Direct Observation of 4-Mercaptopyridine and Bis(4-pyridyl) Disulfide

Langmuir 1998, 14, 3565-3569

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Direct Observation of 4-Mercaptopyridine and Bis(4-pyridyl) Disulfide Monolayers on Au(111) in Perchloric Acid Solution Using In Situ Scanning Tunneling Microscopy Takahiro Sawaguchi,† Fumio Mizutani,*,† and Isao Taniguchi‡ National Institute of Bioscience and Human-Technology, 1-1 Higashi, Tsukuba, Ibaraki 305, Japan, and Department of Applied Chemistry and Biochemistry, Faculty of Engineering, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860, Japan Received January 5, 1998. In Final Form: March 30, 1998 Organic monolayers of 4-mercaptopyridine (PySH) and bis(4-pyridyl) disulfide (PySSPy) self-assembled on an Au(111) electrode in perchloric acid solution were studied using electrochemical scanning tunneling microscopy (STM). Both monolayers formed from PySH and PySSPy exhibited the same surface feature on Au(111), appearing as a ladderlike structure. An ordered structure with intermolecular distances of 0.49 and 1.42 nm was consistently observed for the PySH and PySSPy monolayers describable as Au(111)-SPy, which was explained by a rectangular unit cell of p(5 × x3R-30°). High-resolution STM images revealed the internal structures and micro-orientation of chemisorbed pyridinethiolate molecules. They were oriented with the molecular plane of the pyridine rings mostly perpendicular to the surface, but the molecular axis considerably tilted with respect to the surface normal. It was also found that intermolecular interactions through the sulfur portions were thought to exist between two pyridinethiolate molecules, as is expected to be found in dimer-state adsorption. Thus, we report, for the first time, the detailed investigation of pyridinethiolate monolayers on Au(111) in solution at the molecular level.

Introduction Spontaneously chemisorbed monolayers of thiols and disulfides on gold, so-called self-assembled monolayers (SAM), have been expansively investigated in surface science and interfacial chemistry1-3 because they have wide potential abilities for the design of functionalized surfaces (for understanding of fundamental surface characteristics) and for extension toward various applications. They form stable and highly ordered monolayers by simple immersion of the substrates into a solution containing such molecules. The monolayers of alkanethiols and dialkyl disulfides on gold are the most popular and widely studied materials in elucidating the surface structures and physical and/or chemical properties.3-6 It is generally thought that both thiols and disulfides with long alkyl chains form thiolates as adspecies and therefore construct structurally identical monolayers. In resent years, the application of scanning tunneling microscopy (STM) and its family, scanning probe microscopy (SPM), have opened significant progress in precise determination of the molecular arrangements of the selfassembled monolayers.7-17 For the long-chain alkanethiols, ordered monolayers with (x3 × x3)R30° and sub* To whom correspondence should be addressed. Fax: +81-29854-6161. Phone: +81-298-54-6167. E-mail: [email protected]. † National Institute of Bioscience and Human-Technology. ‡ Kumamoto University. (1) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, MA, 1991. (2) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (3) Ulman. A. Chem. Rev. 1996, 96, 1533. (4) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (5) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (6) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155. (7) Widrig, C. A.; Alves, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2805.

