Sunlight is an inexhaustible source of energy, and using sunlight to generate electricity is one of the cornerstones of renewable energy. More than 40% of the sunlight falling on Earth belongs to the infrared, visible, and ultraviolet spectrum; however, current solar technology primarily utilizes visible and ultraviolet light. Technologies that harness the full spectrum of solar radiation, known as total solar energy, are still in their infancy.
(Left) Single copper-doped tungstic acid nanocrystal; (right) Atomic resolution image of the nanocrystal.
A research team led by Assistant Professors Melbert Jeem and Professor Seiichi Watanabe of the College of Engineering at Hokkaido University synthesized a copper-doped tungstate-based material with full solar energy utilization. Their findings were recently published in the journal Advanced Materials.
Jeem explains: "Currently, the near- and mid-infrared spectrum of solar radiation (800nm to 2500nm) has not been used to generate electricity. Tungstic acid is a candidate for developing nanomaterials that might exploit this spectrum because it possesses a crystal structure with defects that absorb these wavelengths."
Summary of relative light absorption of tungstate crystals from UV to infrared light. 1, 5 and 10 are the copper concentrations that cause the nanocrystals to become photocritical. Image credit: MelbertJeem et al. Advanced materials. July 29, 2023
The scientists used a photofabrication technique they previously developed, underwater photosynthesis of microcrystals, to synthesize tungstate nanocrystals doped with varying concentrations of copper. The structure and light absorption properties of these nanocrystals were analyzed; their photothermal, photoassisted water evaporation and photoelectrochemical properties were measured.
Copper-doped tungsten oxide nanocrystals absorb the entire spectrum of light, from ultraviolet to visible light to infrared; when the copper doping amount is 1%, the infrared light absorption is the largest. 1% and 5% copper-doped nanocrystals show the highest temperature rise (photothermal properties); 1% copper-doped crystals also show the largest water evaporation efficiency, which is approximately 1.0 kilograms per square meter per hour. Structural analysis of 1% copper-doped nanocrystals revealed that copper ions may distort the crystal structure of tungsten oxide, causing the properties observed when light is absorbed.
"Our discovery marks a major advance in the design of nanocrystals capable of synthesizing and harnessing full solar energy," Watanabe concluded. "We have demonstrated that copper doping imparts multiple properties to tungstate nanocrystals through full solar energy utilization. This provides a framework for further research in this area and the development of applications."