American Scientist

What if you discovered an infinitesimally thin material capable of conducting electricity, able to suspend millions of times its own weight, and yet porous enough to filter the murkiest water? And what if this substance was created from the same element as that filling the common pencil?

A growing cadre of scientists aims to make this extraordinary material, graphene, a mainstay technological material by the second half of the 21st century. Not satisfied with that timeline, some entrepreneurial types would like to see widespread adoption of graphene within the next decade.

Graphene is elegant. It is created from a single element, carbon, formed by just one type of bond. Despite graphene’s apparent simplicity, isolating the material was elusive for chemists and physicists alike. Graphene excels at hiding in plain sight, and the techniques and instrumentation perfected in the last two decades have played a pivotal role in its discovery.

Carbon, the sole constituent of graphene, is all around us. The element is the fourth most common in the entire universe. Most people think of materials in terms of atoms and molecules, where molecules are made from defined types and numbers of atoms. With graphene, counting carbon atoms is inconsequential. Merely the way in which the constituent carbons are bound to one another is crucial, with this feature separating graphene from other wholly carbon materials such as diamonds and graphite. At the atomic level, the exclusively carbon graphene resembles a hexagonal “chicken wire” fence, with each carbon atom making up the point of a hexagon. The hexagonal distribution makes graphene’s properties possible, because the distribution allows the individual carbon atoms of graphene to lie flat.

This property of graphene cannot be overlooked. Graphene is a perfect anomaly in the world of chemistry—a flat, two-dimensional molecule, with a single sheet of graphene measuring only one atom thick. You might immediately question the structural integrity of graphene because of its delightfully simplistic construction, but the weaving of the carbon hexagons throughout the structure makes the atomically thin material unexpectedly strong.

You have experienced synthesizing graphene, maybe even earlier today, on a very small scale. The pressure exerted by your hand and fingertips likely created a few layers of graphene the last time you ran a pencil across a notepad, turning humble graphite into graphene as you wrote this week’s grocery list.

Scaling Up

After two researchers in Great Britain, Konstantin Novoselov and Andre Geim, were awarded the Nobel Prize in Physics in 2010, technology magazines everywhere heralded a new era of “wonder materials” based around this atomically thin tessellation of carbon atoms. With its incredibly high strength and almost impossibly low electrical resistance, graphene pulled back a hidden curtain, allowing scientists to catch a glimpse of the marvels that lay beyond.

Early investors were burned, however, by entrepreneurs who over-promised and underdelivered on performance aspects for products (especially composites such as plastics) that had graphene in them but that did not use graphene in a way that made its incorporation worth the added expense. It was, in some cases, just an added bit of snake oil. As the overall volume from new production methods and the quality of the resulting graphene have both increased over time, we are starting to finally see graphene’s true benefits.

If graphene is made from carbon and scientists have known how to isolate the material for more than a decade, why are there so few graphene products on the market?

The roadmap from a fundamental research laboratory to store shelf is never a direct path, although the time that passes between discovery and commercial application is shrinking rapidly.

The graphene flakes on silicon wafers are really just the first droplets in the bottom of a beaker when compared to the revolution that will occur once someone solves the riddle of how to make large-area pristine graphene sheets.

For the last decade or so, Additive Manufacturing (AM) has been all the rage. You might know AM by its more common name, 3D printing. Many early generation AM devices used only plastic, to make interesting 3D renditions of various objects, but the technology has grown significantly more capable.

Additively manufactured structural materials are an obvious place to begin adding graphene flakes. Researchers at the Massachusetts Institute of Technology, using a custom AM machine, printed various 3D objects from graphene and tested them to measure their physical properties compared with more conventionally produced parts. The results were astonishing. Some of the 3D-printed samples had 10 times the strength of steel at 1/20th the mass. They can now print parts and assemblies that may, in some cases, replace custom-manufactured steel parts for increased mechanical strength.

For graphene to make all the revolutionary changes that are predicted (and, in some cases, actually tested), there must be an automated manufacturing process to produce kilograms of graphene per day or tons of the material per year—not just a few grams here and there. Graphite is basically graphene layered upon itself, waiting for someone to separate it out. This is where it gets tricky, however.

