The Synthesis Of N-Doped Graphene Quantum Dots Baesd On One-Step Electron Beam Irradiation Method

As a new member of graphene family, graphene quantum dots(GQDs) have been raised enormous studies in the worldwide, for their unique and novel chemical/physics properties. The band gap and fluorescence(FL) of GQDs can be effectively adjusted by doping heteroatoms. The N atom has been widely applied in chemical doping of carbon nanomaterials which have applications in the fields of bioimaging, supercapacitors, ionsensors, hydrogen storage and so on. This paper focuses on the preparation of N-doped GQDs by high energy electron beam(EB) irradiation, which have 13% of N/C ratio and emit white FL under UV lampexcitation.Firstly, GQDs were synthesized by refluxing carbon fibers(CF) for 24 h in the mixture of strong acid at 80 oC. Their average size is 4.9 nm and average height is 1.5 nm. The interlayer space of GQDs is 0.342 nm, while ID/IG ratio of Raman spectra is 1.07. The results from X-ray photoelectron spectroscopy(XPS) and Fourier Transform infrared spectroscopy(FTIR) show that there are a lot of oxygen-contained functional groupson their surface. However, the FL quantum yield(QY) is low, ca 0.2%. Meanwhile, the FL intensity of GQDs is proportional to the concentration within a limited range, and lower or higher pH value also has a negative impact on the FL intensity of GQDs.Then, whether the reduction process, such as by zinc, by hydrazine hydrate, by EB or by their combination could influence the FL of GQDs was studied. The results show that all these processes can reduce the GQDs effectively, and the C/O ratio can be increased. However, only EB method can enhance the FL intensity and QY of GQDs, and change the FL color from yellow to white. Other mehods have limited effect on the FL of GQDs.Finally, we carefully studied the EB-irradiation effect on GQDs in the aqueous of N,N-dimethylformamide(DMF). The best optimal condition to prepare N-doped GQDs(W-GQDs) is as follow: the concentration of GQDs is 1 mg/ml, with 5% volume fraction of DMF, and the EB irradiation dosage is 560 kGy. The results show that GQDs not only are reduced effectively, but also grafted with N-contained groups, alkylamine-N(CH3)2, in the edge of GQDs sheet and doped with N atoms in the π-conjugated system of GQDs. Furthermore, the mechanism about the white FL emission of W-GQDs has been studied in detail. The results suggest that there are two chromophores in the W-GQDs. The yellow FL emission originated from the de-excited transition in the band edge, while the blue FL emission originated from the de-excited transition in the localized electronic states induced by the newly groups.