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Polymer Films on Electrodes: Investigation of Ion Transport at Poly(3,4-ethylenedioxythiophene) Films by Scanning Electrochemical Microscopy† Nianjun Yang and Cynthia G. Zoski* Department of Chemistry and Biochemistry, New Mexico State UniVersity, Las Cruces, New Mexico 88003-8001 ReceiVed April 28, 2006. In Final Form: July 14, 2006 Electropolymerization, morphology characterization, and ion transport of poly(3,4-ethylenedioxythiophene) (PEDOT) films doped with different counterions (chloride, ferrocyanide (FCN), and poly(p-styrenesulfonate) (PSS-)) on a platinum electrode were investigated using scanning electrochemical microscopy (SECM) during both potential step (chronocoulometric) and cyclic voltammetric scans. An ultramicroelectrode (UME) tip was positioned close to the surface of a PEDOT-modified substrate electrode, and the responses of both electrodes to a substrate potential step or linear sweep were monitored simultaneously. Chloride or ferrocyanide (FCN) ejection during PEDOT reduction was shown to be a function of the reduction potential. The nature of the cation in the bulk solution was not found to be important in the kinetics of ion transport in PEDOT+/FCN- films. Direct evidence for the incorporation of cations of Ru(NH3)63+/2+ in a PEDOT film during its reduction was also obtained by SECM measurements. The adsorption of Ru(NH3)63+ in fully oxidized PEDOT+/PSS- films was observed and attributed to ion exchange between the Na+ co-ion of PSS- and Ru(NH3)63+ in the bulk solution.
Introduction Electrochemical investigations of electronically conjugated conducting polymers (e.g., polyaniline, polypyrrole, polythiophene, and their substitutes) on various electrode substrates continue to be studied extensively1 and reviewed2 as a result of the complexity of fundamental processes such as ion transport and charge transfer in such systems and their use in technological applications including catalysis, electronic devices, electrochromic displays, light-emitting setups, biosensors, and energy conversion and storage.3 There is continuing interest in the nature of charge transfer at polymer-electrolyte interfaces and/or through a polymer layer,4-8 the role of ion transport as a result of charge compensation during the oxidation and reduction of conducting polymers,6-8 the theoretical treatment of parameters determining the rates of both electron and ion transfer,5,6,8 and catalytic properties and processes involving polymer-covered electrodes.4c-e,9 In particular, ion transport has been extensively investigated using various spectroelectrochemical techniques,1-3,7,10 †
Part of the Electrochemistry special issue. * Corresponding author. E-mail:
[email protected].
(1) (a) ElectroactiVe Polymer Electrochemistry; Lyons, M. E. G., Ed.; Plenum Press: New York, 1994; Part 1, p 488. (b) ElectroactiVe Polymer Electrochemistry; Lyons, M. E. G., Ed.; Plenum Press: New York, 1996; Part 2, p 332. (c) Handbook of Conducting Polymers; Skotheim, T. A., Ed.; Marcel Dekker: New York, 1986; Vols. 1 and 2, p 1417. (2) (a) Murray, R. W. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1984; Vol. 13, p 191. (b) Inzelt, G. In Electroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1994; Vol. 18, p 89. (c) Andrieux, C. P.; Saveant, J.-M. In Molecular Design of Electrode Surfaces; Murray, R. W., Ed.; John Wiley & Sons: New York, 1992; Vol. 22, p 207. (d) Oyama, N.; Ohshaka, T. In Molecular Design of Electrode Surfaces; Murray, R. W., Ed.; John Wiley & Sons: New York, 1992; Vol. 22, p 333. (e) Martin, C. R.; Van Dyke, L. S. In Molecular Design of Electrode Surfaces; Murray, R. W., Ed.; John Wiley & Sons: New York, 1992; Vol. 22, p 403. (3) (a) Hillman, A. R. In Electrochemical Science and Technology of Polymers; Linford, R. G., Ed.; Elsevier Applied Science: London, 1987; Vol. 1, p 103. (b) Applications of ElectroactiVe Polymers; Scrosati, B., Ed.; Chapman & Hall: London, 1993; p 353. (4) (a) Oyama, N.; Anson, F. C. Anal. Chem. 1980, 52, 1192. (b) Nowak, R. J.; Schultz, F. A.; Umana, M.; Lam, R.; Murray, R. W. Anal. Chem. 1980, 52, 315. (c) Doblhofcr, K. Electrochim. Acta 1980, 25, 871. (d) Kaufman, F. B.; Engler, E. M. J. Am. Chem. Soc. 1979, 101, 547. (e) Kerr, J. B.; Miller, L. L.; Van De Mark, M. R. J. Am. Chem. Soc. 1980, 102, 3383.
electrochemical quartz crystal microbalance (EQCM),10c,11 electrochemical impedance spectroscopy (EIS),12 scanning electrochemical microscopy (SECM),8,13 and other electrochemical techniques.14 It has been demonstrated that there are two principle limiting mechanisms of electronic charge compensation during the reduction of a polymer film: (a) anion transport corresponding to the ejection of anions initially incorporated into the polymer film during electrodeposition and (b) cation transport corresponding to the injection of cations from the bulk (5) (a) Peerce, P. J.; Bard, A. J. J. Electroanal. Chem. 1980, 112, 97. (b) Peerce, P. J.; Bard, A. J. J. Electroanal. Chem. 1980, 114, 89. (c) Rubenstein, I.; Bard, A. J. J. Am. Chem. Soc. 1981, 103, 5007. (d) Henning, T. P.; White, H. S.; Bard, A. J. J. Am. Chem. Soc. 1981, 103, 3937. (e) Bull, R. A.; Fan, F.-R. F.; Bard, A. J. J. Electrochem. Soc. 1982, 129, 1009. (f) White, H. S.; Leddy, J.; Bard, A. J. J. Am. Chem. Soc. 1982, 104, 4811. (g) Martin, C. R.; Rubinstein, I.; Bard, A. J. J. Am. Chem. Soc. 1982, 104, 4817. (h) Henning, T. P.; White, H. S.; Bard, A. J. J. Am. Chem. Soc. 1982, 104, 5862. (i) Henning, T. P.; Bard, A. J. J. Electrochem. Soc. 1983, 130, 613. (6) (a) Moulton, R. D.; Bard, A. J.; Williams, J. M. Proc. Indian Acad. Sci. 1986, 97, 349. (b) Fan, F.-R. F.; Bard, A. J. J. Electrochem. Soc. 1986, 133, 301. (c) Leddy, J.; Bard, A. J. J. Electroanal. Chem. 1985, 189, 203. (d) Ghosh, P. K.; Bard, A. J. J. Electroanal. Chem. 1984, 169, 113. (e) Bull, R. A.; Fan, F.-R. F.; Bard, A. J. J. Electrochem. Soc. 1983, 130, 1636. (f) Leddy J.; Bard, A. J. J. Electroanal. Chem. 1983, 153, 223. (g) Carlin, C. M.; Kepley, L. J.; Bard, A. J. J. Electrochem. Soc. 1985, 132, 353. (h) Gaudiello, J. G.; Ghosh, P. K.; Bard, A. J. J. Am. Chem. Soc. 1985, 107, 3027. (i) Kaifer A. E.; Bard, A. J. J. Phys. Chem. 1986, 90, 868. (j) Lee, C.; Kwak, J.; Kepley, L. J.; Bard, A. J. J. Electroanal. Chem. 1990, 282, 239. (k) Kim, Y.-T.; Yang, H.; Bard, A. J. J. Electrochem. Soc. 1991, 138, L71. (l) Wang, J.; Bard, A. J. J. Am. Chem. Soc. 2001, 123, 498. (7) (a) Penner, R. M.; Van Dyke, L. S.; Martin, C. R. J. Phys. Chem. 1988, 92, 5274. (b) Paulse, C. D.; Pickup, P. G. J. Phys. Chem. 1988, 92, 7002. (c) Ren, X.; Pickup, P. G. J. Phys. Chem. 1993, 97, 5356. (d) Shimidzu, T.; Ohtani, A.; Honda, K. J. Electroanal. Chem. 1987, 224, 123. (e) Shimidzu, T.; Honda, K. J. Electroanal. Chem. 1988, 251, 323. (f) Baker, C. K.; Qiu, Y.-J.; Reynolds, J. R. J. Phys. Chem. 1991, 95, 4446. (g) Levi, M. D.; Lopez, C.; Vieil, E.; Vorotyntsev, A. Electrochim. Acta 1997, 42, 757. (8) (a) Borgwarth, K.; Heinze, J. In Scanning Electrochemical Microscopy; Bard, A. J., Mirkin, M. V., Eds.; Marcel Dekker: New York, 2001; Part 6, p 227. (b) Quinto, M.; Jenekhe, S. A.; Bard, A. J. Chem. Mater. 2001, 13, 2824. (c) Tsionsky, M.; Bard, A. J.; Dini, D.; Decker, F. Chem. Mater. 1998, 10, 2120. (d) Arca, M.; Mirkin, M. V.; Bard, A. J. J. Phys. Chem. 1995, 99, 5040. (e) Fan, F.-R.; Mirkin, M. V.; Bard, A. J. J. Phys. Chem. 1994, 98, 1475. (f) Scanning Electrochemical Microscopy; Bard, A. J., Mirkin, M. V., Eds.; Marcel Dekker: New York, 2001. (9) (a) Laviron, E. J. Electroanal. Chem. 1980, 112, 1. (b) Andrieux, C. P.; Saveant, J. M. J. Electroanal. Chem. 1978, 93, 163. (c) Laviron, E.; Roullier, L.; Degrand, C. J. Electroanal. Chem. 1980, 112, 11.
