Conductometric formaldehyde gas sensors. A review: From conventional films to nanostructured materials

Abstract This work reviews the up-to-now developed formaldehyde conductometric gas sensors. First, the different methods employed to fabricate the sensing films are explained. They are divided into three groups: thick film and thin film technology based sensors and nanostructured materials. Then, the sensing characteristics of the different devices are presented focusing on the strategies employed to enhance their response. It is shown that regardless of the fabrication technology, ZnO or In2O3 is within the materials with the highest sensitivity to this gas. Furthermore, the sensitivity of ZnO can be improved by doping with proper materials such as Mn, Sn or CdO. Moreover, the effect of the grain size of conventional (thick or thin) films and diameter of the nanofibers is discussed in this review. Furthermore, the results show that nanowire-based sensors can be employed for low temperature detection with fast response. However, despite the high sensitivity reported with some devices, further research is needed to deal with the influence of operating conditions, such as interferents, humidity and long-term stability.