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Functionalized Self-Assembled Alkanethiol Monolayers on Au(111) Electrodes: 1. Surface Structure and Electrochemistry M. J. Esplandiu´,* H. Hagenstro¨m, and D. M. Kolb* Department of Electrochemistry, University of Ulm, 89069 Ulm, Germany Received August 7, 2000. In Final Form: November 7, 2000 Functionalized alkanethiols (-CH3, -NH2, -OH, -COOH, -SH, -SO3H, -CN) of different lengths, self-assembled on Au(111), were studied with cyclic voltammetry and in situ scanning tunneling microscopy. The surface structure was determined under potential conditions at which only a dielectric behavior is expected. In general, the size of the terminal group together with the chemical interaction between neighboring thiol molecules plays a crucial role for the resulting adlayer structure. Shorter-chain thiols with bulky endgroups frequently lead to lower density coverages and more distorted packings.
Introduction During the past decade, the properties of organic molecules which are able to spontaneously assemble on substrates have been intensively investigated. The functionalization of such self-assembled monolayers (SAMs) is an important issue today, because molecular architectures can be built on such templates. The design and construction of novel molecular materials is opening new research areas, where surface physics, chemistry, electrochemistry, biology, and biochemistry meet. A large number of applications and processes are conceivable in areas such as corrosion protection,1 wetting,2,3 friction,4 adhesion,5 microelectronics,6 optics,7 and chemical, biochemical, and electrochemical sensors.8-10 Much work has been published on functionalized, alkanethiol-based SAMs (in general, HS-(CH2)n-X). A number of studies concentrated on the molecular order, stability, and permeability as a function of chain length, endgroup, and solvent.11,12 Some of these properties have also been monitored as a function of electrode potential.13-16 Alkanethiol SAMs on gold with acidic terminal moieties have received special interest because properties such as acid/base equilibria can be controlled by the electrode potential.17 Additionally, these molecules serve * Corresponding authors. E-mail:
[email protected] (M. J. Esplandiu´);
[email protected] (D. M. Kolb). (1) Strattmann, M. Adv. Mater. 1990, 2, 191. (2) Ulman, A. Thin Solid Films 1996, 273, 48. (3) Nuzzo, R. C.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (4) DePalma, V.; Tillman, N. Langmuir 1989, 5, 868. (5) Ferguson, G. S.; Chaudhury, M. K.; Sigal, G. B.; Whitesides, G. M. Science 1991, 253, 776. (6) Chen, J.; Reed, M. A.; Tour, J. M. Science 1990, 286, 1550. (7) Katz, H. E.; Wilson, W. L.; Scheller, G. J. Am. Chem. Soc. 1994, 116, 6636. (8) Malem, F.; Mandler, D. Anal. Chem. 1993, 65, 37. (9) Gooding, J. J.; Praig, V. G.; Hall, E. A. H. Anal. Chem. 1998, 70, 2396. (10) Giz, M. J.; Duong, B.; Tao, N. J. J. Electroanal. Chem. 1999, 465, 72. (11) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (12) Dannenberger, O.; Weiss, K.; Himmel, H. J.; Ja¨ger, B.; Buck, M.; Wo¨ll, Ch. Thin Solid Films 1997, 307, 183. (13) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682. (14) Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. Langmuir 1995, 11, 2237. (15) Sato, Y.; Ye, S.; Haba, T.; Uosaki, K. Langmuir 1996, 12, 2726. (16) Zamborini, F.; Crooks, R. M. Langmuir 1998, 14, 3279.
