1. Center for Nanochemistry, Beijing Science and Engineering Center for Nanocarbons, Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China
2. School of Materials Science and Engineering, Nanyang Technological University, Singapore 639798, Singapore
3. College of Environmental Sciences and Engineering, The Key Laboratory of Water and Sediment Sciences (Ministry of Education), Peking University, Beijing 100871, P. R. China
4. Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China