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In situ study of polypyrrole morphology by STM: effect of the doping state
Journal of Electroanalytical Chemistry 451 (1998) 273 – 277
Short communication
In situ study of polypyrrole morphology by STM: effect of the doping state E. Chaıˆnet *, M. Billon Centre de Recherche en Electrochimie Mine´rale et Ge´nie des Proce´de´s, URA CNRS 1212, ENSEEG, Domaine Uni6ersitaire, BP 75, 38 402 Saint Martin d’He`res, France Received 12 April 1997; received in revised form 9 February 1998
Abstract An electronic conducting polymer is submitted to strong structural variations during the doping (or dedoping) process due essentially to inclusion (or exclusion, respectively) of anions in its structure. The insertion (or exclusion) of ions induces a swelling (or deflation) phase and therefore modifications of the surface morphology. In this article we study these two correlated phenomena, surface morphological variations and swelling process, by scanning tunneling microscopy (STM) of a polypyrrole film as a function of its doping state. In order to account best for the modifications induced by the electrode potential, the STM images are obtained in situ. This allows us to follow the evolution of the swelling process and the surface morphological variations associated with the electrodoping mechanism. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Electronic conducting polymer; Structural variations; Doping/dedoping
1. Introduction Over the last few years, scanning tunnelling microscopy (STM) has become a technique which has been used more and more frequently for the study of electronic conducting polymers (ECP) [1 – 3]. This growing interest can be explained by the power of the STM to investigate the superficial topography of the ECP on a nanometric scale. The first studies were focused on the initial growth mechanisms of ECP [4 – 7]. For example, Evans and co-workers [8] have characterized, by ex situ STM, the structural evolution of a polythiophene layer which occurs during the first steps of the electropolymerization. Following the reports related to the growth mechanism of an ECP film, further studies have been undertaken subsequently in an attempt to evaluate the properties of ECP thin layers [9,10], in particular its * Corresponding lepni.inpg.fr
author.
Fax:
+33
476826777;
e-mail:
0022-0728/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0022-0728(98)00085-0
surface morphology as a function of operating conditions of the electrosynthesis (nature of the supported electrode, of counter-ions, value of the potential or current applied for the electrosynthesis, etc.) [11–14]. The studies of the morphological evolution are generally made by ex situ STM analysis without taking into account the doping state of the polymer layer. In fact, the structure of an ECP film is different depending on whether the polymer is in a conducting or in an insulating state. For this reason, we intend in this paper to give results for in situ STM experiments, in order to observe the morphological evolution of a polypyrrole film during the electrochemical doping (or undoping) process as a function of the electrode potential. In situ STM experiments allow us to take into account the solvation phenomena associated both with the polypyrrole film and with the counter-ion insertion for investigations performed on the same area. Therefore, we can assess simultaneously the morphological modifications of the polymer surface and the bulk swelling
274
E. Chaıˆnet, M. Billon / Journal of Electroanalytical Chemistry 451 (1998) 273–277
process associated with the doping mechanism. In addition, we can modulate these two processes precisely through the electrode potential.
pared by electrochemical etching of a 0.25 mm diameter wire in CaCl2 medium and were insulated with Apiezon Wax 100 resin which is chemically inert.
