Evidence of Chemical Effect on Surface-Enhanced ... - ACS Publications

In this study, polypyrrole (PPy) films were electrochemically deposited on gold substrates roughened by a triangular-wave oxidation−reduction cycle ...
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Langmuir 2002, 18, 174-181

Evidence of Chemical Effect on Surface-Enhanced Raman Scattering of Polypyrrole Films Electrodeposited on Roughened Gold Substrates 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, ROC Received August 24, 2001. In Final Form: October 26, 2001 In this study, polypyrrole (PPy) films were electrochemically deposited on gold substrates roughened by a triangular-wave oxidation-reduction cycle (ORC) in an aqueous solution containing 0.1 N KCl. The results indicate that the intensity of surface-enhanced Raman scattering (SERS) of PPy significantly depends on the ORC scans. Correspondingly, the superficial complex formed on the roughened gold has a strong influence on the SERS effect. The optimum Raman signal of PPy deposited on Au roughened with 25 scans in ORC is ca. 300 times larger than that of PPy deposited on Au without ORC treatment. Meanwhile, the (220) face of Au partly changes into the (111) face with the lowest surface energy after ORC roughening. Toward higher frequency, a shift of CdC backbone stretching of PPy situated at ca. 1575 cm-1 in the SERS spectrum was found which is attributed to interfacial charge transfer from electrodeposited pyrrolylium nitrogen to the roughened gold. Furthermore, this study shows that the SERS enhancement of PPy electropolymerized on roughened Au mainly arises from the chemical effect between PPy and the Aucontaining complex.

Introduction Recently, surface-enhanced Raman scattering (SERS) occurring on roughened metal substrates in principle provides a powerful means of obtaining vibrational information on adsorbate-surface interactions in view of its unique sensitivity and excellent frequency resolution.1-5 Many techniques were used to obtain rough metal substrates, including plasma treatment,6 sputtering coating,7,8 mechanical polishing by using abrasives,9 and vacuum evaporation deposition.10-14 However, a controllable and reproduced surface roughness can be generated through control of the electrochemical oxidation-reduction cycles (ORC) procedure.15 Therein, potential sweep ORC methods15-18 are most popularly used for further * Tel: 886-3-4515811. Fax: 886-2-86638557. E-mail: liuyc@ cc.vit.edu.tw. (1) Oddi, L.; Capelletti, R.; Fieschi, R.; Fontana, M. P.; Ruani, G. Mol. Cryst. Liq. Cryst. 1985, 118, 179. (2) Inoue, T.; Hosoya, I.; Yamase, T. Chem. Lett. 1987, 563. (3) Grasselli, J. G.; Bulkin, B. J. Analytical Raman Spectroscopy; John Wiley & Sons: New York, 1991; Chapter 9, p 295. (4) Takahashi, M.; Niwa, M.; Ito, M. J. Phys. Chem. 1987, 91, 11. (5) Patterson, M. L.; Weaver, M. J. J. Phys. Chem. 1985, 89, 5046. (6) Hesse, E.; Creighton, J. A. Langmuir 1999, 15, 3545. (7) Pockrand, I. Chem. Phys. Lett. 1982, 85, 37. (8) Bandyopadhyay, K.; Vijayamohanan, K.; Venkataramanan, M.; Pradeep, T. Langmuir 1999, 15, 5314. (9) Baibarac, M.; Mihut, L.; Louarn, G.; Mevellec, J. Y.; Wery, J.; Lefrant, S.; Baltog, I. J. Raman Spectrosc. 1999, 30, 1105. (10) Lefrant, S.; Baltog, I.; Chapelle, M. L.; Baibarav, M.; Louarn, G.; Journet, C.; Bernier, P. Synth. Met. 1999, 100, 13. (11) Baibarac, M.; Cochet, M.; Lapkowski, M.; Mihut, L.; Lefrant, S.; Baltog, I. Synth. Met. 1998, 96, 63. (12) Caldwell, W. B.; Campbell, D. J.; Chen, K.; Herr, B. R.; Mirkin, C. A.; Malik, A.; Durbin, M. K.; Dutta, P.; Huang, K. G. J. Am. Chem. Soc. 1995, 117, 6071. (13) Yamada, H.; Nagata, H.; Toba, K.; Nakao, Y. Surf. Sci. 1987, 182, 269. (14) Jung, Y. M.; Sato, H.; Ikeda, T.; Tashiro, H.; Ozaki, Y. Surf. Sci. 1999, 427-428, 111. (15) Pemberton, J. E.; Guy, A. L.; Sobocinski, R. L.; Tuschel, D. D.; Cross, N. A. Appl. Surf. Sci. 1988, 32, 33. (16) Plays, B. J.; Bukowska, J.; Jackowska, K. J. Electroanal. Chem. 1997, 428, 19. (17) Roy, D.; Furtak, T. E. Chem. Phys. Lett. 1986, 124, 299.

