Improved selectivity, response time and recovery time by [0 1 0] highly preferred-orientation silicalite-1 layer coated on SnO2 thin film sensor for selective ethylene gas detection

In this work, sensing response, response time and recovery time for selective ethylene gas detection were improved by coating a layer of [0 1 0] highly preferred-orientation silicalite-1 polycrystals on SnO2 thin film sensors. The sensors were prepared by primarily depositing SnO2 on borosilicate glass substrates using ultrasonic spray pyrolysis technique. Conventional and [0 1 0] preferred orientations of silicalite-1 layers were then directly coated on the SnO2 thin film sensors by hydrothermal crystallization technique with different gel compositions. The silicalite-1/SnO2 thin film sensors were calcined at 550 °C in dry air. The crystal structure and surface morphology of SnO2 and silicalite-1 layers were characterized by XRD and SEM techniques. XANES and TPR were used to verify oxidation state and reduction temperature of the SnO2 thin film sensors, respectively. The interaction of C2H4 and H2O with silicalite-1 was determined by TPD. The sensing performances, such as selectivity, dynamic range, response time and recovery time were evaluated for the C2H4 and H2O. The results showed that the incorporated silicalite-1 layers readily improve the C2H4 selectivity and dynamic range by preferential adsorption of C2H4 molecules on the silicalite-1 filtering layers. The response time and recovery time for the [0 1 0] highly preferred-orientation silicalite-1/SnO2 thin film sensor (t90%, 14 and 144 s) were shorter than those of the conventional one (t90%, 25 and 208 s). It is suggested that the [0 1 0] preferred-orientation silicalite-1, with a pore direction perpendicular to SnO2 thin film surface, can accelerate the molecular diffusion and reduce the diffusion pathway of ethylene to the sensing film.