Electrochemistry of Surface-Confined Mixed Monolayers of 4

Jul 24, 1996 - We report the electrochemistry of surface-confined monolayers of 4-aminothiophenol (4-ATP) and mixed monolayers of 4-ATP and thiophenol...
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Electrochemistry of Surface-Confined Mixed Monolayers of 4-Aminothiophenol and Thiophenol on Au William A. Hayes and Curtis Shannon* Department of Chemistry, Auburn University, Auburn, Alabama 36849-5312 Received September 7, 1995. In Final Form: April 8, 1996X We report the electrochemistry of surface-confined monolayers of 4-aminothiophenol (4-ATP) and mixed monolayers of 4-ATP and thiophenol (TP) on Au surfaces. Cyclic voltammograms of the 4-ATP monolayer in acidic aqueous perchlorate solutions are characterized by an irreversible oxidative wave at 0.730 V vs Ag/AgCl on the first scan and, upon scan reversal, by a persistent, reversible, surface-confined wave centered at approximately 0.500 V and a transient peak at about 0.300 V. We propose an ECE mechanism to account for this electrochemistry: 4-ATP is first oxidized to the cation radical, followed by chemical coupling to form an adsorbed dimer. The dimer is subsequently hydrolyzed in the presence of H2O to yield an adsorbed quinone species that is reversibly electroactive. Grazing angle FTIR spectroscopy was used to identify the product. The transient peak is due to the coupling of desorbed molecules and is consistent with the formation of a phenazine species. We then show that mixed monolayers of 4-ATP and TP can be used to study the coverage dependence of surface-confined reactions. The chemical composition of the mixed systems was determined using two independent Auger electron spectroscopic techniques and grazing angle FTIR spectroscopy. Using 20 min assembly times, we find that the surface concentration of 4-ATP is directly proportional to its mole fraction in solution. Interestingly, TP does not participate in the 4-ATP electrochemistry and functions only to dilute the surface concentration of 4-ATP. We find that the efficiency of the conversion of 4-ATP to product is somewhat higher at low mole fractions of 4-ATP.

Introduction Organized, surface-confined monolayers are commonly used to impart desired chemical or physical properties to surfaces. Alkanethiols adsorbed on Au are the most studied self-assembled monolayer (SAM) systems because of their stability and high degree of organization. A relatively complete picture of the self-assembly process and the resulting structure of the SAMs has emerged over the last decade.1 Simple ω-terminated alkanethiol monolayers, for example, form densely packed, oriented films and can be used to engineer the surface energy of metal surfaces.2 Organic SAMs also have been used as the basis for pH-dependent electrostatic binding of ions,3 for molecular recognition,4 as platforms for further surface chemical modification,5 and for fundamental studies of electron transfer.6 Monolayers containing more than one * Author to whom correspondence should be addressed. Telephone: 334-844-6964. Fax: 334-844-6959. E-mail: shanncg@ mail.auburn.edu. X Abstract published in Advance ACS Abstracts, July 1, 1996. (1) (a) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (b) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (c) Bain, C. D.; Whitesides, G. M. Adv. Mater. 1989, 1, 506. (d) Ulman, A. An Introduction to Ultrathin Organic Films; Academic Press: Boston, 1991. (e) Schinenberger, C.; Sondag-Huethorst, J. A. M.; Jorritsma, J.; Fokkink, L. G. J. Langmuir 1994, 10, 611. (f) Butt, H.-J.; Seifert, K.; Bamberg, E. J. Phys. Chem. 1993, 97, 7316. (g) Hong, T. H.; Davies, P. B.; Bain, C. D. Langmuir 1993, 9, 1836. (h) Poirer, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2859. (i) Poirer, G. E.; Tarlov, M. J.; Rushneier, H. E. Langmuir 1994, 10, 3383. (2) (a) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (b) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3359. (c) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (d) Bertilsson, L.; Liedberg, B. Langmuir 1993, 9, 141. (e) Randall, T.; Carey, R. I.; Biebuyck, H. A.; Whitesides, G. M. Langmuir 1994, 10, 741. (f) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (3) (a) Sun, L.; Johnson, B. J.; Wade, T.; Crooks, R. M. J. Phys. Chem. 1990, 94, 8869. (b) Kepley, L. J.; Crooks, R. M.; Ricco, A. J. Anal. Chem. 1992, 64, 3191. (c) Jones, T. A.; Perez, G. P.; Johnson, B. A.; Crooks, R. M. Langmuir 1995, 11, 1318. (d) Nahir, T. M.; Clark, R. A.; Bowden, E. F. Anal. Chem. 1994, 66, 2595. (e) Decher, G.; Lvov, Y.; Schmitt, J. Thin Solid Films 1994, 244, 772. (f) Jorden, C. E.; Frey, B. L.; Kornguth, S.; Corn, R. M. Langmuir 1994, 10, 3642.