2. Experimental All experiments were performed in ambient atmosphere. The solvent used was water which was purified by a Milli-Q water purification system (Millipore). Lithium perchlorate LiClO4 (Fluka, purum) was used without any pre-treatment. Electrochemical syntheses of polypyrrole films and their characterizations by cyclic voltammetry were made with a PAR 273 potentiostat (from EG&G Princeton Applied Research) controlled by a computer. The electrochemical experiments were carried out in a home-made electrochemical cell in order to operate under the microscope head after the polymer electrosynthesis. The cell, whose capacity was equal to 1.2 cm3, was made with pyrex glass and includes three electrodes arranged, as shown in Fig. 1, in order to perform the electrosynthesis and to polarize the polymer continuously while it is studied by STM. The working electrode was a platinum disk (diameter, 3 mm) embedded in a glass tube. The auxiliary electrode was a platinum ring, set above the working electrode to get a simple current distribution, i.e. a uniform thickness of polymer, and also to allow the surface access for the tip of the microscope. The reference electrode was a saturated calomel electrode (SCE), separated from the medium under study by an Agar– Agar bridge with KCl. All the potentials in the present paper are given relative to this electrode. The electrolyte used for the polypyrrole film elaboration was an aqueous solution containing 0.1 M pyrrole and 0.5 M LiClO4 as supporting electrolyte. We operated only with a freshly prepared solution, which was purged free of oxygen by argon bubbling for 20 min. Then, the polypyrrole film was synthesized by electrolysis at a controlled potential of + 0.8 V until the desired charge was passed. To analyse the polypyrrole film by cyclic voltammetry, the synthesis solution was replaced by a monomer-free solution having 0.5 M LiClO4 as supporting electrolyte. A commercially available Nanoscope II from Digital Instruments was used to collect all the STM images in this paper. The microscope was run in the constant current mode. The best image for each polymer was obtained by using the following set points: 1 nA tunnelling current and 0.16 Hz scan frequencies. The tips employed for the in situ STM studies were of Pt/Ir (80/20) and were polarized at +0.2 V in order to get a zero faradaic current. They were pre-
Fig. 1. Schematic diagram of the electrochemical cell in situ STM: (WE) platinum disk, diameter 3 mm; (CE) platinum ring; (RE) saturated calomel electrode; (SB) Agar – Agar bridge with KCl. Table 1 Parameters obtained from the analysis of in situ STM images (for their definition, see text): zoom of the same area (25.2×25.2 mm) from the images in Fig. 1 taken at different potentials Potential/V
Ra/nm
Rq/nm
Sa/mm2
Sr/%
−0.50 −0.40 −0.30 −0.20 −0.10 0 +0.10
219 211 168 115 76 75 75
460.5 508.6 463.5 337.8 168.7 150.8 156.7
687 682 687 671 654 656 655
108.2 107.5 108.3 105.7 103.1 103.4 103.1
E. Chaıˆnet, M. Billon / Journal of Electroanalytical Chemistry 451 (1998) 273–277
275
Fig. 2. STM images (50× 50 mm, in situ) of a polypyrrole film on a platinum electrode (diameter 3 mm) in a 0.5 M LiClO4 aqueous solution. The polypyrrole film is polarized during its spectroscopic analysis. The values indicated above each image correspond to the potential of polarization vs. SCE. The polypyrrole film is synthesized at a controlled potential of +0.8 V by passing 20 mC cm − 2 from an aqueous solution containing 0.1 M pyrrole and 0.5 M LiClO4.
In order to evaluate the morphological evolution of a polypyrrole film as a function of the doping state, we undertook the in situ STM study as follows. Firstly, the film was characterized by cyclic voltammetry. Afterwards the electrode was polarized sequentially at different potentials beginning from the undoped state. Images were then recorded at each of these stationary electrochemical states without changing the position of the tip, in order to analyse the same area of the polypyrrole film.
3. Results and discussion Electronic conducting polymers such as the polypyrrole, formed by electrosynthesis on a metallic electrode surface pass from an insulator to a conducting state by simple variation of the potential applied to the electrode. The reversible changes of electronic conducting properties associated with the polymer result from the doping process [15]. This phenomenon induces modifications of the polypyrrole structure and, consequently, its superficial morphology is strongly modified on the molecular scale. In order to evaluate the influence of the doping state on the surface morphology, we have studied, in
LiClO4 medium, a polypyrrole film electrosynthesized on a platinum electrode by passing a charge density of 20 mC cm − 2. The doping state of the ECP was modified by the variation of the potential ranging from − 0.5 to + 0.1 V in a stepwise manner, the steps being 0.1 V. For each potential, the in situ STM image of the surface of the polypyrrole film was recorded from which we determine the four parameters listed in Table 1. The first parameter (Ra) is the mean roughness value of the surface relative to the central plane. This value is not characteristic of the profile shape unlike the second parameter (Rq) which gives the standard deviation of the height distribution and accounts for the more or less heterogeneous feature of the topography. Also, two surfaces can possess the same mean surface roughness, i.e. the same value of Ra, but one surface can be more uneven than the other which is reflected by different Rq values. The third parameter (Sa) called ‘surface area’ represents the developed surface of the polymer. The last parameter (Sr), expressed as a percentage, is the ratio of the developed surface to the geometrical area; it is representative of the increase of the polymer area with which we analyse the swelling process during the electrochemical doping phase.