as adhesion layers for biomolecules and cells to study their chemical and electrochemical reactivity at interfaces (this topic has been also extended to other hydrophilic molecules, e.g., for X ) -OH).8-10,18-21 Another field of interest is binary mixed SAMs containing different terminal groups, which allows for a microscopic tuning of macroscopic properties of the interface.22-26 A considerable part of the literature deals with dithiols. Most of it is focused on potential applications in molecular electronics on the basis that dithiols can form SAMs in which only one end of the dithiol is attached to the metal surface, while the other may form a chemical bond with a second metal electrode.27-33 Several groups have used dithiols for covalently attaching nanocrystals,27-29 making use of ionic interaction between dithiols and other species.30,31 Another prospect is applications of aromatic chain SAMs as molecular wires.32,33 In all these papers, loop formation of the dithiol on the metal surface was ruled out. The suggested upright alignment of the hydrocarbon chains was also supported in more recent X-ray photoelectron spectroscopy measurements of 1,8(17) Creager, S. E.; Clarke, J. Langmuir 1994, 10, 3675. (18) Sun, L.; Kepley, L. J.; Crooks, R. M. Langmuir 1992, 8, 2101. (19) Yang, H. C.; Dermody, D. L.; Xu, C.; Ricco, A. J.; Crooks, R. M. Langmuir 1996, 12, 726. (20) Himmel, H. J.; Weiss, K.; Ja¨ger, B.; Dannenberger, O.; Grunze, M.; Wo¨ll, Ch. Langmuir 1997, 13, 4943. (21) Sabapathy, R. C.; Crooks, R. M. Langmuir 2000, 16, 1777. (22) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. Langmuir 1992, 8, 1330. (23) Biebuyck, H. A.; Whitesides, G. M. Langmuir 1993, 9, 1766. (24) Imabayashi, S.; Hobara, D.; Kakiuchi, T.; Knoll, W. Langmuir 1997, 13, 4502. (25) Hobara, D.; Ueda, K.; Imabayashi, S.; Yamamoto, M.; Kakiuchi, T.; Electrochemistry 1999, 67, 1218. (26) Hobara, D.; Sasaki, T.; Imabayashi, S.; Kakiuchi, T. Langmuir 1999, 15, 5073. (27) Bethell, D.; Brust, M.; Schiffrin, D. J.; Kiely, C. J. Electroanal. Chem. 1996, 409, 137. (28) Nakanishi, T.; Ohtani, B.; Uosaki, K. J. Phys. Chem. B 1998, 102, 1571. (29) Colvin, V. L.; Goldstein, A. N.; Alivisatos, A. P. J. Am. Chem. Soc. 1992, 114, 5221. (30) Bandyopadhyay, K.; Vijayamohanan, K. Langmuir 1998, 14, 6924. (31) Brust, M.; Blass, P. M.; Bard, A. J. Langmuir 1997, 13, 5602. (32) Tour, J. M.; Jones, L., II; Pearson, D. L.; Lamba, J. J. S.; Burgin, T. P.; Whitesides, G. M.; Allara, D. L.; Parikh, A. N.; Atre, S. V. J. Am. Chem. Soc. 1995, 117, 9529. (33) Reed, M. A.; Zhou, C.; Muller, C. J.; Burgin, T. P.; Tour, J. M. Science 1997, 278, 253.
10.1021/la001139q CCC: $20.00 © 2001 American Chemical Society Published on Web 01/09/2001
Alkanethiol Monolayers on Au(111) Electrodes
octanedithiol self-assembled on Au surfaces.34 However, the issue of the molecular orientation is still under debate. Structural investigations concentrating on the adsorption mechanism claim that the molecules can also adopt a flat configuration on the substrate, depending on the sample preparation.35-40 SAMs allow control of various chemical/physical interfacial processes. Therefore, they are of interest to electrochemists as a means to influence in a precise way electrochemical processes. One of the challenges in this respect is the metallization of organic thin films, particularly of tailor-made SAMs. Such studies have already been performed by our group with ethanethiol. A detailed structural study of ethanethiol on Au(111), its stability range, and its influence on Cu electrodeposition was performed by cyclic voltammetry and in situ scanning tunneling microscopy.41-43 As a next step, the addition of reactive endgroups with the expectation of modifying the metal deposition process appears very attractive. Even more so, as short-chain thiol layers block the surface to a great extent but do not completely suppress ion transfer, it would be interesting to deal with longer chains that can develop more lateral interactions in the assembly process. However, before metal electrodeposition is considered, a thorough knowledge of the behavior and properties of the surface to start with is required. Therefore, it is the aim of this report to establish the structure of different functionalized SAMs on Au(111) and their stability with respect to the electrode potential. To this end, thiols (HS(CH)nX) of different lengths (n ) 2, 6, 18) and different terminal moieties (X ) -CH3, -OH, -COOH, -SO3H, -SH, -CN, -NH2) will be characterized by in situ scanning tunneling microscopy (in situ STM) and cyclic voltammetry (CV) in a supporting electrolyte. On the basis of the knowledge acquired here, electrochemical metal deposition on such functionalized gold substrates will be discussed in a following communication comprising the second part of this work. Experimental Section Gold Substrates. For cyclic voltammetry, a Au crystal, 4 mm in diameter, oriented better than 1° toward the (111)-face and polished