improving the surface homogeneity.18,19 As shown in the literature,6,11,18,20 the most suitable roughness microstructure for SERS studies ranges from 10 to 100 nm. The mechanism of SERS consists of two major components. The electromagnetic enhancement,17,21 resulting from an apparent increase in the Raman cross section, is quite well understood. However, the chemical enhancement,22,23 concerning the charge transfer on the adsorbatemetal surface, is poorly understood. The chemical mechanism contributing to Raman scattering exaltations is based on an increased polarizability of the molecules that are adsorbed on the metal surface under the influence of incident radiation, as a result of which new chemical bonds with the metal surface are formed.10,24 In chemical enhancement, most studies4-6,8,17,25 were devoted to molecules adsorbed on activated metal surfaces, especially to pyridine adsorbed on roughened silver.13,20,26,27 Baibarac et al.11 studied the polymer film thickness and roughness parameter dependence of SERS spectra on the polyaniline thin films deposited on rough metals produced by an evaporating method. Lefrant et al.10 studied the structural properties of polyaniline and poly(3-hexyl thiophene) films deposited on rough metals produced by an evaporating method via SERS spectroscopy. The results indicated that the type of the rough metallic support can modify SERS (18) Devine, T. M.; Furtak, T. E.; Macomber, S. H. J. Electroanal. Chem. 1984, 164, 299. (19) 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. (20) Schultz, S. G.; Czachor, M. J.; Duyen, R. P. V. Surf. Sci. 1981, 104, 419. (21) Lefrant, S.; Baltog, I.; Baibarav, M.; Louarn, G.; Journet, C.; Bernier, P. Synth. Met. 1999, 101, 184. (22) Lu, P.; Dong, J.; Toshima, N. Langmuir 1999, 15, 7980. (23) Van Duyne, R. P.; Hulteen, J. C.; Triechel, D. A. J. Chem. Phys. 1993, 99, 2101. (24) Gersten, J. I.; Birke, R. L.; Lombardi, J. R. Phys. Rev. Lett. 1979, 43, 147. (25) Tan, X.; Schneider, T.; Buttry, D. A. Langmuir 1994, 10, 2235. (26) Furtak, T. E.; Roy, D. Surf. Sci. 1985, 158, 126. (27) Demuth, J. E.; Christmann, K.; Sanda, P. N. Chem. Phys. Lett. 1980, 76, 201.

