ISSN 1023-1935, Russian Journal of Electrochemistry, 2008, Vol. 44, No. 1, pp. 98–103. © Pleiades Publishing, Ltd. 2008. Original Russian Text © O.V. Levin, V.V. Kondratiev, V.V. Malev, 2008, published in Elektrokhimiya, 2008, Vol. 44, No. 1, pp. 107–112.
Using the Rotating Disk Electrode for Evaluating Film Porosity of Conductive Polymers O. V. Levin, V. V. Kondratiev, and V. V. Malevz St. Petersburg State University, Faculty of Chemistry, Universitetskii pr. 26, Petrodvorets, St. Petersburg, 198504 Russia Received January 16, 2007
Abstract—Conductive polymer (poly-o-phenylenediamine and poly-3-methylthiophene) films were synthesized on a rotating disk electrode. Dependences of the limiting penetration currents on the nature of the polymer and film thickness were obtained in solutions containing electroactive substances (hydroquinone, quinone) reducing or oxidiring at redox potentials beyond the range of polymer electroactivity (selected by adjusting the pH value). The transport of hydroquinone and quinone test species through the pores in polymer films was examined based on the results of these studies, and the degree of film porosity was evaluated. Key words: poly-o-phenylenediamine, poly-3-methylthiophene, rotating disk electrode, degree of porosity DOI: 10.1134/S102319350801014X
INTRODUCTION
The quantitative aspects of this method using a rotating disk electrode (RDE) are considered in our previous work [12].
Conductive polymers are now very promising as materials for constructing various devices with unusual properties such as flexible displays, which are being developed by several companies in the world; hyperfine power supply units, which can be printed on any surfaces with special printers; biological sensors, etc. However, effective practical application of a definite polymer requires knowledge about the mechanisms of charge transfer processes in it and data about the quantitative parameters of these processes.
EXPERIMENTAL Electrochemical synthesis of poly-Ó-phenylenediamine films was conducted in a potentiodynamic mode on a polished glassy-carbon disk 0.3 cm in diameter (Ä = 0.07 cm2) from solutions containing 0.05 M Ó-phenylenediamine and 0.5 M perchloric acid at a potential cycled from –200 to 1200 mV using the procedure of [5]. The potentials were measured (and are given below) relative to a Ag/AgCl electrode in a saturated potassium chloride solution. For preparing the solutions we used bidistilled water and reagent grade regents. Measurements were conducted in a three-electrode cell, which was thermostatted at 25°ë. To remove solute oxygen, argon of high purity was bubbled through the solution. Measurements on a rotating disk electrode were carried out with an AVS-1.1 apparatus (NTF Volta, St. Petersburg, Russia). For electrosynthesis of films we used a PI-50.1 potentiostat-galvanostat with a PR-8 programmer and a Graphite-2 fast-operating interface for numeric measurements of experimental curves. A quinone–hydroquinone system was used as a redox pair for investigating the degree of porosity. The starting (home-made) reagents were recrystallized from hexane before use. Since the redox potential of the given substances depends on pH, all measurements were conducted in a buffer solution with pH 9.18. Under these conditions, the half-wave potential was ~+200 mV for hydroquinone oxidation and –100 mV for quinone reduction.
Poly-Ó-phenylenediamine (PPD) is a redox-conductive polymer from the aniline series, having a ladder structure. The electrochemical properties of this amine are reported in numerous publications [1–11], but many of these works give contradictory or inaccurate data. Therefore, studies on these complex systems are far from being completed. The behavior of electrodes modified by these films is described by various theories. Many of these theories use film porosity to explain the results of experiments. However, there are no procedures for evaluating the degree of film porosity before the experiment. The major procedures for investigating film porosity are either inapplicable to thin films of conductive polymers (mercury injection under pressure, and gas adsorption) or require film drying (various electron microscopy methods), which changes the film structure. In this work we suggest a method for evaluating the degree of film porosity based on investigation of redox processes of soluble redox active substances at potentials corresponding to low conductivity of the polymer. z
Corresponding author, e-mail:
[email protected]
98
USING THE ROTATING DISK ELECTRODE FOR EVALUATING FILM POROSITY I, µÄ 40
99
I, µÄ
0
40
4
30
3
20
2
10 1
1 0
–40 2
–10 –1000 –80
–1000
–500
0
–500
0
500
1000 E, mV
500 E, mV
Fig. 1. CVA of PPD films in solutions of (1) 0.01 M Na2B4O7 · 10H2O; (2) 0.01 M HClO4 + 0.99 M LiClO4.
