Facile conversion of zinc hydroxide carbonate to CaO-ZnO for selective CO₂ gas detection

Tailored synthesis of heterostructures for low temperature (sub 200 °C) CO₂ sensing continues to be a challenging task. The present study demonstrates CO₂ sensing characteristics of CaO-ZnO heterostructures achieved by zinc hydroxide carbonate (Zn₅(CO₃)₂(OH)₆) conversion to ZnO using Ca(OH)₂ at 50 °C. Control samples namely, Zn₅(CO₃)₂(OH)₆, Ca(OH)₂, ZnO, and CaO integrated microsensors exhibited low sensitivity towards CO₂ gas. However, CaO-ZnO heterostructures demonstrated significant sensitivity (26 to 91%) at 150 °C for gas concentration ranging from 100 to 10000 ppm, respectively. In this study, zinc hydroxide carbonate sensitized with 25 wt% Ca(OH)₂ to form CaO-ZnO heterostructures (25CaZMS) displayed a promising sensitivity (77%) and selectivity (98%) towards 500 ppm CO₂ gas. Moreover, the selectivity studies were conducted in the presence of 10 commonly found gases and their sensing performance was compared against CO₂ gas in dry and humid conditions. The developed CaO-ZnO sensor exhibited faster kinetics in comparison to the control samples. Improved sensing performance observed here is attributed to the low-temperature synthesis route which resulted in a large number of active pores and high surface area morphology. Additionally, the high CO₂ adsorption capacity of CaO combined with compatible n-type semiconductors in forming highly dynamic nano-interfaced heterostructure is a promising step towards developing a precise CO₂ gas microsensor.