1. Department of Materials Science and Engineering, Southern University of Science and Technology; Shenzhen, Guangdong 518055, P. R. China
2. Department of Chemical and Biological Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, P. R. China
3. School of Physics and Electronics, Central South University, Changsha, Hunan 410083, P. R. China
4. Energy Institute, and Chinese National Engineering Research Center for Control & Treatment of Heavy Metal Pollution, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, P. R. China