Graphene: The Material That Is Changing The World
Everything You Need to Know About Graphene and Carbon Nanotubes
Graphene may be the next multibillion-dollar material, holding the key to healthcare, energy storage, computing, aircraft and many more exciting applications. It is a material that will fundamentally change the world and it is already being used to revolutionize many different industries today. So what exactly is this amazing material and why is it so important?
Content Outline
- What is Graphene?
- The History Behind Graphene
- The Science Behind Graphene
- Properties of Graphene
- The Production of Graphene
- Carbon Nanotubes
- Applications of Graphene and Carbon Nanotubes
- Potential Drawbacks and Implications
- Conclusion
- TL;DR
What is Graphene?
Graphene (C140H42O20) is one of the most versatile and strongest metals on earth. It is an allotrope of carbon, which consists of a singular layer of atoms that all lie in a plain together, arranged in a 2D hexagonal lattice structure.
Graphene is typically black in color and is referred to as having a “honeycomb” structure, due to the interlocking hexagons that form the material; it contains repeating hexagons of ball-like carbon atoms that extend on the atomic scale for great distances.
The History Behind Graphene
Graphene was a material first theorized to exist in 1947, however it was not actually proven to exist in the real world until the early 2000s.
Graphene was first isolated in 2004 by physicists Andre Geim, Konstantin Novoselov, and other collaborators at the University of Manchester in the UK. The initial question they had before discovering graphene was: Can we make a transistor out of graphite? To investigate this, they extracted thin layers of graphite from a graphite crystal and isolated the graphene by peeling a piece of adhesive Scotch tape off of the graphite.
This yielded a single, one atom thick layer of carbon across the surface of the tape. They then transferred the layers to a silicon substrate and attached electrodes to create a transistor. Geim and Novoselov went on to win the Nobel Prize for physics in 2010 and ever since this discovery, research and applications for graphene have exploded all around the world.
The Science Behind Graphene
Graphene is just like carbon, coal, graphite, or diamond. The reason that graphene is so unique compared to the other allotropes, is the way the carbon atoms are bonded together and the unique shape of the material.
While graphite is its three-dimensional counterpart, the fact that graphene lies on the two-dimensional plain changes the properties of the material. Layers of graphene are held together with crosslinked sp2 carbon-carbon bonds. Since the graphene atoms are arranged in an interlocking hexagonal honeycomb lattice, the covalent bonds between atoms are extremely strong.
The structure of graphene and the way that the carbon atoms are packed together allow for ultra-stable bonds.
In a perfect graphene sheet, the carbon atoms are all sp2-hybridized, and have three in-plane σσ-orbitals and two out-of-plane ππ-orbitals. This means that each carbon atom can form equivalent s-bonds with each of its three neighboring atoms. The bonding energy of one C-C bond in graphene is 4.93 eV. These strong covalent bonds are what give graphene its extraordinary mechanical properties.
Properties of Graphene
What makes graphene so exciting is that it is just one simple material that possesses so many astonishing qualities. The different unique properties of graphene include:
- strength over 200x stronger than steel
- very flexible, similar to fabric
- extremely light weight (lighter than a feather)
- high thermal and electrical conductivity
- light absorption
- renewability
- transparent material
“To put this in perspective, graphene by weight is hundreds of times stronger than steel. I’ve seen one calculation that one sheet of graphene, which is one atom-layer thick, you could put an elephant on it and it wouldn’t break, so that’s amazing.” — Les Johnson, scientist and author of Graphene: The Superstrong, Superthin, and Superversatile Material That Will Revolutionize the World.
Strongest Material
Graphene is covalently bonded to 3 other carbon atoms, which gives the material its incredible strength. The material is stronger than Kevlar and also 200 times stronger than steel. The tensile strength of graphene is 150, 000, 000 psi. Graphene is such a strong material due to the strong covalent bonds and electrostatic forces caused by delocalized electrons that flow through the positively charged carbon atoms. This difference in charge creates a strong electrostatic attraction that holds the graphene together.
