Tin oxide thick film by doping rare earth for detecting traces of CO2: Operating in oxygen-free atmosphere

SnO2 thick films doped with atomic ratios ranging from 0 up to 8 at.% La, 8 at.% Gd, 8 at.% Lu were fabricated, respectively, via hydrothermal and impregnation methods. The crystal phase, morphology, and chemical composition of the SnO2-based nanoparticles were characterized by XRD, FE-SEM, EDX, HRTEM and XPS. Sensing properties of La-SnO2, Gd-SnO2, Lu-SnO2 films, as well as the pure SnO2 film, were analyzed toward CO2 in the absence of O2. It was found that the optimal doping element was La and the best doping ratio was 4 at.%. The maximum response appeared at an operating temperature of 250 °C, on which condition the 4 at.% La-SnO2 exhibited a remarkable improvement of response from 5.12 to 29.8 when increasing CO2 concentration from 50 to 500 ppm. Furthermore, the working mechanism underlying such enhancement in CO2-sensing functions by La additive in the absence of O2 was proposed and discussed.