On the other hand, the second step resulted in a comparable but slightly greater contribution of oxygen functionalities for s-GO (11.7%) than in l-GO (9.1%) or us-GO (9.7%). Further quantification of the functionalisation of GO was achieved by acquiring XPS spectra (Figure ). The survey spectra (Figure and Table , Supporting Information) demonstrated the high chemical purity of GO by evidencing only minute atomic impurities such as nitrogen and sodium (< 0.5%), compared to the typical contributions of the 1s electron of carbon (C1s) and oxygen (O1s) atoms in GO. Moreover, similar C:O ratios were calculated for the three GO types (2.1 -2.2) to indicate similar oxidation degree of these materials.