Thinnest and Lightest Material
At one atom thick, graphene is the thinnest material that we can see. It is 1000 times lighter than one square meter of regular paper and weighs about 0.77 milligrams. To put that into a larger perspective, a single sheet of graphene that is big enough to cover a large football field would weigh less than a gram.
Very Flexible Material
Another amazing property of graphene is that it has the ability to retain its initial size after strain. Graphene sheets suspended over silicone dioxide cavities had spring constants in the region of 1–5 N/m and a Young’s modulus of 0.5 TPa.
High Thermal Conductivity
Graphene the best known conductor of heat. At room temperature, its heat conductivity is (4.84±0.44) × 1⁰³ to (5.30±0.48) × 1⁰³ W·m−1·K−1. Several studies have found graphene to have an unlimited potential for heat conduction based on the size of the sample. This contradicts Fourier’s law of thermal conduction in the micrometer scale. It has also been found that the larger the segment or piece of graphene, the more heat it could transfer as the thermal conductivity increases logarithmically. This means that theoretically, graphene has the ability to absorb an unlimited amount of heat!
High Electrical Conductivity
In graphene, electrons can move faster than in any other material at room temperature and it is the fastest known conductor. Each carbon atom in graphene is connected to three other carbon atoms on a 2D plane. The graphene’s atom contains free ‘delocalized’ electrons (electrons not associated with a specific atom). Each carbon atom has four electrons in its outer shell, however, only three of those electrons are shared with its neighboring three carbon atoms. The remaining electron, known as a pi electron, can move freely in three-dimensional space. This allows it to transmit electrical charges across the sheet of graphene with almost no resistance (this is also what makes the material more lubricant). Recent studies have shown the electron mobility of graphene to be more than 15,000 cm2·V−1·s−1 with a theoretical limit as high as 20, 000. This makes graphene the best conductor of electricity known today. In fact, graphene can even move electrons 10 times faster than silicon using less energy.
Light Absorption
Graphene also has the ability to absorb 2.3% of white light, which is quite remarkable when considering the extreme thinness of the material. Saturable absorption takes place once the optical intensity reaches saturation fluence, which makes it possible for graphene to achieve full-band mode locking.
Renewable
Carbon is known to be the fourth most abundant element in the entire universe according to mass. Due to this abundance, graphene could technically be considered a sustainable and ecologically friendly solution for increasingly complex problems around the world, however there are still problems when it comes to actually producing graphene.
The Production of Graphene
Graphene can actually be found in our everyday pencils, however it is very complicated when it comes to mass-producing the material.
SOURCE: University of Manchester
Graphene was originally made by peeling pieces of adhesive tape off of a block of graphite. This caused flakes of graphene to stick to the surface of the tape, as graphite is essentially a three dimensional stack of graphene layers that are bonded together. The tape would then be dissolved from the graphene using chemicals to produce graphene flakes.
This method was obviously impossible for mass-producing graphene due to the amount of time it would take, so today, there are several different methods where graphene is made in special processes and reactors purpose-built for that task.
Exfoliation
One of the cheapest methods to produce graphene is by mining graphite from the ground and chemically separating the graphene plates, a process known as exfoliation. By using a specific process to mine graphite, the plates of graphene come off the surface of the graphite in a powdery suspension. This can then be isolated and turned into whatever type of graphene product that the manufacturer wants to create. Most of graphene today is produced by exfoliating common graphite in a liquid bath.
This can be done be by either using mechanical exfoliation (using a diamond knife to peel off layers), or electrochemical exfoliation, where chemical reactions and electrostatic repulsion are induced between the layers of graphite to separate them. The only problem with this is that both of these processes are rough on the material and only yield tiny flakes of low-quality multi-layer graphene. This is due to the failure to peel a single layer at a time. These flakes are referred to as nano-platelets. Using extensive mechanical processes can also sometimes cause damage to the crystalline structure.
Chemical Vapor Disposition (CVD)
Chemical vapor deposition, or CVD, in a large single sheet with a high surface area is another process for producing graphene, which is used when scientists take a gas of hydrocarbons along with a metal catalyst. They remove the hydrogen atoms from the hydrocarbon, and only keep the carbon atoms. Then, hopefully these carbon atoms will arrange themselves side by side into the proper graphene lattice form.
