How Jet-Black Metal Converts Sunlight to Steam Power
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Steam power, once a major force behind the Industrial Revolution, could be coming back into fashion, after Chinese researchers designed the world's "darkest metal" that converts sunlight to steam at roughly 90 percent efficiency.

Despite being made from gold, the so-called "plasmonic absorber" is jet black as it absorbs 99 percent of light in the visible to mid-infrared spectrum. Its designers say this is a dramatic improvement over previous metal absorbers and comparable to the world's darkest material, carbon-nanotube (CNT) arrays. Combined with its porous structure, this enables the metal to use solar energy to generate steam at far lower light intensities and temperatures than traditional approaches that concentrate sunlight to very high levels to drive steam turbines.

The device can also assemble itself, which could enable large-scale manufacture of plasmonic absorbers for a host of applications, the researchers said. [10 Technologies That Will Transform Your Life]

"It opens up a lot of possibilities in terms of solar catalysis, water purification, sensors and detectors," said study co-author Jia Zhu, a professor at Nanjing University in China. "Steam can be used to kill bacteria for biomedical applications, others are trying to use steam to run heat engines to generate electricity and steam can also be used as a clean form of water once you condense it. There are a lot of things that can be done and I see huge potential in our absorber."

The new absorber, described in a paper published April 8 in the journal Science Advances, takes advantage of plasmonics, where the free electrons that allow electric current to pass through metals can also be excited by the electromagnetic waves that make up light.

By carefully designing nanoscale metallic structures, it is possible to exploit this effect to absorb the energy from light. At present, these designs are normally effective only at specific wavelengths and building them requires complicated lab techniques such as focused ion beam and e-beam lithography.

For their new absorber, the Chinese researchers employed anodization — a simple process that uses electricity to oxidize the surface of a metal — to create an aluminum oxide template dotted with nanoscale pores. They then introduced a vapor of gold nanoparticles that self-assembled onto the template surface and inside the pores.

The honeycomb shape of the template helps confine light to the absorber by reducing its reflectivity, but Zhu said the secret to its success is randomly sized nanoparticles crammed together.

Typically the light frequency at which electrons become excited depends heavily on the size of the particle, so having various particle sizes means more frequencies are covered. In addition, when the particles are packed tightly, their electrons can work together to interact with light more efficiently and across a wide range of wavelengths, the researchers said.

"Each particle can respond to a different frequency, but when they're closely packed together they also work together," Zhu told Live Science.

To demonstrate the practical effectiveness of the device, the scientists showed it could generate steam by simply floating on water when illuminated with the equivalent of four suns' worth of light, a far lower intensity than other solar-steam generators require. [Top 10 Craziest Environmental Ideas]

According to Zhu, the structure of the absorber also means very little energy is wasted on heating water that is not in contact with the device. "Only the very top surface of the water gets heated up and becomes vaporized immediately," he added. "And the porous structure provides channels for the steam to escape."

Ventsislav Valev, a professor in the Department of Physics at the University of Bath in the United Kingdom, recently built a working nanophotonic steam engine, in which steam is generated using laser-illuminated plasmonic nanoparticles. He said the high absorbance and the broad wavelength range of the team's structure is impressive, and he agreed that it could one day be manufactured on a large scale.

"The issue I see is that, in scaling the production, heat loss to the environment will become an important factor," Valev told Live Science. He said his team found that it was easy to generate steam from small amounts of water, but it became increasingly difficult with larger volumes.

Both the efficiency and bandwidth of the new absorber are similar to those achieved using CNTs, but Zhu said their approach can piggyback on the already well-developed metallurgy industry, while CNT technology is still confined to the lab.

But, he thinks the two approaches to light absorption can be complimentary and both should be investigated. "CNTs have their own unique advantages, but metals are unique, too," Zhu said. "If we can now combine incredible light absorbing in metals with their other properties in things like catalysis or sensing, that would be great."

Zhu said he and his colleagues are currently developing materials and processes that achieve high efficiency with much lower cost and just one sun intensity.

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