1. Beijing Institute of Nanoenergy and Nanosystems, Chinese Academy of Sciences, Beijing 100083, P. R. China.
2. College of Nanoscience and Technology, University of Chinese Academy of Sciences, Beijing 100049, P. R. China.
3. Research Center for Eco-environmental Science, Chinese Academy of Sciences, Beijing, 100085, P. R. China.
4. College of Light Industry and Textile, Inner Mongolia University of Technology, Hohhot, 010051, P. R. China.
5. School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA.