1. School of Materials Science and EngineeringNanyang Technological University Singapore 639798 Singapore
2. College of ChemistryZhengzhou University Zhengzhou 450001 P. R. China
3. College of Materials and Chemical EngineeringKey Laboratory of Inorganic Nonmetallic Crystalline and Energy Conversion MaterialsChina Three Gorges University Yichang Hubei 443002 P. R. China