10.1021/la061167u CCC: $33.50 © 2006 American Chemical Society Published on Web 08/25/2006
Polymer Films on Electrodes
electrolyte.1-14 This kind of ion transport depends significantly on the nature of both co-ions and counterions as well as on electrode potentials.1-14 Poly(3,4-ethylenedioxythiophene) (PEDOT) is polymerized from its thiophene monomer and was developed during the 1980s in the research laboratory of Bayer,15 who functionalized the 3 and 4 positions of thiophene with the cyclic 3,4-dioxy substituent resulting in high regularity of the polymer backbone and a significantly lower oxidation potential for polymer electrodeposition. In the oxidized state, PEDOT is one of the most electrochemically and thermally stable conducting polymers currently available.10e,15b Because of these features, there have been numerous studies related to both the chemical16 and electrochemical (potentiostatic,11g,h,16d,17-19 potentiodynamic,11g,17f,20-22 and galvanostatic11g,12a,b,16d,17f,20f,23,24 modes) synthesis of PEDOT films doped with small (e.g., halide, perchlorate, alkali cations)17,21,22 andlarge(e.g.,surfactants,polyelectrolyte,PSS-)17,18,20,23 ions and/or electroactive molecules (e.g., ferrocyanide)20 in aqueous11g,h,12a,b,17,18,20,23 and organic solutions19,21 as well as in (10) (a) Novak, P.; Kotz, R.; Haas, O. J. Electrochem. Soc. 1993, 140, 37. (b) John, R.; Wallace, G. G. J. Electroanal. Chem. 1993, 354, 145. (c) Bose, C. S. C.; Basak, S.; Rajeshwar, K. J. Phys. Chem. 1992, 96, 9899. (d) Chen, C. C.; Wei, C.; Rajeshwar, K. Anal. Chem. 1993, 65, 2437. (e) Pei, Q.; Zuccarello, G.; Ahlskog, M.; Inganas, O. Polymer 1994, 35, 1347. (f) Gustafsson, J. C.; Liedberg, B.; Inganas, O. Solid State Ionics 1994, 69, 145. (11) (a) Buttry, D. A.; Ward, M. D. Chem. ReV. 1992, 92, 1355. (b) Baker, C. K.; Qiu, Y.-J.; Reynolds, J. R. J. Phys. Chem. 1991, 95, 4446. (c) Hillman, A. R.; Swann, M. J.; Bruckenstein, S. J. Electroanal. Chem. 1991, 291, 147. (d) Naoi, K.; Lien, M.; Smyrl, W. H. J. Electrochem. Soc. 1991, 138, 440. (e) Basak, S.; Bose, C. S. C.; Rajeshwar, K. Anal. Chem. 1992, 64, 1813. (f) Paik, W.; Yeo, I.-H.; Suh, H.; Kim, Y.; Song, E. Electrochim. Acta 2000, 45, 3833. (g) Efimov, I.; Winkels, S.; Schultze, J. W. J. Electroanal. Chem. 2001, 499, 169. (h) Jureviciute, I.; Bruckenstein, S.; Hillman, A. R.; Jackson, A. Phys. Chem. Chem. Phys. 2000, 2, 4193. (i) Bund, A.; Baba, A.; Berg, S.; Johannsmann, D.; Lubben, J.; Wang, Z.; Knoll, W. J. Phys. Chem. B 2003, 107, 6743. (12) (a) Bobacka, J.; Lewenstam, A.; Ivaska, A. J. Electroanal. Chem. 2000, 489, 17. (b) Sundfors, F.; Bobacka, J.; Ivaska, A.; Lewenstam, A. Electrochim. Acta 2002, 47, 2245. (c) Dong, S.; Lian, G. J. Electroanal. Chem. 1990, 291, 23. (13) (a) Kwak, J.; Anson, F. C. Anal. Chem. 1992, 64, 250. (b) Lee, C.; Anson, F. C. Anal. Chem. 1992, 64, 528. (c) Denuault, G.; Trise Frank, M. H.; Peter, L. M. Faraday Discuss. 1992, 94, 23. (d) Trise Frank, M. H.; Denuault, G. J. Electroanal. Chem. 1993, 354, 331. (14) (a) Lee, C.; Kwak, J.; Bard, A. J. J. Electrochem. Soc. 1989, 136, 3720. (b) Pei, Q.; Inganas, O. J. Phys. Chem. 1992, 96, 10507. (c) Gabrielli, C.; Keddam, M.; Perrot, H.; Pham, M. C.; Torresi, R. Electrochim. Acta 1999, 44, 4217. (d) Gabrielli, C.; Garcia-Jareno, J. J.; Pierrot, H. Electrochim. Acta 2001, 46, 4095. (15) (a) Bayer, A. G. Eur. Pat. 339340, 1988. (b) Jonas, F.; Schreder, L. Synth. Met. 1991, 41, 831. (16) (a) Lefebvre, M.; Qi, Z.; Rana, D.; Pickup, P. G. Chem. Mater. 1999, 11, 262. (b) Carlberg, J. C.; Inganas, O. J. Electrochem. Soc. 1997, 144, L61. (c) Fu, Y.; Cheng, H.; Elsenbaumer, R. L. Chem. Mater. 1997, 9, 1720. (d) Zykwinskaa, A.; Domagalaa, W.; Pilawab, B.; Lapkowski, M. Electrochim. Acta 2005, 50, 1625. (e) Xing, K. Z.; Fahlman, M.; Chen, X. W.; Inganais, O. Salaneck,W. R. Synth. Met. 1997, 89, 161. (17) (a) Du, X.; Wang, Z. Electrochim. Acta 2003, 48, 1713. (b) Pigani, L.; Heras, A.; Colina, A.; Seeber, R.; Lopez-Palacios, J. Electrochem. Commun. 2004, 6, 1192. (c) Chen, X.; Xing, K.-Z.; Inganas, O. Chem. Mater. 1996, 8, 2439. (d) Lima, A.; Schottland, P.; Sadki, S.; Chevrot, Chevrot, Synth. Met. 1998, 93, 33. (e) Lisowska-Oleksiak, A.; Kazubowska, K.; Kupniewska, A. J. Electroanal. Chem. 2001, 501, 54. (f) Sakmeche, N.; Aeiyach, S.; Aaron, J.-J.; Jouini, M.; Lacroix, J. C.; Lacaze, P.-C. Langmuir 1999, 15, 2566. (18) (a) Herasa, M. A.; Lupub, S.; Piganic, L.; Pirvub, C.; Seeberc, R.; Terzic, F.; Zanardi, C. Electrochim. Acta 2005, 50, 1685. (b) Yamato, Y.; Ohwa, M.; Wernet, W. J. Electroanal. Chem. 1995, 397, 163. (c) Herasa, M. A.; Lupub, S.; Piganic, L.; Pirvub, C.; Seeberc, R.; Terzic, F.; Zanardi, C. Electrochim. Acta 2005, 50, 1685. (19) (a) Ahonen, H. J.; Lukkari, J.; Kankare, J. Macromolecules 2000, 33, 6787. (b) Bund, A.; Neudeck, S. J. Phys. Chem. B 2004, 108, 17845. (c) Sotzing, G. A.; Redinger, J. L.; Reynolds, J. R.; Steel, P. J. Synth. Met. 1997, 84, 199. (d) Randriamahazaka, H.; Noel, V.; Chevrot, C. J. Electroanal. Chem. 1999, 472, 103. (e) Randriamahazaka, H.; Plesse, C.; Teyssie, D.; Chevrot, C. Electrochem. Commun. 