Functionalized Self-Assembled Alkanethiol ... - ACS Publications

Langmuir 2001, 17, 839-848

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Functionalized Self-Assembled Alkanethiol Monolayers on Au(111) Electrodes: 2. Silver Electrodeposition H. Hagenstro¨m, M. J. Esplandiu´,* and D. M. Kolb* Department of Electrochemistry, University of Ulm, 89069 Ulm, Germany Received August 7, 2000. In Final Form: November 7, 2000 We have studied the early stages of Ag electrodeposition from dilute sulfuric acid solutions on modified Au(111) electrodes with cyclic voltammetry and in situ scanning tunneling microscopy. Several differently functionalized short-, medium-, and long-chain alkanethiols were used for electrode modification. Ag underpotential deposition onto such surfaces is hindered depending on chain length, but the differences between self-assembled monolayers with different endgroups were less significant. Two Ag layers are deposited at the thiol/Au(111) interface in an island growth mechanism at underpotentials. Bulk deposition originates almost exclusively at step edges burying the underlying self-assembled monolayer. Oxidative desorption of the organic film is promoted in contact with a Ag+-containing electrolyte as compared to the metal ion free solution.

Introduction Self-assembled monolayers (SAMs) bear unique properties, as they are easy to prepare, are highly ordered, and offer possibilities of tailoring their chemical and electronic properties. Alkanethiol monolayers, self-assembled on Au(111), represent a model system for the study of this kind of organic-inorganic interface which has been characterized in detail (for reviews, see, for example, refs 1-3). However, in situ structure investigations by scanning probe microscopy at electrode surfaces are scarce, as compared to the work done in ultrahigh vacuum or under ambient conditions. So far, the stability range of SAMs on Au(111) electrodes was confirmed by in situ scanning probe microscopy,4,5 and the process of self-assembly was followed.6-10 Very early, opportunities to create functional surfaces with nanometer thickness were recognized,11 and to date several examples have been given for technological SAMbased applications. Obviously, an alkanethiol coating may inhibit electrode corrosion,12-15 but one could also take advantage of the low dielectric constant of alkanes to create * Corresponding authors. E-mail: [email protected] (M. J. Esplandiu´); [email protected] (D. M. Kolb). (1) Poirier, G. E. Chem. Rev. 1997, 97, 1117. (2) Thin Films: Self-assembled Monolayers of Thiols; Ulman, A., Ed.; Academic Press: San Diego, 1998; Vol. 24. (3) Finklea, H. O. In Electroanalytical Chemistry: a Series of Advances; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol. 19, p 110. (4) Pan, J.; Tao, N.; Lindsay, S. M. Langmuir 1993, 9, 1556. (5) Hobara, D.; Miyake, K.; Imabayashi, S.; Niki, K.; Kakiuchi, T. Langmuir 1998, 14, 3590. (6) Yamada, R.; Uosaki, K. Langmuir 1997, 13, 5218. (7) Xu, S.; Laibinis, P. E.; Liu, G. J. Am. Chem. Soc. 1998, 120, 9356. (8) Dakkouri, A. S.; Kolb, D. M.; Edelstein-Shima, R.; Mandler, D. Langmuir 1996, 12, 2849. (9) Jin, Q.; Rodriguez, J. A.; Lin, C. Z.; Darici, Y.; Tao, N. J. Surf. Sci. 1999, 425, 101. (10) Azzaroni, O.; Andreasen, G.; Blum, B.; Salvarezza, R. C.; Arvia, A. J. J. Phys. Chem. B 2000, 104, 1395. (11) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murray, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932. (12) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682. (13) Scherer, J.; Vogt, M. R.; Magnussen, O. M.; Behm, R. J. Langmuir 1997, 13, 7045. (14) Kim, T.; Chan, K. C.; Crooks, R. M. J. Am. Chem. Soc. 1997, 119, 189. (15) Jennings, G. K.; Munro, J. C.; Laibinis, P. E. Adv. Mater. 1999, 11, 1000.

ultrathin capacitors16 or insulating layers in the design of nanometer-scale electronic devices.17,18 Other fields of interest are the electrical19,20 and optical properties21 of molecules containing aromatic units, which may advance efforts to develop organic electronics.22 A promising approach is selective recognition from solution via specifically modified surfaces. Host-guest interactions of functionalized SAMs were shown to operate as sensors for the detection of ionic,23-26 molecular,27 and biomolecular species.28,29 Metal-organic interfaces and metal-organic-metal sandwich structures obviously have a considerable bearing on applications such as sensing or molecular electronics. Whereas the preparation of metal-organic interfaces (such as SAMs) is straightforward, for molecular electronic devices it is essential to create a metal contact to the endgroup of the SAM. This requirement is more difficult to meet, and three different approaches have been reported in the literature. Metal was deposited onto functionalized alkanethiol SAMs from the vapor phase,30-32 but in many cases the (16) Rampi, M. A.; Schueller, O. J. A.; Whitesides, G. M. Appl. Phys. Lett. 1998, 72, 1781. (17) Collet, J.; Vuillaume, D. Appl. Phys. Lett. 1998, 73, 2681. (18) Okur, S.; Zasadzinski, J. F. J. Appl. Phys. 1999, 85, 7256. (19) Bumm, L. A.; Arnold, J. J.; Cygan, M. T.; Dunbar, T. D.; Burgin, T. P.; Jones, L., III; Allara, D. L.; Tour, J. M.; Weiss, P. S. Science 1996, 271, 1705. (20) Datta, S.; Tian, W.; Hong, S.; Reifenberger, R.; Henderson, J. I.; Kubiak, C. P. Phys. Rev. Lett. 1997, 79, 2530. (21) Dhirani, A.; Lin, P.-H.; Guyot-Sionnest, P.; Zehner, R. W.; Sita, L. R. J. Chem. Phys. 1997, 106, 5249. (22) Chen, J.; Reed, M. A.; Rawlett, A. M.; Tour, J. M. Science 1999, 286, 1550. (23) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988, 332, 426. (24) Turyan, I.; Mandler, D. Anal. Chem. 1996, 69, 894. (25) Moore, A. J.; Goldenberg, L. M.; Ryce, M. R.; Petty, M. C.; Monkman, A. P.; Marenco, C.; Yarwood, J.; Joyce, M. J.; Port, S. N. Adv. Mater. 1998, 10, 395. (26) Flink, S.; Boukamp, B. A.; van den Berg, A.; van Veggel, F. C. J. M.; Reinhoudt, D. N. J. Am. Chem. Soc. 1998, 120, 4652. (27) Weiss, T.; Schierbaum, K. D.; Thoden van Velzen, U.; Reinhoudt, D. N.; Go¨pel, W. Sens. Actuators, B 1995, 26-27, 203. (28) Schmitt, F.-J.; Haüssling, L.; Ringsdorf, H.; Knoll, W. Thin Solid Films 1992, 210/211, 815. (29) Knichel, M.; Heiduschka, P.; Beck, W.; Jung, G.; Go¨pel, W. Sens. Actuators, B 1995, 28, 85. (30) Tarlov, M. J. Langmuir 1992, 8, 80. (31) Smith, E. L.; Alves, C. A.; Anderegg, J. W.; Porter, M. D.; Sirpenko, L. M. Langmuir 1992, 8, 2707.

