Design And Synthesis Of Covalent Triazine Frameworks For Photocatalytic Water Splitting To Produce Hydrogen

Hydrogen energy is a high-calorific value,clean,and green energy source with advantages such as transportability and storage,making it a promising solution to address the energy crisis and environmental pollution.Using solar energy to drive water splitting to produce green hydrogen is not only the cleanest method but also a hot research topic in the field of energy photocatalysis.To promote the development and application of photocatalytic hydrogen production technology,the design and development of efficient photocatalytic materials are crucial.Covalent organic frameworks（CTFs）have gained attention in photocatalytic water splitting due to their strong conjugated structure,high stability,and large specific surface area.However,the application of CTFs in photocatalysis is still limited by high photoinduced charge recombination rates,poor hydrophilicity,weak visible light absorption ability,and a lack of available conjugated monomers.Therefore,in this thesis,we designed and developed a series of high-performance CTFs photocatalytic materials by modifying the functional groups of framework materials and regulating the electron donor-acceptor（D-A）skeleton at the molecular level.The main research contents are as follows:（1）Crystalline CTF-1 prepared by catalyzing the nitriles trimerization reaction using trifluoromethanesulfonic acid（Tf OH）,contains a large amount of unreacted cyano groups.hydrolysis of the cyano groups to amide groups was performed to achieve CTF-1 amidation.The theoretical calculations and experimental results demonstrated that amidation did not alter the crystal structure and skeleton structure of CTF-1 but optimized the energy band structure and enhanced the separation ability of electron-hole pairs,as well as promoting the transfer of photoinduced charges.As a result,the photocatalytic activity of CTFs for water splitting was improved.Visible light photocatalytic test results showed that CTFs with more amide groups exhibited better photocatalytic hydrogen evolution performance,among which CTF-amide-16exhibited a photocatalytic rate of 1133μmol g^-1 h^-1 and good stability.（2）To further uncover the effect of functional group modification on the photocatalytic activity of CTFs materials,this study investigates CTFs functionalized with electron-donating（–OH,–NH₂）and electron-withdrawing（–CN,–COOH）groups.By employing a binary polymerization strategy,modification groups are directly introduced onto the CTF-1 framework structure,and the content of modification groups is adjusted by changing the feed ratio of terephthalonitrile and monomers containing functional groups.The study reveals that a small amount of functional group modification can maintain the high crystallinity of CTFs,while significantly enhancing the catalytic activity of the materials.Theoretical calculations and experiment results demonstrate that CTFs functionalized with electron-donating groups（–OH,–NH₂）have narrower band gaps.In particular,the introduction of hydroxyl groups not only optimizes the energy band structure of CTFs,and enhances visible light absorption but also increases the lifetime of photoinduced carriers,effectively promoting the separation and transfer of photoinduced electrons and holes.Among them,CTF-OH-30 exhibits a visible light photocatalytic hydrogen evolution rate of 4952μmol h^-1 g^-1,an apparent quantum yield of 4.0%at 420 nm,and demonstrates excellent catalytic stability.（3）Based on the previous works,it was found that residual Tf OH was detected in the functionalized CTFs without ammonia treatment,and Tf OH could enhance the photocatalytic activity of CTFs.Among the amino-functionalized CTFs（CTF-NH₂-F）,CTF-NH₂-10-F exhibited the highest photocatalytic hydrogen production performance and good stability,with a visible light hydrogen production rate of up to 4485μmol h^-1 g^-1,which is 2.3 times higher than CTF-NH₂-10（without Tf OH）.The results indicated that residual Tf OH molecules are stable and have a strong interaction with CTF-NH₂ in the CTF-NH₂-F lattice.This interaction not only promotes the separation and transfer of photoinduced charges in CTF-NH₂-10-F but also enhances the affinity of CTF-NH₂-10-F to water,which facilitates the mass transfer between water molecules and the catalytic active sites.（4）To regulate the light absorption and electron transfer properties of D-A materials,1,3,4-oxadiazole conjugated units with electron-withdrawing properties were introduced into the CTF skeleton to construct novel D-A structured CTFs.This allowed for the development of enhanced photocatalytic activity.The results showed that the electron-withdrawing property of the 1,3,4-oxadiazole group widens the light absorption band of the materials,and promotes the separation and migration of photoinduced carriers.Compared with the single 1,3,4-oxadiazole-based element（TCP-OXD）,the stronger electron-withdrawing ability of the bi-1,3,4-oxadiazole-based element（TCP-BOXD）slightly shifts the light absorption band of the material towards the visible light region,but it exhibits better performance in carrier separation efficiency and migration rate.TCP-BOXD exhibits better photocatalytic activity under the full spectrum,with a hydrogen production rate of up to 3000μmol h^-1 g^-1.