Langmuir 2000, 16, 6715-6718
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Electrodeposit Patterns of Polypyrrole in an Asymmetric System Ping Li* and Chunwei Yuan National Laboratory of Molecular and Biomolecular Electronics, Southeast University, Nanjing 210096, Peoples Republic of China Received July 19, 1999. In Final Form: December 31, 1999 The growth morphology of polypyrrole in an asymmetric experimental system is investigated. The distribution of electrical field affects the diffusion of cations and the growth pattern of polymer electrodeposits. Under the combined effects of the electrical field and the magnetic field, the changes of polymer morphologies are studied: the asymmetric pattern disappears due to the convection of solution. It is argued that the morphology patterns depend on the diffusion process.
Introduction Since Mandelbrot proposed the concept of fractals in the 1970s, the formation of fractal patterns in nature has been an ever-fascinating subject of research.1 A twodimensional (2D) growth of polypyrrole (PPy) with a fractal pattern was first prepared in a thin-layer electrochemical cell by Kaufman and co-workers;2 then Fujii reported the diffusion-limited-aggregation (DLA)3 fractal pattern of PPy in a needle-circle electrolytic cell.4-7 In our previous papers, the effects of such electropolymerization conditions, as the intensity of electrical field, the concentration of monomer or electrolyte, and the material of the electrode, etc., on the fractal patterns were studied in detail by using the needle-circle system.8,9 All of this work has broadened understanding of the growth mechanism of the fractal pattern. The above experiments were conducted in a radial symmetric electrical field. In this paper, an asymmetric experimental system is designed to break the symmetrical distribution of the electrical field, so as to investigate the influence of electrical field distribution on the diffusion of the reactant and then on the electrodeposit patterns. The concept also leads us to try to control the growth patterns of PPy by changing the electrical field distribution in a device built for this purpose. The additional magnetic field was exerted in the asymmetric system to study the further morphology changes under the combined effects of electrical field and magnetic field on the growth patterns of PPy. Our research probed the mechanism of the PPy growth, which would contribute to the study of fractals. Experimental Section The experiments were performed in a square glass cell (20 × 20 mm). A working electrode of stainless steel needle (diameter of 0.2 mm) was set in the center of the cell and the tip of the (1) Mandelbrot, B. B. The Fractals, Geometry of Nature, Freeman: San Franciso, 1982. (2) Kaufman, J. H.; Baker, C. K.; Nazzal, A. I.; Flickner, M.; Melroy, O. R.; Kapitulnik, A. Phys. Rev. Lett. 1986, 56, 1932. (3) Witten, T. A.; Sander, L. M. Phys. Rev. Lett. 1981, 47, 1400. (4) Fujii, M.; Yoshino, K. Jpn. J. Appl. Phys. 1988, 27, 457. (5) Fujii, M.; Saeki, Y.; Arii, K.; Yoshino, K. Jpn. J. Appl. Phys. 1990, 29, 2501. (6) Fujii, M.; Arii, K.; Yoshino, K. Synth. Met. 1993, 55-57, 1159. (7) Fujii, M.; Arii, K.; Yoshino, K. J. Electrochem. Soc. 1993, 140, 1838. (8) Shan, J.; Yuan, C.; Zhang, H. Thin Solid Films 1997, 301, 23. (9) Yuan, C.; Li, P. et al. Supermolecular Sci. 1998, V.5 No. 5-6, 751.
Figure 1. Experimental configuration. needle electrode was kept close to the bottom of the cell, to get 2D electrodeposits. Two counter electrodes were copper plates (20 × 5 × 0.1 mm), at opposite sides of the cell. The needle-plate system we used is depicted in Figure 1. The solution consists of 0.1 M monomer pyrrole, 0.1 M sodium dodecylbenzene sulfonate (DBS) as electrolyte, and acetonitrile as solvent. The pyrrole monomer was distilled before the experiments. The height of solution in the cell was controlled between 2 and 4 mm. A glass lid was set above the cell to prevent the volatilization of solvent. At room temperature, the experiments were carried out under constant applied voltage that was supplied by a CHI66 Electrochemical Station. When taking into account the effect of the magnetic field, the electrolytic cell was placed between the magnets. Two samples were supplied as the control in the needle-circle system, where the counter electrode is a copper ring sheet (65 × 10 × 0.2 mm).
Results By using the needle-circle system, the DLA-like growth patterns of PPy electrodeposited on the bottom of circular cell were illustrated in Figure 2 (a), (b). In contrast, Figure
10.1021/la990955z CCC: $19.00 © 2000 American Chemical Society Published on Web 07/08/2000
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Figure 2. The growth patterns of PPy in a needle-circle system (a) 6 V, 0 T, (b) 11 V, 0 T, and in a needle-plate system (c) 7 V, 0 T, (d) 9 V, 0 T.
