Adlayer Structures of Pyridine, Pyrazine and Triazine on Cu(111): an

The molecules are found to form long-range well-ordered adlayers. ... in the double-layer potential region, although the adsorption geometries are dif...
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Langmuir 2002, 18, 5133-5138

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Adlayer Structures of Pyridine, Pyrazine and Triazine on Cu(111): an in Situ Scanning Tunneling Microscopy Study Dong Wang, Qing-Min Xu, Li-Jun Wan,* Chen Wang, and Chun-Li Bai* Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China Received October 24, 2001. In Final Form: April 3, 2002 Adsorption of pyridine, pyrazine, and triazine molecules on a Cu(111) electrode surface was studied in aqueous solution by using cyclic voltammetry and electrochemical scanning tunneling microscopy (STM). High-resolution STM imaging reveals the details of the adlayer structures and internal structure of individual molecules. The molecules are found to form long-range well-ordered adlayers. The same (3 × 3) structures with a coverage of 0.11 are observed on the three adlayers. All three molecules adopt flat-lying orientation in the double-layer potential region, although the adsorption geometries are different. π electrons are believed to be important in stabilizing the molecules in flat-lying orientation. Three structural models are proposed for the three adlayers.

Introduction The orientation and bonding of pyridine (C5H5N), pyrazine (C4H4N2), and triazine (C3H3N3) on transition metal surfaces such as Au, Ag, and Cu are of widespread interest because of their importance in catalysis, metal deposition, coordination, and surface science studies.1-7 The three molecules have structural and electronic similarity characterized by π electrons and one, two, and three nitrogen lone-pair orbitals. Interaction of the molecules with metal surfaces involves both π and lonepair electrons. A large number of results about the orientation and bonding of these molecules with metal substrates have been obtained by using ultrahigh vacuum (UHV) characterization techniques such as low-energy electron diffraction (LEED),6 X-ray photoelectron spectroscopy (XPS),7 electron energy loss spectroscopy (EELS),8,9 and near-edge X-ray-adsorption fine-structure (NEXAFS).10 On the other hand, studies that use in situ techniques in electrolyte solution such as surfaceenhanced Raman scattering (SERS),11,12 FTIR,13,14 scanning tunneling microscopy (STM),15,16 and electrochemical * To whom correspondence may be addressed. Fax: +86-1062558934.E-mail: [email protected];[email protected]. (1) Lipkowski, J.; Stolberg, L. In adsorption of molecules at metal electrodes; Lipkowski, J., Ross, P. N., Ed.; VCH Publishers: New York, 1992; pp 171-238. (2) Dudde, R.; Koch, E. E.; Ueno, N.; Engelhardt, R. Surf. Sci. 1986, 178, 646-656. (3) Hamelin, A.; Morin, S.; Richer, J.; Lipkowski, J. J. Electroanal. Chem. 1989, 272, 241-252. (4) Bandy, B. J.; Lloyd, D. R.; Richardson, N. V. Surf. Sci. 1979, 89, 344-353. (5) Nyberg, G. L. Surf. Sci. 1980, 95, L273-L276. (6) Dudde, R.; Frank, K.-H.; Rocco, M. L. M.; Koch, E. E. Surf. Sci. 1988, 201, 469-480. (7) Davies, P. R.; Shukla, N. Surf. Sci. 1995, 322, 8-20. (8) Chaffins, S. A.; Gui, J. Y.; Lin, C. H.; Lu, F.; Salaita, G. N.; Stern, D. A.; Hubbard, A. T. Langmuir 1990, 6, 1273-1281. (9) Otto, A.; Reihl, B. Surf. Sci. 1986, 178, 635-645. (10) Bader, A.; Haase, J.; Frank, D. F.; Puschmann, A.; Otto, A. Phys. Rev. Lett. 1986, 56, 1921-1924. (11) Huang, Q. J.; Yao, J. L.; Mao, B. W.; Gu, R. A.; Tian, Z. Q. Chem. Phys. Lett. 1997, 271, 101-106. (12) Cai, W. B.; Amano, T.; Osawa, M. J. Electroanal. Chem. 2001, 500, 147-155. (13) Ikezawa, Y.; Sawatari, T.; Terashima, H. Electrochim. Acta 2001, 46, 1333-1337. (14) Ikezawa, Y.; Koda, Y.; Shibuya, M.; Terashima, H. Electrochim. Acta 2000, 45, 2075-2082. (15) Kim, Y. G.; Yau, S. L.; Itaya, K. Langmuir 1999, 15, 7810-7815. (16) Cai, W. B.; Wan, L. J.; Noda, H.; Hibino, Y.; Ataka, K.; Osawa, M. Langmuir 1998, 14, 6992-6998.

methods17,18 were conducted to understand the molecular adsorption at the electrode/electrolyte interface. As a result, it is known that the applied electrode potential and surface crystallography play important roles in the formation of molecular adlayer structures at the electrode/ electrolyte interface. For example, a potential-dependent orientation transition of pyridine molecules from flat-lying to tilt to vertical orientation was observed on Au(100) and Au(111) surfaces, while vertical orientation was observed on Au(110).16-19 In addition, the same molecule could adopt different orientations on different metal electrodes, which is attributed to the electronic interaction between substrate and adsorbates.15,19 The adsorption of these three molecules has been well studied on Au, Pt, and Ag singlecrystal electrodes. However, for the further understanding of the molecular adsorption, it is important to extend the study to other metal surfaces. The ability of electrochemical STM to determine the microstructures at electrode/electrolyte interfaces has been well demonstrated.20-23 The technique has endowed us with a detailed understanding of the bonding and coordination of organic molecules with electrode surfaces in electrolyte solutions. Highly ordered molecular arrays of porphyrin and crystal violet were observed on iodinemodified Au(111), Pt(111), and Rh(111) surfaces.24 The adsorption of aromatic molecules such as benzene, naphthalene, anthracene, pyrene, and perylene on Cu(111) was successfully investigated.25,26 The STM results of pyridine on Au(111) and pyrazine on Pt(111) have been re(17) Stolberg, L.; Lipkowski, J.; Irish, D. E. J. Electroanal. Chem. 1987, 238, 333-353. (18) Stolberg, L.; Lipkowski, J.; Irish, D. E. J. Electroanal. Chem. 1990, 296, 171-189. (19) Lazarescu, V. Surf. Sci. 1995, 335, 227-234. (20) Wan, L. J.; Wang, C.; Bai, C. L.; Osawa, M. J. Phys. Chem. B 2001, 105, 8399-8402. (21) Siegenthaler, H. In Scanning Tunneling Microscopy II.; SpringerVerlag: New York, 1992; pp 7-49. (22) Wan, L. J.; Yau, S. L.; Itaya, K. J. Phys. Chem. 1995, 99, 95079513. (23) Bard, A. J.; Abruna, H. D.; Chidsey, C. E.; Faulkner, L. R.; Feldberg, S. W.; Itaya, K.; Majda, M.; Melroy, O.; Murray, R. W.; Porter, M. D.; Soriaga, M. P.; White, H. S. J. Phys. Chem. 1993, 97, 7147-7173. (24) Itaya, K.; Batina, N.; Kunitake, M.; Ogaki, K.; Kim, Y. G.; Wan, L. J.; Yamada, T. In Solid-Liquid Electrochemical Interfaces; Jerkiewicj, G., Soriaga, M. P.; Vosaki, K., Wieckowski, A., Eds.; ACS Symposium Series 656; American Chemical Society: Washington, DC, 1997; pp 171-188. (25) Wan, L. J.; Itaya, K. Langmuir 1997, 13, 7173-7179. (26) Wang, D.; Wan, L. J.; Xu, Q. M.; Wang, C.; Bai, C. L. Surf. Sci. 2001, 478, L320-L326.

10.1021/la011591p CCC: $22.00 © 2002 American Chemical Society Published on Web 06/01/2002

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ported.15,16,27 Recently, a flat-lying orientation of pyridine was revealed on Cu(100).20 The molecules formed welldefined (2 × 2) structure. In the present paper, we report the first molecularscale study on the adsorption of pyridine, pyrazine, and triazine on Cu(111) by using cyclic voltammetry and in situ STM in 0.1 M HClO4 solutions. The adsorption of the molecules results in a smaller double layer charge in the cyclic voltammograms. Three molecules formed wellordered adlayers. The molecular orientation and registry to Cu(111) lattice were revealed by higher resolution STM images. On the basis of the STM images and previous works, we proposed structural models for the three adlayers. The effect of the underlying lattices on the adsorption was discussed. 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. The details of the sample preparation were the same as described in the literature.25,26 Briefly, after having been mechanically polished with diamond paste successively down to 0.05 µm in particle diameter, the sample was further treated by electropolishing in a phosphoric acid solution (50 mL of 85% H3PO4 and 50 mL of water) at 0.8-1.0 A cm-2 for 3-5 s. The Cu crystal was then rinsed repeatedly with ultrapure water (Millipore-Q). The in situ STM apparatus was a Nanoscope E (Digital Instrument Inc., Santa Barbara, CA). Tunneling tips were prepared by electrochemically etching a tungsten wire (0.25 mm in diameter) in 0.6 M KOH. The ac voltage of 12-15 V was applied until the etching process stopped. The W tips were then coated with a clear nail polish to minimize the faradaic current. The STM images shown here were acquired in the constant-current mode to evaluate corrugation heights of adsorbed molecules. All electrolyte solutions were prepared by diluting ultrapure HClO4, reagent grade pyridine, pyrazine, and triazine (Cica-Merck, Kanto Chemicals) with Millipore water. A homemade electrochemical cell contains a reversible hydrogen electrode (RHE) in 0.1 M HClO4 and a Pt counter electrode. All electrode potentials were reported with respect to the RHE.