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chemical species hold great promise as a scheme for preparing patterns of surface active groups.7 Monolayers formed from aromatic thiols, although less thoroughly studied, are interesting as surface modification agents because they possess, in addition to the properties mentioned above, relatively high electrical conductivities. The first experiments on the adsorption of aromatic thiols on metal surfaces were reported by Hubbard et al.8 Hubbard found that thiophenol and benzyl mercaptan formed complete monolayers with a low degree of longrange order on Pt(111); monolayers possessing long-range order were only observed in the case of thiophenol adsorbed on Ag(111). More recent studies have provided vibrational spectroscopic evidence of the average orientation of adsorbed aromatic thiols and the formation of a metalthiolate bond on Au as well as on Ag, Cu, and Pt surfaces.9 The most complete study to date of aryl thiolate SAMs, (4) (a) Rebek, J. J. Acc. Chem. Res. 1990, 23, 399. (b) Lehn, J.-M. Angew. Chem., Int. Ed. Engl. 1990, 29, 1304. (c) Haeussling, L.; Ringsdorf, H.; Schmitt, F.-J.; Knoll, W. Langmuir 1991, 7, 1837. (d) Rubenstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988, 332, 426. (e) Whitesides, G. M.; Mathias, J. P.; Seto, C. T. Science 1991, 254, 1312. (f) Wulff, G. In Polymeric Reagents and Catalysts; Fort, W. T., Ed.; ACS Symposium Series 308; American Chemical Society: Washington, DC, 1986; p 186. (5) (a) Bent, S. F.; Schilling, M. L.; Wilson, H. E.; Katz, H. E.; Harris, A. L.; Chem. Mater. 1994, 6, 122. (b) Bertilsson, L.; Liedberg, B. Langmuir 1993, 9, 141. (c) Schilling, M. L.; Katz, H. E.; Stein, S. M.; Shane, S. F.; Wilson, W. L.; Buratto, S.; Ungahse, S. B.; Taylor, G. N.; Putvinski, T. M.; Chidsey, C. E. D. Langmuir 1993, 9, 2156. (d) Duevel, R. V.; Corn, R. M. Anal. Chem. 1992, 64, 337. (e) Sun, L.; Thomas, R. C.; Crooks, R. M.; Ricco, A. J. J. Am. Chem. Soc. 1991, 113, 8550. (f) Ulman, A.; Tillman, N. Langmuir 1989, 5, 1418. (6) Bard, A. J.; Abruna, H. D.; Chidsey, C. E.; Faulkner, L. R.; Feldberg, S. W.; Itaya, K.; Majda, M.; Melroy, O.; Murray, R. W.; Porter, M. D.; Soriaga, M. P.; White, H. S. J. Phys. Chem. 1993, 97, 7147. (7) (a) Bain, C. D.; Evall, J.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7155. (b) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M.; Deutch, J. J. Phys. Chem. 1994, 98, 563. (c) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1989, 111, 7164. (d) Laibinis, P. E.; Nuzzo, R. G.; Whitesides, G. M. Langmuir 1992, 8, 1330. (e) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M. J. Phys. Chem. 1992, 96, 5097. (f) Stranick, S. J.; Parikh, A. N.; Tao, Y.-T.; Allara, D. L.; Weiss, P. S. J. Phys. Chem. 1994, 98, 7636. (8) (a) Hubbard, A. T. Chem. Rev. 1988, 88, 633. (b) Gui, J. Y.; Lu, F.; Stern, D. A.; Hubbard, A. T. J. Electroanal. Chem. 1990, 292, 245. (c) Gui, J. Y.; Stern, D. A.; Frank, D. G.; Lu, F.; Zapien, D. C.; Hubbard, A. T. Langmuir 1991, 7, 955.

