Modification of Indium−Tin Oxide Electrodes with Thiophene

The oxidative peak is shifted positive of its position for EDOTCA when simply ...... Groenendaal, L.; Zotti, G.; Aubert, P. H.; Waybright, S. M.; Reyn...
0 downloads 0 Views 1MB Size
Download PDF
1530

Langmuir 2007, 23, 1530-1542

Modification of Indium-Tin Oxide Electrodes with Thiophene Copolymer Thin Films: Optimizing Electron Transfer to Solution Probe Molecules F. Saneeha Marrikar, Michael Brumbach, Dennis H. Evans, Ariel Lebro´n-Paler, Jeanne E. Pemberton, Ronald J. Wysocki, and Neal R. Armstrong* Department of Chemistry, UniVersity of Arizona, Tucson, Arizona 85721 ReceiVed June 26, 2006. In Final Form: October 24, 2006 We describe the modification of indium-tin oxide (ITO) electrodes via the chemisorption and electropolymerization of 6-{2,3-dihydrothieno[3,4-b]-1.4-dioxyn-2-yl methoxy}hexanoic acid (EDOTCA) and the electrochemical copolymerization of 3,4-ethylenedioxythiophene (EDOT) and EDOTCA to form ultrathin films that optimize electrontransfer rates to solution probe molecules. ITO electrodes were first activated using brief exposure to strong haloacids, to remove the top ∼8 nm of the electrode surface, followed by immediate immersion into a 50:50 EDOT/EDOTCA comonomer solution. Potential step electrodeposition for brief deposition times was used to grow copolymer films of thickness 10-100 nm. The composition of these copolymer films was characterized by solution depletion studies of the monomers and atomic force microscopy (AFM), X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy (reflection-absorption infrared spectroscopy (RAIRS)) of the product films. The spectroscopic data suggest that the composition of the copolymer approaches 80% EDOTCA when electropolymerization occurs from concentrated (10 mM) solutions. AFM characterization shows that electrodeposited poly(EDOT)/poly(EDOTCA) (PEDOT/PEDOTCA) films are quite smooth, with texturing on the nanometer scale. RAIRS studies indicate that these films consist of a combination of EDOTCA units with noninteracting -COOH groups and adjacent hydrogen-bonded -COOH groups. The EDOTCA-containing polymer chains appear to grow as columnar clusters from specific regions, oriented nearly vertically to the substrate plane. As they grow, these columnar clusters overlap to form a nearly continuous redox active polymer film. ITO activation and formation of these copolymer films enhances the electroactive fraction of the electrode surface relative to a nonactivated, unmodified “blocked” ITO electrode. Outer-sphere solution redox probes (dimethylferrocene) give standard rate coefficients, kS g 0.4 cm‚s-1, at 10 nm thick copolymer films of PEDOT/PEDOTCA, which is 3 orders of magnitude greater than that on the unmodified ITO surface and approaches the values for kS seen on clean gold surfaces.

Introduction The chemical modification of transparent conducting oxide (TCO) electrodes, such as indium-tin oxide (ITO), affects the performance of electrochemical sensors, photoelectrochemical transducer layers, organic light-emitting diodes (OLEDs), and organic photovoltaic cells (OPVs).1-17 These modifications * To whom correspondence should be addressed. E-mail: nra@ u.arizona.edu. (1) Yang, Y.; Heeger, A. J. Appl. Phys. Lett. 1994, 64, 1245-1247. (2) Karg, S.; Scott, J. C.; Salem, J. R.; Angelopoulos, M. Synth. Met. 1996, 80, 111-117. (3) Cao, Y.; Yu, G.; Zhang, C.; Menon, R.; Heeger, A. J. Synth. Met. 1997, 87, 171-174. (4) Kim, J. S.; Granstrom, M.; Friend, R. H.; Johansson, N.; Salaneck, W. R.; Daik, R.; Feast, W. J.; Cacialli, F. J. Appl. Phys. 1998, 84, 6859-6870. (5) Kugler, T.; Salaneck, W. R. C. R. Acad. Sci., Ser. IV: Phys. Astrophys. 2000, 1, 409-423. (6) Malinsky, J. E.; Veinot, J. G. C.; Jabbour, G. E.; Shaheen, S. E.; Anderson, J. D.; Lee, P.; Richter, A. G.; Burin, A. L.; Ratner, M. A.; Marks, T. J.; Armstrong, N. R.; Kippelen, B.; Dutta, P.; Peyghambarian, N. Chem. Mater. 2002, 14, 30543065. (7) Marks, T. J.; Veinot, J. G. C.; Cui, J.; Yan, H.; Wang, A.; Edleman, N. L.; Ni, J.; Huang, Q.; Lee, P.; Armstrong, N. R. Synth. Met. 2002, 127, 29-35. (8) Book, K.; Bassler, H.; Elschner, A.; Kirchmeyer, S. Org. Electron. 2003, 4, 227-232. (9) Tengstedt, C.; Crispin, A.; Hsu, C. H.; Zhang, C.; Parker, I. D.; Salaneck, W. R.; Fahman, M. Org. Electron. 2005, 6, 21-33. (10) Armstrong, N. R.; Carter, C.; Donley, C.; Simmonds, A.; Lee, P.; Brumbach, M.; Kippelen, B.; Domercq, B.; Yoo, S. Y. Thin Solid Films 2003, 445, 342-352. (11) Heuer, H. W.; Wehrmann, R.; Kirchmeyer, S. AdV. Funct. Mater. 2002, 12, 89-94. (12) Granqvist, C. R. Nat. Mater. 2006, 5, 89-90. (13) Rosseinsky, D. R.; Mortimer, R. J. AdV. Mater. 2001, 13, 783-793. (14) Doherty, W. J.; Wysocki, R. J.; Armstrong, N. R.; Saavedra, S. S. J. Phys. Chem. B 2006, 110, 4900-4907.

address the issues of poor heterogeneous electron-transfer rates on these TCO surfaces,10,18 which translates into poor potentiometric or amperometric sensor responses16,17,19,20 and high series resistances and high onset voltages for both forward and reverse bias organic thin film diodes using the TCO film as the bottom contact.10,18,21 The underlying problem in the use of ITO films in electrochemical and condensed phase device technologies arises from the heterogeneous surface composition and topology of these films, coupled with their chemical reactivity toward atmospheric contaminants.10-13,18-27 For ITO, a clean, stoichio(15) Doherty, W. J.; Armstrong, N. R.; Saavedra, S. S. Chem. Mater. 2005, 17, 3652-3660. (16) McBee, T. W.; Wang, L. Y.; Ge, C. H.; Beam, B. M.; Moore, A. L.; Gust, D.; Moore, T. A.; Armstrong, N. R.; Saavedra, S. S. J. Am. Chem. Soc. 2006, 128, 2184-2185. (17) Ge, C. H.; Doherty, W. J.; Mendes, S. B.; Armstrong, N. R.; Saavedra, S. S. Talanta 2005, 65, 1126-1131. (18) Carter, C.; Brumbach, M.; Donley, C.; Hreha, R. D.; Marder, S.; Domercq, B.; Yoo, S.; Kippelen, B.; Armstrong, N. R. J. Phys. Chem. B, in press. (19) Lindfors, T.; Ivaska, A. J. Electroanal. Chem. 2002, 535, 65-74. (20) Marikkar, F. S. Ph.D. Dissertation, University of Arizona, Tucson, AZ, 2006. (21) Kim, J. S.; Ho, P. K. H.; Thomas, D. S.; Friend, R. H.; Cacialli, F.; Bao, G. W.; Li, S. F. Y. Chem. Phys. Lett. 1999, 315, 307-312. (22) Donley, C.; Dunphy, D.; Paine, D.; Carter, C.; Nebesny, K.; Lee, P.; Alloway, D.; Armstrong, N. R. Langmuir 2002, 18, 450-457. (23) Armstrong, N. R.; Simmonds, A.; Veneman, A.; Xia, W.; Marikkar, F. S.; Brumbach, M.; Schulmeyer, T.; Singh, P. S.; Lee, P. To be submitted for publication. (24) Schulmeyer, T.; Brumbach, M.; Armstrong, N.R. To be submitted for publication. (25) Nuesch, F.; Rothberg, L. J.; Forsythe, E. W.; Le, Q. T.; Gao, Y. L. Appl. Phys. Lett. 1999, 74, 880-882. (26) Nuesch, F.; Forsythe, E. W.; Le, Q. T.; Gao, Y.; Rothberg, L. J. J. Appl. Phys. 2000, 87, 7973-7980.

