Graphene, one-atom thick, two dimensional sp² bonded carbon atoms arranged in a regular hexagonal pattern, has drawn significant attention in recent years owing to its exceptional mechanical, electrical, thermal and optical properties. Graphene-based materials are considered to be good candidates for many useful applications: for example, few layer graphene thin films can serve as transparent conductor, and ‘paper like’ materials consisting of thousands of graphene sheets can functional as mechanical resonator and energy storage material.

Monolayer graphene oxide (GO) sheets with polydispersity in size are synthesized based on a chemical method, and are sorted into four grades with uniform size using a three-step centrifugation. The ultralarge GO (UL-GO) sheets with an average area o...[ Read more ]

Graphene, one-atom thick, two dimensional sp² bonded carbon atoms arranged in a regular hexagonal pattern, has drawn significant attention in recent years owing to its exceptional mechanical, electrical, thermal and optical properties. Graphene-based materials are considered to be good candidates for many useful applications: for example, few layer graphene thin films can serve as transparent conductor, and ‘paper like’ materials consisting of thousands of graphene sheets can functional as mechanical resonator and energy storage material.

Monolayer graphene oxide (GO) sheets with polydispersity in size are synthesized based on a chemical method, and are sorted into four grades with uniform size using a three-step centrifugation. The ultralarge GO (UL-GO) sheets with an average area of 272.2 μm² and small GO (S-GO) with an average area of 1.1 μm² are obtained. GO papers are fabricated by filtration of GO dispersion under vacuum force. The effects of GO size and reduction method on mechanical properties and electrical conductivities of the papers are specifically evaluated in this work. It is found that large size GO sheets can impart a tremendous positive impact on self-alignment, electrical conductivity and mechanical properties of graphene papers. There is a remarkable, more than three-fold improvements in electrical conductivity of the papers made from UL-GO sheets compared to that of the S-GO counterpart. The corresponding improvements in Young’s modulus and tensile strength are equally notable, namely 320% and 280%, respectively. These improvements of bulk properties due to the large GO sheets are correlated to multi-scale elemental and structural characteristics of GO sheets, such as the content of carboxyl groups on the GO edge, C/O ratio and Raman D/G-band intensity ratio of GO on the molecular scale, and the degree of dispersion and stacking behavior of GO sheets on the micro-scale. The graphene papers made from larger GO sheets exhibit a closer-stacked structure and better alignment than those with smaller sheets, as confirmed by the fast Fourier transform analysis, to the benefits of their electrical conductivity and mechanical properties.

Carbon nanofiber (CNF)/GO hybrid papers are also prepared by vacuum filtration with varied CNF to GO weight ratios. The effects of GO content, precursor GO size and reduction on the structure, electrical conductivity and mechanical properties of hybrid papers are studied. The electrical conductivity of hybrid papers shows a decreasing trend before reduction, while it drastically increases after reduction with increasing GO content, a testament to the dominant role played by GO sheets than CNFs. The tensile strength and Young’s modulus of hybrid papers exhibit similar performance with respect to GO content, confirming the importance of GO sheets. The GO sheet size also demonstrates a positive effect on mechanical properties of hybrid papers through better alignment and their role as the substrate.

To further improve the performance, GO papers are modified with MgCl₂. The attachment of Mg²⁺ and Cl^- ions on the basal plane and edges of GO sheets enhances both the interlayer crosslinks and lateral bridging between the edges of adjacent GO sheets by forming the Mg-O bonds. The improved load transfer between the adjacent GO sheets gives rise to a maximum of 200 and 400% increases in Young’s modulus and tensile strength of GO papers. The intercalation of Cl^- ions between the GO layers modifies the properties of GO papers in two different ways, namely, by forming ionic Cl as the electron acceptor and by forming covalent C-Cl bonds. The p-doping effect arising from Cl^- contributes to enhancement in electrical conductivity of GO papers in both the plane and thickness directions. Complemented by the anisotropic electrical conductivities, the GO papers with a layered structure present an exceptionally high dielectric constant of over 60,000 and a dielectric loss of 2.4 at 1k Hz when they are doped with 2 mM MgCl₂, as a result of the combination of MW polarization and dipolar polarization. The excellent mechanical and electrical properties along with unique dielectric performance obtained by the MgCl₂ modified GO papers may open new applications, such as microwave absorbing materials.

Regarding the graphene thin film as transparent conductor, an efficient method is developed to produce transparent conductive graphene films layer-by-layer on a flexible substrate based on the Langmuir Blodgett (LB) assembly technique. GO films are chemically reduced at 90˚C using hydrogen iodide (HI) acid, followed by chemical doping treatments by SOCl₂. The TCFs deliver excellent optoelectrical properties with a low sheet resistance of 1100 Ω/sq at a transmittance 91%. The plasma-treated PET substrate allows interfacial interactions with theGO films: a combination of strong covalent bonds and π-π interactions between the GO and PET is formed.