Chemical Synthesis of PEDOT Nanotubes

We describe a rapid, room temperature, reverse emulsion polymerization method to chemically synthesize bulk quantities of micrometers long nanotubes of electrically conducting poly(3,4-ethylenedioxythiophene) (PEDOT) having tube diameters in the range 50-100 nm. Composites of PEDOT nanotubes with noble metals, metal oxides, etc., can be readily synthesized using postsynthesis and in situ polymerization methods. In addition to its environmental stability and low redox potential, poly(3,4-ethylenedioxythiophene) (PEDOT) is unique among conducting polymers in that its small band gap confers high optical transparency in the doped, conducting state.1 Although it has been extensively investigated for use in antistatic coatings, flexible electronic devices, and transparent electronics,2 its 1D transport properties have been largely unexplored because powders and films of PEDOT obtained possess granular or particulate morphology. Although nanotubes of PEDOT have been electrochemically synthesized in the pores of aluminum oxide template,3 a convenient chemical synthetic route to the bulk quantities of PEDOT nanotubes/fibers has remained a challenge. The PEDOT system is particularly recalcitrant to fibrillar or tubular polymer growth, and techniques such as nanofiber seeding,4 activated seeding,5 and interfacial polymerization6 that have been used to synthesize nanofibers of polyaniline or polypyrrole yield only granular powders when extended to the PEDOT system. In a recent report, polypyrrole nanotubes were synthesized using a hexane/water reverse microemulsion system using sodium bis(2-ethylhexyl) sulfosuccinate (AOT) cylindrical micelles as the template and FeCl3 as the oxidant.7 When applied to the PEDOT system, however, the same system yielded nanorods and not nanotubes.8 Enhanced hydrophobicity and slow kinetics of oxidation of 3,4-ethylenedioxythiophene (EDOT) vs pyrrole were invoked as factors that favor rodlike over tubular polymer growth in the PEDOT system.8 In this study, we have used different synthetic vectors in the above system to synthesize, for the first time, 10-20 μm long PEDOT nanotubes having inner diameter in the 50-100 nm range. In a typical synthesis, a reverse microemulsion was first prepared by dissolving AOT (19.12 mmol) in 70 mL of n-hexane and adding a solution of FeCl3 (10.0 mmol) in 1.0 mL of water to it. The resulting orange-colored mixture was gently stirred for 5 min followed by addition of EDOT monomer (3.52 mmol). After 3 h under gentle magnetic stirring, the blue/black precipitate of PEDOT nanotubes was suction filtered and washed with copious amounts of methanol and acetonitrile. Drying under dynamic vacuum for 12 h at 80 °C yielded a navy blue powder having the elemental composition: C, 45.54; H, 3.52; O, 27.56; Cl, 3.19; S, 19.66; Total: 99.47, consistent with (PEDOT)(Cl)0.15(H2O)0.79 (>80% yield). The water in the sample could be either water of hydration or water trapped in the nanotube pore. The PEDOT nanotubes obtained exhibit a conductivity σRT 3-6 S/cm (compressed pellet). The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images show that the product is composed almost entirely of >10 μm long tubes having outer diameter in the range 300-800 nm and pore diameter in the range 50-100 nm. The dramatic change in morphology from granules to tubes shows not only that the AOT/hexane reverse microemulsion method can be extended beyond the polypyrrole system7 but also that the PEDOT system is itself very sensitive to small changes in experimental conditions. For example, the molar ratio of EDOT/FeCl3/AOT in the previous study (1.0:3.47:6.14) is similar to the present study (1.0:2.84:5.43), and yet, there is a change in morphology from rods to tubes.9 This can be traced to two important differences in our synthetic procedure: (i) our reaction mixture was three times more dilute and not viscous, and (ii) the AOT/FeCl3/hexane mixture was not vigorously stirred to obtain a milky yellow emulsion prior to addition of EDOT monomer. Nanotubes are obtained if the EDOT monomer is added to a gently stirred suspension of AOT/ FeCl3/hexane that still had the initial orange in color from the FeCl3 oxidant (Figure 2A, vial). If, however, the AOT/FeCl3/ hexane mixture is vigorously stirred to yield a light-yellow milky emulsion before adding EDOT monomer like in the previous study, short PEDOT stubs/rods are obtained (Figure 2A, inset).9 The reasons for the change in morphology are unclear, although vigorous stirring could have fragmented the long cylindrical micellar templates resulting in short PEDOT nanorods. Potentialtime profiling10 of the two systems provides evidence consistent with higher oxidation potential and faster reaction rates favoring † Department of Chemistry. ‡ Department of Electrical Engineering. * Corresponding author: Fax (+1)972-883-6586; e-mail sanjeev.manohar@utdallas.edu. Figure 1. Microscopy images of PEDOT nanotubes: (A) SEM; (B) TEM. Inset: magnified image.