Conductometric nitrogen dioxide gas sensors based on sol-gel-prepared hafnium-added indium zinc oxide (Hf-IZO)

We produce a nitrogen dioxide (NO₂) gas sensor based on hafnium-added indium zinc oxide (Hf-IZO). Among the samples of various Hf concentrations (0, 5, 9, 15, 30, and 100 at%), the gas sensor based on the 15 at% Hf-IZO shows the highest response of 79.6 to NO₂ gas of 2.2 ppm, which is about 8.3 times higher than that of the pristine IZO sample. The 15 at% Hf-IZO sensor also exhibits a high selectivity to other gases, a good long-term stability up to 3 months, and a similar gas response even when humidity is varied across a wide range of 0 through 60 %RH measured by a humidity sensor. The microstructural and chemical states of the materials are characterized using X-ray diffraction (XRD), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS) for investigation of the mechanism behind the high sensing performance. The characterization results, combined with the sensor data, disclose a link between the concentration of oxygen vacancies generated in the 15 at% Hf doped IZO and gas response.