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Potential-Assisted Deposition of Alkanethiols on Au: Controlled Preparation of Single- and Mixed-Component SAMs Fuyuan Ma and R. Bruce Lennox* Department of Chemistry, McGill University, 801 Sherbrooke Street West, Montreal, Quebec, Canada H3A 2K6 Received October 1, 1999. In Final Form: February 24, 2000 The commonly used method of preparing RS/Au self-assembled monolayers (SAMs) involves a passive incubation process in a nonabsorbing solvent. Preparation of mixed-component SAMs is particularly problematic under these conditions. The time course of the open circuit potential in the passive adsorption experiment suggests that control of the deposition potential could lead to a faster and ultimately more complete SAM formation process. This is shown to be the case, as both C16 RS/Au SAMs and mixed C16/HOOCC15S/Au SAMs are shown to be readily prepared in approximately 15 min from 5 mM thiol solutions at potentials ranging from 200 to 600 mV (vs Ag/AgCl). The blocking properties of the resulting C16 SAMs are excellent. This technique provides access to mixed-composition SAMs otherwise inaccessible using deposition under open circuit conditions.
Although RSH/Au self-assembled monolayers (SAMs) have been the subject of several hundred scientific reports, the mechanism by which they form is relatively poorly understood and characterized. It is also widely acknowledged by practitioners in the field that it is difficult to obtain SAMs with identical function (i.e. electron-transfer blocking capability1a) or structure (i.e. determined by diffraction1b) although ostensibly identical preparation conditions may have been used. Although it is often stated that SAMs offer a chemist the ability to “engineer” a surface with “tuned” properties, the preparation of both single-component and multicomponent SAMs in a predictable manner is problematic.2 We are very interested in (i) obtaining high-quality, extensively blocked (to aqueous redox couples) SAMs formed from n-alkanethiols and (ii) obtaining binary and higher order SAMs whose composition is both predictable and reproducible. To this end we have found that the chemisorption of the alkanethiol can be controlled electrochemically. A modest anodic potential applied to the gold electrode during RSH adsorption very quickly leads to high-quality SAMs with excellent performance characteristics. This potential deposition process also allows one to precisely control the ratio of components in a model binary SAM made up of C16SH and HSC15CO2H. Alkanethiol monolayer systems have been most commonly prepared either by incubation of a gold substrate in a thiol-containing solution or by deposition from the gas phase.2 Full, close-packed coverage is, however, problematic from the gas phase.2 In neither method is the potential of the gold substrate fixed, and in the solution incubation experiment the complexities arising from a Nernstian redox pinning are not generally acknowledged. Although the self-assembly process is simple, it is often both time-consuming and irreproducible.1a,b In the solu* Corresponding author. E-mail: Bruce•Lennox@ maclan.mcgill.ca. Phone: 514-398-3638. Fax: 514-398-3797. (1) (a) Badia, A. Ph.D. Thesis, McGill University, 1996. (b) Camillone, N.; Leung, T. Y. B.; Scoles, G. Surf. Sci. 1997, 373, 333. (2) Finklea, H. O. In Electroanalytical Chemistry Vol. 19; Electrochemistry of Organized Monolayers of Thiols and Related Molecules on Electrodes; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker Inc.: New York, 1996; pp 110-335.
