Molecular dynamics study of structure and graphitization process of nanodiamonds

Nanodiamond (nD) particles with the shapes of a truncated octahedron and a sphere of an average size of about 23 Å constituted starting configurations for molecular dynamics (MD) simulations using the reactive empirical bond order potential for carbon–carbon interaction. Calculations performed at 300 K showed rebuilding of the surface structure, where trivalent atoms were localised, and in both cases about 72% of atoms localised in the nD interior had tetravalent coordination. The inter-atomic carbon–carbon distances were slightly larger than those for the ideal diamond structure. The results obtained from simulations match quite well the high-energy X-ray diffraction data for explosive diamond nanoparticles recorded at the European Synchrotron Radiation Facility. To study graphitization process of the nDs we performed MD simulations at elevated temperatures up to 1800 K. Transformation of the truncated octahedron nD into onion-like carbon was observed at 1200 K. In this case, bonds between {1 1 1} planes were broken firstly, and then the planes bent forming the onion structure. Graphitization of the spherical nD appeared at 1500 K. The process started at the surface, where fragments of quasi-spherical shells were formed, and evolved into the nD centre. For both nDs the process became faster and more effective with increasing temperature.