Raman spectroscopy is a versatile technique for characterizing the material structure at a molecular level. D band (~1340 cm -1 ) and G band (~1604 cm -1 ) are the two mainly concerned regions in the Raman spectrum of GO and rGO materials (Figure ), where D band corresponds to sp 3 -hybridized carbons, edges and defects, and G band is related to the sp 2 -hybridized carbons. Additionally, a defect activated peak denoted as D+G mode can also be found at ~2920 cm -1 located in the 2D region. The change of D/G ratio (ID/IG) were extracted from the Raman spectrum and plotted in Figure . The linear increase of VGM D/G ratio from 0.96 to 1.21 (Figure )