In the typical hydrothermal synthesis, 0.001 mol Sr(NO3)2 was dissolved into 110 ml distilled water under stirring, then 10 ml 2 M NaOH aqueous solution was introduced into the above solution under continuous stirring for about 10 min, resulting in a white precipitate. Finally, the total 120-ml resultant solution containing white precipitates was transferred into a teflon-lined stainless steel autoclave with a capacity of 150 ml, then sealed and maintained at 200 8C for 20 h. After the hydrothermal process, the autoclave was cooled to ambient temperature naturally, the obtained precipitate was centrifugalized, washed with distilled water for several times to remove the ions possibly remained in the final products, and finally dried at 80 8C in air.