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Langmuir 1996, 12, 5696-5703
Desorption of Spontaneously Adsorbed and Electrochemically Readsorbed 2-Mercaptoethanesulfonate on Au(111) Juan Jose Calvente, Zuzana Kova´cˇova´, M. Dolores Sanchez, Rafael Andreu,† and W. Ronald Fawcett* Department of Chemistry, University of California, Davis, California 95616, and Department of Chemistry, University of Sevilla, Profesor Garcia Gonzalez s/n, 41012 Sevilla, Spain Received February 26, 1996X
The desorption of 2-mercaptoethanesulfonate (MES) spontaneously adsorbed on Au(111) has been studied by using both potential-step and voltammetric experiments. From the amount of gold oxide formed during the oxidation sweep in the fingerprint region it is shown that the adsorption process induces structural changes of the gold surface. It is also shown that together with the reductive desorption of MES ions a concomitant faradaic process occurs. The results suggest that this process is connected to the reduction of solvent on the structurally modified gold electrode. The reductive desorption process of MES undergone at more negative potentials is characterized by a single peak in the voltammetric response and the presence of a maximum in the chronoamperogram. It is shown that the logarithm of the maximum current, the time at which the maximum current appears, and the peak width at half height depend linearly on potential. An experimental protocol for the desorption/readsorption of MES based on a potential-step experiment followed by cyclic voltammetry is outlined as an appropriate tool to analyze simultaneously the desorption of adsorbed and readsorbed MES. A linear dependence between the two desorption peak potentials and the surface concentration of adsorbate was observed. Changes in the environment surrounding the adsorbed moities and in the potential of zero charge of the electrode are shown to be the factors ruling this dependence.
Introduction The spontaneous adsorption of organothiols (SH(CH2)n-X) on metal substrates such as gold and silver has allowed the preparation of well-ordered structures at the molecular level. These chemical systems have been grouped under the name of self-assembled monolayers (SAMs) and because of their promising applications have received considerable attention in the last few years.1-4 One of the more interesting properties of these systems is their portability, allowing the film-covered electrode to be transferred from one solution to another. In addition, in the case of thiols terminated by an acidic group at the other end of the molecule, properties such as acid/base equilibria can be controlled by modulating the electrode potential.5,6 In other systems the kinetics of electron transfer reactions are modified by the presence and structural characteristics of the monolayer.7-12 Some models have been proposed in both fields.13-17 In this †
University of Sevilla. Abstract published in Advance ACS Abstracts, November 1, 1996. X
(1) Ulman, A. An Introduction to Ultrathin Organic Films: From Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991. (2) Ulman, A. Characterization of Organic Thin Films; Butterwoth-Heinemann: Boston, 1995. (3) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (4) Dubois, L.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (5) Bryant, M. A.; Crooks, R. M. Langmuir 1993, 9, 385. (6) Creager, S. E.; Clarke, J. Langmuir 1994, 10, 3675. (7) Chidsey, C. E. D. Science 1991, 251, 919. (8) Acevedo, D.; Abrun˜a, H. D. J. Phys. Chem. 1991, 95, 9590. (9) Miller, C.; Cuendet, P.; Gra¨tzel, M. J. Phys. Chem. 1991, 95, 877. (10) Finklea, H. O.; Hanshew, D. D. J. Am. Chem. Soc. 1992, 114, 3173. (11) Rowe, G. K.; Creager, S. E. J. Phys. Chem. 1994, 98, 5500. (12) Chailapakul, O.; Crooks, R. M. Langmuir 1995, 11, 1329. (13) Smith, C. P.; White, H. S. Langmuir 1993, 9, 1. (14) Fawcett, W. R.; Fedurco, M.; Kova´cˇova´, Z. Langmuir 1994, 10, 2403. (15) Andreu, R.; Fawcett, W. R. J. Phys. Chem. 1994, 98, 12753. (16) Smith, C. P.; White, H. S. Anal. Chem. 1992, 64, 2398.
