Their new method transforms agricultural waste - 100 million tons of rice husk waste is produced per year - into light-emitting diodes in a low-cost, environmentally friendly way.
The research team from the Natural Science Center for Basic Research and Development, Hiroshima University, published their findings in ACS Sustainable Chemistry & Engineering.
“Since typical QDs often involve toxic material, such as cadmium, lead, or other heavy metals, environmental concerns have been frequently deliberated when using nanomaterials,” said Ken-ichi Saitow, lead author and a professor of chemistry at Hiroshima University. “Our proposed process and fabrication method for QDs minimises these concerns,”.
Since porous silicon (Si) was discovered, scientists have explored its uses in applications in lithium-ion batteries, luminescent materials, biomedical sensors, and drug delivery systems. Non-toxic and found abundantly in nature, Si has photoluminescence properties, stemming from its microscopic (quantum-sized) dot structures that serve as semiconductors.
The researchers embarked on finding a new method for fabricating quantum dots that has a positive environmental impact and looked to waste rice husks as they are a good source of high-purity silica (SiO2) and value-added Si powder.
According to Hiroshima University, the team used a combination of milling, heat treatments, and chemical etching to process the rice husk silica. First, they milled rice husks and extracted silica (SiO2) powders by burning off organic compounds of milled rice husks. They then heated the resulting silica powder in an electric furnace to obtain Si powders via a reduction reaction. Third, the product was a purified Si powder that was further reduced to 3nm in size by chemical etching. Finally, its surface was chemically functionalised for high chemical stability and high dispersivity in solvent, with 3nm crystalline particles to produce the SiQDs that luminesce in the orange-red range with high luminescence efficiency of over 20 per cent.
“The present method becomes a noble method for developing environmentally friendly quantum dot LEDs from natural products,” Saitow said in a statement.
The LEDs were assembled as a series of material layers. An indium-tin-oxide (ITO) glass substrate was the LED anode; serving as a good conductor of electricity while sufficiently transparent for light emission. Additional layers were spin-coated onto the ITO glass, including the layer of SiQDs. The material was capped with an aluminium film cathode.
The chemical synthesis method has allowed the team to evaluate the optical and optoelectrical properties of the SiQD light-emitting diode, including the structures, synthesis yields, and properties of the SiO2 and Si powders and SiQDs.
“By synthesising high-yield SiQDs from rich husks and dispersing them in organic solvents, it is possible that one day these processes could be implemented on a large scale, like other high-yield chemical processes,” Saitow said.
The team’s next steps include developing higher efficiency luminescence in the SiQDs and the LEDs. They will also explore the possibility of producing SiQD LEDs other than the orange-red colour they have just created. The scientists suggest their method could be applied to other plants, such as sugar cane bamboo, wheat, barley, or grasses, that contain SiO2.