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J. Phys. Chem. B 2000, 104, 11878-11882
Electrodesorption Kinetics and Molecular Interactions in Well-Ordered Thiol Adlayers On Au(111) M. E. Vela,† H. Martin,‡ C. Vericat,† G. Andreasen,† A. Herna´ ndez Creus,‡ and R. C. Salvarezza*,† Instituto de InVestigaciones Fisicoquı´micas Teo´ ricas y Aplicadas (INIFTA), CIC-CONICET-UNLP, Sucursal 4, Casilla de Correo 16, 1900 La Plata, Argentina, and Departamento de Quı´mica Fı´sica, UniVersidad de La Laguna, Spain ReceiVed: June 13, 2000; In Final Form: August 17, 2000
The electrodesorption of x3×x3 R30° thiol and sulfur lattices on Au(111) has been investigated by in situ STM and electrochemical methods. For thiol and sulfur adlayers, the x3×x3 R30° lattice is desorbed in sharp voltammetric current peaks. The electrodesorption kinetics involves nucleation and growth of holes. From the analysis of the dependence of the peak potential (Ep) on the length of the hydrocarbon chain (n), the thiol-thiol interaction results in ≈3 kJ mol-1/C unit. The value of Ep for n ) 0 indicates that the S-Au(111) bond energy in x3×x3 R30° thiol lattices is ≈19 kJ mol-1 smaller, and the bond has a lesser ionic character than the S-Au(111) bond in the x3×x3 R30° sulfur lattice. Monte Carlo simulations of a desorption model reproduce well the experimental potentiostatic and potentiodynamic results for thiol desorption.
Introduction Self-assembled monolayers (SAMS) of thiols on metals have attracted considerable scientific interest1 due to their possible use to prevent corrosion, to modify wetting and wear properties, to anchor different functional groups to be used as chemical and biochemical sensors, and to develop nanodevices for electronics.2 One of the main problems to understand selfassembly of these fascinating two-dimensional structures arises from the fact that thiol-metal and thiol-thiol interactions are not fully understood. In the case of SAMS on Au, the chemical state of S atoms at the thiol/Au interface has been extensively studied by XPS, but the interpretation of experimental data still remains controversial.3-7 While the nature of the S-Au bond for thiols is a matter of discussion, the understanding of the lateral interactions prevailing in SAMS becomes even more elusive, in particular in aqueous solutions, the most important environment for technological applications. In electrolyte solutions, reductive electrodesorption of SAMS has been used to study thiol-metal and thiol-thiol interactions.8 In fact, SAMS on Au(111)9 and Ag(111)8 are desorbed in sharp voltammetric current peaks whose peak potentials (Ep) shift in the negative direction as the number of C atoms (n) in the thiol molecules increases. On the basis of the shift of Ep with n, and assuming adsorption/desorption equilibrium, intermolecular forces acting at SAMS of thiols on Ag(111) have been estimated.8 On the other hand, it has been proposed that the kinetics of thiol electrodesorption is controlled by counterion transport through SAMS, and accordingly, the shift of Ep with n has been related to changes in the energy barrier for this process.9 Evidence showing that nucleation and growth of holes is rate-determining in thiol desorption from Au(111) and Ag(111) has also been reported.8,10-11 Results from different groups have shown the importance of the substrate, ordering, and neighbor interactions in the desorp* To whom correspondence should be addressed. † Instituto de Investigaciones Fisicoquı´micas Teo ´ ricas y Aplicadas. ‡ Universidad de La Laguna.
