Self-Assembled Monolayers of 4-Mercaptopyridine on Au(111): A

The Journal of Physical Chemistry C 2014 118 (2), 989-993 ... Effect of Chloride Ions on the Adsorption of 3-Mercapto-1-propanesulfonic acid and ... D...
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Langmuir 2004, 20, 2797-2802

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Self-Assembled Monolayers of 4-Mercaptopyridine on Au(111): A Potential-Induced Phase Transition in Sulfuric Acid Solutions Thorsten Baunach, Valentina Ivanova, Daniel A. Scherson,† and Dieter M. Kolb* Abteilung Elektrochemie, Universita¨ t Ulm, 89069 Ulm, Germany Received July 30, 2003. In Final Form: October 17, 2003 In situ scanning tunneling microscopy images of self-assembled monolayers (SAMs) of 4-mercaptopyridine (4-MPy) on Au(111) recorded in neat 0.1 M H2SO4 solutions provided evidence for a potential-induced phase transition over the range 0.40-0.15 V versus saturated calomel electrode. Analysis of the data was consistent with the presence of a (5 × x3) and (10 × x3) superstructure (phase A) at the positive end, that is, 0.40 V, for which the local coverage, θloc, is about 0.2 (two 4-MPy molecules per unit cell), which compresses at the negative end, that is, 0.15 V, to yield a much denser superstructure (phase B, θloc ca. 0.5). This behavior is unlike that reported for the 4-MPy-Au(111) SAM prepared by identical means, in 0.1 M HClO4 (or in sulfate solutions of a much higher pH) for which only the (5 × x3) superstructure was observed over the same potential range. The compression associated with the phase A to phase B transition is attributed to the formation of a hydrogen-bonded network of bisulfate coordinated in turn to the 4-MPy layer via the acidic hydrogens of the pyridinium moieties. Such conditions promote better packing of adsorbed 4-MPy species, which are aided by intermolecular π-π ring interactions, resulting in higher local coverages.

1. Introduction Self-assembled monolayers (SAMs) of organothiols and disulfides on well-defined metal surfaces continue to attract much attention in the surface science community.1-7 More recently, interest has been focused on adsorbates with aryl groups, including phenyl8-14 and heterocyclic rings,15-24 which was prompted, at least in * To whom correspondence should be addressed. Phone: +49731-50-25400. Fax: +49-731-50-25409. E-mail: dieter.kolb@ chemie.uni-ulm.de. † Permanent address: Chemistry Department, Case Western Reserve University, Cleveland, OH 44106, U.S.A. (1) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (2) Ulman, A. Chem. Rev. 1996, 96, 1533. (3) Poirier, G. E. Chem. Rev. 1997, 97, 1117. (4) Hagenstro¨m, H.; Schneeweiss, M. A.; Kolb, D. M. Langmuir 1994, 10, 2435. (5) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (6) Schweizer, M.; Hagenstro¨m, H.; Kolb, D. M. Surf. Sci. 2001, 490, L627. (7) Esplandiu, M. J.; Hagenstro¨m, H.; Kolb, D. M. Langmuir 2001, 17, 828. (8) Sun, L.; Johnson, B.; Wade, T.; Crooks, R. M.; J. Phys. Chem. 1990, 94, 8869. (9) Sabatani, E.; Cohen-Boulakia, J.; Breuning, M.; Rubinstein, I. Langmuir 1993, 9, 2974. (10) Batz, V.; Schneeweiss, M. A.; Kramer, D.; Hagenstro¨m, H.; Kolb, D. M.; Mandler, D. J. Electroanal. Chem. 2000, 491, 55. (11) Go¨lzha¨user, A.; Geyer, W.; Stadler, V.; Eck, W.; Grunze, M.; Edinger, K.; Weimann, T.; Hinze, P. J. Vac. Sci. Technol., B 2000, 18, 3414. (12) Leung, T. Y. B.; Schwartz, P.; Scoles, G.; Schreiber, F.; Ulman, A. Surf. Sci. 2000, 458, 34. (13) Nielsen, J. U.; Esplandiu, M. J.; Kolb, D. M. Langmuir 2001, 17, 3454. (14) Baunach, T.; Kolb, D. M. Anal. Bioanal. Chem. 2002, 373, 743. (15) Gui, J. Y.; Lu, F.; Stern, D. A.; Hubbard, A. T. J. Electroanal. Chem. 1990, 292, 245. (16) Yang, D. F.; Stolberg, L.; Lipkowski, J.; Irish, D. E. J. Electroanal. Chem. 1993, 329, 395. (17) Stolberg, L.; Morin, S.; Lipkowski, J.; Irish, D. E. J. Electroanal. Chem. 1991, 307, 241. (18) Pettinger, B.; Mirwald, S.; Lipkowski, J. Ber. Bunsen-Ges. Phys. Chem. 1993, 97, 395. (19) Alonso, C.; Pascual, M. J.; Salomo´n, A. B.; Abruna, H. D.; Gutierrez, A.; Lo´pez, M. F.; Garcı´a-Alonso, M. C.; Escudero, M. L. J. Electroanal. Chem. 1997, 435, 241.

