Fabrication of NOx gas sensors using In2O3–ZnO composite films

In2O3–ZnO composite films were fabricated and their NOx sensing characteristics were investigated in this study. The content of ZnO in In2O3–ZnO film was controlled by adjusting the Zn2+/In3+ molar ratio (r) during the film preparation. With suitable amount of ZnO incorporated into the In2O3 films, the responses of the composite films to NOx at operation temperatures below 200 °C were greatly improved. However, as the ZnO content was further increased, the grain of the composite film started to merge into big ones, thus decreasing the surface area and resulting in lower sensor responses. The detection limit (S/N = 3) of In2O3–ZnO composite film (r = 0.67) reached 12 ppb at 150 °C. Both pure In2O3 (r = 0) and In2O3–ZnO composite films (r = 0.67) showed no response to CO gas.