tion process of thiols.9,11-12 Structural information about the electrodesorption process of different thiols from the Au(111) surface has also been presented by using in situ STM.12 However, most of the previous studies comparing electrodesorption curves for different thiols have been done without molecular characterization of the SAM structures to be desorbed, a crucial point to compare the electrochemical behavior and molecular interactions present in the adsorbed thiol layers. In this paper, we have followed the electrodesorption of wellordered x3×x3 R30° thiol and sulfur lattices on Au(111) by in situ STM and electrochemical techniques. We have found that desorption of these lattices involves nucleation and growth of holes under charge-transfer control, in agreement with the interpretation given in refs 10 and 11. Thus, the dependence of Ep on n can be explained by considering the activation energy needed to desorb a thiol molecule from the periphery of the growing hole in contact with the electrolyte. We have demonstrated that the Ep value for n ) 0 differs from that found for S desorption indicating that, in contrast to the S-Ag bond for SAMS of thiols on Ag(111), the S-Au bond in SAMS on Au(111) is weaker and has a lesser ionic character than the S-Au(111) bond in adsorbed S. These results are important to understand recent controversial XPS data of thiols.3-7 Monte Carlo simulations of a desorption model reproduce well the experimental potentiostatic and potentiodynamic results for thiol desorption. Experimental Section Au films evaporated on glass (Robax glass, AF Berliner Glas KG, Germany) were used as substrate. After annealing in a hydrogen atmosphere at 650 °C, these films show atomically smooth (111) terraces 40-60 nm in size separated by monatomic high steps.13 Adsorbed layers were prepared by immersion of the substrate for 24 h in 0.05 mM X in ethanolic solution, (X ) propanethiol, hexanethiol, dodecanethiol) and 0.05 mM Na2S in 0.1 M NaOH. Solutions were prepared with high purity chemicals. The adsorption procedure leads to the formation of
10.1021/jp002142l CCC: $19.00 © 2000 American Chemical Society Published on Web 11/22/2000
Electrodesorption Kinetics in Thiol Adlayers On Au(111)
Figure 1. Current density (j) vs E profiles, for the reductive electrodesorption of different adlayers on Au(111) recorded at V ) 5 × 10-2 V s-1 in 0.1 M NaOH. (a) adsorbed S layer. Dashed vertical lines indicate stability domains of the different S surface structures. Insets show the rectangular S8 (≈0.62 × 0.57 nm2 in size) and the x3×x3 R30° S (d ) 0.5 nm) lattice. (b) SAMS of thiols: dodecanethiol (n ) 12), hexanethiol (n ) 6), and propanethiol (n ) 3). The inset shows an Ep(Vf0) vs n plot. The Ep(Vf0) value was calculated by plotting Ep vs V and extrapolating for V ) 0. The slope is 0.035 V/C unit. The experimental point at n ) 0 is the Ep(Vf0) value for S desorption.
well-ordered thiol adlayers as revealed by the STM images. In situ STM imaging was made using a Nanoscope III STM (Digital Instruments Inc.). A large area Au counter electrode and a Pd/H2 reference electrode were used in the STM electrochemical cell. Potentials in the text are referred to the saturated calomel electrode (SCE). STM images were taken in the constant current mode using Pt-Ir tips. Electrodesorption runs were made from -0.5 to -1.3 V at scan rates in the range 5 × 10-3 e V e 5 × 10-2 V s-1 in nitrogen-saturated 0.1 M NaOH. Potentiostatic current transients were recorded for different ∆E values, stepping from a constant potential just before the beginning of the desorption peak (Ei) to a final potential (Ef) for complete desorption. Results and Discussion S-covered Au(111) electrodes placed at a potential E ) -0.6 V in 0.1 M NaOH (Figure 1a) show rectangular S8 species14 0.62 × 0.57 nm2 in size (Figure 1a inset) with a nearest neighbor distance d ≈ 0.3 nm.14c We did not detect d values typical of polysulfide species (d ) 0.20-0.22 nm), showing that disulfide formation is not feasible for adsorbed S atoms on Au(111) in electrolyte solutions. Thus, if S atoms are not able to form disulfide bonds on Au(111) it should be more difficult for S atoms in thiol molecules to form these bonds15 because, in this case, the S-C angle should be distorted to overcome steric constrains due to the presence of the hydrocarbon chains, and an extra energy would be needed. When E is scanned from E ) -0.6 V to E ) -0.8 V, a potential induced transition takes place.14a Thus, 1/2 of the S atoms from S8 species desorb to form soluble sulfide species, while the others rearrange to form the x3×x3 R30° lattice
J. Phys. Chem. B, Vol. 104, No. 50, 2000 11879
Figure 2. Typical current-time transients recorded for the reductive electrodesorption of different adlayers on Au(111). (a) S, (b) dodecanethiol, (c) hexanethiol, (d) propanethiol. The magnitude of the potential step (∆E) for each transient is indicated. Initial potentials (Ei) were -1.05, -0.92, and -0.7 V for dodecanethiol, hexanethiol, and propanethiol, respectively.
