Asymmetrically Patterned Cellulose Nanofibers/Graphene Oxide Composite Film for Humidity Sensing and Moist-Induced Electricity Generation.

The exploration of advanced functional materials from natural resources is significantly important to green and sustainable development. Herein, we design an ultrafast humidity-driven bending response system using asymmetrically patterned cellulose nanofiber (CNF)/graphene oxide (GO) composite films. The CNF/GO composite films are fabricated by vacuum-assisted filtration, followed by a surface imprinting technique. The results reveal that the composite films possess excellent linear response to humidity change and cycle stability in the relative humidity (RH) range from 25 to 85%. The curvature of the film varies from 0.012 to 0.260 cm as the RH changes from 25 to 85%, and the response time is only 3-5 s. The outstanding humidity response is attributed to the addition of GO that actively interacts with water, enhancing the flexibility and humidity sensitivity of the composite films. In addition, asymmetrical patterning improves the water transfer rate by confinement and renders an easy deformation of composite films under the same stress. Molecular dynamics simulation and finite element analysis are used to further elucidate the mechanism therein. Furthermore, this CNF/GO composite film is also an effective hygroelectric generator, with an output voltage as high as 286 mV. This smart CNF/GO film with responsive humidity-driven deformation shows potential applications as a biomimetic leaf, a proximity sensor, and a moisture-driven electricity generator. This work inspires a new approach of smart material design with nanocellulose and GO and promotes their further applications.