Synthesis and Growth of Onion-Like Polyhedral Graphitic Nanocapsules by Thermal Plasma

Graphitic nanocapsules (GNCs) are onion-like carbon structures consisting of concentric polyhedral multilayer shells. GNCs exhibit outstanding physicochemical properties, such as large specific surface area, high electrical conductivity and broad absorption spectra. Their unique structure and interesting properties make them suitable for a range of potential applications. However, current GNCs synthesis methods are hampered by poor yield and/or low purity, which prevents their integration into large-scale applications. In this work, a fast and efficient process for the synthesis of GNCs is presented. Onion-like polyhedral GNCs with diameters between 70 and 300 nm are produced in a one-step process without catalyst by thermal decomposition of methane using a plasma torch. With the present system, GNCs are synthesized semi-continuously at a production rate of ∼ 20 g.h −1 using 1.5 slpm of methane at 82 kPa. The effects of the pressure and the methane flow rate on the morphologies of carbon nanostructures are examined by high-resolution transmission electron microscopy (HR-TEM). The results show a progressive evolution of the morphology from graphene nanoflakes (GNFs) to GNCs with increasing pressure or methane flow rate. We discuss two different nucleation mechanisms to explain the shape of GNFs: the first starts with the curling of a graphene nanoflake while the second involves the delamination of a GNFs into a bowl or a cylinder. Once the first few shells are formed, the growth of the shell-specific crystalline facets occurs with the epitaxial addition of carbon adatoms. Graphical abstract