Research On Preparation And Photocatalytic Properties Of Nitrogen Doped Graphene Quantum Dots Based On The Bottom-up Method

As a new type of luminescent carbon nanomaterials,graphene quantum dots is one of the most important and hot field of research.Compared with the traditional semiconductor quantum dots,graphene quantum dots not have good fluorescence properties they have,but also have the natural non-toxicity of carbon materials.In addition,due to functional groups on the surface of graphene quantum dots,it is beneficial to the dispersion of solvent and modification.Based on excellent properties of graphene quantum dots,it is of great value in various fields,especially in photocatalysis,graphene quantum dots,which is convenient during the synthesis and non-toxic,have stable photoluminescent properties,especially the upconversion luminescence and even can improve the photocatalytic activity of the catalysts.Therefore,graphene quantum dots have lots of advantages in the field of practical application of photocatalysis.Based on the above analysis,in this paper,nitrogen doped graphene quantum dots（NGQDs）are prepared.What’s more,preparation and photocatalytic performance of NGQDs/BiVO₄ composite and NGQDs/Ag/BiVO₄ ternary composite is systematically investigated.The major research works are as follows:（1）Nitrogen doped graphene quantum dot（NGQDs）with high fluorescence quantum yield up to 18.9%were synthesized bythermal decomposition method under hydrothermal condition using citric acid monohydrate as carbon source and urea as nitrogen source.This method was convenient and the conditions were controllable,and then the as-prepared NGQDs exhibited blue fluorescence properties.In addition,we could adjust the size of NGQDs through prolonging the reaction time,so as to control the optical properties.Afterwards,a series of morphologic,structural and optical characterization methods were used to study the relationship between the size and fluorescent characteristic.Besides,we could also study the effect of pH on the fluorescence intensity of NGQDs,and find that the fluorescence intensity of NGQDs was the strongest under neutral condition.Besides,the relationship between the effects of different concentration and solvents and the fluorescence intensity of NGQDs was explored.The experimental results exhibited that it was the different ion concentration that didn’t have great influence on the fluorescence intensity.However,for different solvents,the fluorescence intensity showed a certain degree of quenching.Finally,NGQDs displayed good physical stability by Zeta potential measurement.（2）NGQDs with excellent fluminescent properties was synthesized by direct and simple "bottom-up" method under hydrothermal conditions.The as-prepared NGQDs exhibited an excitation-independent upconversion fluorescence character.Then,the NGQDs/BiVO₄ composite photocatalyst was prepared through hydrothermal reaction and deposition technique.And the NGQDs was successfully loaded on the surface of BiVO₄ from the results of a series of characterization methods.Then hanced photo-activity of NGQDs/BiVO₄ composite could be exhibited by degradating methylene blue（MB）under visible light irradiation.Enhanced photoactivity of NGQDs/BiVO₄ composite could be attributed to up-conversion fluorescence character of NGQDs and lower recombination rate of the electron-hole pairs.In addition,the NGQDs/BiVO₄ composite photocatalyst exhibited excellent stability.The photocatalytic mechanism of NGQDs/BiVO₄ composite photocatalyst was proposed by characterization analysis of UV-Vis DRS,fluorescence,electrochemical detection and trapping experiment method,which revealed electron transfer of material interface and that superoxide radical was the main active material during the photocatalytic degradation of MB.（3）NGQDs/Ag/BiVO₄ ternary composite was prepared by hydrothermal reduction and deposition method.Based on the LSPR effect of Ag,we could build a system of NGQDs/Ag/BiVO₄ ternary plasma type composite with degradating tetracycline hydrochloride（TC·HCl）under visible light irradiation.Due to the LSPR effect of Ag,the photogenerated electrons and holes could be separated effectively,which could directly improve photocatalytic performance of NGQDs/Ag/BiVO₄ composite.The separation of electron-hole pairs should be contributed to electronic storage function of Ag,direct transmission of the photogenerated electrons（DTE）and the plasmon-induced resonant energy transfer（PIRET）caused by the LSPR effect of Ag.Then,the structure and morphology of NGQDs/Ag/BiVO₄ ternary composite were characterized by XRD,XPS,SEM and TEM.In addition,the electrochemical stability and carrier mobility were studied by electrochemical detection.At the same time,trapping experiment and paramagnetic resonance technique（ESR）were used to detect and confirm that ·O2-and ·OH radicals were the most active material in the photocatalytic degradation system.The reaction intermediates of TC·HCl were also studied by mass spectrometry.Based on the above analysis,the possible mechanism of charge transfer in the process of NGQDs/Ag/BiVO₄ ternary composite degradating TC·HCl was systematically analyzed and investigated.