down to 0.03 µm (MaTeck, Ju¨lich, Germany) was used. Before the thiol assembly process, the electrode was flameannealed with a Bunsen burner for 1 min, cooled in air, and put in contact with water after 1 min. For STM measurements, 500 nm thick Au films with (111) orientation, evaporated on heatresistive glass (AF 45, Berliner Glas KG), were employed. The gold substrates were flame-annealed in a hydrogen flame for 2 min and cooled to room temperature in a stream of nitrogen. In some cases, gold films evaporated on mica were used. However, no differences in the structural properties of the SAMs were noted. (34) Rieley, H.; Kendall, G. K.; Zemicael, F. W.; Smith, T. L.; Yang, S. Langmuir 1998, 14, 5147. (35) Kobayashi, K.; Umemura, J.; Horiuchi, T.; Yamada, H.; Matsushige, K. Jpn. J. Appl. Phys. 1998, 37, L297. (36) Kobayashi, K.; Horiuchi, T.; Yamada, H.; Matsushige, K. Thin Solid Films 1998, 331, 210. (37) Kobayashi, K.; Horiuchi, T.; Yamada, H.; Matsushige, K. Appl. Surf. Sci. 1999, 144-145, 435. (38) Cavallini, M.; Bracali, M.; Aloisi, G.; Guidelli, R. Langmuir 1999, 15, 3003. (39) Duwez, A. S.; Yu, L. M.; Riga, J.; Pireaux, J. J.; Delhalle, J. Thin Solid Films 1998, 327-329, 156. (40) Leung, T. Y. B.; Gerstenberg, M. C.; Lavrich, D. J.; Scoles, G.; Schreiber, F.; Poirier, G. E. Langmuir 2000, 16, 549. (41) Hagenstro¨m, H.; Schneeweiss, M. A.; Kolb, D. M. Langmuir 1999, 15, 2435. (42) Hagenstro¨m, H.; Schneeweiss, M. A.; Kolb, D. M. Langmuir 1999, 15, 7802. (43) Hagenstro¨m, H.; Schneeweiss, M. A.; Kolb, D. M. Electrochim. Acta 1999, 45, 1141.
Langmuir, Vol. 17, No. 3, 2001 829 Chemicals. Ethanethiol (HSCH2CH3, abbreviated C2), mercaptoethanol (HS(CH2)2-OH, C2OH), mercaptopropionic acid (HS(CH2)2-COOH, C2COOH), aminoethanethiol (HS(CH2)2NH2, C2NH2), 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 Dr. G. Go¨tz and Prof. P. Ba¨uerle (Department Organic Chemistry II, University of Ulm). Preparation of Functionalized SAMs. Self-assembled monolayers were prepared by immersing the previously annealed gold substrates in millimolar ethanolic or aqueous solutions of the functionalized thiols for 12-16 h. Absolute ethanol (Merck, extra pure) was used as a solvent for ethanethiol, hexanethiol, hexanedithiol, octadecanethiol, and cyanoheptadecanethiol; Milli-Q water was used for all functionalized ethanethiols. After the modification, the samples were removed from the solution, rinsed with the respective solvent, and blown dry with nitrogen. All chemicals were used as received without further purification. Special care was taken in the assembly process of hexanedithiol with respect to the content of oxygen dissolved in the modification solution.44 The samples were prepared by immersion in an ethanolic solution which was previously purged with argon or nitrogen; that is, the assembly process took place in an oxygen-free environment. The samples were kept in the dark during immersion in order to avoid photooxidation.34,45 In this way, the facile oxidation of adsorbed dithiols to disulfides was minimized. Samples not treated in this way formed thiol multilayers during the exposure to the solution. This effect was even stronger when assembling shorter alkyl chain dithiols (not discussed in the present work), as evidenced by clear differences in capacitance measurements. Electrolytic Solutions. Different solutions were used for the study of the SAMs. Acidic solutions (0.1 and 0.05 M) were prepared from H2SO4 (Merck, suprapure) and Milli-Q water (Millipore Corp., USA), and alkaline solutions (0.1 M) were prepared from Merck suprapure NaOH under the same conditions. Cyclic Voltammetry. The voltammograms were obtained with standard electrochemical equipment and a conventional electrochemical cell with separate compartments for reference (SCE, saturated calomel electrode) and counter (platinum wire) electrodes. The electrolyte was thoroughly deaerated by bubbling with nitrogen prior to each experiment. The gold substrate with the thiol monolayer was brought into contact with the electrolyte under potential control. In Situ STM. The STM measurements were performed with a Topometrix TMX 2010 Discoverer, using tungsten tips electrochemically etched from a 0.25 mm diameter wire in aqueous 2 M NaOH. To minimize faradaic currents at the tip-electrolyte interface, the tips were coated with electrophoretic paint when they were used in an acidic medium or with Apiezon wax for alkaline electrolytes. Platinum wires were used in the STM cell as counter electrodes. Ag/AgCl was employed as a reference electrode for alkaline solutions, whereas a platinum wire was used for acidic solutions. All STM images were recorded in the constant-current mode and are displayed here as top views with different gray shades representing different heights (dark areas indicating low parts and bright areas indicating high parts of a surface). All potentials throughout this communication are quoted with respect to the saturated calomel electrode (SCE).