© 1996 American Chemical Society

4-Aminothiophenol and Thiophenol on Au

and the first study of their electrochemistry, was reported by Rubenstein et al.10 In this investigation, contact angle, ellipsometry, cyclic voltammetry, ac impedance, and molecular modeling were used to characterize a series of aromatic thiols on Au. In general, although the degree of order in these monolayers was less than that for alkanethiol SAMs, there was some evidence that a closestpacked structure might be possible for longer aromatic thiols. To the extent that information regarding the electrochemistry of aromatic thiol SAMs has been reported, the focus has been on the electrochemical blocking abilities of the films. However, an area that has received some recent attention is the electrochemical polymerization of self-assembled monolayers to yield two-dimensional polymer films covalently attached to a surface. In general, the monomer precursor is tethered to the surface via an alkyl chain, and a SAM consisting of the monomer is polymerized electrochemically. For example, McCarley11 and Collard12 have independently reported the oxidative polymerization of alkanethiol SAMs containing a pyrrole group at the ω-terminus. Here we report a study of the oxidative electrochemistry of 4-aminothiophenol (4-ATP) monolayers and mixed monolayers of 4-ATP and thiophenol (TP) on Au surfaces. 4-ATP SAMs have received attention because of their ability to electrostatically bind anions at low pH3a and to increase the density of polyaniline thin films grown electrochemically.13 Moreover, there is evidence from STM experiments that 4-ATP forms ordered monolayers in which the S atoms are arranged in a (x3 × x3)R30° configuration on Au(111). We find that 4-ATP is easily oxidized electrochemically and forms an adsorbed product that itself displays reversible, surface-confined voltammetry. In order to determine the mechanism, we studied the behavior of several related systems including mixed monolayers of 4-ATP and TP. A simple ECE mechanism, similar to what is observed in the initial electropolymerization of aniline and analogous organic molecules, is proposed to account for the observed electrochemistry of 4-ATP. In the case of mixed monolayers of 4-ATP and TP, we find that the yield of dimer is a function of the surface concentration of 4-ATP. Experimental Section Chemicals. All thiols (4-aminothiophenol, 4-hydroxythiophenol, and thiophenol) were obtained from Aldrich and were purified by vacuum distillation (thiophenol) or by multiple recrystallization from methanol (4-aminothiophenol and 4-hydroxythiophenol). Unless otherwise noted, the electrolyte was 1 M HClO4. All aqueous solutions were prepared using 18 MΩ Millipore filtered water (Millipore H2O). Perchloric acid was used as received from Fisher. Substrate Preparation. Before each surface modification or electrochemistry experiment, the Au disk working electrode (chronoamperometrically determined surface area, 0.024 cm2) was polished using a 0.05 µm alumina slurry, sonicated in Millipore H2O for 5 min, rinsed thoroughly with Millipore H2O, and repeatedly exposed to hot piranha solution (3:1 H2SO4/30% H2O2). Note: Piranha solution is a strong oxidant and must be used with extreme caution. Afterward, the disk was rinsed with a large volume of Millipore H2O and ethanol (200 proof, Florida (9) (a) Bryant, M. A.; Joa, S. L.; Pemberton, J. E. Langmuir 1992, 8, 753. (b) Carron, K. T.; Hurley, L. G. J. Phys. Chem. 1991, 95, 9979. (10) Sabatani, E.; Cohen-Boulakia, J.; Bruening, M.; Rubenstein, I. Langmuir 1993, 9, 2974. (11) Willicut, R. J.; McCarley, R. L. Langmuir 1995, 11, 296. (12) Sayre, C. N.; Collard, D. M. Langmuir 1995, 11, 302. (13) (a) Sabatani, E.; Redondo, A.; Rishpon, J.; Rudge, A.; Rubenstein, I.; Gottesfeld, S. J. Chem. Soc., Faraday Trans. 1993, 89, 287. (b) Sabatani, E.; Redondo, A.; Rishpon, J.; Rudge, A.; Rubenstein, I.; Gottesfeld, S. J. Am. Chem. Soc. 1990, 112, 6135.

Langmuir, Vol. 12, No. 15, 1996 3689 Distillers) and immediately immersed in the appropriate thiol solution for 20 min. Ethanolic solutions in which the total thiol concentration was 2 mM were used for surface modification. Note: It has been shown that oxidation of Au by UV and ozone at room temperature produces a ca. 2 nm thick oxide layer on Au that is stable to UHV and various rinsing procedures.14 In light of this observation, we wish to comment on the use of piranha solution to prepare Au surfaces for SAMs. First, we observed no improvement in surface coverage, film stability, or reproducibility if piranha solution was not used. Independent experiments on the underpotential deposition of S on Au surfaces also showed no difference between Au surfaces prepared using piranha solution or using electrochemical cycling to remove the oxide.15 Other workers report stable SAMs using an oxygen plasma treatment to prepare the Au surface;10 one report suggests that the thiols may actually dissolve the oxide layer during the assembly process.16 Electrochemistry. All electrochemistry experiments were performed using a Pine AFRDE-5 bipotentiostat, a HP-7015B X-Y recorder, and a single-compartment, three-electrode Teflon cell. All cell and other components that came into contact with the electrolyte were made of Teflon or Kel-F. The Au working electrode was a nominally 1.6 mm diameter Au disk (Bioanalytical Systems, Inc.). In all cases, a platinum wire served as the counter electrode and the reference electrode was Ag/AgCl, to which all voltages are referred. All electrochemistry was carried out under a blanket of UHP Ar after degassing the cell for 15 min. Auger Electron and FTIR Spectroscopy. Glass microscope slides were cleaned in hot piranha solution for about 1 h and were then sonicated for at least 10 min in Millipore H2O and absolute ethanol. The slides were then rinsed with copious amounts of ethanol and blown dry in a stream of UHP Ar. A thin layer of Ni/Cr followed by an approximately 200 nm thick layer of Au were then deposited onto these slides using a Denton DV502A vacuum evaporator. After deposition, samples were rinsed with ethanol and dried in UHP Ar. Samples were soaked for 20 min in the solution to allow the monolayers to self-assemble and dried in a stream of UHP Ar upon removal from solution. Grazing angle FTIR experiments were performed using a Mattson RS-1 spectrometer equipped with a Graseby Specac surface reflectance attachment. The instrumental resolution was 4 cm-1 in all cases. Typically, 500-1000 scans were co-added. The Auger electron spectroscopy apparatus has been previously described.17

