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Langmuir 2006, 22, 3640-3646
Adsorbed Structures of 4,4′-Bipyridine on Cu(111) in Acid Studied by STM and IR Yu-Xia Diao,† Mei-Juan Han,† Li-Jun Wan,*,† Kingo Itaya,*,‡ Taro Uchida,§ Hiroto Miyake,§ Akira Yamakata,§ and Masatoshi Osawa*,§ Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China, Department of Applied Chemistry, Graduate School of Engineering, Tohoku UniVersity, Sendai 980-8579, Japan, and Catalysis Research Center, Hokkaido UniVersity, Sapporo 001-0021, Japan ReceiVed October 13, 2005. In Final Form: February 15, 2006 The adsorption of 4,4′-bipyridine (BiPy) on Cu(111) has been investigated in 0.1 M HClO4 by cyclic voltammetry, electrochemical scanning tunneling microscopy (STM), and surface-enhanced infrared adsorption spectroscopy (SEIRAS). Cyclic voltammetry showed the double layer region extending from -0.2 to 0.26 V and a pair of redox waves superposing on hydrogen evolution wave at more negative potentials. Diprotonated BiPy, BiPyH22+, is adsorbed flat on the Cu(111) (1 × 1) surface and forms a well-ordered monolayer with a (3 × 4) symmetry in the double-layer potential region. At more negative potential, BiPyH22+ is reduced to its monocation radical, BiPyH2•+, and forms another well-ordered structure in which the radicals are stacked in molecular rows with a face-to-face self-dimer as the building unit. The SEIRA spectra of both BiPyH22+ and BiPyH2•+ are dominated by gerade modes which should be IR-inactive for the centrosymmetric species. The breakdown of the selection rule of IR absorption is ascribed to the vibronic coupling associated with charge transfer between BiPyH22+ and the surface and between the radicals.
1. Introduction There have been numerous studies on organic monolayers on metal substrates in recent years because of their importance in fundamental research and industrial applications.1 With the development of scanning probe microscopy, a lot of interfacial phenomena have been revealed at an atomic or submolecular level. Among the various studies, the potential-dependent phase transitions of organic molecular adlayers have been an interesting issue and have been intensively investigated. The examples reported previously include the sulfur-containing molecules (1,3,5-triazine-2,4,6-trithiol,2 thiophene,3 4-mercaptopyridine4), carboxyl-containing molecules (benzoic acid,5 trimesic acid6) and nitrogen-containing molecules (uracil,7 pyridine,8,9 2,2′bipyridine2,10). The common structural feature in these molecules is a planar configuration with two or more electron-rich sites. 4,4′-Bipyridine (BiPy) is widely used in chemistry, material, and electronic research. For example, BiPy acts as a bifunctional * To whom correspondence should be addressed. E-mail: wanlijun@ iccas.ac.cn (L.-J.W.);
[email protected] (M.O.); itaya@ atom.che.tohoku.ac.jp (K.I.). † Chinese Academy of Sciences. ‡ Tohoku University. § Hokkaido University. (1) (a) Lahann, J.; Mitragotri, S.; Tran, T. N.; Kaido, H.; Sundaram, J.; Choi, I. S.; Hoffer, S.; Somorjai, G. A.; Langer, R. Science 2003, 299, 371. (b) Polewska, W.; Vogt, M. R.; Magnussen, O. M.; Behm, R. J. J. Phys. Chem. B 1999, 103, 10440. (2) Wan, L. J.; Noda, H.; Wang, C.; Bai, C. L.; Osawa, M. Chem. Phys. Chem. 2001, 10, 617. (3) Su, G. J.; Zhang, H. M.; Wan, L. J.; Bai, C. L. Surf. Sci. 2003, 531, L363. (4) Baunach, T.; Ivanova, V.; Scherson, D. A.; Kolb, D. M. Langmuir 2004, 20, 2797. (5) Zelenay, P.; Waszczuk, P.; Dobrowolska, K.; Sobkowski J. Electrochim. Acta 1994, 39, 655. (6) (a) Su, G. J.; Zhang, H. M.; Wan, L. J.; Bai, C. L.; Wandlowski, T. J. Phys. Chem. B 2004, 108, 1931. (b) Li, Z.; Han, B.; Wan, L. J.; Wandlowski, T. Langmuir 2005, 21, 6915. (7) (a) Wandlowski, T. Electroanal. Chem. 1995, 395, 83. (b) Wandlowski, T.; Holzle, M. H. Langmuir 1996, 12, 6604. (c) Holzle, M. H.; Wandlowski, T.; Kolb, D. M. J. Electroanal. Chem. 1995, 394, 271. (8) Cai, W. B.; Wan, L. J.; Osawa, M. Langmuir 1998, 14, 6992. (9) Andreasen, G.; Vela, M. Z.; Savarezza, R. C.; Arvia, A. J. J. Electroanal. Chem. 1999, 467, 230. (10) Dretschkow, T.; Wandlowski, T. Electrochim. Acta 1999, 45, 731.
