Preparation of Water-Insoluble Monomer Nanoemulsion Using Tandem Acoustic Emulsification and Its Application to Templated Electropolymerization
Authors: Nakabayashi Koji , Atobe Mahito 
Published: February 2020

Conducting polymer nanowires have attracted great attention because their huge surface area can enhance the performance of devices such as nanosensors and micro- electronics by improving charge-transport rate1. The nanowires have been prepared by various methods, for example, self-assembly2and template synthesis1and so on. In such methods, a templated electropolymerization is an elegant approach for the fabrication of the conducting polymer nanowires. Here, we focus on the electrochemical template method to synthesize conductive polymer nanowires since it facilitates fabrication of one- dimensional nanoscale conducting polymers in a controlled manner. However, in general, the amount of monomers supplied into the nanopores of the substrate is limited in the conventional media like acetonitrile, and therefore the formed nanotube in the pores is brittle due to its hollow structure. Thus, sufficient monomer supply is fundamental issue for the preparation of tightly packed conducting polymer nanowires. Hence, the fabrication of robust nanostructure using template electrochemical deposition has still remained as a challenging target. Our hypothesis was that the use of nanoemulsion of the monomer is outstandingly effective for electrochemical fabrication of the cylindrical conducting polymer material.
Recently, we have reported that a preparation of highly clear and transparent emulsified aqueous solution containing immiscible droplets with average diameters of a few tens nanometers using 20 kHz → 1.6 MHz → 2.4 MHz sequential processing with ultrasonic waves (tandem acoustic emulsification)3-5. This novel technique was found to be adequate for producing emulsion nanodroplets in the absence of any surfactants.
We envisioned that the emulsion droplet prepared by tandem acoustic emulsification method would enter easily into the inside of nanoporous substrate and consequently they would be polymerized to form the solid conducting polymer nanowires directly without the formation of a hollow structure (Fig. 1).
In this work, we have successfully prepared poly(3,4- ethylenedioxythiophene) (PEDOT) nanowires using 20 kHz →1.6 MHz → 2.4 MHz sequential processing with ultrasonic waves (tandem acoustic emulsification). EDOT nanoemulsion solution was prepared according to the previously reported literature3-4; that is, ultrasonication of 20 kHz for 5 min, 1.6 MHz for 5 min, and 2.4 MHz for 5 min were sequentially carried out. The nanoporous alumina membranes (60-μm thick, 200-nm pore size) sputtered on one side with Pt (ca. 200-nm thick) was employed as a template for PEDOT electrodeposition into pores. The polymerization of EDOT was carried out in one-compartment cell equipped with working electrode, Pt plate as a counter electrode, and saturated calomel electrode (SCE) as a reference electrode.
Figure 2 shows SEM photographs of electrodeposited PEDOT nanowires after dissolving the alumina template in 1 M aqueous NaOH solution. In the case of the electrodeposition in nanoemulsion prepared from tandem acoustic emulsification, the well-aligned PEDOT nanowire array was formed as we expected.
In sharp contrast, by the use of 20 kHz sonicated solution, the PEDOT nanowires were not formed. This is because the monomer droplets prepared by 20 kHz ultrasonication would not enter into the pores of the template membrane. Indeed, average EDOT droplet size prepared by 20 kHz ultrasonication was 350 nm, and which is larger than pore size of membrane.
In the presentation, detail experimental condition and the properties of PEDOT nanowires will be discussed.
References
1 C. R. Martin, Science 1994, 266, 1961.
2 L.-W. Yin, Y. Bando, J.-H. Zhan, M.-S. Li, D. Golberg, Adv. Mater., 2005, 17, 1972.
3 K. Nakabayashi, F. Amemiya, T. Fuchigami, K. Machida, S. Takeda, K. Tamamitsu, M. Atobe, Chem. Commun., 2011, 47, 5765.
4 K. Nakabayashi, T. Fuchigami, M. Atobe, Electrochim. Acta, 2013, 110, 593.
5 K. Nakabayashi, M. Kojima, S. Inagi, Y. Hirai, M. Atobe, ACS Macro Lett., 2013, 2, 482.