Room temperature chemiresistive sensing of carbon dioxide using a composite of zinc oxide and nickel oxide

Carbon dioxide (CO2) being a greenhouse gas whilst generally present in the environment, excess concentration leads to health discomfort and even death. So monitoring and detecting CO2 is critical to ensure both personal and environmental safety. Traditional CO2 sensors operate at high temperatures (100-400 degrees C) because thermal energy is needed for better sensitivity. But high temperatures also lead to added complexities and costs and hence there is a preference for developing sensors capable of operating at room temperature. This study reports on the room temperature sensing of carbon dioxide (CO2), based on a composite of n-type zinc oxide (ZnO) and p-type nickel oxide (NiO). The composite is prepared by co-precipitation route and different compositions are investigated along with pure ZnO and NiO for reference. The prepared composites are characterized by a variety of techniques including X-ray diffraction (XRD), scanning electron microscope (SEM), photoluminescence (PL), and UV-Visible spectroscopy. A custom-built setup is used to measure the gas sensing performance and the results show that an optimum composition of 80 % ZnO with 20 % NiO exhibits an excellent response to CO2, with a sensitivity of 61.1% (measured in terms of the resistance change during exposure) for 40 mg/L CO2 concentration. The performance of the sensor is explained by the formation and random distribution of n-p junctions in the composite. The results open up the possibility of further improving the sensitivity by using other heterojunction-based composites. The low processing temperature also makes this approach suitable for flexible substrates.