Figure 1. Cyclic voltammograms of a Cu(111) electrode in 0.1 M HClO4 in the presence of (a) 1 mM pyridine, (b) 1 mM pyrazine, and (c) 1 mM triazine. Scan rate was 50 mV/s.

Cyclic Voltammetry. A Cu(111) electrode was initially examined by cyclic voltammetry (result not shown here) in 0.1 M HClO4 in the absence of organic molecules for comparison with the published result.25,26 After the examination, the sample was transferred into a 0.1 M HClO4 solution containing 1 mM pyridine, pyrazine, and triazine. The cyclic voltammograms (CVs) of Cu(111) in the solutions containing the three molecules were measured. Figure 1a is a CV of Cu(111) in 0.1 M HClO4 + 1 mM pyridine. It is clear that the double layer extends in a wide potential region. The overall shape of the CV is almost the same as that of the bare Cu(111) obtained in 0.1 M HClO4 as described in the literature.25 Only the electric charge involved in the double-layer potential range becomes smaller due to the adsorption of pyridine molecules. The result is very similar to those recorded on Cu(111) in the solutions containing small aromatic molecules, such as benzene, naphthalene, and anthracene,25 and on Cu(100) in the solution containing 1 mM pyridine.20 The CV of Au(111) in 0.1 M HClO4 + 1 mM pyridine was previously reported by Arvia et al.27 With the presence of pyridine, the CV of Au(111) in 0.1 M HClO4 changed. From a comparison of the two CVs, it can be seen that the substrates strongly affect molecular adsorption.

Figure 1b is a CV of Cu(111) in 0.1 M HClO4 + 1 mM pyrazine. The adsorption of pyrazine molecules results in a quasi-reversible redox process at the potential of -0.2 V. This process is related to pyrazine reduction according to the previous study of pyrazine on a Cu electrode in neutral electrolyte.28 Another redox process is seen at -0.1 V. The study on the electrochemical process involved in this peak is in progress. Figure 1c is a CV of Cu(111) in 0.1 M HClO4 + 1 mM triazine. There is no obvious feature in the CV. The double-layer region is extended from -0.35 to 0.2 V with minimized electric charge. Note that the addition of pyrazine and triazine results in an extended double-layer potential region. The anodic currents increase at a positive potential of more than 0.2 V. The details of the adsorption structures of these three molecules will be investigated by in situ STM. In Situ STM. (1) Pyridine Adlayer. The atomic image of the Cu(111)-(1 × 1) structure was routinely discerned on an atomically flat Cu(111) surface in the absence of the molecules in 0.1 M HClO4 solution for the ease of determining the registry of the molecular adlayers with the underlying Cu(111) lattice.25,26 After the atomic resolution image of Cu(111) was achieved, a small amount of pyridine solution was directly injected into the electrochemical cell to form a pyridine adlayer. Figure 2a is an STM image of the pyridine adlayer in 0.1 M HClO4 + 1 mM pyridine. This image was acquired at -0.2 V in a relatively large area. A well-ordered molecular array is seen to extend over a wide atomically flat terrace. The molecular rows cross each other at an angle of either 60° or 120° within experiment error of (2°. From a comparison with the Cu(111)-(1 × 1) atomic image, it is found that all molecular rows are parallel to the 〈110〉 orientation of underlying Cu(111) lattice. The intermolecular distance along the 〈110〉 orientation is measured to be ca. 0.77 nm, which corresponds to three times the lattice distance of Cu(111). On the basis of the orientation of molecular rows and the intermolecular distance, we conclude that the observed structure of the pyridine adlayer is (3 × 3) as shown by the superimposed drawn unit cell in Figure 2b. This structure yields a coverage of 0.11, exactly the same as that of benzene on Cu(111).25 More details of the molecular adlayer were revealed by the higher resolution STM image shown in Figure 2b. A point defect corresponding to a site where a pyridine molecule missed is apparent as indicated by an arrow.

