1. Shenzhen Key Laboratory of Hydrogen Energy and Department of Materials Science and Engineering; Southern University of Science and Technology; Shenzhen 518055 P. R. China
2. School of Physics and Technology; Wuhan University; Wuhan 430072 P. R. China
3. Physical Sciences and Engineering Division; King Abdullah University of Science and Technology; Thuwal 23955-6900 Saudi Arabia
4. School of Electronic Science and Applied Physics; Hefei University of Technology; Hefei 230009 P. R. China