Understanding structure and porosity of nanodiamond-derived carbon onions

Carbon onions derived by thermal annealing of nanodiamonds are an intriguing material for various applications that capitalize the nanoscopic size, high electrical conductivity, or moderately high external surface area. Yet, the impact on synthesis conditions and possible post-synthesis treatment on the pore characteristics lacks a detailed parametric understanding. We present a comprehensive model describing the change of structure, morphology, specific surface area (SSA), and pore size distribution (PSD) of carbon onions derived via thermal annealing of nanodiamonds as a function of synthesis parameters and the effect of physical activation in air. Different heating rates, temperatures, holding times, as well as two different nanodiamond precursors were used. During thermal annealing the increase in SSA occurs along with a loss of surface functional groups and volatile impurities. The sp3-to-sp2 conversion results in a much lower density and an increase in SSA of up to ∼160%. At high temperatures, a sintering and carbon redistribution process limits the increase in SSA and leading to the formation of micrometer-sized graphitic particles with a very low SSA. Oxidation in air is a facile way for the effective removal of predominately amorphous carbon between carbon onion particles and the removal of outer carbon shells.