This can be a very difficult and expensive operation with the graphene layer being grown on a sheet of copper. Another way this can be used us by growing carbon nanotubes, then chemically unbinding and flattening them to form extremely small strips of monolayer graphene. CVD is the most promising way to produce carbon nanotubes on an industrial scale.
Epitaxial Graphene
The third major process for producing graphene is known as epitaxial graphene. This process uses carbon that’s been embedded within silicon carbide. By heating it up to extremely high temperatures, it causes silicon undergo sublimation, transforming it from a solid to gaseous state without passing through the liquid state. This leaves behind a large excess of these carbon atoms that are extremely reactive and want to bond to anything around them, which is what causes them to latch onto something together and create the graphene sheets.
Carbon Nanotubes
By taking a sheet of graphene and rolling it into the shape of a cylinder, you get a carbon nanotube. Carbon nanotubes, also known as buckytubes, are hollow nanoscale tubes that are composed of carbon atoms. They currently have the highest tensile strength of any fibrous material that we know of and are known to be members of the fullerene family.
The cylindrical carbon molecules of the nanotubes feature high aspect ratios; the length-to-diameter values are typically above 103, with diameters from about 1-100 nanometers while the lengths go up to a few millimeters. The unique one-dimensional structure and concomitant properties are what give carbon nanotubes special properties with unlimited potential for nanotechnology-associated applications.
Carbon nanotubes are typically categorized into two main groups according to their graphic shells: single-walled (SWNTs) or multi-walled carbon nanotubes (MWNTs).
Single-Walled Carbon Nanotubes
SWMTs are synthesized using transition-metal catalyzed arc discharge. A SWNT is a long tube that is formed by wrapping a single sheet of graphene into a seamless cylinder with a diameter of around 1 nanometer and ends that are capped by fullerene cages. The surface is formed by fullerene structures that have five alternating hexagons adjacent to one pentagon. The sidewalls of the carbon nanotubes consist of graphene sheets in neighboring hexagonal cells. The method to form a seamless tube includes taking two of the hexagons in a graphene lattice and overlapping them. The vector that connects the centers of the two hexagons is called the chiral vector, which determines the structure of the SWNT.
In order to establish the chiral vector for the SWNT, two atoms in the graphene sheet must be selected. One atom serves as the origin of the vector pointing toward the other atom. Then, the graphene sheet is rolled in a way that allows the two atoms to coincide. As a result of this process, the lengths of the chiral vectors become equal to the circumference and they form a plane that is perpendicular to the longitude direction of nanotubes. There are three main types of SWNTs, named “zigzag” (m = 0), “armchair” (n = m), and “chiral.” The structural variations of these cause them to have differences in electrical conductivity and mechanical strength.
- Mechanical: SWNTs are much stronger than steel; at 1/16th the weight, the tensile strength of an SWNT is over 100 times greater than steel.
- Electrical: an SWNT has the current carrying capacity of 109 amp.cm-2. This means that SWNTs have a greater electron mobility and current carrying capacity than gold, copper, or silicon.
Optical: SWNTs have clear-cut optical absorption and fluorescence response due to the fact that each chirality in the SWNTs exhibits its own characteristic absorption and fluorescence spectrum.
- Thermal: The thermal conductivity of an individual SWNT at moderate temperatures is comparable to that of diamond or in-plane graphite, as these nanotubes are found to display the greatest measured thermal conductivity of any known material at room temperature.
Multi-Walled Carbon Nanotubes
MWNTs are synthesized by arc discharge methods. MWNTs share the same center as SWNT assemblies but have different diameters. The distance between the adjacent shells is about 0.34 nanometer. MWNTs are several tubes in concentric cylinders and have different corresponding properties compared to SMNTs.
- Electrical: When integrated properly into a composite structure, MWNTs are very conductive. The MWNT’s outer wall is the only one that has conductive properties, while the inner walls are not instrumental to conductivity.
- Morphology: The performance and application of MWNTs are due to their high aspect ratio, with lengths that are generally more than 100 times the diameter, as well as the degree of entanglement and the straightness of the tubes.