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metal-endgroup interaction is weak and considerable amounts of the deposit penetrate the organic thin film, leading to short circuits.33,34 Electroless metal deposition from solution was more successful. Strong chemical interactions between an acceptor endgroup and the metal salt can lead to deposition onto the SAM, but the salt has to be chemically reduced in a second step.35-38 Deposition driven by an externally applied electrode potential, commonly referred to as electrodeposition, seems to be the most appropriate technique, because the amount of the deposit and the kinetics of the deposition (and dissolution) process can be largely controlled. Several reports on electrodeposition onto alkanthiol SAMs have been published,39-43 and in some cases in situ probe microscopy has been employed to follow the deposition process.44-48 It was consistently found that SAMmodified electrodes require considerable overpotentials for bulk metal deposition depending on chain length and molecular order. However, the initial stages of metal deposition (θ < 1 ML (monolayer)) are believed to occur at the thiol-substrate interface.40,43,49 Short chains and elevated temperatures are believed to lower the energetic barrier for ion penetration through the SAM. Currently, it is not clear whether functional endgroups influence this behavior in electrochemical environments. This should be the case, because sensing applications reveal such specific interactions (see above) and considerable differences were reported for different endgroups in vapor deposition experiments.32,33 Here, we report on the effects of chain length and functional endgroup of alkanethiol SAMs on Au(111) on electrodeposition of silver. Our group has already performed some studies of copper deposition onto ethanethiol-covered Au(111) but with the disadvantage that this system shows structural transformations leading to some disorder in the film just in the potential range where copper is deposited.48 Therefore, we have chosen silver which is deposited at more positive potentials than Cu; in addition, Ag might be a suitable candidate for a strong interaction with metallophilic thiol endgroups and, thus, for metal deposition onto SAMs. In the first part of this publication, we have characterized (32) Konstantinidis, K.; Zhang, P.; Opila, R. L.; Allara, D. L. Surf. Sci. 1995, 338, 300. (33) Herdt, G. C.; Jung, D. R.; Czanderna, A. W. Prog. Surf. Sci. 1995, 50, 103. (34) Herdt, G. C.; King, D. E.; Czanderna, A. W. Z. Phys. Chem. 1997, 202, 163. (35) Dressick, W. J.; Dulcey, C. S.; Gregor, J. H., Jr.; Calabrese, G. S.; Calvert, J. M. J. Electrochem. Soc. 1994, 141, 210. (36) Grummt, U.-W.; Geissler, M.; Schmitz-Hu¨bsch, T. Chem. Phys. Lett. 1996, 263, 581. (37) Grummt, U.-W.; Geissler, M.; Drechsler, T.; Fuchs, H.; Staub, R. Angew. Chem., Int. Ed. 1998, 37, 3286. (38) Kind, H.; Bittner, A. M.; Cavalleri, O.; Kern, K.; Greber, T. J. Phys. Chem. B 1998, 102, 7582. (39) Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. Langmuir 1995, 11, 4823. (40) Nishizawa, M.; Sunagawa, T.; Yoneyama, H. Langmuir 1997, 13, 5215. (41) Oyamatsu, D.; Nishizawa, M.; Kuwabata, S.; Yoneyama, H. Langmuir 1998, 14, 3298. (42) Oyamatsu, D.; Kuwabata, S.; Yoneyama, H. J. Electroanal. Chem. 1999, 473, 59. (43) Hagenstro¨m, H.; Schneeweiss, M. A.; Kolb, D. M. Electrochim. Acta 1999, 45, 1141. (44) Sun, L.; Crooks, R. M. J. Electrochem. Soc. 1991, 138, L23. (45) Eliadis, E. D.; Nuzzo, R. G.; Gewirth, A. A.; Alkire, R. C. J. Electrochem. Soc. 1997, 144, 96. (46) Gilbert, S. E.; Cavalleri, O.; Kern, K. J. Phys. Chem. 1996, 100, 12123. (47) Cavalleri, O.; Kind, H.; Bittner, A. M.; Kern, K. Langmuir 1998, 14, 7292. (48) Hagenstro¨m, H.; Schneeweiss, M. A.; Kolb, D. M. Langmuir 1999, 15, 7802. (49) Cavalleri, O.; Bittner, A. M.; Kind, H.; Kern, K.; Greber, T. Z. Phys. Chem. 1999, 208, 107.

