What’s stronger than steel, tougher than a diamond and more conductive than copper? It’s graphene.
This one atom thick tissue of carbon has been hailed as a wonder-material since being discovered in 2004. But wait, there’s more. A recent paper in Science has revealed graphene is twice as bullet-proof as Kevlar. And a few days earlier, a paper in Nature showed graphene allows protons – and only protons – through its mesh. This ability might be used to draw hydrogen, a potential fuel, right out of the air, say the Nobel prize winning scientists who discovered it.
Graphene is the crowning achievement of modern day alchemists who’ve spent decades bending and twisting carbon to create weird and wonderful new forms. This most commonplace of elements is a key player in the chemistry of life, and is also proving to be a highly versatile performer in other arenas.
Thirty years ago textbooks listed four varieties of carbon: diamond (the hardest material then known), graphite (better known as pencil lead), amorphous carbon (or soot), and carbon fibre (hard rods of pure carbon that can be used as fillers to increase the strength of plastics).
Then in 1985, English chemist Harry Kroto was inspired by the stars. Examining their spectral signatures, he noticed some carried an unusual form of carbon which he guessed might have the structure of long rods. To test the idea he collaborated with American researchers who reproduced stellar conditions on Earth using a furnace that created a gas of pure carbon at high temperatures. As the carbon cloud cooled it condensed to form a variety of molecules. One of them, made of exactly 60 carbon atoms, turned out to be a sphere of pentagonal and hexagonal panels, like a soccer ball. It also proved to be extremely stable. Kroto and his colleagues named the molecule buckminsterfullerene, in honour of Buckminster ("Bucky") Fuller, the American architect and engineer who used a similar highly stable structure in his visionary designs. Explorations of the new “buckyball” and related structures (known collectively as fullerenes) became the newest buzz in materials chemistry.
By the early 1990s Japanese scientist Sumio Iijima had hopped on the bandwagon. In 1991 he was synthesising fullerenes by sparking an electric current across two carbon electrodes, and found a hard deposit growing on their sides. Investigating further he found long, thin tubes of carbon. Carbon nanotubes turned out to be the strongest materials then known. Calculations showed that, if they could be scaled up and bundled together, they would be strong enough to fulfil futurist author Arthur C. Clark’s dream of a “space-elevator”– a cord thousands of kilometres long that could tether an orbiting satellite to the Earth.
It was later discovered that carbon nanotubes had already earned a place in history. A 2006 paper in Nature reported that carbon nanotubes had been found in the steel of a Damascan sword forged in 17th century Syria, which may explain the legendary reputation these blades attained during the Crusades.
Graphene’s super-strength comes from the chicken-wired carbon atoms in two dimensions – they are bonded even more strongly than in a diamond.
But why are nanotubes so strong?
Scientists guessed the secret lay in the way the carbon atoms bond to each other in a hexagonal chickenwire-like structure. The same structure exists in graphite, which led scientists to ramp up their efforts to isolate the thinnest possible layers of graphite and study its properties. While they wondered how thin they could go, nobody believed it would be possible to achieve a single atomic layer.
Except for Andre Geim, a Russian émigré physicist at the University of Manchester. Geim had a reputation for thinking outside the box. He had won an igNobel Prize in physics for levitating a frog in a magnetic field, and had also once co-authored a paper with his pet hamster. Geim describes his research strategy as the “Lego doctrine” – do something new using whatever equipment is at hand. In 2004 Geim and fellow émigré Kostya Novoselov came up with a new way to shave layers off a block of graphite using sticky tape. After repeatedly shaving the layer using strips of fresh tape, the pair were astonished to find their tape collected a single layer – graphene. For that discovery, they were awarded a Nobel in physics in 2010.
Graphene’s super-strength comes from the chicken-wired carbon atoms in two dimensions – they are bonded even more strongly than in a diamond. Previous strength tests, performed by poking it with a sharp diamond tip, showed graphene to be the strongest material on the planet. It’s so strong that a one square metre hammock (if you could make a graphene sheet that big) would weigh less than a cat’s whisker, but would hold the weight of the cat.
Jae-Hwang Lee at Rice University in Texas decided to test graphene’s mettle against a speeding bullet. Lee and his colleagues fixed multi-layer graphene sheets 10 to 100 nanometers thick across a tiny metal frame to make a membrane, a bit like a micro-scale drum-skin, then fired micro-scale silica bullets into it. The team used high speed cameras to measure the bullets’ speed before and after hitting the graphene, allowing them to calculate how much speed (and energy) was lost in the process – a lot, it turns out.
