Annealing-induced structural changes of carbon onions: High-resolution transmission electron microscopy and Raman studies

The first- and second-order Raman spectra of carbon nano-onions (CNOs), produced via annealing of detonation nanodiamonds with a mean grain size of similar to 5 nm in the argon ambience at the maximal temperature of annealing process (T-MAX) varying from 1500 to 2150 degrees C, are analyzed together with the high-resolution transmission electron microscopy (HRTEM) images. The combined analysis provides a deep insight into the annealing-induced atomic-scale structural modifications of the CNO nanoparticles. The Raman and HRTEM data unambiguously demonstrate the reduction in the number of defects in the CNO structure, as well as indicate the conversion from the diamond sp(3)-bonded carbon phase to the sp(2)-bonded carbon phase with increasing T-MAX and its almost full completion for T-MAX = 1600 degrees C. (C) 2014 Elsevier Ltd. All rights reserved.