Results and Discussion Electrochemistry of Single-Component Monolayers. If the potential of a Au electrode in perchlorate electrolyte is made more positive than about 1.050 V, an oxidative current is observed that corresponds to the formation of gold oxide. On reversing the scan direction, the oxide layer is stripped from the surface at about 0.830 V. If, on the other hand, the scan is reversed at potentials less than about 0.800 V, the voltammetry is essentially featureless (Figure 1A). Similarly, when a thiophenol (TP) modified Au electrode is scanned between 0.000 and 0.700 V, no peaks are observed; the only noticeable difference from the voltammetry of naked Au is the decreased doublelayer capacitance (Figure 1B). This capacitance change is characteristic of organic adsorption and arises because of the low dielectric constant of these materials. If the electrode potential is made significantly more positive than 0.700 V, however, the TP monolayer is no longer stable on the Au surface and the voltammetry of bare Au is recovered after one or two complete cycles. Both of these results are consistent with earlier studies of TP adsorption on polycrystalline Au electrodes.10 The cyclic voltammetry of a 4-ATP monolayer is shown in Figure 2a. If the electrode potential is maintained below (14) King, D. E. In press. As referenced in: Ulman, A. MRS Bull. 1995, 20, 46. (15) Demir, U.; Shannon, C. Unpublished results. (16) Chailapakul, O.; Crooks, R. M. Langmuir 1995, 11, 1329. (17) Bozack, M. J.; Williams, J. R.; Ferraro, J. M.; Feng, Z. C.; Jones, R. E. J. Electrochem. Soc. 1995, 142, 485.

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a

Figure 1. Cyclic voltammograms of naked Au (A) and thiophenol-modified Au (B) recorded between 0.00 and 0.700 V in 1 M HClO4. The scan rate is 100 mV s-1 in both cases.

about 0.700 V, no faradaic current is observed. However, the voltammetry of adsorbed 4-ATP at more positive potentials is characterized by two features, an initial irreversible oxidative wave at about 0.730 V (peak A) and a pair of reversible surface-confined waves centered at approximately 0.500 V (peaks B and B′). In addition, a reductive wave of variable magnitude which does not persist on repeated cycling is observed near 0.300 V (peak C). As expected, there is also a decrease in the doublelayer capacitance relative to that of naked Au due to the presence of the monolayer. After the first complete cycle, the currents in peaks A and C are dramatically reduced; only peaks B and B′ are observed in the steady state voltammogram (Figure 2b). In this experiment, steady state is reached in 3-5 cycles. In addition, the scan rate dependence indicates that peaks B and B′ originate from an adsorbed species (Figure 2c). Peak A occurs at a potential that is positive of the oxidation potential of aniline at the same pH and exhibits the linear scan rate dependence characteristic of a surface-confined molecule (Figure 2c). The position of peak A shifts by 55 mV to more negative potentials as the solution pH is increased by one pH unit. Identical voltammetry is observed in H3PO4 and H2SO4 electrolytes. On the basis of this voltammetric data, we assign peak A to the oxidation of adsorbed 4-ATP in its protonated form to the radical cation. It appears that adsorbed 4-ATP is oxidized intact before the desorption potential is reached, and furthermore, it seems unlikely that this peak corresponds to the oxidative decomposition of 4-ATP via C-S bond scission. Both the value of E°′ and its pH dependence are consistent with the oxidation of the amine moiety. Moreover, the appearance of peaks B, B′, and C after the initial oxidation wave is very similar to what has been observed in the initial stages of the oxidative electropolymerization of aniline and analogous molecules.18 In the case of aniline, the follow-up peaks are explained on (18) (a) Bard, A. J.; Yang, H. J. Electroanal. Chem. 1992, 339 , 423. (b) Diaz, A. F.; Logan, J. A. J. Electroanal. Chem. 1980, 111, 111. (c) MacDiarmid, A. G.; Chiang, J.-C.; Halpern, M.; Huang, W.-S.; Mu, S.L.; Somasiri, N. L. D.; Wu, W.; Yaniger, Si. Mol. Cryst. Liq. Cryst. 1985, 121, 173. (d) Stilwell, D. E.; Park, S.-M. J. Electrochem. Soc. 1988, 135, 2491. (e) Stilwell, D. E.; Park, S.-M. J. Electrochem. Soc. 1988, 135, 2497. (f) Ping, G. J. Electrochem. Soc. 1987, 134, 654C. (g) Gumbs, R. W.; Chandresekhar, P. J. Electrochem. Soc. 1991, 138, 1337. (h) Bacon, J.; Adams, R. N. J. Am. Chem. Soc. 1968, 90, 6596. (i) Wei, Y.; Jang, G.-W.; Chan, C.-C.; Hsueh, K. F.; Hariharan, R.; Patel, S. A.; Whitecar, C. K. J. Phys. Chem. 1990, 94, 7716.

