Synthesis And Photocatalytic Properties Of Covalent Organic Framework Polymers Containing Nitrogen Units

Splitting water to produce hydrogen by photocatalysis or photochemocatalysis is an attractive way to convert intermittent solar radiation into a storable,clean fuel.The rational selection and design of photocatalysts is an important factor affecting the efficiency of photocatalytic water splitting.Since Fujishima first used titanium dioxide as a photoelectrode for water separation in 1972,researchers have put a lot of effort into developing inorganic semiconductor materials such as TiO₂,Zn O,and Cd S.However,inorganic materials also have some defects that cannot be ignored,such as unsatisfactory stability and limited adsorption capacity.Organic semiconductor materials have attracted extensive attention due to their low cost,good stability,broad visible light absorption,and tunable electronic structures.Therefore,it is necessary to study the hydrogen production activity of organic semiconductor materials as photocatalysts.In this dissertation,we designed and synthesized several covalent organic framework polymers containing nitrogen units,and use them as photocatalysts to study their properties through a series of characterization.The main contents are as follows:1.Three kinds of D-A-type CMPs（BTPT-CMP1,BTPT-CMP2,and BTPT-CMP3）based on thiophene-triazine were successfully synthesized by Stille coupling reaction.The effect of crosslinking agent length on photocatalytic hydrogen production was studied by optical and electrochemical characterization.The hydrogen evolution results show that the hydrogen production rate of BTPT-CMP1 with a single thiophene cross-linker reaches5561.87μmol g^-1 h^-1 under visible light,which is 3.02 times and 3.47 times higher than that of BTPT-CMP2 and BTPT-CMP3,respectively.The apparent quantum yield（AQY）of3.8%was obtained at 420 nm.The excellent hydrogen production rate of BTPT-CMP1 is mainly attributed to:（1）larger specific surface area and wider pore-size distribution are beneficial for providing more active sites;（2）little interface charge transfer resistance of BTPT-CMP1 help photo-generated electrons transport,and further activate more reaction substrates.2.Au@TiO₂ and TiO₂-12%Tr Th binary composites and a series of Au@TiO₂-X%Tr Th ternary composites were successfully prepared by in-situ polymerization.At the same time,the content of pure polymer Tr Th in Au@TiO₂-X%Tr Th ternary composite was optimized.Compared with Au@TiO₂ and TiO₂-12%Tr Th binary composites,Au@TiO₂-X%Tr Th ternary composites exhibit very high photocatalytic hydrogen production rate under visible light irradiation.The optical absorption results show that Au@TiO₂-X%Tr Th exhibit a wider absorption region than binary structure composites.The optimal Tr Th loading in Au@TiO₂-X%Tr Th is 12wt%,and the corresponding H₂ production rate is up to 4288.54μmol g^-1 h^-1,which is about 312.3 times and 9.1 times than Au@TiO₂ and TiO₂-12%Tr Th under the same conditions.3.Two covalent organic framework materials（TAPT-COF and TTPA-COF）were successfully synthesized by Schiff base condensation reaction.Compared with TAPT-COF(2028.06μmol g^-1 h^-1),TTPA-COF showed an excellent hydrogen production rate of6485.05μmol g^-1 h^-1,with an apparent quantum yield of 12.2%at 405 nm.At the same time,the photocatalysis stability experiment shows that TTPA-COF as a photocatalyst keeps good stability in the process of hydrogen production.The experimental and theoretical results that the improved photoabsorption,enlarged surface area,and enhanced charge separation efficiency by introduce pyridine nitrogen atoms into the COFs skeleton,which eventually boosting the visible-light photocatalytic water splitting for hydrogen evolution.