bridge ligand in coordination chemistry and as a staple in nanometer-sized supermolecular assembly.11,12 BiPy-metal complexes show interesting photochemical properties, which makes them suitable for applications in solar energy conversion.13 BiPy has also received great attention in bioelectrochemistry as an “electron-transfer promoter”. Electrodes modified with BiPy monolayers accelerate the quasi-reversible electrochemistry of cytochrome c.14 BiPy is useful in fabricating nanoelectronic circuits. The pioneering work was done by Tao et al.15 who measured the resistance of individual BiPy by forming goldBiPy-gold junctions. So far, several AFM and STM studies on the adsorption of BiPy on Au and HOPG surfaces have been reported. Cunha et al.16 observed the perpendicularly oriented BiPy on Au(111) at positive potentials and found the dissolution of the monolayer by lowering the potential. Mayer et al.17 reported three distinctly different perpendicularly oriented BiPy adlayers. They proposed that the monolayer was formed by the coordination of one ring nitrogen atom with the substrate surface, lateral π-π stacking, and hydrogen bonding between the organic molecule and coadsorbed water. Umemura et al.18a and Pinheiro et al.18b found the parallel periodic chains of BiPy adsorbed on Au(111). BiPy (11) (a) Dong, Y. B.; Smith, M. D.; Zurloye, H. C. Inorg. Chem. 2000, 39, 4927. (b) Noro, S.; Kitaura, R.; Kondo, M.; Kitagawa, S.; Ishii, T.; Matsuzaka, H.; Yamashita, M. J. Am. Chem. Soc. 2002, 124, 2568. (12) Liu, Y.; Zhao, Y. L.; Zhang, H. Y.; Song, H. B. Angew. Chem., Int. Ed. 2003, 42, 3260. (13) Schubert, U. S.; Eschbaumer, C. Angew. Chem. 2002, 114, 3016; Angew. Chem., Int. Ed. 2002, 41, 2892. (14) (a) Eddowes, M. J.; Hill, H. A. O. J. Chem. Soc. Chem. Commun. 1977, 3154. (b) Albery, W. J.; Eddowes, M. J.; Hill, H. A. O.; Hillman, A. R. J. Am. Chem. Soc. 1981, 103, 3904. (c) Sagara, T.; Murakami, H.; Igarashi, S.; Sato, H.; Niki, K. Langmuir 1991, 7, 3190. (15) Xu, B. Q.; Tao, N. J. Science 2003, 301, 1221. (16) Cunha, F.; Tao, N. J.; Wang, X. W.; Jin, Q.; Duong, B.; Dagese, J. Langmuir 1996, 12, 6410. (17) (a) Mayer, D.; Dretschkow, T.; Ataka, K.; Wandlowski, T. J. Electroanal. Chem. 2002, 524-525, 20. (b) Wandlowski, Th.; Ataka, K.; Mayer, D. Langmuir 2002, 18, 4331. (18) (a) Umemura, K.; Nishida, N.; Hara, M.; Sasabe, H.; Knoll, W. J. Electroanal. Chem. 1997,438, 207. (b) Pinheiro, L. S.; Temperini, M. L. A. Appl. Surf. Sci. 2001, 171, 89. (c) Pinheiro, L. S.; Temperini, M. L. A. Surf. Sci. 1999, 441, 45.
10.1021/la052765w CCC: $33.50 © 2006 American Chemical Society Published on Web 03/17/2006
Adsorbed Structures of 4,4′-Bipyridine on Cu(111)
was also used as a “chemical marker” to confirm the relative position of fatty acid on HOPG.19 As an extension of 2,2′-BiPy on Cu(111)2 and the supplement of BiPy in neutral electrolyte on Au(111),16,17 we have examined the adsorption of BiPy on a Cu(111) electrode surface in 0.1 M HClO4 by in situ electrochemical STM, cyclic voltammetry, and surface-enhanced infrared adsorption spectroscopy (SEIRAS) in this work. Since the acid dissociation constants of the protonated BiPy in 0.2 M ionic strength solutions are pK1 ) 3.5 and pK2 ) 4.920 and thus BiPy is diprotonated (BiPyH22+) in the acid, BiPy is expected to exhibit adsorption behaviors different from in neutral solutions. It is also known that BiPyH22+ can be reduced to the monocation radical BiPyH2•+ and further to the neutral state BiPyH2.21 In the present experiment, STM provided highresolution images, which allowed us to determine the real arrangement of the molecules and to monitor a potentialdependent phase transition of the adlayer. CV gave the information of the electrochemical behavior of the molecule. SEIRAS that can give detailed information about the molecular structures22 was used to supplement the STM and CV measurements. 2. Experimental Section Scanning Tunneling Microscopy and Cyclic Voltammetry. A commercial Cu(111) single-crystal disk with a diameter of 10 mm (from Mateck) was used as the working electrode for both electrochemical measurement and in situ STM observation. A welldefined (111) single-crystal Cu surface was prepared by mechanical polishing and electropolishing in 50% H3PO4 (Fluka, content g 85%) of the electrode. All solutions were prepared by diluting ultrapure HClO4 (Aldrich Chemical Co., 99.999%, purified by redistillation, and reagent-grade BiPy (Aldrich Chemical Co.) with Millipore water (g18 MΩ‚cm-1, TOC e 3 ppb). The solutions were deaerated with high-purity nitrogen prior to each measurement. A homemade electrochemical cell with a reversible hydrogen electrode (RHE) in 0.1 M HClO4 and a Pt counter electrode was employed for electrochemical measurements. All electrode potentials were reported with respect to the RHE. The cyclic voltammograms (CVs) were measured with an EG&G basic electrochemical system. STM experiments were carried out with a Nanoscope E STM apparatus (Digital Instruments). The tunneling tips used were prepared by electrochemical etching of a tungsten wire (0.25 mm in diameter) in 0.6 M KOH and subsequently coated with clear nail polish to minimize faradaic currents. All STM images shown here were acquired in the constant-current mode. SEIRAS. In situ SEIRAS measurements were carried out in an attenuated total reflection (ATR) mode with a Ge prism/Cu thin film electrode/solution configuration22 by using a Fourier transform spectrometer (Bio-Rad FTS 60A/896) equipped with a MCT detector and a homemade single-reflection accessory (incident angle of 70°). The spectrometer was operated with a spectral resolution of 4 cm-1. Experimental details including the electrochemical cell were described elsewhere.2,23 The Cu film electrode was chemically deposited onto the total reflecting plane of the Ge prism (1 cm in radius and 2.5 cm long) by the same procedure used for the Cu deposition on Si.24 Spectra are shown in the absorbance units defined as -log(R/R0), where R and R0 represent the sample and reference spectra, respectively. The sample spectrum was collected in the supporting electrolyte at a suitable potential in advance, and then sample spectra were collected after the addition of BiPy into the (19) Qian, P.; Nanjo, H.; Yokoyama, T.; Suzuki, T. M.; Akasaka, K.; Orhui, H. Chem. Commun. 2000, 2021. (20) Mugrave, T. R.; Maltson, C. E. Inorg. Chem. 1968, 7, 1433. (21) Lu, T.; Cotton, T. M. Langmuir 1989, 5, 406. (22) (a) Osawa, M. Bull. Chem. Soc. Jpn. 1997, 70, 2861. (b) Osawa, M. Top. Appl. Phys. 2001, 81, 163 (c) Osawa, M. In Vibrational Spectroscopy; Charmers, J., Griffiths, P. R., Eds.; Chichester, 2002; Vol. 1, p 785. (23) Ataka, K.; Yotsuyaanagi, T.; Osawa, M. J. Phys. Chem. 1993, 100, 10664. (24) Miyake, H.; Osawa, M. Chem. Lett., 2004, 33, 278.