sequent refined c(4x3 × 2x3)R30° (short-hand notation, c(4 × 2)) structures have been found on Au(111) substrates.9-11 In addition, a series of STM studies of a butanethiol monolayer by Poirier and co-workers17-19 have revealed that the monolayer exhibits a 2D liquidlike phase which subsequently causes a formation of low-coverage ordered phases with a (p × x3) (p ) 8-10) structure. Similar structures have been observed by the He diffraction method.20,21 Moreover, the existence of a S-S bond with a spacing of 0.22 nm has been found from the results based on an in-depth X-ray diffraction measurement of the decanethiol monolayer on Au(111).22 It was supposed that the thiols stayed in a dimer-state adsorption, where the S-S bond attached to the Au(111) surface, creating the refined c(4 × 2) superlattice. To our knowledge, (8) Alves, C. A.; Smith, E. L.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 1222. (9) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853. (10) Anselmetti, D.; Baratoff, A.; Gu¨ntherodt, H.-J.; Delamarche, E.; Michel, B.; Gerber, Ch.; Kang, H.; Wolf, H.; Ringsdorf, H. Europhys. Lett. 1994, 27, 365. (11) Delamarche, E.; Michel, B.; Gerber, Ch.; Anselmetti, D.; Gu¨ntherodt, H.-J.; Wolf, H.; Ringsdorf, H. Langmuir 1994, 10, 2869. (12) Delamarche, E.; Michel, B.; Biebuyck, H. A.; Gerber, C. Adv. Mater. 1996, 8, 719. (13) Scho¨nenberger, C.; Jorritsma, J.; Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. J. Phys. Chem. 1995, 99, 3259. (14) Scho¨nenberger, C.; Sondag-Huethorst, J. A. M.; Jorritsma, J.; Fokkink, L. G. J. Langmuir 1994, 10, 611. (15) Poirier, G. E.; Pylant, E. D. Science 1996, 272, 1145. (16) Poirier, G. E. Langmuir 1997, 13, 2019. (17) Poirier, G. E.; Tarlov, M. J.; Rushmeier, H. E. Langmuir 1994, 10, 3383. (18) Poirier, G. E.; Tarlov, M. J. J. Phys. Chem. 1995, 99, 10966. (19) Poirier, G. E. Chem. Rev. 1997, 97, 1117. (20) Camillone, N., III; Eisenberger, P.; Leung, T. Y. B.; Schwartz, P.; Scoles, G.; Poirier, G. E.; Tarlov, M. J. J. Chem. Phys. 1994, 101, 11031. (21) Camillone, N., III; Leung, T. Y. B.; Schwartz, P.; Eisenberger, P.; Scoles, G. Langmuir 1996, 12, 2737. (22) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216.

S0743-7463(98)00031-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/30/1998

3566 Langmuir, Vol. 14, No. 13, 1998 Chart 1

however, direct observation of the dimer structure in selfassembled monolayers has not been successful by STM and SPM. 4-Mercaptopyridine (PySH) and bis(4-pyridyl) disulfide (PySSPy) (Chart 1), which are typical aromatic thiol and disulfide compounds used to form self-assembled monolayers as well as alkanethiols, have been recognized as functional molecules to promote a facile electrochemical reaction of metalloproteins such as cytochrome c.23-25 Formation and characterization of the unique surface functionality of the monolayers are of importance in both the fields of interfacial chemistry and biochemistry. Although many promoter molecules have been studied for the electrochemistry of cytochrome c, a molecular level understanding of the mechanism explaining the interaction between cytochrome c and promoter molecules is still unclear. Recently, ex situ STM measurements26,27 and electrochemical28,29 and spectroscopic techniques30,31 have been applied to characterizing the PySH and PySSPy monolayers. However, the detailed monolayer structures have not yet been understood. Regardless of electrochemical and biochemical importance, no in situ STM study has been made on the interfacial structure and molecular arrangement of the monolayers of PySH and PySSPy on a well-defined Au(111) surface in aqueous solution. This paper describes, for the first time, a molecularscale investigation of the self-assembled monolayers of PySH and PySSPy on an Au(111) single-crystal electrode. We performed such a study by using in situ STM operating under the electrochemical potential control. It was found that both molecules formed well-ordered structurally identical monolayers on Au(111) in a perchloric acid solution. The results of high-resolution STM images demonstrate the precise determination of the interfacial structure and molecular arrangements of the self-assembled monolayers. Experimental Section Single-crystal beads were prepared by the crystallization at the end of Au wires (99.99% purity) in a hydrogen-oxygen flame according to the well-established method.32-34 One of Au(111) facets formed on the single-crystal bead was used directly for (23) Armstrong, F. A.; Hill, H. A. O.; Walton, N. J. Acc. Chem. Res. 1988, 21, 407. (24) Taniguchi, I. In Redox Mechanisms and Interfacial Properties of Molecules of Biological Importance; Schults, F. A., Taniguchi, I., Eds.; Electrochemical Soc., Inc.: Pennington, NJ, 1993; p 9. (25) Hawkridge, F. M.; Taniguchi, I. Comments Inorg. Chem. 1995, 17, 163. (26) Mayne, A. J.; Cataldi, T. R. I.; Knall, J.; Avery, A. R.; Jones, T. S.; Pinheiro, L.; Hill, H. A. O.; Briggs, G. A. D.; Pethica, J. B.; Weinberg, W. H. Faraday Discuss. 1992, 94, 199. (27) Hara, M.; Sasabe, H.; Knoll, W. Thin Solid Films 1996, 273, 66. (28) Lamp, B. D.; Hobara, D.; Porter, M. D.; Niki, K.; Cotton, T. M. Langmuir 1997, 13, 736. (29) Taniguchi, I.; Yoshimoto, S.; Nishiyama, K. Chem. Lett. 1997, 353. (30) Bryant, M. A.; Joa, S. L.; Pemberton, J. E. Langmuir 1992, 8, 753. (31) Gui, J. Y.; Lu, F.; Stern, D. A.; Hubbard, A. T. J. Electroanal. Chem. 1990, 292, 245.

