H2 storage by confinement inside germanium nanotubes

Abstract

This work enabled us to compare the behaviour of different types of CNTs and BNNTs with that of various germanium nanotubes with a hydrogen molecule introduced perpendicularly or parallel to their axis. The confinement of the H2 molecule in GeNTs in both cases showed that the confinement energy is large for the smallest diameter, but it becomes close to zero for larger diameters. The main difference with respect to carbon nanotubes is the disappearance of H-H bond activation due to the absence of interactions between the π electrons and the hydrogen atoms. Compared to the BNNTs, we noted elongation of the H-H bond in both orientations for small diameters instead of breaking or shortening. This elongation is due to the strong interaction between the van der Waals radius of hydrogen and the atomic radius of germanium. This leads to a strong attraction between germanium and hydrogen for both orientations. This situation engenders a significant polarization. The most important consequence of the existence of this induced dipole moment is the IR activation of the H-H stretching vibration, which becomes very intense and whose value depends on the diameter of the nanotube. As an application, this vibration frequency can be used to detect and assay H2 contained in nanotubes or deduce the diameter of the host tube. This result shows that it is possible to detect H2 molecules inside GeNTs as well as in CNTs and BNNTs. However, in the germanium nanotubes, the most remarkable result that emerges from this study is that it is possible to detect and clearly differentiate the orientation of the H2 molecule inside the nanotubes, in contrast to other types of nanotubes.