When struck by a projectile travelling at 600 metres per second, Kevlar – which is the gold standard for ballistic armour – can absorb 0.4 megajoules of energy per kilgram of material. Graphene could absorb 0.92 megajoules per kilogram.
To stop the projectile completely, Lee calculated, you’d need a thickness of only 500 nanometres – that’s one hundred times thinner than a human hair.
Graphene does the trick because a bullet’s force rapidly dissipates, rippling through the membrane at speeds of 22 kilometres per second. It’s like dropping a bowling ball on a trampoline that stretches and absorbs the impact, rather than dropping it on concrete that cracks.
And when graphene does eventually crack, the fault lines are predictable. Lee expects that by reinforcing these lines with a polymer layer he could make a composite armour even better than graphene. “I am expecting that commercial graphene armour vests will be possible within a decade,” says Lee.
Scientists also believed graphene would present a barrier to sub-atomic particles. Despite its gaping chicken wire structure, the electron cloud between the carbon atoms was deemed impenetrable. But a team led by Geim has shown that protons can squeeze through.
The team placed a single sheet of graphene between two proton-conducting materials. In theory when they turned up the voltage, no current should have flowed. But current did flow.
The explanation? Despite the electron cloud barrier, “there are, however, areas where [it] is very, very thin”, says Marcelo Lozada-Hidalgo, first author of the paper. Those thinned areas allow protons to squeeze through like minnows through a fishing net but only as long as the graphene net is only one atom thick. Adding even one extra layer of graphene stops the protons squeezing through completely.
Fuel cells unlock the chemical energy in a hydrogen atom by splitting it into electrons and protons.
It’s a “surprising and interesting result”, says Zhe Liu, a materials scientist at Monash University. “In the past, it was believed that graphene was completely impermeable.” For this reason graphene was seen as a good building block for filtration membranes – researchers could drill holes in a single sheet to fit particular atoms or molecules, and be confident nothing else would cross. Finding that intrinsic gaps in graphene will allow protons through, but not hydrogen atoms, could be useful for hydrogen fuel cells, says Liu.
Fuel cells are being developed to power hydrogen cars or life support systems for space travel. They unlock the energy in hydrogen without burning it, by splitting the hydrogen atom into electrons and protons. But in order to produce useful electricity before the particles recombine, the particles must be well segregated: electrons have to go one way round a circuit, and the protons another. One of the leading designs relies on a membrane at the gateway into the circuit – a material that acts like a nightclub bouncer, only letting the VIPs (Very Important Protons) through.
The problem is that the best membrane in commercial use today, DuPont's Nafion polymer, sometimes allows hydrogen molecules through, which wastes fuel. And at tens of microns thick it’s also so bulky that it slows down the flow of protons, reducing power. Employing a single graphene sheet as asleek, highly discerning new “bouncer” could solve both problems at once.
Geim goes a step further suggesting graphene could be used to generate hydrogen fuel directly from the air. Currently hydrogen has to be generated from methane l or splitting water with electricity. Neither are ideal. The first releases carbon dioxide; the seconduses significantly more energy than it produces in the form of hydrogen fuel.
Graphene might provide an alternative, because free hydrogen is floating all around us at about 0.5 parts per million in air. That may seem low, but it still adds up to more than 2 trillion tonnes of free fuel around the globe.
Geim proposes using a catalyst to split the hydrogen into protons and electrons. The protons could then be filtered through the graphene membrane and recombined with electrons on the other side – thereby creating a stream of ultra-pure hydrogen from the air. “It’s speculation,” Geim admitted to Nature, “but before this paper, it would be science fiction.” His team has already used a similar approach to sift hydrogen from water.
Graphene has been wowing us with its marvels for over a decade, and some are slowly trickling in to high-end sports equipment such as skis, bike helmets and tennis rackets. Novak Djokovic and Maria Sharapova, for instance, use HEAD graphene rackets.
While graphene’s not yet used in consumer electronics there have already been more than 7,000 graphene patents filed worldwide, including hundreds by technology giants such as Samsung, Apple and Sony. And where patents lie, products will no doubt soon follow. Liu for one has no doubt, “Graphene will have significant impacts on our daily life in the near future.”
Cathal O'Connell is a science writer based in Melbourne.
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