1. Department of Materials Science and Engineering; College of Engineering; Peking University; Beijing 100871 China
2. State Key Laboratory of Coal Conversion; Institute of Coal Chemistry; Chinese Academy of Sciences; Taiyuan 030001 China
3. University of Chinese Academy of Sciences; Beijing 100049 P. R. China
4. Center for Functional Nanomaterials; Brookhaven National Laboratory; Upton NY 11973 USA
5. BIC-ESAT; College of Engineering; Peking University; Beijing 100871 China
6. School of Materials Science and Engineering and National Institute for Advanced Materials; Nankai University; Tianjin 300350 China