Probing the Thermal Deoxygenation of Graphene Oxide Using High-Resolution In Situ X-ray-Based Spectroscopies

Despite the recent developments in graphene oxide due toits importance as a host precursor of graphene, the detailed electronicstructure and its evolution during the thermal reduction remain largelyunknown, hindering its potential applications. We show that a combina-tion of high-resolution in situ X-ray photoemission and X-ray absorptionspectroscopies offer a powerful approach to monitor the deoxygenationprocess and comprehensively evaluate the electronic structure of gra-phene oxide thin films at different stages of the thermal reduction process.It is established that the edge plane carboxyl groups are highly unstable,whereas carbonyl groups are more difficult to remove. The resultsconsistently support the formation of phenol groups through reaction of basalplane epoxide groups with adjacent hydroxyl groups at moderate degrees ofthermal activation (∼400 C). The phenol groups are predominant overcarbonyl groups and survive even at a temperature of 1000 C. For the first time, a drastic increase in the density of states (DOS) near theFermi level at 600 C is observed, suggesting a progressive restoration of aromatic structure in the thermally reduced graphene oxide.