In Situ Scanning Tunneling Microscopy of Benzene, Naphthalene, and

The naphthalene molecules were preferentially aligned with their long C2 axes along the atom rows of the Cu(111) substrate. .... Langmuir 2009 25 (2),...
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Langmuir 1997, 13, 7173-7179

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In Situ Scanning Tunneling Microscopy of Benzene, Naphthalene, and Anthracene Adsorbed on Cu(111) in Solution Li-Jun Wan† and Kingo Itaya*,‡ Itaya Electrochemiscopy Project, ERATO/JST, The Research Institute of Electric and Magnetic Materials, 2-1-1 Yagiyama-minami, Sendai 980, Japan, and Department of Applied Chemistry, Faculty of Engineering, Tohoku University, Sendai 980, Japan Received June 30, 1997. In Final Form: October 13, 1997X

In situ scanning tunneling microscopy (STM) was used to probe the adlayer structures of benzene, naphthalene, and anthracene on a well-defined Cu(111) electrode surface in aqueous HClO4 solution. These molecules were found to form highly ordered adlayers on a clean Cu(111) surface with a flat-lying orientation. The molecular orientation and packing arrangement were also determined. The ordered benzene adlayer was observed to form a (3 × 3) structure with a characteristic triangular shape for each benzene molecule. The naphthalene adlayer was observed with a (4 × 4) symmetry. The naphthalene molecules were preferentially aligned with their long C2 axes along the atom rows of the Cu(111) substrate. A long range ordered structure was also found for the anthracene adlayer. Straight molecular chains formed along the atomic rows of Cu(111) with a side-by-side configuration.

Introduction The bonding and coordination of organic molecules with metal electrode surfaces are the fundamental issue both in electrochemistry1 and in catalysis in the gas phase.2 The structure and composition features of organic adlayers were traditionally investigated by ultrahigh vacuum (UHV) methods such as low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and electron energy-loss spectroscopy (EELS).2 Hubbard and coworkers have intensively examined various organic adlayers formed on metal electrodes in solution by using these ex situ techniques, the so-called UHV-electrochemical system (UHV-EC), in order to understand electrode/electrolyte interfaces.3,4 However, recent efforts using in situ scanning probe microscopies (SPMs), in particular in situ scanning tunneling microscopy (STM), have established them as invaluable methods for determining the structures of organic adlayers with atomic or molecular resolution both in UHV5-11 and in solution.12-21 * To whom correspondence should be addressed. † The Research Institute of Electric and Magnetic Materials. ‡ Tohoku University. X Abstract published in Advance ACS Abstracts, December 1, 1997. (1) Lipkowski, J., Ross, P. N., Eds. Adsorption of Molecules at Metal Electrodes; VCH Publisher: New York, 1992. (2) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; John Wiley & Sons Inc.: New York, 1994. (3) Hubbard, A. T. Chem. Rev. 1988, 88, 633. (4) Soriaga, M. P. Prog. Surf. Sci. 1992, 39, 525. (5) Ohtani, H.; Wilson, R. J.; Chiang, S.; Mate, C. M. Phys. Rev. Lett. 1988, 60, 2389. (6) Tanaka, H.; Nakagawa, T.; Kawai, T. Surf. Sci. 1996, 364, L575. (7) Stranick, S. J.; Kamna, M. M.; Weiss, P. S. Science 1994, 266, 99. (8) Junk, A. T.; Schlittler, R. R.; Gimzewski, Nature 1997, 386, 696. (9) Weiss, P. S.; Eigler, D. M. Phys. Rev. Lett. 1993, 71, 3136. (10) Chiang, S. In Scanning Tunneling Microscopy I; Wiesendanger, R., Guntherodt, H.-J., Eds.; Springer-Verlag: New York, 1991; pp 181205. (11) Ikai, A. Surf. Sci. Rep. 1996, 26, 261. (12) Siegenthaler, H. In Scanning Tunneling Microscopy II; Wiesendanger, R., Guntherodt, H.-J., Eds.; Springer-Verlag: New York, 1992; pp 7-49.

