Structures and formation mechanisms of some hierarchical and superstructured materials

Abstract

This PhD project concentrates on the investigation of micro and nanostructure of superstructured materials, inclusive of biological-controlled, bio-induced, and artificially synthesised materials, aiming to hypothesise their formation mechanisms. By applying characteristic techniques including scanning electron microscope (SEM) and transmission electron microscope (TEM) etc., the evolution of morphologies and structures can be determined over time so that formation mechanisms could be proposed accordingly.

Shell specimens of the common limpet: Patella vulgata, as a typical biological- controlled material, have been collected and characterised using SEM and TEM. A featured “three-order” architecture of calcium carbonate lamellae was investigated, and the potential growth mechanism of the limpet shell was discussed.

For bio-induced materials, two types of superstructured spherulitic calcium carbonate particles (spherulites) have been investigated, spherulites found in a limpet shell and naturally occurred spherulites in mineral layers. A general formation mechanism of spherulitic growth induced by a dipole field interaction has been proposed. In particular, on the basis of the hypothesised dipole field mechanism, a corresponding biomimetic synthesis has been carried out, using alginate and stevensite as morphology directing agents. The morphological evolution of the biomimetic spherulites was also explicitly exhibited through SEM and TEM images.

Inspired by the biomimetic synthesis, by controlling the ambient conditions e.g. concentration of alginate and carbonate, CaCO₃ nanocrystals with different sizes could be produced. A new potential 3-D interaction was also explored and discussed.

For artificially synthesised materials, two types of layered materials, layered double hydroxide / Sulphur / reduced graphene oxide (LDH/S/rGO) and layered perovskite (LaTaO₄), were characterised using high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and layered structure has been confirmed in both materials. In layered composite LDH/S/rGO, an insertion of sulphur nanoparticles in LDH/rGO layers was assured. In LaTaO₄, an intricate incommensurate modulated superstructure has been found in the unit cell. Schematic diagrams illustrating the respective superstructures are presented accordingly.