The surface functional groups of GO and GQDs were characterized by XPS. GO and GQDs show two apparent peaks centered at 285.0 and 532.0 eV which can attribute to C 1s and O 1s, respectively (Fig. 3(a)). In high-resolution XPS C 1s spectra of GO and GQDs, the presence of Cdouble bond; length as m-dashC (∼284.6 eV), Csingle bondO (hydroxyl and epoxy, ∼286.5 eV), Cdouble bond; length as m-dashO (carbonyl, ∼287.8 eV) species were observed (Fig. 3(b)). Meanwhile, the C 1s spectrum of GQDs was provided in Fig. 3(c). The sum of Csingle bondO and Cdouble bond; length as m-dashO species in GQDs is lower than that of GO as shown in the XPS spectra, the less oxygen content of GQDs than that of GO indicates that GQDs possess less degree of oxidation than GO, due to the partial reduction of GQDs.