Rapid synthesis of γ-Al2O3 nanoparticles in supercritical water by continuous hydrothermal flow reaction system

Highly crystalline γ-alumina (γ-Al2O3) nanoparticle was synthesized hydrothermally in supercritical water by using a continuous flow reaction system from the starting reagent of Al(NO3)3·9H2O. The study was performed under supercritical conditions of water; temperature ranging from 400 to 500 °C, pressures from 25 to 35 MPa and the reaction time was as short as 63 ms to 3 s. Products were characterized by XRD pattern, BET surface areas, transmission electron microscopy (TEM), and dynamic light scattering (DLS). XRD results revealed that γ-Al2O3 particles were obtained at 410 °C or higher through the dehydroxylation reaction in supercritical water, while γ-AlOOH phase was predominant at 400 °C. Primary particle size of γ-Al2O3 was about 4 nm and did not depend on the reaction temperature and time. Furthermore, DLS results revealed that the secondary particle size of γ-Al2O3 dispersed in water increased with increasing reaction temperature and time probably due to particle aggregation. It is noteworthy that secondary particle sizes of γ-Al2O3 dispersed in aqueous solution decreased as the Al(NO3)3·9H2O concentration was increased, since particle aggregation was depressed by high zeta potential with lowering the pH.