Structural Design And Synthesis Of Covalent Triazine Frameworks And Their Photocatalytic Hydrogen Evolution Performance

Hydrogen is considered as the ideal energy type because of the high energy density and environmental friendly combustion product.By virtue of the inexhaustible input energy and green output energy,photocatalytic hydrogen production via water splitting has become the most promising method of hydrogen energy production.With the similar composition of the g-C₃N₄ which is the most explored photocatalyst,covalent triazine frameworks(CTFs)with triazine as linkage show high conjugated system and enhanced light absorption ability.In addition,the diversity of building blocks and the construction methods of CTFs endow more flexiblities and more choices in the design and fabrication of CTFs,and thus CTFs express unique advantages in the field of photocatalytic hydrogen production.Up to now,although the research of CTFs has been achieved primary development in the application of photocatalytic hydrogen production by water splitting and the general problems in photocatalysis still remain to be challenges,that is how to tune the construction structure of photocatalyst and manipulate the interface between photocatalyst and water for suppression of the charge recombination and boosting the charge transfer efficiency inside the catalyst and increasing the photocatalytic activity and apparent quantum efficiency.Therefore,based on the above issues,a series of high photocatalytic activity CTFs with tunable structures and energy levels were constructed at the molecular level with the perspective of design D-A structure in the CTFs skeleton and adoption of sidechain engineering in CTFs and fabrication of the ordered arrangement of the molecular chains in CTFs,respectively.The main contents of this thesis are as follows:(1)A series of heteroatom-controllable D-A CTFs were constructed with the heteroatom doping precisely,which could tune the electron donor ability and the energy levels and the electron transfer path and thus to increase photocatalytic hydrogen production efficiency.in this work,three kinds of fluorene analogues substituted by N,S and O with atomic precision were selected as the building blocks.Theoretical calculation results showed that in these systems triazine ring acts as electron acceptor(A)and fluorene analogues as electron donors(D),and electron donor(D)ability is in the order of carbazole(N)>benzothiophene(S)>benzofuran(O).The experimental results showed that the increased electron donor ability can enhance the visible light absorption range of resulting CTFs.In addition,the stronger donor makes the LUMO energy level more negative in CTFs,resulting in the improvement of the driving force of electron migration.Furthermore,the increased donor ability can bring the strong interaction between electron donor and acceptor,which can suppress the recombination of electrons and holes effectively and the charge transfer efficiency can be enhanced.As a result,the final CTF-N with the strongest electron donor exhibits the highest photocatalytic hydrogen production efficiency,and HER can reach up to 538?mol h^-1,which was much higher than that of most conjugated porous polymers(CPPs)reported at that time.(2)D-A₁-A₂ CTFs were constructed with with the energy level gradient of donor and acceptors to support the excited electrons transfer directional path,and the probability of photoexcited electron transition to ground state was reduced largely and the recombination of electrons and holes can be suppressed effectively,resulting high photocatalytic hydrogen production efficiency.Ternary CTFs were prepared using polycondensation method with introduce of stronger acceptor(A₂)benzothiadiazole(BT)into the D-A CTF(CTF-N).The theoretical calculation results showed that electrons can transfer directionally along with the energy level gradient,thus electrons and holes can be separated effectively with large spatial distance.The experimental results showed that the introduction of A₂ can significantly reduce the recombination of electrons and holes,and more charges can be gathered under the same conditions.The final performance showed that ter-CTF-0.7 had the highest photocatalytic efficiency,and HER can reach up to 966mmol h^-1,which was about two times than that of D-A CTFs.(3)Based on"Bottom-Up"strategy,CTFs with sidechains were constructed to tune the energy levels and affinity towards water bifunctionally,and high photocatalytic hydrogen production efficiency was realized with the large driving force of electron transfer and effective mass transport at the same time.in this work,CTF-tert A with methyl amino sidechain expressed hydrophilic and CTF-C₃ with propyl side chain showed hydrophobic property,and CTF-C₀ without side chain structure was also prepared for comparison.The results showed that the existence of side chains can increase the layer spacing and reduce the conjugation degree of the system,resulting in the lower light absorption capacity.At the same time,with the electron donor ability of the tertiaryamino chain,the LUMO of CTF-tert A becomes more negative,which increase the driving force of electron migration.In addition,CTF-tert A express strong hydrophilicity,which is conducive to interaction between CTF and water.The final performance showed that CTF-tert A had the highest photocatalytic hydrogen production efficiency,and HER can reach up to 544mmol h^-1.(4)An universal strategy to fabricate crystalline CTFs with salt template was developed and the building block number of crystalline CTFs can be expanded,which can provide more choices to fabricate high effective crystalline CTFs in photocatalytic hydrogen production.The lattice of Na Cl can induce the growth of CTFs lattice,and Na⁺further may stabilize the newly formed hexagonal pores and thus Na Cl-template method showed strong universality.In this work 6 crystalline CTFs were successfully prepared with different building blocks.The resulting CTFs were prone to form thinner sheet layer,which is conducive to dispersion in aqueous solution and also to electron transfer.This work extended the monomer scope of crystalline CTFs and discussed the photocatalytic performance of crystalline CTFs.The results showed that the crystallized CTF-HUST-S3had the highest photocatalytic efficiency in photocatalytic simulated seawater,and HER can reach up to 791mmol h^-1.