| The titanium dioxide (TiO2) photocatalyst has been paid attention by scholars at home and abroad because of its excellent photocatalytic performance. However, there were many problems, such as lacking of visible light induced, lower photonic efficiency and selectivity, which imposed restrictions on the industrialized applications of TiO2. In order to improve the photocatalytic activity of TiO2, the modified methods of doping, dye photosensitization and semiconductor compound were researched, and the coupling between photocatalytic and microwavecatalysis, thermocatalysis, and electrocatalysis were also discussed. Unfortunately, industrialized applications of TiO2 were not realized. For the past few years, the research on conducting polymers modified TiO2 photocatalyst has made great progress. The conducting polymers had the extended conjugated double bond, and the electrons of delocalized ?-bond were not restrained by the related atom, so the conducting polymers had the excellent conductivity. On the one hand, the conducting polymers modified could reduce the energy of the exciting electron to widen the visible light response of TiO2, which solved the limitation of the narrow light response. On the other hand, the energy level of conducting polymer and TiO2 could match well to restrain the recombination between hole and electron and to enhance the photonic efficiency. As a result, the presence of conducting polymer improved the photocatalytic performance of TiO2 photocatalyst.In this paper, the TiO2 as the titanium source, aniline (ANI), o-phenylenediamine (oPD) and 2-aminobenzene sulfonic acid (2ABSA) as the monomer of conducting polymer, ammonium persulfate (APS) as the oxidant, the conducting polymers modified TiO2 nanocomposites were synthesized via the 'in situ' oxidative polymerization method. The prepared nanocomposites were used to characterize, to test the photocatalytic performance, and to propose the photocatalytic mechanism. This paper has done the following work:(1) The preparation and characterization of PANIs/TiO2 nanocomposites. PANIs/TiO2 nanocomposites were synthesized via the'in situ' oxidative polymerization method. The influencing factors, such as the molar ratio between monomer and TiO2, the molar ratio between monomer and oxidant, the concentration of hydrochloric acid, were analyzed via photocatalytic degrading methylene blue. TiO2 and PANIs/TiO2 nanocomposites were characterized by scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray diffraction (XRD), elemental analysis (EA), Fourier translation infrared spectrum (FT-IR), ultraviolet-visible diffuse reflectance spectrum (UV-Vis DRS), Photocurrent Test, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). The results of SEM and TEM indicated that the TiO2 and PANIs/TiO2 nanocomposites were polygonal particle, and the grain sizes were about 30-50 nm. The results of XRD, EA and FT-IR indicated that the presence of conducting polymer did not impact on the shape, grain size and lattice structure of TiO2. And the results of UV-Vis DRS, Photocurrent Test, CV and EIS indicated that the presence of conducting polymer enhanced visible light response and photonic efficiency of TiO2, and changed the redox reaction on TiO2 surface.(2) The photocatalytic reactivity test of PANIs/TiO2 nanocomposites. The optimal preparation condition of PANI/TiO2 nanocomposites was that the molar ratio between ANI and TiO2 was 1:5, the molar ratio between ANI and APS was 1:1 and concentration of hydrochloric acid was 1.2 mol/L. The optimal preparation condition of PoPD/TiO2 nanocomposites was that the molar ratio between oPD and TiO2 was 1:4, the molar ratio between opD and APS was 1:1 and concentration of hydrochloric acid was 1.2 mol/L. The optimal preparation condition of P2ABSA/TiO2 nanocomposites was that the molar ratio between 2ABSA and TiO2 was 2:1, the molar ratio between 2ABSA and APS was 1:1 and concentration of hydrochloric acid was 1.2 mol/L. The decolourization ratio of 40 mg/L methylene blue (MB) was 59.3% for PANI/TiO2 nanocomposites,44.6% for PoPD/TiO2 nanocomposites, and 92.0% for P2ABSA/TiO2 nanocomposites respectively, under the nanocomposites prepared with optimal preparation condition as photocatalysts, concentration of PANIs/TiO2 nanocomposites as 1.0 g/L, initial pH value as 3.5±0.5, and reaction time as 180 min.By taking the nanocomposites prepared with the optimal preparation condition as photocatalysts, and methylene blue as target pollutant, the technological parameters were optimized, including initial pH value, PANIs/TiO2 concentration and nanocomposites types. The results indicated that the photocatalytic degradation rate of MB increased with initial pH value increasing from pH=3.5±0.5 to pH=11.5±0.5. The optimum concentrations of PANI/TiO2,PoPD/TiO2 and P2ABSA/TiO2 nanocomposites were 1.0 g/L,1.5 g/L, and 1.5 g/L respectively. The order of photocatalytic performance was P2ABSA/TiO2> PANI/TiO2> PoPD/TiO2> TiO2, and the possible reasons were the energy gap and the energy lever structure of conducting polymer.Through five cycle times, the PANIs/TiO2 nanocomposites were still with excellent photocatalytic performance. It means that the prepared PANIs/TiO2 nanocomposites had the outstanding photocatalytic stability. The good photocatalytic stability of PANIs/TiO2 nanocomposites is depended on their stability of structure.(3) The photocatalytic mechanism of PANIs/TiO2 nanocomposites. Contributions of reactive oxidation species (ROS) were determined by using trapping experiment, with ethylenediamine tetraacetic acid (EDTA) as the h+scavenger, tert-butyl alcohol (TBA) as the ·OH scavenger, and benzoquinone (BQ) as the ·O2-scavenger. The results indicated that the contribution order of ROS was·OH>h+> ·O2- for PANI/TiO2 and P2ABSA/TiO2, and·OH?h+>·O2- for PANI/TiO2.Through the analysis of UV-Vis DRS and photocatalytic mechanism, the reasons for enhancing photocatalytic performance of conducting polymers modified TiO2 nanocomposites were photosensitization-synergetic effect between conducting polymer and TiO2. The photosensitization effect was connected with the band gap of conducting polymer, and the band gap of PANI/TiO2, PoPD/TiO2 and P2ABSA/TiO2 nanocomposites are 2.85 eV,2.96 eV and 2.78 eV respectively. The synergetic effect was connected with the structure and energy level of conducting polymer, and the synergetic factors of PANI-TiO2, PoPD-TiO2 and P2ABSA-TiO2 were 2.29,1.33 and 6.57 respectively.Based on the coloring mechanism and decolorizing mechanism of MB, and combined with literatures and the mass spectrum, two possible photocatalytic degradation pathways of MB were proposed, and the possible photocatalytic degradation pathways of phenol were also proposed. The first degradation pathway of MB tended to degrade the chromophoric group, and the second degradation pathway of MB tended to degrade the auxochrome group. Phenol was oxidized into hydroquinone and catechol due to the -OH as activated group for ortho-position and para-position. Then the benzene ring was opened to produce maleic acid, succinic acid and oxalic acid, and the low-molecular-weight acids were further mineralized to carbon dioxide and water. |
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