10.1021/la011353u CCC: $22.00 © 2002 American Chemical Society Published on Web 12/11/2001

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spectra. Also, Bukowska et al.28 reported that the change of the surface structure via ORC treatment significantly influences the corresponding SERS effect. Meanwhile, many efforts had been made on finding out the optimum ORC procedure.18,21,24,29,30 The corresponding metalcontaining complexes formed on the bulk metals during different ORC procedures might play important roles on the SERS chemical effect, but less work was devoted to them, as shown in the literature. The major limitation of silver used in SERS is its narrower potential window available compared to Au, especially that metal dissolution easily occurs at more positive anodic potentials.30,31 Gold is more suitable from this point of view, since it provides an especially large polarizable potential window of about of 2 V even in aqueous media. Moreover, the corresponding SERS-active moieties on gold are of inherently greater stability than on silver.32,33 Another advantage of gold over silver is its lower chemical reactivity.10 Since many nitrogen-containing heterocycles with five- or six-membered rings are known to give strong SERS spectra, PPy films were originally electropolymerized on Au substrates roughened by a triangular-wave ORC in an aqueous solution to investigate the relationship between the Au-containing complex formed in ORC treatment and the chemical effect on SERS enhancement in this study. Experimental Section PPy Films Deposited on Roughened Au Substrates. All the electrochemical experiments were performed in a threecompartment cell at room temperature, 24 °C, and were controlled by a potentiostat (model PGSTAT20, Eco Chemie). A sheet of gold foil, a 2 × 2 cm platinum sheet, and silver-silver chloride (Ag/AgCl) were employed as the working, counter, and reference electrodes, respectively. Before the electropolymerization of PPy, the gold electrode was polished successively with 1 and 0.05 µm of alumina slurry to a mirror finish and then was roughened by ORC treatment in a separate cell. The electrode was cycled in a deoxygenated aqueous solution containing 0.1 N KCl from -0.28 V (holding 10 s) to 1.22 V (holding 5 s) at 500 mV/s with different scans. Then, the electrochemical synthesis of PPy on roughened gold 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. For different requirements, the charges used in depositing PPy were 10, 10, 10, and 1250 mC cm-2 for SERS, atomic force microscopy (AFM), X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared (FTIR) experiments, respectively. The thickness of PPy was ca. 20 nm in SERS, AFM, and XPS experiments calculated from an empirical formula of 500 mC cm-2 corresponding to 1 µm estimated by a correlation between thickness and charge passed,34 which had been further confirmed in a previous study.35 Characteristics of PPy Deposited on Roughened Au. The crystal orientations of gold substrates before and after roughening treatment were determined via X-ray diffraction (XRD, model Dmax-B, Rigaku) analysis. Raman spectra were obtained using a Renishaw 2000 Raman spectrometer employing a He-Ne laser operating at 632.8 nm of 25 mW and a chargecoupled 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 250 W, and (28) Bukowska, J.; Jackowska, K.; Jaszynski, K. J. Electroanal. Chem. 1989, 260, 373. (29) Aloisi, G.; Funtikov, A. M.; Guidelli, R. Surf. Sci. 1993, 296, 291. (30) Gao, P.; Patterson, M. L.; Tadayyoni, M. A.; Weaver, M. J. Langmuir 1985, 1, 173. (31) Bukowska, J.; Jackowska, K. Electrochim. Acta 1990, 35, 315. (32) Gao, P.; Weaver, J. J. Phys. Chem. 1985, 89, 5040. (33) Gao, P.; Gosztola, D.; Leung, L. W. H.; Weaver, M. J. J. Electroanal. Chem. 1987, 233, 211. (34) Inganas, O.; Erlandsson, R.; Nylander, C.; Lundstrom, I. J. Phys. Chem. Solids 1984, 45, 427. (35) Liu, Y. C.; Hwang, B. J. Thin Solid Films 1999, 339, 233.

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Figure 1. XRD patterns of gold substrates before and after ORC treatments with different scans. Pattern a represents polished Au without further ORC treatment; patterns b-d represent Au roughened with 10, 25, and 50 scans, respectively. 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 C1s neutral carbon peak at 284.6 eV. The complex XPS peaks are deconvoluted into component Gaussian peaks using the “automatic fitting” program provided with the XPS spectrometer. The surface morphology and roughness of roughened gold and deposited PPy films were obtained from AFM (Nanoscope III, Digital Instrument) experiments. The mean roughness was determined from the mean value of the surface relative to the center plane, which is automatically calculated from a program attached to the instrument. The FTIR spectrum was measured by using the mode of attenuated total reflectance (ATR, model DA 8.3, Bomem) with 1 cm-1 resolution.

