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Potential-Induced Structure Changes of Oligopyridine Adlayers on Au(111) Electrodes Yangguang Dai, Christoph Meier, Ulrich Ziener,*,† Katharina Landfester, Corina Ta¨ubert, and Dieter M. Kolb*,‡ Institute of Organic Chemistry III, UniVersity of Ulm, 89069 Ulm, Germany, and Institute of Electrochemistry, UniVersity of Ulm, 89069 Ulm, Germany ReceiVed May 20, 2007. In Final Form: July 26, 2007 The structure of a bisterpyridine-like oligopyridine (abbreviated as 2,4′-BTP) monolayer on Au(111), adsorbed from an acetone solution, was studied by in situ scanning tunneling microscopy and cyclic voltammetry in aqueous 0.1 M H2SO4. Short-range ordered adsorption with an average distance between the individual molecules of about 2 nm was observed only at electrode potentials positive of +0.4 V vs SCE, whereas at more negative potentials, no order could be found. With the help of Cu underpotential deposition, a potential-induced, fast, and fully reversible structure transition within the organic monolayer was identified at about +0.4 V vs SCE. At negative potentials the molecules apparently cluster together and consequently current-potential curves resemble those for a bare gold surface, whereas for E > +0.4 V vs SCE the molecules are spread over the entire surface in a hexagonal, close-packed fashion. This may have interesting consequences for switching between different template structures.
Introduction Self-assembly of organic molecules on structurally well-defined metal substrates is a crucial process for the bottom-up approach toward functional nanodevices. The two-dimensional ordering is based on the information stored in the molecular structures, but can be influenced by an external stimulus like the electric potential. Organothiols and dithiols are among the best investigated compounds, which are employed for self-assembly processes on metals because of the high affinity of sulfur to metal surfaces.1-6 In a further step, organic adlayers can be used as templates to deposit metal clusters electrochemically. The deposition of metals on SAM-covered surfaces is strongly influenced by the structure and the chemical nature of the SAM. Metals like Ag or Cu have been deposited onto Au electrodes covered with alkanethiols,7,8 or mercaptopropionic acid,9 and Te onto monolayers of thiolated β-cyclodextrin on Au.10 Only little research has focused on SAMs of non-sulfur-containing organic molecules in an electrochemical environment. Such monolayers were, e.g., Schiff bases,11 uracil,12,13 cytosine,14 or pyridine and * To whom correspondence should be addressed. (U.Z.) E-mail:
[email protected]. Tel: +49 731 5022884. Fax: +49 731 5022883. (D.M.K.) E-mail:
[email protected]. Tel: +49 731 5025400. Fax: +49 731 5025409. † Institute of Organic Chemistry III. ‡ Institute of Electrochemistry. (1) Ulman, A. Chem. ReV. 1996, 96, 1533. (2) Poirier, G. E. Chem. ReV. 1997, 97, 1117. (3) Hagenstro¨m, H.; Schneeweiss, M. A.; Kolb, D. M. Langmuir 1999, 10, 2435. (4) Schreiber, F. Prog. Surf. Sci. 2000, 65, 151. (5) Schweizer, M.; Hagenstro¨m, H.; Kolb, D. M. Surf. Sci. 2001, 490, L627. (6) Esplandiu, M. J.; Hagenstro¨m, H.; Kolb, D. M. Langmuir 2001, 17, 828. (7) Schneeweiss, M. A.; Hagenstro¨m, H.; Esplandiu, M. J.; Kolb, D. M. Appl. Phys. A 1999, 69, 537. (8) Kolb, D. M.; Engelmann, G. E.; Ziegler, J. C. Solid State Ionics 2000, 131, 69. (9) Petri, M.; Kolb, D. M.; Memmert, U.; Meyer, H. Electrochim. Acta 2003, 49, 183. (10) Woo, D. H.; Choi, S. J.; Han, D. H.; Kang, H.; Park, S. M. Phys. Chem. Chem. Phys. 2001, 3, 3382. (11) Kong, D. S.; Yu, Z. Y.; Yuan, S. L. J. Solid State Electrochem. 2005, 9, 174. (12) Ho¨lzle, M. H.; Wandlowski, T.; Kolb, D. M. J. Electroanal. Chem. 1995, 394, 271. (13) Ho¨lzle, M. H.; Wandlowski, T.; Kolb, D. M. Surf. Sci. 1995, 335, 281.
