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. the single h2 molecule that is absorbed by a na decorated bnnts changes the electrostatic field around na atom, and consequently changes the charge of the participating atoms in the interaction region. according to mulliken population and partial density of state (pdos) analyses, positive charge carried by na atoms decreases. another result of this charge transfer reveals 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. comparing the h2 binding energies of two nanotubes implies that (5,0) bnnt-na is more desirable for the hydrogen molecule adsorption than (3,3) bnnt.