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A conceptual schematic model of the structure of graphene— indicating its basal and edge plane like sites; and an SEM image of a single atomic layer of graphite, known as graphene. Note, in reality the graphene utilised in the majority of work is 1–4+ layers thick. Repro- duced with permission from ref. 41 and 14 respectively. communities.7,8 Many routes for the synthesis of graphene exist, and because not one single method exists for producing graphene which is suitable for all of the potential applications it has to offer, fabrication routes are currently a heavily researched issue.2,6,9 The enormous selection of current methods of graphene synthesis include: dry mechanical exfoliation, suited ideally for the investigation of graphene’s physical properties,2,10,11 chemical exfoliation of graphite where bulk quantities of graphene sheets are created through an intercalation process utilising small molecules (such as tetrabutyl ammonium hydroxide) between the graphene layers in graphite, and consequently the separation of the graphene sheets occurs, caused by intervening atoms or molecules,2,4 the unzipping of CNTs through a variety of ways such as electrochemical, chemical, or physical methods,11 epitaxial growth of graphene,12 and the most recent and novel method of reducing sugar (such as glucose, fructose, or sucrose); this last method is leading the innovation for the cheap-large- scale production of graphene for applications in various fields.13 In reality, it is the method by which graphene is synthesised that defines its properties and consequentially its applications. The applications of electrochemistry span far a field and it is the properties of the electrode itself that are most significant to the performance obtained from these devices. It is therefore vital that research is conducted into possible new materials, and such research can possibly hold the answer to sufficient renewable energy devices and quick, cheap, sensitive and portable detection apparatus. Graphene has already captured the attention of sci- entist’s worldwide with its unique electronic, optical, mechanical, thermal and electrochemical properties that are far superior to its counterparts,2 and although still a relatively new material, liter- ature reports of graphene are rapidly appearing. In this review we highlight the importance of graphene and present an overview of the literature within the field of electro- chemistry. We first present the unique properties of graphene and compare these to other electrode materials such as CNTs, before then detailing the current advancements made in sensing covering the electro-catalysis of graphene, sensing of gaseous species, trace metal analysis, bio-sensing, and other interesting areas of sensing, after which extending into the subject areas of energy storage/generation. Technological advantages of graphene An essential characteristic of an electrode material is its surface area, which is important in applications such as energy storage, biocatalytic devices, and sensors. Graphene has a theoretical surface area of 2630 m2 g1, surpassing that of graphite (10 m2 g1), and is two times larger than that of CNTs (1315 m2 g1).2 Additionally, the electrical conductivity of graphene has been calculated to be 64 mS cm1, which is approximately 60 times more than that of SWCNTs.14,15 Furthermore, graphene’s conductivity remains stable over a vast range of temperatures ranging as low as liquid-helium temperatures4 of which is essential for reliability within many applications. More inter- estingly, graphene is distinguished from its counterparts by its unusual band structure, rendering the quasiparticles in it formally identical to the massless Dirac Fermions. A further indication of graphene’s extreme electronic quality is that it displays the half-integer quantum Hall effect, with the effective speed of light as its Fermi velocity nF z 106 m s1 which can be observed in graphene even at room temperature.16–18 In addition, the charge density of graphene can be controlled by means of a gate electrode,17 for example ultra-high electron mobility has been achieved in suspended graphene,19 where mobilities in excess of 200 000 cm2 V1 s1 at electron densities of 2 1011 cm2 were obtained by suspending a single GNS 150 nm above a Si/SiO2 gate electrode. Graphene’s quality clearly reveals itself with a pronounced ambipolar electric field effect; charge carriers can be tuned continuously between electrons and holes where electron mobility remains high even at high concentrations in both electrically and chemically doped devices, which translates to ballistic transport on the sub-micrometre scale. In comparison the mobility of an electron in silicon is around 1000 cm2 V1 s1 at the maximum—meaning the electron mobility in graphene is more than 200 times higher. This fact suggests that if graphene is used as a channel material, a transistor allowing extremely high- speed operation and with low electric power consumption could be obtained.18 Due to graphene’s unique properties it has been speculated that graphene can carry a super-current,18 and it is clear that its rates are superior to that of graphite and CNTs. The fast charge Fig. 1 This journal is a The Royal Society of Chemistry 2010 Analyst, 2010, 135, 2768–2778 | 2769 View Article Online Published on 04 October 2010. Downloaded by Manchester Metropolitan University on 18/07/2015 16:37:37.PDF Image | Graphene Electrochemistry
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