b

c

Figure 2. (a) Cyclic voltammetry of a 4-aminothiophenol monolayer adsorbed on a Au electrode. The electrolyte was 1 M HClO4 and the CV was recorded at a scan rate of 100 mV s-1. Scan numbers for the first and second scans are indicated. (b) Steady state voltammetry of the product obtained after five cycles showing the persistent peak at 500 mV. (c) Dependence of the current in peaks A (9) and B (b) on scan rate.

the basis of an ECE mechanism19 resulting in the formation of head-to-head and head-to-tail dimers or side products (i.e., hydroquinone) involving reaction with the solvent. The product distribution is variable and depends on experimental conditions, including the pH and the aniline concentration. In the case of adsorbed 4-ATP, once the radical cation is formed on the initial oxidative scan, there are two reasonable possibilities as to its fate. The first is nucleophilic attack by a molecule in the ambient phase, such as the solvent, while the second possibility is reaction with another radical cation or with an unoxidized 4-ATP molecule to yield a dimer. Nucleophilic attack of the 4-ATP radical cation by the solvent might be expected to yield 4-hydroxythiophenol (4-HTP) as the product. The cyclic voltammetry of 4-HTP adsorbed on Au is shown in (19) ECE: sequential heterogeneous electron transfer, chemical reaction, and heterogenous electron transfer. See: Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; John Wiley & Sons: New York, 1980; pp 430-432.

4-Aminothiophenol and Thiophenol on Au

Figure 3. Cyclic voltammetry of adsorbed 4-hydroxythiophenol at a Au electrode. The scan rate is 15 mV s-1.

Figure 4. Single-reflection grazing angle FTIR (GIR) spectra of 4-ATP monolayers before (upper trace) and after (lower trace) electrochemical cycling. The intensity scale is in absorbance units and is the same for both spectra.

Figure 3. The reversible wave at about 0.150 V is observed on the initial scan; it is not necessary to make a potential excursion to positive potentials to observe this peak. The peak current is a linear function of the scan rate, indicating that it arises from a surface-bound species. Comparison of this voltammetry with that of 4-ATP indicates that formation of significant amounts of 4-HTP is not occurring on oxidation of the 4-ATP monolayer. Grazing angle FTIR (GIR) spectra of the 4-ATP monolayer before and after electrochemical cycling are shown in Figure 4. The upper trace shows the GIR spectrum of a freshly prepared 4-ATP monolayer on Au. Three prominent absorptions are observed in this spectrum at 1480, 1590, and 1620 cm-1 and are assigned to the νCC (1480 and 1590 cm-1) and the δNH (1620 cm-1) modes, respectively. The observed frequencies and relative band intensities are essentially identical to what has been observed for 4-ATP on Ag surfaces.20 As noted by these workers for 4-ATP on Ag, all of the bands appearing in the GIR spectrum are of a1 symmetry; in particular, the intense

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b2 ring mode that appears at 1440 cm-1 in the bulk spectrum is entirely absent from the surface spectrum, indicating that the principle axis of adsorbed 4-ATP is oriented along the surface normal. The GIR spectrum shown in the lower trace was obtained from a monolayer subjected to electrochemical cycling until the steady state voltammogram shown in Figure 2b was observed. The most striking difference between the two spectra is the appearance of an intense, sharp band at 1690 cm-1 in the spectrum of the electrochemically cycled monolayer. In addition to this peak, three other 4-ATP bands are also observed, indicating that the efficiency of electrochemical conversion is less than unity. We assign the 1690 cm-1 band to the carbonyl stretch of an adsorbed quinone species. Similar frequencies have been observed previously for surface-confined quinones.21 Moreover, the steady state voltammetric peak at 0.500 V is also characteristic of a hydroquinone/benzoquinone couple pendant to the surface. Hubbard has shown that while contact adsorbed hydroquinones are irreversibly oxidized at metal electrode surfaces, a pendant hydroquinone moiety gives rise to reversible surface-confined voltammetry.22 Taken together, these two results indicate the presence of an adsorbed quinone species in the electrochemically cycled monolayer. To summarize this section, we believe that the oxidation of 4-ATP monolayers results in the formation of a surfaceconfined dimer via a mechanism analogous to the initial electrooxidation of solution phase anilines and that the dimer is subsequently hydrolyzed and converted to a pendant quinone species. The proposed mechanism is shown in Scheme 1. In particular, peak A corresponds to the oxidation of surface-confined 4-ATP to the cation radical which reacts with a neighboring 4-ATP molecule to form a dimer. This strained dimeric species then partially desorbs from the surface and is hydrolyzed by water to yield the final product, which contains the pendant quinone moiety that gives rise to peak B. The hydrolysis of the amine functionality in acid is wellknown;23 furthermore, in acidic solutions, the C-S double bond is readily hydrolyzed to the corresponding ketone.24 An alternative mechanism that is not entirely inconsistent with our data would assign peak A to the oxidation of the C-S bond to yield adsorbed S and a solution phase radical species. The radical would in turn react with an adjacent 4-ATP molecule to yield a dimeric product. The subsequent hydrolysis step would be the same in both schemes. Although the two mechanisms are consistent with the steady state voltammetry (peak B) and FTIR data, we believe that the similarity between peak A and the electrooxidation of aniline supports oxidation of the amine moiety and not C-S bond cleavage. However, some loss of adsorbed material indeed occurs, as evidenced by peak C, and will be discussed in the next section. Composition of 4-ATP/TP Mixed Monolayers. In order to probe the surface electrochemistry of 4-ATP in more detail, we examined the behavior of two-component monolayers formed from co-adsorbed TP and 4-ATP. Surface composition was determined using three independent methods: first, we measured the attenuation of (20) Osawa, M.; Matsuda, N.; Yoshii, K.; Uchida, I. J. Phys. Chem. 1994, 98, 12702. (21) Shannon, C.; Frank, D. G.; Hubbard, A. T. Annu. Rev. Phys. Chem. 1991, 42, 393. (22) Stern, D. A.; Wellner, E.; Salaita, G. N.; Laguren-Davidson, L.; Lu, F.; et al. J. Am. Chem. Soc. 1988, 110, 4885. (23) See, for example: Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications Wiley: New York, 1980; p 430. (24) Ohno, A. In Organic Chemistry of Sulfur; Oae, S., Ed.; Plenum: New York, 1977; Chapter 5, p 201.

