Nanoscale tubules formed by exfoliation of potassium hexaniobate

The exfoliation of acid-exchanged K₄Nb₆O₁₇ with tetra(n-butyl)ammonium hydroxide in water produces a colloidal suspension of individual sheets, which roll into loosely bound tubular structures. The tubule shape can be made permanent via precipitation of the colloid with alkali cations. Atomic force microscopy and transmission electron micrographs reveal that the tubules have outer diameters ranging from 15 to 30 nm and that they are 0.1 to 1 μm in length. The observed curling tendency, preferential folding, and cleavage angles of the individual sheets are interpreted in terms of the crystal structure of the parent solid, K₄Nb₆O₁₇. The driving force for tubule formation appears to be relief of strain that is inherent in the asymmetric single sheets. This driving force is absent in bilayer colloids formed early in the exfoliation process, which are found only as flat sheets. Tubules in colloidal suspensions that have been subjected to turbulence have a tendency to unroll into flat sheets on surfaces, indicating that the forces controlling rolling and unrolling are closely balanced.