Effect of Cd dopant on structural, optical and CO2 gas sensing properties of ZnO thin film sensors fabricated by chemical bath deposition method

Cadmium-doped zinc oxide (Cd-doped ZnO) films were produced by economic facile chemical bath deposition method. The Cd doping content was adjusted as 1%, 3%, 5% and 7%. The structural, morphological and optical properties of the films were characterized by XRD, Raman, SEM and UV–Vis. The response in a carbon dioxide atmosphere was measured by varying the concentration up to 100 ppm at different working temperatures (30–250 °C). XRD measurements demonstrated that all synthesized films have a good crystallite structure with hexagonal wurtzite dominant phase. A large variety of nanostructures are randomly distributed over the films’ surfaces depending on Cd doping content as was manifested by the corresponding SEM images. From the transmittance analysis, an ultraviolet absorption edge corresponding to pure ZnO film undergoes a redshift with the increase in Cd content. The results from Raman spectra are in good agreement with the XRD results. From the gas sensing measurements, a high response toward 100 ppm CO 2 gas was detected by 3% Cd-doped ZnO sensor (88.24% at 125 °C) with an acceptable response of 8.36% at room temperature, which exhibited the lowest response/recovery times as well as highest selectivity, stability and reproducibility. Changes in the CO 2 gas sensing response as a function of Cd doping content are explained based on particle size, optical bandgap and surface images.