Microstructure Dictating Performance: Assembly of Graphene-Based Macroscopic Structures

Abstract

ConspectusA wide range of groundbreaking advancements in electronics, photonics, nanocomposites, etc., have been achieved in the past decades due to the favorable attributes of single-layer graphene, including its record-breaking thermal conductivity, charge carrier mobility, fracture strength, and Young’s modulus. However, the realization of the potentials of macroscale graphene assemblies has remained challenging. The difficulties mainly lie within manipulating graphene sheets into an orderly intermolecular orientation and controlling the macroscopic ordering of the graphitic domains. Controlling the formation of graphene macroscopic structures and eliminating defects on both the nano- and microscales meanwhile optimizing performance is no easy feat. To address these microstructural issues in macroscopic graphene assemblies, multiple chemical, thermal, and mechanical approaches have been developed with a goal of overall performance enhancement. Therefore, in this Account we provide a brief review of our contributions in the microstructure engineering of macroscale graphene assemblies with the focus on graphene fibers (GFs), graphene papers (GPs), and other graphene-based assemblies with graphene oxide (GO) colloids as precursors.Building upon the developments in wet chemistry on assembling individual GO sheets into macroscopic structures, we successfully intercalated large GO sheets with small GO sheets, which increased compactness for wet-spun GFs without disturbing the sheet orientation and alignment of the large GO sheets. Increasing GO compactness during wet assembly allows for the increase of thermal and electrical conductivities as well as the mechanical strength of GFs at a single stroke. A high degree of alignment of graphene sheets with abundant sp2 carbon atoms is also necessary for achieving high thermal and electrical conductivities in graphene assemblies. Fine control of GO sheet alignment and orientation during the wet-spinning assembly is demonstrated through the shape and size confinement of fluidic flow channels. Utilizing this shear-stress-induced self-alignment strategy, the core–shell nonuniformity problem of GFs is addressed. Also, a correlation between the rheological properties and flow patterns of GO during wet-spinning and the microstructure of the GF assemblies is established.In addition to the orientation of graphene sheets, the crystallite size and macroscopic ordering within graphene assemblies also play important roles in determining their performances. Larger and highly oriented graphitic crystalline domains allow for higher thermal and electrical conductivities. By manipulating crystallite domain size and arrangement through high temperature graphitization, the electrical and thermal conductivities and Young’s modulus of GFs can be significantly enhanced. Fine temperature control can also help retain residual covalent cross-links between neighboring graphene sheets, meeting the need for balancing tensile strength, and thermal and electrical conductivities.The enhancement of GF mechanical, thermal, and electrical properties through optimizing the compactness and alignment of graphene sheets and the orientation of graphene crystallite domains have significantly improved the engineering capability of GFs. Such an improvement has been largely beneficial to flexible electronics research and has shown great potential in multifunctional composite materials. Similar strategies have also been emulated on related graphene-based materials such as graphene papers (GPs) and graphene fiber meshes (GFMs), yielding unique and favorable properties and performances.