Graphene Applications

Biological Engineering

Bioengineering will certainly be a field in which graphene will become a vital part of in the future; though some obstacles need to be overcome before it can be used. Current estimations suggest that it will not be until 2030 when we will begin to see graphene widely used in biological applications as we still need to understand its biocompatibility (and it must undergo numerous safety, clinical and regulatory trials which, simply put, will take a very long time). However, the properties that it displays suggest that it could revolutionise this area in a number of ways. With graphene offering a large surface area, high electrical conductivity, thinness and strength, it would make a good candidate for the development of fast and efficient bioelectric sensory devices, with the ability to monitor such things as glucose levels, haemoglobin levels, cholesterol and even DNA sequencing. Eventually we may even see engineered ‘toxic’ graphene that is able to be used as an antibiotic or even anticancer treatment. Also, due to its molecular make-up and potential biocompatibility, it could be utilised in the process of tissue regeneration.

Optical Electronics

One particular area in which we will soon begin to see graphene used on a commercial scale is that in optoelectronics; specifically touchscreens, liquid crystal displays (LCD) and organic light emitting diodes (OLEDs). For a material to be able to be used in optoelectronic applications, it must be able to transmit more than 90% of light and also offer electrical conductive properties exceeding 1 x 106 Ω1m1 and therefore low electrical resistance. Graphene is an almost completely transparent material and is able to optically transmit up to 97.7% of light. It is also highly conductive, as we have previously mentioned and so it would work very well in optoelectronic applications such as LCD touchscreens for smartphones, tablet and desktop computers and televisions.

Currently the most widely used material is indium tin oxide (ITO), and the development of manufacture of ITO over the last few decades time has resulted in a material that is able to perform very well in this application. However, recent tests have shown that graphene is potentially able to match the properties of ITO, even in current (relatively under-developed) states. Also, it has recently been shown that the optical absorption of graphene can be changed by adjusting the Fermi level. While this does not sound like much of an improvement over ITO, graphene displays additional properties which can enable very clever technology to be developed in optoelectronics by replacing the ITO with graphene. The fact that high quality graphene has a very high tensile strength, and is flexible (with a bending radius of less than the required 5-10mm for rollable e-paper), makes it almost inevitable that it will soon become utilized in these aforementioned applications.

In terms of potential real-world electronic applications we can eventually expect to see such devices as graphene based e-paper with the ability to display interactive and updatable information and flexible electronic devices including portable computers and televisions.

Ultrafiltration

Another standout property of graphene is that while it allows water to pass through it, it is almost completely impervious to liquids and gases (even relatively small helium molecules). This means that graphene could be used as an ultrafiltration medium to act as a barrier between two substances. The benefit of using graphene is that it is only 1 single atom thick and can also be developed as a barrier that electronically measures strain and pressures between the 2 substances (amongst many other variables). A team of researchers at Columbia University have managed to create monolayer graphene filters with pore sizes as small as 5nm (currently, advanced nanoporous membranes have pore sizes of 30-40nm). While these pore sizes are extremely small, as graphene is so thin, pressure during ultrafiltration is reduced. Co-currently, graphene is much stronger and less brittle than aluminium oxide (currently used in sub-100nm filtration applications). What does this mean? Well, it could mean that graphene is developed to be used in water filtration systems, desalination systems and efficient and economically more viable biofuel creation.

Composite Materials

Graphene is strong, stiff and very light. Currently, aerospace engineers are incorporating carbon fibre into the production of aircraft as it is also very strong and light. However, graphene is much stronger whilst being also much lighter. Ultimately it is expected that graphene is utilized (probably integrated into plastics such as epoxy) to create a material that can replace steel in the structure of aircraft, improving fuel efficiency, range and reducing weight. Due to its electrical conductivity, it could even be used to coat aircraft surface material to prevent electrical damage resulting from lightning strikes. In this example, the same graphene coating could also be used to measure strain rate, notifying the pilot of any changes in the stress levels that the aircraft wings are under. These characteristics can also help in the development of high strength requirement applications such as body armour for military personnel and vehicles.

Photovoltaic Cells

Offering very low levels of light absorption (at around 2.7% of white light) whilst also offering high electron mobility means that graphene can be used as an alternative to silicon or ITO in the manufacture of photovoltaic cells. Silicon is currently widely used in the production of photovoltaic cells, but while silicon cells are very expensive to produce, graphene based cells are potentially much less so. When materials such as silicon turn light into electricity it produces a photon for every electron produced, meaning that a lot of potential energy is lost as heat. Recently published research has proved that when graphene absorbs a photon, it actually generates multiple electrons. Also, while silicon is able to generate electricity from certain wavelength bands of light, graphene is able to work on all wavelengths, meaning that graphene has the potential to be as efficient as, if not more efficient than silicon, ITO or (also widely used) gallium arsenide. Being flexible and thin means that graphene based photovoltaic cells could be used in clothing; to help recharge your mobile phone, or even used as retro-fitted photovoltaic window screens or curtains to help power your home.

Energy Storage

One area of research that is being very highly studied is energy storage. While all areas of electronics have been advancing over a very fast rate over the last few decades (in reference to Moore’s law which states that the number of transistors used in electronic circuitry will double every 2 years), the problem has always been storing the energy in batteries and capacitors when it is not being used. These energy storage solutions have been developing at a much slower rate. The problem is this: a battery can potentially hold a lot of energy, but it can take a long time to charge, a capacitor, on the other hand, can be charged very quickly, but can’t hold that much energy (comparatively speaking). The solution is to develop energy storage components such as either a supercapacitor or a battery that is able to provide both of these positive characteristics without compromise.

