### Fizzy Graphene

September 25, 2017

One of the few attractions of Upstate New York is the accessibility of venues for outdoor sporting activities. It's a great place to be if you're into hunting, fishing, skiing, hiking, and camping, but one major concern is the number of snowmobile accidents and fatalities.[1] My family lived in Upstate New York when I was a child, and we went on camping excursions summers to some of New York's many state parks.

Walden Pond (left) was made famous by Henry David Thoreau (1817-1862) in his book, Walden; or, Life in the Woods.

Thoreau lived at Walden for two years to experience the spiritual benefits of a simplified lifestyle. This lifestyle, however, would not have been possible without the charity of his friends and family.

(Walden Pond in November, 2009, photo by John Phelan, via Wikimedia Commons

While the swimming was fun, I was further entertained by hanging-out with the other teenagers who were there with their families. These came from all over the state, other states, and Canada, so their customs and particular manner of speech were a little weird. In my city, carbonated beverages were called "soda," while teenagers from the western part of the state called them "pop." This observation of mine is verified in a study by Matthew Campbell and Greg Plumb of East Central University, Oklahoma, who created a map of this near-dichotomy for the entire US.[2]

Whether called "soda," or "pop," such soft drinks are flavored and sweetened carbonated water, which is water infused with carbon dioxide gas. If water doesn't react with the gas, the absorption of a gas in water is described by Henry's Law,
In this equation, the Henry's law constant, kH is equal to the ratio of the concentration of of the gas in water ca to its partial pressure in equilibrium above the water pg; that is, kH = ca/pg. The ΔHsoln is the enthalpy of solution, R is the gas constant, and kH298 is the value at standard temperature, which is actually 298.15°C.

In order to maximize the amount of carbon dioxide dissolved into carbonated water, the process is done at a temperature just above the freezing point of water. When your bottles of soda (or pop) is opened at a higher temperature, some of the gas begins to escape, as can be seen by the bubbly effervescence. While a mild reaction of carbon dioxide with water does produce a small amount of carbonic acid, H2CO3, to produce a slightly sour taste, the main effect of the carbonation is the transport of aroma molecules up your nose, and a tingle on your tongue, a sensation in the category of mouthfeel.

Solubility of carbon dioxide in water.

Not surprisingly, because of Henry's Law, greater partial pressure and lower temperature leads to a higher solubility.

(Graphed using Gnumeric from data in ref. 3. Click for larger image.)[3]

Materials scientists from the Department of Materials Science and Engineering and the Frederick Seitz Materials Research Laboratory of the University of Illinois at Urbana-Champaign have found a method for production of large area graphene sheets using carbonated water at a critical step in the process.[4-5] This blog has a few articles about graphene. Here are the ones from just this year:
• Graphene from Ethylene, June 5, 2017
• Soybean Graphene, March 23, 2017
• Graphene Friction, February 9, 2017
• Graphene Putty, January 16, 2017

Graphene is usually synthesized by chemical vapor deposition onto a copper substrate that acts as a catalyst.[5] A principal problem is the release and transfer of the graphene from the copper to an insulating substrate. That's the critical step addressed by SungWoo Nam, an assistant professor of mechanical science and engineering at Illinois, and his research team, who developed a cleaner and more environmentally friendly method for this release using carbonated water.[5] Graphene transfer allows the reuse of the copper catalyst substrate.[4]

Although they're not scientifically accurate, everyone likes an artist's conception of an idea.

The active phrase here is "de gustibus non disputandum est"(About taste, there can be no question).

(University of Illinois image.)

The carbonic acid in the carbonated water allows an electrochemical reduction of the cuprous oxide (Cu2O) interlayer between the copper catalyst substrate and the graphene, and the subsequent delamination of the graphene.[4-5] Previous methods used less environmentally-friendly salt/alkali-based electrolytes.[4] An important next step is the use of food-grade ethyl cellulose as a transfer layer. Ethyl cellulose is an inexpensive and environmentally-friendly alternative to the typically used polycarbonate or PMMA (poly methyl methacrylate), both of which need toxic solvents for their removal.[4-5]

Says team member, Michael Cai Wang, a PhD student and lead researcher on the project,
"In our case, we are using a bio-mass derived polymer, ethyl cellulose, for the coating... A common and inexpensive polymer often used as a food additive, ethyl cellulose is solvated in just ethanol. 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... After you transfer the graphene, the carbonic acid simply evaporates away as carbon dioxide and water, which doesn’t require any further rinsing."[5]

While graphene has yet to leave the laboratory and become a high-volume electronic material, it's always good to look ahead to the time when the health of workers in a manufacturing environment needs to be protected. Since electronic materials are greatly affected by chemical impurities, this carbon dioxide process is a means to produce extremely pure materials. Carbon dioxide processing might also be a viable alternative for other processes that require delamination.[5]

Funding for this research was provided by the National Science Foundation, the Air Force Office for Scientific Research, NASA's Space Technology Research Grants Program, and the Natural Sciences and Engineering Research Council of Canada.[5]

Schematic diagram of the graphene process, with the end result depicted in the insert.

Cuprous oxide has a large exothermic free energy of formation, so an oxide layer is naturally present at a copper surface.

(University of Illinois image.)

### References:

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