Fig. 4. (a) Multipoint BET nitrogen adsorption–desorption isotherm and (b) corresponding pore size distribution of CuO(A_{24 h}–C_{4 h}) nanoparticles prepared by aging for 24 h followed by the urea-assisted homogenous precipitation at 90 °C for 2 h and subsequent calcination in ambient air at 400 °C for 4 h.