Solvent-free Synthesis of Graphene Oxide

The graphene oxide nanoparticles were initially synthesized by direct heating solvent-free method based on the literature^{35 (link)}. To do this, citric acid was used as an appropriate carbon source. In a typical synthesis, 5.00 g of citric acid (Merk, Germany) was introduced into a beaker, followed by direct heating at 200 °C for about 2.0 h to produce solid black graphene oxide nanoparticles. Notably, after 5.0 min of the reaction, a liquated yellow-colored product was produced. The color was changed to orange within 30.0 min. Finally, after 2 h from the solvent-free heating the orange liquid transferred into the final blackish solid graphene oxide nanoparticles.