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 alpha-Al2O3- Investigation, based on X-ray diffraction pattern, indicates that the transformation of alpha-Al2O3 from gels obtained at high pHgreater than or equal to7, undergoes via formation of boehmite (gel--> AlOOH-->gamma-Al2O3-->alpha-Al2O3). On the other hand, the transformation of alpha-Al2O3 at low pH (less than or equal to 6) follows the gel-->gamma-Al2O3 -->alpha-Al2O3 sequence. The transmission electron microscopy reveals that the particle size of the alpha-Al2O3 decreases from approximate to750 to approximate to70 nm with decreasing pH from 12 to 2.5, respectively. The alpha-Al2O3 with large surface area (approximate to130 m(2)/g) is obtained when it is processed at low pH (2.5). The true powder density of the alpha-Al2O3 sample derived at pH = 2.5 is observed to be 3.92 g/cm (3) after sintering at 1450 degreesC, due to fine particles. The isoelectric point (iep) and xi of the alpha-Al2O3 are found to be 8.7 and 9.2 when synthesized at pH 2.5 and 12, respectively. (C) 2002 Elsevier Science Ltd. All rights reserved.