Thermochemistry of onion-like carbons

The standard enthalpies of formation at 25 C of onion-like carbons (OLC) with different structural ordering have been investigated by high-temperature oxidation calorimetry. In terms of enthalpy and depending on the degree of structural ordering, OLC can be up to 16 kJ mol^-1 less stable than graphite but up to 27 kJ mol^-1 more stable than their fullerene allotropes. Furthermore, OLC are approximately 5-9 kJ mol^-1 less stable than single-wall carbon nanotubes (SWCNTs). The samples prepared at 1800 C are energetically less stable than samples made at 1300 and 1500 C. These changes in energetics may stem from oxygen-containing functional groups bonded to the structure or from the creation of topological defects (polygonization and pentagon formation) whose concentration increases with increasing temperature and whose higher energy is balanced by configurational entropy.