Results and Discussion A. Cyclic Voltammetry. In this section, we will discuss the electrochemical behavior of the SAMs as a function of electrode potential. This will elucidate the blocking properties of these monolayers against permeation of ions and establish the potential range of electrochemical stability. Figures 1-3 show the current-potential profiles (j/E) of thiol-modified Au(111) electrodes in a wide (44) Whittaker, V. P. Biochem. J. 1947, 41, 56. (45) Cooper, E.; Leggett, G. J. Langmuir 1998, 14, 4795.
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Figure 2. Cyclic voltammograms for two medium-chain SAMs on Au(111) in 0.1 M H2SO4. Scan rate, 10 mV s-1. The doublelayer charging region with purely capacitive behavior has been enlarged. Inset: Double-layer capacity of the SAM-covered Au(111).
Figure 1. Cyclic voltammograms of Au(111) covered with five different short-chain SAMs, in 0.1 M H2SO4. Scan rate, 10 mV s-1. The double-layer charging region has been enlarged. Bare Au(111) has been included for comparison (top curve).
potential range, including oxidative desorption. The CVs were recorded in dilute sulfuric acid. As compared to bare Au(111) (top CV in Figure 1), SAMcovered electrodes exhibit markedly reduced double-layer charging currents and an almost potential-independent behavior, which is characteristic for thin layers with a low dielectric constant blocking ion transfer to the electrode. Considering the molecules investigated, the general trend is a decrease of the capacitive current as the chain length is increased. Note especially the strongly reduced current for the octanedecanethiol-modified electrode. Comparing the CVs obtained for shorter alkyl chain molecules with different terminal groups (Figure 1), it is not possible to correlate double-layer capacities with the dielectric properties of the terminal group as was previously suggested.14 Because we are dealing with short alkyl chains, it is much more likely that the capacitance differences stem from variations in the quality of the layer packing as a result of the terminal group sizes and only weak van der Waals interactions.13 No significant differences were seen in the CVs for the medium-chain SAMs of hexanethiol and hexanedithiol
Figure 3. Cyclic voltammograms for two long-chain SAMs on Au(111) in 0.1 M H2SO4 in the double-layer range. Note the different current density scales; the current density of C18 is enlarged by a factor of 10. Scan rate, 10 mV s-1.
measurements (Figure 2). The capacitance values for hexanethiol and hexanedithiol (between 2.5 and 3.5 µF/ cm2, see inset) are in agreement with those reported for C6 in previous reports.46 These data imply that there is no multilayer formation in either system. If multilayers were formed, capacitance values lower than 1 µF/cm2 should have been obtained according to the correlation between the differential capacitances and the chain length of the adsorbates.46 This finding is very important because it means that our dithiol SAM preparation conditions prevent disulfide formation perpendicular to the surface. The smallest currents in our study were found for longchain thiols. However, an interesting aspect is seen in Figure 3: compared to C18, the double-layer current for the nitrile thiol is clearly larger. This observation has (46) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 335, 310.
Alkanethiol Monolayers on Au(111) Electrodes Table 1. Double-Layer Capacities CDL (Estimated from Double-Layer Charging Currents) and Oxidative (QOD) and Reductive (QRD) Desorption Charges for the Functionalized Alkanethiol SAMs on Au(111) alkanethiol
CDL [µF/cm2]
QOD [µC/cm2]
QRD [µC/cm2]
C2 C2OH C2NH2 C2COOH C2SO3H C6 C6SH C18 C17CN
7.4 11.3 20.6 10.0 12.0 2.5-3.5 2.5-3.5 ∼0.1
