Ultrasensitive, Transparent, Flexible, and Ecofriendly NO2 Gas Sensors Enabled by Oxidized Single-Walled Carbon Nanotube Bundles on Cellulose with Engineered Surface Roughness

Cellulose has attracted much attention as an ecofriendlysupport layer forflexible gas sensors. Here, an ultrasensitive, fullytransparent,flexible gas sensor was developed using spray-coatedoxidized single-walled carbon nanotube (Oxy-SWCNT) bundles on acellulose support. Rational oxygen functionalization of the outerSWCNTs in bundles has versatile advantages, such as solutionprocessability and selective adsorption of target gas, while the innerSWCNTs in the sheath-core structure enable an efficient channel forcharge transport. Measurement of the Raman spectra under gasadsorption revealed the sensing mechanism of Oxy-SWCNTs depend-ing on the type of gas molecules (i.e., NO2and NH3). Compared to thesilicon-based Oxy-SWCNT sensor, the cellulose-supported sensor showed an enhanced NO2response with an excellent cross-sensitivity to other gases, mainly due to the polar glycosidic linkages in cellulose. Engineering the surface roughness of the supportingcellulose with halloysite nanotubes further enhanced the NO2response, exhibiting an ultralow detection limit of 0.43 ppb. Theultrasensitive and ecofriendly cellulose-based gas sensor with transparency andflexibility could be an alternative to the high-temperature operating inorganic gas sensor for wearable sensing elements.