High-Speed Morphology Control of Boehmite Nanoparticles by Supercritical Hydrothermal Treatment with Carboxylic Acids

This study demonstrates that the morphology of boehmite (AlOOH) nanoparticles can be controlled over a short timespan by supercritical hydrothermal treatment in the presence of alkyl carboxylic acids including hexanoic, octanoic, decanoic, tetradecanoic, and octadecanoic acids. Boehmite nanoparticles were treated with carboxylic acid in supercritical water at 400 °C and at a water density of 0.35 g/cm³ in a batch-type reactor. When the carboxylic acid was not added, the particles were shaped as rhombic plates. However, the addition of carboxylic acid changed the crystal morphology to hexagonal plates. The aspect ratio (i.e., [length along the a-axis]/[length along the c-axis]) of the rhombic plates increased with a treatment time of 2-30 min, which is a much shorter timespan than that used for conventional hydrothermal crystallization. The aspect ratio of the hexagonal plates increased with increasing concentration of alkyl carboxylic acids. These results clearly indicate that carboxylic acids enhance the dissolution and recrystallization of boehmite. The aspect ratio increased with decreasing length of the alkyl chain of alkyl-carboxylic acid added to the system. Thermogravimetric analysis (TGA) showed that carboxylic acids modified the surface of the boehmite particles. The coverage of the alkyl carboxylic acid on the surface of the nanoparticles was evaluated from the weight loss curve obtained from TGA, and the surface area was evaluated from transmission electron microscopy, which showed that the aspect ratio of the particles increased with increasing the coverage. The results suggest that the carboxylic acid suppresses crystal growth along the shorter axis through surface-capping, thus enhancing dissolution and crystal growth along the a-axis.