S0743-7463(96)00177-1 CCC: $12.00
matter, in spite of their relatively high stability, SAMs are subject to reductive and oxidative desorption processes, which are undesirable from the stability point of view. However, in the previous literature some advantages have been found as a result of the desorptive character of both processes. Weisshaar et al.18 described a new route to form monolayers by the electrodeposition of thiolate previously desorbed in a cathodic scan. In addition, Porter et al.19-21 have used both oxidation and reduction processes as a tool to determine the amount of n-alkanethiol molecules adsorbed on Au and Ag in alkaline media. They stated that the reduction proceeds via the exchange of one electron. In the subsequent positive-going sweep a readsorption of the previously desorbed molecule occurs. The reductive desorption peak potential for the readsorbed molecules is more positive than that corresponding to the spontaneously adsorbed molecules. They related this shift to the penetration of electrolyte ions in the SAM. Schneider and Buttry22 studied the behavior of alkanethiol layers in a nonaqueous solvent by using an electrochemical quartz crystal microbalance. In this case, no reoxidation peak was obtained in the positive-going sweep following the reductive process. However, a readsorption process did occur and was detected by the change in electrode mass during the reverse scan. On the other hand, as pointed out by Krysinski et al.,23 the exact character of the thiolate-gold bond is not yet well-known, although its formation seems to be well (17) Fawcett, W. R. J. Electroanal. Chem. 1994, 378, 117. (18) Weisshaar, D. E.; Lamp, B. D.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 5860. (19) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 365. (20) Walczak, M. M.; Popenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 2687. (21) Weisshaar, D. E.; Walczak, M. M.; Porter, M. D. Langmuir 1993, 9, 323. (22) Schneider, T. W.; Buttry, D. A. J. Am. Chem. Soc. 1993, 115, 12391. (23) Krysinski, P.; Chamberlain, R. V., II; Majda, M. Langmuir 1994, 10, 4286.
© 1996 American Chemical Society
2-Mercaptoethanesulfonate on Au(111)
established. These authors reported a partial charge transfer coefficient varying linearly from 0.3 (at -0.3 V) to 0.45 (at 0.7 V). Previously Wierse et al.24 reported that the electrosorption valency of S2- adsorbed on gold is equal to -2, suggesting that sulfide ions are almost completely discharged during adsorption. Briceno and Chander25 studied the oxidation of hydrosulfide on gold electrodes by cyclic voltammetry and ac impedance. Depending on the potential explored in the oxidation region, several processes were described: underpotential deposition, formation of a monolayer of adsorbed sulfur, and formation of polysulfide intermediates. On the basis of the recent literature, it is clear that the adsorption/desorption mechanism of adsorbed organothiols is worthy of further study. Elucidation of the mechanism should allow a better control of the final structure of the monolayer and of its potential stability region. In this study we report results concerning the desorption/readsorption process undergone by 2-mercaptoethanesulfonate (HS-(CH2)2-SO3- or MES) spontaneously adsorbed on Au(111). In a previous study Nordyke and Buttry26 have used the disulfide derivative of MES, among other compounds, to study the influence of the surface functionalization on the adsorptive behavior of (ferrocenylmethyl)dodecyldimethylammonium. A method to infer the surface coverage of the disulfide derivative was presented. This was based on the amount of redox surfactant desorbed on increasing the concentration of the disulfide reagent in solution. However only relative values can be determined. Experimental Section The sodium salt of 2-mercaptoethanesulfonic acid was purchased from Aldrich and used without further purification. Solutions of 0.01 M HClO4 and 0.01 M NaOH were prepared from redistilled 99.999% pure HClO4 (Aldrich), NaOH (Fisher), and ultrapure water (17.8-18.0 MΩ cm, Barnstead system). The single-crystal gold electrode (111) was purchased from Metals Crystals and Oxides, England. Its preparation involved polishing with different sizes of alumina and diamond paste on a polishing wheel (Buehler), electropolishing in perchloric acid solution, and finally flame annealing as described previously.27 The quality of the single-crystal electrode was checked by measuring a cyclic voltammogram at a scan rate of 20 mV/s in 0.01 M HClO4 before each set of experiments. The real surface area was determined from the charge neccesary to form a monolayer of gold oxide28,29 (386 µC cm-2 ). An area of 0.148 cm2 was obtained for the present electrode. All measurements were carried out in a conventional threeelectrode electrochemical cell at 25 ( 0.5 °C. A gold wire of large surface area was used as counter electrode, and a saturated calomel electrode was used as reference electrode. Modification of Au(111) with the organic molecule was performed in the following way. The electrode was immersed into a solution containing 10 mM of MES in 0.01 M HClO4, and a meniscus formed. In this way only the bottom-working surface of the cylindrical electrode was modified. After 5 min the electrode was removed and washed extensively with 0.01 M NaOH and then transferred to the electrochemical cell, keeping on its surface a drop of liquid. By washing with 0.01 M NaOH, the incorporation of additional species into the electrolyte solution where the desorption process is carried out is avoided. Longer inmersion times up to 2 h in the MES solution did not affect the amount (24) Wierse, D. G.; Lohrengel, M. M.; Schultze, J. W. J. Electroanal. Chem. 1978, 92, 121. (25) (a) Briceno, A.; Chander, S. J. Appl. Electrochem. 1990, 20, 506; (b) 1990, 20, 512. (26) Nordyke, L. L.; Buttry, D. A. Langmuir 1991, 7, 380. (27) (a) Fawcett, W. R.; Fedurco, M.; Kova´cˇova´, Z.; Borkowska, Z. Langmuir 1994, 10, 912; (b) J. Electroanal. Chem. 1994, 368, 265. (28) (a) Rand, D. A. J.; Woods, R. J. Electroanal. Chem. 1971, 31, 29; (b) 1972, 35, 209. (29) Woods, R. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1976; Vol. 9, pp 119-125.
