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Langmuir 1998, 14, 3298-3302
Underpotential Deposition of Silver onto Gold Substrates Covered with Self-Assembled Monolayers of Alkanethiols To Induce Intervention of the Silver between the Monolayer and the Gold Substrate Daisuke Oyamatsu, Matsuhiko Nishizawa, Susumu Kuwabata, and Hiroshi Yoneyama* Department of Applied Chemistry, Faculty of Engineering, Osaka University, Yamada-oka 2-1, Suita, Osaka 565, Japan Received September 2, 1997. In Final Form: April 10, 1998 Underpotential deposition (UPD) of Ag on Au(111)/mica electrodes coated with a self-assembled monolayer (SAM) of propanethiol or octanethiol and reductive desorption of the SAM after conducting UPD have been studied using voltammetry, XPS measurement, and scanning tunneling microscopy (STM). The reductive desorption potential of the SAM was changed by UPD of Ag from a characteristic value obtained at Au to that at Ag, indicating that the UPD of Ag took place through the SAM layer in such a way as to intervene between the SAM and the Au electrode. No significant loss of thiol molecules occurred during the Ag deposition. The rate of UPD through the SAM of propanethiol was so fast as to be completed within 10 s, while that for the SAM of octanethiol took ca. 50 min or more to build up the SAM/Ag/Au structure. Voltammetric results indicated that the UPD of Ag proceeded initially at molecular defects in the SAM of octanethiol and that the resulting Ag islands grew laterally to limiting coverage. Ex situ STM observations showed clearly the presence of such Ag islands on terraces of the Au(111) electrode surface.
Introduction The formation of self-assembled monolayers (SAMs) of organothiols and the underpotential deposition (UPD) of metal atoms provide a useful means for preparing a metal surface with a highly organized single layer of material.1-3 Recently, Jennings and Laibinis4,5 have reported that if SAMs of alkanethiols were prepared on an Ag UPD layer, their stabilities against both thermal desorption and selfexchange with other thiols were higher than those of the SAMs prepared directly on an Au surface. The aim of this paper is to report that such SAM/Ag(UPD)/Au assembly can be fabricated by conducting UPD of Ag “after” assembling short-chain alkanethiols such as propanethiol and octanethiol on Au electrodes, as schematically illustrated in Figure 1. No report has been published on the displacement of SAMs from the Au substrate to the Ag UPD layer. Furthermore, it has not yet been wellestablished that UPD of metals occurs on Au substrates coated with SAMs. Since UPD reactions involving strong adatom-electrode substrate interactions are energetically more favorable than those involving adatom-adatom interactions which occur during bulk electrodeposition, they should not occur unless metal ions reach the underlying electrode surface. Therefore it has been thought that the SAM itself acts as an effective barrier
Figure 1. Schematic illustration of UPD of Ag in the presence of a SAM of alkanethiolate. Though thiolate molecules are thought to tilt in the SAM, such a molecular orientation is omitted in this drawing.
for UPD reactions and that defects in the SAM provide sites for UPD to take place, if any.6 In the present paper, it is reported that atomic islands of Ag are initially deposited at the defects of the SAM and that they grow laterally in such a way as to intervene between the SAM and the Au substrate. We have already reported in a preceding paper7 that Cu UPD occurred through the SAM of propanethiol, though the resulting surface structure remained a little unclear. Experimental Section
* Corresponding author. E-mail address: yoneyama@ ap.chem.eng.osaka-u.ac.jp. Telephone number: +81-6-879-7372. Fax number: +81-6-879-7373. (1) Finklea, H. O. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol. 19, p 109. (2) Kolb, D. M. In Advances in Electrochemistry and Electrochemical Engineering; Gerischer, H., Tobias, C. W., Eds.; Wiley: New York, 1978; Vol. 11, p 125. (3) Bard, A. J.; Abrun˜a, 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. (4) Jennings, G. K.; Laibinis, P. E. Langmuir 1996, 12, 6173. (5) Jennings, G. K.; Laibinis, P. E. J. Am. Chem. Soc. 1997, 119, 5208.