10.1021/la061840f CCC: $37.00 © 2007 American Chemical Society Published on Web 12/13/2006

Modifying ITO Electrodes Via EDOT/EDOTCA Films

metric surface can be created by sputter deposition; however, reactions with atmospheric H2O (hydrolysis to form hydroxide and oxyhydroxide surface products), CO, and CO2 immediately create a surface whose electroactivity (measured by conductivetip atomic force microscopy (C-AFM)) varies widely on the submicron scale.23,24 ITO electrodes also possess a rich acid/ base chemistry and widely varying surface dipoles.25-27 Recent work from our group has shown that as little as 10-50% of the geometric area of the ITO surface can support electron transfer for a chemisorbed redox active molecule.18,22 Pretreatments that increase the electroactive surface area and increase the rates of electron transfer to solution probe molecules are predicted to enhance the performance of either OLED or OPV devices built on the modified ITO surface.10,18 Several successful strategies have been developed for the modification of ITO and related oxide surfaces, including the addition of modifiers through covalent bond formation (e.g., silanes)28-30 and the chemisorption of carboxylic acids, phosphonic acids, and thiols.22,31-33 The recent studies of Zotti and co-workers,34 involving carboxylic acid- and phosphonic acidterminated thiophene monomers on ITO, are particularly notable. Their modification method starts with thiophene monomers that are functionalized at the 2-position with alkyl-COOH or alkylPO3 groups, which provide for attachment at the ITO surface and ensure that polymerization occurs perpendicular to the ITO surface, forming “brush-like” polymer chains. This work is directly related to other surface modification schemes that provide for conducting polymers and/or electrolyte polymer “brushes”.35-37 Another modification alternative involves spin-casting thin films of a dispersion of oligomeric poly(3,4-ethylenedioxythiophene) and higher molecular weight poly(styrene sulfonate) (PEDOT/PSS), which planarizes electrode surfaces, improves both the solid state and solution electroactivity, and may increase the effective work function of ITO electrodes.3,5,38-46 These sulfonic acid-based polymer mixtures may also create an (27) Swint, A. L.; Bohn, P. W. Appl. Phys. Lett. 2004, 84, 61-63. (28) Hatton, R. A.; Day, S. R.; Chesters, M. A.; Willis, M. R. Thin Solid Films 2001, 394, 292-297. (29) Huang, Q. L.; Evmenenko, G. A.; Dutta, P.; Lee, P.; Armstrong, N. R.; Marks, T. J. J. Am. Chem. Soc. 2005, 127, 10227-10242. (30) Yan, H.; Lee, P.; Armstrong, N. R.; Graham, A.; Evmenenko, G. A.; Dutta, P.; Marks, T. J. J. Am. Chem. Soc. 2005, 127, 3172-3183. (31) Brumbach, M.; Armstrong, N. R. Encyclopedia of Electrochemistry; Rubenstein, I., Fujihira, M., Rusling, J., Eds.; Modified Electrodes, Vol. 10; Wiley-VCH: New York, 2006. (32) Kondo, T.; Takechi, M.; Sato, Y.; Uosaki, K. J. Electroanal. Chem. 1995, 381, 203-209. (33) Gardner, T. J.; Frisbie, C. D.; Wrighton, M. S. J. Am. Chem. Soc. 1995, 117, 6927-6933. (34) Zotti, G.; Zecchin, S.; Vercelli, B.; Berlin, A.; Grimoldi, S.; Groenendaal, L.; Bertoncello, R.; Natali, M. Chem. Mater. 2005, 17, 3681-3694. (35) Biesalski, M.; Ruhe, J. Macromolecules 2004, 37, 2196-2202. (36) Zhou, F.; Liu, W. M.; Hao, J. C.; Xu, T.; Chen, M.; Xue, Q. J. AdV. Funct. Mater. 2003, 13, 938-942. (37) Shah, R. R.; Merreceyes, D.; Husemann, M.; Rees, I.; Abbott, N. L.; Hawker, C. J.; Hedrick, J. L. Macromolecules 2000, 33, 597-605. (38) Jonsson, S. K. M.; Salaneck, W. R.; Fahlman, M. J. Electron Spectrosc. Relat. Phenom. 2004, 137-140, 805-809. (39) Higgins, A. M.; Jukes, P. C.; Martin, S. J.; Geoghegan, M.; Jones, R. A. L.; Cubitt, R. Appl. Phys. Lett. 2002, 81, 4949-4951. (40) de Jong, M. P.; van IJzendoorn, L. J.; de Voigt, M. J. A. Appl. Phys. Lett. 2000, 77, 2255-2257. (41) Wong, K. W.; Yip, H. L.; Luo, Y.; Wong, K. Y.; Lau, W. M.; Low, K. H.; Chow, H. F.; Gao, Z. Q.; Yeung, W. L.; Chang, C. C. Appl. Phys. Lett. 2002, 80, 2788-2790. (42) de Kok, M. M.; Buechel, M.; Vulto, S. I. E.; van de Weijer, P.; Meulenkamp, E. A.; de Winter, S. H. P. M.; Mank, A. J. G.; Vorstenbosch, H. J. M.; Weijtens, C. H. L.; van Elsbergen, V. Phys. Status Solidi A: Appl. Res. 2004, 201, 13421359. (43) Yoo, D.; Shiratori, S. S.; Rubner, M. F. Macromolecules 1998, 31, 43094318. (44) Crispin, X.; Marciniak, S.; Osikowicz, W.; Zotti, G.; van der Gon, A. W. D.; Louwet, F.; Fahlman, M.; Groenendaal, L.; De Schryver, F.; Salaneck, W. R. J. Polym. Sci., Part B: Polym. Phys. 2003, 41, 2561-2583.

Langmuir, Vol. 23, No. 3, 2007 1531

acidic microenvironment near the ITO surface, leading to the etching of indium from the ITO surface and impacting the longterm stable operation of devices such as OLEDs.42-44 Chemical activation is often used to pretreat ITO surfaces before further modification and device formation. Oxygen plasma etching in particular appears to be quite effective for many OLED and OPV technologies,5 but still leaves large regions of the ITO surface with less than optimum electronic activity.23 We have recently observed that commercial ITO electrodes can be activated using brief exposures to concentrated solutions of strong acids such as HCl and HI, both of which have been used in the lithographic processing of ITO films.23,47,48 These brief acid exposures remove hydrolysis products and carbonaceous contaminants from the ITO surface, after which both small molecules and polymers can be adsorbed to higher coverages, despite the reformation of some contaminant layers almost immediately. C-AFM studies have shown dramatic differences between the electrical activity before and after acid activation of ITO and some enhancement in electrochemical performance.23 We show in this publication that brief etching of the ITO surface with 5.5M HI, followed by chemisorption of alkylcarboxylic acid derivatives of 3,4-ethylenedioxythiophene (EDOT) and the electrochemical formation of copolymer films of PEDOT and PEDOTCA, results in an ITO surface with optimized electroactivity toward a solution probe molecule such as dimethylferrocene (DMFc), with electron-transfer rates approaching those seen on clean metal (gold) surfaces. Both asreceived and activated/modified ITO electrodes are modeled as partially blocked electrodes using protocols first proposed by Amatore and Save´ant,49-52 and we show that the apparent fraction of the geometric area that is electroactive is increased from less than 1% for the as-received electrodes to well over 50-80% for the acid-activated electrodes with thin PEDOT/PEDOTCA copolymer films. Experimental Materials. Unless otherwise stated, all chemicals were supplied by Aldrich and used without purification. EDOTCA was synthesized at the University of Arizona Chemical Synthesis Facility. Monomer solutions were made in 0.1 M LiClO4 dry acetonitrile. Only 18 mΩ of Millipore water was used in the entire experiment. EDOTCA Synthesis. The compound 6-{2,3-dihydrothieno[3,4b]-1.4-dioxyn-2-yl methoxy}hexanoic acid (EDOTCA) was prepared in 96% yield by the reaction of 2,3-dihydrothieno[3,4-b]-1.4-dioxyn2-yl methanol (EDTM) with excess sodium hydride and 6-bromohexanoic acid in N,N-dimethylformamide (DMF) at room temperature. The above yield is reported after chromatography. All reactions were performed under an atmosphere of argon (Ar) in a glass tube sealed with a threaded poly(tetrafluoroethylene) stopper equipped with a polypropylene O-ring. EDTM (ethylene dioxy-methanol) was prepared by the published method.53 All commercial materials were (45) Jonsson, S. K. M.; Birgerson, J.; Crispin, X.; Greczynski, G.; Osikowicz, W.; van der Gon, A. W. D.; Salaneck, W. R.; Fahlman, M. Synth. Met. 2003, 139, 1-10. (46) Higgins, A. M.; Martin, S. J.; Jukes, P. C.; Geoghegan, M.; Jones, R. A. L.; Langridge, S.; Cubitt, R.; Kirchmeyer, S.; Wehrum, A.; Grizzi, I. J. Mater. Chem. 2003, 13, 2814-2818. (47) Vandenmeerakker, J. E. A. M.; Baarslag, P. C.; Scholten, M. J. Electrochem. Soc. 1995, 142, 2321-2325. (48) Scholten, M.; Vandenmeerakker, J. E. A. M. J. Electrochem. Soc. 1993, 140, 471-475. (49) Amatore, C.; Save´ant, J. M.; Tessier, D. J. Electroanal. Chem. 1983, 147, 39-51. (50) Amatore, C.; Save´ant, J. M.; Tessier, D. J. Electroanal. Chem. 1983, 146, 37-45. (51) Holt, K. B.; Bard, A. J.; Show, Y.; Swain, G. M. J. Phys. Chem. B 2004, 108, 15117-15127. (52) Duo, I.; Fujishima, A.; Comninellis, C. H. Electrochem. Commun. 2003, 5, 695-700. (53) Lima, A.; Schottland, P.; Sadki, S.; Chevrot, C. Synth. Met. 1998, 93, 33-41.

1532 Langmuir, Vol. 23, No. 3, 2007 used as-is without further purification. Dri-Solv DMF, 6-bromohexanoic acid, hexanes, ethyl acetate, diethyl ether (ether), and Geduran Silica Gel 60 (SiO2 for flash chromatography54) were purchased from VWR International. Sodium hydride (95% dry powder) was purchased from Sigma-Aldrich. Deuterochloroform (CDCl3) was obtained from Cambridge Isotopes. Proton nuclear magnetic resonance (1H NMR) spectra were recorded on a Bruker AM-250 instrument using XwinNMR for satellite data processing. Mass Spectra were obtained from the University of Arizona Department of Chemistry Mass Spectrometry and Proteomics Facility. Electrospray ion mass spectra (ESI-MS) were obtained on a Thermoelectron (Finnigan) LCQ Classic instrument. Fast atom bombardment mass spectrometry (FABMS) and high-resolution FABMS (HRFABMS) were obtained on a JEOL HX110A instrument. A solution of 6-bromohexanoic acid (352 mg, 1.8 mM, 1.1 equiv) in dry, argon-purged DMF (5 mL) was treated with sodium hydride (NaH, 95%, 100 mg, 3.96 mM, 1.1 equiv) at room temperature. The mixture was stirred at room temperature until the effervescence ceased. The EDTM (277 mg, 1.61 mM) was dissolved into dry DMF (20 mL) and this solution was added to the suspension of sodium 6-bromohexanoate and unreacted sodium hydride in DMF. The reaction was stirred at room temperature (2 h). The reaction mixture was treated with additional NaH (100 mg) and 6-bromohexanoic acid (25 mg), and the reaction mixture was stirred (72 h). The reaction mixture was poured onto water (50 mL) and was extracted with diethyl ether (1 × 50 mL) The aqueous layer was acidified with 6 N hydrochloric acid and was extracted with ether (3 × 50 mL). The ether layers were combined, washed with brine, dried (MgSO4), and concentrated in vacuo. Flash chromatography (SiO2, 65 g, 2.5 cm × 15 cm; 20% ethyl acetate in hexanes eluent) afforded the product as a colorless oil (490 mg, 495 mg theor, 96%). 1H NMR (250 MHz, CDCl3) δ 6.33 and 6.32 (AB, Jab ) 3.7 Hz, 2H, thiophene -H), 4.33-4.20 (m, 2H), 4.08-4.00 (m, 1H), 3.70-3.55 (m, 2H), 3.49 (t, J ) 6.4 Hz, hexanoic C-6 H), 2.36 (t, J ) 7.4 Hz, 2H, hexanoic C-2 H), 1.71-1.55 (m, 4H), 1.46-1.36 (m, 2H). ESI-MS (negative) m/z 285.1 (base, M-H+), HRFABMS (m-nitrobenzyl alcohol, PEG 300 matrix) m/z: calculated for C13H18O5S, 286.0875; observed, 286.0867. ITO Cleaning and Chemical Activation. Plasma Cleaning. ITO/ glass substrates (Colorado Concept Coatings, LLC, Longmont, CO; sheet resistance ) ∼15 Ω/square) were ultrasonically cleaned for 15 min in a dilute Triton X-100 (Aldrich) aqueous solution, 15 min in water, and 15 min in ethanol, followed by drying in a stream of N2. Plasma etching was performed in a plasma cleaner (Harrick-60 W) with a partial pressure of O2 for 10 min.55 HI Etching. The same initial cleaning steps were used except for plasma cleaning, followed by flooding the surface with 5.5 M HI mounted on a spin coater (Integrated Technologies, Inc.) for 10 s and rinsing with water while spinning at 4000 rpm. These surfaces were immediately modified through the chemisorption of EDOTCA, after drying under a stream of nitrogen.23 ITO Modification. EDOTCA Chemisorption. Activated ITO samples were immersed in 10 mM solutions of EDOTCA in ethanol for the desired chemisorption time. Following chemisorption, the samples were immersed and rinsed thoroughly with acetonitrile and dried under a stream of nitrogen. Electropolymerization. Electropolymerization of PEDOT/PEDOTCA films was performed with a potentiostat/galvanostat (model 283, EG&G Instruments, Princeton, NJ) in a three-electrode Teflon (custom 3-port) electrochemical cell with a Ag/AgCl (in saturated KCl) reference electrode in 0.1 M LiClO4 acetonitrile solutions using various EDOT and EDOTCA monomer concentrations. A coiled platinum wire served as the counterelectrode. For the initial studies, (54) Still, W. C.; Kahn, M.; Mitra, A. J. Org. Chem. 1978, 43, 2923-2925. (55) Marikkar, F. S.; Brumbach, M.; Wang, L.; Carter, C.; Ge, C.; Evans, D. H.; Saavedra, S. S.; Armstrong, N. R. To be submitted for publication. (56) Beamson, G., Briggs, D. High Resolution XPS of Organic Polymers: The Scienta ESCA300 Data Base; John Wiley & Sons: New York, 1992. (57) Khan, M. A.; Armes, S. P.; Perruchot, C.; Ouamara, H.; Chehimi, M. M.; Greaves, S. J.; Watts, J. F. Langmuir 2000, 16, 4171-4179.

Marrikar et al. the area exposed to the electrolyte was 0.78 cm2. Electropolymerization was carried out by voltammetric scans between -0.8 V and 1.1 V versus Fc/Fc+ or chronoamperometry with a potential step at 0.75 V versus Fc/Fc+. Spectroelectrochemical Characterization of PEDOT/PEDOTCA Films. Transmission UV-vis spectra were obtained for PEDOT and PEDOTCA polymer-modified electrodes with a CCD array spectrophotometer (Spectral Instruments, Inc., Tucson, Arizona), while continuously changing the electrode potential at 5 mV/s, using the spectroelectrochemical cell described above with an electrode area of 0.78 cm2, in 0.1 M LiClO4 acetonitrile solutions. AFM Characterization of Electrodeposited Films. The morphology and root-mean-square (rms) roughness of poly(thiophene) films were obtained from phase and height images collected using an atomic force microscope (model Nanoscope, Dimension 3100, Digital Instruments, Santa Barbara, CA). The images were collected in intermittent contact mode with ultrasharp silicon cantilevers (model TESP-7). The particular cantilever used was 125 µm in length with a resonance frequency of 200-400 Hz and a spring constant of 20-100 N/m (Veecoprobes, Santa Barbara, CA). Images were obtained in ambient air at a scan rate of 1.46 Hz. The AFM images reported here are 1 × 1 µm scans. The thickness of the PEDOT/ PEDOTCA copolymer film was obtained from height images by making a “hole”, exposing the underlying ITO surface. The edge of a sharp clean razor blade was used to completely remove the film in this area without scraping the ITO substrate. AFM imaging of bare ITO, scratched using similar experimental conditions, did not show any damage on the surface. It was important to avoid the high ridges formed at the step edges from the buildup of material, during the film removal process, to obtain film thickness. The step height was obtained by making a line scan using the section analysis tool and measuring the vertical distance by locating the arrows in the center of the “hole” on the substrate and a flat region on the film (away from the ridges). The values reported here are averages of several different regions. The minimum thickness measured was at least 3 times greater than the rms roughness of the film and substrate, such that errors due to surface roughness were kept minimal. The surface feature heights were determined from section analysis of height images, while the diameter and widths were obtained from phase images. X-ray Photoelectron Spectroscopic Characterization of PEDOT/PEDOTCA Thin Films. All measurements were conducted in a combined UPS-XPS spectrometer (Kratos Axis-Ultra) with an average base pressure of 10-9 Torr. Several modified samples on ITO were examined in succession during each day of analysis to provide for statistically relevant comparisons between experiments. XPS data were collected with monochromatic Al(KR) radiation (1486.6 eV) with an analyzer pass energy of 20 eV. A 70° takeoff angle was used to enhance the surface sensitivity. The area sampled in XPS analysis is approximately 300 by 600 microns. The surface composition was obtained from a fractional concentration of relevant elements (% A) as calculated below: % A ) [(IA/SA)/

∑ (I /S )] x 100 i i

i

(1)

where I and S are the integrated peak areas and the sensitivity factors, respectively. All XPS data were curve fit using Kratos software using Gaussian peaks. Linear background subtraction was mostly utilized and had to be occasionally replaced by Shirley baseline corrections whenever the former operation was found to be inadequate. Previous comparable XPS polymer characterization studies were utilized to obtain the best possible peak fitting and proposed peak assignments.38,44,56,57 Fourier Transform Infrared (FT-IR) Characterization of PEDOT/PEDOTCA Polymer Films. A dry-air-purged Nicolet 550 spectrometer with a liquid nitrogen-cooled MCT detector was used to obtain FT-IR spectra. Reflection-absorption infrared spectroscopy (RAIRS) spectra were obtained using a FT-80 fixed 80° grazing angle accessory (Spectra-Tech). A wire grid polarizer (Molectron Corp., IGP-225) in the p-polarized geometry was used in the RAIRS analysis of modified ITO electrodes. Thin film PEDOTCA/PEDOT

Modifying ITO Electrodes Via EDOT/EDOTCA Films

Langmuir, Vol. 23, No. 3, 2007 1533

Figure 1. Synthetic scheme for the creation of EDOTCA (2) from EDOT methanol, or EDTM (1). samples on ITO were reduced using a potential step at -0.8 V versus Fc/Fc+ for at least 5 min and removed from 0.1 M LiClO4 acetonitrile solutions under potential control. Reduced samples were immediately analyzed (after rinsing with dry acetonitrile and drying under a stream of nitrogen). These spectra were collected with an aperture setting of 50 at 4 cm-1 resolution, and were the summation of 500 scans. Isotropic samples were made after scraping reduced films off the ITO surface, grinding with KBr, and pressing into pellets for transmission IR analysis. Only 100 scans were required for transmission spectra for an aperture setting of 50 at 4 cm-1 resolution. Determination of Monomer Insertion Ratios. The percentage of incorporated monomer during electropolymerization from the 50% comonomer solution was determined by monitoring the monomer concentration before and after electropolymerization.58 The potential of an ITO electrode (area 0.78 cm2) was stepped to 0.75 V versus Fc/Fc+ in a 0.05 mM solution of EDOT/EDOTCA monomer for 45 s. Changes in monomer concentration were monitored by LC-MS analyses, in the Proteomics Facility, College of Pharmacy, performed on a Finnigan MAT TSQ 7000 (Thermoelectron, San Jose, CA) equipped with an HP 1050 HPLC (HewlettPackard/Agilent Technologies). Separations were obtained using a Vydac C-18 column, 5 µm particle size, at a flow rate of 0.5 mL/ min, going from 5 to 95% solvent B. Solvent B was made up of 0.1% trifluoroacetic acid (TFA) in acetonitrile/water (9:1), and solvent A was made up of TFA in water/acetonitrile (98:2). For quantitation and calibration curve generation, an internal standard of 3,3′bithiophene was added to each sample prior to injection into the column. Detection was carried out at 257 nm with an HP 1050 variable wavelength detector (Hewlett-Packard/Agilent Technologies, Germany). This procedure resulted in baseline-resolved chromatographic peaks with approximate retention times of 11, 15, and 18 min for EDOT, EDOTCA, and 3,3′-bithiophene, respectively. Electron-Transfer Rates Determined from Cyclic Voltammetry. For the determination of electron-transfer rates of solution probe molecules, voltammetry was conducted on a small area ITO electrode (0.04 cm2), in 1 mM solutions of Fc, DMFc, or DECMFc, 0.25 M LiClO4 acetonitrile solutions.59,60 Solution resistance effects were quantified by characterizing the voltammetric peak separations for Fc/Fc+, DMFc/DMFc+, and DECMFc/DECMFc+ on an electrochemically cleaned gold electrode of the same area. The consensus value for the standard heterogeneous rate constant for oxidation of ferrocene at platinum or clean gold electrodes in acetonitrile is ∼1-3 cm/s.61 We therefore used the anodic-cathodic peak separation for the probe redox couples on gold, as a function of sweep rate, to extract a value for the uncompensated solution resistance in this cell, RU ) ∼600 Ω. We also used digital simulations of these redox process on gold, from the theoretical treatment of Nicholson,59 extended to smaller values of rate constants using digital simulation (DigiSim, Bioanalytical Systems), to extract a similar estimate for solution resistance. For the electrode areas used in these experiments and the currents achieved in our voltammetric experiments, we observed that the sheet resistance of the ITO electrodes contributed less than 1 mV corrections to any of the voltammograms shown in this paper. Full details of these correction procedures can be found in ref 20. (58) Doherty, W. J.; Wysocki, R. J.; Armstrong, N. R.; Saavedra, S. S. Macromolecules 2006, 39, 4418-4424. (59) Nicholson, R. S. Anal. Chem. 1965, 37, 1351-1355. (60) Bard, A. J.; Faulkner, L. J. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; John Wiley & Sons: New York, 2001.

Figure 2. (a) Schematic view of the proposed chemisorption and polymerization processes for EDOTCA on ITO. Modes of chemisorption may include (1) hydrogen bonding, (2) chelation and ester formation at metal cation defect sites, and (3) multilayer formation (H-bonded networks). We also indicate the cross-linking of adsorbed EDOTCA monomers, as well as polymer growth from the most electroactive sites. (b) Schematic of the proposed potential step electropolymerization process using 50:50 EDOTCA/EDOT comonomer solutions, in contact with the EDOTCA-modified ITO surface. “Brush-like” polymer growth is proposed from both from short chainlength EDOTCA oligomers and from single-monomer anchors, from the most electroactive regions on the ITO surface.

Results and Discussion Electrochemical Reactions of Adsorbed EDOTCA. The synthetic scheme for the formation of EDOTCA from EDTM is shown in Figure 1, and the details of this synthesis are described in the Experimental Section. A six-carbon alkyl-carboxylic acid group was attached to EDTM to give EDOTCA. EDOTCA was designed to chemisorb to ITO with flexibility and sufficient length in the alkyl chain to provide for cross-linking within the monomer layer (to form PEDOT-like surface oligomers; Figure 2a), or copolymerization with EDOT or EDOTCA monomers in solution (to form PEDOT-like or PEDOT/PEDOTCA copolymers; Figure 2b). (61) Fawcett, W. R.; Opallo, M. Angew. Chem., Int. Ed. Engl. 1994, 33, 21312143.

1534 Langmuir, Vol. 23, No. 3, 2007

Marrikar et al.

Figure 3. Voltammetric characterization of EDOTCA monolayers (25 mV/s in 0.1 M LiClO4 acetonitrile solution) after chemisorption from a 10 mM EDOTCA/EtOH solution. (a) The first, third, fifth, and seventh voltammetric cycles are shown after a 12 h chemisorption period, on air-plasma cleaned ITO, showing increasing electroactivity of chemisorbed EDOTCA as a result of cross-linking. (b) Voltammetric scans obtained after 12 h of chemisorption of EDOTCA on (1) air-plasma-cleaned ITO and (2) HI-etched ITO. (c) Scans obtained after 1 h of chemisorption of EDOTCA on HI-etched ITO before (1) and after (2) monomer incorporation from a 10 µM 50% EDOT/EDOTCA comonomer solution. (d) Scans obtained after 1 h of chemisorption of EDOTCA on ITO (1) before and (2) after monomer incorporation from a 10 mM EDOT solution (one cycle to +1.1 V).

We first demonstrate that EDOTCA can be cross-linked within the adsorbed monolayer to form electroactive oligomeric units (Figure 3). As per previous modification protocols,34,37,62,63 the monomer was chemisorbed from concentrated solutions (10 mM in EtOH), for adsorption times up to 12 h, to ensure that a closepacked monolayer was formed.20 Other chemisorption experiments from this group have suggested that the adsorption of carboxylic acids to air or oxygen-plasma-cleaned ITO surfaces is marked by an initial fast adsorption (∼50% of a monolayer) followed by a slower adsorption process, and involves some etching of the ITO surface, producing trace amounts of free indium in solution.18,64 Figure 3a shows the voltammetric response of adsorbed EDOTCA on detergent/solvent/air-plasma-cleaned ITO, as a function of the number of voltammetric cycles used to achieve cross-linking of the adsorbed monomer. Following 12 h of EDOTCA adsorption (in ethanol) on the cleaned ITO surface, the potential was swept several times to +1.1 V versus Fc/Fc+, in acetonitrile, with no monomer in solution.65 In between these polymerization scans, a scan from -0.5 to -0.2 V was conducted, and Figure 3a shows those voltammograms after one, three, five, and seven polymerization sweeps. Each successive polymerization sweep increased the electroactivity of cross-linked EDOTCA, up to a limiting equivalent coverage of 2.7 × 10-11 mol/cm2 (determined coulometrically, assuming n ) 1), retaining this (62) Meyer, T. J.; Meyer, G. J.; Pfennig, B. W.; Schoonover, J. R.; Timpson, C. J.; Wall, J. F.; Kobusch, C.; Chen, X. H.; Peek, B. M.; Wall, C. G.; Ou, W.; Erickson, B. W.; Bignozzi, C. A. Inorg. Chem. 1994, 33, 3952-3964. (63) Napier, M. E.; Thorp, H. H. Langmuir 1997, 13, 6342-6344. (64) Carter, C. Ph.D. Dissertation, University of Arizona, Tucson, AZ, 2006. (65) Groenendaal, L.; Zotti, G.; Aubert, P. H.; Waybright, S. M.; Reynolds, J. R. AdV. Mater. 2003, 15, 855-879.

electroactivity after multiple voltammetric scans. Simple packing density calculations suggest that we can achieve cross-linking of ∼10-20% of a close-packed monolayer of EDOTCA units to produce the electroactive oligomeric chemisorbed PEDOT monolayer. The chemisorption process was accelerated, and the coverage of cross-linkable EDOTCA was doubled (Figure 3b Scan 2) when the ITO electrode was first etched in 5.5 M HI for 10 s, followed by rinsing in pure water, drying in a stream of nitrogen, and immediate immersion into a 10 mM EDOTCA/EtOH monomer solution. Such an aggressive etch of the ITO surface, when carefully controlled, removes less than ∼8 nm of material, and, in this case, increased the voltammetrically active coverage of EDOTCA to ∼5.8 × 10-11 mol/cm2.23 The potential for the oxidation of this cross-linked material shifted negatively by ∼0.1 V, suggesting a much more electroactive surface, with a lower overpotential for oxidation of the cross-linked oligomers. We next demonstrate the growth of PEDOT/PEDOTCA copolymer on the EDOTCA-modified ITO surface. Figure 3c shows the voltammetric activity for adsorbed EDOTCA on an HI-etched ITO surface, before (scan 1) and after (scan 2) three polymerization scans to +1.1 V, in a dilute (10 µmol) 50:50 EDOT/EDOTCA comonomer solution. The increase in oxidative voltammetric activity (3.4 × 10-10 mol/cm2) suggests the incorporation of either or both monomers into a thin copolymer film (see below). We next created an adsorbed EDOTCA monolayer, without cross-linking, and then explored the electropolymerization with only EDOT monomers in solution (Figure 3d). An HI-etched ITO surface was exposed to a 10 mM EDOTCA/EtOH solution for 1 h. Before electropolymerization, a barely detectable coverage

Modifying ITO Electrodes Via EDOT/EDOTCA Films

Langmuir, Vol. 23, No. 3, 2007 1535

Figure 4. (a,b) Schematic views of a PEDOT dimer and an EDOTCA monomer, with proposed assignments for the carbon and sulfur atoms, which can be distinguished by high-resolution XPS. C(1s) and S(2p) spectra are shown for films grown on ITO substrates for (c,d) 100% PEDOT (100 nm, voltammetrically grown), (e,f) 100% PEDOTCA (50 nm, voltammetrically grown), and (g,h) the mixed polymer PEDOT/ PEDOTCA film (10 nm) grown by potential step to +0.75 V, 45 s from the 50:50 comonomer solution (see text). The peak assignments given for the schematic views in a and b are given following refs 56 and 57. The unassigned C(1s) peak at high binding energy is attributable to a π f π* shake-up. Each S(2p) spectrum has been curve-fit to account for the presence of both the S(2p3/2) and S(2p1/2) peaks. The peaks labeled 8b′ and 8b′′ are believed to arise from oxidized (polaronic) states of the polymer in at least two different domains (ref 57). Overall, these data suggest that the copolymer film, grown from the 10 mM 50:50 comonomer solution, is enriched in EDOTCA units and remains partially oxidized after immersion from the electrochemical cell.

was noted (Scan 1 - 1.7 × 10-11 mol/cm2). One voltammetric cycle to +1.1 V was next conducted with only 10 mM EDOT in solution, which increased the electroactive coverage (scan 2) to 4.2 × 10-10 mol/cm2 of electroactive polymer. The oxidative peak is shifted positive of its position for EDOTCA when simply adsorbed to the HI-etched ITO surface, and there is a small shoulder at approximately -0.35 V, suggesting that the electroactivities of the adsorbed, partially cross-linked monolayer and the polymer layer attached to it are energetically resolved. Further voltammetric cycling to +1.1 V, or potential steps to potentials between +0.75 and +1.1 V provided for much thicker copolymer films. Characterization of PEDOT, PEDOTCA, and PEDOT/ PEDOTCA Films. The composition of single-component and copolymer films was characterized using films created both by voltammetric and potential step polymerization. The voltammetric scans for an electropolymerization of ∼50-200 nm films of 100% PEDOT, 100% PEDOTCA, and copolymers of PEDOT/ PEDOTCA are shown in the Supporting Information, Figure S1. The onset potential for polymerization and the i/V behavior are strongly dependent upon the solution monomer composition.20 Single voltammetric sweeps to +1.1 V, followed by coulometric analysis of the voltammetric response from +0.5 to -0.75 V, shows surface coverages of electroactive polymer ranging from 19 nmol/cm2 for polymerization from the 10 mM 100% EDOT solution, down to 4 nmol/cm2 for polymerization from the 10 mM 100% EDOTCA solution. Voltammetric polymerization from the 50:50 (5 mM EDOT/5 mM EDOTCA) solution produced an electroactive polymer surface coverage of 17 nmol/cm2. From the point of view of their electrochemical activity, however, better quality films were obtained by potential step polymerization, at potentials where electropolymerization was kinetically con-

trolled. For these experiments, the clean ITO electrode was stepped from 0.0 to +0.75 V for 45 s (Figure S1, Supporting Information), using 50:50 (5 mM EDOT/5 mM EDOTCA) solutions. At this potential, the rates of PEDOTCA and PEDOT film formation appeared to be nearly equal. Studies of monomer consumption from EDOT/EDOTCA acetonitrile comonomer solutions were conducted as described recently for the formation of other PEDOT-like copolymers.58 Potential step polymerization was carried out in small volumes of 0.05 mM EDOT/EDOTCA solutions, such that a measurable monomer depletion could be detected by HPLC of the electrolyte solution. HPLC analysis suggested that EDOT and EDOTCA are incorporated with equal efficiency into the PEDOT/ PEDOTCA polymer;20 however, these studies had to be done at low comonomer concentrations, in contrast to the 10 mM comonomer concentrations that produced films that optimized the redox activity for solution probe molecules, discussed below. XPS and FT-IR (transmission and RAIRS) experiments on films deposited by potential step from 10 mM comonomer solutions, however, suggest that the EDOTCA mole fraction in the copolymer is higher than 50%. Figure 4 show high-resolution XPS data for both the C(1s) and S(2p) regions of 100% PEDOT, 100% PEDOTCA, and PEDOT/PEDOTCA copolymer films. The spectral resolution was sufficient to see the contribution made by the C(1s) peaks attributable to the alkyl side chain carbons and the carbon atoms in or adjacent to the carboxylic acid portion of EDOTCA in addition to the carbon expected in the actual thiophene polymer chain.20,56 Each C(1s) peak was fit with at least three major components using the assignments suggested in refs 57 and 66 as a guide. In the C(1s) spectra, Component 1 (and, where appropriate, the unresolved Component 4 - 285.5 eV) originates from a combination of the carbon

1536 Langmuir, Vol. 23, No. 3, 2007

atoms adjacent to sulfur in the polymer and the carbon atoms in the alkyl chain; Component 2 (and, where appropriate, Components 5 and 6 - 286.8 eV) arises from carbon atoms in the thiophene core adjacent to the ether oxygen atoms and from carbon atoms in the alkyl tail adjacent to oxygen atoms. Component 3 (and, where appropriate, Component 7 - 288.3 eV) are assumed to arise from carbon atoms in the ethylenedioxy chain and carbon adjacent to the carboxylic acid in the side chain. The weak feature at 290.1 is attributed to a π f π* “shakeup” signal at the thiophene ring.56,57 The S(2p) spectral region was complicated by the presence of the poorly resolved S(2p1/2,3/2) doublet.57 Three sets of doublet peaks were required to fit the S(2p) envelope (Figure 4d,f,h). Khan and co-workers assigned the binding energies of the S(2p) signal from 164-165.2 eV and 165.8-167.2 eV to the neutral S and cationic S+, respectively (Components 8a and 8b′, respectively).66-69 Fitting of these peaks was more easily achieved if an additional set of higher binding energy S(2p) peaks was assumed (Component 8b′′) suggesting additional heterogeneity in the oxidized (doped) regions of the polymer thin film. On the basis of these assignments, the fractional concentration of polaronic states is highest for the copolymer film (∼66%), versus the 100% PEDOT film (∼40%) and the 100% PEDOTCA film (∼33%) (Table S1, Supporting Information). From consideration of the integrated spectral intensities in both the C(1s) and S(2p) envelopes for pure PEDOT,57 pure PEDOTCA, and the PEDOT/ PEDOTCA copolymer, we conclude that the fraction of EDOTCA (x) in the potential step-deposited PEDOTy/PEDOTCAx copolymer (x + y ) 1) is 0.5-0.8.20 Figure 5 shows the FT-IR characterization for all three polymers deposited on HI-etched ITO (RAIRS data, using p-polarized radiation; Figure 5a-c,f) and for powders harvested from electropolymerized thin films (Figure 5d,e). The strong asymmetric and symmetric νC-H bands appear both from the alkyl-side arm and the EDOT ring at ∼2950 cm-1 and ∼2860 cm-1, respectively.67,70,71 The absorbance bands between ∼1250 and 1527 cm-1 mainly arise from νC-C and νCdC stretching vibrations in the EDOT ring.72,73 The ethylene-dioxy group dominates the transitions at ∼920-1186 cm-1.72-75 Table S2 in the Supporting Information summarizes these peak assignments. Examining the intensities of the symmetric and asymmetric νC-H region near 2800-3000 cm-1 and the resolved νCdC region near 1527 cm-1 in the isotropic transmission spectra (details in ref 20), we conclude once again that the mixed polymer is at least 50% EDOTCA units. The unresolved nature of most of the peaks prevent us from accurately determining the average chain orientations within the polymer films. Figure 5d,e shows the νC-O bands attributable to free -COOH groups and hydrogen-bonded -COOH groups, along with νC-O (66) Hwang, J.; Tanner, D. B.; Schwendeman, I.; Reynolds, J. R. Phys. ReV. B 2003, 67, 115205-1-115205-10. (67) Garreau, S.; Louarn, G.; Buisson, J. P.; Froyer, G.; Lefrant, S. Macromolecules 1999, 32, 6807-6812. (68) Kvarnstrom, C.; Neugebauer, H.; Blomquist, S.; Ahonen, H. J.; Kankare, J.; Ivaska, A.; Sariciftci, N. S. Synth. Met. 1999, 101, 66-66. (69) Kvarnstrom, C.; Neugebauer, H.; Ivaska, A.; Sariciftci, N. S. J. Mol. Struct. 2000, 521, 271-277. (70) Meng, H.; Perepichka, D. F.; Bendikov, M.; Wudl, F.; Pan, G. Z.; Yu, W. J.; Dong, W. J.; Brown, S. J. Am. Chem. Soc. 2003, 125, 15151-15162. (71) Socrates, G. Infrared and Raman Characteristic Group Frequencies: Tables and Charts, 3rd ed.; John Wiley & Sons: New York, 2001. (72) Hernandez, V.; Ramirez, F. J.; Otero, T. F.; Navarrete, J. T. L. J. Chem. Phys. 1994, 100, 114-129. (73) Louarn, G.; Mevellec, J. Y.; Buisson, J. P.; Lefrant, S. Synth. Met. 1993, 55, 587-592. (74) Damlin, P.; Kvarnstrom, C.; Ivaska, A. J. Electroanal. Chem. 2004, 570, 113-122. (75) Murugan, A. V.; Quintin, M.; Delville, M. H.; Campet, G.; Vijayamohanan, K. J. Mater. Chem. 2005, 15, 902-909.

