Origin of working temperature in H2S sensing process of SnO2-CuO thin bilayer: A theoretical macroscopic approach

Abstract

Resistive sensors composed of SnO2 and CuO, are known to be highly efficient in detection of detrimental H2S gas in terms of sensitivity, selectivity and speed. Recently, dependency of electrical response of the sensor toward H2S gas concentration has been related to the selective mechanism (formation of CuS) by a theoretical model. Another important factor in design of gas sensors is the working temperature which so far has not been explicitly explained for H2S sensing process of SnO2-CuO system. In present study, origin of this temperature for SnO2-CuO thin bilayer based on the selective mechanism has been theoretically interpreted. For this purpose, Poisson, Laplace and continuity equations were solved for a classical p-type/n-type semiconductor junction in air and a metal/n-type semiconductor junction in gas to obtain response of the sensor as function of temperature. Results show that existence of the optimum sensing temperature is the outcome of competition between Schottky barrier at SnO2/CuS interface and ohmic resistivity of metallic Covellite. Depending on electron density, electron mobility and thickness of SnO2 layer, the working temperature changes between 150°C and 230°C which is within the range of experimentally reported values. Finally, theoretical normalized response-temperature curve is compared to previous experimental ones. Despite some differences in curvature, theoretical curve explains the origin of optimum H2S sensing temperature for SnO2-CuO thin bilayer, relatively well.