Structural Determination of Thermally and Hydrazine Treated Graphene Oxide Using Electron Spectroscopic Analysis

Here we use UV photoelectron spectroscopy (UPS) and metastable induced electron spectroscopy (MIES) to determine the valence electron structure of graphene oxide (GO) and hydrazine modified graphene oxide, or so-called reduced GO (rGO). We show that pristine GO has a low density of states (DOS) in the 2pπ region. Upon thermal treatment, under vacuum, to 200 °C, the DOS in the 2pπ region increase. The change in the DOS is also reflected in a change in the functional groups attached to the GO. These changes are followed by X-ray photoelectron spectroscopy (XPS). Based on the XPS measurements, the GO is described as highly carboxylated, much akin to pyromellite, with incorporated benzoquinone moieties. After heat treatment to 100 °C, we propose the close proximity of the pendant carboxyls undergoes a condensation reaction to form dianhdydrides such that the graphene sheet now consists of a mixture of dianhydride, phthalate-type carboxyl, and benzoquinone moieties with the work function increasing to 4.8 ± 0.1 eV. With further heating to 200-300 °C, these groups are cleaved and the work function decreases back to 4.2 ± 0.1 eV. Lastly, the rGO is shown to form pyrazoline-type bonds with an interrupted 2pπ graphene network which results in a decreased work function to 3.4 eV.