(Figure 1a, inset).14c Scanning E from -0.8 to -1.0 V, the x3×x3 R30° lattice is removed at a sharp current peak located at -0.92 V (Figure 1a), leaving uncovered Au terraces and some S atoms adsorbed at step edges.14c The charge density (q) involved in the desorption of the x3×x3 R30° lattice is q ) 150 ( 10 µC cm-2, which for a S coverage of 1/3 implies a charge transfer of two electrons per S atom. Therefore, the reductive desorption of S from the x3×x3 R30° lattice can be written as14,16
2e + Au(111)-S + H2O ) HS- + OH- + Au(111) (1) Desorption of the remaining S atoms from step edges takes place at more negative potentials, preceding the hydrogen evolution reaction (HER). Current-time transients recorded in the potential range of the x3×x3 R30° desorption peak show current maxima (jp) (Figure 2a) that increase one decade in current/ 0.09 V. The presence of current maxima in the potentiostatic current transients indicates that desorption kinetics involves nucleation and growth of two-dimensional holes. On the other hand, the potential dependence of jp indicates that the desorption process is under charge-transfer control.17 Large scale STM images of dodecanethiol-covered Au(111) surfaces taken in the range -0.5 V > E > -1.0 V show wellordered domains of the c(4 × 2) superlattice (Figure 3a). At higher resolution, domains of the x3×x3 R30° lattice can also be detected on the Au(111) surface (Figure 3b). In the x3×x3 R30° lattice, thiol molecules are placed at sites of the Au(111) surface with a nearest neighbor distance d ) 0.5 nm.1 Theoretical calculations have shown that the adsorption energy is greater at hollow hcp or fcc sites and depends on the Au-S-C angle.18 Recent data from X-ray Standing Waves (XSW) for thiol adsorption on the Cu(111) face reveal an equivalent occupation of fcc and hcp hollow sites.19 On the other hand, the origin of the c(4 × 2) thiol superlattice is not clear. The different contrast of the thiol molecules in the STM images has been assigned either to the presence of hydrocarbon chains with different tilts20 or to the displacement of a row of thiol molecules from hollow to nearest bridge sites.21 This yields alternating rows of molecules placed at hollow and bridge sites, leading to different
11880 J. Phys. Chem. B, Vol. 104, No. 50, 2000
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Figure 3. STM images (raw data) at E ) -1.0 V of the Au(111) surface in 0.1 M NaOH covered by ordered domains of dodecanethiol lattices. (a) (47 × 31 nm2) image of the dodecanethiol adlayer, where rows corresponding to the c(4 × 2) lattice are clearly seen. (b) STM image at a higher resolution (20 × 12.5 nm2), where domains of the x3×x3 R30° can be resolved. Upper and lower insets show details of the zigzag c(4 × 2) (5.0 × 3.0 nm2) and the x3×x3 R30° (2.3 × 1.3 nm2) lattices found in the ordered domains, respectively (see arrows).
