Controlled Electrochemical Synthesis of Conductive Polymer Nanostructures and Electrochromism Property Study
Lee, Sang Bok
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Conductive polymers have attracted the attention of many scientists in the whole world since Australians DE Weiss and coworkers reported high conductivity in oxidized iodine-doped polypyrrole, a polyacetylene derivative. In recent years, conductive polymers have served as the basis for many different new technologies such as electrochemical power sources, electrochromic display devices, photovoltaic devices and biosensors. For all of these applications, it is necessary to maintain ultra-fast charge and discharge rates in the conductive polymer layers in order to obtain high energy capacity, fast sensing speed or color change rate. For conductive polymer films there is always a problem for the slow charge/discharge ratio due to the diffusion of counter-ions into and out of the conductive polymer layers. To solve this problem, one-dimensional conductive polymers are ideal choices. We have investigated the electrochemical synthetic mechanism for conductive polymer nanotubes in a porous alumina template using poly(3,4-ethylenedioxythiophene) (PEDOT) as a model compound. As a function of monomer concentration and potential, electropolymerization leads either to solid nanowires or hollow nanotubes and it is the purpose of these investigations to uncover the detailed mechanism underlying this morphological transition between nanowire and nanotube. Electrochemically-synthesized PEDOT nanostructures were characterized using transmission electron microscopy to measure the extent of nanotubular portion in the PEDOT nanostructure. The study on potential dependency of nanotubular portion shows that nanotubes are grown at a low oxidation potential (< 1.2 V vs. Ag/AgCl) regardless of monomer concentration. This phenomenon is explained by the annular base electrode shape at the pore bottom of a template and further supported by an electrochemical study on a flat-top electrode. We investigate the mechanism by taking into account the effect of electrolyte concentration, temperature, and template pore diameter on PEDOT nanostructures. This mechanism is additionally used to control the nanotube dimensions of other conductive polymers such as polypyrrole and poly(3-hexylthiophene). The electrochromic properties and applications of PEDOT nanostructures are also addressed.