Sawaguchi et al. STM experiments. A (111) plane exposed by mechanical polishing of the bead with successively finer grades of alumina paste was used for electrochemical measurements. Both electrodes were annealed in a hydrogen-oxygen flame and quenched quickly into ultrapure water saturated with hydrogen. Then the electrode was transferred to the site of the next experimental procedures with a droplet of ultrapure water to protect the surface from contamination.33,34 Ultrapure HClO4 solution (Cica-Merk) was used to prepare a 0.05 M HClO4 solution with water purified from a Milli-Q water system (Milli-Q SP, Millipore Co.). 4-Mercaptopyridine (PySH) and bis(4-pyridyl) disulfide (PySSPy) were purchased from Aldrich and used without further purification. The formation of self-assembled monolayers was performed by contacting the Au(111) electrodes with freshly prepared aqueous solutions of 20 µM PySH or PySSPy at room temperature (20 ( 1 °C). Especially, for PySH modification, a freshly prepared solution is strongly recommended to obtain reproducible results. The immersion times were typically 2 min for the PySH monolayer and 1 h for PySSPy. To remove excess molecules from the surface, the modified electrodes were thoroughly rinsed with pure water under supersonication for a few minutes. In situ STM measurements were carried out with a Nanoscope E (Digital Instruments, Santa Barbara, CA). The tunneling tip was prepared from electrochemically etched W or Pt/Ir (80:20) wires, and their side walls were insulated with transparent nail polish to minimize the faradaic current in electrolyte solutions. The tip was previously soaked in the electrolyte solution to remove soluble contaminants before STM measurements. All images were acquired in the constant current mode to evaluate corrugation heights of adsorbed molecules. A Pt wire was used for counter electrode, and all the electrode potentials are reported with respect to the reversible hydrogen electrode (RHE).

Results and Discussion The spontaneous chemisorption of PySH readily produces the self-assembled monolayer on the Au(111) electrode. The Au(111) electrode treated with PySH typically indicates an open circuit potential (OCP) of about 0.8 V vs RHE in 0.05 M HClO4. The Au(111) which was modified in the PySSPy solution also gave a similar value of OCP, suggesting that the both PySH and PySSPy molecules are considered to form the same type of monolayer in the sense of surface adspecies, i.e., a gold thiolate.24,25 Similarly to the case of alkanethiol monolayers,1,2 thiolates are expected to exist on the surface to form the pyridinethiolate-gold bond, described as Au(111)-SPy. The voltammetry of the Au(111)-SPy electrode in 0.05 M HClO4 solution presents no characteristic peak in the potential range of the so-called double layer region between 0.1 and 1.0 V, while a bare Au(111) draws a broad peak near 0.55 V (close to the potential of zero charge peculiar to the Au(111) electrode). Noticeable electrochemical reaction such as reductive desorption of adsorbed pyridinethiolate does not occur at the potentials adopted in STM measurements. An in situ STM imaging was first initiated in 0.05 M HClO4 under potential control at 0.8 V vs RHE, as much as the OCP value mentioned above. The tip potential was held at 0.5 V vs RHE, and this imaging condition corresponds to 0.3 V bias voltage (sample bias positive). Immediately after introduction of Au(111)-SPy into the perchloric acid solution, the Au(111)-SPy formed from PySSPy revealed wide (111) terraces separated by single atomic steps crossing at an angle of 60 or 120°, which resulted from the hexagonally arranged atoms of the Au(111) substrate. However, no molecular feature was (32) Itaya, K.; Sugawara, S.; Sashikata, K.; Furuya, N. J. Vac. Sci. Technol. 1990, A8, 515. (33) Honbo, H.; Sugawara, S.; Itaya, K. Anal. Chem. 1990, 62, 2424. (34) Sawaguchi, T.; Yamada, T.; Okinaka, Y.; Itaya, K. J. Phys. Chem. 1995, 99, 14149.

PySH and PySSPy Monolayers on Au(111)

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Figure 1. In situ STM image of the 4-pyridinethiolate monolayer on Au(111) in 0.05 M HClO4 solution. The potentials of the sample and tip were 0.8 and 0.5 V vs RHE, respectively, and this imaging condition corresponds to 0.3 V bias voltage (sample bias positive). The tunneling current was 2 nA.

observed on the terraces in initial STM imaging, whereas an atomic image of (1 × 1) hexagonal structure with an interatomic distance of 0.29 nm was routinely observed on a bare Au(111) in 0.05 M HClO4, which was used to determine the relative orientation of the adsorbates with respect to the Au(111) substrate. This is indicative that the pyridinethiolate molecules adsorb on the Au(111) electrode surface but do not form an ordered monolayer at the initial stage just after introduction of the modified electrode into perchloric acid solution. In continuous STM imaging of Au(111)-SPy at 0.8 V, it is found that small patches of molecularly ordered portions began to appear on the featureless (111) terrace. The small patches were gradually growing to construct larger domains, indicating that the adsorbed pyridinethiolates were slowly rearranged to form the molecularly ordered phases on the surface. Figure 1 shows an example of in situ STM images of the ordered pyridinethiolate monolayer on Au(111), which was acquired 0.5 h after the initial STM scan of the surface. It is obviously seen that the entire Au(111) surface was covered with many translational and rotational domains and each domain consisted of ordered molecular rows appearing as double stripes at a glance. The direction of each molecular row is parallel to a [1 h1 h 2], the so-called x3 direction, which is perpendicular to the direction of the Au(111) atomic row, i.e., the [11 h 0] direction. The molecular rows in adjacent domains cross each other at a rotation angle of exclusively 60°, exactly following the 3-fold symmetry of the underneath Au(111). The size of the individual domains increases with observation time and falls in the range of 30-50 nm wide. Although the domain growth seems to be very slow at this potential, the molecular organization process depends on the applied potential of the electrode. In fact, when the electrode potential was slightly shifted negatively, 0.8 V to 0.7 V, the surface organization of the adsorbed pyridinethiolate proceeds even faster, exhibiting a relatively smooth formation of larger molecularly ordered domains. It is noted that a similar trend for the ordering process and the domain structure with molecular rows was observed on Au(111)-SPy prepared in PySH solution. Interestingly, domain boundaries and molecular defects are easily found to exist at the intersection of the grown ordered monolayer domains. As presented in Figure 1,

Figure 2. High-resolution STM image (a) and height-shaded view (b) of the 4-pyridinethiolate monolayer on Au(111). The superimposed rectangular indicates the p(5 × x3R-30°) unit cell. The imaging condition was the same as that in Figure 1

the typical sizes of the defects are about 1.5-5.0 nm, which corresponds to the width for one to several molecular rows of the monolayer. On the other hand, the depth of the molecular defects (ca. 0.21 nm) is almost identical to the height of a single-atom step for the Au(111) surface (0.24 nm), independent of the defect size. These results are consistent with monatomic pit formation in the uppermost layer of Au(111) during self-assembling process in solution, since similar defects or depletion have been reported for self-assembled monolayer systems of alkanethiols.14,16,19 The orientation of pyridinethiolates in the monolayer were revealed in detail by high-resolution STM images of 5 × 5 nm2 area shown in Figure 2a,b. The images were acquired specifically under condition with minimal thermal drift in order to determine the monolayer structure as accurately as possible. The STM image shown in Figure 2a revealed that each molecular row observed as the double stripe in Figure 1 is indeed composed of linearly arranged spots with different brightness, exhibiting a distinctive molecular arrangement such as a ladderlike structure. The elliptic brightest spots are aligned along the x3 direction at the both sides of the ladderlike molecular rows, which are linked by slightly darker block spots elongated along the [11 h 0] direction. The long axes of the elliptic spots are off by a rotation angle of ca. 45° with respect to the x3 direction. These ladderlike molecular

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Figure 3. Cross section profiles of the 4-pyridinethiolate monolayer on Au(111) along the directions indicated by the arrows shown in Figure 2a.

