An Experimental Study of Electromechanical Actuation of Graphene and Reduced Graphene Oxide : Physical Mechanisms and Applications

Graphene and reduced graphene oxide (rGO) structures have been reported to have high actuation potential in recent computational and experimental studies. However, despite their promising actuation strain performances and various actuation behaviours, the lack of in-depth experimental analysis and the current technological limitations have led to a scarcity of fundamental knowledge on the physical mechanisms and governing parameters. This has become a major hindrance for further advances of graphene based actuators and their implementations in practical applications. With the aim of gaining further insight to these actuation phenomena of graphene and rGO structures, this study presents an experimental investigation of electromechanical actuation due to charge injection in 2D and 3D graphene and rGO structures.
   
   Enhancement of actuation strain performance has been achieved in both 2D and 3D rGO structures via rational material synthesis and smart designing of actuators. The changes of the interlayer spacing and the intrinsic oxygen functionality on the in-plane actuation of 2D rGO paper immersed in aqueous electrolyte has been investigated. Attributing the presented actuation phenomenon to the electrical double layer (EDL) the convenient synthesis process conducted in this study has led to achieve a maximum in-plane actuation strain of 0.2%. An inversely proportional relationship of capacitance/stiffness ratio has been identified to govern the actuation in rGO papers. This investigation highlighted the importance of identifying the governing parameters and the detailed understanding of carbon-oxygen molecular structure of the synthesised rGO for its coherent function as an actuator.
   
   Secondly, a subsequent study on 2D rGO paper demonstrates corona treatment as a cost effective and a convenient surface treatment technique to achieve further enhancement of actuation strain. This approach was found to produce a maximum in-plane actuation strain of 0.3% which is about 200% enhancement of the original rGO paper. Investigation of the actuation strain performance for varying corona treatment times exhibited a quadratic behaviour for electron and hole injection as opposed to the linear behaviour in previous reports for oxygen plasma treatment [19, 20]. This study further clarified the need to attribute the actuation strain performance to the analytical parameters involved in order to distinguish the physical mechanisms governing the actuation behaviour.
   
   Thirdly, an investigation of the actuation due to charge injection in 3D rGO elastomer in aqueous electrolytes was conducted. Its superelastic feature leads to an unconventional actuation response upon electron and hole injection. Owing to the favourable combination of enhanced charge accumulation at highly compressed states and its porous structure, the 3D rGO elastomer produces the highest actuation strain reported in a graphene driven actuator (4.3% at 1 V). Taking advantage of its effective ion adsorption due to its highly porous cellular structure, the actuation strain can be further enhanced for a given voltage and a charging frequency via the selection of an electrolyte with high diffusivity. This investigation has provided an opportunity to design an actuator with high actuation strain performance, tuned via mechanical compression.
   
   Finally, in order to promote the potential of its practical implementations, tentative investigations of quantum mechanical actuation of graphene actuators was conducted. This proposes a concept design that employs the resonance frequency variations of the graphene actuator upon charge injection to detect the quantum mechanical actuation behaviour.