Fabrication of onion-like carbon from nanodiamond by annealing

Onion-like carbon (OLC) was synthesized by annealing nanodiamond in low vacuum of 1 Pa and at annealing temperatures from 500°C to 1400°C. The high-resolution transmission electron microscope (HRTEM) images, X-ray diffraction (XRD) and Raman spectrum of the OLC showed that there was no OLC when the annealing temperature was lower than 900°C. Moreover, the fragment amorphous carbon existed on the surfaces of the nanodiamond particles. At the annealing temperature of 900°C, the OLC particles began appearing and the size of the OLC particles was smaller than 5 nm. When the annealing temperature was increased from 900°C to 1400°C, the nanodiamond was transformed into OLC gradually. At the annealing temperature of 1400°C, all the nanodiamond particles were transformed into OLC completely. The OLC exhibited similarity to the original nanodiamond particles in shape. A mechanism for the OLC synthesis by annealing was provided. The graphitization started at the surfaces of the nanodiamond particles. The formation process of the OLC includes formation of graphite fragments, connection and curvature of graphite sheets between diamond (111) planes and closure of the graphite layers.