Marrikar et al.

bands that could probably be attributable to -COO- forms of the carboxylic acid group (overlapped with -OH bending modes due to entrapped H2O).76-80 The release of two protons per monomer during the heterocycle polymerization may increase the acidity within the film, thus preventing the deprotonation of -COOH groups, and therefore adsorbed moisture may be the main contributor in this region.58 The free perchlorate counteranions within the films are known to have two IR active bands at 1102 cm-1 (asymmetric stretch) and 624 cm-1 (asymmetrical bending).81 Additional bands are introduced after intermolecular bonding interactions, which is highly probable in oxidized PEDOT films due to counterion stabilization.82 However, these contributions from ClO4- can be masked by the ethylene-dioxy group transitions in the same region. The p-polarized RAIRS data for a 100% PEDOTCA film in Figure 5f show that the fraction of νC-O bands attributable to H-bonding interactions has decreased, suggesting that these interactions occur mainly in the substrate plane, and are therefore not easily detected in the RAIRS experiment. These H-bonding interactions would appear, therefore, to be an important factor in the stabilization of “columns” or “wires” of PEDOTCA and PEDOT/PEDOTCA copolymer films, as they grow out from the ITO surface. AFM Characterization of ITO with Adsorbed EDOTCA and Ultrathin Films of Copolymerized PEDOT/PEDOTCA. Figures 6 and 7 summarize the tapping-mode AFM images (height and phase contrast) of clean ITO (both detergent-cleaned/airplasma-etched and HI-etched) and HI-etched ITO surfaces over which we electropolymerized ultrathin films of PEDOT/PEDOTCA. Figure 6a,b focuses on detergent-cleaned and acidetched ITO films. Figure 6c-e focuses on HI-etched ITO electrodes with cross-linked adsorbed EDOTCA monolayers and ultrathin copolymer films created by potential step polymerization. Figure 7a-c focuses on comparisons of thicker voltammetrically polymerized PEDOT, PEDOTCA, and PEDOT/PEDOTCA copolymer films, compared with an electrode that is representative of optimized potential step polymer growth conditions (Figure 7d). The AFM images of the clean ITO samples (Figure 6a) consist of crystalline grains with an average diameter of ∼36 nm, with occasionally larger grains up to ∼60 nm in diameter, and an rms roughness on a 1 × 1 µm scale of ∼3.0 nm. The etching of these ITO films with 5.5M HI for ∼10 s (Figure 6b) decreases the ITO thickness by no more than 8 nm, and the rms roughness in this case was nearly unchanged. Other ITO samples we have explored, with initial surface roughnesses of ∼1 nm, increase in surface roughness after HI etching to ∼2-3 nm. The adsorption of EDOTCA from 10 mM EDOTCA/ethanol solutions for periods of 12 h on the HI-etched ITO surface (Figure 6c) creates a somewhat smoother surface (rms roughness of 2 nm), with larger apparent grain sizes (up to ∼86 nm in diameter). Figure 6d is the AFM image for an HI-etched ITO surface exposed to a 10 mM EDOTCA/ethanol solution for 1 h, followed by potential step electropolymerization in acetonitrile to cross-link the adsorbed EDOTCA monomer. Figure 6e is the AFM image from (76) Smith, E. L.; Alves, C. A.; Anderegg, J. W.; Porter, M. D.; Siperko, L. M. Langmuir 1992, 8, 2707-2714. (77) Gershevitz, O.; Sukenik, C. N. J. Am. Chem. Soc. 2004, 126, 482-483. (78) Dong, J.; Tsubahara, N.; Fujimoto, Y.; Ozaki, Y.; Nakashima, K. Appl. Spectrosc. 2001, 55, 1603-1609. (79) Chen, P. J.; Wallace, R. M.; Henck, S. A. J. Vac. Sci. Technol., A 1998, 16, 700-706. (80) Wu, N. Q.; Fu, L.; Su, M.; Aslam, M.; Wong, K. C.; Dravid, V. P. Nano Lett. 2004, 4, 383-386. (81) Cvjeticanin, N. D.; Mentus, S. Phys. Chem. Chem. Phys. 1999, 1, 51575161. (82) Ramaswamy, S.; Sridhar, B.; Ramakrishnan, V.; Rajaram, R. K. Acta Crystallogr., Sect. E: Struct. Rep. Online 2001, 57, O1149-O1151.

Modifying ITO Electrodes Via EDOT/EDOTCA Films

Langmuir, Vol. 23, No. 3, 2007 1537

Figure 5. RAIRS spectra for electrochemically grown polymer films (all potential steps to 0.75 V, 180 s): (a) 100% PEDOTCA, (b) the mixed polymer grown from the 50:50 comonomer solution, and (c) 100% PEDOT. All films grown on ITO electrodes to a thickness of ∼60 nm. The regions indicated with an (*) indicate the CR ) Cβ transition at ∼1527 cm-1 arising from the thiophene ring, while the νC-H region near 2800-3000 cm-1 has contributions from both the EDOT ring and the alkyl chain20 (see text). (d,e) Expanded views of the transmission FT-IR spectral region for electrochemically grown polymer films: (d) 100% PEDOTCA and (e) the mixed polymer grown from the 50:50 comonomer solution. (f) RAIRS (p-polarized) spectrum of the 100% PEDOTCA film.76-78

an HI-activated ITO surface exposed for 1 h to a 10 mM EDOTCA solution in ethanol, followed by potential step electropolymerization (180 s at 0.75 V) in the presence of 50:50 EDOT/EDOTCA (10 mM). The average grain size seen in both the height and phase channels does not appear to vary significantly; however,

there is some “texturing” of the surface of each grain, consistent with an ultrathin film of polymer at the termination of each grain. Figure 7a shows the crystalline, “layered” features found in 100% PEDOT films (rms roughness ) 88 nm), where the layering

1538 Langmuir, Vol. 23, No. 3, 2007

Marrikar et al.

Figure 7. AFM images (tapping mode in air; 1 × 1 µm; left column ) height images, right column ) phase (60°) images), for voltammetrically polymerized films of (a) 100% PEDOT (200 nm height scale), (b) 100% PEDOTCA (50 nm height scale), and (c) PEDOT/PEDOTCA films grown from a 10 mM 50:50 comonomer solution (100 nm height scale). The films in a-c were grown by four voltammetric cycles to +1.1 V vs Fc/Fc+. (d) As for Figure 6e, a PEDOT/PEDOTCA copolymer film grown from a 10 mM EDOT/ EDOTCA solution on an HI-etched ITO surface, by a potential step to +0.75 V for 45 s (50 nm height scale); this protocol provided the PEDOT/PEDOTCA-modified ITO surface with the lowest rms roughness (2-3 nm) and the highest rates for electron transfer for the DMFc/DMFc+ redox couple (see text).

Figure 6. AFM (tapping mode, in air) of clean and modified ITO (1 × 1 µm) 25 nm height scale (left) and 60° phase scale (right). The inset in the phase contrast image is a zoomed region (0.25 × 0.25 µm). (a) ITO sample cleaned by ultrasonic cleaning in detergent (H2O)/ethanol followed by cleaning in an air plasma (60 W, 10 min). (b) ITO electrode cleaned by ∼10 s exposure to 5.5 M HI/ water. (c) ITO electrode treated as that in b, followed by exposure to 10 mM EDOTCA/EtOH solution for 12 h, before electrochemical cross-linking of the adsorbed monomers as discussed in Figure 3b, scan #2. (d) ITO treatment as in b with exposure to the EDOTCA/ EtOH solution for 1 h, followed by a potential step to 0.75 V vs Fc/Fc+. (e) ITO treatment as in d with polymerization in the presence of a 10 mM 50:50 EDOT/EDOTCA comonomer solution; potential step duration ) 180 s (see also Figure 7d).

of these films is especially noticeable in the phase image.83-86 This type of “layering” of the PEDOT film is especially

pronounced when voltammetric cycles (versus potential steps) are used to produce this thin film. Figure 7b shows that 100% PEDOTCA films produce a much smoother surface (rms roughness ) ∼2 nm). Figure 7c shows that the voltammetrically grown copolymer film exhibits some of the roughness and layering of the 100% PEDOT films. Figure 7d, which was grown in the same manner as the film in Figure 6e, shows that a potential step polymerization (+0.75 V), immediately after immersion in the (83) Sakmeche, N.; Aeiyach, S.; Aaron, J. J.; Jouini, M.; Lacroix, J. C.; Lacaze, P. C. Langmuir 1999, 15, 2566-2574. (84) Aasmundtveit, K. E.; Samuelsen, E. J.; Pettersson, L. A. A.; Inganas, O.; Johansson, T.; Feidenhans, R. Synth. Met. 1999, 101, 561-564. (85) Breiby, D. W.; Samuelsen, E. J.; Groenendaal, L.; Struth, B. J. Polym. Sci., Part B: Polym. Phys. 2003, 41, 945-952. (86) Kim, J.; Kim, E.; Won, Y.; Lee, H.; Suh, K. Synth. Met. 2003, 139, 485-489.

