Study On Fabrication And Optical Properties Of Graphene Quantum Dots By Citric Acid Pyrolysis Method

Graphene quantum dots (GQDs) are graphene nanosheets smaller than10nm in width, which combine excellent properties of graphene and quantum dots. GQDs possess unique properties of graphene and other special properties because of its quantum confinement effect and boundary effect. GQDs have attractive application prospects in biological imaging, optoelectronic ’devices, photocatalysis, antibacterial system and environmental detection and so on. In this thesis, we report a pyrolytic method to synthesize GQDs by pyrolysing citric acid, to investigate the influence of different process parameters on the structure and optical properties of GQDs. The main results included the following three aspects:1. The optimum process parameters of pyrolysing citric acid are at200℃for15minutes. In this condition, we got a large number of GQDs with good dispersity and crystallinity. In the range of180～220℃, the average size of GQDs increases gradually from2.7nm to8.5nm as the increase of pyrolytic temperature, the growth rate of GQDs is first slowly and then become faster, accordingly the shape of GQDs is first round shape gradually tend to be oval. Overhigh pyrolytic temperature or overlong pyrolytic time can increase the degree of carbonization of citric acid. Overlong pyrolytic time may still come into being a lot of composite structure of GQDs/graphene existed in the product.2. GQDs prepared by pyrolysing citric acid are composed of multilayer graphene. Pyrolytic time affects the crystallinity of GQDs. In the range of180～220℃, the crystallinity of GQDs improves gradually as the extension of pyrolytic time. Two kinds of lattice fringe0.21nm and0.24nm corresponding to the graphene (1100) and (1120) surface can be observed by using TEM, which can be used to confirm that GQDs with high crystallinity and good dispersibility are contained in the product of pyrolysing citric acid. The edge structures of GQDs with structural defects are given priority to zigzag, and their edges are oxidized and introduced a large number of oxygen-containing functional groups so that they have good solubility in the water. 3. With the improvement of pyrolytic temperature, the size of GQDs increases gradually, whose absorption peak and fluorescence peak show a red shift. When the pyrolytic temperature increase from180℃to200℃, the absorption peak position of GQDs shifts from349nm to359nm; And when the pyrolytic temperature is up to220℃, GQDs appear two absorption peak near350nm and460nm, which shows that the energy gaps of GQDs are related to their size and shape, which change with different pyrolytic temperature. GQDs prepared by pyrolysing citric acid at200℃for15minutes have a strong absorption peak near360nm, and appear the strongest fluorescence peak at474nm, which emit blue-green fluorescence.