rows are arranged at translational positions along the [11 h 0] direction, which form a moleculaly ordered phase. From the STM observation, it is expected that each elliptic spot corresponds to the pyridine ring of individual pyridinethiolate molecules and the slightly darker spots between the pyridine rings are probably originated from the sulfur portions of the pyridinethiolates. The molecular feature as shown in Figure 2a was consistently observed on the (111) terrace treated with both PySH and PySSPy. Figure 3 shows cross section profiles of the pyridinethiolate monolayer on Au(111) along two directions of [11 h 0] and [1 h1 h 2] indicated by the arrows A and B shown in Figure 2a. The intermolecular distance along the arrow A was found to be 1.42 nm (Figure 3a), which corresponds to 5 times the lattice parameter of Au(111) (0.2885 nm). The corrugation height for this direction appeared to be 0.09 nm as a maximum value. The cross section along the arrow B presents an intermolecular distance of 0.49 nm and corrugation height of 0.04 nm (Figure 3b). On the basis of the intermolecular distances for two directions, we concluded that the pyridinethiolate monolayer was composed of rectangular unit cells, namely, p(5 × x3R-30°), as shown by the unit cell superimposed in Figure 2a. The lattice distances of 5 and x3 on Au(111) correspond to 1.44 and 0.50 nm, respectively, which are consistent with our experimental values. Surprisingly, a careful inspection of the image enables us to determine the internal structure and micro-orientation of the pyridinethiolate molecules adsorbed on Au(111). The ladderlike molecular rows observed in Figure 2a also revealed that pyridinethiolate molecules are oriented with the molecular plane of the pyridine ring mostly perpendicular to the (111) surface, but the molecular axis through the nitrogen and sulfur atoms of the pyridinethiolate is considerably tilted with respect to the surface normal. These features can be more clearly seen in the height-shaded surface plot as shown in Figure 2b. The large corrugation height (0.09 nm) obtained from Figure 3a supports this vertical orientation of the pyridine ring with the tilted molecular axis, and the molecular

Sawaguchi et al.

Figure 4. Real space model for the structure of the 4-pyridinethiolate monolayer on Au(111). The rectangular p(5 × x3R-30°) unit cell was overlaid in the model.

orientation is good agreement with previous reports for the monolayers of pyridinethiolates28-31 and a related system, thiophenol.35-41 Furthermore, it is expected that an intermolecular interaction at the sulfur portions might exist between the two pyridinethiolates. These interactions are observed as the slightly darker block spots located at the center of the ladderlike structure. Although individual sulfur spots were not clearly resolved in this study, the approximate interatomic distance corresponding to the S-S region can be estimated to be 0.22 nm. It is important to note that the S-S interactions observed in STM imaging are essentially similar to those found in grazing incidence X-ray diffraction study for the dimer states of the decanethiol monolayer reported by Fenter et al.22 They suggested the existence of disulfide species with a S-S separation distance of ∼0.22 nm in a (x3 × x3)R30° structure of the long chain molecules. To explain the observed STM images, we propose the structural model for the pyridinethiolate monolayer on Au(111) as is shown in Figure 4. In this model, the sulfur atoms of the adsorbed pyridinethiolates sit near the 3-fold hollow sites of the (111) surface and two neighboring pyridinethiolates are located symmetrically around the x3 direction, where the molecular arrangement resembles the dimer-state adsorption such as the PySSPy molecule. Each adsorbed pyridinethiolate possesses the molecular orientation with the vertical molecular plane and the tilted molecular axis of the pyridine rings with respect to the surface. The p(5 × x3R-30°) rectangular (35) Carron, K. T.; Hurley, L. G. J. Phys. Chem. 1991, 95, 9979. (36) Sabatani, E.; Cohen-Boulakia, J.; Bruening, M.; Rubinstein, I. Langmuir 1993, 9, 2974. (37) Sandroff, C. J.; Herschbach, D. R. J. Phys. Chem. 1982, 86, 3277. (38) Gui, J. Y.; Stern, D. A.; Frank, D. G.; Lu, F.; Zapien, D. C.; Hubbard, A. T. Langmuir 1991, 7, 955. (39) Stern, D. A.; Wellner, E.; Saraita, G. N.; Laguren-Davidson, L.; Lu, F.; Batina, N.; Frank, D. G.; Zapien, D. C.; Walton, N.; Hubbard, A. T. J. Am. Chem. Soc. 1988, 110, 4885. (40) Tao, Y.-T.; Wu, C.-C.; Eu, J.-Y.; Lin, W.-L.; Wu, K.-C.; Chen, C.-H. Langmuir 1997, 13, 4018. (41) Dhirani, A.-A.; Zehner, R. W.; Hsung, R. P.; Guyot-Sionnest, P.; Sita, L. R. J. Am. Chem. Soc. 1996, 118, 3319.

