adding 1/3 solution B dropwise to solution A under stirring for 30 min, then the
remaining solution B was slowly added dropwise to form solution D after stirring 4 h at 80 C. Solution C was prepared by adding certain amount of polyvinylpyrrolidone to isopropanol, or just composed of isopropanol. Solution C was slowly added to the solution D with further stirring for 4 h, yellowish sol can be obtained, put the sol in draught cupboard for evaporation process, then the sol turned to stick gel. The gel was heat treated at 110 C for 2 h, then the fluffy precursor could be obtained. Finally the prepared fluffy precursor was ground to fine powder and calcined at different temperature for 2 h in muffle furnace. The process steps are illustrated in Fig. 1. Table 1 shows the different experimental
synthesis conditions.