Modulation Of Carbon Nitride Structures And Enzyme-like Activity Studies

Covalently bonded carbon nitride(CN)has been widely applied in the fields of photocatalysis,photoelectrochemistry and biosensors due to its excellent optical and electronic properties,facile synthesis and low cost.From structural view of point,carbon nitride is rich in unsaturated N atoms,which can provide abundant lone pairs of electrons for metal coordination.Along this line,these nitrogen atoms in the tri-s-triazine units are widely regarded as ideal sites for the inclusion of metal elements.For this reason,metal-doped carbon nitride(M-CN)materials have been investigated intensively.The doping of metals in the form of ions or single atoms greatly modified the electron structure of CN,not only improving the photocatalytic performance by accelerating the charge transfer,but also extending the application of CN to the fields of electrocatalysis,organic synthesis,and nanozymes.In terms of molecular structure of M-CN,CN can also be considered as a solid ligand for metal ions,similar to the M-N_x structure widely found in heme of natural enzymes.Accordingly,as an emerging new type of nanozyme,M-CN has drawn increasing attention in mimicking peroxidase for many bioassays.In natural enzymes,the chemical structure and spatial environment provided by ligands is crucial for the activity of the enzyme and the interaction with the substrate.However,the structural regulation of CN ligands has not been involved in M-CN nanozymes.As known,controlling polymerization degree of CN via condensation temperature is one of the simplest and most effective strategies for engineering its structure at the molecular level.CN with high degree of polymerization has a high graphitization degree and less-NH_x terminal group on the surface,whereas CN with low degree of polymerization has more defective moieties.In this context,the combination of copper and urea was selected from five metals(Mn,Fe,Co,Ni,Cu)and four nitrogen-containing precursors,then CN materials with different polymerization degree would provide a facile way to engineer the coordination environments for metal centers.In general,two strategies have been developed to prepare M-CN:one is bottom-up synthesis,and the other is the post-synthetic route.For the former,metals participate in the high temperature calcination process,which is more conducive to making full use of the sites to anchor metals.Nonetheless,for the latter,the solid ligand of CN could not move freely in principle,making the interaction between metal center and ligands greatly hindered,and thus probably limiting the further improvement of the activity of M-CN.It is envisioned that from the material preparation the great potential of M-CN nanozyme system would be unlocked by regulating the structure of CN ligands and comparing different synthesis pathways.The main contents of this thesis are as listed as follows:(1)Using urea as precursor,regulation of CN structures was realized by controlling the condensation temperature.The CN with different degree of polymerization showed broad differences in many aspects,such as crystal structure,functional groups and C/N ratio,providing various ligands for the further interaction between metals and CN.A series of Cu-CN materials with different degree of polymerization were prepared by post-synthesis physical adsorption method and bottom-up mixing calcination method respectively.(2)The peroxidase-like properties of Cu-CN materials prepared by the above two methods were compared.It is found that the Cu-CN prepared by bottom-up strategy not only provided more anchor sites for metal ions,but also endowed a stronger interaction with metals,thus showing superior peroxidase-like catalytic activity.Moreover,it is worth mentioning that samples synthesized at 400?were unique in both structure and properties.Cu-CN under the polymerization at 400?demonstrated the highest nanozymatic performance in the series of samples.Distinct to CN that were generally condensed at 550-600?for photocatalysis or nanozymatic reactions in previous works,it strongly disclosed that a minimum condensation degree already endowed an excellent activity in mimicking natural enzymes.This work would greatly pave the rational synthesis and application of M-CN nanozymes with a task-specific structure modulation.