Synthesis of α-Al2O3 at mild temperatures by controlling aluminum precursor, pH, and ethylenediamine chelating additive

This study demonstrates the synthesis of α-Al2O3 by sol–gel method according to various reaction parameters. Various Al2O3 phases were synthesized by a simple sol–gel method using three different aluminum precursors (aluminum isoporoxide (AIP), Al(OH)3, and AlO(OH)) and pHs (3, 7, and 9). Thermally treating of the synthesized powders at 1200 °C produced rhombohedral structure α-Al2O3. When AIP was used as an aluminum precursor, α-Al2O3 was synthesized at all pH levels by calcination at 1200 °C. The structure was easily changed to α-Al2O3 by the addition of ethylenediamine as a chelating additive at the lower temperature of 1000 °C. In contrast, no α-Al2O3 structure was obtained by using Al(OH)3 or AlO(OH) precursors at higher pH in spite of thermal treatment at 1200 °C. The specific surface areas were larger in α-Al2O3 synthesized using AIP precursor compared with that using Al(OH)3 and AlO(OH) precursors. Electrophoretic light scattering (ELS) measurement in aqueous solution at pH=7 revealed positive surface charges in the α-Al2O3 synthesized using AIP precursor, but negative charges in that synthesized using Al(OH)3 and AlO(OH) precursors. Most significantly, the α-Al2O3 synthesized with the ethylenediamine chelating additive had a negative charge, despite the use of AIP precursor, with a higher mobility and larger aggregated particle diameter.