NO2 response of In2O3 thin film gas sensors prepared by sol–gel and vacuum thermal evaporation techniques

In2O3 thin films have been prepared by high vacuum thermal evaporation (HVTE) and by sol–gel (SG) techniques. The deposited HVTE and SG films have been annealed at 500°C for 24 and 1 h, respectively. After annealing at 500°C, the films are highly crystalline cubic In2O3. XPS characterization has revealed the formation of stoichiometric In2O3 (HVTE) and nearly stoichiometric In2O3−x (SG). SEM characterization has highlighted substantial morphological differences between the SG (highly porous microstructure) and HVTE (denser) films. All the films show the highest sensitivity to NO2 gas (0.7–7 ppm concentration range), at 250°C working temperature. Negligible H2O cross has resulted in the 40–80% relative humidity range. Only 1000 ppm C2H5OH has resulted in a significant cross to the NO2 response.