How the Diameter and Structure of (OH)₃Al₂O₃Si_<i>x</i>Ge_1–<i>x</i>OH Imogolite Nanotubes Are Controlled by an Adhesion versus Curvature Competition

Abstract

Imogolites are natural aluminosilicate nanotubes displaying an astonishing monodispersity in diameter. The diameter is controlled by the structure and composition of the nanotube wall and can be tuned by several chemical manipulations. It has recently been discovered that the structure of imogolite nanotubes can change from single-walled (SW) to double-walled (DW) when Si is replaced by Ge during synthesis. Starting from the pure Ge composition, we show that the transition between DW and SW structures can be induced by the incorporation of a small quantity of Si in the synthesis. At that point, the suspension contains a mixture of structures with a nearly constant average diameter. In particular, we found evidence for the presence of a few nanoscrolls. Above 25% Si, SW nanotubes become more stable and present a continuously decreasing diameter with increasing Si. A model is proposed to explain the stability of these different nanotubes and, more generally, the structures of other organic or inorganic nanotubes as a balance between rigidity, surface tension, and adhesion competitive energies.