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Effect of Argon Plasma Treatment on Surface-Enhanced Raman Spectroscopy of Polypyrrole Deposited on Electrochemically Roughened Gold Substrates Yu-Chuan Liu*,† and Chee-Chan Wang‡ Departments of Chemical Engineering and Cosmetic Science, Vanung UniVersity, 1, Van Nung Road, Shuei-Wei Li, Chung-Li City, Taiwan ReceiVed: October 13, 2004; In Final Form: January 21, 2005
In this work, polypyrrole (PPy) films were electrodeposited on electrochemically roughened gold substrates modified by argon plasma treatment. First, a gold substrate was roughened by a triangular-wave oxidationreduction cycle (ORC) in an aqueous solution containing 0.1 N HCl. Then the roughened gold substrate was further treated by argon plasma. Encouragingly, the surface-enhanced Raman scattering (SERS) spectroscopy of polypyrrole electrodeposited on this roughened gold substrate modified by argon plasma treatment exhibits a higher intensity by 8-fold, as compared with the SERS of PPy electrodeposited on an unmodified roughened gold substrate. Meanwhile, the electropolymerization for pyrrole monomers occurring on the modified roughened gold substrate is easier. Also, the nucleation and growth of electropolymerization of pyrrole monomers on the modified and unmodified gold substrates are different.
Introduction In recent spectroscopic analyses, Raman spectroscopy has been employed to evaluate the structure situation of heterogeneous individual and single- and double-walled carbon nanotubes.1-3 Nevertheless, only poor information can be provided due to weak signal or interference from noise.4,5 In collecting the Raman signals of organic compounds that are present in a system at very low concentration levels, surface-enhanced Raman scattering (SERS) spectroscopy has been employed to enhance the detection.6-8 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.9,10 As shown in the literature, many techniques have been developed to obtain rough metal substrates. They include plasma treatment,11,12 sputtering coating,13,14 mechanical polishing by use of abrasives15 vacuum evaporation deposition,16 and electrochemical and laser deposition.17 However, controllable and reproducible surface roughness can be generated through control of the electrochemical oxidation-reduction cycles (ORC) procedure.18,19 Currently, some systems including metal/metal alloy colloids,20,21 metal-coated metal colloids,22,23 and metal/adsorbate/ metal sandwiches24,25 have been developed to further improve SERS performance. Rivas et al.22 studied the effects on SERS of Ag-coated Au and Au-coated Ag colloidal particles, prepared by deposition of Ag or Au through chemical reduction on Au or Ag colloids, respectively. The results indicated that the coverage of Au with Ag induces an increase of the enhancement factor corresponding to Au, although it does not reach the value corresponding to Ag. The Ag coverage by Au represents an improvement of the SERS activity of the deposited metal. Zhang et al.24 studied the SERS of a sandwich structure of self* Corresponding author: Tel 886-3-4515811, ext 540; fax 886-286638557; e-mail
[email protected]. † Department of Chemical Engineering. ‡ Department of Cosmetic Science.
assembled monolayers of functionalized azobenzene thiols on gold to be further covered with a film of Ag mirror. The results revealed that the enhancement correlates to both the silver islands above and the gold substrate underneath. To our knowledge, combined methods to obtain roughened metal substrates for SERS studies have not yet been investigated. Since many nitrogen-containing heterocycles with five- or sixmembered rings are known to give strong SERS spectra, polypyrrole (PPy) films were originally electrodeposited on electrochemically roughened gold substrates modified by argon plasma treatment to investigate the contribution of the modification by plasma treatment to the increased SERS enhancement in this study. Also, the distinguishable nucleation and growth mechanisms between pure PPy films electrodeposited on the roughened Au substrates with and without further modification by plasma treatment are discussed. Experimental Section Chemical Reagents. Pyrrole (Py) was triply distilled until a colorless liquid was obtained and was then stored under nitrogen before use. HCl and LiClO4 were used as received without further purification. The reagents (p.a. grade) were purchased from Acros Organics. All of the solutions were