2,2′-bipyridine.15,16 The Schiff base molecules adsorb on Au(111) with a flat-lying orientation and form an adlayer with well-defined order.11 2,2′-Bipyridine offers a conformatively higher flexibility than the monocyclic nucleobases. In solution, the transoid conformation N-C-C-N is preferred, whereas on Au(111) at positive charge densities, it forms an ordered monolayer of molecular chains, determined by the coordination of the lone electron pairs of both ring nitrogen atoms with the substrate surface and lateral π-stacking interactions.15,16 At negative charge densities under acidic conditions 2,2′-bipyridine is monoprotonated and adopts the transoid conformation with the protonated nitrogen atom pointing to the gold surface and the other to the solution.16 In previous works, we could show that the bisterpyridine-like 2,4′-BTP (see Figure 1) forms highly ordered monolayers at the organic solution/highly oriented pyrolytic graphite (HOPG) interface17,18 as well as on HOPG under ultrahigh vacuum (UHV) conditions.19 Similar ordering is found on various metal substrates like Au(111) under UHV conditions. The driving force for the self-assembly is dominated by weak intermolecular hydrogen bonds C-H‚‚‚N with hardly any influence of the substrate on the arrangement. Self-assembly on HOPG from organic solution shows solvent-induced morphological changes. Besides the high tendency toward 2D ordering, these molecules offer the possibility of strong metal complexation acting as bistridentate ligands in solution for octahedrally binding metal ions like Co2+, Fe2+, or Zn2+.17,20 This affinity to metals was already proven for the 2D case where Cu was deposited onto a monolayer of 2,4′-BTP on HOPG under UHV conditions forming a metal-organic network.19 (14) Wandlowski, T.; Lampner, D.; Lindsay, S. M. J. Electroanal. Chem. 1996, 404, 215. (15) Cunha, F.; Tao, N. J. Phys. ReV. Lett. 1995, 75, 2376. (16) Dretschkow, T.; Lampner, D.; Wandlowski, T. J. Electroanal. Chem. 1998, 458, 121. (17) Ziener, U.; Lehn, J.-M.; Mourran, A.; Mo¨ller, M. Chem. Eur. J. 2002, 8, 951. (18) Meier, C.; Ziener, U.; Landfester, K.; Weihrich, P. J. Phys. Chem. B 2005, 109, 21015. (19) Breitruck, A.; Hoster, H. E.; Meier, C.; Ziener, U.; Behm, R. J. Surf. Sci. 2007, in press. (20) Ruben, M.; Ziener, U.; Lehn, J.-M.; Ksenofontov, V.; Gu¨tlich, P.; Vaughan, G. B. M. Chem. Eur. J. 2005, 11, 94.
10.1021/la701479r CCC: $37.00 © 2007 American Chemical Society Published on Web 09/19/2007
Structure Changes of Oligopyridine Adlayers
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Figure 1. Structure formula of the bisterpyridine-like oligopyridine 2,4′-BTP in the energetically preferred N,N-transoid conformation of adjacent heteroaromatic rings.
Here, we report on the self-assembly process of 2,4′-BTP on Au(111) under potential control in acidic aqueous medium and on potential-induced structure changes in the adlayer. Furthermore, copper deposition in the underpotential region was employed to monitor the state of oligopyridine adsorption, because this deposition reaction is particularly sensitive to the chemical state of Au(111). Experimental Section The Au(111) electrodes were single-crystal discs (MaTeck, Juelich, Germany), 4 mm in diameter and in height for electrochemical experiments and 12 mm in diameter and 2 mm thick for use in the STM. The electrodes were flame-annealed before each experiment according to standard procedures in our laboratory,21 which led to clean and well-ordered surfaces. The oligopyridine monolayer was prepared by immersing the freshly annealed gold crystal into a saturated solution of 2,4′-BTP in acetone for a certain length of time (ranging from minutes to overnight). After removal from the modification solution, the electrode was thoroughly rinsed with deionized water and transferred to the electrochemical cell. We mention in passing that the very same modification procedure was also performed with 1,2,4trichlorobenzene as solvent, which led to practically identical results in cyclic voltammetry. Therefore, all results reported in the following refer to acetone as modification solvent (Merck, >99%). The oligopyridine 2,4′-BTP was prepared inhouse.17 All solutions were made from H2SO4 (Merck, suprapure), CuSO4 (Merck, p.a.), and Milli-Q water (Millipore Corp.USA; 18 MΩcm). Cyclic voltammetry was performed in a three-compartment glass cell with standard electrochemical equipment. A saturated calomel electrode (SCE) served as reference electrode. The STM measurements were performed with a Topometrix TMX 2010 Discoverer, using tungsten tips (0.25 mm diameter wire) electrochemically etched in 2 M NaOH. The tips were coated with an electrophoretic paint to reduce the faradaic current at the tipelectrolyte interface to less than 50 pA. Pt wires were used as quasireference electrode and as counter electrode. However, all potentials are quoted with respect to the saturated calomel electrode (Pt vs SCE ) +0.55 V). All STM images were obtained in the constantcurrent mode with tunnel currents of typically 2 nA.
