Modification of Electrochemically Roughened Au (111) by

In this study, electrochemically roughened Au(111) is modified with underpotential deposition (UPD) silver for the first time. The formation of self-a...
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Modification of Electrochemically Roughened Au(111) by Underpotentially Deposited Silver and Its Effect on the Subsequent Electrodeposition of Polypyrrole Yu-Chuan Liu* Department of Chemical Engineering, Van Nung Institute of Technology, 1, Van Nung Road, Shuei-Wei Li, Chung-Li City, Tao-Yuan, Taiwan, Republic of China Received March 13, 2003. In Final Form: May 28, 2003 In this study, electrochemically roughened Au(111) is modified with underpotential deposition (UPD) silver for the first time. The formation of self-assembled monolayers of pyrrole monomers and subsequently further autopolymerization are found on the UPD-Ag-modified gold surfaces. The presence of UPD Ag improves the stability of polypyrrole (PPy) monolayers against desorption in a sonicating bath at elevated temperature. Furthermore, the nucleation and growth mechanism for PPy electropolymerized on UPDAg-modified Au is distinguishable from that on unmodified Au. The corresponding conductivity and the Raman intensity of PPy electropolymerized on UPD-Ag-modified Au increase by about 2-fold and 4-fold, respectively.

Introduction At metal or metal oxide surfaces, two methods of the formation of self-assembled monolayers (SAMs)1,2 and the modification of underpotential deposition (UPD)3,4 have provided useful means for functionalizing the surface and tailoring its properties. The ease and flexibility of the selfassembly process provide a convenient method for altering the properties of the metal as an electrode.3,4 On these metals, thiols and related molecules form a densely packed, oriented monolayer.5,6 In general, the lack of stability of these films against both desorption at elevated temperature and molecular exchange with thiol-containing solutions results in some restrictions to their application.9,10 Strategies to improve the stability of SAMs include achieving multiple gold/sulfur interactions11 or incorporating polymerizable groups12 or aromatic moieties.13 There is growing evidence that the bond between the sulfur and the UPD atom is stronger than the bond between the sulfur and gold atoms, which results in more stable SAMs.4,11,14 In addition, the UPD metal has a more noble * Tel: 886-3-4515811 ext 540. Fax: 886-2-86638557. E-mail: [email protected]. (1) Protsailo, L. V.; Fawcett, W. R. Langmuir 2002, 18, 8933. (2) Huang, K.; Wan, M. Chem. Mater. 2002, 14, 3486. (3) Rooryck, V.; Reniers, F.; Herman, C. B.; Attard, G. A.; Yang, X. J. Electroanal. Chem. 2000, 482, 93. (4) Lin, S. Y.; Chen, C. H.; Chan, Y. C.; Lin, C. M.; Chen, H. W. J. Phys. Chem. B 2001, 105, 4951. (5) Vericat, C.; Lenicov, F. R.; Tanco, S.; Andreasen, G.; Vela, M. E.; Salvarezza, R. C. J. Phys. Chem. B 2002, 106, 9114. (6) Lin, S. Y.; Tsai, T. K.; Lin, C. M.; Chen, C. H. Langmuir 2002, 18, 5473. (7) Whelan, C. M.; Smyth, M. R.; Barnes, C. J. Langmuir 1999, 15, 116. (8) Schoenfisch, M. H.; Pemberton, J. E. J. Am. Chem. Soc. 1998, 120, 4502. (9) Carg, N.; Carrasquillo, M. E.; Lee, T. R. Langmuir 2002, 18, 2717. (10) Valiokas, R.; Ostablom, M.; Svedhem, S.; Svensson, S. C. T.; Liedberg, B. J. Phys. Chem. B 2002, 106, 10401. (11) Jennings, G. K.; Laibinis, P. E. J. Am. Chem. Soc. 1997, 119, 5208. (12) Kim, T.; Crooks, R. M.; Tsen, M.; Sun, L. J. Am. Chem. Soc. 1995, 117, 3963. (13) Wang, M. C.; Liao, J. D.; Weng, C. C.; Klauser, R.; Frey, S.; Zharnikov, M.; Grunze, M. J. Phys. Chem. B 2002, 106, 6220. (14) Zamborini, F. P.; Campbell, J. K.; Crooks, R. M. Langmuir 1998, 14, 640.