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Figure 1. Cyclic voltammograms of Cu(111) in 0.1 M HClO4 without (a) and with 10-4 and 2 × 10-3 M BiPy (b and c, respectively). Potential scan rate was 50 mV‚s-1. electrolyte solution. To facilitate the assignments of the observed bands, vibrational frequencies of BiPyH22+ and other related molecules were calculated by density function theory (DFT) using GAUSSIAN 03 (B3LPY method with 6-31G basis set).25
3. Results 3.1. Electrochemical Behavior of Cu(111) in 0.1 M HClO4 Containing BiPy. Figure 1a shows a typical cyclic voltammogram of Cu(111) in 0.1 M HClO4. A featureless double-layer region is observed between ca. -0.35 and 0.15 V, consistent with former reports.26,27 After confirming the Cu(111) surface being clean, we recorded CVs of Cu(111) in 0.1 M HClO4 containing BiPy molecules with different concentrations (Figure 1b and c). Since no new information could be obtained at slower scan rates, here we show the results of CV at a scan rate of 50 mV‚s-1. The first scan of each CV was made in the negative direction from the open circuit potential (ca. 0.23 V). With the addition of BiPy, the electric charge in the double-layer potential region becomes smaller and the double layer region extends to more positive potentials, suggesting the adsorption of BiPy on the single-crystal surface. The CV in 0.1 M HClO4 + 10-4 M BiPy (Figure 1b) shows a cathodic peak at ca. -0.30 V. The small wave corresponds to the one-electron reduction of BiPYH22+ to monocation radical BiPYH2•+, as will be shown later. With an increase in the BiPy concentration, the cathodic peak disappears as shown in Figure 1c, probably due to negative shift to merge with the hydrogen evolution current. The sharply rising anodic peak at -0.32 V in (25) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision B.05; Gaussian, Inc.: Wallingford, CT, 2004. (26) Wan, L. J.; Itaya, K. Langmuir 1997, 13, 7173. (27) (a) Lukomska, A.; Sobkowski, J. J. Electroanal. Chem. 2004, 567, 95. (b) Lukomska, A.; Smolinski, S.; Sobkowski, J. Electrochim. Acta 2001, 46, 3111.
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Figure 2. (a) Large-scale STM top view of BiPy on Cu(111). The images were obtained at -0.20 V with a tunneling current of 8.6 nA. (b) High-resolution STM image from (a). The inset in (b) shows an image of the Cu(111) (2.2 nm × 2.2 nm) observed before injecting BiPy. (c) The HOMO and LUMO of BiPyH22+.
the reverse positive potential sweep suggests the oxidation of BiPYH2•+ to BiPYH22+. 3.2. STM Measurement. We have investigated the structures of the Cu(111) surface in 0.1 M HClO4 by changing electrode potential, bias, and tunneling current. A Cu(111) (1 × 1) structure was consistently revealed in the double-layer potential region. After the examination of the clean Cu(111) (1 × 1) surface, a small amount of BiPy solution was directly injected into the electrochemical cell for STM observation in the double-layer potential region (ca. 0.1 V). The concentration of BiPy in 0.1 M HClO4 was ca. 1 mM. A few minutes later, an ordered organic adlayer formed on the substrate. The adsorbed species is BiPyH22+, as will be shown later. A typical STM image of the BiPyH22+ adlayer is shown in Figure 2a, acquired at -0.20 V in a relatively large area. A well-defined molecular array is seen to extend over the atomically flat terrace. The molecular rows cross each other at an angle of either 60° or 120° within the experimental error of (2°. Higher-resolution STM imaging reveals further details of the molecular adlayer including the internal molecular structure, orientation, and packing arrangement in the ordered adlayer. In Figure 2b, each molecule appears in a shape like the number eight, which is similar to the HOMO of BiPyH22+ (Figure 2c). The length of the molecule is measured to be approximately 0.7 nm, in agreement with the dimension of BiPyH22+.28 The observed results strongly suggest a flat-lying orientation of BiPyH22+ adsorbed on Cu(111) surface. By comparing with the underlying Cu(111) lattice shown in the insert of Figure 2b, the intermolecular distances and molecular row orientation with substrate lattice can be decided. A parallelogram unit cell is outlined in the STM
Figure 3. (a) Large-scale STM image of BiPy adlayer on Cu(111). Imaging conditions are E ) -0.36 V and tunneling current 8.2 nA. (b) High-resolution STM top view of BiPy adlayer on Cu(111). (c) Cross-sectional profile along the A-B direction in (b) showing the corrugation height difference in a molecular row.
