Energetics and Structural Properties of Nitrogen Molecules Confined in a Carbon Capsule
The energetics and structural properties of N2 molecules confined in a closed-ended (5,5) carbon capsule are investigated by means of semiempirical and density functional theory approaches. Similar to previous works addressing the encapsulation of molecular nitrogen in the endohedral cavities of carbon nanotubes we obtain, with increasing the number of encapsulated molecules, the formation of various non bonded molecular conformations namely, linear chains, zig–zag arrays, and several tubular helicoidal configurations. However, our density functional theory calculations reveal that the interaction potential between the encapsulated nitrogens and the internal surface of the carbon structure is not always repulsive for all our considered intermolecular distances. In fact, we have obtained the presence of a second energy minima in the N2-wall interaction energy curve, located near the inner surface, suggesting the possible existence of a more reactive carbon wall. For a single encapsulated nitrogen molecule, energy barriers of the order of 4 eV need to be overcome in order to achieve N2 adsorption to the carbon structure. However, we have found that with the close proximity of neighboring co-adsorbed nitrogens (a fact that naturally occurs at high densities of stored N2) these barriers are considerably lowered, favoring the existence of reaction paths through which adsorption to the internal surface could occur. Finally, by analyzing the electronic structure of our nitrogen containing carbon capsule we observe, with increasing the number of stored N2, the existence of sizable energy shifts and changes in the orbital nature of the highest-occupied and lowest-unoccupied molecular orbitals, which favours the possibility of having a sizable charge transfer and subsequent bond formation between the nitrogen species and the carbon wall. We believe that our results shed some new light into the properties of small molecules under extreme confinement and should be taken into account when analyzing the maximum storage capacity as well as the transport properties of nitrogen molecules encapsulated in carbon hollow nanostructures.
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Document Type: Research Article
Publication date: 2008-11-01
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