Nanoscale structural and morphological features of kaolinite nanoscrolls

Abstract

One-dimensional kaolinite nanoscrolls have been arousing great interest due to their applicability in advanced materials for adsorption and slow release of reagents, as well as in nanoscale reactors and carriers. Production of high-quality halloysite-like nanoscrolls with controlled morphology, however, remains challenging as there is a lack of understanding of the curling process of kaolinite layers on the atomic scale. In the present work, the nanoscrolls were efficiently produced from the readily available natural kaolinite using a two-pot solvothermal exfoliation of the kaolinite-dimethyl sulfoxide, kaolinite-urea, and kaolinite-N-methylformamide precursors. The structures, sizes, shapes and crystallographic properties of the produced halloysite-like nanoscrolls were characterized using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM) and electron tomography in scanning transmission mode (STEM). In order to avoid structural disintegration of the highly electron beam-sensitive nanoscrolls, we used a low imaging current (70–240 e/A2); as a result, the acquisition of structure images was possible. In contrast to the triclinic symmetry of kaolinite, electron diffraction patterns suggest a hexagonal symmetry of exfoliated layers. Both HRTEM and STEM tomography show partially and completely rolled up layers, with the axis of curvature being parallel to either the a or b axes. The methanol-grafted nanoscrolls have a variable but larger basal spacing (from 0.76 to 0.90 nm) than the value in ordered kaolinite (0.72 nm). Their external diameters range from 22 to 75 nm, lengths from 218 to 2287 nm, aspect ratios from 5 to 74, and the scrolls have recognizable chirality. Crystallographic image processing of HRTEM structure images suggest that the tetrahedral sheet can be on either the outer or the inner sides of the nanoscrolls. Molecular simulation results for the curled kaolinite layers are consistent in their details with the experimental observations, suggesting that the layers may roll up either way.