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the SAM structure of various thiols on Au(111) in the base electrolyte with in situ scanning tunneling microscopy (STM) and cyclic voltammetry.50 Silver electrodeposition on SAMs of ethanethiol functionalized with hydroxy and carboxylic acid endgroups, hexanethiol, 1,6-hexanedithiol, octadecanethiol, and 17-cyanoheptadecanethiol, is reported in this part of the communication, making use of the same techniques. Experimental Section Chemicals. Ethanethiol (HSCH2CH3, abbreviated C2), Mercaptoethanol (HS(CH2)2-OH, C2OH), meraptopropionic acid (HS(CH2)2-COOH, C2COOH), hexanethiol (HS(CH2)5CH3, C6), and octadecanethiol (HS(CH2)17CH3, C18) were purchased from commercial suppliers (Aldrich, Alfa, Fluka, Merck; purum, >98%). 17-Cyanoheptadecanethiol (HS(CH2)17CN, C17CN) and 1,6-hexanedithiol (HS(CH2)5-SH, C6SH) were kindly provided by G. Go¨tz and P. Baüerle (Department Organic Chemistry II, University of Ulm). Electrolyte solutions contained 0.1 M H2SO4 and 0.2-2 mM Ag2SO4 and were prepared from H2SO4 (Merck, suprapure), Ag2SO4 (Fluka, puriss. p.a.), and Milli-Q-water (Millipore Corporation, USA). All chemicals were used as received without further purification. Sample Preparation. The gold samples for the STM studies consisted of 500 nm thick films evaporated onto a special glass (AF 45, Berliner Glas KG) with a 2 nm Cr adhesion layer or of 200 nm thick films evaporated directly onto freshly cleaved and baked out mica sheets (Plano GmbH). The samples were annealed in a hydrogen flame (Au/glass for 2 min, Au/mica for 2 s) at yellow heat to yield large, atomically flat (111) terraces.51,52 For the electrochemical experiments, the Au(111) electrode was a single-crystal disk (MaTeck, Ju¨lich) with a diameter of about 4 mm and a gold wire attached to the back for better handling. The gold crystal was annealed for 2 min in a Bunsen burner flame, cooled in air, and then brought into contact with the thiol modification solution. Sample Modification. All SAMs were prepared by immersing the annealed gold sample in a 1 mM solution of the respective thiol for 12-20 h (overnight). Absolute ethanol (Merck, extra pure) was used as a solvent for ethanethiol, hexanethiol, hexanedithiol, octadecanethiol, and cyanoheptadecanethiol and Milli-Q water was used for all functionalized ethanethiols. Special care was taken in the preparation of the dithiol SAMs, because the thiol endgroups are easily oxidized. The samples were prepared by immersion in ethanolic solution previously bubbled with argon or nitrogen to reduce exposure to ambient oxygen. The preparation was carried out in the dark in order to avoid photooxidation. Dithiol SAMs not prepared in this way showed multilayer formation during modification. Hexanethiol and hexanedithiol SAMs were prepared in the same way for better comparison. For STM measurements, the samples were then taken from the modification solution, washed with the respective solvent, dried in a stream of nitrogen, and finally introduced into the electrochemical STM cell. For the electrochemical measurements, after modification at room temperature the sample was thoroughly rinsed with Milli-Q water and transferred to the electrochemical cell. The modified crystal was then brought into contact with the deaerated electrolyte under potential control. Cyclic Voltammetry. The cyclic voltammograms were obtained with standard electrochemical equipment and a conventional electrochemical cell with separate compartments for the reference and counter electrodes. All potentials are quoted versus the saturated calomel electrode (SCE), which served as the reference electrode. In Situ STM. The STM measurements were performed with a Topometrix TMX 2010 Discoverer equipped with an electrochemical cell and a bipotentiostat. Pt/Ir tips were prepared by electrochemically etching a 0.25 mm diameter wire in 3.5 M NaCN (50) Esplandiu´, M. J.; Hagenstro¨m, H.; Kolb, D. M. Langmuir 2001, 17, 828. (51) Will, T.; Dietterle, M.; Kolb, D. M. In Nanoscale Probes of the Solid/Liquid Interface; Gewirth, A. A., Siegenthaler, H., Eds.; NATO ASI Series E, Vol. 288; Kluwer: Dordrecht, 1995; p 137. (52) DeRose, J. A.; Thundat, T.; Nagahara, L. A.; Lindsay, S. M. Surf. Sci. 1991, 256, 102.

Alkanethiol Monolayers on Au(111) Electrodes

Figure 1. Cyclic voltammogram for the Ag underpotential deposition on Au(111) in 0.1 M H2SO4 + 1 mM Ag2SO4. The scan rate is 1 mV s-1, but the current density has been scaled to 10 mV s-1 for better comparison with the other CVs. Dotted vertical lines mark the Nernst potential at 0.38 V and the most positive UPD peak at 0.91 V vs SCE. and subsequently coating them with electrodeposition paint to minimize faradaic currents at the tip/electrolyte interface. Two types of reference electrodes were used for the STM measurements; either a silver wire (Go¨tze Berlin, >99.9%), which is a convenient, low-noise reference electrode in Ag+-containing electrolytes, or a homemade microreference electrode (Hg/HgSO4, EHg/HgSO4 ) +0.41 V vs ESCE) was used. Because varying Ag+ concentrations were employed, all potentials in the STM images will be quoted with respect to the Ag/Ag+ Nernst potential (EAg/Ag+ ) 0.381 V vs SCE at a Ag+ concentration of 1 mM). All STM images were recorded in the constant-current mode and are displayed as top views with different shades of gray representing different heights (dark areas indicating low parts and light areas indicating high parts of the surface).

