Sol-gel processed TiO₂-based thin films as innovative humidity sensors

Active elements for humidity sensors based on alkali-doped and undoped TiO₂ films have been prepared by the sol--gel technique. TiO₂-based films are deposited onto Al₂O₃ substrates with comb-type Au electrodes by dip-coating, from the appropriate solutions, to obtain four different compositions of 1, 3, 6, and 10 at.% Li and K, with respect to the alkali metal/Ti system. The humidity-sensing electrical properties of the films are evaluated using d.c. and a.c. measurements. The electrical characterization of TiO₂ films doped with 10 at.% K shows that this material has some unique properties. Resistence versus relative humidity(rh) values show variations as high as seven orders of magnitude in the humidity range tested. Very high humidity sensitivity is also observed at the lowest rh values (4-10% rh), at 40°C, when the resistence is measured at frequencies lower than 1 Hz. The results also show great stability over time. The possibility of modifying the rh sensitivity of TiO₂, films doped with 10 at.% K by varying the frequency at which the resistance is measured is a characteristic that allows this material to be described as smart.