For at least the last decade, “solar thermal” technologies, in which sunlight is used to convert water into steam that runs electric turbines or performs desalination, has been a kind of darling of the investment community. About six years ago, nanoparticles started to get into this solar-thermal game when Rice University researchers added some nanoparticles to cold water and were able to make steam when they exposed the combination to sunlight.
Since then, a lot of work in what is now termed photothermal conversion has turned to the field of plasmonics, which exploits the wave of electrons that is produced when photons strike a metallic surface. However, producing plasmonic nanostructures is certainly not as straightforward as just adding some nanoparticles to water.
Now, researchers in China have combined the ease of adding nanoparticles to water with plasmonics to create a photothermal conversion process that exceeds all plasmonic or all-dielectric nanoparticles previously reported.
Researchers at Sun Yat-sen University in China demonstrated in the journal Science Advances what they claim is the first material that simultaneously has both plasmonic-like and all-dielectric properties when exposed to sunlight.
The key to achieving this combination is the use of tellurium (Te) nanoparticles, which have unique optical duality, according to G. W. Yang, professor at Sun Yat-sen University and coauthor of the research.
By dispersing these nanoparticles into water, the water evaporation rate is improved by a factor of three under solar radiation. This makes it possible to increase the water temperature from 29 degrees to 85 degrees Celsius within 100 seconds.
Thermal images show the difference in solar radiation absorbed by a bare silicon wafer (left) and a Te nanoparticle (right). Image: Science Advances
“The Te nanoparticles perform like a plasmonic nanoparticle when it is smaller than 120 nanometers (nm) and then as a high-index all-dielectric nanoparticle when those nanoparticles are larger than 120 nm,” said Yang.
The Te nanoparticles are able to achieve this duality because they have a wide size distribution (from 10 to 300 nm). This enhanced absorption can cover the whole solar radiation spectrum.
Another property of the Te nanoparticle is that when it is excited by sunlight, the excitation energy is transferred entirely to the carriers (electrons and holes). This pushes the carriers out of equilibrium and into special states of momentum with higher temperatures.
Yang explains that as the system evolves toward equilibrium, these carriers relax. As the carriers scatter, it leads to a phenomenon known as Coulomb thermalization, which forms a hot gas of thermalized carriers that couple with phonons and transfer their excess energy to the lattice. This results in the efficient heating of the Te nanoparticles.
For this approach to work for commercial desalination, Yang acknowledges that the current method of producing the Te nanoparticles with nanosecond laser ablation in liquid is limited. “Now, we are trying to prepare the Te nanoparticles by other methods,” he added.
But because the Te nanoparticles have a unique optical duality, Yang envisions other applications for the technology. “We want to apply them in sensors or nanoantennas,” he said.