The evolution of hydrogen induced defects and the restoration of pi-plasmon as a monitor of the thermal reduction of graphene oxide

In this article, we study the modification of the optical, chemical and electronic properties of graphene oxide (GO) during thermal reduction in ultra-high-vacuum by combining the results of several electron spectroscopies. We find that the fraction of oxygen moieties on the surface, as deduced from the evolution of C is core level in photoemission, is progressively reduced upon increasing the annealing temperature from 150 to 650 degrees C. The intensity of the CH stretching mode, associated with CH defects on GO surface and measured in the low energy region of electron energy loss spectra (EELS), decreases as a function of the annealing temperature. The removal or the reduction of such hydrogen or oxygen defects induces a restoration of sp(2) carbon hybridization. The presence of such hybridization is confirmed by the capability to excite pi-plasmon as observed in the EELS spectra. In particular we find a critical annealing temperature (T-ann = 300 degrees C) at which pi-plasmon excitation via electron scattering is accessible suggesting the formation of graphene-like domains with size comparable with the plasmon wavelength (lambda(p)similar to 5 nm). The linear dispersion of pi-band close to Fermi level, as measured in UPS, confirms the formation of graphene-like domains.