1. a Department of Applied Chemistry , Birla Institute of Technology , Mesra , Ranchi , India
2. b Microscopy and Microanalysis Unit, University of the Witwatersrand , Johannesburg , South Africa
3. c Molecular Sciences Institute, School of Chemistry, University of the Witwatersrand , Johannesburg , South Africa
4. d Advanced Materials Division, Mintek , Randburg , South Africa