Search

We have applied density functional calculations to study the structure, stability and hydrogen storage properties of Li-coated C 70 fullerenes. Our results show that among different possibilities for the geometry of Li-coated C 70 fullerenes, Li atoms prefer to occupy exohedral and endohedral positions on top of the pentagons. Among isomers of Li 2 C 70 and Li 6 C 70 , those in which one of the Li atoms occupy the endohedral position of one of the polar pentagonal rings and the other ones situate on the top of the pentagons around the opposite polar pentagon, have higher binding energies. Charge transfer is occurred from Li to C 70 , putting extra electrons in the vicinity of the Li atom. This results in the binding of hydrogen in atomic form to the on-top C site that is nearest to the Li atom in the preferred configuration of hydrogenated Li-coated C 70 fullerene. Therefore, there are two types of hydrogen binding in Li 6 C 70 . One group of hydrogen bind to Li atoms in quasi-molecular form, which will desorb at a lower temperature, and the other group of H atoms bind to C in atomic form, which will desorb at higher temperatures. [ABSTRACT FROM AUTHOR]

Abstract: We applied density functional calculations to investigate the 7‐5‐5‐7 defects obtained by the addition of an X2 dimer (X = B, Al, Ga, C, Si, Ge, N, P, As, O, S, and Se) to the zigzag (6, 0), (7, 0), and (8, 0) and armchair (4, 4), (5, 5), (6, 6) single‐walled carbon nanotubes (SWCNTs). Two different orientations for X2 ad‐dimer defects were considered. According to our results, defect formation energies depend strongly on the nanotube diameter and the orientation of the defect. Moreover, it was found that defect formation energies of the X2 ad‐dimer‐defective SWCNTs depend upon the nature of X–X, being in the series of B/C/N/O (first row atoms) < Ga/Ge/As/Se (third‐row atoms) < Al/Si/P/S (second‐row atoms). X2 ad‐dimer defects reduce the HOMO‐LUMO gaps (Eg) of the zigzag (n, 0) SWCNTs by only about 1.06%‐8.53%, while decrease the Eg of the armchair (n, n) SWCNTs by about 10.03%‐42.29%, in comparison with those of perfect SWCNTs. Our results indicated that tube diameter has a very slight effect on the adsorption energy of two hydrogen atoms on the exterior sidewalls of XAD‐defective SWCNTs. Moreover, the adsorption of hydrogen atoms on the XAD‐defective armchair SWCNTs is stronger than on the XAD‐defective zigzag ones, depending strongly on the type of ad‐dimer dopants. A decreasing trend is observed for Er/H values as the absolute values of the natural charge of the X atoms increases. [ABSTRACT FROM AUTHOR]