Results Cyclic Voltammetry in 0.1 M H2SO4. In Figure 2 are shown the cyclic current-potential curves for Au(111) in 0.1 M H2SO4 before and after modification with 2,4′-BTP. Comparison with 2,2′,6′,2′’-terpyridine shows that 2,4′-BTP as a terpyridine derivative can be assumed being protonated under acidic conditions.22,23 All of the characteristic features for a high quality surface of Au(111) in 0.1 M H2SO4 (in particular the pronounced (21) Kibler, L. A. Preparation and Characterization of Noble Metal Single Crystal Electrode Surfaces. http://www.uni-ulm.de/echem/ekat/downloadpage. html. (22) James, B. R.; Williams, R. J. P. J. Chem. Soc. 1961, 2007. (23) El-Gahami, M. A.; Ibrahim, S. A.; Fouad, D. M.; Hammam, A. M. J. Chem. Eng. Data 2003, 48, 29.
Figure 2. Cyclic current-potential curves for bare (‚‚‚) and 2,4′BTP covered (s) Au(111) in 0.1 M H2SO4. Scan rate: 5 mV s-1.
current spike at +0.78 V vs SCE) are no longer present in the cyclic voltammogram for the SAM-covered electrode. The curve is now dominated by a pair of broad peaks around +0.5 and +0.6 V vs SCE, which is typical for many SAM-covered Au(111) electrodes in sulfate solutions and which signals a structure transition within the SAM in such an electrolyte.24 Not surprisingly, these peaks are conspicuously absent in the currentpotential curve for the very same electrode in 0.1 M HClO4 (not shown). It may be interesting to note that the cyclic voltammogram for E < +0.4 V vs SCE has a close resemblance with the curve for the bare, but unreconstructed Au(111) electrode in 0.1 M H2SO4,25 i.e., the surface appears to be SAM-free, but the potential-induced surface reconstruction is blocked. In addition, the double-layer charging of the gold electrode at negative potentials signals an adsorbate-free surface that is roughly 20% smaller than the total gold surface. We shall come back to this observation later. The curve for the SAM-covered electrode in Figure 2 also shows that the adlayer is stable between -0.3 to +0.9 V vs SCE, where the electrode potential can be cycled many times without any significant change in shape or magnitude of the voltammogram. In Situ STM Measurements in 0.1 M H2SO4. 2,4′-BTPcovered Au(111) electrodes were imaged in 0.1 M H2SO4 at various potentials after the gold crystals had been modified in acetone for about 14-17 h. Although at potentials negative of +0.4 V vs SCE no sign of the molecules on the surface could be detected in STM, the adlayer was clearly imaged with molecular resolution for E > +0.4 V vs SCE. This is shown in Figure 3a, where the individual molecules are seen as bright dots, resembling a close packed structure, albeit without much long-range order. From a Fourier analysis of such an image (Figure 3b), an average intermolecular distance of 2.0 ( 0.2 nm was inferred, which is in good agreement with 2,4′-BTP nextneighbor distances on HOPG in 1,2,4-trichlorobenzene and in 1-phenyloctane.18 A most surprising result is shown in Figure 4, where the structure change upon stepping the electrode potential between +0.25 and +0.45 V vs SCE is imaged while scanning the surface. The transition is almost instantaneous and fully reversible. A characteristic feature of the structure at +0.25 V compared to +0.45 V vs SCE is an apparently higher mobility of the adsorbed (24) Baunach, T.; Ivanova, V.; Scherson, D. A.; Kolb, D. M. Langmuir 2004, 20, 2797. (25) Kibler, L. A.; Kleinert, M.; Kolb, D. M. J. Electroanal. Chem. 1999, 467, 249.
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Figure 4. STM image of 2,4′-BTP covered Au(111) in 0.1 M H2SO4. The electrode potential was stepped from +0.25 to +0.45 V vs SCE and back (see arrows).
