1. School of Resources Environment and Materials Guangxi University 530004 Nanning P. R. China
2. Center of Nanoenergy Research School of Physical Science and Technology Guangxi University 530004 Nanning P. R. China
3. Beijing Institute of Nanoenergy and Nanosystems Chinese Academy of Sciences 101400 Beijing P. R. China
4. School of Nanoscience and Technology University of Chinese Academy of Sciences 100049 Beijing P. R. China