Mechanistic Study on Electrochemical Deposition of Conjugated

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J. Phys. Chem. B 2008, 112, 9311–9317

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Mechanistic Study on Electrochemical Deposition of Conjugated Polymers in Centrifugal Fields Atsushi Murotani, Toshio Fuchigami, and Mahito Atobe* Department of Electronic Chemistry, Tokyo Institute of Technology 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8502, Japan ReceiVed: February 14, 2008; ReVised Manuscript ReceiVed: May 29, 2008

This work presents a detailed study of the mechanism for the electrochemical deposition of conjugated polymers on anodes in centrifugal fields. The rate of the electrochemical deposition of polyaniline was affected significantly by a centrifugal acceleration force of 315 g. However, no centrifugal effects were observed on the electrochemical deposition of poly(3,4-ethylenedioxythiophene). It was found that the degree of the centrifugal effect generated depended greatly on the size of the oligomer aggregates just before their deposition. To further confirm the influence of size on electrochemical systems under centrifugal fields, we also carried out an electrochemical redox reaction using various sizes of vinylferrocene-immobilized polystylene latex particles. 1. Introduction

2. Experimental Section

Recently, considerable attention, both fundamental and practical, has been directed toward conjugated polymer films formed electrooxidatively on anodes, because they exhibit properties such as electroconductivity, semiconductivity, redox, doping, electrochromism, etc.1–4 Generally, the properties of polymers originate from their chemical (molecular) and physical (morphological) structures. Therefore, it follows that the structures of conjugated polymer films should be able to be controlled in order to tailor them to the purposes of their utilization. Their chemical structures can be controlled by changing the molecular structures of the corresponding monomers and by selecting electrolytic conditions and procedures for electrochemical polymerization.1–4 On the other hand, many methods for controlling their physical structures have been also reported. Solvent, electrolyte, electrode material, concentrations, temperature, additives, and electrochemical protocol have all been shown to play a role for this purpose.5,6 The use of ionic liquids7–9 and supercritical fluids10,11 as electrolytic media are also effective for controlling polymer morphologies. Martin et al. developed a template electrochemical polymerization to obtain cylindrical conjugated polymers.12,13 Rotation of the electrode during electrochemical polymerization results in changes in morphology of polymer films.14–16 In addition, ultrasonication to electropolymerization allows the formation of a smooth film of conjugated polymers.17–20 In our previous work, we demonstrated the use of centrifugal fields to control physical structures of polyaniline, polythiophene, polypyrrole, and polyphenylene films deposited electrochemically on anodes.21–23 In addition, the rate of polymer deposition increased under an applied gravitational force. However, the mechanism for these significant centrifugal effects has not yet been clarified. This paper provides guidelines for and a preliminarily discussion of the mechanism for centrifugal forces to affect the electrochemical deposition of conjugated polymers.

2.1. Electrochemical Depositions of Polyaniline and Poly(3,4-ethylenedioxythiophene) under a Centrifugal Field. 2.1.1. Centrifugal Facilities. Figure 1 shows centrifuge facilities equipped with a cylindrical electrolytic cell made of polytetrafluoroethylene resin, 14 mm in diameter and 7 mm in length. The cell consists of three usual electrodes (two platinum disk electrodes for working and counter, and Ag|AgCl wire or Ag wire as a reference electrode) in a single compartment, and the electrical contacts between the electrodes and the potetiostat/galvanostat were made of silver rotating rings and carbon brushes. The entire cell assembly was suspended from the lid of a centrifuge tube with a polyethylene line. The surfaces of the working and counter electrodes faced inward and outward, respectively, to the centrifugal acceleration force. The working electrode was covered with an insulating sheet remaining bare in the central part (6 mm diameter, 0.28 cm2 area). 2.1.2. Procedures for Electrochemical Depositions of Polyaniline and Poly(3,4-ethylenedioxythiophene). Electrodepositions of polyaniline and poly(3,4-ethylenedioxythiophene) (PEDOT) were carried out using a cyclic potential scanning method under the following electrolytic conditions: potential scanning range 0.1-1.1 V vs Ag|AgCl (for the polymerization of aniline) and from -1.0 to +1.6 V vs an Ag quasireference electrode (for the polymerization of 3,4-ethylenedioxythiophene (EDOT)); scanning rate 0.1 Vs1-; monomer concentration 0.1 M; electrolytes 4.0 M HCl aqueous solution (for the polymerization of aniline); and 0.1 M tetrabutylammonium tetrafluoroborate ((TBA)BF4) in propylenecarbonate (for the polymerization of EDOT). After the polymerizations, cyclic voltammograms of polyaniline and PEDOT films were measured within the ranges of +0.1 to +1.1 V vs Ag|AgCl and -1.0 to +1.2 V vs SCE at 0.1 Vs1- in 4.0 M HCl aqueous solution and propylenecarbonate solution containing 0.1 M (TBA)BF4, respectively. Furthermore, polyaniline and PEDOT films deposited on the anode were cathodically reduced at + 0.1 V (vs Ag|AgCl) and 0.0 V (vs SCE), respectively, and then observed using a scanning electron microscope (SEM).

