Self-microstructuring by hydrothermal synthesis with controlled crystal growth

Abstract

Hydrothermal synthesis is a process that combines material synthesis driven by the reaction of aqueous solutions and the crystal growth at elevated temperature and pressure. Various materials and structures have been produced aiming at new sensors or actuators. However, the estimation of the structure or morphology after the synthesis is difficult because crystal growth and dissolution proceeds in parallel after indefinite nucleation. This study proposes an improved hydrothermal process to produce regular structures with controlled nucleation and crystal growth. First example is zinc oxide (ZnO) urchin-like structures. ZnO particles of φ100 nm were self-assembled on a substrate with dip coating method. Then, hydrothermal synthesis was carried out using these particles as nuclei. Urchin-like structures or hollow spheres were obtained. Second example is titanium dioxide (TiO₂) rods vertically aligned on a substrate. In this case, the substrate crystal structure is the key. Fluorine doped tin oxide (FTO) can be used not only for transparent electrode but for the special substrate for hydrothermal synthesis because the lattice constant of FTO is similar with that of TiO₂.