Turning water to steam, no boiling required

A new, extremely black material can turn water into steam using only sunlight, without the need to bring the water to a boil. Made of gold nanoparticles tens of billionths of a meter wide affixed to a scaffold pocked with tiny channels, or “nanopores,” the material is a deep black color because it reflects very little visible light. It is 99 percent efficient at absorbing light in the visible spectrum and parts of the infrared spectrum, researchers report April 8 in Science Advances.

Thanks to its highly porous structure, the material floats on the surface of water, allowing it to soak up the sun’s rays. When light of a certain wavelength hits a gold nanoparticle inside one of the nanopores, it stirs up the electrons on the surface, sloshing them back in forth in an oscillation known as a plasmon. These plasmons produce localized, intense heating, which vaporizes the water nearby.
The wavelength of light that excites a plasmon depends on the size of the nanoparticle. So in order to take advantage of as much of the sun’s output as possible, the group interspersed a variety of sizes of gold nanoparticles in the pores, which could therefore absorb a range of wavelengths.
It’s not the first time scientists have produced steam with plasmonic materials, but the new material improves the efficiency of the process, converting up to 90 percent of the light’s energy into steam, says materials scientist Jia Zhu of Nanjing University in China, a leader of the research group.

“They have really come out with a very intriguing solution,” says mechanical engineer Nicholas Fang of MIT, who was not involved in the research. The efficiency isn’t quite as high as scientists have achieved with certain other types of materials, like carbon nanotubes, Fang says. But the new material should be cheaper to manufacture.

Efficient steam generation could be useful for desalination, producing freshwater from salty water, says Zhu. Other potential applications range from sterilization to running steam engines. “Steam can be used for many other things,” he says. “It is a very useful form of energy.”