Carbon nano-onion as next-generation functional nanomaterial: Synthesis methods and practical applications

Carbon nano-onion (CNO) (also known as onion-like carbon, OLC), exhibiting multiple enclosed fullerene shell structures, as one of the most promising nanoforms, has attracted worldwide attention during the past decades due to its exceptional chemical and physical properties such as non-toxicity, high chemical stability, large sufficient surface area with low density, and superior high electronic and thermal conductivities, visible photoluminescence, etc. Nowadays, functional CNOs have been applied in energy storage devices, supercapacitors, photovoltaics, light-emitting diodes and bio-imaging technology. Since the first observation of CNOs by transmission electron microscopy as a byproduct in the synthesis of carbon black in 1980, numerous experimental and theoretical studies including expressive practical applications of CNOs have been intensively developed in modern chemistry. With respect to synthetic techniques, the high-temperature annealing of nano diamond, detonation of high explosive molecules, arc discharge of graphite, chemical vapor deposition, laser ablation, thermal pyrolysis, hydrothermal carbonization, and microwave pyrolysis have been reported. It has been addressed that the synthesis approach plays a key role in determining the structure of CNOs and resultant properties. This paper reviewed the developments of CNOs through major synthesis methods utilized for a selected wide spectrum of applications, by covering both the past and current progress. The contents outlined in the current review will offer readers comprehensive insights into the design and development of CNO materials.