tion-based process, the surface coverage increases with increased incubation time3-5 and fully covered, highly ordered monolayers may require days or even weeks to prepare.3-6 Furthermore, it is difficult or impossible to prepare binary (or higher order) SAMs with predictable composition using a solution-mediated, passive adsorption process. The more stable thiol gradually replaces the second component8,9 via an ill-defined exchange mechanism. We have reasoned that control of SAM formation, including binary-component SAMs, requires conditions which lead to a kinetically rather than thermodynamically controlled deposition process. Several studies discuss how an applied potential affects an already-formed SAM. For example, Sondag-Huethorst et al.10-13 and Stolberg et al.14 observed that a potential applied to a RS/Au SAM can influence the monolayer structure. Other reports note that the SAM structure changes with applied potential in nonaqueous media, but not in aqueous media.15-17 Potential control of the SAM/ (3) Stranick, S. T.; Parikh, A. N.; Tao, Y.-T.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 7636. (4) Stranick, S. J.; Parikh, A. N.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 11136. (5) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301. (6) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (7) Hahner, G.; Woll, Ch.; Buck, M.; Grunze, M. Langmuir 1993, 9, 1955. (8) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M.; Deutch, J. J. Phys. Chem. 1994, 98, 563. (9) (a) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164. (b) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. Langmuir 1992, 8, 1330. (10) Sondag-Huethorst, J. A. M.; Fikkink, L. G. J. Langmuir 1992, 8, 2560. (11) Sondag-Huethorst, J. A. M.; Fikkink, L. G. J. J. Electroanal. Chem. 1994, 367, 49. (12) Sondag-Huethorst, J. A. M.; Fikkink, L. G. J. Langmuir 1995, 11, 2237. (13) Sondag-Huethorst, J. A. M.; Fikkink, L. G. J. Langmuir 1995, 11, 4823. (14) Stolberg, L.; Lipkowski, J.; Irish, D. E. J. Electroanal. Chem. 1987, 238, 333. (15) Popenoe, D. D.; Deinhammer, R. S.; Porter, M. D. Langmuir 1992, 8, 2521. (16) Anderson, M. R.; Gatin, M. Langmuir 1994, 10, 1638.
10.1021/la9913046 CCC: $19.00 © 2000 American Chemical Society Published on Web 06/22/2000
Potential-Assisted Deposition of Alkanethiols
electrode also influences the wettability of a RSH/Au SAM once it has formed.18,19 The redox nature of the chemisorption process has been demonstrated by the considerable (80%) degree of electron transfer from preformed C18SH Langmuir monolayers when they are transferred as Langmuir-Blodgett (LB) films to an Au substrate.20 Ab initio calculations21 and Kelvin probe measurements22,23 suggest that the extent of charge transfer in the gold thiolate ranges from 0.4 to 0.7 depending on the geometry of the adsorption site. Stratmann and co-workers24 have shown that applied potentials slightly positive of the potential of zero charge (pzc) cause the thiol molecule to be “attracted” to the gold surface during SAM formation. Adsorption of the thiolate (rather than the usual thiol) was reported by Porter and co-workers, but the functional qualities of the resulting SAMs were not described.25 Very recently, Ron and Rubinstein reported that RS/Au SAMs can be prepared in a rapid fashion in strongly oxidizing conditions (1.45 V vs SCE) in anhydrous ethanol.26 A complex interplay betwen oxidative adsorption and desorption is noted. The “gold standard” assessment of SAM integrity (blocking of electron transfer from a water soluble redox couple) was not, however, reported for SAMs prepared in this manner. We reasoned that the potential shifts which inevitably arise during an uncompensated (i.e. open circuit) electrontransfer process may modulate the progress and extent of SAM formation. This in fact is evident on monitoring the open circuit potential (OCP) of a gold substrate during SAM formation. Buildup of excess charge on the gold retards subsequent thiol deposition (Figure 1). Injection of thiol (C12SH) to a solution containing a clean polycrystalline (ca. 70% (110) and 30% (111))33 gold electrode causes an immediate cathodic shift in the OCP from +300 mV to -100 mV (vs Ag/AgCl). This potential slowly shifts from -100 mV to +100 mV in 15 min and then gradually shifts to approximately +200 mV over the next several hours (see inset in Figure 1). These characteristics suggest that the slowness of the chemisorption process under the commonly used OCP deposition conditions arises because (17) Everett, W. R.; Welch, T. L.; Reed, L.; Fritsch-Faules, I. Anal. Chem. 1995, 67, 292. (18) Abbott, N. L.; Gorman, C. B.; Whitesides, G. M. Langmuir 1995, 11, 16. (19) Gorman, C. B.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1995, 11, 2242. (20) Krysinski, P.; Chamberlain, R. V., II; Majda, M. Langmuir 1994, 10, 4286. (21) Sellars, H.; Ulman, A.; Shnidman, Y.; Eilers, J. E. J. Am. Chem. Soc. 1993, 115, 9389. (22) Evans, S. D.; Ulman, A. Chem. Phys. Lett. 1990, 170, 462. (23) Evans, S. D.; Urankar, E.; Ulman, A.; Ferris, N. J. Am. Chem. Soc. 1991, 113, 4121. (24) (a) Losch, R.; Stratman, M.; Viefhaus, H. Electrochim. Acta 1994, 3912. (b) Weldige, K.; Rohwerder, M.; Vago, E.; Viefhaus, H.; Stratmann, M. Fresenius J. Anal. Chem. 1995, 353, 320. (c) Rohwerder, M.; De Weldige, K.; Vago, E.; Viefhaus, H.; Stratmann, M. Thin Solid Films 1995, 264, 240. (25) Weisshaar, D. E.; Lamp, B. D.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 5860. (26) Ron, H.; Rubinstein, I. J. Am. Chem. Soc. 1998, 120, 1344413452. (27) The capacitance is measured from cyclic voltamograms when the charging current is independent of applied potential (ref 28). (28) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559. (29) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (30) Miller, C. J.; Cuendet, P.; Gratzel, M. J. Phys. Chem. 1991, 95, 877. (31) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335. (32) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (33) Badia, A.; Back, R.; Lennox, R. B. Angew. Chem., Int. Ed. Engl. 1994, 33, 2332.
