Adsorption of Carboxylic Acids on Gold by Anodic Reaction

Woon-kie Paik*, Subong Han, Woonsup Shin, and Yousung Kim. Department of Chemistry, Sogang University, Seoul 121-742, Korea. Langmuir , 2003, 19 (10),...
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Adsorption of Carboxylic Acids on Gold by Anodic Reaction Woon-kie Paik,* Subong Han, Woonsup Shin, and Yousung Kim Department of Chemistry, Sogang University, Seoul 121-742, Korea Received November 13, 2002. In Final Form: March 17, 2003 Adsorption of carboxylic acids from liquid solutions on gold was investigated by electrochemical experiments including voltammetric measurements and quartz crystal microgravimetry. The adsorption was found to occur at gold surfaces held at relatively high electrical potentials, and the adsorption step was associated with anodic currents. From the experimental evidences, we propose that the carboxylic acid molecules adsorb on a gold surface through an anodic reaction that is analogous to the previously studied adsorption reaction of organic sulfur compounds forming self-assembled monolayers. The potentials at which the carboxylic acids adsorb to appreciable extents were much higher than the potentials at which thiols start to adsorb. In all of the adsorption reactions of thiols, disulfides, and carboxylic acids, oxidation of the metal surface, assisted by the adsorbate-metal interaction, appears to be a common requirement for the adsorption.

Introduction Various molecules form self-assembled monolayers (SAMs) by first adsorbing on solid surfaces due to special interactions between the molecules and the surface and by subsequent organization by interactions among the adsorbed molecules. The orderly structures and interesting properties of these SAMs have been attracting an enormously large number of investigators in the past two decades.1-12 However, the nature of the moleculesubstrate interaction and the processes leading to the adsorbate-substrate bonding have in most cases, especially in the case of carboxylic acids, eluded unambiguous clarification. Even for the much-studied adsorption of thiols and disulfides, the mechanism of the process has not been clear for some time, and certain misguided explanations have been prevailing. We have reported that thiols were found to adsorb on gold or silver from liquid solutions through an electrochemical mechanism, in which the metal surface is oxidized as the metal-sulfur bond is formed.13-16 The hydrogen originally attached to the sulfur atom is released as a hydronium ion instead of the molecular hydrogen supposedly released in the previously * Corresponding author. Present address: Department of Chemistry, Pohang University of Science and Technology, Pohang, 790784 Korea. Telephone: +82-11-9786-1319. Fax: +82-2-701-0967. E-mail: [email protected]. (1) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (2) Schlenoff, J. B.; Li, M.; Ly, H. J. Am. Chem. Soc. 1995, 117, 12528. (3) Duan, C. M.; Meyerhoff, M. E. Mikrochim. Acta 1995, 117, 195. (4) Finklea, H. O. In Electroanalytical Chemistry: a Series of Advances, Vol. 19; Bard, A. J., Rubinstein, I., Ed.; Marcel-Dekker: New York, 1996; Vol. 19, pp 109-335. (5) Ulman, A. Chem. Rev. 1996, 96, 1533. (6) Arnold, S.; Feng, Z. Q.; Kakiuchi, T.; Knoll, W.; Niki, K. J. Electroanal. Chem. 1997, 438, 91. (7) Hong, S. H.; Zhu, J.; Mirkin, C. A. Science 1999, 286, 523. (8) Nishiyama, K.; Ueda, A.; Tanoue, S.; Koga, T.; Taniguchi, I. Chem. Lett. 2000, 930. (9) Zhao, J. W.; Uosaki, K. Langmuir 2001, 17, 7784. (10) Lee, S. B.; Martin, C. R. Anal. Chem. 2001, 73, 768. (11) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418. (12) Uosaki, K.; Kondo, T.; Okamura, M.; Song, W. B. Faraday Discuss. 2002, 121, 373. (13) Eu, S.; Paik, W. Chem. Lett. 1998, 405. (14) Eu, S.; Paik, W. Mol. Cryst. Liq. Cryst. 1999, 337, 49. (15) Paik, W.; Eu, S.; Lee, K.; Chon, S.; Kim, M. Langmuir 2000, 16, 10198. (16) Chon, S.; Paik, W. Phys. Chem. Chem. Phys. 2001, 3, 3405.

