Facial dissociations of water molecules on the outside and inside of armchair single-walled silicon nanotubes: theoretical predictions from multilayer quantum chemical calculations

Abstract

The possible reaction pathways of dissociative adsorption of a single water molecule on the sidewalls of armchair (n, n) (n = 4–10) single-walled silicon nanotubes (SWSiNT) have been investigated by the multilayer models. Both the simplified fragment embedding and ONIOM calculations were carried out to study the diameter dependence of reactivity for the dissociation of water on SWSiNTs. The active fragments with different cluster sizes, such as Si16H10, Si30H16, and Si10mH4m (m = 4–10), were used for the multilayer calculations. The employment of the medium-sized Si30H16 cluster is able to reach a good balance between the computational efficiency and accuracy for the large-sized reaction system. In comparison with those full B3LYP/LANL2DZ calculations for Si(4,4) and Si(5,5) nanotubes, the approximate multilayered models can give reasonable predictions on the optimized geometries, activation energies, and exothermic energies with significant reduction in computational cost. The external complexes of the dissociative adsorption of H2O on SWSiNTs were predicted to be more stable than those internal complexes. The convex surfaces of SWSiNTs were also more reactive to H2O with the smaller activation barrier energies (10–13 kcal/mol) than those (15–22 kcal/mol) on the concave side. Both the activation barriers and exothermic energies of dissociative adsorptions of H2O on the internal (external) sidewalls of armchair SWSiNT were found to be insensitive to the tube curvature. The passivation of the outer surface and the removal of water molecules may be crucial for the experimental preparation of the single-walled silicon nanotubes.