First of all, we should probably rule out mass production of graphene using the method by which it was originally isolated. While it is amusing to imagine a cavernous room filled with people using adhesive tape to separate graphene sheets from piles of pencil lead, it is simply not practical. Perhaps someone can figure out how to automate this particular process, but, even then, it doesn’t appear likely to scale well to the mass production needed. In other words, don’t invest your retirement savings in adhesive tape futures!

Researchers at Rutgers University are making sheets of graphene out of ordinary graphite flakes and some sulfuric or nitric acid. The addition of the acid oxidizes the graphene sheets that make up the graphite, and forcing oxygen atoms between the sheets of graphene causes them to split apart, forming graphene oxide sheets suspended in acid and water. Next, the liquid is filtered out, leaving flakes of graphene oxide to clog up the filter. The sum of all the clogs across the filter eventually makes up a paperlike sheet of graphene oxide. This paperlike sheet can then be removed from the filter by dissolving the filter away using a solvent that doesn’t react with graphene oxide. The last step is to remove the oxygen, which is done by using hydrazine, leaving only a pure graphene coating.

This resulting material is called reduced graphene oxide, or RGO for short. In this instance, “reduced” refers to a chemical use of the word, where the oxidation state of each graphene carbon has been decreased through the removal of the oxygen by hydrazine. In this case, hydrazine is a reducing agent, which is oxidized by its reaction with the graphene oxide.

Methane, a carbon-rich gaseous compound with which we humans are very familiar, can be reacted with copper at high temperatures to produce graphene. Simply heat the copper to about 1,000 degrees Celsius and expose it to the methane gas. Layers of graphene will form on the copper’s surface from the plentiful carbon atoms in the methane gas, a process called chemical vapor deposition (CVD). There are two big problems with this method: It takes a long time to make even a little graphene, and the quality of the graphene produced is not very good.

David Boyd at the California Institute of Technology, along with his research collaborators, has found a way to improve on the CVD approach so that it will work with lower temperatures and produce a higher quality graphene. They, too, use copper and methane, but they add a bit of nitrogen to improve the layering of the graphene on the copper. In this method, energy still needs to be added, but not nearly as much. The reaction goes forward at a “mere” 420 degrees. Global industry has considerable experience with CVD, so it should be possible to eventually automate the process on a large scale; the goal is to produce centimeters or even meters of high-quality graphene at a time.

For the remarkable wonders of graphene to be realized, it must be produced in  massive amounts—cheaply.

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Are dangerous chemicals, complex machines, and multistep chemical reactions and processes too complex for your tastes? Then consider this approach, discovered at Kansas State University, where they produced graphene by creating an explosion. Have you ever built a spud gun? Basically, if you take a one- to two-meter-long PVC pipe, create a combustion chamber at one end using a spark plug and a quick-sealing endcap, stuff a potato in the other end, and fill the now sealed combustion chamber with a flammable vapor (hair spray is good), then you have a spud gun. Once the potato is in place, the chamber fueled with hair spray and then sealed, you can point the far end of the PVC pipe toward your target and discharge your battery to cause the spark plug to spark. The resulting small explosion creates a pressure wave that dislodges the potato from the end of the combustion chamber, moving it up the nozzle of the PVC pipe, and into the air—often launching it tens of meters into the distance. The physics of what happens in the combustion chamber is very similar to the method that scientists at Kansas State University used to create graphene, in what may become a scalable process that could be a step toward mass production.

Interestingly enough, graphene wasn’t what the scientists were trying to make. Instead, they were trying to make something called a carbon soot aerosol gel for use in insulation and water purification systems. These gels were suddenly forgotten when they realized that their soot wasn’t what they were looking for, but graphene. And not just a little bit of graphene. They claim that their process is the least expensive so far for potentially mass-producing graphene, and that it doesn’t require much input energy. Granted, nothing is ever that simple, but this approach sounds like a good one to pursue in conjunction with other methods.

