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Revisiting Graphene Oxide Chemistry

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In the last decade, graphene oxide (GO) has attracted a widespread interest for its mechanical strength, tunable optoelectrical properties, simple processability and its potential as precursor for a low-cost and large-scale production of graphene (reduced graphene oxide, RGO). Nevertheless, after more than 150 year since its discovery, the type and distribution of oxygen functional groups in graphene oxide (GO) and reduced graphene oxide (RGO) remain still a subject of great debate. Currently, the most acknowledged model for GO corresponds to a random functionalization of the carbon basal plane with epoxide and hydroxyl groups, forming graphitic and partially oxidized domains. However, no definitive evidence of this model has been reported and local analytic techniques are required to access the chemistry of these materials at a nanometric scale. Electron energy loss spectroscopy in a scanning transmission electron microscope can provide the suitable resolution, but GO and RGO are extremely sensitive to electron irradiation. To solve this issue we employ an adapted experimental setup to reduce electron illumination below damage limit. GO oxygen maps obtained at a few nanometers scale show separated domains with different oxidation levels. The C/O ratio varies from about 4:1 to 1:1, the latter corresponding to a complete functionalization of the graphene flakes. In RGO the residual oxygen concentrates mostly in regions few tens of nanometers wide. Specific energy-loss near-edge structures are observed for different oxidation levels. By combining these findings with first-principles simulations we propose a model for the highly oxidized domains where graphene is fully functionalized by hydroxyl groups forming a 2D-sp3 carbon network analogous to that of graphane.

A. Tararan, A. Zobelli, A. Benito, W.K. Maser and O. St├ęphan, Chem. Mat. 28, 3741 (2016)