Results Silver Deposition on Bare Au(111): Cyclic Voltammetry. We recently reported on Ag underpotential deposition (UPD) on bare Au(111).53 The cyclic voltammogram (CV) of silver UPD on Au(111) in a mixture of 0.1 M H2SO4 and 1 mM Ag2SO4 is shown in Figure 1. The two vertical dashed lines mark the Nernst potential at 0.38 V versus SCE and the onset of Ag UPD around 0.92 V versus SCE. These lines are preserved in all the following CVs for comparison. Approximately two monolayers of silver are deposited at underpotentials before a very fast layer-by-layer bulk growth starts with virtually no overpotential. Deposition of the first monolayer occurs between the first UPD peak and the second, minor one around +0.5 V versus SCE. Several phase transitions related to the increasing silver coverage were observed by in situ STM to proceed between these two more anodic UPD peaks. The growth of the second monolayer proceeds in a single step at the third UPD peak and is clearly separated from the silver bulk deposition region. Short-Chain SAMs: Cyclic Voltammetry. In Figure 2, we show the CVs of Ag UPD on C2-, C2OH-, and C2COOH-modified Au(111). The negative-going potential scan was started at 0.55 V versus SCE, that is, between the two major UPD peaks, because short-chain SAMs are slowly oxidized at potentials positive of 0.9 V.50,54 This makes it impossible to hold the modified electrode at potentials, where no Ag is adsorbed, for more than a few (53) Esplandiu´, M.-J.; Schneeweiss, M. A.; Kolb, D. M. Phys. Chem. Chem. Phys. 1999, 1, 4847. (54) Hagenstro¨m, H.; Schneeweiss, M. A.; Kolb, D. M. Langmuir 1999, 15, 2435.

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Figure 2. Cyclic voltammogram for Ag underpotential deposition on short-chain SAM modified Au(111) in 0.1 M H2SO4 + 0.5 mM Ag2SO4: (a) ethanethiol SAM, 10 mV s-1; (b) mercaptoethanol SAM, 1 mV s-1; (c) mercaptopropionic acid SAM, 1 mV s-1. All current densities are scaled to a scan rate of 10 mV s-1.

seconds without irreversibly damaging the SAM. The CVs are significantly changed as compared to that obtained on bare gold, but in all cases the main features for Ag deposition are still seen. The peak current densities are reduced and broadened (note the current density scaling), and no separate UPD peak can be found near the Nernst potential, as was the case for the bare electrode. Instead, there is a steady increase in current directly leading to bulk deposition. However, the currents in the double-layer charging region are bigger than expected for thiol-covered surfaces and approach the values obtained for the thiolfree surface, which may be related to slow faradaic currents from Ag penetrating the SAM. The shape of the CVs returning from the anodic potential limit should be regarded with care. Some damage to the SAM can be expected in this potential range, because the following cycles (not shown here) indicate less inhibited Ag deposition. For example, the small cathodic peak in the C2OH CV around 0.62 V was not observed in the preceding cycle, which was limited to 0.8 V. Generally, only minor changes were introduced upon repeated cycling in the lower UPD range. Similar results were found for SAMs of aminoethanethiol (HS(CH2)2-NH2) and mercaptoethanesulfonic acid (HS(CH2)2-SO3H), which are not shown here for the sake of brevity. These observations also agree with results from Yoneyama and co-workers for propanethiol (C3).41,42 Altogether, it should be noted that Ag UPD is clearly affected by short-chain SAMs but considerably less than in the case of Cu UPD. Especially, no major shifts in the peak potentials were observed, as was found for Cu.48 Short-Chain SAMs: In Situ STM. First, we discuss the in situ STM results for C2, before comparing them with those for C2OH and C2COOH. Again, the starting potential for these experiments was in the Ag UPD range (around 0.3 V vs Ag/Ag+). Figure 3a shows the typical appearance of modified SAMs at these potentials. This image presents an atomically flat terrace with a step originating at a screw dislocation in the left part of the image. No molecular order could be resolved for any of the systems under study, when in contact with the silvercontaining electrolyte. Some of the characteristic substrate

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Figure 3. In situ STM images of the Ag deposition onto ethanethiol-modified Au(111) in 0.1 M H2SO4 + 1 mM Ag2SO4. IT ) 1 nA. (a) Surface structure between the two UPD peaks; (b) the small islands from (a) have grown and merged to form an almost complete layer around the Nernst potential; (c) 3D deposit at overpotentials, rapidly growing onto the SAMcovered, nucleation-inhibiting terraces. Note the different surface texture of the cluster and the substrate terraces. All potentials are given versus Ag/Ag+ in that solution.

vacancy islands55,56 can be found, but the coverage is reduced with respect to the base electrolyte.50 This finding


