Fig. 1. SEM images of (a–c) CuO(A_{24 h}–C_{2 h}), (d) CuO(A_{2 h}–C_{2 h}), (e) CuO(A_{72 h}–C_{2 h}) and (f) CuO(A_{24 h}–C_{4 h}) nanoparticles prepared by aging the reaction mixture for different times followed by the urea-assisted homogenous precipitation (UAHP) at 90 °C for 2 h and subsequent calcination at 400 °C in ambient air.