(Invited) Electrochemical Synthesis of Carbon Nano Onions
Authors: Bian Yixuan , Li Meixian 
Published: May 2020

Carbon nano-onions (CNOs) as one of the carbon allotropes, possessing peculiar structure, are composed of multilayered concentric graphitic shells. CNOs can be classified into two types, including hollow polyhedral CNOs and core–shell structured CNOs. CNOs have attracted a great deal of attention because of their unique chemical and physical properties. Dependent on these properties they show potential applications in many areas such as solid lubricants, electrode materials in capacitors, as anode materials in lithium-ion batteries, and as catalyst support in fuel cells as well as biomedical field as pharmaceutical delivery platforms 1. However, to produce CNOs in quantity would be extremely challenging, especially hollow polyhedral carbon onions. At present, the preparation methods of CNOs reported included annealing of nanodiamonds at high temperature 2, thermal treatment of laser pyrolysis carbon blacks and co-sputtering 3, chemical vapour deposition 4 and so on. The common characteristic of these methods was the need for high temperature. Due to lack of controllable preparation methods, little research on hollow polyhedral CNOs has been reported. Here, we describe a simple electrochemical method to prepare high quality hollow polyhedral CNOs without the use of high temperature equipment.
Electrochemical synthesis was carried out in a mixed solution of acetonitrile and toluene containing 0.10 M tetra-n-butylammonium perchlorate by using a three-electrode electrochemical system with a platinum plate electrode as the working electrode, a platinum plate electrode as the counter electrode and a Ag rod as the quasi reference electrode in a glove box, as shown in Figure 1a. Multi-potential steps electrolysis were employed. The obtained solid products after electrolysis for a few hours were dispersed in N-cyclohexylpyrrolidone by ultrasonication, and then the solid spherical CNOs and the hollow polyhedral CNOs were separated by gradient centrifugation, as shown in Figures 1b and 1c by high resolution transmission electron microscope characterization. Figure 1d displays TEM oblique image of a hollow polyhedral CNO. A series of images obtained from different tilt angles proved that CNOs were polyhedral hollow structures. Hollow carbon nano onions have an average diameter of 20 nm. The synthetic method is economical and environmentally friendly. Because the CNOs obtained by different methods owned different structures and properties, the properties and applications of the hollow polyhedral CNOs are under way.
ACKNOWLEDGEMENT
This work was supported by the National Natural Science Foundation of China (No 21675003 and 21974003).
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