Nanostructured Catalytic Filter-Assisted Nanotube Array Sensor for Selective NO₂ Detection

Gas sensing is crucial in fields such as environment monitoring, smart homes, healthcare, and public safety. Among various gas sensors, metal oxide-based chemiresistive sensors offer advantages like high sensitivity, low cost, and miniaturization. However, they struggle with poor selectivity due to the broad sensitivity of metal oxides to multiple gases. While integrating a catalytic filter can help to address this issue, if often results in large device size, signal interference, and humidity sensitivity.
Wide bandgap metal oxides, such as ZnO, SnO₂, TiO₂, are widely used in gas sensors due to their unique electrical, chemical, and thermal properties, which make them highly sensitive to gas molecules. In this study, we propose a ZnO nanosheet filter-assisted gas sensor. A Pd/SnO₂ sensing layer is deposited on large surface area of a porous alumina membrane (PAM) substrate, realizing excellent NO₂ sensitivity. A catalytic filter consisting of ZnO nanosheets is grown on the sensor surface using a solution method. Benefiting from the through channels of PAM, top-bottom sensing electrodes can be used to isolate signal interference from filter.
The growth time of the ZnO nanosheets is controlled as 75 minutes to ensure complete pore coverage. The filter-assisted sensor shows a response of 9.15 to 1 ppm NO₂ at an optimal working temperature of 250 ^oC, without interference from the other gases like ethanol, toluene, formaldehyde, and CO at the same concentration. Additionally, the sensor’s NO₂ response is less affected by humidity compared to the bare sensor, reducing the response change rate in 80% relative humidity from 93% to 37%. The theoretical limit of detection (LOD) is 1.86 ppb, meeting NO₂ detection requirements across various scenarios.
To further demonstrate the robustness of the filter-assisted sensor in real environment, we developed a portable sensor module. The sensor exhibits a trend similar to that of a commercial electrochemical NO₂ sensor module, with an average absolute error of only 0.94 ± 0.67 ppb. Finally, its miniaturization and mass production capabilities are validated through a compatible photolithography process on a PAM with adjustable sizes ranging from 10 to 100 µm. All the above functionality positions the filter-assisted sensor as an advanced tool for NO₂ detection, promising significant improvements in environmental monitoring for public, industrial, and domestic use.