One‐step synthesis of flower‐like η‐Al 2 O 3 via supercritical hydrothermal method

In this Letter, flower‐like η‐Al 2 O 3 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(NO 3 ) 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 N 2 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‐Al 2 O 3 phase. Moreover, the morphology of η‐Al 2 O 3 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.