We have carried out a systematic first-principles study of the H-induced unzipping phenomenon (scission of a row of C-C bonds along the tube axis via chemisorption of H atoms on selective sites exterior to the nanotube) of single-walled armchair and zigzag nanotubes. The calculations reveal for the first time that for the armchair tubes, there is a maximum radius beyond which the tubes can not be unzipped by H atoms. More specifically, the smaller (5,5), (6,6), (7,7), (8,8), (9,9) tubes exhibit the unzipping effect, whereas the larger (10,10) and (11,11) tubes resist the H-induced unzipping. In sharp contrast, the (7,0), (9,0) and (10,0) zigzag tubes are more stable and can not be unzipped by H. We select the (6,6) armchair nanotube as a representative case to demonstrate the generic unzipping mechanism, for which a detailed analysis is carried out. Our theoretical calculations lend strong support to the recent experimental observations of H-activated coalescence of armchair nanotubes, revealing the underlying mechanism in the electronic structure responsible for this remarkable phenomenon. Fig. 1(a) shows the relaxed atomic structure with the H atom adsorbed at the exterior to the tube. We label the nearest-neighbor C atom to the H by C1, and the nearest-neighbor C atom to C1 at the same layer by C2. Fig. 2(a) shows the valence charge density contour plot in the presence of a single H for a plane containing the C atoms on the same layer. One can observe the directional sp2-like covalent bonding between the C atoms, the - sp3 like bonding between C1 and H, and the weakening of the C1-C2 sp2 bond.
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