Is Graphene the Wonder Material it Claims to be? By Jack Corbett.
Table of Contents
Introduction 2
Graphene’s discovery 4
Methods of isolating graphene 5
What’s so special about graphene though? 8
The applications of graphene 10
Types of Graphene 12
Graphene’s electrical applications 14
Graphene coatings and composites 17
Graphene barriers and filters 20
Applications of graphene’s strength 21
What’s been happening recently? 24
Graphene: The Not-So-Wonder Material? 27
Conclusion 28
Introduction
In recent years science has evolved to such a point where some theoretical materials can become a reality. An example of this is graphene.
Graphene is a single atom thick layer of purely carbon atoms arranged in a hexagonal/ honeycomb format. This hexagonal format means that each carbon atom is held together by 3 bonds. Graphene sheets are technically a 2D structure due to them being 1 atom layer thick.
One of the most incredible properties of graphene is the wide variety of allotropes (different forms in which an element can exist) it can be formed into. This includes:
Graphene sheets
Buckminsterfullerene (Bucky balls)
Carbon nanotubes
Graphite
Diamonds
Graphene’s discovery
The first theoretical study for graphene was started in 1947 by physicist Philip R. Wallace where the theoretical properties of graphene were first identified and researched. However, due to technological limitations in this time period, it was impossible to isolate graphene until 57 years later…
Graphene was first isolated at the University of Manchester in 2004 by Professor Andre Geim and Professor Konstantin Novoselov. Graphene was isolated using the mechanical exfoliation method of recovery, using sticky/ scotch tape to remove layers of graphite from a larger source, sticking and peeling apart the tape to slowly remove individual layers of graphene. This method was initially used to isolate only small flakes of graphene from a larger source of graphite until different isolation methods could be discovered. These small isolated flakes were used to prove the existence of the at that point still theoretical material graphene. Research into this material began and many incredible properties were discovered.
For their work on graphene, Professor Andre Geim and Professor Konstantin Novoselov won the Nobel Prize for Physics in 2010.
Methods of isolating graphene
There are 4 principle methods used to collect graphene:
Chemical Vapour Deposition (CVD)
• This is done by combining gases which contain graphene inside a reaction chamber set at an ambient temperature. A heated sheet of substrate (often nickel or copper) is then added. A reaction occurs which causes a thin film of graphene to deposit onto the substrate. This process is very slow and the thickness of the graphene produced is usually measured in microns per hour.
• This process is however very popular due to the graphene produced from it being at a very high quality whilst remaining relatively inexpensive.
Mechanical exfoliation
• This method of recovery was used to first discover graphene.
• This method involves using sticky tape and a lot of patience to remove layers of graphene from a bulk mass of graphite. The layers are then stuck to a substrate such as silicon and when the tape is removed, a single layer of graphene with a large crystal size will be deposited.
• A problem with this method is the purity and quality of the resulting graphene
Liquid exfoliation
• This process involves blasting raw, bulk material of graphite with ultrasonic energy into small graphene fragments. Powdered graphite is usually sonicated in a solvent for hours to remove single layers of graphene.
Electrochemical exfoliation
• This involves electrodes made out of graphite being introduced to a solvent. If the graphite is the positive electrode then graphite oxide is produced. If the graphite is the negative electrode then non-oxidised graphene flakes are produced.
Despite there being many new techniques and technologies used to isolate graphene, these techniques can only produce very small quantities and so graphene is not yet industrially viable. Many of these techniques cannot be effectively upscaled to support industrial use. There is ongoing research into new and more industrially viable methods of obtaining graphene (some of the properties which graphene has shown make it into a potentially incredibly useful substance).
What’s so special about graphene though?
Despite graphene’s incredibly simple molecular structure, it has some amazing properties which warrant the difficulty to retrieve it. These include:
Graphene is 200x stronger than steel – 130,000 MPa (Mega Pascals) tensile strength. Steel has a tensile strength of 650 MPa and Kevlar has a tensile strength of 3750 MPa.
1 million times thinner than a human hair
The world’s most conductive material both electrical and thermal
Is technically classed as 2D – 3 million layers of graphene stacked on top of each other to be only 1 mm in height
Stretchable
Transparent
Flexible
Impermeable
Incredibly lightweight – a sheet of graphene covering an entire football field would weigh <1 gram , graphene weighs just 0.77 mg/m2!