2(c), (d) shows the dramatic morphology changes of PPy in the square cell. The main branches spread out from the center point to the opposite sides of the square cell, where the copper plate electrodes are set; there are few branches growing toward the sides with no copper plate. The whole growth pattern in Figure 2 (c), (d) reflects the heterogeneous distribution of the electrical field in the electrolyte cell. After exerting the magnetic field, which was applied perpendicular to the current, the electrodeposit of PPy was not only affected by the distribution of the electrical field, but also by the magnetic field. The electrodeposit patterns under 0.05 T can be seen from Figure 3 (a), (b), which were produced at constant voltage 7 V and 9 V, respectively. The branches grew, not only extending to the two opposite sides where the counter electrodes were set, but also to the other two sides where there was no electrode, although the growth had higher priority to the former two sides. At a higher magnetic field of 0.1 T, the directional growth of PPy disappeared, but grew in almost every direction symmetrically, as in Figure 3(c), (d). And the morphology distributions were denser compared to those under no magnetic field (Figure 2(c), (d).) The curves presenting the changes of currents at different magnetic field intensities are shown in Figure 4. It is found that there is a sharp increase in current after exerting the magnetic field, and the current rises steadily with the higher magnetic field. As can be seen from curve b, there is a sharp increase in current after
exerting the magnetic field: the current climbed rapidly from 5.64 × 10-4 Å at 0 T to 25.70 × 10-4 Å at 0.05 T; the curve rose steadily to 35.60 × 10-4 Å with the higher magnetic field. Discussion The distribution of electrical field in the needle-circle electrode system is symmetric and only the effects of the intensity of the electrical field can be reflected in the morphology.9 Once the symmetry of the magnetic field is disturbed, the effects of distribution of electrical field on the morphology can be investigated. In the symmetric electrical field, the diffusion of reactant is homogeneous, so the random walks of cations in solution produce the regular DLA-like pattern, as in Figure 2(a), (b). While in the asymmetric electrical field the motional cations of monomer or oligomer tend to diffuse to the area with higher electrical intensity, where they produce polymer easier and electrodeposit first. By comparison, few conductive polymers deposit at the area with weak distribution of electrical field. The ions diffuse at higher velocity to the area with strong electrical field and deposit first, resulting in the long branches spreading to the sides with counter electrodes, seen in Figure 2(c), (d). In this way, the fractal patterns changed into an irregular DLA-like pattern. By designing a specially distributed electrical field, we can induce the directional diffusion of the cation and then control the electrodeposit patterns of PPy. In future work, more purpose-built electrical fields will be applied to study
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Figure 3. The growth pattern of PPy under magnetic field (a) 7 V, 0.05 T, (b) 9 V, 0.05 T, (c) 7 V, 0.1 T, (b) 9 V, 0.1 T.
how to control the growth pattern more efficiently and to a more microscopic degree. After introducing the magnetic field, the effect of asymmetric distribution of electrical field on the growth pattern is different from that on the growth pattern without a magnetic field. The Lorentz force acts on the moving ions and induces the lateral drifts of the ions. In macroscopic effect, the lateral drifts of motional ions form the convection of solution around the central anode. It is the convection that carries the monomer or oligomer cations to the area with lower electrical field intensity, where they are electrodeposited. As is clearly illustrated in Figure 3(a): the upward two and downward two branches point to the sides with no electrodes. This disturbed the limitation of polymerization at the area with higher electrical field intensity, and also proved convincingly that the rate-determining process is still diffusion, although affected by the magnetic field. At higher magnetic field, the increased convection counteracted the effect of electrical field intensity, which almost induced the same diffusion possibility at every direction. As a result, the asymmetric diffusion of ions under the asymmetric electrical field disappears and the asymmetric morphology changed to symmetric because of the effect of the magnetic field, shown in Figure 3(c), (d). Because the convection in solution brings about a thinner diffusion layer, the rate of diffusion of ions in the diffusion layer is increased, which is well-known as the magnetohydrodynamic effect (MHD).10 Due to this effect, it can be seen from Figure 4 that there is an increase in (10) Fahidy, T. Z. J. Appl. Electrochem. 1983, 13, 553.
Figure 4. The currents at different magnetic fields: (a) at 7 V, (b) at 9 V.
current that reflects the velocity of reaction. This is also an indirect evidence that the reaction rate is dependent on the diffusion process. The morphology structure has a close relationship with the thickness of the diffusion layer:11 with a thinner diffusion layer, the screening effect is reduced, so the distance between branches is decreased and the morphology is denser. In summary, by using an asymmetric system, it is found that the distribution of the electrical field influences the (11) Nagatani, T.; Safues, F. J. Phys. Soc. Jpn. 1990, V.59. No. 10., 3447.
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electrodeposit patterns of PPy, resulting in the asymmetric pattern. Also, in this way we can achieve the goal of controlling the patterns of electrodeposits. Because of the excellent conductivity of PPy, the controllable PPy branch growth would widen its applications in electronic devices. With the introduction of a magnetic field, the diffusion of ions to the low electrical field intensity area was induced by convection, and the symmetric growth pattern appeared
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in the asymmetric electrical field. This again argues that it is the diffusion process that determines the morphology of the electrodeposit. Acknowledgment. This work is financially supported by NSFC (No. 59773016). LA990955Z