contrast in the STM image.21 Note that there are two types of c(4 × 2) superlattices: the zigzag (inset in Figure 3) and the rectangular (Figures 4-5). In both cases, the rows of bright spots are separated by ≈1 nm, but distances between nearest bright spots in the row are ≈0.5 and 1.0 nm for the zigzag and rectangular c(4 × 2) superlattices, respectively. When E is scanned in the negative direction (Figure 1b) down to -1.3 V, the x3×x3 R30° and c(4 × 2) lattices are removed at a current peak located at -1.15 V, i.e., 0.23 V more negative than for S (Figure 1a). In situ STM images taken immediately after desorption reveal the formation of nanometer-sized thiol aggregates similar to those observed in ref 12. These aggregates arise from the low solubility of thiols in aqueous media. The charge density involved in the electrodesorption process involves 1/2q, indicating a one-electron-transfer desorption process. Therefore, this process can be written as9a,22
1e + Au(111)-SR ) R-S- + Au(111)
(2)
Current-time transients recorded in the potential range corresponding to dodecanethiol desorption (Figure 2b) are similar to those described for the x3×x3 R30° S lattice. Desorption of the c(4 × 2) and x3×x3 R30° lattices (Figure 4a,b) from ordered domains of hexanethiol-covered Au(111) takes place at the current peak at -0.99 V (Figure 1b). On the other hand, the c(4 × 2) and x3×x3 R30° (Figure 5a,b) propanethiol lattices are desorbed at the current peak located at -0.82 V (Figure 1b). In both cases, nanometer-sized thiol aggregates are detected on the Au(111) surface by in situ STM. Again, current-time transients for hexanethiol and propanethiol desorption (Figure 2c,d) are similar to those previously described
Figure 4. STM images (raw data) at E ) -0.85 V of the Au(111) surface in 0.1 M NaOH covered by ordered domains of hexanethiol lattices. (a) (47 × 31 nm2) image of the hexanethiol adlayer where rows corresponding to the c(4 × 2) lattice are clearly seen. (b) STM image at a higher resolution (20 × 12.5 nm2) where domains of the c(4 × 2) and x3×x3 R30° can be resolved. Upper and lower insets show details of the rectangular c(4 × 2) (5.6 × 3.0 nm2) and the x3×x3 R30° (2.1 × 1.2 nm2) lattices found in the ordered domains, respectively (see arrows).
for S. Note that the time scale for the x3×x3 R30° and c(4 × 2) lattices desorption is too small for the process to be followed in real time by in situ STM. The analysis of the voltammetric peaks for thiol and S desorption reveals that as V decreases, Ep shifts in the positive direction with a slope 0.04 V/decade, as expected for a process involving nucleation and growth.23 The Ep(Vf0) vs n plot (Figure 1b, inset) for the three thiols leads to a straight line with a slope ≈ 0.035 V mol-1/C atom unit. This value is close to those previously found for thiol desorption from Ag(111)8 (0.04 V mol-1/C atom unit) and Au(111) (0.025 V mol-1/C atom unit).9c The small differences in the slope value could be originated in different adsorption times for SAM preparation, scan rates, or electrolyte composition for SAM electrodesorption. Note that for n ) 0 the result is Ep,n)0 ) -0.69 V, which reflects the S-Au bond energy in a thiol environment but without any lateral interactions. Our results show that the rate-limiting step of the reductive desorption of thiols from x3×x3 R30° and c(4 × 2) lattices on Au(111) involves nucleation and growth of holes. Reaction proceeds by removal of thiol molecules from the edge of a hole leading to an increase in the area of the holes with time, thus producing maxima in the current transients.17 Note that the role of defects in the electrodesorption of thiol layers has been recognized in previous papers.9,11-12,24 Therefore, the rate of desorption of thiol molecules from x3×x3 R30° and c(4 × 2) lattices should involve an energy barrier resulting from different contributions: adsorbate-substrate interactions (EA-S), adsorbateadsorbate interactions (EA-A), the energy to overcome hydrophobic forces between hydrocarbon chains and water (EH), and
Electrodesorption Kinetics in Thiol Adlayers On Au(111)
J. Phys. Chem. B, Vol. 104, No. 50, 2000 11881
Figure 5. STM images (raw data) at E ) -0.7 V of the Au(111) surface in 0.1 M NaOH covered by ordered domains of propanethiol lattices. (a) (40 × 27 nm2) image of the propanethiol adlayer, where rows corresponding to the c(4 × 2) lattice are clearly seen. (b) STM image at a higher resolution (20 × 12.5 nm2), where domains of the c(4 × 2) and x3×x3 R30° can be resolved. Upper and lower insets show details of the rectangular c(4 × 2) (4.5 × 2.5 nm2) and x3×x3 R30° (2.1 × 1.3 nm2) lattices found in the ordered domains, respectively (see arrows).
