Chemistry and Structure of Graphene Oxide via Direct Imaging.

Graphene oxide (GO) and reduced GO (rGO) are the only variants of graphene that can be manufactured at the kilogram scale, and yet the widely accepted model for their structure has largely relied on indirect evidence. Notably, existing high-resolution transmission electron microscopy (HRTEM) studies of graphene oxide report long-range order of sp(2) lattice with isolated defect clusters. Here, we present HRTEM evidence of a different structural form of GO, where nanocrystalline regions of sp(2) lattice are surrounded by regions of disorder. The presence of contaminants that adsorb to the surface of the material at room temperature normally prevents direct observation of the intrinsic atomic structure of this defective GO. To overcome this, we use an in situ heating holder within an aberration-corrected TEM (AC-TEM) to study the atomic structure of this nanocrystalline graphene oxide from room temperature to 700 °C. As the temperature increases to above 500 °C, the adsorbates detach from the GO and the underlying atomic structure is imaged to be small 2-4 nm crystalline domains within a polycrystalline GO film. By combining spectroscopic evidence with the AC-TEM data, we support the dynamic interpretation of the structural evolution of graphene oxide.