Synthesis Of Carbon Nitride Materials And Investigation On Their Properties

Non-metallic semiconductor catalytic materials have drawn wide attention in solar energy,hydrogen energy and pollutant treatment,mianly due to their excellent physicochemical stability and non-toxic property.In semiconductor materials,carbon nitride materials have excellent optical and physicochemical properties,which make them very promising catalytic materials.As representative of carbon nitride materials,graphitic carbon nitride（g-C₃N₄）and N-doped graphene have shown excellent catalytic performance in many areas.Though g-C₃N₄ has excellent optical and physicochemical properties,which can be used as catalyst in many areas,its catalytic performance is not high enough,and more current researches are focusing on preparing a composite material but paying less attention to studying pure g-C₃N₄.Besides,few studies have reported to use g-C₃N₄ as an intermediate template to prepare other high-performance materials.Here we prepared g-C₃N₄ with high performance by pre-treatment or post-treatment to get morphology controlled g-C₃N₄ material and activated g-C₃N₄ material.In addition,we used the g-C₃N₄ as a template to prepare N-doped graphene with high performance,which broadened the application of g-C₃N₄.In this paper,we used high-temperature calcination method to perpare g-C₃N₄ or N-doped graphene through controlling precursor ratio and selecting oxidant and carbon sources.After exploring the experimental conditions and optimizing the preparation processes,we finally obtained high-performance materials.The main findings are as follows:（1）Melamine,cyanuric acid and urea were used as raw materials to prepare supramolecular precursors by mixing and sonicating in ethanol solution.By changing the molar ratio of melamine to urea,the ratio of hydrogen bonds to ionic bonds in precursor supramolecular can be controlled,resulting in different supramolecular system defects.After calcining the precursors at a high temperature,g-C₃N₄ materials can be obtained.Besides,the morphologies of g-C₃N₄ can be tailored from hollow to worm-like through adjusting the molar ratio of melamine to urea from 3:1 to 1:3.The as prepared g-C₃N₄ materials have high surface areas and excellent catalytic performance,expecially the worm-like g-C₃N₄,of which photocatalytic performance is 13times of the bulk g-C₃N₄.（2）Bulk g-C₃N₄ material was firstly obtained from calcination of melamine.A mixed solution of hydrogen peroxide and ammonia was employed to chemically activate g-C₃N₄.The activation process did not change the crystal structure,functional group,morphology and specific surface area of pristine g-C₃N₄,but it introduced H and O elements into the CN framework of g-C₃N₄,resulting in a broader optical absorption range,higher light absorption capability and more efficient separation of photogenerated electrons and holes.The photoactivity was investigated by the degradation of rhodamine B under visible light irradiation.It was found that the RhB dye in the activated g-C₃N₄ was mainly oxidized by the photogenerated holes.It is believed that sufficient holes account for the two-step degradation process because they would significantly improve the efficiency of the N-de-ethylation reaction of RhB.（3）The fabrication of N-doped graphene for oxygen reduction consists of a two-step calcination strategy and usesα-hydroxy acids（AHAs）as carbon source and melamine as nitrogen source.Three different AHAs,malic acid,tartaric acid,and citric acid were chosen as the carbon sources.In the first calcination,g-C₃N₄ was obtained from the thermal condensation of melamine and carbon intermediate was obtained fromα-hydroxy acids.Through the second calcination,g-C₃N₄ decomposed while carbon retained its layered structure to form graphene.In addtion,some N atoms inserted into C framwork,thus produced N-doped graphene.The as prepared N-doped graphene materials have high surface areas and high nitrongen contents,which show excellent catalytic porformance in oxygen reduction reaction（ORR）.