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Our recent paper demonstrated that the adlayer structures of benzene were visualized with an extraordinarily high resolution on Rh(111) and Pt(111) in HF solution.15 On Rh(111), two potential-dependent structures of benzene adlayers with c(2x3 × 3)rect and (3 × 3) symmetries were identified in solution,15 while similar structures had previously been found to form in UHV conditions.2,5 The fact that the adlayer structures of benzene on Rh(111) are similar in UHV and in HF solution suggests that the adsorbate-substrate interaction is predominant for hydrophobic molecules such as benzene, and the role of water molecules is surprisingly minor in the formation of benzene adlayers in solution. On the other hand, it is known that benzene does not form an ordered structure on Pt(111) in UHV,2 while a well-ordered (x21 × x21)R10.9° structure was found at cathodic potentials in HF solution.15 This result indicates that the nature of electrified interfaces can play an important role in the ordering processes of adsorbed organic molecules. We recently reported that in situ STM allowed us to visualize the adlayers of larger aromatic molecules such as naphthalene and anthracene.16 Naphthalene yielded a long range ordered (3x3 × 3x3)R30° structure on Rh(111) in HF, whereas anthracene produced a disordered structure. On Pt(111), disordered structures were found for both naphthalene and anthracene.16 These results indicate that aromatic molecules are more strongly adsorbed on Pt(111) than on Rh(111), resulting in a lesser (13) Tao, N. J.; Cardenas, G.; Cunha, F.; Shi, Z. Langmuir 1995, 11, 4445. (14) Cuhan, F.; Tao, N. J. Phys. Rev. Lett. 1995, 75, 2376. (15) Yau, S.-L.; Kim, Y.-G.; Itaya, K. J. Am. Chem. Soc. 1996, 118, 7795. (16) Yau, S.-L.; Kim, Y.-G.; Itaya, K. J. Phys. Chem. 1997, 101, 3547. (17) Kunitake, M.; Batina, M.; Itaya, K. Langmuir 1995, 11, 2337. (18) Batina, N.; Kunitake, M.; Itaya, K. J. Electroanal. Chem. 1996, 405, 245. (19) Kunitake, M.; Akiba, U.; Batina, N.; Itaya, K. Langmuir 1997, 13, 1607. (20) Ogaki, K.; Batina, N.; Kunitake, M.; Itaya, K. J. Phys. Chem. 1996, 100, 7185. (21) Itaya, K.; Batina, N.; Kunitake, M.; Ogaki, K.; Kim, Y.-G.; Wan, L.-J.; Yamada, T. In Solid-Liquid Electrochemical Interfaces; Jerkiewicz, G., Soriaga, M. P., Uosaki, K., Wieckowski, A., Eds.; ACS Symposium Series 656; American Chemical Society: Washington, DC, 1997; p 171.

