ARTICLE pubs.acs.org/Langmuir
Gradient Doping of Conducting Polymer Films by Means of Bipolar Electrochemistry Yutaka Ishiguro, Shinsuke Inagi,* and Toshio Fuchigami* Department of Electronic Chemistry, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502 Japan ABSTRACT: In this paper, we report a novel electrochemical doping method for conducting polymer films based on bipolar electrochemistry. The electrochemical doping of conducting polymers such as poly(3-methylthiophene) (PMT), poly(3,4-ethylenedioxythiophene) (PEDOT), and poly(aniline) (PANI) on a bipolar electrode having a potential gradient on its surface successfully created gradually doped materials. In the case of PEDOT film, the color change at the anodic side was also observed to be gradually transparent. PANI film treated by the bipolar doping gave a multicolored gradation across the film. The results of UVvis and energy dispersive X-ray analyses for the doped films supported the distribution of dopants in the polymer films reflecting the potential gradient on the bipolar electrode. Furthermore, the reversibility of the bipolar doping of the PMT film was demonstrated by a spectroelectrochemical investigation.
’ INTRODUCTION Recently, a number of reports have been published concerning gradient soft matters due to their unique physical properties potentially applicable as functional surface and biointerface. A variety of fabrication processes for molecular and macromolecular gradients make it possible to produce useful functionalities.1,2 Among them, the use of distributed electric field to electrosynthesis is one promising approach to obtain the gradient materials. Bohn and his co-workers reported that the gradient electric field on a plate electrode induced by a bipotentiostat worked well to form the desired gradient material.35 The bipolar electrochemistry is also a powerful technique to generate a gradual electric field on a wireless conducting object. When an isolated conducting substrate in a solution is subjected to a parallel electric field, it can become a bipolar electrode, that is, an electrode that simultaneously acts as both anode and cathode. This type of electrochemistry has made possible the study of chemical reactions without physical contact to a circuit, for instance, in electrogenerated chemiluminescence applications.610 Recently, Bj€orefors and co-workers successfully created molecular gradients on a bipolar electrode.11,12 For example, selfassembled monolayers on gold substrate were converted to a gradient surface by the cathodic desorption of thiols. They also demonstrated that electrodeposition of copper and electropolymerization of pyrrole on a bipolar electrode.12 Although the pioneering works1119 utilizing bipolar electrochemistry to materials science are very interesting and promising, they have all been based on deposition or desorption of organic/polymeric molecules or inorganic materials involved at the surface of a conducting substrate. This facile system driven without r 2011 American Chemical Society
attachment of the substrates to an electrical circuit has extreme potential to explore the novel gradient materials. Our target substrate in this research is π-conjugated polymers, which are well-known as conducting polymers.2022 π-Conjugated polymers on electrode are usually electroactive and easily positively charged when anodically oxidized, accompanying incorporation of counteranion (dopant) presented in the electrolyte. The bandgap of the partially doped polymer is likely to be narrow in comparison to that in the neutral state; consequently, the absorption band shifts to the near-infrared region. For example, in the case of poly(3,4-ethylenedioxythiophene) (PEDOT),2325 the absorbance around the visible region greatly decreases in the doped state to a colorless (transparent) appearance. Furthermore, the electron conductivity of conjugated polymer is enhanced by doping. These interesting features of conjugated polymers triggered us to create novel materials by means of bipolar electrochemistry. To attain gradual doping of conjugated polymer film, namely, to make a composition-gradient soft matter in-plane is quite challenging. In our preliminary work, we successfully demonstrated the bipolar patterning of poly(3-methylthiophene) (PMT) film.26 As the work extended, we report herein the detailed studies on bipolar doping of PMT and other conducting polymers such as PEDOT and poly(aniline) (PANI) (Chart 1), including the evidence of gradient composition by optical and spectroelectrochemical studies and local elemental analysis. Received: February 3, 2011 Revised: April 1, 2011 Published: May 16, 2011 7158
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EDX Analysis. EDX analysis was performed with Genesis XM2 (Keyence). A line analysis was employed to quantify the amounts of element included at different positions of polymer films. Absorption and Transmittance Analyses. The polymer film on ITO substrate was placed in a Shimadzu UV-1800 spectrophotometer. A line analysis was performed at each position of the film using a slit. Spectroelectrochemical Study. A thin U-type cell equipped with wire driving electrodes was subjected to a UVvis spectrophotometer. The PMT film on ITO was placed in the cell, and the bipolar doping was carried out by passing a pulsed alternating current ((0.5 mA) for 30 s intervals. The absorbance at 475 and 800 nm was measured at the fixed position (6 mm from initial cathodic edge) and was recorded with electrolysis time.
