Study In Novel Polymer Photocatalytic Materials

The increasingly critical energy crisis and environmental pollutions are two major obstacles to achieving sustainable development,and photocatalytic technology which converts clean solar energy into chemical energy has shown great prospects in scientific and application aspects because of its huge potential to solve these two problems concurrently.In recent years,conjugated polymer semiconductors as photocatalytic materials have become an emerging and highly attractive research topic because of several advantages such as structural tunability,plentiful sources,easy to process and low production costs,low density and free of toxic metal elements.Our research group previously developed and improved a kind of polyimide(PI)based photocatalyst for the first time in the world which is active in photocatalytic degradation of dyes and hydrogen evolution from water splitting.While the weak light absorption in visible region due to relative low degree of conjugation and wide bandgap of PI backbone,plus the high recombination rate of photoinduced electrons and holes limit the photocatalytic efficiency of PI under visible light.So it is necessary to broaden the light absorption and enhance the charge separation of PI photocatalysts,and develop non-imide based novel polymer photocatalysts with higher degree of conjugation.Based on the above background,in this dissertation the following researches have been carried out:1.A series of porphyrin-containing PI materials was developed by using meso-tetrakis(4-aminophenyl)porphyrin,melamine and pyromellitic dianhydride precursors via a one-step solvothermal method previously developed by our group for PI photocatalysts.The structures of materials were characterized with various methods and the results revealed that porphyrin was uniformly coupled into the PI framework through covalent bonding and the visible-light(400?700 nm)absorption was greatly improved,while steady-state photoluminescence and time resolved fluorescence decay tests demonstrated efficient energy transfer between PI and porphyrin.The photocatalytic activities of materials were evaluated by the photodegradation of methyl orange dye(MO)under visible light(?>420 nm),and the activity of porphyrin-containing PIs was enhanced significantly,with the highest pseudo-first-order rate constant 11 times that of neat PI and 35 times that of neat porphyrin.The photocatalytic mechanism was explored and the degradation of MO was mainly driven by·O₂^-via the reduction of O₂ by photoinduced electrons.Theoretical calculations revealed that the HOMO level of porphyrin-containing PI was lower than porphyrin while higher than neat PI,So the oxidation capability of holes of porphyrin-containing PI was also intermediate,and the influence of the energy level of porphyrin over the photocatalytic activity was further confirmed through a series of control samples with porphyrin adsorbed on the PI surface.This work not only succeeded in promoting the photocatalytic activity of PI by introducing dye molecules,but also provided an insight into multiple effects of dye molecules.2.A new hydrazone-linked conjugated polymer THT-TA was constructed with simple building blocks 2,4,6-trihydrazino-1,3,5-triazine(THT)and terephthalaldehyde(TA)via polycondensation in solution,and the material was carefully analyzed by various characterizations and theoretical calculations.The structural characterizations demonstrated that the chemical structure of the polymer agree with expectations,and TEM images clearly revealed the 2D sheet-like morphology.XRD measurements revealed semi-crystallinity nature of the polymer,and the analysis of diffraction pattern suggested that THT-TA may contain a novel parallelogram framework,rather than common hexagonal framework.Multiple reaction systems were explored and highest crystallinity of the product was achieved with dimethyl sulfoxide as solvent and pyrrolidine as catalyst.The optical band gap of THT-TA was 2.50 e V and the absorption edge is at 513 nm,while the frontier orbitals were delocalized across the framework from DFT calculation,validating that the polymer with higher degree of conjugation was achieved from small precursors which can only absorb UV light,and further calculation studies suggested that impurity levels may stem from active terminal groups.The photocatalytic hydrogen evolution was successfully achieved with Pt as co-catalyst and sodium ascorbate as sacrificial agent under visible light(?>420 nm),and the sample showed considerable stability in the long-term test for more than 200 h.This study diversifies the family of organic polymers synthesized from simple monomers with photocatalytic activity under visible light.3.Based on PI photocatalyst synthesized via solvothermal method,we constructed a series of P3HT-PI heterojunction photocatalysts by impregnating PI with poly(3-hexylthiophene)(P3HT)synthesized through chemical oxidation method,and investigated the interaction of P3HT and PI and photocatalytic activity at various loading amount.The energy transfer between PI and P3HT was confirmed by spectroscopy studies,while the chain packing of P3HT and the details of energy transfer varied with loading amount of P3HT.The photocatalytic hydrogen evolution tests demonstrated negligible hydrogen evolution activity over P3HT,and when the mass fraction of P3HT to PI is 1.5%,P3HT-PI exhibited optimal hydrogen evolution rate which is 3.0 times under full-arc light(?>300 nm)and 6.3 times under visible light(?>300 nm)as high as PI.The structure of PI exhibited no obvious difference before and after the impregnation,while p-type polymer semiconductor P3HT with narrow bandgap not only enhanced visible light absorption,but also can form type II heterojunction with n-type polymer PI and promoting the charge separation of the material,which both resulted in the enhancement of photocatalytic activity.The hydrophobic alkyl side chains of P3HT may be one of the cause of lower photocatalytic activity at higher loading amount,since P3HT and P3HT-PI exhibited higher charge-transfer resistance than PI from electrochemical impedance spectroscopy.This study demonstrated that PI based polymer-polymer heterojunction can be efficient photocatalyst.