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degree of surface diffusion of adsorbed molecules and hence more disordered phases on Pt(111). We have also previously revealed an important fact that relatively large organic molecules such as porphyrin formed an ordered adlayer on an iodine-modified Au(111), while they formed a disordered adlayer in the absence of the iodine monolayer.17-21 The iodine-modified Au(111)17-19 and Ag(111)20 electrodes were found to be suitable substrates on which to form highly ordered adlayers of larger molecules, demonstrating that the interaction between molecules and substrates is an important factor in the processes of ordering of molecular adsorbates on substrates. Van der Waals type interactions between the hydrophobic iodine adlayer and the organic molecules allow molecular motion on the surface to form the ordered phase.19, 21 The adsorption of organic molecules on Cu electrodes is an interesting subject, because of relatively weaker interactions between aromatic molecules and Cu.1,2 It is also well-known that benzotriazole is an effective corrosion inhibitor for Cu electrodes. Note that Behm and his co-workers recently reported adlayer structures of benzotriazole adsorbed on Cu(100).22 Here, we describe, for the first time, results of an in situ STM study of adlayer structures of simple aromatic molecules such as benzene, naphthalene and anthracene directly attached to a well-defined Cu(111) in aqueous HClO4 solution. A Cu(111)-(1 × 1) structure was observed on an atomically flat Cu(111) surface in a double-layer potential range in the absence of organic molecules. The molecules of benzene, naphthalene, and anthracene were found to adsorb on Cu(111) to form ordered layers with (3 × 3), (4 × 4), and (4 × 5) structures, respectively. The molecular orientation and packing arrangement for each adlayer were derived from high-resolution STM images. Experimental Section A bulk Cu(111) single-crystal disk with a diameter of 10 mm (from MaTeck) was used as a working electrode for both electrochemical measurement and in situ STM observation. After having been mechanically polished with successively finer grades of diamond paste with particle sizes down to 0.05 µm, the Cu(111) sample was annealed at ca. 600-700 °C in a quartz tube under hydrogen atmosphere for 2-3 h to remove the damaged layer produced by the mechanical polishing. The Cu(111) surface was then electropolished in a phosphoric acid solution (50 mL of 85% H3PO4 and 50 mL water) at 0.8-1.0 A cm-2 for 3-5 s. The Cu crystal was rinsed repeatedly with ultrapure Millipore water (Millipore-Q). A droplet of water was left on the electrode surface to protect it from contamination during transfer to the electrochemical cell. The Cu sample was then mounted in the electrochemical cell for either cyclic voltammogram measurement or in situ STM observation. A well-defined single-crystal surface was prepared in situ by an anodic dissolution. It is shown below that the anodic dissolution in pure HClO4 was found to proceed in the layer-by-layer mode, exposing atomically flat terraces over large areas. A solution of 0.1 M HClO4 was prepared by diluting ultrapure HClO4 (Cica-Merck, Kanto Chemicals) with ultrapure Millipore water. Reagent grade benzene, naphthalene, and anthracene were from Kanto Chemicals Co. Ltd. The solubilities of benzene and naphthalene in water were reported to be ca. 9 and 0.23 mM at room temperature, respectively.23 The solutions containing these organic molecules were prepared at a specific concentration (benzene) or at saturated concentrations (naphthalene and anthracene). Although the solubility of anthracene was expected to be much lower than that of benzene or naphthalene, it was found in this study that the formation of a monolayer of anthracene was easily established in a saturated solution within (22) Vogt, M. R.; Polewska, W.; Magnussen, O. M.; Behm, R. J. J. Electrochem. Soc. 1997, 144, L113. (23) Dean, J. A., Ed. Lange’s Handbook of Chemistry; McGraw-Hill: New York, 1985.

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Figure 1. Cyclic voltammograms of Cu(111) in (a) 0.1 M HClO4, (b) 0.1 M HClO4 + 1 mM benzene, (c) 0.1 M HClO4 + naphthalene, and (d) 0.1 M HClO4 + anthracene. The potential scan rate was 50 mV s-1. a few minutes. The home-made electrochemical cell contained a reversible hydrogen electrode (RHE) in 0.1 M HClO4 and a Pt counter electrode. All electrode potentials are reported with respect to the RHE. The in situ STM apparatus used was a Nanoscope III (Digital Instrument Inc., Santa Barbara, CA). The tunneling tips were prepared by electrochemically etching a tungsten wire (0.25 mm in diameter) in 0.6 M KOH. An ac voltage of 12-15 V was applied until the etching stopped. The W tips were then coated with clear nail polish to minimize the Faradaic current.15 During STM observation the potential of the tunneling tip was carefully adjusted at a value more anodic than the Cu deposition potential to avoid Cu deposition on the tip, as described in the literature.24,25 Most of the STM images shown here were acquired in the constant-current mode to evaluate the corrugation heights of the Cu(111) substrate and the adsorbed molecules.