prepared with deionized 18 MΩ‚cm water. Preparation of Roughened Au Substrates Modified by Argon Plasma Treatment. All of 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 gold foil with a bare surface area of 0.238 cm2, a 2 × 2 cm2 platinum sheet, and silver/silver chloride (Ag/AgCl) were employed as the working, counter, and reference electrodes, respectively. Before the ORC treatment, the gold electrode was mechanically polished (model Minimet 1000, Buehler) successively with 1 and 0.05 µm of alumina slurry to a mirror finish. Then the gold substrate was cycled in a deoxygenated 0.1 N HCl aqueous solution from 0.28 V (holding 10 s) to +1.22 V (holding 5 s) versus Ag/AgCl at
10.1021/jp045313l CCC: $30.25 © 2005 American Chemical Society Published on Web 02/26/2005
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500 mV/s for 25 times, which corresponds to the optimum roughening procedure for SERS.26 After the ORC treatment, the roughened Au electrode was further treated by argon plasma. Plasma treatment of the substrates were effected by mounting them in a plasma unit (model PD-2, Kyoto), in which a discharge was excited by a 13.56 MHz 60 W radio frequency generator at a gas pressure of 1 mTorr. The substrates were exposed to Ar plasma for 5 min. Electrodeposition of Polypyrrole on Modified Roughened Au Substrates. Electrochemical polymerization of PPy on the roughened Au substrate modified by argon plasma treatment was carried out at a constant anodic potential of 0.85 V vs Ag/ AgCl in a deoxygenated aqueous solution containing 0.1 M pyrrole and 0.1 N LiClO4. For comparison, PPy was also electrodeposited on the roughened Au substrate without the modification of argon plasma treatment under the same preparation conditions. Characteristics of Modified Roughened Au and Electrodeposited PPy. Raman spectra of PPy films were obtained on a Renishaw 2000 Raman spectrometer employing a He-Ne laser of 1 mW radiating on the sample operating at 632.8 nm and a charge-coupled device (CCD) detector with 1 cm-1 resolution. The surface morphologies of gold substrates were examined by scanning electron microscopy (SEM, model S-4700, Hitachi). The surface roughness of the roughened Au substrates was obtained from atomic force microscopy (AFM; Nanoscope III, Digital Instruments) 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 experiments regarding the surface mean roughness and the SERS enhancement were performed twice. The error is controlled within 5%. Results and Discussion Surface Morphologies of Au Substrates. Figure 1 reveals the surface morphologies of the mechanically polished Au substrates with and without the further modification by argon plasma treatment. Basically, the two substrates are similar due to a lower power of plasma employed in this study. Under this condition, argon plasma treatment, as generally used in roughening metal substrates, demonstrates less influence on the bulk Au substrates. However, it shows a significant effect on the surface morphology of an electrochemically roughened Au substrate, as discussed below. Figure 2a shows the surface image of the roughened Au substrate without modification by argon plasma treatment. It is a typical aspect of a rough surface, but somewhat thin metal islands are exhibited, with good Raman activity, which demonstrates a microstructure smaller than 100 nm.27,28 With the treatment of argon plasma on the roughened Au substrate, as shown in Figure 2b, the surface morphology of the modified roughened Au substrate become thicker and finer. Moreover, it demonstrates a more even surface with closepacking nanoparticles. Further AFM experiments indicate that the surface mean roughness is 143 and 48 nm for the roughened Au substrates with and without the modification of plasma treatment, respectively. Since the molecules located between two metallic nanoparticles display the greatest SERS enhancement,29,30 the interesting surface morphology of the roughened Au substrate modified by argon plasma treatment would contribute to the enhanced SERS effect, as discussed later. Electrodeposition of PPy on Roughened Au Substrates. As shown in the literature,31,32 there are two kinds of nucleation, namely, instantaneous and progressive, and two types of growth, two-dimensional (2D) and three-dimensional (3D). The number
Figure 1. SEM images of different gold substrates: (a) mechanically polished gold substrate; (b) mechanically polished gold substrate modified by argon plasma treatment.