RESULTS AND DISCUSSION Poly-Ó-phenylenediamine, which can be doped with hydrogen ions, loses its activity at large positive potentials [13]. The conductivity of this polymer can be diminished by using weak-alkali electrolytes. Figure 1 shows the cyclic voltammogram of a PPD film in 0.01 M sodium tetraborate (pH 9.18) and the same in 0.01 M çëlO4. It can be seen that the peak intensity of film redox processes in alkali solutions is much lower, and the peaks are shifted toward negative potentials. The anode segment of electric inactivity in alkali solutions increased appreciably and in fact determined the choice of pH for these solutions. On the RDE not modified with a polymer film, the oxidation wave is irreversible, and its limiting current is proportional to the hydroquinone concentration and the square root of the electrode rotation frequency (for rotation rates of 1000–2500 rpm). Thus, the known Levich equation is satisfied: 0
I lim = nFADc R /δ = 0.62nFAD
2 ⁄ 3 –1 ⁄ 6
ν
ω
1⁄2 0 cR,
where Ä is the electrode surface; n is the number of electrons transferred in a redox reaction; ν is kinematic viscosity; D is the diffusion coefficient; ω is the angular disk rotation rate (ω = 2πf, f is the disk rotation rate); 0 and c R is the concentration in solution. Figure 2 shows the cyclic curves of I vs E (CVA) (scan rate 50 mV/s) of hydroquinone electrooxidation in the absence of a poly-Ó-phenylenediamine film on the rotating electrode. It can be seen that the process occurs at positive potentials corresponding to the nonRUSSIAN JOURNAL OF ELECTROCHEMISTRY
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Fig. 2. CVA of hydroquinone electrooxidation on an RDE in a solution of 0.01 M Na2B4O7 · 10H2O with a C6H4(OH)2 addition at concentrations of M: (1) 0, (2) 2 × 10–4, (3) 4 × 10–4, and (4) 6 × 10–4.
conductive state of PPD (cf. Figs. 1 and 2). As follows from Fig. 3, the polymer film considerably decreases the hydroquinone electrooxidation current, but the process still occurs in the same range of potentials that correspond to oxidation on the film-free electrode. Also, it follows that changes in the range of electric activity of PPD films on the CVA curves are insignificant when hydroquinone is added to the solution. This suggests that in the case of the modified electrode, electrooxidation of hydroquinone also occurs on the glassy-carbon surface. Therefore, treatment of experimental data below uses Eqs. (1)–(6) from [12]. Before starting data treatment, we note that at low concentrations of hydroquinone, the anode segment of the oxidation curve on the modified electrode contains a peak; the current at this peak almost coincides with that on a pure glassy-carbon disk at the same concentration of hydroquinone (Fig. 3a). These oxidation rate maxima gradually vanished at higher concentrations of hydroquinone (Figs. 3b, 3c). A possible explanation to the appearance of this peak is the polymer memory effect. Due to this effect, the film remains conductive for some time after its transition from the reduced to oxidized state even at rather high positive potentials because of slower oxidation of polymer chains. Thus, oxidation of hydroquinone occurs not only on the free surface of the electrode, but also in the conductive regions of the film. These partially oxidized chains can only transfer a limited amount of charge; at sufficiently high concentrations of hydroquinone, the contribution of bulk oxidation becomes insignificant, the dominant process being hydroquinone diffusion in pores or incorporation in the bulk of the film (see [12] and Eq. (1) below). No. 1
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LEVIN et al.