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E. Chaıˆnet, M. Billon / Journal of Electroanalytical Chemistry 451 (1998) 273–277
Fig. 3. (a) Evolution of the cross-section profile of the polymer strands as a function of the applied potential to the film. These cross-section profiles are issued from the same line taken on the STM images of Fig. 2. (b) Schematic description of the swelling process in accordance to the morphological evolution deduced from the parameters in Table 1.
The film surface presents a general aspect which is relatively well ordered on a large scale whatever its doping state, as shown in Fig. 2. The surface is composed of molecular strands that are more or less linear and parallel to each other. They are themselves com-
posed of nodules and can be compared to strings of pearls. A careful analysis of the images taken at different potentials reveals important variations of surface morphology between a doped and an undoped film, as attested by the values given in Table 1. We notice that
E. Chaıˆnet, M. Billon / Journal of Electroanalytical Chemistry 451 (1998) 273–277
the mean surface roughness is divided by a factor of three during the doping process. A similar evolution is observed for the Rq parameter which represents a strong decrease of the heterogeneous feature associated with the surface morphology. The doping process induces a notable evolution of the surface morphology, which becomes smoother. The Sa and Sr parameters decrease when the potential applied to the polypyrrole film changes from −0.5 to 0.1 V but, in comparison with Ra and Rq, their relative variation is smaller (of the order of 4.5%). Also the small variations of Sa and Sr indicate that the magnitude of the swelling associated with the doping process does not induce an overall increase in the surface area; therefore, the ECP swelling is not homogeneous but affects only some parts of its surface. On the other hand, in comparison with the morphological evolution of the cross-section profile of the polymer film versus the imposed potential (Fig. 3a), the regions undergoing the most significant variations seem to be preferentially the cavities, corresponding to the strength of relaxation in the polymer bulk. As we analyse a thick layer, the bulk swelling predominates over the surface relaxation. So we observe a superficial smoothing due to the growth of the bulk which is represented schematically in Fig. 3b, where the swelling process takes into account both the modification of the surface morphology and the low variation of the surface area. This interpretation is also supported by the observations made by comparing the images recorded in Fig. 2. During the doping phase, the swelling process seems to occur as follows. Only the volume of some polymer nodules composing the molecular strands grows in an heterogeneous manner, in privileged directions perpendicular to the film surface in the direction of the electric field. We have undertaken a similar study in the same electrolyte for a polypyrrole film synthesized by passing a charge density of 5 mC cm − 2 in order to evaluate the qualitative effect of film thickness on the morphology. By comparing these results with those obtained previously, we notice that the surface topography of the polypyrrole film is the same in both cases. As for the thicker film, the doping process induced the preferential swelling of the ECP in the direction of the electric field. But, unlike the thicker film, we observe only a small variation in the surface morphology between an undoped and a doped film. We notice, however, in the present case that when the polypyrrole film is undoped, its surface is smoother for the thin film (5 mC cm − 2) than for the thick film (20 mC cm − 2); on the contrary, when the polypyrrole is doped, its surface is smoother for the thick film than for the thin layer. This different behaviour between the thick and the thin layer can be explained by the relaxation of mechanical stresses depending on the structural nature of the polymer. Indeed, as observed by Evans et al. [14], the thin film is made mainly of molecular strands directly anchored on the electrode, this structure being more
277
ordered unlike the amorphous structure of the thick film. When the electrode potential is modified in the reverse direction, the polypyrrole film passes from a conducting to an insulating state (i.e. from a doped to an undoped state) and we recover the original morphology of the thin layer, which gives proof of mechanical reversibility of the swelling process.
4. Conclusion We have shown in the present paper that the in situ STM characterization under potential control of an electronic conducting polymer, such as the polypyrrole, can be a good tool for following the evolution of morphology during the doping and dedoping steps. We have reported an in situ STM study of the morphological evolution of a polypyrrole film as a function of its electrochemical doping. It appears that the morphological variations are heterogeneous and do not affect all of the polymer film surface uniformly. The morphological modifications are induced by a swelling process due to the insertion of counter-ions in the polypyrrole matrix during the electrochemical doping stage. We observe in this case that the swelling of the polymer matrix is in fact a swelling of some nodules of which the polymer film is composed. Furthermore, the heterogeneous swelling is preferentially orientated along the electric field.
