Liquid Crystalline Behaviors of Reduced Graphene Oxide

Abstract

Graphene, a honeycomb lattice made of carbon atoms has a large surface area, high mechanical strength, high intrinsic mobility, extreme thermal conductivity, and many other ultimate properties for prospective applications such as transparent electrodes, flexible devices, energy storage, biomedicine, etc. It is generally recognized as a hydrophobic material and has poor solubility in any solvent. On the other hand, graphene oxide (GO) that can serve as the precursor of graphene is well-dispersed in aqueous environments because graphene sheets are covered with carboxyl, epoxides and hydroxyl groups. GO flakes would form liquid crystals (LCs) in water and organic solvents because of its tremendously high aspect ratio and high solubility. In the recent studies, GO LC can also be controlled by electric fields and has a very large Kerr coefficient. Therefore they can be very useful for electro-optical devices, such as low-power consuming display device that is a crucial component for next generation wearable IT devices. However, it is generally acknowledged that GO undergoes spontaneous modification and reduction in the room condition. So, it is remain challenge that GO LC also undergoes reduction while they are using in electro-optical device. Here, we show that aqueous reduce graphene oxide (r-GO) dispersions also can have the liquid crystalline behavior. In this study, r-GO was synthesized by reducing GO with L-ascorbic acid which generally called vitamin C as a reducing agent. And we applied hexadecyltrimethylammonium bromide (CTAB) as a surfactant to prevent the aggregation of r-GO during the reduction process. R-GO dispersion medium with surfactant can also have LC phase that react by external electric field and they could be a good LC phase for electro-optical devices. So we can call r-GO dispersion medium with surfactant as r-GO LC. Optical birefringence induced by external electric field was observed in the r-GO LC and the birefringence of r-GO LC was higher than GO LC in same concentrations. This means that to get the retardation point that some electro-optical devices wanted, less electrical energy needed when the r-GO LC used than GO ones. And it was shown that the color of the GO LC was changed gray at the first time to black in the room condition after 1 month, meaning that reduction of GO LC. But in case of r-GO LC, since r-GO LC was already reduced and well dispersed with surfactant, they were very stable in the room conditions. So there was no color change by time in case of r-GO LC. It is apparent that staying stable state is required for making high quality electro-optic device so r-GO LC are more adequate material than GO LC.