Results and Discussion
3.1 Hydrogen adsorption in single Al atom decorated (8,0) CNT
Single walled carbon nanotube (CNT) with a chirality of (8,0) has been considered for the study. A large supercell with two units along c-axis has been used and this is essential to minimize the adatom-adatom interaction. The size of the triclinic supercell used is 28.83Å×28.83Å×8.52Å with α=β=90° and γ= 120°. The average C-C bond lengths in the CNT supercell vary from 1.41 to 1.44 Å. The optimized structure of (8,0) CNT has a diameter of 6.20 Å .
Initially the (8,0) CNT decorated with single Al atom has been investigated to understand the mechanism of hydrogen adsorption. Among the three possible ...view middle of the document...
H2 molecules were added one by one to the Al-decorated CNT and the structures were optimized. The optimized structures of H2 adsorbed Al-decorated (8,0)CNT is shown in Figure 1 (a-f). The adsorption distances, change in H2 bond length, and the binding energy of H2 adsorption are listed in Table 1.
The first H2 adsorbs on Al-decorated CNT with a weak binding energy of 0.09 eV. For the subsequent H2 molecules, the change in binding energy is plotted in Figure 2. Adsorption binding energy of second and third H2 molecules is higher compared to first H2 molecule and this nature is attributed to the H2-H2 interaction. The H2-H2 interaction is repulsive at a smaller distance between molecules (<0.34 Å) and shows a weak attractive potential at larger distances . The increase in H2 adsorption binding energy is observed also by Ao et al and has been confirmed by the band broadening mechanism of H2 below the Fermi energy [15,41]. Up to 6 H2 molecules adsorb with an average adsorption binding energy of 0.201 eV/H2 and this is suitable for ambient condition applications [18,19].
So far a periodic CNT has been considered for the hydrogen adsorption study. In general, a non periodic nanotube terminated with hydrogen atoms is used to simulate periodic nanostructures. Is there any significant change in the hydrogen adsorption behavior or storage capacity when the tube is terminated with H2? To answer this question, the hydrogen adsorption binding energies in the hydrogen terminated non periodic supercell have been calculated. The adsorption binding energies of 1-6 H2 molecules corresponding to the hydrogen terminated (8,0) CNT is listed in Table 1. In the hydrogen terminated (8,0) CNT, the first H2 adsorbs with a binding energy of 0.139 eV. The binding energies slightly vary compared to the periodic supercell. The average adsorption binding energy when 6 molecules are adsorbed in this case is 0.185 eV/H2.
Figure 3 shows the Projected Density of States (PDOS) corresponding to the pure Al atom and decorated Al atom. The occupied s orbital of Al, just below the Fermi energy (upper panel), is absent in the lower panel and this depicts the charger transfer from Al atom to the CNT. There is a strong chemical interaction between Al and the CNT which is due to the charge transfer between the Al to the CNT. The Hirshfeld charge analysis show that a charge of -0.340 e is transferred to the CNT and is confirmed by the decreased peak of Al when it is decorated on CNT (Figure 3). A fraction of charge of -0.194e from Al is distributed to the carbon atoms on which Al is bonded. Charge on these carbon atoms vary from -0.011e to -0.077e. This charge transfer is due to the difference in electron affinity of Al and the carbon atoms and is responsible for the enhanced interaction between the H2 and Al-decorated CNT. In the decorated Al, the unoccupied s and p orbitals are shifted slightly towards higher energy levels. The empty s orbital is split in to two peaks due...