Figure 5. Cyclic current-potential curves for bare (‚‚‚) and 2,4′BTP covered (s) Au(111) in 0.1 M H2SO4 + 0.5 mM CuSO4. The solid curve represents first and second cycle. Scan rate: 10 mV s-1. Figure 3. (a) STM image of 2,4′-BTP covered Au(111) in 0.1 M H2SO4 at E ) +0.55 V vs SCE. (b) Fourier analysis of STM image (a) yielding r ) 2.0 ( 0.2 nm.
molecules, expressed in the more diffuse contrast in the +0.25 V image parts in Figure 4. The origin of the increase in mobility refers to a decrease in molecule-substrate interactions with a more negative electrode potential. A similar behavior was reported for a mixed CuOEP/CoPc adlayer on Au(111) in HClO4.26 For this system, a potential switch from +0.85 to +0.35 V vs RHE resulted in a nonreversible phase separation of the molecular components at +0.35 V. This phase separation of the mixed adlayer was attributed to a higher mobility of the adsorbed molecules at a more negative electrode potential. Whereas the state of the SAM at positive potentials as a densely packed adlayer seems reasonable, the results so far yield no information about the adlayer at potentials negative of +0.4 V vs SCE. Although apparently invisible for the STM, the 2,4′BTP molecules must be on the surface, as they cannot escape into the aqueous solution. It is suggested (i) that they are too mobile to be imaged by STM due to a less pronounced adherence (26) Yoshimoto, S.; Higa, N.; Itaya, K. J. Am. Chem. Soc. 2004, 126, 8540.
of the positively charged (protonated) molecules on the SO42adlayer or (ii) that they cluster in islands which are found only occasionally by STM (see below). In the following, we describe the use of Cu underpotential deposition, i.e., the formation of a Cu monolayer which requires direct contact to gold sites, as a means to shed light on the structure of the SAM-covered Au(111). Cu Deposition onto a Bis(terpyridine)-Covered Au(111) Surface. The formation of the first Cu monolayer on Au(111) in sulfuric acid solutions at potentials positive of the Nernst potential for Cu/Cu2+ (in the so-called underpotential region) is among the best studied reactions in electrochemical surface science.27 The cyclic voltammogram (dashed curve in Figure 5) reveals that monolayer formation occurs in two energetically well-separated steps: formation of a (x3 × x3) R30° structure at θCu ) 2/3 (honeycomb structure of the Cu adlayer with sulfate nested in the honeycombs) around +0.2 V vs SCE, and formation of a full, pseudomorphic Cu layer around 0 V vs SCE. This two-peak structure is a fingerprint of Cu deposition onto bare (27) Kolb, D. M. In AdVances in Electrochemical Science and Engineering; Alkire, R. C., Kolb, D. M., Eds.; Wiley-VCH: Weinheim, Germany, 2002; Vol. 7, pp 107.
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Figure 7. STM image (20 × 20 nm2) of 2,4′-BTP covered Au(111) in 0.1 M H2SO4 + 0.01 mM CuSO4 at E ) +0.10 V vs SCE. The image shows the well-known (x3 × x3) R30° structure of Cu on Au(111) and about 0.2 nm high islands.
Figure 6. (a) STM image (50 × 50 nm2) of 2,4′-BTP covered Au(111) in 0.1 M H2SO4 + 0.01 mM CuSO4 at E ) +0.45 V vs SCE. (b) Fourier analysis of the STM image, yielding r ) 1.9 ( 0.1 nm.
Au(111). The corresponding curve for Cu underpotential deposition onto a 2,4′-BTP-modified Au(111) electrode is shown in Figure 5 (solid curve). As is frequently observed for SAM-covered gold surfaces,28 Cu deposition is initially hindered during the potential scan in the negative direction, in the case of a 2,4′-BTP adlayer down to about +0.15 V vs SCE, where a cathodic current reflects the onset of the deposition process. However, quite surprisingly, on the positive potential sweep, the current response unequivocally signals the presence of an essentially SAM-free gold surface. The charge under the anodic current peaks in the cyclic voltammograms (Figure 5) is only 5% smaller for the 2,4′-BTP-covered Au(111) than for the bare surface, indicating that roughly 95% of the total electrode surface must be devoid of organic material. Another interesting observation is identical current-potential curves for first and second cycle, indicating an absolutely reversible structural rearrangement with the 2,4′(28) Hagenstro¨m, H.; Schneeweiss, M. A.; Kolb, D. M. Electrochim. Acta 1999, 45, 1141.