was the same for all the functionalized alkanethiols studied in this work. In addition, there are a number of small islands (0.2-0.3 nm high) that grow when the potential is lowered, which causes the substrate vacancy islands to disappear. In Figure 3b, the electrode was scanned slightly negative of the Ag Nernst potential (-0.02 V). The islands have merged, forming a nearly closed monolayer of ca. 0.25 nm height. However, at this stage growth on the terraces proceeds slowly and low overpotentials are required to close the monolayer. After polarization around -0.15 V for about 10 min, a large cluster in the top left part of Figure 3c is seen (the area shown in Figure 3b is located slightly below the area in Figure 3c). The image is shaded to enhance the contrast. One clearly recognizes the closed monolayer with a large number of irregular small islands and decorated step edges. The terraces of the large cluster show a different texture/corrugation than the former “substrate” terraces, and the cluster rapidly grows at more negative potentials. Additional nucleation on the substrate terraces is much slower. We therefore conclude that these two types of terraces can be assigned to differently composed surfaces. The smoother-growing cluster may arise from nucleation and growth at SAM defects that allow a very fast growth (via a mushrom-shaped deposit) with the subsequent burial of the thiols, whereas the rougher terrace of the remaining surface contains Ag buried at the Au/thiol interface. In Figure 4, a sequence of images taken at potentials positive of ENernst is shown for the ethanethiol-modified Au(111) electrode. Starting at 0.41 V versus Ag/Ag+, a similar state is found as in Figure 3a. When the potential reaches 0.56 V versus Ag/Ag+ (Figure 4b), holes emerge because of Ag dissolution, grow, and form channels until one Ag layer has dissolved (Figure 4c). The surface is left with many featureless bumps, and the terrace edges equal those at the onset of Au oxidation.57 The inset in Figure 4c shows a small ordered sulfate structure domain typical for bare Au(111), between the featureless islands, which proves the surface to be free of any further Ag adsorbate after the dissolution process presented in Figure 4a-c. The islands may be remaining oxidation products of the SAM. From the in situ STM images displayed in Figure 5, it can be inferred that mercaptopropionic acid SAMs do not significantly differ from C2 SAMs in their effect on Ag UPD. The island density in the first image is higher, but the electrode potential is also 0.1 V more negative than in Figure 3a. Islands evolve with potential in analogy to C2 (Figure 5b). Likewise, at positive potentials dissolution of one Ag layer and some residue islands are also observed (Figure 5c). Mercaptoethanol SAMs show a similar response (Figure 6) as C2 and C2COOH. The first image (Figure 6a) is recorded after polarization at low overpotentials. Here, the second UPD layer is not completely filled, but nevertheless bulk growth has started at step edges. The bulk deposit can be clearly distinguished by its smooth appearance from the small Ag islands residing on the terraces. In addition, these UPD islands are much more stable against positive potentials. In Figure 6b, the same surface area is shown at 0.07 V versus Ag/Ag+ as in the first image (Figure 6a) and the islands remain, but the bulk deposit at the steps has dissolved. At 0.59 V versus (55) Haüssling, L.; Michel, B.; Ringsdorf, H.; Rohrer, H. Angew. Chem., Int. Ed. Engl. 1991, 30, 569. (56) Scho¨nenberger, C.; Sondag-Huethorst, J. A. M.; Jorritsma, J.; Fokkink, L. G. J. Langmuir 1994, 10, 611. (57) Schneeweiss, M. A.; Kolb, D. M. Solid State Ionics 1997, 94, 171.


Figure 4. In situ STM images acquired during dissolution of UPD Ag from ethanethiol-modified Au(111) in 0.1 M H2SO4 + 2 mM Ag2SO4. IT ) 2 nA. (a) Surface structure before stripping of the first Ag UPD layer; (b) channel structure emerging during the stripping process; (c) surface structure after Ag stripping (and oxidative SAM desorption), showing oxidized step edges and remainders of the oxidized SAM. The inset shows the ordered sulfate structure between the remaining islands; scan range 8 × 8 nm2, IT ) 3 nA, E ) 0.55 V vs Ag/Ag+ () 0.95 V vs SCE).

Ag/Ag+, one layer is stripped in much the same way as for the other two short-chain SAMs. As already mentioned, mercaptoethylamine and mercaptoethanesulfonic acid

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Figure 5. In situ STM images of Ag UPD onto mercaptopropionic acid SAMs on Au(111); 0.1 M H2SO4 + 0.4 mM Ag2SO4. IT ) 1.2 nA. (a) Surface structure between the two UPD peaks; (b) growing Ag islands close to the Nernst potential; (c) channel structures emerging during Ag UPD stripping and oxidative SAM desorption. All potentials are versus Ag/Ag+.

SAMs were also studied, but the results are not shown, because no major deviations from the other short-chain SAMs were found. Medium-Chain SAMs: Cyclic Voltammetry. The first and second cycles of Ag UPD on hexanethiol- and

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Figure 7. Cyclic voltammograms for Ag UPD onto mediumchain SAM modified Au(111) in 0.1 M H2SO4 + 1 mM Ag2SO4. (a) Hexanethiol SAM, 1 mV s-1; (b) 1,6-hexanedithiol SAM, 1 mV s-1. The current density was scaled to 10 mV s-1 for better comparison with the other CVs. The first cycle started at + 0.44 V vs SCE in the negative direction.

Figure 6. In situ STM images of Ag UPD onto mercaptoethanol SAMs on Au(111); 0.1 M H2SO4 + 0.4 mM Ag2SO4. IT ) 1.4 nA. (a) Remainders of bulk deposit at the Nernst potential; (b) same area as in (a) after bulk stripping; (c) channel structures emerging during Ag UPD stripping and oxidative SAM desorption. All potentials are versus Ag/Ag+.

1,6-hexanedithiol-modified Au(111) are displayed in Figure 7. The CVs of the methyl- and mercapto-terminated SAMs are quite similar to the previous ones and on first sight do not differ greatly from their short-chain analogues. The peak current densities and double-layer capacities

