Adlayer Structures of Benzene and Pyridine Molecules on Cu(100) in

Guo-Kun Liu, Bin Ren, De-Yin Wu, Sai Duan, Jian-Feng Li, Jian-Lin Yao, ... Gui-Jin Su, Hui-Min Zhang, Li-Jun Wan, Chun-Li Bai, and Thomas Wandlowski...
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J. Phys. Chem. B 2001, 105, 8399-8402

8399

Adlayer Structures of Benzene and Pyridine Molecules on Cu(100) in Solution by ECSTM Li-Jun Wan,*,† Chen Wang,† Chun-li Bai,*,† and Masatoshi Osawa‡ Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China, and Catalysis Research Center, Hokkaido UniVersity, Sapporo 060, Japan ReceiVed: June 5, 2001

Adsorption of benzene (C6H6) and pyridine (C5H5N) on Cu(100) has been investigated in electrolyte solution by using cyclic voltammetry and in situ scanning tunneling microscopy (STM). Both molecules adsorbed on Cu(100) surface in HClO4 solution and formed well-defined (2 × 2) structures with a flat-lying orientation. Structural models were proposed to interpret the bonding and coordination of the two molecules with Cu(100) surface.

The adsorption of organic molecules on solid surfaces continues to raise interests in both fundamental study and industrial application.1-3 The adlayer structure and conformation of molecules at interfaces is of particular importance in surface chemistry and catalysis science. As model molecules, benzene and pyridine have been intensively investigated on various single-crystal substrates mainly by ex situ ultrahigh vacuum (UHV) techniques during past decades.4-10 The results obtained by these indirect methods have helped understanding the coordination between adsorbed molecules and underlying metal surfaces. The development of scanning probe microscopy (SPM), in particular scanning tunneling microscopy (STM), has added real space information to the adsorption of benzene and other organic molecules.11,12 Recently, electrochemical STM (ECSTM), combining electrochemistry and STM, has made it possible to observe an atom, an anion, and an organic molecule at electrode/solution interface with atomic resolution.13-16 For example, sulfate (bisulfate) was observed on Ir(111) and Pd(111) surfaces;17,18 a benzene adlayer was observed on Rh(111), Pt(111), and Cu(111) surfaces in solution.19,20 The ordered benzene adlayer was found to form a (3 × 3) structure with a characteristic triangular shape for each benzene molecule, implying the 3-fold binding site. The benzene molecule was proposed to adsorb on electrode surfaces with its π electrons, i.e., as flat-lying. The result was consistent with that observed in the UHV environment.6,7 On pyridine adsorption, the interaction of the molecule with metal surface is involved in π and lone-pair electrons. Different orientation with respect to metal surfaces is expected on pyridine adlayer.3,22-25 The success in the investigation of benzene and pyridine on fcc(111) surface by using ECSTM encouraged us to explore the adsorption of benzene and pyridine on fcc(100). Here, we report the preliminary result of an in situ ECSTM observation of benzene and pyridine adlayers on Cu(100). To the best of our knowledge, no in situ STM results have been reported on the two molecules on Cu(100) in solution, although the importance of the research has been realized for a long time. In the present study, benzene and pyridine molecules were found to form an ordered adlayer on Cu(100) in 0.1 M HClO4 solution. The bonding and coordination of the molecules to Cu atoms * Corresponding authors. E-mail: [email protected]; clbai@ infoc3.icas.ac.cn. Fax: +86-10-62558934 † Chinese Academy of Sciences. ‡ Hokkaido University.

were determined by high-resolution STM images. The study is expected to extend the findings obtained by the ex situ method and leads to a new insight into the nature of the electrode/ solution interface. Experimental Section A procedure similar to that used to expose atomically flat Cu(111) and Cu(110) surfaces was used to prepare the Cu(100) sample.20,21 Briefly, after having been polished mechanically with successively finer grades of diamond paste, with particle sizes down to 0.05 µm, the Cu(100) surface was 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. The so-prepared Cu(100) was mounted in a homemade electrochemical cell for either electrochemical measurement or in situ STM observation. To produce an atomically flat Cu(100) surface, the freshly polished surface was further treated in a surface annealing process. The sample was immersed in a 0.1 M HClO4 (Cica-Merck, Kanto Chemicals) solution at the open circuit potential, and the electrode potential was scanned negatively to the hydrogen evolution region and then back to the Cu dissolution region. After 3-5 cycles, the electrode was held at a potential near the onset of the anodic dissolution. Benzene (Kanto Chemicals) and pyridine (Wako Chemicals) were used as received. Electrolyte solutions were prepared with Millipore water. All electrode potentials were reported versus a reversible hydrogen electrode (RHE) in 0.1 M HClO4. The microscope used was a Nanoscope E (Digital Instruments). All STM images were collected in the constant-current mode by using an electrochemically etched tungsten wire in a diameter of 0.25 mm coated with clean nail polish. Results and Discussion (1) Electrochemistry. Steady-state cyclic voltammograms (CVs) of Cu(100) in (a) 0.1 M HClO4, (b) 0.1 M HClO4 + 1m M benzene, and (c) 0.1 M HClO4 + 1 mM pyridine were obtained by using the so-called hanging meniscus method. The first scan of each CV was made in the negative direction from the open circuit potential (OCP; ca. 0.24 V). The CVs are shown in Figure 1. It can be seen that all three CVs have almost the

