Aluminium oxide nanoparticles prepared by water-in-oil microemulsions

A new water-in-oil microemulsion, consisting of the non-ionic surfactant Triton X-114, cyclohexane as the oil phase and 1.0 M AlCl3 aqueous solution as the aqueous phase, was developed and used to synthesise aluminium oxide (Al2O3) nanoparticles. The partial phase diagram of this ternary system was also determined at 38 uC. Syntheses of Al2O3 nanoparticles were carried out in a range of microemulsions with a fixed oil : surfactant ratio of 70 : 30 (w/w), but with different aqueous phase contents. For the purposes of comparison, Al2O3 particles were also synthesised in the corresponding emulsion and by direct precipitation from aqueous solution. Nano-sized Al2O3 particles were only obtained via microemulsions, whereas the emulsion route led to hollow ball-shaped particles with sub-micron apparent sizes and the direct precipitation process resulted in angular particles about 250 nm in dimension. The nanoparticles synthesised in the microemulsion with 20 wt% aqueous phase content have smaller particle sizes (5–15 nm) and a lower transformation temperature into a-form Al2O3 crystallites. Pure a-Al2O3 nanocrystals were obtained by calcination at 1000 uC for 12 h, while the nano-sized dimension of the particles retained though the apparent particle size increased to some extent. This transformation temperature is about 200 uC lower than that for the Al2O3 particles synthesised by direct precipitation.