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 O-2. It was found that the optimal doping element was La and the best d Dping 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 O-2 was proposed and discussed. (C) 2014 Elsevier Ltd. All rights reserved.