1. State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, 100029 Beijing, P. R. China
2. State Key Laboratory of Vanadium and Titanium Resources Comprehensive Utilization, ANSTEEL Research Institute of Vanadium & Titanium (Iron & Steel), Chengdu 610031, China
3. Department of Chemistry, City University of Hong Kong, Hong Kong SAR, Hong Kong
4. Department of Applied Physics, The Hong Kong Polytechnic University, Hong Kong