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 not only grow on their former positions on the bare substrate surface but also on new nuclei that form smaller nuclei particles and the surface morphology is flatter. The current maximum (im) for electropolymerization of pyrrole on different electrodes obtained from the chronoamperometric curves are compared with the theoretical curves of 2D and 3D nucleation and growth obtained from those equations derived by Harrison and Thirsk32 for current-time relation, as shown in Figure 3. It is clear that before and after nuclei overlapping (im) the experimental curve for PPy deposited on a roughened Au substrate modified by plasma treatment is consistent with the theoretical curve of the 3D instantaneous nucleation. It can be ascribed to the deposition of PPy on more even and closer-packing nanoparticles, as discussed before in SEM. However, a positive deviation of i/im from the theoretical curve of 3D instantaneous nucleation is observed for PPy deposited on an unmodified roughened Au substrate. Also, the nucleation and growth rate of PPy deposited on the modified
Improved Electropolymerization for Pyrrole on Gold
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Figure 3. Dimensionless plots of I-t curves for pyrrole polymerized on electrochemically roughened gold substrates with (2) and without (4) further argon plasma treatment in 0.1 N pyrrole and 0.1 N LiClO4 at 0.85 V vs Ag/AgCl, as compared with theoretical models for nucleation. Symbols represent (2) pure PPy and (4) the PPy/clay composite, respectively. Curves a and b represent 3D instantaneous and progressive models (---), respectively. Curves c and d represent 2D instantaneous and progressive models (s), respectively.
Figure 2. SEM images of different roughened gold substrates: (a) electrochemically roughened gold substrate; (b) electrochemically roughened gold substrate modified by argon plasma treatment.
roughened Au substrate is quite slow (times to im are 1.0 and 0.7 s for PPy deposited on the modified and unmodified roughened Au substrates, respectively). Generally, the position of nucleation is favorably located on the defective surface, like an edge or step on a surface, with a higher surface energy.33 Thus, much time is necessary for the nucleation of PPy deposited on the modified roughened Au substrate with thicker nanoparticles on it. Figure 4 shows the cyclic voltammograms of pyrrole oxidized on different roughened Au substrates. They indicate that ca. 0.55 and 0.65 V vs Ag/AgCl were the onset potentials of pyrrole polymerized on the modified and unmodified roughened Au substrates, respectively. It clearly explicates that the roughened Au substrate modified by plasma treatment demonstrates a catalytic electroxidation pathway. Generally, at the same applied anodic potentials, the higher polymerization overpotential takes advantage of a higher oxidation level and conductivity of PPy obtained. The modified roughened Au substrate can provide a more suitable surface for PPy electrodeposited on it. Enhanced SERS Spectra of PPy. Figure 5 shows the Raman spectra of PPy electrodeposited on roughened Au substrates with and without the modification of argon plasma treatment.
Figure 4. I-E curves for pyrrole polymerized on electrochemically roughened gold substrates with (curve a) and without (curve b) further argon plasma treatment in 0.1 N pyrrole and 0.1 N LiClO4 at 0.85 V vs Ag/AgCl.
Obviously, a PPy spectrum obtained on the modified roughened Au substrate exhibits a higher intensity, more than 8 times greater magnitude. This increase in intensity is significant in comparison with the reports of conductive polymers deposited on various rough metals.8,34 This phenomenon of a higher intensity PPy spectrum obtained on the modified roughened Au substrate is consistent with the knowledge that analyte molecules located between two metallic nanoparticles display the greatest SERS enhancement. Surfaces with closer packing nanoparticles indeed provide more possibilities for molecules deposited on the boundaries of nanoparticles. Conclusion By combining the methods of electrochemical ORC and argon plasma treatments used in roughening metal substrates for
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Figure 5. SERS spectra of PPy electrodeposited on different substrates: (a) electrochemically roughened gold substrate modified by argon plasma treatment; (b) electrochemically roughened gold substrate (the intensity was magnified 8-fold).
obtaining a SERS effect, enhanced SERS spectroscopy of PPy, which was electrodeposited on a roughened Au substrate modified by argon plasma treatment, by 8-fold was observed, as compared with the normal SERS of PPy electrodeposited on an unmodified roughened Au substrate. The mechanism of nucleation and growth of PPy deposited on the modified roughened Au substrate obeys the 3D instantaneous nucleation. Meanwhile, the surface of the modified roughened Au substrate can provide a catalytic electroxidation pathway for PPy polymerization. The present method can be extended to other SERS studies of S- or N-containing heterocycles with five- or sixmembered rings and to other metal substrates. Acknowledgment. We thank the National Science Council of the Republic of China (NSC-92-2214-E-238-001) and Vanung Universiry for their financial support. References and Notes (1) Andersson, M.; Osterlund, L.; Ljungstrom, S.; Palmqvist, A. J. Phys. Chem. B 2002, 106, 10674.
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