## 3 Results and Discussion  
### Characterization and Formation Mechanism of GQDs  
Oidation and hydrothermal treatment modified the structure of GO. Different characterizations were performed to explain the formation mechanism of GQDs. Figure 1 shows the surface morphologies of GO, O-GO, and R-O-GO. Dried GO, O-GO, and R-O-GO exist in sheets, and O-GO and R-O-GO both present metallic appearance. The SEM images show that there are wrinkles at the surface of GO, O-GO and R-O-GO (Figure 2). HR-TEM images (Figure 3) show that GQD1 and GQD2 are nanosheets with small size and circular shape. GQD1 are distributed with diameters ranging from 2.25 to 5.03 nm (3.38 nm in average) and GQD2 1.20 nm to 2.98 nm (2.03 nm on average). FT-IR analysis of GO, O-GO, and R-O-GO (Figure 4) shows that there are four characteristic peaks at \(\sim\)3430, \(\sim\)2360, \(\sim\)1630, and \(\sim\)1070 cm\({}^{-1}\), corresponding to -OH, C=O, C=C, and C-O-C  
Figure 1: Photographs of (**a**) GO, (**b**) O-GO and (**c**) R-O-GO taken under visible light. GO: graphene oxide; O-GO: oxidized graphene oxide; R-O-GO: reduced O-GO. Figure 2: Scanning electronic microscopy (SEM) images of (**a**) GO, (**b**) O-GO, (**c**) R-O-GO dispersed in water. groups, respectively [30]. The absorption intensity of -OH (3437 cm\({}^{-1}\)) and C-O-C (1070 cm\({}^{-1}\)) groups weakens in R-O-GO, suggesting that -OH and C-O-C groups disappear gradually during the hydrothermal treatment. Furthermore, chemical oxidation can create a lot of disordered structures in GO, which is revealed by the Raman spectra (Figure 5). The G band is associated with E\({}_{\rm 2g}\) vibrational modes of the sp\({}^{2}\) carbon atom, and the D band represents the edge defect and amorphous graphitic system [31; 32]. The intensity ratio \(I_{\rm D}/I_{\rm G}\), \"disorder structure\" to the \"crystalline structure\" is adopted to evaluate the structure of different oxidized GO. An increase in the \(I_{\rm D}/I_{\rm G}\) means an increase in the amount of topological disorder in the graphite layer and a decrease in the size of nanocrystalline graphite [30]. The Raman Spectra of GO, O-GO and R-O-GO given in Figure 5 show that the D and G bands are  
Figure 4: Fourier transform infrared (FT-IR) spectra of GO, O-GO and R-O-GO. Figure 3: High resolution transmission electron microscopy (HR-TEM) images of (**a**) GQD1 and (**b**) GQD2. The insets in (**a**,**b**) are the corresponding representative images of individual GQD1 and GQD2. The right column shows the diameter distribution of GQD1 and GQD2. 1345 and 1588 cm\({}^{-1}\), respectively. The intensity ratio \(I_{\rm D}/I_{\rm G}\) decreases from 1.02 (GO) to 0.96 (O-GO), which is contrary to the general rule. We deduce that after the treatment of GO by HNO\({}_{3}\), the obtained O-GO is of relatively high quality. However, the O-GO exhibits a broader D band, which indicates that the intercalation of N atoms and oxygen-containing groups leads to somewhat disordered structures. This result is similar to a previously-reported conclusion [20]. It was predicted that the intensity ratio \(I_{\rm D}/I_{\rm G}\) of GO would significantly decrease after reduction as the sp\({}^{2}\) structure restored. However, the \(I_{\rm D}/I_{\rm G}\) was found to be 1.15 in R-O-GO after hydrothermal treatment, which is larger than that in O-GO. Similar results have also been reported elsewhere in the literature [33; 34]. We deduce that the increase of the \(I_{\rm D}/I_{\rm G}\) ratio after chemical reduction can be attributed to the decrease in the average size of the sp\({}^{2}\) domains upon reduction of O-GO, in which newly-created graphitic domains are more numerous in number, but smaller in size, than those present in O-GO. Besides that, the increased fraction of graphene edges could also contribute to the increase in the \(I_{\rm D}/I_{\rm G}\) ratio, which is in accordance with the reported results [33; 35]. The chemical structure and the type of oxygen-containing groups of GO changed after oxidation and hydrothermal treatment.