Fabrication and Rapid Gas-Sensing Response of NiO/ZnO p-n Heterojunctions for n-Propanol Gas

In this study, NiO/ZnO heterojunction materials were prepared by calcining metal–organic frameworks (MOFs). The structural and morphological characteristics of the NiO/ZnO composite were investigated using various characterization methods, including X-ray diffraction, X-ray photoelectron spectroscopy, and scanning electron microscopy. Gas-sensing tests showed that at the operating temperature of 190 °C, the NiO/ZnO heterojunction (with a molar ratio of 1:1) exhibited the highest response value (Ra/Rg = 201.7) and good selectivity toward 100 ppm n-propanol. Compared to pure ZnO and NiO, the response of NiO/ZnO was significantly improved (ZnO: 6, NiO: 14.6), with increases of 33.5-fold and 13.8-fold, respectively. The response and recovery times were 92 and 30 s, respectively. Additionally, to enable rapid identification of n-propanol gas concentrations, this study developed and validated a method by training and predicting response curves using a random forest algorithm, achieving identification of n-propanol gas at different concentrations (2–100 ppm) within 7 s. Finally, the enhanced sensing performance was mainly attributed to the formation of the interfacial p-n heterojunction between NiO and ZnO, together with increased surface active sites, oxygen vacancies, and chemisorbed oxygen species.