Langmuir 2002, 18, 1713-1718
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In Situ Analysis of Electropolymerization of Aniline by Combined Electrochemistry and Surface Plasmon Resonance Xiaofeng Kang, Yongdong Jin, Guangjin Cheng, and Shaojun Dong* State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China Received August 13, 2001. In Final Form: November 5, 2001 The combination of electrochemistry with surface plasmon resonance (SPR) has been used to characterize the growth of polyaniline (PAn) on a gold electrode surface during potential cycling. Potential-modulated SPR characteristics of the PAn film were also revealed. The potential switch between the oxidized and reduced states of PAn can lead to a large change of SPR response due to the variation in the imaginary part of the dielectric constant of PAn film resulting from the transition of the film in conductivity. The redox transition of the PAn film during potential cycling is very profitable to the SPR measurements. Two modes of SPR measurement, SPR angular scan (R-θ) and the time evolution of the reflectivity change at a fixed angle (R-t), were displayed to study the growth process of the PAn film. The angle shift of the resonance minimum recorded at each cathodic limit of cyclic potential scanning allows for the unambiguous measurement of the film growth. During cyclic potential scanning, the R-t curve was repeatedly modulated with the direction of the potential ramp as a result of the redox switch of the PAn film, and the amplitude of potential-modulated reflectivity change was well correlated with the cyclic number. The time differential of the R-t curve permits continuous monitoring of the film growth process. These results illustrate that the combined technique is suitable for studying the electropolymerization process of a conducting polymer.
Introduction Surface plasmon resonance (SPR) spectroscopy has emerged as a powerful tool for in situ real-time characterization of a solid/liquid interface. SPR refers to the optical excitation of surface plasmon oscillations at a metal dielectric boundary by evanescent waves from the base of a prism. It is a collective oscillation of charges at a metal boundary which propagates along the interface. The electromagnetic field associated with the motion of these charges is a maximum at this interfacial boundary and decays exponentially in both directions perpendicularly to it. SPR is sensitive to the modification occurring at this interface. During the past decade, the SPR technique has gained rapid development. The SPR-related publications have exponentially grown since 1992.1 Its main applications deal with the analysis of biomolecular interactions,2-9 monitoring the assembly process of molecules,10-15 and studies of the conformational change of protein mol* Corresponding author: e-mail,
[email protected]; fax, +86431-5689711. (1) Homola, J.; Yee, S. S.; Gauglitz, G. Sens. Actuators B 1999, 54, 3-15. (2) Jung, L. S.; Shumaker-Parry, J. S.; Campbell, C. T.; Yee, S. S.; Gelb, M. H. J. Am. Chem. Soc. 2000, 122, 4177-4184. (3) Mernagh, D. R.; Janscak, P.; Firman, K.; Kneale, G. G. Biol. Chem. 1998, 379, 497-503. (4) Brecht, A.; Gauglitz, P. Anal. Chim. Acta 1997, 347, 219-233. (5) Servers, A. H.; Schasfoort, R. B. M.; Salden, M. H. L. Biosens. Bioelectron. 1993, 8, 185-189. (6) Nelson, B. P.; Grimsrud, T. E.; Liles, M. R.; Goodman, R. M.; Corn, R. M. Anal. Chem. 2001, 73, 1-7. (7) Malmqvist, M. Nature 1993, 361, 186-187. (8) Georgiadis, R.; Peterling, K. P.; Peterson, A. W. J. Am. Chem. Soc. 2000, 122, 3166-3173. (9) Guedon, P.; Livache, T.; Martin, F.; Lesbre, F.; Roget, A.; Bidan, G.; Levy, Y. Anal. Chem. 2000, 72, 6003-6009. (10) Mack, J.; Leipert, D.; Badia, A.; Knoll, W.; Jung, G. Adv. Mater. 1999, 11, 809-814. (11) Cooper, M. A.; Try, A. C.; Carroll, J.; Ellar, D. J.; Williams, D. H. Biochim. Biophys. Acta 1998, 1373, 101-111. (12) Badia, A.; Arnold, S.; Scheumann, V.; Zizlsperger, M.; Mack, J.; Jung, G. Sens. Actuators, B 1999, 54, 145-165.