are reduced at least by a factor of 2 as compared to shortchain thiols. Additionally, the cathodic scan exhibits a shift of the first UPD peak to more negative values (around 0.72 V vs SCE), which could be understood in terms of an increased kinetic hindrance of Ag UPD by the increased film thickness. The charges involved in the first UPD peak are 83 and 81 µC/cm2 for hexanethiol and hexanedithiol, respectively, which represents slightly more than onethird of the gold surface coverage. Close inspection of the double-layer current shows a much lower current in the beginning of the first cycle at 0.45 V than upon completion of the cycle at the same potential. This could be due to a slow faradaic process, which was also assumed for shortthiol-modified electrodes. Remarkably, the Ag stripping peak near 0.9 versus SCE for both medium-chain SAMs was found at more negative potentials than on bare gold surfaces. Repeated cycling in the potential range between 0.36 and 0.95 V versus SCE obviously damages the organic monolayer, which can be concluded from potential shifts of all UPD peaks toward values characteristic of bare Au(111) (Figure 7, dashed curves). For hexanethiol and hexanedithiol SAMs, the charge under the UPD stripping peak is 5-20% higher than the deposited UPD charge. Furthermore, a splitting of the deposition peak is observed in the second UPD cycle, indicating partial desorption of the SAM. Medium-Chain SAMs: In Situ STM. Figure 8 shows Ag dissolution from medium-chain SAM covered electrodes starting at -0.04 V versus Ag/Ag+ (Figure 8a-c, hexanethiol and Figure 8d-f, hexanedithiol). Between the Nernst potential and -0.04 V, the growth of twodimensional Ag islands could be monitored starting from defects, such as step edges and grain boundaries. In this potential region, the laterally growing islands could not complete the monolayer. New layers nucleate and grow on top of the UPD layer islands. The adsorbed SAM obviously hinders lateral Ag growth. This impeded layerby-layer growth finally results in a rather smooth film topography (cf. Figure 8a). The bulk deposit is dissolved at 0.2 V versus Ag/Ag+ (Figure 8b) and only small islands with heights around 0.3 nm remain on the terrace. No


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Figure 8. In situ STM images of Ag UPD onto medium-chain SAMs on Au(111); 0.1 M H2SO4 + 1 mM Ag2SO4. IT ) 1 nA. Electrode potentials vs Ag/Ag+ as indicated in the images. Top row (a)-(c) hexanethiol; bottom row (d)-(f) 1,6-hexanedithiol; (a) and (d) bulk deposition at low overpotentials; (b) and (e) surface morphology between the two UPD peaks (note the low density of vacancy islands); (c) and (f) channel structures (smaller than for short-chain SAMs) emerging during Ag UPD stripping and oxidative SAM desorption.

molecular order was resolved in the silver-containing solution in contrast to the base electrolyte environment. Striking features are (i) the very low density, if not complete absence, of vacancy islands in the whole potential range under study and (ii) the monolayer stripping at positive potentials, creating considerably smaller channel structures than with the shorter chains (see Figure 8c). Figure 8d-f presents the STM results of hexanedithiolmodified Au(111). Round clusters with heights up to 1 nm could be perceived at the first steps of bulk deposition. These clusters exhibit a narrow size distribution at low overpotentials. For higher overpotentials, the cluster shapes became blurred and a rough surface is obtained. This rough appearance is very different from the smooth one that results from silver bulk deposition on bare gold. Positive of the Nernst potential, the film topography largely resembles that of the methyl-terminated SAM (cf. parts e and b of Figure 8 taken at 0.2 V and parts f and c of Figure 8 at 0.6 V vs Ag/Ag+, respectively). Long-Chain SAMs: Cyclic Voltammetry. The longchain molecules obviously have a stronger effect on Ag electrodeposition than short and medium alkyl chains. The current densities are reduced by 2 orders of magnitude with respect to the short-chain SAMs, and only small currents are registered at very positive potentials. These films withstand cycling to very positive potentials, revealing only minor losses in blocking properties. On the other hand, the observed overpotential for bulk deposition is still small. At about 50 mV negative of the Nernst potential, the current rises into diffusion limited growth.

This is a remarkably low overpotential. It has been reported that Cu deposition can be blocked for more than 0.3 V beyond the Nernst potential by long-chain SAMs.39 Two aspects of the curves do not allow a clear-cut explanation. The first one is the rather large double-layer current, and the second is the weakly tilted background, which is even different for the two curves. Experimental artifacts cannot be entirely excluded, but both features have been consistently observed in all the experiments and one should bear in mind that the overall current density is extremely low. However, some minor variations in film quality were observed despite identical conditions during modification. Long-Chain SAMs: In Situ STM. Large, flat Ag terraces instead of isolated islands were found on the terraces of long-chain-modified electrodes in the overpotential deposition (OPD) range (Figure 10a). Already at low overpotentials layered growth was observed. In agreement with the CV results, bulk Ag growth is only weakly shifted to more negative values, although longchain SAMs are generally believed to present a significant barrier for transfer processes at electrochemical interfaces. Of course, the current densities are strongly affected by the presence of the long-chain SAM (see Figure 9), but this may not be reflected in the STM images because of the longer time scale of the STM experiment. In the anodic scan, islands remain as an intermediate state (Figure 10b). At very positive potentials, vacancy islands become visible (Figure 10c) and the surface resembles that of the Ag-free SAM on Au(111) at potentials (e.g., at 0.75 V vs Ag/Ag+

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corrugated than the remaining parts of the terrace. The deposit gets smoother when the islands coalesce near the Nernst potential (Figure 10e) and finally forms a closed monolayer (Figure 10f). At the same time, the terrace diameter has increased (parts e and f of Figure 10 display the same area), indicating that bulk deposition has started at step edges. No further nucleation on terraces was observed. The layer that evolved near the Nernst potential is easily dissolved above 0.05 V versus Ag/Ag+. Further dissolution could not be clearly imaged, but a roughening of the surface was again observed around 0.6 V (not shown). It should be noted that only one UPD layer was clearly observed to grow for both long-chain SAMs. Discussion

Figure 9. Cyclic voltammograms of Ag UPD onto long-chain SAM modified Au(111) in 0.1 M H2SO4 + 2 mM Ag2SO4: (a) octadecanethiol SAM, 1 mV s-1; (b) 17-cyanoheptadecanedithiol SAM, 10 mV s-1. Current densities were scaled to 10 mV s-1.

