Temperature-induced modification on the structural, optical and morphological properties of Zn_0.96Cu_0.04O nanoparticles

Zn_0.96Cu_0.04O nanoparticles synthesized by co-precipitation method were annealed at different temperatures, 400, 500, 600 and 700 °C for 2 h in air atmosphere. Crystalline phases and optical studies of the nanoparticles were studied by X-ray diffraction (XRD) and ultraviolet (UV)–visible photo-spectrometer. Elemental composition was studied by the energy dispersive X-ray (EDX) analysis and the microstructure was examined by scanning electron microscope. The XRD showed that the prepared nanoparticles had different microstructure without changing a hexagonal wurtzite structure. The average crystallite size increased from 28 to 60 nm when the annealing temperatures increased from 400 to 700 °C. The EDX analyses confirmed the presence of Cu in ZnO system and the weight percentage was nearly equal to their nominal stoichiometry within the experimental error. The optical band gap was varied between 3.75 and 3.86 eV and found maximum, 3.86 eV at 500 °C. Existence of functional groups and bonding were analyzed by fourier transform infrared spectra. The observed blue shift in UV emission from 400 to 500 °C in photoluminescence spectra was due to the intrinsic and extrinsic impurities whereas the red shift after 500 °C was due to the increase of crystalline size and relaxation of tensile strain. The reduction in intensity of green band emission with temperature was due to the reduction of intrinsic and extrinsic defects in Zn–O–Cu lattice.