Electrochemical Formation of a Polyaniline-Analogue Monolayer on a

Eugen P. Koval'chuk , Volodymyr M. Ogenko , Oleksandr V. Reshetnyak , Orest B. Pereviznyk , Natalya Davydenko , Iryna E. Marchuk. Electrochimica Acta ...
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Langmuir 1999, 15, 6807-6812

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Electrochemical Formation of a Polyaniline-Analogue Monolayer on a Gold Electrode Susumu Kuwabata,*,† Ryozo Fukuzaki,† Matsuhiko Nishizawa,† Charles R. Martin,‡ and Hiroshi Yoneyama*,† Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamada-oka 2-1, Suita, Osaka 565-0871, Japan, and Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523 Received December 15, 1998. In Final Form: June 8, 1999 Electrochemical preparation of polymerized self-assembled monolayers on an Au(111) substrate has been attempted by using aminobenzenethiol and 3-aminophenethylthiol. A self-assembled monolayer of aminobenzenethiol was easily polymerized if it consisted of ortho and meta isomers in their molar ratio of 1:1 but not at a single component monolayer of each isomer. In contrast, a self-assembled monolayer of a single component of 3-aminophenethylthiol allowed its easy electrochemical polymerization. The polymerized monolayers gave electrochemical activities very similar to those of conventional polyaniline and its derivatives. Furthermore, the self-assembled monolayer of these thiols got high stability against reductive desorption in an alkaline solution when polymerized. Underpotential deposition of Cu gave Cu coverage of 41% and 65% on the electrodes coated with the polymerized monolayers of aminobenzenethiol and 3-aminophenethylthiol, respectively.

Introduction A self-assembled monolayer (SAM) of alkanethiol provides a convenient way to attach special functions to the electrode substrates such as Au, Ag, and Cu.1-3 Especially much attention has been paid to attachment of redox activities to the electrodes3-5 and fabrication of electrode surfaces allowing control of electrochemical reactions such as deposition of conducting polymers.6-8 As recognized from these studies, SAMs can be electrochemically utilized, but the potential window where the SAM-coated electrodes are useful is limited by oxidative or reductive desorption of thiols.3,9-13 Crooks et al. recently reported that photopolymerized SAM made of thiols having diacetylene groups was very durable against desorption,14 suggesting that the polymerization of the SAM provides reinforcement of durability of the SAM. Polymerization of SAMs was also attempted by McCarley * To whom correspondence should be addressed. Telephone and Fax: +81-6-6879 7374. E-mail: [email protected]. † Osaka University. ‡ Colorado State University. (1) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: San Diego, 1991. (2) Bard, A. J.; Abruna, H. D.; Chidsey, C. E.; Faulkner, L. R.; Feldberg, S. W.; Itaya, K.; Majda, D.; Murray, R. W.; Porter, M. D.; Soriaga, M. P.; White, H. S. J. Phys. Chem. 1993, 97, 7147. (3) Finklea, H. O. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol. 19, pp 109335 and references therein. (4) Shimazu, K.; Yagi, I.; Sato, Y.; Uosaki, K. J. Electroanal. Chem. 1994, 732, 117. (5) Rowe, G. K.; Carter, M. T.; Richardson, J. N.; Murray, R. W. Langmuir 1995, 11, 1797. (6) (a) Rubinstein, I.; Rishpon, J.; Sabatani, E.; Redondo, A.; Gottesfeld, S. J. Am. Chem. Soc. 1990, 112, 6136. (b) Rubinstein, I.; Gottesfeld, S.; Sabatani, E. U.S. Patent No. 5,108,573, 1992. (7) Hayes, W. A.; Kim, H.; Yue, X.; Perry, S. S.; Shannon, C. Langmuir 1997, 13, 2511. (8) Turyan, I.; Mandler, D. J. Am. Chem. Soc. 1998, 120, 10733. (9) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335 (10) Schneider, T. W.; Buttry, D. A. J. Am. Chem. Soc. 1993, 115, 12391. (11) Everett, W. R.; Fritsch-Faules, I. Anal. Chim. Acta 1995, 307, 253. (12) Zhong, C. J.; Porter, M. D. J. Electroanal. Chem. 1997, 425, 147. (13) Hobara, D.; Miyake, K. Imabayashi, S.; Niki, K.; Kakiuchi, T. Langmuir 1998, 14, 3590.

et al.,15 Collard et al.,16 and Smela et al.17 with the use of alkylthiols having pyrrole units. Nonaka et al.18 reported that a SAM of o-aminobenzenethiol could be polymerized by electrochemical oxidation if the SAM was formed on a poly-Au substrate under ultrasonic irradiation. In this paper, we report that electrochemical polymerization of a SAM of aminobenzenethiol formed on an Au(111) substrate occurs easily only if the SAM is composed of ortho (o-ABT) and meta (m-ABT) isomers, whereas that of 3-aminophenethylthiol having two methylene groups between the benzene ring and the terminal thiol group occurs easily. In addition, underpotential deposition of Cu on the polymerized SAM-coated electrodes was investigated to obtain information on coarseness of the polymerized monolayers. Experimental Section Aminobenzenethiols (ABT) of ortho, meta, and para isomers were purchased from Aldrich and purified by distillation prior to use. 3-Aminophenethylthiol (3-APT) was prepared with the use of 3-nitrophenethyl bromide as a starting material. 3-Nitrophenethyl bromide (2.5 g) and 0.83 g of thiourea were added to 50 mL of ethanol, and the solution was refluxed overnight. Addition of 0.44 g of NaOH gave a pale yellow precipitation of nitrophenethylthiol, which was collected by filtration. The resulting powder was put in 20 mL of 1 mol dm-3 HCl solution containing 0.6 g of suspended Sn metal powder to reduce chemically a nitro group, resulting in a crude product of aminophenethylthiol, which was obtained by removing residual Sn and evaporation of the solvent. The resulting product was purified by recrystallization in acetonitrile, and the final yield was 0.81 g. Water used for preparation of electrolyte solutions (14) (a) Kim, T.; Chan, K. C.; Crooks, R. M. J. Am. Chem. Soc. 1997, 119, 189. (b) Kim, T.; Ye, Q.; Sun, L.; Chan, K. C.; Crooks, R. M. Langmuir 1996, 12, 6065. (c) Kim, T.; Crooks, R. M. Tetrahedron Lett. 1994, 35, 9501. (d) Chan, K. C.; Kim, T.; Schoer, J. K.; Crooks, R. M. J. Am. Chem. Soc. 1995, 117, 5877. (15) (a) Willicut, R. J.; McCarley, R. L. J. Am. Chem. Soc. 1994, 116, 10823. (b) Willicut, R. J.; McCarley, R. L. Langmuir 1995, 11, 296. (c) Willicut, R. J.; McCarley, R. L. Anal. Chim. Acta 1995, 307, 269. (16) Sayre, C. N.; Collard, D. M. Langmuir 1995, 11, 302. (17) (a) Smela, E.; Ingana¨s, O.; Lundstro¨m, I. Science 1995, 268, 1735. (b) Smela, E.; Kariis, H.; Yang, Z.; Mecklenburg, M.; Liedberg, B. Langmuir 1998, 14, 2984. (18) Sato, N.; Nonaka, T. Chem. Lett. 1995, 805.

10.1021/la981719b CCC: $15.00 © 1999 American Chemical Society Published on Web 08/13/1999

