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Nanocrystals of undoped and nickel-doped zinc oxide (ZnNiO, where x = 0.00-0.05) were synthesized by the coprecipitation method. Crystalline size, morphology, and optical absorption of prepared samples were determined by X-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscope (SEM), and UV-visible spectrometer. XRD and SEM studies revealed that Ni-doped ZnO crystallized in hexagonal wurtzite structure. Doping of ZnO with Ni was intended to enhance the surface defects of ZnO. The incorporation of Ni in place of Zn provoked an increase in the size of nanocrystals as compared to undoped ZnO. Crystalline size of nanocrystals varied from 10 to 40 nm as the calcination temperature increased. Enhancement in the optical absorption of Ni-doped ZnO indicated that it can be used as an efficient photocatalyst under visible light irradiation. Optical absorption measurements indicated a red shift in the absorption band edge upon Ni doping. The band gap value of prepared undoped and Ni-doped ZnO nanoparticles decreased as annealing temperature was increased up to 800 °C. [ABSTRACT FROM AUTHOR]