part, by the ability of these species to promote reversible heterogeneous electron transfer between electrodes and large solution-phase species of biological significance, particularly cytochrome c.25,26 Indeed, there exists a rather large body of literature regarding electrochemical and structural aspects of 4-mercaptopyridine (4-MPy) SAMs assembled on well-defined Au single-crystal surfaces using scanning tunneling microscopy (STM)27-30 and surfaceenhanced IR absorption spectroscopy.31-33 Our efforts have been directed toward elucidating the role of anions in controlling the structure and electrochemical properties of these SAMs. This work describes electrochemical and structural aspects of a potential-induced phase transition observed for a 4-MPy SAM on Au(111) in 0.1 M H2SO4. As will be shown, in situ STM images revealed the presence of a new superstructure, denoted as phase B, at potentials close to the onset of 4-MPy reductive desorption (+0.15 V vs saturated calomel electrode, SCE), for which the local (20) Brolo, A. G.; Irish, D. E.; Lipkowski, J. J. Phys. Chem. B 1997, 101, 3906. (21) Turyan, I.; Mandler, D. Anal. Chem. 1997, 69, 894. (22) Hebert, P.; Le Rille, A.; Zheng, W. Q.; Tadjeddine, A. J. Electroanal. Chem. 1998, 458, 5. (23) Madueno, R.; Pineda, T.; Sevilla, J. M.; Bla´zquez, M. Langmuir 2002, 18, 3903. (24) Bryant, M. A.; Joa, S. L.; Pemberton, J. E. Langmuir 1992, 8, 753. (25) Armstrong, F. A.; Hill, H. A. O.; Walton, N. J. Acc. Chem. Res. 1988, 21, 407. (26) Lamp, B. D.; Hobara, D.; Porter, M. D.; Niki, K.; Cotton, T. Langmuir 1997, 13, 736. (27) Wan, L.-J.; Hara, Y.; Noda, H.; Osawa, M. J. Phys. Chem. B 1998, 102, 5943. (28) Jin, Q.; Rodriquez, J. A.; Li, C. Z.; Darici, Y.; Tao, N. J. Surf. Sci. 1999, 425, 101. (29) Yoshimoto, S.; Yishida, M.; Kobayashi, S.; Nozute, S.; Miyawaki, T.; Hashimoto, Y.; Taniguchi, I. J. Electroanal. Chem. 1999, 473, 85. (30) Sawaguchi, T.; Mizutani, F.; Yoshimoto, S.; Taniguchi, I. Electrochim. Acta 2000, 45, 2861. (31) Ataka, K.; Hara, Y.; Osawa, M. J. Electroanal. Chem. 1999, 473, 34. (32) Taniguchi, I.; Yoshimoto, S.; Sunatsuki, Y.; Nishiyama, K. Electrochemistry 1999, 67, 1197. (33) Taniguchi, I.; Yoshimoto, S.; Yoshida, M.; Kobayashi, S.; Miyawaki, T.; Aono, Y.; Sunatsuki, Y.; Taira, H. Electrochim. Acta 2000, 45, 2843.

