The Humidity Sensing Behaviors Of Sn⁴⁺, La³⁺ And Ce⁴⁺ Doped Titania Nanofilms

In this paper, the sol-gel method was used to prepare pure titania, Sn⁴⁺-doped titania, and La³⁺/ Ce⁴⁺-doped TiO₂-20wt%SnO₂ films on quartz glass plates, whose surface was pre-printed with comb-like Ag electrode by the dip-coating route. A series of humidity sensing assemblies were obtained by sintering at different temperatures. The physical parameters such as impedance and capacitance of films were measured by using intelligent LCR equipment and the humidity sensing properties were analyzed. The effects of Sn⁴⁺, La³⁺ and Ce⁴⁺ion doping, sintering temperature, working frequency and coating number on humidity sensing behaviors were studied. The surface morphology of the films was investigated by atomic force microscope, and the doping mechanism was studied.Experimental results show that, the films appear better humidity sensitivity at lower frequency and lower sintering temperature. With increasing frequency and sintering temperature, the humidity sensitivity of films becomes worse. Humidity sensitivity of films doped with Sn⁴⁺, La³⁺ and Ce⁴⁺ is better than pure titania. The intrinsic impedance decreases by the RE ions doping.The impedance of TiO₂-20wt%SnO₂ film with single-layer and sintered at 400℃decreases from 10⁹Ωto 10⁶Ωin the humidity range of 25-95 RH%; The impedance of 0.5wt% of La₂O₃ doped film sintered at 400℃decreases from 10⁸Ωto 10⁶Ωin the humidity range of 45-95 RH%; the impedance of 0.2wt% of CeO₂ doped film sintered at 500℃decreases from 10⁸Ωto 10⁶Ωin the humidity range of 25-95 RH%.Humidity sensitivity of films doped with RE ions becomes worse, the reason is that RE ions filled in the crystal boundary of TiO₂ and suppressed the adsorption and ionization of outer molecular water.