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Magneto-electropolymerization of conducting polypyrrole
Physica B 246—247 (1998) 412—415
Magneto-electropolymerization of conducting polypyrrole I. Mogi*, K. Watanabe, M. Motokawa Institute for Materials Research, Tohoku University, Katahira, Sendai 980-77, Japan
Abstract Magnetic fields were applied to the electropolymerization process of an organic conducting polymer polypyrrole. The magneto-electropolymerization was carried out in a cryocooler-cooled superconducting magnet and a Bitter magnet. The magnetic fields shifted the cathodic peak to more negative potentials in the cyclic voltammogram of the polypyrrole film. Such an effect was remarkable at higher anodic polymerization potentials. The magneto-electropolymerized film exhibited a relaxation phenomenon in the cyclic voltammogram during immersion of the film in an aqueous solution. These results demonstrated that the magneto-electropolymerization allows control of the electrochemical properties of polypyrrole. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: Magneto-electropolymerization; Polypyrrole; Cyclic voltammogram
1. Introduction Recent development of a cryocooler-cooled superconducting magnet [1] is expanding the applications of magnetic fields dramatically, particularly to chemical processing of materials. It is well known that most organic polymers have such a large anisotropy in diamagnetic susceptibility that they are subject to the diamagnetic orientation, resulting in morphological changes in magnetic fields [2—5]. Organic conducting polymers have attracted much attention as potential materials with many applications to electronics devices. Redox (reduction—oxidation) reactions of some conducting polymers, such as polypyrrole (PPy) (see Fig. 1) and polythiophene, accompany doping and * Corresponding author. Fax: #81 22 215 2016; e-mail: [email protected].
Fig. 1. Molecular structure of polypyrrole.
undoping processes of anions, and thus their electrochemical properties considerably depend on their morphology. We made attempts to prepare PPy by magneto-electropolymerization (electropolymerization in magnetic fields) and found that the magnetic fields drastically change its growth morphology [6,7]. In this paper we report that magneto-electropolymerization allows control of the redox potential of PPy films and also report a relaxation phenomenon in the magneto-electropolymerized films.
0921-4526/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved PII S 0 9 2 1 - 4 5 2 6 ( 9 7 ) 0 0 9 4 9 - 6
I. Mogi et al. / Physica B 246–247 (1998) 412—415
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2. Experimental All the electrochemical measurements were done at 20°C by using an electrochemical analyzer BAS100B/W. The electrode system consisted of a platinum disk (H1.6 mm) as a working electrode, an Ag/AgCl electrode as a reference one, and a platinum plate as a counter electrode. The PPy films were prepared on the platinum disk electrode by oxidative electropolymerization [8], which produces the anion X~ doped polypyrrole (PPy/X~). The electropolymerization was carried out by passing a charge of 1.0 C cm~2 (&2.5 lm thickness) at a constant potential in a 0.1 M (1 M"1 mol dm~3) pyrrole aqueous solution containing 0.1 M sodium p-toluenesulfonate (TsONa) as a supporting electrolyte. A cyclic voltammogram of the PPy/TsO~ film, which represents a faradaic current i versus a sweeping potential E, was measured to examine the redox properties. Magnetic fields were generated up to 5 T by a cryocooler-cooled large-bore (220 mm) superconducting magnet and up to 12 T by a Bitter magnet. The fields were applied perpendicularly to the surface of the working electrode and parallel to the faradaic current to eliminate the magnetohydrodynamic effect [9]. The schematic diagram of the experimental setup was described elsewhere [10].
3. Results and discussion The PPy/TsO~ films were electropolymerized at various constant potentials of 0.85, 1.0 and 1.2 V. Fig. 2 shows the cyclic voltammograms of the films prepared in 0 and 5 T, where the voltammograms were measured in a 0.1 M TsONa aqueous solution in the absence of a magnetic field. Characteristic cathodic peaks around !0.9 V in Fig. 2a represent the film reduction process accompanied with the undoping of the TsO~ ions. The cathodic peak of the 5 T film has a more negative potential, and this negative shift is prominent in the films electropolymerized at 1.0 V (Fig. 2b) and 1.2 V (Fig. 2c): The peak shift is 70 mV at 0.85 V and it surprisingly goes up to ca. 200 mV at 1.0 and 1.2 V. The cyclic voltammograms were examined for the PPy/TsO~ films electropolymerized at 1.0 V in
Fig. 2. Cyclic voltammograms of the PPy/TsO~ films in a 0.1 M TsONa aqueous solution with a potential sweep rate of 50 mV s~1. The films were electropolymerized in 0 and 5 T at potentials of (a) 0.85 V, (b) 1.0 V and (c) 1.2 V.
