Interplay of Solid–Liquid Interactions and Anisotropic Aggregation in Solution: the Case Study of Γ-Alooh Crystallites

The development of original nanoparticle size and shape requires a detailed knowledge and rationalization of the formation mechanism. A consolidated mechanism of gamma-AlOOH boehmite nanorod formation in aqueous hydrothermal conditions is proposed based on an advanced characterization of the final product, kinetic studies, and surface energy calculations. Boehmite nanorods with a width of about 10 nm and more than 100 nm in length are obtained in slightly acidic conditions from Al(III) molecular precursors at temperature higher than 150 degrees C. The identification of protoboehmite as an intermediate in the kinetic study combined with the low solubility of aluminum in reacting conditions and the presence of stacking defects in the rods is indicative of an aggregation mechanism. Electron diffraction studies show that the crystallites are oriented in the nanorods with a [001] preferential growth direction. Using DFT calculations, we demonstrated that the nanorod morphology is not the expected thermodynamically stable one in these experimental conditions. An explanation of the oriented aggregation of primary particles is proposed based on an eased depletion of the solvation water molecules of the (001) surface compared to the other surfaces.