10.1021/jp012154m CCC: $20.00 © 2001 American Chemical Society Published on Web 08/09/2001

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Figure 1. Cyclic voltammograms of Cu(100) in (a) 0.1 M HClO4, (b) 0.1 M HClO4 + 1m M benzene, and (c) 0.1 M HClO4 + 1 mM pyridine. The potential scan rate was 50 mV s-1.

Figure 2. STM image of a Cu(100)-(l × 1) structure in 0.1 M HClO4. Scanning rate was 16.28 Hz. Tunneling current was 5 nA.

same features. The CV of Cu(100) in 0.1 M HClO4 in Figure 1a is very similar to those of Cu(111) and Cu(110) in the same electrolyte solution.20,21 A double layer region extends from -0.3 to 0.1 V. At potentials more positive than 0.15 V, copper dissolution takes place with an abruptly increasing anodic current. At potentials more negative than -0.35 V, hydrogen evolution occurs. With the addition of benzene and pyridine in solution, the overall shape of each CV in Figure 1b and 1c is almost the same as that in Figure 1a. Only the electric charge involved in the double layer potential region becomes smaller due to adsorption of the molecules. No obvious current peak corresponding to phase transition is observed. (2) Cu(100)-(1 × 1). A Cu(100)-(1 × 1) structure was acquired on the terrace area of Cu(100) surface. Figure 2 is an STM image showing the Cu(100)-(1 × 1) structure recorded

Wan et al. at -0.2 V in 0.1 M HClO4. The atomic resolution STM image of Cu(100)-(l × 1) has not yet been achieved in HClO4 solution, although a nearly identical image was reported from an AFM observation in HClO4 solution and an STM observation in H2SO4 solution.26,27 A square lattice with an atomic spacing of ca. 0.26 nm is clearly seen in Figure 2. The identical (1 × 1) structure was consistently observed in the double layer potential region. No evidence was found for the oxidation of Cu(100) as described in an in situ AFM experiment in which the Cu(100) surface exhibited a (2 × 2)R45° structure of overlayer with adsorbed O or OH-.26 The observed results indicate that the Cu(100) surface retains a clean and unreconstructed (1 × 1) structure in the present experiment. (3) Benzene Adlayer. After acquiring the Cu(100)-(l × 1) structure at -0.2 V, a small amount of benzene solution was added directly into the electrochemical cell and formed an average concentration of 0.1 M HClO4 + 1 mM benzene. Figure 3a shows a large scale STM image of a benzene adlayer at -0.2 V. It can be seen that benzene molecules adsorb on the Cu(100) surface with a long-range ordered structure. The molecular rows cross each other at an angle of 90° within experimental error, showing a 4-fold rotational symmetry. From a comparison with the crystal orientation of Cu(100)-(1 × 1) lattice, we found that the molecular rows of benzene are parallel to the underlying close-packed Cu(100) atomic rows. Higher resolution STM image made it possible to reveal the details of the benzene adlayer. A typical STM image is shown in Figure 3b. It is clearly seen that each spot corresponding to a benzene molecule in the lower resolution image shown in Figure 3a is now split into two bright lobes, forming a characteristic “dumbbell” shape. The orientation of the central lines of all dumbbells is along the same [001] direction. The intermolecular distance along the close-packed directions of Cu(100)-〈110〉 is measured to be ca. 0.52 nm, which corresponds to twice the lattice distance of Cu(100) (0.256 nm). From the intermolecular distance and crystallographic direction, a Cu(100)-(2 × 2)-C6H6 structure can be concluded. A unit cell for this structure is outlined in Figure 3b. The structure and registry of benzene on fcc(100) surfaces such as Pd(100), Ni(100) and Pt(100) were well investigated by experiment and theoretical calculation.4,8,9 For example, an atomic superposition electron delocalization molecular orbital method was applied to the study of favored bonding sites for benzene on Ni(100) surface.28 The calculated result predicted that benzene molecule adsorbed on Ni(100) surface with its aromatic ring parallel to the substrate surface. Each benzene molecule is aligned with the carbon containing C2 axes along the 〈010〉 azimuth. The recent ex situ studies also indicated that benzene adsorbs more weakly on the Cu surface with the molecular plane parallel to the substrate surface.10,12,29 On the other hand, the molecular shape of a benzene appearing in an STM image is dependent on its bonding site.3,7,19,30 Three distinct types of STM images for isolated benzene molecules located at 3-fold hollow, atop, and 2-fold bridge sites on Pt(111) at 4 K were reported.7 More recently, the effect of bonding sites on molecular appearance in STM image was revealed on Pt(111), Rh(111), and Cu(111) in electrolyte solution at room temperature.19,20 On the basis of the previous results, we proposed a structural model for benzene on Cu(100) shown in Figure 3c. In Figure 3c, benzene molecules are located at hollow sites with flat-lying orientation. Each molecule orients its carbon-containing C2 axis along the [001] direction of underlying Cu(100). The two lobes marked by circles in a molecule are positioned on the two sides of a hollow site.