Results and Discussion Cyclic Voltammetry. Steady state cyclic voltammograms of Cu(111) were obtained by using the so-called hanging meniscus method26 in the absence and presence of benzene, naphthalene, or anthracene in 0.1 M HClO4. The first scan of each CV was made in the negative direction from the open circuit potential (OCP; ca. 0.23 V). Figure 1(a) shows a CV of Cu(111) in the absence of organic molecules. It can be seen that the double-layer region extends from -0.35 to 0.15 V. At ca. 0.15 V, the anodic dissolution of Cu started, and an abrupt increase in anodic current commenced at ca. 0.25 V. The cathodic peak at 0.2 V observed upon reversal of the potential scan corresponds to the electrodeposition of Cu metal from the Cu2+ ions formed during the anodic scan. The cathodic current beginning at ca. -0.35 V is ascribed to hydrogen evolution. Note that although a similar voltammogram was reported in a HCl solution,24 a small broad cathodic (24) Suggs, D. W.; Bard, A. J. J. Am. Chem. Soc. 1994, 116, 10725. (25) Suggs, D. W.; Bard, A. J. J. Phys. Chem. 1995, 99, 8349. (26) Yamada, T.; Batina, N.; Itaya, K. J. Phys. Chem. 1995, 99, 8817.

Aromatics Adsorbed on Cu(III) in Solution

peak previously observed in HCl before the hydrogen evolution reaction did not appear in a HClO4 solution in the absence of chloride ions. The CVs obtained in the presence of benzene, naphthalene, and anthracene are shown in Figure 1b,c, and d. The overall shape of each CV is almost the same as that in Figure 1a obtained in the absence of organic molecules. Only the electric charge involved in the double-layer potential range becomes smaller due to adsorption of the molecules. Our previous results have shown that benzene, naphthalene, and anthracene were chemisorbed on Rh(111) and Pt(111) electrode surfaces, resulting in blockage of the hydrogen adsorption and desorption reactions.15,16 The voltammograms shown in Figure 1 appear to suggest that the organic molecules are more weakly adsorbed on Cu(111). In general, it is known that the adsorption of small aromatic molecules such as benzene on metals of group IB elements, i.e., Cu, Ag, and Au, has the character of physisorption.1,2 A recent infrared spectroscopic study on the adsorption of benzene on Pt and Cu in UHV clearly indicates that benzene is adsorbed on Cu more weakly than on Pt.27 The results mentioned above strongly encouraged us to use a Cu electrode to investigate the adsorption of the aromatic molecules, including relatively large molecules such as anthracene. We expected the formation of highly ordered adlayers of the molecules on Cu(111) in solution based on our previous results obtained on iodine-modified electrodes,15-21 because of weaker interactions between Cu and the molecules. In Situ STM. (1) Cu(111)-(1 × 1). To produce an atomically flat Cu(111) surface for STM observation, a freshly polished surface was first immersed in 0.1 M HClO4 at the OCP in the STM electrochemical cell, and the electrode potential was swept negative at the scan rate 50 mV/s to the hydrogen evolution region and then back to the Cu dissolution region. After three or five such cycles, the electrode was finally held at a potential near the onset of the anodic dissolution. Although atomically rough surfaces were consistently observed soon after the electropolishing, well-defined terrace-step features were found to develop gradually, indicating that the anodic dissolution proceeded by a layer-by-layer mode in HClO4. The similar layer-by-layer dissolution was recently found for Cu in HCl,24,25 S-modified Ni,28,29 I-modified Pd,30 and I-modified Ag.31 The terrace width further increased when the electrode potential was held in the double-layer region for at least 30-60 min. This observation suggests that surface annealing also takes place on Cu(111). Figure 2a is a typical large scale STM image of a soprepared Cu(111) surface acquired at -0.1 V. Monatomic steps (0.22 nm) are seen to run nearly parallel or at an angle of ca. 60° to each other, as expected for surfaces with a threefold symmetry. However, it is clear that the steps include many defects such as kinks because the step lines were not atomically straight, as can be seen in Figure 2a. The terraces are atomically flat with a low defect density, suggesting that the Cu(111) surface has a welldefined structure in HClO4. High-resolution STM imaging made it possible to observe individual Cu atoms on the terraces. To our knowledge, atomic resolution STM images have not yet (27) Haq, S.; King, D. A. J. Phys. Chem. 1996, 100, 16957. (28) Suzuki, T.; Yamada, T.; Itaya, K. J. Phys. Chem. 1996, 100, 8954. (29) Ando, S.; Suzuki, T.; Itaya, K. J. Electroanal. Chem. 1996, 412, 139. (30) Sashikata, K.; Matsui, Y.; Itaya, K.; Soriaga, M. P. J. Phys. Chem. 1996, 100, 20027. (31) Teshima, T.; Ogaki, K.; Itaya, K. J. Phys. Chem. 1997, 101, 2046.