Results and Discussion Characteristics of Redeposited Au in the ORC Procedure. To investigate the differences of crystalline orientations of gold before ORC treatment and redeposited gold after ORC treatment, the redeposited gold powders were carefully scraped down from the gold substrates. Then, the scraped powders were analyzed by XRD. Figure 1 shows these results. The peaks located at 38.2°, 44.4°, 64.6°, and 77.5° are assigned to the (111), (200), (220), and (311) faces of Au, respectively. After ORC treatments of 0, 10, 25, and 50 scans, the ratios of the intensities of the (111) face to the intensities of the (220) face of the identical Au substrate increase from 0.396 to 0.640, 0.794, and 0.990, respectively. Similarly but less significantly, the ratios of the intensities of the (200) face to the intensities of the (311) face increase from 0.996 to 1.06, 1.10, and 1.15, respectively, with increasing scans in ORC. Chen et al.36 reported an in situ electrochemical scanning tunneling microscopy study of the structural changes of silver surfaces following an ORC. The result indicates that the low index plane (111) has the lowest surface energy. The growth of this crystal plane becomes apparent after an ORC. Since gold and silver have the same forms of crystal lattices, an increase in the intensity of the (111) orientation of gold after ORC treatment shown in Figure 1 is a reasonable phenomenon. Figure 2 shows the surface images of roughened Au substrates. The structural (36) Chen, J. S.; Devine, T. M.; Ogletree, D. F.; Salmeron, M. Surf. Sci. 1991, 258, 346.

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Figure 3. XPS Cl 2p core-level spectra of Au substrates roughened with different scans in ORC. Curves a-e represent 5, 10, 15, 25, and 50 scans, respectively.

Figure 2. AFM images of gold substrates before and after ORC treatments with different scans: (a) polished Au before ORC treatment; (b) roughened Au with 25 scans in ORC; (c) roughened Au with 50 scans in ORC.

features of redeposited Au demonstrate dimensions of ca. 100 nm, which are suitable for the SERS study.6,20 The

mean roughness of Au roughened with 0, 25, and 50 scans in ORC are 3.1, 145, and 113 nm, respectively. This means that a highly roughening procedure is not necessary for obtaining higher surface roughness. As reported in the literature,6,9,15,30,37 anion-containing complexes of metals, which were distinguishable from bulk metals, were easily formed during the ORC treatment. It was also found that the SERS activity is much more stable when the constituents of the complex are simultaneously present.26 Furthermore, it was believed that the SERS effect chiefly comes from the roughening procedure in ORC7 and the corresponding complex formed at the interface of roughened metal.9 However, less effort had been made to confirm the existence of the complex form on the roughened metal and its corresponding relationship with the roughening procedure and its contribution to the SERS effect. Figure 3 shows the XPS Cl 2p core-level spectra of Au substrates roughened with different scans in ORC. The main peaks of the chloride-containing complexes formed on Au substrates are located at 208 and 199 eV. These chloride peaks at 208 and 199 eV are assigned to Cl (+7) and Cl (-1), respectively,38 which represent the complexes of Au(ClO4)4- and AuCl4- formed on roughened Au substrates. In ORC treatment, the ClO4complex may come from the oxidation of Cl- and the binding with O2- in the anodic scan. The O2- is provided from the reduction of oxygen, which is evolved at the anodic vertex, in the cathodic scan. The complexes formed on Au roughened with different scans in ORC are quite different. Correspondingly, the colors of redeposited gold are bright and pale tan for 25 and 50 scans, respectively. Cai et al.19 reported the similar observation that the colors of roughened Pt electrodes are different in different roughening conditions. Figure 4 shows the depth distribution of chloride contained in the complex of Au roughened with 25 scans in ORC. It reveals, combined with the phenomena shown in Figure 3, that the formation of AuCl4- is prior to the formation of Au(ClO4)4- in sequential ORC treatment. Cai et al.19 reported that the surface signal of the (37) Plieth, W. J. J. Phys. Chem. 1982, 86, 3166. (38) Schmeisser, D.; Naaemann, H.; Gopel, W. Synth. Met. 1993, 59, 211.

