**Acknowledgements**

The authors would like to thank professor Xuanke Li for his good suggestion and professional advice. This work was supported by the National Natural Science Foundation of China (Grant Nos. 91016003 and 51372177), the Hubei Provincial Department of Education Science Research Project (Grant No. Q20141104), the Key Laboratory of Hubei Province for Coal Conversion and New Carbon Materials (Grant No. WKDM201701), and the China Scholarship Council Fund (201808420114).
