Conducting Polymers Developed on Ice Surfaces with Enhanced Electrochemical Properties

Abstract

Conducting polymers have been studied extensively in the past few decades due to various advantages such as high electrical conductivity, optical transparency, chemical stability, flexibility, and low cost. Recently, research to improve the electrical properties of conducting polymers for high performance in electronic devices are intensively conducted. Various studies have been reported to improve the conductivity of conducting polymers by controlling various factors such as the type of dopant, doping level, and conjugate length in the synthesis of conducting polymers. More recently, improving conductivity through the formation of nanostructure of conducting polymers has been attracted great attention. In order to synthesize conducting polymer nanostructures, various methods using hard templates and soft templates as well as conventional chemical oxidation polymerization have been reported. In particular, among them, the two-dimensional nanostructure has become important because it is advantageous for high-density integration of next-generation electronic devices to exhibit improved electron transport properties. Nevertheless, the synthesis of conducting polymers having two-dimensional nanostructure with high electrical conductivity have mainly based on methods using graphene oxide as templates. However, this approach showed difficulty in removing the template after synthesis, which made hard to obtain pure conducting polymer. It was also difficult to obtain reliable electrical properties over a large area due to the non-uniform distribution of functional groups of graphene oxide. To overcome this limitation, I synthesized conducting polymers using the ice surface as a template, where the ice template is easily removed in the next step. Consequently, pure two-dimensional conducting polymer nanosheets with high conductivity were obtained. In this dissertation, the role of crystallinity of the ice surface on the ice-templated conducting polymers are investigated, and new strategies for utilizing the ice-templated conducting polymers as the electrocatalyst, the virus filtration membrane are proposed. Chapter 1 provides an overall overview of ice-assisted chemistry for synthesizing functional materials using ice, and introduces current research trends on conducting polymers prior to introducing ice-templated conducting polymers. Research on conducting polymer-metal composites and porous conducting polymers are also included. Furthermore, research motivation and objectives of this dissertation are presented to develop the current research based on ice-assisted chemistry. Chapter 2 covers the study of the nanostructure and electrical properties of the ice-templated conducting polymers, and the study of a role of the crystallinity of the ice surface. Although the recently established ice-templated synthetic method of two-dimensional conducting polymers is evaluated as an environmentally friendly and easy-fabrication technology, the role of the crystallinity of the underlying ice surface remains unclear in determining the physicochemical and electrical properties of conducting polymers. In this study, crystallinity of ice is systematically controlled, and the electrical properties of conducting polymer nanosheets grown on the ice surface and the packing structure of conducting polymer crystals are studied in depth. Intriguingly, the crystallinity of the conducting polymer nanosheets resembled that of the ice surface, and it was confirmed that the higher the crystallinity of ice, the more predominantly anisotropic growth of the conducting polymer nanosheets in the face-on orientation. In addition, it was turned out that highly crystalline conducting polymer nanosheets led to more efficient charge transport due to improved degree of backbone ordering due to the pre-organized aniline moieties on the ice surface and strong polaron delocalization with the extended chain conformations. These results suggest that controlling the crystallinity of ice is simple but effective to control the electrical properties of conducting polymers. Chapter 3 describes the study of conducting polymer-platinum (Pt) nanoparticle composites based on ice-templated conducting polymers. Pt nanoparticles are well known as the most effective electrocatalysts for various electrochemical reactions. However, for commercially available Pt catalyst supported on carbon such as Pt/C catalyst, a high reaction temperature is generally required. And the particle agglomeration cannot be prevented, so that the catalytic activity decreases rapidly over time. Besides, carbon nanostructures such as graphene are even expensive. Thus, in this study, a new approach for the synthesis of uniform, high areal density Pt nanocrystals supported by ice-templated conducting polymers is presented. The key strategy is the use of ice-templated conducting polymers at the air-water interface as a platform to promote the nucleation of platinum. Highly crystalline Pt nanoparticles with a narrow size distribution of about 2.7 nm and a high electrochemically active surface area of 94.57 m2 g-1 were obtained. It showed good durability and excellent carbon monoxide tolerance. This approach suggests potential applications for the production of various other electrocatalysts with enhanced catalytic activity. Chapter 4 covers the study of the synthesis of two-dimensional porous conducting polymer nanosheets synthesized on the ice surface. In this study, the directional freezing is performed to the microplastics-containing solution and subsequently ice-templated synthesis is conducted on this microplastics-containing ice. This process leads to easy removal of microplastics in aqueous solution by detaching the synthesized conducting polymer nanosheets from the ice. The removal efficiency of microplastics was over 97 %. More importantly, while microplastics are being removed, porous two-dimensional conducting polymer nanosheets are synthesized with a solvent treatment process for removing the microplastics from the detached conducting polymer nanosheets. Intriguingly, the size of the pores could be easily tuned depending on the size of the microplastics. Then, porous conducting polymer nanosheets with 100 nm pores were utilized as the virus filtration membranes for coronavirus (SARS-CoV-2). As a result, high rejection rate up to 96.3 % was achieved. This approach is important in that it can remove microplastics in aqueous solution and simultaneously make porous conducting polymers with various pore sizes, which suggests a potential for variety of applications. Chapter 5 provides conclusions and perspectives based on these studies. Through these studies, the role of ice in ice-templated synthesis was investigated in more depth. 2D nanostructure of the ice-templated conducting polymers could be realized owing to the confinement provided by the quasi-liquid layer (QLL) on the ice surface. It was confirmed that the electrical properties and packing structure of the conducting polymers are greatly affected by the crystallinity of ice surface. Moreover, new advanced nanomaterials with high functionality and improved electrochemical properties were synthesized by utilizing ice-assisted chemistry, which showed high potential for application in various fields. Nevertheless, studies on the utilization of QLL on the ice surface are still lacking, which is ascribed to the difficulty in accurate identification and precise control of QLL. Thus, further studies are required to understand and utilize QLL on the ice surface, which may include the in-situ transmission electron microscopy and ultrafast X-ray scattering experiments. Such future studies will contribute to developing new advanced materials and expanding the scope of the ice-templated synthesis to various applications.