Preparation of graphene oxide and characterisation using electron spectroscopy

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 sp(2) and sp(3) hybridisation. The G-O3 and G-PP graphene oxide samples contain a low amount of C sp(3) 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 sp(3) and significantly bulk C sp(3) contributions, indicating intercalation by oxygen groups. The intensity ratio of the REELS pi + sigma C sp(2) bulk to the C sp(2) 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 sp(3) 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. (C) 2014 Elsevier B.V. All rights reserved.