The enthalpies of formation of carbon nanomaterials as a key factor for understanding their structural features

Carbon nanomaterials (CNMs), such as carbon nanotubes (CNTs), carbon nanoflakes (CNFs) and their N-doped derivatives, show sufficient correlations between structure, composition and properties. In this study, the relationships between structure and composition of different types of CNMs as well as their enthalpies of formation are analyzed based on original experimental results and earlier published data. The adiabatic bomb calorimetry technique is used for the determination of these values, together with elemental and thermal analyses, electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy, which are applied for structure and composition analysis. The contributions of surface (ΔfH0298(surf.)) and bulk (ΔfH0298(bulk)) components in the values of enthalpies of formation ΔfH0298 for different CNMs are estimated for the first time. It is shown that ΔfH0298(surf.) is highly influenced by SBET, whereas ΔfH0298(bulk) is defined by the number and homogeneity of inner layers conformable to the graphite structure. In the case of nitrogen-doped CNMs, ΔfH0298 is influenced not only by the nitrogen content but also by the coordination of the heteroatom; substitutional nitrogen demonstrates a higher effect compared to pyrrolic and pyridine-like nitrogens.