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was purified by a Milli-Q Gradient A10. All other chemicals were of reagent grade and used without further purification. An Au electrode substrate was prepared by vacuum evaporation of gold on a freshly cleaved natural mica sheet (Nilaco Co.) which was heated at 350 °C. It is well established that a vapordeposited Au film on a heated mica has predominantly a (111) surface crystallinity and serves as a quasi-Au(111) electrode.3,19 The prepared electrode was immersed in a piranha solution, which is a mixture of concentrated H2SO4 and 30% H2O2 solution (3:1 by volume), for 5 min and rinsed thoroughly with water. Some etching effect of the piranha solution is known to give a well-organized Au(111) surface.3,19 The electrode coated with a single component monolayer of o-ABT, m-ABT, p-ABT, or 3-APT was prepared by immersing the Au(111)/mica substrate in an ethanol solution containing one of those thiol derivatives in 1 mmol dm-3 for 30 min. The resulting electrode will be denoted as o-ABT/Au, m-ABT/Au, p-ABT/Au, or 3-APT/Au, respectively. The electrode coated with a monolayer of o-ABT and m-ABT mixture was prepared using an ethanol solution containing both isomers in 0.5 mmol dm-3 each, and the resulting electrode will be denoted as a mixed-ABT/Au. The electrode prepared was placed at a bottom hole of an electrochemical cell with a Teflon-coated O-ring (0.68 cm i.d.). The effective surface area of the electrode was determined to be 0.4 ( 0.02 cm2 by measuring the charges involved in oxidation of chemically adsorbed iodine.20 A Pt foil and an Ag/AgCl in a KCl-saturated aqueous solution were used as the counter and reference electrodes, respectively, and electrochemical measurements were carried out with the use of a BAS-100B electrochemical analyzer. Reflection absorption of FTIR spectroscopy (RAS-FTIR) measurements was conducted by using a Nicolet MAGNA-IR 750 equipped with a Harrick surface reflectance attachment having a fixed grazing angle of 75°. Its measurement chamber was always filled with dry air using a pure gas generator.

Results and Discussion Electrochemical Polymerization Behavior. Parts a-c of Figure 1 show cyclic voltammograms obtained in the initial three potential sweeps of o-ABT/Au, m-ABT/ Au, and mixed-ABT/Au electrodes in 0.5 mol dm-3 HClO4 aqueous solution, respectively. Anodic currents were observed at potentials positive of ca. 0.6 V for all cases, and the first cycle of the potential scan gave especially remarkable currents as given by broken curves. Successive second and third cycles caused a decrease in anodic currents and an appearance of anodic and cathodic waves at potentials ranging between 0.4 and 0.6 V. If the same experiment was made by using a naked Au electrode, any significant increase in oxidation currents was not observed at the potential range given in Figure 1, but a small undulation of the base currents appeared as for the case of Au in an electrolyte solution of propylene carbonate.17b Therefore, it was a little hard to judge whether the current waves observed at potentials between 0.4 and 0.6 V in voltammograms a and b of Figure 1 were ascribed to redox reaction of the surface-confined species or effects of the electrode substrate. In the case of voltammograms shown in Figure 1c, however, oxidation and reduction waves appeared after the first potential cycle can be regarded as a couple of redox reaction. The obtained electrochemical behavior is quite similar to those of anodic polymerization of aniline and ring-substituted anilines dissolved in acidic solution.21 It is well-known that polyaniline and its derivatives show two sets of redox couples. The first redox couple of the polyaniline film, which appears at more (19) (a) Chidsey, C. E. D.; Loiacono, D. N.; Sleator, T.; Nakahara, S. Surf. Sci. 1988, 200, 45. (b) Golan, Y.; Margulis, L.; Rubinstein, I. Surf. Sci. 1992, 246, 312. (c) Walczak, M M.; Alves, C. A.; Lamp, B. D.; Porter, M. D. J. Electroanal. Chem. 1995, 396, 103. (20) (a) Walczak, M. M.; Alves, C A.; Lamp, B. D.; Porter, M. D. J. Electroanal. Chem. 1995, 396, 103. (b) Rodriguez, J. F.; Mebrahtu, T.; Soriaga, M. P. J. Electroanal. Chem. 1987, 233, 283.

Figure 1. Cyclic voltammograms of o-ABT/Au (a), m-ABT/Au (b), mixed-ABT/Au (c), and 3-APT/Au (d) electrodes taken at 100 mV s-1 in 0.5 mol dm-3 HClO4 aqueous solution for initial three potential sweeps.

