May 30 2019
MIT researchers have recently discovered that ingredients like detergent, table salt, and baking soda are amazingly effective for cooking up carbon nanotubes (CNTs).
Sodium-containing compounds, such as those found in common household ingredients like detergent, baking soda, and table salt, are surprisingly effective ingredients for cooking up carbon nanotubes, new MIT study finds. (Image credit: Christine Daniloff, MIT)
In a study recently reported in the journal Angewandte Chemie, the researchers have reported that compounds made of sodium are often found in standard household ingredients and they can catalyze the growth of CNTs at temperatures much lower than those required by conventional catalysts.
According to the team, sodium could enable the growth of CNTs on a wide range of lower-temperature materials, like polymers, which usually melt under the elevated temperatures required for the conventional growth of CNTs.
In aerospace composites, there are a lot of polymers that hold carbon fibers together, and now we may be able to directly grow CNTs on polymer materials, to make stronger, tougher, stiffer composites. Using sodium as a catalyst really unlocks the kinds of surfaces you can grow nanotubes on.
Richard Li, Study Lead Author and Graduate Student, Department of Aeronautics and Astronautics, MIT
Li’s MIT co-authors are postdocs Estelle Kalfon-Cohen, Erica Antunes, Kehang Cui, and Luiz Acauan; alumni Andrew Liotta ’16, Akira Kudo PhD ’16, and Ananth Govind Rajan SM ’16, PhD ’19; professor of aeronautics and astronautics Brian Wardle, and professor of chemical engineering Michael Strano, along with collaborators at Harvard University and the National Institute of Standards and Technology.
Peeling onions
When CNTs are viewed under a microscope, they look like empty cylinders of chicken wire. Every tube is developed from a rolled up lattice of hexagonally assembled carbon atoms. Moreover, the bond present between carbon atoms is extremely strong, and when these carbon atoms are patterned as a tube, such as a CNT, or into a lattice, such as graphene, these structures can have incredible strength and stiffness, and also exhibit special chemical and electrical characteristics. As such, scientists have attempted coating a variety of surfaces with CNTs to create tougher, stiffer, and stronger materials.
A process known as chemical vapor deposition (CVD) is generally used by researchers to grow CNTs on many different materials. A material of interest, like carbon fibers, is coated in a catalyst, which is often an iron-based compound, and then mounted in a furnace, via which carbon dioxide gas and other carbon-containing gases are able to flow. At temperatures of around 800 ºC, the iron begins to extract carbon atoms from the gas, which attach to the iron atoms and to one another, ultimately creating vertical tubes of carbon atoms around separate carbon fibers. Scientists then dissolve the catalyst by using a wide range of methods, and pure carbon nanotubes are ultimately left behind.
Li and his team were working on various ways to grow CNTs on an array of surfaces by coating them with diverse solutions of iron-containing compounds, and while doing so, they found that the ensuing CNTs appeared different from what they had anticipated.
The tubes looked a little funny, and Rich and the team carefully peeled the onion back, as it were, and it turns out a small quantity of sodium, which we suspected was inactive, was actually causing all the growth.
Brian Wardle, Professor, Department of Aeronautics and Astronautics, MIT
Tuning sodium’s knobs
On the whole, iron has been the conventional catalyst for growing CNTs. This is the first time investigators have noticed that sodium also has an equivalent effect, stated Wardle.
“Sodium and other alkali metals have not been explored for CNT catalysis,” stated Wardle. “This work has led us to a different part of the periodic table.”
To ensure that their initial observation was not simply a coincidence, the researchers tested a variety of sodium-containing compounds. Initially, they experimented with commercial-grade sodium, which was in the form of detergent pellets, table salt, and baking soda and was bought from the campus convenience store. However, ultimately, the researchers upgraded to purified forms of those compounds, which they subsequently dissolved in water. Next, they dipped a carbon fiber in each solution of the compound, coating the whole surface in sodium and at last transferred the material to a furnace and performed the usual steps involved in the CVD process to grow CNTs.
At large, the researchers observed that, while iron catalysts were able to form CNTs at up to 800 ºC, the sodium catalysts formed dense and short forests of CNTs at relatively lower temperatures of about 480 ºC. In addition, after the surfaces spent approximately 15 to 30 minutes in the furnace, the sodium merely vaporized away, leaving behind hollow CNTs.
A large part of CNT research is not on growing them, but on cleaning them—getting the different metals used to grow them out of the product. The neat thing with sodium is, we can just heat it and get rid of it, and get pure CNT as product, which you can’t do with traditional catalysts.
Brian Wardle, Professor, Department of Aeronautics and Astronautics, MIT
According to Li, upcoming work may concentrate on enhancing the quality of CNTs grown with the help of sodium catalysts. The team also noticed that although sodium was able to create short, dense forests of CNTs, the tubes’ walls were not suitably aligned in seamlessly hexagonal patterns. CNTs get their distinctive strength only from these crystal-like configurations. Within the CVD process, Li intends to “tune various knobs”, altering the temperature, timing, and environmental conditions so as to enhance the quality of CNTs grown using sodium.
There are so many variables you can still play with, and sodium can still compete pretty well with traditional catalysts. We anticipate with sodium, it is possible to get high quality tubes in the future. And we have pretty high confidence that, even if you were to use regular Arm and Hammer baking soda, it should work.
Richard Li, Study Lead Author and Graduate Student, Department of Aeronautics and Astronautics, MIT
According to Shigeo Maruyama, professor of mechanical engineering at the University of Tokyo, the possibility to cook up CNTs from such a standard ingredient as sodium should provide a deeper understanding of how the remarkably robust materials are able to grow.
“It is a surprise that we can grow carbon nanotubes from table salt!” stated Maruyama, who was not part of the study. “Even though chemical vapor deposition (CVD) growth of carbon nanotubes has been studied for more than 20 years, nobody has tried to use alkali group metal as catalyst. This will be a great hint for the fully new understanding of growth mechanism of carbon nanotubes.”
The study was partly supported by Boeing, Airbus, Embraer, Lockheed Martin, ANSYS, Saab AB, TohoTenax, and Saertex through MIT’s Nano-Engineered Composite aerospace STructures (NECST) Consortium.