Fig. 2. Characterization of the distribution of different oxygenated groups on various GQDs. O1s (a) and N1s (b) XPS analysis of various GQDs. The atomic percentage of different oxygenated groups on various GQDs are calculated via the deconvolution of the XPS spectra (c). PL spectra of c-GQDs (d) and h-GQDs (e) under different excitations. Inset: photograph of the c-GQD (left) and h-GQD (right) aqueous solutions obtained under UVA light. UV-Vis-NIR spectra of c-GQDs and h-GQDs (f).