Improving humidity sensing properties of nanoporous TiO2–10 mol% SnO2 thin film by co-doping with La3+ and K+

La3+ and K+ co-doped nanoporous TiO2–10 mol% SnO2 thin films have been prepared by sol–gel method with Pluronic P123 as the organic template on alumina substrates, and their humidity sensing properties have been investigated. It was found that the addition of rare earth ion La3+, alkali ion K+ and tri-block copolymer Pluronic P123 is beneficial for improving the humidity sensitive properties of the samples. The sensing behaviors of thin films were inspected at different sintering temperatures by constructing a humidity-impedance measuring system. It is found that nanoporous TiO2–SnO2 doped with 0.3 mol% La3+ and 50 mol% K+ sintered at 500 °C for 4 h shows the best humidity sensing properties. This thin film achieves a high sensitivity with the impedance changed five orders of magnitude from 109 Ω to 104 Ω with excellent linearity in the relative humidity (RH) range of 11–95% at 25 °C. The response and recovery time of this thin film are about 11 and 18 s, respectively. High sensitivity, narrow hysteresis loop, rapid response and recovery, prominent stability and good reproducibility are obtained. The effects of dopant content on humidity sensitivity have also been discussed in terms of segregation of rare earth ions at grain boundaries and granularity of crystals, and influence of K+ on the decrease in the intrinsic resistance of the materials and direct contribution to the conduction.