Microstructural, crystallographic and optical characterizations of Cu-doped ZnO nanoparticles co-doped with Ni

In the present study Ni, Cu co-doped ZnO nanoparticles have been successfully synthesized by sol–gel method. Hexagonal wurzite structure of Cu-doped ZnO was not affected by Ni-doping. Cubic NiO phase was detected at high Ni content (Ni = 6 wt%). The enhanced crystallite size at lower Ni content is due to the existence of surface Ni ions and the presence of more defects on the grain surface reduced the crystallite size. The change in lattice parameters and ‘d’ value was discussed based on the micro-strain, volume of the unit cell and the secondary phases. The green band absorption between 493 and 484 nm confirmed the presence of oxygen related defects and intrinsic defects. The red shift of energy gap (ΔEg ≈ 0.12 eV) below Ni = 2wt% is due sp–d spin-exchange interactions between the band electrons and the localized d electrons of transition- metal ion but after Ni = 2wt%, energy gap was increased by the secondary phases like NiO. The shift of vibration frequencies towards higher frequency side revealed the incorporation of Ni in the octahedral and tetrahedral sites existing in hexagonal-wurtzite structure.