the energy due to water adsorption on bare Au at the growing hole (ES-W). EA-S and ES-W do not depend on the length of the hydrocarbon chain. Therefore, the shift in Ep with n should reflect mainly EA-A and EH, both hindering desorption. We have made two-dimensional Monte Carlo simulations for the desorption of the x3×x3 R30° thiol lattice on Au(111). The substrate surface was simulated on a matrix formed by 62500 particles arranged in a (111) lattice including periodic boundary conditions. The Monte Carlo time unit was computed every time all particles of the matrix were selected at random. Selected particles were removed with a probability P ) Pe PN. The term Pe represents the desorption probability that depends on electrochemical parameters through the Butler-Volmer equation17
Pe ) A[exp(-RczFη/RT) - exp(RazFη/RT)]
(3)
where the first term corresponds to desorption and the second to adsorption. In eq 3, η is the overpotential defined as E Ep,n)0, F is the Faraday constant, z is the number of electrons, R is the gas constant, T is the absolute temperature, Rc and Ra are the cathodic and anodic transfer coefficients,17 respectively, and A is a preexponential factor. On the other hand, PN represents a particle desorption probability that depends on the number of nearest neighbors, N,
PN ) B exp(-NEo/RT)
(4)
In eq 4, Eo includes both EA-A and EH, and B is a preexponential factor. According to the model, the removal of particles implies the appearance of vacancies at the monolayer. Thus, the surface
Figure 6. (a) Snapshots of the x3×x3 R30° lattice desorption at η ) 0.1 V and different Monte Carlo time units: (i) 0, (ii) 25, (iii) 50. Note that desorption proceeds through holes. (b) Voltammograms at V ) 5 × 10-2 V s-1 for the reductive electrodesorption of different thiols obtained from Monte Carlo simulations using equations [3-4] and Rc and Ra ) 0.5, z ) 1, and Eo ) 3 kJ mol-1/C unit. (c) Current-time transients obtained as indicated in panel b at different ∆E values. Current and Monte Carlo time in the voltammograms (b) and current transients (c) were scaled with the experimental charge. (d) Ep(Vf0) vs n plot. The straight line shows a slope 0.035 V/C unit.
coverage, θ, of vacancies can be defined. The simulation proceeds through the formation of holes (Figure 6a) until θ f 1, i.e., a total desorption of particles is attained. From the time dependence of θ and η, and using Rc ) Ra ) 0.5, z ) 1, and Eo ) 3 kJ mol-1/C unit, voltammograms (Figure 6b), current transients (Figure 6c), and an Ep(Vf0) vs n plot (Figure 6d) similar to those observed in the experiments can be reproduced. Note that the value of Eo is in the same order of magnitude as hydrophobic (≈2 kJ mol-1/C unit)25 and van der Waals forces for thiols (≈3 kJ mol-1/C unit).26 Note, however, that our model does not include the diffusion of the desorbed thiol molecules away from the substrate or the formation of thiol agglomerates on the electrode for thiols with longer chain lengths due to their low solubility. Diffusion of thiol molecules from the Au(111) surface to the solution could be rate-determining at high overpotentials, as already suggested.11 Despite this drawback, the model presented here reproduces well the electrodesorption curves and current transients around the current peak potential related to thiol desorption from the x3×x3 R30° and c(4 × 2) lattices. In fact, in the potential range of our measurements, the j vs t-1/2 decay expected for a diffusion-controlled process was not observed in the current