I, µÄ 20
Ip, µÄ –5 0
(a) 4 3
10
5
0 –10
(a)
4
15 1
3
25 –20
2
35
2 –30
1 45
30
(b)
4 3
0
2
0
6
4 (b)
10 2 3
10 20
0 1
–10
30
–30
1
40
2
0 40
3
20
2 1
0
–20
–1000
–500
0
500
1000 E, mV
Fig. 3. Effects of the PPD film on the oxidation wave of hydroquinone. 0.01 M Na2B4O7 · 10H2O was used as electrolyte; (1) pure glassy-carbon disk without any hydroquinone additions, (2) glassy-carbon disk coated with a PPD film without any hydroquinone additions, (3) glassycarbon disk coated with a PPD film in solutions with hydro0
quinone at concentrations c R indicated below, and (4) pure 0
glassy-carbon disk in solutions with hydroquinone at c R , M: 0
0
4
6 cR0 × 104, å
4
(c)
–1500
2
0
(a) c R = 2 × 10–4, (b) c R = 4 × 10–4, and (c) c R = 6 × 10–4.
Fig. 4. (a) Dependences of the limiting penetration current on the hydroquinone concentration on a (1) pure glassy-carbon electrode and (2)–(4) PPD-coated electrode for film thicknesses s (nm) of (2) 150, (3) 330, and (4) 390; (b) the same dependences on a (1) pure glassy-carbon electrode, (2) electrode with a freshly synthesized PPD film 390 nm thick, and (3) electrode with a PPD film 390 nm thick after drying in air.
The limiting diffusion currents of hydroquinone oxidation, recorded on the modified electrode, are proportional to the hydroquinone concentration. The proportionality coefficient, however, is smaller than for the pure glassy-carbon electrode (Fig. 4). This agrees with Eq. (5) from [12] for the limiting penetration current Ip, I p = nFAD f kc 0 /s { 1 + k D f δ/Ds }
(1)
= nFADc 0 / { δ + Ds/k D f }.
Here, D is the diffusion coefficient of reactant species in solution; Df is the diffusion coefficient of these species in a film; k is the film porosity factor (in the case of diffusion of the test species in the pores of the film, or the distribution coefficient of these species between the film and the solution (when the species penetrate in the
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USING THE ROTATING DISK ELECTRODE FOR EVALUATING FILM POROSITY
bulk of the film); s is the film thickness; and the value of the Ds/kDf parameter is comparable to the thickness δ of the diffusion layer in the vicinity of the RDE. When the film thickness increased, the slope of the concentration dependence of the limiting penetration current (Fig. 4a) decreased. This confirmed that the Ds/kDf term was significant compared to the thickness of the diffusion layer δ (see Eq. (1)). After the electrosynthesized polymer films were dried, their surface generally acquired microfractures. For PPD films used in this work, this can be seen on microphotographs obtained with a scanning electron microscope (Fig. 5). Therefore, the PPD film dried in air should be more porous than the freshly synthesized film. Indeed, the 0 slope ratio of the dependence Ip( c R ) is higher for the dry film, in agreement with the increased degree of its porosity (curves 2 and 3, Fig. 4b). The limiting penetrating currents measured also increased with the electrode rotation speed. In this case, the data are conveniently treated by constructing a dependence of the limiting reverse penetration current on the reciprocal root from the disk rotation speed [12]: 1/I p = s/k D f nFAc 0 + 1.61D
1⁄3 1⁄6
ν
ω
–1 ⁄ 2
(2)
/nFADc 0 .
