AN EIS STUDY OF THE HUMIDITY-SENSITIVE ELECTRICAL-CONDUCTION OF ALKALI-DOPED TIO2 FILMS

This paper reports the study of the humidity-sensitive electrical prop erties of 10at% Li- and K-doped TiO2 films, which have been proposed a s active materials for integrated humidity sensors, in comparison with pure TiO2 films. Prototype sensors have been prepared by depositing w ith a sol-gel technique dense TiO2-based films onto Al2O3 substrates w ith comb-type Au electrodes. The humidity-sensitive electrical behavio ur 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 an d RH has been evaluated to confirm the conduction mechanism. The pure TiO2 films show a moderate variation of the impedance with RH, while t he alkali-doped films show a very large decrease in resistance and a n early constant capacitance with increasing RH. The humidity sensing me chanism for pure TiO2 films is due to the proton hopping between water molecules adsorbed on oxide surfaces. A novel sensing mechanism is pr oposed for alkali-doped TiO2 films. Alkali ions directly participate i n their PH-sensitive conduction, with a different mechanism for K+ and Li+ ions.