Researchers at Monash University, in Victoria, Australia, have found a way to coat fibers with titanium dioxide nanocrystals, which break down food and dirt in sunlight. The researchers, led by organic chemist and nanomaterials researcher Walid Daoud, have made natural fibers such as wool, silk, and hemp that will automatically remove food, grime, and even red-wine stains when exposed to sunlight.
Daoud and his colleagues coat the fibers with a thin, invisible layer of titanium dioxide nanoparticles. Titanium dioxide, which is used in sunscreens, toothpaste, and paint, is a strong photocatalyst: in the presence of ultraviolet light and water vapor, it forms hydroxyl radicals, which oxidize, or decompose, organic matter. However, says Daoud, "these nanocrystals cannot decompose wool and are harmless to skin." Moreover, the coating does not change the look and feel of the fabric.
"When you burn something, you oxidize it," says Jeffrey Youngblood, a materials engineering professor at Purdue University, who is developing self-cleaning materials that repel oil. "This [titanium dioxide coating] is just burning organic matter at room temperature in the presence of light."
Titanium dioxide can also destroy pathogens such as bacteria in the presence of sunlight by breaking down the cell walls of the microorganisms. This should make self-cleaning fabrics especially useful in hospitals and other medical settings. Daoud says that "self-cleaning property will become a standard feature of future textiles and other commonly used materials to maintain hygiene and prevent the spreading of pathogenic infection, particularly since pathogenic microorganisms can survive on textile surfaces for up to three months."
The idea of using titanium dioxide to make self-cleaning surfaces is not new. Titanium dioxide powder is added to paints and as a transparent coating (roughly 10 nanometers thick) on glass to make self-cleaning windows.
To make self-cleaning wool, Daoud and his colleagues use nanocrystals of titanium dioxide that are four to five nanometers in size. In the past, the researchers have made self-cleaning cotton by coating it with these nanocrystals. But coating wool, silk, and hemp has proved more difficult. These fibers are made of a protein called keratin, which does not have any reactive chemical groups on its surface to bind with titanium dioxide.
The researchers chemically modify the surface of wool fibers, adding chemical groups called carboxylic groups, which strongly attract titanium dioxide. Then they dip the fibers in a titanium dioxide nanocrystal solution. The researchers have outlined this process in a paper that recently appeared online in the journal Chemistry of Materials.
In the paper, the researchers show how the material stands up to red-wine stains, which are notoriously difficult to remove. Titanium-dioxide-coated wool shows almost no sign of the red stain after 20 hours of exposure to simulated sunlight, while the untreated wool remains boldly stained. Other stains disappear faster: coffee stains fade away in two hours, while blue-ink stains disappear in seventeen hours.
Different types of self-cleaning materials that incorporate nanoparticles have been developed in the past. Stain-repellant fabrics and paints that are currently on the market typically have a nanoparticle or nanofiber coating that causes drops of liquid to roll off instead of getting absorbed into the material. The liquid drops take small particles of dirt and grime with them.
More materials are in the research stage. These include microstructured, Teflon-like materials that bounce oil off their surface. (See "No More Thumbprints.") Purdue's Youngblood has made a material that changes its structure depending on whether it's in contact with oil or water, causing water to spread out into a thin film and oil to bead up so that it runs off or is easily wiped off with water. (See "Self-Cleaning, Fog-Free Windshields.")
All of these materials are based on making the surface oil or water repellant, says Youngblood. This is a concept that is completely different from that of the new titanium dioxide coating. "We're controlling wettability and surface interaction," he says. Titanium dioxide coatings, on the other hand, degrade organic matter. "It has nothing to do with surface wettability whatsoever. Here, you're not removing what's on the surface: you're burning it off."
Each of these techniques to make self-cleaning materials has its own limitations. Superhydrophobic materials, which repel water, are typically good at removing dirt particles but "don't deal with oils well," Youngblood says. Materials that repel oil, such as the one that he has developed, might not work with certain types of oil. The titanium-dioxide-coated materials, on the other hand, will not work unless they are exposed to sunlight for hours.
The sunlight requirement has not stopped the technology from getting commercial interest. Several wool manufacturers have suggested that they'd like to evaluate the technology, Daoud says. He expects self-cleaning wools to be available in the market within two years, once sufficient laboratory and industrial trials have been completed.