It is thought that these oxygen functionalities are present in defected sp 3 sites within the sp 2 lattice of graphene, as a result of the oxidation of graphite. Consistent with previous studies, all three GO materials had a negligible 2D peak (~2660 cm -1 ) compared to what was previously observed for the starting graphite source , giving rise to the first order D band (~1338 cm -1 ). Broad D and G bands were rather observed in the Raman spectra of the three GO samples (Figure ), describing the introduction of defects in the graphene lattice . The ID/IG ratio did not significantly differ among the three GO materials, implying that the sonication time did not further increase the amount of defects in the graphene lattice with the introduction of additional oxygen functionalities, which would otherwise increase the ID/IG ratio.