Preparation, Microwave Absorption And Electrochemical Properties Of Polyaniline Nanocomposites

Polyaniline (PANI) as a typical conducting polymer, has been applied in microwave absorption, electromagnetic wave sheilding and supercapacitor electrodes for its ease of synthesis, excellent physical and chemical performance as well as environmental stability. However, it still belongs to a class of semiconductors even after doped with protonic acids, the drawback of low electric conductivity restricts its applications. In order to overcome this problem, PANI is usually combined with some other nanomaterials with high electric conductivity such as carbon nanotubes or graphene nanosheets. In addition, nanocomposites of PANI combined with magnetic nanomaterials are expected to have good electric as well as magnetic properties. Carbon quantum dots (CDs), as a new kind of carbon-based nanomaterials, refer to tiny carbon nanoparticles in size of sub-10 nm, and composed by sp2-hybridized carbon atoms. In recent years, CDs have attracted more and more attentions in the area of cell imaging, biosensor and photocatalysis applications, due to their low cost, ease to synthesize, excellent flourescence and very low cytotoxicity. More importantly, it has been reported that CDs are both good electron donors and electron receptors. In this thesis, CDs were prepared through one-pot pyrolysis method by using konjac flour as a new carbon resource. Two ternary composites of PANI/ferrocene/CDs and PANI/CoS/CDs were synthesized by in situ polymerizations respectively. The morphology, microstructure, capacitance and microwave absorption properties of both ternary composites have been studied. In particular, the influence of an applied magnetic field (MF) on morphology, microstructure, capacitance and microwave absorption properties of PANI/CoS/CDs has been investigated. The main works are included as follows:1. CDs with a size distribution of 1-2 nm were prepared by using konjac flour as a carbon source. A ternary composite of PANI/ferrocene/CDs was synthesized by in situ polymerization of aniline, CDs and ferrocene. The morphology, microstructure and compositions of PANI/ferrocene/CDs have been characterized by using scanning electron microscope (SEM), transmission electron microscope (TEM), hydrogen nuclear magnetic resonance (1H NMR) spectrum, photoluminescence (PL) spectrum and thermogravimetric (TG) analysis. In the meantime, microwave absorption property of PANI/ferrocene/CDs has also been studied. Obviously, compared to PANI or binary composite of PANI/CDs, the ternary composite of PANI/ferrocene/CDs shows a much stronger microwave absorption, for example, the maximum reflection loss can reache -25.4 dB at the frequency of 5.4 GHz. The result of electromagnetic parameters measurement indicates that the enhanced microwave absorption property of PANI/ferrocene/CDs arises mainly from the increased dielectric loss. Two Cole-Cole semicircles in Debye relaxation curves reveal that there are dual dielectric relaxation losses:one is dipolar relaxation and the other is interfacial polarization, in which a major contribution may come from the enhanced interfacial polarization owing to smaller size and higher dielectric constant of CDs than those of PANI.2. The ternary composite of PANI/CoS/CDs-0.5T with a hollow-spherical morphology was synthesized by in situ polymerization of aniline, CDs, CoCl2 and Na2S under an applied MF of 0.5 T. Compared to PANI/CoS/CDs-OT that was synthesized without a MF, the crystallinity of PANI/CoS/CDs-0.5T has been greatly improved. Besides, PANI/CoS/CDs-0.5T shows a much better microwave absorption than all of PANI-0.5T, PANI/CoS-0.5T and PANI/CoS/CDs-OT. And the maximum reflection loss of -24 dB at 14 GHz can be achieved when molar ratio of aniline, Na2S, and CoCl2 equals to 3:1:1. Moreover, PANI/CoS/CDs-0.5T exhibits a weak ferromagnetic property whereas PANI/CoS/CDs-OT shows no ferromagnetism. Therefore, the additional magnetic loss must have contribution to the enhanced microwave absorption of PANI/CoS/CDs-0.5T compared to that of PANI/CoS/CDs-0T. Besides, the presence of several Cole-Cole semicircles suggest that not only Debye relaxation, but also some other mechanisms such as Maxwell-Wagner relaxation (mainly interfacial polarization) and electron polarization exist in the composite of PANI/CoS/CDs-0.5T.3. The electrochemical performance of PANI/CoS/CDs-0.5T and PANI/CoS/CDs-OT has been studied with/without an applied MF, respectively. The results reveal that the capacitance of both PANI/CoS/CDs-0.5T and PANI/CoS/CDs-OT tested under an applied MF is always higher than that tested without an applied MF. Moreover, the capacitance of PANI/CoS/CDs-0.5T is a bit larger than that of PANI/CoS/CDs-OT, which can be attributed to larger spectific surface area as well as better crystallinity of hollow spherical composite of PANI/CoS/CDs-0.5T than irregular-shaped composite of PANI/CoS/CDs-OT. The MF-induced orientation of PANI chains leads to more orderly packing chains and thus an improved crystallinity. As for the electrodes made of either PANI/CoS/CDs-0.5T or PANI/CoS/CDs-OT, EIS results indicate that an applied external MF can reduce the charge transfer resistance at electrode/electrolyte interface, which lead to their larger magnetocapacitances than both tested without a MF.