Effects of hydrothermal synthetic conditions on the particle size of γ-AlO(OH) in sub and supercritical water using a flow reaction system

In the present study, we synthesized boehmite (γ-AlO(OH)) fine particles hydrothermally at 350 °C and 400 °C using a flow reaction system. The influences of the synthetic parameters including temperature, pressure, starting Al(NO3)3 concentration, and pH of the starting solution, on the boehmite particle size were investigated. Products were characterized by X-ray diffractometry and transmission electron microscopy. According to XRD analyses, all the product were crystalline γ-AlO(OH). From the results of TEM observation, the morphology of γ-AlO(OH) particles obtained was rhombic or hexagonal plates with 70–470 nm of the average particle size. The particle size increased with an increase in the reaction temperature and the starting Al(NO3)3 concentration, while that decreased with an increase in the pH of the starting solution. Moreover, at 400 °C the particle size enlarged from 170 nm to 300 nm with increasing reaction pressure from 25 MPa to 40 MPa, while there is no dependency of reaction pressure on the particle size at 350 °C. Furthermore, the particle formation and growth mechanisms are discussed based on the solubility of γ-AlO(OH) under hydrothermal conditions.