Want to make a super strong material from nanoscale building blocks? Start with the highest quality building blocks, right?
Wrong - at least when working with "flakes" of graphene oxide (GO).
A new study from Northwestern University researchers shows that better GO "paper" can be made by mixing strong, solid GO flakes with weak, porous GO flakes. The finding will aid the production of higher quality GO materials, and it 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, Northwestern Engineering professor of materials science and engineering, 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, three other groups participated, led by Horacio Espinosa, professor of mechanical engineering at the McCormick School of Engineering; SonBinh Nguyen, professor of chemistry at Northwestern; and Tae Hee Han, a former postdoc researcher at the University who's now a professor of organic and nano engineering at Hanyang University, South Korea.
Controlled oxidative etching of GO yields single layers with tunable porosity. a Schematic models of GO and porous GO. Red dots represent sites with oxygen-containing functional groups, which are preferentially etched, leaving holes on the graphene sheet. HR-TEM images of GO sheets b before and after being etched for c 1 h, and d 3 h, respectively. Pores are highlighted in blue color. The corresponding lower-magnification TEM images are shown in the insets. e A low-magnification TEM image of the 5-h-etched GO sample, showing extensive formation of large pores. f Size distribution and g number density of pores found on the starting and etched GO sheets, based on a survey over an area of 500 nm2 in HR-TEM images. Error bars represent the standard deviation (SD).
(© Nature Communications)
When a solution of GO flakes is poured onto a filter and the water removed, a thin "paper" is formed, usually a few inches in diameter with a thickness less than or equal to 40 micrometers. Intermolecular forces hold the flakes together, nothing more.
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, 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 percent weaker than solid flakes, but paper made from those flakes was only 10 percent 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 percent of the weakest flakes strengthened it by about 95 and 70 percent, respectively.
"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 two-dimensional 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.
Northwestern University. Posted: Aug 15, 2019.