redox potential than the corresponding bulk metal and allows an expanded potential window in cyclic voltammetry.11 Burgess et al.15 reported octadecyl mercaptan (OM) submonolayers on silver electrodeposited on gold quartz crystal microbalance (QCM) electrodes. It was found that electrodeposition of a silver monolayer onto the surface of gold QCM electrodes affords a surface at which the OM self-assembly reaction can be easily controlled. Also, a selected submonolayer coverage can be reliably prepared in about 10 min. Zamborini et al.14 reported the study of UPD Cu corrosion and passivation with SAMs of organomercaptan. A methyl-terminated aromatic SAM shifts the potential for Cu-UPD oxidation more positive than a hydroxyl-terminated aromatic SAM. For longer chain n-alkanethiol SAMs, the presence of the Cu-UPD layer markedly improves the stability of the SAM compared to when it is adsorbed directly on the Au surface. Lin et al.4 reported the study of SAMs of alkanoic acids on gold surfaces modified by UPD metals. The metal adlayers promote the anchoring of carboxylate headgroups and the assembly of ω-alkanoic acids, which would otherwise exhibit no chemisorption on bare gold. The binding scheme is different from that of SAMs of alkanethiols on gold and alkanoic acids on silver or copper surfaces. Also, measurements from X-ray photoelectron spectroscopy (XPS) suggest that Au/Ag(upd) is not prone to oxidation in air, as reported by Jennings and Laibinis.11 Moreover, as shown in the literature,16,17 the polymerized SAMs with a terminal pyrrole exhibit increased stability toward thermal desorption and greater resistance to exchange with solution thiols. Furthermore, the chemical and physical properties of conducting polymers (CPs) such as discharging behavior,18,19 adhesion to the electrode surface,16,20 surface roughness and conductivity21,22 can be (15) Burgess, J. D.; Hawkridge, F. M. Langmuir 1997, 13, 3781. (16) Willicut, R. J.; McCarley, R. L. J. Am. Chem. Soc. 1994, 116, 10823. (17) Willicut, R. J.; McCarley, R. L. Anal. Chim. Acta 1995, 307, 269. (18) Rubinstein, I.; Rishpon, J.; Sabatani, E.; Redondo, A.; Gottesfeld, S. J. Am. Chem. Soc. 1990, 112, 6135. (19) Sabatani, E.; Gafni, Y.; Rubinstein, I. J. Phys. Chem. 1995, 99, 12305. (20) Kowalik, J.; Tolbert, L.; Ding, Y.; Bottomley, L.; Vogt, K.; Kohl, P. Synth. Met. 1993, 55, 1171.

10.1021/la0344276 CCC: $25.00 © 2003 American Chemical Society Published on Web 07/04/2003

