Density functional study of the structure and water adsorption activity of an Al$_{30}$O$_{30}$ star-shaped alumina nanocage


Abstract: Molecular and electronic structures of a novel Al$_{30}$O$_{30}$ star-shaped alumina nanocage (SANC) were studied using the recently developed CAM-B3LYP density functional method. Comparison of the stretching vibrational modes of this compound with the corresponding modes related to an Al$_{20}$O$_{30}$ perfect cage and Al$_{50}$O$_{75}$ tubular alumina nanomaterials showed a shift to lower frequencies, while the bending modes moved to higher frequencies. The highest occupied molecular orbital (HOMO) of the SANC had 65% nonbonding character, whereas the lowest unoccupied molecular orbital (LUMO) was 72% antibonding. The HOMO and LUMO of the SANC arose mostly from Al 3s and 2p atomic orbitals. The theoretically estimated energy gap for this compound was 4.4 eV, which is lower than those for the alumina nanocage (ANC) and nanotube (ANT). The SANC with internal and external diameters of 5.7 and 6.2 Å had potential to interact with water molecule from sites Al(I) in the openings of the cage, Al(II) in the internal pore, and Al(III) in the external arms. The relative water adsorption activity of these sites was Al(I) $>$ Al(III) $>>>$ Al(II). The SANC can be introduced as a novel alumina nanostructure with lower stability and higher activity than well-known alumina materials.

Keywords: Alumina, nano, HOMO, LUMO, DFT

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