assumed mechanisms. Organic disulfides were also found to adsorb through an electrochemical step in which oxidation of the gold surface is involved, although a net reduction current is observed due to the reductive dissociation of the S-S bond. Carboxylic acids and derivatized carboxylic acids are among the compounds that form SAMs by adsorption on the surfaces of metals and metal oxides. They adsorb readily on oxides of aluminum, silver, and copper17 and on indium-tin oxide.18,19 Carboxylic acids and derivatized carboxylic acids adsorb on copper group metals including silver and gold.20-26 The ionizable hydrogen atom of the acid is reported to leave from the carboxylic group with the resulting carboxylate ion adsorbing with the two oxygen atoms attached to the metal surface.17,19-21 This is analogous to the adsorption of thiols, where the hydrogen atom is removed from the S-H group of the thiols as the S-Au bond is formed.27-31 Tao et al. reported that forced protonation with vapor phase HCl caused serious structural rearrangement of alkanoate SAMs on Ag and Cu to different (disordered) structures of acid.32 Despite the (17) Tao, Y. T. J. Am. Chem. Soc. 1993, 115, 4350. (18) Yan, C.; Zharnikov, M.; Golzhauser, A.; Grunze, M. Langmuir 2000, 16, 6208. (19) Oberg, K.; Persson, P.; Shchukarev, A.; Eliasson, B. Thin Solid Films 2001, 397, 102. (20) Zelenay, P.; Waszczuk, P.; Dobrowolska, K.; Sobkowski, J. Electrochim. Acta 1994, 39, 655. (21) Han, S. W.; Ha, T. H.; Kim, C. H.; Kim, K. Langmuir 1998, 14, 6113. (22) Li, H. Q.; Roscoe, S. G.; Lipkowski, J. J. Electroanal. Chem. 1999, 478, 67. (23) Han, S. W.; Seo, H.; Chung, Y. K.; Kim, K. Langmuir 2000, 16, 9493. (24) Zhang, Z. J.; Imae, T. Nano Lett. 2001, 1, 241. (25) Zhang, Z. J.; Yoshida, N.; Imae, T.; Xue, Q. B.; Bai, M.; Jiang, J. Z.; Liu, Z. F. J. Colloid Interface Sci. 2001, 243, 382. (26) Zhao, X. Y.; Zhao, R. G.; Yang, W. S. Langmuir 2002, 18, 433. (27) Kwon, C. K.; Kim, K.; Kim, M. S.; Lee, S. B. Bull. Korean Chem. Soc 1989, 10, 254. (28) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 3629. (29) Bryant, M. A.; Pemberton, J. E. J. Am. Chem. Soc. 1991, 113, 8284. (30) 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. (31) Li, Y.; Huang, J.; McIver, R. T., Jr.; Hemminger, J. C. J. Am. Chem. Soc. 1992, 114, 2428. (32) Tao, Y. T.; Hietpas, G. D.; Allara, D. L. J. Am. Chem. Soc. 1996, 118, 6724.

10.1021/la026836s CCC: $25.00 © 2003 American Chemical Society Published on Web 04/16/2003

