Improved young's modulus of graphene papers made from large graphene oxide sheets

Graphene, a two-dimensional, one-atomic thick carbon material, is the versatile building block for many carbon-based strong and stiff materials given its exceptional mechanical properties [1]. Graphene oxide (GO) is the oxygenated derivative of graphene covered with hydroxyl and epoxy groups on the basal plane as well as carboxyl groups at the edges [2]. These functional groups provide GO with excellent solubility in water, allowing one to produce a large quantity of graphene sheets from the natural graphite. GO nanosheets have been used to fabricate various new class of materials, such as GO papers, GO/carbon nanotube hybrid films and GO/polymer nanocomposites. GO papers can be easily fabricated by stacking GO sheets via flow-directed filtration of aqueous GO dispersion. This new material outperforms other paper-like materials in terms of mechanical properties, with its Young’s modulus significantly higher than that of carbon nanotube bucky papers or inorganic vermiculite films [3]. The combination of such exceptional mechanical properties with other intriguing functional properties, such as thermal stability [4], high electrical conductivity [5] and biocompatibility [6], makes GO papers a very promising candidate for many technological applications: e.g., free-standing flexible electrodes for Li-ion batteries and supercapacitors [7-8], biomedical applications such as inclusion in heart valves and drug delivery [9], transparent conducting films [10-11] and nanocomposites [12-13]. Among the many parameters that affect the Young’s modulus of GO papers, the lateral dimension of precursor GO sheets plays an important role. Large area GO sheets are ideally suited in a number of applications: e.g. 3D graphene-based networks in self-assembled hydrogels [14], formation of liquid crystals in an aqueous solution [15], 2D aligned structure in polymer-based composites [12] and conductive thin films for optoelectronic devices [1011]. However, it is necessary to understand as to how and to what extent the size of GO sheets can affect the properties of GO papers, including the Young’s modulus, as well as the underlying mechanisms. In this paper, molecular dynamics simulations (MDSs) are used to predict the Young’s modulus of GO papers consisting of different GO sizes. The alignment of the GO sheets in GO papers is quantitatively assessed using the fast Fourier transform (FFT) method. The interactions between GO sheets of different sizes are evaluated at atomic level to identify the dominant mechanisms underlying the GO size-property relationship.