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significantly enhanced by depositing them on an electrode surface which has first been modified with an appropriate thiol or disulfide SAM. Moreover, the oxidation-reduction cycle (ORC) procedure generally used for roughening metal substrates in surface-enhanced Raman scattering (SERS) studies can also provide an easy and reliable way to modify the metal substrates.23,24 The electrochemical ORC procedures for roughening substrates are generally composed of two different ways. One is a triangular-wave ORC,25,26 and the other is a square-wave ORC.27,28 For producing a controllable surface roughness and a homogeneous surface, the former has an advantage over the latter.29,30 Freeman et al.31 reported a method to prepare Ag-clad Au nanoparticles. Very small amounts of Ag (5%) lead to an increase in SERS intensity, but further increases lead to complete loss of signal. Also, measurements from XPS suggest that Au/UPD Ag is not prone to oxidation in air.11 Since SAMs, UPD metals, and ORC roughening can be individually used to modify metal substrates, the combination of these methods and ideas would multiply the corresponding effects. In this study, Au(111) substrates electrochemically roughened by a triangular-wave ORC in an aqueous solution were modified by UPD Ag, which is original and not reported before. By extension of the idea of SAMs, the characteristics of autopolymerization and electropolymerization of pyrrole on these UPD-Agmodified Au substrates were examined. Experimental Section Chemical Reagents. Pyrrole (Py) was triply distilled until a colorless liquid was obtained and was then stored under nitrogen before use. H2SO4, Ag2SO4, LiClO4, and KCl were used as received without further purification. The reagents (p.a. grade) were purchased from Acros Organics. All the solutions were prepared using deionized 18 MΩ cm water. Preparation of Roughened Au Substrates. All the electrochemical experiments were performed in a three-compartment cell at room temperature, 22 °C, and were controlled by a potentiostat (model PGSTAT30, Eco Chemie). A sheet of singlecrystal Au(111) foil with bare surface area of 0.238 cm2 and a 2 × 2 cm platinum sheet were employed as the working and the counter electrodes, respectively. Silver-silver chloride (Ag/AgCl) and a high-purity silver wire were employed as the reference electrodes for the polymerization of polypyrrole (PPy) and the deposition of UPD Ag, respectively. Before the ORC treatment, the Au(111) electrode was mechanically polished (model Minimet 1000, Buehler) successively with 1 and 0.05 µm alumina slurry to a mirror finish. Then the electrode was cycled in a deoxygenated aqueous solution containing 0.1 N KCl from -0.28 to +1.22 V versus Ag/AgCl at 500 mV/s with 25 scans. The durations at the cathodic and anodic vertexes are 10 and 5 s, respectively. After the ORC treatment, the roughened Au electrode (called the unmodified Au substrate) was rinsed throughout with deionized (21) Willicut, R. J.; McCarley, R. L. Langmuir 1995, 11, 296. (22) Wurm, D. B.; Brittain, S. T.; Kim, Y. T. Langmuir 1996, 12, 3756. (23) Sonnichsen, C.; Franzl, T.; Wilk, T.; Von Plessen, G.; Feldmann, J.; Wilson, O.; Mulvaney, P. Phys. Rev. Lett. 2002, 88, 774021. (24) Sanchez-Cortes, S.; Domingo, C.; Garcia-Ramos, J. V. Langmuir 2001, 17, 1157. (25) Bukowska, J.; Jackowska, K. Electrochim. Acta 1990, 35, 315. (26) Liu, Y. C. Langmuir 2002, 18, 174. (27) Gao, P. M.; Patterson, L.; Tadayyoni, M. A.; Weaver, M. J. Langmuir 1985, 1, 173. (28) Taylor, C. E.; Pemberton, J. E.; Goodman, G. G.; Schoenfisch, M. H. Appl. Spectrosc. 1999, 53, 1212. (29) Devine, T. M.; Furtak, T. E.; Macomber, S. H. J. Electroanal. Chem. 1984, 164, 299. (30) Cai, W. B.; Ren, B.; Li, X. Q.; She, C. X.; Liu, F. M.; Cai, X. W.; Tian, Z. Q. Surf. Sci. 1998, 406, 9. (31) Freeman, R. G.; Hommer, M. B.; Grabar, K. C.; Jackson, M. A.; Natan, M. J. J. Phys. Chem. 1996, 100, 718.