References [1] S. Wakabayashi, H. Kato, M. Tomitori, O. Nishikawa, J. Appl. Phys. 76 (1994) 5595. [2] C. Bai, C. Zhu, G. Huang, J. Yang, M. Wan, Ultramicroscopy 42 – 44 (1992) 1079. [3] R. Nyffenegger, C. Gerber, H. Siegenthaler, Synth. Met. 55–57 (1993) 402. [4] J. Lukkari, L. Heikkila, M. Alanko, J. Kankare, Synth. Met. 55 – 57 (1993) 1311. [5] F. Chao, M. Costa, C. Tian, Synth. Met. 53 (1993) 127. [6] J. Lukkari, M. Alanko, L. Heikkila, R. Laiho, J. Kankare, Chem. Mater. 5 (1993) 289. [7] G. Caple, B.L. Wheeler, R. Swift, T.L. Porter, S. Jeffers, J. Phys. Chem. 94 (1990) 5639. [8] R. Yang, D.F. Evans, L. Christensen, W.A. Hendrickson, J. Phys. Chem. 94 (1990) 6117. [9] R. Yang, W.H. Smyrl, D.F. Evans, L. Christensen, W.A. Hendrickson, J. Phys. Chem. 96 (1992) 1428. [10] G. Dietler, M. Heuberger, S. Tresch, P. Bujard, Synth. Met. 67 (1994) 211. [11] J. Li, E. Wang, Synth. Met. 66 (1994) 67. [12] T. Yano, L.A. Nagahara, K. Hashimoto, A. Fujishima, J. Vac. Sci. Technol. 12 (1994) 1596. [13] C. Froeck, A. Bartl, L. Dunsch, Electrochim. Acta 40 (1995) 1421. [14] R. Yang, K. Naoi, D.F. Evans, W.H. Smyrl, W.A. Hendrickson, Langmuir 7 (1991) 556. [15] T.J.L. Bre´das (Ed.), Handbook of Conducting Polymers, Marcel Dekker, New York, 1986.
Short communication
In situ study of polypyrrole morphology by STM: effect of the doping state E. Chaıˆnet *, M. Billon Centre de Recherche en Electrochimie Mine´rale et Ge´nie des Proce´de´s, URA CNRS 1212, ENSEEG, Domaine Uni6ersitaire, BP 75, 38 402 Saint Martin d’He`res, France Received 12 April 1997; received in revised form 9 February 1998
Abstract An electronic conducting polymer is submitted to strong structural variations during the doping (or dedoping) process due essentially to inclusion (or exclusion, respectively) of anions in its structure. The insertion (or exclusion) of ions induces a swelling (or deflation) phase and therefore modifications of the surface morphology. In this article we study these two correlated phenomena, surface morphological variations and swelling process, by scanning tunneling microscopy (STM) of a polypyrrole film as a function of its doping state. In order to account best for the modifications induced by the electrode potential, the STM images are obtained in situ. This allows us to follow the evolution of the swelling process and the surface morphological variations associated with the electrodoping mechanism. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Electronic conducting polymer; Structural variations; Doping/dedoping
1. Introduction Over the last few years, scanning tunnelling microscopy (STM) has become a technique which has been used more and more frequently for the study of electronic conducting polymers (ECP) [1 – 3]. This growing interest can be explained by the power of the STM to investigate the superficial topography of the ECP on a nanometric scale. The first studies were focused on the initial growth mechanisms of ECP [4 – 7]. For example, Evans and co-workers [8] have characterized, by ex situ STM, the structural evolution of a polythiophene layer which occurs during the first steps of the electropolymerization. Following the reports related to the growth mechanism of an ECP film, further studies have been undertaken subsequently in an attempt to evaluate the properties of ECP thin layers [9,10], in particular its * Corresponding lepni.inpg.fr
author.
Fax:
+33
476826777;
e-mail:
0022-0728/98/$19.00 © 1998 Elsevier Science S.A. All rights reserved. PII S0022-0728(98)00085-0
surface morphology as a function of operating conditions of the electrosynthesis (nature of the supported electrode, of counter-ions, value of the potential or current applied for the electrosynthesis, etc.) [11–14]. The studies of the morphological evolution are generally made by ex situ STM analysis without taking into account the doping state of the polymer layer. In fact, the structure of an ECP film is different depending on whether the polymer is in a conducting or in an insulating state. For this reason, we intend in this paper to give results for in situ STM experiments, in order to observe the morphological evolution of a polypyrrole film during the electrochemical doping (or undoping) process as a function of the electrode potential. In situ STM experiments allow us to take into account the solvation phenomena associated both with the polypyrrole film and with the counter-ion insertion for investigations performed on the same area. Therefore, we can assess simultaneously the morphological modifications of the polymer surface and the bulk swelling
274
E. Chaıˆnet, M. Billon / Journal of Electroanalytical Chemistry 451 (1998) 273–277
process associated with the doping mechanism. In addition, we can modulate these two processes precisely through the electrode potential.
pared by electrochemical etching of a 0.25 mm diameter wire in CaCl2 medium and were insulated with Apiezon Wax 100 resin which is chemically inert.
2. Experimental All experiments were performed in ambient atmosphere. The solvent used was water which was purified by a Milli-Q water purification system (Millipore). Lithium perchlorate LiClO4 (Fluka, purum) was used without any pre-treatment. Electrochemical syntheses of polypyrrole films and their characterizations by cyclic voltammetry were made with a PAR 273 potentiostat (from EG&G Princeton Applied Research) controlled by a computer. The electrochemical experiments were carried out in a home-made electrochemical cell in order to operate under the microscope head after the polymer electrosynthesis. The cell, whose capacity was equal to 1.2 cm3, was made with pyrex glass and includes three electrodes arranged, as shown in Fig. 1, in order to perform the electrosynthesis and to polarize the polymer continuously while it is studied by STM. The working electrode was a platinum disk (diameter, 3 mm) embedded in a glass tube. The auxiliary electrode was a platinum ring, set above the working electrode to get a simple current distribution, i.e. a uniform thickness of polymer, and also to allow the surface access for the tip of the microscope. The reference electrode was a saturated calomel electrode (SCE), separated from the medium under study by an Agar– Agar bridge with KCl. All the potentials in the present paper are given relative to this electrode. The electrolyte used for the polypyrrole film elaboration was an aqueous solution containing 0.1 M pyrrole and 0.5 M LiClO4 as supporting electrolyte. We operated only with a freshly prepared solution, which was purged free of oxygen by argon bubbling for 20 min. Then, the polypyrrole film was synthesized by electrolysis at a controlled potential of + 0.8 V until the desired charge was passed. To analyse the polypyrrole film by cyclic voltammetry, the synthesis solution was replaced by a monomer-free solution having 0.5 M LiClO4 as supporting electrolyte. A commercially available Nanoscope II from Digital Instruments was used to collect all the STM images in this paper. The microscope was run in the constant current mode. The best image for each polymer was obtained by using the following set points: 1 nA tunnelling current and 0.16 Hz scan frequencies. The tips employed for the in situ STM studies were of Pt/Ir (80/20) and were polarized at +0.2 V in order to get a zero faradaic current. They were pre-
Fig. 1. Schematic diagram of the electrochemical cell in situ STM: (WE) platinum disk, diameter 3 mm; (CE) platinum ring; (RE) saturated calomel electrode; (SB) Agar – Agar bridge with KCl. Table 1 Parameters obtained from the analysis of in situ STM images (for their definition, see text): zoom of the same area (25.2×25.2 mm) from the images in Fig. 1 taken at different potentials Potential/V
Ra/nm
Rq/nm
Sa/mm2
Sr/%
−0.50 −0.40 −0.30 −0.20 −0.10 0 +0.10
219 211 168 115 76 75 75
460.5 508.6 463.5 337.8 168.7 150.8 156.7
687 682 687 671 654 656 655
108.2 107.5 108.3 105.7 103.1 103.4 103.1
E. Chaıˆnet, M. Billon / Journal of Electroanalytical Chemistry 451 (1998) 273–277
275
Fig. 2. STM images (50× 50 mm, in situ) of a polypyrrole film on a platinum electrode (diameter 3 mm) in a 0.5 M LiClO4 aqueous solution. The polypyrrole film is polarized during its spectroscopic analysis. The values indicated above each image correspond to the potential of polarization vs. SCE. The polypyrrole film is synthesized at a controlled potential of +0.8 V by passing 20 mC cm − 2 from an aqueous solution containing 0.1 M pyrrole and 0.5 M LiClO4.