’ RESULTS AND DISCUSSION
Figure 1. (a) Schematic illustrations of U-type electrolytic cell containing a driving anode and cathode pair and a bipolar electrode. (b) Photographs of the PMT film produced by bipolar doping (0.02 C) in 0.005 M Bu4NPF6/acetonitrile and the amounts of phosphorus (P) and fluorine (F) contained at each distance from the cathodic edge of the doped film.26
’ EXPERIMENTAL SECTION Materials. All reagents and dehydrated solvents were purchased from commercial sources and used without further purification. Electrolytic solutions were deaerated by intensive N2 bubbling just before use. General Procedure for Electrochemical Polymerization. Potential sweep electropolymerization of 3-methylthiophene (0.1 M) was carried out with an ALS 600A electrochemical analyzer using an ITO working electrode (5 mm 20 mm) and platinum counter electrode (20 mm 20 mm), and SCE reference electrode in 0.1 M Bu4NPF6/ acetonitrile at a sweep rate of 0.1 V/s. After potential sweep between 0.3 and 2.0 V (2 cycles), the polymer was deposited on the ITO electrode. The polymer was then dedoped by potentiostatic electrolysis at 0.5 V for 25 min in monomer-free 0.005 M Bu4NPF6/acetonitrile to give a red-colored PMT film. Film thickness was estimated as ca. 11 μm by a Keyence laser focus displacement meter LT-8100. PEDOT film was prepared similarly to the method above. The potential sweep range was between 0.8 and 1.5 V (2 cycles). The dedoping potential was 0.8 V. PANI film was also prepared similarly to the method above in 0.1 M H2SO4. The potential sweep was carried out between 0.2 and 0.8 V (50 cycles). The dedoping of the film at 0.5 V for 5 min in 0.005 M H2SO4 gave a transparent film.
General Procedure for Electrochemical Doping on Bipolar Electrode. The PMT film on ITO electrode was subjected to a U-type
electrolytic cell equipped with a platinum anode (10 mm 10 mm) and a platinum cathode (10 mm 10 mm) connected to a Hokutodenko HABF-501A as illustrated in Figure 1. The distance between the bipolar electrode and the platinum electrodes was kept constant (2.5 cm). The electrochemical doping of the PMT film was carried out by constant current (1 mA) electrolysis in 0.005 M Bu4NPF6/acetonitrile. After the charge was passed, the film was washed with acetonitrile and dried in vacuo. The bipolar doping for PEDOT and PANI films was also carried out using the U-type cell in 0.005 M Bu4NPF6/acetonitrile and 0.005 M H2SO4, respectively.
