Investigation of Sm_1−xCa_xFe_0.7Co_0.3O₃ films (x = 0.0–0.3) prepared by modified sol–gel spin-coating method for carbon monoxide and propane gas sensing

Sm_1−xCa_xFe_0.7Co_0.3O₃ thin film gas sensors with different calcium concentrations (x = 0.0–0.3) were prepared using a modified sol–gel spin-coating method and then annealed at 650 °C for 2 h in air. The thin films were characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), scanning electron microscopy (SEM), and electrical resistance measurements at concentrations from 1 to 200 ppm for carbon monoxide (CO) and from 1 to 500 ppm for propane (C₃H₈) in the temperature range of 25–300 °C for both gases. The resulting XRD data reveal that a pure phase with perovskite structure was obtained for each substitution in x and corresponds to the orthorhombic system with a Pbnm space group. Topography and roughness of the coating were obtained by AFM; roughness results of the sample do not significantly affect the response of the sensor, but the Ca²⁺ content does. Morphology of the nanoparticles and the thickness of each doping in calcium were observed by SEM, and the thickness was around 300 nm. The gas-sensing characteristics of the Sm–Ca–Fe–Co–O system were investigated in CO and C₃H₈ gases. It was found that the electrical resistance and gas-sensing characteristics were influenced by the substitution of Sm²⁺ by Ca²⁺ ions. When x was greater than or equal to 0.2 and CO concentration was 200 ppm and operating temperature was 300 °C, there was a higher response of the sensor, reaching up to 100%. For operating temperatures below 100 °C, the response of the Sm_1−xCa_xFe_0.7Co_0.3O₃-based sensors to CO or C₃H₈ was < 5% regardless of the degree of substitution of Ca. On the other hand, at temperatures above 100 °C, the response depended on the gas to be sensed. In addition, the partial substitution of Ca in the structure increased the response of the sensor in relation to CO and C₃H₈ concentrations. It is suggested that this is due to the process of electric valence compensation.