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Figure 2. STM top views of Cu(111) in 0.1 M HClO4. (a) Large scale STM image showing an atomically flat Cu(111) surface. (b) Cu(111)-(1 × 1) structure. The set of arrows point out the close-packed directions of Cu(111). Scanning rates were (a) 8.1 and (b) 21.7 Hz, respectively. Tunneling currents were (a) 2 and (b) 20 nA.

been achieved on bare Cu(111) in solution,24 although atomic force microscopy (AFM) showed a Cu(111)-(1 × 1) structure.32 Our first atomic STM image of a Cu(111) surface is shown in Figure 2b. The hexagonal close-packed structure can be seen with an interatomic distance of ca. 0.26 nm (the diameter of a Cu atom is 0.256 nm), indicating that the Cu(111) surface has a (1 × 1) structure. The arrows point out the directions of close-packed Cu(111) determined by the crystallographic orientation of the crystal. The corrugation amplitude was too low to be measured accurately, i.e., ca. 0.01-0.015 nm, which is significantly smaller than that observed on Au, Pt, Rh, or Ag, typically 0.02-0.03 nm.12,15,31 The identical (1 × 1) structure was consistently observed in the potential range between -0.35 and 0.15 V. No evidence was found for the oxidation of Cu(111) in HClO4, in contrast to the report based on in situ AFM experiments that the Cu(100) surface might be covered with adsorbed O or OH-, exhibiting a (x2 × x2)R45° structure, even in HClO4.32 The image shown in Figure 2b demonstrates that, under the present conditions, the Cu(111) surface retains a clean and unreconstructed (1 × 1) structure. (2) Benzene Adlayer. After the atomic resolution image of the Cu(111)-(1 × 1) structure shown in Figure 2b was (32) Cruickshank, B. J.; Sneddon, D.; Gewirth, A. A. Surf. Sci. Lett.1993, 281, L308.

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Figure 3. (a and b) High-resolution STM images of a Cu(111)(3 × 3)-C6H6 adlattice acquired at -0.3 V. Tunneling current was 20 nA. Scanning rate was 9.04 Hz. (c) Real space structural model. All molecules are assigned to threefold hollow sites.

observed, a small amount of benzene solution was directly injected into the electrochemical cell at -0.3 V. The average concentration of benzene was ca. 1 mM. Figure 3a is a large scale STM image revealing the molecular feature of the benzene adlayer. This image was acquired at -0.3 V. Although several single molecular defects are apparent, it can be seen that a well-ordered adlayer of molecules with a characteristic molecular shape and packing arrangement is extended over wide atomically flat terraces. The molecular rows cross each other at an angle of either 60° or 120° within an experimental error of (2°. From a comparison with the crystal orientation