2004, 6, 299. (f) Randriamahazaka, H.; Plesse, C.; Teyssie, D.; Chevrot, C. Electrochem. Commun. 2003, 5, 613. (g) Noel, V.; Randriamahazaka, H.; Chevrot, C. J. Electroanal. Chem. 2003, 542, 33. (h) Randriamahazaka, H.; Noel, V.; Chevrot, C. J. Electroanal. Chem. 2003, 556, 35. (i) Randriamahazaka, H.; Noel, V.; Chevrot, C. J. Electroanal. Chem. 2002, 521, 107. (j) Noel, V.; Randriamahazaka, H.; Chevrot, C. J. Electroanal. Chem. 2003, 558, 41. 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ionic liquids.22,24 Properties of the resulting PEDOT-film-covered electrodes including the growth mechanism,17b,g,19d,20c-f,22 conductivity,17a,18a,19a,h,j,k charge transfer,11g,12a,b,17e,18a,19a,e,h,i mass change,11g volume swelling,16a,d and stability11g,18b,19k during redox cycling have been explored. Applications of PEDOT films in ion-selective electrodes,23b in sensors,18a,b,20c as supercapacitors,17e,18a and as electronic devices16b,e have also been investigated. Comparatively fewer investigations have focused on ion transport in PEDOT films.11f,17e,19b,e,g,m,20a,g Several research groups using EQCM have found that ion transport into and out of PEDOT films is greatly affected by the synthesis mode, the ions doped in the film initially and those existing in the solution during redox cycling, and the solvents used.11f,17e,19b,20a,g For example, Paik et al.11f found that macrocyclic anions (phthalocyanine and porphyrin derivatized with sulfonate anionic groups) were not expelled from PEDOT films whereas heteropolytungstate ions were partially released during redox cycling when PEDOT films were prepared potentiostatically or galvanostatically. However, PEDOT films synthesized potentiodynamically showed significantly diminished permeability toward anions. LisowskaOleksiak et al.17e characterized lithium ion transport in oxidized PEDOT films using an effective diffusion coefficient. Bund et al.19b reported anion transport and solvent exchange. Niu et al. proposed a model based on EQCM results that incorporated mixed ion transfer together with solvent exchange.20g Randriamahazaka et al. reported simultaneous cation ejection and anion incorporation during the oxidation of PEDOT films in ionic liquids using EIS and cyclic voltammetry.19e,g,m Despite these studies, the potential dependence of ion transport in PEDOT films is not well understood. EQCM, EIS, and cyclic voltammetry give mostly indirect information on the relative contributions of cations and anions in the overall charge-transport process because these methods deal with property changes (e.g., mass change, resistance, conductivity, and intensity of adsorption bands) of the polymer film itself rather than for the transported ions.11 For example, in EQCM measurements it is difficult to separate solvent motion from cation and anion transport in the mass changes that are recorded.11 SECM8f is a versatile technique that has been used to explore ion activity at polymer-modified substrate electrodes by bringing an ultramicroelectrode (UME) tip close to the polymer surface8,13 and monitoring the activity across the polymer/solution interface by measurements made exclusively at the UME tip or alternatively at both the UME tip and the polymer-covered substrate. For example, SECM has been used to (1) selectively monitor the flux of electroactive ions leaving or entering a Nafion-coated electrode;13a,b (2) monitor changes in Cl- concentration near a polyaniline (PANI)-modified substrate;13c,d and (3) quantitatively study the redox behavior of the conducting polymer polypyrrole (PPy) where the expulsion of bromide and ferrocyanide anions accompanying the reduction of PPy were studied at different (20) (a) Syritski, V.; Gyurcsanyi, R. E.; Opik, A.; Toth, K. Synth. Met. 2005, 152, 133. (b) White, A. M.; Slade, R. C. T. Electrochim Acta 2004, 49, 861. (c) Vasantha, V. S.; Chen, S.-M. Electrochim. Acta 2005, 51, 347. (d) LisowskaOleksiak, A.; Nowak, A. P.; Jasulaitiene, V. Electrochem. Commun. 2006, 8, 107. (e) Lima, A.; Schottland, P.; Sadki, S.; Chevrot, Chevrot, Synth. Met. 1998, 93, 33. (f) Sakmeche, N.; Aaron, J.-J.; Aeiyach, S.; Lacaze, P.-C. Electrochim. Acta 2000, 45, 1921. (g) Niu, L.; Kvarnstrom, C.; Ivaska, A. J. Electroanal. Chem. 2004, 569, 151. (21) (a) Lapkowski, M.; Pron, A. Synth. Met. 2000, 110, 79. (b) Zykwinska, A.; Domagala, W.; Lapkowski, M. Electrochem. Commun. 2003, 5, 603. (22) Damlin, P.; Kvarnstrom, C.; Ivaska, A. J. Electroanal. Chem. 2004, 570, 113. (23) (a) Sundfors F.; Bobacka J. J. Electroanal. Chem. 2004, 572, 309. (b) Bobacka, J. Anal. Chem. 1999, 71, 4932. (24) Hass, R.; Garcya-Canadas, J.; Garcia-Belmonte, G. J. Electroanal. Chem. 2005, 577, 99.
