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  • Recycling: From Leftovers to LED Lights

Photonics Spectra
Dec 2015
Light-emitting diodes (LEDs), such as those found ubiquitously in holiday lights, television sets and other devices, now have been fabricated from food and soft-drink waste. Researchers at the University of Utah in Salt Lake City have found a method to turn food waste such as discarded pieces of tortilla into carbon dots (CDs), a type of quantum dot (QD), which are then turned into a material for use in LEDs. Their synthesis process could be an environmentally sustainable alternative to current methods of LED production.

A small LED fabricated by carbon dots.
A small LED fabricated by carbon dots. Photo courtesy of Prashant Sarswat.


Not only could the recycling method take advantage of the abundant, largely unused resource of food and beverage waste, it could also reduce the need to produce LEDs from elements that are costly and potentially toxic to the environment.

In a February 2014 report on food loss, the U.S. Department of Agriculture noted that in 2010, an estimated 31 percent of food produced was not available for human consumption at the retail and consumer levels.

“With food and beverage waste that are already there, our starting material is much less expensive. In fact, it’s essentially free,” said assistant professor Prashant Sarswat, who codeveloped the process with professor Michael Free.

LEDs can be produced from QDs, which are nanocrystals with luminescent properties. QD particles can be made with a variety of materials, some of which are expensive and may be harmful to the environment, including the compound cadmium selenide.

In recent years, scientists have focused on the use of CDs to create LEDs. The carbon particles — which can be derived from food and beverage waste — are less toxic, less expensive and can be used in a broader variety of applications than other types of QDs.

“QDs derived from food and beverage waste are not based on common toxic elements such as cadmium and selenium, which makes their processing and disposal more environmentally friendly than it is for most other QDs. In addition, the use of food and beverage waste as the starting material for QDs allows for reduced waste and cost to produce a useful material,” Free said.

The work is published in Physical Chemistry Chemical Physics (doi: 10.1039/c5cp04782j).

Using a solvothermal synthesis process, the scientists placed food and beverage waste into a solvent under pressure and high temperature until CDs were formed. Soft drinks and pieces of bread and tortilla were used in the experiment. Each type of waste was heated directly and indirectly for 30 to 90 minutes.

“Synthesizing and characterizing CDs derived from waste is a very challenging task. We essentially have to determine the size of dots, which are only 20 nanometers or smaller in diameter, so we have to run multiple tests to be sure CDs are present and to determine what optical properties they possess,” Sarswat said.

After finding traces of the CDs from the synthesis, the researchers illuminated the particles to monitor their formation and color. To determine the particles’ optical and material properties, they used techniques including Fourier transform IR spectroscopy, x-ray photoelectron spectroscopy and atomic force microscopy imaging.

They measured the size of the CDs, which correlates with the intensity of the particles’ color and brightness. Among their findings, they determined that sucrose and D-fructose dissolved in soft drinks were the most effective sources for producing the particles.

At the end of the process, the CDs were suspended in epoxy resins, heated and solidified for use in LEDs.

The researchers hope to continue studying the stability and performance of LEDs produced from food and beverage wastes.


GLOSSARY
quantum dots
Also known as QDs. Nanocrystals of semiconductor materials that fluoresce when excited by external light sources, primarily in narrow visible and near-infrared regions; they are commonly used as alternatives to organic dyes.  
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