Design,Synthesis And Properties Of Novel Carbon Nitride Materials

As a kind of organic semiconductor material with two-dimensional（2D）layered structure,carbon nitride has great application potential in many fields such as photoelectrocatalysis,energy storage,environmental remediation,sensing and biomedicine because of its simple synthesis,non-toxic,good physical and chemical stability,unique electronic structure.In recent years,carbon nitride has become a kind of star material widely concerned by scientific researchers.However,most researches are still limited to the traditional g-C₃N₄,it is of great significance to design and synthesize new carbon nitride materials.In this paper,carbon nitride materials are divided into three categories,C_xN_y,C_xN_yH_z and N-doped carbon nitride materials.In this paper,the precursors are designed and 5 kinds of new carbon nitride materials have been successfully prepared by simple synthesis methods,including 2 kinds of C_xN_y and 3 kinds of C_xN_yH_z carbon nitride materials.Their structures were confirmed and application directions were preliminarily explored.The work of this paper is as follows:1.A new polymer C_xN_y carbon nitride—C₃N₃ was mass synthesized on the surface of Cu foil by Ullmann polymerization with cheap cyanuric chloride as the precursor at a non-vacuum low temperature（260°C）.Before acid washed,C₃N₃ showed good crystallinity although there were Cu Cl and Cu Cl₂ impurities in the pores of the structure.After removing Cu impurities by acid washed,the chemical composition of C₃N₃ was proved,but strong acid introduced a lot of defects,its crystallinity decreased sharply.In the study of electrocatalytic hydrogen evolution reaction（HER）,C₃N₃ before acid washed showed 306 m V vs RHE over potential at the current density of 10 m A/cm²,and lower over potential of 256 m V vs RHE under the light,indicating it is a good photoelectrochemistry（PEC）HER catalyst.2.Based on the experience of obtaining C₃N₃ by Ullmann polymerization catalyzed by Cu,using Zn instead of Cu,C₃N₃ was successfully obtained on the surface of Zn foil under the same reaction conditions,which is easy to distinguish,named C₃N₃-Zn,and the product catalyzed by Cu was named C₃N₃-Cu.Compared with the insoluble Cu Cl impurity,the main by-product of Zn catalytic reaction is Zn Cl₂,which is easily soluble in water.Therefore,we obtained relatively pure C₃N₃ whose internal structure was not damaged by strong acid.Chemical characterizations confirm that C₃N₃-Zn and C₃N₃-Cu have the same structure.When C₃N₃-Zn is added between Sn O₂ electron transport layer and perovskite absorption layer as the interface modification layer at a lower concentration（0.09 mg/m L is the best）,the photoelectric conversion efficiency of perovskite solar cell device is increased by 1.91%,and the comprehensive performances are greatly improved,which indicates that C₃N₃-Zn has great application prospect in perovskite solar cells.3.Based on the precursor of cyanuric chloride,an ionic C_xN_yH_z carbon nitride material—C₃NH₂-（Cl）,was prepared by mechanical ball milling by nucleophilic substitution reaction between cyanuric chloride and 4,4’-bipyridine.The obtained C₃NH₂-（Cl）ionic polymer is highly water soluble.The structure of C₃NH₂-（Cl）was analyzed by liquid NMR and GPC,and its average molecular weight M_w is 129443.Although the BET specific surface area of C₃NH₂-（Cl）is less than 10 m² g^-1,it has excellent iodine vapor adsorption performance with a limit unit adsorption capacity of 2.48 g g^-1.This excellent iodine adsorption performance is mainly attributed to the highly conjugated structure,abundant N affinity sites in C₃NH₂-（Cl）,as well as the electrostatic interaction between the positive charges in the large cations and I3^-.4.Based on C₃NH₂-（Cl）with high iodine vapor adsorption,4,4’-bipyridine was replaced with 2,4,6-tris（4-pyridine）-1,3,5-triazine,and a new ionic C_xN_yH_z carbon nitride material—C7N3H4-（Cl）was successfully prepared in solvent using simple nucleophilic substitution.The chemical composition of C₇N₃H₄-（Cl）and highly crystalline structure were proved by a series of characterization.According to the N₂ adsorption and desorption result,C7N3H4-（Cl）’s BET specific surface area is 17.001 m² g^-1with a limit unit adsorption capacity for iodine vapor of3.38 g g^-1,which is higher than C₃NH₂-（Cl）,mainly attributed to a large number of highly conjugated triazine and pyridine rings,abundant N affinity sites,and the electrostatic interaction between the high density of positive charge carried by the large cations and I₃^-.5.From the solvent reaction of 4,4’-bipyridine and phosphonitrilic chloride trimer,two kinds of 4,4’-bipyridine ionic salt—C₁₀N₁₀H₂-（Cl）,a small ionic molecule C_xN_yH_z carbon nitride material were unexpectedly obtained,instead of an ionic polymer.The two crystals have the same chemical composition of C₁₀N₁₀H₂-（Cl）,which was revealed by single crystal X-ray diffraction.The crystal 2 obtained from the mixed solvent of acetonitrile and DMF can be exfoliated to ultra-thin 2D nanosheets（≈2 nm）by aqueous dispersion ultrasound.The Density Functional Theory（DFT）calculation and Natural Bond Orbital（NBO）analysis indicate that its stability is from a strong hydrogen bond assisted ionic interaction,which can maintain the independent existence of 2D ionic salts.Crystal 2 shows red fluorescence in both solution and solid state,especially in solution with a large stokes shift（≈386 nm）,which lays a foundation for its application in biomedical field.This work is not only conducive to the construction of new 2D materials,but also helps to understand the basic intermolecular interaction.