1. Department of Chemical Technology, University of Johannesburg, P.O. Box 17011, Doornfontein 2028, South Africa
2. Department of Chemistry, Vaal University of Technology, Private Bag X021, Vanderbijlpark 1900, South Africa
3. Nanotechnology Innovation Centre, Advanced Materials Division, Mintek, Private Bag X3015, Randburg, Johannesburg 2125, South Africa