In order to evaluate the morphological evolution of a polypyrrole film as a function of the doping state, we undertook the in situ STM study as follows. Firstly, the film was characterized by cyclic voltammetry. Afterwards the electrode was polarized sequentially at different potentials beginning from the undoped state. Images were then recorded at each of these stationary electrochemical states without changing the position of the tip, in order to analyse the same area of the polypyrrole film.
3. Results and discussion Electronic conducting polymers such as the polypyrrole, formed by electrosynthesis on a metallic electrode surface pass from an insulator to a conducting state by simple variation of the potential applied to the electrode. The reversible changes of electronic conducting properties associated with the polymer result from the doping process [15]. This phenomenon induces modifications of the polypyrrole structure and, consequently, its superficial morphology is strongly modified on the molecular scale. In order to evaluate the influence of the doping state on the surface morphology, we have studied, in
LiClO4 medium, a polypyrrole film electrosynthesized on a platinum electrode by passing a charge density of 20 mC cm − 2. The doping state of the ECP was modified by the variation of the potential ranging from − 0.5 to + 0.1 V in a stepwise manner, the steps being 0.1 V. For each potential, the in situ STM image of the surface of the polypyrrole film was recorded from which we determine the four parameters listed in Table 1. The first parameter (Ra) is the mean roughness value of the surface relative to the central plane. This value is not characteristic of the profile shape unlike the second parameter (Rq) which gives the standard deviation of the height distribution and accounts for the more or less heterogeneous feature of the topography. Also, two surfaces can possess the same mean surface roughness, i.e. the same value of Ra, but one surface can be more uneven than the other which is reflected by different Rq values. The third parameter (Sa) called ‘surface area’ represents the developed surface of the polymer. The last parameter (Sr), expressed as a percentage, is the ratio of the developed surface to the geometrical area; it is representative of the increase of the polymer area with which we analyse the swelling process during the electrochemical doping phase.
276
E. Chaıˆnet, M. Billon / Journal of Electroanalytical Chemistry 451 (1998) 273–277
Fig. 3. (a) Evolution of the cross-section profile of the polymer strands as a function of the applied potential to the film. These cross-section profiles are issued from the same line taken on the STM images of Fig. 2. (b) Schematic description of the swelling process in accordance to the morphological evolution deduced from the parameters in Table 1.
The film surface presents a general aspect which is relatively well ordered on a large scale whatever its doping state, as shown in Fig. 2. The surface is composed of molecular strands that are more or less linear and parallel to each other. They are themselves com-
posed of nodules and can be compared to strings of pearls. A careful analysis of the images taken at different potentials reveals important variations of surface morphology between a doped and an undoped film, as attested by the values given in Table 1. We notice that
E. Chaıˆnet, M. Billon / Journal of Electroanalytical Chemistry 451 (1998) 273–277
the mean surface roughness is divided by a factor of three during the doping process. A similar evolution is observed for the Rq parameter which represents a strong decrease of the heterogeneous feature associated with the surface morphology. The doping process induces a notable evolution of the surface morphology, which becomes smoother. The Sa and Sr parameters decrease when the potential applied to the polypyrrole film changes from −0.5 to 0.1 V but, in comparison with Ra and Rq, their relative variation is smaller (of the order of 4.5%). Also the small variations of Sa and Sr indicate that the magnitude of the swelling associated with the doping process does not induce an overall increase in the surface area; therefore, the ECP swelling is not homogeneous but affects only some parts of its surface. On the other hand, in comparison with the morphological evolution of the cross-section profile of the polymer film versus the imposed potential (Fig. 3a), the regions undergoing the most significant variations seem to be preferentially the cavities, corresponding to the