As shown inFigure 1(a), as-prepared GO sheets are nearly
transparent under electron irradiation, indicating that the
GO sheets are quite thin. As shown inFigure 1(b),the
C1s XPS spectrum of GO indicates the presence of four
components: the C in C=C bonds (284.5 eV), the C in C-O
bonds (286.6 eV), the C in C=O bonds (287.7 eV), and the
C in O–C=O bonds (288.7 eV). It indicates the considerable
degree of the oxidation existing in GO material [39, 40]. GO
sheets are a chemically modified graphene with suspended
hydroxyl, epoxyl, and carboxyl functional groups [41], which
result in the hydrophilic nature of GO