1. School of Science, Inner Mongolia University of Technology, Hohhot, 010051, P. R. China
2. Inner Mongolia Key Lab of Nanoscience and Nanotechnology, Inner Mongolia University, Hohhot 010021, P. R. China
3. State Key Laboratory of Baiyunobo Rare Earth Resource Research and Comprehensive Utilization, Baotou Research Institute of Rare Earths, Baotou 014030, P. R. China