Preparation And Surface Activity Application Of Nitrogen-doped Carbon Quantum Dots

Carbon Quantum Dots（CQDs）,as a kind of carbon nanomaterials,have the advantages of good water solubility,strong fluorescence,small size and low biological toxicity.They have been widely used in sensors,ion detectors,capacitors and photocatalysts.However,CQDs prepared by conventional methods have single fluorescence color,low quantum yield and single hydrophilicity.It is difficult to study in an immiscible solution system,which greatly limits its application range.In response to the above problems,this paper proposed to enhance the fluorescence of carbon quantum dots by doping with nitrogen atoms,modify the surface of CQDs with functional groups to obtain amphiphilic nitrogen-doped carbon quantum dots,discussed its surface activity and properties.And for the unique structure of the modified substance,the application of it as an auxiliary agent to assist the liquid phase exfoliation of graphene in an aqueous solution was studied.（1）Using citric acid as the carbon source and 3-dimethylaminopropylamine as the nitrogen doping agent,Nitrogen-doped Carbon Quantum Dots（N-CQDs）were prepared under hydrothermal conditions at 180°C,and through the characterization results of infrared absorption spectroscopy（FTIR）and hydrogen nuclear magnetic spectroscopy（¹H-NMR）proved that N-CQDs contained a large number of chemical functional groups such as amides,tertiary amines,primary amines and other nitrogen-containing elements,which proved that nitrogen doping was successfully carried out;ultraviolet-absorption visible spectroscopy（UV-vis）and fluorescence spectroscopy（PL）tests showed that N-CQDs have good optical properties and have blue fluorescence;transmission electron microscopy（TEM）results showed that N-CQDs are spherical in shape and have good dispersibility in aqueous solutions,with a size in the range of 2-7nm,and have the small size characteristics of quantum dots.（2）By modifying the surface functional groups of N-CQDs,the amphiphilic substances R-（N-CQDs）and R-（N-CQDs）-L were synthesized.The structure characterization and surface activity tests showed that R-（N-CQDs）-L has a longer hydrophobic carbon long chain structure and stronger surface activity.Specifically,the surface tension value of the R-（N-CQDs）-L aqueous solution at the critical micelle concentration（1.0g/L）is 24.33m N/m,and the two-phase stratification time after emulsification t(V_H2O=5.0m L)=510s,after 5minutes of foaming,the foam volume can still reach the original 95%,and after compounding with sodium dodecyl sulfate（SDS）,the synergy of the system is more significant,providing a new way for the application of CQDs.（3）Based on the special structure of R-（N-CQDs）-L and its excellent performance in surface activity,it was used as a dispersing aid to study the preparation of graphene by liquid phase exfoliation of graphite with the assistance of ultrasound,which further expanded the scope of application of amphipathic N-CQDs.The influence of dispersant dosage,ultrasonic stripping time and initial graphite concentration on the concentration of graphene dispersion was studied,the morphology and structure of graphene were characterized,and the mechanism of R-（N-CQDs）-L liquid exfoliated graphene was analyzed.The results showed that under the condition of centrifugation at 4000r/min for 30min,when the concentration of R-（N-CQDs）-L is 0.5g/L,the ultrasonic time is 3h,and the initial graphite material is 8mg/m L,the concentration of graphene dispersion can reach 0.924mg/m L and has good stability.Using scanning electron microscope（SEM）,transmission electron microscope（TEM）and atomic force microscope（AFM）to analyze the morphology of graphene,the results showed that the surface of graphene is flat and smooth,and the size is mainly in the range of a few hundred nanometers to 2μm,and the thickness is less than 2nm,most of them belong to double-layer and few-layer graphene.The results of Raman spectroscopy（Raman）,X-ray powder diffraction（XRD）and FTIR showed that compared with raw graphite,the surface of graphene is not oxidized excessively,has fewer defects and a more complete structure.