Instead of PVC pipe, the scientists used a more robust chamber for their combustion event. They replaced the hair spray with acetylene or ethylene gas mixed with oxygen. They did use a spark plug to create the combustion, just as we did with our spud gun. The fuel, the acetylene or ethylene gas, was turned into graphene and some other carbon detritus.

Then there is the soybean oil method—as in, the same stuff you can use at home when you cook. A research team in Australia found a way to use everyday soybeans to produce single-layer graphene sheets on top of a nickel substrate—potentially making sheets with large areas all at one time. The process is a variation of the CVD process described previously, but with a significant difference: This one is done in ambient air (no specialized vacuum chambers, etc.) and the required energy is not as great as is for other CVD processes.

The secret is in the nickel foil catalyst used and in carefully controlling the temperature of the process to prevent, as much as possible, the formation of carbon dioxide. Voilà: In goes soybean oil—out comes graphene. It is worth mentioning that the team investigated other metal foils, including copper, and no others promoted the formation of graphene. Only nickel did.

When all else fails, why not just go home and use your blender to make the wonder material of the 21st century? That’s essentially what Jonathan Coleman of Trinity College, Dublin, did when he and his team put some graphite in a blender, added an over-the-counter dishwashing liquid, and hit the start button. With only a little more processing required to separate the newly formed graphene sheets, Coleman and his colleagues found that they could produce several hundred grams per hour using a fairly modest set of mixing equipment in a 10,000-liter vat. It isn’t yet clear, however, whether this method can provide high-quality graphene.

A search of the scientific literature reveals a myriad of techniques that can produce graphene of varying quality. What they have in common is complexity, energy, and the fact that they can only achieve the production of small quantities of graphene, which then needs to be separated out from the other reaction products. To date, there is no simple production technique that results in large quantities of high-quality graphene. For the truly remarkable wonders of graphene to be realized, it must be produced in massive amounts—cheaply.

Quality Control

Would you like to buy a 10 millimeter x 10 millimeter monolayer of graphene flakes on a silicon substrate? $146. How about a 60 millimeter x 40 millimeter piece of monolayer graphene on copper? $172. There are companies specializing in graphene that will sell individual users samples at very reasonable prices. In fact, for $124 and up they will sell you a small bit of graphene on your own custom substrate.

Making graphene, though, is not trivial. The best mass-market graphene comes from chemically exfoliated, natural, mined graphite, and companies that own interests in graphite mines are already establishing themselves as players in this graphene revolution, leveraging their preferential access to raw materials in order to increase share prices.

But without agreement in the market or regulation, how would buyers determine which so-called graphene product would be best for their needs?

The Center for Advanced 2D Materials (CA2DM) at the National University of Singapore has established seven different tests by which it measures graphitic materials to establish quality and identity. Unfortunately, only a few of these tests are within the reach of a typical company laboratory; the others require expensive equipment that needs to be run and maintained by specially trained technicians.

The three cheapest tests to perform determine the size of a particular flake, the degree of defects within a given sample, and the elemental makeup of a sample. The size of a flake is determined by an optical microscope, whereas a graphene/graphite sample on a backing surface is measured by a typical light microscope. A camera and computer are able to measure the rough dimensions of a graphene/graphite particle and report roughly how big the resulting flakes are.

Because graphene’s electronic properties are very sensitive to defects in the flakes, the degree of these defects is an important parameter to measure. This measurement is made with what is called Raman spectroscopy, which measures vibrational patterns in the sample. Oxidation of the carbon-carbon bonds in graphene by oxygen opens up graphene to environmental degradation, and the introduction of other atoms onto the graphene surface causes various properties to change dramatically. For example, adding even a single hydrogen atom to the graphene structure causes the graphene to become magnetic.

The defect measurements would be supported by elemental analysis, particularly the Carbon-Nitrogen-Hydrogen-Sulfur (CNHS) analysis. Mined graphite would contain residues of the formerly living matter from which it was created, and these elements would ultimately detract from the quality of the graphene through one mechanism or another. Unfortunately, CNHS analysis is a destructive technique. Part of the sample must be burned for the components to be analyzed. Although this would be useful for batch-to-batch control of relatively cheap industrially exfoliated graphite, it will not be acceptable for samples of graphene produced by other methods.