strength of relaxation in the polymer bulk. As we analyse a thick layer, the bulk swelling predominates over the surface relaxation. So we observe a superficial smoothing due to the growth of the bulk which is represented schematically in Fig. 3b, where the swelling process takes into account both the modification of the surface morphology and the low variation of the surface area. This interpretation is also supported by the observations made by comparing the images recorded in Fig. 2. During the doping phase, the swelling process seems to occur as follows. Only the volume of some polymer nodules composing the molecular strands grows in an heterogeneous manner, in privileged directions perpendicular to the film surface in the direction of the electric field. We have undertaken a similar study in the same electrolyte for a polypyrrole film synthesized by passing a charge density of 5 mC cm − 2 in order to evaluate the qualitative effect of film thickness on the morphology. By comparing these results with those obtained previously, we notice that the surface topography of the polypyrrole film is the same in both cases. As for the thicker film, the doping process induced the preferential swelling of the ECP in the direction of the electric field. But, unlike the thicker film, we observe only a small variation in the surface morphology between an undoped and a doped film. We notice, however, in the present case that when the polypyrrole film is undoped, its surface is smoother for the thin film (5 mC cm − 2) than for the thick film (20 mC cm − 2); on the contrary, when the polypyrrole is doped, its surface is smoother for the thick film than for the thin layer. This different behaviour between the thick and the thin layer can be explained by the relaxation of mechanical stresses depending on the structural nature of the polymer. Indeed, as observed by Evans et al. [14], the thin film is made mainly of molecular strands directly anchored on the electrode, this structure being more
277
ordered unlike the amorphous structure of the thick film. When the electrode potential is modified in the reverse direction, the polypyrrole film passes from a conducting to an insulating state (i.e. from a doped to an undoped state) and we recover the original morphology of the thin layer, which gives proof of mechanical reversibility of the swelling process.
4. Conclusion We have shown in the present paper that the in situ STM characterization under potential control of an electronic conducting polymer, such as the polypyrrole, can be a good tool for following the evolution of morphology during the doping and dedoping steps. We have reported an in situ STM study of the morphological evolution of a polypyrrole film as a function of its electrochemical doping. It appears that the morphological variations are heterogeneous and do not affect all of the polymer film surface uniformly. The morphological modifications are induced by a swelling process due to the insertion of counter-ions in the polypyrrole matrix during the electrochemical doping stage. We observe in this case that the swelling of the polymer matrix is in fact a swelling of some nodules of which the polymer film is composed. Furthermore, the heterogeneous swelling is preferentially orientated along the electric field.
References [1] S. Wakabayashi, H. Kato, M. Tomitori, O. Nishikawa, J. Appl. Phys. 76 (1994) 5595. [2] C. Bai, C. Zhu, G. Huang, J. Yang, M. Wan, Ultramicroscopy 42 – 44 (1992) 1079. [3] R. Nyffenegger, C. Gerber, H. Siegenthaler, Synth. Met. 55–57 (1993) 402. [4] J. Lukkari, L. Heikkila, M. Alanko, J. Kankare, Synth. Met. 55 – 57 (1993) 1311. [5] F. Chao, M. Costa, C. Tian, Synth. Met. 53 (1993) 127. [6] J. Lukkari, M. Alanko, L. Heikkila, R. Laiho, J. Kankare, Chem. Mater. 5 (1993) 289. [7] G. Caple, B.L. Wheeler, R. Swift, T.L. Porter, S. Jeffers, J. Phys. Chem. 94 (1990) 5639. [8] R. Yang, D.F. Evans, L. Christensen, W.A. Hendrickson, J. Phys. Chem. 94 (1990) 6117. [9] R. Yang, W.H. Smyrl, D.F. Evans, L. Christensen, W.A. Hendrickson, J. Phys. Chem. 96 (1992) 1428. [10] G. Dietler, M. Heuberger, S. Tresch, P. Bujard, Synth. Met. 67 (1994) 211. [11] J. Li, E. Wang, Synth. Met. 66 (1994) 67. [12] T. Yano, L.A. Nagahara, K. Hashimoto, A. Fujishima, J. Vac. Sci. Technol. 12 (1994) 1596. [13] C. Froeck, A. Bartl, L. Dunsch, Electrochim. Acta 40 (1995) 1421. [14] R. Yang, K. Naoi, D.F. Evans, W.H. Smyrl, W.A. Hendrickson, Langmuir 7 (1991) 556. [15] T.J.L. Bre´das (Ed.), Handbook of Conducting Polymers, Marcel Dekker, New York, 1986.