image. Four BiPyH22+ ions locate at the four corners of the unit cell. The parameters of the unit cell are measured to be a ) 0.8 ( 0.1 nm and b ) 1.0 ( 0.1 nm, respectively. Namely, a (3 × 4) structure is defined. The ordered structure was consistently observed in the potential range from -0.25 to ca. 0.2 V near the onset of copper oxidation. No orientation change was revealed in this potential range. A potential-dependent phase change was found in the BiPyH22+ adlayer by STM imaging. With the change of electrode potential to more negative region, a new structure different from the flatlying structure appeared. Figure 3a shows a typical large-scale STM image acquired at ca. -0.36 V where the reduction peak is observed in CV. The image emerged several minutes after scanning the potential to this value, indicating a slow dynamic process. Small nuclei of the ordered phase appeared initially and grow to cover all terraces within several minutes (see Supporting Information). Molecular domains marked as A and B are observed on the same terrace. The two domains cross each other at an angle, R, of ca. 150°. The higher-resolution STM image in Figure 3b shows that the adlayer is composed of the alternative rows of type I and II. The distance between the same type rows is measured to be 2.2 ( 0.2 nm. The molecular alignment in row I is almost parallel to the 〈121〉 direction of the underlying Cu(111) lattice, while that
Adsorbed Structures of 4,4′-Bipyridine on Cu(111)
in row II is along the 〈110〉 direction. An angle, β, between the alignment directions in two neighboring molecular rows is 150° ( 2°, which may have some correlation with R ) 150°. The size of each molecule is ca. 0.68 nm in length, which is close to the length of BiPyH22+ (ca. 0.7 nm) and suggests a perpendicular orientation with the long axis parallel to the surface. The intermolecular distance in the same row is measured to be ca. 0.4 nm, close to the typical stacking distance of heterocyclic aromatic molecules.29 A careful observation reveals that molecules in each row are observed with alternatively different brightness. A cross-sectional profile measured along the direction of A-B in Figure 3b is shown in Figure 3c. The corrugation height difference is measured to be ca. 0.03 nm. Adsorbed molecules were found to desorb gradually with time (Supporting Information), and finally clear terraces of the substrate emerged again. 3.3. SEIRAS Measurement. Infrared spectroscopy can provide rich information about the compositions and structures of molecules and is suitable to supplement the electrochemical and STM measurements described above. SEIRAS in the ATR mode was used in the present study due to its higher sensitivity and much less interference from the solution background than in IR reflection-absorption spectroscopy (IRAS).22 The chemically deposited Cu electrode used in the SEIRAS study was polycrystalline, but the CVs for this electrode were nearly identical to that for the Cu(111) electrode shown in Figure 1. Figure 4a shows a series of SEIRA spectra of the Cu electrode in 0.1 M HClO4 + 1 mM BiPy recorded sequentially at every 10 s during a potential sweep from +0.1 to -0.4 V and back to +0.1 V at a sweep rate of 1 mV‚s-1. The reference spectrum was collected at -0.2 V (the midway of the potential range of the SEIRAS measurement) before injecting BiPy into the cell. To see the spectral features in more detail, selected spectra at several potentials are shown in Figure 4b. In addition to the negative broad band around 1650 cm-1 and the positive broad band around 1100 cm-1 assigned to the bending mode of water repelled from the interface by the adsorption of BiPyH22+ and a Cl-O stretching mode of perchlorate, respectively,23 several weak bands are observed in the spectra. The weak bands change their intensities almost reversibly with respect to the potential sweep, indicating that these bands arise from adsorbed species. On the other hand, the perchlorate band shows an irreversible behavior. The intensity of this band sharply increases around -0.4 V and keeps nearly constant on the reverse positive potential sweep. It is worth noting that the spectrum of the adsorbed species changes between -0.3 and -0.4 V where the redox peaks appear in the CV and the phase change of the adsorbed molecular layer is observed by STM. The irreversible behavior of the perchlorate band may be caused by the reduction and phase change. For comparison, the same measurement was also carried out in a neutral solution (0.1 M NaClO4). The spectra observed (not shown) were nearly identical to those observed on an Au(111) electrode in neutral solution17 and largely different from those observed on the Cu electrode in acid (Figure 4). The spectral change was found to occur at pH 3-5. Recalling pK1 ) 3.5 and pK2 ) 4.9 for BiPy,20 the results suggest that the adsorbed species in the acid is BiPyH22+ (and its reduced species at E < -0.3 V). The 1643 cm-1 band clearly indicates protonation of BiPy.21 Further supports of the adsorption of the positively charged species are given by the perchlorate band. The positive peak of the perchlorate band implies the increase of the perchrolate con(28) Weakley, T. J. R. Acta Crystallogr. 1987, C43, 2144. (29) Weck, M.; Dunn, A. R.; Matsumoto, K.; Coates, G. W.; Lobkoysky, E. B.; Grubbs, R. H. Angew. Chem., Int. Ed. 1999, 38, 2741.
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Figure 4. (a) 3D plot of SEIRA spectra of a chemically deposited Cu thin film electrode in 0.1 M HClO4 + 1 mM BiPy acquired sequentially at every 10 s during a potential sweep from +0.1 to -0.4 V and back to +0.1 V at a sweep rate of 1 mV‚s-1. (b) Selected spectra at several potentials indicated in the figure. The spectra were offset for clarity. The reference potential was -0.2 V in the supporting electrolyte without BiPy.
centration at the interface compared with that in pure electrolyte. This is explained by the electrostatic interaction between positively charged adsorbed species and perchlorate anion. The doublet feature of the perchlorate band also suggests the interaction. An independent SEIRAS experiment in 0.1 M H2SO4 revealed that (bi)sulfate is adsorbed on the Cu electrode at potentials more positive than -0.4 V, indicating the potential of zero charge (pzc) of the electrode locates around -0.4 V in acid. The charge compensation by perchlorate anion may facilitate the adsorption of BiPyH22+ at the positively charged electrode surface. The frequencies and relative intensities of the IR bands observed at E > -0.3 V are listed in Table 1 along with those for BiPyH22+ (IR and Raman spectra of its dichloride salt and Raman spectrum in an acidic solution21). The right-most column lists the frequencies calculated by DFT and assignments (in parentheses) assuming D2h symmetry. The calculated frequencies were scaled to fit the Raman data of acidic solution of BiPy.21 The calculation is in reasonable agreement with the literature data. The vibrational frequencies of BiPyH22+ adsorbed on an Ag electrode observed by surface-enhanced Raman spectroscopy (SERS) in a pH 1.8 aqueous solution of 0.1 M KCl21 were also listed in the table (second column) for comparison. As can be found in the table, the SEIRA spectrum is largely different from the normal IR spectrum of BiPyH22+ in the solid state and rather resembles the
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Table 1. Vibrational Frequencies (cm-1) of BiPyH22+ Adsorbed on the Cu Electrode (-0.3 < E < +0.1 V), in Crystal and Acidic Solution, and Calculated by DFTa BiPyH22+ adsorbed BiPyH22+ SEIRAS (-0.3 to +0.1 V) 1643 (s)
1633 (s)
1610 (w) 1597 (w) 1521 (s)
on Ag
1612 (m)
IR
Raman
Raman
1646 (vs) 1630 (w) 1616 (w) 1594 (w)
1652 (vs)
1629 (m) 1620 (m) 1593 (s)
1525 (m) 1502 (w)
1531 (m)
1555 (w) 1522 (m)
1490 (s) 1409 (w)
acidic solutiona
solid (dichloride) SERSb
1476 (m) 1410 (w)
1481 (s) 1469 (sh) 1359 (m)
1333 (s) 1290 (m) 1290 (w)
1295 (vs) 1250 (w)
1212 (w)
1214 (w)
1293 (vs) 1274 (sh)
1298 (vs) 1256 (m)
1220 (w)
1220 (w) 1204 (m)
1232 (m) 1193 (m) 1122 (m) 1072 (m) 1053 (w) 1017 (w)
1012 (a) 1001 (w) 981 (w)
a
1072 (w) 1013 (vs)
1648 (ag) 1643 (b1u) 1620 (b3g) 1620 (b2u) 1533 (b3g) 1530 (ag) 1500 (b1u) 1489 (b2u) 1377 (b2u) 1361 (b3g) 1324 (b3g) 1306 (b2u) 1261 (ag) 1253 (b3g) 1252 (b2u) 1243 (ag) 1215 (b1u) 1123 (b2u) 1108 (b3g) 1070 (ag) 1056 (b1u) 1035 (b1u) 1011 (ag) 996 (b1u) 987 (b3u)
vs ) very strong, s ) strong, m ) medium, w ) weak, sh ) shoulder. b Data taken from ref 21.
Table 2. Vibrational Frequencies (cm-1) of the Adsorbate on Cu (E < -0.3 V), and Self-Dimers of Methyl- and Heptyl-Viologen Monocation Radicals ((MV•+)2 and (HV•+)2, Respectively) adsorbate on Cu (E < -0.3 V)
(MV•+)2a
1643 1597 1501 1336 1220
1605 1511 1340 1184
(HV•+)2b 1633 1596 1508 1335 1240 1184 1165 1025
980 a
1097 (w) 1079 (w) 1060 (vw) 1031 (m) 1011 (vs) 992 (w)
calcd (assignment in D2h symmetry)
Data taken from ref 30. b Data taken from ref 31.
