Development of nanocrystalline TiO₂-Er₂O₃ and TiO₂-Ta₂O₅ thin film gas sensors:Controlling the physical and sensing properties

A systematic comparison of single and binary metal oxide TiO2, TiO2-Er2O3 and TiO2-Ta2O5 thin film gas sensors with nanocrystalline and mesoporous microstructure, prepared by sol-gel route, was conducted. The gas sensitivity was increased by secondary phase introduction into TiO2 film via two mechanisms, firstly due to the inhibition of anatase-to-rutile transformation, since the anatase phase accommodates larger amounts of adsorbed oxygen, and secondly due to the retardation of grain growth, since the higher surface area provides more active sites for gas molecule adsorption. The binary metal oxide gas sensors exhibited a remarkable response towards low concentrations of CO and NO2 gases at low operating temperature of 200 °C, resulting in improving the thermal stability of sensing films as well as reducing their power consumption. TiO2-Ta2O5 sensor with molar ratio of TiO2:Ta2O5 = 50:50 (TT11) showed the highest response towards all CO concentrations operated at 200 °C, whereas TiO2-Er2O3 sensor with molar ratio of TiO2:Er2O3 = 75:25 (TE31) had the highest response towards all NO2 concentrations at the same operating temperature. The response magnitude of 10.5 and 11.7 was achieved for TE31 and TT11 sensors towards 400 ppm CO, respectively. In addition, the response magnitude of 4.5 and 3.8 was achieved for TE31 and TT11 sensors towards 10 ppm NO2, respectively. The calibration curves revealed that all sensors followed the power law (S = A[gas]B) (where S is sensor response, coefficients A and B are constants and [gas] is gas concentration) for the two kinds of gases. The response magnitude of the sensors obtained in this work is superior to TiO2-based sensors reported in previous studies.