The Effect of Thermal Reduction on the Photoluminescence and Electronic Structures of Graphene Oxides

Electronic structures of graphene oxide (GO) and hydro-thermally reduced graphene oxides (rGOs) processed at low temperatures (120-180 degrees C) were studied using X-ray absorption near-edge structure (XANES), X-ray emission spectroscopy (XES) and resonant inelastic X-ray scattering (RIXS). C K-edge XANES spectra of rGOs reveal that thermal reduction restores C = C sp(2) bonds and removes some of the oxygen and hydroxyl groups of GO, which initiates the evolution of carbonaceous species. The combination of C K-edge XANES and K-alpha XES spectra shows that the overlapping pi and pi* orbitals in rGOs and GO are similar to that of highly ordered pyrolytic graphite (HOPG), which has no band-gap. C K-alpha RIXS spectra provide evidence that thermal reduction changes the density of states (DOSs) that is generated in the pi-region and/or in the gap between the pi and pi* levels of the GO and rGOs. Two-dimensional C K-alpha RIXS mapping of the heavy reduction of rGOs further confirms that the residual oxygen and/or oxygen-containing functional groups modify the pi and sigma features, which are dispersed by the photon excitation energy. The dispersion behavior near the K point is approximately linear and differs from the parabolic-like dispersion observed in HOPG.