The Preparation,characterization And Reaction Mechanism Study Of Graphene Quantum Dots

Because of good physical/chemical properties,excellent optical properties and superb biocompatibility,graphene quantum dots(GQDs)have broad prospects for applications in biological imaging,drug delivery and optoelectronic devices.Traditional methods for preparing GQDs always introduce a large amount of ionic impurities,or use complex operation processes and expensive equipment,which greatly increases the time and cost of GQDs' preparation,and limits the development of GQDs.On the other hand,the formation mechanism of GQDs proposed in existing preparation methods is too simple.People pursuit wonderful properties of GQDs too much but ignore the chemical process in preparing GQDs.In this thesis,based on the preparation of GQDs,novel hydrothermal and solvothermal methods using hydrogen peroxide as oxidant were proposed,and the properties of products were characterized by various equipment.Beside,nitrogen doped graphene quantum dots(N-GQDs)with high prepartion yield and high fluorescence yield were prepared by polymerizing small molecules,and the formation process was also deduced.The specific research methods are as follows:First of all,graphene oxide(GO)was prepared by Hummer' method.Hydroxyl radical decomposed by hydrogen peroxide under high temperature and high pressure was used to react with oxygen-containing functional groups on the surface of GO,which realized the process of breaking C-C bond.By controlling the reaction time,GQDs products with different sizes and different colors of fluorescence can be obtained.GQDs with green fluorescence had an average size of 100 nm and a fluorescence quantum yield of 5.3%.And GQDs with blue fluorescence had an average size of 50 nm and a fluorescence quantum yield of 3.6%.In order to remove excess hydrogen peroxide after reaction,a simple catalyze-dialysis method was proposed to simplify the purification process and reduce the time cost.The measurements showed that oxygen content decreased with reaction proceeded.High resolution X-ray photoelectron spectrum indicated that the content of hydroxyl groups decreased obviously while the content of carbonyl and carboxyl groups increased slightly,which mean that the forming process may be a reaction between free radical and hydroxyl/epoxy groups and generating carbonyl/carboxyl groups.And then,we discussed the specific chemical process,the stability of free radicals and the rearrangement process involved in the reaction,which made a clear explanation to the mechanism of reaction process.Beside,we also give a qualitative explanation to the multi fluorescence color and the inside mechanism.Secondly,utilizing the characteristic that expandable graphite is a precursor in preparation of GO with low cost and close to GO in functional group,expandable graphite was used to prepare expanded graphite first and reacted with hydrogen peroxide in N,N-dimethylformamide(DMF)to prepare GQDs then in a solvothermal method.DMF was used to improve dispersion of expanded graphite and enhance contact between reactants.By using this method,GQDs with average size of 30nm,15.2%of quantum yield and excellent water solubility can be obtained.Because the reaction did not introduce ionic impurities which were difficult to remove,purified GQDs product can be obtained through filtration-evaporation-redissolution process,which avoid using dialysis process and reduce the cost of time and money.Finally,considering the current method to prepare N-GQDs require another reagent to act as nitrogen source with low yield,we used nitrilotriacetic acid(NTA)as the main body and nitrogen source at the same time,to prepare gram scale N-GQDs with average size of 30nm and quantum yield of 45.8%successfully.Transmission electron microscopy(TEM),atomic force microscopy(AFM),X-ray photoelectron spectroscopy(XPS)and transient fluorescence spectroscopy were used to analysis the morphology,element composition,fluorescence lifetime and fluorescence quantum yield.The aqueous and oil-like fluorescent ink was prepared by GQDs' aqueous solution and ethanol solution which were applied in pens and ball pens respectively.No writing can be observed under sunlight while the handwriting is clearly visible under 365nm ultraviolet light.In addition,through theoretical analysis and computer simulation based on the density functional theory(DFT),we analysis the charge distribution of NTA,and a reaction mechanism consisted of intermolecular dehydration and nucleophilic addition-intramolecular dehydration was proposed.From this mechanism,the structure of N-GQDs was deduced from one NTA molecule step by step.By DFT simulation,it can be found that the degree of planarization in product increased with reaction proceeded.Alternate single and double bond formed around nitrogen atoms,which made sp2 hybrid orbits formed in nitrogen atoms and two lone pair electrons participated in the forming of ?-delocalized bond.This process leaded to a higher electron cloud density which explained the reason for high fluorescence quantum yield.At the same time,the spatial dynamics in the reaction was also analyzed.It was found that after a NTA molecule coupled to the N-GQDs,other active site on this NTA could easily form a stable hexatomic ring by rotation of single bond with less resistance.Finally,the reaction mechanism and the equipment characterization were compared.It was found that the structure change in mechanism was consistent with the results of equipment characterization,which can be an auxiliary proof to confirm the reaction mechanism.