The combination of these properties create an incredible material which is multidisciplinary in nature as it has so many varied properties.
The applications of graphene
Graphene’s unique structure and amazing properties allow it to be used in a variety of different situations. These include:
Graphene batteries
Super-fast circuit boards
Flexible phones
Biomedical applications
Graphene paint to coat surfaces and make them waterproof, strong and light
Water filters
Air filters to remove harmful greenhouse gases
Super sensitive sensors
Food packaging which will prevent bacterial infection
Graphene based armour including possible subdermal applications
Smart technologies
Types of Graphene
Because graphene can be used in so many different applications, different forms of graphene are produced to best suit these functions.
There are currently 6 main forms of graphene:
Graphene
• This is a single atom thick sheet of pure graphene which is either freely suspended or adhered to a substrate.
• Monolayer graphene is graphene at its purest form and so is useful for high frequency electronics.
• Bi- and tri-layer graphene show different qualities as the number of layers increase as well as becoming cheaper to produce.
Few-layer graphene (FLG) and Multi-layer graphene (MLG)
• A 2D sheet as either a standing flake or substrate-bound coating consisting of multiple well-defined layers of graphene.
• MLG is useful for composite materials and as a mechanical reinforcement.
Graphene Oxide (GO)
• Chemically modified graphene prepared by oxidation and exfoliation.
• Graphene oxide is a monolayer material with a high oxygen content.
• This thin membrane allows water to pass through but blocks other materials.
Reduced graphene oxide (rGO)
• rGO is graphene oxide which has been processed by chemical, thermal, microwave, photo-chemical, photo-thermal or microbial/ bacterial methods to reduce its oxygen content.
• Conductive inks are one potential use for rGO.
Graphite Oxide
• This is the precursor to Graphene oxide.
• It is a bulk solid formed by the oxidation of graphite.
• Graphite oxide can be exfoliated in solution to form monolayer graphene or few-layer graphene oxide.
Graphite nanoplatelets, graphite nanosheets, graphite nanoflakes
• 2D graphite materials with a thickness and/or lateral dimension of less than 100 nanometres.
• These terminologies are used to distinguish new ultrathin forms of finely milled graphite powders.
• These are excellent for electrically conductive composites.
Graphene’s electrical applications
Graphene has a remarkably high electron mobility at room temperature when compared to silicon-based semiconductors.
Graphene also has an incredibly low electron-mobility resistance, 35% less than copper at room temperature which is the current least resistive material. Because of these two factors and graphene’s relatively tiny size, graphene can be used to create incredibly fast and powerful computer chips and cables which can be used in both new and old technologies from improving planes and buildings to new microdevices.
Graphene could be used to make phones smaller and faster and include even more functionalities. Improved computer chips and cables can be used in aircraft to improve their energy efficiency and technological capabilities.
Due to graphene’s size, it can also be used in micro-technologies such as nanobots which could be used in medical applications to deliver medicine to specific parts of someone’s body.
Due to graphene’s unique structure, it is extremely strong and flexible. This combined with new graphene circuit boards, would allow for bendable phones which could be used to create wearable technology or computers built into clothing.
Graphene has an incredibly high thermal conductivity. This means that graphene could be used in circuit boards as both electrical components and as heat sinks. This would be very useful in high end and high strain electronics as the graphene would allow them to work faster for longer without being damaged by the heat produced by such powerful components. This cooling ability would also be extremely useful as it would save energy in the circuit by absorbing any energy lost as heat.
One of the few applications for graphene which has actually come into mass production are Graphene LEDs. Graphene LEDs are LED lightbulbs where the filament has been coated in graphene, this allows heat to be quickly dissipated from the filament. This means that lower wattage bulbs can produce the same amount of light as a traditional LED bulb due to the increased energy efficiency as less is lost from heat. These graphene LEDs will also be able to last ~10% longer as the heat would damage the filament less.
