One-step synthesis of flower-like η-Al2O3 via supercritical hydrothermal method

Abstract

In this Letter, flower-like η-Al2O3 nanostructure was synthesised via hydrothermal method without any surfactant or further calcination. A molten salt bath rose the reactor temperature as much as 500°C to achieve the supercritical condition. Such a high temperature caused a rapid one-step synthesis in two minutes. Al(NO3)3 and NaOH were the only precursors. Synthesised powders were investigated by X-ray powder diffraction and scanning electron microscope, and the specific surface area was also determined by N2 adsorption-desorption analysis. Increasing reaction time >2 min caused undesired Ostwald ripening. So instead of giving more time, a small amount of NaOH was added to form the η-Al2O3 phase. Moreover, the morphology of η-Al2O3 changed to flower-like by increasing the density of the supercritical fluid. Finally, the probable mechanism of flower-like morphology formation was proposed in two steps.