Synthesis of rGO-CuO/ZnO nanocomposites for humidity tolerant room temperature NO₂ gas sensor

In this study, we have synthesized CuO, ZnO, and CuO@ZnO composites with different mole ratios of CuO and ZnO, like CZ11 (1:1), CZ21 (2:1), and CZ12 (1:2) for NO₂ gas sensing. Furthermore, the synthesized CuO, ZnO, and CuO@ZnO composites have been mixed with 10 wt% rGO. For electrical measurements, thin films of the synthesized samples have been fabricated by drop casting method which is abbreviated as CuO/rGO, ZnO/rGO, CZG11, CZG21, and CZG12. The measured percentage response values of CuO/rGO, ZnO/rGO, CZG11, CZG21, and CZG12 for 10 ppm NO₂ gas at room temperature are 30, 45, 56, 55 and 77, respectively. Thus, among these samples, CZG12 has the best percentage response, approximately 2.6 and 1.7 times greater than CuO/rGO and ZnO/rGO sensors, with a response time of 13 s. XPS results confirm that this sample has more oxygen vacancies as compared to bare CZ12. Moreover, we investigated the sensing behavior of the CZG12 sensor at different temperatures (27–100 °C) and relative humidity (RH) levels (20–95%). The sensor shows a better percentage response at room temperature and for intermediate humidity (∼50%) levels. Additionally, at room temperature, the sensor shows negligible response to other gases such as NH₃, CO, H₂S, and Cl₂, proving its excellent selectivity towards NO₂ gas. Moreover, it has been observed that after 100 days, the response is almost constant, suggesting the long-term stability of this sensor. The remarkable gas sensing capabilities of CZG12 sensor strongly imply its immense potential for environmental surveillance and regulation.