In the following, the effect of duration of the electrochemical process was investigated by fixing the voltage at -1.8 V. With increasing time, the atomic percentage of Cr slightly increased within the rGO film (Figure ). Moreover, more trivalent chromium was found inside the rGO stack with increasing synthesis time (Figure ). Figure shows that the significant part of the synthesis throughout the thickness of the rGO film occurs in the first 15 mins with a maximum atomic percentage of chromium at ~30%. Fitting of the Cr spectral regions reveals that for duration < 5 min or low voltage samples (-1.1 V) a certain percentage of Cr inside the laminated rGO structure is metallic. However, for -1.8V-15min samples, the Cr 3+ /Cr ratio is > 0.99 for depth up to 300 nm and >0.95 for 500 nm. This indicates that only trace amount of Cr is in its metallic valence state for this synthesis procedure. Figure and S4b indicate that the rise in O content follows a similar trend as the Cr 3+ content with matching stoichiometry ratio of Cr2O3. All XPS spectra and the corresponding curve fittings are displayed in Figure . S5-S10. In agreement with that, Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) further confirms that Cr2O3 is homogeneously created throughout the depth of the rGO films (Figure ). Furthermore, FTIR spectrometry of original GO (F. S12) and the -1.8V-15min samples confirm the reduction of functional groups. Hence, the oxygen detected in XPS is not due to functional groups on the GO plane.