negative potential, is located at around 0.15 V vs Ag/AgCl, but positive shifts of the redox potential to 0.3-0.5 V vs Ag/AgCl are seen for cases of the ring-substituted polyanilines.21c,d Accordingly, it is strongly suggested from the electrochemical behaviors of the mixed-ABT/Au electrode shown in Figure 1c that the aniline units of ABT were oxidized in the first potential scan to give the corresponding polymers. It is also noteworthy that the mixed-ABT monolayer gave much larger anodic currents and redox waves than the single-component monolayer of o-ABT or m-ABT. (21) (a) Diaz, A. F.; Logan, J. A. J. Electroanal. Chem. 1980, 88, 277. (b) Huang, W. S.; Humphrey, B. D.; MacDiarmid, A. G. J. Chem. Soc., Faraday Trans. 1 1986, 82, 2385. (c) Cattarin, S.; Boubova, L.; Mengoli, G.; Zotti, G. Electrochim. Acta 1988, 33, 1077. (d) Bidan, G.; Genis, E. M.; Penneau, J. F. J. Electroanal. Chem. 1989, 271, 59.

Polyaniline-Analogue Monolayer on Au Electrode

Figure 2. Schematic illustrations of electrochemical polymerization of mixed-ABT (a) and 3-APT (b).

If a SAM is prepared with the use of a deposition bath containing two kinds of thiols, a composition of the resulting mixed SAM is often inconsistent with that in the solution. In general, thiols having longer alkyl chains tend to occupy a higher coverage than shorter thiols when the deposition bath contains the thiols in the same concentration.22 This tendency becomes more marked with increasing the soaking time in the mixed thiol solution. However, it is believed in the case of the mixed-ABT/Au that the composition of SAM is not greatly different from that in the deposition bath, because both ABT species have the same molecular size and the soaking time of the Au(111)/mica of 30 min chosen here would not cause significant exchange reactions, if any. Then, it may be assumed that o-ABT and m-ABT molecules are adsorbed alternatively on average. As widely accepted, the electrochemical polymerization of aniline takes place through head and tail coupling at the para positions. If the same is true for the polymerization of aniline units in the ABT monolayer, the reaction can occur with a scheme as illustrated in Figure 2a for the case of alternatively adsorbed m-ABT and o-ABT. However, it is not easy for the single component monolayer of o-ABT or m-ABT to be polymerized through head and tail coupling because adsorption of ABT with such geometric configuration as shown in Figure 2a seems difficult due to absence of any alkyl chain between the aminobenzene and the sulfur group. If the alkyl chains are available, the adsorbed molecules could be tilted so as to come close to the para (22) (a) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M.; Deutch, J. J. Phys. Chem. 1994, 98, 563. (b) Offord, D. A.; John, C. M.; Griffin, J. H. Langmuir 1994, 10, 761.

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Figure 3. Cyclic voltammograms of mixed-ABT/Au (a) and 3-APT/Au (b) electrodes taken at 100 mV s-1 in 0.5 mol dm-3 KOH aqueous solution before (- - -) and after (s) polymerization.

positions of an aniline units of the adjacent ABT, the degree being dependent on the carbon number of the alkyl chain. Support for this view is seen in the cyclic voltammetric behavior of 3-APT SAM, which is shown in Figure 1d. Large anodic currents appeared in the first potential scan, and redox waves having comparable magnitude to those obtained in the voltammograms for the mixed-ABT/Au electrode (Figure 1c) appeared in the succeeding potential scans, although the monolayer was composed of only metaposition isomer. Adsorbed 3-APT can be polymerized with a scheme shown in Figure 2b where it is noticed that the amino group of an adsorbed APT can be made close to the para-position of the aniline unit of the adjacent APT. Figure 3 shows voltammograms taken in deaerated 0.5 mol dm-3 KOH of mixed-ABT/Au and 3-APT/Au electrodes before and after oxidation of the monolayer. The sharp reduction peaks indicating reductive desorption of thiols appeared at -0.67 and -0.86 V for the mixed-ABT/Au and 3-APT/Au electrodes, respectively, before the oxidative polymerization. The difference in the peak potentials between two monolayers seemed to be well reflected by the general tendency of reductive desorption of the alkanethiol SAMs; the peak potential shifts negatively as the molecular length is larger.9 The amount of charges involved in the reductive desorption of ABT/Au was 79.0 µC cm-2 and that of 3-APT/Au was 80.1 µC cm-2, from which the amounts of mixed-ABT and 3-APT adsorbed on the Au substrate were evaluated to be 8.2 × 10-10 and 8.3 × 10-10 mol cm-2, respectively. In contrast, no reductive desorption peak was observed if the polymerized electrodes were used. The amount of charges involved in the initial anodic wave of the voltammogram of the mixed-ABT/Au which