ecules.16-19 Recently, the application of SPR is extending to the electrochemical field. In electrochemical SPR, the gold film on the glass slide is used as both the surface plasmon medium and the working electrode of electrochemistry. As surface plasmons are located on the electrode surface, an electrochemical process such as the adsorption of species, the modification of electrode surface, etc., can affect the excitation of surface plasmons. Therefore, SPR is suitable for the study of the electrode surface process. To date, several works have been devoted to the SPR study of electrochemical problems. These include the determination of the electric field profile within the multilayer films of zirconium phosphonate,20 detecting trace metals,21,22 studies of the diffusion and adsorption processes on electrode surface,23-25 determination of the surface coverage of hydrated proteins at a fixed potential,26 study of the electrochemical doping/dedoping process27,28 (13) Evans, S. D.; Flynm, T. M.; Ulman, A. Langmuir 1995, 11, 38113814. (14) Jordan, C. E.; Frutos, A. G.; Thiel, A. J.; Corn, R. M. Anal. Chem. 1997, 69, 4939-4947. (15) Kurth, D. G.; Osterhout, R. Langmuir 1999, 15, 4842-4846. (16) Boussaad, S.; Pean, J.; Tao, N. J. Anal. Chem. 2000, 72, 222226. (17) Sota, H.; Hasegawa, Y.; Iwakura, M. Anal. Chem. 1998, 70, 20192024. (18) Schlereth, D. D. J. Eletroanal. Chem. 1999, 464, 198-207. (19) Salamon, Z.; Wang, Y.; Brown, F.; Macleod, H. A.; Tollin, G. Biochemistry 1994, 33, 13706-13711. (20) Hanken, D. G.; Corn, R. M. Anal. Chem. 1997, 69, 3665-3673. (21) Chinowsky, T. M.; Saban, S. B.; Yee, S. S. Sens. Actuators, B 1996, 35-36, 37-43. (22) Jung, C. C.; Saban, S. B.; Yee, S.S.; Darling, R. B. Sens. Actuators, B 1996, 32, 143-147. (23) Iwasaki, Y.; Horiuchi, T.; Morita, M.; Niwa, O. Surf. Sci. 1999, 427-428, 195-198. (24) Iwasaki, Y.; Horiuchi, T.; Morita, M.; Niwa, O. Sens. Actuators, B 1998, 50, 145-148. (25) Iwasaki, Y.; Horiuchi, T.; Morita, M.; Niwa, O. Elecroanalysis 1997, 9, 1239-1241. (26) Terrettaz, S.; Stora, T., Duschl, C.; Vogel, H. Langmuir 1993, 9, 1361-1369.
10.1021/la0155303 CCC: $22.00 © 2002 American Chemical Society Published on Web 01/30/2002
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Figure 1. Schematic of electrochemical SPR setup.
and developing an electrochemical SPR sensor.29,30 Herein, we attempt to further extend the scope of the SPR technique to the analysis of electrochemical growth of an organic conducting polymer. The electrochemical method, especially cyclic voltammetry, is an effective deposition method for PAn film which can give finer control of the film growth and is very convenient to combine with other spectroscopic methods for in situ characterizing the growth and property of the film. In this paper, we combine SPR with cyclic voltammetry to investigate the electrochemical growth process of a PAn film on a gold electrode surface. One of our purposes is to demonstrate the feasibility of monitoring the film growth in situ using an electrochemical SPR method. Two modes of SPR measurement, SPR angular scan and the time evolution of reflectivity change at a fixed angle, are displayed how to monitor the film growth. In addition, the electrochemical SPR characteristics of PAn film during potential cycling are presented and explained reasonably. To our knowledge, this work is the first example of an electrochemical SPR technique for the study of electrochemical growth and potential-modulated SPR property of the organic conducting polymer. The plentiful information obtained with this in situ real-time technique has the potential to give insight into the growth process of a PAn film. It is a novel way to extend the SPR technique in the electrochemical field. Experimental Section Reagents. The chemicals were of all analytical reagent grades. Aniline (Aldrich, 99.7%) was used after distilling. The aniline aqueous solution was stored in the dark at a low temperature. H2SO4 was used as received. All of the aqueous solutions were prepared with doubly distilled deionized water. Setup of the Electrochemical SPR System. Electrochemistry and SPR measurements were performed with a homebuilt electrochemical SPR system using Kretschmann optical configuration. As shown schematically in Figure 1, the setup of the electrochemical SPR cell was designed specially to drive SPR and electrochemistry “in situ”. The evaporated gold film (thickness 46 nm) on the glass microscope slide (n ) 1.562 at 650 nm) was used as both surface plasmon medium and the working electrode. The evaporated chromium as an adhesion layer (thickness 1.5 nm) was used to avoid the peeling of gold during electrochemical SPR experiments. The glass slide with the evaporated gold on the opposite side was pressed on a BaK4 cylindrical glass prism (n ) 1.566 at λ ) 650 nm) via an index matching liquid (n ) 1.5652). The gold surface of the slide was covered with a silicone rubber sheet with a hole for the electrolyte (27) Georgiadis, R.; Peterlinz, K. A.; Rahn, J. R.; Peterson, A. W.; Grassi, J. H. Langmuir 2000, 16, 6759-6762. (28) Kang, X.; Jin, Y.; Cheng, G.; Dong, S. Submitted for publication in Anal. Chem. (29) Kang, X.; Cheng, G.; Dong, S. Electrochem. Commun. 2001, 3, 489-493. (30) Koide, S.; Iwasaki, Y.; Horiuchi, T.; Niwa, O.; Tamiya, E.; Yokoyama, K. Chem. Commun. 2000, 741-743.
Kang et al. contact. The exposed gold surface area was 0.8 cm2. The linearly p-polarized light from a laser diode (650 nm, 5 mW) was directed through the prism onto the gold electrode. The intensity of the reflected light was measured by a photodiode with a chopper lock-in amplifier technique. The cell was provided with a platinum wire auxiliary electrode and a KCl saturated Ag/AgCl reference electrode. Potentials reported here were relative to this reference electrode. The three-electrode system was connected to EG&G PAR-174A polarographic analyzer, EG&G PARC-175 universal programmer, and X-Y recorder for the electrochemical control and measurements. There were also an inlet and a outlet in the cell connected to a IFIS-C type of FIA system (Ruike electron Ltd, Xian, China) for delivering the solution. Method. The gold substrate was cleaned prior to use by immersing it for 2 min in a freshly made piranha solution (75% H2SO4/25% H2O2), followed by rinsing with doubly distilled deionized water and spectrophotometric grade ethanol. (Caution: piranha solution is a powerful oxidizing agent and reacts violently with organic compounds. It should be handled with extreme care.) This treatment was repeated until the gold surface was fully wetted by water. After being cleaned, the gold-coated glass slide was assembled to the electrochemical SPR cell as mentioned above. Subsequently, a deoxygenated 0.05 M H2SO4 solution containing 0.05 M aniline was added to the cell by the FIA system. Polyaniline film was then deposited by cycling the potential between -0.2 and 0.9 V. After a cycle was scanned, the anodic limit was adjusted to 0.7 V to lower the production of unwanted byproducts. The growth process of PAn film was monitored by SPR measurements, and simultaneously cyclic voltammograms were recorded. Both modes for SPR measurements, recording the SPR angular scan curves at the cathodic and anodic extremes of each cyclic potential scan (R-θ) and SPR kinetic curves at a fixed angle (R-t) were chosen to study the growth process of the PAn film. After the film growth, the cell was washed and refilled with 0.05 M H2SO4. The electrode potential was scanned repeatedly between -0.2 and 0.7 V for five cycles in order to complete the polymerization of any traces of aniline. SPR angular scans were then carried out after fixing the potential at -0.2 or 0.7 V for 5 min to study the effects of electrode potential on SPR response. The optical constants for the oxidized and reduced PAn film were extracted by nonlinear least-squares fitting to the full Fresnel equations as described previously.28,29