various magnetic fields of up to 12 T, and the cathodic peak potential E of the voltammograms is 1# plotted as a function of the magnetic field in Fig. 3. The peak potential drastically changes around 1 T, and the potential shift gets saturated above 5 T. Our previous paper [6,7] showed that magnetic fields drastically change the growth morphology of electropolymerized PPy/TsO~ in a quasi twodimensional space. PPy/TsO~ grows into a diffusion-controlled fractal pattern in 0 T and a kineticcontrolled closed pattern in 0.5 T. The micrograph of the latter showed that the organization of the polymer is dense aggregates of needle-like branches,
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I. Mogi et al. / Physica B 246–247 (1998) 412—415
Fig. 3. Cathodic peak potential E in the cyclic voltammogram 1# of the PPy/TsO~ films versus the magnetic field B in which the films were polymerized.
resulting from the diamagnetic orientation of the aromatic planes. The magneto-electropolymerized PPy/TsO~ film is thus considered to be so rigid that the mobility of the dopant anion within the film gets smaller and a more negative potential is necessary for the undoping process, resulting in a negative shift of the cathodic peak. Such a magnetic field effect depends on the polymerization potential, as shown in Fig. 2. Osaka et al. [11,12] studied the surface morphology of the electropolymerized PPy films and showed that the film formed at a higher anodic potential is rougher than that formed at a lower potential. Because the magneto-electropolymerization makes the PPy film dense and rigid, such an effect is more effective for the rough film formation at higher anodic potentials. We examined a change in the cyclic voltammograms of the 0- and 5 T films during immersion of the films in a 0.1 M TsONa aqueous solution in zero magnetic field and found a relaxation phenomenon in the 5 T film. Fig. 4 shows the immersion time dependence of the voltammogram of the 5 T film formed at 1.0 V, where the voltammograms were measured in the same solution after the immersion. The cathodic peak shifts to more positive potentials and the peak current rises upon increasing the immersion time up to 16 h. The voltammogram was also measured after the 28 h immersion,
Fig. 4. Cyclic voltammograms of the magneto-electropolymerized (5 T) PPy/TsO~ films in a 0.1 M TsONa aqueous solution with a potential sweep rate of 50 mV s~1. The voltammograms were measured after immersion of the film in the TsONa solution for 2, 4 and 16 h in the absence of a magnetic field.
and it was almost the same as that at 16 h. Such relaxation behavior implies that the rigid organization in the 5 T film is released and the film recovers flexibility during the immersion.
Acknowledgements The experiments with magnetic fields were done in the High Field Laboratory for Superconducting Materials, Tohoku University. This work was supported in part by the Magnetic Science Project of the Japan Science and Technology Corporation.
References [1] K. Watanabe, S. Awaji, T. Fukase, Y. Yamada, J. Sakuraba, F. Hata, C.K. Chong, T. Hasebe, M. Ishihara, Cryogenics 34 (1994) 639. [2] G. Maret, K. Dransfeld, in: F. Herlach (Ed.), Strong and Ultrastrong Magnetic Fields and their Applications, Springer, Berlin, 1985, p. 143. [3] K. Akagi, S. Katayama, H. Shirakawa, Synth. Met. 17 (1987) 241. [4] A. Yamagishi, T. Takeuchi, T. Higashi, M. Date, Physica B 164 (1990) 222.
I. Mogi et al. / Physica B 246–247 (1998) 412—415 [5] T. Kimura, T. Maeda, H. Sata, M. Yamato, E. Ito, Polym. J. 27 (1995) 247. [6] I. Mogi, M. Kamiko, J. Crystal Growth 166 (1996) 276. [7] I. Mogi, M. Kamiko, Bull. Chem. Soc. Japan 69 (1996) 1889. [8] R.A. Bull, F.F. Fan, A.J. Bard, J. Electrochem. Soc. 129 (1982) 1009.
415
[9] R. Aogaki, K. Fueki, T. Mukaibo, Denki Kagaku 43 (1975) 504. [10] I. Mogi, Bull. Chem. Soc. Japan 69 (1996) 2661. [11] T. Osaka, K. Naoi, H. Sakai, S. Ogano, J. Electrochem. Soc. 134 (1987) 285. [12] Y. Yatsuda, H. Sakai, T. Osaka, J. Chem. Soc. Japan (1985) 1331.
