vs log(υ) slope. As shown in Figures 4-7, many aspects of the experimental CV response observed during the hydrogen-assisted desorption of catechol-derived adlayers are controlled by nucleation and growth. Specifically, jpeak, FWHM, and ∆Epeak do scale with υx, υ1-x, υ1-x in the limit of slow scan rates.44 However, the value of x does not coincide with the simplest limiting condition of nc ) 0 and x ≈ 0.6. In fact the experimentally determined value
Catechol 2D Phase Transition at Pt(111)
Figure 8. In situ absorbance spectra obtained for Pt(111) electrode immersed in a 2 mM o-H2Q aqueous solution containing 0.1 M HClO4. Reference potential: 0.04 V. 1000 interferograms were collected at each potential with a resolution of 8 cm-1 with p-polarized light. Bottom spectrum: ATR-IR spectra of aqueous solution of 0.2 M o-H2Q and 0.1 M HClO4 (reduced by 0.01) as reference.
of x is higher in all cases. This is consistent with the expectation that the number of molecules forming critically sized nuclei (nc) should be considerably higher than zero. For cases where nc > 0, x will tend toward higher values, such that x > 0.6. Additionally, for a nearly flat and polar molecule, the effect of molecular orientation during collision might not be negligible even in the slow scan rate regime and may have to be considered. The experimental CV response is therefore controlled by collisionnucleation-growth phenomena, consistent with the theoretical expectations for a 2D electrochemical phase transition. 3.3. SNIFTIR and ATR-IR Spectroscopy of the HydrogenAssisted Reductive Desorption of o-Q(ads) and Oxidative Chemisorption of o-H2Q(aq) in 2 mM o-H2Q(aq) Solutions: Chemical Characterization of the 2D Phase Transition. To obtain more information about the nature of the species involved in the adsorption/desorption processes taking place at the Pt(111) electrode surface in the catechol-containing solutions, in situ infrared spectroscopy experiments were performed. These experiments were carried out in perchloric instead of sulfuric acid solutions in order to avoid interferences from the S-O stretching bands, thus achieving a wider spectral window. It should be noted that this change in the supporting electrolyte has no significant influence in the overall voltammetric behavior. A set of potential-difference spectra obtained at different sample potentials is shown in Figure 8. The reference potential was chosen as 0.04 V, a potential negative enough to ensure the reductive desorption of catechol from Pt(111) and the absence of absorbates other than adsorbed hydrogen. The bottom spectrum in Figure 8 is the ATR-IR spectrum for a 0.2 M catechol + 0.1 M HClO4 solution, which can be used as a reference for the assignment of the bands in the in situ spectra. Main bands in the ATR-IR spectrum appear at 1200, 1261, 1276, 1373, 1470, and 1515 cm-1. Assignments for this absorption bands are summarized in Table 1. Most of the absorption bands observed in the ATR-IR spectrum for the catechol solution appear as negative-going bands in the in situ spectra, provided that the sample potential is above 0.07 V. Since these negative-going bands are also observed when the spectra were collected with s-polarized light, it can be stated that they originate from dissolved catechol molecules that are depleted
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Figure 9. In situ absorbance spectra obtained for a Pt(111) electrode immersed in a 3 mM catechol + 0.1 M HClO4 solution in deuterium oxide. Reference potential: 0.02 V. Each spectrum was colleted with p-polarized light and results from the coaddition of 1000 interferograms with a resolution of 8 cm-1. Bottom spectrum: ATRIR spectrum of a 0.2 M o-H2Q and 0.1 M HClO4 solution in deuterium oxide solution (reduced by 0.04) as reference. Table 1. Vibrational Frequencies for Catechol Observed at ATR-IR Spectra in Aqueous Solutions of 0.2 M Catechol + 0.1 M HClO4 and Their Assignmentsa wavenumber, cm-1
assignment
references
1200 1261 1276 1373 1470 1515
δ(OH) ν(CO) ν (CO) ν(CC) + δ(OH) ν(CC) + δ(CH) ν(CC) + δ(CH)
33, 53-55 33, 53-55 33, 53-55 55-57 53, 55 53, 55
a
δ:bending, ν:stretching vibration.
from the thin-layer solution when the electrode potential is stepped from the reference to the sample potential. In other words, the desorption of catechol takes place to form catechol molecules in the aqueous solution; neither the ring nor its aromaticity is broken as a result of the reductive desorption process. Because of the interference from the absorption by uncompensated water, the in situ spectra reported in Figure 8 do not show clear positivegoing bands that could be ascribed to (adsorbed) species formed at the sample potential. Broad features appearing at ca. 1580 and 1322 cm-1 in the spectrum collected at 0.5 V could be related to species formed at the sample potential. Decreasing the sample potential makes the corresponding bands shift downward. This potential-dependent frequency is a typical feature for absorbed species which suggests that the observed bands could correspond to absorbed species formed from catechol at the sample potential. The presence of adsorbate bands in the spectra collected in the catechol-containing solution can be corroborated by using deuterium oxide solutions, thus avoiding the interference of the O-H bending band from water, which can distort any band appearing around 1600 cm-1. The in situ spectra reported in Figure 9also shows negative-going bands at 1505, 1458, and 1273 cm-1 which fit with the frequency of the ATR-IR spectra of catechol in deuterium oxide solution (bottom spectrum). These bands in the in situ spectra, which are also observed in the spectra collected with s-polarized light, can be related to the depletion of catechol molecules in solution upon adsorption at the sample potential. The positive-going bands, appearing at 1623 and 1395
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cm-1 in the spectrum collected at 0.60 V, can only be observed in the spectra collected with p-polarized light. According to the surface selection rule,58 this behavior is typical for adsorbed species. The potential-dependent band frequency of the band around 1600 cm-1 is consistent with this latter statement. The observation of bands for the adsorbed species indicates the existence of a dipole moment perpendicular to the surface. Thus, adsorption with the adsorbate lying flat can be discarded. The assignment of the bands at 1623 cm-1 and 1395 cm-1 is not straightforward. The bands do not fit with those reported for TiO2 (anatase) nanoparticles, for which the adsorpion of catecholate in a chelate configuration was deduced from the in situ infrared spectra.53 An absorption band at ca. 1640 cm-1 has been assigned by Zhao et al.30 to the CdO stretching mode of 1,4-benzoquinone. An adsorbed quinone can be tentatively proposed (vide infra) as the species formed during the oxidative adsorption of catechol. A similar proposal was made in the case of the adsorbate formed upon the oxidative adsorption of 1,4hydroquinone on Pt(111) electrodes. Consideration of Physical Phenomena Influencing the Macroscopic Electrochemical Response during the 2D Phase Transition of Q(ads)/H2Q(aq) at Pt(111) Electrodes. The differences between the CV responses observed during the reductive desorption of o-H2Q and p-H2Q at Pt(111)41 surfaces are intriguing. Both molecules share similarities in their solutionphase reactivity because they possess similar functional groups. Physical differences born out of differences in chemical structure also exist. o-H2Q is a polar molecule while p-H2Q is nonpolar. o-H2Q can undergo intramolecular hydrogen bonding31 while p-H2Q does not. Instead, a solid-state charge-transfer (CT) crystal involving intermolecular hydrogen-bonding, i.e., quinhydrone, tends to be readily formed between p-H2Q and p-quinone.27 A quinhydrone analogue for catechol is not as readily formed. Both molecules can form stable paramagnetic semiquinone species, with the electronic charge residing primarily in the oxygen atoms. Like the electric charge, the spin in the semiquinone is delocalized but resides mostly in the carbon atoms that are not bound to oxygen. From a fundamental standpoint, the o-Q/o-H2Q/Pt(111) system may serve as a model platform to study how long-range supramolecular interactions, known to occur in 3D solid-state quinonoid compounds13,26 and related to the physical properties mentioned above, become manifested as adsorbate-adsorbate or adsorbate-substrate interactions at 2D electrochemical interfaces of quinone-derived adlayers. Some aspects of the electrochemical response observed during the 2D phase transition, such as collision-nucleation-growth phenomena, could emanate from the physical properties outlined above. As an example, the possible effect of donor-acceptor interactions and valence tautomerism between o-Q(ads) and o-H2Q(aq) is considered. This chemical degree of freedom could help interrelate the dramatic differences between the electrochemical reactivity observed in 0 mM vs 2 mM o-H2Q(aq) solutions and the emergence of the phase transition. A recent review discussing valence tautomerism and related subjects is available.25 At first, only the effect of electron CT interactions will be considered in the near-zero dimensional (0D) limit implicit in the coordination model, i.e., CT interactions between a single surface adduct, Pt-o-Q(ads), referred herein as the acceptor (A) molecule and a donor (D) molecule arriving from the solution, i.e., o-H2Q(aq). For simplicity, the closely related subject of proton transfer will be considered later.27,59,60 The discussion will not (58) Greenler, R. G. J. Chem. Phys. 1966, 44 (1), 310-315. (59) Laviron, E. J. Electroanal. Chem. 1983, 146 (1), 15-36. (60) Laviron, E. J. Electroanal. Chem. 1984, 164 (2), 213-227.
Rodriguez-Lopez et al. Scheme 1
Scheme 2
distinguish between partial or full electron CT. The electrode potential, an important external stimulus that can be experimentally controlled, will be assumed to be at a near-zero, but positive, value with respect to the potential of zero free charge of the electrode, where a full catechol-derived adlayer is known to be present. Interactions of the van der Waals type between A and D are further assumed to exist. Upon collision, if the neutral interacting molecules (AD) exhibit a valence instability, electron CT will lead to the formation of A-D+, an ionic pair. In the case at hand, two attractive interactions are expected, one electrostatic due to the difference in charge, and one magnetic due to the spin interaction, which will be assumed to be antiferromagnetic and maximized in this example. Based on the chemical structure of the original molecules, one (arbitrary) possible configuration is that the negatively charged oxygen atoms in A- may chelate the surface while the spin density points away from the surface and into the solution phase. D+ will be assumed to be in the solution phase with the positively charged (protonated) oxygen atom(s) pointing away from the surface species, A-, so that a strong spin-spin interaction will be present and maximized between the pair. If this hypothetical 0D scenario is extended to 2D, a magnetic and electric double-layer is obtained as in Scheme 1. Strong attractive magnetic and Coulombic interactions are present. The possibility of proton transfer was neglected for simplicity but should be considered in a realistic dynamic scenario such as the electrochemical interface. If proton transfer59,60 and/or proton tunneling27 are now allowed, several different types of 2D phase transitions become possible. For simplicity, only proton transfer will be considered next and electron CT will be forbidden. The double-layer that results from the ionic-to-neutral phase transition (due to proton transfer from D+ to A-) is now the antiferromagnetic double-layer shown in Scheme 2. Strong attractive magnetic interactions are now present. It can be argued that the minimum number of molecules (nj) required to initiate a true 2D phase transition according to this spin pairing mechanism would be four (nj > 4) in the case of monolayer formation. Any number of permutations is possible with this chemical degree of freedom, i.e., donor-acceptor interactions through electron charge transfer or proton transfer. For example, Scheme 1 and Scheme 2 may as well describe DA interactions taking place at the edge of a truly 2D island of A molecules, as in the top view shown in Scheme 3. Such islands might be present at sub-monolayer coverages during the reductive desorption/oxidative chemisorption process. The most important implication of the model is that it changes the description of attractive adsorbate-adsorbate interactions in this system from the traditional short-range intermolecular description (van der Waals-type) to a description that permits consideration of long-range supramolecular effects (e.g., through Madelung energy concepts or band-formation models) that are known to exist in solid-state compounds. Extended, quasi-onedimensional analogues, of the DADADADADA-type, are documented to take place in solid-state compounds26 containing quinoidal moieties and lead to the formation of molecular metals,
Catechol 2D Phase Transition at Pt(111) Scheme 3
Langmuir, Vol. 24, No. 7, 2008 3561
to potentials rivaling those of molecular hydrogen via chemically reversible, nondissociative interactions, leading to a phase transition. Further study of such systems is of interest, especially the role of adsorbate-substrate vs adsorbate-adsorbate interactions in the emergence of the macroscopic behavior observed for the catechol/Pt(111) system. Such knowledge might aid in the design of analogous redox systems, preferably involving nonprecious metals, to be used in renewable energy storage applications. In such applications, phase transition behavior is desirable because it can prevent concentration-induced power losses.
4. Conclusions
molecular superconductors, magnets, ferroelectrics, etc.13 Those are known to exhibit phase transitions originating from neutralionic valence instability. Related phenomena have also been described for quinhydrone27 including solid-state proton-transfer effects, under high-pressure conditions. To our knowledge, similar behavior has not been documented at electrochemical interfaces and may be present in the catechol/Pt(111) system. Further experimental and theoretical studies would be needed to ascertain unequivocally if they are or not present. The experimental manifestations of phenomena such as that depicted in Schemes 1-3 are difficult to interpret through a theoretical framework limited to coordination models, which are almost 0D. A prediction of their macroscopic manifestation during electrochemical experiments may have to be considered using simplified bandformation models61 from which extended 1D and 2D concepts emerge more naturally. Such concepts may be useful in the interpretation and description of adsorbate-adsorbate interactions at the electrochemical interface, particularly in such applications as molecular electronics and spintronics where a detailed understanding of the electronic structure may be needed. Figure 2 shows two redox pairs; from left to right these are the o-Q(ads)/o-H2Q(aq) pair (ca. 0.08 V), ascribed to the surface adsorption/desorption process, and the o-Q(aq)/o-H2Q(aq) (ca. 0.8 V) pair, ascribed to the solution process. The result demonstrates that in aqueous solutions it is thermodynamically feasible to shift the formal oxidation potential of catechol-metal adducts (61) Hoffmann, R. Solids and Surfaces: A Chemist’s View of Bonding in Extended Structures; Wiley-VCH: New York, 1988.
The electrochemical hydrogen-assisted desorption of catecholderived adlayers takes place, in the presence of 2 mM o-H2Q(aq), as an electrochemically reversible 2D phase transition controlled by collision-nucleation-growth phenomena. The results demonstrate that it is thermodynamically feasible to shift the formal oxidation potential of catechol-metal adducts to potentials near those exhibited by molecular hydrogen in aqueous solutions via chemically reversible, nondissociative interactions, leading to a 2D phase transition. Adsorbate-adsorbate interactions play a controlling role in the emergence of the phase transition behavior observed for the catechol/Pt(111) system. The 2D phase transition is not observed during hydrogen-assisted desorption of catecholderived adlayers in clean supporting electrolyte. The behavior may be related to the existence of donor-acceptor interactions. An important implication of the model is that it describes attractive adsorbate-adsorbate interactions in this system as long-range supramolecular interactions. Future work will aim at unambiguously identifying (i) the role, if any, of valence-tautomerism and donor-acceptor interactions in the emergence of the observed behavior and (ii) the role of electrode surface structure and composition using model Pt(hkl) and Pt nanoparticle surfaces. Acknowledgment. A.C.J. and M.R.L. thank (i) the Pontifical Catholic University of Puerto Rico, (ii) the University of Puerto Rico-Mayaguez, and (iii) NASA-IDEAS, NSF-LSAMP and NSFEPSCoR for support as well as (iv) the Alicante group for their enormous scientific generosity. The experimental assistance by Eugenio Mun˜oz (PCUPR) is gratefully acknowledged. Partial contribution of MEC (Spain) through FEDER projects CTQ200604071 and CTQ2006-09868 is gratefully acknowledged. LA702654V
