Functional Modification And Photocatalytic Performance Of Graphite Carbon Nitride Nanosheets

Since the industrial revolution in the 18th century,with the rapid improvement of science and technology,global environmental pollution and energy shortages have become increasingly prominent,which has severely limited the sustainable development of human society.Compared with chemical methods,photocatalytic technology has the advantages of economy,cleanliness,safety and reproducibility,and has attracted wide attention from the scientific community.However,many common photocatalysts cannot be used due to their harmful to the environment,narrow band gap,and poor chemical stability.Graphite carbon nitride(g-C3N4),as a metal-free photo catalyst,has quickly become a popular material in this field due to its advantages such as cheapness and availability,good physical and chemical stability,suitable band structure,and easy modification.However,g-C3N4 prepared by the thermo polymerization method has a small specific surface area,photo-generated carriers are easily recombined,and quantum efficiency is low,which greatly limits its photocatalytic performance.Therefore,in recent years,researchers have devoted themselves to designing various nanostructures,constructing heterojunctions,and doping various elements to improve the photocatalytic performance.However,in practical applications,there are still many problems with the modified g-C3N4,such as poor dispersion in the organic phase,which is not conducive to catalyzing organic reactions;lack of active sites for nitrogen fixation reactions,it is difficult to improve nitrogen fixation performance;without suitable carriers,it is difficult to recycle the powder;traditional modification methods have complicated processes and high costs.The main content of this dissertation is to perform functional modification based on graphite phase carbon nitride nanosheets(CNNS)to effectively solve the above problems.CNNS is made by alkali treatment from g-C3N4.The thickness is extremely thin.The surface has a large number of amino groups and hydroxyl groups and has good hydrophilic ity.We modified the 4-trifluoromethylfluorenyl group to the surface of CNNS by covalent grafting reaction.Its hydrophobic ity and electron-withdrawing properties can optimize the lipophilicity and band structure of CNNS,which is good for catalyzing organic reactions at the phase boundary.At the same time,grafting some chelates can also introduce metal ions into the catalytic system,acting as active sites,and ultimately achieving efficient nitrogen fixation performance.In addition,the study shows that the hydrophilic of CNNS also has great application potential.Through hydrogen bonding,CNNS is stably supported on the surface of textiles,realizing self-cleaning of textiles and efficient degradation of formaldehyde.1.The surface of CNNS prepared by the alkali treatment has a large number of amino groups and hydroxyl groups,which not only improves the hydrophilicity,but also is easy to modify.The group covalently grafted 4-trifluoromethylbenzyl(TFMB)onto the surface of CNNS through a nucleophilic substitution reaction of amino and fluorenyl bromide groups,and successfully synthesized 4-trifluoromethylbenzyl-modified carbon nitride nanosheets(TFMB-CNNS).The TFMB group is highly hydrophobic.Under the combined action of hydrophilic hydroxyl groups,TFMB-CNNS is amphiphilic and can stay at the oil/water two-phase interface stab lily.This special dispersion property enables TFMB-CNNS to drive the phase boundary catalysis reaction.The research group designed experiments to realize the application value of TFMB-CNNS by photocatalytic reduction of nitrobenzene in the organic phase,conversion to aniline,and purification and enrichment in the water phase TFMB-CNNS exhibits highly efficient and repeatable photocatalytic performance for reducing nitrobenzene,and is universal.In addition,the mechanism of TFMB-CNNS phase boundary photocatalysis was deduced through experiments and calculations.It was concluded that the electron-withdrawing ability of TFMB group can promote the separation of photo-generated carriers of CNNS and improve its photocatalytic performance2.After the covalent grafting reaction to modify the CNNS was initially effective,we tried to graft EDTA groups and introduce transition metal chelates on the surface of the CNNS as the active site of the nitrogen-fixing reaction.Experiments show that EDTA groups can be stably grafted onto the surface of CNNS by dehydration condensation reaction of amino and carboxyl groups,and then the coordination ability of EDTA units can be used to prepare carbon nitride nanosheets containing a large number of highly dispersed active sites of iron(Fe-EDTA-CNNS).Combined with theoretical calculations,it can be seen that the iron chelation site can be used as the active site of the nitrogen fixation reaction,which can effectively adsorb and activate nitrogen molecules and greatly reduce the reaction energy barrier.Experiments have proved that Fe-EDTA-CNNS has excellent and repeatable photocatalytic nitrogen fixation performance.At the same time,chelation is more stable compared to electrostatically adsorbed iron ions.In addition,the chelating unit has a certain electron withdrawing ability,which inhibits the recombination of photo-generated carriers inside the CNNS and improves its photocatalytic performance.In addition,through simulation and calculation,it was verified that the hydrogen evolution reaction was suppressed as a competitive reaction.This modification method can be used to improve the performance of the photocatalyst by creating a new active site for a certain reaction,and provides a new modification idea for the functionalization of the photocatalyst.3.After the functional modification of CNNS was achieved through covalent grafting based on amino group s,we realized that the amino and hydroxyl groups on the surface of CNNS were the key points of functional modification.After combining with the actual production and life needs,we tried to find a suitable carrier for CNNS.On the one hand,the specific surface area of CNNS was increased by the carrier,thereby improving its light utilization rate,and it also solved the problem of difficult recovery of powder CNNS;In terms of CNNS modification,the carrier can give full play to its photocatalytic performance,and this green technology can be applied in practice.Experiments show that the surface of cotton textiles with a cellulose matrix has a large number of hydroxyl groups,which can form hydrogen bonds with hydroxyl groups and amino groups on the surface of CNNS.The CNNS colloid is dispersed on the surface of cotton textiles by a simple spraying method.After drying,the CNNS can still stably absorb after being washed several times.The load capacity is about 1g/m2.CNNS modified textiles have good photocatalytic self-cleaning properties,and can degrade a variety of dyes and stains commonly found in life.In addition,the large specific surface area of cotton textiles also improves the performance of CNNS photocatalytic degradation of formaldehyde gas.Even under low-intensity visible light,its degradation effect on indoor formaldehyde gas is very considerable.This work enables the expansion of photocatalytic technology from experiments to applications.