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Graphene has been hailed as a phenomenally versatile material capable of revolutionizing a wealth of technologically vital areas such as energy, materials science, and biomedical science to name just a few. The unique material possesses a range of desirable faculties such as high carrier mobility – meaning electrons move quickly through the material - optical transparency and exceptional mechanical and electronic properties.
Despite the ‘revolutionary’ claims, graphene isn’t without a few pitfalls. Graphene occurs as a densely packed, single layer of carbon atoms arranged in a hexagonal lattice, and is one million times smaller than the diameter of a single human hair. It is usually found as part of graphite and is easily obtained, as Andre Geim and Konstantin Novoselov demonstrated in 2004 when they isolated the material using sticky tape.
More often than not, when we mention graphene, we are actually referring to graphene oxide which has been reduced either chemically or thermally to yield graphene. The presence of oxygen can make the material easier to work with, but this graphene is not pure, or ‘pristine’ – it is still riddled with defects, and oxidation and reduction of the material leads to a degradation of graphene.
Pristine graphene - that is graphene in its original, pure, unoxidized form - enjoys superior properties to its oxidized counterpart, but pristine graphene isn’t easy to come by and its lack of abundance has held back the development of graphene-based functional devices.
Research has shown graphene to be a hundred times stronger than steel, and incredibly lightweight, making it ideal for a number of uses including composites and coatings, and sensor and electronics. Despite this, its thin nature means it is prone to ripping and tearing. In 2014, James Tour, a chemist at Rice University in Houston, Texas, developed a method of reinforcing graphene with carbon nanotubes, and rebar graphene was born. Akin to the reinforcement bars found in concrete, which serve to augment the concrete’s strength and durability, these nanoscale reinforcements improve the properties of graphene, making it twice as strong as pristine graphene.
Rebar graphene is made by spin-coating single-walled carbon nanotubes onto a copper substrate before growing graphene above them through chemical vapor deposition. Experiments in which the graphene underwent stress testing have illustrated that the carbon nanotube rebars divert rips and bridge cracks that would otherwise have spread in unreinforced graphene.
The nanotubes help graphene stay stretchy and pliable, and reduce the effects of cracks, properties which are desirable in flexible electronics and electrically active wearables for example. The carbon reinforcements don’t however, prevent ultimate failure; if enough force is applied to tear the graphene, it will still tear, but rather than follow a straight rip, the carbon nanotubes force the cracks to zig-zag as they propagate.
The team has also employed carbon nanotubes to reinforce graphene foam, a material which can be molded into any desired shape and support a massive 3,000 times its own weight, before bouncing back to its former height.
So, graphene might be a revolutionary material, but there is still work needed before it can truly transform certain industries. While it may be the most desirable form of graphene – possessing superior properties compared to its less pure counterparts – pristine graphene is not abundant enough for it to be considered a ‘useful’ material. Instead, we will have to make do with processing graphene oxide to a form that can be used, and augmenting that where necessary. Rebar graphene is one such example and may find uses as all-carbon transparent electrodes – which have been shown to be mechanically and chemically stable – and in flexible electronics.
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