1. Centre for Materials Science, College of Science, Engineering and Technology, University of South Africa 1 , Johannesburg 1710, South Africa
2. Institute for Nanotechnology and Water Sustainability (iNanoWS), College of Science, Engineering and Technology, University of South Africa 2 , Johannesburg 1710, South Africa
3. Department of Physics, College of Science, Engineering and Technology, University of South Africa 3 , Johannesburg 1710, South Africa