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of 〈110〉 determined by the Cu(111)-(1 × 1) atomic image as indicated by the arrows, it can be easily concluded that all molecular rows are almost perfectly parallel to the underlying Cu(111) atomic rows. All benzene molecules exhibit a corrugation height of ca. 0.02-0.025 nm. STM images acquired with even higher resolution allowed us to determine the internal structure and orientation of each benzene molecule adsorbed on Cu(111). The image shown in Figure 3b is an example acquired in an area including molecular defects. It is clear that each benzene molecule appears as a set of three spots with almost the same corrugation height. The intermolecular distance along the close-packed directions of Cu(111) is measured to be ca. 0.77 nm, which corresponds to three times the lattice distance of Cu(111). The molecular rows cross each other at an angle of either 60° or 120° within experimental error. A clear dip exists at the center of each triangle with three lobes. The difference in corrugation height between the spot and the dip is ca. 0.006 nm. Therefore, we conclude that the benzene adlayer forms a (3 × 3)-C6H6 structure with a surface coverage of 0.11, as illustrated in Figure 3c. Each benzene molecule appeared with three isolated spots in a triangular configuration. Similar three spots have been found for benzene on Rh(111) in UHV5 and in solution,15 suggesting that each benzene molecule is also located on the threefold hollow site on Cu(111). It is rather surprising that all features observed for benzene adsorbed on Cu(111) are almost identical to those found on Rh(111), as described in our previous paper.15 However, there is a difference in detail between the benzene adlayer on Cu(111) and that on Rh(111). Each unit cell of the benzene adlayer on the Rh(111) surface was always accompanied by a weaker additional spot, which was attributed either to coadsored CO in UHV5,10 or to a water molecule or a hydronium cation in aqueous solution.15 On the other hand, no such additional spot was found on Cu(111), as can be seen in Figure 3b. Although many factors should be taken into account for the weak additional spots,5,15 the interatomic distance of Cu on Cu(111), 0.256 nm, is smaller than that of Rh, 0.268 nm. It is reasonably expected that the benzene adlayer is more closely packed on Cu(111) than on Rh(111), resulting in the disappearance of hydronium cations from the unit cell. It is also important to note that the adsorbed benzene monolayer on Cu(111) is desorbed in UHV in the temperature range between 150 and 240 K.33 At room temperature, all benzene molecules except those adsorbed at defect sites undergo complete desorption from the Cu(111) surface.33 This result obtained in UHV is in contrast to the present in situ STM result. The benzene adlayer on Cu(111) is stable at room temperature in solution, suggesting that water molecules play an important role in stabilizing the adlayer. On the other hand, the nature of electrode/electrolyte interfaces is also expected to be one of the most important factors. The ordered (3 × 3) structure was consistently observed in the potential range from -0.35 to -0.2 V. At potentials more positive than -0.2 V, the ordered structure became unclear and produced noisy images, suggesting that either desorption or reorientation took place at the positive potentials. No clear peak corresponding to the phase transition between the ordered and disordered phases appears in CV, as shown in Figure 1, indicating that charge transfer processes are not involved in the phase transition. Note that no peak was also observed for benzene on (33) Xi, M.; Yang, M. X.; Jo, S. K.; Bent, B. E. J. Chem. Phys. 1994, 101, 9122.

