Fabrication and Characterization of Electrically Conducting Polypyrrole Micro/nanostructures

Abstract

Conducting polymer micro/nanomaterials have attracted much attention due to their extraordinary optical and electrical properties, improved processability and promising applications in „plastic electronics‟. Although many methods for synthesizing conducting polymer micro/nanostructures have been proposed and well-established, in this thesis, new systematic and modified approaches to the fabrication of conducting polypyrrole (PPy) micro/nanostructures possessing improved structural and electrical properties, together with variety of morphologies are provided. The formation mechanism of the resulting PPy micro/nanostructures was studied by spectroscopical and microscopical analysis techniques in detail. In addition, the produced PPy micro/ nanostructures were used as chemical sensors for toxic gas detection and catalytic counter electrode materials for dye-sensitized solar cells (DSSCs) to explore the feasibility of practical applications. In chapter 2, PPy hexagonal microplates (PHMs) with a quasicrystalline structure and high electrical conductivity (up to 400 S/cm) were simply fabricated by organic single-crystal surface-induced polymerization (OCSP) method using single crystals of 4-sulfobenzoic acid monopotassium salt (KSBA). In OCSP process, KSBA crystals, for the first time, have been adopted as sacrificial templates for synthesis of PHMs by electrostatic interaction between the anionic surface of the KSBA crystal and the cationic PPy chain. Synthetic time-resolved PPy morphology dynamics reveals that the fabrication process of PHMs constituted of PPy nanostructures combines a shape-copying process for forming a PPy preform that imitates the shape of KSBA crystal and the self-assembly process of PPy on the preform. The PHMs exhibited the enhanced thermal stability at an elevated temperature as well as improved pi-stacking and bipolaron structure compared to conventional PPy. The strong pi-stacks among PPy rings of bipolaron structures lead to a high quasicrystalline structural order and the metallic conduction. A general strategy for fabricating diverse types of PPy micro/ nanostructures by OCSP method using crystals of a series of sulfobenzoic acid salt forms with various cations (K+, Na+, Li+, or NH4 +) in different positions (para, meta, or ortho) of the sulfonate group on the benzene ring was proposed in chapter 3. The electrical properties were dictated by the molecular structures of the organic salt molecules used. While the highest conductivity was observed in PPy using crystals of para-linked 4- sulfobenzoic acid monopotassium salt, the lowest conductivity was observed in PPy prepared in the presence of crystals of ortho-linked 2- sulfobenzoic acid monoammonium salt. In chapter 4, ultra-thin PPy nanosheets (UPNSs) were fabricated by OCSP process using hydrated crystals of sodium decylsulfonate (C10SO3Na) below the Krafft temperature. The hydrated C10SO3Na crystals were used as templates through electrostatic binding of the cationic PPy chains oxidized by FeIII ions on the anionic C10SO3Na crystal surface. The resulting UPNSs composed of a single PPy domain had a single layer thickness of ~21. Moreover, the UPNSs exhibited higher conductivity and longer conjugation lengths than the PPy nanoparticles prepared using emulsion polymerization. We systematically investigated the UPNSs as gas sensors for detecting and quantifying toxic gases such as HCl and NH3. The UPNSs exhibited much higher gas sensitivity and faster response time compared with the PPy nanoparticles. In last chapter, monodisperse PPy nanospheres (~85 nm) were fabricated by chemical oxidative synthesis within micelles composed of myristyl trimethyl ammonium bromide (MTAB) and decyl alcohol as nanoreactors. Pt nanoparticles (~3.2 nm) supported on PPy nanospheres (PPy–Pt) were fabricated using a microwave-assisted polyol process. Colloidal dispersions containing PPy and PPy–Pt nanoparticles were dropcast directly onto fluorine-doped tin oxide (FTO) glass to use as counter electrodes for DSSCs. The DSSC made of HCl-doped PPy/FTO counter electrode exhibited power conversion efficiency ~7.7%. The postdoped and highly oxidized PPy allowed the electrons to move into the PPy layer readily and facilitated the electrocatalytic reaction of the I3 –/I– redox couple, giving enhanced cell performance. PPy–Pt was used for the first time as a catalytic counter electrode for DSSCs. The DSSC using PPy–Pt counter electrode with higher electrocatalytic activity and mesoporosity showed enhanced power conversion efficiency (8.6%) compared with that of the conventional Pt-based DSSC (8.1%).