An EIS study of the humidity-sensitive electrical conduction of alkali-doped TiO2 films

This paper reports the study of the humidity-sensitive electrical properties of 10at% Li- and K-doped TiO2 films, which have been proposed as active materials for integrated humidity sensors, in comparison with pure TiO2 films. Prototype sensors have been prepared by depositing with a sol-gel technique dense TiO2-based films onto Al2O3 substrates with comb-type Au electrodes. The humidity-sensitive electrical behaviour of the films has been investigated using electrochemical impedance spectroscopy (EIS) at relative humidity (RH) values ranging from 4-87%. The variation of dielectric parameters as a function of frequency and RH has been evaluated to confirm the conduction mechanism. The pure TiO2 films show a moderate variation of the impedance with RH, while the alkali-doped films show a very large decrease in resistance and a nearly constant capacitance with increasing RH. The humidity sensing mechanism for pure TiO2 films is due to the proton hopping between water molecules adsorbed on oxide surfaces. A novel sensing mechanism is proposed for alkali-doped TiO2 films. Alkali ions directly participate in their PH-sensitive conduction, with a different mechanism for K+ and Li+ ions.