Water was purified by double distillation of deionized water. n-Propanethiol purchased from Wako pure chemicals and noctanethiol purchased from Nakalai tesque were reagent grade chemicals and were used as received. All other chemicals used were of reagent grade. Au/mica electrode substrates having quasi (6) (a) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988, 332, 426. (b) Sun, L.; Crooks, R. M. J. Electrochem. Soc. 1991, 138, L23. (c) Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. Langmuir 1995, 11, 4823. (d) Gilbert, S. E.; Cavalleri, O.; Kern, K. J. Phys. Chem. 1996, 100, 12123. (7) Nishizawa, M.; Sunagawa, T.; Yoneyama, H. Langmuir 1997, 13, 5215.
S0743-7463(97)00984-0 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/21/1998
UPD of Silver onto Gold Substrates
Figure 2. Cyclic voltammograms for deposition and stripping of Ag on bare Au (solid line), PT-SAM/Au (dashed line), and OT-SAM/Au (dotted and dashed line), taken in an aqueous solution containing 1 mM Ag2SO4 and 0.1 M H2SO4 at the scan rate 20 mV s-1. (111) surfaces were prepared by vacuum evaporation of Au on freshly cleaved natural mica sheets (Nilaco Co.) maintained at 280 °C.7 The electrode coated with SAMs of n-alkanethiol was prepared by immersing the Au/mica substrate in a 1 mM alkanethiol/ethanol solution overnight (13-15 h) at room temperature. The electrode substrates prepared were placed at the bottom hole of an electrochemical cell with a Teflon-coated O-ring (the apparent electrode area was 0.36 cm2). The effective surface area of the electrode was determined using the reported method in which anodic oxidation of chemically adsorbed iodine is utilized.8 After the electrode was mounted in an electrolytic cell, an aqueous solution containing 1 mM KI and 1 M H2SO4 was poured into the cell, followed by standing still for 5 min. The electrolytic cell was washed with distilled water several times and finally filled with 1M H2SO4 aqueous solution to take a voltammogram of the electrode at the scan rate 3 mV s-1. By integrating an anodic wave due to oxidation of the adsorbed iodine which appeared at 1.2 V vs Ag/AgCl, the effective surface area of 0.40 cm2 was determined. Underpotential deposition of Ag was conducted by holding the electrode potential at +50 mV against an Ag wire reference electrode in an aqueous solution containing 1 mM Ag2SO4 and 0.1 M H2SO4 (+470 mV vs Ag/ AgCl). The reductive desorption8a,9 of SAMs of alkanethiols was conducted in 0.5 M KOH by linear sweep voltammetry at the scan rate 100 mV s-1 using an Ag/AgCl reference electrode. Anodic stripping of the deposited Ag was performed in 0.1 M H2SO4 with use of an Ag/AgCl reference electrode by linear sweep voltammetry at the scan rate 20 mV s-1. XPS measurements were performed by using ESCA-1000 XPS apparatus (Shimadzu) which allowed integration of photoelectron signals obtained by repeated spectra measurements. STM observations were carried out in air with the use of a Nanoscope III STM apparatus (Digital Instruments, Inc.) equipped with a Pt-Ir tip. Images were obtained in the constant current mode with a tip current of 1 nA and a bias voltage of 0.1 V.
Results and Discussion Underpotential Deposition of Ag on Au Electrodes Covered with n-Alkanethiol. Figure 2 shows cyclic voltammograms obtained in an acidic Ag2SO4 solution of a bare Au electrode, a Au electrode coated with a SAM of propanethiol (PT-SAM/Au), and a Au electrode coated with a SAM of octanethiol (OT-SAM/Au). The measurements were performed using a Ag/Ag+ electrode as a reference (8) (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. (9) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335.