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Figure 1. Current-potential relationship of a C12SH monolayer formed by potential deposition. The applied potential, Eprep, refers to the electrode in deposition mode (5 mM C12SH/ 0.1 M LiClO4/ethanol solution). The current measured refers to the interrogation mode (using 5 mM K3Fe(CN)6/1 M KBr at 0.1 V vs AgAgCl). Deposition time ) 10 min. The thiol is injected into the stirred solution coincident with application of the potential. The inset tracks the open circuit potential of a clean gold electrode in stirred ethanol/0.1 M LiClO4 upon injection of C12SH. Oxygen was not excluded from either type of experiment.
the gold electrode is driven to a potential where continuation of the chemisorption process is energetically unfavorable. These kinetics are similar to those reported by ex situ techniques where the adsorption process has a fast phase (ca. 80% coverage) and a very slow, asymptotic phase (20%).6,7 The observations in Figure 1 show that rapid and complete chemisorption may in fact be accessible under controlled potential conditions, perhaps even at modest positive potentials (>200 mV). To this end, we have prepared a series of RSH/Au SAMs under potential control and evaluated them using the redox probe methodology. The attenuation of the rate of electron transfer from water soluble redox couples at long chain SAMs is very sensitive both to defects and to the nature of the terminal group of the RSH chain. An electrode prepared under potential control (Eprep ) 600 mV vs AgAg/Cl) shows an Fe(CN)63--linked Faradaic current which is suppressed 3-fold compared to that for the best SAM obtained by the passive, OCP incubation process. The capacitance values27 of these two SAMs are not, however, meaningfully different and are typical of a well-ordered monolayer28-32 (1.1 µF/cm2 for C12SH in 1 M KBr). On the other hand, when Eprep < 0.20 V, the resulting SAM is relatively poorly blocking toward Fe(CN)63- (Figure 1) and has the characteristics of a SAM prepared under OCP conditions. The comparison of the time course and potential values in Figure 1 (inset) and the Eprep values which are effective in SAM formation suggest that formation of a good blocking SAM could indeed be problematic at OCP electrodes. Neither contact angle measurements (advancing, water drop, 112°) nor the monolayer thickness (16 Å) as measured by ellipsometry is significantly different in these monolayer preparations whereas the barrier properties clearly are. The potential-assisted deposition is very rapid (complete in ca. 15 min) whereas OCP deposition requires hours to days1a,37 to produce a high-quality RSH/Au SAM (Figure 2). In the early stages of each deposition process (i.e. 1 and 5 min, respectively), neither SAM is fully formed and the Fe(CN)63--linked current is large. This advantage probably arises for two reasons. First, an applied potential which is sufficiently oxidizing may assist the chemisorption of RSH via control of the charge-transfer process in
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Figure 2. Current observed in 5 mM K3Fe(CN)6/1 M KBr/0.1 V versus Ag/AgCl of a C12SH/Au monolayer prepared by two protocols monitored as a function of deposition time: (a) SAMs formed at a fixed potential (0.6 V) in 5 mM C12SH/0.1 M LiClO4/ ethanol solution; (b) SAMs formed by passive OCP incubation in a 5 mM C12SH/ethanol solution. Lines are drawn to assist data visualization.
