Selective filter for SnO2-based gas sensor: application to hydrogen trace detection

The main drawback of the SnO2-based gas sensors is their low selectivity. In this study, we present a highly selective hydrogen sensor with minimum cross sensitivity to ethanol, methane, carbon monoxide and hydrogen sulfide. Thick film SnO2 sensors are treated by hexamethyldisiloxane (HMDS) at high temperature (500-600 °C). After this treatment, the electrical properties of sensors are greatly modified: the sensitivity towards hydrogen is markedly increased and a high selectivity to hydrogen is achieved. In the meantime, transient responses curves to H2 trace are altered: the response and recovery times are longer for treated sensors than for the untreated one. Nevertheless, if the temperature of the sensor is increasing from 450 °C to 550 °C, the response time notably decreases and the detection of hydrogen trace as low as 250 ppb (v/v) is quite possible with a good stability. HMDS-treated material has been characterized with different analytical methods: SiO2 formation is confirmed by Fourier transform infrared transmission spectroscopy (FTIR) and scanning electron microscopy (SEM) equipped with X probe. Moreover, temperature-programmed desorption (TPD) experiments reveal that surface hydroxyl groups are involved in the HMDS-SnO2 interaction. It could be supposed that a weakly porous Si-based coating film overlaps the treated SnO2 material and acts as a molecular sieve. Hydrogen can pass easily through the dense filter whereas the diffusion of other gases, and especially oxygen, is considerably reduced. The increase in sensitivity is explained with a simplified model based on an inversely proportional relation between the conductance of the SnO2 material and the concentration of adsorbed oxygen ions.