(27) Andreasen, G.; Vela, M. E.; Salvarezza, R. C.; Arvia, A. J. Langmuir 1997, 13, 6814-6819.

(28) Lazarescu, V.; Gutu, A.; Totir, N.; Hamelin, A. J. Electroanal. Chem. 2000, 486, 16-22.

Results and Discussion

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Figure 2. (a) STM top view of pyridine adlayer on Cu(111) in 0.1 M HClO4 + 1 mM pyridine at -0.2 V. Tunneling current was 10 nA. Scanning rate was 13 Hz. (b) High-resolution STM image of pyridine adlayer. Tunneling current was 10 nA. Scanning rate was 13 Hz. A defect is indicated by the arrow. A dashed line triangle is used to indicate the internal structure of STM images with the substrate orientation. (c) A schematic representation for the (3 × 3)-C5H5N structure. The dashed line triangle is superimposed in the model to compare with Figure 2b. Two gray circles between two Cu atoms represent the two bright spots in STM images, while the open circle represents the weak spot. (d) STM top view of pyridine adlayer on Cu(111) at 0.1 V. Tunneling current was 10 nA. Scanning rate was 13 Hz.

The high-resolution STM image can reveal the internal structure of each pyridine molecule. It is clear that each spot in Figure 2a now appears as a set of three spots with almost the same distance of 0.25 nm but with different corrugation. A clear dip exists at the center of each triangle formed by three spots. The sides of the triangle are in the orientation of 〈211〉 for the Cu(111) substrate (indicated by dashed lines). This result reveals the micro-orientation of a pyridine molecule and indicates a special coordination and bonding of the molecule with Cu(111) lattice. The orientation and registry of pyridine on various single crystal surfaces were previously studied by using STM. It was reported that the pyridine molecule takes a tilt or vertical orientation on Cu(110) and Cu(100) in UHV,30,31 while it takes a flat-lying orientation on Cu(100) in solution.20 The adsorption of pyridine on Au(111) was investigated in acidic and neutral solutions, respectively.16,27 The potential-dependent orientations from flatlying to tilt to vertical were found in neutral solution, whereas the vertical orientation was found in acidic solution.16,27 An ex situ study indicated that pyridine bonds at clean Cu(111) with its C2 axis parallel to the surface.7 The results show that the adsorption of pyridine is very complicated and related to many factors including elec(29) Yau, S. L.; Kim, Y. G.; Itaya, K. J. Am. Chem. Soc. 1996, 118, 7795-7803. (30) Lauhon, L. J.; Ho, W. J. Phys. Chem. A 2000, 104, 2463-2467. (31) Lee, J. G.; Ahner, J.; Yates, J. T. J. Chem. Phys. 2001, 114, 1414-1419.

trodes, electrolytes, potentials, and so on. However, the STM image features of the molecule in different orientations were well issued. On the other hand, benzene, with a similar molecular structure to pyridine, has been clearly imaged on Cu(111), Pt(111), and Rh(111).25,29,32 On the basis of the previous results, the observed feature of pyridine in the present study strongly suggests that the molecule takes a flat-lying orientation. To propose a structural model for pyridine in the present study, we have considered the micro-orientations of a molecule appearing in the STM image and the possible adsorption sites on Cu(111) such as top-site, bridge site, and hollow site. A structural model is proposed in Figure 2c. According to the micro-orientations of pyridine with Cu(111) resolved in Figure 2b, each pyridine molecule is assumed to bond at the 2-fold site with respect to the underlying Cu(111) lattice. The three spots for one pyridine molecule appeared in STM images are expected to locate on the Cu(111) surface as indicated by the three small circles. The two brighter spots are located between the Cu atoms in the 2-fold sites indicated by two gray circles, while the weak one corresponding to the nitrogen atom on atop site is indicated by an open circle. The corrugation difference between three spots reflects a combination of electronic and position effects. The sides of the triangle formed by a pyridine molecule are parallel to the directions of 〈211〉 of Cu(111), consistent with the molecular feature ap(32) Chiang, S. Chem. Rev. 1997, 97, 1083-1096.

