Structural, optical and gas sensing properties of Cu2O/CuO mixed phase: effect of the number of coated layers and (Cr + S) co-Doping

Thin films of Cu2O/CuO mixed phase have been deposited on pre-cleaned glass substrate by a spin-coating technique. The influence of Cr-doping, (Cr + S) co-doping and the number of coated layers on the structures and optical behaviors of the films were investigated by X-ray diffraction (XRD), Fourier transform infrared (FTIR), Raman, and UV-Visible spectroscopies. From XRD, FTIR and Raman results; the films are composed of polycrystalline monoclinic CuO and cubic Cu2O phases with crystallite sizes ranged from 10.05 to 23.08 nm. Increasing the thickness improves the films’ crystallinity and decreases the defects level in the films. Cr and S incorporation encourages the growth of CuO phase at the expense of Cu2O one and affects the preferred growth direction. The doping with Cr and S blue shifted the Bg mode and multi-phonon transitions. The direct and indirect optical band gaps decreased from 2.25 eV and 1.60 eV to 2.10 eV and 1.20 eV by growing the number of deposited layers from 2 to 8 layers. The film sensitivity towards CO2 at different gas flow rate was studied and compared with those of similar systems. Also; response time, recovery time, detection limit, and limit of quantification are estimated.