As can be seen in Fig. 6, the slope of these curves for PPD films of varying thickness coincides with the slope of the corresponding curve for the pure glassy-carbon electrode. Along with the coincident oxidation potential regions (see above), this validates interpretation of the observed limiting currents as penetration currents, caused either by diffusion of the (hydroquinone) test species in the film pores or by their penetration in the bulk substance with further oxidation on the glassy-carbon support of the RDE [12]. In the same figure, it can be seen that the parameter s/kDf nFAÒ0 increases with the film thickness; it seems that this occurs mainly due to the decrease in the coefficient k, but this growth is not proportional to growth of thickness. This behavior of the dependence is evidence in favor of the transport of (hydroquinone) test species over the film pores because variation of the distribution coefficient as a function of film thickness is unlikely (when the process occurs by the inclusion mechanism). The hypothetical decrease in porosity for thicker films agrees with the data of [14] on electrosynthesis of PPD films. Growth of the film thickness is retarded with time by increased screening of the electrode surface with the newly synthesized layers of the film. Thus, synthesis time is 5 min for a 150 nm film, 12 min for a 330 nm film, and 25 min for a 390 nm film. It is reasonable to assume that as a result of prolonged synthesis, the new polymer layers partially fill RUSSIAN JOURNAL OF ELECTROCHEMISTRY
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X4000 Fig. 5. SEM micrograph of the PPD film surface.
the pores that formed during the preceding stages of film synthesis. Therefore, the films obtained by prolonged synthesis have a much lower degree of porosity, as reflected by the data of Fig. 6 (cf. curves 2–4). From Fig. 6 it also follows that extrapolation of the values of the reverse limiting current on a pure glassy-carbon electrode to the infinite disk rotation frequency gives a nonzero initial segment. This is probably caused by the uncertainty of determinations and was taken into account in further porosity evaluations. If the diffusion coefficient in pores is taken to be the same as in solution (Df = D), the porosity coefficient k can be estimated from the initial 1/Ip × 10–4, Ä–1 4
6
4 3 2
2
0
0.01
0.02 0.03 f –1/2, (rpm/min)–1/2
Fig. 6. Dependences of the limiting penetration current on the RDE frequency. The hydroquinone concentration is 6 × 10–4 M; (1) pure glassy-carbon electrode, (2)–(4) PPD-coated electrode, film thickness s, nm: (2) 150, (3) 330, (4) 390. No. 1
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LEVIN et al. I, µÄ 10
I, µÄ 60
0
1
–10
2
(a) 1
40
4 –20 20 2 –30
3 0
–500
0
500 E, mV 0
Fig. 7. Effects of the P3MeT film on the quinone reduction wave. 0.01 M Na2B4O7 · 10H2O was used as electrolyte; (1) pure glassy-carbon electrode, (2) P3MeT-coated glassy-carbon disk in solution without quinone additions, (3) pure glassy-carbon disk with 4 × 10–4 M quinone in solution, and (4) P3MeT-coated glassy-carbon disk in a solution with the same quinone concentration.
segments of the dependence 1/Ip = ϕ(f–1/2). Estimations show that for low film thicknesses (150–330 nm), the coefficient k is almost constant, 0.14–0.19, and decreases to 0.02 when the film thickness increases to 390 nm. Similar results could also be obtained for other polymer films. Poly-3-methylthiophene (P3MeT) was synthesized by the procedure of [15] in an anhydrous medium and then transferred to an aqueous solution of a borate buffer, pH 9.18. As the given polymer films were electroactive within the positive range of potentials, the study of these films used quinone reduction with a half-wave potential of the order of –100 mV. Within the range of the potentials under study, there was no pronounced electrochemical response of the P3MeT film itself that contacted the aqueous solution. After the film was transferred to an acetonitrile solution of LiClO4, reproducibility of its CVA was 10% compared to that of the freshly synthesized film investigated in the same solution, which indicated that the coating was stable. Figure 7 presents the curves of I vs E recorded on glassy-carbon rotating disk electrodes, both pure and coated with a P3MeT film in a solution containing 4 × 10–4 M quinone, pH 9.18. It can be seen that poly-3methylthiophene affects the intensity of quinone reduction, but not the half-wave potential of this process. As for PPD, for poly-3-methylthiophene films the dependence of the limiting penetration current on the concentration of the test particle is linear, the slope ratio for the modified electrode being much lower than for the pure glassy-carbon disk (Fig. 8a).