There are many ways to determine the number of layers in a given graphite flake. One such test, called atomic force microscopy (AFM), uses a hair-thin needle mounted on a small springboardlike lever to measure the atomic forces between the needle and a sample. A laser reflects off the top of the lever, which is able to measure the amount of deflection, up or down, that the needle experiences in its interaction with the surface. The readout gives the thickness measured, and because graphite flakes stack at a constant distance from one another, you can do the math to determine the number of layers. AFM is able to create an image from many scans, because it adds successive one-dimensional lines together to display a sample’s topography. In effect, it creates a height map of a surface.

It all might be made possible by the most abundant, most versatile, and most essential of all elements, carbon.

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Scanning electron microscopy and transmission electron microscopy are methods of looking at what a flake of graphene looks like, but on a much finer level than optical microscopy is capable of. These two analyses have a much higher magnification resolution and are therefore able to find rips, tears, and other punctures in a flake; such punctures may be naturally existing or may have formed during the graphene's isolation or handling. These two analyses combined with AFM would give the most complete 3D picture of a graphene/graphite sample overall.

The last major analysis performed by CA2DM is X-ray photoelectron spectroscopy (XPS). XPS determines the chemical makeup of a sample nondestructively, and so would give you all of the information that CNHS provides while still allowing you to recover your sample. In this technique, X-rays are fired at the graphene surface, and some of the X-rays are absorbed by electrons in the sample. The electrons are ejected from the sample with an energy characteristic of the element in the sample, which tells you what elements are present and in what amounts.

Means of Growth

Other than the Scotch tape method and chemical exfoliation, what could our options be for making graphene in large amounts? Is there any way that we might print or grow something into graphene? Mechanical exfoliation may be used to peel hunks of graphite from the surface of a larger graphite hunk, with successive peelings carried out to isolate a few monolayer sheets. This process has been dramatically improved over the years, and indeed, special tapes are now used, which can dissolve in water or other solvents more easily than can office tape. That makes depositing graphene flakes even easier than before.

The second method, chemical exfoliation, has a history going back to the late 1800s. As with the mechanical exfoliation process, researchers have added to the field by developing new exfoliation parameters. Generally they are less harsh on the graphite and so minimize damage to the graphene surfaces. Perhaps the method uses recyclable materials, which would be tremendously important for any company that wants to produce literally tons of graphene per year. Some of the improvements improve the yield of pristine monolayer flakes, which is the most important optimization of all.

Graphene can also be grown from silicon carbide to produce what is called epitaxial graphene.

Graphene layer growth from the decomposition of silicon carbide is now an extremely complicated process, in which the silicon is sublimed at high temperature but the atmosphere above the surface layer is variable. Tailoring the environment above the surface allows researchers to produce graphene at better efficiencies than with an open-air atmosphere. A 2009 Nature Materials editorial by Peter Sutter described an advance in epitaxial growth that involved removing air from above the silicon carbide surface and replacing it with an inert noble gas atmosphere. Since then, research has turned back toward reactive atmospheres.

In a twist, three groups from across Germany devised a method in which they glued a plastic made from many aromatic benzene hexagons onto a silicon carbide surface and found that this plastic actually drastically improved the size and quality of graphene monolayers produced from the silicon sublimation. This work was inspired by an earlier paper, which fused CVD with epitaxial growth to improve the graphene yield. It seems that somehow the combination of these two processes creates a product that is leagues better than either isolated method. If time shows that this combination turns out to be repeatable and economical, it could set the stage for graphene’s everyday importance to skyrocket. What’s more, it could even force out natural, mined graphite from high-tech graphene uses. That could spell disaster for graphite mining companies that are betting their futures on selling to graphene consumers. This will be a development to keep close tabs on.

