Hydrogen Adsorption on (5,0) and (3,3) Na-decorated BNNTs

The storage capacity of hydrogen on Na-decorated born nitride nanotubes (BNNTs) is investigated by using density functional theory within Quantum Espresso and Gaussian 09. The results obtained predict that a single Na atom tends to occupy above the central region of the hexagonal rings in (5,0) and (3,3) BNNT structures with a binding energy of -2.67 and -4.28 eV/Na-atom respectively. When a single H2 molecule is absorbed by a Na decorated BNNT, electrostatic field around Na atom and consequently charge of the participating atoms in the interaction region also undergo change. According to Mulliken population and partial density of state (PDOS) analyses, it is observed that positive charge carried by Na atom decreases.Another result of this charge transfer is revealed as an increase in the magnitude of the dipole moment of BNNT-Na-1H2 with respect to BNNT-Na. The results of charge density, separation distance of atoms in adsorption region as well as the H2 binding energy show the hydrogen molecule adsorption is a physical adsorption (physisorption). In the adsorption process, a single sodium atom with losing about 0.4e of its net charge can adsorb up to six H2 molecules with the binding energy of -0.35 and -0.32 eV/ H2 molecule on (5,0) and (3,3) BNNTs respectively. A comparison of the H2 binding energy of two nanotubes implies that the (5,0) BNNT-Na is more favorable for the hydrogen molecule adsorption.