Poly(3-methylthiophene) Film. A poly(3-methylthiophene) (PMT) film was prepared on an indium thin oxide (ITO) working electrode by the potential sweep electropolymerization in 0.1 M Bu4NFPF6/acetonitrile and then placed into a U-type cell equipped with a platinum anode and a platinum cathode connected to a constant-current power supply (Figure 1a).26 On the bipolar electrode, one side facing the cathode acts as an anodic surface and the other side as a cathodic surface. Electrochemical doping of the neutral PMT film on the bipolar electrode was carried out in a non-nucleophilic Bu4NPF6/ acetonitrile electrolytic solution. In a 0.005 M solution of the electrolyte, a part of the PMT film on the bipolar electrode exhibited a blue color with the passage of 1 mA current. The color change was derived from the formation of ionic states, namely, polaron (radical cation) and bipolaron (dication), in the polymer backbone.27 The doping of the PMT film gradually proceeded from the anodic side of the bipolar electrode after the passage of 0.02 C (Figure 1b). The PMT film could also be cathodically n-doped at the cathodic side on the bipolar electrode; however, no color change was observed during electrolysis. The main reaction at the cathodic side on the bipolar electrode is probably reduction of a trace amount of contaminating water. The anodically doped PMT film was stable and kept its charged state and the dopant even when rinsed with acetonitrile under open-circuit conditions. To quantify the doping, energydispersive X-ray (EDX) analysis of the doped PMT film on ITO was performed after rinsing with acetonitrile. The observed amounts of elements comprising the PF6 anion, which was included as a dopant, were plotted at each position (Figure 1b). Next, we conducted the UVvis absorption analysis of the PMT film (doped for 0.03 C). The absorption spectra of the PMT film at each position were shown in Figure 2a. The spectra for neutral area clearly showed an absorption maximum at 475 nm due to the ππ* transition of the conjugated polymer chain. On the other hand, in the spectra for the doped area, the absorption the ππ* transition decreased and an absorption band at a higher wavelength newly appeared. The observed absorbance at 475 and 800 nm at each position was plotted in Figure 2b. Both profiles corresponded to the appearance of the film and the results of EDX analysis. This spectroscopic measurement strongly supports the in-plane distribution of dopant on the PMT film. The reversibility of the bipolar doping of the PMT film using the U-type cell was investigated by simply switching the polarity of the driving electrodes. When we applied an opposite electric 7159
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Figure 4. Photographs of the PEDOT film produced by bipolar doping after charging with (a) 0.005 and (b) 0.01 C in 0.005 M Bu4NPF6/ acetonitrile.
Figure 2. (a) UVvis absorption spectra of the gradually doped PMT film (for 0.03 C), measured at each distance from the cathodic edge. (b) Photographs of the PMT film doped on the bipolar electrode (0.03 C) and plots of absorbance measured at 475 and 800 nm at each position.
Figure 5. (a) Transmittance spectra of the gradually doped PEDOT (for 0.01 C), measured at each distance from the cathodic edge. (b) Photographs of the PEDOT film doped on the bipolar electrode (for 0.01 C) and the transmittance profile measured at 520 nm at each position.
Figure 3. (a) Spectroelectrochemical profiles of the PMT film showing the time course of observed absorbance at 475 and 800 nm, measured at 6 mm position from the initial cathodic edge on the bipolar electrode. (b) Photographs of the PMT film during the first cycle of the pulsedbipolar doping.
field for the gradually doped PMT film on the bipolar electrode, the polarity on the bipolar electrode was totally changed and thus the blue color of the doped PMT turned to be red, together with the simultaneous color change from red to blue on the new anodic side. In order to survey the reversibility spectroscopically, the spectroelectrochemical study of the bipolar doping for PMT film was carried out using another thin U-type cell equipped with wire driving electrodes for this measurement. The focused position was fixed at 6 mm from the initial cathodic edge of the bipolar electrode. The absorbance at 475 and 800 nm at the fixed position was recorded by a UVvis spectrophotometer during the electrolysis. The neutral PMT film on the bipolar electrode was then passed with an alternating current ((0.5 mA) for 30 s intervals. Figure 3a represents the results of the spectroelectrochemical investigation. The absorbances at both 475 and 800 nm were constant in the steady state through the repeating pulse-electrolysis. Although the absorption magnitudes decrease during increased numbers of cycles, the detachment of the PMT film from the ITO substrate did not occur. The aspect of the PMT film during the first cycle is shown in Figure 3b. The thicker film of PMT (made by more sweep times) took a longer period to gain a steady-state absorbance as is usually observed in conventional doping. This would affect a loss of good reversibility. Poly(3,4-ethylenedioxythiophene) (PEDOT) Film. Poly(3,4-ethylenedioxythiophene) (PEDOT) is one of the most intensively investigated conducting polymers due to its very high
conductivity and excellent environmental stability.2325 Furthermore, its interesting property of transparency in the p-doped state is potentially applicable to organic transparent electrodes and electrochromic materials. The localized and gradient doping for PEDOT film by the bipolar technique would also be interesting. A PEDOT film was prepared on an ITO working electrode by the potential sweep electropolymerization in 0.1 M Bu4NPF6/ acetonitrile. After further dedoping of the film, the substrate was used as a bipolar electrode in the U-type cell shown above. The as-prepared (neutral) PEDOT film was a dark blue color with low transmittance. The bipolar doping of the PEDOT film with 0.5 mA gave rise to a distinct color change at the anodic surface to transparency depending on the charge passed (Figure 4). The UVvis transmittance analysis for the gradually doped PEDOT (0.01 C) was carried out at each position and the results were summarized in Figure 5a. At the cathodic surface (0.8 V. The gradual change of the spectra at 12 and 14 mm positions is attributed to the gradient potential between 0.3 and 0.8 V. The foregoing combination of bipolar patterning of conducting polymer film and its spectroelectrochemical investigation reveals the actual potential distribution across the bipolar electrode.
