Spectroscopic and Conductivity Studies of Doping in Polythiophene and Poly(3,4-Ethylenedioxythiophene) and their Applications as Gas Sensors

Abstract

The studies in this project involve investigations of doping in polythiophene and poly(3,4-ethylenedioxythiophene). Techniques including Raman, infrared, x-ray photoelectron and electron paramagnetic resonance spectroscopy, elemental analysis and conductivity measurement are used to characterize polythiophene and poly(3,4- ethylenedioxythiophene) samples. The main purpose of these studies is to search the most suitable materials for preparing conducting polymer gas sensors based on polythiophene or its derivatives. Polythiophene samples prepared either electrochemically or chemically are examined and the reactions between the dedoped polythiophene and iodine, a commonly used oxidizing agent are studied using both iodine solutions in acetonitrile and iodine vapour. Ion-exchange reactions are also attempted by treating pre-doped polythiophene powder with solutions in acetonitrile containing copper(Il) complex anions. Studies of doping in poly(3,4-ethylenedioxythiophene) cover a wide range of dopants: First, a study of secondary doping using iodine is carried out. Here Raman spectroscopy with 514.5 nm excitation is used to investigate the level of I3- dopant in the polymer, while Raman spectroscopy with 785 nm excitation is used to investigate the change in the ratio of the intensity of the symmetric Cα=Cβ stretching vibration associated with the oxidized structure and the reduced structure respectively with changes in the polymer doping level. Other spectroscopic analyses .are also carried out to examine the changes in the physical structures of the polymer and the conductivity is measured to see whether secondary doping has a significant impact on the electronic structure. Second, a study of dopant-exchange using copper(II) complex anions in aqueous and acetonitrile solvents is carried out. Here electron paramagnetic resonance spectroscopy and elemental analysis are combined to determine whether the incorporation of copper(Il) complex anions in poly(3,4-ethylenedioxythiophene) is due to doping or due to adhesion; conductivity of the samples is also measured to see whether there is any change in the electrical properties upon dopant-exchange / anion-adhesion. Third, a study of dopant effects in poly(3,4-ethylenedioxythiophene) using an electrochemical doping method is carried out. Four commonly used dopants, BF4-, Cl-, SO42- and poly(styrene-4-sulfonate), are investigated using Raman spectroscopy with 785 nm excitation to study the shift in the position of the symmetric Cα=Cβ stretching vibrations when the polymer incorporating one of the four chosen dopants is subjected to a progressive increase in the applied potential. The doping levels achieved in these electrochemically prepared samples are estimated from a pre-determined correlation equation that relates the ratio of the intensity of the symmetric Cα=Cβ stretching vibrations and the polymer doping level. Finally, a study of poly(3,4-ethylenedioxythiophene) incorporating anionic polyelectrolyte dopants and their applications as gas sensors is carried out. Here poly(3,4-ethylenedioxythiophene) incorporating poly(2-acrylamido-2-methyl- 1-propane sulfonate) is studied for the first time and its properties are compared to those of poly(3,4-ethylenedioxythiophene) incorporating poly(styrene-4-sulfonate), which have already been well-studied by various research groups. These materials are prepared into gas sensing units either electrochemically or chemically to investigate their abilities in the detection of gases and solvent vapours.