Diameter monodispersity of imogolite-like nanotube: a density functional theory study

Abstract

The diameter monodispersity and the surface charge distribution of three imogolite-like nanotubes (not substituted (IMO), substituted by NH2 (IMO-NH2), substituted by F (IMO-F) are investigated using self-consistent periodic density functional theory, and the phenomenon of the monodispersity is explained qualitatively in terms of bond length. We assume that the axial length of the nanotube is constant and confirm it; the energetic minimum axial lengths of the three nanotubes increase in the sequence IMO_NH2 IMO IMO_F, and are respectively 8.61, 8.62 and 8.66 . Then the energies for different nanotubes and lamellar structures are calculated. A series of strain energy curves of IMO, IMO_NH2 and IMO_F are plotted based on calculations, and the results show that the energetic minimum diameters of these three nanotubes increase in the sequence of IMO IMO_NH2 IMO_F, and are respectively N= 9, 10 and 11. In order to explain the diameter monodispersity, we have calculated the bond lengths of SiO, AlO and AlOH three nanotubes and plotted the curves of length against diameter. Results show that the monodispersity can be attributed to the interaction between the energy increase resulting from the stretching of the SiO, AlO bonds in the inner wall, and the energy decreases caused by the shortening of the AlOH bond in the outer wall. In a word, with the increase of tube diameter, the SiO and AlO bonds increase while the AlOH bond decreases monotonically. Additionally, we have also calculated the Mulliken charge distributions of the three nanotubes with different diameter and analysed their surface charges. On this basis, we summarize the effect of diameter on surface charge. Results show that the main positive charges are accumulating on the outer surface while the negative charges are located on the inner region, and the outer surface charge increases gradually with the increase of the diameter of the nanotubes. The study indicates that the internal surface functional group has an effect on the axial length, diameter and surface charge of the imogolite-like nanotubes. We can control the nanotube diameter and surface charge distribution by changing different functional substitutes in the inner surface; it is significant in the molecular design and application of imogolite-like materials.