Humidity sensing properties of La~(3+)/Ce~(3+)-doped TiO_2-20 wt.% SnO_2 thin films derived from sol-gel method

The humidity sensing properties of La³⁺/Ce³⁺-doped TiO₂-20 wt.%SnO₂ thin films were studied. Sol-gel method was employed to prepare the films on alumina substrates. By constructing a humidity-impedance measuring system, the sensing behaviors were inspected for the films sintered at different temperatures. Experimental results showed that, 0.5 wt.% of La₂O₃ or Ce₂O₃ doped films sintered at 500 °C for 2 h had the best humidity sensing properties, the impedance decreasing from 10⁹ Ω to below 10⁴ Ω in the humidity range of 15–95 RH%. Moreover, Ce³⁺-doping had better humidity sensing properties than La³⁺-doping. The doping mechanism was discussed in terms of phase composition, granularity of crystalline and segregation of rare earth ions at grain boundaries.