Enhancement of NO2 sensing properties of In2O3-based thin films using an Au or Ti surface modification

Metal oxide gas sensors using In2O3 are able to detect low concentrations of NO2. The gas sensitive properties of In2O3 can be improved by adding catalysts or promoters. In the present work, the surface modification of reactively RF-sputtered In2O3 films (cubic phase) using Ti or Au is presented. The surface modified In2O3 films are investigated with respect to physical and gas sensitive properties. Fine distributed Au particles can be formed on top of the In2O3 surface. This enhances the NO2 sensitivity significantly by a factor of 9 at 350°C and by a factor of 3.6 at 400°C. A low cross-sensitivity toward several gases, such as SO2, NH3, CO and H2, is found. The response time τ50 at 400°C is decreased from 4.0 to 2.2 min. On the other hand, particles of amorphous TiO2 can be produced on top of the In2O3 surface. The Ti additive improves the signal shape of the response toward traces of NO2. The response time τ50 at 400°C is decreased from 4.0 to 1.8 min. For the Au surface modification a possible sensing mechanism is discussed.