Investigation on the cross sensitivity of NO 2 sensors based on In 2O 3 thin films prepared by sol-gel and vacuum thermal evaporation

Abstract

In 2O 3 thin films have been prepared from commercially available pure In 2O 3 powders by high vacuum thermal evaporation (HVTE) and from indium iso-propoxide solutions by sol-gel techniques (SG). The films have been deposited on sapphire substrates provided with platinum interdigital sputtered electrodes. The as-deposited HVTE and SG films have been annealed at 500°C for 24 and 1 h, respectively. The film morphology, crystalline phase and chemical composition have been characterised by SEM, glancing angle XRD and XPS techniques. After annealing at 500°C the films’ microstructure turns from amorphous to crystalline with the development of highly crystalline cubic In 2O 3− x (JCPDS card 6-0416). XPS characterisation has revealed the formation of stoichiometric In 2O 3 (HVTE) and nearly stoichiometric In 2O 3− x (SG) after annealing. SEM characterisation has highlighted substantial morphological differences between the SG (highly porous microstructure) and HVTE (denser) films. All the films show the highest sensitivity to NO 2 gas (0.7–7 ppm concentration range), at 250°C working temperature. At this temperature and 0.7 ppm NO 2 the calculated sensitivities ( S= R g/ R a) yield S=10 and S=7 for SG and HVTE, respectively. No cross sensitivity have been found by exposing the In 2O 3 films to CO and CH 4. Negligible H 2O cross has resulted in the 40–80% relative humidity range, as well as to 1 ppm Cl 2 and 10 ppm NO. Only 1000 ppm C 2H 5OH has resulted to have a significant cross to the NO 2 response.