In A First, LED Lights Made Using Rice Husk. See Details

Researchers working on quantum dots have devised a method to recycle rice husks to create the first silicon quantum dot (QD) light-emitting diode (LED) lights.

Scientists at Hiroshima University have developed world's first LED light using rice husk | Credit: ACS Chemistry and Engineering/Hiroshima University

New Delhi: Every year, milling of rice to separate the grain from the husks produces about 100 million tonnes of rice husk waste. Researchers working on quantum dots have devised a method to recycle rice husks to create the first silicon quantum dot (QD) light-emitting diode (LED) lights. The research marks the first time LED lights have been made using rice husks.

What Are Quantum Dots?

Quantum dots are man-made nanocrystals which can transport electrons. When quantum dots are stimulated by an external source as ultraviolet light, they can emit light of various colours. Quantum dots have unique chemical and physical properties due to their size and highly compact structure. These artificial semiconductor nanoparticles have found applications in composites, solar cells, and fluorescent biological labels. 

The researchers' new method to develop light-emitting diode (LED) lights using rice husks transforms agricultural waste into state-of-the-art light-emitting diodes in a low-cost, environmentally friendly way. 

The study, led by scientists from the Natural Science Center for Basic Research and Development, Hiroshima University, Japan, was recently published in the American Chemical Society journal ACS Sustainable Chemistry & Engineering.

In a statement issued by Hiroshima University, Ken-ichi Saitow, the lead author of the study, said that environmental concerns have been frequently deliberated when using nanomaterials. This is because typical quantum dots often involve toxic material such as cadmium, lead, or other heavy metals. 

The proposed process and fabrication method for quantum dots minimise these concerns, according to the researchers.

Porous Silicon (Si) was discovered in the 1950s. Since then, researchers have explored the uses of porous silicon in lithium-ion batteries, luminescent materials, biomedical sensors, and drug delivery systems. Silicon is non-toxic and found abundantly in nature, and has photoluminescence properties. This is because Silicon has microscopic or quantum-sized dot structures which serve as semiconductors.

Rice Husks Are An Excellent Source Of High-Purity Silica

Scientists are aware of the environmental concerns surrounding the current quantum dots, because of which they decided to search for a new method for fabricating quantum dots which has a positive effect on the environment. The team found that waste rice husks are an excellent source of high-purity silica or SiO2, and value-added Silicon powder.

Process Of Developing LEDs From Rice Husk

In order to process the rice husk silica, the researchers used a combination of milling, heat treatments, and chemical etching. 

First, they milled rice husks. Then, they burned off organic compounds of milled rice husks to extract silica. 

After this, the researchers heated the resulting silica powder in an electric furnace to obtain silicon powders through a reduction reaction.

Then, a purified Silicon powder was obtained. This was further reduced to three-nanometre in size by chemical etching. 

Finally, the surface of the powder was made functional for high chemical stability and high dispersivity in solvent, using three-nanometre crystalline particles. These produced the Silicon Quantum Dots which luminesce in the orange-red range with high luminescence efficiency of over 20 per cent.

Saitow said 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.

He further said that the present method becomes a noble one for developing environmentally friendly quantum dot LEDs from natural products. 

According to the study, the LEDs were assembled as a series of material layers. For instance, an indium-tin-oxide (ITO) glass substrate was the LED anode, which is a good conductor of electricity while sufficiently transparent for light emission.

The scientists spin-coated additional layers onto the ITO glass. These included the layer of Silicon Quantum Dots. They capped the material with an aluminium film cathode. 

The researchers also developed a chemical synthesis method which allowed them to evaluate the optical and optoelectrical properties of the Silicon Quantum Dots LED. 

Saitow said that by synthesising high-yield Silicon Quantum Dots from rich husks and dispersing them in organic solvents, it is possible that one day, these processes could be implemented on a larger scale, like high-yield chemical processes.

What Do The Scientists Aim To Do Next?

The researchers' next steps include developing higher efficiency luminescence in the Silicon Quantum Dots and the LEDs, Nx exploring the possibility of producing Silicon Quantum Dots LEDs other than the orange-red colour they have just created.

The researchers suggest that the method they have developed could be applied to other plants, such as sugarcane, bamboo, wheat, barley, or grasses which contain silica.

The natural products and their wastes may hold the potential for being transformed into non-toxic optoelectronic devices. 

The scientists hope for the commercialisation of the eco-friendly approach to create luminescent devices from rice husk waste.