Graphene May Be a Wonder Material, But Don’t Hold Your Breath
Not long after graphene was discovered back in 2004, the prospect of exciting new possibilities began to arise. Stronger, cheaper and simply better in every way when compared to existing materials, it was thought that graphene would soon take the world by storm. However, we have yet to experience a graphene industrial revolution so much as steady advances in its research. We explore how graphene is made and the challenges it faces at a commercial scale.
Table of Contents
What is Graphene?
Carbon is one of the most abundant yet versatile elements available on our planet, making it nature’s favorite building block. While some carbon-based molecules are incredibly complex, graphene is simply a sheet of repeating carbon units that is just one atom thick. Due to the nature of bonding, the carbons in graphene arrange themselves into a hexagonal pattern with wonderful symmetry.
Graphene itself is not a new molecule, having been hypothesized to exist as graphite – a mass of graphene sheets. However, it wasn’t till 2004 when the real deal, single-layer graphene, was isolated and characterized1. These single-atom-thick sheets were found to be surprisingly stable at room temperature, while possessing some very interesting properties.
It was a big achievement for the scientists working in the field as well as for the wider community. For years, it was theorized that harnessing the potential of graphene’s unique properties could transform the landscape of technology and electronics. With its isolation, it would be only a matter of time before this wonder material took over the world.
Graphene exhibits high thermal and electrical conductivity, it is stronger and lighter than any metal alloy in existence, while having the added benefit of being both transparent and flexible. To find out why graphene behaves in such a fascinating matter, we need to study its electrons. After all, chemistry is governed by the comings and goings of electrons.
In graphene, all the electrons from the carbon atoms are delocalized – they can move freely over the entire structure. In addition, these electrons possess no mass due to quantum effects, making them extremely quick. This leads to the material being an extremely conductor of heat and electricity. Due to the nature of its carbon bonding, graphene is also the strongest material ever discovered, surpassing the tensile strength of even Kevlar.
One key area in which graphene has been earmarked for success is within the semiconductor industry. Billions of silicon transistors are manufactured every day, finding their way into the electronic devices we can’t seem to live without. Some forms of graphene, such as nanoribbons, can be transformed into a semiconducting material that is many times more efficient than silicon.
Since carbon is such a good building block, it is also possible to react graphene with other atoms to change its structure and properties. For example, graphene can react with palladium nanoparticles, making it sensitive to tiny molecules such as hydrogen2. These ‘nanosensors’ can detect analytes that are too small or too low in concentration for traditional sensors.
Even in biology and medicine, graphene has found potential applications. Its high surface area makes it an ideal candidate as a drug delivery system, transporting drugs to targeted areas in our bodies where they can do their job. In tissue engineering, graphene has been seen to promote the growth of neurons from stem cells, making them a potential treatment in neurodegenerative diseases3.
How is Graphene Made?
Significant advances in graphene manufacturing have been made in the past two decades. However, producing precise graphene sheets reliably and consistently has been a roadblock for researchers. It is extremely challenging to control the size and shape of atomic-scale structures, which in turn can influence the properties of the material.
Exfoliation – Simple Yet Effective
Since graphite is so abundant, the fact that we can obtain sheets of graphene from it is extremely beneficial. Pencil ‘lead’ is made of graphite; when we write, we are actually shearing away at the layers of carbon and transferring them onto a surface. In fact, one of the most used methods to obtain graphene is the use of simple adhesive tape to extract it from graphite!
Other shearing or ‘exfoliation’ methods such as liquid-phase exfoliation involve more complicated equipment, but the principle of mechanical separation remains the same. Liquid phase exfoliation involves using a specialized solvent to disperse graphite in, followed by sonication and centrifugation to separate the layers4. This is one of the most common methods to produce graphene in industrial quantities.
If we can efficiently extract single-layer sheets from bulk graphite, it will undoubtedly be the most cost-effective source of graphene. However, it would be impractical for many of these methods to be adopted for the mass production of high yield, high quality graphene.
Unzipping Carbon Nanotubes
An ingenious way of producing graphene is to take a roll of it and unzip it along its length5. These hollow tubes, known as carbon nanotubes, are easily obtainable. The carbon nanotubes are oxidized by potassium permanganate, which produces an opening in the structure. This enhances the reaction of the next carbon, continuing along until the entire tube is ‘unzipped’.
Using atomic force microscopy (AFM) and other imaging techniques, it was found that the graphene structures made this way are both pure and stable. However, high temperatures (up to 1200 °C) are required for producing the initial carbon nanotubes, meaning the reaction is not exactly ‘green’6.
Building from Bottom-Up
To get around having to sift through piles of soot for the perfect sheet, some researchers have turned to producing graphene using a ‘bottom-up’ approach. By constructing graphene from scratch using known building blocks, precise and pure sheets can be made.
Researchers have been able to produce graphene structures by joining small organic molecules together, using heat to assist the formation of C-C bonds7. Compared to other techniques, the relatively low temperatures used (up to 450 °C) in these reactions are more favorable for scaling up.
What’s Stopping the Graphene Revolution?
There are several products available today that incorporate this material, such as this tennis racket from Head. Samsung has also been rumored to release a graphene battery by 2021, which is exciting (watch this space)! But despite all the promise and potential surrounding graphene, it hasn’t quite been the catalyst for a technological revolution.
Unsurprisingly, the main reason behind this is money. Despite its superiority over traditional materials, the cost to benefit ratio of incorporating graphene into consumer products remains unclear. Just as in the case of drug discovery, most companies would rather stick to tried and tested methods rather than risk failure.
A handful of companies simply throw in graphene into their products for the novelty and advertising potential. Instead of fully harnessing graphene’s unique properties, they bank on extensive marketing and exaggerated news coverage to help increase sales. But they cannot be blamed entirely; part of the reason for this is the lack of reliable sources of high-quality graphene.
Challenges in Quality, not Quantity
Graphene nanoflakes are the most common graphene product on the market, mass produced using liquid-phase exfoliation techniques8. The worldwide production capacity of these nanoflakes is estimated to be over 2000 tons per year as of 2017, with this number likely much higher at present.
However, studies have found that many of these graphene nanoflakes contain less than 50% real graphene, with high levels of contamination that makes it unsuitable for many applications9. Furthermore, there are the persistent inconsistencies in thickness, size and shape of the material that further limits its usefulness.
If industrial production can improve to the point where these challenges can be addressed and rectified, such as using a grading system to guide improvements in quality and consistency, there could yet be a graphene revolution.
A Graphene-Based Future
There are other encouraging signs that industries are investing in the wonder material. Since its discovery, there has been a year-on-year, exponential increase in the number of patents filed related to graphene. Some of these detail specialized uses for graphene; DNA sequencing, filtration systems, novel touchscreens. A breakthrough in any of these areas could give graphene the commercial push it needs.
Although the hype surrounding a graphene-based future has somewhat faded over the years, there are encouraging signs that we are on the right track. Research into more efficient methods to produce graphene will help reduce costs and improve the quality of the final product, prompting more industries to attempt to harness its power. Graphene could well be the wonder material of the future, but it might take us a little while to get there.
Cover image © 2014 magicalhobo. Licensed under CC-BY.
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