) 1.1 V vs SCE) where the short- and medium-chain SAMs are readily damaged.50,54 The underpotential growth of Ag on the 17-cyanoheptadecanthiol-modified electrode is displayed in Figure 10d-f. Small islands emerge on terraces below 0.2 V versus Ag/Ag+. The distribution of the islands is rather inhomogeneous, and the surroundings of the nuclei appear more

A number of questions arising from the above presented results will be discussed in this section, also in relation to results from the literature. The central questions are the following: What is the interface structure in the various potential ranges? Where is the metal deposit located with respect to the SAM? Is there an endgroup effect on electrodeposition? The discussion will be divided into three parts, referring to the three main potential regions: OPD, low UPD, and high UPD. High Underpotential Range. A process that was common to all short- and medium-chain SAMs under study was the complete stripping of one silver adlayer around 0.9 V. For the long-chain SAMs, changes are also observed in that region, but the images did not reveal a distinct

Figure 10. In situ STM images of Ag UPD onto long-chain SAMs on Au(111); 0.1 M H2SO4 + 1 mM Ag2SO4. Top row (a)-(c) octadecanethiol, IT ) 2 nA; bottom row (d)-(f) 17-cyanoheptadecanedithiol, IT ) 1 nA; (a) bulk deposition at low overpotentials; (b) surface morphology between the two UPD peaks with monatomic high Ag islands; (c) Ag-free SAM after Ag UPD stripping; (d) C17CN, surface morphology between the two UPD peaks; (e) deposition of second Ag UPD layer close to the Nernst potential; (f) completed second UPD layer at the Nernst potential.


dissolution mechanism. At first sight, it seems natural to ascribe these changes to Ag desorption, as a peak is found in the CVs in this potential region with or without a SAM. However, several points have to be considered. The short- and medium-chain SAMs on Au(111) are heavily damaged after potential excursions into this anodic potential range. CV curves and STM images (not shown) reveal that Ag deposition is hardly blocked during the second cycle as compared to the bare gold electrode. Additionally, the cathodic counter charges of the stripping peaks are smaller, which leads us to conclude that partial oxidation of the SAM is involved in all cases. However, the extent of damage scales with chain length, being smaller for the long chains. Figure 10c shows a native C18 SAM, where no Ag is adsorbed. Such a state cannot be obtained for the shorter-chain SAMs, where the film structure is severely disrupted during the dissolution of the Ag underpotential deposit. The potential range around 0.9 V versus SCE was earlier identified as a region for oxidative desorption of C2 (cf. ref 54), but the appearance of this process is vastly altered in the presence of Ag and the surface is more strongly affected during desorption. The charge under the anodic peak (∼135 µC cm-2 for C2 and ∼300 µC cm-2 for C2OH) is much smaller than the charge associated with oxidative desorption of C2 SAMs on Ag-free Au(111) (∼500 µC cm2) but larger than mere Ag desorption from Au(111) (∼100 µC cm-2). Furthermore, the structural appearance of C2 and C6 stripping is similar; the average channel width of the medium-chain SAMs is considerably smaller. This feature points toward a higher stability of the C6 and C6SH films as compared to the shorter chains. The long chains are such a good barrier that the film stays largely intact and no channel formation is observed. Although oxidative desorption of short-chain SAMs takes place at roughly the same potential with or without Ag, the C6 and C6SH SAMs desorb or aggregate in the presence of Ag around 0.9 V, whereas they are stable at that potential in the supporting electrolyte. These observations indicate that Ag is directly involved in and promotes the desorption process of the short- and mediumchain SAMs at positive potentials. We take this as evidence that UPD Ag is located between gold and the adsorbed thiolate and weakens the thiol bond. The fact that anodic desorption of the SAM is facilitated in Ag electrolytes could also explain why the dissolution peak potentials are shifted negative for all short- and medium-chain SAMs. The long-chain SAMs on the other hand seem to be able to compensate the disorder brought about by Ag desorption with intermolecular forces, which permits preservation of the monolayer integrity. The solubility of these molecules in aqeuous solution is also lower, of course. It should be noted that UPD layers of Ag and Cu were reported to enhance the stability of SAMs toward elevated temperatures58 and reductive desorption.40 Obviously, the stability of a SAM sensitively depends on the mechanism for attacking the thiolate bond. Low Underpotential Range. The conclusion that a layer of Ag is stripped at 0.9 V versus SCE requires that it was present on the electrode at lower potentials. This raises questions about the coverage and structure of UPD Ag underneath or next to the SAM. Around 0.7 V versus SCE (∼0.3 V vs Ag/Ag+), the SAM appears undisturbed, but no order was resolved with STM and the roughness is rather high. In this potential range, the Ag coverage for the SAM-free gold electrode is about 0.44, slowly increasing when the potential is lowered. The first monolayer is (58) Jennings, G. K.; Laibinis, P. E. Langmuir 1996, 12, 6173.

