The first artificial material that follows sunlight may upgrade solar panels

Dubbed SunBOTs, the devices can capture much more solar energy than stationary devices

SunBOT

SunBOTs (in the foreground of this illustration) are the first artificial material that can maneuver like sunflowers to face the sun as it moves across the sky.

Yusen Zhao, Yousif Alsaid and Ximin He

As the sun moves across the sky, sunflowers continually orient themselves to soak up the most light (SN: 8/4/16). Now a type of human-made material can do that, too.

This is the first artificial material capable of phototropism, researchers report November 4 in Nature Nanotechnology. Stemlike cylinders of the material, dubbed SunBOTs, can maneuver to capture about 90 percent of available sunlight, even when the sun comes in at an oblique angle, materials scientist Ximin He of UCLA and her colleagues found. The technology could someday be used to optimize solar panels, desalinate water or move robots, the researchers say.

Other scientists have made artificial substances that can bend toward light, but those materials stop arbitrarily. SunBOTs can self-regulate, moving into the optimal position needed to absorb the sun’s rays, then making small adjustments to stay there as the sun shifts.

That ability comes from a SunBOTs’ configuration: a stemlike polymer about 1 millimeter in diameter embedded with a nanomaterial that responds to light. The nanomaterial absorbs light and converts it into heat; the polymer shrinks in response to increased temperatures.

When He and colleagues trained a beam of light on one of these artificial stems, the illuminated side heated up and contracted. That caused its top to bend toward the light. The newly shaded underside of the stem then cooled, stopping the SunBOT’s movement in a position best oriented to soak up the light. The process repeated as the angle of the light beam changed.

SunBOT
SunBOTs are designed to bend to meet different angles of a beam of light (arrows indicate the direction of incoming light), and can shift position as the light moves. That ability lets the devices capture more solar energy than stationary devices.Xiaoshi Qian, Yusen Zhao, Yousif Alsaid and Ximin He

To build their initial SunBOTs, the researchers used gold nanoparticles and a hydrogel. But tests with other materials — such as carbon black nanoparticles and liquid crystalline polymers — revealed that the components could be mixed and matched.

“If we have this big repertoire of materials working with the same principle … scientists can use it in different environments for different applications,” says Seung-Wuk Lee, a bioengineer at the University of California, Berkeley who was not involved in the study. For instance, hydrogel SunBOTs work in water, He’s team found.

SunBOTs can be lined up in rows to cover an entire surface, creating a “mini sunflower forest,” she says. Coating surfaces with this material could solve one of the biggest problems in solar energy: As the angle of direct sunlight changes as the sun moves overhead, conventional materials can’t keep up.

Materials that stay in one position — like solar cells on a solar panel — capture about 24 percent of available energy from the sun. Simply by moving to face the sun, He says, an array of SunBOTs can harvest about 90 percent of energy from sunlight.

By creating a material that can follow sunlight, the researchers were able to keep near-maximum solar absorption as the sun moved overhead, Lee says. “That is a major thing that they achieved.”

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