1. School of Physics and Information Technology, Shaanxi Normal University, Xi’an 710062, P. R. China
2. State Key Lab of Loess and Quaternary Geology (SKLLQG), Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, P. R. China
3. School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, P. R. China