Preparation of graphene oxide and characterisation using electron spectroscopy

Highlights: • Graphene oxide by ozonation and cold glow air plasma from 8 nm graphene platelets. • XPS, REELS for characterising the surface chemical composition and the structure. • C sp³ and carbon–oxygen groups content smaller contrary to wet chemical methods. • REELS shows decreasing number of graphene layers: platelets > ozone > plasma. • Oxidation separates graphene layers via incorporating interlayer oxygen-H₂O groups. - Abstract: Graphene oxide was prepared from commercial graphene powder (G) platelets of 8 nm thickness by oxidation in ozone (G-O3) and low-energy (cold) glow air plasma in a pink region (G-PP). X-ray photoelectron spectroscopy (XPS) was applied for chemical characterisation of the atomic content and for obtaining quantitative information on C hybridisation. Reflection electron energy loss spectroscopy (REELS) was used for characterising the surface structure including the content of surface and bulk carbon atoms of sp² and sp³ hybridisation. The G-O3 and G-PP graphene oxide samples contain a low amount of C sp³ bonds and carbon–oxygen groups in comparison to graphene oxide prepared by the “wet” chemical methods. Oxidation of commercial graphene platelets powder, especially in air plasma, leads to increasing surface C sp³ and significantly bulk C sp³ contributions, indicating intercalation by oxygen groups. The intensity ratio of the REELS π + σ C sp² bulk to the C sp² surface energy loss peaks, decreasing in the order G>G-O3>G-PP, indicates exfoliation of layers in G-O3 and G-PP by oxygen functional groups and water with decreasing average number of layers in graphene oxide nanostructures due to oxidation. Although, the “wet” chemical methods are more effective for oxidation leading to a larger amount of C sp³ and oxygen groups, the proposed methods of oxidation by ozonation and in air plasma are inexpensive, safe, effective, environmentally friendly and do not result in toxic chemical waste products