Structural Design,morphology Regulation And Photocatalytic Application Of Covalent Triazine Framework Synthesized Via Polycondensation Of Amidine And Aldehyde

Covalent triazine frameworks（CTFs）represent an exciting new type of porous organic material,constructed by organic monomers via cyclic trimerization reaction,possessing characteristics like high conjugated skeleton,rich nitrogen content,good stability,and high specific surface area,that make it promising for diverse applications including optoelectronic,adsorption,catalysis,energy storage,etc.,especially for the photocatalytic applications.Until now,great progress has been made in photocatalytic water splitting and CO₂ reduction,but challenges still exist.Most of CTFs reported consist of benzene rings in their chemical structure and lack of suitable chelating metal sites,then it is difficult for the metal-based co-catalyst to disperse on CTFs and to support more active catalytic sites.Moreover,the poor crystallinity of CTFs affects the separation and transport efficiency of the photo-carrier,leading to its recombination.Additionally,the powder-type CTFs are not easy to be processed and realize the device of photocatalyst to achieve recovery and continuous operation as well as scaled-up application.To solve problems of CTFs with less functionality,low crystallinity,and poor processability,systematic research has been done in this work,including structure design of CTFs,synthesis strategy for crystalline CTFs,and preparation of CTFs film with controllable thickness and size.The main points are elucidated as follows:（1）CTFs with bipyridine group named CTF-Bpy,prepared from aldehyde monomer and amidine containing bipyridine through polycondensation reaction,were designed and synthesized for solving the problem of co-catalysts dispersion and used in photocatalytic CO₂ reduction.The CTF-Bpy gains high crystallinity and high surface area up to 900 m²g^-1,and exhibits a good CO₂ adsorption capacity of 12 wt%.Furthermore,high activity photocatalytic CO₂ reduction system（CTF-Bpy-Co）was constructed by fixing Co²⁺in the bipyridine sites.As the X-ray photoelectron spectroscopy and synchrotron radiation near side absorption spectrum confirmed that the distribution of Co²⁺in CTF-Bpy-Co is single-site.The CO evolution rate of the CTF-Bpy-Co system was 1200μmol h^-1 g^-1under≥420 nm light irradiation in the presence of Ru（Bpy）₃Cl₂ as a photosensitizer and triethanolamine as sacrificial agent,and this system keeps high activity in four cycles.（2）A novel strategy for the preparation of crystalline CTFs was developed deriving from aliphatic amine mediated synthesis method.The low reversibility of CTFs polymerization reaction makes it difficult to prepare highly crystalline CTFs.Aiming at enhancing the reversibility of polymerization reaction,in this part,a strategy that through Schiff base intermediate generated by aliphatic amines reacting with aldehyde monomer,to decrease the concentration of aldehyde monomer in solution and realize the controllability of reversible CTFs polymerization was developed.This method is benefit to form CTF-1 with high crystallinity and large specific surface area(774 m² g^-1).Different morphologies like nanoparticles（CTF-1-C3）,nanoribbons（CTF-1-C6）,and nanosheets（CTF-1-C18）were obtained by regulating the length of aliphatic amine segment.When potassium tert-butanol was used as a base catalyst,hollow tubular morphology was formed.And among these materials,CTF-1-C3 shows the best photocatalytic hydrogen evolution activity(H₂:10000μmol g^-1 h^-1),which is six times of amorphous CTF-1.（3）A new method was developed to prepare CTFs film and improve the processability of CTFs assisted by aliphatic amine.To cope with the poor processibility of CTFs,a Schiff base intermediate generated by n-hexamine reacting with p-benzaldehyde spread on the surface of dimethyl sulfoxide and polymerize with the surface confined effect to form CTFs film,which is transparent,free-standing,and can be easily transferred.Grazing incidence X-ray diffraction and X-ray diffraction analysis confirmed the film is partly crystallized.The thickness of the film can be manipulated from 30 nm to 500 nm by changing the concentration of monomers.what’s more,the size of the film can be tuned according to the size of the reactor,the maximum lateral size was up to 18 cm,and the area was up to 250 cm².In this work,an immobilized photocatalytic system was prepared by transferring the film to a glass substrate.The HER rate of the immobilized photocatalysts was high up to 5.4 mmol h^-1 m^-2 under visible light and maintained stable H₂ release activity for a long time（100 h+50 h）.