Adlayer Structures of Benzene and Pyridine Molecules

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(c) Figure 3. (a and b) High-resolution STM images of a Cu(100)-(2 × 2)-C6H6 adlattice in 0.1 M HClO4. Tunneling current was (a) 5 nA and (b) 10 nA. Scanning rate was (a) 13.02 Hz and (b) 16.28 Hz, respectively, (c) A schematic illustration for the Cu(100)-(2 × 2)C6H6 structure.

The ordered benzene adlayer was observed consistently in the potential region from -0.25 to 0.1 V. At potentials more than 0.1 V, the ordered structure became unclear and produced noisy images similar to the adsorption of benzene on Cu(111),20 suggesting that either desorption or reorientation took place at the positive potentials. (4) Pyridine Adlayer. The adsorption of pyridine on an Au(111) surface has been investigated by in situ STM.23,24 The

Figure 4. (a and b) STM top views of pyridine adlayer on Cu(100) surface in 0.1 M HClO4 showing a Cu(100)-(2 × 2)-C5H5N structure. Scanning rate was 13.02 Hz. Tunneling current was 10 nA. (c) A proposed model for the (2 × 2)-C5H5N structure.

8402 J. Phys. Chem. B, Vol. 105, No. 35, 2001 pyridine molecules were found to adsorb on an Au(111) surface and form an ordered structure. In the present study, the adsorption of pyridine is investigated on Cu(100). The electrode potentials applied to Cu(100) electrode were restricted to the negative potential region near -0.3 V. Figure 4a is an STM image showing a pyridine adlayer on Cu(100) in a solution of 0.1 M HClO4 + 1 mM pyridine. This image was acquired at -0.3 V. The appearance of the observed adlayer is different from the Cu(100) lattice and benzene adlayer shown in Figures 2 and 3. Several defects where pyridine molecules are missing can be observed in Figure 4a indicated by arrows. It is assumed that one defect corresponds to a pyridine molecule. Each molecule appears in an elongated shape. A careful observation found that the molecular rows are parallel to the close-packed direction of the underlying Cu(100) lattice. The intermolecular distances along close-packed 〈110〉 directions are measured to be ca. 0.52 nm, as long as twice the lattice distance of Cu(100). Therefore, the adlayer of pyridine can be assigned to a (2 × 2) structure. A unit cell is outlined in Figure 4a. A lowpass filtered STM image shown in Figure 4b is used to reveal the details of the pyridine adlayer. It can be seen in this image that each molecule adsorbs on the Cu(100) surface along the [001] direction. The elongated shape corresponding to a molecule outlined in the image shows two spots. The distance from the center of the two spots is ca. 0.27 nm, close to the size of chemical structure of a pyridine ring (0.28 nm). There exists a brightness difference in a pyridine molecule, indicating a different corrugation height shown as a and b. The average value of the corrugation height difference is measured to be ca. 0.015 nm from a cross-sectional profile shown in the lower part of Figure 4b. On the other hand, the effect of nitrogen atoms on STM images has been found on various molecular adlayers on Au(111). For example, the two elongated blobs of each 1, 10′ phenanthroline molecule were attributed to the electron tunneling that flowed through the two nitrogen atoms.31 From the features observed in the STM image of Figures 4a and b, a structural model for the pyridine adlayer is proposed in Figure 4c. In the model, pyridine molecules are located in hollow positions with their pyridine rings parallel to the Cu(100) surface. The nitrogen atom of a pyridine is bound to an atop site of Cu atom. Each molecule orients along nitrogencontaining C2 axis with Cu(100)-[001]. The brightness difference of a pyridine molecule in STM image is dominated by the nitrogen atom. Note that the structural models of both benzene and pyridine in the present study are proposed on the basis of the in situ STM observation and the results reported in the literature. The flat-lying molecular orientations proposed here are in a limited potential region. However, other adsorption configurations such as a side-on or tilt orientation for the molecule are also possible. A side-on orientation with the molecular plane perpendicular to the surface was reported previously in ex situ studies.10,12,32 In the electric charged solid/ liquid interface, the molecular reorientation is well found,22,24 especially with the variation of electrode potentials. It is obvious