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is given by a hatched area in Figure 1c was estimated to be 177 µC cm-2, which is believed to be the sum of the charges for polymerization of the mixed-ABT monolayer and those for oxidation of the resulting polymer which simultaneously occurs. The amount of the latter charges was evaluated to be 35.7 µC cm-2 by integration of the anodic wave of the redox couple obtained in the third potential scan. Then, the amount of charges involved in the oxidative polymerization reactions is determined to be 177-35.7 ) 141.3 µC cm-2, which was about four times as large as that of charges consumed in the oxidation of the polymer. Since two electrons are involved in the polymerization reaction of one ABT molecule, the results suggest that ca. 0.5 mol of electrons was involved in the redox reaction of 1 mol of aniline unit in the polymerized monolayer, being in good agreement with those expected for the redox reaction of polyaniline and its derivatives.21,23 From the polymerization charges obtained, the amount of ABT molecules adsorbed on the Au substrate is evaluated to be 7.3 × 10-10 mol cm-2. The value obtained is in good accordance with the expected amount of thiols adsorbed on Au(111) with a (x3 × x3)R30° structure which is 7.7 × 10-10 mol cm-2, but it was a little smaller than that obtained by the reductive desorption experiment of the adsorbed ABT as mentioned above. However, the observed discrepancy is not unreasonable. Since the double layer capacity of a thiol monolayer coated electrode is smaller than that of the naked electrode substrate, charging currents in the double layer are superposed on the faradic currents during the course of the desorption reaction.9-13 It is very hard to estimate precisely the amount of charges due to changes in the double layer capacity because a dielectric constant of the monolayer and potential of zero charge of both SAM-coated electrode and the naked one are needed for the estimation, but their precise values cannot be easily obtained. Then, the amount of the adsorbed ABT estimated by charges consumed by the polymerization reaction is more reliable since it is not required to consider changes in the double layer capacity. As for the 3-APT monolayer, the amount of charges involved in the anodic oxidation of the monolayer and the oxidation of the resulting polymer were 173 and 33 µC cm-2, respectively, from which the amount of 3-APT molecules adsorbed and that of electrons involved in the redox reaction of 1 mol 3-APT in the polymerized monolayer were estimated to be 7.1 × 10-10 mol cm-2 and 0.48, respectively, the values being comparable to those obtained for the mixed-ABT/Au. RAS-FTIR Measurements of Polymerized Monolayer. Figure 4 shows RAS-FTIR spectra of the mixedABT/Au and polymerized mixed-ABT/Au, the latter of which was prepared by subjecting the mixed-ABT/Au to the above-mentioned cyclic potential sweeps in 0.5 mol dm-3 HClO4 aqueous solution. A spectrum obtained for p-ABT/Au, which was subjected to the same potential sweeps, is also given in the figure for comparison. The spectra of the mixed-ABT/Au before and after polymerization were essentially the same, and absorption peaks assignable to stretching mode of C-C bond (νC-C) of aromatic rings appeared at 1480 and 1590 cm-1 and those assignable to stretching mode of C-N (νC-N) bond and bending mode of N-H (δN-H) appeared at 1250 and 1620 cm-1, respectively. As can be seen, there is good accordance between spectra a and b, providing evidence that the oxidative desorption of the adsorbed thiols did not occur (23) (a) Kobayashi, T.; Yoneyama, H.; Tamura, H. J. Electroanal. Chem. 1984, 177, 281. (b) Hirai, T.; Kuwabata, S.; Yoneyama, H. J. Chem. Soc., Faraday Trans. 1 1989, 85, 969. (c) Kuwabata, S.; Kihira, N.; Yoneyama, H. Chem. Mater. 1993, 5, 604.