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Figure 3. Chronocoulometry for Ag deposition onto bare Au (solid line), PT-SAM/Au (dashed line), and OT-SAM/Au (dotted and dashed line), taken in an aqueous solution containing 1 mM Ag2SO4 and 0.1 M H2SO4.. The electrodes were polarized at 50 mV vs Ag/Ag+ (at 470 mV vs Ag/AgCl).
electrode (+420 mV vs Ag/AgCl in saturated KCl), but the potentials are given in reference to Ag/AgCl in this figure because the Ag/AgCl electrode was used in other electrochemical measurements. The results obtained at the bare Au electrode showed typical waves due to UPD of Ag and stripping of the Ag deposited on the Au(111) electrode.10 The Au electrodes coated with thiols showed broad cathodic waves in the potential region between ca. 0.7 and ca. 0.9 V vs Ag/AgCl for PT-SAM/Au and in the potential region between ca. 0.45 and ca. 0.6 V vs Ag/AgCl for OT-SAM/Au, while anodic waves appeared at about 0.8-0.9 V vs Ag/AgCl for both electrodes. Since the cathodic waves appeared at potentials more positive than the threshold potential for bulk deposition of Ag, which is ca. 0.40 V vs Ag/AgCl, they are believed to have resulted from UPD of Ag. Then the UPD of Ag was undertaken at the constant potential 0.47 V vs Ag/AgCl (50 mV vs Ag/Ag+) (given by the arrow in Figure 2) in 0.1 M H2SO4 aqueous solution containing 1 mM Ag2SO4. Figure 3 shows results obtained by chronocoulometry of the UPD of Ag on bare Au, PT-SAM/Au, and OT-SAM/ Au electrodes. It was found that the charges involved in the Ag UPD on the naked Au(111) electrode became constant at 83.2 µC with electrode polarization for 1 min. If the Ag UPD occurs on an ideal surface of a Au(111) substrate in such a way as to form a (1 × 1) structure, 89.2 µC is expected for a full monolayer coverage of the electrode area of 0.4 cm2. Accordingly, the Ag UPD was saturated at 93% of its full coverage, which was a little higher than the reported value of 85%4 obtained with the use of XPS spectra of Ag UPD on Au(111) prepared by polarization of the Au(111) electrode at +100 mV vs Ag/Ag+ in the same electrolyte solution as that used in this study. As shown in the figure, the rate of Ag UPD was retarded by covering the Au electrode substrate with SAMs. In the case of using the PT-SAM/Au electrode, the constant charge 75.8 µC was obtained after polarization for longer than 10 min, giving the limiting coverage of 85.0%, but an increase in the deposition charges was observed even at 20 min of polarization for the OT-SAM/Au electrode. In the latter case, the limiting coverage of 82.8% (73.9 µC) was obtained by polarization for ca. 50 min. Figure 4 shows XPS spectra of Ag 3d and Au 4f obtained for the (10) Chen, C.; Vesecky, S. M.; Gewirth, A. A. J. Am. Chem. Soc. 1992, 114, 451.
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Figure 5. Linear sweep voltammograms for reductive desorption of a PT-SAM formed on (a) naked and (b) Ag UPDcovered Au electrodes, taken in 0.5 M KOH at the scan rate 0.1 V s-1. Voltammogram c was obtained after application of Ag UPD to a PT-SAM-coated Au electrode. The deposition of Ag was conducted by holding the electrode potential at 50 mV vs Ag/Ag+ for 1 min in an aqueous solution containing 1 mM Ag2SO4 and 0.1 M H2SO4.
Figure 4. XPS data of (a) Ag 3d and (b) Au 4f obtained for the OT-SAM/Au electrode which was polarized for 1 h at 50 mV vs Ag/Ag+ in a 0.1 M H2SO4 aqueous solution containing 1 mM Ag2SO4. The spectra of Ag and Au were obtained by integration of 150 and 10 signals, respectively.
