Experimental details
2.1. Samples preparation
Aluminum and calcium co-doped zinc oxide nanopowders (ZnO:(Ca,Al)) were prepared by a modified sol-gel method, following the preparation procedure already adopted by El Mir et al. [34], using 16 g of zinc acetate dehydrate [Zn(CH3COO)2·2H2O] as a precursor in 112 ml of methanol.