Renewable Sources

Expensive, rare, or otherwise valuable starting materials will generate significant demand for those starting materials, which would limit graphene’s use in everyday materials. Therefore, it is absolutely imperative to find a way that graphene can be made reliably from a cheap (or free) resource. If graphene could be made from things that would otherwise go to waste, this would significantly decrease the long-term price of graphene so that anyone could have access to it.

If such a process were available, those who invented it would be regarded as highly as Fritz Haber, who won the Nobel Prize in Chemistry in 1918 “for the synthesis of ammonia from its elements.” Haber took nitrogen from the air and hydrogen from methane gas, combined them under high pressure and temperature over a metal catalyst to speed up the reaction, and boom! Ammonia came out of the reaction, ready to be put into fertilizer. Haber’s invention quite literally feeds the world.

What starting material could we use for carbon as a feedstock that would not unduly tax typical sources of carbon, such as fossil fuels or natural gas? Certainly, one option is to harvest carbon dioxide from the air and reduce it back to C. That is an extremely energy-intensive process, however, and no technological advances within the known laws of physics will reduce that energy demand.

If graphene could be made from things that would otherwise go to waste, this would significantly decrease the long-term price of graphene.

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That leads us back to thinking about something that is abundant, all around us, makes efficient use of capturing carbon, and can capture this carbon without direct energy input from humans: plants. Plants take in passive solar light and carbon dioxide from the atmosphere and grow in most places of their own accord. Huge trees are carbon sinks made possible by photosynthesis. Lots of plant waste is generated per year, which might go toward creating graphene if it would otherwise take up space within a landfill. Invasive species of plants, such as kudzu and bamboo in the southeastern United States, can serve as a feedstock.

James Tour took this to a logical extreme in 2011 on a bet. Tour had been thinking about the ways to use the carbon already free around us in the environment. He had been successful in converting Plexiglas (polymethylmethacrylate) to graphene, and table sugar was his next target. After having turned table sugar into pyrolysis-CVD graphene flakes on a piece of copper foil, one of his colleagues perked up, and dared Tour to make graphene out of six different carbon-based materials: cookies, chocolate, grass, polystyrene (Styrofoam), roaches, and dog feces. This result is interesting, as the Australian laboratory mentioned above failed when using a copper foil substrate for their soybean oil conversion process. What these conflicting stories mean, however, is that there is vast room for improvement in our understanding of the way graphene forms from gaseous molecules.

Using the same method employed with the table sugar, all of the proposed unusual carbon sources produced small flakes of high-quality graphene. Tour and his coworkers stressed that no preparation or purification of these weird materials was necessary. In other words, a roach leg could be dropped on the foil, heated up, and come out as graphene. You can’t even make a cake with that much ease. Tour’s 2011 finding, combined with the CVD-epitaxy findings from the German team in 2016, could provide a clear route to making large, cheap, defect-free graphene samples.

Outer Space

At the moment, NASA is researching ways to process waste carbon dioxide from astronauts’ breath on the International Space Station into graphene. This improvement to the life-support system would have a twofold bonus. For one, a waste material such as carbon dioxide otherwise requires sequestration with special chemicals that need to be shipped up with special deliveries from Earth. Processing the carbon dioxide into graphene would mean that fewer resupply missions would be necessary.

Turning carbon dioxide into graphene provides another benefit as well: The resulting graphene could be incorporated into new solar cells, or could be put to use in the water purification systems, or a thousand other possibilities, rather than trying to eject it out the airlock. This possibility helps to lengthen the umbilical cord between the station and Earth. Eventually we need to cut that umbilical entirely, if we are to ever send humans on extended missions to other planets and beyond.

Luckily, there is a side benefit for us Earthlings as well. A process like this would also be able to take carbon dioxide from the atmosphere and turn our own breath into organic electronics or a million other things in which uses for graphene could be found. Although turning carbon dioxide into graphene would not be cost-effective or energy efficient on Earth (right now), abundant power from solar cells aboard the International Space Station could provide the kick necessary to strip oxygen from the carbon dioxide. Companies could “mine” the atmosphere to take carbon dioxide from processes that can’t help but produce it, and turn the waste gas into a raw material for further products. The “waste not, want not” principle that every hiker and explorer knows well means that a system designed for reuse will ultimately increase the chances of a mission’s success (whether it be on Earth or in space), while also minimizing environmental impact. Redundancy on Earth can only be a good thing. In outer space, it is an absolute requirement.