’ CONCLUSIONS In summary, we have successfully demonstrated the gradient electrochemical doping of conducting polymers such as PMT, PEDOT, and PANI on bipolar electrode using the U-type cell. Both the distribution of the dopant and the absorbance profiles in the polymer films treated by the bipolar doping clearly reflected the potential gradient induced on the bipolar electrode in the U-type cell. The bipolar doping of the PMT film was found to be reversible when passing the alternating current, evidenced by the spectroelectrochemical investigation. It is noteworthy that this bipolar technique is driven without attachment of the polymer to an electrical circuit. This advantage makes this method be quite new, simple, and powerful for fabricating composition-gradient materials. Therefore, the method has the potential as a novel electrochromic application involving simultaneous multicolored imaging. ’ AUTHOR INFORMATION
Figure 8. UVvis absorption spectra of the gradually doped PANI film (for 0.05 C), measured at each distance from the cathodic edge.
changes diversely from transparent yellow to green, and further to blue depending on the applied potential.30 The electrochemical polymerization and doping of PANI are usually conducted in a water system. That intrigued us to survey the scope and limitation of the bipolar technique in water. A PANI film was prepared on an ITO working electrode by the potential sweep electropolymerization in 0.1 M H2SO4. The obtained PANI film was further dedoped in 0.005 M H2SO4 to transparency. This was placed in the U-type cell as the bipolar electrode. Then, the bipolar doping of the film with 4 mA in 0.005 M H2SO4 for 0.05 C resulted in the formation of multicolored film with gradation as shown in Figure 7. This phenomenon is highly important not only to utilize this method as a novel electrochromic application, but also to estimate the applied potentials on each position of the bipolar electrode. Then, we measured absorption spectra at each position of the PANI film. Figure 8 represents the absorption spectra of the film measured at 2 mm intervals from the cathodic edge of the bipolar electrode. Previously, the spectroelectrochemical study on PANI film was reported30 showing that (1) there is almost no absorption around the visible region at 0.2 V; (2) the supply of more anodic potential (0.30 V) induced the absorption at the near-IR region with the absorption maximum (λmax) at 810 nm; (3) then,
Corresponding Author
*Department of Electronic Chemistry, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502 Japan. Tel: þ81-45-924-5406, Fax: þ81-45-924-5406, E-mail:
[email protected] (S. Inagi),
[email protected] (T. Fuchigami).