Magneto-electropolymerization of conducting polypyrrole I. Mogi*, K. Watanabe, M. Motokawa Institute for Materials Research, Tohoku University, Katahira, Sendai 980-77, Japan
Abstract Magnetic fields were applied to the electropolymerization process of an organic conducting polymer polypyrrole. The magneto-electropolymerization was carried out in a cryocooler-cooled superconducting magnet and a Bitter magnet. The magnetic fields shifted the cathodic peak to more negative potentials in the cyclic voltammogram of the polypyrrole film. Such an effect was remarkable at higher anodic polymerization potentials. The magneto-electropolymerized film exhibited a relaxation phenomenon in the cyclic voltammogram during immersion of the film in an aqueous solution. These results demonstrated that the magneto-electropolymerization allows control of the electrochemical properties of polypyrrole. ( 1998 Elsevier Science B.V. All rights reserved. Keywords: Magneto-electropolymerization; Polypyrrole; Cyclic voltammogram
1. Introduction Recent development of a cryocooler-cooled superconducting magnet [1] is expanding the applications of magnetic fields dramatically, particularly to chemical processing of materials. It is well known that most organic polymers have such a large anisotropy in diamagnetic susceptibility that they are subject to the diamagnetic orientation, resulting in morphological changes in magnetic fields [2—5]. Organic conducting polymers have attracted much attention as potential materials with many applications to electronics devices. Redox (reduction—oxidation) reactions of some conducting polymers, such as polypyrrole (PPy) (see Fig. 1) and polythiophene, accompany doping and * Corresponding author. Fax: #81 22 215 2016; e-mail: [email protected].
Fig. 1. Molecular structure of polypyrrole.
undoping processes of anions, and thus their electrochemical properties considerably depend on their morphology. We made attempts to prepare PPy by magneto-electropolymerization (electropolymerization in magnetic fields) and found that the magnetic fields drastically change its growth morphology [6,7]. In this paper we report that magneto-electropolymerization allows control of the redox potential of PPy films and also report a relaxation phenomenon in the magneto-electropolymerized films.
0921-4526/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved PII S 0 9 2 1 - 4 5 2 6 ( 9 7 ) 0 0 9 4 9 - 6
I. Mogi et al. / Physica B 246–247 (1998) 412—415
413
2. Experimental All the electrochemical measurements were done at 20°C by using an electrochemical analyzer BAS100B/W. The electrode system consisted of a platinum disk (H1.6 mm) as a working electrode, an Ag/AgCl electrode as a reference one, and a platinum plate as a counter electrode. The PPy films were prepared on the platinum disk electrode by oxidative electropolymerization [8], which produces the anion X~ doped polypyrrole (PPy/X~). The electropolymerization was carried out by passing a charge of 1.0 C cm~2 (&2.5 lm thickness) at a constant potential in a 0.1 M (1 M"1 mol dm~3) pyrrole aqueous solution containing 0.1 M sodium p-toluenesulfonate (TsONa) as a supporting electrolyte. A cyclic voltammogram of the PPy/TsO~ film, which represents a faradaic current i versus a sweeping potential E, was measured to examine the redox properties. Magnetic fields were generated up to 5 T by a cryocooler-cooled large-bore (220 mm) superconducting magnet and up to 12 T by a Bitter magnet. The fields were applied perpendicularly to the surface of the working electrode and parallel to the faradaic current to eliminate the magnetohydrodynamic effect [9]. The schematic diagram of the experimental setup was described elsewhere [10].
3. Results and discussion The PPy/TsO~ films were electropolymerized at various constant potentials of 0.85, 1.0 and 1.2 V. Fig. 2 shows the cyclic voltammograms of the films prepared in 0 and 5 T, where the voltammograms were measured in a 0.1 M TsONa aqueous solution in the absence of a magnetic field. Characteristic cathodic peaks around !0.9 V in Fig. 2a represent the film reduction process accompanied with the undoping of the TsO~ ions. The cathodic peak of the 5 T film has a more negative potential, and this negative shift is prominent in the films electropolymerized at 1.0 V (Fig. 2b) and 1.2 V (Fig. 2c): The peak shift is 70 mV at 0.85 V and it surprisingly goes up to ca. 200 mV at 1.0 and 1.2 V. The cyclic voltammograms were examined for the PPy/TsO~ films electropolymerized at 1.0 V in
Fig. 2. Cyclic voltammograms of the PPy/TsO~ films in a 0.1 M TsONa aqueous solution with a potential sweep rate of 50 mV s~1. The films were electropolymerized in 0 and 5 T at potentials of (a) 0.85 V, (b) 1.0 V and (c) 1.2 V.
