Development of Zn2SnO4 thin films deposited by spray pyrolysis method and their utility for NO2 gas sensors at moderate operating temperature

The Zn2SnO4 thin film based NO2 sensors are deposited onto the glass substrates using the spray pyrolysis method and effect of substrate temperature on NO2 response is studied. The crystallinity, morphology and topography of the films were studied by XRD, SEM and AFM techniques, respectively. The structural analysis shows that films are polycrystalline and crystallize in cubic inverse spinel crystal structure. SEM studies show that grain size increases with increase in substrate temperature and shape changes from spherical to irregular cubes. AFM studies show that, roughness and hence the exposed surface area is higher for the film deposited at 375 °C substrate temperature. Highest sensor response at moderately low (200 °C) operating temperature is 2.66 for 200 ppm NO2 concentration. Pd sensitization enhanced response to 3.31 along with decreased response and recovery times. The sensor response and selectivity in relation to, operating temperature, and the gas concentration is systematically studied. Films are highly selective towards NO2 with a 25 times gas response as compared to other gases. The selectivity of Zn2SnO4 thin films towards the gases varied as NH3 < CO < LPG < SO2 < H2S < NO2. Gas response falls nearly to 80% of its initial value after 240 days indicating durability of the films. Annealing film at 400 °C restored the gas response to a value that observed initially.