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Figure 4. RAS-FTIR spectra of mixed-ABT/Au before (a) and after (b) being subjected to cyclic voltammetry for polymerization as shown in Figure 1 and that of p-ABT/Au taken after being subjected to cyclic voltammetry under the same conditions as the case of mixed-ABT/Au (c).

in its polymerization. A lack of absorption peaks due to stretching mode of SdO, which should appear at 11001350 cm-1, also supports the stability of the adsorbed SAM. It has been reported for the SAM of p-ABT that the adsorbed p-ABT molecules were oxidatively dimerized and one molecule of the resulting dimer was converted to quinone species which was desorped from the substrate.24,25 The same results were reproduced in this study. Spectrum c shows a decrease in the absorption peak due to δN-H and appearance of absorption peaks assignable to the stretching mode of the CdO bond at 1720 and 1750 cm-1 with repeating cyclic potential sweeps of p-ABT/Au under the same conditions as that employed in the polymerization of the mixed-ABT/Au. Electrochemical Properties of Polymerized Monolayers. Figure 5 shows cyclic voltammograms of the polymerized mixed-ABT/Au electrode taken in 0.5 mol dm-3 NaClO4 solution, the pH of which was adjusted to various values by adding appropriate amounts of 0.02 mol dm-3 phosphate buffer. The following two features are noticeable in the obtained voltammograms: (i) Both oxidation and reduction peak potentials were shifted negatively with increasing pH at the rate of ca. 60 mV per pH; (ii) The redox activity became poor with increasing pH and eventually disappeared at around pH 5. Similar redox behavior was also obtained with the use of the polymerized 3-APT/Au electrode. Since the electrochemical properties observed here are very close to those of the first redox waves of conventional polyaniline films, the redox reaction mechanism proposed for polyaniline and its derivatives21,23 seems to be valid in the redox reaction of the polymerized monolayer of mixed-ABT and 3-APT. To get information on coarseness of the polymerized films, underpotential deposition (UPD) of Cu was attempted on the Au substrate through the polymerized mixed-ABT and the polymerized 3-APT/Au coated on the Au electrode substrates. Cyclic voltammograms taken in (24) Hayes, W. A.; Shannon, C. Langmuir 1996, 12, 3688. (25) Lukkari, J.; Kleemola, K.; Meretoja, M.; Ollonqvist, T.; Kankare, J. Langmuir 1998, 14, 1705.

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Figure 5. Cyclic voltammograms of the polymerized mixedABT/Au electrode taken at 100 mV s-1 in different pHs. The solution contained 0.5 mol dm-3 NaClO4 and an appropriate amount of 0.02 mol dm-3 phosphate buffer. Figure 7. Schematic illustration of Cu UPD on polymerized 3-APT/Au (a) and polymerized mixed-ABT/Au (b) electrodes.

Figure 6. Cyclic voltammograms of naked Au (a), polymerized mixed-ABT/Au (b), and polymerized 3-APT/Au (c) electrodes taken at 2 mV s-1 in 50 mmol dm-3 H2SO4 containing 1 mmol dm-3 CuSO4 (A) and time course of electrolysis charges obtained at the above-mentioned electrodes by application of a potential step from 0.5 to 0.02 V vs Ag/AgCl in the same CuSO4 electrolyte solution (B).

50 mmol dm-3 H2SO4 containing 1 mmol dm-3 CuSO4 are shown in Figure 6A where a voltammogram obtained for the naked Au is also given. The naked Au(111) gave two separated reduction waves due to Cu UPD at 0.32 and 0.08 V vs Ag/AgCl. When the polymerized mixed-ABT/Au and the polymerized 3-APT/Au electrodes were used, Cu UPD was greatly hindered as recognized from the voltammograms b and c shown in Figure 6A. It is, however,