Wan et al. that more straightforward experimental techniques such as infrared spectroscopy must be applied to obtain further structural information for both benzene and pyridine on Cu(100) in solution. After imaging the adlayer at -0.3 V, the electrode potential was scanned in both positive and negative directions. In the negative direction, the molecular desorption was observed. In the positive potential region, different images from Figure 4 were found. The detailed study is now in progress. Acknowledgment. This work is supported by the National Natural Science Foundation of China (No. 20025308) and the Chinese Academy of Sciences. We thank Q. M. Xu for her help in preparing the figures. References and Notes (1) Jung, T. A.; Schlittler, R. R. Gimzewski, J. K. Science 1997, 386, 696. (2) Kaman, M. M.; Stranick, S. J.; Weiss, P. S. Science 1996, 274, 119. (3) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; John Wiley & Sons: New York, 1994. (4) Gland, J. L.; Somorjai, G. A. Surf. Sci. 1973, 38, 157. (5) Lin, R. F.; Koesttner, R. J.; Van Hove, M. A.; Somorjai, G. A. Surf. Sci. 1983, 134, 161. (6) Ohtani, H.; Wilson, R. J.; Chiang, S.; Mate, C. M. Phys. ReV. Lett. 1988, 60, 2389. (7) Weiss, P. S.; Eigler, D. M. Phys. ReV. Lett. 1993, 71, 3139. (8) Bertolini, J. C.; Malmai-Imelik, G.; Rousseau, J. Surf. Sci. 1977, 67, 478. (9) Hofmann, P.; Horn, K.; Bradshaw, A. M. Surf. Sci. 1981, 105, L260. (10) Lauhon, L. J.; Ho, W. J. Phys. Chem. A 2000, 104, 2463. (11) Chiang, S. Chem. ReV. 1997, 97, 1083. (12) Lauhon, L. J.; Ho, W. Surf. Sci. 2000, 451, 219. (13) Kolb, D. M. Angew. Chem., Int. Ed. Engl. 2001, 40, 1162. (14) Gewirth, A. A.; Niece, B. K. Chem. ReV. 1997, 97, 1129. (15) Itaya, K. Prog. Surf. Sci. 1998, 58, 121. (16) Moffat, T. P. Scanning Tunneling Microscopy Studies of Metal Electrodes. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1999; Vol. 21, p211-316. (17) Wan, L.-J.; Hara, M.; Inukai, J.; Itaya, K. J. Phys. Chem. B 1999, 103, 6978. (18) Kim, Y. G.; Soriaga, J. B.; Soriaga, M. P. J Colloid Interface Sci. 2000, 227, 505. (19) Yau, S.-L.; Kim, Y.-G.; Itaya, K. J. Am. Chem. Soc. 1996, 118, 7795. (20) Wan, L.-J.; Itaya, K. Langmuir, 1997, 13, 7173. (21) Wan, L.-J.; Itaya, K. J. Electroanal. Chem. 1999, 473, 10. (22) Lipkowski, J.; Ross, P. N., Eds. In Adsorption of Molecules at Metal Electrodes; VCH Publishers: New York, 1992. (23) Andreasen, G.; Vela, M. E.; Salvarezza, R. C.; Arvia, A. J. Langmuir 1997, 13, 6814. (24) Cai, W. B.; Wan, L.-J.; Noda, H.; Osawa, M. Langmuir 1998, 14, 6992. (25) Hoon-Koshla, M.; Fawcett, W. R.; Goddard, J. D.; Tian, W.-Q.; Lipkowski, J. Langmuir 2000, 16, 2356. (26) Cruickshank, B. J.; Sneddon, D. D.; Gewirth, A. A. Surf. Sci. 1981, 105, L260. (27) Vogt, M. R.; Lachenwitzer, A.; Magnussen, O. M.; Behm, R. J. Surf. Sci. 1998, 399, 49. (28) Grimm, F. A.; Huntley, D. R. J. Phys. Chem. 1993, 97, 3800. (29) Triguero, L.; Pettersson, L. G. M.; Minaev, B.; Agren, H. J. Chem. Phys. 1998, 108, 1193. (30) Sautet, P.; Bocquet, M.-L. Phys. ReV. B 1996, 53, 4910. (31) Cunha, F.; Jin, Q.; Tao, N. J.; Li, C. Z. Surf. Sci. 1997, 389, 19. (32) Bridge, M. E.; Connolly, M.; Lloyd, D. R.; Somers, J.; Jakob, P.; Menzel, D. Spectrochim. Acta 1987, A43, 1473.