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Figure 4. XPS Cl 2p core-level spectra of the Au substrate roughened with 25 scans in ORC, before and after sputtering with Ar+ for different times: (a) before sputtering; (b) sputtering for 1 min; (c) sputtering for 2 min.

adsorbate does not increase monotonically with increasing the surface roughness, which causes a reduction of the apparent enhancement. This reveals that the Ramanactive sites may disappear at extreme roughening conditions. Therefore, in a highly roughening procedure of 50 scans, the decomposition of ClO4- into Cl- and O2 would occur. This phenomenon of ClO4- decomposition was also observed at high temperature and in air.38-40 It results in AuCl4- existing on the surface of Au roughened with 50 scans in ORC. Figure 5 shows the XPS O 1s spectra of complexes formed on Au roughened with different scans in ORC. The main peaks locate at ca. 532.8, 532.2, and 532.8 eV for 10, 25, and 50 scans, respectively. The results of further deconvolution show that there are three peaks in this region. The peak of the low binding energy component at ca. 531.4 eV is associated with the formation of negatively charged oxygen, such as ClO4-.41,42 The ratios of this component associated with ClO4- in the O 1s region are 0.080, 0.186, and 0.083 for 10, 25, and 50 scans, respectively. These results are consistent with the chloridecontaining complexes formed on Au roughened with different scans in ORC, as discussed before. SERS Effect of PPy Films Deposited on Roughened Au. As shown in the literature,12,20,26,43 the laser excitation wavelength should be far removed from direct electronic resonance with an electronic transition localized on the adsorbate in the SERS study. The visible adsorption spectrum of PPy shows that the laser excitation wavelength of 632.8 nm used in this study is well outside the strongest wavelength adsorption band. This assures that no interference comes from resonance-enhanced Raman spectroscopy (RRS) in the SERS study. Figure 6a shows the Raman spectra of PPy films deposited on roughened Au substrates with different scans in ORC. In this study, the optimum condition for obtaining the SERS effect is based on the strongest peak of CdC (39) Samuelson, L. A.; Druy, M. A. Macromolecules 1986, 19, 824. (40) Truong, V. T.; Ennis, B. C.; Forsyth, M. Polymer 1995, 36, 1933. (41) Kang, E. T.; Neoh, K. G.; Ong, Y. K.; Tan, K. L.; Tan, B. T. G. Polymer 1991, 32, 1354. (42) Benseddik, E.; Makhlouki, M.; Bernede, J. C.; Lefrant, S.; Pron, A. Synth. Met. 1995, 72, 237. (43) Garreau, S.; Louarn, G.; Buisson, J. P.; Froyer, G.; Lefrant, S. Macromolecules 1999, 32, 6807.

Figure 5. XPS O 1s core-level spectra of Au substrates roughened with different scans in ORC: (a) roughened Au with 10 scans in ORC; (b) roughened Au with 25 scans in ORC; (c) roughened Au with 50 scans in ORC.

backbone stretching of PPy located at ca. 1560-1630 cm-1 shown in SERS.10,31,44,45 The normalized Raman intensity

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Figure 6. (a) SERS spectra of PPy films deposited on Au substrates roughened with different scans in ORC. Curves a-d represent 0, 10, 25, and 50 scans, respectively. (b) Normalized Raman intensity of PPy films deposited on Au substrates roughened with different scans in ORC.

