Boron Added to Carbon Nanotubes Produce Super Oil Sponge

Daniel Hashim with samples of nanotube sponges (Rice University)

Materials scientists at Rice University in Houston have devised a material that can absorb large quantities of oil by adding the element boron to carbon nanotubes. The Rice researchers, joined by colleagues in the U.S., Belgium, Japan, Spain, and Mexico published their findings Friday in the journal Scientific Reports.

Rice graduate student Daniel Hashim (pictured right), the paper’s lead author, and colleagues devised a process of doping carbon nanoscale tubes with boron, an abundant element in nature that combines with other elements. Bonding boron to carbon nanotubes, the team discovered, results in bends and kinks in the tubes as they grow.

These elbows in the tubes produce a sponge-like material with a combination of unusual properties. Being 99 percent air, the material is very light. The researchers also discovered the material is superhydrophobic, which means it repels water. The combination of light weight and superhydrophobia means it floats easily.

The team found as well that while the material may repel water, it is very attracted to oil. And being made from carbon nanotubes, the material is a conductor of electricity and can be manipulated with magnets.

Hashim’s lab director Pulickel Ajayan says nanotube materials that soak up oil have been made before, but they had to be derived from an existing solid sheet of nanotubes. In this case, the carbon nanotubes grew us normal on a substrate via chemical vapor deposition, but the boron that covalently bonded with carbon at the atomic level induced the normally straight tubes to grow with bends and kinks.

Growing the carbon nanotubes with these attributes creates the material in a one-step process. “The advantage here,” says Ajayan, “is that the material is directly created during growth and comes out as a cross-linked porous network.”

Hashim demonstrated the oil-absorption ability of the material, by dropping a piece of sponge into a dish of water with used motor oil floating on top. After the sponge soaked it up, Hashim put a match to the material, burned off the oil, and returned the sponge to the water to absorb more. He says the sponges described in the paper can absorb more than a hundred times their weight in oil.

The robust sponge can be used repeatedly and stands up to abuse; one of the lab samples remained elastic after about 10,000 compressions. “They’re super-low density, so the available volume is large,” says Hashim, “That’s why the uptake of oil can be so high.” The material also holds the oil, which means the oil can be retrieved, if needed, or burned off.

The material still needs further work before it can be ready for commercial use in oil remediation — now underway in Ajayan’s lab — including production of the material in large sheets or being able to weld sheets together. The co-authors of the paper have suggested other applications, including scaffolds for bone-tissue regeneration, and light composites for the automobile and aircraft industries.

In the following video, Hashim demonstrates the properties of this material.

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