OT-SAM/Au electrode after polarization for 1 h at 50 mV vs Ag/Ag+ in a 0.1 M H2SO4 aqueous solution containing 1 mM Ag2SO4. The spectra were obtained by integration of 150 and 10 Ag and Au signals, respectively, to give high signal-to-noise ratios. If the coverage of the deposited Ag was evaluated from the intensities of the Ag 3d5/2 and Au 4f7/2 peaks of the spectra shown in this figure using the same procedures as those described by Jennings and Laibinis,5 82% of the coverage was obtained, in good accordance with the values obtained both by chronocoulometry and by anodic stripping of the deposited Ag, the latter of which will be described in a later section. These results confirm that not bulk deposition of Ag but UPD of Ag only occurred on PT-SAM/Au and OT-SAM/Au electrodes with cathodic polarization at 50 mV vs Ag/Ag+. Reductive Desorption of SAMs of n-Alkanethiol. Voltammogram a of Figure 5 shows the reductive desorption of a PT-SAM formed on a Au electrode, giving a cathodic peak at -0.79 V vs Ag/AgCl. On the other hand, the desorption of PT-SAM adsorbed on a Au electrode which was previously covered with a Ag UPD layer occurred at -1.2 V vs Ag/AgCl, being 0.41 V negative of that at the Au electrode, as shown by voltammogram b. The Ag UPD adlayer was prepared by polarization at 50 mV vs Ag/Ag+. It seems important to note that the peak potential of voltammogram b of Figure 5 was the same as that reported for a PT-SAM formed on an evaporated Ag(111) film electrode.9 The amount of adsorbed PT molecules was estimated to be 8.5 (( 1.4) × 10-10 mol cm-2 by integrating the current peaks of voltammograms a and b, the value being roughly equal to the coverage expected for a (x3 × x3)R30° overlayer structure of alkanethiols on a Au(111) surface.8a Voltammogram c of Figure 5 shows a reductive desorption wave of a PT-SAM formed on a Au electrode after polarizing the electrode at 50 mV vs Ag/Ag+ in a 0.1 M H2SO4 aqueous solution containing 1 mM Ag2SO4 for
1 min. The integration of the current peak of voltammogram c gave the coverage 8.0 × 10-10 mol cm-2, indicating that no significant loss of the adsorbed PT molecules occurred during the course of the Ag deposition. It was found that the reductive desorption potential was almost the same as that of the PT-SAM prepared on the Ag UPD layer (voltammogram b), suggesting that such a SAM/Ag(UPD)/Au structure as illustrated in Figure 1 was formed by the Ag UPD on the PT-SAM-coated Au electrode. The reductive desorption behavior of the PT-SAM was not changed by shortening the period of Ag deposition at 50 mV vs Ag/Ag+ from 1 min to 10 s, indicating that the formation of the bilayer structure was completed within this short deposition time of Ag. For the formation of the SAM/Ag(UPD)/Au structure, the SAM layer has to be displaced from the Au substrate to the Ag adlayer during the Ag UPD. Such an exchange of adsorbates has been observed for the Ag UPD on an iodine-coated Au electrode.11,12 Furthermore, Tarlov13 has recently reported that if a Ag monolayer was vacuum-deposited on a SAM of octadecanethiol adsorbed on Au substrates, Ag atoms migrated under the SAM to form a SAM/Ag/Au structure. Ex situ STM images of 200 nm × 200 nm of the Ag-deposited PT-SAM electrode surface did not show any noticeable difference from that of a naked Au/mica electrode, suggesting that the Ag adlayer deposited homogeneously on the surface of the PT-SAM electrode without formation of Ag islands. However, we discovered that Ag islands were formed at the OT-SAM/Au electrode, as will be discussed below. Figure 6 shows reductive desorption waves of an OTSAM. Voltammograms a and b were obtained for OTSAMs prepared on a naked Au electrode and on a Ag (UPD)-coated Au electrode, respectively. It was found that reductive desorption of an OT-SAM adsorbed on a naked Au electrode occurred at -1.0 V, while that of an OT-SAM prepared on the Ag-covered Au electrode was at -1.33 V. Voltammogram c of Figure 6 is for the OT-SAMcoated electrode polarized at 50 mV vs Ag/Ag+ for 3 min. Interestingly, the reductive desorption of the OT-SAM occurred at two different potentials (-1.07 and -1.34 V), (11) Sugita, S.; Abe, T.; Itaya, K. J. Phys. Chem. 1993, 97, 8780. (12) Hubbard, A. T.; Stickney, J. L.; Rosasco, S. D.; Soriaga, M. P.; Song, D. J. J. Electroanal. Chem. 1983, 150, 165. (13) Tarlov, M. J. Langmuir 1992, 8, 80.
UPD of Silver onto Gold Substrates
Figure 6. Linear sweep voltammograms for reductive desorption of an OT-SAM formed on (a) naked and (b) Ag UPDcovered Au electrodes, taken in 0.5 M KOH at the scan rate 0.1 V s-1. Voltammogram c was obtained after application of 3 min of Ag UPD to an OT-SAM-coated Au electrode. Other details were the same as those given in Figure 5.
Figure 7. Linear sweep voltammograms for reductive desorption of an OT-SAM after conducting Ag UPD at 50 mV vs Ag/Ag+ for the periods given in the figure. Other details were the same as those given in Figure 5.
these potentials being almost the same as the desorption potentials of an OT-SAM from Au and Ag surfaces, respectively. The appearance of the well-separated two peaks suggests that the Ag deposition proceeded in such a way as to give Ag islands on the electrode surface. This is reasonable because the Ag UPD reaction would take place mainly at the molecular defects14 in the SAM which originate from irregularities of alkyl chain alignments and/or organizations. Influence of Ag-UPD on the Reductive Desorption of an OT-SAM. Figure 7 shows desorption waves of an OT-SAM from the electrode substrate after polarization at 50 mV vs Ag/Ag+ in a 0.1 M H2SO4 aqueous solution containing 1 mM Ag2SO4 for the periods given in the figure. The desorption at the more negative potential, which is due to the desorption of OT from Ag, became dominant by extending the Ag deposition time. It is of no doubt that the amount of octanethiol molecules adsorbed on Ag increased with increasing deposition time of Ag and eventually gave such a SAM/Ag(UPD)/Au structure as shown in Figure 1. (14) (a) Becka, A. M.; Miller, C. J. J. Phys. Chem. 1992, 96, 2657. (b) Poirier, G. E.; Tarlov, M. J. Langmuir 1994, 10, 2853.
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Figure 8. Linear sweep voltammograms taken in 0.1 M H2SO4 at the scan rate 20 mV s-1 for anodic stripping of Ag from the four kinds of Ag-deposited OT-SAM electrodes after conducting Ag UPD at 50 mV vs Ag/Ag+ (at 470 mV vs Ag/ AgCl) for given periods. The voltammograms were taken after reductively desorbing the SAM.
The amount of Ag deposited on the OT-SAM/Au was estimated from the charges involved in anodic stripping of the deposited Ag. Since the anodic stripping behaviors of Ag are largely influenced by the presence of an OTSAM, as shown in Figure 2, it is necessary to remove the SAM prior to the anodic stripping experiments in order to determine precisely the amount of deposited Ag. Then, the stripping waves of Ag as shown in Figure 8 were obtained by the following procedures. After UPD of Ag on the Au electrode coated with an OT-SAM was conducted for given periods under the same conditions as mentioned above, the electrolyte solution was replaced with a 0.5 M KOH aqueous solution and the electrode was polarized at -1.4 V vs Ag/AgCl for 1 min under agitation to induce reductive desorption of the SAM layer. The electrolyte solution was replaced again with a 0.1 M H2SO4 aqueous solution, and then linear sweep voltammetry was conducted by scanning the applied potential from 0.2 to 1.1 V vs Ag/AgCl at 20 mV-1. As shown in Figure 8, the anodic waves due to oxidation of the deposited Ag became great with an increase of the deposition time, as expected. The amount of charges involved in the anodic stripping was determined by integrating the broad peaks, as given by the hatched area in the stripping waves, for example, for 10 min. Figure 9 shows the amount of Ag deposition evaluated in three different ways, i.e., chronocoulometry for Ag deposition, anodic stripping of deposited Ag after desorption of OT, and cathodic stripping of OT adsorption on Ag. In all cases, Ag deposition was not completed with polarization for 10 min. However, we found that the stripping charge of the deposited Ag showed a saturation tendency with prolonged cathodic polarization and that finally 74.0 µC was obtained. It was found that results obtained by chronocoulometry and by anodic stripping of Ag agreed fairly well, while a little larger discrepancy is noticed for the results obtained by cathodic stripping of OT adsorbed on Ag. Also shown in this figure is the coverage of the deposited Ag, which was evaluated with the assumption that 89.2 µC gave a monolayer coverage of Ag on Au(111), as described above. The Ag atoms deposited at the defects of the OT-SAM would crawl under the SAM and cause further defects at which Ag UPD can proceed, resulting in a lateral growth of Ag monolayer islands. The Ag-deposited OT-SAM/Au electrode with a Ag coverage of ca. 0.5 was prepared by polarizing the OT-SAM/Au at 50 mV vs Ag/Ag+ in a 0.1 M H2SO4 aqueous solution containing 1 mM Ag2SO4 for
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Figure 9. Coverage of Ag as a function of Ag UPD time, estimated from anodic stripping waves of Ag (O) from chronocoulometry (s) given in Figure 3, and from reductive desorption of the OT-SAM (b) given in Figure 7. Least-squares fittings are made for the results obtained from anodic stripping of Ag (- - -) and reductive desorption of the OT-SAM (- ‚ -).
