Chinese scientists say they have figured out a way to turn carbon nanotubes into a superabsorbing and reusable sponge for organic materials. They predict their CNT sponge material may be particularly valuable in applications such as oil spills on ocean, lake and river surfaces because the material won’t absorb water. Their work is reported in Advanced Materials.
Led by Peking University’s Anyuan Cao and Tsinghua University’s Dehai Wu, the group writes in the journal that “the sponges have new properties that integrate the merits of fragile aerogels with their high surface area [the lowest density solid material known is an aerogel], and conventional soft materials with their robustness and flexibility.”
Cao and Wu claim that their CNT material is rugged, elastic, lightweight and can absorbing up to 180 times its own weight in organic matter. This compares very favorably with commercial cellulose-, resin-, polymer- or ceramic-based absorbent materials that typically absorb a fraction of that amount and, furthermore, have to go through heat or some other chemically processing to release the oils or solvents they soak up. They say their sponge material can literally release its contents with simple squeezing, allowing the sponge to be reused and the contents recovered.
According to their paper, Cao and Wu’s sponges are basically CNTs 30–50 nanometers in diameter and tens to hundreds of micrometers long. They exploit the nanotubes’s surface affinity for organic materials and hydrophobicity, and the low density allows the sponge material to float easily on water.
The CNTs are formed via chemical vapor deposition and self-assembly. They say the secret is allow the creation of random arrays of long nanotubes. This lets the nanotubes glide by each other, and maintain flexibility and elasticity. They also say the sponge effect works best if they are first primed with a solvent and then squeezed down. Material given this densification treatment (see video below) can be reduced to a pellet size. When put into use, even at the edge of a spill, they report that, “A small densified pellet of sponge can quickly remove a spreading diesel oil film with an area up to 800 times that of the sponge.”
They report some example capacities: 143 times the sponge’s weight for diesel oil and 175 for ethylene glycol.
Cao imagines many other applications. “The nanotube sponges can be used as filters, membranes, or absorbents to remove bacteria or contaminants from liquid or gas. They could also be used as noise-absorption layers in houses, and soldiers might benefit by using these sponges in impact energy absorbing components while adding little weight. Thermally insulated clothing is also possible.”
Here is a video of how the flexible CNT material can be squeezed down: