In-Plane Growth of Poly(3,4-ethylenedioxythiophene) Films on a

Apr 17, 2018 - Alternating current (AC) bipolar electropolymerization of 3,4-ethylenedioxythiophene (EDOT) using a gold (Au) wire as a bipolar electro...
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Letter Cite This: ACS Macro Lett. 2018, 7, 551−555

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In-Plane Growth of Poly(3,4-ethylenedioxythiophene) Films on a Substrate Surface by Bipolar Electropolymerization Tempei Watanabe,† Masato Ohira,‡ Yuki Koizumi,‡ Hiroki Nishiyama,† Ikuyoshi Tomita,† and Shinsuke Inagi*,† †

Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8502, Japan ‡ Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226-8502, Japan S Supporting Information *

ABSTRACT: Alternating current (AC) bipolar electropolymerization of 3,4-ethylenedioxythiophene (EDOT) using a gold (Au) wire as a bipolar electrode (BPE) on a substrate surface resulted in gradual growth of the corresponding poly(3,4-ethylenedioxythiophene) (PEDOT) thin film from the terminals of the Au wire on the substrate. Studies to clarify the polymerization behavior were conducted under various electrolytic conditions, including monomer concentration, applied frequency, monomer structure, and substrate material. This method could be used to draw conducting polymer films on a nonconductive substrate, guided by an applied external electric field, and thus has potential for circuit patterning in organic electronic devices.

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of a BPE, and subsequent dendritic propagation of the PEDOT deposition is due to the electrophoretic effect of the charged PEDOT in the direction of the external electric field. The PEDOT fiber morphology could be controlled from the dendritic form to the linear (one-dimensional) form by employing a microspace surrounding BPEs to limit the monomer supply to the active sites of the BPEs.14 During the development of this interesting methodology, a surprising phenomenon, namely, the in-plane growth of PEDOT films from BPEs on a glass substrate, was observed when operated in a higher concentration of monomers (Figure 1d). This is notable, in that conducting polymer films are deposited on a nonconductive substrate and grow in-plane on the substrate under the influence of an external electric field. PEDOT microfiber growth was achieved by electropolymerization of an EDOT monomer under AC-bipolar electrochemical conditions with the setup shown in Figure 2a. In a low concentration of supporting electrolyte (5 mM tetrabutylammonium perchlorate, Bu4NClO4) in acetonitrile (MeCN), a Au wire (10 mm long, 30 μm diameter) behaves as a BPE when a sufficient voltage is applied between feeder electrodes.15 The solution potential distribution generated in the electrolyte induces a potential difference at the interfaces of the solution and the metal wire at its terminals. The potential is sufficiently

xidative electrochemical polymerization (or electropolymerization) has been a well-known process to provide conducting polymer films on a working electrode, where an aromatic monomer undergoes anodic oxidation to generate its radical cation, followed by repeated coupling reactions (Figure 1a).1 The methodology is very useful to obtain a conducting polymer film on an electrode surface because it is generally difficult to prepare a thin film of such polymers with long π-conjugation by solution processes due to their low solubility in solvents. In the oxidative electropolymerization of aromatic monomers, site-selective deposition of conducting polymers on a part of an electrode material has been achieved2−9 using bipolar electrochemistry,10−12 in which a “wireless” conductive material behaves as an electrode, i.e., a bipolar electrode (BPE), that involves anodic polymerization at one edge and sacrificial cathodic reaction at the other edge driven by application of an external electric field between a pair of feeder electrodes (Figure 1b). Such asymmetrical modification of conductive materials (e.g., metal wire, metal plate, and carbon nanotubes) is of significant interest in various applications. We have recently developed a new methodology to prepare conducting polymer microfibers of poly(3,4-ethylenedioxythiophene) (PEDOT) from terminals of a gold (Au) wire BPE by application of an alternating-current (AC) voltage between feeder electrodes (Figure 1c).13 In the AC-bipolar electropolymerization of 3,4-ethylenedioxythiophene (EDOT), Au wire terminals act as platforms for the initial deposition of PEDOT. The highly conductive PEDOT participates in a part © XXXX American Chemical Society

Received: February 28, 2018 Accepted: April 6, 2018

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DOI: 10.1021/acsmacrolett.8b00170 ACS Macro Lett. 2018, 7, 551−555

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ACS Macro Letters

was first investigated. Under optimized conditions (150 mM monomer, 15 mM BQ, 3 mM Bu4NClO4/MeCN, 5 Hz, 60 s), PEDOT films appeared at the edges of the BPE and propagated in-plane on the surface of the glass substrate in a dendritic manner, as monitored using optical microscopy (Figure 2c). Scanning electron microscopy (SEM) and laser microscopy observations of the PEDOT film on the glass substrate revealed film thicknesses of ca. 2.5 μm maintained for a long distance (Figures S2 and S3). Different from the conventional electropolymerization giving a thicker film with reaction time (Figure 1a), this method exhibited longer in-plane growth of thin films. The concentration of EDOT monomer particularly affected the behavior of film formation. The applied frequency of the AC voltage was also important to determine whether PEDOT propagates as a film or as fibers from the BPE. At lower frequency (1 and 5 Hz), PEDOT thin films were formed, whereas at higher frequency (15 and 50 Hz), only PEDOT fiber growth was evident (Figure 2c). Considering these results, a plausible mechanism for the inplane growth of PEDOT films is suggested (Figure 3). In the AC-bipolar electropolymerization of EDOT, both ends of the Au wire (BPE) are initially activated for electrolytic reactions. One side generates radical cation forms of the EDOT monomer to undergo polymerization by repeated oxidative coupling, whereas the sacrificial reduction of BQ proceeds at the other end. Under application of an AC voltage to the feeder electrodes, the polarities of the BPE terminals alter repeatedly to afford PEDOT around both ends of the BPE. The influence of the external electric field on the electrophoresis of generated PEDOT, which is already doped (cationic charged), is very important to afford anisotropic growth of PEDOT as fibers or films after precipitation from the electrolyte.16 The key factor that determines the polymer morphologies (fiber or film) is the concentration of EDOT monomer in the electrolyte. When the concentration of EDOT monomer is low (50 mM), PEDOT

Figure 1. Oxidative electropolymerization methods. (a) Conventional method to form a polymer film on an anode surface. (b) Bipolar method to form a polymer cluster at the anodic edge of the BPE. (c) AC-bipolar method (low concentration of monomer) to afford polymer microfibers from both edges of the BPE. (d) AC-bipolar method (high concentration of monomer) to afford polymer films from both edges of the BPE on a nonconductive substrate surface.

large to simultaneously support anodic and cathodic reactions, i.e., the oxidative electropolymerization of EDOT and the sacrificial reduction of benzoquinone (BQ) to hydroquinone (HQ) at both ends of the wire (Figure 2b). In the present study, the effect of the monomer, benzoquinone, and electrolyte concentrations on the growth behavior of PEDOT

Figure 2. (a) Schematic illustration of a cell setup for AC-bipolar electrolysis. (b) Schemes for the oxidative polymerization of EDOT and sacrificial reduction of BQ at a Au wire as a BPE set in between Pt feeder electrodes. (c) Optical microscopy images of PEDOT films and fibers obtained with different applied frequencies. 552

DOI: 10.1021/acsmacrolett.8b00170 ACS Macro Lett. 2018, 7, 551−555

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Figure 4. Illustration of the configuration of a tilted BPE on a glass substrate, and optical microscopy images of the resultant PEDOT films and fibers formed by AC-bipolar electropolymerization.

(Figure 5). In all cases with tetrabutylammonium (Bu4N+) perchlorate (ClO4−), tetrafluoroborate (BF4−), and hexafluorFigure 3. Plausible mechanism for the formation of PEDOT film by AC-bipolar electropolymerization in a high concentration of EDOT monomer.

fibers are obtained due to repeated PEDOT generation and deposition at the growing tips of the fibers, as reported previously.13 On the other hand, a higher concentration of EDOT monomer (150 mM) generates a large amount of PEDOT initially at the terminal of the Au wire, followed by the deposition of PEDOT film on the glass substrate in contact with the Au wire tip. In the next step, the frontiers of the PEDOT films behave as BPEs, where further electropolymerization of EDOT proceeds and results in continuous film growth. Therefore, the PEDOT morphology is dependent on the applied frequencies; i.e., only lower frequencies afforded film growth due to the high concentration of PEDOT generated at each pulse. Although the similar but uncontrolled film or fiber structures of conducting polymers can be obtained when conventional electropolymerization of monomers is carried out at a pair of thin electrodes connected with the AC power supply (Figure S4),17−19 our bipolar system has advantages such as its wireless nature, which realizes the interconnection of wireless objects by controlling the direction of an external electric field.13,14 To support this mechanism, the AC-bipolar electropolymerization of EDOT was examined using the BPE configuration as shown in Figure 4, where the extremities of the Au wire were on and off the surface of the glass substrate with a small piece of 80 μm thick Au spacer. With this configuration, the resultant PEDOT was a film formed at one end and fiber formed at the other end. PEDOT films were successfully formed on the glass substrate at the contacted end. However, at the floating end, PEDOT fibers propagated, similar to the case with a low concentration of the EDOT monomer. The electrogenerated PEDOT deposited not as a film on the substrate but at the Au tip, due to the gap between the Au tip and the surface of the glass substrate (>80 μm), which led to fiber formation. This result supports the proposed mechanism for PEDOT film formation as shown in Figure 3. The effect of counterions, which compensate for the cationic state of the electropolymerized PEDOT, on the film morphology was investigated using various supporting electrolytes for the AC-bipolar electropolymerization of EDOT

Figure 5. (a,c,e) Optical microscopy images of PEDOT films obtained by AC-bipolar electropolymerization using various supporting electrolytes. (b,d,f) SEM images of the corresponding PEDOT films.

ophosphate (PF6−) counterions, PEDOT thin films grew in a similar manner, as shown in the optical microscope images (Figure 5). However, the propagation rate was different depending on the salts used as observed in the fiber formation in our previous work.13 The surface morphologies of these films were different and exhibited flatness (ClO4−), wrinkles (BF4−), and micrograins (PF6−). The polymerization rate and surface morphologies of conducting polymer films obtained on an electrode surface by the conventional electropolymerization method are dependent on the size and properties such as ionic conductivity of the counterions used.20,21 The AC-bipolar polymerization of other EDOT derivatives and thiophenes was examined next. In our previous study, we reported that EDOT-C1 and EDOT-C10, which have a methyl group and decyl group, respectively, afforded fiber-like morphologies when processed in a monomer concentration of 50 mM.13 Under the present conditions with a higher monomer concentration (150 mM), PEDOT-C1 and PEDOT553

DOI: 10.1021/acsmacrolett.8b00170 ACS Macro Lett. 2018, 7, 551−555

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ACS Macro Letters C10 were obtained as a film form and as fibers, respectively (Figure 6). Chloromethyl-substituted EDOT (EDOT-Cl) and

(PVC), polypropylene (PP), polyethylene (PE), poly(tetrafluoroethylene) (PTFE), and poly(ethylene terephthalate) (PET). In-plane PEDOT film growth successfully occurred on the surfaces of PVC, PP, PE, and PET under the optimized conditions (Figure 7). However, PEDOT fibers were

Figure 7. Optical microscopy images of PEDOT films and fibers grown on insulating substrate surfaces: (a) glass, (b) PVC, (c) PP, (d) PE, (e) PTFE, and (f) PET.

obtained at the Au tip when PTFE was used as a substrate. The use of oleophobic PTFE resin as a substrate prevented film deposition, probably due to the incompatibility with deposited PEDOT. On the other hand, PEDOT-C10 grew as the fiber form, regardless of the substrate material. When a higher voltage was applied, PEDOT-C10 was obtained as a film on the PTFE surface (Figure S6). In conclusion, the in-plane growth of conducting polymer films by the AC-bipolar electropolymerization method was successfully demonstrated under optimized conditions. The concentration of monomers, the applied frequency, and the monomer structures were investigated to determine those conditions for film formation, i.e., high monomer concentration, low frequency, and high conductivity of the corresponding polymers. The selection of substrate materials was also important to obtain PEDOT films. From these trials, we have proposed a mechanism for film formation. This technique could be used to draw conducting polymer patterns on a nonconductive substrate, guided by an applied external electric field, and thus could have potential for the patterning of conducting polymer circuits in electronic devices.

Figure 6. (a,c,e,g) Optical microscopy and (b,d,f,h) SEM images of PEDOT-C1, PEDOT-C10, PEDOT-Cl, and PolyBT, respectively.

bithiophene (BT) were polymerized as fibers under the same conditions (Figure 6). To elucidate the reason for this behavior, electropolymerized films were prepared using each monomer with the conventional potential sweep method, and their electrical conductivities (four-probe method) were compared (Table S1). The measured conductivities of PEDOT and PEDOT-C1 were higher than those of PEDOT-C10, PEDOTCl, and polybithiophene (PolyBT) by 1 order of magnitude. According to the proposed film growth mechanism, the inplane growth of polymer films in AC-bipolar polymerization requires a high concentration of generated polymers. Thus, the electrical conductivity of a polymer itself is important for it to behave as a BPE, in which redox reactions occur at the propagating edges. Although the measured conductivities of the polymer films in Table S1 were not identical to those of the growing films in AC-bipolar electropolymerization, the tendency of the conductivities could explain the morphological dependence of the polymers on the monomer structures used. The PEDOT films grown from the Au wire have good electrical conductivity; therefore, they can behave as BPEs for subsequent bipolar electropolymerization of other monomers. After replacement of the electrolytic solution of EDOT with that of EDOT-C1, further bipolar electropolymerization was conducted to successfully introduce a second segment of PEDOT-C1 film from the edges of the first segment of the PEDOT film (Figure S5). The versatility of this method was also surveyed using other insulating substrate materials, such as poly(vinyl chloride)

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00170. Experimental details and additional supporting figures (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Shinsuke Inagi: [email protected]. 554

DOI: 10.1021/acsmacrolett.8b00170 ACS Macro Lett. 2018, 7, 551−555

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ACS Macro Letters ORCID

(17) Curtis, C. L.; Ritchie, J. E.; Sailor; Michael, J. Fabrication of conducting polymer interconnects. Science 1993, 262, 2014−2016. (18) Fujii, M.; Arii, K.; Yoshino, K. Neuron-type polypyrrole device prepared by electrochemical polymerization method and its properties. Synth. Met. 1995, 71, 2223−2224. (19) Thapa, P. S.; Yu, D. J.; Wicksted, J. P.; Hadwiger, J. A.; Barisci, J. N.; Baughman, R. H.; Flanders, B. N. Appl. Phys. Lett. 2009, 94, 033104. (20) Melato, A. I.; Mendonca, M. H.; Abrantes, L. M. Effect of the electropolymerisation conditions on the electrochemical, morphological and structural properties of PEDOTh films. J. Solid State Electrochem. 2009, 13, 417−426. (21) Moradi, A.; Emamgolizadeh, A.; Omrani, A.; Rostami, A. A. Electropolymerization and characterization of 3,4-ethylenedioxy thiophene on glassy carbon electrode and study of ions transport of the polymer during redox process. J. Appl. Polym. Sci. 2012, 125, 2407−2416.

Ikuyoshi Tomita: 0000-0003-3995-5528 Shinsuke Inagi: 0000-0002-9867-1210 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by a Kakenhi Grant-in-Aid (No. JP17H03095) from the Japan Society for the Promotion of Science (JSPS) and research grants from The Murata Science Foundation and Casio Science Promotion Foundation.

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REFERENCES

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DOI: 10.1021/acsmacrolett.8b00170 ACS Macro Lett. 2018, 7, 551−555