Oyamatsu et al.
Figure 11. STM images of a OT-SAM-coated Au electrode after conducting Ag UPD for 1 h. Other details were the same as those given in Figure 10.
observed over the entire surface with monatomic step lines of Au(111). It is important to note that the microscope tip is thought to be positioned within the hydrocarbon layer under the tunneling conditions we used, and therefore the image would result from electron tunneling between the tip and the surface of the electrode substrate.15 The cross-sectional view of the STM image was obtained with use of Nanoscope III software for line A-B shown in the STM image, and the result is shown in Figure 10b. It is recognized that the height of that protrusion ranged between 0.2 and 0.3 nm, the value being roughly equal to the spacing of the lattice plane of Ag(111). Figure 11 shows an STM image of the OT-SAM-coated electrode which was subjected to UPD of Ag for 1 h. The polarization time chosen here was long enough to achieve the limiting coverage of Ag, as already described above. The picture shows that the average diameter of the Ag island increased to ca. 10 nm with the polarization for 1 h and that some islands make contacts to the neighboring islands. The ratio of the sum of areas occupied by the Ag islands to the whole area of the STM image, which was also obtained using Nanoscope III software, was estimated to be around 0.8, which was in fairly good agreement with the coverage of Ag obtained by the chronocoulometry and the XPS measurements of the UPD of Ag. Conclusion
Figure 10. (a) Constant current STM images of a 75 × 75 nm2 section of an OT-SAM-coated Au electrode after conducting Ag UPD at 50 mV vs Ag/Ag+ for 3 min. The images were recorded with a bias voltage of 0.1 V and a 1-nA tip current. (b) Crosssectional view of the STM images taken along line A-B given in part a.
3 min, and the prepared electrode was immersed in 0.1 M H2SO4 overnight. The linear sweep voltammetry of the resulting electrode taken in a 0.5 M KOH solution gave well-separated reductive desorption waves of the OTSAM which were essentially the same as those shown in Figure 6c, suggesting that Ag atoms deposited beneath SAMs do not diffuse over the electrode surface at room temperature unless further electrochemical deposition of Ag is undertaken. STM Observations. Figure 10a shows an STM image of the OT-SAM coated electrode which was previously polarized at 50 mV vs Ag/Ag+ in a 0.1 M H2SO4 aqueous solution containing 1 mM Ag2SO4 for 3 min. A number of protrusions having an average diameter of 4 nm are
It has been discovered in the present study that the reductive desorption potential of a SAM of thiol formed on Au is changed negatively with UPD of Ag. The change of the desorption potential of the SAM occurs for a fraction of the SAM whose amount is equal to the amount of UPD of Ag. We believe that Ag UPD onto an alkanethiol-coated Au electrode proceeds basically with the mechanism discussed here, that is, that the UPD of Ag is initiated at defects of SAMs of thiols and then grows laterally with intervention of Ag between thiol molecules and the Au substrate. Acknowledgment. This work was supported by a Grant-in-Aid for Priority Area “Electrochemistry of Ordered Interfaces” No. 09237104 from the Ministry of Education, Science, Culture and Sports, Japan. LA970984E (15) (a) Widrig, C. A.; Alves, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2805. (b) Delamarche, E.; Michel, B.; Biebuyck, H. A.; Gerber, C. Adv. Mater. 1996, 8, 719.