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similarity between the adsorption of thiols and carboxylic acids, most of the carboxylic acids were reported to adsorb on gold only when highly positive electrical potentials are applied to the metal,20,22,33 unlike thiols that readily adsorb on gold surfaces in the absence of potential control. The reported radioactive tracer and IR measurements under applied potentials indicated that adsorption of carboxylic acids to appreciable coverage starts at a certain electrode potential of gold. The coverage reached its maximum values at more positive potentials after sharply increasing with increasingly positive potential (between ∼0.3 and 1.0 V vs SCE for acetic acid;33 between ∼0.8 and 1.2 V for benzoic acid20). These reported potential ranges are much higher than those for adsorption of thiols, which include the range of uncontrolled rest potentials. Kim et al. reported that a quinone acid, anthraquinone-2-carboxylic acid (AQCA), was found to adsorb on gold spontaneously whereas numerous other monocarboxylic acids did not adsorb on gold although they readily adsorbed on silver.34 The fact that the carboxylic acids adsorb on gold at high positive potentials suggests that the adsorption process may involve an anodic reaction that is similar to the case of thiol adsorption. The above-mentioned report that only the quinone acid AQCA spontaneously adsorbed on gold was also intriguing because the oxidizing nature of the quinone moiety might have helped the oxidative adsorption. In this article we report results from our experimental study on the electrochemical nature of the adsorption process of carboxylic acids on gold, and the results were discussed in light of the previous findings on the adsorption of organic sulfur compounds. An electrochemical adsorption scheme is presented. Experimental Section Organic acids of highest purity grade were purchased from Sigma-Aldrich, and reagent grade perchloric acid was purchased from Duksan Chemical. These chemicals were used as obtained. Water used as the solvent was distilled and deionized water of resistivity greater than 17 MΩ cm. Dimethylformamide (DMF) used as the solvent was reagent grade purchased from SigmaAldrich and was used as obtained. Gold-coated quartz crystal oscillator electrodes supplied by International Crystal Manufacturing or by Elchema were used for the electrochemical and electrochemical quartz crystal microgravimetry (EQCM) experiments. In EQCM, one of the goldplated exciting oscillator electrodes of the quartz crystal served as the working electrode of the electrochemical cell. The cells were equipped with a Pt counter electrode and a reference electrode. The reference electrode was Ag/AgCl in 3 M NaCl, separated from the rest of the cell in a tube with a Luggin capillary ending. The potential values presented throughout this paper refer to this reference electrode. A digital potentiostat (BAS 100) with low current module was used for the potential control or for recording of the open-circuit potential. The change in the oscillation frequency of the quartz crystal, considered to reflect the mass change on the surface, was monitored after addition of the organic acids to the solution. The electrochemical quartz crystal microbalance from Elchema in combination with an Auplated 10-MHz quartz crystal oscillator electrode had a 1-Hz resolution and gave 4.42 ng cm-2 Hz-1 sensitivity. Thus, the measurement limit in the mass change was a few nanograms. A gold flag (99.99% purity) of about 1 cm2 surface area was used as the substrate electrode in the open-circuit potential measurements. The electrode surfaces were precleaned by immersion in hot “piranha solution” (H2O2-H2SO4 1:3 mixture) and by repeatedly scanning the potential between the hydrogen evolution (33) Corrigan, D. S.; Krauskopf, E. K.; Rice, L. M.; Wieckowsky, A.; Weaver, M. J. J. Phys. Chem. 1988, 92, 1596. (34) Han, S. W.; Joo, S. W.; Ha, T. H.; Kim, Y.; Kim, K. J. Phys. Chem. B 2000, 104, 11987.

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Figure 1. Cyclic voltammogram of gold in 0.1 M HClO4 in the absence of organic acid (dotted line) and after addition of n-hexanoic acid (full line) to 1 mM concentration. The scan rate was 50 mV s-1. and the oxygen evolution regions in dilute sulfuric acid. The geometric surface area of the EQCM electrode was 0.20 cm2. However, the plating of gold was made on a roughened crystal surface and the gold surface was further roughened by the cleaning and the cyclic scans of potential before the experiments with added organic acids. The roughness factor of the electrode after the above treatments was estimated to be approximately between 3 and 4 by the measured reduction charge of surface oxide on gold. The electrolyte solutions used were aqueous 0.01 or 0.1 M HClO4 or 0.1 M HClO4 in dimethylformamide (DMF). For most of the experiments, the solutions were deaerated with purified (99.999%) nitrogen to avoid oxygen contamination. Bubbling nitrogen through the solution vigorously also helped homogenizing of the solution when the adsorbate was injected. Samples of organic acids (or DMF solution in the case of AQCA) were dispensed from a microsyringe to the electrolyte solutions to attain the desired concentration of the resulting solution, mostly 10 mM.

