First-principles calculations were carried out to compare hydrogen absorption in pristine metallic and semiconductor carbon nanotubes (CNTs) with the situation in their Ge- and GeO2-doped counterparts. We found out that the pristine carbon nanotubes have low absorption efficiency ( -1.53 eV in the metallic, and -2.06 eV in the semiconductor carbon nanotube). When Ge was doped into both carbon nanotubes, the hydrogen absorption was enhanced to -5.29 eV in the metallic and -3.99 eV in the semiconductor carbon nanotubes. Investigating the Partial density of states proved that there was considerable overlap between Ge 4p and hydrogen 1s orbitals in both CNTs. When CNTs were doped with GeO2, hydrogen atoms were bound to oxygen atoms, due to high electronegativity of oxygen atom. The hydrogen absorption was found to be increased remarkably in the metallic carbon nanotube ( -6.59 eV). In order to compare the binding energy of Ge and GeO2 doped metallic and semiconductor carbon nanotubes, the partial density of states and the magnetization of the samples were studied.

Keywords