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coverage normalized to the gold surface is θloc ) 0.5 (i.e., 0.69 × 1015 molecules cm-2), which is considerably higher than that associated with the well-known (5 × x3) superstructure (or phase A), that is, θloc ) 0.2, found at more positive potentials, for example, +0.4 V versus SCE. No evidence for phase B was obtained for the 4-MPy Au(111) SAM in perchloric acid of the same pH (or in mildly acidic sodium sulfate solutions), which always yielded STM images consistent with phase A over the entire potential range, as has been reported in the literature.27,29,30,34 2. Experimental Section The gold single-crystal electrode used in these measurements was a solid cylinder (MaTeck, Ju¨lich, Germany, 0.4 cm diameter, polished down to 0.03 µm) with one of its ends oriented to better than 1° along the (111) plane. A Au wire was attached to the back of the specimen for better handling. Prior to each experiment, the Au crystal was annealed for 5 min in a propane or hydrogen flame, allowed to cool in air, and then immersed into a 4-MPy solution for a prescribed length of time (see the following). Immediately thereafter, the electrode was removed from the solution, rinsed thoroughly with ultrapure water (USF Elga, Germany), and then introduced into either a conventional electrochemical cell or a STM cell (see the following) filled with either 0.1 M H2SO4 (Merck, suprapure), 0.1 M HClO4 (Merck, suprapure), 0.1 M NaOH (Merck, suprapure), or unbuffered 0.25 M (or 0.1 M) Na2SO4 (Merck, suprapure) solutions in ultrapure water. For the purely electrochemical experiments, the electrolyte was deaerated by purging with N2. For both types of measurements, rapid potential control was achieved by setting the potentiostat in the open mode at the desired value, Estart, prior to contacting the modified electrode with the solution. SAMs of 4-MPy (Aldrich, as received) were prepared by immersing the freshly annealed Au(111) electrode without potential control for a certain period of time, tmod, in a 0.1 mM 4-MPy aqueous solution prepared with deareated water to prevent thiol oxidation. Problems associated with the concomitant adsorption of sulfide (an impurity often found in commercial 4-MPy) could be overcome provided tmod was no longer than a few minutes,33 affording an expedient and reliable means of preparing 4-MPy-Au(111) layers for subsequent electrochemical and structural studies. Cyclic voltammograms were recorded with standard electrochemical equipment. In situ STM measurements were performed with a Topometrix TMX 2010 Discoverer using an all Kel-F custom-made cell35 with either tungsten or Pt/Ir (80:20) tips electrochemically etched in 2 M NaOH or 3.4 M NaCN, respectively. The tips were coated with an electrophoretic paint to minimize Faradaic currents. Tunneling currents for image acquisition were in the range of 0.5-1.0 nA. Two Pt wires, cleaned by annealing in a hydrogen flame, were used as the counter and pseudo-reference electrodes (EPt ) +0.55 ( 0.05 V vs SCE), respectively; however, all electrode potentials are quoted with respect to SCE.

3. Results and Discussion Cyclic voltammetric curves recorded for 4-MPy-Au(111) SAMs (tmod ) 10 min) in 0.1 M NaOH solutions (not shown here) yielded in the scan in the negative direction a peak at about -0.53 V versus SCE. This peak was attributed to the one-electron reductive desorption of 4-MPy, that is,

RSAu + e- f RS- + Au

(1)

and provides a basis for determining surface coverages.36 (34) Sawaguchi, T.; Mizutani, F.; Taniguchi, I. Langmuir 1998, 14, 3565. (35) Kolb, D. M. Adv. Electrochem. Sci. Eng. 2002, 7, 107. (36) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335.

Figure 1. Cyclic voltammetry curves for 4-MPy-Au(111) surfaces in 0.1 M H2SO4 (solid line) and 0.1 M HClO4 (dotted line). Scan rate: v ) 10 mV/s, Estart ) +0.15 V. Also shown for comparison is the corresponding voltammogram for bare Au(111) in 0.1 M H2SO4 recorded under otherwise identical conditions (dashed-dotted line). The arrow indicates the onset of reductive desorption.