Results and Discussions SPR Angular Scan Curves of the Electropolymerization Process. Polyaniline film was grown on a gold electrode surface by repeated cyclic potential scans. SPR angular scan curves of R-θ were recorded at the cathodic (Figure 2A) and anodic (Figure 2B) limits of each cyclic potential scan, respectively. In addition to the angle of resonance minimum in two sets of SPR curves showing a shift toward higher angle with increasing cyclic number, we note that the SPR curves recorded at the anodic limit (0.7 V) (Figure 2B) also show a very distinct change in shapesboth the resonance depth (or peak reflectivity) and the widthsrelative to that recorded at the cathodic limit (-0.2 V) (Figure 2A). With the increase of PAn film thickness by repeated cyclic potential scans, an apparent increase of peak reflectivity in Figure 2B is observed. In principle, the shift in the angle of resonance minimum can readily be associated with the change in refractive index and the increase in polyaniline film thickness; the shape change of the SPR curve is mainly determined by the variation in imaginary part of the dielectric constant of the film, i.e., the extinction coefficient K, which is a measurement of the light absorption in a medium.31 Thus the shape change of SPR curves may be related to the redox property of the PAn film. Figure 3 displays representative cyclic voltammograms of the polymeriza(31) Salamon, Z.; Macleod, H. A.; Tollin, G. Biochim. Biophys. Acta 1997, 1331, 117.
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Figure 3. Representative cyclic voltammograms recorded during the growth of PAn film on gold electrode in 0.05 M H2SO4 solution containing 0.05 M aniline at a scan rate of 20 mV‚s-1. The curves represent the 2nd, 4th, 6th, 8th, and 10th cycles from the inside to the outside.
Figure 2. SPR angular scan curves of reflectivity versus incident angle recorded at the open circuit after potential scanning to each cathodic (A) and anodic (B) limits in 0.05 M H2SO4 solution containing 0.05 M aniline. The range of cyclic potential for the first cycle was set between -0.2 and 0.9 V. The anodic limit for subsequent cycles was set to 0.7 V. A total of 11 voltammetric cyclic curves were recorded from the left to the right. The left most curve in part A is corresponding to the bare gold film.
tion reflecting the smooth growth of the film with cyclic numbers. In Figure 3, we observe a couple of redox peaks (Ea ) 0.06 V, Ec ) 0.14 V), and the peak potentials remain essentially constant with increasing film thickness. This is an indication that the PAn film retains a similar property regardless of the film thickness. Many previous works32-38 demonstrated that the redox is corresponding to the redox transition between an insulation state, leucoemeraldine, and a conducting state, emeraldine. On the other hand, optical constants are related to film conductivity. At optical frequencies, a conducting material can be characterized by a complex dielectric constant, . For a light with wavelength λ that is incident on material with conductivity σ, the dielectric constant can be expressed as27
) real + (2σλ/c)i where c is the speed of light, the imaginary part of the dielectric constant is imag ) 2σλ/c, which is related to the optical absorption of the film. It can be seen from the (32) Dinh, H. N.; Vany´sek, P.; Birss, V. I. J. Electrochem. Soc. 1999, 146, 3324-3334. (33) Hatchett, D. W.; Josowicz, M.; Janata, J. J. Electrochem. Soc. 1999, 146, 4535-4538. (34) Kilmartin, P. A.; Wright, G. A. Electrochim. Acta 1998, 43, 30913103. (35) Bartlett, P. N.; Astier, Y. Chem. Commun. 2000, 105-112. (36) Nunziante, P.; Pistoia, G. Electrochim. Acta 1989, 34, 223-228. (37) Hatchett, D. W.; Josowicz, M.; Janata, J.; Baer, D. R. Chem. Mater. 1999, 11, 2989-2994. (38) Rubinstein, I.; Rishpon, J.; Sabatani, E.; Redondo, A.; Gottesfeld, S. J. Am. Chem. Soc. 1990, 112, 6135-6136.