Figure 8. STM image (50 × 50 nm2) of 2,4′-BTP covered Au(111) in 0.1 M H2SO4 + 0.01 mM CuSO4, while stepping the electrode potential from +0.15 to +0.45 V vs SCE.
BTP-layer. The same conclusions are reached on the basis of in-situ STM measurements, as is shown in the following. Figure 6a shows an STM image of the 2,4′-BTP-modified Au(111) electrode in 0.1 M H2SO4 + 0.01 mM CuSO4 at +0.45 V vs SCE. Apparently, this is about the same structure as the one observed in a Cu2+-free solution at +0.45 V (Figure 4), with a next-neighbor distance of approximately 1.9 nm as derived from a Fourier analysis (Figure 6b). However, when stepping the electrode potential to +0.1 V, i.e., into the onset of Cu underpotential deposition, this structure disappears almost instantaneously. The result is shown in Figure 7: For most of the surface the well-known (x3 × x3) R30° structure, typical for Cu upd on bare (i.e., SAM-free) Au(111) is seen, with islands of about 0.2 nm in height, often arranged in a domain-wall like fashion and covering roughly 10% of the total surface. The reversible structure transition is demonstrated once more in Figure 8, where the potential was stepped from +0.15 to +0.45 V vs SCE during a surface scan. While at +0.15 V islands
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assigned to clustered 2,4′-BTP and the (x3 × x3) R30° structure for Cu on “bare” gold can be seen, the densely packed adlayer structure appears immediately at +0.45 V, uniformly spread over the whole surface. Not unexpectedly, the oligopyridine islands disappear instantaneously, as they provide the material for the adlayer.
Discussion The experimental results provide evidence for a potentialdriven reversible structure transition for 2,4′-BTP on Au(111) in acidic solution, where the molecules are protonated (see above). Whereas these molecules, albeit unprotonated, form a highly ordered superstructure at the HOPG/solution interface,17,18 we observe a hexagonal densely packed arrangement without much long-range order in 0.1 M H2SO4 only at electrode potentials positive of +0.4 V vs SCE. The order on HOPG is mainly based on intermolecular weak C-H‚‚‚N hydrogen bonds17,18 which are suppressed by protonation under acidic conditions. Thus the missing long-range order is not too surprising at the Au(111)/ H2SO4 interface. At more negative potentials, the individual molecules are no longer detectable in STM images presumably due to a higher molecule mobility and cyclic voltammetry in essence signals the presence of an adsorbate-free gold surface. This transition appears to be fast and fully reversible. Both voltammograms, with and without Cu2+ in solution, show the characteristics of “bare” Au(111) for E < +0.4V vs SCE, although with a surface reduced by some 5-20%. Since the molecules upon the structure transition will still be on the surface, the question of the adlayer structure in that potential region arises. We are led to conclude on the basis of voltammetry that the 0.2 nm high islands which are seen in STM images at E < +0.4 V vs SCE covering roughly 10% of the total surface, represent
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clustered organic material. Conceivably the molecules are in an upright position,16 thus allowing most of the gold surface to be free of 2,4′-BTP. Note that the total amount of organic molecules in the SAM for E > +0.4V vs SCE corresponds to only a few percent of a full monolayer, the latter being defined by the number of gold atoms in the surface layer. It also appears from cyclic voltammetry that there is a significant interaction between the Cu2+ ions in solution and the 2,4′-BTP molecules, which cause the molecules clustering at negative potentials to be slightly slower than in pure 0.1 M H2SO4. This would explain the rather asymmetric form of cathodic and anodic branches of the currentpotential curve (Figure 5) in contrast to the symmetric shape in 0.1 M H2SO4 (Figure 2). However, a chemical interaction between Cu2+ and 2,4′-BTP does not seem unreasonable. In conclusion, the potential-driven structure change in the bis(terpyridine)-like 2,4′-BTP SAM on Au(111) in 0.1 M H2SO4, which is between flat-lying molecules spread over the whole surface and upright standing molecules clustered together, may have interesting consequences for a potential-controlled and reversibly switchable template structure: For E > +0.4 V vs SCE, the electrode surface is SAM covered, i.e., organic in nature, and for E < +0.4 V vs SCE, it is in essence that of bare gold. Acknowledgment. This work was supported by the Deutsche Forschungsgemeinschaft through SFB 569. Note Added after ASAP Publication. This article was released ASAP on September 19, 2007. Author name Corinna Ta¨ubert was corrected to Corina Ta¨ubert, and the article was reposted on October 8, 2007. LA701479R
