The Regulation Of The Optical Properties Of Graphene Quantum Dots Based On Co-doping

As a type of zero-dimensional carbon nanomaterials,graphene quantum dots(GQDs)have significant quantum confinement effect and boundary effect.They have extensive application in the fields of optoelectronic devices,biological imaging,electrocatalysis,fluorescent probes and so on.At present,there are many methods to prepare GQDs,which can be generally divided into top-down method and bottom-up method.The band gap of GQDs can be effectively changed by doping impurity atoms,thus regulating the photoluminescence(PL)performance of quantum dots.The GQDs prepared by refluxing oxidation method contain a large number of oxygen-containing functional groups.The GQDs containing a large number of oxygen-containing functional groups are called graphene oxide quantum dots(GOQDs).The main contents of this paper are as follows:1.Preparation and properties of GOQDsGOQDs were prepared by refluxing oxidation with VXC-72 conductive carbon black as raw material and concentrated nitric acid as a strong oxidant.The average particle size of the prepared GOQDs is about 3 nm,and the samples containing a large number of oxygen-containing functional groups have a good dispersion ability in water.Under the light excitation of a certain wavelength range,the PL emission wavelength of the prepared GOQDs is mainly concentrated at 524 nm.The GOQDs sample does not have the property of fluorescence dependent on the excitation wavelength.Under the irradiation of 365 nm UV lamp,the GOQDs sample emits bright yellow-green fluorescence.2.Preparation and PL properties of nitrogen and sulfur co-doped graphene oxide quantum dots by thermochemical methodUsing the prepared GOQDs as raw material,urea as nitrogen source and3-mercaptopropionic acid as sulfur source,nitrogen and sulfur co-doped graphene oxide quantum dots(N,S-GOQDs)were prepared by thermochemical method.Separate GOQDs,urea and 3-mercaptopropionic acid,place them in Teflon inner cups respectively,and heat them in a high-temperature drying oven.Set the reaction temperature as 220 ? and the reaction time as 20 h.The obtained N,S-GOQDs samples were characterized and analyzed.The results show that the maximum blue shift of PL emission peak of N,S-GOQDs samples is 144 nm compared with GOQDs under 340 nm excitation light,indicating that the incorporation of N and S atoms significantly increases the band gap of GOQDs.This is mainly due to the structural changes of doped pyridine like nitrogen,pyrrole like nitrogen atoms and quantum dots surface defects in N,S-GOQDs.N,S-GOQDs have the property of fluorescence dependent excitation wavelength change.Compared with N-GOQDs and S-GOQDs,N and S diatomic doping play a good synergistic effect,which greatly changes the band gap of GOQDs than single atom doping,thus affecting the optical properties of GOQDs.3.Preparation and PL properties of nitrogen and fluorine co-doped graphene oxide quantum dots by a thermochemical methodNitrogen and fluorine co-doped graphene oxide quantum dots(N,F-GOQDs)were prepared by the thermochemical method with GOQDs as raw material,urea as nitrogen source and xenon difluoride as fluorine source.Separate GOQDs,urea and xenon difluoride,place them in Teflon inner cups respectively,and heat them in a high-temperature drying oven.Set the reaction temperature to 230 ? and the reaction time to 22 h.The obtained N,F-GOQDs samples were characterized and analyzed.The results show that compared with GOQDs,the maximum blue shift of PL emission peak of N,F-GOQDs is 134 nm under the excitation light of 320 nm,indicating that the incorporation of N and F atoms greatly increases the band gap of GOQDs,resulting in the blue shift of PL emission peak.Under 320 nm excitation light,the maximum Stokes shift of N,F-GOQDs samples is 200 nm.Compared with N-GOQDs and F-GOQDs,N and F diatomic doping play a better synergistic effect,which greatly changes the band gap of GOQDs than single atom doping,thus affecting the PL performance of GOQDs.4.Preparation of nitrogen and chlorine co-doped graphene oxide quantum dots by hydrothermal method and their PL propertiesNitrogen and chlorine co-doped graphene oxide quantum dots(N,Cl-GOQDs)were prepared by hydrothermal method with GOQDs as raw material,ammonia as nitrogen source and hydrochloric acid as chlorine source.Separate GOQDs,ammonia and hydrochloric acid,place them in Teflon inner cups respectively,and heat them in a high-temperature drying oven.The reaction temperature is 200 ?,220 ? and240 ? respectively,and the reaction time is 22 h.Different N,Cl-GOQDs samples were prepared by adjusting the reaction temperature.The obtained N,Cl-GOQDs samples were characterized and analyzed.The results show that under 440 nm excitation light,compared with GOQDs,the maximum blue shift of PL emission peak of N,Cl-GOQDs is 47 nm,indicating that the incorporation of N and Cl atoms increases the band gap of GOQDs,resulting in the blue shift of PL emission peak.In order to better study the effect of N and Cl atom doping on the PL properties of GOQDs,change the doping order of N and Cl atoms and study their effect on the PL properties of quantum dots.It is found that changing the doping order of N and Cl atoms has a great influence on changing the band gap of GOQDs.In addition,the binding energy of atoms has a great influence on the PL properties of N,Cl-GOQDs samples.Compared with N-GOQDs and Cl-GOQDs,N and Cl diatomic doping play a better synergistic effect,which greatly changes the band gap of GOQDs than single atom doping,thus affecting the optical properties of GOQDs.