Theoretical investigation of methane adsorption onto boron nitride and carbon nanotubes.

Methane adsorption onto single-wall boron nitride nanotubes (BNNTs) and carbon nanotubes (CNTs) was studied using the density functional theory within the generalized gradient approximation. The structural optimization of several bonding configurations for a CH4 molecule approaching the outer surface of the (8,0) BNNT and (8,0) CNT shows that the CH4 molecule is preferentially adsorbed onto the CNT with a binding energy of -2.84 kcal mol-1. A comparative study of nanotubes with different diameters (curvatures) reveals that the methane adsorptive capability for the exterior surface increases for wider CNTs and decreases for wider BNNTs. The introduction of defects in the BNNT significantly enhances methane adsorption. We also examined the possibility of binding a bilayer or a single layer of methane molecules and found that methane molecules preferentially adsorb as a single layer onto either BNNTs or CNTs. However, bilayer adsorption is feasible for CNTs and defective BNNTs and requires binding energies of -3.00 and -1.44 kcal mol-1 per adsorbed CH4 molecule, respectively. Our first-principles findings indicate that BNNTs might be an unsuitable material for natural gas storage.