A coulometric analysis for such 4-MPy-Au(111) SAMs yielded a charge under the reduction peak of 50 ( 5 µC cm-2, which is similar to that found by other workers,27,34 that is, 44.4 µC cm-2, and is consistent with a (5 × x3) superstructure, θloc about 0.2 (two 4-MPy molecules per unit cell or, more precisely, 0.28 × 1015 molecules cm-2. 3.1. Sulfuric Acid Solutions. Shown in Figure 1 is the cyclic voltammogram (first cycle) recorded in 0.1 M H2SO4 aqueous solutions for a 4MPy-Au(111) SAM after tmod ) 5 min (solid line). The corresponding curves for bare Au(111) and for the modified surface in 0.1 M HClO4 are shown for comparison. The changes in the voltammetric behavior induced by the presence of 4-MPy SAMs on Au(111) are, indeed, significant. Particularly prominent is the presence of a quasi reversible peak (A and A′) at about 0.4 V, for which the charge (corrected by the rather featureless background) was estimated to be about 20 µC/ cm2. The onset of a negative current at 0.1 V versus SCE (see arrow) is, in all likelihood, related to the onset of reduction of adsorbed 4-MPy. Attention will be focused in the following on changes in the SAM associated with the peak at about 0.4 V, using STM as an in situ structural probe. In situ STM images obtained for 4-MPy-Au(111) at 0.40 V for tmod ) 5 min yielded a striped structure denoted hereafter as phase A (Figure 2a). The distance between the 4-MPy molecules along the stripes, where each bright spot can be ascribed to a S in 4-MPy bound to the Au,36 was estimated to be 0.48 ( 0.05 nm, whereas the distances between the stripes were either 1.3 ( 0.13 nm (see R, Figure 2a) or 2.9 ( 0.3 nm (β, Figure 2a), consistent with a (5 × x3) and a (10 × x3) superstructure, respectively. A very different STM image, reported here for the first time, was observed upon polarizing the electrode at +0.15 V versus SCE for about 30 min (Figure 2b). The surface in this case exhibits numerous domains with a maximum characteristic length of about 10 nm, rotated 60° with respect to one another. Analysis of this image was consistent with a rearrangement of the adsorbed 4-MPy molecules into a much denser superstructure than that in phase A, referred to hereafter as phase B. Unfortunately, it was not possible to obtain an internal calibration based on the known interatomic distances of the underlying substrate [or intermolecular distances for an adsorbed species, as in the (x3 × x7)R19.1° superstructure for sulfate on Au(111)] by removing the monolayer from the

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Figure 3. High-resolution STM image of a 4-MPy modified Au(111) electrode in 0.1 M HClO4 showing phase A.

Figure 2. In situ STM images of a 4-MPy-Au(111) electrode (tmod ) 5 min) in 0.1 M H2SO4 recorded (a) at +0.40 V showing phase A and (b) at +0.15 V (phase B). The insets show highresolution images of both phases.

surface.14 Because the onset of reductive desorption of the SAM coincides with hydrogen evolution, and the bubbles thus created stick to the STM tip, acquisition of highresolution images is nearly impossible. Within these uncertainties, the distance between the stripes for phase B (Figure 2b) was estimated to be 0.55 ( 0.05 nm, and that between the bright spots along the stripes, for which the corrugation height was very small, was 0.32 ( 0.05 nm. Also present in these images was a large number of holes about 0.2-0.3 nm deep. This potential-induced phase transition was found to be reversible. Larger domains, necessary to observe the structure by STM, appeared on the surface only after polarization of the electrode for about 30 min. In agreement with other workers,27,30,34,37 measurements performed in 0.1 M HClO4 for a 4-MPy SAM on Au(111) yielded only phase A (Figure 3), that is, (5 × x3), in the potential range under consideration (0.0-0.5 V). Also, in harmony with published data, the voltammogram of a tmod ) 5 min 4-MPy-Au(111) SAM electrode in 0.1 M HClO4 (see dotted curve, Figure 1) did not show evidence (37) Wan, L.-J.; Noda, H.; Hara, Y.; Osawa, M. J. Electroanal. Chem. 2000, 489, 68.

