Grow your own nanotubes

Baking soda, table salt, and detergent are surprisingly effective ingredients for cooking up carbon nanotubes. A team of MIT researchers found that such sodium-containing household ingredients can catalyze the growth of carbon nanotubes, or CNTs, at much lower temperatures than traditional catalysts require, although a home oven wouldn’t be hot enough to make it work. The discovery may enable CNTs to grow on a host of materials that would melt at higher temperatures, such as polymers and other substances, offering a way to make those materials stronger.

Under an electron microscope, carbon nanotubes resemble hollow cylinders of chicken wire. Each tube is made from a rolled-up lattice of hexagonally arranged carbon atoms. The bond between carbon atoms is extraordinarily strong, and when patterned into a flat lattice (as in graphene) or a tube-shaped lattice (as in a CNT), such structures can have exceptional stiffness and strength.

Researchers typically grow CNTs through chemical vapor deposition, in which a material such as a silicon wafer is coated in a catalyst—usually an iron-based compound—and placed in a furnace, through which carbon-containing gases flow. At temperatures near 800 °C, the iron starts to draw carbon atoms out of the gas, forming vertical nanotubes.

The MIT researchers were experimenting with ways to grow CNTs when they noticed that one batch produced results different from what they expected. “The tubes looked a little funny,” says aero--astro professor Brian Wardle. “It turns out a small quantity of sodium, which we suspected was inactive, was actually causing all the growth.” 

They tested a range of sodium--containing compounds, including baking soda, table salt, and detergent pellets—ingredients they could have purchased at LaVerde’s, the convenience store in the student center. Eventually, they upgraded to purified versions of those compounds.

While iron catalysts form carbon nanotubes at around 800 °C, they found that sodium catalysts did so at around 480 °C. And after about 15 to 30 minutes in the furnace, the sodium simply vaporized away, leaving hollow carbon nanotubes behind.

“We anticipate with sodium, it is possible to get high-quality tubes in the future,” says aero-astro grad student Richard Li, the study’s lead author. “Even if you were to use regular Arm & Hammer baking soda, it should work.”