1. Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, South Korea
2. Research Institute of Advanced Materials, Seoul National University, Seoul 151-744, South Korea
3. School of Advanced Materials Engineering, Kookmin University, Seoul 136-702, South Korea
4. Chemical and Bioscience Center, National Renewable Energy Laboratory Golden, Colorado 80401
5. Department of Ceramic Engineering, Kangnung National University, Kangnung, 210-702, South Korea
6. Department of Mechanical Engineering and Materials Science, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
7. School of Chemical Engineering, Sungkyunkwan University, Suwon, 440-746, South Korea
8. Center for Energy Materials, Materials Science and Technology Division, Korea Institute of Science and Technology (KIST), Seoul 136-791, South Korea