Currently, scientists are working on enhancing the capabilities of lithium ion batteries (by incorporating graphene as an anode) to offer much higher storage capacities with much better longevity and charge rate. Also, graphene is being studied and developed to be used in the manufacture of supercapacitors which are able to be charged very quickly, yet also be able to store a large amount of electricity. Graphene based micro-supercapacitors will likely be developed for use in low energy applications such as smart phones and portable computing devices and could potentially be commercially available within the next 5-10 years. Graphene-enhanced lithium ion batteries could be used in much higher energy usage applications such as electrically powered vehicles, or they can be used as lithium ion batteries are now, in smartphones, laptops and tablet PCs but at significantly lower levels of size and weight.

What is Graphene Oxide

Today’s graphene is normally produced using mechanical or thermal exfoliation, chemical vapour deposition (CVD), and epitaxial growth. One of the most effective way of synthesised graphene on a large scale could be by the chemical reduction of graphene oxide.

Since the first report on mechanical exfoliation of monolayer graphene in 2004, interest in graphite oxide (which is produced by oxidation of graphite) has increased dramatically as people search for a cheaper, simpler, more efficient and better yielding method of producing graphene, that can be scaled up massively compared to current methods, and be financially suitable for industrial or commercial applications.

While graphite is a 3 dimensional carbon based material made up of millions of layers of graphene, graphite oxide is a little different. By the oxidation of graphite using strong oxidizing agents, oxygenated functionalities are introduced in the graphite structure which not only expand the layer separation, but also makes the material hydrophilic (meaning that they can be dispersed in water). This property enables the graphite oxide to be exfoliated in water using sonication, ultimately producing single or few layer graphene, known as graphene oxide (GO). The main difference between graphite oxide and graphene oxide is, thus, the number of layers. While graphite oxide is a multilayer system in a graphene oxide dispersion a few layers flakes and monolayer flakes can be found.

Properties of Graphene Oxide

One of the advantages of the gaphene oxide is its easy dispersability in water and other organic solvents, as well as in different matrixes, due to the presence of the oxygen functionalities. This remains as a very important property when mixing the material with ceramic or polymer matrixes when trying to improve their electrical and mechanical properties.

On the other hand, in terms of electrical conductivity, graphene oxide is often described as an electrical insulator, due to the disruption of its sp2 bonding networks. In order to recover the honeycomb hexagonal lattice, and with it the electrical conductivity, the reduction of the graphene oxide has to be achieved. It has to be taken into account that once most of the oxygen groups are removed, the reduced graphene oxide obtained is more difficult to disperse due to its tendency to create aggregates.

Functionalization of graphene oxide can fundamentally change graphene oxide’s properties. The resulting chemically modified graphenes could then potentially become much more adaptable for a lot of applications. There are many ways in which graphene oxide can be functionalized, depending on the desired application. For optoelectronics, biodevices or as a drug-delivery material, for example, it is possible to substitute amines for the organic covalent functionalization of graphene to increase the dispersability of chemically modified graphenes in organic solvents. It has also been proved that porphyrin-functionalized primary amines and fullerene-functionalized secondary amines could be attached to graphene oxide platelets, ultimately increasing nonlinear optical performance.

In order for graphene oxide to be usable as an intermediary in the creation of monolayer or few-layer graphene sheets, it is important to develop an oxidization and reduction process that is able to separate individual carbon layers and then isolate them without modifying their structure. So far, while the chemical reduction of graphene oxide is currently seen as the most suitable method of mass production of graphene, it has been difficult for scientists to complete the task of producing graphene sheets of the same quality as mechanical exfoliation, for example, but on a much larger scale. Once this issue is overcome, we can expect to see graphene become much more widely used in commercial and industrial applications.

What is Graphene?

In simple terms, graphene, is a thin layer of pure carbon; it is a single, tightly packed layer of carbon atoms that are bonded together in a hexagonal honeycomb lattice.

In more complex terms, it is an allotrope of carbon in the structure of a plane of sp2 bonded atoms with a molecule bond length of 0.142 nanometres. Layers of graphene stacked on top of each other form graphite, with an interplanar spacing of 0.335 nanometres.

It is the thinnest compound known to man at one atom thick, the lightest material known (with 1 square meter coming in at around 0.77 milligrams), the strongest compound discovered (between 100-300 times stronger than steel and with a tensile stiffness of 150,000,000 psi), the best conductor of heat at room temperature (at (4.84±0.44) × 10^3 to (5.30±0.48) × 10^3 W·m−1·K−1) and also the best conductor of electricity known (studies have shown electron mobility at values of more than 15,000 cm2·V−1·s−1). Other notable properties of graphene are its unique levels of light absorption at πα ≈ 2.3% of white light, and its potential suitability for use in spin transport.

Bearing this in mind, you might be surprised to know that carbon is the second most abundant mass within the human body and the fourth most abundant element in the universe (by mass), after hydrogen, helium and oxygen. This makes carbon the chemical basis for all known life on earth, so therefore graphene could well be an ecologically friendly, sustainable solution for an almost limitless number of applications. Since the discovery (or more accurately, the mechanical obtainment) of graphene, advancements within different scientific disciplines have exploded, with huge gains being made particularly in electronics and biotechnology already.

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