Solid-state synthesis and effect of temperature on optical properties of Cu-ZnO, Cu-CdO and CuO nanoparticles

► Undoped and Cu-doped nanoparticles have been synthesized using PEG 400. ► The effect of temperature on the structure and optical properties was studied. ► The intensity of peaks and crystanillity were increased with increase in temperature. ► The optical band gaps were decreased. ► Modulation of band energies offers new ways to improve efficiency of the solar cell.