Reactively sputtered MoO3 films for ammonia sensing

We report the gas-sensing properties of ion-beam sputter deposited MoO 3 thin-films. The change in the DC conductivity was measured in dry N 2 with 10% O 2 in the presence of up to 490 ppm of NH 3 , NO, NO 2 , C 3 H 6 , CO and H 2 . At ∼440 °C the film was found to be very sensitive to NH 3 , with 490 ppm increasing the conductivity by approximately a factor of 70. This was approximately 17 times greater than the response to the other gases. The NH 3 response was strongly affected by the accompanying levels of O 2 , NO 2 and H 2 O. For example, changing the accompanying O 2 levels from 1% to 20% decreased the NH 3 response by approximately a factor of 20. Similarly, the presence of 100 ppm NO 2 (in 10% O 2 ) decreased the NH 3 response by approximately a factor of three, and 1% water vapor decreased it by more than a factor of two. The NH 3 response, however, was relatively unaffected by 100 ppm of accompanying NO, C 3 H 6 , CO or H 2 . XPS measurements show that the increased conductivity in the presence of NH 3 was also accompanied by a partial reduction of the surface MoO 3 . We observed an increase in the resistance of the films after extended time at elevated temperatures.