Sensitive and selective CO2 gas sensor based on CuO/ZnO bilayer thin-film architecture

• Synthesis of CuO/ZnO (C/Z) bilayer thin film via sol gel spin coating technique. • Improved CO 2 response of C/Z bilayer thin film with respect to CuO and ZnO thin films. • CO 2 adsorption modeling of C/Z bilayer thin film. • Estimation of activation energy, E A , and heat of adsorption, Q for CO 2 adsorption on C/Z bilayer thin film. • Description of CO 2 sensing mechanism in CuO, ZnO and C/Z bilayer thin films. A CuO/ZnO (C/Z) bilayer thin film was fabricated with a porous top CuO layer to facilitate a sensitive and selective response towards CO 2 gas. Such a sensor architecture allowed optimum oxygen and CO 2 gas adsorption in the interfacial region. The C/Z thin-film sensor exhibited a good response (47%) for 2500 ppm CO 2 at 375 °C as opposed to CuO (15%) at 300 °C and ZnO (16%) at 350 °C. The sensor was selective to CO 2 in respect of CO and CH 4 gases at 375 °C with selectivity factor κ CO2 ∼ 5 and ∼ 8 for CO and CH 4 respectively. By analyzing the conductance-time transients for the gas, the adsorption behavior of CO 2 on the heterogenous C/Z bilayer thin-film sensor was established. CO 2 obeyed an extended Freundlich model of adsorption. Theoretical analysis of the said adsorption model was performed through which the activation energy (E A ) and heat of adsorption (Q) of CO 2 gas were estimated. A complementary relationship between E A and Q was established. It was shown that E A decreases with increasing concentration from 123.95 to 108.36 kJ/mol for 1000–2500 ppm CO 2 for energetically heterogeneous surfaces. Alternatively, Q values increase with increasing concentration from 59.73 to 71.65 kJ/mol for 500–2500 ppm CO 2 . The CO 2 sensing mechanism was elucidated based on surface defects for CuO and ZnO. CO 2 sensing in the C/Z bilayer thin-film sensor was controlled by the adsorption of oxygen forming a space charge layer at the surface and interface of the p-n heterojunction and by band-bending as a result of the change of electron concentration across the junction.