noticed that reduction current peaks appeared at 0.16 and 0.05 V vs Ag/AgCl for the polymerized mixed-ABT/ Au and the polymerized 3-APT/Au, respectively, indicating that Cu UPD on these electrodes took place with applications of significant overpotentials. The obtained results suggest strongly that the polymerized monolayers were porous enough to allow penetration of Cu2+ ions because the UPD reaction occurs only when Cu2+ ions reach the Au electrode surface. It is thought that when the penetration of Cu2+ ions occurred, the monolayers had no charges, because the Cu UPD took place at such negative potentials as to induce reduction of the polymerized monolayer to give a leucoemeraldine base which is not protonated in aqueous solution of pH > -0.2.21b The finding that larger overpotentials were required to induce the Cu UPD on the polymerized 3-APT/Au than on the polymerized mixedABT/Au seemed to reflect well difference in resistance of the monolayers resulting from difference in length of the thiol molecules. Quantitative evaluation of the amount of Cu deposited on the electrode was made by chronocoulometry measurements with application of a potential step from 0.5 to 0.02 V, which induced only the UPD of Cu. Results obtained are shown in Figure 6B. The amount of charges obtained at the naked electrode was 440 µC cm-2, which corresponds to deposition of 2.28 × 10-9 mol cm-2 Cu, and is in good accord with the surface density of Au atoms of the Au(111) surface which is 2.31 × 10-9 mol cm-2, suggesting that the deposition of Cu took place with a full surface coverage. Interestingly, the amount of charges for Cu UPD obtained at the polymerized 3-APT/Au (280 µC cm-2) was larger than that obtained at the polymerized mixed-ABT/Au (178 µC cm-2) although the peak currents of Cu UPD observed in cyclic voltammograms were comparable for two electrodes as shown in Figure 6A. These results suggested that the magnitude of the peak currents in voltammograms was not determined by amount of Cu deposition but by other factors including diffusion rates of Cu2+ in the monolayers. The Cu coverage of 65% and 41% of full Au atoms on the surface was estimated for the former and the latter cases, respectively. We reported previously that the underpotential deposition of Cu on an Au(111) coated with a SAM of propanethiol

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occurs on a thiol-free Au surface which occupies two-thirds of Au atoms on the naked Au(111).26 Considering this fact, the amount of the deposited Cu obtained in this study suggests that the deposition at the polymerized 3-APT/ Au took place on almost all the thiol-free deposition sites of the Au(111) as schematically shown in Figure 7a. At the polymerized mixed-ABT/Au, however, the amount of Cu deposition was insufficient to cover all the thiol-free Au sites. The polymerized mixed-ABT/Au does not contain any alkyl chain between aminobenzene and terminal sulfur, and with this reason, deposition sites of Au (111) located right below the bridging amino groups would not be available for the UPD of Cu, as suggested from a side view shown in Figure 7b. Then the Cu atom deposition seems to take place on such Au sites that are not covered with the polymerized mixed-ABT, resulting in the smaller amount of deposition than on the polymerized 3-APT/Au. Conclusion All electrochemical data obtained in this study indicated that SAMs having aminobenzene groups at their terminals were polymerized by electrochemical oxidation in the same manner as the electrochemical polymerization of aniline and its derivatives. Unfortunately, however, spectroscopic evidence showing the polymerization could not be obtained (26) Nishizawa, M.; Sunagawa, T.; Yoneyama, H. Langmuir 1997, 13, 5215.

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even by RAS-FTIR. It is basically difficult to extract information on fine structures from spectroscopic measurements of the conventional polyaniline film and its derivatives,27 much less from those of its monolayer. However, our investigation is still under way to seek further confirmation of the polymerization with the use of other methods including scanning tunneling microscopy. The data, which allowed us to believe firmly polymerization of the aminobenzene units, are reinforcement of durability against desorption of the SAMs after their oxidation as shown in Figure 3. This is an advantage of extension of the electrochemical applications of SAM. In the case of the polyaniline-analogue monolayers prepared in this study, further substitution of other functional groups is possible at benzene rings or N of amino groups, allowing preparation of durable monolayers having desired functions. Acknowledgment. This work was supported by Grant-in-Aid on Priority Area of “Electrochemistry of Ordered Interfaces” No. 09237104 and No. 11118242 from the Ministry of Education, Science, Sports and Culture. LA981719B (27) (a) Ohkuni, Y.; Matsuda, H.; Ohsaka, T.; Oyama, N. J. Electroanal. Chem. 1983, 158, 55. (b) Ohsaka, T.; Ohnuki, Y.; Oyama, N. J. Electroanal. Chem. 1984, 161, 399.