Transformation of nanodiamond into carbon onions: A comparative study by high-resolution transmission electron microscopy, electron energy-loss spectroscopy, x-ray diffraction, small-angle x-ray scattering, and ultraviolet Raman spectroscopy

Abstract

The structural properties of both nanodiamond particles synthesized by detonation and the products of their transformation into carbon onions via vacuum annealing at 1000 and 1500°C have been studied using high-resolution transmission electron microscopy (HRTEM), electron energy-loss spectroscopy, x-ray diffraction (XRD), small-angle x-ray scattering (SAXS), and Raman spectroscopy. The advantages of UV Raman spectroscopy over visible Raman spectroscopy for the analysis of these carbon nanomaterials are demonstrated. It was found that the synthesized nanodiamond particles have a composite core-shell structure comprising an ordered diamond core covered by a disordered (amorphous) outer shell formed by the mixed sp2∕sp3 bonding of carbon atoms. The observed structure of the nanodiamond particles are comparable with the structure of the bucky diamond clusters comprising a diamond core and a reconstructed surface which stabilizes the cluster at the average diameter of ∼30Å, as predicted recently from theoretical studies. Assuming a spherical shape for the particles and employing a two-step boundary model of electron density distribution developed in this work to describe the SAXS patterns produced by the core-shell structure of the nanodiamond particles, it was evaluated that the average diameter of the core is ∼30Å and the average thickness of the shell is ∼8Å; values which are in agreement with results obtained from HRTEM and XRD measurements. A discrepancy between these results and average diamond crystallite size obtained from Raman spectra by applying the phonon confinement model (35–45Å) is discussed. It is hypothesized from analysis of broadening of the XRD diamond peaks that at the nanoscale under influence of the particle shape, which is not strictly of a cubic (or spherical) symmetry, a slight hexagonal distortion of the cubic diamond structure appears in the nanodiamond particles. The transformation of the nanodiamond into carbon onions proceeds from the amorphous outer shell of the particles inwards towards the particles’ diamond core. UV Raman spectroscopy effectively senses the initial stage of the transformation revealing a reconstruction of the mixed sp2∕sp3 bonding of carbon atoms located in the outer shell, into sp2-bonded carbon atoms similar to those in nanocrystalline graphite. It is shown that intershell distance in carbon onions formed from nanodiamonds depends on the temperature of the transformation and relates to the linear thermal expansion coefficient of the graphite structure along the stacking direction of the graphene layers (the c axis). In accordance with SAXS results, there is evidence for an increase of the average particle size of the synthesized nanodiamond [48(3)Å] after transformation into carbon onions [58(10)Å].