Transformation of graphene nanoflakes into onion-like carbon during spark plasma sintering

Highlights: • Onion-like carbon were produces from graphene nanoflakes by SPS at 1100 °C and 30 MPa. • Sintered pellets can be easy oxidized by HNO₃ vapors without their destruction. • The oxygen content increased up to 28.8 at.% (oxidation time was 6 h). • The possible mechanism of OLC formation during SPS are proposed. • Applications of SPS for carbon phase transformations are discussed. Graphene nanoflakes (GNFs) were consolidated by spark plasma sintering (SPS) at 1100 °C and 30 MPa. The sintered material was characterized by TEM, BET and Raman spectroscopy. Along with unmodified GNFs it contained graphene sheets and onion-like carbon (OLC) particles. Phase separation and functionalization of the pellet surface were performed by oxidation with nitric acid vapors. According to the XPS data the oxygen content in the oxidized sintered material increased from 17.8 up to 28.8 at. % with increasing the oxidation time from 3 up to 6 h. At the same time graphene sheets and OLC disappeared. Gas phase oxidation prevented the destruction of sintered pellets. The possible mechanism of OLC formation during SPS was proposed. Potential applications of SPS for carbon phase transformations were discussed.