Sheets of tangled nanowires can be used to draw oil from water.
Researchers have created a sheet of entwined nanowires that sop up oil but leave water behind.
The sponge, which resembles paper, can absorb 20 times its weight in oil, a feature that may make the technology useful in cleaning up spills and separating contaminants from water.
Although the spaghetti-like mess of nanowires has not yet been put to the test outside the lab, researchers say that the material should be easy to produce in large quantities. The sponge can be reused after heating, which evaporates off any oil or organic solvent the mesh has absorbed.
Several materials can absorb such substances from water, but this one is unique in that it also repels almost 100% of water, says study author Francesco Stellacci of the Massachusetts Institute of Technology in Cambridge. This is important, he says, “because in many situations you absorb a lot of water before you can even get to absorbing oil”. The results are published online today in Nature Nanotechnology1.
To make the nanowire membrane, Stellacci and his colleagues adapted an existing method to make nanowire sheets2. They produced wires made from manganese oxide that were roughly 20 nanometres in diameter and naturally assembled into a wool-like tangle. The team added a layer of silicone to the membrane to make it hydrophobic, or water repellent.
Because the membrane is mostly air, it functions like a sponge, drawing in liquid through capillary action. But because the surfaces are unfavourable to water, the membrane selectively absorbs hydrophobic solvents such as oil.
Strangely, the membrane seemed to absorb some solvents more readily than others, even when they were expected to have similar levels of hydrophobicity on the basis of their electric properties. The result might hold some application in extracting solvents from difficult-to-separate mixtures.
The unusually high rejection of water and absorption of oil means that the technology shouldn't take long to make it into the field. "It’s something that’s scalable," says chemist Joerg Lahann of the University of Michigan in Ann Arbor. "They have shown paper-sheet sizes, which for nanotechnology is pretty impressive."
Previous studies suggest nanomaterials made from manganese oxide may show some toxic effects in mammals. But the membranes seem to be mechanically robust, meaning the material may be unlikely to degrade in large quantities and enter the environment. Stellacci adds the same selective adsorption behavior might be replicated with other materials.
Yuan, J. et al. Nature Nanotech advance online publication doi:10.1038/nnano.2008.136 (2008).
Yuan, J., Laubernds, K., Villegas, J., Gomez, S. & Suib, S. L. Adv. Mater. 16, 1729–1732 (2004).