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Molecular Adsorption at Well-Defined Electrode Surfaces: Hydroquinone on Pd(111) Studied by EC-STM† Youn-Geun Kim, Jack H. Baricuatro, and Manuel P. Soriaga* Department of Chemistry, Texas A&M UniVersity, College Station, Texas 77843 ReceiVed April 30, 2006. In Final Form: July 31, 2006 The interaction of hydroquinone (H2Q) with well-defined Pd(111) surfaces at preselected potentials in dilute H2SO4 has been studied by molecule-resolved electrochemical scanning tunneling microscopy (EC-STM). H2Q spontaneously undergoes oxidative chemisorption to benzoquinone (Q), which adopts a slightly tilted parallel orientation. Evidently, the surface coordination is through the quinone π-electron system. At potentials within the double-layer region, a close-packed well-ordered Pd(111)-(3 × 3)-Q adlattice was formed. A potential excursion to 0.7 V, a potential at which the solution-phase Q/H2Q redox reaction takes place, introduced disorder into the organic adlayer; this positivepotential-induced order-to-disorder phase transition is reversible because the ordered (3 × 3)-Q adlattice was regenerated when the potential reverted to 0.4 V. When the potential was poised at 0.2 V, a potential at which hydrogen evolution was initiated, an appreciable fraction of Q was (hydrogenatively) desorbed; the remnant Q molecules were agglomerated in small islands that retained the (3 × 3) symmetry of the full adlayer. Two possible structural models of the Pd(111)-(3 × 3)-Q adlattice are described.
Introduction Studies on the interaction of organic molecules with electrode surfaces help establish the surface science of electrocatalysis.1-3 For instance, the initial step in the anodic oxidation or cathodic hydrogenation invariably involves the chemisorption of the organic reactant. In this regard, we previously investigated the adsorption of various diphenolic compounds at clean and surfacemodified Pd(hkl) electrode surfaces; experiments were based upon a combination of electrochemistry (EC), molecule-resolved scanning tunneling microscopy (STM), and ultrahigh vacuum (UHV) surface spectroscopy.4-8 The choice of diphenols as model compounds was dictated by their facile quinone/diphenol electrontransfer reactions in aqueous solutions. Because the energetics of such reactions are drastically altered when surface coordination is via the aromatic ring, the quinone/diphenol reaction thus serves as a convenient “electrochemical marker” to differentiate between chemisorbed and unadsorbed species. We recently reported results from high-resolution electron energy loss spectroscopy (HREELS) that demonstrated that, on bare metal, diphenols are oxidatively chemisorbed as quinines.5 Although the quinone molecules are coordinated to the substrate through their π-electron system, thus enforcing a flat orientation, the appearance of a weak in-plane C-H stretching peak in the HREEL spectrum, in apparent violation of the metal-surface dipole selection rule, indicated that they were not oriented completely parallel to the surface but had a slight tilt.5 In a †
Part of the Electrochemistry special issue. * To whom correspondence should be addressed. E-mail: soriaga@ mail.chem.tamu.edu. (1) Hubbard, A. T. Chem. ReV. 1988, 88, 633. (2) Itaya, K. Prog. Surf. Sci. 1998, 58, 121. (3) Soriaga, M. P. Prog. Surf. Sci. 1992, 39, 325. (4) Kim, Y.-G.; Soriaga, M. P. In Interfacial Electrochemistry; Wieckowski, A., Ed.; Marcel Dekker: New York, 1999; pp 249-268. (5) Soto, J. E.; Kim, Y.-G.; Soriaga, M. P. Electrochem. Commun. 1999, 1, 135. (6) Soto, J. E.; Kim, Y.-G.; Chen, X.; Park, Y.-S.; Soriaga, M. P. J. Electroanal. Chem. 2000, 500, 374. (7) Kim, Y.-G.; Soriaga, M. P. Phys. Chem. Chem. Phys. 2001, 3. 3303. (8) Kim, Y.-G.; Soto, J. E.; Chen, X.; Y.-S.; Soriaga, M. P. J. Electroanal. Chem. 2003, 167, 554.
subsequent investigation, we combined HREELS and EC-STM to ascertain whether the slightly tilted flat orientation was unique to the quinonoid moiety or was also a characteristic of the aromatic ring. The experimental measurements indicated that benzene also adopts a tilted-parallel orientation.8 In this article, we describe results from studies based upon molecule-resolved EC-STM on the influence of applied potential on the interfacial structure of the adlayer formed from the interaction of dilute acid solutions of H2Q with well-ordered Pd(111) electrodes. Experimental Section Atomically smooth single-crystal surfaces of palladium were prepared by the Clavilier method9 in which the tip of a polycrystalline Pd wire was melted in a hydrogen-oxygen flame to form a bead gilded with single-crystal (111), (110), and (100) facets;10-12 for this work, only the (111) facet was employed. The bead electrodes were cooled slowly in a stream of argon and then immersed in Milli-Q Plus water (Millipore Systems, Houston, TX) saturated with argon. With a protective thin film of water, the electrode was quickly transferred to the EC-STM cell with one of the (111) facets positioned under the STM tip. Larger surface areas were required for the voltammetric experiments; hence, the (111) facets were metallographically polished and crystallographically oriented as described in detail elsewhere.10-12 EC-STM was performed using a Nanoscope E microscope (Digital Instruments, Santa Barbara, CA) equipped with a custom-built Kel-F electrochemical cell. The tips were prepared by electrochemically etching a 0.25 mm tungsten wire in 1 M KOH at 15 V ac. Transparent nail polish was then applied to the tip to minimize faradaic currents.4-8,10-12 Current-potential curves were acquired with a CV-27 voltammograph (Bioanalytical Systems, West Lafayette, IN). Unless otherwise indicated, all potentials were referenced against the (9) Clavilier, J.; Faure, R.; G. Guinet, G.; Durand, R. J. Electroanal. Chem. 1980, 107, 205. (10) Kim, Y.-G.; Soriaga, M. P. J. Phys. Chem. B 1998, 102, 6188. (11) Kim, Y.-G.; Soriaga, J. B.; Vigh, G.; Soriaga, M. P. J. Colloid Interface Sci. 2000, 227 505. (12) Kim, Y.-G.; Soriaga, M. P. J. Colloid Interface Sci. 2001, 236, 197.
10.1021/la0611920 CCC: $33.50 © 2006 American Chemical Society Published on Web 10/03/2006
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Figure 1. Steady-state cyclic voltammograms for Pd(111) in (a) 0.05 M H2SO4 solution and (b) 1 mM hydroquinone in 0.05 M H2SO4 solution. Potential sweep rate, 5 mV s-1.
reversible hydrogen electrode (RHE). The sulfuric acid supporting electrolyte was prepared from double-distilled grade H2SO4 (Aldrich Chemicals, Milwaukee, WI) and Milli-Q Plus water. Solutions of hydroquinone (Aldrich Chemicals, Milwaukee, WI) were prepared from freshly sublimed crystals dissolved in 0.05 M H2SO4. Solutions were thoroughly deaerated with high-purity argon prior to use. The dilute aqueous H2Q solution was injected into the EC-STM cell only after an atom-resolved EC-STM image was obtained for the Pd(111)-(1 × 1) substrate.7,12,13
Results and Discussion Electrochemistry. Figure 1 shows the steady-state cyclic voltammograms of Pd(111) in 0.05 M H2SO4 solution before (Figure 1a) and after (Figure 1b) the addition of 0.1 mM H2Q. In agreement with published results,10,11,14 the following voltammetric features were obtained for the H2Q-free electrolyte: (A) sharp redox peaks at ca. 0.25 V associated with the adsorption-desorption of hydrogen atoms and bisulfate anions; (B) a rapid increase of the cathodic current at potentials more negative than 0.24 V attributed to hydrogen absorption and evolution; and (C) a sharp anodic peak at 1.1 V due to the oxidation of the Pd(111) surface. A comparison of Figure 1a and b indicates that the adsorption of H2Q-derived species suppresses the adsorption-desorption features at 0.25 V for the organic-free (uncoated) electrode. The well-defined redox peak at 0.65 V arises from the Q/H2Q redox couple. In acidic media, this redox couple is Nernstian and hence is insensitive to the nature of the electrode surface; in effect, the current-potential curve in Figure 1b resembles the voltammetric (13) Kim, Y.-G.; Yau, S.-L.; Itaya, K. Langmuir 1999, 15, 7810. (14) Okada, J.; Inukai, J.; Itaya, K. Phys. Chem. Chem. Phys. 2001, 3, 3297. (15) Yau, S.-L.; Kim. Y.-G.; Itaya, K. J. Am. Chem. Soc. 1996, 118, 7795. (16) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; John Wiley and Sons: New York, 1994.
Figure 2. (A) Wide-area EC-STM image of flame-annealed Pd(111) in 0.05 M H2SO4 at 0.25 V. Bias voltage, 200 mV; tunneling current, 2 nA. (B) High-resolution EC-STM image of a Pd(111)-(1 × 1) electrode surface in 0.05 M H2SO4 at 0.25 V; the close-packed [11h0] directions are indicated by the arrows. Bias voltage, 60 mV; tunneling current, 20 nA.
profile of the Q/H2Q redox reaction at an iodine-coated Pd [Pd(111)-(x3 × x3)R30°-I] electrode.12 EC-STM. The general topography of a freshly prepared Pd(111) facet is shown by the large-scale EC-STM image in Figure 2A. This image was obtained at 0.25 V where, in 0.05 M H2SO4, the Pd surface may be subjected to small amounts of electrochemically generated hydrogen; the surface hydrogen atoms, however, are transparent to STM. The surfaces prepared in this study exhibited extended terraces with a few step-lines within a scan area of 300 nm × 300 nm; these distinct monatomic steps were aligned along the close-packed [11h0] direction of the Pd(111) substrate; the hexagonal [11h0] directions are indicated by the arrows in Figure 2B. Terraces with widths larger than 100 nm were common. The long-range atomically flat terraces allowed the high-resolution visualization (Figure 2B) of hexagonal structures with an interatomic distance of ca. 0.27 nm; such atom-resolved images served to confirm the Pd(111)-(1 × 1) surface structure. Chemisorption of Hydroquinone. After the existence of a well-ordered Pd(111) substrate was verified (Figure 2B), 0.1 mM H2Q was introduced into the EC-STM cell. As previously proven,5 the chemisorbed adlayer actually consists of benzoquinone, instead of hydroquinone, in an oxidative-chemisorption process. Ordered molecular arrays immediately emerged although the completion of the full-coverage film took at least 2 min because of diffusional mass-transport limitations. Figure 3 shows
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Figure 3. (A) Unfiltered wide-area EC-STM image (bias voltage, 120 mV) after the formation of an atomically flat hydroquinone adlattice in 0.05 M H2SO4 at 0.4 V. (B) Dynamic tunneling current EC-STM image of the (3 × 3)-Q adlayer. The top half was obtained at a tunneling current of 50 nA (which reveals the substrate structure), whereas the bottom half was generated at a tunneling current of 20 nA (which shows only the Q adlayer).
EC-STM images of the ordered Q adlattice after a 180 s exposure. In the 30 nm × 30 nm image in Figure 3A, a wide, atomically flat terrace with a long-range hexagonal pattern is easily discernible. A so-called dynamic tunneling current STM image may be obtained when, halfway through a scan, the tunneling current is abruptly switched from a high (e.g., 50 nA) to a low (e.g., 20 nA) value. At the higher current, only the substrate underlayer is observed; at the lower current, only the overlayer is imaged. The results from such an experiment are depicted in Figure 3B. The top portion, for the 50 nA scan, shows the Pd(111)-(1 × 1) substrate atoms; the lower half, for the 20 nA scan, displays the benzoquinone adlayer. It is important to highlight the fact in Figure 3 that all of the molecular rows of the Q adlayer are almost perfectly parallel to the atomic rows of the underlying Pd(111) surface. Such an ordered adlayer, in registry with the hexagonal close-packed substrate atoms, was consistently observed in the potential range from 0.25 to 0.6 V. An unfiltered high-resolution EC-STM image was acquired at 0.5 V to provide configurational details of the quinone molecules within the ordered adlayer. As shown in Figure 4A, a molecular array of hexagonal symmetry in registry with the Pd(111) substrate was obtained in which the Q molecular entities are equidistantly separated by 0.82 nm. The latter intermolecular separation is 3 times longer than the substrate nearestneighbor distance; hence, it establishes the organic adlattice to be (3 × 3)-Q, with a surface coverage, Θ ≡ ΓQ/ΓPd, of 0.11.
Kim et al.
Figure 4. Unfiltered high-resolution EC-STM image of the (3 × 3)-Q adlayer on Pd(111) at 0.5 V. (A) Normal view. (B) Enlarged view. Bias voltage, 120 mV; tunneling current, 30 nA.
Figure 5. Unfiltered high-resolution EC-STM image of (3 × 3)-Q islands on Pd(111) at 0.2 V. Bias voltage, 120 mV; tunneling current, 30 nA.