Industry Standards

Graphene is composed of pure carbon as a single sheet in a flat hexagon pattern. Any changes to this structure mean that the resulting chemical is no longer technically graphene; instead it is a graphene derivative. Graphene behaves very differently from graphene oxide, and both behave differently from lithium-doped graphene.

Take, for instance, the difference between two samples of exfoliated graphite from two different companies. One sample could have been exfoliated by a process that is rather harsh, so that the exfoliation added defects of oxygen atoms or alcohol groups to the graphene flakes. The second sample could have been exfoliated more gently, in a way that preserves the carbon-only structure without adding holes or tears in the flakes. Which is better than the other? How can you tell them apart? Both manufacturers slapped “Graphene” on the bottle and sold it to you at an exorbitant price; they must be indistinguishable in a product formulation and therefore you can just go with the cheaper option, right? Not so. The source of the graphene and how it was prepared have tremendous implications for its performance. A device might not work at all, or it may just work worse than expected.

Standards do not exist yet for graphene production, and not all companies are on board with establishing standards at all. These standards could take many possible forms and do not necessarily mean legal regulation. That would be quite obviously an extreme measure and would be unenforceable in other countries. Considering the international playing field for graphene, this would be a significant hindrance. Nobody wants that. However, at this point in the game, most products labeled “graphene” on the market are not actually graphene. Rather, they are thin flakes of graphite that can be up to a few hundred layers thick. Some manufacturers are able to produce flakes with a high yield of monolayer graphene, and these companies will gladly tell you that they produce a guaranteed percentage of monolayer graphene, with most of the rest of the sample consisting of flake aggregates between two and ten layers thick. A word to those of you who are interested in using true graphene for an application—ask about these flake thicknesses from your supplier. It is absolutely critical to take what they say to an independent lab for verification to establish a definitive level of trust.

Ideally, standards set forth should grade graphene, taking into account parameters such as the yield of monolayer flakes, the size of those flakes, and the elemental analysis of the sample (at a minimum). That way, a vendor can stand behind the production cost of their so-called graphene sample, rather than jacking up the cost for some graphite that has been pulverized in a kitchen blender. Caveat emptor. On the other hand, if a vendor is selling high-surface-area epitaxially grown graphene with a repeatable or verifiable certificate of analysis, then you may have a justification to pay more for that sample.

Carbon Everywhere

Graphene’s potential to change the course of innumerable industries is only limited by the imagination and cunning of business leaders who share a vision with a knowledgeable chemist, engineer, or physicist. Bolder, more enterprising technologies will develop by adding different molecules to graphene, treating it as a scaffold onto which biomolecules can be grafted, perhaps as passive sensors for chemical and biological weapons.

Graphene as a coating material could even change industries in the short term. Because graphene is mostly nonreactive and very hydrophobic, any surface coated in a layer of graphene would move through water with decreased friction from water-metal surface tension. A graphene layer on tanker ships would make worldwide shipping more effective. Adding a graphene layer onto a windshield would create a surface that was not only transparent (because graphene itself is transparent) but would naturally repel water and increase driver safety in rainstorms. Want to reduce air drag on a high-performance car? Ensure that its shell is perfectly atomically flat by encasing it in graphene. Maybe an especially talented engineer in the future will design a vehicle with perfectly smooth and regular flow over the car’s body, eking out a few more horsepower from the engine and a few more miles per gallon from the tank.

And all these things might be made possible by one of the most abundant, most versatile, and most essential of all elements, carbon—the same carbon that forms the basis of all known forms of life on Earth and enables graphene to be formed: graphene—the superstrong, superthin, and superversatile material that will revolutionize the world.

This article is excerpted and adapted from Graphene: The Superstrong, Superthin, and Superversatile Material that will Revolutionize the World (Prometheus Books, 2018). Reprinted with permission from the publisher.