normal Raman spectrum of BiPyH22+ in acid and the SER spectrum of BiPyH22+ adsorbed on an Ag electrode. The SEIRA bands, except for the bands at 1597 and 1490 cm-1, have their counterparts in the Raman spectra assigned to in-plane ring modes (gerade modes classified into either ag or b3g symmetry). The bands at 1597 and 1490 cm-1 are likely b2u modes, but the assignment is not very certain, as will be discussed later. At E < -0.3 V, four new bands emerge at 1597, 1501, 1336, and 1001 cm-1 associated with the reduction peak around -0.3 V in the CV. Slight peak shifts are observed for other bands. The spectral feature was largely different from that of BiPYH2•+ calculated by DTF but is almost identical to those for the selfdimers of (or one-dimensionally stacked) monocation radicals of methyl- and heptyl-viologens (N,N′-dialkyl-4,4′-bipyridium cations) as tabulated in Table 2. The radicals interact with each other via π orbitals with a face-to-face configuration in the self-
dimers.30,31 This result strongly suggests that BiPyH22+ is reduced to the monocation radical, BiPYH2•+, and forms a face-to-face self-dimer (or one-dimensional stack) on the electrode surface. The reduction of BiPyH22+ to BiPYH2•+ has been suggested also on Ag in acid from cyclic voltammetry.21 Since the spectral change around -0.3 V is reversible against the potential change, the reaction can be written as
2BiPyH22+ + 2e- T (BiPyH2•+)2
(1)
The reduction and dimerization via strong π-π interaction between the radicals is believed to cause the phase change observed by STM at about -0.3 V.
4. Disscussion 4.1. Adsorbed Structure of BiPyH22+ (E > -0.3 V). The SEIRAS spectrum at E > - 0.3 V strongly argues that the adsorbed species on Cu is BiPyH22+. STM revealed the BiPyH22+ adlayer has (3 × 4) symmetry (Figure 2). Figure 5a shows a proposed structural model for the (3 × 4) symmetry observed. Each pyridine ring in the BiPyH22+ is temporarily assumed to locate at a 3-fold hollow site of the substrate lattice. On the basis of the high-resolution image shown in Figure 2b, BiPyH22+ is assumed to be oriented flat on the surface with its long axis along 〈121〉 direction. In SEIRAS, molecular vibrations having transition dipole components perpendicular to the surface are selectively observed.22 If we apply the surface-selection rule, the observed SEIRA spectrum conflicts with the STM observation because (30) Ito, M.; Sasaki, H.; Takahashi, M.J. Phys. Chem. 1987, 91, 3932. (31) Osawa, M.; Yoshii, K. Appl. Spectrosc. 1997, 51, 512.
Adsorbed Structures of 4,4′-Bipyridine on Cu(111)
Figure 5. (a) Schematic representation for a (3 × 4) BiPy structure. (b) The proposed structural model for the π-π stacking arrangement consisting of BiPyH2•+ dimers.
the flat-lying BiPyH22+ should not show any in-plane modes in the SEIRA spectrum. We explored possible orientations that fit with the obtained SEIRA spectra but could not fined any suitable ones. For example, if we assume the 1597- and 1490-cm-1 bands to be b2u modes that have transition dipole moments along the short axis of BiPyH22+, a perpendicular orientation with its long axis parallel to the surface can be conceived as a possible orientation. If so, the 1597-cm-1 band should be observed as strong as the 1490-cm-1 band and also other b2u modes at 1359, 1232, and 1122 cm-1 should be observed with medium intensity as in the solid-state spectrum. However, the 1597-cm-1 band is observed only weakly and the latter three modes are totally missing in the SEIRA spectrum. Regarding this issue, it should be noted again that the SEIRA spectrum is largely different from the normal IR spectrum of BiPyH22+ in the solid state and resembles the normal Raman spectrum of BiPyH22+ in acid and SER spectrum of BiPyH22+ adsorbed on an Ag electrode (Table 1). Some of the observed IR bands are apparently assigned to gerade modes. The result conflicts with the selection rule of IR absorption: IR absorption and Raman scattering are exclusive each other for centrosymmetric molecules and Raman-active (gerade) modes should be IR-inactive (vice versa). This selection rule holds true for IR and Raman spectra of the dichloride salt of BiPyH22+ with a few exceptions that may arise from crystal field effect (that is, the molecular structure is approximated by D2h symmetry). One possible explanation for the results might be the lowering of the symmetry by adsorption. If the molecular symmetry were reduced, however, many IR bands should appear in this spectral range independent of molecular orientation. The missing of the most IR-active ungerade modes cannot be explained by either symmetry reduction or molecular orientation.
