Synthesis And Gas Sensing Properties Of Nitrogen-contained Covalent Organic Polymers

Covalent organic polymers are a class of porous organic materials with wide application value.They have the advantages of structural diversity,high porosity,good modifiability,and strong stability.They are widely used in the fields of gas adsorption,heterogeneous catalysis,and gas sensing.Nitrogen is one of the important constituent elements of covalent organic polymers.In covalent organic polymers with carbon as the backbone,nitrogen usually acts as a functional group,which has a significant impact on the properties of covalent organic polymers.The configuration of nitrogen element is divided into four configurations of primary nitrogen,secondary nitrogen,tertiary nitrogen and quaternary nitrogen according to the valence bond connection of nitrogen atom.In this paper,nitrogen-containing covalent organic polymers are constructed and applied in the field of gas sensing.From the perspective of gas sensing,the differences in the properties of nitrogen elements in different configurations are revealed,and the mechanism of the sensing response generated by the interaction between nitrogen-containing groups and gas molecules is deduced in combination with related characterizations.In Chapter 2,we constructed four viologen-containing covalent triazine frameworks（Vio-CTFs）by ionothermal polymerization.Vio-CTFs have good sensing performance for NO₂,among which Vio-CTF-450 has a response value of 43%for50ppm NO₂,a response time of 12s,and a recovery time of 116s.With the increase of synthesis temperature,part of the structure of Vio-CTFs decomposes.The results of in situ UV-Vis experiments showed that the triazine ring was the active site for sensing NH₃,and the viologen group was the active site for sensing NO₂.It can be known by DFT calculation that when NO₂ interacts with the viologen group,some electrons are transferred to the viologen group.Combined with the Mott-Schottky test results,Vio-CTFs are n-type semiconductors,and the response of Vio-CTFs to NO₂ has a negative effect,thus further confirming that the viologen group is the sensing active site of NO₂.In Chapter 3,we constructed four covalent organic polymers containing a single nitrogen configuration.Among them,Amino-COP,Caz-COP and CTF have sensing responses to NH₃,and Vio-COF has sensing responses to NO₂.Characterization and calculation results show that there are charge transfers between NH₃ and primary nitrogen,secondary nitrogen and tertiary nitrogen groups,and between NO₂ and quaternary nitrogen groups to generate sensing responses.In the in situ UV-visible spectroscopy experiments,it was confirmed that there is an interaction between NH₃and primary nitrogen,secondary nitrogen,and tertiary nitrogen,and that there is an interaction between NO₂ and quaternary nitrogen.The mode of action of nitrogen groups.In Chapter 4,we synthesized one-dimensional pyridine polymers（PPy）.PPy has excellent sensing performance to NH₃,its response value to 100ppm NH₃ is 87%,the response time is 15s,and the recovery time is 254s.PPy has high sensing selectivity for NH₃.The baseline resistance and response value of PPy decreased with the increase of temperature.The response of PPy to humidity increased with the increase of relative humidity,and the response to humidity was difficult to recover,and the net response of PPy to NH₃ decreased with the increase of relative humidity.