Understanding the ethanol and acetone sensing behaviour of CuO thin films through elements of gas diffusion theory

Highlights: • Synthesis of CuO thin film via sol gel spin coating technique. • Ethanol and acetone sensing characteristics of CuO thin film. • Application of gas diffusion theory to p-type CuO thin film. • Variation of gas response with film thickness. • Irreversible sensing of ethanol and acetone. A general gas diffusion equation was used to predict the variation of sensor response with operating temperature and thickness for p-type CuO thin films for ethanol and acetone vapours. Assuming Langmuir-Hinshelwood mechanism and non-linear variation of sensor conductance with gas concentration, the response transients were modelled for a wide concentration range under isothermal conditions and fitted with extended Freundlich and Langmuir isotherms for ethanol and acetone respectively. Maximum responses of 144% and 168% were obtained for 300 ppm ethanol and acetone respectively at 300 °C for 240 nm CuO thin films. For CuO thin films of thickness 120–240 nm, response (S) versus temperature (T) exhibited a bell-shaped dependence with peak maxima of ethanol and acetone varying only slightly. Irreversible type gas response was obtained for a concentration range of 300–50 ppm for both ethanol and acetone vapours wherein response time systematically decreases with an increase in target gas concentration.