Langmuir, Vol. 19, No. 17, 2003 6889 water and finally dried in a vacuum-dryer with dark atmosphere for 1 h at room temperature for subsequent use. Preparation of UPD-Ag-Modified Roughened Au Substrates. The cathodic deposition potentials of UPD Ag and bulk Ag deposited on the electrochemically roughened Au substrates were obtained from the cyclic voltammograms from -0.50 to +0.50 V versus Ag+/° at 20 mV/s in 1.0 mM AgSO4 and 0.1 M H2SO4 aqueous solutions. The substrates were then cycled in this range at 20 mV/s until the UPD Ag and bulk Ag waves became well-defined. Finally, the scan was stopped at 0.10 V versus Ag+/° which is just prior to the onset of bulk Ag deposition, as shown later. The prepared Au/UPD Ag substrate (called the UPD-Ag-modified Au substrate) was immediately removed and rinsed throughout with deionized water and finally dried in a vacuum-dryer with dark atmosphere for 1 h at room temperature for subsequent use. Autopolymerization and Electropolymerization of Pyrrole on the UPD-Ag-Modified Au Substrates. A deoxygenated aqueous solution containing 0.5 M pyrrole was instantly dropped onto the as-prepared UPD-Ag-modified Au substrate. Then it was placed in a desiccator with nitrogen and dark atmosphere for 2 h. Finally the sample (called autopolymerized PPy) was rinsed throughout with deionized water and dried in a vacuumdryer with dark atmosphere at room temperature before testing. The electrochemical polymerization of PPy on the UPD-Agmodified Au substrate (called electropolymerized PPy) was carried out at a constant anodic potential of 0.85 V versus Ag/AgCl in a deoxygenated aqueous solution containing 0.1 M pyrrole and 0.1 M LiClO4. Before electropolymerization, a wait of ca. 20 min is necessary to reach a steady value of the open circuit potential (OCP). The charge passed was 5000, 25, and 200 mC cm-2 for conductivity, SERS, and scanning electron microscopy (SEM) measurements, respectively. For comparison, PPy was also electrodeposited on the unmodified Au substrate by using the same preparation conditions. Characteristics of UPD-Ag-Modified Au Substrates and Prepared PPy Films. Before conductivity measurements, the PPy-based films were stripped from the electrodes with clear adhesive tape. They had a mechanical stability that made them well suited for the measurements. The conductivities of PPy films were determined by using a four-probe technique with a direct current (dc) measurement at room temperature.39 The surface morphologies of the UPD-Ag-modified Au substrate, unmodified Au substrate, and prepared PPy films were obtained using scanning electron microscopy (model S-4700, Hitachi). Raman spectra were obtained using a Renishaw 2000 Raman spectrometer employing an Ar laser of 1 mW radiating on the sample operating at 514.5 nm and a charge-coupled device (CCD) detector with 1 cm-1 resolution. For the XPS measurements, a Physical Electronics PHI 1600 spectrometer with monochromatized Mg KR radiation, 15 kV and 250 W, and an energy resolution of 0.1-0.8% ∆E/E was used. To compensate for surface charging effects, all XPS spectra are referred to the C 1s neutral carbon peak at 284.6 eV.

Results and Discussion Characteristics of the UPD-Ag-Modified Au Substrate. In an ORC treatment, the chloride electrolyte was selected; this facilitates the metal dissolution-deposition process that is known to produce SERS-active roughened surfaces.32 As shown in previous studies,33,34 Au- and Cl-containing nanocomplexes with microstructures smaller than 100 nm were formed on the roughened Au substrates after an ORC treatment. Figure 1a shows the typical voltammetric curve obtained at 20 mV s-1 on electrochemically roughened gold in 1.0 mM Ag2SO4 and 0.1 M H2SO4. Figure 1b obtained in 0.1 M H2SO4 is demonstrated as a reference. In a comparison of curves a and b of Figure 1, it is clearly found that the cathodic waves centered at (32) Chang, R. K.; Laube, B. L. CRC Crit. Rev. Solid. State Mater. Sci. 1984, 12, 1. (33) Liu, Y. C. Langmuir 2002, 18, 9515. (34) Liu, Y. C.; Jang, L. Y. J. Phys. Chem. B 2002, 106, 6748.