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A most plausible explanation is IR activation of Raman-active modes through vibronic coupling of molecular vibrations with charge transfer (CT) between the metal surface and the adsorbed BiPyH22+. The breakdown of the selection rule of IR absorption by vibronic coupling has been well established for CT complexes.32 The IR absorption of vibronically coupled gerade modes is strongly enhanced, and original IR-active ungerade modes are quenched by the coupling.32 The vibronic coupling has been suggested also for molecules adsorbed on metal surfaces.33 In the latter case, the surface-selection rule mentioned above must be modified slightly. Since the direction of the oscillating dipoles produced via the vibronic coupling is perpendicular to the molecular plane and the electric field that excites adsorbed species is perpendicular to the surface,22 the IR activation of gerade modes can occur effectively for BiPyH22+ adsorbed flat on the surface with π orbitals. The vibronic coupling mechanism can explain both the STM and SEIRAS results without any conflictions except for the bands at 1597 and 1490 cm-1. We speculate that these two bands arise from the molecules adsorbed at defect sites on the surface. 4.2. Adsorbed Structure of BiPyH2•+ (E < -0.3 V). The spectral similarity between the observed SEIRA spectrum at E < -0.3 V and the IR spectra of face-to-face self-dimers of viologen monocation radicals strongly suggests the dimerization of BiPyH2•+ (or one-dimensional stacking with the dimer as the building unit). In the case of viologen radicals, monomer and dimer can clearly be distinguished from largely different spectral features, which has been discussed extensively, and the IR bands characteristic to dimmer are assigned to the totally symmetric (ag) modes which are activated via vibronic coupling via the CT between the two radicals.30,31 The vibronically excited modes are out-of-phase vibrations of the two monomers, Q- ) (Q1 Q2)/x2. By analogy, we assign the observed bands for adsorbed BiPyH2•+ to ag modes activated via the vibronic coupling. On the basis of the SEIRA spectrum, the driving force of the phase transition from the flatly lying (3 × 4) phase to the onedimensional stacked phase around -0.3 V is believed to be the strong π-π interaction between the radicals. A possible structural model for the ordered monolayer observed at -0.35 V (Figure 3) is proposed in Figure 5b. In this model, the molecules align along the 〈121〉 direction in row I and along the 〈110〉 direction in row II. Molecules are assumed to locate alternatively on-top and bridge sites, by which alternative difference in brightness can be explained. The angle between the molecular alignment directions in two neighboring rows of 150° is consistent with the observation. BiPyH2•+ appears to be oriented perpendicularly to the surface with its long axis parallel to the surface in the highresolution STM image of the adsorbed. However, the molecular planes should be inclined to some extent so that the oscillation dipoles produced by the vibronic coupling (perpendicular to the molecular planes) can interact with the surface IR field (perpendicular to the surface) to give the vibronically coupled vibrations. Therefore, a slightly tilted orientation as the side view in Figure 5b shows is more likely. Unfortunately, the tilting angle of the molecular plane from the surface cannot be estimated from neither STM nor SEIRAS.
Conclusion We have investigated the adsorption of BiPy molecules on Cu(111) surface in 0.1 M HClO4 with cyclic voltammetry, STM, and SEIRAS. A clean and atomically flat Cu(111) surface could be prepared by electropolishing. Cyclic voltammetry and SEIRAS (32) Ferguson, E. E.; Matsen, F. A. J. Am. Chem. Soc. 1960, 82, 3268. (33) Devllin, J. P.; Consani, K. J. Phys. Chem. 1981, 85, 2597.
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revealed that BiPy is adsorbed on the electrode surface in its diprotonated form, BiPyH22+, in the double-layer potential region and is reduced to its monocation radical, BiPyH2•+, in more negative region overlapping with hydrogen evolution. Two different adlayer structures were imaged in the two potential regions. At the positive potential region, BiPyH22+ is adsorbed on the surface with a flat-lying orientation and forms an ordered monolayer with (3 × 4) symmetry. Two pyridine rings in a molecule appears in a “8” shape. In the negative-potential region, BiPyH2•+ adopts a π-π stacking arrangement with a tilted orientation. In the SEIRA spectrum of flat-lying BiPyH22+, most of the observed bands were IR-inactive modes for free BiPyH22+. The results appeared to conflict with the selection rule of IR absorption and the surface-selection rule but were explained reasonably without any conflictions by assuming vibronic coupling of Raman-active modes via charge transfer between BiPyH22+ and the surface. The SEIRA spectrum of BiPyH2•+
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was also dominated by the Raman-active modes vibronically excited by the charge transfer between the face-to-face selfdimer (as the bulding unit) of the radical. Acknowledgment. This work was supported by the National Natural Science Foundation of China (20520140277, 20575070, and 20121301), Chinese Academy of Sciences, Japan Science and Technology Agency (CREST), and the Ministry of Education, Culture, Sports, Science and Technology of Japan (Grant-in-Aid for Basic Research No. 14205121 and for Scientific Research on Priority Areas 417). We thank Dr. Y. Okinaka for his help in the writing of this manuscript. Supporting Information Available: Process of phase transitions. This material is available free of charge via the Internet at http://pubs.acs.org. LA052765W