Results and Discussion To study the effect of organic acids on the electrochemical properties of gold and to see whether adsorption of carboxylic acids is accompanied by electrochemical oxidation, we performed cyclic voltammetry of a gold electrode in dilute solutions of carboxylic acids. In Figure 1 the dotted line is the cyclic voltammogram (CV) of gold obtained in the background electrolyte 0.1 M HClO4 solution, and the solid line is the CV obtained after a small amount of n-hexanoic acid was introduced into the solution to the concentration of 1 mM. The clear difference between the CV traces with and without the carboxylic acid in the positive range of potential shows that the electrode is affected by adsorbed carboxylic acid at least in the range of potential where the difference is marked. The usual oxide formation on the Au surface beginning from about 1.0 V is suppressed by the presence of the carboxylic acid. The oxidation current starts at a higher potential (∼1.1 V). The reduction peak near 0.9 is smaller than that obtained without the organic acid, which indicates that the amount of surface oxide of gold formed during the anodic scan is smaller in this case due to the suppression of oxide formation by the adsorbed carboxylate. At higher potentials oxide formation seems to replace the adsorbed acids. Similar effects of adsorbed carboxylic acid on CV traces of gold and platinum were reported by Zelenay et al.20 and by Corrigan et al.33 There is also a subtle difference in the CVs near 0.6 V. Figure 2 provides an expanded view of this region. A small

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Figure 2. Expanded view of the cyclic voltammogram near 0.6 V.

reversible pair of anodic and cathodic peaks is visible. This pair, which appears only when the carboxylic acid is present, indicates that an anodic oxidation occurs on the surface of the Au electrode with adsorption of the acid and a cathodic reduction can occur later on reversing the potential. The acid itself cannot be oxidized at this potential, and consequently, the oxidation must be of the gold surface, assisted by the carboxylic acid molecules adsorbing. In previous studies we revealed that adsorption of thiols on gold or silver occurs through an electrochemical oxidation:13-16

RSH + M f RS-M + H+ + e-(M) (M ) Au, Ag) (1) where the metal surface (M) is oxidized simultaneously with the adsorption of the RS group. The above reaction scheme was supported by evidences including negative potential shifts of the metal substrates and also by generation of anodic currents observed at fixed potentials, similarly to the experimental findings from the present study (see below). Reversible desorption of the thiolates at very negative potentials was reported by many authors.35-41 The reversible desorption makes the oxidative adsorption mechanism more plausible because the adsorption reaction is simply the reverse of the desorption reaction. The appearance of the reversible current peak in Figure 2 indicates that the adsorbed carboxylate desorbs at negative potentials. These similarities between the cases of thiols and carboxylic acids lead us to propose a similar anodic mechanism of adsorption of carboxylic acids as follows:

RCOOH + Au f RCOO-Au + H+ + e-(Au) (2) This reaction, generating electrons, should lower the potential of gold when the flow of current is not allowed, and consequently, a continuous reaction would be retarded (35) Weisshar, D. E.; Lamp, B. D.; Porter, M. D. J. Am. Chem. Soc. 1992, 114, 5860. (36) Schneider, T. W.; Buttry, D. A. J. Am. Chem. Soc. 1993, 115, 12391. (37) Calvente, J. J.; Kovacova, Z.; Andreu, R.; Fawcett, W. R. Langmuir 1996, 12, 5696. (38) Hatchett, D. W.; Stevenson, K. J.; Lacy, W. B.; Harris, J. M.; White, H. S. J. Am. Chem. Soc. 1997, 119, 6596. (39) Hagenstrom, H.; Schneeweiss, M. A.; Kolb, D. M. Langmuir 1999, 15, 2435. (40) Wano, H.; Uosaki, K. Langmuir 2001, 17, 8224. (41) Yang, D. F.; Morin, M. J. Electroanal. Chem. 1998, 441, 173.

