Synthesis And Optical Properties Of Carbon Nanodots And Their Composites Based On Zeolite-like Porous Materials

Porous materials are a class of materials with a certain number and size of pore structures,which are already widespread in nature,such as wood,coral reefs,sponges,charcoal,etc.With the development of modern industry,much more porous materials with unique properties have been prepared,such as zeolites/molecular sieves,mesoporous oxides,metal-organic frameworks（MOFs）,porous organic framework materials（POFs）,etc.Among them,zeolites are a kind of inorganic microporous crystal materials with regular pore structure,which are constructed by TO₄（T=Si,Al,P,etc.）tetrahedra connected by bridge oxygen atoms,and their pore sizes are generally less than 2nm.Zeolites have the characteristics of diverse framework structures and framework compositions,regular and adjustable nanopores,high specific surface area and adjustable active centers,and functional primitives.As an important class of catalytic,adsorption and separation materials,they are widely applied in petroleum refining,such as manufacturing,petrochemical and fine chemicals.Since the artificial synthesis of zeolites in 1940,the synthesis methods and structure types of zeolite materials have been greatly developed.There are hundreds of synthetic zeolites and inorganic microporous materials.However,due to the limitation of many factors（such as pore structure and stability）,only few kinds have been applied in industrial applications.Therefore,in addition to the traditional catalysis,adsorption and separation functions,the development of new properties and novel applications of zeolite porous materials has become an important current research.At the same time,zeolite is an excellent host material.Its porous structure can assemble different guest functional molecules（such as metal or metal clusters,carbon nanomaterials,metal complexes,semiconductor quantum dots,etc.）according to functional requirements.By changing types of host-guest materials and controlling assembly methods（assembly,grafting,modification,etc.）,the prepared host-guest assembly materials with specific properties could be applied in different fields,such as catalysis,energy storage,medicine,sensing,etc.Carbon dots,as a new type of carbon-based nanomaterials,have been one of the research hotspots in recent years.Its unique optical properties,photoelectric effect,low toxicity,simple preparation and other advantages make its widely applications in biology,chemistry,optoelectronics and other fields.Carbon dots were first discovered in 2004 when using gel electrophoresis to strip and purify carbon nanotubes.Generally,carbon dot refers to a zero-dimensional carbon particle with a particle size less than 10nm,which is composed of an amorphous or crystalline carbon core and functional groups（such as amino,hydroxyl,carboxyl,carbonyl,etc.）on the surface.According to the structures of carbon core,it can be roughly divided into carbon nanodots（CNDs）,carbon quantum dots（CQDs）and carbonized polymer dots（CPDs）,etc.The properties of carbon dots are closely related to their synthesis precursors and synthesis methods.Therefore,photoluminescence from ultraviolet to near-infrared or efficient light-induced electron/hole separation can be achieved by controlling factors such as chemical structure,size,surface functional groups,doping heteroatoms,et al.These excellent chemical properties make them high application potentials in many fields.However,like other luminescent molecules,carbon dots also have problems such as aggregation quenching and increased surface defects.In order to solve these problems,the carbon dots can be introduced into the host matrix,and their optical and electrical properties can be further optimized through the support and fixation of the matrix.Among various matrices,porous materials benefiting from their controllability and tenability can design and synthesize a series of carbon dot@porous material composite materials with improved performance or new properties.This thesis designs and synthesizes a series of CDs and CDs-based composites based on the zeolite-like porous materials,and studies their luminescence properties.The main research results are as follows:1.Chapter 1 mainly introduces the development history and application of various porous materials（e.g.microporous,mesoporous,macroporous,etc.）,as well as the preparation,mechanism and application of carbon dots.2.Chapter 2 mainly focuses on the regulation of the luminescence on carbon dot composites based on the host-guest energy transfer strategy.In the carbon dot-based composite material,energy transfer could be occurred between guest carbon dots and heteroatoms on the porous framework under suitable conditions.Based on this,the luminescence rule of carbon dots made by different precursors under hydrothermal conditions was systematically studied,and the effect of different coordination states of heteroatom on the emission of composite materials was studied.Therefore,by pre-designing the precursors of carbon dot（energy donor）and the coordination states of manganese atoms（energy acceptor）,luminescence could be modulated based on energy transfer.As a result,CDs@Mn-LEV and CDs@Mn APO-CJ50 with red room temperature phosphorescence emission,and CDs@Mn APO-tren with yellow-green phosphorescent emission were prepared.These composite materials have excellent stability and may be applied in anti-counterfeiting and LED lighting fields.3.Chapter 3 mainly introduces the method of preparing carbon dots by pyrolysis of organic templates occluded in the pores of zeolites.First,zeolite Mg APO-44 with CHA topology was prepared using N-methylpiperidine（NMD）as template in a hydrothermal system.Then,the zeolite crystal product was calcined and etched to prepare excitation-dependent blue photoluminescence CDs.The CDs had uniform particle size,showed good dispersion and stability in water and ethanol,which could be used as fluorescent probes to detect nitro explosives such as picric acid（PA）.The detection limit（LOD）and quenching equation constant are 0.36μM and 1.30×10⁵.In addition,the concentration and p H values of the aqueous solutions will have a certain impact on the detection results.4.On the basis of Chapter 3,the further study found that the carbon dot solution prepared by calcination has multi-center emissions at a certain low concentration.In Chapter 4,after further thin-layer chromatography,these different luminescent centers are separated and characterized by steady-state/transient spectroscopy to study their luminescence mechanism.Three main components are cyan luminescent CD-1 with high polarity,blue luminescent CD-2 with low polarity and green luminous CD-3 with medium polarity.Characterizations have shown that the uneven calcination and oxidation lead to different carbonization degrees of the organic templates in the zeolite,resulting in the formation of three different carbon dots.The photoluminescence of multi-center carbon dots may originate from the competition between conjugated fluorophores and surface functional groups fluorophores.This research provides a foundation for further understanding of the luminescence mechanism of carbon dots.5.Chapter 5 mainly summarizes the main research results of this paper,and proposes the prospects of future design,synthesis and applications of the CDs@porous material composites.