is calculated from the ratio of the intensity of PPy deposited on roughened Au to that of PPy deposited on polished Au without further ORC treatment. Thus, no correction to the normal Raman scattering intensity is necessary to account for differences in sampling geometry and scattering phenomena.46 Figure 6b shows the influence of ORC cycles on the SERS effect. It suggests that the optimum scans for roughening the Au substrate in ORC treatment is 25 scans, in which the signal is ca. 300 times larger than the signal obtained on Au without ORC treatment. This degree of difference in intensity at different scans is significant in comparison with the reports of conducting polymers chemically deposited on various rough metals.9-11 (44) Zhong, C. J.; Tian, Z. Q.; Tian, Z. W. J. Phys. Chem. 1990, 94, 2175. (45) Bukowska, J.; Jackowska, K. Synth. Met. 1990, 35, 143. (46) Taylor, C. E.; Pemberton, J. E.; Goodman, G. G.; Schoenfisch, M. H. Appl. Spectrosc. 1999, 53, 1212.

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As demonstrated in Figure 6a, both much lower intensity and worse resolution are obtained from the Raman spectrum of PPy deposited on an Au substrate without ORC treatment. The poor non-SERS spectra of PPy films were also reported in the literature.45,47-49 Cai et al.19 reported that the surface signal of pyridine adsorbed on rough Pt does not increase monotonically with increasing the degree of surface roughening, which causes a reduction of the apparent enhancement, since it may produce an aged Pt electrode for over-roughening. Chen et al.36 reported that the SERS-active sites on Ag disappear with increasing the ORC scans. Lefrant et al.10 also found that the SERS intensity of polyaniline chemically deposited on rough Au obtained via the vacuum evaporation technique increases with decreasing the metallic film thickness. That is why the optimum scans for obtaining the most excellent SERS spectrum of PPy is 25 rather than 50 scans in this study. Combining the results shown in Figure 3, the ClO4- complex existing at the Au-PPy interface plays an important role on the SERS effect. As shown in Figure 6a, the intensity of the peak shown at the higher frequency side of the double peaks of C-H in-plane deformation of PPy50 located at about 1052 and 1083 cm-1, which is a characteristic of the oxidized PPy,51 consistently increases with increasing the Raman intensity of PPy films deposited on Au roughened with different scans in ORC. This phenomenon can reflect on the difference in conductivities of PPy films deposited on different Au substrates. Before conductivity measurements, the PPy films were stripped from the electrodes with Scotch tape and 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. The results indicate that the conductivities of 4000 mC cm-2 PPy films deposited on Au roughened with 0, 10, 25, and 50 scans in ORC are 23.7, 62.5, 90.1, and 49.3 S cm-1, respectively. Moreover, the peak of the CdC backbone stretching of PPy44 deposited on Au without ORC treatment shifts from 1575 cm-1 to higher frequencies of 1610, 1604, and 1610 cm-1 when PPy films were deposited on Au roughened with 10, 25, and 50 scans in ORC, respectively. This similar phenomenon was also found in the literature,4,5,9 which was attributed to the charge transfer at the interface of adsorbate-metal surface. Meanwhile, this PPy deposition-induced frequency shift to a higher one can be interpreted as the donation of the nonbonding electrons of nitrogen atoms of PPy to the roughened Au.4 This explanation is consistent with the proposed charge-transfer interaction involving the deprotonated pyrrolylium nitrogen by Kang et al.,41 which will be further confirmed via XPS studies. Certainly, this frequency shift of the CdC backbone stretching can be also attributed to the difference in electric characteristics of the ORC-treated Au surfaces, which would reflect on the characteristics of PPy deposited on them, as later shown in Figures 7 and 8. Furthermore, the peak position of CdC backbone stretching shifts to the lower frequency side with increasing the doping level of PPy,52 which demonstrates a positive effect on the corresponding conductivity. Thus, this peak shift to higher frequency occurring on PPy deposited on Au roughened with 25 scans (47) Cheung, M. K.; Bloor, D.; Stevens, G. C. Polymer 1988, 29, 1709. (48) Sun, B.; Schweinsberg, D. P. Synth. Met. 1994, 68, 49. (49) Ohtsuka, T.; Wakabayashi, T.; Einaga, H. Synth. Met. 1996, 79, 235. (50) Furukawa, Y.; Tazawa, S.; Fujii, Y.; Harada, I. Synth. Met. 1988, 24, 329. (51) Liu, Y. C.; Hwang, B. J. Synth. Met. 2000, 113, 203. (52) Tian, B.; Zerbi, G. J. Chem. Phys. 1990, 92, 3892.