PySH and PySSPy Monolayers on Au(111)

unit cell, 1.44 nm × 0.5 nm, was indicated in the model. This model fits with our STM observations and provides a clear evidence for the ladderlike structure of the adsorbed pyridinethiolates on Au(111). It is noteworthy to mention that the packing density for the p(5 × x3R-30°) structure of Au(111)-SPy is much less than those reported for the (x3 × x3)R30° alkanethiol monolayers. As shown in the model, the p(5 × x3R-30°) unit cell contains two pyridinethiolate molecules, which yields a surface coverage of 0.2 with respect to the atomic density of Au(111), corresponding to a packing density of 4.61 × 10-10 mol/ cm2. On the basis of the reductive desorption of thiolates assuming an one-electron reduction process,42 several experimentally estimated values for packing densities were reported for the monolayers of pyridinethiol28 and thiophenol.40,43 These values are in the range of (4.45.4) × 10-10 mol/cm2, much less than those of the (x3 × x3)R30° structure of alkanethiol monolayers on Au(111), which are experimentally evaluated to be (7-8) × 10-10 mol/cm2.44-46 These lower values for the simple aromatic thiol monolayers are good agreement with our results for the p(5 × x3R-30°) structure of Au(111)-SPy. Finally we should note that the p(5 × x3R-30°) structure of pyridinethiolates observed in our STM measurements would be a final monolayer state for the Au(111)-SPy. The pyridinethiolate monolayers do not form an ordered phase at initial stage and are followed by slow reorientation process to construct the p(5 × x3R-30°) structure, as described above. The initial disordered monolayer is thought to be a 2D liquidlike phase, which is similar to that for the butanethiol monolayer observed by Poirier et al.17-19 There is another possibility that protonation of the pyridinyl group might cause the disorder phase. Since Bryant and Crooks47 estimated a surface pKa of 4.6 ( 0.5 for Au-SPy on the (42) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335. (43) Zhong, C.-J.; Porter, M. D. J. Am. Chem. Soc. 1994, 116, 11616. (44) Weisshaar, D. E.; Lamp, B. D.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 5860. (45) Walczak, M. M.; Alves, C. A.; Lamp, B. D.; Porter, M. D. J. Electroanal. Chem. 1995, 396, 103.

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basis of the differential interfacial capacitance measurements in various pH solutions, a large amount of the adsorbed pyridinethiolates is expected to be protonated in our acidic condition. Therefore, the pH effect might be also related to the ordering process of the monolayer. A more detailed study for the disorder-order process, which seems to be greatly associated with electrode potentials applied, is in progress. Conclusion We presented, for the first time, a successful in situ STM imaging of the highly ordered monolayers of chemisorbed 4-mercaptopyridine (PySH) and bis(4-pyridyl) disulfide (PySSPy) on the Au(111) electrode in HClO4 solution. High-resolution STM images revealed that both monolayers formed from PySH and PySSPy exhibited the identical molecular arrangement on Au(111), appearing as a ladderlike structure. The ordered structure with intermolecular distances of 1.42 and 0.49 nm was consistently observed, which was explained by a rectangular unit cell of p(5 × x3R-30°) for the Au(111)-SPy. The in situ STM imaging of the internal structure of the pyridinethiolate monolayers clearly suggested that the pyridinethiolates are oriented with mostly vertical orientation of the molecular plane of pyridine rings, but the molecular axis of the pyridinethiolate is considerably tilted with respect to the surface normal. It is also found that the intermolecular interaction through the sulfur portions exists between two pyridinethiolate molecules in the monolayer, as is expected from a dimer-state adsorption. Acknowledgment. This work was supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan and Grant-in-Aid for Scientific Research (No. 09237106) on Priority Area of Electrochemistry of Ordered Interfaces from Ministry of Education, Science, Sports, and Culture of Japan (for I.T.). LA980031W (46) Zhong, C.-J.; Zak, J.; Porter, M. D. J. Electroanal. Chem. 1997, 421, 9. (47) Bryant, M.; Crooks, R. M. Langmuir 1993, 9, 385.