various magnetic fields of up to 12 T, and the cathodic peak potential E of the voltammograms is 1# plotted as a function of the magnetic field in Fig. 3. The peak potential drastically changes around 1 T, and the potential shift gets saturated above 5 T. Our previous paper [6,7] showed that magnetic fields drastically change the growth morphology of electropolymerized PPy/TsO~ in a quasi twodimensional space. PPy/TsO~ grows into a diffusion-controlled fractal pattern in 0 T and a kineticcontrolled closed pattern in 0.5 T. The micrograph of the latter showed that the organization of the polymer is dense aggregates of needle-like branches,
414
I. Mogi et al. / Physica B 246–247 (1998) 412—415
Fig. 3. Cathodic peak potential E in the cyclic voltammogram 1# of the PPy/TsO~ films versus the magnetic field B in which the films were polymerized.
resulting from the diamagnetic orientation of the aromatic planes. The magneto-electropolymerized PPy/TsO~ film is thus considered to be so rigid that the mobility of the dopant anion within the film gets smaller and a more negative potential is necessary for the undoping process, resulting in a negative shift of the cathodic peak. Such a magnetic field effect depends on the polymerization potential, as shown in Fig. 2. Osaka et al. [11,12] studied the surface morphology of the electropolymerized PPy films and showed that the film formed at a higher anodic potential is rougher than that formed at a lower potential. Because the magneto-electropolymerization makes the PPy film dense and rigid, such an effect is more effective for the rough film formation at higher anodic potentials. We examined a change in the cyclic voltammograms of the 0- and 5 T films during immersion of the films in a 0.1 M TsONa aqueous solution in zero magnetic field and found a relaxation phenomenon in the 5 T film. Fig. 4 shows the immersion time dependence of the voltammogram of the 5 T film formed at 1.0 V, where the voltammograms were measured in the same solution after the immersion. The cathodic peak shifts to more positive potentials and the peak current rises upon increasing the immersion time up to 16 h. The voltammogram was also measured after the 28 h immersion,
Fig. 4. Cyclic voltammograms of the magneto-electropolymerized (5 T) PPy/TsO~ films in a 0.1 M TsONa aqueous solution with a potential sweep rate of 50 mV s~1. The voltammograms were measured after immersion of the film in the TsONa solution for 2, 4 and 16 h in the absence of a magnetic field.
and it was almost the same as that at 16 h. Such relaxation behavior implies that the rigid organization in the 5 T film is released and the film recovers flexibility during the immersion.
Acknowledgements The experiments with magnetic fields were done in the High Field Laboratory for Superconducting Materials, Tohoku University. This work was supported in part by the Magnetic Science Project of the Japan Science and Technology Corporation.
References [1] K. Watanabe, S. Awaji, T. Fukase, Y. Yamada, J. Sakuraba, F. Hata, C.K. Chong, T. Hasebe, M. Ishihara, Cryogenics 34 (1994) 639. [2] G. Maret, K. Dransfeld, in: F. Herlach (Ed.), Strong and Ultrastrong Magnetic Fields and their Applications, Springer, Berlin, 1985, p. 143. [3] K. Akagi, S. Katayama, H. Shirakawa, Synth. Met. 17 (1987) 241. [4] A. Yamagishi, T. Takeuchi, T. Higashi, M. Date, Physica B 164 (1990) 222.
I. Mogi et al. / Physica B 246–247 (1998) 412—415 [5] T. Kimura, T. Maeda, H. Sata, M. Yamato, E. Ito, Polym. J. 27 (1995) 247. [6] I. Mogi, M. Kamiko, J. Crystal Growth 166 (1996) 276. [7] I. Mogi, M. Kamiko, Bull. Chem. Soc. Japan 69 (1996) 1889. [8] R.A. Bull, F.F. Fan, A.J. Bard, J. Electrochem. Soc. 129 (1982) 1009.
415
[9] R. Aogaki, K. Fueki, T. Mukaibo, Denki Kagaku 43 (1975) 504. [10] I. Mogi, Bull. Chem. Soc. Japan 69 (1996) 2661. [11] T. Osaka, K. Naoi, H. Sakai, S. Ogano, J. Electrochem. Soc. 134 (1987) 285. [12] Y. Yatsuda, H. Sakai, T. Osaka, J. Chem. Soc. Japan (1985) 1331.