equation that the variation of the PAn film in conductivity can cause a large change in imaginary part of the dielectric constant, imag. This corresponds to the change between the zero value of the reduced PAn film (a transparent film with a real dielectric constant) and the nonzero value of the oxidized PAn film (an absorbing film with nonzero imaginary part of the dielectric constant). Therefore, this redox transition can lead to a large change in shape of the SPR curves. We observe from Figure 2B that the shape change becomes more distinct for a thicker PAn film. This type of reversible switch behavior is a characteristic of PAn and other conducting polymers.27 The redox transition of the PAn film during cyclic potential scanning is very profitable to the SPR measurements of the film growth. Although the angle of resonance minimum in the SPR curve recorded at the anodic limit (0.7 V) also shifts gradually toward higher angle with the continuous increase of film thickness, the curve broadening with the growth of PAn film has a serious influence on the accurate measurement of the resonance angle. The advantage of the cyclic potential growth method is that the PAn film becomes transparent on each cycle as the electrode potential crosses the cathodic limit, so that the shift in the angle of resonance minimum, corresponding to the change in the film thickness, is accessible to unambiguous measurements at each cathodic limit. Here, an additional shift of 0.3° is corresponding to a PAn layer thickness of 1.25 nm as determined previously from EQCM measurements.28,29 Figure 4 shows the thickness of the polymer film for each sequential deposition from cyclic number 1 to 11. A linear dependence of the film thickness on cyclic number is observed, indicating an even growth of PAn film on the gold electrode. We have displayed that the shift in the angle of resonance minimum recorded at each cathodic limit allows the monitoring of the film growth and no interference from potential switch. However, this method cannot provide in real time information about the film growth during cyclic potential scanning. Potential-Modulated SPR Characteristics of the PAn Film. It is very important to study the potentialmodulated SPR characteristics of the PAn film for obtaining information during the cyclic potential scan. Figure 5 depicts the typical R-θ curves obtained by the scan angle for bare gold and PAn film before and after electrochemical oxidation in 0.05 M H2SO4 solution. As expected for the redox switch between the transparent
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Figure 4. Correlation between the total PAn film thickness and the voltammetric cycle number indicating that the growth of PAn film is linear.
Figure 5. SPR angular scan curves for PAn film (A) and bare gold (B) electrode obtained before and after electrochemical oxidation in 0.05 M H2SO4 solution. The thickness of PAn film is 9 nm. Solid lines in Figure 5A are the results of the Fresnel calculation; the points are for experimental data. The parameters used for gold are permittivity -0.167 + 3.447i and thickness 47.5 nm. The dielectric constants for reduced and oxidized PAn film are 1.59 and 1.56 + 0.252i at 650 nm, respectively.
PAn film and absorbing PAn film, the R-θ curve for the oxidized PAn film occurs with a very distinct change in both the resonance depth and the width relative to that of the reduced PAn film (Figure 5A). At the same time, the control experiment measuring the SPR response of the bare gold surface to the applied potential (Figure 5B) shows that the change in Figure 5A is not due to the underlying gold. This shape change should be ascribed to the change of PAn film in the optical property resulting from the conductivity change of the film. The optical constants, for the oxidized and reduced PAn film can be extracted by nonlinear least-squares fitting to a four-layer Fresnel optical mode that accounts for the refractive constant and thickness of the gold and PAn layer.28,29 We first fitted the experimental data on the bare gold layer in water to determine the initial thickness and permittivity of gold. After depositing a PAn film, we fitted the SPR curves to extract the optical constants of the PAn film by setting the gold parameter constants. The pertinent data for the fits are listed in Figure 5. The optical constants are found to be 1.59 and 1.56 + 0.252i for the reduced and
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Figure 6. Typical SPR kinetic curves and its time differential curve. The SPR kinetic curves of PAn film (thickness 9 nm) were taken by in situ monitoring the change of the reflectivity with the potential at 66.3° in 0.05 M H2SO4 solution. Potential scan rate is 20 mV‚s-1. (The two dotted arrows indicate Y-axis in curves, and the short solid arrows show the directions of scan.)
