1. Department of Chemistry, Faculty of Science, The University of Bamenda, P. O. Box 39, Bambili-Bamenda, Cameroon
2. Department of Chemistry, Faculty of Science, University of Buea, P. O. Box 63, Buea, Cameroon
3. School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, China
4. Lunan Research Institute, Beijing Institute of Technology, Zhengtai Road, Tengzhou 888, China
5. Department of Chemistry, Faculty of Science, University of Dschang, P. O. Box 67, Dschang, Cameroon