Synthesis and characterization of nanocapsules with shells made up of Al13 tridecamers

The synthesis of samples by the sol-gel method with aluminum tri-sec-butoxide as cation precursor, 2-propanol as solvent, and sulfuric acid as hydrolysis catalyst gave rise to nanocapsules with an average diameter of 20 nm and a shell thickness of 3.5 nm. The analysis of the X-ray diffraction patterns and the 27Al MAS NMR spectra showed that the shell of the nanocapsules was made up of Al13 tridecamers ordered in a noncrystalline symmetry. The interaction between the capsule's shells opened the capsule structure, producing curved fibers, but maintaining the atomic local order. This opening of the capsules favored the reordering of the atomic local order of Al13 tridecamers into the one of crystalline boehmite, when the sample was aged at room temperature for several days; it also increased the pore volume and the specific surface area of the sample. The crystallization transformed the curved fibers into rods made of small crystalline boehmite bars. The capsule morphology was preserved after calcining the nonaged sample at 700 degrees C, indicating that the transformation of the phase made up of ordered Al13 tridecamers into a noncrystalline alumina was pseudomorphic. We describe and partially explain one of the possible atomic ordering evolutions from the one of an isolated Al13 tridecamer, to the phase forming the nanocapsules shell, until eventually coming to the ordering corresponding to boehmite crystalline rods.