Design,Synthesis And Photocatalytic H₂ Evolution Properties Of Porous Materials Based On Covalent Organic Frameworks

Covalent organic frameworks（COFs）is a class of crystalline porous polymer materials.It has shown strong potential for photocatalytic solar energy conversion and H₂ production due to its multiple advantages such as modular synthesis,tunable band gap,high surface area,excellent visible light absorption,high carrier mobility and good physical and chemical stability.However,the shortcomings of photogenerated electron-hole pairs,which are prone to compounding,make them less effective in photocatalytic H₂ production.For this reason,many efforts have been made to solve this problem,for example,through morphological modulation,introduction of suitable structural units or linkers in their backbone,chemical doping,co-catalyst deposition or the construction of composite semiconductor materials.On this basis,this paper investigates the effects of imine COFs frameworks containing fluorine electron-absorbing groups,imide bonds,imine bonds containing hydrogen bonds,imine bonds on photocatalytic activity using imine COFs with excellent photocatalytic activity,and taking covalent triazine organic frameworks（CTFs）as the research object to investigate the effects of CTFs-based composite organic semiconductor materials on the photocatalytic activity.The main studies are as follows:1.The preparation of highly fluorinated imine COFs and investigation of their photocatalytic H₂ evolution properties.We prepared TFTA-COF materials containing fluorine substituents and TPAL-COF materials containing hydrogen substituents using 2,4,6-tris（4-aminophenyl）-1,3,5-triazine（TAPT）as the main precursor and reacted with 2,3,5,6-tetrafluoro-p-dibenzaldehyde（TFTA）and terephthalaldehyde（TPAL）,respectively,using a solvothermal method.The chemical composition and chemical structure,crystal structure,morphology,photoelectrochemical properties of the photocatalysts and their photocatalytic performance were analysed using a series of characterisation and tests.The results of the characterisation tests show that the fluorinated TFTA-COF has excellent photoelectrochemical properties.Photocatalytic tests showed that the H₂ evolution rate of the fluorinated TFTA-COF reached 80μmol·h^-1·g^-1,which was much higher than that of the non-fluorinated TPAL-COF.2.Investigation of the linkage bonds between structural moieties on the H₂ evolution properties of COFs photocatalytic materials.Using TAPT as the main monomer,we prepared PMDA-COF,hydrogen-bonded imine-linked DHTA-COF and TPAL-COF using a solvothermal reaction with 1,2,4,5-benzenetetracarboxylic anhydride（PMDA）,2,5-dihydroxy-p-benzaldehyde（DHTA）and TPAL,respectively.DHTA-COF with hydrogen bonding and TPAL-COF with amine bonding were prepared.The chemical composition and structure,morphology,crystal structure and photoelectrochemical properties of the photocatalytic materials were analysed using a series of characterisation and tests,and the effect of the linkage bonds between structural moieties on the photocatalytic activity was analysed using a typical photoreactor.The photochemical properties results show that PMDA-COF linked by imide bonds confers stronger conjugation effect,wider visible light absorption range and higher charge carrier mobility to the framework.The photocatalytic results showed that the H₂ evolution activity of PMDA-COF was 435.6μmol·h^-1·g^-1,which was about 8 and 64 times higher than the activity of DHTA-COF(56.2μmol·h^-1·g^-1)and TPAL-COF(6.8μmol·h^-1·g^-1),respectively.3.The preparation of CTF-1/GO-x composite organic photocatalytic material and investigation of their photocatalytic H₂ evolution performance.We have synthesized the organic compound semiconductor CTF-1/GO-x（x is the percentage of GO）by adding different mass ratios of graphene oxide（GO）during the synthesis of CTF-1 using a simple room temperature synthesis method.The differences in the visible light-catalyzed H₂evolution performance of CTF-1/GO-x are presented.The chemical composition and chemical structure,morphology,physical phase,photoelectrochemical properties of the composite photocatalysts and their H₂ evolution activity were analysed by a series of tests,physicochemical characterisation and photocatalytic experiments.It was shown that a strongπ-πconjugation effect between the CTF-1/GO-x composites was generated,which resulted in more superior structure and properties.The photocatalytic activity showed a significant enhancement of CTF-1/GO-x activity after GO modification,with an optimum H₂ evolution rate of 2262.4μmol·h^-1·g^-1,which was about 9 times higher than that of pure CTF-1(256μmol·h^-1·g^-1).