Using carbonated water, graphene synthesized by chemical vapor deposition is easily removed from a metal substrate by under-etching delamination, allowing the substrate to be reused multiple times. Image: University of Illinois Department of Mechanical Science and Engineering.Researchers at the University of Illinois at Urbana-Champaign have discovered a new use for carbonated water in the manufacture of one of the world's thinnest, flattest and strongest materials – graphene.
As the uses for graphene grow, the speed and quality with which it can be manufactured will be paramount. With that in mind, the research group of SungWoo Nam, assistant professor of mechanical science and engineering at Illinois, has developed a cleaner and more environmentally friendly method to isolate graphene using carbon dioxide (CO2) in the form of carbonic acid. Their findings are published in a paper in the Journal of Materials Chemistry C.
Nam, an expert in the field of two-dimensional materials, is especially interested in using graphene to produce sensors or flexible devices – for instance, a wearable patch that, when placed directly on skin, is so thin and transparent that it isn't noticeable. Nam currently has projects with industry for making wearable graphene sensors.
Graphene is generally synthesized via chemical vapor deposition onto a metal substrate, typically copper foil. One particularly tricky aspect of this process is how to separate the atomically thin material from the metal substrate for integration into useful devices. At the moment, this typically involves either dissolving away the high-purity metal or delaminating the graphene from the substrate, both of which require the use of harsh chemicals that leave stubborn residues. The ultra-thin graphene also needs to be coated with a support layer of polycarbonate or PMMA (poly methyl methacrylate), which requires the use of often toxic and carcinogenic solvents.
"In our case, we are using a biomass derived polymer, ethyl cellulose, for the coating," explained Michael Cai Wang, Nam's PhD student and lead researcher on the project. "A common and inexpensive polymer often used as a food additive, ethyl cellulose is solvated in just ethanol.” They then use carbonic acid to delaminate the graphene from the substrate.
"This not only makes our graphene transfer process more environmentally friendly, it is now also compatible with a variety of polymeric and soft biological materials such as common plastics and hydrogels that would otherwise not tolerate harsh solvents," added Wang.
"After you transfer the graphene, the carbonic acid simply evaporates away as carbon dioxide and water, which doesn't require any further rinsing," Nam noted. "We're thus saving both water and time by eliminating the conventional need for the repetitive and tedious rinsing process. In using electrolytes such as sodium hydroxide or sodium chloride, for example, the sodium tends to remain on the graphene, which is very difficult to completely get rid of."
"By delaminating the graphene off from the copper foil using carbonic acid, we are also able to reuse the growth substrate multiple times instead of expending it, realizing significant material and cost savings" Wang said.
"I think scientifically what we are bringing to the community is to really motivate people to think about a cleaner way for making graphene," Nam said. "We are trying to improve upon the well-established protocols so that industry can easily adopt our techniques. Because a lot of devices are contaminated by these previously used chemicals, it inevitably affects the property of graphene."
"Graphene is just starting to mature from the laboratory and into commercial applications," explained Wang. "Once you start large-scale manufacturing, workers' health is also a major consideration, another benefit of our greener process."
The group also hopes the scientific community might be inspired by this work to find novel ways to utilize CO2 for practical applications. They envision extending the useful lifecycle of carbon while diverting and mitigating its emission into the atmosphere. Nam believes this method will not only have an impact on the production of graphene, but also provide a green and affordable technique to use for etching and processing other materials as well.
"If you are interested in making the best transistor in the world, you have to have the cleanest, purest material that you can synthesize and transfer," he said. "Here we provide that opportunity to the community. In addition, a lot of people are trying to measure the intrinsic properties of other materials as well. Our approach will help them do that."
This story is adapted from material from the University of Illinois at Urbana-Champaign, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.