Modifying ITO Electrodes Via EDOT/EDOTCA Films

Langmuir, Vol. 23, No. 3, 2007 1539

Figure 8. (a-c) Voltammetric responses for 1 mM DMFc/DMFc+ in CH3CN on ITO surfaces at various sweep rates after modification with (a) cross-linked monomer films of chemisorbed EDOTCA (as in Figures 3a and 6c); (b) films of PEDOTCA/PEDOT grown by potential step electropolymerization from 10 mM 50/50 comonomer solutions (45 s, +0.75 V, as in Figure 7d); (c) a thin film of spin-cast PEDOT/PSS (∼60 nm thickness);10 and (d) the same PEDOT/PEDOTCA copolymer film on ITO as in b, showing the voltammetric response for a 1 mM solution of Fe(CN)6-4. (e) The voltammetric peak potential separation in a 1 mM DMFc 0.1 M LiCLO4 acetonitrile solution as a function of scan rate for various ITO surface pretreatments, and modification with PEDOT/PEDOTCA copolymer films after correction for solution resistance: (1) detergent/EtOH/air-plasma-cleaned ITO (correlates with Figure 6a); (2,3,5) HI-etched ITO followed by a 1-h exposure to 10 mM EDOTCA (EtOH) solution, followed by a potential-step polymerization in acetonitrile in the presence of a 10 mM 50:50 comonomer solution of EDOTCA/EDOT (+0.75 V; (2) ) 45 s, (3) ) 90 s, (5) ) 180 s (as for Figure 6e)); (4) an HI-etched (only) ITO electrode; (6) the most optimized PEDOT/PEDOTCA/ITO electrode: HI-etched ITO, immersed immediately in a 50:50 10 mM comonomer solution, followed by a 45 s potential step polymerization of PEDOT/PEDOTCA (correlates with the AFM image in Figure 7d).

comonomer solution, produces the thinnest, smoothest, and most uniform copolymer films (∼10 nm, as determined by AFM imaging of a scratched polymer film). The AFM images in Figures 6e and 7d are representative of several 1 × 1 µm electrode areas sampled at least 50 microns apart, on several modified ITO electrodes. Electron Transfer of Solution Probe Molecules at PEDOT/ PEDOTCA-Modified ITO Surfaces. The highest rates of electron transfer to a solution probe molecule, occurring through conducting polymer thin films, are expected to be seen in the potential region where the PEDOT/PEDOTCA copolymer

achieves its most conductive (“polaron”) state.66,87,88 The transmission spectroelectrochemical characterization of optimized PEDOT/PEDOTCA films show that this state is achieved in the potential region from -0.6 to 0.0 V versus Fc/Fc+ (Figure S2, Supporting Information). Some enhancement in the electrontransfer rate coefficient, kS, was obtained for ITO electrodes modified only with cross-linked EDOTCA monolayers (as discussed in Figure 3), and these data are shown in the Supporting (87) Chen, X. W.; Inganas, O. J. Phys. Chem. 1996, 100, 15202-15206. (88) Chiu, W. W.; Travas-Sejdic, J.; Cooney, R. P.; Bowmaker, G. A. Synth. Met. 2005, 155, 80-88.

1540 Langmuir, Vol. 23, No. 3, 2007

Marrikar et al.

Figure 9. Schematic view of the ITO activation processes using acid etching and PEDOT/PEDOTCA copolymer film growth. (a) Solventcleaned ITO electrode, without activation, showing a few well-separated electrically active sites at which DMFc/DMFc+ redox chemistry occurs. (b) Copolymer “brush-like” growth from these active sites increases the electrochemical activity toward DMFc/DMFc+ minimally. (c) Activation of the ITO electrode with HI produces a higher surface coverage of electrochemically active sites. (d) We propose that the growth of the PEDOT/PEDOTCA ultrathin films from such an activated ITO surface facilitates the overlap of polymer chains (which may further interact through interchain H-bonding), forming a nearly continuous electroactive surface.

Information, Figure S3. We examined the redox behavior for Fc/Fc+, DMFc/DMFc+, and DECMFc/DECMFc+, and found that the DMFc/DMFc+ redox couple, with E° ) -0.11 V vs Fc/Fc+, showed the greatest enhancement in the electron-transfer rate following ITO modification with PEDOT/PEDOTCA copolymer films, consistent with the fact that the redox chemistry for DMFc occurs in the potential region where these polymer films achieve doping levels of 80-100%, but are not overoxidized. Figure 8 summarizes the enhanced voltammetric behavior for DMFc/DMFc+ as a function of modification of the ITO surface, by haloacid activation and by optimization of the potential step copolymerization of PEDOT/PEDOTCA thin films. In order to estimate solution resistance contributions to peak potential separation (∆Ep), we first recorded voltammograms for DMFc at clean gold electrodes of the same area as for the ITO experiments, and, assuming the kS for DMFc/DMFc+ is 1-4 cm‚s-1 at clean gold and platinum electrodes,61 we estimated a solution resistance for our electrochemical cell configuration of

∼600 Ω. We determined this value of RS by both measuring the distortion in the DMFc/DMFc+ voltammograms at various sweep rates and by comparing those voltammograms with voltammetric data generated using the program DigiSim. Good agreement was achieved for both methods. Once the solution resistance was known, all values of ∆Ep used in determinations of kS were corrected for this effect. Small electrode areas were also used in these studies to minimize currents and the effects of ITO sheet resistance, solution resistance, contact resistance, and so forth.89-91 Figure 8a-c shows the voltammetric responses for DMFc/ DMFc+ on a 100% PEDOTCA/ITO surface, an optimized PEDOT/PEDOTCA copolymer film on ITO, and a spin-cast PEDOT/PSS film on ITO. Figure 8d shows the voltammetric response for Fe(CN)6-3/Fe(CN)6-4 on the PEDOT/PEDOTCA film from Figure 8b. Figure 8e shows values of ∆Ep (solution resistance corrected) versus sweep rate (V) for various DMFc/ (89) Harrar, J. E.; Pomernac, C. L. Anal. Chem. 1973, 45, 57-79. (90) Britz, D. J. Electroanal. Chem. 1978, 88, 309-352. (91) Strojek, J. W.; Kuwana, T. J. Electroanal. Chem. 1968, 16, 471-483.

Modifying ITO Electrodes Via EDOT/EDOTCA Films

Langmuir, Vol. 23, No. 3, 2007 1541

Table 1. Electron-Transfer Rate Coefficient, kS, for the DMFc/DMFc+ Redox Couple as a Function of Modification Conditions for the Creation of PEDOT/PEDOTCA Copolymer/ITO Surfaces apparent electron-transfer rate constant, kS (cm/s) × 103 at various sweep rates treatmenta

0.01(V/s)

0.1(V/s)

1.0(V/s)

1.5(V/s)

30.0

30.0

ITO detergent/ethanol/air-plasma cleaned 0.5 ,0.5 0.6

bare ITO (A) (as for Figure 6c, prior to cross-linking) (B) (as for A, followed by voltammetric cross-linking, as for Figure 3a). (C) exposed to 10 mM 50:50 comonomer solution with a potential step to +0.75 V, 45 s

bare ITO after HI-etch as for B above (D) 1 h chemisorption in EDOTCA, followed by potential step polymerization in a 1 mM 50:50 comonomer solution, for 45 s (E) as for D, potential step polymerization in a 10 mM 50:50 comonomer solution, for 45 s as for E, potential step polymerization for 90 s as for E, potential step polymerization for 180 s (as shown in Figure 6e) as for (C) electropolymerization for 45 s, starting with the HI-etched ITO as for (C) polymerization by potential step in a 10 mM 100% EDOT solution, 45 s PEDOT:PSS (∼40 nm) spin-cast PEDOT:PSS (∼12 nm) spin-cast

(reversible)

30.0

ITO-HI etched 4.1 1.4 1.6 2.5 5.0 20.0

20.0

(reversible)

(reversible)

(reversible)

g 750.0-400.0 (reversible)

4.0 25.0

4.2 25.0

3.8 25.0

0.9 4.4 (reversible)

a A ) 12 h exposure of the cleaned ITO electrode to a 10 mM EDOTCA/ethanol solution. B ) the same as A followed by voltammetric cross-linking (10 voltammetric scans, up to +1.1 V)

