Templated electrochemical synthesis of conducting polymer nanowires from corresponding monomer nanoemulsions prepared by tandem acoustic emulsification

Abstract

A new approach to the preparation of solid conducting polymer nanowires is proposed, involving templated electrochemical polymerization of a corresponding monomer nanoemulsion formed by tandem acoustic emulsification.

---

Conducting polymer nanowires have attracted much attention because their large surface area can enhance the performance of devices such as nanosensors and micro-electronics by improving the charge-transport rate.¹ These nanowires have been prepared by various methods, but synthesis is usually complex and requires delicate control of experimental parameters, such as the concentration and reaction time. Templated synthesis was invented in the 1990s by Martin and co-workers,^2–4 where the synthesis of nanotubes and nanowires composed of conducting polymers such as polypyrrole, polythiophene and polyaniline, as well as metals and inorganic compounds, was conducted using electrochemical or chemical methods.^3,5–7 To date, these have been recognized as the most simple and straightforward methods for the preparation of conducting polymer nanowires. However, monomer diffusion into the nanopores of a template with high aspect ratio conduits is generally limited for conventional monomer solutions, resulting in insufficient filling of the nanopores to obtain solid nanowires. In this case, the formed nanowires are typically brittle and have a hollow structure.⁸ Thus, nanostructure fabrication using templated electrochemical polymerization still remains a challenge.

In this work, we successfully demonstrate the preparation of solid conducting polymer nanowires using the templated electrochemical polymerization in a corresponding monomer nanoemulsion. In this approach, as shown in Fig. 1, the monomer is polymerized electrochemically, not in a solution, but in nanodroplets of the emulsion densely packed in the nanopores of the template. Consequently, solid nanowires are obtained after removal of the template, and hollow structures are not formed. However, with this method, the emulsion nanodroplets must be sufficiently smaller than the pores in the template.

Fig. 1 Preparation of nanocylindrical PEDOT using template electrochemical polymerization in tandem acoustic emulsified solution in a template.

Ultrasonication provides stable emulsions without the need for surfactants as a result of mechanical forces generated from acoustic cavitation at liquid/liquid phase boundaries.^9–11 This process has been termed acoustic emulsification and is regarded as a powerful method for the rapid and sustainable production of emulsions. Emulsion droplets prepared using general-use ultrasonic devices with frequencies ranging from 20 kHz to 1.0 MHz are typically between 100 and 1000 nm in diameter.^12,13 However, the pore size of templates used for templated synthesis is generally between 20 and 200 nm.^6,14,15 Therefore, it can be expected that monomer droplets prepared by a single ultrasonication process in the kilohertz range would be too large to enter into the nanopores of the template membrane.

Recently, we have reported the preparation of very clear and transparent emulsified aqueous solutions containing immiscible droplets with average diameters of a few tens of nanometers using sequential processing with ultrasonic waves with frequencies of 20 kHz, 1.6 MHz and 2.4 MHz (tandem acoustic emulsification).^16–18 This novel technique was capable of producing emulsion nanodroplets in the absence of surfactants. We envisioned that the use of such small droplets for template electrochemical polymerization would enable their introduction and dense packing into the pores of the template.

In the present work, 3,4-ethylenedioxythiophene (EDOT), which is only slightly soluble in an aqueous electrolyte, was used as the monomer. A nanoemulsified solution of EDOT was prepared according to a the previously reported procedure.¹⁶ Briefly, a mixture of EDOT and aqueous electrolyte was sequentially ultrasonicated at 20 kHz for 5 min, 1.6 MHz for 5 min, and 2.4 MHz for 5 min to form an EDOT nanoemulsion with an average droplet size of 82 nm. A commercially available nanoporous alumina membrane (60 μm thick, 200 nm pore size, Anodisc 13 Membrane Disc, Whatman) sputtered on one side with Pt (ca. 200 nm thick) was employed as a template electrode for poly(3,4-ethylenedioxythiophene) (PEDOT) electrodeposition into the nanopores. The polymerization of EDOT monomer nanodroplets was conducted in a one-compartment cell equipped with the template electrode, a Pt plate counter electrode, and a saturated calomel electrode (SCE) as a reference electrode.

Fig. 2 shows current–time curves for the electropolymerization of EDOT droplets recorded at a template electrode of 1.4 V vs. SCE. The current corresponding to monomer oxidation in the emulsified solution prepared by a single ultrasonication step at 20 kHz was almost the same as that for the background, which indicates that the EDOT droplets could not enter into the pores of the template membrane. The average EDOT droplet size was 350 nm, which is larger than the pore size of the template membrane. The small Faradaic current observed in curve b could be ascribed to the oxidation of a small amount of dissolved EDOT in the aqueous phase. Fig. 2c, shows that the saturated EDOT aqueous solution gave a comparable result to that obtained for the solution sonicated at 20 kHz. On the other hand, a higher oxidation current was observed in the tandem emulsified solution (Fig. 2a). The EDOT droplets formed by tandem acoustic emulsification were sufficiently smaller than the pores in the template used, so that they could easily enter into the pores, and be directly electrooxidized to form PEDOT.

Fig. 2 Current–time curves for constant-potential electropolymerization in (a and b) emulsified solutions and (c) saturated EDOT aqueous solution. Emulsification conditions were (a) 20 kHz, 5 min → 1.6 MHz, 5 min → 2.4 MHz, 5 min and (b) 20 kHz, 5 min.

The template electrodes were observed after PEDOT electrodeposition. Fig. 3 shows cross-sectional optical micrographs of nanoporous alumina membranes filled with PEDOT (dark areas) electrodeposited in the saturated EDOT aqueous solution and the acoustically emulsified solutions. Although the PEDOT-filled volume increased with deposition time in all media, the deposition rate was higher in the tandem sonicated solution than in the other media. The EDOT nanodroplets prepared by tandem ultrasonication could easily enter into the pores of the template, so that oxidation of EDOT in the pores of the template contributed to the high deposition rate.

Fig. 3 Cross-sectional optical micrographs of PEDOT-filled porous alumina membranes. Electrodeposition was conducted at 1.4 V vs. SCE for 800 s in (a) saturated EDOT aqueous solution and (b and c) acoustically emulsified solutions.

Fig. 4 shows SEM micrographs of electrodeposited PEDOT nanowires after the nanoporous alumina template was dissolved in an aqueous solution of 1 M NaOH. Electrodeposition in the saturated solution and the solution sonicated at 20 kHz resulted in low-density hollow nanowires (Fig. 4b and d). Furthermore, the nanowires were not sufficiently strong to be self-supporting and fell down after removal of the alumina template (Fig. 4a and c). A mechanism for the formation of such hollow structures was proposed by Martin,² in which the nascent conducting polymer is preferentially deposited as a thin layer on the pore walls due to solvophobic and electrostatic interactions between the polymer and the pore walls. In addition, monomer diffusion into the nanopores of the template is generally limited in the monomer solution, so that insufficient filling with the conducting polymer occurs. Therefore, the formed nanowires are usually brittle and have a hollow structure (Fig. 5).

Fig. 4 SEM images of cylindrical PEDOT nanostructures synthesized in (a and b) saturated EDOT aqueous solution and (c–f) acoustically emulsified solutions. Emulsification conditions were (c and d) 20 kHz, 5 min and (e and f) 20 kHz, 5 min → 1.6 MHz, 5 min → 2.4 MHz, 5 min.

Fig. 5 Schematic representation of template electrochemical polymerization of saturated EDOT and 20 kHz sonicated solutions.

In contrast, the use of the EDOT nanoemulsion reaction medium prepared by tandem acoustic emulsification gave a well-aligned PEDOT nanowire array, as shown in Fig. 4e. Moreover, the rod-like structure of the nanowires was confirmed by higher-magnification SEM observation (Fig. 4f). In this case, the EDOT monomer was electropolymerized not from an EDOT solution, but from nanodroplets of EDOT densely packed in the nanopores of the template. Therefore, a solid PEDOT “nano-brush” could be obtained after removal of the template.

In conclusion, we have demonstrated that a monomer nanoemulsion prepared by tandem acoustic treatment can be used as an electrolytic medium for templated electropolymerization with nanoprecise filling with a conducting polymer such as PEDOT and the formation of robust PEDOT nanowires after removal of the porous alumina template.

The present approach expected to open a new path to nanostructure fabrication using conducting polymers.

Notes and references

1. Handbook of Conducting Polymers, ed. T. A. Skotheim, R. L. Elsenbaumer and J. R. Reynolds, Marcel Dekker, New York, 1998.
2. C. R. Martin, Science, 1994, 266, 1961.
3. C. R. Martin, L. S. Van Dyke, Z. Cai and W. Liang, J. Am. Chem. Soc., 1990, 112, 8976.
4. C. R. Martin, Adv. Mater., 1991, 3, 457.
5. R. M. Penner and C. R. Martin, J. Electrochem. Soc., 1986, 133, 2206.
6. L. S. Van Dyke and C. R. Martin, Langmuir, 1990, 6, 1118.
7. R. V. Parthasarthy and C. R. Martin, Chem. Mater., 1994, 6, 1627.
8. M. Acik and G. Sonmez, Polym. Adv. Technol., 2006, 17, 697.
9. M. K. Li and H. S. Fogler, J. Fluid Mech., 1978, 88, 499.
10. M. K. Li and H. S. Fogler, J. Fluid Mech., 1978, 88, 513.
11. S. R. Reddy and H. S. Fogler, J. Phys. Chem., 1980, 84, 1570.
12. Encyclopedia of Surface and Colloid Science, ed. K. Kamogawa and M. Abe, Marcel Dekker, New York, USA, 2002.
13. T. Sakai, Curr. Opin. Colloid Interface Sci., 2008, 13, 228.
14. S. I. Cho and S. B. Lee, Acc. Chem. Res., 2008, 41, 699.
15. M. Fu, Y. Zhu, R. Tan and G. Shi, Adv. Mater., 2001, 13, 1874.
16. K. Nakabayashi, F. Amemiya, T. Fuchigami, K. Machida, S. Takeda, K. Tamamitsu and M. Atobe, Chem. Commun., 2011, 47, 5765.
17. K. Nakabayashi, T. Fuchigami and M. Atobe, Electrochim. Acta, 2013, 110, 593.
18. K. Nakabayashi, M. Kojima, S. Inagi, Y. Hirai and M. Atobe, ACS Macro Lett., 2013, 2, 482.

---

Footnote

† Electronic supplementary information (ESI) available: Materials and additional details for the tandem acoustic emulsification treatment, templated electrochemical polymerization, and SEM observation. See DOI: 10.1039/c4ra02976c

---