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Scheme 1. Proposed ECE Mechanism for the Electrooxidation of 4-Aminothiophenol Monolayersa

a Peak A corresponds to the oxidation of 4-ATP to the cation radical. This species then couples with an adjacent 4-ATP molecule to yield a dimer. This strained molecule then partially desorbs from the surface and is hydrolyzed to the quinone species shown. This species gives rise to the steady state reversible wave at 0.500 V and the 1690 cm-1 band in the GIR spectrum.

the Au Auger peak at 69 eV by the adsorbed monolayers; second, we quantified the intensity of the S Auger peak at 152 eV as a function of monolayer composition; third, we used GIR to obtain the vibrational spectra of the monolayers as a function of composition. In preliminary Auger spectroscopy experiments on mixed monolayer samples, we observed that the intensity of the Au peak at 69 eV decreased monotonically as the concentration of 4-ATP in the assembly solution was increased. In addition, there was a direct correlation between the intensity of the S peak at 152 eV and the 4-ATP mole fraction.25 These results suggested that the molecular orientations, and hence packing densities, of TP and 4-ATP adsorbed on Au are different. Two additional experiments confirm this supposition. First, the relative molecular packing densities of the two species can be directly calculated from the Auger spectra of pure TP and pure 4-ATP monolayers. The ratio of the S Auger intensity measured for the 4-ATP monolayer to that of the TP monolayer is 2.83 ( 0.47. This result demonstrates convincingly that the packing densities of the two mol(25) The intensity of the N Auger transition at 379 eV was not analytically useful due to its relatively low crosssection, which is a factor of 3 less than that of the S 152 eV transition. We took the following precautions to minimize the effects of beam damage to the films (see: Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. J. Chem. Phys. 1993, 98, 678). Experiments were carried out so that the beam current densities were as small as possible and so that the spectra were collected in the minimum possible time. Auger coverage data are based on averaging intensities obtained from several spots on a given sample and at least three repeat runs. There was no evidence of significant beam damage.