transients. Now we turn to the meaning of (Ep,n)0) (Figure 1b, inset). The values of Ep,n)0 should correspond to the energy barrier to desorb a S atom of the x3×x3 R30° and c(4 × 2) lattices without lateral interactions from hydrocarbon chains. However, our experimental data for S show that desorption takes place at a potential ≈0.2 V more negative than Ep,n)0. It can be argued that the presence of the c(4 × 2) lattice of thiols, not observed for adsorbed sulfur, turns desorption easier. In fact, it has been suggested that the c(4 × 2) structure involves an alternating arrangement of thiols in hollow and bridge sites.21 However, the energy difference for thiol adsorption at these sites is ≈4 kJ mol-1,18 too small to explain a shift of ≈0.2 V, which for z
11882 J. Phys. Chem. B, Vol. 104, No. 50, 2000 ) 1 is close to 19 kJ mol-1. This indicates that the chemical state of S in the x3×x3 R30° and c(4 × 2) thiol lattices and in the x3×x3 R30° S lattice on Au(111) is different, i.e., the S-Au bond for atomic sulfur is stronger than the S-Au bond for thiols. In fact, we have made XPS measurements for the x3×x3 R30° S lattice on Au(111) and have obtained the S 2p3/2 peak at 161.3 eV, while the thiolate bond yields the S 2p3/2 peak at 162 eV,4 revealing a greater ionic character of the S-Au bond in adsorbed sulfur. It should be noted that in some cases a small S 2p3/2 peak at ≈161 eV is observed in XPS spectra of adsorbed thiols on Au(111). The origin of this peak is a matter of discussion.3-7 Results from this paper about the chemical state of S in adsorbed S and thiols on Au(111) support the idea that the peak at 161 eV is related to S impurities present in the adsorbed thiol layers, as recently suggested.6 Our conclusion about the chemical state of S in the x3×x3 R30° thiol lattice and in the x3×x3 R30° S lattice on Au(111) contrasts with observations for S and thiols adlayers on Ag(111). In this case, electrodesorption data for the x7×x7 R10.9° lattice of different thiols from Ag(111) lead to a (Ep,n)0) value that agrees with the Ep value for the electrodesorption of the x7×x7 R10.9° S lattice.27 This suggests that the chemical state of S atoms in both S and thiol adlayers on Ag(111) is similar. In fact, electroadsorption valency28 and XPS29 data indicate that the S-Ag bond in thiol adlayers has a greater ionic character than the S-Au bond in thiols adsorbed on Au. Finally, we have also noted that Ep,n)0 lies in the potential range where S8 species are stable (see Figure 1a). Therefore, we believe that the chemical state of S atoms in adsorbed thiols would be similar to that of S in S8 surface structures. Conclusions We have demonstrated that the desorption of x3×x3 R30° and c(4 × 2) lattices for S and thiols involves nucleation and growth of holes. On the basis of the analysis of the dependence of the peak potential on the hydrocarbon chain length we can also estimate that the hydrocarbon chain interactions in these lattices contribute 3 kJ mol-1/C unit to the energy barrier for thiol desorption. Monte Carlo simulations of a desorption model reproduce well the experimental potentiostatic and potentiodynamic results. Finally and most important, we have found an “unexpected” result, (considering previous results for thiols on Ag): the S-Au bond for adsorbed thiols in the x3×x3 R30° adlayer is less ionic than that found for S in the same lattice. The energy difference introduced by this “extra” ionic character is about 19 kJ mol-1. Acknowledgment. We thank Agencia Nacional de Promocio´n Cientı´fica y Tecnolo´gica (PICT 97-1993) and CONICET (PIP-0897) (Argentina) and PI1999/128 Gobierno Canario
