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

Abstract

A CuO/ZnO (C/Z) bilayer thin film was fabricated with a porous top CuO layer to facilitate a sensitive and selective response towards CO2 gas. Such a sensor architecture allowed optimum oxygen and CO2 gas adsorption in the interfacial region. The C/Z thin-film sensor exhibited a good response (47%) for 2500 ppm CO2 at 375 °C as opposed to CuO (15%) at 300 °C and ZnO (16%) at 350 °C. The sensor was selective to CO2 in respect of CO and CH4 gases at 375 °C with selectivity factor κCO2 ∼ 5 and ∼ 8 for CO and CH4 respectively. By analyzing the conductance-time transients for the gas, the adsorption behavior of CO2 on the heterogenous C/Z bilayer thin-film sensor was established. CO2 obeyed an extended Freundlich model of adsorption. Theoretical analysis of the said adsorption model was performed through which the activation energy (EA) and heat of adsorption (Q) of CO2 gas were estimated. A complementary relationship between EA and Q was established. It was shown that EA decreases with increasing concentration from 123.95 to 108.36 kJ/mol for 1000–2500 ppm CO2 for energetically heterogeneous surfaces. Alternatively, Q values increase with increasing concentration from 59.73 to 71.65 kJ/mol for 500–2500 ppm CO2. The CO2 sensing mechanism was elucidated based on surface defects for CuO and ZnO. CO2 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.