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potentials and direct proof of cation incorporation was obtained by the use of electroactive cations.8d Measurements from both SECM and EQCM have been used to study ion transport in ferrocyanide (FCN)-doped PEDOT films where EQCM mass measurements provided support for ion exchange between FCN in the polymer and chloride ions from the solution during redox cycling and SECM tip current measurements monitored FCN release during the reduction of the polymer film.20a Here we present a more quantitative approach to the study of the redox behavior of PEDOT films based on an analysis of tip currenttime (i-t) curves and cyclic voltammograms recorded by either a step or linear sweep of the substrate potential. The expulsion of chloride and ferrocyanide anions accompanying the reduction of PEDOT was studied at different potentials, and direct proof of cation incorporation was obtained by the use of electroactive cations of hexaammineruthenium(III) chloride. Experimental Section Chemicals. 3,4-Ethylenedioxythiophene (EDOT), poly(sodium p-styrenesulfonate) (NaPSS, MW ≈ 70 000), ferrocenemethanol (FcMeOH, Aldrich Chemical Co., Milwaukee, WI), potassium chloride (KCl), sodium chloride (NaCl, Fisher Scientific, Fair Lawn, NJ), hexaammineruthenium(III) chloride (Ru(NH3)6Cl3), potassium sulfate (K2SO4), silver perchlorate (AgClO4), sodium sulfate (Na2SO4, Alfa Aesar, Ward Hill, MA), lithium perchlorate (LiClO4), potassium nitrate (KNO3), potassium hexacyanoferrate(II) (K4Fe(CN)6, Fluka Chemical Co., Ronkonkoma, NY), and tetraethylammonium chloride (TEACl, 99%, Sigma, St. Louis, MO) were used as received. Aqueous solutions were prepared with Milli-Q water (18.2 MΩ, Millipore Co., Bedford, MA). Electrodes and Instrumentation. SECM microelectrode (UME) tips with RG ) 5.1 were constructed by heat sealing 25-µm-diameter “hard” Pt wires (Goodfellow, Cambridge Science, Park Cambridge CB4 DJ, England) in glass capillaries under vacuum, followed by polishing and sharpening as previously described.8f,25 The UME tip was characterized using SECM in 1.0 mM FcMeOH/0.1 M KCl solutions. Its diameter (i.e., the diameter of the Pt microwire) was determined to be 25.0 µm by comparing experimental positive feedback approach curves of FcMeOH with theoretical ones according to standard SECM tip fabrication methods described in the literature.8f,25 The dimensions of the glass sheath surrounding the Pt microwire are reflected in the RG value (equal to the ratio of the diameter of the Pt wire + glass assembly and the diameter of the Pt microwire), which was determined to be 5.1 by comparing experimental and theoretical negative feedback approach curves.8f,25 The UME tip diameter of 25.0 µm and RG ) 5.1 were also confirmed by optical microscopy (Olympus BX51). The RG value was checked frequently and resharpened if changes in this value resulted from UME polishing between experiments. Thus, all reported data were recorded on a UME tip with a diameter of 25.0 µm (corresponding to the diameter of the Pt wire) and RG ) 5.1. A 2-mm-diameter Pt inlaid disk electrode (CH Instruments, Austin, TX), polished with 0.3, 0.1, and 0.05 µm alumina slurry consecutively and cleaned with water in an ultrasonic bath for 3 min after each polishing step, was used as a substrate for the electropolymerization of PEDOT. SECM measurements were performed on a PC-controlled CHI model 900B SECM (CH Instruments, Austin, TX) that uses a combination of stepper motors and an XYZ piezo block in order to position the UME tip. An SCE reference electrode (CH Instruments, Austin, TX) jacketed in a glass sleeve with supporting electrolyte served as the reference electrode. The counter electrode was a 1.00-mm-diameter platinum wire (Alfa Aesar, Ward Hill, MA). All potentials are reported with respect to SCE. Throughout this article, anodic current is negative, and cathodic current is positive. Preparation of PEDOT Films. PEDOT films loaded with chloride ions (PEDOT+/Cl-) were electropolymerized potentiostatically at (25) (a) Bard, A. J.; Fan, F.-R. F.; Kwak, J.; Lev, O. Anal. Chem. 1989, 61, 1794. (b) Bard, A. J. In Scanning Microchemical Microscopy; Bard, A. J., Mirkin, M. V., Eds.; Marcel Dekker: New York, 2001.
Yang and Zoski 1.05 V on a 2-mm-diameter Pt inlaid disk electrode in a 0.1 M KCl solution containing 10 mM EDOT. PEDOT films loaded with lithium perchlorate (PEDOT+/ClO4-), ferricyanide (PEDOT+/FCN-), or poly(p-stryrenesulfonate) (PEDOT+/PSS-) were prepared in the same manner at 1.05 V in a 0.1 M LiClO4 + 10 mM EDOT, 0.1 M K4(FeCN)6 + 10 mM EDOT, or 0.1 M NaPSS + 10 mM EDOT solution, respectively. The polymerization of EDOT in the presence of KCl or FCN was stopped when the total charge reached 50 mC (20 min for PEDOT+/Cl-; 10-15 min for PEDOT+/FCN-). The polymerization of PEDOT+/PSS- proceeded comparatively more slowly, reaching a plateau of 48 mC after 30 min, which was maintained up to 40 min when the polymerization was stopped. The coverage of the resulting film was then inspected with an Olympus BX 51 optical microscope (Olympus America Inc., Melville, NY). PEDOT films doped with Ru(NH3)62+/3+ were obtained by the partial reduction of PEDOT+/Cl- or PEDOT+/PSS- films by stepping the substrate potential from 0.2 V to 0, -0.3, -0.4, -0.5, or -0.6 V for 50 s in a 0.1 M Ru(NH3)6Cl3 solution. The films were dried in air after polymerization. All solutions for film polymerization were bubbled with argon for 10 min in a 3 mL SECM Teflon cell and kept under an inert atmosphere of argon. The thickness of the PEDOT+/Cl- films (13 µm) were determined using scanning electrochemical microscopy (SECM), as will be described in the next section. The PEDOT+/FCN- and PEDOT+/ PSS- films were much thinner, and SECM could not be used in determining the thickness of these films. For PEDOT+/PSS- films, the thickness was determined by using a published relationship between the total polymerization charge, 14 mC cm-2, and the measured thickness, 0.10 µm for PEDOT+/PSS- films.12a From this relationship, a thickness of 1.7 µm was calculated. For PEDOT+/ FCN- films, the thickness was determined by using a published relationship between the total polymerization charge, 1 C cm-2, and the measured thickness, 0.8 µm for polypyrrole+/FCN- films.12c From this relationship, a thickness of 1.3 µm was calculated. Scanning Electrochemical Microscopy. To investigate the morphology, thickness (of Cl- doped films), and ion transport in the PEDOT films, the UME tip was positioned using the reduction reaction of oxygen naturally present in 0.1 M K2SO4 at the UME tip.26a The tip potential was set to -0.8 V, and that of the polymerfilm-coated substrate was unbiased. Using the resulting negative feedback approach curve, the tip-substrate distance was set to 5 µm to observe the morphology of the PEDOT film. The supporting electrolyte solution was replaced with a 0.4 mM Ru(NH3)63+/0.1 M KCl solution. The tip was then held at -0.45 V and scanned in the x and y planes at a rate of 2 µm s-1. The substrate was unbiased. To determine the thickness of PEDOT films doped with chloride ions, the oxygen reduction reaction was carried out at the tip as it approached the insulated region of the substrate or approached the PEDOT film. This procedure for determining film thicknesses has been reported previously.9f,26d When the tip current reached 1.2 nA during an approach to the insulated region of the substrate, the tip approach was stopped, and from the tip current-distance curve, the distance (l0) between the tip and substrate was calculated through fitting with theoretical negative-feedback approach curves; l0 was calculated to be 3.1 ( 0.2 µm. A similar tip approach was made to the PEDOT-film-covered substrate and stopped when the tip current was 1.2 nA. The distance (l) between the tip and the film-covered substrate was calculated to be 16.3 ( 0.2 µm. The thickness of the PEDOT film was calculated as (l0 - l) ) 12.8 µm. Investigations of ion transport into and out of the PEDOT film were carried out with the tip-substrate distance set to 13 µm as described above. After the tip was positioned, the electrolyte solution was then deoxygenated for 10 min before performing SECM measurements. A distance of 13 µm (i.e., within a distance of about one tip diameter26b,c) was found to be large enough to avoid UME tip/polymer film contact due to changes in the film volume as the (26) (a) Horrocks, B. R.; Mirkin, M. V.; Pierce, D. T.; Bard, A. J.; Nagy, G.; Toth, K. Anal. Chem. 1993, 65, 1213. (b) Kwak, J.; Bard, A. J. Anal. Chem. 1989, 61, 1794. (c) Kwak, J.; Lee, C.; Bard, A. J. J. Electrochem. Soc. 1990, 137, 1481. (d) Mirkin, M. V.; Arca, M.; Bard, A. J. J. Phys. Chem. 1993, 97, 10790.
