Recent research into graphene-based materials is largely focused on graphene quantum dots (GQDs) and their optical properties. A facile method has been developed to extract GQDs from reduced graphene oxide (RGO) by the ozonation pre-oxide method. The as-prepared GQDs, which were 2-5 nm in diameter, exhibited strong fluorescence activity ranging from $355 nm to $440 nm. The prepared GQDs possessed strong fluorescence with quantum yields from 3.18% to 9.48%. What's more, the fluorescence properties of the GQDs could be determined by tuning the pH of the ozonation system. We speculated the mechanisms of ozonation, thermal and hydrothermal treatment. We found that pyrocatechol could lead to fluorescence quenching of the GQDs, which might produce novel potential for the detection of targets.

Absorption and fluorescence spectra of graphene quantum dots (GQDs) have been computed by using time-dependent density functional theory (TDDFT). Different functionals, including PBE, TPSSh, B3LYP, PBE0, CAM-B3LYP, and LC-ωPBE, have been tested and B3LYP/6-31G(d) has been proven to be the most accurate method for our work. The bulk solvent effects of toluene and dichloromethane have been assessed by using the polarizable continuum model (PCM). The absorption wavelength of GQDs in solvents is red-shifted compared with that in the gas phase. Edge functionalization effects analysis shows that a small number of substituted groups on GQDs induce a small redshift whereas a large redshift occurs when the edges of GQDs are all decorated. Little difference in the fluorescent emission was observed in solvents and in the gas phase. Molecular orbital transition and transition density matrix analysis have been performed. The electronic transition mainly occurs in the middle part of the structure of C132. The strong absorption of C132 corresponds to a S0 →S3 transition and the fluorescence emission is ascribed to a S1 →S0 transition, which indicates that Kasha's rule is obeyed.

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We took ozonation as an effective method to re-oxidize graphene oxide (GO) and discussed the behaviour of ozone in ozonation process of graphene oxide according to the changes of optical properties, compositions and structure. The results indicated that the ozonation process may involve the oxidation stage and decomposition stage. The fluorescence shifted blue with prolonging ozonation time.

Graphene quantum dots (GQDs) play a critical role in many applications in the electrical and optical fields. We develop a simple three-step hydrothermal etching method to prepare GQDs by adopting graphene oxide (GO) as a precursor and nitric acid as an oxidant. We discuss the formation mechanism of GQDs by the characterization of products and intermediates with Scanning electronic microscopy (SEM), Transmission electron microscopic (TEM), Raman, Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). Two kinds of GQDs have been obtained after the treatment of GO with different concentrations of nitric acid. The sizes of GQDs are small, with diameters of 3.38 nm and 2.03 nm on average, respectively. When excited with 365 nm UV light, the two kinds of GQDs exhibit green and yellow luminescence; the different optical properties can be attributed to the differences in degree of oxidation and nitrogen doping. The result is important for GQDs in synthesizing and optical field.