Computer Simulations of the Structure of Nanoporous Carbons and Higher Density Phases of Carbon

Abstract

The most stable form of solid carbon is graphite, a stacking of graphene layers in which the carbon atoms show $sp^2$ hybridization which leads to strong intra-layer bonding. Diamond is a denser phase, obtained at high pressure. In diamond the carbon atoms show $sp^3$ hybridization. Metastable solid carbon phases can be prepared also with lower density than graphite (in fact, densities lower than water); for instance the carbide-derived carbons. These are porous materials with a quite disordered structure. Atomistic computer simulations of carbide-derived carbons indicate that the pore walls can be viewed as curved and planar nanographene ribbons with numerous defects and open edges. Consequently, the hybridization of the carbon atoms in the porous carbons is $sp^2$. Because of the high porosity and large specific surface area, nanoporous carbons find applications in gas adsorption, batteries and nanocatalysis, among others. We have performed computer simulations, employing large simulation cells and long simulation times, to reveal the details of the structure of the nanoporous carbons. In the dynamical simulations the interactions between the atoms are represented by empirical many-body potentials. We have also investigated the effect of the density on the structure of the disordered carbons and on the hybridization of the carbon atoms. At low densities, typical of the porous carbide-derived carbons formed experimentally, the hybridization is $sp^2$. On the other hand, as the density of the disordered material increases, a growing fraction of atoms with $sp^3$ hybridization appears.

Work dedicated to Professor N. H. March.