Polymer Films on Electrodes
Langmuir, Vol. 22, No. 25, 2006 10341
ization mechanism for PEDOT doped with chloride (PEDOT+/ Cl-) potentiostatically can be written as
Figure 1. Cyclic voltammograms of 0.1 M KCl (-) and 10 mM EDOT/0.1 M LiClO4 (- -) on the 2-mm-diameter platinum electrode at a scan rate of 0.01 V s-1. polymer film is oxidized and reduced.16a,d SECM was then used in substrate generation/tip collection (SG/TC) mode where the potential of the PEDOT-film-covered substrate was scanned using cyclic voltammetry from 0.2 or 0.4 to -0.6 V and then returned to the initial potential, while the UME tip potential was simultaneously held at a constant value sufficient to collect the redox species of interest ejected from the PEDOT film as it was reduced and then reoxidized. Thus at the tip, chloride ions were oxidized at 1.2 V, Ru(NH3)63+/2+ ions were reduced/oxidized at -0.45/-0.1 V, and Fe(CN)63-/4- ions were reduced/oxidized at -0.1/0.3 V. The substrate and UME tip currents were monitored simultaneously as a function of substrate potential. Freshly prepared films were used for each tip/substrate voltammogram, with the tip being positioned each time. SECM chronoamperometry experiments were also performed where the substrate potential was stepped from 0.2 V to 0, -0.3, -0.4, or -0.6 V while the potential of the UME tip positioned 13 µm above the PEDOT-film-covered substrate was held at 1.2 V to completely oxidize chloride ions collected at the UME tip. Freshly prepared films were used for each SECM chronoamperometry experiment, with the tip being positioned each time.
Results and Discussion Electropolymerization of PEDOT. Prior to depositing a PEDOT film doped with chloride ions, the voltammetric behavior of the EDOT monomer and chloride ions was first studied on a 2-mm-diameter platinum substrate. Figure 1 shows the cyclic voltammograms of chloride ions (solid line) and EDOT monomer (dashed line) on the Pt substrate. There is no oxidation current due to chloride ions until approximately 1.1 V, after which the current increased quickly and a sharp anodic wave at about 1.3 V appeared. On the reverse scan, a broad cathodic wave at about 0.9 V was observed. Similar behavior was observed for the oxidation of chloride ions on a 25-µm-diameter Pt inlaid disk microelectrode. Increasing the chloride concentration led to a proportional increase in the anodic peak current. The anodic peak current was also found to be proportional to the square root of the potential scan rate. These proportionalities indicate that the anodic current is due to the diffusion-controlled chloride ions and that no complexation of platinum with chloride ions occurs. The cathodic wave on the Pt disk substrate results from the reduction of the oxidation products formed at 1.3 V. In contrast, the oxidation current of EDOT began at 0.9 V, with a broad anodic wave appearing at 1.1 V and the second anodic wave at 1.65 V. It has been reported that the anodic current beginning at 0.9 V is due to the oxidation of EDOT and the anodic wave at 1.65 V results from the over-oxidation of EDOT in aqueous solutions.17 In our experiments, EDOT was polymerized potentiostatically in the presence of chloride ions over the potential range from 0.9 to 1.1 V where chloride ions are not oxidized, as can be observed by comparison of the voltammogram for chloride ions with that of EDOT in Figure 1. An advantage of potentiostatic electropolymerization is that the reduction or oxidation state of the polymer is clearly known. The polymer-
where Cl- ions from the supporting electrolyte were incorporated into the PEDOT film. The polymerization (oxidation) potential and current due to EDOT (10 mM) were found to be dependent on the concentration of chloride ions. Concentrations of chloride ions of 1.15 V and larger currents, but no stable films were formed on the electrode surface. This is probably due to the oxidation of chloride ions to Cl2(g) at the higher polymerization potential, which prevents the growth of the PEDOT film on the platinum substrate. However, concentrations of >0.2 M resulted in polymerization potentials of -0.5 V because Cl- diffuses into the bulk solution and some re-enters the PEDOT film. The constant UME tip current in the potential range of 0.2 to -0.2 V suggests an unchanging concentration of chloride ions in the tip-substrate gap, which may be due to the steady ejection of chloride ions as the polymer is partially reduced. On a second and succeeding cycles (not shown), the UME tip current remained at approximately the steady limiting current of -0.5 nA that was obtained in the range of 0.2 to -0.2 V during the first cycle. As further evidence of Cl- ejection, a white precipitate (AgCl) appeared when drops of a silver perchlorate solution were mixed with an aliquot of the KNO3 solution in which PEDOT+/Cl- was reduced at -0.6 V for 10 min in a separate experiment. In the absence of film reduction, there was no visible AgCl precipitation on addition of silver perchlorate solution. Additionally, a PEDOT+/ClO4- film was polymerized onto the Pt disk substrate. Figure 4B shows the substrate cyclic voltammogram (solid line) recorded in 0.1 M K2SO4 over the potential range of 0.2 to -0.6 V and the UME tip current (dashed line) (ET ) 1.2 V) as a function of this substrate potential. The cyclic voltammogram of PEDOT+/ClO4- shows a shape similar to that of PEDOT+/Cl-, indicating their similar redox behavior. A slight difference in the capacitance-like current in the anodic process may be due to the effect of the doped ions in the polymer
Polymer Films on Electrodes
Langmuir, Vol. 22, No. 25, 2006 10343 Table 1. Effective Collection Efficiency (g ) QTAS/QSAT) of Chloride Ions at the Tip Electrode When a PEDOT+/ Cl--Covered Substrate Was Stepped from 0.2 V to Different Potentialsa potential step from/to vs SCE QT × 108 C QS × 103 C
Figure 5. SECM current-time curves at the substrate (-) and tip (- -) in a 0.1 M K2SO4 solution after the application of a potential step to the substrate coated with PEDOT+/Cl-. The substrate potential step was (A) from 0.2 to -0.6 V and (B) from 0.2 to 0 V while the tip potential was fixed at 1.2 V to collect the oxidation current of Cl-. The tip-substrate distance was 13 µm, and the film thickness was 13 µm.
films.2,12,17-19 Although a reduction wave appeared for the reduction of PEDOT+/ClO4- (solid line in Figure 4B), the UME tip current (dashed line in Figure 4B) showed only a background current and no current due to Cl- ejection from the PEDOT+/ ClO4- film. These results demonstrate that the current response at the UME tip in Figure 4A is due to the release of chloride ions from the PEDOT+/Cl- film in order to maintain charge balance in the film as it is reduced. To investigate the extent of Cl- ejection from PEDOT+/Clfilms, 13-µm-thick, freshly polymerized oxidized PEDOT+/Clfilms were reduced at potentials corresponding to partial (ES > -0.48 V) or complete (ES < -0.48 V) reduction, and both substrate and UME tip currents were recorded in the SG/TC SECM mode. The same UME tip, positioned 13 µm above each film and held at 1.2 V, recorded the oxidation current of the ejected chloride ions as a function of time. Figure 5 shows the current-time curves from PEDOT+/Cl- films (solid line) and of the tip (dashed line) when the substrate potential was stepped from 0.2 V (the open circuit potential of the PEDOT film) to -0.6 V (A) and from 0.2 to 0.0 V (B). The substrate current decayed to zero at times greater than 3 s (A) and 8 s (B), and the tip current decayed to constant, nonzero values of ∼ -1.4 nA (A) and ∼ -1.2 nA (B) in less than 2 s (both A and B); the nonzero steady-state tip current is attributed to the convergent diffusion of Cl- to the UME tip. In contrast, for a bromide-doped polypyrrole film (PPy+/Br-) of 10 µm thickness, the PPy+/Br-covered platinum substrate and the UME tip required ∼20 s to reach zero current and steady-state current, respectively, when ES was stepped to a potential more negative than the PPy+/Brreduction potential.8d These results suggest that the release of chloride ions during the reduction of PEDOT+/Cl- is more facile than the release of bromide ions during the reduction of PPy+/ Br- despite the longer diffusion distance for chloride ions (13 µm) compared to that for bromide ions (5 µm) in reaching the UME tip positioned above the respective polymer films. This difference in release time between the two polymers will depend on the polymer structure and the film thickness and morphology (e.g., resistance). However, because of the non-cross-linking (open) structure of PEDOT,16 it appears that PEDOT is more
g
average g for the given potential step
0.2/-0.6 V
26.0 35.8 33.3 32.5 39.1
1.36 1.29 1.08 1.42 1.35
1.22 1.78 1.97 1.44 1.88
1.66
0.2/-0.4 V
20.2 19.2 18.3
0.85 0.88 0.91
1.52 1.40 1.29
1.40
0.2/-0.3 V
1.55 1.58 1.23
0.68 0.83 0.49
0.15 0.12 0.16
0.14
0.2/0 V
0.34 0.33 0.31
0.21 0.28 0.17
0.10 0.08 0.12
0.10
a
QS values represent the difference between the charge due to the potential step at the polymer-film-covered electrode and that at the corresponding bare Pt electrode in 0.1 M K2SO4. At the bare Pt electrode, charges of 25.3 µC (0.2 to -0.6 V), 21.9 µC (0.2 to -0.4 V), 20.9 µC (0.2 to -0.3 V), and 1.35 µC (0.2 to 0 V) were calculated by integrating the current-time curves over 2 s.
easily reduced than polypyrrole; consequently, the ejection of chloride ions from the PEDOT film occurs more easily. The substrate and UME tip current scales in Figure 5A are approximately 10 times larger than those in part B for the substrate and the tip, indicating that more chloride ions were released when the substrate potential was stepped to -0.6 V and PEDOT+/ Cl- was fully reduced than when it was stepped to 0 V and only partially reduced. These results are consistent with the significant increase in the anodic tip current as the substrate potential is scanned at values negative of -0.48 V, as shown in Figure 4A. The extent of reduction of PEDOT+/Cl- films can be quantified by normalizing the amount of charge collected at the tip (QT) by the amount of charge passed at an equivalent area on the substrate surface (QS*AT/AS), where QS, AT, and AS represent the amount of charge collected at the substrate, the surface area of the tip, and the surface area of the substrate, respectively. Thus, the effective collection efficiency can be defined as g ) QTAS/ QSAT.8d A series of g values was calculated by integrating currenttime curves similar to those in Figure 5 over 2 s, the time required for the tip current to reach a constant value. Table 1 contains the results for four values of the reduction potential: 0.2 V to -0.6, -0.4, -0.3, and 0 V. QS, QT, and g values were found to be essentially constant for a given magnitude of each potential step. Values of g ) 1.66 and 1.40 were obtained at -0.6 and -0.4 V, respectively, corresponding to the complete reduction of PEDOT+/Cl- and reduction in the vicinity of the reduction potential of ∼ -0.48 V, respectively; the larger-than-unity values for the effective collection efficiency g at these potentials is attributed to convergent diffusion to the UME tip. These g values indicate that there is a significant release of Cl- ions when the reduction potential is within the vicinity of the reduction potential of the polymer and then quickly falls to the much lower values of 0.14 at -0.3 V and 0.10 at 0 V, suggesting that electroneutrality is primarily maintained by anion transport when the potential is stepped to a sufficiently negative value in the vicinity of the polymer reduction potential (eq 2). The dramatic decrease in the effective collection efficiency (i.e., at -0.3 or 0 V) suggests that significant cation incorporation into the PEDOT+/Cl- film occurs at these less-negative potentials (eq 3). Therefore, both cation
10344 Langmuir, Vol. 22, No. 25, 2006
Yang and Zoski
Figure 6. SECM cyclic voltammograms of (A) PEDOT+/FCN- in a 0.1 M K2SO4 solution at a scan rate of 0.005 V s-1 and the corresponding dependences of the tip current vs substrate potential where the tip potentials were held at 0.3 V (B) to record the oxidation of FCN and at -0.1 V (C) to record the reduction of FCN- ejected from the film. The tip-substrate distance was 13 µm, and the film thickness was estimated to be 1.3 µm according to the relationship between the polymerization charge and the thickness of the film.12c
incorporation (eq 3) and anion ejection (eq 2) occur during PEDOT film reduction:20g
PEDOT+/Cl- + e- f PEDOT + Cl-
(2)
PEDOT+/Cl- + e- + K+ f PEDOT-K+Cl-
(3)
Cation incorporation during reduction has been observed at PEDOT films doped with other ions17,19 and in other polymers.7-11 These results are similar to those reported for polypyrrole films.8d Cation incorporation in a partially reduced PEDOT film (potential step of 0.2 to -0.4 V) will be discussed in a later section. Ferrocyanide (FCN) was also employed as a doping reagent during the polymerization of PEDOT to investigate anion ejection during the reduction of an initially oxidized PEDOT+/ferricyanide (PEDOT+/FCN-) film. Figure 6 shows the substrate cyclic voltammogram (A) of PEDOT+/FCN- in a 0.1 M K2SO4 solution and the corresponding UME tip current (B, C) as a function of substrate potential when the potential of the UME tip, positioned 13 µm above the PEDOT film, was held at 0.3 V (B) to oxidize ejected ferrocyanide or at -0.1 V (C) to reduce ejected ferricyanide. The substrate cyclic voltammogram (A) exhibits two cathodic peaks as the substrate potential is scanned negatively from 0.4 to -0.6 V and two anodic peaks in the reverse scan. The pair of peaks at 0.08/0.15 V is due to the reduction/ oxidation of ferricyanide incorporated into the PEDOT+/FCNfilm. The second pair of peaks at the more negative potentials of -0.43/-0.35 V is due to the PEDOT+/PEDOT redox reaction. Charge compensation for both reductions could be achieved by