Figure 3. Current peaks on injection of hexanoic acid on a Au electrode at (A) 900 mV, and (B) 600 and 500 mV in aqueous 0.1 M HClO4 solution. The resulting concentration of hexanoic acid was 1 mM. The points of injection are set to time zero.

unless the electrons are consumed elsewhere. If the potential of the electrode immersed in an electrolyte solution is fixed in the range where the above reaction is possible and a carboxylic acid is injected into the system, an anodic current should be observed, as was in the case of thiols adsorbing on gold or silver. Figure 3 represents such observations. At 900 mV (Figure 3A), a clearly defined current peak over the background current level was visible in the figure drawn in the expanded current scale. At potentials below 600 mV (Figure 3B), the current peaks appearing after injection of the acid were extremely small,42 whereas, in the previously studied case of thiols, the current was quite large even at a few hundred millivolts below 500 mV. The tendency of current peaks increasing with rising electrical potential is exactly what is expected of an anodic reaction. Because the adsorption process is an anodic reaction, it naturally occurs faster (larger current) to greater extents. The carboxylic acids appear to adsorb on gold only at relatively high anodic potentials, and to lesser extents than thiols if the potential is not high enough. This difference of carboxylic acids from the sulfur compounds in the adsorption affinity is due to lack of the strong interaction such as the bonding of sulfur to gold or silver. The approximate integrated anodic current of the CV around 0.55 V (Figure 2) and the integrated area under the current peak at 900 mV of Figure 3A are about 3 × 10-5 C cm-2, which is on the order of (42) Both the background current and the noise levels at 900 mV in Figure 3 were higher than those at the lower potentials. The background current, which we were unable to suppress on repeated trials with newly prepared solutions, was apparently diffusion controlled and, hence, showed the effect of stirring by nitrogen gas bubbling. The measurements at lower potentials were made with a low current module that had a noise filtering facility.

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Figure 4. Response of open circuit potential of gold on injection of hexanoic acid.

Figure 5. Response of current to potential program and stirring of the solution after injection of AQCA solution in DMF solution.

monolayer coverage. The integrated currents at 500 and 600 mV are one to 2 orders of magnitude smaller, indicating that extents of adsorption at those potentials are much smaller than monolayer. All the above estimations of charges are not quite accurate due to the current levels that are not sufficiently larger compared to the background currents or the double layer charging currents. The anodic and cathodic current peaks in the CV are reversible counterparts to each other. The desorption reaction is the reverse of reaction 2. It would be necessary to know the surface structure of carboxylate-covered gold to calculate the amount of adsorbed carboxylate at monolayer coverage and the associated charge. The dense adlayer of thiols on gold(111) with (x3 × x3)R30° structure contains 7.8 × 10-10 mol cm-2 of thiol which consumes 75 µC cm-2 of anodic charge on adsorption. The adsorption number density of the carboxylate ions may be close to that of thiol molecules, or it could be somewhat smaller considering the fact that each of the carboxylate ions having two oxygen atoms is bulkier than a sulfur atom. The adsorption charge from the integrated anodic peak at 900 mV, ∼30 µC cm-2, is smaller than the adsorption charge of thiols, 75 µC cm-2, but is of the same order of magnitude. The numbers extracted from the literature data are 1.8 × 10-10 mol cm-2 (AQCA by Han et al.34), ∼5 × 10-10 mol cm-2 (acetic acid by Weaver et al. from radiotracer measurement;33 benzoic acid by Sobkowsky et al.20), and 18 × 10-10 mol cm-2 (acetic acid by Weaver et al. from IR33) at their conditions of maximum adsorption. If the above adsorption reaction occurs with the substrate cut off from the electrochemical control circuit, the electrical potential of the substrate, the open-circuit potential, should shift to the negative direction due to the electrons generated. The response of the open-circuit potential of gold to injection of the carboxylic acids was, however, not as markedly clear as in the case of thiols. It was not easy to observe the response in the open-circuit potential with carboxylate adsorption, because it is hard to maintain the gold electrode at the relatively high potentials necessary for the adsorption to occur, due to inevitable drift of the open circuit potential when oxygen was purged out of the solution. Figure 4 shows the potential excursion before and after hexanoic acid was added. The sudden negative shift of the potential is evident with hexanoic acid, although it is somewhat obscured by the natural drift. In separate experiments to record possible potential shift with adsorption of benzoic acid, we added sodium benzoate into the perchloric acid solution. Usually