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Figure 7. I-E curves for pyrrole polymerized on Au substrates roughened with different scans in ORC. Curve a represents polished Au without further ORC treatment; curves b and c represent Au roughened with 25 and 50 scans, respectively.

in ORC, which demonstrates the strongest SERS effect, is reduced due to the frequency downshift coming from the effect of its higher conductivity. Figure 7 shows the cyclic voltammograms of pyrrole oxidized on rough and flat Au substrates. They indicate that ca. 0.8, 0.6, and 0.7 V versus Ag/AgCl were the onset potentials of pyrrole polymerized on Au substrates roughened with 0, 25, and 50 scans in ORC, respectively. It clearly explicates that the roughened Au substrate demonstrates a catalytic electroxidation pathway, especially for the Au surface treated with 25 ORCs. Therefore, at the same applied anodic potentials, the higher polymerization overpotential takes the advantage of a higher oxidation level and conductivity of PPy obtained. On the other hand, the higher oxidation level also results from the electron transfer from PPy to Au(ClO4)4- or AuCl4-, forming on the Au substrate during roughening, as discussed in Figure 7. Combining the intensity performance shown in Figure 6, it can be concluded that the overall Raman intensity increases with increasing the polymerization overpotential used in depositing PPy. Figure 8 shows the surface images of PPy films deposited on roughened Au substrates. The image of PPy deposited on the surface treated with 25 ORCs shows a film that is more uniform than the other two. This can be attributed to the different electric characteristics of the ORC-treated Au surfaces on which PPy films were deposited. The mean roughnesses of PPy films deposited on Au roughened with 0, 25, and 50 scans in ORC are 44.4, 47.6, and 143.2 nm, respectively. However, this increase in PPy roughness with increasing ORC scans in roughening the Au substrate does not reflect on the corresponding Raman intensity, as shown in Figure 6. It is well-known that the overall SERS enhancement of 106 is the product of a 104 enhancement from a roughness-independent chemical mechanism and a 102 enhancement from a roughness-dependent electromagnetic mechanism.13,20 Therefore, the chemical effect predominantly contributes to the SERS effect of PPy electrodeposited on roughened Au. Figure 9 demonstrates the IR spectra of PPy films deposited on Au substrates with and without ORC treatment. It is encouraging that the broad peak, occurring at 3100-3400 cm-1 corresponding to the N-H stretching in the PPy ring,53,54 markedly appears for PPy deposited on Au roughened with 25 scans in ORC. To observe the

Figure 8. AFM images of PPy films deposited on Au substrates roughened with different scans in ORC: (a) polished Au without further ORC treatment; (b) roughened Au with 25 scans in ORC; (c) roughened Au with 50 scans in ORC.

chemical effect of PPy deposited on roughened Au, the FTIR spectrum was measured by using the ATR mode.

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Figure 9. FTIR spectra of PPy films deposited on polished Au (upper) and on Au roughened by ORC for 25 scans (lower).

