1. School of Chemistry and Chemical Engineering, Northwestern Polytechnical University, Xi’an 710129, P. R. China
2. College of Chemistry and Chemical Engineering, Xi’an University of Science and Technology, Xi’an 710021, P. R. China
3. Key Laboratory of Coal Resources Exploration and Comprehensive Utilization, Ministry of Natural Resources, Xi’an 710021, P. R. China
4. Queen Mary University of London Engineering School, Northwestern Polytechnical University, Xi’an, Shaanxi 710129, P. R. China
5. School of Advanced Materials and Nanotechnology, Xidian University, Xi’an, Shaanxi 710071, P. R. China