1. Department of Chemistry, Faulty of Science, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore
2. Field of Environment and Energy, School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi 923-1292, Japan
3. Joint School of National University of Singapore and Tianjin University, International Campus of Tianjin University, Binhai New City, Fuzhou 350207, P. R. China