* To whom correspondence should be addressed. Telephone and Fax: 81-45-924-5407. E-mail: [email protected].

10.1021/jp8013145 CCC: $40.75  2008 American Chemical Society Published on Web 07/16/2008

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Figure 1. Centrifuge facilities equipped with an electrolytic cell.

Figure 2. Schematic illustration of the experimental procedure for the filtration experiment.

Figure 3. Cyclic voltammograms of polyaniline films prepared at (A) 1g and (B) 315g.

2.2. Filtration Experiment. Figure 2 shows a schematic illustration of the filtration experiment for the detection of oligomer aggregates. In this experiment, electropolymerizations of aniline and EDOT were carried out by the galvanostatic method under the following conditions: electrolytes 4.0 M HCl aqueous solution (for the polymerization of aniline) and 0.1 M (TBA)BF4 propylene carbonate solution (for the polymerization of EDOT); current density 1.0 mA cm-2; and charge passed 10 C. Following the electrochemical polymerizations of aniline and EDOT, in which oligomers before deposition on the anode were still contained, electrolytes were filtered through a 200-nm-poresize membrane filter. After filtering each oligomer solution, the

membrane filter was observed using an SEM to determine the presence of oligomer aggregates bigger than 200 nm on the filter. Furthermore, UV-vis spectroscopy was performed on the electrolytic solutions before and after filtration. 2.3. Estimation of the Effect of Deposit Size on Electrochemical Processes under a Centrifugal Field. 2.3.1. Synthesis of VFcPS Latex Particles. Three different sizes (2 µm, 500 nm, and 200 nm) of VFcPS latex particles were prepared according to a procedure published in the literature.24–26 2.3.2. Electrochemical Redox Reaction of VFcPS Latex Particles under a Centrifugal Field. Voltammetry of VFcPS particles under a centrifugal field was carried out using the

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Figure 4. Cyclic voltammograms of PEDOT films prepared at (A) 1g and (B) 315g.

Figure 5. SEM photographs of the surfaces of PEDOT films prepared at (A) 1g and (B) 290g.

Figure 7. (A) An SEM image of the filter surface after filtration of the aniline oligomer solution. (B) Absorption spectra of aniline oligomer solutions before and after filtration.

Figure 6. Mechanism for the electrooxidative deposition of conjugated polymer under a centrifugal field.

same electrolytic cell as described in section 2.1.1, but a modified electrode was used for a working electrode in this experiment. In order to force the VFcPS particles to be oxidized by electrode reactions, a nitrobenzene-coated electrode was constructed using a boron-doped diamond electrode (5000 ppm B/C, DiaChem). The diamond electrode was washed with concentrated H2SO4 and rinsed with distilled water in an ultrasonic bath. The electrode was covered with an insulating sheet remaining bare in the central part (3 mm diameter, 0.07 cm2 area). The nitrobenzene (NB) film was prepared by carefully syringing 1.0 mm3 of a NB +/acetone

(v/v 1/9) solution including 0.1 M tetrabutylammonium perchlorate onto a fresh surface of the diamond electrode, making sure that the solution covered the surface completely. The acetone included in the film was allowed to evaporate at room temperature. The VFcPS particles on the NB film-modified electrode suspended in water (0.5 wt %) containing 0.1 M NaClO4 were treated with cyclic voltammetry under a centrifugal force at 69 g. The voltammetry was measured at a specific centrifugation time. 3. Results and Discussion 3.1. Electrochemical Depositions of Polyaniline and Poly(3,4-ethylenedioxythiophene). In our previous work, it was found that morphologies of polyaniline, polythiophene, polypyrrole, and polyphenylene films formed electrochemically on

