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In-plane porosity strengthens multilayered graphene-oxide paper

By Rachel Nuwer September 30, 2019
(a) Graphene oxide sheets are wrinkled during processing, disrupting the packing of neighboring sheets; (b) if wrinkle-free sheets are tightly stacked, they become more rigid, preventing further packing. Both result in architectural defects (red circles) in the multilayer films. Thus, the softer, porous graphene oxide sheets effectively serve as a binder, allowing more compliant packing in the multilayer films, and leading to higher stiffness.

When researchers want to make a strong material with superior mechanical properties, normally they seek to build it from individual units that are also as strong as possible. A new paper-like multilayered graphene oxide film shows that this is not always the case, however. As recently reported in Nature Communications, when researchers combine strong, solid pristine graphene oxide sheets with weaker, porous ones, they produce an even stiffer film than those made with strong components alone.

“The old, common-sense approach is that you want to have as strong a unit as possible when constructing something,” says Jiaxing Huang, a professor of materials science and engineering at Northwestern University who led the collaborative work. “The unexpected part of this study is that when we intentionally weaken some of the units in a bulk material, we actually get better overall performance.”

“It’s counterintuitive in the beginning, but eventually it makes sense and is really straightforward,” he adds.  

Bulk graphene oxide films are made out of stacked two-dimensional sheets that are held together by intermolecular forces, including van der Waals interactions and hydrogen bonding. Oftentimes, wrinkles form in stacked sheets. This breaks contact between the sheets and impedes bond formation. “Each sheet will feel some mechanical stress—it’s like a big piece of newspaper,” Huang says. To avoid such defects, researchers try to create perfect stacking, but this can cause problems, generating very stiff local domains connected by weaker ones, which makes the material prone to failure.

Huang and his collaborators at Northwestern and at Hanyang University in South Korea did not originally set out to solve this issue. Instead, they were investigating how etching holes in single-layer graphene oxide sheets using ammonia and hydrogen peroxide weakens the sheets’ properties. They found that the holes significantly reduced the amount of stress individual sheets could handle—an expected result.

The surprise arrived, however, when they created bulk papers made entirely of hole-ridden sheets. They assumed the resulting papers would be weak and soft, but they found that the modulus did not decrease as much as expected. For example, while a single sheet weakened in the etching formula for one hour was 70% weaker than an unetched one, a stacked paper made of multiple etched sheets was just 13% weaker than a paper made from unetched sheets. Looking at a cross section of the etched paper, they found a smooth composition demonstrating uniform interlayer stacking free from large wrinkles.

Mixing in weakened graphene oxide sheets with strong ones, they realized, could cause better connections to form between domains. “The soft units are weaker, but they get along better with the other parts, so they contribute to a better overall mechanical property,” Huang says. “In technical terms, having soft sheets drastically improves the load transfer.” 

The team confirmed their hypothesis by undertaking tensile tests with films built from different compositions of strong and weak sheets. They found that including a mix of 10% weaker units in the films’ composition almost doubled the elastic modulus when compared to just using strong sheets. Including 20% weaker units caused a 75% increase in the modulus. In experiments applying shear force to the films, the researchers found that those with soft layers exhibited a stronger interlayer load transfer than films composed solely of strong layers, thereby exhibiting stronger shear strength.

“The [researchers] elegantly designed the material systems and characterization methods to demonstrate how a more defective and weaker, holey graphene oxide can synergistically team up with pristine graphene oxide to make their hybrids even stronger,” says Dan Li, an Australian Laureate Fellow in materials science and engineering at the University of Melbourne, who was not involved in the research. “This work will stimulate new strategies to make stronger graphene-based bulk materials.”

Graphene oxide remains the model system for constructing bulk materials out of two-dimensional single layer dispersion, but Huang believes that their method will also apply to other 2D materials as their single layers become available in bulk quantities. “There’s nothing special about graphene oxide’s chemical properties in this work—we’re really talking about the shape here,” Huang says. “I’m certain that … other materials will be developed as people put more effort into this, and …what we discovered today will be very important.”

Read the article in Nature Communications.