Mesoporous Co₃O₄/SnO₂ Nanostructure-Based Heterojunctions for NO₂ Sensors

Over a decade, the application of p–n heterojunction-based sensing garnered tremendous interest in the domain of gas sensors. This interest is mainly attributed to the exceptional electronic band alignment of the constituent materials and their unparalleled features. This includes the provision of a greater number of gas interaction sites with the analyte gas molecules, superior charge transport, and decreased operational temperature of the sensor. Herein, we synthesized p-type cobalt oxide/n-type tin oxide (p-Co 3 O 4 /n-SnO 2 ) heterojunction by a simple hydrothermal method. The molar ratios of Sn were varied with three different concentrations (10, 20, and 30%). The structural, morphological, chemical compositions, and specific surface area of the as-prepared samples were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM), Brunauer–Emmett–Teller (BET), and X-ray photoelectron spectroscopy (XPS) analyses. The formation of the nanostructured heterojunction between Co 3 O 4 and SnO 2 was confirmed with XPS and HR-TEM results. The high surface area and oxygen vacancy of the prepared Co 3 O 4 /SnO 2 samples were evident from the BET and XPS analyses. The gas selectivity of the prepared samples was evaluated against nitrogen dioxide, hydrogen sulfide, ammonia, sulfur dioxide, nitrous oxide, and carbon dioxide. The prepared 10% Co 3 O 4 /SnO 2 nanostructured sensor exhibited the maximum response toward 100 ppm of NO 2 at a low operating temperature of 150 °C. Moreover, the proposed sensor exhibited a higher band bending of about 0.15 eV. The enhanced gas-sensing performance of the 10% Co 3 O 4 /SnO 2 sensor with a low operating temperature is the result of the depletion region formed at the interface of Co 3 O 4 /SnO 2, high surface area, and a large number of oxygen vacancies of the heterojunction samples. This approach paves the way for a plausible application of the Co 3 O 4 /SnO 2 heterojunction for monitoring the NO 2 gas in atmospheric air.