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Figure 8. (A) An SEM image of the filter surface after filtration of the EDOT oligomer solution. (B) Absorption spectra of EDOT oligomer solutions before and after filtration.

anodes were significantly affected by centrifugal forces.21–23 In addition, the deposition rate of the polymers was also increased by the application of a gravitational force. Figure 3 shows cyclic voltammograms of polyaniline films deposited on the anode surface at 1g (Earth’s gravity) and 315g. Although their shapes seem to be almost identical, the oxidative and reductive charges of the film deposited at 315 g (oxidative charge: 4.60 C, reductive charge: 4.50 C) were apparently higher than those of the film deposited at 1g (oxidative charge: 3.71

Murotani et al. C, reductive charge: 3.67 C). This fact indicates that the amount of polyaniline formed at 315g was more than that formed at 1g, and thus that the polymerization of aniline was accelerated by the centrifugal force. However, as shown in Figure 4, the charges of poly(3,4ethylenedioxythiophene) (PEDOT) deposited on the anode surface at 315g (oxidative charge: 9.84 C, reductive charge: 9.64 C) were nearly equal to those of the deposit at 1 g (oxidative charge: 9.87 C, reductive charge: 9.73 C). Furthermore, the morphologies of the films obtained at 1 and 315g were similar (see Figure 5). Thus, the polymerization of 3,4ethylenedioxythiophene (EDOT) was not influenced exceptionally by the application of a gravitational force. 3.2. Filtration Experiment. The proposed mechanism for the electrodeposition of conjugated polymers is indicated in Figure 6.27 Oxidation of monomers on the anode surface results in the formation of radical cations, and their coupling leads to the formation of oligomers which nucleate and precipitate on the surface and are later deposited on the anode as polymers (see route A in Figure 6). According to this mechanism, the effective sedimentation of the produced oligomers to the anode under centrifugal fields may result in an increase in the deposition rate and the morphology of deposits. It is well-known that the sedimentation velocity given by Stokes equation is governed mainly by the size of sediments rather than any difference in density between sediments and media because size is squared in the equation.28 Hence, the influence of sediment size on centrifugation has been well summarized so far. On the basis of this knowledge, centrifugal fields at several hundreds g affect only sediments of size greater than micrometer-order, and therefore that centrifugation at several hundred g cannot separate a macromolecule like an oligomer from a solution, since its size is generally less than 100 nm. However, the conjugated macromolecules frequently form aggregates with sizes greater than micrometerorder, which is mainly caused by interchain interactions29 (see route B in Figure 6). Thereby, the effective sedimentation of the oligomer aggregates to the anode can be induced even by centrifugal forces ranging up to several hundred g. To confirm the formation of oligomer aggregates, we filtered electrolytes including oligomers after electrochemical

Figure 9. Schematic illustration of the electrochemical redox reaction of VFcPS.

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Figure 10. SEM photographs of (a) 2 µm, (b) 500 nm, and (c) 200 nm VFcPS latex particles.

Figure 11. UV-vis spectra of dichloromethane solutions of vinylferrocene (a), VFcPS latex particles (b-d), and 2 µm unmodified polystyrene latex particles (e). Size of VFcPS latex particles: (b) 2 µm, (c) 500 nm, and (d) 200 nm.

