This work was supported by the Major State Basic Research Development Program of China (2007CB210200) and by the
Fundamental Research Funds for the Central Universities, HUST (2010MS039). The authors thank Analytical and Testing Center of Huazhong University of Science and Technology for thermogravimetric experiments. We thank Professor Baeyens Jan (University of Warwick, School of Engineering, Coventry, U.K.) for his guidance while preparing the paper.