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Stiffening of graphene oxide films by soft porous sheets, Mao, et. Al., Espinosa, Han, Nguyen, Huang, Nature Communications, 2019


Counter-intuitively, adding some porous graphene oxide (GO) sheets can significantly enhance the modulus of the bulk multilayer GO films compared with those constructed by pristine ones (twice the tensile modulus), although the individual porous GO sheets are drastically weaker than the pristine ones.

This reveals that the mechanical properties of pristine GO sheets do not scale up by simply stacking into bulk multilayer films, and introducing sheets containing porosity-defects represents an alternative stiffening way, a new consideration in making bulk materials from 2D nanomaterials.

Scientific question:

How the properties of the bulk, multilayer GO films scale from the constituent nanoscale 2D sheets, and how the inclusion of porous weak sheets stiffen the overall multilayer films?

Key of how:

Stacking 2D GO sheets along hardly generate densely and conformally packed multilayer films, thus decreasing overall mechanical properties. The co-assembly of pristine and etched GO sheets yielding stiffer films is attributed to that the more compliant porous sheets act as a binder to improve interlayer packing and load transfer, thereby enhancing stiffness.

Major points:

1. Among 2D sheets making bulk materials, GO sheets are a model system to study how materials properties scale from the constituent nanoscale building blocks to their overall structures.

2. Porous GO sheets are made by oxidative etching (mixture of ammonia and hydrogen peroxide), with the etched parts and thus pores preferentially at the oxygen-containing functional group sites. The pore size (mostly 0.5-1.0 nm) and number can be tuned by etching time, and the etching does not significantly reduce the GO sheets.

3. Nanoscale membrane-deflection tests on single GO sheets (pristine and etched) by a diamond AFM probe show that from pristine to etched ones, the modulus (283 GPa v.s. 85 & 36 GPa) and rupture force decreases significantly, but the deflection at rupture increases, due to the presence of the nanopores.

4. Tensile tests on pure pristine films and etched GO films reveal that the etching-induced porosity deteriorates greatly the stiffness of single-layer GO, but not as pronounced for multilayer films (from pristine to etching1h 3h 5h, stiffness is 30% 13% 0% for single GO sheet, but is 87% 62% 28% for multilayer GO films).

The tensile behavior of multilayer films depends on the properties of the constituent sheets and also the interaction/load transfer between the sheets. It is suggested from SEM observation that although the porosity weakens significantly single sheet, porous GO sheets can pack tighter and allow more effective load transfer, thus offsetting the porosity-induced stiffness decrease in single sheets.

5. Tensile tests on mixed pristine and etched multilayer GO films show that the films containing 50% 5h-etched GO are as stiff as the pure pristine GO films, and those containing 10% have modulus that are nearly twice the pure pristine GO films.

The mechanisms involve that 2D sheets are unlikely to pack densely for effective load transfer (wrinkled sheets disrupt the interlay packing while unfolded stiff sheets generate spaces/pores in between), and the porous compliant GO serves as filler and binder between pristine GO sheets to improve interlayer load transfer.

6. Lap-shear tests of GO films (all pristine, all 5h-etched, 25% etched) reveal that all etched ones show highest shear strength with a simple elastic behavior, which indicates a uniform shear deformation and a tight binding between porous sheets. The pristine GO films have lowest shear strength, and its stress-strain curves imply partial interlayer sliding and rearrangement at the initial stage before a tightly bound state. The 25% etched GO films have an intermediate shear strength and a plastic-to-elastic transition in stress-strain behavior.

7. Structural observation of the delaminated surfaces corroborates that the all etched GO films possess strong interlayer load transfer (terrace-like fracture structures), while the all pristine GO ones poor shear load transfer (cellular patterns of wrinkles, not bonding well). The mixed films show both cellular wrinkles and terraces.


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