polymerization in the form of an oligomer solution through a 200-nm-pore-size membrane filter, and then we observed the filter’s surface after filtration using a scanning electron microscope (SEM). Furthermore, oligomer solutions both before and after filtration were subjected to UV-vis spectroscopy. We applied this method to the electrolytes after the electrochemical polymerization of aniline and EDOT, with regard to which centrifugal effects were and were not observed, respectively. Figure 7A shows an SEM image of the filter surface after filtration of the aniline oligomer solution. Large amounts of oligomer aggregates were visible on the filter. Figure 7B shows the absorption spectra of aniline oligomer solutions before and after filtration. Filtration with a 200-nm-pore-size membrane filter led to the disappearance of the absorbance of the long wavelength region in the solution’s spectrum. Actually, the aniline oligomer solution was green before filtration and colorless after filtration (see inset of Figure 7B). Hence, it can be stated that almost all oligomers in the electrolyte after polymerization were made up of aggregates larger than 200 nm, and that these oligomers were separated from the electrolyte by filtration. Therefore, it may be possible to induce the of the oligomer aggregates to the anode via the application of a gravitational force. However, no oligomer aggregates were visible on the filter surface after filtration of the EDOT oligomer solution, as shown in Figure 8A. In addition, no remarkable variation was observed in the UV-vis spectrum of the filtrate (see Figure 8B). Actually, the color of the EDOT oligomer solution was not changed by filtration, and the filtrate was also green in color (see inset in Figure 8B). These results suggest that almost all of the EDOT oligomers in the electrolyte passed through the membrane filter. For this oligomer solution, the sedimentation of oligomers is not expected to be effectively caused by the application of a gravitational force ranging up to several hundred g. Thus, the sizes of aniline and EDOT oligomer aggregates in the electrolytes are quite different, and this difference determined

whether or not any centrifugal effects were generated, as demonstrated in section 3.1. 3.3. Estimation of the Effect of Deposit Size on Electrochemical Processes under Centrifugal Fields. In the previous section, it was found that oligomer aggregate size is quite an important factor in the effects of centrifugal forces on the electrochemical deposition of conjugated polymers. Subsequently, we estimated the effect of size on deposits in electrochemical processes under centrifugal fields more quantitatively using the following model reaction. Aoki et al. synthesized various sizes of vinylferrocenemodified polystylene (VFcPS) latex particles and examined an electrochemical reaction of their suspension on a nitrobenzene (NB) film-modified electrode in aqueous electrolytes.24 VFcPS latex particles were oxidized via four steps (see Figure 9): (a) deposition of the VFcPS particles in their suspension on the NB film-modified electrode; (b) liberation of the vinylferrocene moiety as poly(vinylferrocene) from the polystylene matrix (NB film is required for this liberation); (c) diffusion of poly(vinylferrocene) in the NB film; and (d) electrochemical oxidation of poly(vinylferrocene) to poly(vinylferrocenium) at the electrode surface. Because these were cascading steps, the current value of the electrochemical redox reaction of poly(vinylferrocene)/poly(vinylferrocenium) in step d were to have been increased if the deposition step a had been accelerated by the centrifugation. In this work, we synthesized three different sizes of VFcPS latex particles by changing the volume ratio of 2-propanol to water, monomer concentration, initiator concentration, and stabilizer concentration.26 Figure 10 shows SEM images of the synthesized particles. The diameters of three kinds of VFcPS latex particles were evaluated to be ca. 2 µm, 500 nm, and 200 nm, respectively. Absorption at 440 nm assigned to the vinylferrocene moieties was visible in the UV-vis spectra of all dichloromethane solutions of the VFcPS latex particles (see Figure 11), and vinylferrocene moieties were hence found to be successfully modified in all the particle samples. Figure 12 shows cyclic voltammograms of VFcPS latex particles under a centrifugal field. In the case of the 2-µm particles, the oxidation and reduction peak currents corresponding to the redox reaction of the vinylferrocene moieties increased remarkably with an increase in centrifugation time. This is probably because the sedimentation of the particles onto the electrode surface proceeded effectively under the application of a gravitational force. A similar time-course was observed for the effect of a centrifugal force on the currents in the case of the 500-nm particles, although the rate of increase in the peak currents along with centrifugation time was smaller than that in the case of the 2-µm particles. Furthermore, no changes in the currents with time were observed in the case of the 200-nm particles. These results

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Figure 12. Cyclic voltammograms of (a) 2 µm, (b) 500 nm, and (c) 200 nm VFcPS latex particles at various centrifugation times.

proved that the size of VFcPS latex particles as deposits is associated strongly with the degree of centrifugal effect generated, and that a deposit size of more than several hundred nanometers is required to induce centrifugal effects on electrochemical deposition under an acceleration force ranging up to several hundred g. 4. Conclusion We have studied the mechanism for the effect of centrifugation on the electrochemical deposition of conjugated polymers on anodes. It was found that the size of the oligomer aggregates just before their deposition on the anode was quite an important factor in their effect. Additionally, the formation of aggregates more than several hundred nanometers in size was required to induce centrifugal effects on the electrochemical deposition

under acceleration forces ranging up to several hundred g. The influence of deposit size thus conjectured was supported strongly by the model electrochemical process using vinylferrocenemodified polystylene (VFcPS) latex particles under centrifugal fields. Acknowledgment. This work was financially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology, Japan. References and Notes (1) Kaner, R. B. In Electrochemical science and technology of polymers 2; Linford, R. G. Ed.; Elsevier Applied Science Publishers: London and New York, 1987; Chapter 3. (2) Heinze, J. In Organic Electrochemistry, 4th ed.; Lund, H.; Hammerich, O.; Eds.; Marcel Dekker, Inc.: New York, 2001;Chapter 32.