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pearing in Figure 2b. The proposed structural model is in agreement with the results of higher resolution STM images. The ordered (3 × 3) structure was consistently observed in the potential range from -0.35 to 0 V which is near the potential of zero charge of Cu(111).33 At potentials more positive than 0 V, the well-ordered adlayer became disordered. Small molecular clusters shown in Figure 2d are clearly seen on Cu(111) surface at 0.1 V. The orientational transition may be responsible for the disordered adlayer. It is clear that other in situ experimental techniques could supply further structural information on the order-disorder transition of the pyridine adlayer. (2) Pyrazine Adlayer. Pyrazine adlayer was imaged in 0.1 M HClO4 + 1 mM pyrazine. Figure 3a shows a typical STM image of pyrazine adlayer in a relatively large area acquired at -0.2 V. A well-defined adlayer similar to that of pyridine shown in Figure 2 is observed. The molecular rows of adlayer are parallel to the substrate 〈110〉 orientation by comparing the pyrazine adlayer with the underlying Cu(111)-(1 × 1) lattice. The intermolecular distance is measure to be 0.77 nm, about three times that of the lattice distance of substrate. On the basis of the orientation of molecular rows and the intermolecular distance, we conclude that the observed structure of the pyrazine adlayer is also (3 × 3) symmetry as shown by the outlined unit cell in Figure 3a. Figure 3b is a higher resolution STM image acquired from the same area as Figure 3a. The details of the molecule can be clearly seen in the image. Each pyrazine molecule looks like a dumbbell with two parallel ellipsoid lobes. Furthermore, there exists a valley parallel to the 〈211〉 direction of substrate between two lobes of dumbbellshaped pyrazine. The features are indicated in Figure 3b. The corrugation height between the valley and lobes of each pyrazine is measured to be ca. 0.025 nm. The molecular appearance is similar to that of benzene molecule adsorbed on Rh(111).29 The adsorption of pyrazine on Au(111), Ag(111), Pt(111), and other metal electrode surfaces has been well studied.15,19 It was confirmed that pyrazine adsorbs on a Au(111) surface in both flat and vertical orientations, while on Ag(111) it adsorbs in a flat orientation.19 This difference is attributed to the different electronic effect between the pyrazine and metal electrodes. From the present in situ STM result, a flat orientation of pyrazine on Cu(111) is proposed. However, The exact adsorption geometry of pyrazine on Cu(111) surface is still open to discussion. In a recent ECSTM study, the pyrazine molecule on Pt(111) was proposed to occupy 3-fold hollow sites because a triangular appearance was revealed in STM images.15 Here, the different STM image features indicate a different adsorption geometry of pyrazine on Cu(111). As a comparison, it is known from the theoretical and experimental results that benzene gives a weak 2-fold bump shape when adsorbed on the bridge site of Pt(111).32,34 In the Rh(111)c(2x3 × 3)rect-benzene adlayer, benzene was proposed to occupy a 2-fold site to explain the observed dumbbell shape in the STM image.29 A recent theoretical result showed that the bridge 〈211〉 site was an energetically preferable site for benzene on Ni(111).35 On the basis of the previous results and the present STM images, a structural model was tentatively proposed in Figure 3c. The pyrazine is located on the bridge 〈211〉 site. The dumbbell shape that (33) Hamelin, A.; Sevastyanov, E.; Popov, P. J. Electroanal. Chem. 1983, 145, 225-264. (34) Sautet, P. Chem. Rev. 1997, 97, 1097-1116. (35) Yamagishi, S.; Jenkins, S. J.; King, D. A. J. Chem. Phys. 2001, 114, 5765-5773.

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Figure 3. (a) STM top view of pyrazine adlayer on Cu(111) in 0.1 M HClO4 + 1 mM pyrazine at -0.2 V. Tunneling current was 10 nA. Scanning rate was 15 Hz. (b) Higher-resolution STM image of the pyrazine adlayer. Tunneling current was 10 nA. Scanning rate was 15 Hz. A characterized dumbbell shape of pyrazine is outlined in the image. (c) A schematic representation for the (3 × 3)-C4H4N2 structure. Two gray ellipsoids were superimposed in the model to indicate the observed dumbbell shape in the STM image of (b).

appeared in STM images is expected to locate perpendicular to the bridge sites. The feature is schematically described by two gray ellipsoids in Figure 3c. The valley between two ellipsoids is parallel to the 〈211〉 direction consistent with the direction in the STM image of Figure 3b.

