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 nanosheets and graphitic hollow polyhedrons. The graphene quantum dots were firstly formed by the dehydration of citric acids and then grew and stacked into graphitic nanosheets, 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. Carbon nano-onions (CNO), also named as onion-like carbon, showed a structure of concentric multiple fullerene-shells with high specific surface area up to 600 m 2 /g, diameters of 105 0 nm and excellent potentials in energy storage and conversions, like supercapacitors, lithium ion batteries and electrocatalysis. [1-3] The CNOs were firstly observed by S. Iijima using transmission electron microscopy (TEM) in 1980 [1] and attracting the worldwide research interests by the report of D. Ugarte in 1992, [2] where the in-situ formation process of CNOs from amorphous soot were revealed by TEM. The reported CNOs were usually synthesized under harsh conditions, such as electron irradiation of carbon soot under vacuum, [2] arc discharge of graphite electrodes in water [4] and heat treatment of nano diamonds at high temperature 1500°C, [5] which impeded the practical applications of CNOs. Although some new synthesis methods of CNOs at relative low temperature (200~500°C) were reported, [6-8] the formation and evolution processes of the synthesized CNO were still not clear. For example, the carbonization of tomatoes at 240°C using

We present a critical review of CNOs regarding the structure and synthesis process, elaborating the recent reports on soft-chemistry methods under mild conditions. In particular, solubility and interlayer spacing are discussed.

We report an environmentally friendly strategy for the synthesis of Fe3C/Fe/graphitic carbon based on hydrothermal carbonization and graphitization of carbon spheres with potassium ferrate (K2FeO4) at 800 °C.

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