Study On Regulation Of The Distribution Of Degree Of Polymerization In Graphitic Carbon Nitride And Its Photocatalytic Activity

As a newly developed photocatalyst,graphitic carbon nitride?g-CN?has attracted worldwide attention due to its metal-free feature,facile synthesis,appealing band structures,and extraordinary chemical ability.However,the polymerization process during the thermal condensation is still poorly understood,and the synthesized samples show a complex structure and low specific surface areas,with photocatalytic activities far from being optimized.In this dissertation,we presented a new understanding on the thermal condensation process of g-CN from the point view of polymerization reaction kinetics,and proposed a quasi-sealed condensation route to regulate the distribution of the degree of polymerization in the final product.Both the crystallinity and surface chemical states of the synthesized g-CN have been greatly improved,leading to a substantially enhanced photocatalytic activity.The research contents of this dissertation mainly include:For the first time,we pointed out that the thermal condensation of g-CN is a molecular crystal-based,solid-state reaction.The effect of the crystal topochemical environments on the polymerization reaction kinetics has been studied,and the condensation process was found to be more kinetically favorable to conduct a linear polymerization.The produced melon chains with different degrees of polymerization?DPs?are assembled in a zigzag fashion,between them the classic hydrogen bonds can be formed and generate a layered structure.We found that the residual NH₃ concentration in the reaction environment is a key kinetic factor to affect the deamination polymerization process of g-CN,and proposed a quasi-sealed thermal condensation method to regulate the DP distribution in the synthesized g-CN.Graphitic carbon nitride samples were prepared by thermal condensation of melamine under the quasi-sealed condition,in air and flowing nitrogen atmospheres,respectively.The crystal structure,morphology,composition and chemical bonding states have been systematically characterized,and their photocatalytic activities have been extensively evaluated by photocatalytic hydrogen evolution and photodegradation of organic dyes,respectively.Compared with the conventional g-CN,the samples produced by quasi-sealed thermal condensation possess a more uniform DP distribution,which makes the final product shows better crystallinity and higher specific surface areas with fruitful uncondensed-HN₂ groups.The newly synthesized g-CN show much better water dispersibility,which promotes the fast charge carrier transport in aqueous solution and give rise to a remarkably improved photocatalytic activity.The experimental results indicated that compared to the conventional counterpart prepared in flowing nitrogen,the photocatalytic activity of g-CN synthesized in quasi-sealed condition is 4.98 times higher for hydrogen evolution and 7.59 times higher for photodegradation of rhodamine B.Furthermore,the synthesized g-CN photocatalyst showed an impressive catalytic stability.