Cellulose Nanofiber/Carbon Nanotube Dual Network-Enabled Humidity Sensor with High Sensitivity and Durability

Abstract

Humidity sensors have been widely used for humidity monitoring in industrial fields, while the unsatisfactory flexibility, time consumption, and expensive integration process of conventional inorganic sensors significantly limit their application in wearable electronics. Using paper-based humidity sensors is considered a feasible method to overcome these drawbacks because of their good flexibility and roll-to-roll manufacturability, while they still face problems such as poor durability and low sensitivity. In this study, we report a high-performance paper-based humidity sensor based on a rationally designed bilayered structure consisting of a nanoporous cellulose nanofiber/carbon nanotube (CNF/CNT) sensitive layer and a microporous paper substrate. The vast number of hydrophilic hydroxyl groups on the surface of CNF and paper fibers enables fast water molecule exchange between the humidity-sensitive material and the external environment via hydrogen bonding, endowing the paper-based sensor with an excellent humidity responsive property. The obtained sensor displays a maximum response value of 65.0% (ΔI/I 0) at 95% relative humidity. Furthermore, the mechanical interlocking structure formed between the CNF/CNT layer and the paper layer provides the sensor with strong interlayer adhesion. Benefiting from the unique structure, the sensor also exhibits outstanding bending (with a maximum curvature of 22.2 cm–1) and folding durability (up to 50 times). Finally, as a proof of concept, a simple humidity-measuring device is assembled, which demonstrates an excellent responsive property toward human breath and the change of air humidity, indicating a great potential of our paper-based humidity sensor toward practical applications.

Respiratory Monitoring by Ultrafast Humidity Sensors with Nanomaterials: A Review

“…The carrier doping and the induced electric field can change n and μ , and thus, σ is modulated by the environmental humidity. This mechanism commonly appears in semiconductor and metal nanomaterial devices (e.g., graphene oxide [ 54 , 55 , 56 , 57 ], carbon nanotube/nanocoil [ 58 , 59 ], MXene nanosheet [ 60 , 61 ]) and is similarly used to detect other gases with metal oxide semiconductors, such as oxygen using SnO 2 [ 62 ].…”

“…Water molecules interact strongly with a hydrophilic surface. As this can increase the output sensitivity to humidity change, most of the reported nanomaterials in fast-response humidity sensors are hydrophilic such as oxide nanomaterials (graphene oxide [ 54 , 102 , 103 , 105 , 143 , 144 , 145 ], silicon dioxide [ 16 , 39 , 47 , 52 ], titanium oxide [ 48 , 51 , 53 , 110 , 111 , 120 , 129 ]) and polymers [ 41 , 42 , 43 , 59 , 64 , 67 , 146 ]. Decreasing the adsorption energy of water molecules on the surface can be important for desorbing the molecules spontaneously when the environmental humidity decreases.…”

Tailoring the Surface Properties of Micro/Nanofibers Using 0D, 1D, 2D, and 3D Nanostructures: A Review on Post‐Modification Methods

“…Several sensors have been developed based on nanofibers modified by post‐modification methods, which can respond to a wide variety of stimuli for different applications. For instance, nanofibers‐based sensors have been applied in physical and chemical sensors for detecting pressure, [ 304 ] humidity, [ 305 ] body movements, [ 306 ] breath, [ 307 ] and sweat composition, [ 308 ] to cite a few. Characteristics such as high surface area‐to‐volume ratio, high porosity, and the possibility to control the pore size and distribution as well as the multiple options for functionalization are some of the advantages of applying nanofibers in sensors.…”

“…Additionally, electrospun nanofiber mats have been investigated for electrochemical sensing applications, once they can provide higher active surface area without the need to produce large amounts of material. [ 308,318,334 ] In this direction, another report [ 305 ] combined cellulose nanofiber modified with carbon nanotube (CNF/CNT) by electrospinning on paper, forming a sensitive layer on a flexible substrate. The proposed paper‐based configuration was presented as an alternative to overcome conventional inorganic sensor limitations for wearable electronics applications.…”

Fabrication and the barrier characterization of the cellulose nanofibers/organic montmorillonite/poly lactic acid nanocomposites

“…CNF is superior as packaging material due to the advantages while using as the barrier material 10 . However, the surface hydroxyl on CNF is numerous, resulting in the poor adhesion towards the substrate especially in the weak polar solution 11 . Therefore, it is necessary to undergo hydrophobic modification to improve the dispersion property and the reaction yield before utilization.…”