Transformation of carbon onions to diamond by low-temperature heat treatment in air

We have demonstrated the transformation of spherical carbon onions to diamond by low-temperature heat treatment. Carbon onions were prepared by 1700 degrees C annealing of diamond nanoparticles. High-resolution transmission electron microscopy (HRTEM) and X-ray diffraction showed that diamond nanoparticles transformed into the carbon onions and no trace of diamond was detected after the annealing Carbon onions were subsequently heat treated at 500 degrees C in air. HRTEM images showed that diamond particles several tens of nanometers in diameter as well as the carbon onions coexist after the heat treatment in air. The results suggest that the onions transformed to diamond at a tow-temperature under atmospheric pressure without electron and ion irradiation. From detailed HRTEM and electron energy-loss spectroscopy studies, sp(3) sites in the onions and oxygen during the heat treatment are considered to play important roles in the transformation without any irradiation. (C) 2000 Elsevier Science S.A. All rights reserved.