Fabrication and Plastic Deformation of Graphene Oxide Paper
Authors: Liu Siyu , Hu Kaiwen , Cerruti Marta , Barthelat Francois 
Published: May 2020

Graphene oxide (GO) derives from graphite through oxidation, with subsequent dispersion and exfoliation in suitable solvents [1]. GO contains various oxygen-containing functional groups, mainly including hydroxyl, carboxyl, carbonyl and epoxy groups [2]. These oxygen-containing groups make GO highly hydrophilic and can form stable aqueous colloids to assemble paper-like materials by simple and inexpensive solution processes [3]. A monolayer GO nanosheet has high Young’s modulus (207.6 GPa [4]) and ultimate strength (63 GPa [5]). However when individual GO nanosheets are assembled into mesoscopic sheets or “paper”, these mechanical properties decrease significantly because GO sheets are only weakly connected by hydrogen bonds and van der Waals interactions [6]. GO paper also has low plasticity due to the limited movement of dislocation dipoles in a hexagonal lattice [7-8], so it is very brittle and cannot form complex 3D structures. The fabrication of GO paper thus needs to be optimized to achieve better mechanical properties and wider applications.

In this study we fabricated ultra-stiff and strong GO paper with controlled thicknesses by directed-flow vacuum filtration. Young’s modulus and ultimate strength were improved by three methods: (i) borax cross-linking, to form a combined hydrogen and covalent bonding system; (ii) thermal annealing, to evaporate inter-sheet water to make GO paper more compact; (iii) sonication, to disperse GO nanosheet more uniformly before filtration. Mechanical tests were performed by both flexion (three-point bending) and uniaxial tension. We found that the flexural modulus of GO paper is significantly lower than the tensile modulus because of interlayer shearing, micro-buckling and delamination during flexural deformation. The stiffest material we made has a tensile modulus of 109.9 GPa and a flexural modulus of 45.7 GPa, which is among the strongest and stiffest GO papers in the open literature (Fig.1) [9].

Another challenge is to make GO paper plastically deformable, a requirement to form 3D structures from flat paper-like materials. Here we developed a low-cost and eco-friendly method to “plasticize” GO paper by the addition of a slurry of cellulose fibers. After mixing 25 wt.% cellulose slurry with GO suspension, the filtrated composite paper can retain 85% of Young’s modulus but shows around three times larger fracture strain than pure GO paper. Using the GO paper reinforced by office paper slurry, we successfully formed semi-spheres with smooth and compact surfaces using an embossing method. The stiffness of the deformed structure was further improved by immersion cross-linking in borax solution. The stiff and lightweight semi-spheres can be used as mechanical structures such as acoustic diaphragms and protective layers. After reduction either by chemical or thermal method, the application can be further expanded to supercapacitors, actuators and electrode materials.

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Figure 1