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Figure 1. Cyclic voltammograms at 20 mV/s of the first scan for electrochemically roughened gold electrodes in different electrolytes. (a) In 1.0 mM Ag2SO4 and 0.1 M H2SO4. (b) In 0.1 M H2SO4. The insert of the figure displays the deposition and the dissolution of UPD Ag in the range of 0.14-0.50 V vs Ag+/°.

0.224 V (clearly shown in the inserting figure) and -0.030 V versus Ag+/° correspond to the UPD and bulk depositions of Ag, respectively, which can be further confirmed from the deposition time-resolved characteristic curves for UPD Ag and bulk Ag. As shown in the literature,35 the formation of an epitaxial monolayer of Ag onto Au(111) would correspond to a charge density of 222 µC/cm2. The coverage of UPD Ag onto Au in this study determined by averaging the cathodic and anodic charges11 of 61.1 and 26.7 µC/cm2, respectively, is 0.20, which is close to the value of 0.19 calculated from the XPS analysis. The anodic and cathodic charges of UPD Ag were determined by using a program of peak analysis provided with the potentiostat. Also, less charge dissolved in the anodic scan, as shown in the inserting plot of Figure 1, may indicate that a stronger interaction presents between UPD Ag and electrochemically roughened Au and an alloying process exists during the deposition of UPD Ag on Au. Figure 2a shows the surface morphology of UPD Ag film deposited on the roughened Au substrate. Comparing this morphology with that of unmodified Au shown in Figure 2b, a fluffy and thin layer of UPD Ag coated on the nanoparticles of roughened Au without changing their microstructures can be nebulously found. To make certain of the UPD Ag deposited on roughened Au, atomic force microscopy (AFM) with the mode of friction force was performed. The result indicates that the morphology of UPD Ag on roughened Au can be distinguished from that of roughened Au, as different extents of brightness were shown in AFM. The existence of UPD Ag on Au can be further confirmed from the XPS analyses. Curves a and b of Figure 3 show the XPS Ag 3d5/2-3/2 core-level spectra of UPD Ag and bulk Ag deposited on electrochemically roughened Au substrates, respectively. The bulk Ag was prepared by using a deposition procedure similar to that performed on the UPD Ag, as described in the Experimental Section, but the scan was stopped at the cathodic vertex of -0.40 V versus Ag+/° at which the deposition of bulk Ag is just completed, as shown in Figure 1a. As reported by Jennings and Laibinis,11 the binding energy of UPD Ag deposited on flat polycrystalline Au is lower (35) Takami, S.; Jennings, G. K.; Laibinis, P. E. Langmuir 2001, 17, 441.

Figure 2. SEM images of different substrates. (a) UPD Ag deposited on electrochemically roughened gold. (b) Electrochemically roughened gold.

Figure 3. XPS Ag 3d5/2-3/2 core-level spectra of different forms of silver deposited on electrochemically roughened gold substrates. (a) UPD Ag. (b) Bulk Ag.

than that of the corresponding bulk Ag by 0.6 eV. However, when Figure 3a is inspected and compared with Figure 3b, the Ag 3d5/2 main peak of the UPD Ag is located at

Modification of Electrochemically Roughened Au

Figure 4. Raman spectra of PPy layers on different substrates with and without sonicating treatments at various temperatures. (a) Autopolymerized PPy on the UPD-Ag-modified Au substrate without sonicating treatment at room temperature of 22 °C. (b) Autopolymerized PPy on the UPD-Ag-modified Au substrate with sonicating treatment for 30 min at an elevated temperature of 70 °C. (c) Autopolymerized PPy on the unmodified Au substrate with sonicating treatment for 30 min at an elevated temperature of 70 °C for reference.

