Milling rice to separate the grain from the husk produces around 100 million tons of rice husk waste per year worldwide.
Scientists from Hiroshima University’s Center for Basic Research and Development of Natural Sciences, in search of a scalable method of manufacturing quantum dots, have developed a way to recycle rice husks to create the first silicon quantum dot (QD) LED light. His new method transforms agricultural waste into state-of-the-art light-emitting diodes in a cost-effective and environmentally friendly way.
Since typical QDs often include toxic materials, such as cadmium, lead, or other heavy metals, environmental concerns have often been discussed when using nanomaterials. Our proposed QD manufacturing process and method minimizes these concerns.
Ken-ichi Saitow, lead author of the study and professor of chemistry at Hiroshima University.
Since the discovery of porous silicon (Si) in the 1950s, scientists have explored its applications in lithium-ion batteries, luminescent materials, biomedical sensors and drug delivery systems. Si, which is non-toxic and abundant in nature, has photoluminescent properties, derived from its microscopic (quantum-sized) dot structures that serve as semiconductors.
Recognizing the environmental concerns surrounding quantum dots today, researchers set out to find a new method of making quantum dots that would have a positive environmental impact. It turns out that rice husk waste is an excellent source of high-purity silica (SiO2) and value-added Si powder.
The team used a combination of grinding, heat treatments and chemical etching to process the rice husk silica:
- First, they crushed rice husks and extracted silica powder (SiO2) by burning the organic compounds in the crushed rice husks.
- Second, they heated the resulting silica powder in an electric furnace to obtain Si powders through a reduction reaction.
- Third, the product was purified Si powder which was then reduced to 3 nanometers by etching.
- Finally, its surface has been chemically functionalized to achieve high chemical stability and high dispersivity in solvent, with 3 nm crystal particles to produce SiQDs that luminescent in the orange-red range with high luminescence efficiency over 20 %.
This is the first research to develop an LED from rice husk waste. The non-toxic quality of silicon makes it an attractive alternative to currently available semiconductor quantum dots.
The present method becomes a noble method for developing environment-friendly quantum dot LEDs from natural products.
Kenichi Saitow
The LEDs were assembled in a series of layers of material.
An indium tin oxide (ITO) glass substrate was the anode of the LED; it is a good conductor of electricity and transparent enough to emit light. Additional layers were applied to the ITO glass, including the layer of SiQD. The material was covered with an aluminum foil cathode.
The chemical synthesis method developed by the team made it possible to evaluate the optical and optoelectric properties of the SiQD light-emitting diode, including the structures, synthesis yields and properties of SiO2 and Si powders and SiQDs.
By synthesizing high-yield SiQDs from rich shells and dispersing them in organic solvents, it is possible that these processes can one day be applied on a large scale, like other high-yield chemical processes.
Kenichi Saitow
The team’s next steps include developing higher-efficiency luminescence in SiQDs and LEDs.
They will also explore the possibility of producing SiQD LEDs other than the red-orange color they just created.
For the future, the scientists suggest that the method they have developed could be applied to other plants, such as bamboo from sugar cane, wheat, barley or grasses, which contain SiO2. .
These natural products and their residues could have the potential to be transformed into non-toxic optoelectronic devices.
Ultimately, scientists would like to see the commercialization of this eco-friendly approach to creating luminescent devices from rice husk residue.
More information: pubs.acs.org (English text).
Going through www.hiroshima-u.ac.jp