1. School of Materials Science and Engineering Xi'an University of Science and Technology Xi'an 710054 P. R. China
2. Department of Chemistry City University of Hong Kong Kowloon 999077 Hong Kong P. R. China
3. Department of Materials Science and Engineering City University of Hong Kong Kowloon 999077 Hong Kong P. R. China
4. Department of Chemistry College of Science Korea University Seoul 136-713 (Republic of Korea
5. Department of Materials Science and Engineering University of Washington Seattle Washington 98195-2120 USA