2
1/Ip × 10–4, Ä–1 10
4
6 cR0 × 104, å
(b)
2
8
6
4
2
1
0.020
0.024
0.028
0.032 f –1/2, (rpm)–1/2
Fig. 8. Effects of the P3MeT film on the limiting penetration current. (a) Dependences of the limiting penetration current on the quinone concentration and (b) dependences of the reverse limiting penetration current on the RDE rotation frequency for (1) pure glassy-carbon disk and (2) P3MeTcoated glassy-carbon disk.
As for PPD, for P3MeT films the dependence of the reverse limiting penetration currents on the electrode rotation speed is linear to the power –1/2. However, the slope of this curve is slightly smaller than theoretical (Fig. 8b), which is probably caused by partial degradation of the films during prolonged measurements. The deviations from the linear dependence within the high concentration ranges of quinone are evidently caused by the appearance of a pH gradient near the electrode. For a P3MeT film 1.1 µm thick, porosity was evaluated in an assumption that Df = D and proved to be ~0.12. To summarize, the conclusion can be drawn about porosity of conductive films of both n (PPD) and p-doped (P3MeT) polymers.
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ACKNOWLEDGMENTS This work was financially supported by the Russian Foundation for Basic Research, grant nos. 04-03-33018 and 05-03-33252. REFERENCES 1. Chiba, K., Ohsaka, T., Ohnuki, Y., and Oyama, N., J. Electroanal. Chem., 1987, vol. 219, p. 117. 2. Oyama, N., Ohsaka, T., Chiba, K., and Takahashi, K., Bull. Chem. Soc. Jpn., 1988, vol. 61, p. 1095. 3. Wu Ling-Ling, Luo Jin, and Lin Zhong-Hua, J. Electroanal. Chem., 1996, vol. 471, p. 53. 4. Komura, T., Ito, Y., Yamaguti, T., and Takahashi, K., Electrochim. Acta, 1998, vol. 43, no. 7, p. 723. 5. Komura, T., Yamaguti, T., and Takahashi, K., Electrochim. Acta, 1996, vol. 41, no. 18, p. 2865. 6. Komura, T., Funahashi, Y., Yamaguti, T., and Takahashi, K., J. Electroanal. Chem., 1998, vol. 446, p. 113.
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7. Goyette, M.-A. and Leclerc, M., J. Electroanal. Chem., 1995, vol. 382, p. 17. 8. Ogura, K., Kokura, M., Yano, J., and Shiigi, H., Electrochim. Acta, 1995, vol. 40, no. 17, p. 2707. 9. Yano, J. and Nagaoka, T., Synth. Met., 1997, vol. 84, p. 271. 10. Martinusz, K., Inzelt, G., and Horanyi, G., J. Electroanal. Chem., 1995, vol. 395, p. 293. 11. Chiba, K., Ohsaka, T., and Oyama, N., J. Electroanal. Chem., 1987, vol. 217, p. 239. 12. Malev, V.V. and Levin, O.V., Elektrokhimiya, 2008, vol. 44, pp. 91. 13. Levin, O., Kondratiev, V., and Malev, V., Electrochim. Acta, 2005, vol. 50, nos. 7-8, p. 1573. 14. Pisarevskaya, E.Yu. and Levi, M.D., Elektrokhimiya, 1994, vol. 30, p. 50. 15. Kondrat’ev, V.V., Tolstopyatova, E.G., and Malev, V.V., Elektrokhimiya, 2002, vol. 38, p. 663.
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