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 ecofriendly support layer for flexible gas sensors. Here, an ultrasensitive, fully transparent, flexible gas sensor was developed using spray-coated oxidized single-walled carbon nanotube (Oxy-SWCNT) bundles on a cellulose support. Rational oxygen functionalization of the outer SWCNTs in bundles has versatile advantages, such as solution processability and selective adsorption of target gas, while the inner SWCNTs in the sheath–core structure enable an efficient channel for charge transport. Measurement of the Raman spectra under gas adsorption revealed the sensing mechanism of Oxy-SWCNTs depending on the type of gas molecules (i.e., NO2 and NH3). Compared to the silicon-based Oxy-SWCNT sensor, the cellulose-supported sensor showed an enhanced NO2 response with an excellent cross-sensitivity to other gases, mainly due to the polar glycosidic linkages in cellulose. Engineering the surface roughness of the supporting cellulose with halloysite nanotubes further enhanced the NO2 response, exhibiting an ultralow detection limit of 0.43 ppb. The ultrasensitive and ecofriendly cellulose-based gas sensor with transparency and flexibility could be an alternative to the high-temperature operating inorganic gas sensor for wearable sensing elements.