1. Hefei National Research Center for Physical Sciences at the Microscale, CAS Key Laboratory of Materials for Energy Conversion, Department of Materials Science and Engineering, University of Science and Technology of China, No. 96 Jinzhai Road, Hefei, Anhui 230026, P. R. China
2. Department of Environmental Science and Engineering, University of Science and Technology of China, Hefei 230026, P. R. China
3. Key Laboratory of Photovoltaic and Energy Conservation Materials, CAS, Institute of Solid State Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei, Anhui 230031, China