for peaks.31 Furthermore, the capacity was 24 ( 2 µF cm-2, that is, close to that for bare Au(111) at potentials negative of the potential of zero charge. On these bases, it can be concluded that the phase transition is intrinsic to the 4-MPy-Au(111) SAM in sulfuric acid. Additional measurements performed in 0.25 M Na2SO4 (see section 3.3) also yielded only phase A, indicating that even in solutions containing sulfate, the pH plays a key role in promoting the phase transition. 3.2. On the Nature of the Phase A to Phase B Transition. Various aspects pertaining to Phase A (which are common both in sulfuric and perchloric acid) have been discussed in detail in the literature;27,30,34,37 hence, only issues concerning phase B will be considered here. (i) Specifically, if it is assumed that every 4-MPy species is bound to the same type of site on the surface, the distances for the measured unit cell would fit best a (x3 × 1) superstructure, for which the distance between molecules in a row is 0.29 nm. This value is smaller than the van der Waals diameter of sulfur, that is, 0.37 nm,38 larger than a typical S-S bond (0.2 nm),39 but well within the value measured experimentally, that is, 0.32 ( 0.05 nm. It must be emphasized that in the case of alkanethiols on Au(111), mostly a (x3 × x3) superstructure is observed,40,41 for which the S-S distance is 0.49 nm and, thus, much longer than that observed for phase B. It thus follows that other interactions must be involved to account for the dense packing. (ii) The stacking distance found for phase B of 4-MPyAu(111), that is, 0.32 ( 0.05 nm, is in line with the corresponding values observed for the (nonthiol) 2,2′bipyridine and 4,4′-bipyridine SAM on Au(111),42,43 that is, 0.31-0.43 nm, where the dense packing was attributed to intermolecular π-π ring interactions. As is wellknown,44 such interactions are derived from London dispersion forces among π-electron systems, such as aromatic heterocycles. It thus seems quite plausible that the same type of phenomena may also operate for phase B, especially because typical van der Waals distances for aromatic heterocycles, either in the solid lattice or in solution, are usually around 0.34 nm.45 (38) Pauling, L. The Nature of the Chemical Bond; Cornell University Press: New York, 1945. (39) Beagley, B.; McAloon, K. T. Trans. Faraday Soc. 1971, 67, 3216. (40) Demir, U.; Shannon, C. Langmuir 1994, 10, 2794. (41) Tachibana, M.; Yoshizawa, K.; Ogawa, A.; Fujimoto, H.; Hoffmann, R. J. Phys. Chem. B 2002, 106, 12727. (42) Dretsckow, T.; Lampner, D.; Wandlowski, T. J. Electroanal. Chem. 1998, 458, 121. (43) Cunha, F.; Tao, N. J.; Wang, X. W.; Jin, W.; Duong, B.; D’Agnese, J. Langmuir 1996, 12, 6410. (44) Saenger, W. Principles of Nucleic Acid Structure; Springer: Heidelberg, 1984. (45) Ts’o, P. O. P. In Basic Principles in Nucleic Acid Chemistry; Ts’o, P. O. P., Ed.; Academic Press: New York, 1974; Vol. 1, p 453.

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Figure 4. Schematic diagram of the proposed structure of phase B. For simplicity, the pyridine rings are assumed to stand normal to the electrode surface and the orientation along the surface direction are assumed to be the same. The left side panel displays a top view of the hydrogen-bonded network, where in the bottom row the elongated white areas represent the 4-MPy fragment, the tetrahedral dark area is the sulfate ion, and the protons are shown as gray circles. Shown in the right panel is a side view of phase B emphasizing the hydrogen bonding between bisulfate and the pyridinium ion.

Although the previous arguments would support the existence of phase B, two additional observations indicate that another, more subtle, and not as yet identified factor plays a key role, namely, phase B is not observed for pH of about 5 (see the following) nor in solutions of a lower pH devoid of sulfate. In particular, the pKa of the 4-MPy SAM on Au(111) in NaClO4 is on the order of 4.6 at +0.2 V versus Ag/AgCl.46 On this basis and under the conditions upon which the experiments described herein were conducted, that is, pH ) 1, all pyridine rings would be protonated leading to net intermolecular repulsive interactions. One possible means of counterbalancing this effect, thereby allowing adsorbate molecules to come in close proximity, is by coordinating bisulfate ions to the positively charged pyridinium ring(s) via hydrogen bonding. Such an arrangement is depicted in Figure 4, where, for simplicity, the 4-MPy molecules have been drawn with their planes normal to the Au(111) surface. Although somewhat speculative, such electrostatically bound bisulfate can in turn form hydrogen-bonded networks (as depicted in the figure), stabilizing even further the proposed arrangement. The fact that coadsorbed bisulfate is not observed in the STM images may not be too surprising. Experiments involving in situ IR spectroscopy might help clarify the role of bisulfate adsorption in the phase transition. A potential shift in the positive direction leads to the formation of phase A. One conceivable pathway for this transition involves the specific adsorption of (bi)sulfate on bare Au(111), which becomes increasingly available as the transition from phase B to phase A ensues, at least for the (10 × x3) superstructure. As seen in the respective STM images (Figure 2a), the (10 × x3) structure of phase A supplies a substantial number of bare Au(111) sites for sulfate to adsorb. Although subject to some uncertainties, this phenomenon could well account for the peak observed in the cyclic voltammogram in sulfuric acid (solid curve, Figure 1), which is not found in perchloric acid (dotted line in Figure 1). 3.3. Additional Voltammetric and Structural Aspects of the SAM in Sodium Sulfate. The voltammetric curves obtained for the 4-MPy SAM on Au(111) in 0.25 M (46) Bryant, M. A.; Crooks, R. M. Langmuir 1993, 9, 385.

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Figure 5. Voltammetric curve for the 4-MPy SAM on Au(111) in 0.25 M Na2SO4 showing a small cathodic peak B at -0.03 V and its anodic counterpart B′ at +0.02 V. Estart ) -0.2 V (see text for details).

Figure 6. (a) STM image of 4-MPy Au(111) recorded in 0.1 M Na2SO4 (pH 5) at +0.35 V (tmod ) 5 min), showing numerous islands of one gold step high. (b) STM image obtained upon subsequent electrode polarization at -0.05 V versus SCE. The corresponding cross sections show the height of the islands compared to a step height.

Na2SO4 (see Figure 5) revealed a small cathodic peak (B at -0.03 V) and its anodic counterpart (B′ at +0.02 V) on a background current equivalent to a double layer capacity of about 52 ( 5 µF cm-2 (at +0.20 V). Unlike the results obtained in sulfuric acid, the voltammetric feature in this case was not related to a phase transition. In particular, STM images of 4-MPy Au(111), immersed at +0.35 V into 0.1 M Na2SO4 (pH ca. 5), were characterized by the presence of numerous islands of only one gold step high (Figure 6a and line scans in this figure) covering an area of about 30%. Images obtained after a potential jump from +0.35 to -0.05 V yielded, after about 10 min, islands of a much smaller characteristic diameter at much lower densities, that is, 8% (Figure 6b), for which the height was twice as large as that observed for a freshly made SAM immersed at +0.35 V. This process is not reversible, that is, increasing the potential from -0.05 to +0.35 V does not lead to changes in the surface. Also clearly seen in the images (see Figure 7) are small areas of phase A with precisely the same metrical parameters as those for the same phase in sulfuric acid, that is, 1.52 ( 0.15 nm, and an intermolecular distance within the rows of 0.47 ( 0.05 nm consistent with a (5 × x3) superstructure (see inset of Figure 7). These images persisted down to the onset of reductive desorption. As indicated therein, the domains become rather small with regions in which the layer does not appear to be well ordered.

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Figure 7. STM images of a tmod ) 5 min 4-MPy SAM on Au(111) in 0.25 M Na2SO4 showing phase A, that is, a (5 × x3) superstructure.

