Comparative study on micromorphology and humidity sensitive properties of thin-film and thick-film humidity sensors based on semiconducting MnWO4

Based on our experience of developing thick-film MnWO4 humidity sensors, a thin-film humidity sensor with nano-sized MnWO4 grains has been fabricated using the sol–gel technique. The thin-film sensor shows smaller humidity sensitivity than that compared to a thick-film sensor. However, it exhibits a fast response to humidity change and also has a very low temperature coefficient within the temperature range of 20–60°C. The absence of capillary pores in the well defined thin-films makes water condensation, and hence electrolytic conduction, impossible. The physically adsorbed multi-layered water molecules on the surface of the thin sensing film play a dominating role for the humidity sensing mechanism. Studies on thin-film sensors based on MnO and WO3 indicate that the W+6 sites in the hubnerite material contribute strongly to the humidity sensing mechanism.