Electrochemical Deposition of Conjugated Polymers (3) Roncali, J. Chem. ReV. 1997, 97, 173. (4) Stenger-Smith, J. D. Prog. Polym. Sci. 1998, 23, 57. (5) Sadki, S.; Schottland, P.; Brodie, N.; Sabouraud, G. Chem. Soc. ReV. 2000, 29, 283. (6) Schopf, G.; Kossmehl, G. Polythiophenes: Electrically ConductiVe Polymers; AdVances in Polymer Science, Springer-Verlag: Berlin, 1997; Vol. 129. (7) Sekiguchi, K.; Atobe, M.; Fuchigami, T. Electrochem. Commun. 2002, 4, 881. (8) Sekiguchi, K.; Atobe, M.; Fuchigami, T. J. Electroanal. Chem. 2003, 557, 1. (9) Pringle, J. M.; Efthimiadis, J.; Howlett, P. C.; Efthimiadis, J.; MacFarlane, D. R.; Chaplin, A. B.; Hall, S. B.; Officer, D. L.; Wallace, G. G.; Forsyth, M. Polymer 2004, 45, 1447. (10) Atobe, M.; Osuka, H.; Fuchigami, T. Chem. Lett. 2007, 36, 1448. (11) Atobe, M.; Iizuka, S.; Fuchigami, T.; Yamamoto, H. Chem. Lett. 2007, 36, 1448. (12) Penner, R. M.; Martin, C. R. J. Electrochem. Soc. 1986, 133, 2206. (13) Martin, C. R. Science 1994, 266, 1961. (14) Loganathan, K.; Pickup, P. G. Langmuir 2006, 22, 10612. (15) Oetero, T. F.; Rodriguez, J. J. Electrochem. Soc. 1991, 310, 219. (16) Lin, Y.; Wallace, G. G. Electrochim. Acta 1994, 39, 1409. (17) Osawa, S.; Ito, M.; Tanaka, K.; Kuwano, J. Synth. Met. 1987, 18, 145.

J. Phys. Chem. B, Vol. 112, No. 31, 2008 9317 (18) Osawa, S.; Ito, M.; Tanaka, K.; Kuwano, J. J. Polym. Sci., Part B: Polym. Phys. 1992, 30, 19. (19) Atobe, M.; Kaburagi, T.; Nonaka, T. Electrochemistry 1999, 67, 1114. (20) Atobe, M.; Tsuji, T.; Ryosuke, A.; Fuchigami, T. J. Electrochem. Soc. 2006, 153, D10. (21) Atobe, M.; Hitose, S.; Nonaka, T. Electrochem. Commun. 1997, 1, 278. (22) Atobe, M.; Murotani, A.; Hitose, S.; Suda, Y.; Sekido, M.; Fuchigami, T.; Chowdhury, A.-N.; Nonaka, T. Electrochim. Acta 2004, 50, 977. (23) Murotani, A.; Atobe, M.; Fuchigami, T. Electrochemistry 2006, 74, 590. (24) Chen, J.; Xu, C.; Aoki, K. J. Electroanal. Chem. 2003, 546, 79. (25) Xu, C.; Aoki, K. Langmuir 2004, 20, 10194. (26) Tuncle, A.; Kahraman, R.; Piskin, E. J. Appl. Polym. Sci. 1994, 51, 1485. (27) Bade, K.; Tsakova, V.; Schultze, J. W. Electrochim. Acta 1992, 37, 2255. (28) Leng, W. W. -F. Industrial Centrifugation Technology; McGrawHill: New York, 1998. (29) Nguyen, T.-Q.; Doan, V.; Schwartz, B. J. J. Chem. Phys. 1999, 110, 4068.

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