368.5 eV with a positive shift of 0.5 eV with respect to that of bulk Ag. The difference can be ascribed to the fact that the UPD Ag is deposited onto the roughened Au(111) with Au- and Cl-containing nanocomplexes33,34 in this study, not on a flat polycrystalline Au. A charge transfer occurs from UPD Ag to the roughened Au. It results in a stronger interaction between them. XPS analyses of our previous study of UPD Cu on PPy36 and UPD Cu on Pt reported in the literature37 also show positive shifts of 0.50 and 0.95 eV, respectively, with respect to those of bulk Cu. Clearly, the UPD Ag deposited on electrochemically roughened Au in this study is well-defined. It shows in a type of valence Ag. Autopolymerization of Pyrrole on UPD-Ag-Modified Au Substrates. Figure 4a shows the SERS spectrum of the autopolymerized PPy film deposited on the UPDAg-modified Au substrate. It is a typical Raman spectrum of polypyrrole deposited on metal substrates, as shown in the literature.26,38 The peak shown at about 1600 cm-1 is assigned to the CdC backbone stretching.39 The double peaks shown in the range of 1300-1400 cm-1 are assigned to the ring stretching of neutral and oxidized PPy.40 Meanwhile, the peak shown at the higher frequency side of the double peaks of C-H in-plane deformation of PPy41 located at about 1052 and 1083 cm-1 is a characteristic of the oxidized PPy.42 As described in the Experimental Section, the sample is free of pyrrole monomers in the SERS analysis. Thus, it certifies that the oxidized PPy can be synthesized, which is distinguishable from the known chemical or electrochemical method, on the UPDAg-modified Au due to the special activity of the complexes. Furthermore, the peak located at 934 cm-1 is assigned to (36) Liu, Y. C.; Yang, K. H.; Ger, M. D. Synth. Met. 2002, 126, 337. (37) Hammond, J. S.; Winograd, N. J. Electroanal. Chem. 1977, 80, 123. (38) Wan, X.; Liu, X.; Xue, G.; Jiang, L.; Hao, J. Polymer 1999, 40, 4907. (39) Tian, B.; Zerbi, G. J. Chem. Phys. 1990, 92, 3892. (40) Liu, Y. C. J. Solid State Electrochem. 2002, 6, 490. (41) Furukawa, Y.; Tazawa, S.; Fujii, Y.; Harada, I. Synth. Met. 1988, 24, 329.

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Figure 5. Dimensionless plots of i-t curves for pyrrole polymerized on different Au substrates in 0.1 M pyrrole and 0.1 M LiClO4 at 0.85 V vs Ag/AgCl, as compared with theoretical models for nucleation. Solid and hollow triangles represent PPy films electrodeposited on UPD-Ag-modified Au and unmodified Au substrates, respectively. Curves a and b represent 3D instantaneous and progressive models (dashed lines), respectively. Curves c and d represent 2D instantaneous and progressive models (solid lines), respectively.

the C-H out-of-plane deformation of PPy.43 It demonstrates a markedly enhanced intensity compared to that of the PPy spectrum generally shown in the literature.26,38 According to surface selection rules of Raman and SERS, vibrations along the direction perpendicular to the surface should be more enhanced than those along the parallel direction.44,45 Therefore, pyrrole monomers first orderly adsorb to the UPD-Ag-modified Au substrate to form SAMs, followed by autopolymerization and further oxidation occurring on the substrate due to its special activity. The deposited PPy on the UPD-Ag-modified Au substrate is parallel to the surface to form a densely packed monolayer, which would reflect on the characteristics of the nucleation and growth mechanism and the layer-by-layer deposition in the subsequent electropolymerization of PPy on the UPD-Ag-modified Au substrate, as discussed later. The main problems of SAMs on unmodified substrates are the lack of stability of these films during long-term exposure to the atmosphere or to higher temperatures9,10 and the easy exchange with a new thiol or related molecules. To examine the stability of autopolymerized PPy on the UPD-Ag-modified Au substrate, desorption tests were performed in a sonicated bath at 70 °C for 30 min. In an inspection of curves a and b of Figure 4, before and after tests, respectively, the Raman spectra of PPy are quite similar. This means that the deposited PPy on the UPD-Ag-modified Au is stable. This may be ascribed to a stronger interaction between UPD Ag and PPy. In contrast, the autopolymerized PPy on unmodified Au almost desorbs from the substrate after testing, as shown in Figure 4c. Electropolymerization of Pyrrole on UPD-AgModified Au Substrates. As shown in the literature,46,47 (42) Liu, Y. C.; Hwang, B. J. Synth. Met. 2000, 113, 203. (43) Cheung, K. M.; Bloor, D.; Stevens, G. C. Polymer 1988, 29, 1709. (44) Moskovits, M. J. Chem. Phys. 1982, 77, 4408. (45) Gao, X. P.; Davies, J. P.; Weaver, M. J. J. Phys. Chem. 1990, 94, 6858. (46) Hwang, B. J.; Santhanam, R.; Lin, Y. L. J. Electrochem. Soc. 2000, 147, 2252.