Adlayer Structures on Transition Metals

The ordered (3 × 3) structure was consistently observed in the potential range from -0.2 to 0.2 V. At potentials more positive than 0.2 V, the adlayer could not be clearly seen. The possibility of the change in molecular orientation with electrode surface potential cannot be simply excluded. (3) Triazine Adlayer. Figure 4a shows an STM image of the triazine adlayer in 0.1 M HClO4 + 1 mM triazine acquired at -0.2 V in a relative large area. A well-ordered molecular array similar to that of pyridine and pyrazine is seen. The molecular rows align in the orientation parallel to the 〈110〉 direction of Cu(111) substrate. The result presents a sharp contrast to the result of the ex situ study of triazine on Pt(111), where no chemisorbed triazine was detected.8 The higher resolution STM image shown in Figure 4b gives us more details of the molecular adlayer and internal structure of triazine molecules. In Figure 4b each triazine molecule appears as a set of three spots with the same distance of ca. 0.25 nm. A clear dip exists in the center of a molecule indicated by a dashed triangle with three spots. Moreover, these three spots have almost the same corrugation height, indicating the same tunneling probability on these three locations. This feature is similar to that of the adsorbed benzene on Cu(111) and Rh(111).25,29,32 It is not unexpected because adsorbed triazine and benzene all have D3v symmetry. In addition, the intermolecular distance is measured to be ca. 0.77 nm, about three times the lattice distance of Cu(111). On the basis of the above features, we conclude that the observed structure of the triazine adlayer is a (3 × 3) as outlined in Figure 4b. This structure yields coverage of 0.11. It is worth noting that with the increase of nitrogen atoms in the three heterocyclic aromatic molecules, the adlayer symmetry is almost the same, indicating that the inter-adsorbate interaction seems less important than adsorbate-substrate interaction in stabilizing the adlayers, although the coordination between adsorbate and Cu(111) substrate varies. A structural model is tentatively proposed in Figure 4c. The triazine molecule is arranged on the hollow site. The C-N bond of triazine is arranged to align in the 〈110〉 orientation of the Cu lattice. The model is similar to that of the benzene adlayer on Cu(111) and Rh(111).25,29 In the present arrangement, all three N atoms are located on the near-top positions. The bright spots in high-resolution STM image are thought to locate between the Cu atoms, as indicated by three small circles superimposed in Figure 4c. Further fine structure characterization and theoretical simulation should be useful to understand the bonding and coordination relation between adsorbates and substrate thoroughly. The ordered (3 × 3) structure was consistently observed in the potential range from -0.35 to 0.2 V. No orientation change was revealed in this potential range. At potentials more positive than 0.2 V the disordered images were recorded, indicating either desorption of triazine or other electrode processes occur. Conclusion In summary, we have investigated the adlayer structures of pyridine, pyrazine, and triazine on Cu(111) by in situ STM. (3 × 3) adlayer structures, the same as the adlayer structure of benzene on Cu(111), are revealed, which can be attributed to similar π electrons existing in these molecules. The interaction between π electrons and the substrate is thought to play an important role in the formation of ordered adlayers. All three molecules are proposed to adopt the flat-lying orientation in the doublelayer potential region. However, the presence of one, two,

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Figure 4. (a) STM top view of triazine adlayer on Cu(111) in 0.1 M HClO4 + 1 mM triazine at -0.2 V. Tunneling current was 10 nA. Scanning rate was 15 Hz. (b) High-resolution STM image of triazine adlayer. Tunneling current was 10 nA. Scanning rate was 15 Hz. A triangle in dashed line is used to indicate the internal structure of STM images. (c) A schematic representation for the (3 × 3)-C3H3N3 structure. A set of gray circles represents bright spots in a higher resolution STM image of (b).

and three nitrogen heteroatoms has a special effect on the surface coordination of the adsorbates with the substrate. For example, pyridine and pyrazine are proposed to occupy 2-fold bridge sites and triazine to occupy 3-fold hollow site based on the high-resolution STM image. The

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potential-dependent adlayer change is also investigated. The results presented in this paper will provide the fundamental understanding of surface coordination and bonding of heteroatomic molecules with a Cu electrode and give further evidence of the special importance of adsorbate-substrate electronic interaction in determining adsorption behavior. Further studies using other experimental techniques such as capacitance measurement and in situ IR may supply further structural information on

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the adsorption of pyridine, pyrazine, and triazine adlayers on Cu(111). Acknowledgment. This work was supported by the National Natural Science Foundation of China (No. 20025308, No. 20177025), the National Key Project on Basic Research (Grant G2000077501), and the Chinese Academy of Sciences. LA011591P