1. Ecomaterials and Renewable Energy Research Center (ERERC), Department of Physics, Nanjing University, 22 Hankou Road, Nanjing 210093, P. R. China, Department of Materials Science and Engineering, Nanjing University, 22 Hankou Road, Nanjing 210093, P. R. China, National Laboratory of Solid State Microstructurcs, Nanjing University, Nanjing 210093, P. R. China, and Photocatalytic Material Center (PCMC), National Institute for Materials Science (NIMS), 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan