Understanding the structural transformation of carbon black from solid spheres to hollow polyhedra during high temperature treatment.

Understanding the structural transformation of carbon black during high temperature treatment and the underlying mechanism are very important because of the correlation with nanocarbon species such as fullerenes and carbon onions. Herein, we find that carbon black nanoparticles exhibit a solid skin-core structure constructed from small graphene flakes. The skin shows a more ordered structure than the core. During heating treatment in an inert atmosphere, the small graphene flakes coalesce together to form large-area lamellae at 1600 °C. Then, the solid spherical nanoparticles completely transform to hollow polyhedra at 1800 °C with significantly improved crystallinity. What's more, the inner cores of carbon black can be removed through simple oxidation in air, demonstrating that the cores are more disordered and reactive than the skin. Accordingly, the structural transformation mechanism is ascribed to well-ordered graphitic shells being preferentially formed by coalescing ordered small graphene flakes in the skin region of carbon black nanoparticles. The multilocular hollow structure is subsequently formed by reconstruction of highly disordered and twisted inner cores in a confined space.