Graphene: What Is It, and What Is It Used For?

One evening in late 2004, two researchers at the University of Manchester were holding one of their regular “Friday Night Experiments” sessions in the lab. On these nights, they would play around with ideas that fell outside their regular research scope.

Little did Profs. Andre Geim and Konstantin Novoselov know that the wacky experiment they would perform that night – using sticky tape to peel thin flakes away from a graphite surface – would lead to one of the most highly-cited scientific papers of all time, kick-starting a brand-new era for materials science.

What is graphene?

Graphene is a one-atom-thick monolayer of carbon atoms, all tightly bound to each other in a two-dimensional, hexagonal honeycomb-like lattice.

2D materials like graphene were once thought impossible. Scientists had noticed that the melting point of thin films decreases rapidly as they get thinner, becoming more and more unstable. So, it was believed that a one-atom-thick layer of material could only exist if supported by a larger 3D structure.

That was until 2004, when Geim and Novoselov managed to experimentally synthesize, isolate and characterize the first flakes of graphene. The pair would go on to be jointly awarded the 2010 Nobel Prize in Physics for their “groundbreaking experiments regarding the two-dimensional material graphene.”

Graphene’s unusual bonding structure makes it mechanically a very strong material – stronger than steel – while also remaining flexible and stretchable. Additionally, its π-bonding network allows electrons to easily flow through graphene sheets, making it a superior conductor of electricity.

Are there different types of graphene?

Most typically, the word “graphene” refers to a one-atom-thick sheet of bonded carbon atoms. However, several other forms of graphene are also of interest to the scientific community.

Multi-layer graphene

Producing single monolayers of pristine graphene can be a very challenging and expensive task – often prohibitively so. As the number of layers in a graphene flake increases, its electronic, mechanical and thermal properties will become affected. However, scientists believe that multi-layer graphene (≤ 10 layers) and few-layer graphene (≤ 5 layers) flakes could still exhibit unique properties that would be useful for biomedical devices.

Graphene oxide

By subjecting graphite to an oxidation process, oxygen-containing functional groups become attached to the graphite surface. After sonication, single-layer or few-layer sheets will break off of the graphite – creating graphene oxide.

Graphene oxide is easier to synthesize than pristine graphene. Its structure is very similar, but with the addition of chemically reactive epoxy (-O-), hydroxyl (-OH) and carboxylic acid (-COOH) functional groups to its surface. These functional groups make graphene oxide more soluble in water and polar solvents compared to graphene, allowing for further functionalizing graphene oxide to develop novel materials.

The addition of these oxygen-containing functional groups severely impacts the electrical properties of graphene oxide, making it an electrical insulator. To restore its electrical conductivity to some degree, graphene oxide can be partially reduced to form reduced graphene oxide – a form of graphene oxide that has fewer functional groups and a restored π-bonding network.

Graphene nanoribbons

Graphene nanoribbons are narrow strips of graphene, measuring just a few nanometers to tens of nanometers in width. Due to these dimensions, nanoribbons are sometimes referred to as being a 1D or quasi-1D material.

Graphene nanoribbons can be produced either by directly slicing up graphene or carbon nanotubes (“top-down” fabrication) or through the polymerization of organic molecules on metal substrates (“bottom-up” fabrication).

These nanoribbons are of interest to scientists as they possess most of the exceptional properties of graphene sheets, while also exhibiting unique characteristics due to their extreme narrowness. For example, fine-tuning width and edge properties of a graphene nanoribbon can result in it acting as a semiconductor.

Graphene aerogels

Graphene aerogel, also known as aerographene, was first developed in 2013 by a team of scientists at Zhejiang University, China.

Under the leadership of Professor Chao Gao, the team was investigating the synthesis of macroscopic materials made of graphene. By combining large sheets of graphene oxide with freeze-dried carbon nanotube solutions and then reducing the graphene oxide, the team successfully created a 3D, macroscopic graphene-based material.

The resultant material – aerographene – is a conductive, elastic, “lighter than air” foam-like material that has an ultra-low density. Due to this extremely low density, aerographene foams are highly absorbent, able to absorb up to 900 times their weight in oil.

What is graphene used for?

The unique physical and electronic properties of graphene (and its derivatives) have seen it rise to the status of a “wonder material” – a disruptive technology sought after by different industries as a potential solution to various issues in their sector.

Energy

Demand for higher-performance rechargeable lithium-ion batteries is driving the battery sector to explore new battery designs and chemistries. The superior electrical conductivity and ion mobility of graphene, combined with its high stability and large surface area, make it a very attractive material for the battery industry.

