Study On Synthesis And Catalytic Properties Of Fluorescent Carbon Dots And Their Composite Triazine Covalent Frameworks

While the rapid development of society and the high development of human material civilization,energy and environmental problems have become more and more prominent.According to the carbon peaking and carbon neutrality strategy proposed by the st ate,the direct use of coal for power generation and heat supply through combustion will be limited,and it is of great significance to convert it into raw materials with high added value to achieve clean utilization of coal.Coal and its by-products contain a large number of ultra-small sp²hybrid nanocrystals,which meet the natural structural characteristics and components required by fluorescent carbon dots（CDs）.Based on this,our research group has developed the technology of large-scale preparation of fluorescent carbon dots from coal and coal pitch,and the current realization of the macro application of coal-based fluorescent carbon dots is the key to promoting the formation of the industrial chain of coal-based new materials.CDs are zero-dimensional carbon nanomaterials with multicolor fluorescence emission and semiconducting properties,generally less than 10 nm in diameter,graphite carbon nuclei with sp² hybridization,and widely distributed surface functional groups.The edge defects in CDs,different components and surface states give them tunable luminescence,easy functionalization,excellent photocatalysis and photothermal effects,and can be compounded with other materials for energy conversion and utilization.Ultrasonic catalysis has the characteristics of environmental friendliness,cavitation effect to accelerate chemical reaction,etc.,and can effectively remove organic pollutants,but new highly responsive acoustic sensitizers need to be developed to improve catalytic efficiency.CDs are small nanoparticles with abundant functional groups on the surface,therefore,as a sonosensitizer,more cavitation effects can be formed,and the extreme conditions generated by ultrasound can split the groups on the surface of CDs,which is expected to provide more reactive oxygen species for the removal of organic pollutants.As a liquid energy,hydrogen peroxide（H₂O₂）is used in Fenton reaction,chemical synthesis,medical disinfection,environmental remediation and other fields,and the market demand is huge.Although a variety of catalytic materials for the preparation of H₂O₂ have been developed,among which covalent triazine f rameworks（CTFs）stand out in the photocatalytic production of H₂O₂ due to their unique structure and good stability,there are still problems such as low utilization of visible light and rapid carrier recombination.With the help of interface effects,CDs can provide intermediate energy levels and improve the utilization of visible light,while CDs act as donors/acceptors and reservoirs for charge,promoting charge transfer and inhibiting phot ocarrier recombination.Therefore,CDs were used as a new sonosensitizer to degrade organic pollutants by ultrasonic catalysis,and a new ultrasonic mechanism was discovered on the basis of good catalytic effect.At the same time,the composite photocatalysts prepared by CDs and CTFs significantly improved the separation capacity of carriers,and greatly improved the photocatalytic yield of H₂O₂.The specific research content of this paper is as follows:（1）Using coal pitch carbon dot as acoustic sensitizer,the effect and reaction mechanism of ultrasonic catalytic degradation of Rhodamine B were studied.It was found that compared with the typical sonosensitizer Ti O₂,the ultrasonic catalytic activity of carbon dots was almost twice that of Ti O₂,and showed a different catalytic mechanism.Experiments show that ultrasonic cavitation can break the oxygen-containing groups on the surface of CDs,and also have pyrolysis and sonoluminescence characteristics.The study revealed that the cleavage of the CDs oxygen-containing group during cavitation bubble collapse produces a transient active material,oxygen radical（?O^-）,followed by a rapid conversion reaction between?O^-and H⁺and H₂O from?O^-to?OH and?O₂^-,promoting the production of reactive oxygen species（ROS）,resulting in high-performance ultrasonic catalysis.At the same time,these conversion reactions are reversible and can be adjusted by the p H value of the aqueous solution,which determines the generation and evolution of?O^-under ultrasonic radiation,which is expected to achieve the control of ROS yield in the microenvironment.（2）CDs and monomeric terephthalonitrile were synthesized by organic interface to synthesize CDs@CTFs composite catalyst,and transmission electron microscopy characterization and comparative experimental results showed that CDs@CTFs was a unique nanoscale sandwich structure.Experiments show that CDs induce rapid nucleation and growth of CTFs on their surfaces,which can increase productivity by 13.5 times per hour,significantly improve the production of photocatalytic H₂O₂,the efficiency is 22.6 times that of pure CTFs,and the efficiency of solar-chemical energy conversion（SCC）reaches 0.16%.The results show that CDs can extract and store photoexcitation holes in CTFs in time,expand long-wavelength light absorption,and create an available intermediate band.In addition,CDs can adsorb alkali metal ions to adjust the surface polarizat ion electric field of CTFs induced by them,further promote the separation and migration of carriers,which is conducive to the oxidation of water and increases the production rate of H₂O₂.Therefore,the CDs@CTFs could be utilized for the photosynthesis of H₂O₂ in seawater without incurring the burden of fresh water resource shortage and material high cost.（3）In order to further improve the efficiency of photocatalytic production of H₂O₂,a highly crystalline composite catalyst CDs@CTFs was prepared by a mild metal-free solvent-free method.Characterization and experimental results showed that CDs still exist by calcination and form an intercalation structure with CTFs.Compared with CTFs alone,CDs@CTFs catalytic performance is improved by 2.3 times.The introduction of CDs in the catalyst system not only expands the range of light absorption,but also promotes the dissociation of excitons and the separation of carriers,forming a redox center of spatial separation,thereby promoting the dual-channel production of H₂O₂ by oxygen reduction reaction（ORR）and water oxidation reaction（WOR）.In addition,it is proved that the combination of alkali metal ions and CDs can help CDs extract photogenerated holes from CTFs to drive artificial photosynthesis,increasing the photocatalytic efficiency by 34%,and this conclusion is verified by simulation calculation.The results show that the H₂O₂ yield of the composite catalyst is 2464μmol^-1g^-1h^-1 and the SCC efficiency is 0.90%,which is the highest photocatalytic material in the particle photocatalytic（PC）system reported so far.What’s more,it can work continuously under natural sunlight and still has good stability after100 days of recycling and storage.