Synthesis and structural characterization of alumina nanoparticles

Alumina nanoparticles were synthesized by thermal dehydration of Al(OH)(3) (gibbsite), which shows the sequential formation of AlOOH (bohemite), gamma, delta, theta, and alpha-alumina phases upon heat treatment in the temperature range: 150-1150 degrees C. Alumina nanoparticles were characterized by X-ray diffraction and Al-27 MAS-NMR studies to investigate the structural transitions, in particular, the changes in Al-O speciation as a function of heat treatment. gamma-alumina is produced by annealing bohemite at 400 degrees C, and transforms into alpha-alumina via the formation of delta and theta-alumina as the intermediate phases. The formation of theta and alpha-alumina starts at 1020 degrees C and the concentration of these phases increases upon annealing at 1100 degrees C, while the concentrations of gamma and delta-alumina decreases steadily. The heat treatment at 1150 degrees C produces alpha-phase (84%) and the remnant theta-phase (16%). Al-27 MAS-NMR revealed the existence of two types of Al-O speciation i.e. AlO4 and AlO6 structural units whose relative concentration depends upon the transition alumina phases.