In the case of pure ZnO, however, weobserve an increase of response with increasing operating temper-ature in the range of the temperature chosen for the test. At 400◦C,the response is 85% which is 2.2 times less than that of the sen-sor based on Ni0.9Zn0.1O/ZnO at 300◦C. From these results it canbe observed that the sensor based on Ni0.9Zn0.1O/ZnO comparedto Ni0.9Zn0.1O and ZnO, shows a strong increase of response to300 ppm of CO and a drop in the maximum operating temperature.This suggests an influence of the formation of heterojunction on thegas sensing response and on the optimal operating temperature.The existing of the heterojunction can be proved by the dynamicresponse to CO at 300◦C for the three sensors (Fig. 4(b)), whereit can be seen that the baseline resistance in air of the compositematerial (Rair= 60 M) is far higher than that of the single com-ponents (ZnO: Rair= 750 k, Ni0.9Zn0.1O: Rair= 131 ). This kind ofheterojunction effect has also been reported for other heterojunc-tion systems like CuO-NiO [49] and Fe2O3-NiO [50].