Hayes and Shannon

ecules are significantly different.26 If we assume that TP is bonded in a roughly horizontal configuration so that its phenyl ring is nearly parallel to the Au surface (molecular area ) 9.28 × 10-2 nm2) and that 4-ATP adopts a more nearly vertical orientation with its principle axis normal to the surface (molecular area ) 3.59 × 10-1 nm2), we calculate a packing density ratio of 3.87. Due to its orientation, the molecular packing density of 4-ATP is higher than that of TP, and this is reflected in a larger S Auger signal. There are several reasons why the experimentally determined ratio is less than the calculated value. In the case of vertically oriented 4-ATP, a small fraction of the S Auger intensity will be attenuated by the adsorbed hydrocarbon layer. It is also likely that the actual orientations deviate somewhat from the limiting values used in our calculation. GIR data for 4-ATP and TP two-component monolayers, which will be discussed in the next section, indicate that the two molecules are oriented differently on the Au surface. Although it is relatively routine to obtain the grazing angle spectrum of 4-ATP and possible to detect this molecule at submonolayer concentrations, we have not been able to measure an IR absorption spectrum for adsorbed TP.27 There are two reasons for this. The first is related to the well-known surface selection rule: the IR active modes of 4-ATP are observed in the GIR experiment because they lie along the surface normal; TP vibrational modes are not observed because they (or a significant projection) are essentially parallel to the plane of the surface. The second reason is that the infrared absorption cross sections for the polarizable TP molecule are substantially smaller than that of the highly polar 4-ATP molecule. Carron et al.9b studied the adsorption of TP on Cu, Ag, and Au using SERS and were able to measure both the molecular orientation (i.e., the tilt angle of the principle axis) as well as the orientation of the phenyl ring relative to the surface plane. They found a tilt angle of 15° with respect to the surface normal in the case of TP on Au. However, the SERS data also indicated that the plane of the phenyl ring was oriented parallel to the Au surface as opposed to the edge-on configuration adopted on Ag and Cu surfaces. The packing density of TP is lower in the ring-parallel orientation, for which we estimate a packing density ratio of about 2. This is reasonably close to the value we observe in our AES experiments. Conventionally, the attenuation of electrons emitted from a surface due to the presence of an adsorbed monolayer can be described by a simple exponential function.28 If the monolayer is adsorbed uniformly across the surface, an effective film thickness can be calculated using known values of the electron mean free path. In a simple extension of this model, the effective thickness of a mixed monolayer is taken to be the weighted average of the two single-component thicknesses, where the weighting factors are related to the mole fractions of each component of the mixed monolayer. Using this approach, which is based on a method previously reported by Whitesides,29 we calculated the mole fraction of 4-ATP on the surface using the Au attenuation data. (26) In the case of both molecules, electrochemical blocking experiments indicate that, for single-component films, nearly a complete monolayer is formed. See also ref 9. (27) The presence of TP on the surface was verified by Auger spectroscopy. (28) (a) Seah, M. P. In Practical Surface Analysis by Auger and X-Ray Photoelectron Spectroscopy; Briggs, D., Seah, M. P., Eds.; Wiley: New York, 1983; Chapter 5. (b) For a slightly different, although similar, approach, see: Batina, N.; Frank, D. G.; Gui, J. Y.; Kahn, B. E.; Lin, C.-H.; Hubbard, A. T.; et al. Electrochim. Acta 1989, 34, 1031. (29) Folkers, J. P.; Laibinis, P. E.; Whitesides, G. M.; Deutch, J. J. Phys. Chem. 1994, 98, 563.

4-Aminothiophenol and Thiophenol on Au

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Figure 5. Composition of thiophenol/4-aminothiophenol mixed monolayers on Au. The mole fraction of adsorbed 4-aminothiophenol, X4-ATP(surf), which was calculated using the three independent techniques outlined in the text, is plotted as a function of the mole fraction of 4-aminothiophenol in solution, X4-ATP(soln).

The Auger electron current, Ii, originating from an adsorbed layer is expected to depend linearly on the adsorbate packing density, Γ, according to the following equation30

Ii ) IpφGΓ

Figure 6. Single reflection grazing angle FTIR (GIR) spectra of 4-aminothiophenol/thiophenol mixed monolayers as a function of X4-ATP(soln). The value of X4-ATP(soln) is given for each spectrum.

(1)

where Ip is the primary beam current, φ is the collection efficiency of the detector, and G is the Auger electron yield factor. If the packing densities of the single-component monolayers are known, it should be possible to calculate the effective packing density of molecules in the twocomponent monolayers using the weighted average of the TP and 4-ATP packing densities. Our third technique for determining the composition of the mixed films was GIR. We measured the FTIR spectra of mixed monolayer films and constructed a Beers’ law plot of the integrated intensity of the ring CC mode of 4-ATP at 1580 cm-1. Figure 5 summarizes the results of the three analyses. We plot the mole fraction of 4-ATP in the mixed monolayers, X4-ATP(surf), as a function of the concentration in the ethanolic thiol solution, X4-ATP(soln). It is clear that all three methods are in excellent agreement and that there is an approximately linear relationship between the fraction of 4-ATP in solution and its mole fraction on the surface. This is as expected for a nonequilibrium SAM in which the transfer ratios of the two adsorbates are roughly equal. In Figure 6 we present the GIR spectra of 4-ATP/TP mixed monolayers in the 1400-1700 cm-1 region. The assignments of the three characteristic bands have been previously discussed.31 Significantly, the integrated intensities of all three bands scale almost exactly with the 4-ATP mole fraction, indicating that all of the observed intensity arises from absorption by 4-ATP and that TP contributes essentially nothing to the total absorbance. This supports our earlier assertion regarding the tilt angle difference between TP and 4-ATP. Furthermore, the relative peak intensities of the three vibrational bands do not change appreciably in the three spectra, indicating that the molecular orientation of 4-ATP is not a strong function of coverage. (30) Schoeffel, J. A.; Hubbard, A. T. Anal. Chem. 1977, 49, 2330. (31) Infrared band assignments were based on ref 20 and the following: (a) Lin-Vien, D.; Colthup, N. B.; Fateley, W. G.; Grasselli, J. G. The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules; Academic Press, Boston, 1991; Chapters 10 and 17. (b) Green, J. H. S. Spectrochim. Acta 1968, 24A, 1627.

Figure 7. Dependence of IpB/IpA, on X4-ATP(surf) showing how the efficiency of product formation scales with the coverage of 4-aminothiophenol. The dependence of IpA on mole fraction is shown in the inset.

Electrochemistry of Mixed Monolayers. We turn now to a discussion of the electrochemistry of the two-component films. Cyclic voltammetry experiments were carried out in exactly the same manner as for single-component monolayers. On the initial scan, we observed the three characteristic surface-confined waves that were seen in the case of the pure 4-ATP monolayers; after 3-5 cycles, we recovered the identical steady state voltammetry. E°′ was found to vary by up to (15 mV from experiment to experiment; however, average E°′ values were the same as what was seen for pure 4-ATP. Thus, there was no systematic trend in E°′ as a function of 4-ATP mole fraction. On the other hand, all of the peak currents were found to scale with the mole fraction of 4-ATP. This suggests that TP is not electrochemically active and acts only to dilute the surface concentration of the electroactive 4-ATP. We have also studied the behavior of octadecanethiol/4-ATP mixed monolayers. The surface electrochemistry of these systems is identical to what is reported here. This indicates that coupling between the 4-ATP radical cation and TP does not occur to a measurable extent and that TP indeed acts only as a diluent. The surface electrochemistry of two-component monolayers is summarized in Figure 7. We plot the current ratio Ip,B/Ip,A, which is related to the efficiency of product formation, vs

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the mole fraction of 4-ATP.32 The growth of peak A with mole fraction is shown in the inset. Whereas there is a linear increase of the current in peak A with 4-ATP mole fraction, we observe a weak monotonic decrease in Ip,B/Ip,A as the surface concentration of 4-ATP increases. At the highest 4-ATP coverages, the efficiency of dimer formation reaches a limiting value of about 0.25. One might expect that the current observed in peaks B and B′ would be small until some critical surface concentration was achieved where cation radicals had some probability of finding themselves adjacent for coupling. On the other hand, if islands of 4-ATP were forming, one might expect the efficiency to be coverage independent, with no onset behavior at low mole fraction. On this basis, our data are consistent with the formation of homogeneous mixed films. However, if the average island size were relatively large, the onset behavior might be difficult to observe experimentally. The nature of the coverage-dependent behavior at low mole fraction is not completely understood. The increased currents in peak B relative to peak A may be due to desorption of small amounts of TP from the surface as the potential is swept to its positive limit. This would lower the total surface coverage and lead to increased surface mobilities for the cation radicals and the remaining 4-ATP molecules, thereby increasing the probability of coupling. We would expect the number of desorbed TP molecules to be proportional to the initial TP coverage at a fixed scan rate, which would account for the observed increase in efficiency at low 4-ATP mole fraction. Of course, additional factors, such as the onset of competing reaction pathways or the formation of an electrochemically silent product, could also contribute to this effect. The observation of peak C in the initial scan (Figure 2a) which then disappears in the steady state voltammetry indicates that some desorption of material is occurring. We note that the formal potential of peak C/C′ is very similar to the oxidation potential of phenazine (0.28 V), which is known to form during the initial phase of polyaniline growth.33 (32) The same result is obtained if one plots the integrated peak areas.

Hayes and Shannon

Conclusions The electrochemistry of surface-confined 4-ATP monolayers has been investigated. Cyclic voltammetry and FTIR spectroscopy demonstrate that the electrooxidation proceeds via an ECE mechanism which yields a desorbed product (Figure 2a, peak C) and an adsorbed quinonecontaining species that displays reversible surfaceconfined electrochemistry (Figure 2a, peak B). Our proposed mechanism is analogous to the initial steps in the electrooxidation of solution phase anilines. In terms of 4-ATP electrochemistry, our most important finding is that 4-ATP can be oxidized intact to the cation radical at the Au surface. Mixed monolayer techniques were used to probe the surface electrochemistry as a function of 4-ATP coverage. For short assembly times, the surface concentration of 4-ATP is directly proportional to the concentration of 4-ATP in the assembly solution. Therefore, the chemical composition of the two-component monolayers can be easily controlled. We find that the efficiency of product formation is somewhat dependent on the 4-ATP mole fraction and measured a limiting efficiency of about 0.25 for pure 4-ATP monolayers. Finally, although dimerization and hydrolysis reactions predominate when the reaction is carried out in pure electrolyte, the stability of the 4-ATP cation radical on Au suggests that it should be possible, through the use of mixed monolayer techniques, to carry out highly localized surface chemical transformations electrochemically. Acknowledgment. Partial financial support of this work by the Society of Analytical Chemists of Pittsburgh and the Auburn University Grant In Aid Program is gratefully acknowledged. The authors wish to acknowledge Prof. M. J. Bozack for performing the Auger electron spectroscopy experiments. LA9507390 (33) Shim, Y.-B.; Won, M.-S.; Park, S.-M. J. Electrochem. Soc. 1990, 137, 538.