Optical and gas sensing properties of Cu-doped ZnO nanocrystalline thin films for sensor applications

Undoped and Cu-doped ZnO thin films were prepared by spray pyrolysis technique. The prepared samples were characterized by X-ray diffraction (XRD), Field emission Scanning electron microscopy (FESEM), EDX, and UV-Vis absorption spectroscopy. Room temperature NH3 gas sensing properties of films were investigated for 5 ppm ammonia concentration. The X-ray diffraction patterns indicate a hexagonal wurtzite structure with polycrystalline nature in all the samples. A decrease in crystallite size with an increase in Cu concentration has been observed. UV-Vis spectroscopy evidences a decrease in transmittance with Cu concentration. Tauc's plot shows a decrease in energy band gap values with doping. Nanoflake morphology was observed from the FESEM analysis. It has been noticed that the sharpness of the structure of the nanoflakes decreases with the intensification of the copper content in the ZnO lattice. The stoichiometry of Zn, Cu, and O is confirmed by EDX. Gas sensing analysis of the thin-film samples is reported in this work. The highest ammonia sensing response with the lowest recovery time is obtained for 2% Cu-doped ZnO with a response value of 98%. It is observed that with further doping the gas sensing response has been decreased. Based on the gas sensing study, the Cu-doped ZnO thin films is an efficient gas sensing active material.