Energetic stability of hydrogen-chemisorbed carbon nanotube structures

To optimize the performance of carbon nanotubes in the area of hydrogen storage we explore stable structures that can form upon hydrogenation. Through free energy calculations using first-principles DFT we find that at room temperature the trans-hydride structure, i.e., one in which H-atoms chemisorbed on nearest-neighbor sites alternate between outside and inside of the nanotube is the most stable structure under moderate hydrogen dosage for tubes with diameter >1.2 nm. Such structures are energetically stable with respect to recombination into H2 or decomposition into hydrocarbons like benzene or ethylene. We also find that the IR spectrum of the trans-hydrides display striking dependence on nanotube chirality.