the ejection of Fe(CN)64- or the incorporation of K+ ions. We were also interested in the role of Fe(CN)63- in charge compensation during the redox cycling of the PEDOT+/FCNfilm. When the UME tip potential was held constant at 0.3 V in Figure 6B, the flat baseline of the tip voltammogram on the cathodic scan shows that no appreciable ejection of ferrocyanide occurs at substrate potentials more positive than 0.2 V. At approximately 0.2 V where the reduction of ferricyanide begins to occur in the PEDOT+/FCN- film, the Fe(CN)64- oxidation tip current began to increase, reached a steady-state value at approximately 0 to -0.3 V, and increased again from -0.3 to -0.48 V, which is the peak potential of the PEDOT+/FCNreduction, where a second steady-state current was observed until the substrate potential was reversed at -0.6 V. On the reverse scan, the Fe(CN)64- oxidation tip current decreased back to zero current without reaching a steady-state value because Fe(CN)64- diffuses into the bulk solution and some is probably incorporated into the PEDOT film. In Figure 6C, the UME tip potential was held at -0.1 V to reduce any Fe(CN)63- that may be ejected as the substrate potential was cycled from 0.4 to -0.6 V. The Fe(CN)63- reduction tip current initially increased from a substrate potential of 0.4 to 0.2 V, which is the beginning of the reduction wave for Fe(CN)63- in the initial PEDOT+/FCNfilm where a maximum Fe(CN)63- reduction tip current was reached, and then decreased to and remained at zero at approximately 0 to -0.6 V as Fe(CN)63- was reduced to Fe(CN)64- in the PEDOT film. On the reverse scan from -0.6 V, the Fe(CN)63- reduction tip current remained at approximately zero until 0.05 V, where Fe(CN)64- began oxidation to Fe(CN)63- and IT increased, reaching a maximum value at 0.2 V, and then decreased as ES continued to increase positively. The UME tip voltammograms in Figure 6B and C demonstrate that during the reduction of initially oxidized PEDOT/FCNfilms Fe(CN)63- ions are ejected as the film is partially reduced until the reduction wave of Fe(CN)63- to Fe(CN)64- in the PEDOT+/FCN- film commences at 0.2 V. After this potential, Fe(CN)63- ions cease to be ejected from the now partially reduced film, and the Fe(CN)63- tip reduction current decreases to zero. Instead, the more negatively charged Fe(CN)64- ions are ejected as they are formed in the PEDOT film. This result is consistent with those reported for potentiodynamically polymerized PEDOT films doped with ferrocyanide20a and potentiostatically polymerized polypyrole (PPy) films also doped with Fe(CN)64- (PPy+/ FCN-), thus forming PPy+/FCN- films on a Pt substrate.8d Of these studies, the former also monitored the release of FCN- and concluded that only FCN release occurs. In the studies reported here, both Fe(CN)64- and Fe(CN)63- ejections were monitored at the UME tip. Unique to the studies reported here is the appearance of two steady-state waves in the UME tip voltammogram as the PEDOT+/FCN- film is reduced and Fe(CN)64is ejected and detected at the UME tip. These steady-state current plateaus occur as a result of the approximately 550 mV separation that exists between the reduction peaks of Fe(CN)63- and PEDOT+ in the PEDOT+/FCN- film. Figure 6B and C clearly shows that in a PEDOT+/FCN- film, Fe(CN)64- ejection begins approximately 100 mV before the FCN-/FCN redox potential of 100 mV and continues until the reversal potential of -600 mV is reached. In contrast, Fe(CN)63- ejection has already begun 300 mV before the FCN-/FCN redox potential, reaches a maximum 100 mV before the FCN-/FCN redox potential, and then decreases to zero as ES is scanned negatively beyond the FCN-/FCN redox potential. Very similar substrate CVs and tip voltammograms were obtained in solutions of electrolytes such as Na2SO4, KCl, NaCl, and TEACl. This is in contrast to
Polymer Films on Electrodes
Figure 7. SECM substrate (A) and tip (B, C) cyclic voltammograms of PEDOT films initially containing Ru(NH3)63+ in a 0.1 M K2SO4 solution at a scan rate of 0.01 V s-1. (A) PEDOT film prepared at 1.05 V in a solution containing 0.1 M KCl and then partially reduced at 0 V in a solution containing Ru(NH3)63+. The tip current (B) is due to the reduction of Ru(NH3)63+ ejected from PEDOT; ET ) -0.45 V. The tip current (C) is due to the oxidation of Ru(NH3)62+ ejected from the film; ET ) -0.1 V. The tip-substrate distance was 13 µm, and the film thickness was 13 µm.
polypyrole, where the nature of the cation (TEA+ vs K+) was found to have a strong effect on the release of Fe(CN)64- during the reduction of the PPy+/FCN- polymer film.8d Cation Transport during Redox Cycling of PEDOT. Cation incorporation into PEDOT+/Cl- films during reduction was investigated by partially reducing PEDOT+/Cl- films in a 0.1 M Ru(NH3)63+ solution at 0 V for 50 s by stepping ES from 0.2 to 0 V:
PEDOT+/Cl- + e- + 1 /3Ru(NH3)63+ f PEDOT[Ru(NH3)63+]1/3Cl- (4) The resulting electrode with a composition of PEDOT[Ru(NH3)63+]1/3Cl- was rinsed and used as the SECM substrate in a solution of 0.1 M K2SO4. Figure 7 shows the substrate cyclic voltammogram (A) over the potential domain of -0.1 to 0.3 V. The UME tip was biased at -0.45 V (B) to observe the diffusioncontrolled reduction of Ru(NH3)63+ ejected from the PEDOT film rather than the oxidation of chloride. We were also interesting in confirming that there was no Ru(NH3)62+ present in the polymer film over this potential range that could be ejected from a similarly prepared polymer film by holding the tip potential at -0.1 V (C) to record the diffusion-controlled oxidation of Ru(NH3)62+. A capacitance-like current was observed in Figure 7A as the CV of the substrate was recorded at a sweep rate of 0.01 V s-1 between -0.1 and 0.3 V. The Ru(NH3)63+ tip reduction current in Figure 7B increased with substrate potential from -0.06 to 0.30 V, reaching a maximum at 0.2 V on the reverse scan, and then decreased on sweeping back to the -0.16 V starting potential, demonstrating that a considerable amount of Ru(NH3)63+ was incorporated during the partial reduction of the PEDOT film. This Ru(NH3)63+ tip reduction current in B was found to increase with an increase in the reduction time in the step experiment to allow more Ru(NH3)63+ loading and was also enhanced when
Langmuir, Vol. 22, No. 25, 2006 10345
ES assumed more negative values (e.g., < -0.4 V) and decreased when ES assumed more positive values for the partial reduction loading of Ru(NH3)63+ relative to the reduction potential of the PEDOT+/Cl- film. These results demonstrate that cations are incorporated into the PEDOT film during reduction. In Figure 7C, the tip potential was set to -0.1 V to oxidize any Ru(NH3)62+ that may be present in and ejected from the PEDOT[Ru(NH3)63+]1/3Cl- polymer film. The small background current observed in Figure 7C indicates that no Ru(NH3)62+ cations were involved in charge compensation under these experimental conditions. We also looked at Ru(NH3)63+ cation adsorption on the PEDOT+/Cl- film when immersed in Ru(NH3)63+ solution. The PEDOT+/Cl- film was immersed in a 0.1 M Ru(NH3)63+ solution for 50 s without partial reduction, and the resulting electrode was used in SECM measurements in the same manner as the partially reduced films. No redox current for the Ru(NH3)62+/3+ redox pair was observed either at the PEDOT+/ Cl- substrate or at the UME tip, indicating that there was no adsorption of Ru(NH3)62+ onto PEDOT+/Cl- films. PEDOT films were also polymerized in the presence of the large polyanion poly(p-styrenesulfonate) (PEDOT+/PSS-), which is strongly bound to the polymer matrix so that cation transport dominates.17e The PEDOT+/PSS- film was then reduced at -0.4 V in a 0.1 M Ru(NH3)63+ solution for 50 s by stepping ES from 0.2 to -0.4 V:
PEDOT+/PSS- + 3/2e- + /2Ru(NH3)63+ f PEDOT[Ru(NH3)62+]1/2PSS- (5)
1
