Tikalon Blog by Dev Gualtieri

Graphene from Ethylene

June 5, 2017

As a young scientist in elementary school, most of my scientific knowledge came from library books, the newspaper, and such magazines as Life and Popular Science. It was in Popular science that I read how some fruits could be ripened by exposure to ethylene gas. This ripening process works only on climacteric fruits, such as tomato, apple, melon and banana, and it doesn't work on non-climacteric fruits such as citrus, grapes, and strawberries.

While some might consider this process to be another insidious way for corporate agriculture to fool both us and Mother Nature, we would not have such a variety of fruits available to use without it. Using this process, fruits can be harvested before ripening, shipped long distances, and appear at our local supermarket in a form that we enjoy eating. This is most apparent for bananas, which need to travel extreme distances to reach Tikalon's Northern New Jersey home, so they're picked while still green and exposed to ethylene somewhere in transit.

Ethylene.

Elthylene, which is also called ethene, is the smallest alkene molecule.

(Modified Wikimedia Commons image by Origami-Kranich.)

As you likely guessed, ethylene is the reagent for making polyethylene. More than half of ethylene production is used to make polyethylene, the most widely used polymer. High-density polyethylene (HDPE), a material used in many of my grandchildren's toys, has a high ratio of strength to density. This arises from the low degree of branching in the polyethylene polymer that results in larger intermolecular forces between the polymer chains. HDPE is commonly used in food storage containers, plastic bottles, and Tyvek barrier films used in construction.

Graphene, a material formed as a monolayer of carbon atoms, has been a popular nanoscale material to research in the last few decades. Its potential as a useful electronic and structural material is underscored by the short span of time between its discovery and the award of a Nobel Prize to its discoverers, Andre Geim and Konstantin Novoselov. These physicists first published a paper on graphene in 2004, and they received the 2010 Nobel Prize in Physics.

I've written quite a few articles on this wonder material, the most recent of which can be viewed here (Soybean Graphene, March 23, 2017). That article discussed a novel technique for graphene synthesis developed by Australian scientists from CSIRO Manufacturing (Lindfield, New South Wales, Australia), the University of Sydney (Sydney, Australia), the University of Technology (Sydney, Australia), and Queensland University of Technology (Brisbane, Australia). Their process used soybean oil as a precursor to graphene.

While graphene is typically grown using chemical vapor deposition techniques, an international research team with members from the Technische Universität München (Garching, Germany), the University of St. Andrews (St. Andrews, United Kingdom), and the Georgia Institute of Technology (Atlanta, Georgia) has examined production of graphene by assembly of adsorbed molecules of ethylene on the (111) crystal facet of rhodium. They report their results in an open access article in the Journal of Physical Chemistry C.[1-3]

This graphene synthesis technique is inspired by the common process of coking in which organic compounds will transform to carbon when adsorbed onto metal surfaces.[1] While coking is important in steelmaking, it's also a nuisance in catalysis, since it poisons the catalytic surface.[1] The present process utilizes this coking process on a catalyst in which heated one-dimensional polyaromatic hydrocarbons (1D-PAH) are converted into two-dimensional molecules. Surface diffusion allows coalescence of these molecules into graphene.[1]

Reactions leading to the formation of graphene from ethylene

Surface reactions leading to graphene on the (111) crystal face of rhodium. The reactions start with ethylene, which is converted at various temperature stages, as shown. Some early steps up to 520 K are omitted for clarity. (Georgia Institute of Technology image by F. Esch, R. Schaub, and U. Landman.)

Some earlier efforts to produce graphene from simple hydrocarbon precursors created soot, as would be expected in a coking process, rather than graphene.[2] The trick was to heat the ethylene in stages, finally to a higher temperature than before. This resulted in pure layers of graphene on the rhodium surface.[2] As ethylene lost hydrogen atoms, the remaining carbon atoms self-assembled into the honeycomb bonding of graphene.[2]

On initial heating above room temperature, the ethylene links into one-dimensional chains of polyaromatic hydrocarbons. Additional heating caused these chains to crosslink into two-dimensional molecules that surface-diffuse and coalescence into high purity graphene.[2] As Uzi Landman, a professor of physics at the Georgia Institute of Technology who led the theoretical component of this research,

"The temperature must be raised within windows of temperature ranges to allow the requisite structures to form before the next stage of heating... If you stop at certain temperatures, you are likely to end up with coking."[2]

To understand the process, the research team used scanning-tunneling microscopy, high-resolution electron energy loss spectroscopy, and thermally programmed desorption to observe and characterize the surface components at each process step.[2] Dehydrogenation was an important step, but not all hydrogen is removed at once. Some of the hydrogen remains, and it aids the bond-breaking process that detaches the larger molecule precursors and allows them to become mobile.[2]

Graphene formation from ethylene on a rhodium (111) surface

Scanning-tunneling microscope images of graphene formation from ethylene (top), and atomic models of what's happening (bottom). (Image by R. Schaub, licensed under the Creative Commons Attribution 4.0 International license.)

While this graphene synthesis technique is simple and potentially a lower cost alternative to chemical vapor deposition, the problem remains that the graphene is attached to the rhodium substrate and must be removed.[2] This research was funded by the Air Force Office of Scientific Research and the Office of Basic Energy Sciences of the U.S. Department of Energy.[2]

Graphene from Ethylene

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