Flexible Sensoring And Actuating Properties Of Folded Graphene Oxide Prepared Through Wrinkling On Curved Surfaces

Surface topographies of materials play a significant role in their physical and chemical properties,so the controllable fabrication of surface patterns is a hot topic in the field of nanoscience and nanotechnology.Compared with traditional etching approach to the preparation of micropatterns,surface wrinkling-based micro-patterning is more advantageous in the low cost,high efficiency and easy operation.Up to now,many studies have been carried out in the fabrication of micro/nanopatterns using wrinkling technology,however,wrinkling on curved substrate are still in obvious deficiencies.Here,we made a great progress in this field by developing new materials,new methods and new applications,which is designed and inspired from the natural wrinkling topographies on curved surface.Before the experiment,graphene oxide(GO)and commercial latex balloons were selected as the wrinkling film and the substrate,respectively.There are rich functional groups on the nanosheets of GO,offering it good solution-based film-forming properties.Furthermore,GO can also be easily modified via chemical reductions or modifications.The strain of the latex balloon substrate can be easily controlled through inflating or deflating.The tensile strain of the balloon substrate used here could reach up to 486%,much larger than the conventional polydimethylsiloxane(PDMS)substrate,laying a solid foundation for the generation of highly folded GO film.In addition,the conductive properties of reduced graphene oxide(RGO),as well as the swelling properties of latex substrate toward certain organic solvents,can further extend their applications in the form of a bilayer system.Based on the fact that most of wrinkling patterns are on curved biological surfaces(e.g.,the cerebral cortex and the paulownia bark),two contrary wrinkling approaches were developed using a spherical balloon and a cylindrical latex balloon.The two methods are named as isotropic shrinking-induced wrinkling method on spherical surfaces(ISWMonSS)and anisotropic shrinking-induced wrinkling method on cylindrical surface(ASWMonCS).Cerebral cortex-like,and paulownia bark-like highly folded GO patterns were prepared successfully after the operations of inflation,coating,drying,and deflation.Since the pre-tensile strain of the substrate could be controlled easily by inflating,one-step controllable preparation of multi-level bionic folds could be realized.The topographies of the wrinkling patterns can be controlled by inflating various volumes of gas into the balloons or adjusting the thickness of GO films.The shrinking-induced residual stress exists in the contracted bilayer system,giving it potential deformation ability.Finally,the highly-folded GO/latex bilayer systems were explored as solvent-responsive actuators and flexible sensors,based on their excellent flexible and highly stretchable properties.Driven by the residual stress,the highly-folded GO/latex bilayer system self-assembled into a tubular structure and exhibited excellent bi-directional,reversible and large curved deformation under the stimulation of some organic solvents.The maximum curvature changes in THF could reach up to 2.75 mm-1,which is the highest value in this field,far more than all the other similar actuators.Based on its excellent solvent-responsive properties,the highly-folded GO/latex bilayer actuator could be used in the fields of bionic actuation and oil spill collecting.Paulownia bark-like folded GO was reduced in situ using hydrazine hydrate vapor,and multi-scale folded RGO with good conductivity could be achieved.The contacts or breaks between the folds of the RGO are sensitive to very tiny tensile strains,leading to the detectable resistance change.So,strain sensors with high sensitivity(~75),wide strain response range(0~300%)and excellent stability can be designed and assembled for gesture recognition and human body monitoring.