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.
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