Researchers print fully recyclable electronics that replace toxic chemicals with water

The team developed a cyclical process in which the device is rinsed with water, dried in relatively low heat, and printed on again.
Deena Theresa
Carbon nanotubes.
Carbon nanotubes.

Duke University  

In the electronics industry, placing several layers of components on top of each other to develop complex devices is no easy task. And with printed electronics, the task is more complicated.

"If you’re making a peanut butter and jelly sandwich, one layer on either slice of bread is easy," Aaron Franklin, the Addy Professor of Electrical and Computer Engineering at Duke, said in a statement. "But if you put the jelly down first and then try to spread peanut butter on top of it, forget it, the jelly won’t stay put and will intermix with the peanut butter. Putting layers on top of each other is not as easy as putting them down on their own — but that’s what you have to do if you want to build electronic devices with printing."

Franklin and his team previously demonstrated the first fully recyclable printed electronics. This time they've produced the same that replaces the use of chemicals with water in the fabrication process, in what can be called a first.

Recyclable printed electronics use three carbon-based inks: semiconducting carbon nanotubes, conductive graphene, and insulating nanocellulose.

Water-based approach to make environmentally-friendly electronics

To make a water-based ink in which the carbon nanotubes don't clump together, a surfactant similar to detergent is added. However, the output does not create a layer of carbon nanotubes dense enough for a high current of electrons to travel across.

"You want the carbon nanotubes to look like al dente spaghetti strewn down on a flat surface," said Franklin. "But with water-based ink, they look more like they’ve been taken one by one and tossed on a wall to check for doneness. If we were using chemicals, we could just print multiple passes again and again until there were enough nanotubes. But water doesn’t work that way. We could do it 100 times and there’d still be the same density as the first time," he added.

This happens because the surfactant prevents additional layers from adhering to the first. In traditional processes, the surfactants would be removed using high temperatures or harsh chemicals, which can pose human and environmental health risks.

Franklin and his group developed a cyclical process in which the device is rinsed with water, dried in relatively low heat, and printed on again. "When the amount of surfactant used in the ink is also tuned down, the researchers show that their inks and processes can create fully functional, fully recyclable, fully water-based transistors," the release said.

Franklin has already proven that nearly 100 percent of the carbon nanotubes and graphene used in printing can be recovered and reused in the same process. And nanocellulose can be recycled or biodegraded like paper. Though the process does use a lot of water, it’s not nearly as much as what is required to deal with the toxic chemicals used in traditional fabrication methods.

The current fabrication technology is high-energy and relies on hazardous chemicals and toxic gases. According to Franklin, his approach could be used in the manufacturing of other electronic components like the screens and displays that are now ubiquitous to society. He intends to "signal to the rest of the field that there is a viable path toward making some electronics manufacturing processes much more environmentally friendly".

The research appeared online on Feb. 28 in the journal Nano Letters.

Study Abstract:

Printing thin-film transistors (TFTs) using nanomaterials is a promising approach for future electronics. Yet, most inks rely on environmentally harmful solvents for solubilizing and postprint processing the nanomaterials. In this work, we demonstrate water-only TFTs printed from all-carbon inks of semiconducting carbon nanotubes (CNTs), conducting graphene, and insulating crystalline nanocellulose (CNC). While suspending these nanomaterials into aqueous inks is readily achieved, printing the inks into thin films of sufficient surface coverage and in multilayer stacks to form TFTs has proven elusive without high temperatures, hazardous chemicals, and/or lengthy postprocessing. Using aerosol jet printing, our approach involves a maximum temperature of 70 °C and no hazardous chemicals─all inks are aqueous and only water is used for processing. An intermittent rinsing technique was utilized to address the surface adhesion challenges that limit film density of printed aqueous CNTs. These findings provide promising steps toward an environmentally friendly realization of thin-film electronics.

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