Selective SO_2 detection at low concentration by Ca substituted LaFeO_3 chemiresistive gas sensor: A comparative study of LaFeO_3 pellet vs thin film

摘要

A comparative study of La_(1-x)Ca_xFeO_3 (0.4 ≤ x ≤ 0.8) pellets and La_(0.6)Ca_(0.4)FeO_3 thin films for the selective detection of low concentration SO_2 gas has been performed. The La_(0.6)Ca_(0.4)FeO_3 thin films exhibited superior gas sensing performance than La_(1-x)Ca_xFeO_3 (0.4 ≤ x≤ 0.8) pellets in terms of sensitivity, operating temperature, detection limit, response, and recovery time. The HRTEM images confirm the nanoparticle nature of the La_(1-x)Ca_xFeO_3(0.4 < x < 0.8) samples. The material crystallizes in the orthorhombic crystal system with the Pbnm space group. Further, occupancy refinement of neutron diffraction data indicates the presence of oxygen vacancies which was also supported by XPS analysis. Among the various compositions of La_(1-x)Ca_xFeO_3(0.4 ≤ x ≤ 0.8), the x = 0.4(La_(0.6)Ca_(0.4)FeO_3) was found to exhibit the best gas sensing performance and chosen for thin film preparation. The gas sensing studies of thin films, prepared by the DC magnetron sputtering method, confirm the enhanced gas sensitivity towards 3 ppm SO_2 gas at a low operating temperature of 120 °C. The selectivity studies show that La_(1-x)Ca_xFeO_3(0.4 ≤ x ≤ 0.8) preferentially detects SO_2 gas at the given operating condition over other gases such as CH_4, CO_2, and CO. The gas sensing mechanism and high sensitivity of the La_(1-x)Ca_xFeO_3sensor towards the detection of SO_2 gas were explained by DFT calculations which give evidence for the strong binding of SO_2 molecules on the 121 surface of the sensor.