For comparison, the C 1s spectrum of asprepared GO is exhibited in Fig. 4f. Except non-oxygenated carbon
(C¼C), three oxidation states can be clearly observed, the carbon
in C-O bond at 285.2 eV, the carbonyl carbon at 286.9 eV and the
carboxylate carbon (OC¼O) at 288.6 eV