- Physical: MWNTs (defect–free) have a great tensile strength and when integrated into a composite like a thermoplastic or thermoset compound, the strength can significantly increase.
- Thermal: Due to the level of defects, MWNTs have a thermal stability above 600 °C. This can also be a result of a residual catalyst in the product that accelerates decomposition.
- Chemical: MWNTs have a high chemical stability, as they are an allotrope of sp2 hybridized carbon, similar to graphite and fullerenes. The nanotubes, however, can be made to enhance the strength of the composites.
Applications of Graphene and Carbon Nanotubes
With its special structure and properties, graphene and carbon nanotubes are the perfect material to use in many different industries for various applications.
Superconductors
Graphene has been turned graphene into a superconductor that is capable of carrying electricity with no resistance. This was discovered by squishing two layers of graphene together and offsetting them by twist angles of about 1.1°, the ‘magic angle.’ The electronic band structure of the twisted bilayer graphene that was created exhibited flat bands of almost zero Fermi energy, which resulted in correlated insulating states at half-filling.
By using electrostatic doping to separate the graphene material away from these correlated insulating states, scientists found zero-resistance states that had a critical temperature of up to 1.7 kelvin. This means that when cooled to almost absolute zero, two sheets of graphene pushed together that are offset by exactly 1.1 degrees becomes a superconductor. Superconductors could provide a source of unlimited energy, as they do not constantly need to be resupplied with current.
Clean Energy/Energy Storage
Graphene has reported advantages for electrochemical energy generation/storage applications. Due to the fact that graphene is the world’s thinnest material, it also has an extremely high surface-area to volume ratio, which makes graphene a very promising material for use in batteries and supercapacitors. Graphene can enable batteries, supercapacitors, and even fuel-cells that can store more energy and charge faster.
The use of graphene for energy applications can be classified into two main categories: energy storage devices and energy conversion devices. The graphene-based devices can provide clean energy with theoretically zero waste emission.
Batteries and Supercapacitors: Graphene can be used in batteries to improve charge, discharge cycling performance and overall capacity. Graphene can also be utilized in supercapacitors to enhance power density and rate performance.
Solar Cells: Solar cells can be manufactured using graphene to enhance power conversion efficiency. Although the electrical properties of graphene in solar cells were not a breakthrough, a solar cell that can be installed on any kind of surface such as cars, clothes, paper, and cell phones, that is flexible and transparent has been developed with graphene.
There are even scientists trying to find out if graphene solar cells can generate energy from raindrops, which theoretically might actually be possible.
Graphene Oxide Water Filters
A graphene oxide nanofilter is composed of graphene-coated nanomembranes that are more applicable in water filtration than any other methods. Graphene is partial to water molecules, mechanically sturdy, chemically dormant and non-permeable to gases or liquids. The graphene is embedded with carbon nanotubes that serve as nanofilters, which are more useful for dye rejection in water waste, removal of salt ions and the dichlorination of water.
“Graphene oxide sheets arrange in such a way that between them there is room for exactly one layer of water molecules. If another atom or molecule tries the same trick, it finds that graphene capillaries either shrink in low humidity or get clogged with water molecules.” — Dr Rahul Nair
Graphene is oxidized to form tiny pores. Using amide coupling between carboxyl groups of graphene oxide and carboxyl groups of a polyamide active layer, graphene oxide is irreversibly bound to the membrane of a nanofilter. It draws in water to capillaries formed by several layers, while blocking other molecules, which can even include gas molecules.
Graphene oxide membranes have been shown to be effective at removing contaminants from water at an exceptional level. Direct contact of bacteria with graphene oxide on the membrane surface results in 65% bacterial inactivation after 1 h of contact time. This bactericidal effect is imparted to the membrane without any detrimental effect to the original membrane transport properties.
Material Composites
Graphene can be added to metals, polymers or ceramics to create more conductive composites that are resistant to heat and pressure. Graphene composites have many potential applications and there is currently a lot of research being done to create new unique and innovative materials.
