Hydrothermal Synthesis of Carbon Nano-Onions from Citric Acid.

Carbon nano-onions with concentric fullerene-like carbon shells were synthesized by hydrothermal carbonization of citric acids at 180 °C and formed through the intermediates of graphene quantum dots, graphitic nano-sheets and graphitic hollow polyhedrons. The graphene quantum dots were firstly formed by the dehydration of citric acids and then grew and stacked into graphitic nano-sheets, which curved into graphitic hollow polyhedrons due to the interface energy. The graphitic hollow polyhedrons eventually transformed into spherical carbon nano-onions with the lowest interface energy in the hydrothermal solution by disordering and rearrangement of the carbon shell.