Heterogeneous Metal Oxide-Graphene Thorn-Bush Single Fiber as a Freestanding Chemiresistor

The development of freestanding fiber-type chemiresistors, having high integration ability with various portable electronics including smart clothing systems, is highly demanding for the next-generation wearable sensing platforms. However, critical challenges stemming from the irreversible chemical sensing kinetics and weak reliability of the freestanding fiber-type chemiresistor hinder their practical use. In this work, for the first time, we report on the potential suitability of the freestanding and ultraporous reduced graphene oxide fiber functionalized with WO₃ nanorods (porous WO₃ NRs-RGO composite fiber) as a sensitive nitrogen dioxide (NO₂) detector. By employing a tunicate cellulose nanofiber (TCNF), which is a unique animal-type cellulose, the numerous mesopores are formed on a wet-spun TCNF-GO composite fiber, unlike a bare GO fiber with dense surface structure. More interestingly, due to the superior wettability of TCNF, the aqueous tungsten precursor is uniformly adsorbed on an ultraporous TCNF-GO fiber, and subsequent heat treatment results in the thermal reduction of a TCNF-GO fiber and hierarchical growth of WO₃ NRs perpendicular to the porous RGO fiber (porous WO₃ NRs-RGO fiber). The freestanding porous WO₃ NRs-RGO fiber shows a notable response to 1 ppm NO₂. Furthermore, we successfully demonstrate reversible NO₂ sensing characteristics of the porous WO₃ NRs-RGO fiber, which is integrated on a wrist-type wearable sensing device.