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.

Graphical abstractWe report one step synthesis of γ-Al2O3 at 410 °C or higher temperatures in supercritical water. The γ-Al2O3 particle size in solution decreased with an increase of the reactant concentration and with the decreasing of pH according to DLS results. Formation of γ-Al2O3 is probably due to rapid reaction promoted by supercritical water.Figure optionsDownload full-size imageDownload as PowerPoint slide