Langmuir, Vol. 17, No. 3, 2001 847

completed around 0.5 V, and a second UPD layer is deposited on Au(111) around 0.4 V (0.03 V vs Ag/Ag+).53 If a full monolayer is intercalated at the SAM/substrate interface in the high UPD range (0.92-0.6 V vs SCE), one may ask why we did not find order in the SAM as was found for alkanethiols on Ag(111)59 and in ex situ studies of SAMs on Ag UPD layers.42,60 The reason may be found in the experimental conditions. First, Ag is deposited through the SAM and it is known to have a high exchangecurrent density. These dynamic conditions could prevent the system from settling into an ordered state. This may also account for variations in island density and layer roughness at a fixed potential between different SAMs. Second, lowering the potential results in the deposition of more Ag, which is again introduced at the interface, forming the islands that can be seen in the intermediate UPD range. On one hand, it is difficult to imagine two open layers of Ag forming a stable bilayer with a SAM on top, but on the other hand the charges recorded in the CVs do not account for the underpotential deposition of two complete monolayers. Because of the insufficient stability of the short- and medium-chain SAMs at potentials positive of 0.9 V versus SCE, one can only follow the stripping of the first layer (around 0.9 V) and the growth of the second one close to the Nernst potential. It was not possible to study in situ the very first stages of Ag UPD on the freshly prepared SAM-covered Au(111) electrode. Therefore, we cannot clarify the structure of the two silver layers, that are detected on short- and medium-chain SAMs in the underpotential range. Yoneyama and co-workers concluded from ex situ STM experiments that vacancy islands would be filled first during Ag UPD on SAM-covered Au(111).41,42 A reduced density of vacancy islands was indeed observed in this study, but this was found to depend on chain length. Longand medium-chain SAMs displayed no vacancy islands in the Ag UPD range, whereas some small pits were found in the short-chain SAMs. Close inspection indicates that those can persist even during island growth of the second UPD layer. Frequently observed variations between samples of the same molecular species prohibited assignment of observed differences between differently functionalized SAMs to special endgroup-adsorbate interactions. At the same time, the endgroups had a strong impact on the order formed on Au(111) in the base electrolyte. Clearly, differences were observed in completion of the second Ag UPD layer (compare Figures 3c, 6a, 8f, and 10f), but the SAMs proved to be extremely sensitive to sample history (which is reflected in the CVs) and the differences do not indicate an obvious trend. We therefore restrain ourselves from a mechanistic interpretation in this respect. We can only state that it is not too surprising to find no significant endgroup effects, especially with the short-chain alkanethiols. As was discussed in the first part of this work,50 short-chain alkanethiols do not yield a highly packed monolayer that conforms to most of the SAM models. With this in mind, one should not expect big effects of the terminal group. We can state, however, that Ag UPD is remarkably less inhibited than Cu UPD, where significant shifts in deposition potential were found even for C2 SAMs.43 This could be understood by considering the different exchange current densities or, as was earlier pointed out by Yoneyama and co-workers,42 the different size of the solvation shells of Cu2+ and Ag1+. Nevertheless, (59) Heinz, R.; Rabe, J. P. Langmuir 1995, 11, 506. (60) Hsieh, M.-H.; Chen, C. Langmuir 2000, 16, 1729.

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the kinetics in the UPD range is strongly influenced as the current densities are reduced with increasing chain length. Overpotential Range. Overpotential deposition is hindered, but as soon as three-dimensional growth starts, it proceeds rapidly. However, it should be noted that bulk growth on SAM-covered Au(111) extends much slower laterally than on the bare surface, leading to a rougher bulk deposit. Nucleation is almost exclusively found at step edges and grain boundaries, and formation of threedimensional Ag clusters on neighboring SAM-covered terraces is very slow. The surface texture of the bulk deposit is much smoother than that of the UPD-covered terraces. We therefore conclude that the underpotential deposit is found under the SAM, whereas the bulk deposit buries the SAM in a mushroom type of growth. However, a clear-cut distinction between under- and overpotential growth cannot be based exclusively on the location of the deposit. The transition from (slow) layer-by-layer to (fast) cluster growth is mainly determined by the properties of the organic monolayer. By comparison of the corresponding STM data (Figures 3c, 8a,d, and 10a,f), it is obvious that layer-by-layer growth can continue beyond the two monolayers of the underpotential deposit. Layered overpotential growth is found especially at step edges proceeding as slow as the underpotential deposit. Strikingly, this effect increases with chain length. Whereas only small islands grow from short-chain-covered steps (cf. Figure 3c), large areas of flat deposits are recognized on the medium-chain samples (cf. Figure 8a). Long-chain SAMs lead to flat overpotential growth even on terraces (cf. Figure 10a,f), but bulk growth was not studied systematically in these cases. The dithiol SAM represents an interesting exception from this behavior. No two-dimensional growth was observed. Small clusters are found instead in the low overpotential range. As the growth mode substantially deviates from all other systems investigated, we believe that the second thiol endgroup (which is not adsorbed on the Au surface) affects the initial stages of overpotential deposition. We explain this by the strong interaction between silver and the functional group which results in the formation of Ag-thiolate moieties.


Summary Cyclic voltammetry and in situ STM were employed to study the effects of different chain lengths and endgroups of alkanethiol SAMs on Au(111) on Ag electrodeposition. The results indicated no pronounced differences in the interaction of the different endgroups with the Ag deposit, except for the 1,6-hexanedithiol SAM, which modified the onset of bulk deposition. It was found that two layers of Ag are deposited at underpotentials, although in some cases the deposition of the second layer was not completed. The structure and density of those two layers could not be unequivocally determined. There is strong evidence that both layers are inserted at the thiol/Au interface. No molecular order was resolved with in situ STM, which might be understood by the fact that Ag is deposited through the SAM during imaging, thereby disturbing the interface state. More time might be necessary to reach an ordered SAM again. Deposition is kinetically hindered, depending on the chain length, which can be inferred from cyclic voltammograms. Bulk deposition originates at substrate defects. The different surface appearance, growth rate, and growth mode of the underpotential and bulk deposit substantiates our interpretation that at high overpotentials the SAM is buried by the bulk deposit, whereas the UPD layer is located between the SAM and Au. Short- and medium-chain SAMs are damaged when the first Ag UPD layer is stripped around 0.9 V versus SCE, although in a Ag+-free electrolyte the SAMs are stable at that potential. Oxidative desorption is clearly changed when the electrolyte contains Ag+. Only long-chain SAMs are able to resist the disturbances brought about by desorption of UPD Ag which obviously is involved in the thiolate bond. Acknowledgment. We are indebted to Dr. G. Go¨tz and Professor P. Baüerle (Department Organic Chemistry II, University of Ulm) for the synthesis of 17-cyanoheptadecanethiol and monoprotected 1,6-hexanedithiol. M.J.E. gratefully acknowledges a stipend from the Alexandervon-Humbolt Stiftung, and H.H. thanks the Deutsche Forschungsgemeinschaft for a grant through Graduiertenkolleg 328. LA001140P

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