Preparation Of Conducting Polymer Nanomaterials In Room Temperature Ionic Liquids

Conducting polymers have attracted considerable attention because of their application in electrochemical catalysis, sensor, capacitor, etc."Green solvent"room temperature ionic liquids are environmentally benign, and exhibit many excellent physical and chemical properties. They play an important part in green chemistry. This dissertation is concentrated on the preparations of nano-sized conducting polymers and conducting polymer composites by using electrochemical, chemical and photo-induced polymerization in room temperature ionic liquid solutions or microemulsions composed of room temperature ionic liquid and water. Their micrographs, structure, properties and applications have been investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), infrared absorption spectra (IR), ultraviolet visible absorption spectra (UV–vis), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS), etc. The main points of this dissertation are summarized as follows:(1) Three room temperature ionic liquids 1-ethyl-3-methylimidazolium hexafluorophosphate ( [emim]PF₆ ) , 1-butyl-3-methylimidazolium hexafluorophosphate ( [bmim]PF₆ ) and 1-octyl-3-methylimidazolium hexafluorophosphate ([omim]PF₆) were prepared. Their electrochemical stabilities were investigated by CV, and corresponding electrochemical windows was 4.14 V, 3.40 V, and 3.28 V, respectively. Longer the substituted group on room temperature ionic liquid cation is, weaker the effect of impurities on the electrochemical stability of room temperature ionic liquids is.(2) The electrochemical behavior of CoCl₂ in [bmim]PF₆ was investigated by CV. The well-defined cyclic voltammograms were obtained from electrochemical measurement under different temperatures, and the reversible behavior for Co²⁺/Co³⁺ redox couple on glassy carbon electrode in [bmim]PF₆ was confirmed by the characteristic of the peak currents. The diffusion coefficients (about 10^-11 m²Â·s^-1) of Co²⁺ in [bmim]PF₆ under different temperatures were evaluated from the dependence of the peak current density on the potential scan rates in cyclic voltammograms. It is found that the diffusion coefficient increases with increasing temperature. Diffusion activation energy of Co²⁺ in [bmim]PF₆ is also calculated to be 23.4 kJ?mol-1 according to the relationship between diffusion coefficient and temperature. It is confirmed that [bmim]PF₆ is a good solvent for electrochemical reactions.(3) The electropolymerization of benzotriazole on an Au electrode was investigated via CV and chronoamperometry in a room temperature ionic liquid medium, [bmim]PF₆ containing glacial acetic acid. The chronoamperometric investigation revealed that the instantaneous nucleation predominated the potentiostatic electropolymerization of benzotriazole at the oxidation peak potential. SEM image indicated that the polymer film was compact and relatively smooth and IR result suggested the polymer chains were formed mainly via coupling of the unsaturated nitrogen atoms. The polymer was found to be highly electroactive, showing a quasi-reversible and stable pair of redox peaks centering at 0.9 V versus Ag/AgCl in 0.1 mol?L-1 H2SO4 solution.(4) Room temperature ionic liquid microemulsion, a new way to synthesize the controllable size and shape of nano-scale materials, has received increasing attention. Polyaniline (PAN) has been prepared by CV in room temperature ionic liquid-in-water (IL/W) microemulsion. The electrochemical properties of the PAN prepared in IL/W microemulsion were compared with that of the PAN polymerized in conventional acidic aqueous solution. Also, the effects of the potential cycle number on the specific capacitance and electrochemical characteristics of the PANI film electrode were discussed in detail. The result shows that the specific capacitance of the PAN film obtained by 50-cycle electropolymerization is 334 F·g^-1 at the charge-discharge current density of 2 mA·cm^-2. The SEM image indicates that the PAN film presented cauliflower like morphology, and the EDS elemental analysis result suggests the polymer were co-doped with SO₄^2- and PF₆^-.(5) Chronopotentiometry was employed to prepare polyaniline-silver (PAN-Ag) nanocomposite films in water-in-room temperature ionic liquid (W/IL) microemulsion and room temperature ionic liquid-in-water (IL/W) microemulsion. The resulted nanocomposites were characterized by SEM, TEM, HRTEM, and XRD. It is demonstrated that the PAN-Ag nanocomposite prepared in W/IL microemulsion is nanofibrous and the Ag nanocrystals with 5 nm diameter are dispersed homogeneously, whereas the morphology of the PAN-Ag nanocomposite prepared in IL/W microemulsion exhibits dendritic structure and the diameter of Ag nanocrystals is 50-100 nm. Further, the effects of different microemulsion systems and electrochemical synthesis conditions on the electrochemical properties of the nanocomposite films were studied by CV and EIS. The pure PAN films were also made for comparative purpose. It is found that the special structures of the PAN-Ag nanocomposite result in more excellent electrochemical activity than that of the pure PAN.(6) Three nano-sized PAN powders doped with room temperature ionic liquid [bmim]PF₆ and dodecyl benzene sulfonic acid (DBSA) or hydrochloric acid (HCl) have been prepared in room temperature ionic liquid-in-water microemulsion system. The oil-phase room temperature ionic liquid was used as both monomer solvent and doped counterion. The effects of different counterions on the properties (molecular weight, electrical conductivity, glass transition temperature, electrochemical activity) of PAN were investigated. The PAN co-doped with [bmim]PF₆ and DBSA shows the highest molecular weight (81104 g·mol^–1), the highest electrical conductivity (1.85 S·cm^–1) the lowest glass transition temperature (181°C) and the highest redox reaction current density, while the PAN doped with [bmim]PF₆ only exhibits the lowest conductivity (0.018 S·cm^–1) and lower redox reaction current density. The PAN co-doped with [bmim]PF₆ and HCl shows higher conductivity. Also they exhibit good electrochemical stability and charge/discharge performance in neutral medium. These indicate that co-doping of different counterions under acidic condition could improve oxidation degree and doping ratio of PAN, and results in high electrical conductivity and good electrochemical properties.(7) Photo-induced polymerization was employed to prepare PAN nanoparticles in room temperature ionic liquid for the first time. Photons and photoactive room temperature ionic liquid cations replaced conventional oxidants and metal complexes to promote the polymerization of aniline monomer. The diameter of the resulted PAN is confirmed in nano-scale by SEM. With increase of protonic acid in medium, the yield of the PAN increased, the UV absorption of the PAN strengthened, and a blue shift of the p-polaron absorption was observed. And the conductivity of the PAN also increased with the acid content in medium. The potential mechanism of photo-induced polymerization of aniline was proposed. Moreover, after the room temperature ionic liquid was separated from the reaction mixture and reused for five times, no obvious decrease in catalytic activity could be found in photo-induced polymerization of aniline. The method may open a new pathway to prepare nano-scale conducting polymers with sunlight.