Aromatics Adsorbed on Cu(III) in Solution

Rh(111), as described previously.15 The potential of zero charge (pzc) was reported to be ca. -0.25 V vs. SCE for Cu electrodes.34 Recently, more negative values were obtained on Cu(111).35 The interaction energy between aromatic molecules and Cu electrodes should be dependent on the charge on the electrode. We expect that the positive charge on the Cu(111) electrode decreases the interaction between benzene molecules and Cu to form an ordered adlayer in solution. However, further studies using different techniques such as capacitance measurement and in situ IR are needed to understand the nature of the benzene adlayer at potentials more positive than -0.2 V. A similar potential dependence was also found for naphthalene but not for anthracene, as described below. (3) Naphthalene Adlayer. Figure 4 shows high-resolution STM images of a naphthalene adlayer at -0.3 V. A long range ordered adlayer can be clearly seen over the large area (20 × 20 nm), as shown in Figure 4a. Naphthalene molecules uniformly cover the Cu(111) surface with a small number of defects. Each spot has an elongated feature corresponding to an individual naphthalene molecule. The molecular rows were found to be parallel to the 〈110〉 direction of the underlying Cu(111) lattice. This ordered adlayer was consistently observed in the potential range from -0.35 to -0.15 V. At more positive potentials, the ordered adlayer disappeared, resulting in noisy STM images, which was observed also for the adsorption of benzene as described above. The STM image shown in Figure 4b shows more details of the packing arrangement and the internal structure of the naphthalene adlayer. In this image the two-ring structure of the naphthalene molecule appears as two main spots with additional details of the internal structure. The distance between the two main spots is ca. 0.56 nm, as expected from the molecular model of naphthalene. It is clear that each molecule obviously bonds to Cu(111) with a flat-lying orientation. The naphthalene molecules are aligned with the longer molecular axis (C2) in the same direction along the Cu rows. The observed distance between the nearest neighbor molecules is 1.0 ( 0.02 nm, which is nearly equal to four times the Cu lattice distance of 0.256 nm. All features in the STM images indicate that the structure of the naphthalene adlayer can be defined as Cu(111)-(4 × 4)-C10H8. A unit cell is outlined in Figure 4a and b. This (4 × 4) structure results in a surface coverage of 0.0625. It was reported in our recent paper that naphthalene formed a highly ordered structure with (3x3 × 3x3)R30° symmetry on Rh(111) in HF solution.16 The internal structure of naphthalene was more clearly discerned on Rh(111) than on Cu(111), which allowed us to determine a micro-orientation of naphthalene molecules along the molecular rows.16 Although the image shown in Figure 4b can fairly accurately determine the symmetry of the adlayer, (4 × 4), the determination of the micro-orientation was not as easy as that on Rh(111) even though optimum imaging conditions were searched by varying the tunneling current (1-50 nA) and the bias voltage (20-300 mV). The (3x3 × 3x3)R30° structure found on Rh(111) results in a surface coverage of 0.11, which is nearly twice as large as that found on Cu(111). This difference suggests that naphthalene is more strongly attached on Rh(111) than on Cu(111). The repulsive interaction between adjacent naphthalene molecules seems to be a predominant factor for the formation of the (4 × 4) structure. A tentatively proposed ball model for the (4 × 4)-C10H8 structure is (34) Hamelin, A.; Sevastyanov, E.; Popov, P. J. Electroanal. Chem. 1983, 145, 225. (35) Hartinger, S.; Doblhofer, K. J. Electroanal. Chem. 1995, 380, 185.

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Figure 4. (a and b) High resolution STM images of a Cu(111)(4 × 4)-C10H8 adlayer acquired at -0.3 V. Tunneling current was 20 nA. Scanning rate was 10.85 Hz. The close-packed directions of the Cu(111) lattice are indicated by a set of arrows. (c) Schematic representation for the (4 × 4) structure.

shown in Figure 4c. In this model each naphthalene molecule is placed onto two neighboring twofold sites with its C2 axis aligned along 〈110〉 of the Cu(111) substrate. A similar binding site was assumed for naphthalene on Rh(111), as described previously.16 In this open adlayer structure, however, one can expect that molecular motion is not completely prohibited in the unit cell. We anticipate

