New Insight into H₂S Sensing Mechanism of Continuous SnO₂–CuO Bilayer Thin Film: A Theoretical Macroscopic Approach

SnO₂–CuO is one the most promising systems for detection of detrimental H₂S gas. Although previous experimental research has suggested a sulfidation reaction to explain selectivity toward H₂S, little is known about the origin of change of electrical response of this system by changing the H₂S gas concentration. In this study the relation between sensing response of continuous SnO₂–CuO bilayer thin film and H₂S gas concentration is computed based on changeability of chemical composition of covellite Cu_<i>x</i>S. For this purpose, chemical activity of sulfur as a function of atomic fraction in covellite copper sulfide is estimated using Gibbs energies of formation and chemical thermodynamics. By considering equilibrium between covellite and H₂S, a relationship between chemical composition of the copper sulfide and concentration of the gas at 150 °C is obtained. Utilizing electronic physics and finite difference method, electrical response of the sensor as a function of H₂S concentration is computed. Results show that increasing H₂S concentration from 0.053 ppm to saturation value of 455 ppm changes chemical composition of the upper conductive covellite layer from Cu_1.5S to CuS, which increases the response of the system. The model explains why SnO₂–CuO systems can detect H₂S gas from sub-ppm levels to hundreds of ppm without involving oxygen adsorption/desorption phenomena. Finally, the theoretical response–concentration curve is compared with the previous experimental curve. Despite a few differences, the theoretical curve matches relatively well with the experimental one