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Figure 7. SEM images of PPy films electrodeposited on the UPD-Ag-modified Au substrate (a) and on the unmodified Au substrate (b).

Figure 6. Mass change-charge plot for the growth of PPy films electrodeposited on the UPD-Ag-modified Au substrate (a) and on the unmodified Au substrate (b).

there are two kinds of nucleation, namely, instantaneous and progressive, and two types of growth, such as twodimensional (2D) and three-dimensional (3D). The number of nuclei in the instantaneous nucleation mechanism is constant, and they grow on their former positions on the bare substrate surface without the formation of new nuclei. Hence the radii of the nuclei are larger and the surface morphology is rougher. In progressive nucleation, the nuclei grow not only on their former positions on the bare substrate surface but also on new nuclei which form smaller nuclei particles and the surface morphology is flatter. The current maxima (im) for the electropolymerization of pyrrole on different electrodes obtained from the chronoamperometric curves are compared and fitted with the theoretical curves of 2D and 3D nucleation and growth obtained from those equations derived by Harrison and Thirsk47 for the current-time relationship, as shown in Figure 5. It is clear that before and after nuclei (47) Harrison, J. A.; Thirsk, H. R. Electroanalytical Chemistry; Marcel Dekker: New York, 1971; Vol. 5, p 67.

overlapping (im), the nucleation and growth mechanisms for PPy electropolymerized on UPD-Ag-modified Au and unmodified Au substrates are quite different. Before nuclei overlapping, the mechanisms obey the 2D instantaneous and 3D instantaneous nucleation and growth for the former and the latter, respectively. During the period of waiting for a steady OCP before starting electropolymerization, an ordered layer of autopolymerized PPy, as discussed before, may have been formed on the UPD-Agmodified Au substrate. These autopolymerized PPy units can then serve as nucleation sites for the subsequent electrochemical growth of PPy. This 2D nucleation process results in a more compact layer, which is consistent with a flatter surface morphology obtained in the following 3D progressive nucleation process after nuclei overlapping for PPy deposited on the UPD-Ag-modified Au substrate. This implies a more ordered nucleation process for the polymer formed on the modified electrode, since nucleation occurs at the sites of ordered autopolymerized PPy. This is also evidence of random nucleation followed by 3D growth on the unmodified substrate as compared to a specific nucleation followed by an ordered growth for the modified electrode before nuclei overlapping. As shown in the literature,48-51 a denser and more compact morphology (48) Liu, Y. C.; Hwang, B. J. J. Electroanal. Chem. 2001, 501, 100.