There appear to be limitless applications for graphene composites, as one type of graphene-polymer can prove to be a light, flexible and excellent electrical conductor, while another dioxide-graphene composite was found to have interesting photocatalytic efficiencies, and there are many more possibilities to expand on in the future. Graphene composites have the potential to be applied in medical implants, engineering materials for aerospace, renewables and more.
The military is also interested in pursuing graphene composites, as a two-layer epitaxial graphene film can withstand perforation by a diamond tip. The goal of developing this material would be to create ultrathin body armor that could be light as a feather and simultaneously strong as diamond. This type of armor would be capable of stopping a bullet dead in its tracks. Only two sheets of graphene appear to maintain the ultra-hardening effect, as extra layers seem to have a detrimental effect.
Electronics
Graphene can be used to coat and improve current touch screens for phones and tablets. The amazing properties can also be used to make the circuitry for computers, greatly improving their speed. While these are just a couple of examples of the applications currently being used for graphene in electronics, graphene also has the potential to spark the next-generation of electronics:
Wearable technology: Indium-tin oxide is currently used for touch screens as it conducts well but it is brittle. Due to the flexible nature of graphene as well as it’s mechanical properties and conductivity, the future could include a smart phone that people wear on their wrists or a tablet they could roll up like a newspaper; flexible, wearable devices.
Graphene Transistors: The smaller the size of the transistor, the better they perform within circuits. One of the fundamental challenges facing the electronics industry in the next 20 years is the further miniaturization of technology, which is a challenge that graphene could be instrumental in solving. In fact, at the University of Manchester, the world’s smallest transistor has been created by researchers using graphene.
Medical Applications
1. Tracking Health: At the University of Texas at Austin, researchers have created graphene-based temporary tattoos that are capable of tracking a person’s vital signs, such as their skin temperature and hydration.
Researchers at the University of Illinois at Chicago have discovered that graphene can help detect cancer cells. In an experiment, researchers at the university took brain cells from mice and placed them onto a graphene sheet. They found that the graphene sheet was able to distinguish between a single cancerous cell (glioblastoma or GMB cell) and a normal cell from the mice brain cells.
2. Graphene in Cancer Treatment: While graphene can detect cancer cells in the early stages of the disease, it can also stop them from growing any further in most types of cancer. This is possible because the graphene intervenes with the correct formation of the tumor or causes autophagy, which leads to the death of cancer cells and allows for new healthier cells to regenerate.
3. Graphene in Drug Delivery: Graphene can also be used for drug delivery to carry chemotherapy drugs to tumors for cancer patients. Graphene based carriers target cancer cells better, as well as reduce and decrease the toxicity of the effected healthy cells. Graphene in drug delivery is not only limited to cancer treatment; anti-inflammatory drugs have also been carried by graphene with chitosan combinations, which yielded promising results.
Graphene in Food Packaging
Another great application for graphene is that it can be used as a coating material to prevent the transfer of water and oxygen, as a membrane in food or pharmaceutical packaging. The three main properties of graphene that help contain foods include: increased strength, barrier effect and conductivity.
- Graphene can create containers that are stronger and more flexible, protected from UVA rays due to the graphene composites in the form of Masterbatches (PE, PP, ABS, POM).
- Containers and packaging that have been manufactured with a graphene composite are extremely resistant, completely waterproof and antibacterial. This is possible because the graphene forms an impermeable layer to any gas, substance or bacteria.
- In addition to these amazing applications, biodegradable, anti-theft and intelligent sensor prototypes are being worked on with graphene as well.
The graphene membrane keeps the food and medicines fresh for a longer period of time, and while this may seem a simple application, it can actually dramatically reduce the amount of food waste that people throw away every day.
There are also plenty of other really interesting applications of graphene that are currently being utilized, which can be further investigated on this website.
Potential Drawbacks and Implications
Studies have pointed out environmental, social and economic impacts that graphene can have, however, graphene still requires further in-depth research on the material. There are still many problems when it comes to implementing the wide-spread use of this material and there are indications of the potentially harmful effects of graphene on the ecosystem.
