Ionothermal Synthesis of Carbon Nanostructures: Playing with Carbon Chemistry in Inorganic Salt Melt

Solution growth of sp carbon nanostructures from molecular precursors has been a tremendous challenge as the operation temperature windows offered by most solvents used in wet chemistry synthesis are too low to allow the complete condensation and carbonization of normal simple molecules, such as sugar. Inorganic ionic salt melt has recently enabled direct solution growth of carbon nanostructures with tailored heteroatom doping and porosity, which are indispensable for most functionalities of carbon materials. Compared with molecular solvents (i.e., water and most organic solvents), the ionic salt melt not only provides a wider and higher operation temperature window, but also strong solvation power that ensures the carbonization and precipitation in a controllable manner. Starting from molecules like sugar, this ionothermal process can produce disordered carbon, graphene-like two-dimensional carbon, highly porous carbon nanosheets, etc., depending on the synthetic conditions. Doping of heteroatoms, such as nitrogen, into the resultant carbon can be made by using oxysalt additives such as nitrate, in which N atoms are reduced and finally incorporated into the sp carbon network. The ionothermal process also can produce nanostructured carbon in a templated fashion in which both salt particles and molecular precursors can serve as the hard or soft template. Due to the favorable microstructure and chemical composition, the ionothermal-derived carbon nanostructures have found diverse applications, as exemplified here by electrocatalysis for oxygen reduction and capacitive energy storage. Further development in ionothermal process will focus on the scalable, green production of carbon nanostructures orientated for commercial applications.