’ ACKNOWLEDGMENT We thank Prof. Mahito Atobe at Yokohama National University for EDX measurements. This study was financially supported by the Ogasawara Foundation for the Promotion of Science and Engineering, and Kato Foundation for Promotion of Science (for S.I.). ’ REFERENCES (1) Morgenthaler, S.; Zink, C.; Spencer, N. D. Soft Matter 2008, 4, 419–434. (2) Genzer, J.; Bhat, R. R. Langmuir 2008, 24, 2294–2317. (3) Terrill, R.; Balss, K.; Zhang, Y.; Bohn, P. W. J. Am. Chem. Soc. 2000, 122, 988–989. (4) Balss, K.; Coleman, B.; Lansford, C.; Haasch, R. T.; Bohn, P. W. J. Phys. Chem. 2001, 105, 8970–8978. (5) Wang, X.; Bohn, P. W. J. Am. Chem. Soc. 2004, 126, 6825–6832. (6) Arora, A.; Eijkel, J. C. T.; Morf, W. E.; Manz, A. Anal. Chem. 2001, 73, 3282–3288. 7161
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(7) Zhan, W.; Alvarez, J.; Crooks, R. M. J. Am. Chem. Soc. 2002, 124, 13265–13270. (8) Chow, K. F.; Mavre, F.; Crooks, R. M. J. Am. Chem. Soc. 2008, 130, 7544–7545. (9) Mavre, F.; Chow, K. F.; Sheridan, E.; Chang, B. Y.; Crooks, J. A.; Crooks, R. M. Anal. Chem. 2009, 81, 6218–6225. (10) Fosdick, S. E.; Crooks, J. A.; Chang, B. -Y.; Crooks, R. M. J. Am. Chem. Soc. 2010, 132, 9226–9227. (11) Ulrich, C.; Andersson, O.; Nyholm, L.; Bj€orefors, F. Angew. Chem., Int. Ed. 2008, 47, 3034–3036. (12) Ulrich, C.; Andersson, O.; Nyholm, L.; Bj€orefors, F. Anal. Chem. 2009, 81, 453–459. (13) Bradley, J.; Ma, Z. Angew. Chem., Int. Ed. 1999, 38, 1663–1666. (14) Bradley, J.; Babu, S.; Ndungu, P. Fullerenes, Nanotubes, Carbon Nanostruct. 2005, 13, 227–237. (15) Bradley, J.; Chen, H.; Crawford, J.; Eckert, J.; Ernazarova, K.; Kurzeja, T.; Lin, M.; McGee, M.; Nadler, W.; Stephens, S. Nature 1997, 389, 268–271. (16) Warakulwit, C.; Nguyen, T.; Majimel, J.; Delville, M.; Lapeyre, V.; Garrigue, P.; Ravaine, V.; Limtrakul, J.; Kuhn, A. Nano Lett. 2008, 8, 500–504. (17) Ramakrishnan, S.; Shannon, C. Langmuir 2010, 26, 4602–4606. (18) Ramakrishnan, S.; Shannon, C. Langmuir 2011, 27, 878–881. (19) Bouchet, A.; Descamps, E.; Mailley, P.; Livache, T.; Chatelain, F.; Haguet, V. Small 2009, 5, 2297–2303. (20) Skotheim, T. A., Reynolds, J. R., Eds. Handbook of Conducting Polymers, 3rd ed.; CRC Press: Boca Raton, FL, 2007. (21) Inzelt, G. Conducting Polymers; Springer: Heidelberg, 2008. (22) Heinze, J.; Frontana-Uribe, B. A.; Ludwigs, S. Chem. Rev. 2010, 110, 4724–4771. (23) Groenendaal, L.; Jonas, F.; Freitag, D.; Pielartzik, H.; Reynolds, J. R. Adv. Mater. 2000, 12, 481–494. (24) Heywang, G.; Jonas, F. Adv. Mater. 1992, 4, 116–118. (25) Kirchimeyer, S.; Reuter, K. J. Mater. Chem. 2005, 15, 2077–2088. (26) Inagi, S.; Ishiguro, Y.; Atobe, M.; Fuchigami, T. Angew. Chem., Int. Ed. 2010, 49, 10136–10139. (27) Beaujuge, P. M.; Reynolds, J. R. Chem. Rev. 2010, 110, 268–320. (28) Spencer, H. J.; Skabara, P. J.; Giles, M.; McCulloch, I.; Coles, S. J.; Hursthouse, M. B. J. Mater. Chem. 2005, 15, 4783–4792. (29) Li, D.; Huang, J.; Kaner, R. B. Acc. Chem. Res. 2009, 42, 135–145. (30) Watanabe, A.; Mori, K.; Iwasaki, Y.; Nakamura, Y.; Niizuma, S. Macromolecules 1987, 20, 1793–1796.
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