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Figure 8. Raman spectra of PPy films electrodeposited on the UPD-Ag-modified Au substrate (a) and on the unmodified Au substrate (b); the intensity of the latter was magnified by 5-fold.

would reflect on the enhancement of conductivity stability in the aging behavior of PPy films exposed to an oxygen and water atmosphere. Moreover, an encouraging phenomenon is that the conductivity of PPy deposited on the UPD-Ag-modified Au substrate can be enhanced from 95.3 to 284 S cm-1. The growth of PPy electropolymerized on the UPD-Agmodified Au substrate was also monitored by electrochemical quartz crystal microbalance (EQCM).52 Figure 6a shows the mass change-charge plot during the electropolymerization of PPy on the UPD-Ag-modified Au. Unlike the case on the unmodified Au shown in Figure 6b, the mass change (y) and the corresponding charge passed (x) shows a linear relationship of y ) 367.7x + 7.382 with a correlation coefficient of 0.9999. This quite linear relationship represents a typical pattern for a layer-bylayer growth model, as proposed by Wurm et al.53 The corresponding surface morphology of PPy should be denser and more compact than that of PPy deposited on unmodi(49) Liu, Y. C.; Hwang, B. J. Thin Solid Films 2000, 360, 1. (50) Neoh, K. G.; Young, T. T.; Kang, E. T.; Tan, K. L. J. Appl. Polym. Sci. 1997, 64, 519. (51) Dhanalakshmi, K.; Saraswathi, R.; Srinivasan, C. Synth. Met. 1996, 82, 237. (52) Liu, Y. C.; Hwang, B. J. Thin Solid Films 1999, 339, 233. (53) Wurm, D. B.; Zong, K.; Kim, Y. H.; Kim, Y. T.; Shin, M.; Jeon, C. J. Electrochem. Soc. 1998, 145, 1483.

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fied Au, as shown in Figure 7. Meanwhile, because the slope of the mass versus charge curve is positive, the doped PPy film prepared at 0.85 V versus Ag/AgCl in an aqueous solution containing 0.1 M pyrrole and 0.1 M LiClO4 is in an anion dynamics region.54 This is consistent with the result that only ClO4- anions present in the oxidized PPy film, as revealed from the XPS analysis. Figure 8 shows the Raman spectra of PPy films deposited on different gold substrates. Obviously, a PPy spectrum obtained on the UPD-Ag-modified Au substrate exhibits higher intensity, more than 4 times. This increase in intensity is significant in comparison with the report of polyaniline chemically deposited on various rough metals by Baibarac et al.55 This phenomenon of higher intensity can be explicated from the chemical effect of SERS, since charge transfer readily occurs between pyrrolylium nitrogen and UPD Ag. A similar report was also shown in the literature.56 Moreover, the oxidation degree57 increases from 0.51 to 0.57 calculated from the double peaks of C-H in-plane deformation at 1052 and 1083 cm-1, and the CdC backbone stretching39 shifts from 1600 to 1589 cm-1 for PPy deposited on the UPD-Agmodified Au substrate. Both of them indicate that the conductivity of PPy is enhanced. Conclusion In this study, valence Ag is originally and successfully deposited on electrochemically roughened Au(111) substrates. Pyrrole monomers were found to adsorb onto the UPD-Ag-modified Au substrate to form orderly SAMs and can be further autopolymerized due to the special activity of the nanocomplexes on the electrode. The stability of SAMs in adhesion is significantly improved due to the presence of UPD Ag. Furthermore, electropolymerized PPy on the UPD-Ag-modified Au demonstrates some distinguished properties. The conductivity and the intensity on SERS are enhanced by 2-fold and 4-fold, respectively. Acknowledgment. The author thanks the National Science Council of the Republic of China (NSC-91-2214E-238-001) and the Van Nung Institute of Technology for their financial support. Help from undergraduate student Hsien-Der Fu is acknowledged. LA0344276 (54) Hepel, M. Electrochim. Acta 1996, 41, 63. (55) Baibarac, M.; Cochet, M.; Lapkowski, M.; Mihut, L.; Lefrant, S.; Baltog, I. Synth. Met. 1998, 96, 63. (56) Plays, B. J.; Bukowska, J.; Jackowska, K. J. Electroanal. Chem. 1997, 428, 19. (57) Liu, Y. C.; Tsai, C. J. Chem. Mater. 2003, 15, 320.