The solid line in Figure 8A shows the substrate cyclic voltammogram of the resulting polymer film in a 0.1 M K2SO4 solution. Beginning at a substrate potential of -0.6 V, the film is in the reduced form PEDOT[Ru(NH3)62+]1/2PSS- as demonstrated in eq 5. The dashed line in Figure 8A shows the cyclic voltammogram of a PEDOT+/PSS--film-coated substrate in a 0.1 M K2SO4 solution where the anodic/cathodic waves at -0.24/-0.29 V are due to the oxidation/reduction, respectively, of the PEDOT+/PSS- film. Thus, the wave at approximately -0.28 and -0.29 V in the solid line in Figure 8A corresponds to the co-oxidation/reduction of PEDOT/PSS- with the Ru(NH3)62+/3+ redox couple, and these results confirm the presence of Ru(NH3)62+/3+ cations in the PEDOT+/PSS- film on reduction. Figure 8B shows the UME tip current as a function of the substrate potential where the tip potential was held at -0.45 V to reduce ejected Ru(NH3)63+. The Ru(NH3)63+ cathodic tip current is zero at -0.6 V as expected because only Ru(NH3)62+ is present in the PEDOT[Ru(NH3)62+]1/2PSS- film at this substrate potential. This Ru(NH3)63+ reduction tip current remains at zero until -0.35 V, where it begins to increase as the oxidation wave of PEDOT/PSS-/[Ru(NH3)62+]1/2 is approached. Ru(NH3)63+ is produced in the film and ejected in order to maintain charge balance within the now partially oxidized film. This tip current increase continues though the reversal potential at 0.20 V, until 0.05 V on the reverse scan where it decreases to zero as the substrate potential is scanned back to -0.6 V. Figure 8B compared to Figure 7B shows about a 4-fold increase in the Ru(NH3)63+ tip reduction current and a lower substrate current demonstrating that extensive cation incorporation occurred during the reduction of PEDOT+/PSS- at a potential as negative as -0.4 V. In Figure 8C, the tip potential was held at -0.1 V, and the ejection of Ru(NH3)62+ was recorded as a function of substrate potential. A Ru(NH3)62+ oxidation tip current was observed at the initial potential of -0.6 V, slightly decreased as ES approached
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Figure 8. SECM substrate (A) and tip (B, C) cyclic voltammograms of PEDOT films (initially containing PSS- (- -, A), PSS-, and Ru(NH3)62+ (-, A)) in a 0.1 M K2SO4 solution at a scan rate of 0.01 V s-1. (A, -) PEDOT film prepared at 1.05 V in a solution containing 0.1 M NaPSS and then reduced at -0.4 V in a solution containing 0.1 M Ru(NH3)63+. (A, - -) PEDOT film prepared in a solution containing 0.1 M NaPSS. The tip current (B) is due to the reduction of Ru(NH3)63+ ejected from PEDOT; ET ) -0.45 V. The tip current (C) is due to the oxidation of Ru(NH3)62+ ejected from the film; ET ) -0.1 V. The tip-substrate distance was 13 µm. The thickness of PEDOT+/PSS- was estimated to be 1.7 µm according to the relationship between the polymerization charge and the thickness of the film.12a
-0.35 V, and then rapidly decreased toward zero as the oxidation peak of PEDOT/PSS-/[Ru(NH3)62+]1/2 at -0.3 V was approached and passed. The film was oxidized to PEDOT+/PSS-/ [Ru(NH3)63+]1/3, and no Ru(NH3)62+ was available for ejection. The Ru(NH3)62+ oxidation tip current remained at zero from -0.2 to 0.2 V and from 0.2 to -0.3 V on the reverse scan, whereafter the Ru(NH3)62+ tip oxidation current increased as the substrate potential was swept through the reduction potential of the PEDOT+/PSS-/[Ru(NH3)63+]1/3 film and Ru(NH3)62+ was ejected. The tip current voltammograms shown in Figure 8B and C correspond to the detection of ejected Ru(NH3)63+ at positive substrate potentials and Ru(NH3)62+ at negative substrate potentials, respectively. This is in contrast to the behavior of PEDOT+/Cl- films, which predominantly eject anions when reduced at negative potentials. Unlike PEDOT+/Cl- films, PEDOT+/PSS- films adsorb Ru(NH3)63+ and behave similarly (Figure 9A-C) to the reduced PEDOT+/PSS- films in 0.1 M Ru(NH3)63+ solution shown in Figure 8A-C. However, the tip currents at such films with adsorbed Ru(NH3)63+ are approximately 10 times smaller than those shown in Figure 8A-C. This adsorption is probably due to an ion exchange between the Na+ co-ion of PSS- (which may also be incorporated into PEDOT+/PSS- films during electropolymerization) with Ru(NH3)63+ in the bulk solution. This ion exchange (Figure 9) is different from ion incorporation (Figure 8) that is induced by charge compensation during reduction or oxidation of the polymer films.
Yang and Zoski
Figure 9. SECM substrate (A) and tip (B, C) cyclic voltammograms of a PEDOT film initially containing PSS- in a 0.1 M K2SO4 solution at a scan rate of 0.01 V s-1. (A) PEDOT film prepared at 1.05 V in a solution containing 0.1 M NaPSS and then dipped into 0.1 M Ru(NH3)63+ for 50 s. The tip current in B is due to the reduction of Ru(NH3)63+ ejected from PEDOT; ET ) -0.45 V. The tip current in C is due to the oxidation of Ru(NH3)62+ ejected from the film; ET ) -0.1 V. The tip-substrate distance was 13 µm, and the thickness of PEDOT+/PSS- was estimated to be 1.7 µm according to the relationship between the polymerization charges and the thickness of the films.12
These results demonstrate that (i) cation incorporation occurred during the partial reduction of PEDOT+/Cl- at potentials as positive as 0 V; (ii) extensive cation incorporation occurred during the reduction of PEDOT+/PSS- at a potential as negative as -0.4 V; (iii) the release of Ru(NH3)62+/3+ ions is substratepotential-dependent, depending on both the redox potential of the Ru(NH3)62+/3+ couple and the redox potentials of PEDOT+/ Cl- and PEDOT+/PSS-; (iv) Ru(NH3)63+/Ru(NH3)62+ electron transfer occurs within the PEDOT+/PSS- film; and (v) the adsorption of Ru(NH3)63+ in fully oxidized PEDOT+/PSS- films was observed and attributed to ion exchange between the Na+ co-ion of PSS- and Ru(NH3)63+ in the bulk solution.
Conclusions Ion transport into and out of PEDOT films investigated by SECM shows cation incorporation and anion release that depended on the potential applied to the polymer films, the structure of the polymer, and the redox behavior of doped ions in the polymer films during redox cycling. The main ion transport of PEDOT+/Cl- and PEDOT+/FCN- is the incorporation of cations rather than the expulsion of anions at potentials less negative than -0.2 V vs SCE. Significant anion ejection occurs at more negative potentials in the vicinity of the reduction potential of the polymer. Fe(CN)64- anions were released in two steps: as the peak potential for the reduction of Fe(CN)63- in the polymer was approached and then again as the peak potential for the reduction of PEDOT+ was approached. Fe(CN)63- anions were released at positive potentials until the reduction potential of Fe(CN)63- was reached, and reduction to Fe(CN)64- occurred within the polymer film. Similar behavior was observed for solutions containing a variety of different electrolytes (Na2SO4, NaCl, KCl, and TEACl).
Polymer Films on Electrodes
The incorporation of hexaammineruthenium(III) cations in PEDOT/Cl- accompanying a potential step from 0.2 to 0 V vs SCE was demonstrated for PEDOT doped with a small anion (chloride) and with a potential step from 0.2 to -0.4 V vs SCE for a large polyanion (poly(p-styrenesulfonate)). Ru(NH3)63+ cations were released in one step during a subsequent oxidation step (due to the overlap of the oxidation potentials for Ru(NH3)62+ and PEDOT) and monitored with an SECM tip electrode positioned above the polymer film. We also observed the release
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of Ru(NH3)62+ cations during a subsequent reduction step as the reduction potentials of Ru(NH3)63+ and PEDOT+/PSS- were approached. Acknowledgment. The support of this work by a grant from the National Science Foundation (CHE-0210315; C.G.Z.) is gratefully acknowledged. Preliminary experiments by Kothandam Krishnamoorthyare are acknowledged. LA061167U