DMFc+ voltammograms, on bare and modified ITO electrodes, showing the dramatic differences achievable as the growth of the PEDOT/PEDOTCA copolymer film is optimized. In Figure 8a, the voltammetric responses for DMFc correlate to the ITO surfaces created in the same manner as those in Figures 3a and 6c (kS g 1.4 × 10-3 cm‚s-1). Figure 8b (kS g 0.4 cm‚s-1) correlates with ITO surfaces modified as those in Figures 7d and 8e (plot #6). Rates of electron transfer for DMFc/DMFc+ on this surface are nearly as high as those seen on clean gold. A schematic view of the stages of ITO activation and PEDOT/ PEDOTCA copolymer film growth is shown in Figure 9. Low kS values seen for the ITO surface with only chemisorbed and cross-linked layers of EDOTCA (Figure 8a) may arise due to monomer cross-linking predominantly at the active sites on the ITO surface, without substantially increasing the effective electrode area and/or the number of available mediating sites, compared to the bare ITO electrode. We believe the changes in kS for DMFc/DMFc+ at ITO surfaces with cross-linked monolayers of EDOTCA in the presence of EDOT/EDOTCA comonomer solutions in Figure 8e (scans 2, 3, and 5) are due to the formation of “transition surfaces” between cross-linked patches and true brush-like polymer growth. As a comparison to other ITO electrodes modified with commercially available, doped thiophene polymers, we tested HI-etched ITO electrodes over which we spin-cast 60 nm films of commercial PEDOT/PSS (Figure 8c and Table 1). There is a clear enhancement of kS for such modified surfaces (0.4 × 10-2 cm‚s-1 e kS e 2.5 × 10-2 cm‚s-1), depending upon PEDOT/ PSS film thickness, but the enhancement is not a great as that achieved on our optimized growth PEDOT/PEDOTCA copolymer films. We also explored the voltammetric behavior for 100% PEDOT films on HI-etched ITO, of comparable thickness (Table 1), and here we were able to achieve kS values near 10-3 cm‚s-1.20 These lower rates may arise from ineffective wiring to the active sites

(random polymer precipitation in the absence of chain anchors34,92) and a decrease in the charge diffusion rate through the film (decrease in the free volume within the polymer chains relative to the alkyl-substituted EDOTCA93). Although activation of ITO surfaces with strong haloacids produces the most uniform PEDOT/PEDOTCA films and the greatest enhancement in kS for solution redox probes, we have recently observed that this enhancement can be significantly impacted by the near-surface composition of the as-received commercial ITO.20, 23 Several different batches of ITO have been recently explored, all nominally doped to identical Sn/In ratios of 1:10. XPS characterization of these electrodes, however, showed that they were tin-rich in their near-surface regions, with Sn/In ratios varying from Sn/In ) 0.15 to Sn/In ) 0.19. The larger Sn/In ratio ITO samples, even when only cleaned by detergent/water and ethanol, consistently produce higher values for kS for the DMFc/DMFc+ redox couple (kS ) 0.8 × 10-3 cm‚s-1 versus kS ) 2.5 × 10-5 cm‚s-1) and higher densities of PEDOT/PEDOTCA “growth features” after a potential step polymerization process (4 × 107 cm-2 versus 2 × 106 cm-2). One must conclude from these studies that the near-surface tin concentration is critical in controlling the electrochemical properties of an ITO electrode, especially toward electrodeposition of conducting polymers. Indeed, C-AFM studies, on the ∼20 nm2 distance scale, show that electrical activity is substantially enhanced for these higher Sn/In ratio ITO surfaces.23 PEDOT/ PEDOTCA copolymer films grown from nonactivated ITO electrodes, with the higher Sn/In near-surface compositions, showed substantial enhancements in kS for DMFc/DMFc+ (kS ) 1.7 × 10-2 cm‚s-1 versus 2.5 × 10-3 cm‚s-1), as would be expected for an array of “nanoelectrodes” produced on otherwise (92) Ho, K. C.; Yeh, W. M.; Tung, T. S.; Liao, J. Y. Anal. Chim. Acta 2005, 542, 90-96. (93) Gaupp, C. L.; Welsh, D. M.; Rauh, R. D.; Reynolds, J. R. Chem. Mater. 2002, 14, 3964-3970.

1542 Langmuir, Vol. 23, No. 3, 2007

Marrikar et al.

blocked electrode surfaces. Further characterization of these ITO electrodes with variable Sn/In near-surface compositions will be reported elsewhere. Estimation of the Apparent Activation at the Partially Blocked ITO Electrode. Copolymer growth observed at the least activated ITO electrodes discussed above forms a classic partially blocked electrode that can be modeled with a protocol first proposed by Amatore and Save´ant for electrodes composed of an array of microscopic electroactive regions.49-51,94,95 When the average distance between two electroactive sites (2Ro) is small compared to the diffusion layer thickness (δ), that is, Ro , δ, the voltammograms obtained in the presence of probe molecules in solution will change from a sigmoidal to a normal peak shape,96,97 and the measured standard rate coefficient for electron transfer, kS, will be related to the maximum possible electron-transfer rate (e.g., at a clean noble metal electrode), kET, and the fraction of the electrode surface that is electrically active (1 - θ):

kS ) kET(1 - θ)

(2)

Assuming a maximum electron-transfer rate for DMFc/DMFc+ of ∼1.0 cm‚s-1, (1 - θ) for each type of ITO electrodes was estimated. The detergent/ethanol-cleaned ITO surface (Figure S4a, Supporting Information, before copolymer modification) showed (1 - θ) ) ∼0.0008, increasing to (1 - θ) ) ∼0.004 after HI-etching (Figure 8e, #4). The addition of PEDOT/ PEDOTCA copolymer films to these ITO surfaces, regardless of pretreatment, gives values of (1 - θ) ) ∼0.003. After PEDOT/ PEDOTCA modification of detergent/ethanol-cleaned ITO, (1 - θ) ) 0.003, and after PEDOT/PEDOTCA modification of air-plasma-cleaned ITO, (1 - θ) ) 0.03. Finally, potential step growth of PEDOT/PEDOTCA copolymer on the HI-etched ITO surface (Figures 7d and 8b) gives values of 0.4 e (1 - θ) e 0.75. To our knowledge, these are the largest values of kS yet determined for an outer-sphere redox probe on modified or unmodified TCO electrodes.

Conclusions Chemisorbed monolayers of carboxy-terminated modifiers, such as PEDOTCA, on activated ITO surfaces can clearly impart a significant improvement in the electrochemical properties of electrodes, which have heretofore given significantly lower electron-transfer rates to outer-sphere redox processes, such as the one-electron oxidation/reduction of DMFc. These studies also show that the normal, as-received ITO samples, even those (94) Rubinstein, I. Physical Electrochemistry Principles, Methods, and Applications; Marcel Dekkar, Inc.: New York, 1995. (95) Fujishima, A.; Masuda, H.; Honda, K.; Bard, A. J. Anal. Chem. 1980, 52, 682-685. (96) Ji, X. P.; Jin, B. K.; Ren, J. J.; Jin, J. Y.; Nakamura, T. J. Electroanal. Chem. 2005, 579, 25-31. (97) Davies, T. J.; Compton, R. G. J. Electroanal. Chem. 2005, 585, 63-82.

with high dopant densities, should be considered as “partially blocked” electrodes, with as little as a few percent of the ITO surface capable of participating in electron-transfer events with low concentrations of solution probe molecules. Figure 9 summarizes the various ITO activation procedures utilized to enhance charge-transfer rates to probe molecules such as DMFc. The ITO surfaces obtained with only a detergent/ethanol cleaning process (Figure 9a) show the lowest rates of electron transfer, and induce the growth of a few isolated conducting polymer “nanoelectrodes” (Figure 9b and Figure S4, Supporting Information) that are spaced apart at least by ca. g 500 nm (Figure 9a,b). The formation of electroactive “wires and cables” may occur at these active sites, since EDOT or EDOTCA monomer electropolymerization, starting with adsorbed EDOTCA, will be most efficient at these active sites. HI etching increases the active site density and induces overlap of polymer “clusters” protruding from adjacent active sites (Figures 9c,d), exposing a continuous active surface, to the approaching redox active molecules in solution. Conducting polymer growth from these activated surfaces, however, must still be optimized with respect to polymer film thickness in order to achieve optimized values of kS for probe molecules like DMFc. This suggests that the internal resistance of the polymer “wires” may be significant, and that continuous polymer films with thicknesses less than 10-20 nm are required for both optimized electroanalytical applications and condensed phase devices such as OLEDs and OPVs. Recent applications of ITO electrodes in OLED and OPV devices also indicate a less than optimum electronic activity, without activation, and that, even after activation or creation of ITO in an ultrahigh vacuum environment, the composition quickly changes to introduce tunneling barriers and passive regions.22,23 Our long-term goal is to create modification protocols for ITO and related TCO surfaces that mitigate these compositional problems without resorting to polymer thin films with low effective pKa’s, which may further destabilize the oxide surface. The formation of “brush-like” PEDOT/PEDOTCA copolymers, on HI-activated ITO surfaces, appears to be a viable first step toward that goal. These approaches also appear to be viable for the electrochemical “wiring” of electroactive dendrimers and ligand-protected nanoparticles to TCO surfaces.14,15 Acknowledgment. This work was supported by a grant from the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Research, U.S. Department of Energy, DE-FG03-02ER15378, the Office of Naval Research, the National Science Foundation (NRA ) CHE 0517963; DHE ) NSF CHE0347471), and the NSF-Center for Materials and Devices for Information Technology, DMR-0120967. Supporting Information Available: Additional figures further characterizing the polymer films discussed in this paper. This material is available free of charge via the Internet at http://pubs.acs.org. LA061840F