with the benzoate addition, although there was not a clear sudden shift of potential, the potential drift showed clear inflections indicating an additional down swing due to adsorption (not shown). The reproducibility of this behavior was poor, giving different shapes and magnitudes of the inflection, but the tendency of down swing was consistent. The benzoate added will be protonated to benzoic acid because the background electrolyte was acidic. There may have been some disturbance in potential due to a small temperature fluctuation caused by the protonation reaction of benzoate ions.43 These negative potential shifts also support the anodic reaction scheme for adsorption of the acids. However, the potential shift is much less distinguished in comparison to the case of the sulfur compounds,13-16,44 which is most probably due to the fact that the anodic reaction with the carboxylic acids proceeds at slower rates and to smaller extents than that with the sulfur compounds at comparable potentials. The electric current response of adsorption of anthraquinone-2-carboxylic acid (AQCA) was tested with a DMF solution of the acid and a 0.1 M LiClO4 solution in DMF as the electrolyte. Figure 5 shows the potential scan program applied to cleanse the electrode before the addition of the acid solution and the current response before and after injection of a DMF solution of AQCA. Here, no current was observed right after the acid addition. However, as the solution was vigorously stirred by a magnetic stirrer, a small anodic current appeared, as can be seen in the figure. The current subsided when the stirring was stopped. AQCA appears to adsorb on gold also by anodic reaction when the molecules are brought in contact with the gold surface by convection and diffusion. The high viscosity values of the acid and the electrolyte solutions seemed to have prevented rapid mixing and diffusion of the acid molecules to reach the gold surface in the absence of stirring. The original hypothesis we made on the role of the quinone group in helping the oxidative adsorption as mentioned in the Introduction part could not be borne out in the present study. The reversible potential of the quinone-hydroquinone pair was somewhat lower than the potential necessary to drive the adsorption of a carboxylic acid. However, it is not possible either to rule out the role of the oxidizing property of the (43) In these experiments we added sodium benzoate instead of benzoic acid into the HClO4 solution for better solubility. We used acidic solution in order to avoid oxide formation or oxygen evolution at high potentials. (44) Zhong, C. J.; Woods, N. T.; Dawson, G. B.; Porter, M. D. Electrochem. Commun. 1999, 1, 17.

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anthraquinone moiety, possibly in conjunction with other factors, such as the interaction between the carboxylate and gold, in driving the adsorption. In fact, the IR spectrum of Kim et al. in their study of adsorption of AQCA on gold34 showed a decreasing peak of the central ring CdO stretching on adsorption, indicating reduction of the quinone into hydroquinone. The electrode frequency shifts in the EQCM experiment with cyclic potential scans were recorded before and after addition of a carboxylic acid, as in Figure 6. The frequency shifts are converted into changes in the surface mass according to the Sauerbrey equation, assuming that the frequency shifts are mostly due to the mass changes.45,46 A distinguishing feature of the mass curve obtained in the carboxylic acid solution is that the mass increase occurs at lower potential than that of the oxide formation in the absence of the acid. The adsorption is shown to occur on the anodic sweep at about 0.6 V, in agreement with the findings from the CV experiments (Figure 1) and the anodic peak observations at various potentials (Figure 3). Our attempts to obtain quantitative estimation of adsorbed carboxylic acids by EQCM experiments were mostly unsuccessful. In many cases the frequency of the oscillating crystal drifted upward after injection of the organic acids at a constant potential. If the frequency is governed primarily by the mass change on the surface, the frequency should have decreased with the adsorption of the acids. One possible cause of this anomalous upward drift of the frequency may be corrosive removal of the surface layer of gold, similar to the lifting of a gold surface atomic layer suspected as the cause of pit-formation in the SAMs of thiols on gold.47-50 Carboxyl acids are known to assist corrosion of metal surfaces due to the anionic property as ligands to the metal ions.51,52 Viscoelastic effects at the electrode-electrolyte interface or displacement of adsorbed ions is considered not to impart a large effect, such as the unexpected frequency behavior. Now that we have examined the electrochemical oxidative steps involved in the adsorption of thiol and carboxylic acid molecules, a general overview of adsorption reactions of the self-assembling sulfur compounds and carboxylic acids is in order. In our previous studies dialkyl disulfide molecules were found to adsorb on Au and Ag by a reaction that gives a net cathodic current:13-16

RSSR′ + M + e-(M) f RS-M + R′S- (M ) Au, Ag) (3) followed by R′S- + M f R′S-M + e-(M)

(4)

It should be noted that the net observed current from the above two sequential reaction steps is cathodic because only part of the thiolate R′S- ions generated by reaction 3 is consumed in the following reaction 4 due to diffusion of the thiolate ions away from the electrode surface. Also to be noted is that reaction 3 is a reductive dissociation of the S-S bonds that require 2-electron transfer and at (45) Buttry, D. A.; Ward, M. D.Chem. Rev. 1992, 92, 1355. (46) Bruckenstein, S.; Hillman, A. R. In The Handbook of Surface Imaging and Visualization; Hubbard, A. T., Ed.; CRC: Boca Raton, FL, 1995. (47) Poirier, G. E.; Tarlov, M. J. J. Phys. Chem. 1995, 99, 10966. (48) McDermott, C. A.; McDermott, M. T.; Green, J.-B.; Porter, M. D. J. Phys. Chem. 1995, 99, 13257. (49) Edinger, K.; Grunze, M.; Woll, C. Ber. Busen-Ges. 1997, 101, 1811. (50) Yamada, R.; Uosaki, K. Langmuir 1997, 13, 5218. (51) Persson, D.; Leygraf, C. J. Electrochem. Soc. 1995, 142, 1468. (52) Lopez-Delgado, A.; Cano, E.; Bastidas, J. M.; Lopez, F. A. J. Electrochem. Soc. 1998, 145, 4140.

Figure 6. Mass change of a Au electrode with potential scans before and after addition of hexanoic acid in 0.01 M HClO4.

the same time a 1-electron oxidation of gold surface atoms, as can be written in the following two equations:

RSSR′ + 2e-(M) f RS- + R′S-

(3a)

RS- + M f RS-M + e-(M)

(3b)

Therefore, it becomes evident that, in all of the adsorption reactions of thiols and disulfides and also in adsorption of carboxylic acids, surface oxidation of the substrate metal is a common requirement for the adsorption. The surface oxidation is undoubtedly assisted by S-M bond formation in the case with the sulfur compounds, hence easy adsorption of the sulfur compounds at lower electrical potentials than those for the carboxylic acids. Silver and copper are more readily oxidized at lower potentials than that for gold. That seems to be one reason silver and copper can adsorb carboxylic acids readily

RCOOH + M f RCOO-M + H+ + e- (M ) Ag, Cu) (2′) whereas gold cannot adsorb carboxylic acid via the above reaction without manipulation of the potential. In fact the “clean” silver and copper surfaces are in an usual environment in the oxidized state by reaction with oxygen. Carboxylic acids may adsorb on such surfaces by replacement of the oxide ions by the carboxylate ions, which amounts to an acid-base reaction: RCOOH + 1/2M2O f RCOO-M + 1/2H2O (M ) Ag, Cu). It can be speculated that the interaction between the metal surface and the carboxylate is partly ionic whereas the bond between the metals and sulfur is mostly of covalent nature. Understanding the correct mechanism of adsorption is profoundly important in many respects. The electrochemical mechanism dictates that the adsorption steps are strongly influenced by the electrical potential of the substrate or the presence of the substance in the solution that can influence the potential. Any meaningful measurement of the rate of adsorption or the adsorption equilibrium should be made under potential control. Kinetic measurements made under potential control, however, have been scarce except for a few electrochemical impedance studies.53,54 A potential-controlled adsorption (53) Riepl, M.; Mirsky, V. M.; Wolfbeis, O. S. Mikrochim. Acta 1999, 131, 29. (54) Subramanian, R.; Lakshminarayanan, V. Electrochim. Acta 2000, 45, 4501.

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process or adsorption in the presence of an oxidizing substance could be used beneficially for fast formation of SAMs of better quality. Acknowledgment. The Korea Science and Engineering Foundation supported this research. Part of the data

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acquisition facilities was made by Heung Cho Ko. This manuscript was prepared using the library facilities of Kumamoto University and Pohang University of Science and Technology. LA026836S