The first artificial material that follows sunlight may upgrade solar panels

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, maneuvered to capture about 90 percent of available light shining onto a surface at a 75-degree angle in lab tests, 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.

To build their initial SunBOTs, the researchers used gold nanoparticles and a hydrogel. But tests with other materials — such as reduced graphene oxide 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 22 percent of available solar energy, He says.

By creating a material that can follow sunlight, the researchers may have opened the door to devices that are able to maximize solar absorption, even as the sun moved overhead, Lee says. “That is a major thing that they achieved.”