Vela et al. (Spain) for the financial support of this work. H.M. thanks to Gobierno Canario (Spain) for a fellowship grant. References and Notes (1) Ulman, A., Chem. ReV. 1996, 96, 1533. (2) Haag, R.; Rampi, A. M.; Holmlin, R. E.; Whitesides, G. M. J. Am. Chem. Soc. 1999, 121, 7895. (3) Zubragel, C.; Deuper, C.; Schneider, F.; Neumann, M.; Grunze, M.; Schertel, A.; Woll, C. Chem. Phys. Lett. 1995, 238, 308. (4) Castner, D.; Hinds, K.; Grainger, D. W. Langmuir 1996, 12, 5083. (5) Ishida, T.; Hara, M.; Kojima, I.; Tsuneda, S.; Nishida, N.; Sasabe, H.; Knoll, W. Langmuir 1998, 14, 2092. (6) Zhong, C.-J.; Brush, R. C.; Anderegg, J.; Porter, M. D. Langmuir 1999, 15, 518. (7) Ishida, T.; Choi, N.; Mizutani, W.; Tokumoto, H.; Kojima, I.; Azehara, H.; Hokari, H.; Akiba, U.; Fujihira, M. Langmuir 1999, 15, 6799. (8) Hatchett, D. W.; Uibel, R. H.; Stevenson, K. J.; Harris, J. M.; White, H. S. J. Am. Chem. Soc. 1998, 120, 1062. (9) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335. (b) Walczak, M.; Alves, C. A.; Lamp, B. D.; Porter, M. D. J. Electroanal. Chem. 1995, 396, 103. (c) Zhong, C.-J.; Porter, M. D. J. Electroanal. Chem. 1997, 425, 147. (10) Calvente, J. J.; Kova´cova´, Z.; Sa´nchez, M. D.; Andreu, R.; Fawcett, W. R. Langmuir 1996, 12, 5696. (11) Yang, D.-F.; Morin, M. J. Electroanal. Chem. 1997, 429, 1. (12) Hobara, D.; Miyake, K.; Imabayashi, S.; Niki, K.; Kakiuchi, T. Langmuir 1998, 14, 3590. (13) Andreasen, G.; Vela, M. E.; Salvarezza, R. C.; Arvia, A. J. Langmuir 1997, 13, 6814. (14) Gao, X.; Zhang, Y.; Weaver, M. J. J. Phys. Chem. 1992, 96, 4156. (b) McCarley, R. L.; Kim, Y.-T.; Bard, A. J. J. Phys. Chem. 1993, 97, 211. (c) Vericat, C.; Andreasen, G.; Vela, M. E.; Salvarezza, R. C. J. Phys. Chem. B 2000, 104, 302. (15) Fenter, P.; Eberhardt, A.; Eisenberger, P. Science 1994, 266, 1216. (16) Hamilton, I. C.; Woods, R. J. Appl. Electrochem. 1983, 13, 783. (b) Briceno, A.; Chander, S. J. Appl. Electrochem. 1990, 20, 506 (c) Gao, X.; Zhang, Y.; Weaver, M. J. Langmuir 1992, 8, 668. (17) Thirsk, H. R.; Harrison, J. A. A Guide to the Study of Electrode Kinetics; Academic Press: London, 1972. (18) Beardmore, K. M.; Kress, J. D.; Bishop, A. R.; Grombech-Jensen, N. Synth. Met. 1997, 84, 317. (19) Jackson, G. J.; Woodruff, D. P.; Jones, R. G.; Singh, N. K.; Chan, A. S. Y.; Cowie, B. C. C.; Formoso, V. Phys. ReV. Lett. 2000, 84, 119. (20) Anselmetti, D.; Baratoff, A.; Guntherodt, H. J.; Delamarche, E.; Michel, B.; Gerber, C.; Kang, H.; Wolf, H.; Ringsdorf, H. Europhys. Lett. 1994, 27, 365. (21) Tera´n, F.; Vela, M. E.; Salvarezza, R. C.; Arvia, A. J. J. Chem. Phys. 1998, 109, 5703. (22) Yang, D.-F.; Al-Maznai, H.; Morin, M. J. Phys. Chem. B 1997, 101, 1158; Hagenstrom, H.; Schneeweiss, M. A.; Kolb, D. M. Langmuir 1999, 15, 2435. (23) Noel, M.; Chandrasekaran, S.; Ahmed, C. J. Electroanal. Chem. 1987, 225, 93. (24) Yang, D.-F.; Wilde, C. P.; Morin, M. Langmuir 1996, 12, 6570. (25) Israelachvili, J. N. In Intermolecular and Surface Forces; Academic Press: London, 1994; pp 353, 408. (26) Nuzzo, R. G.; Dubois, L. H.; Allara, D. L. J. Am. Chem. Soc. 1990, 112, 558. (27) Azzaroni, O.; Vela, M. E.; Andreasen, G.; Salvarezza, R. C., in preparation. (28) Hatchett, D. W.; Stevenson, K. J.; Lacy, W. B.; Harris, J. M.; White, H. S. J. Am. Chem. Soc. 1997, 119, 6596. (29) Bensebaa, F.; Zhou, Y.; Deslandes, Y.; Kruus, E.; Ellis, T. H. J. Electroanal. Chem. 1998, 405, L472.
