Facile synthesis of nano-crystalline alpha-alumina at low temperature via an absolute ethanol sol–gel strategy

Facile synthesis of nano-crystalline α-Al2O3 at low temperature was systematically investigated. An easily accessible and reproducible sol–gel strategy utilizing aluminum isopropoxide, absolute ethanol and nitric acid (or hydrochloric acid) as raw materials was designed. The thermal decomposition of the precursor (gel) and subsequent formation of α-Al2O3 were characterized by X-ray diffraction (XRD), nitrogen adsorption and desorption measurements, thermogravimetric-differential scanning calorimetry (TG-DSC) and Fourier transform infrared spectra (FTIR). During the process of subsequent low temperature heat treatment of precursor gel, the formation of α-Al2O3 was firstly observed at temperature as low as 400 °C without the appearance of other transition phases. However, the additional γ-Al2O3 diffraction peaks appeared at 800 °C unexpectedly and then gradually disappeared in the further calcination stage up to 1000 °C. These phenomena could not be illustrated by the traditional alumina phase transformation theories. Herein, a hypothetical scheme was put forward to explain these unique phenomena met with in our investigation. Besides, the critical role of the acid modifier in lowering the phase transformation temperature of α-Al2O3 was also carefully studied. The results indicated that the increasing molar ratio of acid to aluminum isopropoxide and using nitric acid as modifier were in favor of the phase transformation from γ- to α-Al2O3.