Graphene in macroscopic order: liquid crystals and wet-spun fibers

Acc Chem Res. 2014 Apr 15;47(4):1267-76. doi: 10.1021/ar4002813. Epub 2014 Feb 20.

Abstract

In nanotechnology, the creation of new nanoparticles consistently feeds back into efforts to design and fabricate new macroscopic materials with specific properties. As a two-dimensional (2D) building block of new materials, graphene has received widespread attention due to its exceptional mechanical, electrical, and thermal properties. But harnessing these attributes into new materials requires developing methods to assemble single-atom-thick carbon flakes into macroscopically ordered structures. Because the melt processing of carbon materials is impossible, fluid assembly is the only viable approach for meeting this challenge. But in the meantime, researchers need to solve two fundamental problems: creating orientational ordering in fluids and the subsequent phase-transformation from ordered fluids into ordered solid materials. To address these problems, this Account highlights our graphene chemistry methods that take advantage of liquid crystals to produce graphene fibers. We have successfully synthesized graphene oxide (GO) from graphite in a scalable manner. Using the size of graphite particles and post fractionation, we successfully tuned the lateral size of GO from submicron sizes to dozens of microns. Based on the rich chemistry of GO, we developed reliable methods for chemical or physical functionalization of graphene and produced a series of functionalized, highly soluble graphene derivatives that behave as single layers even at high concentrations. In the dispersive system of GO and functionalized graphenes, rich liquid crystals (LCs) formed spontaneously. Some of these liquid crystals had a conventional nematic phase with orientational order; others had a lamellar phase. Importantly, we observed a new chiral mesophase featuring a helical-lamellar structural model with frustrated disinclinations. The graphene-based LCs show ordered assembly behaviors in the fluid state of 2D colloids and lay a foundation for the design of ordered materials with optimal performances. Using the wet-spinning assembly approach, we transformed prealigned liquid crystalline dopes into graphene fibers (GFs) with highly ordered structures. We extended the wet-spinning assembly strategy to polymer-grafted or mixed graphene LCs to obtain hierarchically assembled, continuous nacre-mimetic fibers and hybridized graphene fibers. Both the neat GFs and the composite fibers are strong, flexible, electrically conductive, and chemically resistive. Multifunctional fibers that are both flexible and modular could be a key for applying atomically thin graphene in real-world materials and devices such as supercapacitors and solar cells. Therefore, we have opened a brand-new avenue for transforming mineral graphite into high performance, multifunctional GFs and offered an alternative strategy for the fabrication of carbon fibers. We hope that this Account and further efforts in the field will guide researchers toward the macroscopic assembly of graphene and its real-world applications.