Synthesis of onion-like carbon nanoparticles by flame pyrolysis

Spherical carbons with an onion-like structure can be synthesized using a facile flame pyrolysis (FP) method. The methodology has been known for centuries and in the 20th century, the technique was developed into a process to make ‘lamp black’ carbons. In more recent years, the method of using flames to make carbon soot has been exploited by using many new carbon starting materials. Many different names have been given to these spherical carbons, and in this review, we have referred to the spherical carbons made by FP as onion-like carbon nanomaterials (OLCNs). They generally have a quasi-spherical shape with size ≤ 60 nm and are made up of turbostratic layered domains/platelets stacked on top of each other to give the quasi-spherical structure. Recent developments in the area have involved the use of different reactor designs and different carbon sources to make the OLCNs as well as in situ and post doping/functionalization to modify the OLCN surface. Further, it has been shown that these OLCNs can also be made from waste oils, indicating their role in waste disposal approaches to carbon synthesis. Numerous uses have been found for these OLCNs (such as in supercapacitors), which has driven research in this area.