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elongated spot. The image was acquired at -0.25 V in an anthracene-saturated 0.1 M HClO4 solution. Figure 5b shows a high-resolution STM image showing details of an anthracene adlayer. The image of each molecule shows details of the internal structure of anthracene. It is also clear that the anthracene molecules are preferentially aligned with their long C2 axes along one of the close-packed directions of the Cu(111) lattice, as indicated by the set of arrows. The side-by-side configuration is more clearly seen than that for naphthalene shown in Figure 4b. The intermolecular distance along the straight molecular rows is ca. 1.25 ( 0.05 nm, which corresponds to five times the lattice parameter of Cu. A unit cell and a model of the packing arrangement are shown in Figure 5b and c, respectively. The distance on the shorter side is ca. 1.0 ( 0.05 nm. The two directions in the unit cell are parallel to the Cu(111) lattice for all molecules observed. These features strongly suggest that the structure of the observed molecular adlayer is (4 × 5)-C14H10 with a coverage of 0.05, as shown in Figure 5c. We tentatively propose that each anthracene molecule occupies nearly three two-fold bridge sites with its long C2 axis along the Cu(111) lattice. The center ring of anthracene might be exactly located in the center position of a two-fold bridge site, whereas two other rings are slightly shifted from the center position. A detailed inspection of the STM image shown in Figure 5b revealed the nonequivalent appearance of the three rings of anthracene. It is noteworthy that the anthracene adlayer could be seen in the potential range between -0.35 and 0 V. As described above for benzene and napthalene adlayers, those molecules could not be seen at potentials more positive than ca. -0.15 V. On the other hand, a clear image similar to that shown in Figure 5b was observed with anthracene even at 0 V, which is more positive than the pzc. This indicates that the interaction between anthracene and the Cu(111) surface is stronger than that of benzene or naphthalene with Cu(111), because of the larger cross sectional area of anthracene. Finally, the results shown here strongly encouraged us to explore the potential applications of in situ STM to investigate various organic molecules. Behm and his coworkers recently reported adlayer structures of benzotriazole adsorbed on Cu(100).22 Benzotriazole is known to be an effective corrosion inhibitor for Cu electrodes. Conclusion

Figure 5. (a and b) High-resolution STM images of a Cu(111)(4 × 5)-C14H10 adlayer acquired at -0.3 V. Tunneling current was 10 nA. Scanning rate was 10.85 Hz. (c) Schematic representation for the (4 × 5) structure.

that this type of motion decreased the resolution in STM imaging on Cu(111). (4) Anthracene Adlayer. It was surprising to find an extraordinarily ordered adlayer of anthracene on Cu(111) because anthracene forms completely disordered adlayers on Rh(111) and Pt(111), as described in our recent paper.16 Even in a large area of 30 × 30 nm, the highly ordered molecular array can be seen with molecular resolution as shown in Figure 5a. Each molecule appeared as an

An atomically flat Cu(111) single-crystal electrode surface could be exposed by in situ anodic dissolution in HClO4. The Cu(111)-(1 × 1) structure was consistently observed on the well-defined Cu(111) surface in the doublelayer potential range. High-resolution STM images were presented of molecules of benzene, naphthalene, and anthracene adsorbed on a Cu(111) single-crystal electrode surface in aqueous HClO4 solution. The packing arrangement and the internal molecular structure of the molecules were determined by in situ STM. All of the three molecules formed long range ordered adlayers on Cu(111) with flat-lying orientation. In situ STM revealed a (3 × 3)-C6H6 structure with a coverage of 0.11. Each benzene molecule appeared as a triangular shape, implying its three-fold registry. A (4 × 4)-C10H8 adlayer structure with a coverage of 0.0625 was observed in a 0.1 M HClO4 + naphthalene solution. The naphthalene molecule was found to adsorb on the electrode surface with its long C2 axis along the 〈110〉 direction of the Cu(111) lattice. The spots corresponding to the two-ring structure were clearly discerned in high-resolution STM images.

Aromatics Adsorbed on Cu(III) in Solution

Each naphthalene molecule was assumed to be located on a row of Cu atoms across two neighboring twofold sites. Anthracene molecules also formed a long range ordered adlayer on a Cu(111) surface. A (4 × 5)-C14H10 structure was revealed. The present results show that highresolution in situ STM images can supply direct information on the adsorption of organic molecules on active metallic electrode surfaces. The internal structures of

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benzene, naphthalene, and anthracene adlayers on Cu(111) were successfully probed. Acknowledgment. We thank the ERATO project for financial support and Dr. Y. Okinaka for his help in the writing of this manuscript. LA9706936