Graphene coatings and composites
In recent years, graphene has been used in paints and coatings. These graphene-based paints have flakes and sheets of graphene suspended in them and so when they are painted or air brushed onto surfaces they deposit a thin film of graphene. This film of graphene is extremely good at preventing water or other molecules to get through the paint to affect the material beneath due to the crystal lattice structure of the graphene sheets. Graphene’s strength also helps with these paints as it can help reinforce the material and will make it corrosion resistant as the corrosive substance will not be able to get through the graphene layers. The increased strength will prevent wear and tear on the material under the paint and so can prevent scuffs or wearing on the material. e.g. an iron bucket could have a thin layer of graphene paint airbrushed onto it, which would prevent water and air getting to the iron to prevent rusting from occurring. This film would also reduce the amount of damage caused to the bucket by dropping or bashing the bucket.
These graphene-based paints could be modified to create graphene-based lubricants. These lubricants could be used in industry to create a highly effective lubricant for use in heavy machinery. Graphene would work excellently as a lubricant as the individual layers and flakes would be able to slide over one another with little to no friction.
Another application of graphene paints and coatings could take advantage of its high thermal conductivity. This means that graphene could be used in a paint to create a heatsink around components in electrical and industrial applications.
Graphene’s thermal properties mean that it could be used in a lubricant or other liquid to act as a better coolant than water and could be a reusable coolant unlike water by transferring heat away in a continually flowing liquid which is then passed off to a graphene heat sink further down the system away from the component producing the heat. This graphene coolant liquid can then be cooled and reused.
Recent applications of graphene have combined nano-sheets of graphene with different polymers to improve its capabilities. One of the most prominent was a nanocomposite made up of graphene and polystyrene, where by the addition of polystyrene into graphene’s composition increased its tensile strength in tests by a 57-70% increase.
By combining graphene’s varied properties into other materials, we can create new stronger and lighter variations on current materials. A predicted use of this will revolutionise the world of aviation, graphene composites could be used to create stronger and lighter plastics which could be used to construct the body and wings of an aircraft, this would make the plane much lighter which would hugely increase the planes fuel efficiency making it fly further for longer and making it better for the environment. These graphene composite plastics would also increase the strength of the wings but due to graphene’s flexibility, would allow the wings to still bend and not be incredibly brittle. Graphene could be used to replace the carbon fibre in other technologies due to its similar composition but increased strength and reduced weight.
Graphene barriers and filters
Since its discovery, graphene has been theorised to be able to act as the world’s best and most effective barriers which can block the movement of even some of the least dense gases such as helium. This could be used as a barrier in industry to prevent the movement of materials and in food packaging to increase the shelf life of foods by preventing oxygen from reaching the food to stop rotting.
Graphene oxide can be used as a perfect membrane to separate water from other molecules; due to the oxygen molecules in the graphene sheets acting like a water molecule-shaped hole in a graphene fence.
Graphene oxide is formed by sucking a mixture of pure water and graphene oxide (created by liquid exfoliation) through an aluminium based filter. The pure water would be sucked through the filter but the graphene oxide would be deposited on top of the aluminium based filter. This graphene oxide deposit is dried and can be peeled off of the filter. This sheet of graphene oxide is amber in colour and allows only pure water to pass through it.
Due to these sheets of graphene oxide only allowing water to pass through, they can be used as water filters to desalinate water or to remove dissolved materials from the water to create drinking water.
Applications of graphene’s strength
Despite graphene being only 1 atom layer thick, it has an incredibly high tensile strength (the tensile strength is a materials elastic potential before something can break through it e.g. Kevlar has a higher tensile strength than steel and so more force has to be applied to penetrate through it when the same amount of material is used)
Recent tests into graphene have shown that it can absorb blows which would punch through steel and can withstand blows that are twice as strong as the ones which Kevlar can absorb. This could make it perfect for body armour used by the police or the military due to graphene’s incredible strength and low weight.
These tests were conducted at the University of Massachusetts-Amherst by Jae-Hwang Lee and his results were publicised by Science, volume 321 on 18th July 2008.
Lee conducted an experiment in which he used laser pulses to super- heat gold filaments until they vaporised and acted like gunpowder to fire a micrometre-size glass bullet into sheets of graphene at 3 kilometres per second (that’s approximately 3x the speed of a bullet fired from an M16 rifle!).
Lee’s team found out that graphene can withstand blows from these micro bullets by stretching into a cone shape at the point of impact and spreading out the kinetic energy of the impact by cracking radially outwards. This outward cracking completely dissipates the impact across the entire graphene sheet which slows the bullet down and prevents penetration.