An enlargement of the EC-STM image (Figure 4B) provides important details of the individual Q molecules within the (3 × 3)-Q adlattice. Although the carbon atoms in the aromatic ring cannot be distinguished individually, the outline of the asymmetric Q molecule can be sufficiently discerned to the extent that one is able to locate the para-oxygen atoms. The Q molecule appears as a set of two conjoined spots of uneven brightness separated by elongated features that are also of uneven brightness. The latter elongated features are separated by 0.55 nm, the same distance that separates the two para-oxygen atoms in benzoquinone. On the basis of this de-
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Figure 6. Two possible real-space structures of Pd(111)-(3 × 3)-Q. The oxygen atoms occupy atop sites in structure A but reside on 2-fold bridging sites in B.
tail, a representation of the Q molecule is superimposed on one of the EC-STM “spots” to illustrate the structure of an individual benzoquinone molecule within the adlayer. Not depicted in the idealized rendition of the Q molecule is the fact that, as indicated by the uneven brightness of the individual images, the chemisorbed Q molecule, which is predominantly oriented flat on the surface, is slightly tilted with the 1,2 positions slightly closer to the surface than the 4,5 positions. It may also be noted from Figure 4 that the molecular C2 axis that connects the para-oxygen atoms is rotated 30° with respect to the hexagonal close-packed [11h0] direction of the Pd(111) substrate. This result is in contrast to what was previously observed with hydroquinone sulfonate in which the O-O axis of the Q moiety was aligned along the [11h0] direction of the Pd(111) substrate, presumably dictated by the presence of the bulky sulfonate group.6,7 Although the (3 × 3)-Q structure remained highly ordered at potentials within the double-layer region, it was unstable at more positive potentials. An excursion to 0.7 V, at which the Q/H2Q redox reaction of solution-phase species transpired but before anodic oxidation of the chemisorbed organic species was initiated, introduced disorder into the Q adlayer. Such disorder, coupled with the onset of faradaic currents, inhibited the acquisition of high-resolution EC-STM images at this potential. However, once the potential reverted to 0.4 V, the sharp STM images shown in Figures 3 and 4 reemerged to signify the regeneration of the original ordered (3 × 3)-Q adlattice. In another set of experiments, EC-STM images were obtained when the potential was shifted to a lower (less positive) value, 0.2 V, inside the hydrogen evolution region. A typical result is shown in Figure 5 where it can be seen that (i) a major fraction, but not all, of the Q molecules were (hydrogenatively) desorbed, (ii) the remnant Q species were agglomerated in small islands, each of which retained the original ordered (3 × 3)-Q structure, and (iii) the shapes of the individual Q images were the same as those for the more densely packed surface. When the potential reverted to 0.4 V, chemisorbed Q was replenished from the solution-borne H2Q species, and the full Pd(111)-(3 × 3)-Q was restored. Structural Models. Two structural models for the (3 × 3)-Q adlattice consistent with the present results are feasible; both are depicted in Figure 6. In the first (Figure 6A), the O atoms occupy
atop sites, and in the second (Figure 6B), the O atoms are located at 2-fold bridge sites. Unfortunately, the EC-STM images obtained in this study are unable to differentiate between the two possible models. In both structures, the center of the quinonoid ring is located on a 2-fold site. The observation that the shapes of the individual Q images are independent of the potential suggests that the placement of the quinonoid ring is always on a 2-fold site whether the (3 × 3)-Q structure is for the full monolayer at 0.4 V (Figures 4) or for the smaller islands at 0.2 V (Figure 5). Comparison with Benzene. EC-STM studies on Pd(hkl) surfaces have hitherto focused only on unsubstituted aromatic compounds such as benzene and naphthalene. The results presented here serve to help advance the ongoing endeavor to examine the influence of various functional groups on the chemisorption and interfacial structure of aromatic compounds at electrocatalyst surfaces. The influence of the O-H substituents on the interfacial properties of aromatic molecules may be (partially) gleaned from a comparison of the results described here and those recently reported by us for benzene.8 For example, at potentials within the double-layer region, the chemisorption of benzene yielded a well-ordered Pd(111)-c(2x3 × 3)-rectC6H6 adlattice in which the adsorbed benzene molecules are oriented flat but with a slight tilt. When the potential was decreased to values just inside the hydrogen evolution region, about onethird of the chemisorbed molecules were desorbed, and the leftover molecules were reconstructed to a Pd(111)-(3 × 3)-C6H6 adlayer that encompassed the entire surface and is not just restricted to small island domains; in this less densely packed phase, the aromatic molecules were oriented more closely parallel to the surface. The appearance of the EC-STM images was also potential-dependent: dumbbell-shaped images were observed for the Pd(111)-c(2x3 × 3)-rect-C6H6 adlattice in the doublelayer region whereas triangular-shaped images were noted for the Pd(111)-(3 × 3)-C6H6 adlayer in the hydrogen evolution region. In the former, the center of the aromatic ring was postulated to reside on a 2-fold site; in the latter, it was thought to exist on a 3-fold hollow site. Acknowledgment. Acknowledgment is made to the Robert A. Welch Foundation and the National Science Foundation (CHE9703521) for support of this research. LA0611920