oxidized forms of PAn film, respectively. Similar optical constant values of 1.6 and 1.54 + 0.25i for the reduced and oxidized PAn film have been obtained by ellipsometry at 633 nm.39 It can been seen from these optical constants and Figure 5A that the change of PAn film in conductivity mainly influences the change in the shape of SPR curve. We observe that the potential switch between the reduced and oxidized forms of PAn causes a very large change of reflectivity at fixed angle 66.3° (marking ∆R in Figure 5A) relative to the shift of the resonance minimum (marking ∆θ in Figure 5A). In the SPR measurement, two modes of recording the shift of the resonance minimum angle and the reflectance change at the fixed angle can be chosen to study different surface processes. Obviously, in the present system, the latter is favorable to pursue the sensitive SPR response. Figure 6 displays the typical SPR kinetic curve taken by in situ monitoring of the change of the reflectance with the potential at 66.3° (∆R-E) and its time differential curve (∆R/∆t-E). The observed reflectivity at 66.3° for the oxidized PAn film is larger than that of the reduced PAn film due to the change of the film conductivity. The time differential curve of reflectivity change is similar to cyclic voltammograms, also yielding a couple of peaks, centered at the redox formal potential of the PAn film, corresponding to the transformation between the oxidized and reduced states of the PAn film. These results show that electrochemical SPR is sensitive to the change of PAn film in electrooptical property. The effect of an applied electric field on SPR response is one of the most important points to be considered in electrochemical SPR. In recent works, Hanken and Corn20,40 used electrochemical SPR to monitor the electric field profile inside the multilayer film of zirconium phosphate by applied a external ac voltage of 30 V at 10 kHz. Tao’s group41 studied the effect of electric field on the absorption spectra of molecules using SPR. Generally, in cyclic voltammetric SPR, the effect of electrode potential (∆V) on SPR response originated from three aspects, a (39) Greef, R.; Kalaji, M.; Peter, L. M. Faraday Discuss. Chem. Soc. 1989, 88, 277-289. (40) Hanken, D. G.; Jordan, C. E.; Frey, B. L.; Corn, R. M. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1998; Vol. 20, pp 141-225. (41) Wang, S.; Boussaad, S.; Wong, S.; Tao, N. J. Anal. Chem. 2000, 72, 4003-4008.
Electropolymerization of Aniline
Figure 7. SPR kinetic curves in each cyclic potential scan process. After each cyclic potential scan, the angle position was refixed at each resonance minimum position of R-θ curves in Figure 2A. The electropolymerization conditions are the same as those in Figure 2.
change in dielectric constant (∆), a change in average thickness of the molecular layer (∆d), and a change in surface charge density of the electrode (∆q). The ∆ change upon modulation of an electrode potential reflects the change in electronic states of the deposited molecules. The change in reflectivity (∆R) caused by the modulation of the complex dielectric function ) real + imagi is expressed as42
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Figure 8. Time evolution curve of reflectivity change. The fixed angle is 66.3° and the angle position holds unchanged during the whole continuous cyclic potential growth. Other conditions are as shown in Figure 7.
∆R ) (∂R/∂real)∆real + (∂R/∂imag)∆imag In the present system, the potential-induced change in the refractive index (∆real) is very small; ∆R arises mainly from variation in the imaginary part of the dielectric constant of the PAn film. This is due to the redox transformation of PAn film that leads to a large change of the film conductivity. The potential-induced change in the thickness (∆d) is a piezoelectric effect and usually very small. ∆q is related to the surface capacitance. The interfacial region between a metal and an electrolyte is known as the electrical double layer. Electrochemical reactions occur if the potential of electrode is sufficient to oxidize or reduce a species close to the electrode. In such a case, the double layer region behaves as a capacitor. Changing the potential of electrode will modify the electron concentration near the metal surface. It is known that surface plasmon resonant frequency strongly depends on the surface charge density.41 Therefore, the electrode potential can influence the SPR response. According to ∆q ) CDL∆V, the potential-induced change of the reflectivity (∆R) should be proportional to the surface capacitance that varies as the thickness change of the PAn film. Because the current reflects the surface charge density in unit time, the time differential of ∆R (∆R/∆t) should be correlated with the current in cyclic voltammograms. SPR Kinetic Curves of the Electropolymerization Process. Recording the potential-modulated change in the reflectivity allows us to monitor the electrochemical growth process of PAn film. To compare the change in the reflectivity between each cyclic potential scans, we employ a strategy to reset the fixed angle at resonance minimum position of R-θ curves after each cyclic potential scan (see Figure 2A). A series of SPR kinetic curves are recorded in Figure 7. Two major changes are observed in Figure 7: (42) Ko¨tz, R.; Kolb, D. M.; Sass, J. K. Surf. Sci. 1977, 69, 359-364.
Figure 9. Time differential curve of Figure 8.
(I) The reflectivity is repeatedly modulated with the direction of the potential ramp as a result of redox switch of PAn film. (II) The amplitude of potential-modulated reflectivity oscillation (∆R) increases linearly with cyclic number. Therefore, the electrochemical growth of PAn film can be monitored by recording the change of reflectivity. This method, compared with the usual SPR method of monitoring the angle shift, can afford a larger SPR response. However, it needs to reset the fixed angle after each cyclic potential scan and, thus, could not provide continuous information concerning the electropolymerization process. To continuously monitor the growth process of PAn film, we fix the angle at 66.3° before electropolymerization, and the angle position holds unchanged during the whole continuous cyclic potential scans. The time evolution of reflectivity change is recorded in Figure 8. When the SPR electrochemical cell is filled with 0.05 M aniline + 0.05 M H2SO4 (marking 1 in Figure 8), during an equilibration time, no distinct change in the reflectivity is observed at the open circuit, indicating that polymerization of aniline monomer on a bare gold electrode does not occur at the open circuit. Upon the repeated potential cycling (marking 2 in Figure 8), sequential oscillations of SPR kinetic curve with a periodicity responding to that of cyclic potential scan are observed during the film growth. The reflectivity
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above results have demonstrated that this method is viable for in the real time continuous monitoring of the growth and properties of the PAn film. Conclusion
Figure 10. Linear correlation plot between the peak height of the time differential of reflectivity change and the peak current in cyclic voltammograms.
at a given potential (e.g., at the cathodic or anodic potential limits) increases continuously in the whole process of polymerization, which is an indication of the film growth. However, as represented in Figure 2, the increase in the reflectivity with the film thickening is accompanied with the shift of R-θ curve toward higher angle, thus the change in the reflectivity with cyclic number at a given potential is not a simple linear relationship. It is difficult to obtain a direct quantified measure the film thickness by using the change of reflectivity at the fixed angle 66.3°. Here, we use the time differential of reflectivity (∆R/∆t) to monitor the film growth. Figure 9 is the time differential curve of Figure 8. We observe that the peak height of the time differential of reflectivity is linearly dependent on the cyclic number and, thus, the film thickness. And it is well correlated with peak current in cyclic voltammograms. Figure 10 displays the correlated plot of the anodic branch between the time differential height and peak current with a linear correlation coefficient of 0.997. The
In this paper, we successfully used the combined technique of SPR with electrochemistry to investigate the growth process of PAn film on a gold electrode surface. Potential modulated SPR characteristics of PAn film were also revealed. Some interesting results have been obtained. First, we observe that SPR angular scan curve shifts gradually to higher angles as the PAn film thickens. It allows us to monitor the change in the film thickness by accurately recording the shift in the angle of resonance minimum at each cathodic limit of cyclic potential scanning. Second, the potential switch between the oxidized and reduced states of PAn can cause a large change in the reflectivity. This is ascribed to the transition of PAn film in conductivity that results in the variation in the imaginary part of the dielectric constant of PAn film. Such an effect can be used to monitor the film growth during potential cycling. In addition, it is possible to use a PAnenhanced SPR response to develop a PAN-based electrochemical SPR transducer.29 Third, during continuous cyclic potential scanning, sequential oscillations of the R-t curve with a periodicity corresponding to that of the cyclic potential scanning were observed. Most importantly, the time differential of the R-t curve makes it possible to continuously monitor the film growth. The obtained results have demonstrated that this technique is a viable in situ method for studying the electropolymerization process. It can provide plentiful information on the film growth as well as the optical and electrochemical properties of PAn film by using different kinds of studying modes of electrochemical SPR. Acknowledgment. This work was supported by the National Natural Science Foundation of China (No. 29835120). LA0155303
