Carbon Nanotube Sponges

Scientists have invented a carbon-based sponge that can soak up organic pollutants, such as oils and solvents, from the surface of water. No water is absorbed and the sponge can then be wrung out and reused, like an ordinary household sponge. Absorbing up to 180 times its own weight in organic matter, the sponge is light and tough and has the potential to dramatically enhance oil spill cleanup. Professors Anyuan Cao (Peking University) and Dehai Wu (Tsinghua University), who are publishing their breakthrough in Advanced Materials, say “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.” Current commercial absorbents for oil spill recovery and industrial use tend to be based on cellulose or polypropylene. These materials can absorb only up to 20 times their own weight and are impractical for large spills, where dispersants are used. Dispersants allow the oil to become diluted, but it remains in the water. Other materials based on porous oxide-based materials or other polymers can absorb up to twice as much pollutant per weight, but generally need to be heated to remove the organic material. High-temperature heating is not practical on small scales or on ships, and a clear advantage of a squeezable sponge is that the oil can be readily recovered and reused. For other applications including solvent cleanup, the sponges can be heated to remove the pollutant, without affecting the properties of the sponges. Cao and Wu’s sponges are made from interconnected carbon nanotubes– tiny, strong and hollow cylinders of interconnected carbon atoms. In this instance the tubes are 30–50 nanometres across and tens to hundreds of micrometers long (a nanometre is 10–9 metres, or one millionth of a millimetre; a micrometre is 1000 times as long). The surface of the tubes is naturally hydrophobic (water-hating), therefore no further modification is needed for the sponges to repel water. At the same time, they love to absorb oil on their surface. As the sponges are over 99% porous or empty, they float on water and there is a lot of room for oil to be absorbed, leading to the extremely high capacity for retention – for example, 143 times the sponge’s weight for diesel oil and 175 for ethylene glycol. Lateral thinking was the key to the scientists’ breakthrough. A major ambition among carbon nanotube researchers is to look for ways to make large lined-up arrays of the tubes. Cao and Wu, however, searched for a method that would make long tubes that were completely disordered. This randomness allows the tubes to slide past each other, allowing the sponge to be manually reduced in size by 95%, and bent or twisted without breaking. As the sponge is squeezed, any oil or solvent in the cavities and on the surface of the tubes is expelled. To gain the best effect, the sponges first have to be filled with solvent and then compressed gently in a process called densification, but after this they are extremely robust and can be used potentially thousands of times. They swell to recover their original dimensions when exposed to oil or solvent and “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”, as illustrated in the accompanying figure. This effect occurs even if the sponge is placed at the edge of the spill. Potential applications reach beyond oil spill recovery. According to Cao, “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.” Large-scale production is currently being investigated. A. Cao et al., Adv. Mater. ; DOI: 10.1002/adma.200902986 Source

Nano Sponge For Oil Spills

Monday, June 02, 2008

A nanowire membrane that sops up oil while repelling water could be used for cleaning up oil spills.

By Prachi Patel-Predd

Credit: Francesco Stellacci, MIT, and Nature Nanotechnology

A thin membranes made from a web of nanowires might become a promising tool for cleaning up oil spills and removing toxic contaminants from groundwater. When dipped into a mixture of water and oil, the 50-micrometer-thick membrane absorbs the oil, swelling to 20 times its weight.

Typically, oil spills are cleaned up using the same basic technology used 20 years ago. This includes using absorbent materials to sop up traces of oil. Natural sorbents such as hay and cellulose can soak up between 3 and 15 times their weight in oil, while synthetic polymer-based sorbents can absorb up to 70 times their weight. But these materials tend to absorb water as well.

The new membrane absorbs oil and solvents and is superhydrophobic, which means it strongly repels water. "If you were to put it in water for a month and take it out it would still be dry," says Francesco Stellacci, the MIT Materials science and engineering professor who led the work, published online in Nature Nanotechnology. Stellaci says the material should not be too expensive to make in large quantities and can be easily reused many times, although the researchers haven't measured how many times yet.

Michael Rubner, an MIT materials science and engineering professor who was not involved in the project, says that the membrane's reusability is its most distinctive feature. Other hydrophobic structures have typically been made from organic materials. The inorganic nanowires can handle temperatures up to 600 degrees Celsius, where organic materials would degrade. "If the membrane becomes foul with oil or you have to remove the oil, ... [you] can basically cook it and clean it up and, in principle, use it over and over again," Rubner says.

The membrane is a mat of potassium manganese oxide nanowires, each about 20 nanometers wide. Stellacci and his colleagues assemble the mats using a method similar to one used to produce paper: they make a suspension of nanowires and dry it on a substrate. They have made membranes that are 27 centimeters on each side, but Stellacci says they could be made in larger mats.

Two important characteristics give the membrane its exceptional oil-absorbing and water-repelling properties. First, the nanowire mesh has tiny pores--10-nanometers wide on averagecapable of wicking water and other liquids up into the membrane. To keep water away, researchers coat the membrane with water-repelling silicone. The result: water rolls off the surface of the membrane while oil travels quickly up the pores. Stellacci and his colleagues tested the membrane with mixtures of different organic solvents and oils, including motor oil, gasoline and toluene.

The researchers also found that the membrane can separate nearly identical solvents; when dipped into a mixture of benzene and toluene, the membrane absorbs only the toluene. "That's pretty amazing, because they're quite similar molecules," says Joerg Lahann, a chemical engineering professor at the University of Michigan. This property could open up other applications, such as purifying or separating chemicals and solvents.

Researchers hope that the nanomembrane could reduce waste and lower the cost of cleaning oil spills from boats and in the petroleum industry. But Doug Helton, a scientist with the National Oceanic and Atmospheric Administration, says that it might be too early to say whether the nanomembrane might be practical in cleaning up large oil spills. The heating technique needed to clean the membrane might prove "a fairly onerous process," he says.

Plus, the membrane's oil-sopping capacity might diminish at a real spill. "Oil spills are pretty messy," Helton says. "There might be a lot of debris. That might reduce the efficiency of the sorbent." For now, Helton thinks the membrane could be good for removing water contaminants at factories or cleaning up smaller oil spills--in garages and machine shops.

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