The Preparation And Photoluminescent Properties Of Fluorine And Chlorine Doped Graphene Oxide Quantum Dots

As a type of zero-dimensional carbon nanomaterials,graphene quantum dots(GQDs)have attracted wide attention due to their stable and bright photoluminescence(PL)properties,good biocompatibility,excellent catalytic activity and good chemical stability.At present,the application of GQDs depends largely on their PL,to tune the PL of GQDs is very important for expanding their application.Actually,the PL properties of GQDs is affected by many factors,such as the size,shape,boundary type,the types and numbers of functional groups existing on the specific surface,the type and concentration of doped heteroatoms atoms and so on.Even in different environments such as PH value or temperature,the PL of GQDs may be changed.Therefore,to adjust the PL properties of GQDs,on the one hand,GQDs with different properties should be prepared by different methods.On the other hand,GQDs can be chemically modified to improve their PL properties,such as to prepare doped graphene quantum dots.The fluorine atom with strong electronegativity can be combined with carbon atom to form multi-orbital or singleorbital hybrid C-F bond,which affects the electronic distribution of the conjugate plane of GQDs,and a series of properties of GQDs will be changed accordingly.The tunable band gap of GQDs can be obtained by fluorinating GQDs.In addition,chlorine and fluorine are in the same main group and they have similar chemical properties,the structure and properties of chlorine doped graphene quantum dots are also worth studying.The main contents of this paper are as follows:1.Preparation of graphene oxide quantum dots from two different raw materials by reflux oxidation.The first method is to prepare graphene oxide quantum dots(GOQDs)by reflux oxidation using VXC-72 spherical carbon black as raw material and concentrated nitric acid as oxidant.GOQDs were prepared by mixing 5g carbon black raw material with 500 ml nitric acid oxidant at 135 ? for 24 hours.The quality of the obtained GOQDs was 3.3862 g,and the yield was 67.7%.The GOQDs emit 525 nm stable and bright fluorescence at a certain excitation wavelength.The second method is to prepare graphite oxide from natural flake graphite by improved Hummers' method,then the graphite oxide were cut into GOQDs by reflux oxidation using concentrated nitric acid as oxidant.14.2g dry graphite oxide was prepared from 8g natural flake graphite.GOQDs with mass of 1.1g can be prepared by reflux oxidation of 3g graphite oxide.The results show that there is no essential difference in the structure and properties of GOQDs prepared from the two raw materials,but the PL spectra of GOQDs are slightly different.we provide an important way to prepare GOQDs with high yield using graphite as the parent material of GQDs.2.Study on hydrothermal synthesis of fluorinated graphene oxide quantum dots and their PL properties.Using GOQDs as raw material and xenon difluoride(chemical formula: XeF2)as fluorine source,fluorinated graphene oxide quantum dots(F-GOQDs)were prepared by hydrothermal method.When XeF2 and GOQDs were mixed and transferred to the autoclave at 200? for 24 hours,F-GOQDs with different PL properties could be obtained.By changing the mass ratio of XeF2 to GOQDs,F-GOQDs with different fluorination degree can be obtained.The binding types and fluorine concentration in GOQDs were studied by various characterization methods,and the effects of different binding types and fluorine concentration on the PL properties of F-GOQDs were explored.It was found that the band gap of F-GOQDs depended on the concentration of fluorine,which indicated that the fluorine concentration of F-GOQDs could be tuned by adjusting the mass ratio of XeF2 to GOQDs?Accordingly,F-GOQDs with variable bandgap can be obtained.3.Study on photochemical synthesis of F-GOQDs and their PL properties.GOQDs as raw material and XeF2 as fluorine source,F-GOQDs were prepared by photochemical fluorination.After mixed XeF2 with GOQDs in a quartz boat and transferred to a photochemical reaction chamber,F-GOQDs can be rapidly prepared under ultraviolet light produced by high pressure Hg lamp.Similar to hydrothermal method,F-GOQDs with different fluorination degree can be obtained by changing the mass ratio of reactant XeF2 to GOQDs.The highest fluorine concentration of the obtained F-GOQDs can reach 53.17 at% by photochemical fluorination,but F-GOQDs with such high fluorine concentration can not be prepared by hydrothermal method.It was found that the excessive mass ratio of XeF2 to GOQDs would lead to a sharp decrease in the yield of F-GOQDs when using hydrothermal method.It was found that the-OH(hydroxyl)and C=O(carbonyl)of GOQDs decreased during photochemical fluorination.which indicated that some oxygen-containing functional groups of GOQDs were replaced by fluorine atoms in the process of fluorination.When irradiated time was changed to 5 minutes,10 minutes,30 minutes,50 minutes respectively,we found that F-GOQDs prepared with five minutes irradiated time also had high fluorine concentration,but their PL spectra did not show obvious blue shift.However,the fluorine concentration of F-GOQDs wiht 50 minutes irradiated was not significantly different from that of F-GOQDs after 10 minutes irradiated,which indicated that 10 minutes of irradiated time was sufficient,while 5 minutes of irradiated time was obviously not long enough.Through comparative analysis on the PL of GOQDs and F-GOQDs,we found that the F-GOQDs prepared by photochemical method is similar to that hydrothermal method,the band gap of F-GOQDs is also depended on the concentration of fluorine.The higher the fluorine concentration of F-GOQDs,the larger the blue shift of PL spectrum.Based on this,we propose a fast method to prepare F-GOQDs by photochemical fluorination,and the maximum blue shift of PL spectrum of F-GOQDs can reach 77 nm.Therefore,the band gap of GOQDs can be effectively tuned by photochemical fluorination.However,the PL mechanism of F-GOQDs is quite complex,and the mechanism of PL blue shift of F-GOQDs relative to GOQDs is worth to further study.Based on the experimental results of F-GOQDs prepared by photochemical fluorination,the fluorination model of F atom replacing OH(hydroxyl)and C=O(carbonyl)in the fluorination process of GOQDs was established.Using first principles which based on density functional theory(DFT),the absorption spectra(equivalent to optical emission spectra)of GOQDs and F-GOQDs were calculated.The results show that the blue shift of PL in the caculated absorption spectra will occur when part of OH or C=O are replaced by F atoms.The results of DFT calculation are support our experimental results,which provide strong evidence for explaining the PL blue shift mechanism.4.Study on photochemical synthesis of chlorine doped graphene oxide quantum dots and their PL properties.High active chlorine free radicals can be produced by irradiating chlorine in photochemical reaction chamber with strong ultraviolet radiation produced by high pressure Hg lamp,and chlorine doped graphene oxide quantum dots(Cl-GOQDs)can be obtained by the contact reaction of chlorine free radicals with GOQDs.Cl-GOQDs with chlorine content of 3.5 at% can obtained after 20 minutes irradiated.By analyzing the PL spectrum of Cl-GOQDs,it is found that the PL spectrum of Cl-GOQDs do not change significantly when compared with GOQDs.However,the PL of F-GOQDs with 3.1% fluorine concentration have changed considerably.This shows that the PL changes caused by introducing chlorine atoms into GOQDs can not achieve the same effect as that caused by introducing fluorine atoms with the same concentration,which is similar to that of graphene.In this paper,we provided a feasible method for preparing Cl-GOQDs by photochemical.It is necessary to explore new conditions and methods for doping Cl-GOQDs with higher chlorine concentration.In addition,the effect of chlorine on the band gap of GOQDs is worth to further study.