Tuning of energy gap, microstructure, optical and structural properties of Cr doped Zn0.96Cu0.04O nanoparticles

Zn0.96 − xCu0.04CrxO nanoparticles with different Cr concentrations from 0 to 4% were successfully synthesized by co-precipitation method. The X-ray diffraction pattern showed the crystalline nature with hexagonal wurtzite structure. The diminution in activation energy and the enhanced strain by Cr-doping from 0 to 3% reduced the average crystal size from 27 to 23.8 nm while the formation of secondary phase such as spinel ZnCr2O4 enhanced it to 25.6 nm after Cr = 3% via increasing the activation energy. The solubility limit of Cr in the present system is fixed as 3%. The energy dispersive X-ray spectra confirmed the presence of Cu and Cr in Zn-O. The optical absorption spectra showed the three distinct bands corresponding to UV region (305–324 nm) due to electronic transition between the bands; blue region (382–386 nm) mainly due to the Zn/Cr interstitials and bluish green (481–482 nm) region due oxygen vacancies and interstitials. The noticed poor transmittance throughout the region and the broader absorption bands in Cr = 1% doped Zn0.96Cu0.04O confirmed that it had more defects such as oxygen vacancies, Zn interstitials and other defects. The blue shift of energy gap from 3.68 eV (Cr = 0%) to 3.71 eV (Cr = 1%) was due to the creations of more defects. The red shift of Eg from 3.71 eV (Cr = 1%) to 3.6 eV (Cr = 3%) was explained by sp–d exchange interactions between the band electrons and the localized d-electrons of the Cr3 + ions. Presence of chemical bonding was confirmed by FTIR spectra.