1. Energy Materials, Materials Science and Manufacturing, Council for Scientific & Industrial Research (CSIR), Pretoria 0001, South Africa
2. Department of Physics, Institute of Applied Materials, SARChI Chair
in Carbon Technology and Materials, University of Pretoria, Pretoria 0002, South Africa
3. Department of Physics, University of the Free State, Bloemfontein, Bloemfontein 9300, South Africa
4. Department of Chemistry, University of Pretoria, Pretoria 0002, South Africa