1. Key Laboratory of Applied Surface and Colloid ChemistryShaanxi Normal UniversityMinistry of Education Xi'an 710062 P. R. China
2. Shaanxi Engineering Lab for Advanced Energy TechnologySchool of Materials Science and EngineeringShaanxi Normal University Xi'an 710119 P. R. China
3. School of PhysicsNorthwest University Xi'an 710069 P. R. China