This diagram illustrates how large single layers of GO wrinkle easily, whereas small, hard flakes don’t integrate well. Both leave gaps that weak flakes can fill. Image: Northwestern University.
This diagram illustrates how large single layers of GO wrinkle easily, whereas small, hard flakes don’t integrate well. Both leave gaps that weak flakes can fill. Image: Northwestern University.

If you want to make a super-strong material from nano-scale building blocks, then you should start with the highest quality building blocks, right? Wrong – at least when working with ‘flakes’ of graphene oxide (GO).

A new study from researchers at Northwestern University shows that better GO ‘paper’ can be made by mixing strong, solid GO flakes with weak, porous GO flakes. This finding will aid the production of higher-quality GO materials and also sheds light on a general problem in materials engineering: how to build a nano-scale material into a macroscopic material without losing its desirable properties.

"To put it in human terms, collaboration is very important," said Jiaxing Huang, a professor of materials science and engineering at Northwestern University, who led the study. "Excellent players can still make a bad team if they don't work well together. Here, we add some seemingly weaker players and they strengthen the whole team."

The research was a four-way collaboration; in addition to Huang's group, three other groups participated. These were led by: Horacio Espinosa, professor of mechanical engineering at the McCormick School of Engineering at Northwestern; SonBinh Nguyen, professor of chemistry at Northwestern; and Tae Hee Han, a former postdoc researcher at Northwestern who's now a professor of organic and nano engineering at Hanyang University in South Korea. The researchers report their findings in a paper in Nature Communications.

GO is a derivative of graphite that can be used to make the two-dimensional (2D) super material graphene. Since GO is easier to make than pristine graphene, scientists study it as a model material. It generally comes as a dispersion of tiny flakes in water; from one end to the other, each flake is only 1nm thick.

When a solution of GO flakes is poured onto a filter and the water removed, this produces a thin ‘paper’, usually a few inches in diameter with a thickness less than or equal to 40µm. Intermolecular forces hold the flakes together, nothing more.

Scientists can make strong GO in single layers, but layering the flakes into a paper form doesn't work too well. While testing the effect of holes on the strength of GO flakes, Huang and his collaborators discovered a possible solution to this problem.

Using a mixture of ammonia and hydrogen peroxide, the researchers chemically ‘etched’ holes in the GO flakes. Flakes left soaking for one to three hours were drastically weaker than un-etched flakes. After five hours of soaking, the flakes became so weak they couldn't be measured.

Then, the team found something surprising: paper made from the weakened flakes was stronger than expected. At the single-layer level, one-hour-etched porous flakes, for example, were 70% weaker than solid flakes, but paper made from those flakes was only 10% weaker than paper made from solid flakes.

Things got even more interesting when the team mixed solid and porous flakes together, Huang said. Instead of weakening the paper made solely from solid flakes, the addition of 10% or 25% of the weakest flakes strengthened it by about 95% and 70%, respectively.

If GO sheets can be likened to aluminum foil, Huang explained, making GO paper is just like stacking the foil up to make a thick aluminum slab. If you start with large sheets of aluminum foil, chances are good that many will wrinkle, impeding tight packing between sheets. On the other hand, smaller sheets don't wrinkle as easily. They pack together well but create tight stacks that don't integrate well with other tight stacks, creating voids within the aluminium slab or GO paper where it can easily break.

"Weak flakes warp to fill in those voids, which improves the distribution of forces throughout the material," Huang said. "It's a reminder that the strength of individual units is only part of the equation; effective connection and stress distribution is equally important."

This finding will be directly applicable to other 2D materials, like graphene, Huang said, and will also lead to the design of higher-quality GO products. He hopes to test it out on GO fibers next.

This story is adapted from material from Northwestern University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.