Thus, both intensity and resolution are not comparable with the adsorption spectrum with samples dispersed in KBr pellets. Plays et al.16 also found that three peaks markedly appear in the 3100-3500 cm-1 N-H stretching region in the study of oxidation of 1,8-diaminonaphthalene on roughened copper. Therefore, the appearance of N-H stretching of PPy means that charge transfer indeed occurs from the deposited pyrrolylium nitrogen to the metalcontaining complex formed on bulk metal. It increases the possibility of transition of the N-H stretching mode in exciting by the laser beam, and hence it results in the strong enhanced Raman intensity obtained. As explained in Figure 6, charge transfer could take place through the nitrogen atom with a lone pair of electrons to the metal complex.4,22,27,41 Figure 10 shows the resulting XPS N 1s spectra of PPy films deposited on Au roughened with 0, 25, and 50 scans in ORC. With further deconvolution of the N 1s spectra into three component peaks, the positively charged nitrogen (-N+H-) species with the higher binding energy (BE) tail (BE > 401 eV) can be used to define the oxidation level of PPy. This oxidation level is calculated from the ratio of the peak area of N+ (BE > 401 eV) to that of the total N 1s shown in the XPS spectrum.55,56 Generally, the doping level is equal to the oxidation level of PPy. The results show that the oxidation level of PPy deposited on Au without ORC pretreatment increases from 25.1 to 33.2 and 29.5 for PPy deposited on Au roughened with 25 and 50 scans in ORC, respectively. This is attributed to an electron transfer from pyrrolylium nitrogen to the metal. Correspondingly, the conductivity and the doping level of PPy also increase,10,41 and hence the enhanced Raman spectra is obtained due to the charge-transfer effect,13 which is consistent with the phenomenon observed in Figure 6. In this situation of PPy deposited on roughened Au, the polarizability becomes much higher than that of free molecules57 since charge transfer can lower the electronic excitation energy of the deposited PPy.58 (53) Iroh, J. O.; Wood, G. A. J. Appl. Polym. Sci. 1996, 62, 1761. (54) Jones, R. A. Heterocyclic Compounds, Pyrroles; Wiley: New York, 1990; Vol. 48, Part 1, p 61. (55) Kan, E. T.; Neoh, K. G.; Ong, Y. K.; Tan, K. L.; Tan, B. T. Macromolecules 1991, 24, 2822. (56) Eaves, J. G.; Kopelove, A. B. Polym. Commun. 1987, 28, 38.

Figure 10. XPS N 1s core-level spectra of PPy films deposited on Au substrates roughened with different scans in ORC: (a) polished Au without further ORC treatment; (b) roughened Au with 25 scans in ORC; (c) roughened Au with 50 scans in ORC.

Polypyrrole Films Electrodeposited on Gold

Conclusion In this study, PPy films were electrochemically deposited on gold substrates roughened with different scans in ORC. After roughening, the (220) face of Au partly changes into the (111) face with the lowest surface energy. The results indicate that the intensity of the SERS spectrum of PPy significantly depends on the ORC scans and the corresponding superficial complex formed on the roughened gold. The main complexes are Au(ClO4)4- and AuCl4for Au substrates roughened with 25 and 50 scans in ORC, respectively. The optimum Raman signal of PPy deposited on Au roughened with 25 scans in ORC is ca. 300 times (57) Otto, A.; Mrozek, I.; Grabhorh, H.; Akermann, W. J. Condens. Matter 1992, 4, 1143. (58) Burstein, E.; Chen, Y. J.; Chen, C. Y.; Lundquist, S.; Tosatti, E. Solid State Commun. 1979, 29, 567.

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larger than that of PPy deposited on Au without ORC treatment. Meanwhile, a higher frequency shift of ca. 30 cm-1 of CdC backbone stretching of PPy situated at ca. 1575 cm-1 in the SERS spectrum was found which is attributed to interfacial charge transfer from electrodeposited pyrrolylium nitrogen to the roughened gold. Conclusively, the SERS enhancement of PPy electropolymerized on roughened Au mainly arises from the chemical effect between PPy and the Au complex. Acknowledgment. The author thanks the National Science Council of the Republic of China (NSC-89-2214E-238-001) and the Van Nung Institute of Technology for their financial support. LA011353U