Figure 8. First (solid line) and second (dashed line) cyclic voltammetric scan for a 4-MPy-Au(111) SAM in 0.1 M H2SO4 over the range 0.0-1.4 V versus SCE; (dashed-dotted line) recorded in 0.25 M Na2SO4.

3.4. Oxidation of the SAM. The first (solid line) and second (dashed line) cyclic voltammetric scans obtained for a 4-MPy-Au(111) SAM over the range 0.0-1.4 V in sulfuric acid are shown in Figure 8. Coulometric analysis of these curves indicates a much larger charge under the oxidation peak C′ for the first compared to the second cycle. This excess charge, that is, corrected by the oxidation of the Au substrate, amounts to 480 ( 50 µC cm-2. Because the curve obtained during the second cycle appears in good agreement with that reported in the literature47 for bare Au(111) in this electrolyte, such a single oxidative scan leads to the complete desorption of 4-MPy from the surface. Further insight into some aspects of the oxidation was gained from STM images obtained in 0.1 M Na2SO4. According to voltammetric measurements performed in 0.25 M Na2SO4, the onset of decomposition occurs at about 0.8 V. Despite this information, in situ STM images indicate that the film begins to decompose at potentials as negative as +0.45 V. This is clearly shown in Figure 9a by the doubling of the periodicity of the stripes as a result of partial desorption, as originally suggested by Osawa et al.,37 and by the partial disappearance of ordered domains (Figure 9b). At even more positive potentials, that is, +0.55 V, the ordered domains nearly totally disappear (Figure 9c). At +1.05 V, which corresponds to the peak potential observed in the cyclic voltammogram (Figure 8), both the film and the gold surface become oxidized (Figure 9d), as indicated by the appearance of small islands and pits. Finally, when the potential is further increased, the surface completely roughens (Figure 9e). Subsequent reduction at +0.35 V (Figure 9f) leads to the appearance of pits and islands, which are typical of (47) Hamelin, A. J. Electroanal. Chem. 1996, 407, 1.

Figure 9. Series of STM images recorded at the specified potentials for a tmod ) 5 min 4-MPy SAM on Au(111) in 0.1 M Na2SO4 (see text).

the behavior of bare gold under this potential protocol.48 This sequence shows the destruction of the surface at high positive potentials, which is also typical for acidic solutions, and demonstrates that the SAM is removed from the surface under those conditions. 4. Conclusions A new reversible peak at about +0.35 V has been identified in the cyclic voltammetry of SAMs of 4-MPy on Au(111) surfaces in 0.1 M H2SO4 solutions. In situ STM images obtained at +0.15 and +0.40 V versus SCE revealed the presence of two distinctly different superstructures, denoted as phases A and B. Analysis of these images was consistent with a (x3 × 1) superstructure for phase B and (5 × x3) and (10 × x3) superstructures for phase A. Although phase A is also observed in perchloric acid and mildly acidic sodium sulfate, phase B is only found in 0.1 M H2SO4 solutions. A model has been proposed to explain this unique, dense superstructure, which invokes forma-tion of a hydrogen-bond network of bisulfate ions coordinated in turn to the pyridinium moieties in 4-MPy via hydrogen bonding. This arrangement allows for the rings to come in close proximity, aided by intermolecular π-π ring interactions, yielding local (48) Kolb, D. M.; Dakkouri, A. S.; Batina, N. In Nanoscale Probes of the Solid/Liquid Interface; Gewirth, A. A., Siegenthaler, H., Eds.; Nato ASI Series, Kluwer: Dordrecht, 1995; Vol. E 288, p 263.

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coverages of about 0.5 (normalized to the gold surface), that is, much larger than the 0.2 value associated with phase A. Acknowledgment. T.B. and V.I. gratefully acknowledge a stipend from the Deutsche Forschungsgemeinschaft through Graduiertenkolleg 328 (Molecular Organization

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and Dynamics at Interfaces) and Sonderforschungsbereich 569 (Hierarchic Structure Formation and Function of Organic-Inorganic Nano Systems). D.S. gratefully acknowledges a research award from the Alexander-vonHumboldt Stiftung. LA035389T