some fashion. XPS,34 NMR,35 and IR36 studies, for example, are consistent with the formation of a gold-thiolate bond (RS-Au) when thiols or disulfides adsorb to gold. It is important to note that the modest potentials associated with excellent SAM formation (200-600 mV vs Ag/AgCl) cannot directly oxidize RSH in ethanol or create a surface mediator in the form of a gold oxide. The applied potential may however assist in organizing the monolayer, as has been shown for already-formed SAMs.10-17 Temporal1,37 and/or potential annealing of as-formed SAMs may be in fact an important factor in the formation of SAMs which are excellent blockers. The controlled potential deposition technique also provides a very useful entry into the preparation of mixed monolayers with well-defined composition. It is not possible in the OCP process to precisely control the ratio of components in the resulting mixed monolayer, as one component invariably displaces the other.8,9 The composition of a mixed monolayer is thus not the same as the composition of the incubating solution and is incubationtime dependent.8,9 This problem is a serious impediment to the use of the self-assembly process to produce “engineered” surfaces with known composition and properties. The formation of mixed monolayers under potentialcontrolled conditions was assessed using two thiols (C16SH and HSC15CO2H) and two redox probes (Fe(CN)63and Ru(NH3)63+). A C16SH-modified electrode is blocking toward both Fe(CN)63- and Ru(NH3)63+ whereas, at neutral (34) (a) Nuzzo, R. G.; Fusco, F.; Allara, D. L. J. Am. Chem. Soc. 1987, 109, 2358. (b) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733. (35) Badia, A.; Demers, L.; Dickinson, L.; Morin, F. G.; Lennox, R. B.; Reven, L. J. Am. Chem. Soc. 1997, 119, 11104. (36) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (37) Forouzan, F.; Bard, A. J.; Mirkin, M. V. Isr. J. Chem. 1997, 37, 155. (38) Takehara, K.; Takemura, H.; Ide, Y. Electrochim. Acta 1994, 39, 817.
Ma and Lennox
Figure 3. Dependence of SAM composition on the composition of the preparation solution, expressed as the percent HSC15CO2H (Xi) in C16SH. Current was measured at -0.3 V versus Ag/AgCl in a 2 mM Ru(NH3)6Cl3/1 M KBr solution for SAMs formed by potential deposition (at 0.6 V for 10 min; open squares) and passive incubation (24 h; closed circles). Lines are drawn to assist data visualization.
pH, a Au/SC15CO2- -modified electrode blocks Fe(CN)63current but does not block Ru(NH3)63+-associated current.38 This differential blocking allows us to assess the extent of HSC15CO2- loading in a C16SH monolayer. The current at mixed-composition (10-90% HSC15CO2H) SAMs was first measured in an Fe(CN)63- solution (pH 7) to establish that the electrode was well blocked. Electrodes prepared by both potential-assisted and OCP adsorption processes exhibit negligible (0.01-0.03 mA/ cm2) Fe(CN)63- -related current when the percent HSC15CO2H (Xi) ranges from 0 to 100%. The current was then measured in a Ru(NH3)63+ solution (Figure 3), where, for Xi ) 10-90%, the current change is clearly dependent upon the percent of HSC15CO2H in the incubation solution. In contrast, the current measured at SAMs formed by OCP self-assembly does not vary for Xi ) 10-90%, suggesting that the carboxylate is excluded from the SAM during the monolayer process. The potential-assisted deposition process clearly provides controlled access to SAMs otherwise inaccessible through OCP self-assembly. Moreover, the success of this process demonstrates that one can kinetically control the SAM formation process. In conclusion, we have demonstrated that (i) a modest anodic potential applied to the electrode surface facilitates the chemisorption of RSH to a clean gold surface; (ii) an excellent blocking monolayer can be formed in a very short time (minutes) compared to the hours to days used in passive self-assembly methods; and (iii) the composition of a binary monolayer of C16SH and HSC15CO2H can be controlled by the potential deposition methodology whereas it is not controlled in the more commonly used OCP, passive incubation process. Detailed studies exploring the mechanistic details of this potential-assisted control are currently underway in our labs. Acknowledgment. We are grateful to the Natural Sciences and Engineering Research Council of Canada for financial support of this work. LA9913046
