Highly selective and efficient room temperature NO₂ gas sensors based on Zn-doped CuO nanostructure-rGO hybrid

In the present work nanostructures of Zn-doped CuO with nominal compositions Cu_1−xZn_xO (x = 0, 0.03, 0.05, 0.07, 0.10, 0.15) have been synthesized via wet chemical method. The field-emission scanning electron microscope (FESEM) and transmission electron microscope (TEM) results show the formation of 1-D nanochain type morphology in pristine CuO and the same is retained up to Zn doping of 7% (x = 0.07). However, for higher Zn doping (x > 0.07) microflower type morphology is observed. The thin films of the as-synthesized pristine and Zn-doped CuO-reduced graphene oxide (rGO) hybrid materials have been fabricated by drop casting method on glass substrates to study their electrical and gas sensing behavior. The temperature dependent resistance measurements confirm semiconducting behavior of the hybrid films. The gas sensing performances of all hybrid films for NO₂ gas have been systematically investigated. The results demonstrate that Zn doping in CuO remarkably increases the gas sensing response as compared to pristine CuO. For example, 5% Zn-doped CuO-rGO hybrid sensor shows percentage response of ~ 54.5, whereas pristine CuO-rGO hybrid sensor shows percentage response of ~ 19.6. Furthermore, sensing performance of hybrid films initially increases with increasing x up to x = 0.07 and after this it starts decreasing with x. The measurements of sensing response for x = 0.05 in the temperature range 296–343 K for 40 ppm NO₂ exhibit maximum response at room temperature (296 K) and the lowest detection limit of ~ 6 ppm NO₂. Moreover, the hybrid sensors exhibit almost negligible response to other gases like CO, NH₃, H₂S and Cl₂ at room temperature, indicating their excellent selectivity towards NO₂ gas. The detail correlations between the microstructural characteristics of Zn doped CuO nanostructures and gas sensing behavior of the corresponding hybrid films have been discussed and described in this paper.