Scientists from Hiroshima university created world’s first LED light by using rice husks
and chemically obtained products.
Milling rice to separate the grain from the husks produces about 100 million tons of rice
husk waste globally each year.
Scientists searching for a scalable method to fabricate quantum dots have developed a
way to recycle rice husks to create the first silicon quantum dot (QD) LED light.
Their new method transforms agricultural waste into state-of-the-art 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 the American Chemical Society journal 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.
The proposed process and fabrication method for QDs minimizes these concerns.
Since porous silicon (Si) was discovered in the 1950s, 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.
Aware of the environmental concerns surrounding the current quantum dots, the researchers
set out to find a new method for fabricating quantum dots that has a positive environmental
impact.
Waste rice husks are an excellent source of high-purity silica (SiO2) and value-added
Si powder.
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.
Second, they 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 3 nanometer in
size by chemical etching.
Finally, its surface was chemically functionalized for high chemical stability and high dispersivity
in solvent, with 3 nm crystalline particles to produce the SiQDs that luminesce in the
orange-red range with high luminescence efficiency of over 20%.
The researcher says that this is the first research to develop an LED from waste rice
husks, adding that the non-toxic quality of silicon makes them an attractive alternative
to current semiconducting quantum dots available today.
The LEDs were assembled as a series of material layers.
An indium-tin-oxide (ITO) glass substrate was the LED anode; it is 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 aluminum film cathode.
The chemical synthesis method the team developed has allowed them 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.
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
color they have just created.
Looking ahead, the scientists suggest that the method they have developed could be applied
to other plants, such as sugar cane bamboo, wheat, barley, or grasses, that contain SiO2.
These natural products and their wastes might hold the potential for being transformed into
non-toxic optoelectronic devices.
Ultimately, the scientists would like to see commercialization of this eco-friendly approach
to creating luminescent devices from rice husk waste.