Langmuir, Vol. 12, No. 23, 1996 5697
Figure 1. Cyclic voltammograms for a MES-coated Au(111) electrode in 0.01 M NaOH for two different scan rates: (a) 20 mV/s and (b) 100 mV/s. Solid and dotted lines represent the first and second scan, respectively. of adsorbed MES ions, as determined from the reductive desorption charge. Prior to measurements the solution was deareated by passing argon for 8 min. Cyclic voltammetry and chronoamperometry were performed using a EG&G 273 potentiostat connected to a 80286 PC computer via an IEEE-488 interface.
Results and Discussion Cyclic Voltammetry. The desorption process undergone by MES spontaneously adsorbed on Au(111) has been studied by using cyclic voltammetry and chronoamperometry. Figure 1 shows two successive cyclic voltammograms for the modified electrode after transferring to a solution containing 0.01 M NaOH. For similar compounds the peak observed in the first scan (peak A) has been assigned to the reductive desorption (RD) process:19-21
Au-SR + 1e- f Au + RS-
(1)
The small peak that appears at more positive potentials in the second scan (peak B) is ascribed to the desorption of the MES readsorbed during the first anodic scan. This is supported by the fact that the charge associated with peak B increases with increasing the scan rate from 2.2 µC cm-2 for 20 mV/s to 10 µC cm-2 for 100 mV/s. When the time scale over which diffusion is operating is smaller, more molecules can be readsorbed in the anodic scan. As expected the charge associated with peak A does not depend on the scan rate. On the other hand the more positive potential of peak B reveals that the readsorbed MES is desorbed easier than the MES spontaneously adsorbed during the immersion procedure. This can be a consequence of changes in the microenvironment and/ or the potential of zero charge as the amount of adsorbate decreases. In the following text we will use the symbols MESSA and MESRA for the spontaneously adsorbed and electrochemically readsorbed MES ions, respectively. It should be noted that after the third scan there is no longer evidence for any desorption peak. However, the resulting voltammogram does not coincide with that corresponding to the background electrolyte (Figure 2). Its features do not match those expected for the capacitative response of a coated or uncoated electrode. The forward and reversal scans are not symmetrical with respect to the zero line, and the cathodic current is larger than that observed for the background electrolyte. These observations seem to indicate the onset of a faradaic process, which presumably also operates together with the desorption of MES. Due to the proximity of the potential region where the hydrogen evolution reaction takes place, the underlying process can be identified with the reduction of the solvent. Ohmori et al.30 studied the hydrogen evolution reaction on gold in alkaline solutions. (30) Ohmori, T.; Enyo, M. Electrochim. Acta 1992, 37, 2021.
5698 Langmuir, Vol. 12, No. 23, 1996
Figure 2. Comparison of cyclic voltammograms for (a) a clean Au(111) electrode and (b) an MES modified Au(111) electrode after five cycles in the RD region in 0.01 M NaOH. Scan rate 20 mV/s.
They showed that at high concentration (0.01-0.5 M) of NaOH and KOH, underpotential deposition (upd) of the alkali metal at the level of a submonolayer takes place concomitantly with hydrogen evolution at potentials around -1.14 V vs SCE. For the present system it was observed that lowering the scan rate results in the presence of a loop in the cyclic voltammogram at the more negative potentials (
