The high-volume synthesis of two-dimensional (2D) materials in the form of platelets is desirable for various applications. While water is considered an ideal dispersion medium, due to its abundance and low cost, the hydrophobicity of platelet surfaces has prohibited its widespread use. Here we exfoliate 2D materials directly in pure water without using any chemicals or surfactants. In order to exfoliate and disperse the materials in water, we elevate the temperature of the sonication bath, and introduce energy via the dissipation of sonic waves. Storage stability greater than one month is achieved through the maintenance of high temperatures, and through atomic and molecular level simulations, we further discover that good solubility in water is maintained due to the presence of platelet surface charges as a result of edge functionalization or intrinsic polarity. Finally, we demonstrate inkjet printing on hard and flexible substrates as a potential application of water-dispersed 2D materials.

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wileyonlinelibrary.commolecules, so GO is hydrophilic and can easily be dispersed in water. [ 2 ] The freestanding laminated GO membranes that are prepared from GO solutions can play an important role in many technological applications, including surface coatings, [ 3 ] ionic and molecular sieving, [4][5][6] hydrogen storage, [ 7,8 ] transparent and fl exible electronics, [9][10][11][12] composites, [ 13,14 ] micro-and nanoscale devices, [ 15 ] and biology and medicine. [ 16,17 ] GO papers require certain mechanical properties to provide adequate resistance to the mechanical loads and harsh environments that arise in commercial applications and must retain structural integrity over their lifetimes.Dikin et al. investigated the mechanical properties of GO papers with thicknesses varying from 2.5 to 25 µm with tensile testing. [ 2 ] Kang et al. used nano-indentation on a dynamic contact module system to measure the mechanical properties of 50-and 60-nm-thick GO fi lms. [ 18 ] Park et al. characterized the mechanical properties of one, two, and three overlapped layers of GO platelets using atomic force microscopy (AFM). [ 19 ] The Young's moduli measured with nanoresonators consisting of thin, stacked GO fi lms were found to surpass values obtained in previous measurements. [ 20 ] These results suggest that the mechanical properties of GO fi lms or papers, such as stiffness and fracture strength, might vary with thickness; however, no systematic study of this issue has been carried out to date.Graphene oxide (GO) papers are candidates for structural materials in modern technology due to their high specifi c strength and stiffness. The relationship between their mechanical properties and structure needs to be systematically investigated before they can be applied to the broad range fi elds where they have potential. Herein, the mechanical properties of GO papers with various thicknesses (0.5-100 µm) are investigated using bulge and tensile test methods; this includes the Young's modulus, fracture strength, fracture strain, and toughness. The Young's modulus, fracture strength, and toughness are found to decrease with increasing thickness, with the fi rst two exhibiting differences by a factor of four. In contrast, the fracture strain slightly increases with thickness. Transmission electron, scanning electron, and atomic force microscopy indicate that the mechanical properties vary with thickness due to variations in the inner structure and surface morphology, such as crack formation and surface roughness. Thicker GO papers are weaker because they contain more voids that are produced during the fabrication process. Surface wrinkles and residual stress are found to result in increased fracture strain. Determination of this structure-property relationship provide improved guidelines for the use of GO-based thin-fi lm materials in mechanical structures.

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