These results showed that graphene has a tensile strength of 130,000 MPa! And that only a 100-nanometre thick layer of graphene was needed to stop the micro bullets.
This data shows that multiple layers of graphene could be woven into clothing or other materials to form a graphene composite body armour.
Another theoretical use for graphene could be the world’s first subdermal body armour. Subdermal graphene body armour is made from layers of graphene which are implanted under the skin, in a similar fashion to a tattoo, and so act as permanent body armour meaning that soldiers or police officers wouldn’t be required to wear bulky armour plates which only cover parts of their body. Graphene would also be perfect for this application when compared to other substances as it is made of pure carbon and so the human body wouldn’t react to the graphene being implanted whereas other materials would suck up some of the bodies water or be attacked by the body’s cells, e.g. another material which was proposed was nano-cellulose, this however couldn’t be used as due to it being made from plant matter, it absorbed the bodies water and bloats, whereas graphene does not react with anything in the body due to it being purely carbon atoms. Another reason why graphene is a perfect choice as it’s flexibility wouldn’t inhibit any of the user’s movement when performing any complex movements such as running or jumping.
What’s been happening recently?
Graphene research is still being undertaken and here are some of the most recent results:
23rd January 2018 – Proton transport in graphene
• New findings demonstrated an increase in the rate at which the material conducts protons when it is illuminated by sunlight. This has been dubbed the ‘Photo-proton’ effect. This effect could be exploited to design devices which are able to directly harvest solar energy to produce hydrogen gas, which could then be used as a promising green fuel.
• To test how light affects the behaviour of protons permeating through the carbon sheet, a team led by Dr Marcelo Lozada-Hidalgo and Professor Sir Andre Geim (one of the two scientists who first isolated graphene) fabricated graphene membranes and placed platinum nanoparticles on one side. The Manchester scientists discovered that the proton conductivity of these membranes was enhanced x10 when illuminated with sunlight.
13th November 2017 – Graphene filters
• Manchester scientists have used graphene water filters to turn whiskey clear.
• Previously graphene oxide membranes were shown to be completely impermeable to everything but water, however this study has now shown that we can tailor the molecules that pass through these membranes by simply making them ultrathin.
• A research team led by Professor Rahul Nair have tailored this membrane to allow all solvents to pass through but without compromising its ability to sieve out the smallest of particles.
• In the newly developed ultrathin membranes, graphene oxide sheets are assembled in a way so that pinholes form produce an atomic-scale sieve which allows the large flow of solvents through the membrane.
• These graphene-based membranes can be used for organic solvent nanofiltration (OSN). OSN technologies can be used to separate charged or uncharged organic compounds from an organic solvent, for example a graphene-based OSN could be used to separate various organic dyes dissolved in methanol.
• To test out the capabilities of this, the Manchester scientists decided to filter whiskey through the membrane and found that the membrane allowed the alcohol to pass through but removed the larger molecules, including the amber colouring.
Graphene: The Not-So-Wonder Material?
Graphene may be seen as this amazing new material with limitless possibilities but the reality is that the world is already moving on.
There have been huge leaps in the production and research of graphene with the new graphene centre being built in Manchester but graphene may not be as good as everyone thinks.
Whilst on paper graphene is the perfect material, its main drawback is collecting defect-free graphene. Producing even small amounts of defect-free graphene is difficult but as the sheets are increased in size, the rate of defects occurring is hugely increased and so is the cost and so producing large amounts of defect-free graphene is virtually impossible and extremely expensive. So far, all of the processes to collect graphene have only been viable for lab use as they can only produce a maximum of a few small effect-free flakes in which to test and so far upscaling these production methods has proved to be incredibly difficult and expensive and shows that any significant volume of graphene production is still a distant dream.
‘Scientists are like children with new toys’ …
A new carbon-based material called Carbyne has been recently discovered which is theoretically even stronger than graphene. Carbyne is a ‘Carbon nano-rope/rod’ and has so far only been isolated in a lab once by making a ‘Thermos flask’ of graphene for the carbyne to sit inside.
Conclusion
Despite all of these huge developments, most of these applications are still only theoretical and haven’t yet been put into practice due to graphene being incredibly difficult to mass produce and so very difficult to test. This also means that currently graphene cannot be commercially viable and so wide scale production of graphene is still in the distant future until a viable mass production method is available.
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