Rechargeable lithium-ion batteries – currently the most common battery chemistry – routinely use graphite as an anode. But the superior electronic properties of graphene, compared to graphite, have led to new waves of research investigating the potential use of graphene nanosheets and nanocomposites as battery anodes and cathodes, as well as using graphene as a filler to improve the properties of solid polymer electrolytes.

Some are also investigating the concept of an “all-graphene-battery” – a battery that would use functionalized graphene cathodes and graphene oxide anodes together to realize very high power densities, making it an ideal solution for large-scale energy storage.

Graphene supercapacitors are another application of graphene for energy storage. Supercapacitors are similar to batteries; where batteries excel in delivering good energy densities, supercapacitors deliver better power density, making them an ideal solution for more niche applications where a large volume of energy is needed in a shorter timeframe.

Electronics

Since its discovery, graphene has been heralded as a material that could enable significant advances in the world of electronics.

One of the main drivers of improving electronics in recent decades has been our ability to develop ever-smaller silicon components. But now, scientists are reaching an impasse, with silicon-based components suffering from current leakage and other performance issues when the size of the transistor approaches just a few nanometers. Graphene, with its high electron mobility and conductivity, was proposed as a material that could help overcome some of these problems with silicon-based computing. Now, researchers have reported to construction of a graphene-based transistor with a gate length of just 0.34 nm.

Graphene’s high electron mobility and ability to interact strongly with photons (“trapping” light) have led to its inclusion in optoelectronic devices, such as photodiodes, photodetectors and optical receivers.

Touchscreens and flexible electronics are another area where researchers are investigating the application of graphene. The light transmittance and conductivity of monolayer graphene are very close to that of indium tin oxide (ITO) – a transparent conductor that is used in touchscreens, smart windows and OLED displays. One of the major limitations of ITO is its brittle nature, which makes it unsuitable for flexible electronics.  ITO is also very expensive to manufacture and relies on the supply of indium, an expensive and rare metal. By comparison, graphene sheets are far more flexible with a similar set of electronic properties.

Medicine

Another important property of graphene and graphene oxide is their good biocompatibility. Combined with their large surface area and chemical stability, this has made graphene-based materials and interesting new frontier in drug delivery.

Compared to similar carbon-based materials that are already established in the drug delivery field, such as carbon nanotubes, graphene oxide is available at a lower cost and can be more easily modified or functionalized. The high surface area of graphene oxide also allows for ultra-high drug-loading efficiency.

Elsewhere in the biomedical space, graphene oxide has been employed as a key component of wound-healing membranes and dressings, oxygen delivery systems, coatings for biomedical implants, and scaffolds for bone tissue engineering and neuronal regeneration.

Graphene-based materials – including pristine graphene, graphene oxide and reduced graphene oxide – are also a useful component for biosensors. The large surface area of pristine graphene offers an expanse of active sites for biomolecular interactions, while graphene oxide and reduced graphene oxide can be functionalized to bind to DNA, enzymes, nanoparticles, proteins, antigens, antibodies and other molecules of interest.

Composite materials

Graphene itself is an interesting material with a wealth of potential applications for its unusual properties. But nanosheets of graphene and graphene oxide can also be added to other materials to form high-performance composites that are tougher, stronger and more conductive than before.

Graphene additives have been applied as a key component of anti-corrosion coatings, thermal and acoustic insulation materials, cement and fire-fighting materials. In the United Kingdom, a trial of graphene-enhanced road asphalt is currently underway, with the graphene additive helping to provide stiffness and resistance to an asphalt mix that already contains high amounts of reclaimed asphalt.

While a so-called “wonder material” might be expected to find its way into the medical, energy, electronics and engineering sectors, there is one field where its appearance is perhaps more surprising – competitive sports.

Studies have found that incorporating graphene nanoparticles into textiles for sportswear can improve the wear resistance and breathability of the clothes, while also enhancing their antimicrobial properties. Graphene-enhanced tennis rackets, skis, ski boots, hockey sticks and walking/running shoes are all currently commercially available, with the addition of graphene normally being done to reduce weight while boosting durability and shock absorption.

The future outlook for graphene science

The number of potential applications for graphene and graphene-based materials continues to grow with each year that passes. Now, twenty years on from its discovery and first synthesis, graphene shows promise for drug delivery, biosensors, composite materials, batteries, electronics and much more.

However, it cannot be denied that the uptake of graphene from the research sector to direct commercial use has been slower than many might have predicted.

There are still several important hurdles that must be cleared before graphene breaks through into the mainstream. Chief among these are the advancements needed in graphene production – improving quality control, uniformity, scalability and ease of fabrication for graphene materials will be key.