A critical role of pH in the colloidal synthesis and phase transformation of nano size α-Al2O3 with high surface area

This paper demonstrates the critical role of pH in the colloidal (i.e. sol-gel) synthesis of nano size α-Al2O3. Investigation, based on X-ray diffraction pattern, indicates that the transformation of α-Al2O3 from gels obtained at high pH⩾7, undergoes via formation of boehmite (gel→AlOOH→γ-Al2O3→α-Al2O3). On the other hand, the transformation of α-Al2O3 at low pH (⩽6) follows the gel→γ-Al2O3 →α-Al2O3 sequence. The transmission electron microscopy reveals that the particle size of the α-Al2O3 decreases from ≈750 to ≈70 nm with decreasing pH from 12 to 2.5, respectively. The α-Al2O3 with large surface area (≈130 m2/g) is obtained when it is processed at low pH (2.5). The true powder density of the α-Al2O3 sample derived at pH=2.5 is observed to be 3.92 g/cm3 after sintering at 1450 °C, due to fine particles. The isoelectric point (iep) and ξ of the α-Al2O3 are found to be 8.7 and 9.2 when synthesized at pH 2.5 and 12, respectively.