Preparation Of Conducting Polymer/Semiconductor Nanocomposites And Their Photocatalytic Activities

Environmental, energy sources and materials sciences are the three key projects of 21th century. One of them is the photocatalysis using oxide semiconductor, which associated tightly with those topics. Since 1972, numerous research groups have paid much attention to the photocatalytic technique and many achievements have been obtained. However, enhancing the photoconversion efficiency, maximizing the rate of photoinduced charge separation, and extending the photoresponse of the semiconductor catalyst into the visible range continue to be still pose a major challenge to the scientific community. Though there are many advantages for the enhanced photocatalytic activity, the semiconductor nano-material cannot be widely used because of its high surface energy and easy aggregation.Since 1977, conducting polymers (CPs) have drawn considerable interest because of their unusual electrical, optical and photoelectrical properties and their numerous applications in various fields. Recently, the preparation for the nanocomposites of CPs with inorganic nanoparticles has attracted many researchers' attention, aiming to obtain the materials with synergetic or complementary behaviors between the CPs and the inorganic nanoparticles. Because of their semiconductor energy level structure, broad absorption spectra and very high stability under irradiation of solar light, CPs can be used to photosensitize semiconductor oxides and to obtain the novel photocatalysts response to the visible light.In this paper, several conducting polymer/semiconductor nanocomposies were prepared. TiO₂, ZnO and SnO₂ were chose as the representative of semiconductor oxides. PANI, PTh and PPy were chose as the representative of conducting polymers. The crystal structure, surface morphology, absorption spectra and the thermic stability of the products were investigated by TG-DTA, XRD, XPS, TEM, SEM, AFM, FT-IR, UV-Vis, DRS, and GPC techniques. The photocatalytic activities of the nanocomposites were evaluated by different process. The photosensitive mechanism was also studied. The synthesis approaches used in this work include sol-gel method, solid-state technique, microwave irradiation means, in-suit polymerization and emulsion polymerization. The main results from those studies are summarized as following:1. PANI/TiO₂-Fe³⁺ nanocomposite was synthesized by sol-gel and in-suit polymerization methods. The photocatalytic activity of products was evaluated by the degradation of methyl orange.(1) The TG-DTA and XRD analysis indicated that pure and well crystalline anatase TiO₂ can be obtained at 500℃. Doping Fe³⁺ can improve the aggregation and inhibit the growth of TiO₂ nanoparticles. The average grain sizes were 18 nm and 10 nm for TiO₂ and TiO₂-Fe³⁺ nanopowders, respectively. The TEM and AFM shown that PANI/TiO₂-Fe³⁺ nanocomposite had a core-shell structure with TiO₂ as core and PANI as shell. The average grain size was 25 nm.(2) Compare with pure PANI, the IR absorbability of PANI/TiO₂-Fe³⁺ shifted slightly to the lower wavenumber, and the heat decomposition temperature of PANI/TiO₂-Fe³⁺ raised 180℃.(3) In the presence of PANI/TiO₂-Fe³⁺ nanocomposite as photocatalyst, the degradation rate of methyl orange was 70.3% under sunlight irradiation within 30 min, and the apparent rate constant was 5.64×10^-2 which was better than that of the P25.(4) Higher conduction band potential of PANI than TiO₂ was the main reason for the enhanced photocatalytic activity.2. A PANI/TiO₂ composite film deposited on the glass surface was prepared by sol-gel dip-coating technique and in-suit polymerization method. The photocatalytic activity of the products was evaluated by the degradation of rhodamine-B.(1) The AFM images show that the TiO₂ film consisted of cuboid shape and anatase phase TiO₂ nanoparticles. The average grain size of TiO₂ in the film is about 20 nm. After coating with PANI, the shape of the particle changed into irregular sphericity and the size is increased up to about 35 nm in diameter.(2) UV-vis spectroscopy analysis indicated that the absorption edges of the thin films were 370 nm for TiO₂, and 390 nm for PANI/TiO₂, respectively. A movement of approximately 20 nm towards the longer wavelength region was obtained from the coating of PANI on the surface of TiO₂. The band gap of PANI/TiO₂ film was 3.18 eV. XPS analysis showed that the ration of N⁺/N in PANI/TiO₂ was 0.63, which was higher than that of 0.5 in bulk PANI. (3) In the presence of PANI/TiO₂ nanocomposite film as photocatalyst, the degradation rate of rhodamine-B was 67.1 % under sunlight irradiation within 120 min and the activation energy of the reaction reduced 3.415 kJ·mol^-1.3. PTh/ZnO nanocomposite was synthesized by solid-state reaction method. The photocatalytic activity of the products was evaluated by the degradation of polyethylene film.(1) ZnO, PTh and PTh/ZnO nanopowders were prepared by solid-state reaction method withing 30 min, respectively. The shapes of the products were sphericity, whiskers and rodlike particles for ZnO, PTh and PTh/ZnO nanopowders, respectively.(2) Compare with the pure PTh, the heat decomposition temperature of PTh/ZnO raised 153℃. PTh/ZnO had stronger light absorbability than ZnO and PTh between 200 nm and 600 nm. The DRS analysis shown that the polymerization degree of PTh in nanocomposite was 9.(3) In the presence of PTh/ZnO nanocomposite as photocatalyst, the mass loss of PE films was 31.2% under UV irradiation (λ=253.7 nm) for 240 h. The AFM micrograph of the PE film after photodegradation showed a large number of pits on the film surface and finally disintegrated into powders.4. PPy/SnO₂/CA nanocomposite film was synthesized by microwave irradiation and emulsion polymerization methods and using cellulose acetate as film former.(1) The SnO₂ nanoparticles was prepared rapidly by microwave irradiation method and the average grain size was 13 nm. The TEM observation showed that PPy/SnO₂ nanocomposite had a core-shell structure with SnO₂ as core and PPy as shell. The thickness of the shell was 17 nm.(2) Compare with pure PPy, the heat decomposition temperature of PPy/SnO₂ raised 100℃. The UV absorbability of PPy/SnO₂ shifted slightly to the lower wavenumber.(3) In the presence of PPy/SnO₂/CA nanocomposite film as photocatalyst, the polymerization of MMA was happened under fluorescence light irradiation. The number-average molecular weight of the PMMA was 1.3×10⁵ and the distribution index was 1.06. Higher conduction band potential of PPy than SnO₂, donor strength of SO₃^2- and acceptor strength of H⁺ induced the polymerization of MMA.