Scalability: Difficult to Mass-Produce
Due to the extremely small nature of graphene, it can be very difficult to mass-produce. While there are certain aspects of graphene such as graphene platelets, and reduce graphene oxide that is scalable as powders and dispersed on substrates, the effective uses of graphene still have problems with scaling. There remain problems with connecting nanowire graphene to circuits at scale and with scaling nano-pores in graphene. These problems are slowing down the creation of sensors, wearable electronic devices and superior water filtration technology. Commercial applications and scaling are still a little disjointed, however this is fairly typical for a materials science advancement and there are already numerous applications being applied today with breakthroughs in graphene production every day.
Environmental Concerns
Graphene nanoparticles have been discovered to be dangerous for water resources. Research by the University of California Riverside laboratory has found that a difference in movement and stability of graphene nanoparticles occurs when placed in groundwater and surface water.
The study shows that while graphene remains stable in surface water, as it has more natural organic matter that keeps the graphene nanoplatelets stable, graphene particles become less stable in groundwater. This is due to the fact that groundwater usually has more hardness and less natural organic matter, which may cause the graphene nanoplatelets to settle or even get removed entirely.
There is still limited information about how artificially-engineered materials such as graphene affect different ecosystems; more research needs to be done to study how graphene reacts with external environments and the potential repercussions that this may cause.
Human Exposure
Studies at the University of Edinburgh have shown that nanoparticles of graphene can lead to severe respiratory problems, such as lung cancer. Through the use of a pharyngeal aspiration model, small graphene flakes have been seen to enter into the respiratory system of humans. Upon entering into the breathing system, nanoplatelets of graphene can become lodged deep within the lungs. Since the human immune system does not have a mechanism to remove an inert material like graphene, the graphene nanoparticles can stay permanently inside the lungs and cause severe physiological harm to the tissue at the cellular level. This means that graphene nanoparticles have the potential to cause severe inflammation within the lungs, leading to acute respiratory problems. The small graphene platelet lodged within the lung can eventually develop a tumor and lead to lung cancer.
Exposure to carbon nanotubes in particular, has been associated with mesothelioma (cancer of the lung lining). Nanotubes can scar lung tissues if they are inhaled, which is concerning as nanotubes are already being used in numerous common products, including bicycle frames, automobile bodies, and tennis rackets. While these potential health risks are relevant to the people involved in manufacturing, they can affect the general public as well. Not a lot of research has been conducted to determine whether or not there are risks to human health when products containing nanotubes are crushed or incinerated in a waste dump, which means that there may be other potentially harmful affects of graphene and carbon nanotubes on human health.
Conclusion
While there is certainly a lot more research that needs to be conducted to determine further uses and implications, graphene is a material that is already changing the world every day, and there are still so many opportunities to expand on for future applications. With its incredible properties, graphene is an exciting material full of possibility that it is already on its way to revolutionizing the world.
TL;DR
- Graphene is a very powerful, versatile and strongest metal, consisting of a singular layer of atoms that are arranged in a 2D hexagonal lattice structure (carbon atoms arranged in a “honeycomb” shape).
- Graphene was first isolated in 2004 by physicists Andre Geim, Konstantin Novoselov who used adhesive tape to pull off graphene flakes from graphite.
- Graphene is just carbon, like coal, or graphite, or diamond. The difference that sets graphene apart is how the carbon atoms are bonded together and the unique shape of the material’s atomic structure.
- Graphene is a material that has many amazing properties, such as being 200x stronger than steel, very flexible (similar to fabric), extremely lightweight (lighter than a feather), and having high thermal and electrical conductivity.
- There are three main processes for producing graphene: exfoliation, chemical vapor disposition (CVD), and epitaxial graphene.
- Carbon nanotubes are nanoscale hollow tubes that are composed of carbon atoms, created by taking a sheet of graphene and rolling it into the shape of a cylinder.
- There are two types of carbon nanotubes: single-walled nanotubes (SWNTs) and multi-walled carbon nanotubes (MWNTs).
- There are many applications utilizing graphene and carbon nanotubes today such as in superconductors, electronics, water filters, clean energy, material composites, energy storage and multiple medical applications.
- Potential drawbacks of graphene include difficulty in production on a wide-spread scale, environmental concerns and human toxicity that can lead to severe respiratory problems.
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